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Metal–organic framework composites Cite this: DOI: 10.1039/c3cs60472a

Qi-Long Zhu and Qiang Xu* Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), synthesized by assembling metal ions with organic ligands have recently emerged as a new class of crystalline porous materials. The amenability to design as well as fine-tunable and uniform pore structures makes them promising materials for a variety of applications. Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites/hybrids, which exhibit new properties that are superior to those of the individual components through the collective behavior of the

Received 24th December 2013 DOI: 10.1039/c3cs60472a

functional units. This is a rapidly developing interdisciplinary research area. This review provides an overview of the significant advances in the development of diverse MOF composites reported till now with special emphases on the synergistic effects and applications of the composites. The most widely

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used and successful strategies for composite synthesis are also presented.

National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, 563-8577, Japan. E-mail: [email protected]; Fax: +81-72-751-9629; Tel: +81-72-751-9562

metal ions or clusters of metal ions, their structures can be designed according to targeted properties.8–10 A key structural feature of MOFs is the ultrahigh porosity (up to 90% free volume) and incredibly high internal surface areas, extending beyond a Langmuir surface area of 10 000 m2 g1,11–13 which play a crucial role in functional applications, typically in storage and separation,14–17 sensing,18–21 proton conduction22–27 and drug delivery.28–30 Generally, porous MOFs show microporous characters (o2 nm) whereas the pore sizes could be tuned from several angstroms to several nanometers by typically controlling the length of the bi- or multipodal rigid organic linkers. In addition, versatile framework functionalities beyond their accessible porosity can

Qi-Long Zhu was born in Fujian, P. R. China in 1984. He received his PhD degree from the Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Sciences (CAS) in 2012 under the supervision of Prof. Xin-Tao Wu. He then joined Prof. Qiang Xu’s group at the National Institute of Advanced Industrial Science and Technology (AIST, Japan) as a postdoctoral fellow and became Qi-Long Zhu a JSPS (Japan Society for the Promotion of Science) fellow in 2013. His research currently focuses on the development of nanostructured materials and MOF composites for applications, especially in catalysis and energy storage.

Qiang Xu received his PhD degree in Physical Chemistry in 1994 from Osaka University, Japan. After one year as a postdoctoral fellow at Osaka University, he started his career as a Research Scientist in Osaka National Research Institute in 1995. Currently, he is a Chief Senior Researcher at the National Institute of Advanced Industrial Science and Technology (AIST, Japan) and adjunct professor at Qiang Xu Kobe University. He received the Thomson Reuters Research Front Award in 2012. His research interests include porous and nanostructured materials and related functional applications, especially for clean energy. He has published more than 280 papers in refereed journals.

1. Introduction Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), which are emerging as a class of very promising crystalline microporous materials promoted by the use of a set of well-established principles of coordination chemistry, have received great interest.1–7 Based on the geometries of the organic linkers and coordination modes of the inorganic

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arise from the metal components (e.g. magnetism, catalysis), organic linkers (e.g. luminescence, nonlinear optics (NLO), chirality) or a combination of both.31–36 A great deal of research efforts during the past two decades have mostly been aimed at preparing new MOF structures and exploring their various applications,37–43 and selected MOFs are now commercialized. Nevertheless, MOFs exhibit a few weak points such as poor chemical stability that impede the use of their full potential. In order to satisfy the realistic applications of MOFs, it is desirable to further enhance the properties and introduce new functionalities. Fortunately, combining MOFs with a variety of functional materials has been proposed recently to combine the merits and mitigate the shortcomings of both the components.44–47 The research on MOF composites provides fabrication protocols for high-performance composites with sophisticated architectures. MOF composites/hybrids are materials composed of one MOF and one or more distinct constituent materials, including other MOFs, with properties noticeably different from those of the individual components. In composite materials, the advantages of both MOFs (structural adaptivity and flexibility, high porosity with ordered crystalline pores) and various kinds of functional materials (unique optical, electrical, magnetic and catalytic properties) can be combined effectively, and therefore, new physical and chemical properties and enhanced performance that are not attainable by the individual components may be accessed.48–50 Consequently, the remarkable features of composites resulting from the synergistic combination of both MOF and other active components make them suitable for a wide range of applications. The choice of the appropriate MOF can be achieved by taking advantage of the existing library of porous crystals; alternatively, simulation tools can be used as an efficient screening method.51–53 To date, MOF composites have been successfully made with active species, including metal nanoparticles/nanorods (NPs/NRs), oxides, quantum dots (QDs), polyoxometalates (POMs), polymers, graphene, carbon nanotubes (CNTs), biomolecules and so on, resulting in a performance unattainable by the individual constituents (Scheme 1).54–61 Moreover, they offer the great advantage of a flexible design, that it, one can tailor-make the materials as per the specifications of optimum design. Through their perfect mastery of compositional component, porosity, functionality

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and morphology, each MOF-based composite represents a new material with specific functional properties. With these additional advantages from active materials, it is desirable to harness the useful properties through the incorporation of various kinds of functional materials into MOFs. This new research field provides chemists with opportunities for creative expression, and the remarkable new properties and multifunctional nature of the emerging MOF composites will stimulate the emergence of innovative industrial applications in a diverse range of technologically important fields. These composites may be used directly as innovative advanced materials or as precursors of novel inorganic solids, providing promising applications in functional and protective coatings, storage and separation, heterogeneous catalysis, sensing and biology.30,48,62,63 This paper reviews the recent significant progress in the development of MOF composites, classified on the basis of the kinds of composites. The components, structures and properties of composites are discussed, with particular emphasis placed on the synergistic effects of the composites on their performance in various applications. We sincerely hope that this review will inspire the interest and enthusiasm of chemists in the investigation of MOF composites, who are advised to further read the cited articles.

2. MOF–metal nanoparticle composites Metal nanoparticles (MNPs) have been extensively investigated because of their unique physiochemical properties different from those of their bulk counterparts and because of their wide range of potential and actual applications.64–66 However, it is well known that free MNPs have high surface energies and tend to aggregate and fuse; as a result the intriguing properties observed for the MNPs disappear and difficulties arise for longterm storage, processing and application. The encapsulation of metal nanoclusters/nanoparticles in systems with confined void space, for example, in mesoporous and microporous solids, including metal oxides, zeolites, mesoporous silica and activated carbons, appears to be an efficient way of preventing aggregation.67 Porous MOFs are thermally robust and have permanent nanoscale cavities or open channels. Given the similarity to zeolites, MOFs can be utilized as supports for metal nanoparticles with controlled sizes inside the pores, thereby circumventing the common issue of nanoparticle aggregation and benefiting their utilization for applications such as catalysis, since MOFs provide powerful confinement effects.68 2.1

Scheme 1

The composites of MOFs and functional materials.

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Methodologies

As in the case for other supports, there are two main approaches for the immobilization of MNPs in MOFs. The first and most widely used approach, known as ‘‘ship in bottle’’ approach, involves embedding MNPs in a MOF matrix, entailing the impregnation of metal precursors using various techniques such as solution infiltration, vapour deposition and solid grinding for introducing the metal precursors, followed by reduction of


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metal precursors to metal atoms. Another approach, also known as ‘‘bottle around ship’’ or template synthesis approach, involves the synthesis of MNPs individually first and subsequent addition of suitable chemicals to assemble the MOF around the MNPs. Although great efforts have been made in this field, general and facile methods that can easily introduce the metal precursors inside the pores of the MOF and control the nucleation and growth of MNPs with high uniformity and small particle size are still imperative. 2.1.1 Solution infiltration method. This method is mostly used to prepare MNPs immobilized in MOFs, in which a solution of the metal precursors, usually in the form of common inorganic salts, is adoptable. When the desolvated porous MOFs are soaked in the metal precursor solution, the precursors are infiltrated into the pores of MOFs by capillary force, which is followed by reduction with reducing agents, typically H2 gas and NaBH4, to obtain MNPs deposited in MOFs. By impregnating a MOF, MIL-100(Al) ([Cr3F(H2O)3O(btc)2], btc = 1,3,5-benzenetricarboxylate), with the tetrachloropalladinic acid solution, Zlotea et al. synthesized dispersed Pd NPs embedded in the MOF with a mean size of 2.5 nm.69 The framework of MIL-100(Al) has two types of large cavities (internal diameters: 2.1 and 2.5 nm) with a smaller window opening size of 8.7 Å. The corresponding Pd/MIL-100(Al) composite showed a high metal content of 10 wt% without degradation of the porous host. The appreciable decreases in the BET surface area and the total pore volume of the composite indicated that insertion of Pd NPs altered the pore structure of MIL-100(Al). Our group reported the stabilization of core–shell structured bimetallic NPs on a zeolitic imidazolate framework, ZIF-8 (Zn(MeIM)2, MeIM = 2-methylimidazole), by a facile successive deposition– reduction method.70 The desolvated ZIF-8 was sequentially immersed in aqueous solutions of Au and Ag precursors, with respective reduction and drying to yield Au@Ag core–shell NPs on the MOF matrix, while the reversed deposition sequence of Ag and Au yielded Au@AuAg NPs attributed to a galvanic replacement reaction. The particles of the samples have similar sizes of 2–6 nm, indicating some aggregation. A brighter core coated with a darker shell in each particle observed in HAADFSTEM images and the corresponding EDS results of both line scan and point analyses unambiguously demonstrated the Au-rich core and the Ag-rich shell. In order to facilitate the infiltration of the metal precursors, it is necessary to improve the interactions between the metal precursors and pores or channels of MOF supports. Fortunately, the existence of coordinatively unsaturated metal sites (CUSs) in some MOFs provides the possibility that the pores or channels can be chemically functionalized.71–73 Through grafting of amine molecules onto coordinatively unsaturated sites of the dehydrated MOF, the amine-grafted MIL-101 ([Cr3F(H2O)2O(bdc)3], bdc = 1,4-benzene dicarboxylate) was obtained, which can be used to encapsulate noble metals with the help of the ionic reactions between the positively charged surface ammonium groups and anionic noble metal salts ([PdCl4]2, [PtCl6]2 and [AuCl4]) (Fig. 1).74 Using a similar strategy, bimetallic Au–Pd NPs were successfully immobilized in the ethylenediamine (ED)-grafted MIL-101.75


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Fig. 1 Site-selective functionalization of MIL-101 with unsaturated metal sites: (a) perspective view of the mesoporous cage of MIL-101 with hexagonal windows; (b) evolution of coordinatively unsaturated sites from chromium trimers in mesoporous cages of MIL-101 after vacuum treatment; (c) surface functionalization of the dehydrated MIL-101 through selective grafting of amine molecules onto coordinatively unsaturated sites; (d) selective encapsulation of noble metals in the amine-grafted MIL-101. Reprinted with permission from ref. 74.

One of the disadvantages of this simple liquid impregnation method is the poor control of the loading amount of the metal. To circumvent this drawback, the incipient wetness method, one of the impregnation techniques, has been proposed, in which the solution of metal precursors, whose volume is the same as the total pore volume of the MOF support, is used.76,77 When the defined amount of solution is added to the guest-free solid support, capillary force leads to the incorporation of the solution into the pores. The successive reduction reaction of the metal precursors generates MNPs in the support. In this method, the loading amounts of the metal are feasibly controllable by varying the concentrations of the precursor solution.78 Sabo et al. introduced this method to the preparation of 1 wt% Pd@MOF-5 (MOF-5 = [Zn4O(bdc)3]) by using a solution of [Pd(acac)2] (acac = acetylacetonate) in CHCl3 and the subsequent thermal hydrogenolysis of the intermediate [Pd(acac)2]x@MOF-5 (x = 0.074) material.76 The total volume of chloroform solution of [Pd(acac)2] was calculated from the pore volume of the desolvated MOF (1.18 cc g1). However, details on the Pd particle size and the distribution over the matrix were not given by the authors. In a recent study, our group presented mono- and bimetallic polyhedral metal nanocrystals (MNCs) immobilized in MIL-101 by the incipient wetness method in combination with a CO-directed reduction at the solid–gas interface.79 When the metal acetylacetonate, M(acac)2 (M = Pt, Pd or Pt/Pd), which was impregnated into MIL-101 by the incipient wetness method, was reduced by using a CO–H2–He mixture as the gas-phase reducing agent, cubic Pt, tetrahedral Pd and octahedral PtPd NCs deposited onto MIL-101 with average sizes of 8.0, 8.5 and 10.5 nm, respectively, were surprisingly formed (Fig. 2). The formation of Pt and Pd polyhedra was directed by preferential binding of CO on their (100) and (111) facets. Meanwhile, because of the higher adsorption enthalpy of CO on Pt than on Pd in the CO-directed reduction process, PtPd bimetallic

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Fig. 2 (a) Schematic illustration of the formation of cubic Pt, tetrahedral Pd, and octahedral PtPd NCs on the MIL-101 support in the presence of CO and H2. STEM images of (b) Pt/MIL-101, (c, d) Pd/MIL-101, and (e) PtPd/MIL-101. The inset in (e) shows a single octahedron. Reprinted with permission from ref. 79.

nanocrystals showed metal segregation, leading to a Pd-rich core and a Pt-rich shell. Nevertheless, the use of H2–He in place of CO–H2–He under the same reduction conditions only resulted in small spherical Pt NPs, suggesting the importance of the reduction method. MOFs have been utilized as supports for MNPs since they provide a powerful confinement effect to limit the growth of MNPs; however, the precursor compounds and products can actually deposit on the external surface of MOFs to form the MNPs with aggregation on the external surface of MOFs. Recently, we developed a double solvent method (DSM) for avoiding MNP aggregation on the external surface of MOFs.80–82 This method is based on a hydrophilic solvent (water) and a hydrophobic solvent (hexane). The former containing the metal precursors with a volume set equal to or slightly less than the pore volume of the adsorbent can be absorbed within the hydrophilic adsorbent pores, while the latter, in a large amount, plays an important role in suspending the adsorbent and facilitating the impregnation process. Since the inner surface area of a MOF is much larger than the outer surface area, the small amount of aqueous precursor solution can go inside the hydrophilic pore, and the deposition of metal precursors on the outer surface can be greatly minimized. By using this method combined with the hydrogen reduction at a relatively low temperature of 200 1C, ultrafine Pt, Pd, Au and Rh NPs were successfully immobilized inside the pores of MIL-101 without aggregation on the external surface of the framework.80,83 In addition, various amounts of Pt NPs were loaded in MIL-101 by varying the concentration of the Pt precursor. However, the hydrogen reduction method is

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not suited for the preparation of non-noble metal-based NPs because of contradictions between the high reduction temperatures of non-noble metals and the low thermal stability of MOFs. To solve this problem, very recently, we exploited an overwhelming reduction (OWR) approach with a high-concentration NaBH4 solution combined with DSM to introduce non-noble metal-based AuNi NPs into the pores of MIL-101, which was based on the assumption that when the metal precursors deposited in the pores of MOF can be reduced completely by a pore-volume amount of NaBH4 solution that can be incorporated into the pores by capillary force, the aggregation of MNPs on the external surface will be avoided to a great extent.81 Otherwise, when a low-concentration NaBH4 solution was used, the reduction of the precursor inside the pores cannot be completed, and a part of the precursor would re-dissolve and diffuse out of the pores, resulting in the aggregation of MNPs on the outer surface of MOF (Fig. 3). TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine AuNi NPs with an average particle size of 1.8 nm throughout the interior cavities of MIL-101. Contrary to the conventional methods, Suh’s group developed a particular fabrication method for MNPs by using the redox active MOFs, which can avoid using reducing agents.84–87 They used redox active MOFs, which are constructed from the redox active organic ligand or metal building blocks such as Ni(II) macrocyclic complexes, as supports for MNPs and immersed them in the solutions of noble metal salts such as AgNO3, NaAuCl4 and Pd(NO3)2. As soon as metal ions were diffused into the pores of the redox active MOFs, they can be reduced to metallic nanoparticles and redox active building blocks in the MOFs are oxidized. The amount of MNPs loaded in MOFs can be controlled by the immersion time of the host solid in the

Fig. 3 (a) Schematic representation of immobilization of the AuNi nanoparticles by the MIL-101 matrix using the double solvent method (DSM) combined with a liquid-phase concentration-controlled reduction strategy. TEM images of AuNi@MIL-101 obtained by reduction using NaBH4 solutions of (b) 0.6, (c) 0.4 and (d) 0.2 M, respectively. Reprinted with permission from ref. 81.


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Fig. 4 Reaction of [(NiL1)3(bpdc)3]2pyridine6H2O (1) incorporating the redox active Ni(II) square-planar macrocyclic complex with Ag+ ions to generate Ag NPs. Reprinted with permission from ref. 84.

precursor solution and the maximum amount of MNPs formed can be predicted according to the stoichiometric redox reactions between the redox active networks and metal ions. They first employed a redox active Ni(II) square-planar macrocyclic complex, [Ni(C10H26N6)]2+ (NiL12+), to construct a redox active MOF, [(NiL1)3(bpdc)3]2pyridine6H2O (1, bpdc = biphenyldicarboxylate). 1 immersed in AgNO3 methanolic solution at room temperature afforded the Ag/MOF nanocomposite with Ag NPs of size B3 nm (Fig. 4).84 Besides, Au and Pd NPs were also fabricated in other redox active Ni-MOFs by using the same method.85–87 Moreover, with a similar strategy, they also prepared a redox active [Zn3(ntb)2(EtOH)2]4H2O (2) with Zn(NO3)2 and a redox active organic ligand, 4,40 ,400 -nitrilotrisbenzoate (ntb3), which can be readily oxidized to the amine radical.88 Pd NPs of size 3.0 0.4 nm were formed in the channels (aperture size 7.7 Å) of the MOF after immersing the MOF in the acetonitrile solution of Pd(NO3)2 for 30 min. The EPR spectrum indicated that the MOF was oxidized to the positively charged network with the nitrogen radical. The deposition of MNPs inside the MOF can also be achieved by the photochemical process. In early 2012, Lin and co-workers synthesized photoactive MOFs, which were constructed from linear [Ir(ppy)2(bpy)]+-derived dicarboxylic acids H2L1 (ppy = 2-phenyl-pyridine; bpy = 2,2 0 -bipyridine; L12 = 2,2 0 -bipyridine5,50 -dicarboxylate) and H2L2 (L22 = 2,20 -bipyridine-5,50 -dibenzoate) and the Zr6(m3-O)4(m3-OH)4(carboxylate)12 secondary building units (SBUs) and featured a network the same as that of UiO-66 ([Zr6O4(OH)4(bdc)6]).89,90 The resultant MOFs, Zr6(m3-O)4(m3-OH)4(bpdc)5.94(L1)0.06 (3) and Zr6(m3-O)4(m3-OH)4(L2)664DMF (4), have the ability to produce Pd NPs by triethylamine (TEA)-mediated photo-reduction. The precursor K2PtCl4 was loaded by mixing with MOF powder in a mixed solvent of THF–TEA–H2O under N2 bubbling. Under irradiation with visible light from a Xe-lamp with a 420 nm cut-off filter, TEA can reductively quench the photoexcited [Ir(ppy)2(bpy)]+* to generate the reduced radical [Ir(ppy)2(bpy )], which can reduce K2PtCl4 to form Pt NPs within the host framework. The other solution methods, such as colloidal deposition with a surfactant91,92 and urea deposition-precipitation,93 have also been developed to obtain MNP–MOF composites. However, these methods usually resulted in the formation/aggregation of MNPs on the surface of MOF crystals. 2.1.2 Gas phase infiltration method. The incorporation of noble MNPs within porous MOFs by means of metal organic chemical vapor deposition (MOCVD) initially developed by Fischer and co-workers is an effective way, with which a series of MNP@MOF nanocomposites have been prepared since 2005.78,94–99 In this


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technique, a desolvated porous MOF is exposed to the vapor of a volatile metal precursor in a tightly sealed Schlenk tube under a static vacuum, and then the precursor is introduced into the pores of the MOF at an appropriate temperature depending on the vapor pressure of the metal precursor under a static vacuum. Metal NPs embedded in the MOF are formed after being treated by hydrogen reduction or simple thermal decomposition. Fischer and co-workers firstly employed MOF-5 as a host framework and acquired the nanocomposite systems, MNP@MOFs (M = Pd, Au and Cu), with the MNPs primarily embedded inside the MOF pores by hydrogenolysis of the embedded precursors ((Z3-C3H5)Pd(Z5-C5H5), (CH3)Au(PMe3) and (Z5-C5H5)Cu(PMe3) for Pd, Au and Cu, respectively) (Fig. 5).94 Then, Ru NPs with a size range of 1.5–1.7 nm encapsulated in intact MOF-5 frameworks were also successfully obtained via gas phase loading of MOF-5 with the volatile compound Ru(cod)(cot) (cod = 1,5-cyclooctadiene, cot = 1,3,5-cyclooctatriene) followed by hydrolysis of the Ru compound inside the pores.96 However, the MOF-5 is particularly labile when traces of water cannot be rigorously excluded and this property possibly limits its applicability as a support.100 In addition, the window opening of the cavities with 7.8 Å rules out a number of MOCVD precursors with larger dimensions for inclusion and subsequent MNPs inside MOF-5.94 Thus, they then shifted their studies from MOF-5 to highly porous MOF-177 ([Zn4O(btb)2], btb = 1,3,5-benzenetribenzoate),95 which is less sensitive to traces of water and possesses a wider window of the pores accessible for larger metal organic precursors.101 A series of volatile precursors were loaded to desolvated MOF-177 via the gas phase loading method, and as representative examples, the inclusion compounds [(Z3-C3H5)Pd(Z5-C5H5)]10@MOF-177 and [(Z5-C5H5)CuL]2@MOF-177 (L = PMe3, CNtBu) were converted into the composite materials Pd@MOF-177 and Cu@MOF-177 by photolysis and thermal hydrogenolysis, respectively.95 Detailed characterizations show that the Pd and Cu NPs are around 3 nm and the MOF-177 matrix remained unchanged during the NP formation. Recently, they chose chemically robust ZIF-8 (Zn(MeIM)2, IM = imidazolate) and ZIF-90 (Zn(ICA), ICA = imidazolate-2-carboxyaldehyde) as host matrices to load Au NPs via gas phase infiltration of Au(CO)Cl followed by hydrogen reduction.99

Fig. 5 MOF-5 cage (blue/yellow) with four incorporated (Z3-C3H5)Pd(Z5-C5H5) precursors (red). The elemental cell of the crystalline MOF-5 contains eight cavities of this kind. Reprinted with permission from ref. 94.

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Fig. 7 Au/CPL-2 nanocomposite via solid grinding of CPL-2 with Me2Au(acac) followed by H2 reduction. Reprinted with permission from ref. 108. Fig. 6 Schematic illustration of preparation of Ni@ZIF-8 from ZIF-8 via gas phase loading of nickelocene followed by hydrogen reduction. Reprinted with permission from ref. 102.

Low-dose HRTEM and electron tomography revealed a homogeneous distribution of Au NPs throughout the ZIF matrix. The functional groups of ZIF-90 directed the anchoring of intermediate Au species and stabilized drastically smaller and quite monodispersed Au NPs in contrast to the unfunctionalized ZIF-8. In addition, our group prepared Ni@ZIF-8 by introducing nickelocene (Ni(cp)2) into evacuated ZIF-8 via gas phase loading followed by hydrogen reduction (Fig. 6).102 The mean diameter of Ni NPs was 2.7 0.7 nm. Similarly, Kim and co-workers have obtained Ni@mesoMOF (mesoMOF = Tb16(TATB)16, TATB = triazine-1,3,5-tribenzoate) with a size range from 1.4 (Ni147) to 1.9 (Ni309) nm by using the gas phase infiltration method.103 Kempe and co-workers succeeded in the syntheses of Pt@MOF177 with 2–5 nm Pt NPs embedded in the unchanged MOF-177 framework via gas phase loading of Me3PtCp0 (Cp0 = methylcyclopentadienyl) and subsequent hydrogenolysis.104 To address the instability of the MOF-177 host to the presence of water generated in the catalytic reactions, they switched to the relatively water stable MIL-101 and prepared Pd@MIL-101 by loading MIL-101 with a Pd precursor compound (Z3-C3H5)Pd(Z5-C5H5).105 Loadings higher than 50 wt% could be accomplished. In particular, a clear correlation between the reduction protocol and the resultant particle size can be observed. Then, they synthesized the PdxNiy@ MIL-101 systems with an exact adjustment of the Pd to Ni ratio by applying the simultaneous loadings of (Z3-C3H5)Pd(Z5-C5H5) and Ni(Z5-C5H5)2.106 Larger particles at the outer surface of the MIL101 crystals were observed at high Ni content. Fortunately, this problem can be solved by increasing the temperature and reducing the pressure of H2 during reduction and cavity-conform bimetallic NPs were generated under optimized conditions. Very recently, Suh’s group reported for the first time Mg nanocrystals (NCs) in a MOF.107 The vapor of bis(cyclopentadienyl) magnesium (MgCp2) was deposited in SNU-90 0 ([Zn4O(atb)2], atb = aniline2,4,6-tribenzoate) at 80 1C by the gas phase infiltration method, and the resulted MgCp2@SNU-90 0 was thermally decomposed at 200 1C under an argon atmosphere, which gave rise to hexagonaldisk shaped Mg NCs embedded in SNU-90 0 . The hexagonal Mg nanodisks had a diagonal length of 44–88 nm with a thickness of 16–61 nm. The structure of SNU-90 0 was maintained even after the formation of Mg NCs. 2.1.3 Solvent-free solid grinding method. The solvent-free solid grinding of MOFs with volatile organometallic precursors,

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which have a certain degree of vapor pressure at room temperature, is a facile but surprisingly effective technique for depositing MNPs onto MOFs. During grinding, the sublimated vapor of the volatile precursor readily infiltrates into the cavities of a MOF, leading to the uniform distribution of the precursor molecules. The metal NPs embedded within the MOF are obtained by treating the adsorbed species with H2 gas at a mild temperature. Till now, only the volatile organogold complex, dimethyl Au(III) acetylacetonate (Me2Au(acac)), has been studied as an Au precursor. Haruta and co-workers deposited gold clusters on several porous MOFs with 1-D channels, CPL-1 ([Cu2(pzdc)2(pyz)]n, pzdc = pyrazine-2,3-dicarboxylate, pyz = pyrazine), CPL-2 ([Cu2(pzdc)2(bpy)]n, bpy = 4,4 0 -bipyridine), MIL-53(Al) ([Al(OH)(bdc)]), MOF-5 and HKUST-1 ([Cu3(btc)2(H2O)3]), by a solid grinding method followed by H2 reduction.108–110 Au/CPL-2 gave small Au NPs in a nearly uniform cluster size of 2.2 0.3 nm (Fig. 7). However, the CVD method yielded larger Au NPs with particle size of 3.1 1.9 nm. In addition, they found that the size of Au NPs strongly depended on the kind of MOFs with different structures, pore sizes and surface nature, and MIL-53(Al) could support Au clusters with the smallest size of 1.5 0.7 nm.108 We prepared Au@ZIF-8 nanocomposites with various Au loadings via solid grinding of Me2Au(acac) and ZIF-8.111 The sizes of Au NPs showed a slight increase from 1 wt% (3.4 1.4 nm) to 5 wt% (4.2 2.6) Au loadings. More recently, we prepared a 3-D porous MOF, [Cd2(L)(H2O)0.5H2O] (5, L = 4,40 -(hexafluoroisopropylidene)di-phthalate), with hydrophobic and hydrophilic channels (7.8 and 6.3 Å, respectively).112 By the facile solid grinding method, subnano Au clusters were incorporated into the porous structure, which were unobservable by TEM and HAADF-STEM. Nevertheless, EDS unambiguously showed Au signals and XAFS (X-ray absorption fine structure) data analyses demonstrated the presence of Au clusters with an average atom number of 2.5.112 2.1.4 Template synthesis method. This method involves coating the MNPs with a MOF by introducing pre-synthesized MNPs into a synthetic solution containing molecular building blocks of MOF. Usually, pre-synthesized MNPs are stabilized by surfactants, capping agents or even ions. The NPs do not occupy the cavities of the MOF, but instead are surrounded by grown MOF materials. By using this method, the general problems of the aggregation of MNPs on the external surface and the damage to the MOF nanostructures during the post-reduction process can be restricted. In addition, the size, composition and morphology of incorporated MNPs can be easily controlled. However, controllable growth of MOFs on the surface of MNPs rather than self-nucleation


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Fig. 8 Scheme of the controlled encapsulation of nanoparticles in ZIF-8 crystals. Through surface modification with surfactant PVP, nanoparticles of various sizes, shapes and compositions can be encapsulated in a welldispersed fashion in ZIF-8 crystals, which are formed by assembling zinc ions with imidazolate ligands. Reprinted with permission from ref. 113.

in solution is the biggest challenge due to possible lattice mismatch between the MOFs and MNPs. Hupp, Huo and co-workers reported a controlled encapsulation strategy that enabled polyvinylpyrrolidone (PVP)-capped nanoparticles of various sizes, shapes and compositions to be enshrouded by the framework of ZIF-8.113 After mixing PVP-capped Au NPs with methanolic solutions of zinc nitrate and 2-methylimidazole at room temperature, the pink solid Au@ZIF was collected by centrifugation. The encapsulation process relies on the successive adsorption of PVP-modified NPs on the continuously forming fresh surfaces of the growing ZIF-8 spheres until the particles are depleted. The spatial distribution of incorporated NPs within ZIF-8 crystals can also be controlled by their addition sequence. Spatial distributions as a single type of nanoparticles in the central areas or off the central areas of the MOF crystals, and as two types of nanoparticles in the central areas or one type in the central area but the other type in the transition layers of the MOF crystals have been achieved (Fig. 8). The composite assembly strategy was successfully extended to other PVP-capped nanostructured objects such as Pt, CdTe, Fe3O4 and lanthanide-doped NaYF4 NPs, Ag cubes, polystyrene spheres, b-FeOOH rods and lanthanide-doped NaYF4 rods. Sada and co-workers reported the fabrication of composite crystals of a MOF, [Zn4O(bpdc)3] (6), embedding Au nanorods (NRs) by heating the mixture of 11-mercaptoundecanoic acid (MUA)-capped AuNRs, bpdc, and Zn(NO3)26H2O in N,N-diethylformamide (DEF) at 80 1C for 12 h.114 The resultant purple cubic crystals indicate that AuNRs were embedded homogeneously in the MOF crystals. TEM observations of AuNR@6 nanocomposites showed partial aggregation of the embedded AuNRs. Tsuruoka et al. synthesized the Au@HKUST-1 with a similar thermal approach, where the Au NPs were localized around the center of the crystals as confirmed by HRTEM and XPS.115 Zhu, Qiu and co-workers fabricated an Au@MIL-101(Fe) core–shell composite with controllable MIL-101(Fe) shell thickness by using a versatile stepwise layer-by-layer (LBL) method.116 The composite was prepared by repeated cycles of dispersing the mercaptoacetic acid (MAA)-functionalized Au NPs into the


Review Article

ethanol solution of FeCl3 and H3btc separately at 70 1C. The MOF shell thickness can be rationally controlled by altering the number of assembly cycles. Extensive efforts have been devoted to the fabrication of MOF composites with dispersed MNPs; nevertheless, the synthesis of core–shell metal@MOF NPs with a single metal NP core coated with a uniform MOF shell still presents significant challenges. He et al. reported the core– shell Au@MOF-5 NPs with a single Au NP core coated with a uniform MOF-5 shell.62 Different from the conventional twostep method to synthesize MOF-MNP composites, the core– shell Au@MOF-5 NPs were prepared by directly mixing both the Au and MOF precursors (HAuCl4, Zn(NO3)26H2O, and H2bdc) in the reaction solution containing DMF, PVP and ethanol. It was found that HAuCl4 was first reduced to Au NPs by DMF within a short time; subsequently, MOF-5 formed and spontaneously grew on the surface of the PVP-capped Au NPs. By simply changing the added amount of the Au precursor in the reaction solution, the shell thickness can be controlled. Khaletskaya et al. achieved a delicate core–shell composite of AuNR@Al(OH)(1,4-ndc) (7, 1,4-ndc = naphthalenedicarboxylate) composed of an individual AuNR core and an MOF shell.47 To ensure an accurate localization of the MOF crystal nucleation onto Au NRs, the Au NRs were first coated with a hydrated amorphous alumina layer that acted as a localized aluminium source. Then the Al-modified NRs were used as reactive seeds. Dissolution of the amorphous alumina coating during a microwave treatment in the presence of 1,4-ndc promoted the nucleation of 7 on the surface of the Au NRs. It is effective to assemble MOFs on MNPs by using cappingagent-stabilized MNPs. However, the capping agents, which are difficult to remove from the composites, may have a negative impact on the performance of the MNPs. Thus, it is imperative to avoid using additional capping agents during the template synthesis process. Wang et al. reported the incorporation of highly dispersed uncapped Pt NPs within the nanostructures of ZIF-8 by using the 2-methylimidazole-stabilized noble Pt NPs.117 The imidazole used as the organic linker of the ZIF-8 framework also acted as an efficient stabilizing agent for metal NPs. Addition of a methanol solution of zinc nitrate into the methanol solution of Pt NPs and 2-methylimidazole at room temperature led to the hetero-nucleation of ZIF-8 on the Pt NPs. HRTEM images of the resultant Pt@ZIF-8 clearly showed the uniform distribution of Pt NPs with a size of B2.0 nm in the network of ZIF-8. Recently, yolk–shell nanomaterials, which represent a new class of special core–shell structures with a distinctive core@void@shell configuration, generally denoted as A@B, have attracted a great deal of attention from scientists due to their appealing structures and tunable physical and chemical properties.118,119 Tsung and co-workers firstly introduced MOF into the yolk–shell nanostructures.120 They developed a new synthetic strategy in which metal nanocrystals (NCs) were first coated with a layer of Cu2O as the sacrificial template and then a layer of polycrystalline ZIF-8. The clean Cu2O surface assisted in the formation of the ZIF-8 coating layer and was etched off spontaneously and simultaneously by the protons generated during the formation of ZIF-8. With the strategy, nanocrystal@ZIF-8 yolk–shell nanocomposites with different metal NC cores have

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Fig. 9 Metal nanocrystal@ZIF-8 yolk–shell nanostructures. (a) SEM and (b, c) TEM images, and (d) schematic sketch of the yolk–shell nanostructure. Reprinted with permission from ref. 120.

been constructed. TEM results of Pd octahedra@ZIF-8 showed that the morphology of the Pd NCs was preserved after the coating and the ZIF-8 shell was polycrystalline with a thickness of B100 nm (Fig. 9). 2.2

Functional applications

MNPs possess distinctly different physicochemical properties from their bulk counterparts due to the large surface to volume ratio, making them particularly attractive candidates for many applications. Depending on the nature of the metal and MOF supports, MNP@MOF composites have been extensively applied in hydrogen storage, heterogeneous catalysis and sensing by taking advantage of the growth limitation of MNPs in MOF matrices as well as the large surface areas and size selectivity of MOFs. 2.2.1 Hydrogen storage. Hydrogen, which is considered as an environmentally attractive energy carrier, will enable a secure and clean energy future. Controlled storage and release of hydrogen in a safe and efficient way remain one of the most difficult challenges. Porous MOFs have attracted great attention as potential hydrogen storage materials because of their high porosity and internal surface areas, and some MOFs exhibit high hydrogen storage capacities (47 wt%) at 77 K and high pressure.121,122 However, at room temperature, the hydrogen uptake capacities of MOFs are less than 1 wt% due to the low interaction energies (typically 3–10 kJ mol1) between the frameworks and H2.122 Recently, the spillover mechanism123,124 proposed by Yang and co-workers has been adopted to prepare MNP-doped MOFs for enhanced hydrogen storage application. There have been reports that MOF-5 and 2 impregnated with Pd led to an increase of reversible hydrogen storage from 1.15 to 1.86 wt% and 1.03 to 1.48%, respectively, at 1 bar and 77 K.76,88 The enhanced storage capacity was attributed to the spillover effect of Pd NPs and the MOFs acting as a spillover receptor.76 Zlotea et al. modified MIL-100(Al) with B2.0 nm Pd NPs, resulting in significant changes in gas sorption properties.69

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The loss of excess hydrogen storage at low temperature can be correlated with the decrease of the specific surface area and pore volume after Pd impregnation. However, at room temperature, the hydrogen uptake in the composite Pd/MIL-100(Al) was almost twice that of the pristine material, which was partially accounted by Pd hydride formation and the spillover mechanism. The room-temperature hydrogen adsorption of Pt@MOF-177 was investigated in a gravimetric fashion (magnetic suspension balance), which showed almost 2.5 wt% in the first cycle at 25 1C and 144 bar whereas it sharply decreased down to 0.5 wt% in consecutive cycles.104 The hydrogen uptake was not completely reversible, but it became completely reversible in the second and third cycles after the drop in hydrogen uptake. The authors proposed that the loss in uptake was attributed to the formation of palladium hydrides that were not desorbed at room temperature. Alternatively, doping the MOF with Mg NPs can substantially enhance H2 uptake with both physi- and chemi-sorption. Recently, Suh’s group reported MgNC@SNU-90 0 as a hybrid hydrogen storage material.107 This composite material had both physisorption at low temperature and chemisorption at high temperature, and exhibited synergistic effects to increase the isosteric heat of H2 physisorption and to decrease the temperatures for chemisorption/desorption of H2. The zero-coverage isosteric heat of H2 adsorption increased as the amount of Mg increased, up to 11.6 kJ mol1 for Mg(10.5 wt%)@SNU-90 0 from 4.55 kJ mol1 for SNU-90 0 . Contrary to the physisorption, the H2 chemisorption capacities of the material increased as the temperature and the amount of loaded Mg increased. At 473 K under 30 bar, the H2 uptake capacity of Mg@SNU-90 0 reached 0.71 wt% (Fig. 10), which corresponded to 7.5 wt% H2 adsorption in Mg alone, suggesting 99% conversion of Mg to MgH2 upon H2 adsorption. Temperature-programmed desorption mass spectroscopy (TPD-MS) analysis showed that desorption of H2 occurred at T 4 523 K and 1 atm; the temperature was much lower than that of bare Mg NCs. 2.2.2 Heterogeneous catalysis 2.2.2.1 Catalytic hydrogen generation from chemical hydrides. Chemical hydrogen storage which involves storing of hydrogen in the form of chemical bonds is one of the safe and efficient alternatives to physical hydrogen storage. One of the promising hydrogen storage techniques relies on liquid-phase chemical storage materials, in particular, aqueous ammonia borane (NH3BH3, AB), hydrazine (N2H4) and formic acid (HCOOH, FA), which have the hydrogen capacities of 19.6, 12.6 and 4.4 wt%, respectively.125–129 The hydrogen generation from these chemical hydrides can be controlled by the use of metal nanocatalysts. Several reports have been found on the catalytic hydrogen generation from chemical hydrides in the presence of metal nanoparticle catalysts immobilized in MOFs. In 2011, we reported bimetallic AuPd NPs immobilized in MIL-101 and ethylenediamine (ED)-grafted MIL-101 (ED-MIL-101) as highly active catalysts for the conversion of formic acid to highquality hydrogen at a convenient temperature.75 ED-MIL-101 with the electron-rich functional group was used to improve the interactions between the metal precursors and the MIL-101 support,


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Fig. 11 Plots of time versus volume of hydrogen generated from AB (1 mmol in 5 mL water) hydrolysis at room temperature catalyzed by the Au@MIL-101, Ni@MIL-101 and AuNi@MIL-101 catalysts (50 mg, (Au + Ni)/AB (molar ratio) = 0.017). Reprinted with permission from ref. 81.

0

Fig. 10 Chemical absorption of H2 in Mg(10.5 wt%)@SNU-90 . (a) H2 absorption kinetics at 325 K and 80 bar. Inset: data with a magnified time scale. (b) H2 absorption kinetics at 415 K and 40 bar. (c) H2 absorption isotherms at 473 K in Mg(6.52 wt%)@SNU-90 0 (green) and Mg(10.5 wt%)@SNU-90 0 (red). (d) TPD-MS data; m/z = 2, measured under argon. Reprinted with permission from ref. 107.

which exhibit improved immobilization of smaller alloy metal NPs. As a result, Au-Pd/ED-MIL-101 exhibited a much higher catalytic activity than AuPd/MIL-101. Additionally, the strong bimetallic synergistic effects between Au and Pd provided a higher tolerance with respect to CO poisoning than monometallic Au and Pd counterparts. In the presence of 20 mg of AuPd/ED-MIL-101 (20.4 wt%, Au:Pd = 2.46) catalyst, 140 mg of formic acid can be completely converted to H2 and CO2 in 65 min at 90 1C. Then, our group reported highly dispersed Ni NPs immobilized by the framework of ZIF-8 via chemical vapor deposition (CVD) and chemical liquid deposition (CLD) approaches followed by H2 reduction to obtain CVD-Ni/ZIF-8 and CLD-Ni/ZIF-8, respectively, which showed high catalytic activity for hydrogen generation from the hydrolysis of aqueous AB at room temperature.102 For CLD-Ni/ZIF-8 with a particle size of 4.5 1.0 nm, the hydrolytic dehydrogenation of AB can be completed (H2/AB = 3.0) in 19 min (Ni/AB = 0.016), giving a turnover frequency (TOF) value of 8.4 min1. The Ni NPs in the sample prepared by using the CVD approach showed a smaller mean diameter of 2.7 0.7 nm and a higher activity, over which the reaction can be completed in 13 min (Ni/AB = 0.019), corresponding to a TOF value of 14.2 min1. In order to avoid the deposition of the metal precursors on the outer surface of MOF, we recently reported the use of a double solvent method combined with H2 reduction to introduce ultrafine Pt NPs (1.8 0.2 nm) in the pores of MIL-101, which were highly active for the hydrolysis of AB.80 With 2 wt% Pt@MIL-101 (Pt/AB (molar ratio) = 0.0029), the dehydrogenation reaction was complete within only 2.5 min, making it the most active Pt catalyst for this reaction reported so far. Furthermore, the pyrolytic dehydrogenation of AB confined in the pores of Pt@MIL-101 was also tested, and showed two H2


evolution peaks at 88 and 130 1C, respectively, much lower than those for the pristine AB. The decrease of AB dehydrogenation temperature and the effective depression of volatile byproducts and material foaming during the pyrolysis process were observed for AB/Pt@MIL-101, demonstrating the synergistic effect of the Pt NP catalysis and nanoconfinement of the MIL-101 framework. More recently, we fabricated ultrafine non-noble metal-based AuNi NPs (1.8 0.2 nm) throughout the interior pores of MIL-101 by the double solvent method in combination with an overwhelming reduction approach.81 H2 was released rapidly from aqueous AB in the presence of AuNi@MIL-101 with a Au/Ni atomic ratio of 7 : 93 due to the synergistic effect between Au and Ni (Fig. 11), giving a TOF value of 66.2 min1, which is much higher than that of the most active non-noble metal-based catalysts reported and even higher than that of most Pt-, Rhand Ru-related catalysts. The hydrogen generation from the decomposition of N2H4 in aqueous solution catalyzed by MOF-supported metal nanocatalysts has also been exploited. Highly dispersed bimetallic NiPt NPs with an average diameter of 2.2 0.3 nm supported on ZIF-8 via a simple impregnation method followed by co-reduction exerted high catalytic activity and durability for selective decomposition of N2H4 to H2 in aqueous alkaline solution.130 The activity of the catalysts strongly depended on the NiPt composition, and the catalyst Ni0.8Pt0.2/ZIF-8 showed the highest activity to convert N2H4 to N2 and H2 in 26 min at 323 K (metals/ N2H4H2O = 1 : 15), giving the highest TOF value of 90 h1 among the reported catalysts. 2.2.2.2 Photocatalytic hydrogen production. The photochemical reduction of water into hydrogen molecules using a photocatalyst does not rely on fossil fuels, and therefore is an ideal method for producing clean energy. MOFs provide a versatile and tunable platform to hierarchically integrate functional components for solar energy utilization. Effective hydrogen evolution from water containing a sacrificial electron donor under visible-light irradiation can be achieved by the Pt-loaded MOFs constructed from light-harvesting organic linkers. The Pt@MOF assemblies prepared by loading Pt NPs in photoactive MOFs, Zr6(m3-O)4(m3-OH)4(bpdc)5.94(L1)0.06 (3) and Zr6(m3-O)4(m3-OH)4(L2)664DMF (4), were found to be effective

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Fig. 12 (a) Scheme showing the synergistic photocatalytic hydrogen evolution process via photoinjection of electrons from the light-harvesting MOFs (3 and 4) into the Pt NPs. (b) Diffuse reflectance spectra and a photograph of suspensions of these samples. (c) Relationship between the amount of K2PtCl4 added in the reaction solution and the amount of Pt deposited inside the MOF. (d) Time-dependent hydrogen evolution curves of samples. Reprinted with permission from ref. 89.

photocatalysts for hydrogen evolution by synergistic photoexcitation of the frameworks and electron injection into the Pt NPs.89 Both MOFs were built from two [Ir(ppy)2(bpy)]+-derived dicarboxylate ligands. The radicals, [Ir(ppy)2(bpy )], generated in the MOFs by TEA-mediated photoreduction can transfer electrons to the entrapped Pt NPs to reduce protons of water (Fig. 12a). Under the illumination of a 450 W Xe-lamp with a 420 nm cutoff filter for 48 h, Pt@3 and Pt@4 photocatalysts gave the hydrogen evolution turnover numbers (TONs) of 3400 and 7000 based on Ir phosphors (Fig. 12d), 1.5 and 4.7 times the values afforded by the homogeneous control [Ir(ppy)2(bpy)]Cl/K2PtCl4 under their respective conditions. Moreover, the catalysts could be recycled and reused at least three times. Matsuoka and co-workers prepared Ti-MOF–NH2 consisting of titanium–oxo clusters and 2-aminobenzenedicarboxylic acid organic linkers.131 Then, Pt NPs as cocatalysts were deposited onto Ti-MOF–NH2 via a photodeposition process. The resultant composite Pt/Ti-MOF–NH2 exhibited efficient photocatalytic activity for hydrogen production from an aqueous solution containing 0.01 M triethanolamine as a sacrificial electron donor under visible-light (l 4 420 nm) irradiation. The total evolution of hydrogen after 9 h irradiation reached 33 mmol in the presence of the photocatalyst Pt/Ti-MOF–NH2 (10 mg) at room temperature. Without the deposition of Pt NPs, Ti-MOF–NH2 showed much lower activity than that of Pt/Ti-MOF–NH2, suggesting that efficient charge separation caused by the presence of Pt NPs played a role in enhancing the efficiency of the hydrogen production reaction. 2.2.2.3 Catalytic CO oxidation. The heterogeneously catalyzed oxidation of carbon monoxide (CO) is one of the ‘‘ever-greens’’ in catalysis research due to its relevance in practical applications, such as purification of air, gas sensors for the detection of trace amounts of CO, automotive exhaust gas treatment and polymer

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electrolyte fuel cells.132–134 The potential of MOF-supported metal NPs as efficient catalysts for CO oxidation has aroused tremendous interest since the work of our group.111 We reported the first example of an active catalyst in CO oxidation by using MOF-supported noble metal NPs.111 The CO oxidation activity over Au@ZIF-8 was improved with increasing Au loading from 0.5 to 5 wt%, as evidenced by the decrease of the temperature from 225 to 175 1C for 50% conversion of CO. Both the ZIF-8 host framework and Au NPs remained stable under the harsh reaction conditions based on PXRD and TEM examinations. Recently reported 5 wt% Pt@MIL-101 with ultrafine Pt NPs showed a higher CO oxidation activity.80 MIL-101 exhibited no catalytic activity for CO oxidation reaction in the whole temperature range. The Pt@MIL-101 started to show the activity at 50 1C, while CO to CO2 conversion increased suddenly at 100 1C with complete conversion at 150 1C. The catalyst showed stable activity, keeping 100% CO conversion for 150 min at 175 1C. Studies also showed that cubic Pt, tetrahedral Pd, and octahedral PtPd NCs on the MIL-101 support exhibited similar activities.79 EL-Shall et al. reported high CO oxidation activities over Pd, Cu and PdCu NPs supported on MIL-101.135 Pd@MIL-101 with 2.9 wt% Pd loading showed the highest catalytic activity with a full conversion at 107 1C among the catalysts with various loadings of Pd. The authors proposed that most of the activity comes from the small Pd NPs embedded within the pores. The PdCu catalyst showed significant enhancement over the Cu catalyst, while its activity was lower than that of Pd catalyst due to the larger size of the NPs. Wu et al. immobilized Au NPs on UiO-66 as catalysts with 1.5–4.0 wt% Au loading, which exhibited high catalytic and stability for gas-phase CO oxidation with 50% conversion at temperature ranging from 155–175 1C.136 2.2.2.4 Catalytic organic reactions. Owing to the confinement of the substrate in a MOF as well as the limitation of the growth of the constrained MNPs during the catalytic reactions, MNP@MOFs are one of the most promising heterogeneous catalysts, and some of them exhibit excellent catalytic performance in organic reactions. The selection of suitable MOF supports and the preparation method to obtain small-size MNPs is the most important factor for the catalytic performance. The MOFs with specific chemical and thermal stability should be selected according to different requirements for different organic catalytic systems. For example, the crystal lattice of thermally stable MOF-5 is extremely sensitive to moisture, and therefore can be destroyed due to the formation of H2O during some reactions, typically the catalytic alcohol oxidation reaction.100 Instead, MIL-101 possesses a robust framework combined with long-term stability in water, organic solvents and acidic media,137 while ZIF-8 exhibits high stability (up to 550 1C), and remarkable chemical resistance to boiling alkaline water and organic solvents.138 In addition, the sizes of the pores/channels and the nature of the inner surface in MOFs could tune unique product selectivity.108 Recently, the organic reactions using MNPs embedded within MOFs as heterogeneous catalysts have been well reviewed,139 and here we mostly introduce recent advances published since 2012.


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Scheme 2 Representative catalytic (a) aerobic oxidation, (b) hydrogenation reactions and (c) C–C coupling reactions over MNP@MOF catalysts.

The representative catalytic reactions over MNP@MOFs are aerobic oxidation, hydrogenation and C–C coupling reactions (Scheme 2). Catalytic aerobic oxidation. The catalytic oxidation reaction is one of the most studied reactions over MNP@MOF catalysts.91,96,99,104,108 Li and co-workers reported the excellent performance of Au catalysts with MIL-101(Cr) and MIL-53(Cr) supports for selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA-oil) by molecular oxygen in the absence of solvent and initiator.93 Both framework Cr ions and Au NPs in catalysts were believed to act as effective catalytic sites for this reaction, leading to an enhanced conversion (B30%) and KA-oil selectivity (480%). In addition, Au/MIL101(Cr) with smaller Au size presented a slightly higher selectivity than Au/MIL-53(Cr). Similarly, AuPd/MIL-101, which was prepared by deposition of AuPd-PVP colloids onto MIL-101, was active and selective for the oxidation of a variety of saturated (including primary, secondary and tertiary) C–H bonds with molecular oxygen.92 Cyclohexane conversion exceeding 40% was achieved (TOF = 19 000 h1) with 480% selectivity to KA-oil under mild solvent-free conditions. Higher activity and selectivity of AuPd/MIL-101 in cyclohexane aerobic oxidation compared with those of their pure metal counterparts and an Au + Pd physical mixture was observed, which may be correlated to the synergistic alloying effect of bimetallic AuPd NPs. Catalytic hydrogenation reaction. Hydrogenation is a key transformation in industry. The Pd, Ru and Ni NPs immobilized by MOFs have been reported to be very effective in the hydrogenation of a wide range of substrates including alkenes,76,103 alkynes,77 aromatics,96 nitro-aromatics70,116 and aryl alkyl ketones.105 The Ir NPs loaded in ZIF-8 via the gasphase loading method were applied for the hydrogenation of


Review Article

cyclohexene and phenylacetylene, and provided high activities in the hydrogenation of neat cyclohexene to cyclohexane (initial TOF = 370 h1) at complete conversion and phenylacetylene to styrene with high selectivity (Z90%) at complete conversion under mild conditions.140 Kempe and co-workers reported that the bimetallic NiPd NPs loaded into the host structure of MIL-101 are highly active in the hydrogenation of cyclic ketones and dialkyl ketones, which are much more difficult to be hydrogenated than aryl alkyl ketones.106 Results revealed that the pronounced synergistic effect between Ni and Pd greatly promoted the hydrogenation of cyclohexanone, cycloheptanone and 3-heptanone with greater than 80% conversion. With the combination of the catalytic properties of MNPs and the molecular sieving capability of the pore apertures (3.4 Å) in ZIF-8, the substantially encapsulated MNPs in the internal frameworks of ZIF-8 can show interesting size selectivity for catalytic hydrogenation of ethylene/n-hexene versus cyclooctene. MNP@ZIF-8 (M = Pd, Pt) with the absence of MNPs on the outer surface of the composites showed high catalytic activity for ethylene/n-hexene hydrogenation, while large molecules such as cyclooctene (5.5 Å) could not access the pores, resulting in no conversion.113,117,120 After chiral modification, MNP@MOFs may also exhibit high activity with good enantioselectivity for the asymmetric reactions. Pt/MIL-101 catalyst chirally modified with cinchona alkaloid was tested for the chiral hydrogenation of a-ketoesters and proved effective for the asymmetric hydrogenation of ethyl pyruvate (4469 h1 TOF with 76.5% ee value) and ethyl 2-oxo-4-phenylbutyrate (42000 h1 TOF with 76.8% ee value).141 Of particular note was that the catalyst could be reused at least four times without distinct loss in activity and enantioselectivity. Catalytic C–C coupling reaction. C–C bond forming reactions such as Suzuki-Miyaura, Ullmann, Heck, and Sonogashira coupling reactions are among the most interesting and important synthetic transformations in organic chemistry. Pd is probably the most active metal in promoting these reactions, and Pd NPs incorporated in MOFs have been examined as C–C coupling catalysts.74,142–146 The MIL series and their surface modified MOFs were mainly used as supports for Pd NPs due to their high chemical stability in water and organic solvents. Pd@MIL-101, which was obtained by a simple solution infiltration method followed by H2 reduction, had high activity and selectivity for the direct C2 arylation of various indoles with aryl boronic acids using O2 or 2,2,6,6tetramethylpiperidine N-oxyradical as an external oxidant in acidic media under mild conditions.147 The Pd@MIL-101 afforded 98% yield of the direct arylation of N-methylindole with phenyl boronic acid, which was much higher than the 19% conversion provided by a commercial Pd/C. Zhang et al. supported Pd NPs on a nanoscale MOF (ScBTC) by using a impregnation process followed by the microwave-assisted reduction method, which was represented as a highly active catalyst for the Suzuki cross-coupling reaction between aryl/heteroaryl halides and arylboronic acids.148 Excellent yields of different crosscoupling products were approached and these reactions can be repeated easily with the similar high yields, indicating the

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excellent performance of the catalyst. It also revealed that, compared to the corresponding bulk MOFs and conventional active carbon materials, the nanoscale ScBTC particles were a superior support in the catalytic reactions. 2.2.3 Sensing. Surface-enhanced Raman scattering (SERS), which produces an enormous enhancement in the Raman intensity when a molecule is in the vicinity of metal (usually Ag and Au) nanostructures, provides an invaluable tool as a reliable, high-resolution detection technique for extremely minute quantities of target molecules. A prerequisite for making use of the SERS effect is the adsorption of the detected molecules at the metallic surface. Some attempts have been made to coat metal nanostructures with microporous materials such as alumina149 and silica.150 However, the incorporation of metal nanostructures into porous MOFs gives some unique strength due to the well-defined pores of MOFs with controllable size (from several angstroms to nanometers) and functionality, large surface areas and selective adsorption properties for some specific molecules. Allendorf and co-workers infiltrated HKUST-1, MOF-508 ([Zn(bdc)(bpy)0.5]), and MIL-68(In) ([In(OH)(bdc)]) using an aqueous-ethanolic solution of AgNO3 without the degradation of frameworks.151 The Ag+ within the pores can be completely reduced to Ag in the presence of EtOH. The resultant Ag@MOFs produced an apparent SERS effect compared to the empty frameworks, and the trend in the Raman enhancement appeared to be correlated with the MOF pore size. SERS-active AuNR@6 has also been fabricated by the growth of 6 on protected Au nanorods, which enabled in situ monitoring of guest diffusion (Fig. 13).114 Time-dependent SERS spectra are observed from the fixed crystal of AuNR@6/CHCl3 sealed in a glass capillary with DEF, which were monitored at constant intervals to monitor exchange of the guest molecules from CHCl3 to DEF. The increase of the characteristic band for DEF and the decrease for CHCl3 occurred simultaneously during the exchange of the guest molecules. This time-dependent phenomenon clearly demonstrated that incorporation of Au NRs did not lead to further resistance for guest molecules to transfer in nanopores and come close to Au NRs. Taking advantage of the selective adsorption properties of some MOFs, composites of MOFs with nanostructures can be expected to function as sensors for the selective detection of specific molecules. Since the MOF-5 has the ability to selectively

Fig. 13 Schematic illustration of SERS-active MOFs embedding Au NRs. Reprinted with permission from ref. 114.

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capture CO2 from flue gas, the selective detection of CO2 in a gas mixture was realized by the integration of MOF-5 and Au NPs into well-defined core–shell NPs with a single metal NP core coated with a uniform MOF shell.62 The core–shell composites with a MOF-5 shell thickness of 3.2 0.5 nm showed unique SERS activity toward CO2 in the gas mixture. Such sensitive SERS detection can be easily applied to other analytes including DMF and ethanol by using the core–shell Au@MOF-5 and Au@ZIF-8 NPs, respectively. Very recently, the size-selectivity of core–shell AuNR@MOF-5 on SERS has been investigated by using various sizes of pyridine derivatives.152 2.3

Conclusion

Incorporating metal NPs within MOFs is an important and effective way to achieve unique functions originating from the synergistic effect of metal NPs and MOFs for multifunctional applications. According to the significant progress described above, it is clear that the fabrication of various metal NPs in MOFs will be a highly promising field for the future. However, the design and construction of MNP@MOF composites for potential applications remain at a nascent stage, and some major challenges must be overcome in order to utilize the potential of the new multifunctional materials. The development of general preparative methods for the incorporation of ultrafine MNPs within the pores of MOFs without agglomeration of MNPs on the external surface, two-or-more-component-MNP@MOFs, well-defined MNP@MOF core–shell nanostructures, and the distinctive MNP@MOF nanostructures such as yolk–shell or multi-shell structures is highly desirable.

3. MOF–metal oxide composites Metal oxide nanomaterials with controllable shape, size, crystallinity and functionality are widely employed in applications like electronics, optics, electrochemical energy conversion and storage, solar energy harvesting, catalysis, and so on.153 In order to further improve the properties and introduce new functionalities, some attempts to integrate metal oxides, especially those with magnetic or semiconducting properties, and MOFs into core–shell nanostructures have been undertaken. In principle, the preparation methods of metal-oxide-NP@MOF nanocomposites are similar to those of MNP@MOF materials. One is the generation of metal oxides within the cavities of MOFs via the oxidative annealing or decomposition process of preloaded precursors.154,155 The other one is the encapsulation of presynthesized metal oxide NPs inside the MOF matrices.113 In the latter approach, the NPs are usually decorated with suitable surface functional groups (amine and carboxylic acid) that can improve the affinity between the NPs and MOFs to promote a controlled crystal growth.156 Pre-treating the metal oxide cores with a material with increased compatibility toward a MOF, such as PVP, can also facilitate the growth of the MOF around metal oxide NPs.113,157 Alternatively, the use of metal oxide NPs as both the template and metal precursors for the formation of MOFs, namely self-template synthetic strategy,


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would also be a wise choice to acquire well-defined core–shell nanostructures.158 Besides the core–shell metal-oxide-NP@MOF nanostructures, MOFs can be used as templates for the synthesis of metal oxides with specific morphologies.159,160


3.1

Metal-oxide-NP@MOF nanocomposites

Fischer and co-workers have reported the solvent-free two-step syntheses of dispersed metal oxide nanomaterials inside MOFs by using the gas phase infiltration of the volatile metal precursors, followed by oxidative annealing of the loaded metal precursors. They synthesized nanosized ZnO and TiO2 hosted inside MOF-5 from ZnEt2 and Ti(O-iPr)4, respectively.161,162 Based on the results, doubly loaded Cu/ZnO@MOF-5 and Au/MOx@MOF-5 (M = Zn, Ti; x = 1, 2) samples were prepared by gas phase loading of MOx@MOF-5 with the corresponding volatile organometallic molecules and thermally activated hydrogenolysis. In a subsequent study, ZIF-8 was used to load ZnO throughout the whole host matrix.163 It was shown that the surface of ZnO nanoparticles was dominated by polar O–ZnO and Zn–ZnO facets as well as by defect sites, which all exhibited high reactivity towards CO2 activation forming various adsorbed carbonate and chemisorbed CO2d species. Alternatively, transition-metal nitrates, which generally exhibit lower decomposition temperatures than alkali metals,164 can be used as metal precursors to produce metal oxides. In addition, the confinement effect of MOFs provides limited growth of metal oxide NPs. Therefore, the direct pyrolysis of metal nitrates accommodated in the pores of MOFs could generate uniform and small NPs. Wang et al. firstly employed ZIF-8 as the host to prepare hexagonal Co3O4 NPs via the thermolysis of cobalt nitrate incorporated in the MOF host at a low temperature of 200 1C.155 The Co3O4 NPs were highly dispersed in the well-retained MOF networks and exhibited excellent catalytic activity for CO oxidation. Complete conversion of CO can be achieved at 80 1C by the Co3O4@ZIF-8 with good cycling and long-term stability. Moreover, the MOF host could be removed to yield neat Co3O4 NPs with a mean diameter of 18 nm, while the excellent catalytic activity of Co3O4 can be retained. This preparation method can be easily extended to the fabrication of other metal oxide NPs with a variety of metal centers. The combination between MOFs and magnetic materials such as ferromagnetic metal NPs (e.g. Co, Ni and Fe) and superparamagnetic metal oxide NPs (e.g. Co3O4, g-Fe2O3 and Fe3O4) appears to be of tremendous interest owing to the versatility of the collective functionalities.45 The ability to achieve precise positioning and collection of this type of composites due to an external magnetic field could offer ‘‘on demand’’ catalysis, sensing, selective sequestration and targeted drug delivery, enabling new applications within miniaturized platforms.113,165–169 In the recent work of Kaskel and co-workers, the magnetic functionalization of aluminium- and copper-based MOFs with extrinsic superparamagnetic g-Fe2O3 was explored.170 They used iron oxide NPs grafted with the carboxylate groups of organic linkers as seeds for the growth of MOFs. Superparamagnetic functionalization enabled rapid separation of the MOF catalyst from the reaction media in a static magnetic field. Significantly, the possibility for


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controllable drug (ibuprofen) release triggered by magnetically induced heating was realized by this magnetic composite. The release rate of ibuprofen from the network can be apparently accelerated at higher temperature. Based on a similar process, another two magnetic composites, Fe3O4@HKUST-1171 and Fe3O4@ZIF-8,157 were successfully obtained based on the Fe3O4 NPs decorated with pyridine groups and modified with an anionic polyelectrolyte, respectively. The pre-treated magnetic cores can adsorb metal ions to initiate nucleation on the surface of magnetic particles. Fe3O4@HKUST-1 was tested for the preconcentration of palladium in environmental samples, while Fe3O4@ZIF-8 microspheres as catalysts could be easily loaded/ unloaded into/out of a capillary microreactor with the help of an external magnetic field. Electrospun MFe2O4 (M = Co, Ni) nanofibers possessing superparamagnetic properties were mixed with the precursor solution of zinc nitrate and terephthalic acid, producing the desired magnetic composites under solvothermal conditions.172 The resultant composites MFe2O4@MOF-5 showed their potential for the sequestration of polycyclic aromatic hydrocarbons (PAH), with the advantage of controlling the recovery, without using filtration or centrifugation systems (Fig. 14). Recently, a liquid phase epitaxy (LPE) process, also known as the LBL method, has been introduced in the field of MOFs, which involves the use of an appropriately functionalized organic surface as a nucleation template and the addition of the organic ligand and the metal precursor in a step-by-step fashion.173 Qiu and co-workers firstly conducted the LPE process to rationally design and fabricate the core–shell architecture with a magnetic core and a designable MOF shell, in which the MOF shell thickness can be finely controlled by tuning the stepwise assembly process, while the structure, composition and function of the MOF shell can be tailored by choosing different framework building blocks.156 By using this strategy, Fe3O4@HKUST-1 and Fe3O4@MIL-100(Fe) core–shell microspheres with different MOF shell thicknesses have been synthesized. The application of Fe3O4@MIL-100(Fe) to the magnetic solid-phase extraction of polychlorinated biphenyls at trace levels in environmental

Fig. 14 SEM images of (a) CoFe2O4 fibers and (b, c) CoFe2O4/MOF-5 composite. (d) Real time emission spectra of a 1,2-benzanthracene solution in the presence of CoFe2O4/MOF-5. (e) Plot of time vs. the molar percentage during PAH sequestration. Reprinted with permission from ref. 172.

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water samples has been developed.174 In a similar experiment, a well-defined number of HKUST-1 layers were grown on COOH terminated silica magnetic beads by using the LPE process.175 The potential of the resultant composite as a high-pressure liquid chromatography (HPLC) medium has been demonstrated by the separation of toluene, xylene and pyridine. Another significant advancement has been further made by integrating zero/one-dimensional metal oxides (such as ZnO) with semiconducting properties into MOFs. Predictably, such MOF-semiconductor composites would be beneficial for potential applications in chemical sensing, photocatalysis and other optoelectronic devices. The first example demonstrated by Wang and co-workers is the growth of MOF-5 thin films on ordered ZnO NR arrays using a slow diffusion approach combined with molecule self-assembly technology.176 The MOF-5 crystallized with good quality and bound well to the hexagonal-patterned ZnO arrays to form the ZnO@MOF-5 hybrid films with a core– shell structure. The photoluminescence spectra of the hybrid films displayed a blue shift in green emission accompanying an intensity reduction in UV emission compared to pure ZnO arrays, disclosing the application potential of such composites in sensors, micro/nanodevices and screen displays. More recently, the freestanding ZnO@ZIF-8 NRs as well as vertically standing arrays (including nanorod arrays and nanotube arrays) have been fabricated on the basis of a self-template strategy, where ZnO NRs not only acted as the template but also provided Zn2+ ions for the formation of ZIF-8 (Fig. 15).158 Notably, the as-prepared ZnO@ZIF-8 nanorod arrays displayed distinct photoelectrochemical (PEC) response to hole scavengers

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with different molecular sizes (e.g., H2O2 and ascorbic acid (AA)). As shown in Fig. 15f, the photocurrent response of the ZnO@ZIF-8 NR arrays displayed opposite changes with the addition of H2O2 or AA. The different photocurrent responses were assigned to the limitation of the aperture (B3.4 Å) of the ZIF-8 shell, which only allowed H2O2 to arrive in the ZnO surface through the shell of ZIF-8 and scavenge the photogenerated holes, leading to the enhancement of photocurrent response. In contrast, the pores of ZIF-8 could be obstructed by AA with larger molecular size than the aperture size of ZIF-8. Therefore, the diffusion of H2O that contributed to the photocurrent at blank situation was restrained, thereby resulting in the reduction of photocurrent response. However, without ZIF-8, the photocurrent response of the ZnO NR arrays can be markedly enhanced by the addition of both H2O2 and AA. The result indicates that such semiconductor–MOF composites could be developed into many types of optical/electric sensors with high sensitivity and specific selectivity, if the semiconductor cores and the MOF shells are judiciously chosen. With the ability to excite electrons to the conduction band or to generate holes in the valence band, metal oxide semiconductors can be used to perform photocatalytic reactions.177 Zn2GeO4, an important member of semiconductor photocatalysts, has been intensively investigated in the degradation of organic contaminants,178 water splitting,179 and photoreduction of CO2.180 Although the photoreduction of CO2 on Zn2GeO4 has been researched for several decades, the conversion efficiency is still low. One factor can be contributed to the weak CO2 adsorption. Liu et al. demonstrated that some MOFs such as ZIF-8 can effectively adsorb CO2 dissolved in water, and promote photocatalytic activity of a semiconductor catalyst in CO2 reduction into liquid fuels in an aqueous medium.181 They grew the ZIF-8 NPs on the Zn2GeO4 NRs to yield Zn2GeO4/ZIF-8 hybrid NRs, which exhibited 3.8 times higher dissolved CO2 adsorption capacity than the bare Zn2GeO4 NRs, resulting in a 62% enhancement in the photocatalytic conversion of CO2 into liquid CH3OH fuel. 3.2

Fig. 15 SEM images of (a, c) the ZnO nanorod and nanotube arrays grown on the fluorine-doped tin oxide (FTO) substrate and (b, d) the as-prepared ZnO@ZIF-8 nanorod and nanotube arrays. (e) Schematic diagram of the PEC sensor with selectivity to H2O2. (f) Photocurrent responses of the ZnO@ZIF-8 nanorod array in the presence of H2O2 and AA with different concentrations. Reprinted with permission from ref. 158.

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MOF-templated metal oxide composites

In addition to the metal-oxide-NP@MOFs with core–shell nanostructures, some metal oxides with well-controlled size, shape and structure have been obtained by using a MOF-templated strategy, owing to the versatile template effect of MOFs.159,160 In this strategy, both the polyhedral and smooth external surfaces and controllable internal pores/channels of MOFs could possess the template action. Moreover, the template can be readily removed by chemical or calcination as needed. Lin and co-workers coated the nanoscale MIL-101(Fe) particles with an amorphous octahedral shell of titania by acid-catalyzed hydrolysis and condensation of titanium(IV) bis(ammonium lactato)dihydroxide (TALH) in water.159 The prepared composite showed a similar morphology to the original MIL-101(Fe) particles and the thickness of the TiO2 coating could be controlled by adjusting the concentration of acid and the reaction time. Only peaks of MIL-101(Fe) appeared in the PXRD pattern for the coated particles, indicating that the MOF did not decompose and the titania shell was amorphous. More importantly, the core–shell composite can be calcined to


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produce crystalline Fe2O3@TiO2 nanoparticles, which in combination with K2PtCl4 enabled visible light-driven hydrogen production from water. Another important investigation was made by Kondo, Mallouk and co-workers. High surface area micro/mesoporous brookite phase titania with surface areas up to 270 m2 g1 has been made from the HKUST-1 template by dehydration, infiltration with Ti(O-iPr)4 and subsequent hydrothermal treatment at 200 1C.160 Etching the MOF with 1 M aqueous HCl followed by 5% H2O2 yielded a titania replica that retained the morphology of parent HKUST-1 crystals and contained partially ordered micropores (B0.6 nm) derived from the MOF template as well as disordered mesopores (B6.0 nm) arising from incomplete filling of the pores of the template by Ti(O-iPr)4. It is the first example of using MOFs as templates to make porous, crystalline metal oxides.

4. MOF–silica composites Silica nanoparticles and nanostructures, which provide unprecedented material platforms to accomplish many nanoscale functions, have attracted considerable attention because of their extensive applications in catalysis, separation, and drug release.182–185 Many of the advances in silica nanochemistry are based on its porosity, stability, dielectric properties and opportunities for introducing multiple functionalities. Integrating the functionalities of silica nanomaterials with the high surface area and porosity of MOFs would combine the unique properties of both materials and lead to novel applications. There are currently two main types of MOF–silica composites (SiO2@MOFs and MOFs@SiO2, respectively), both of which will be reviewed here. The former involves incorporation of dispersed silica nanoparticles within the pores/channels of MOFs or growth of a MOF shell on a pre-formed silica sphere in MOF precursor solutions, while the latter exploits the advantages of a silica shell as a surface coating or the mesoporous properties and processability of silica supports to promote the growth of microporous MOF particles throughout the porous silica supports. In addition, there are two intriguing reports that SiO2 nanoparticles can be used as nucleating agents to boost the reaction rate of MOF-5,186 while SBA-15 mesoporous silica was utilized as a directing agent for oriented growth of MOF-5.187 4.1

Review Article

was strictly constrained, the resultant silica showed a drastic decrease of its crystallization temperature because of its nanoscale size.189 Notably, the hydrophilic silanol moieties on the surface of silica particles were increased, and therefore the remarkable affinity of the resultant SiO2@MOF composites to hydrophilic molecules was obtained.190 Compared to the original MOFs, the composites exhibited enhanced adsorption of water and selective adsorption of hydrophilic molecules, because of the strong interaction between the doped silica and adsorbates. Besides, the CPL-5 with one-dimensional channels showed more significant gate effects on selectivity for the adsorbates. Due to their optical transparency, ease of production with narrow size distribution and low cost, the merge of presynthesized silica spheres with MOFs is another way to extend the scope of the utilization of these materials. The properties of the SiO2@MOF core–shell structures can be easily tailored by varying the organic building blocks and metal nodes in the shell portions for specific applications. Both seed coating and chemical modification of the surface of silica spheres are effective methods for directing the initiation and growth of MOFs on the surface of silica spheres to produce unique SiO2@MOF core–shell microspheres, because the heterogeneous growth of MOFs on the support surface is usually poor.191,192 Usually a carboxylate-terminated surface induces a regular growth of MOFs. Nevertheless, simply mixing MOF precursors and bare silica spheres only resulted in self-nucleation of MOFs in solution and non-uniformity of products.193,194 The fabrication of monodispersed SiO2@MOF core–shell microspheres with an In-MOF195 shell by the controllable growth of MOF shells on spherical carboxylate-terminated SiO2 cores has been demonstrated by Oh and co-workers (Fig. 16).196–198 The thickness of the MOF shell within SiO2@In-MOF microspheres can be rationally controlled from 65 to 295 nm by both regulating the amount of MOF precursors and altering the amount of silica spheres added in the reactions.196 Moreover, by means of a multi-step MOF growth process, well-defined multi-layered core–shell structures with various MOFs can be

SiO2@MOF composites

Silanol moieties on the silica surface greatly contribute to the hydrophilic property of silica, where the number of surface silanols increases with decreasing size of silica.188 Thus, nanosized silica with a large surface area may show significant alternations of the adsorption behavior after the formation of nanocomposites with host materials. Sol–gel polycondensation of tetramethoxysilane performed in the nanochannels of porous HKUST-1 and CPL-5 ([Cu2(pzdc)2(dpe)], dpe = 1,2-di(4-pyridyl)ethylene) which possess three-dimensional interconnecting and one-dimensional channels, respectively, resulted in the formation of subnanosized silica dispersed homogeneously within the host frameworks.189,190 Since the growth of silica


Fig. 16 (a) Formation of SiO2@In-MOF core–shell microspheres. (b) TEM images of the SiO2@MOF microspheres with an average diameter of 1.37 0.04 mm. (c) EDX spectrum profile scanning of core–shell particles obtained in STEM mode. Reprinted with permission from ref. 196.

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constructed, in which the composition of the multilayer was controlled by changing the metal centers of the isoreticular MOFs and altering the sequence of the multi-step MOF growth process.197 The unique features of high porosity, shape selectivity and multiple active sites make MOFs promising as novel stationary phases for high-performance liquid chromatography (HPLC). However, the irregular shapes and wide size distribution of MOF particles lead to difficulty in column packing and low column efficiency. The fabrication of monodispersed SiO2@MOF core–shell microspheres as stationary phases for HPLC provides an effective way to overcome the drawbacks.199,200 In SiO2@ZIF-8, the ZIF-8 density was easily controlled by changing the number of growth cycles of ZIF-8, and after three cycles of ZIF-8 growth, a 400 nm thick ZIF-8 shell was grown on the SiO2 microspheres.199 The slurry-packed SiO2@ZIF-8 column was investigated for fast and efficient HPLC separation of endocrine-disrupting chemicals and pesticides, and showed that baseline separation of these targets was achieved within 7 min with high resolution, good column efficiency and precision, indicating the synergistic effects of the good column packing properties of the silica spheres and the separation ability of the ZIF-8 crystals. Spheres-on-sphere silica particles modified with –COOH and NH2 groups were also studied as support to grow HKUST-1, and the obtained core– shell composite microspheres can be packed into columns for the separation of toluene/ethylbenzene/styrene and toluene/ o-xylene/thiophene.200 Some MOFs are labile and can be broken by increasing the concentration of H+ or OH, which can be used as coating agents for pH-responsive controllable drug delivery. Che and co-workers designed and synthesized a drug delivery vehicle based on MOF-coated mesoporous silica nanoparticles (MSNs), which served as a pH-responsive nanoreservoir for efficient targeted drug delivery to cancer cells (Fig. 17).201 The silica surface was functionalized with amino groups to allow preferential epitaxial growth of the MOF of zinc and 1,4-bis(imidazol-1-ylmethyl) benzene (BIX-Zn). After loading the drug Topotecan (TPT), the mesoporous SiO2 NPs were capped by a shell of BIX-Zn, giving rise to the MSN-NH2-TPT@BIX-Zn architecture. Drug release was triggered by H+ cleavage of the coordination bond of the BIX-Zn nanolayer. Moreover, MOF-coated MSNs showed a remarkably enhanced efficiency in killing HeLa cells (human cervical carcinoma), because they were more readily internalized through an endocytosis mechanism due to the positively charged BIX-Zn layer and released TPT in the cytoplasm. The intracellular uptake of MSN materials in cancer cells was confirmed by confocal laser scanning microscopy (CLSM) and TEM. The step-by-step method, which has been introduced above for the fabrication of MOF shells on metal NPs and metal oxide NPs,116,156 was also used for preparing uniform layers of MOFs with controllable thickness on silica nanostructures.202,203 Silica colloidal crystals with sub-micrometer dimensions in photonic arrays are capable of reflecting light at a specific wavelength (stop band) due to their periodic structure.204 Because the stop band is sensitive to changes in the effective refractive index and/or lattice spacing, silica colloidal crystals

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Fig. 17 TEM images of (a) MSN–NH2, (b) MSN–NH2–TPT and (c) MSN– NH2–TPT@BIX-Zn. Release profiles of TPT from (d) MSN–NH2–TPT at pH 7.4 and (e) MSN–NH2–TPT@BIX-Zn at different pH values. (f) CLSM images of HeLa cells after incubation with 100 mg mL1 MSN–NH2– TPT@BIX-Zn for 4 h. Reprinted with permission from ref. 201.

have been previously investigated as sensors.205 Hupp and co-workers recently combined HKUST-1 with silica nanospheres configured as thin-film colloidal crystals to achieve signal transduction in response to molecular sorption by the MOF.202 HKUST-1 was grown on the carboxylate-functionalized silica colloidal crystals via the step-by-step method. After 24 growth cycles the array of silica spheres maintained hexagonal order and was covered homogeneously with intergrown 50–100 nm MOF crystals. Visible and near-infrared extinction spectra revealed a linear red shift of the stop band of the colloidal crystal with the number of MOF-grown cycles. 4.2

MOF@SiO2 composites

Nanoscale MOFs (NMOFs) with particle dimensions in the tens to hundreds of nanometers range have the potential to be used as nanocarriers for imaging agents and drug molecules.206 However, because of the poor biocompatibility and the intrinsic instability of many NMOFs under physiological conditions, surface modification is necessary to optimize their in vivo performance. The surface modification of NMOF with a silica shell offers several advantages, which can not only improve their water dispersibility, biocompatibility and affinity to target specific cells or tissues, but also slow down the degradation of NMOFs, thus preventing the premature release of cargoes.30 Lin and co-workers first stabilized NMOF particles by encapsulating them within a silica shell.207–210 Generally, NMOF@SiO2 nanocomposites are prepared by first modifying NMOFs with a hydrophilic polymer such as PVP to keep the NMOF particles well dispersed, and then coating with a silica shell in a silica precursor solution. Firstly, the NMOF of Ln(BDC)1.5(H2O)2 (8, Ln = Eu3+, Gd3+, or 3+ Tb ) was coated with PVP and then treated with tetraethyl orthosilicate (TEOS) in basic ethanol to afford the 8@silica


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nanocomposite. The silica shell thickness can be controlled by adjusting the reaction time or reactant concentrations.207 Gd3+ release experiments in a slightly acidic environment demonstrated that the silica shell significantly increased the stability of NMOF cores and allowed for the controlled release of the metal constituents. NMOF@SiO2 can be further functionalized with a silylated Tb-EDTA monoamide derivative for the luminescence sensing of dipicolinic acid, which is a major constituent of many pathogenic spore-forming bacteria. This method was also expanded to coat a NMOF constructed from Pt-based drugs, and two Mn-based NMOFs ([Mn(bdc)(H2O)2] (9) and [Mn3(btc)2(H2O)6] (10)).208,209 The Pt-containing NMOFs were stabilized with shells of amorphous silica to prevent rapid dissolution and to effectively control the release of the Pt species, the anticancer efficacy of which was demonstrated on multiple cancer cell lines in vitro.208 Since paramagnetic Mn2+ ions have been shown to be potential magnetic resonance imaging (MRI) contrast enhancement agents with low toxicity,211 the Mn-based NMOF@SiO2 composites with a thin silica shell and surface functionalization with an organic fluorophore and a cell-targeting peptide had an ability to enhance their delivery to cancer cells for target-specific MRI.209 Subsequently, an amino-functionalized nanoscale NH2-MIL-101(Fe) synthesized by incorporating 2-aminoterephthalic acid was coated with silica by an alternative method using sodium silicate as the silica source because of the instability of the NH2-MIL-101(Fe) particles under previous basic conditions.210 Such aminofunctionalization of the NMOFs allowed for the loading of a fluorophore (BODIPY) and an anticancer drug (ESCP) via covalent modifications, and therefore the composites can be used as a nanodelivery vehicle for imaging contrast agents and anticancer drugs by slow release of the cargoes via NMOF degradation (Fig. 18). Intact BODIPY-grafted NMOF was not fluorescent due to iron quenching, but it exhibited strong fluorescence upon NMOF decomposition. Confocal microscopy of this composite on HT-29 human colon adenocarcinoma cells showed the dose-dependent cellular localization and fluorescence. More recently, Lin and co-workers were able to create Zr-based NMOFs containing a phosphorescent Ru(bpy)32+ (bpy = 2,2 0 -bipyridine) derivative as the bridging ligand, which has a dye loading of 57.4%.212 The Zr-based NMOFs were further stabilized with a silica coating and functionalized with poly(ethylene glycol) (PEG) and a targeting molecule (PEG-anisamide) for in vitro optical imaging of cancer cells. Confocal microscopy studies using human lung cancer cells demonstrated increased uptake of the targeted nanoparticles, which was confirmed by ICP-MS analysis. Different from the above-mentioned three-step method developed by Lin and co-workers to synthesize MOF@SiO2 core–shell composites by adding a PVP-functionalized MOF into the SiO2 precursors,207–210 a facile short-time and lowcost route for one-step synthesis of HKUST-1@SiO2 nanoscale particles under ultrasonic irradiation at room temperature has been presented recently.213 In this approach, the HKUST-1@SiO2 core–shell composite was prepared by directly mixing both the MOF and silica precursors in the ultrasonic reaction solution, where the HKUST-1 nanocrystals in the size range of 200–400 nm


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Fig. 18 (a) Schematic representations of the covalent grafting of BODIPY or ESCP, and the surface functionalization with a thin shell of silica on the NMOF of NH2-MIL-101(Fe). Confocal fluorescence images of HT-29 cells incubated with (b) no particles, (c) 0.19 mg mL1 and (d) 0.38 mg mL1 of BODIPY-NMOF@silica particles. The bars represent 25 mm. Reprinted with permission from ref. 210.

were first generated within a short time, and subsequently SiO2 formed and spontaneously grew on the surface of the assynthesized MOF nanocrystals. The silica shell thickness could be fine-tuned in the size range of 12–60 nm for various reaction times. Due to the accessible semiconductor quantum dots of the binuclear copper paddle wheels in HKUST-1, the nanocomposite can be fabricated into thin films on the glass substrates by sol–gel spin coating as a photocatalyst for the photocatalytic degradation of phenol under visible light. The films as photocatalyst showed higher photocatalytic performance with good durability compared with commercial TiO2 and bare HKUST-1 particles. Besides the well-defined core–shell structures, incorporation and immobilization of nanosized MOF crystals in confined spaces, such as the interior of mesoporous silica matrices, lead to another type of NMOF@SiO2 composites. The composites possess hierarchically porous systems combining both micro- and mesoporosity, and thus would provide unique MOF-intrinsic structural and chemical properties. Like the crystalline zeolites, ordered mesoporous silica have emerged as a novel class of silica solids, such as SBA-15, MCM-41 (with a hexagonal arrangement of the mesopores) and MCM-48 (with a cubic arrangement of the mesopores) characterized by very large specific surface areas, ordered pore systems and welldefined pore radius distributions,214–216 which have been used as supports to deposit NMOFs within the mesopores.217–219 In addition, the incorporation of NMOFs in other suitable silica matrices including silica beads, monoliths, foams and aerogels has also been investigated.220–222

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Fig. 19 (a) Schematic illustrations of the crystal structures of MOPs and MOP 3 dispersed in silica nanopores. (b) TGA curves of SBA-15, MOP 3 and MOP 3@SBA-15 (M3S-x, x represents the mass fraction of MOP 3) samples. (c) H2 adsorption capacities of M3S and MOP 3 samples at 77 K. The adsorption capacities of M3S samples were calculated by subtracting the uptake of the SBA-15 support from the measured uptake. Reprinted with permission from ref. 225.

Among MOFs dispersed within the porous silica materials, HKUST-1 has been intensively studied. The in situ preparation of MOF-5 and HKUST-1 in the SBA-15 matrix has been presented, which is based on the direct dispersion of MOF precursors into the pores of SBA-15 to form MOF@SBA-15 composites under solvothermal conditions.217,218 The ratios of the micropore/ mesopore volume in the MOF@SBA-15 composites can be tuned by controlling the initial concentration of precursors in reaction solutions. MOF-5@SBA-15 exhibited interesting gas adsorption behaviors and enhanced moisture stability since MOF-5 crystals were confined and isolated inside the mesopores of the rigid SBA-15 support,217 while HKUST-1@SBA-15 showed a higher ethanol adsorption rate at 303 K compared to bulk HKUST-1 crystals.218 To avoid rapid nucleation and growth of MOF crystals at the outer surface of the supports, the use of DMSO for the impregnation of HKUST-1 precursors within the silica supports followed by solvothermal treatment was preferred over the classical routes involving a mixture of BTC in ethanol and copper in aqueous solution.220,221 Sequential incorporation of copper nitrate and BTC in the pores of silica supports is another feasible way to avoid the self-nucleation of HKUST-1 on the external surface of supports.219,222 The monodispersed silica beads (B3 mm) incorporated with nanosized HKUST-1 crystals can be employed as an HPLC stationary phase, which combined the good column packing properties of the silica and the separation ability of the HKUST-1.220 Due to the high mass transport efficiency of the silica monoliths, HKUST-1 inside the mesopores of silica monoliths operated efficiently as a catalytic microreactor for continuous flow applications with low back pressure.221 The LBL method has also been employed for the growth of crystalline MOF films in mesoporous silica foam by Eddaoudi and co-workers.223 Using this stepwise method, homogeneous MOF thin films of HKUST-1 and ZIF-8 on confined surfaces have been obtained. For a comparison, in situ crystallization has also been studied, which only resulted in the inhomogeneity in the

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growth and the presence of additional isolated microsized MOF crystals not attached to the foam. Ulker et al. reported a novel nanostructured composite of silica aerogel embedded with HKUST-1 nanoparticles, which was prepared in the presence of pre-formed MOF particles using a slightly modified version of the conventional sol–gel method used to synthesize silica aerogels.224 Recently, three metal–organic polyhedra (MOPs) with identical geometries but different ligand functionalities (namely tert-butyl, hydroxyl, and sulfonic groups) have been incorporated into the twodimensional hexagonal nanopores of SBA-15 by impregnation with a methanol solution of MOPs (Fig. 19).225 In comparison with bulk MOP materials in the solid state, MOPs confined in silica nanopores can be well-dispersed, leading to accessible open windows and active sites in the MOPs. The results also showed that the highly dispersed MOPs exhibited enhanced thermal stability and obviously superior H2 adsorption capacity compared with aggregated ones.

5. MOF–organic polymer composites Organic polymers possess many unique attributes, including ease of production, light weight, and good thermal and chemical stability, which are desirable to integrate with other functional materials to make composites.226 In particular, confined polymers at nanometer scales exhibit fascinating and unexpected properties different from those in the bulk state.227 A new methodology for controlling polymer synthesis within porous MOFs has therefore been established. MOF–organic polymer composites formed from various combinations of MOFs and organic polymers constitute a class of composite materials with combined properties. 5.1

Organic polymer@MOF composites

Polymer inclusion in crystalline porous hosts with regulated and tunable nanochannel structures has been attracting much attention. In the pioneering work of Kitagawa and co-workers, various porous MOFs with nanochannels of different sizes, shapes, dimensions and surface functionalities have been used as nano-reactors for confined polymerization.228–231 Especially, MOFs bearing active sites on pore surfaces are capable of acting as both nanomolds and catalysts for polymer synthesis.231–233 Polymers studied thus far vary from polystyrene to conducting polymers like polypyrroles. As an example, the first polymerization of styrene (St) in the nanochannels (7.5 7.5 Å2) of [M2(bdc)2(ted)] (11, M = Cu or Zn; ted = triethylenediamine) was performed in 2005.228 Quantitative recovery of the accommodated polystyrene (PSt) with a molecular weight of B55 000 could be attained by decomposition of the host framework in NaOH solution. The recovered PSt showed much narrower molecular weight distribution than those synthesized in bulk and solution. In the subsequent studies, the efforts in radical polymerization were further directed to controlling the molecular weight,234 tacticity,235,236 dimensions,231,237 and copolymer sequence238 of the resulting polymers, which have been extensively summarized in several reviews57,239,240 and will not be discussed herein in detail. Besides, our group has reported the


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Fig. 20 Molecular structures of (a) DVTP ligand and (b) monomers. (c) Schematic diagrams of host–guest cross-polymerization. Reprinted with permission from ref. 244.

polymerization of furfuryl alcohol (FA) in MOFs and the carbonization of the resultant PFA@MOF composites for porous carbon preparation.241–243 Very recently, an interesting example of host–guest crosspolymerization within MOFs has been presented. It is wellknown that highly ordered crystalline packing of polymer chains is difficult to achieve. Uemura, Kitagawa and co-workers have disclosed a strategy that relied on ‘‘ordered crosslinks’’ to produce polymeric materials that exhibited a crystalline arrangement (Fig. 20).244 In this strategy, divinyl crosslinkers, 2,5-divinylterephthalate (DVTP), were used as substitutional ligands to prepare a MOF, [Cu(DVTP)x(bdc)1x(ted)0.5] (12x) where x is the molar amount of DVTP. Subsequent polymerization of vinyl monomers inside the channels of the host MOF generated highly ordered PSt chains crosslinked by the divinyl species. Crosslinking among aligned polymer chains connects together the polymer chains accommodated in adjacent channels, and thus ordered alignment of the polymer can be maintained after selective dissolution of the MOF matrix. The crystalline arrangement of the resulting PSt was confirmed by PXRD measurements and it was found that the diffraction intensity grew coherently with crosslink density, being sharp and intense for PSt from 120.06. TEM examination of PSt from 120.06 clearly demonstrated the presence of parallel lattice fringes with a period of 4.9 Å, in agreement with the lateral distance of PSt chains observed in PXRD. Aside from the significant progress in polymer synthesis in coordination nanospaces, the conjunction of porous MOFs with organic polymer particles for the induction of core–shell structures from MOFs has also been reported.245,246 To facilitate the coating of polymer surfaces with MOFs, carboxylate-terminated


Review Article

Fig. 21 SEM and TEM images of (a, b) carboxylate-terminated polystyrene spheres, (c, d) polystyrene@ZIF-8 core–shell microspheres by conducting two cycles of the ZIF-8 growth process, and (e, f) hollow ZIF-8 microspheres. Reprinted with permission from ref. 245.

polystyrene spheres can be used. Polystyrene@ZIF-8 core–shell particles were prepared under solvothermal conditions, where ZIF-8 layer thickness can be prudently controlled by altering the number of growth cycles (Fig. 21).245 It is known that template synthesis is one of the most-used strategies to prepare hierarchically hollow microspheres, especially for pores inside the shells. With this in mind, a subsequent etching process on polystyrene@ZIF-8 by DMF to remove polystyrene cores resulted in a unique hollow ZIF-8. This simple approach provides a vital path for the preparation of hollow structures of MOFs. In a similar way, polystyrene@MIL101(Fe) and hollow MIL-101(Fe) spheres have been synthesized with tailored structures and tunable MOF shell thickness using the LBL self-assembly strategy.246 5.2

MOF@organic polymer composites

Similar to the structures of MOF@SiO2 composites, both growth of an organic polymer shell on the surface of NMOFs to form the welldefined core–shell nanostructures and dispersion of MOF particles within the macroporous polymers configured as monoliths and beads will be described. However, MOF–polymer mixed-matrix membranes will not be discussed. In addition to silica encapsulation, coating NMOFs with an organic polymer layer is another effective strategy for the modification of NMOF surfaces to enhance biocompatibility and retard framework dissolution.29,247,248 The polymer molecule should have an end group which can bind to the NMOF particles through either vacant metal coordination sites on the particle surface, electrostatic attraction, or covalent attachment to the bridging ligands. Boyes and co-workers modified the surface of a Gd-based NMOF, [Gd(bdc)1.5(H2O)2] (13), with a range of polymers

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through the thiol end groups of polymers which can bind to vacant orbitals of the Gd3+ ions on the surface of the MOF.247,248 The polymer coatings slowed down the release of Gd3+ ions. The homopolymer modified Gd-MOF nanoparticles were shown to be biocompatible and can be utilized as a positive contrast agent in magnetic resonance imaging. The longitudinal relaxivity rates of these nanocomposites were easily tuned by changes in the molecular weight and chemical structures of the polymers.247 Gd-MOF nanoparticles functionalized with copolymers as multifunctional nanomedicines had cancer cell targeting, bimodal imaging, and disease treatment capabilities.248 Horcajada, Gref and co-workers prepared a series of porous Fe-based NMOFs as non-toxic nanocarriers for the delivery of drugs.29 For biological applications, the NMOF surfaces were engineered by coating with amino or carboxylate terminated polyethylene glycol (PEG) during the course of the synthesis process, which prevented aggregation of the nanoparticles. These composites have been applied to encapsulate previously challenging antitumoral and antiviral drugs, as well as cosmetic agents, and progressive release was obtained under simulated physiological conditions. Additionally, the potential of these composites as contrast agents was demonstrated. Zhou and co-workers have reported a porous Cu(II)-based coordination nanocage covered with alkyne groups and its surface functionalization by grafting with azide-terminated PEG through click chemistry (Fig. 22).249 Because the coordination nanocages are hydrophobic, which greatly limits their application in aqueous conditions, surface functionalization with hydrophilic polymers can tune the nanocages into colloids and enhance their water stability. Measurements showed that the nanocage cores were grafted with 1-4 PEG chains. Given their proven composition and water stability, the PEG-modified nanocages were used as a nanocarrier for drug release of 5-fluorouracil (5-FU), which is a widely used anticancer drug. Pure 5-FU was dialyzed as a control experiment, in which nearly 90% of the total drug was released within 7 hours. However, around 20% of the loaded drug was released in the initial 2 hours from the nanocarrier. After that, there was a much flatter release curve up to 24 h. The initial burst release may come from the drug that was imbedded within the outer polymeric corona. The rest of the drug, which was loaded within the cavities of the nanocarrier with strong interaction between Lewis acid sites of nanocages and the base site in 5-FU, was released very slowly. The immobilization of a MOF in a pre-formed macroporous polymer used as a monolithic support could give hierarchical porosity across the macro- and microporous length scales. The first report of this type of MOF–polymer composite prepared by the deposition of HKUST-1 within the open-cell structure of polyHIPE came from Kaskel and co-workers.250 The polyHIPE monolith prepared from 4-vinylbenzyl chloride and divinylbenzene was hydrophilized by introducing hydroxyl groups to promote MOF growth throughout the porous support. The HKUST-1@polyHIPE composite was prepared through impregnation with HKUST-1 precursor solution and hydrothermal treatment. The MOF loading level of the composite was tunable depending on the number of impregnation steps. SEM images

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Fig. 22 (a) The scheme of click reaction. (b) Gel permeation chromatography curves of the PEG (blue) and PEG-modified nanocage (black). (c) Matrix assisted laser desorption ionization-mass spectrometry (MALDI-MS) of PEG-modified nanocage (dashed lines represent the theoretical molecular weight of nanocages grafted with various PEG chains). (d) TEM image of PEG-modified nanocages. (e) The release of 5-FU from control (square) and PEG-modified nanocages (circle). Reprinted with permission from ref. 249.

revealed HKUST-1 crystals of varying sizes distributed within the interconnected macropores of the polyHIPE matrix while conserving the primary architecture of the monolith. The amount of HKUST-1 incorporated in the polymer material was up to 62.3 wt% by means of thermogravimetric analysis. Similar observations were reported for the growth of HKUST-1 and UiO-66 into macroporous polyacrylamide (PAM) beads and polyurethane (PU) foams using solvothermal methods, respectively.251,252 The mm-sized HKUST-1@PAM composite beads were prepared in a single step from hydrophilic PAM with the terminal amide groups on the surface and displayed enhanced mechanical stability and handling over bulk MOF phases in heterogeneous reaction systems.251 The direct synthesis of UiO-66 on open cell PU foams resulted in the UiO-66@PU material.252 Both composites maintained the macrostructures and flexibility of the polymer supports and exhibited the microporosity and high surface areas of MOFs. An alternative to the preparation of hierarchical MOF@polymer composites is to form the monolithic polymers in the presence of pre-formed MOF NPs. The fabrication of ZIF-8/polysulfone (ZIF-8/PS) composite spheres from a polymer solution containing ZIF-8 nanocrystals using a single orifice spinneret has been recently reported.253 For preparing such a composite, different amounts of ZIF-8 powder were mixed with polymer solution containing a surfactant as a surface pore-forming agent, and


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toward the o/w interface to form stable Pickering emulsions, consistent with the hydrophobic nature of ZIF-8. However, for some other MOFs, such as MIL-101 and UiO-66, the modulation of the surface hydrophobicity to improve emulsion stability and capsule formation is necessary. SEM images of composite capsules prepared with a ZIF-8 : organics ratio of 1 : 1.75 revealed the formation of intact spherical structures (41 13 mm) with homogeneously distributed ZIF-8 NPs embedded in the PS shell (Fig. 23). The ZIF-8@PS capsules appeared effective for the retention of encapsulated dye molecules.

6. MOF–quantum dot composites Fig. 23 (a) SEM image of intact ZIF-8@PS microcapsules. (b) SEM image (and inset) of a single broken capsule showing embedded ZIF-8 and the hollow interior. (c) Reconstructed z-stack of CLSM images of ZIF-8@PS capsules loaded with oil red O (ORO). (d) Time vs. release curves of ORO from ZIF-8@PS (black squares) and PS only capsules after acid dissolution of the ZIF-8 (red circles) determined by monitoring the characteristic ORO absorption at 518 nm. Reprinted with permission from ref. 255.

the resulting solution was injected through a syringe tip to a water tank to form composite spheres via solvent/water exchange. Nitrogen and carbon dioxide sorption analyses indicated that the ZIF/PS composite spheres had similar absorption properties as the pure ZIF-8. The group of Yan reported the incorporation of a MOF, [Al4(OH)2(OCH3)4(NH2-bdc)3] (CAU-1), into polymethyl methacrylate (PMMA) to produce the CAU-1@PMMA composite, and the fabrication of CAU-1@PMMA coated capillary for open tubular capillary electrochromatography.254 The composites were synthesized by the polymerization of methyl methacrylate in the DMSO solution of CAU-1 in the presence/absence of capillary. SEM and TEM images of the inner surface of the capillary column revealed CAU-1 nanocrystals homogeneously dispersed within the polymer. The incorporation of CAU-1 into PMMA not only reduced separation time and increased column efficiency and loading capacity for the separation of acidic compounds, but also improved the resolution of the sulfa drugs and structurally related peptides. Bradshaw and co-workers demonstrated a facile synthetic method for the preparation of MOF–polymer composite microcapsules by immobilizing a series of NMOFs located at the oilin-water (o/w) interface of emulsion droplets in a polystyrene (PS) membrane.255 The emulsions were formed by application of high shear forces to biphasic mixtures of dodecane and aqueous dispersions of NMOFs. When the organic inner phase contains monomers, cross-linkers and an initiator, these stable ‘‘MOF somes’’ can act as polymerisation microreactors to form the desired composite microcapsules via polymer-induced phase separation of a cross-linker polystyrene membrane. Capsulate formation was dependent on the surface properties of the MOF NPs and the phase separation behavior of the polymer component. Aqueous dispersion of ZIF-8 colloids possessing a zeta potential of 23.7 0.8 has sufficient tendency


Quantum dots (QDs) are high quality quantum scale semiconducting materials with unique optical and electrical properties. They offer the advantageous features of photostability, high molar extinction coefficients and luminescent quantum yields, size-dependent optical properties and low cost. Currently, the applications of QDs in light-emitting devices and solar photon conversion devices are attracting a great level of interest.256,257 Moreover, recent evidence suggests that QDs can be used in biological imaging as they display superior fluorescent properties compared with conventional chromophores and contrast agents.258 Since QDs have many desirable properties, the versatility of functional MOFs can be extended by introducing highly luminescent semiconductor QDs within the frameworks of MOFs. In QD@MOF composites, QDs can be stabilized against photochemical degradation through the deposition of a nanometre MOF shell, whilst retaining their valuable optical properties. The synergistic combination of luminescent QDs and the controllable porosity of MOFs in the QD@MOF composites could lead to many applications in selective molecular sensing, light harvesting, photochemical synthesis, and so on. The desire to maintain the size, composition and morphology of QDs requires methods for the encapsulation of pre-formed QDs in MOFs, by which the unique optical and electrical properties of QDs in composites would be retained. The first report of a QD@MOF composite through simple conjugation of fluorescent QDs with the MOF precursors was presented by Maspoch and coworkers.167 The QDs in the reaction mixture were mechanically trapped into the MOF during the growth of the MOF. The encapsulation of the QDs in the interior of the MOF particles can be confirmed by the fluorescence microscopy and TEM studies. Due to the weak interactions between the MOF precursors and the surface of QDs, the surface modification of QDs is necessary to direct the growth of the coordination network shell. Hupp, Huo and co-workers demonstrated the encapsulation of the PVP-modified lanthanide-doped NaYF4 rods and CdTe QDs in ZIF-8. Both composites emitted green light.113 The emission of NaYF4@ZIF-8 arises from upconversion of near-infrared radiation, while for CdTe@ZIF-8, the emission is a consequence of absorption of ultraviolet photons. The emission wavelength of the CdTe@ZIF-8 was red-shifted slightly to that for free CdTe NPs. Note that the emission of CdTe QDs encapsulated in ZIF-8 crystals can be selectively quenched by 2-mercaptoethanol,

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while bulky cyclohexanethiol was excluded by the small aperture of ZIF-8. The QDs with anionic surfaces could have the ability to accumulate the metal cations on the surface via complexation, which serve as starting points for the succeeding assembly of MOFs around QDs. For example, adaptive self-assembly of nucleotide monophosphates and lanthanide ions on the surface of carboxyl-modified CdSe/ZnS QDs to form the core–shell nanocomposites has been presented recently.259 The carboxyl-QDs possess carboxylate groups on the surface, which can promote the preferential formation of coordination networks on the anionic surface. The average thickness of the coordination shell was tunable depending on the concentration of QDs used. For comparison, amine-modified QDs were also employed under the same synthetic conditions. As a result, MOF particles and amino-QDs were observed separately and core–shell structures were not obtained. The results indicated that surface preorganization of lanthanide ions was an essential step in adaptive self-assembly. Buso, Falcaro and co-workers introduced the incorporation of highly luminescent multishell CdSe/CdS/ZnS QDs within cubic MOF-5 crystals through appropriate surface functionalization.260 The modification of QDs with hydrophilic 5-amino-1-pentanol can facilitate their dispersion in the typical solvents used in the synthesis of MOF-5, e.g., DMF or DEF, permitting the dispersion of QDs within the evolving framework during the reaction. A simple solvothermal reaction resulted in random incorporation of QDs within the MOF-5 crystals. Compositional and optical characterization of QD@MOF-5 samples obtained in DMF revealed the nonuniform distribution of the QDs within the cubic MOF-5 crystals, while QDs were homogeneously distributed throughout the crystals when using DEF suspension. The quenching effect of the thiols was studied on the composites and the observed degree of quenching varied markedly between two thiols of different sizes, which suggested that such composites can be used as size-selective optical sensors for small molecules by harnessing the QD emission as a probe and the sieving properties provided by the MOF. Porphyrin-based MOFs as light-harvesting antenna have attracted much attention due to their efficient energy-transport properties. Studies on the porphyrin-based MOFs have shown that the photogenerated excitons can migrate over 10–30 porphyrin struts within their lifetime, with a high anisotropy along a preferred direction.261 However, their absorption bands provide limited coverage in the visible spectral range for solar energy conversion. To solve this problem, Wiederrecht, Hupp and co-workers presented a strategy of modifying porphyrinbased MOFs with CdSe/ZnS core–shell QDs for the enhancement of light harvesting via energy transfer from the QDs to the MOFs (Fig. 24).262 The QDs were coated with a monolayer of an amphiphilic polymer with amine functional groups and a monolayer of PEG, and then were sensitized on the exterior surface of two zinc-centered porphyrin-based MOFs through the amine–Zn coordination. The photon-generated excitons in the QDs can transfer to the MOFs through resonance energy transfer, and thereby the broad absorption band of the QDs in the visible region offered greater coverage of the solar spectrum by the MOF–QD composites. Time-resolved fluorescence measurements

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Fig. 24 (a) Schematic diagram of using QDs to enhance light harvesting by MOFs through energy transfer from the QDs to the porphyrin-based MOFs. (b) Optical microscopy image of a plate-shaped MOF particle. (c, d) Fluorescence decays of CdSe/ZnS core–shell QDs of different sizes (QD550 and QD620) on glass and two porphyrin-based MOFs. Reprinted with permission from ref. 262.

showed that the lifetimes of QDs were shortened because of the energy transfer from the QDs to the MOFs, and the energy transfer with quantum efficiencies of up to 84% was achieved by tuning the size of the QDs. This sensitization approach can result in a 450% increase in the number of photons harvested by a single monolayer MOF structure with a monolayer of QDs on the surface of the MOF. Graphene quantum dots (GQDs), a fascinating class of recently discovered carbon dots that comprise discrete and quasi-spherical nanoparticles with sizes below 10 nm, are superior in terms of chemical inertness, low cytotoxicity and excellent biocompatibility compared to conventional semiconductor quantum dots.263 Recently, a report has appeared on the encapsulation of luminescent GQDs in ZIF-8 without a capping agent.264 The encapsulation was due to the surface adsorption of GQDs having polar functional groups (–COOH, –OH, etc.) with continuously forming faces of the ZIF-8 nanocrystals. The GQDs (ca. 5 nm) were well confined, dispersed in an ordered manner and had a profound impact on modulating the shape and size of the ZIF-8 nanocrystals in relation to the molar ratios of the ZIF-8 precursors. A higher ratio of GQDs to ZIF-8 tended to direct the GQD@ZIF-8 towards spherical particles with a smaller size. Stabilizing GQDs inside the ZIF-8 nanocrystals resulted in tailoring the luminescence emission of the GQD@ZIF-8 composite with a red shift of ca. 32 nm, which remained even after three months under normal laboratory conditions. Also the water adsorption capacity increased for the composite as compared to the pristine ZIF-8.

7. MOF–polyoxometalate composites Polyoxometalates (POMs), which are discrete anionic metal– oxygen clusters, form a large and distinctive class of molecular inorganic compounds with unrivalled diversity in composition, size and shape.265 They play an important role in various fields, such as catalysis, medicine, electrochemistry, photochromism


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and magnetism.266 Particularly, the catalytic applications of POMs in acid and oxidation reactions are garnering increasing attention.267 However, their low specific surface area and low stability under catalytic conditions are unfavourable. Immobilizing POMs in porous solid materials is a promising approach to stabilize POMs and optimize their catalytic performance. Among these solid supports, porous MOFs offer significant advantages of high surface area and porosity over the traditional solid supports. The dispersion of POMs within MOFs prevents the POMs from conglomerating and deactivating, enhancing their catalytic properties. On the one hand, POMs, on account of their compositional diversity and structural versatility, have been found to be extremely versatile building blocks for the construction of coordination supramolecules.268–270 In such POM-based MOFs, the organic ligands substitute the oxo groups of POMs to covalently link the metallic centers. They are traditionally prepared by cluster assembly approaches that lack predictability and controllability. On the other hand, POMs can be encapsulated in the pores of MOFs through host–guest interactions to form POM@MOF composites, where no covalent bonds are shared between the two components.271,272 Till now, a large number of POM-containing MOFs have been synthesized and have been well summarized in reviews.273 Here, we will put our emphasis on POM@MOF host–guest composites. Two methods have been employed to prepare POM@MOF host–guest composites. The first is a straightforward approach which involves the direct impregnation of POM moieties into the pores of MOFs. This impregnation method was firstly ´rey and co-workers to incorporate Keggin-type explored by Fe POMs into MIL-101.137 Considering the size of the K7PW11O40 ion, the polyanions could only diffuse into the larger cages of MIL-101, which allowed about five POM moieties per large cage. A similar approach has been followed by using the same MIL-101 host, and the impregnated POM@MIL-101 materials showed good catalytic activity and selectivity towards oxidation reactions.274–277 However, this approach suffers from drawbacks, including a low loading amount of POM, low homogeneity and leaching during reactions. Another effective approach is based on the template effect of POMs, in which POMs act as noncoordinating anionic templates to construct the three-dimensional MOFs. The self-assembly of POM@MOF composites is usually performed under hydrothermal conditions. By using this onepot approach, POMs could be readily encapsulated inside the MOFs in large amounts, and the confinement effect of the MOF pores does not lead to leaching, avoiding agglomeration and deactivation of the active species.278 During the last few years, many POM@MOF host–guest structures have been synthesized via self-assembly reactions that are mainly driven by the coordination chemistry of each component.279–282 Sun et al. prepared a series of POM@HKUST-1 composites based on HnXM12O40 (X = Si, Ge, P, As; M = W, Mo) from simple one-pot hydrothermal reactions (Fig. 25).283,284 X-ray single-crystal analysis indicated that Keggin polyanions were alternately arrayed as guests in the cages of HKUST-1, and only about half of the pores in the host were occupied.


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Fig. 25 Templated self-assembly of POM@HKUST-1 composites by simply mixing MOF precursors and polyanions. (a) Keggin polyanion. (b) Three-connected node and hexagonal face (blue) defined by a BTC ligand linked to six adjacent Cu2+ ions. (c) SBU and square face (green) defined by four Cu2+ ions. (d) Cube of eight sodalite-like truncated-octahedral cages sharing square faces. Reprinted with permission from ref. 284.

The catalytic performance of PW12O40@HKUST-1 with accessible catalytically active sites was evaluated by the hydrolysis of esters in excess water, which showed high catalytic activity and sizeselectivity. With smaller esters like methyl esters and ethyl esters, very high catalytic activity was observed. The catalyst can be recycled 15 times without activity loss and leaching. Particularly noteworthy is that PW12O40@HKUST-1 displayed its potential application in the elimination of nerve gas, with encapsulated polyanions as catalytically active centers for decomposition of dimethyl methylphosphonate (DMMP).284 This composite showed extraordinarily good adsorption capability for DMMP with 15.5 molecules per formula unit. The conversion of DMMP to methyl alcohol was 34% at room temperature, and increased gradually with temperature, reaching 93% at 50 1C. Besides, the effect of particle size of the SiW12O40@HKUST-1 composite on the catalytic activity for the dehydration of methanol to dimethyl ether has been fully studied, demonstrating a negative effect of the diffusion limitation.285 A similar effect of diffusion limitation was also found in the esterification reaction of acetic acid with 1-propanol catalyzed by [email protected] Hill and co-workers further explored POM@HKUST-1 by incorporating Cu containing Keggin-type polyoxometalate units [CuPW11O39]5.287 It was observed that the stability of both components was mutually enhanced due to the synergistic effect between POM and MOF. The interactions also dramatically increased the catalytic activity of the POM for air-based oxidative decontamination of various toxic sulfur compounds including H2S to S8. The turnover number, based on H2S consumed and POM units present in the composite, reached ca. 4000 in less than 20 h in aqueous solution. In addition, sizeand shape-dependent selectivity was clearly observed in the oxidation of the evaluated mercaptans to disulfides. The encapsulation of phosphotungstic acid (PTA) in the MIL-101 host as heterogeneous catalyst has also been investigated.288–290 PTA can be encapsulated in both the large cages and small pores of MIL-101 by the one-pot synthesis method to

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during POM encapsulation.292 The molecular steps involved in the self-assembly of HKUST-1 around the POM molecules were unravelled by using ex situ NMR spectroscopy, small-angle X-ray scattering, near-IR spectroscopy, and dynamic light scattering. During the formation of HKUST-1, Cu2+ ions first attached to the surface of PTA through the complexation of Cu2+ ions with POM, and then the prearranged Cu2+ ions were linked with benzenetricarboxylate groups resulting in the POM@HKUST-1 structure. Direct evidence of strong interaction between Cu2+ and POM ions was provided by 31P, 183W and 17O NMR spectra, which showed significant shifts (Fig. 26). Thus, this study could open a new supramolecular approach for successful incorporation of a variety of POMs into different MOFs. In the latter work, the template effect of the Keggin polyanion during the synthesis of NH2-MIL101(Al) was further investigated by means of in situ small- and wide-angle X-ray scattering (SAXS/WAXS), and kinetic analysis demonstrated the role of PTA as a nucleation site.293

8. MOF–carbon composites

Fig. 26 31P NMR spectra of (a) a solution of H3PMo12O40xH2O, and a solution of Cu(NO3)23H2O and H3PMo12O40xH2O (b) before and (c) after BTC linker addition. The signal at d = 0 ppm is due to the external reference. Reprinted with permission from ref. 292.

increase the loading amount of POMs. Interconnection of the cages through the pentagonal and hexagonal windows does not lead to the diffusion of POMs out of the host. PTA@MIL-101 exhibited the highest activity reported to date for the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate at 313 K with TOFs exceeding 600 h1.288 Besides, a remarkable activity in two acidcatalyzed reactions of the esterification of n-butanol with acetic acid and the dimethyl ether production from methanol was presented by the catalysts. In contrast, in the catalysts prepared by impregnation, the strong interaction between POM and the support deteriorates the acidity of the resulting solid, and hence the catalytic activity. Subsequently, PTA@MIL-101 was also assessed as a potential catalyst for many other reactions including the selective dehydration of fructose and glucose to 5-hydroxymethylfurfural,289 the Baeyer condensation of benzaldehyde and 2-naphthol, the three-component condensation of benzaldehyde, 2-naphthol, and acetamide, and the epoxidation of caryophyllene by hydrogen peroxide.290 More recently, PTA@NH2-MIL-101(Al) has been used as a support to accommodate highly dispersed Pt clusters, which can be used as catalyst for CO oxidation and toluene hydrogenation.291 The use of POMs as templates is a common synthesis strategy for preparing POM@MOF composites. Nevertheless, very few studies have been done on understanding the template effects of POMs during the formation of MOFs, which is of crucial importance for the rational design of new generations of POM@MOF materials, even other MOF composites.292,293 Bajpe et al. first studied the molecular-level mechanism of such template effect

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Carbon, which possesses various allotropes (graphite, fullerenes, nanotubes and diamond), microtextures with different degrees of graphitization, dimensionality from 0D to 3D, and existence forms (powder, fiber, foam, fabric and composite), represents a very attractive material for many applications.294 As the most promising candidates for functional applications, nanocarbons, especially graphene and carbon nanotubes (CNTs), are gaining increasing attention owing to their appeal arising from the combination of the outstanding properties of in-plane graphite with a high surface area.295 Graphene is a monolayer of carbon atoms arranged in a 2D honeycomb lattice and can be seen as an individual atomic plane pulled out of bulk graphite. Graphene oxide (GO) is a graphene derivative derived from the oxidative exfoliation of graphite, which is solution-dispersible and can act as the precursor of graphene after reduction.296 On the other hand, CNTs are well-ordered, high aspect ratio allotropes of carbon. The two main variants, singlewalled carbon nanotubes (SWCNTs, diameters of 0.4–2 nm) and multi-walled carbon nanotubes (MWCNTs, diameters of 2–100 nm), both possess a high tensile strength, ultra-light weight, and excellent chemical and thermal stability.297 The exceptionally mechanical, electrical and thermal properties of graphene and CNTs commend them as valuable nanostructured fillers in MOF composites. This new class of MOF composites, which combines nanocarbons with functional inorganic materials resulting in a combination of the individual properties, is even more predestined for applications concerning sustainable energy and environment. To date, numerous MOF–nanocarbon composites have been successfully made with activated carbons, carbon monoliths, GO and CNTs, and intensively explored for diverse applications. 8.1

Incorporation of MOFs into porous carbons

Porous carbon materials are considered as one of the most promising candidates for gas applications due to their low cost, high thermal and chemical stability, high adsorption capacity


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and facile regeneration ability. Recent studies have revealed that incorporation of MOF particles into the meso-/macropores of carbon materials to obtain hierarchically porous systems can optimize the structural features, and thereby enhance the ńior and co-workers described adsorption properties.298,299 Ju a composite material with MOFs inside activated carbon (AC) pores, which proved to be efficient in adsorbing aldicarb in rats.298 The composite was synthesized under hydrothermal conditions by adding AC in different proportions in situ during the synthesis of Ln-succinate (Ln = Tb and Eu). SEM micrographs and EDS analyses clearly showed that the AC pores were filled with Tb-MOF. A biological test of the Tb-MOF@AC composite was performed on rats to evaluate the adsorption of aldicarb, which is one of the most toxic pesticides registered. In an acidic medium, the average percentages of aldicarb adsorption observed in vitro in 10 min were 46% and 41% for activated carbon and Tb-MOF, respectively. The values were much lower than the values of 78% and 77% for Tb-MOF@AC composites containing 40% and 50% of AC, which is clearly an important result in cases of poisoning, because the composite begins to remove aldicarb from the stomach with an acidic environment, significantly reducing the risk of it entering the ileum and then the innermost tissues. The studies on the injuries caused by aldicarb demonstrated that treatment with the composite containing 50% of AC maintained the tissue integrity totally, indicating the effective protective action of the composite. By following a similar method, the study of incorporating hierarchical porous carbon monoliths (HCM) with HKUST-1 through a repeat impregnation and crystallization strategy to enhance the volumetric based CO2 capture capability of the original HCM has been reported.299 HCM was used as a matrix for in situ synthesis of HKUST-1. The resulted HKUST-1@HCM composite retained the monolithic shape and exhibited unique hybrid structure features of both HCM and HKUST-1. A high CO2 uptake of 22.7 mL cm3 on a volumetric basis was achieved by this composite, which was almost twice that of original HCM. The dynamic gas separation measurement of HKUST-1@HCM, using 16% (v/v) CO2 in N2 as feedstock, illustrated that CO2 can be easily separated from N2 under ambient conditions and achieved a high separation factor for CO2 over N2 ranging from 67 to 100, reflecting its strong competitive CO2 adsorption (Fig. 27). Meanwhile, it underwent a facile CO2 release on purging with argon at 25 1C. 8.2

MOF–carbon nanotube composites

Carbon nanotubes (CNTs), molecular-scale tubes of graphitic carbon, are among the stiffest and strongest fibers known, with Young’s moduli as high as 1 TPa and tensile strengths of up to 63 GPa.300 They also have remarkable adsorption and metallicand semi-conductive electronic properties depending on their structures and diameters. Many of these outstanding properties can be best exploited by incorporating CNTs into some form of matrix and the preparation of CNT-containing composite materials is now a rapidly growing subject.301,302 In particular, CNTs have been considered as useful composite fillers in H2 storage research. Currently, there is great interest in the preparation


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Fig. 27 (a) Breakthrough curve of HKUST-1@HCM using a stream of 16% (v/v) CO2 in N2 at 25 1C. (b) Recycle runs of CO2 adsorption–desorption on HKUST-1@HCM at 25 1C, using a stream of 16% (v/v) CO2 in N2, followed regeneration by argon flow. Reprinted with permission from ref. 299.

of CNT@MOF composites to obtain enhanced thermal and chemical stability, gas storage capacity and mechanical properties owing to the synergistic effects. The discrete and selectable type and loading amount of the filled CNTs enable a good structure–property relationship. A commonly used method for preparing CNT@MOF composites involves mixing nanotube dispersions with MOF precursor solutions and then in situ incorporating CNTs during the formation of MOF crystals. To facilitate solubilisation of CNTs and heteronucleation of MOFs, CNTs are often pretreated chemically. CNTs have been considered as useful composite fillers in H2 storage research. The incorporation of CNTs into MOF crystals was first exploited by Park and co-workers.50 The CNT@MOF-5 composite was synthesized by adding acid-treated MWCNTs dispersed in DMF to the MOF-5 synthesis mixture. The obtained CNT@MOF-5 had the same crystal structure and morphology as those of virgin MOF-5, and the presence of MWCNTs admixed with MOF-5 crystals was confirmed by TEM (Fig. 28). Compared to the pristine MOF-5, the stability of CNT@MOF-5 upon exposure to electron beam irradiation and atmospheric moisture was dramatically improved. The good stability during electron beam exposure was presumably due to the fact that the incorporated MWCNTs sustained the composite crystal structures by dissipating electrostatic charges accumulating under electron beam irradiation. A similar enhancement of structural stability was also observed when MOF-5 was confined within the interior channels of MWCNTs.303 Significantly, the as-prepared composite exhibited a much greater Langmuir specific surface area increasing from 2160 to 3550 m2 g1, and a great increase in H2

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Fig. 28 (a) HRTEM micrograph of a CNT@MOF-5 crystal (white arrows indicate MWCNT lattices). The inset shows the SEM micrograph of a crystal. (b) Enlarged view of the boxed area in (a), and (c) typical selected area electron diffraction patterns of (a). Reprinted with permission from ref. 50.

storage capacity from 1.2 to 1.52 wt% at 77 K and 1 bar, and from 0.3 to 0.61 wt% at 298 K and 95 bar. The increased H2 storage capacity should be attributed to the increased ultramicropores at the interface and the improved structural integrity of the MOF component. Subsequently, they prepared a composite of Pt-loaded MWCNT@MOF-5 by using Pt-loaded MWCNTs under the same solvothermal condition, where 1.5 and 4.2 times enhancement in H2 uptake was observed compared with the pure MOF at 77 K and under higher pressures at 298 K, respectively, indicating the presence of hydrogen spillover.304 Hydrogen-storage enhancement was also found very recently in an interpenetrated MWCNT@MOF-5 composite with high mesoporosity.305 Prasanth et al. have reported enhanced hydrogen sorption at high pressure in SWCNT@MIL-101, which was synthesized by adding various amounts of purified SWCNTs in situ during the synthesis of MIL-101.306 A layered morphology for the composite crystals observed by TEM suggested the growth of MIL-101 on the functionalized surface of SWCNTs. The incorporation of SWNTs into MIL-101 was further revealed by the decrease in mesopores as well as the increase in ultramicropores for the composite in N2 adsorption measurements. By incorporating SWCNTs, H2 storage capacities of MIL-101 were observed to increase from 6.37 to 9.18 wt% at 77 K up to 60 bar and from 0.23 to 0.64 wt% at 298 K up to 60 bar, which was ascribed to the decrease in the pore size and enhancement of micropore volume of MIL-101 by nanotube incorporation. Recent experimental and theoretical studies indicate that doping alkali-metal ions, in particular Li+ ions, to the frameworks of MOFs can enhance not only the H2 adsorption capacities but also the CO2 uptake owing to the strong affinity of Li+ ions towards H2 and CO2 molecules.307–310 Motivated by

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Fig. 29 (a) CO2 and (b) CH4 adsorption isotherms of HKUST-1 (I), Li+@ HKUST-1 (II), CNT@HKUST-1 (III) and CNT-Li+@HKUST-1 (IV) at 298 K. Solid and open symbols represent adsorption and desorption, respectively. Reprinted with permission from ref. 311.

these results, Cao, Yang and co-workers have reported the Li+-doped CNT@HKUST-1 composite, which was prepared by heterogeneous nucleation and growth of MOF on the carboxylic surface of acid-treated MWCNTs and subsequent introduction of Li+ ions into the frameworks.311 Powder X-ray diffraction indicated that the incorporation of CNTs and Li+ ions did not disrupt or destroy the crystal structure of HKUST-1. TEM revealed that the CNTs were highly admixed with HKUST-1 with two distinctive lattices of the two components. Although the incorporation of CNTs and Li+ ions significantly decreased the BET specific surface area (SSA) of the framework, the pore volume of the composite was nearly identical to that of the unmodified HKUST-1, suggesting that there were ultramicropores in the interface region between CNTs and the frameworks. Gas storage capacity was enhanced in the presence of CNTs or Li+ ions over the pristine HKUST-1, which was further increased for the Li+-doped CNT@HKUST-1 composite by about 305% and 200% for CO2 and CH4 uptake per effective SSA, respectively (Fig. 29). The isosteric heat of CO2 adsorption calculations indicated that the gas adsorption enhancement was attributed to the strong affinity of Li+ ions for gas molecules. Synergistic effects on porosity and chemistry properties can lead to significant improvements in adsorption capacity and selectivity. Since the incorporation of CNTs into MOFs has been proven to be capable of enhancing adsorption capacity for CO2,311,312 the applications of CNT@MOF composites can be reasonably expanded to gas separation. Recently, Xiang et al. evaluated the potential of CNT@HKUST-1 and a series of representative MOFs including MOF-177, UMCM-1 ([Zn4O(bdc)(btb)4/3]), ZIF-8, MIL-53(Al) and HKUST-1 in separation and purification of CO2 from the CO2–CH4 mixture.313 The dual-site Langmuir


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Freundlich (DSLF)-based ideal adsorption solution theory (IAST) was used to predict adsorption of each component in the CO2–CH4 mixture. The IAST-predicted results showed that the CNT@HKUST-1 composite exhibited the greatest selectivity among these materials in the range of 5.5 to 7.0 for equimolar CO2–CH4 at the pressure between 1 and 20 bar. Moreover, the selectivity of this composite for the CO2–CH4 mixture did not change with the composition of CH4, which is one of the excellent properties as a promising separation material. A more recent study showed that the hierarchically structured bulk composites fabricated by confined growth of HKUST-1 into the interspace of the vertically aligned carbon nanotube (VACNT) arrays exhibited enhanced adsorption selectivity of CO2 over N2 in all the pressure range, owing to the synergistic effect between é et al. prepared a defect-free HKUST-1 and VACNTs.314 Dume ZIF-8/CNT membrane via growing ZIF-8 homogeneously within the interstitial pores of the CNT bulky-paper supports to form a continuous and dense network, which showed a high selectivity of N2 over CO2 of 7 for the 50 : 50 N2–CO2 mixture.315 In addition to the applications of gas storage and separation, CNT@MOF composites have also been developed as novel hybrid electrode materials for the determination of nano-molar levels of lead,316 and used as a precursor for the carbonization synthesis of CNT@mesoporous carbon composites.317 8.3

MOF–graphene composites

Graphite oxide (GO) is prepared by the oxidation of graphite with strong oxidation agents. This oxidative treatment results in the decoration of various functional groups such as hydroxyl and epoxide groups on the basal plane of GO, and carboxylate groups mainly located at the edges of the layers, increasing its hydrophilicity and solution dispersibility. The coexistence of ionic groups and aromatic sp2 domains allows GO to act as structural nodes and participate in the bonding interactions in MOFs. In addition, the functionalization of the graphene plane with carboxylate or pyridine groups could enhance the coordination bonds and thus guide the growth of MOFs. The concept of MOF–GO composites has been first developed by Bandosz and co-workers in 2009.318 By virtue of the facile delamination and functional nature of the GO surface for composite preparation, a number of MOF–GO composites including MOF-5,63 HKUST-1,319 and MIL-100(Fe)320 have been synthesized by simply dispersing GO powder into the MOF synthesis mixtures. It is expected that the incorporation of GO into the frameworks of MOFs will enhance the stability, porosity and electron-conductive properties of MOFs. Strong chemical bonds existing between the MOFs (MOF-5 and HKUST-1) and GO as a result of the coordination between the GO oxygen groups and the metallic centers of MOFs lead to the formation of MOF–GO composite materials.318,321 It is proposed that GO/MOF-5 composites are comprised of a sandwich-like structure based on the alternation of GO sheets with layers of MOFs.318 As shown by the SEM images in Fig. 30, the morphology of GO/MOF-5 composites changes as GO content increases. In the hypothesized building process, MOF-5 blocks attach to a graphene layer by reaction with epoxy groups on GO,


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Fig. 30 SEM images for the parent (A) MOF-5, (B) GO and GO/MOF-5 composites at GO loadings of (C, D) 5 wt%, (E) 10 wt% and (F) 20 wt%. Reprinted with permission from ref. 318.

similar to the interaction of water with MOF-5, and then an alternation between the attachment of GO layers and MOF-5 blocks takes place. No collapse of the MOF-5 structure occurred owing to the limited number of epoxy groups and their restricted spatial location compared to free molecules of water. However, at a high loading of GO, preferential MOF interaction with the carboxylate groups at the edges of the GO layers led to the formation of wormlike structures as shown in Fig. 30F, because of the higher number of carboxylic groups on the edges of the GO layers and the greater affinity of metallic centers for these groups. The porous structures of the MOFs were well preserved in their corresponding composites as demonstrated by PXRD and adsorption studies. Besides the micropores, GO/MOF-5 and GO/HKUST-1 composites showed small amounts of mesopores, which increased with an increase in the content of GO. Composite formation with graphene materials, on the one hand, is dependent on the degree of oxidation, crystallite size, and the functional groups at the GO surface.322,323 The study of GO/HKUST-1 composites revealed that the lack of functional groups on graphite only resulted in the formation of a simple physical mixture.322 On the other hand, the specific network geometry of MOFs also plays an important role in the production of MOF–GO composites based on the relative orientation of metal coordination sites on the MOF available for GO binding.320,324 Owing to the cubic shape of unit cells in both MOF-5 and HKUST-1, the coordination sites are located within parallel planes or perpendicular planes (Fig. 31a and b). This specific structural feature favours the attachment of the graphene layers to the MOF units and the subsequent growth of the crystal. Unlike MOF-5 and HKUST-1, MIL-100(Fe) is not an ideal candidate to build MOF–GO composites since the metal coordination sites lie along irregular planes, and the attachment of the GO layers to the spherical cages of MIL-100(Fe) prevents the proper

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Fig. 31 Schematic comparison of the coordination between GO carbon layers and the MOF units for different types of MOF networks: MOF-5, HKUST-1 and MIL-100. For MOF-5 and HKUST-1: atoms involved in coordination are indicated. For MIL-100(Fe): the red pyramids represent supertetrahedra units made of trimers of iron octahedra linked by molecules of BTC. Reprinted with permission from ref. 320.

formation of the MIL-100(Fe) structure (Fig. 31c). As a result, a more irregular texture and a higher amount of an amorphous phase were observed for GO/MIL-100(Fe).320 The performance in ammonia adsorption by MOF–GO composites has been evaluated and found to be strongly dependent on the porosity and chemical nature of MOF, and the synergy of the two components. Both GO/MOF-5 and GO/HKUST-1 composites exerted some synergy enhancing ammonia adsorption capacity and retention at ambient temperature in comparison with the parent MOFs and the hypothetical physical mixtures of the components.63,325 A schematic view of ammonia adsorption for the GO/HKUST-1 composite is presented in Fig. 32. Indeed, physical adsorption forces in MOFs may not be strong enough to retain small molecules such as ammonia in ambient conditions. In GO/MOF-5 and GO/HKUST-1 composites, the surface interactions between GO and MOF induce the formation of a new pore space in the interface between the carbon layers and the MOF units, and provide strong dispersive forces, which enhances

Fig. 32 Visualization of two sites of ammonia adsorption in the composites with (1) physisorption at the interface between graphene layers and HKUST-1 units and (2) binding to the copper centers. Ammonia molecule is represented by the dark gray circles. Reprinted with permission from ref. 325.

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the physical adsorption capacity and the retention of ammonia molecules.63,325 In the case of GO/MIL-100(Fe), the situation was opposite as the poor interface between MIL-100(Fe) and GO decreased the porosity and crystallization of the composites.320 The specific nature of the MOF also has a significant influence on the ammonia adsorption. The GO/MOF-5 composite offered higher dispersive forces than MOF-5 alone, and thus an enhancement in ammonia adsorption was observed. Nevertheless, the collapse of the MOF-5 structure in the presence of humidity caused the retained ammonia to be progressively desorbed from the materials, which limits the environmental application of the synthesized composites.63 Active HKUST-1 and MIL-100(Fe) have unsaturated metal sites arising from the removal of coordinated water molecules bound to metal centers, which can act as additional reactive sites for ammonia adsorption in the composites.325–327 In GO/HKUST-1, ammonia molecules are adsorbed via coordination to these open metal centers. Further reaction with the framework irreversibly leads to the formation of complexes, which is accompanied by the collapse of the HKUST-1 structure.326,328,329 This reactive adsorption is visible through two successive changes of the adsorbent’s color during the adsorption process. Unlike GO/MOF-5, ammonia adsorption and retention in GO/HKUST-1 composites are enhanced in moist conditions owing to the dissolution of ammonia in a water film present in the pore system.325 For GO/MIL-100(Fe), the main mechanism of ammonia retention is via the Lewis interactions between ammonia and the metal ¨nsted interactions between ammonia and centers and Bro the water molecules present in composites.320 NO2 and H2S adsorption by GO/HKUST-1 composites, which follows the similar multi-step reactive adsorption behavior, has also been thoroughly investigated.324,330–332 The presence of oxygen functionalities on either side of the GO layer allows it to act as a structure-directing agent in MOF assembly. In GO/MOF-5, the role of GO as a structure-directing agent for MOF growth was clearly observed.318 The morphology of GO/MOF-5 composites changed from layered distribution to disordered wormlike structure as the loading of GO increased. More recently, the tuning of the shape and particle size of ZIF-8 was found to be modulated by the concentration of GO in ZIF-8/ GO composites, where the ZIF-8 nanoparticles were stabilized on the GO surfaces through functional groups.333 With increasing GO content from 1 to 20 wt%, the transformation from hexagonal morphology with the particle size in the range of B100 to 150 nm to a homogeneous distribution of uniform nanospheres of B4 nm was observed. Thus, the functional GO sheets acted as both structure-directing and size-controlling agents for ZIF-8 nanocrystals. The composites exhibited remarkably increased CO2 storage capacity compared to the parent ZIF-8 and can be used as precursors to prepare GO@ZnS nanocomposites. Liu et al. also discussed the influence on the HKUST-1 nanocrystal size induced by the incorporation of GO into the composites.334 It was demonstrated that epoxy groups on the GO layers can act as seed sites to prevent aggregation of crystallites and increase dispersion, which led to the formation of nanosized and welldispersed HKUST-1 crystallites on the layers of GO. The obtained


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composites exhibited about a 30% increase in both CO2 and H2 storage capacity, which was primarily attributed to a synergistic effect between the two components. As an alternative to the monodentate epoxyl linking in aforementioned MOF–GO composites, the bridging bidentate coordination ability of carboxylate groups favors a higher degree of connectivity and stronger coordination bonds. However, the density of carboxylate groups on the basal plane of the GO layer is low; rather, this functionality is mainly located at the edges. The Loh group functionalized the reduced GO (rGO) with benzoic acid using the diazonium grafting method, which resulted in a 3-fold increase in carboxylate functionality over GO.335 The benzoic acid functionalized graphene (BFG) was subsequently used as a structure-directing template and framework linker for MOF-5 growth. It was observed that BFG led to more pronounced changes in the structure of MOF-5 compared to pure GO because of the high density of carboxylate groups on BFG. At a low loading level of 1 wt%, a similar sandwich structure as reported for GO/MOF-5 (5 wt%) was observed. More interestingly, BFG/MOF-5 synthesized with 5 wt% BFG revealed a dramatic transformation into unusual nanowire morphology (Fig. 33). The nanowire grew along the [220] direction due to strong metal–carboxylate binding interactions, and had a uniform shape with a diameter of B300 nm. A kinetically controlled template-directed growth model was proposed for this composite,

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where the BFG acted as the nucleation template. Micro-Raman spectroscopy and AFM images indicated that the BFG sheets having a diameter coincident with that of the composite nanowires were intercalated within the structure. As derived from the N2 adsorption data, intercalated graphene slightly increased the surface area in porous MOF-5. The electrical properties of single BFG/MOF-5 nanowire have been studied. The generally insulating nature of the MOF–BFG system at low field was observed, confirming the spatial separation of graphene by MOF. The current increased sharply at voltage 44 V due to thermionic emission and this can be further enhanced under white-light illumination due to photoinduced charge transfer. Moreover, the modification of BFG/MOF-5 with organic dyes introduced photoexcitation and charge injection to produce a more significant enhancement in the photocurrent. Subsequently, the same group synthesized another composite by adding different weight percentages of rGO sheets functionalized with a pyridine-terminated dye (G-dye; 5, 10, 25, 50 wt%) to the iron-porphyrin MOF (Fe-P MOF).336 Graphene sheets decorated by pyridine groups on either side of the sheets are analogous to pillar connectors. Studies revealed that pyridine-functionalized graphene sheets can also influence the crystallization process of MOF. Accompanying the increased lattice distortion of MOF, the morphology of the Fe-P MOF crystal changed from a plate shape to a rod shape when an increasing amount of G-dye (5 and 10 wt%) was added. The incorporation of G-dye not only increased the porosity of composite, but also enhanced the electrocatalytic properties. The presence of graphene and pyridinium linkers acted synergistically with the porphyrin catalysts to afford the facile 4-electron oxygen reduction reaction (ORR) pathway. The composite also possessed a much higher selectivity for ORR and a significantly reduced methanol crossover effects compared to Pt catalyst, which make it possible to be used as a promising Pt-free cathode in alkaline direct methanol fuel cells (DMFCs).

9. MOF thin films on substrates

Fig. 33 (a) Raman spectra of MOF-5 and BFG/MOF-5 (5 wt%). The inset shows an SEM image of an individual BFG/MOF-5 nanowire. (b–d) Raman mapping of BFG/MOF-5 nanowires. Left: optical image. Middle: Raman maps integrated by Raman band at 1609 cm1. Right: Raman maps integrated by the D band of graphene. Reprinted with permission from ref. 335.


Recently, the deposition of patterned thin films of MOFs on a substrate has paved the way for the nanotechnological applications of MOF-based devices. Thus, the fabrication of MOF thin films with defined porosity, combined with chemical functionality, on various solid substrates is highly desirable in view of their potential use as smart membranes, catalytic coatings, chemical sensors, and so on.337–339 This field is nascent but rapidly developing, and a large number of important contributions have been published since the first report by Fischer in 2005.340–346 Also, several excellent reviews on MOF thin films and surface/interface chemistry of MOFs related to this topic have appeared recently.56,347–351 Therefore, we shall not describe here in detail but just briefly summarize the most successful techniques developed for the growth of MOF thin films. Several ways to deposit thin films of MOFs on substrates have been reported, of which some are restricted to conductive substrates, such as the microwave-assisted or electrochemical syntheses.344,345,352 Generally, two fabrication methods have been distinguished for the direct growth/deposition of MOF

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thin films. The first one involves the addition of the substrate to a MOF synthesis solution under ambient or solvothermal conditions.353,354 Growth takes place on the surface of the substrate and sometimes in solution at the same time. In a modification of this method, not the virgin reaction mixtures, but the aged precursor solutions containing the MOF nuclei, from which the pre-precipitated bulk phase is removed by filtration if required, are used.191,340,355 This growth leads to the formation of polycrystalline films where crystals are attached to the substrate surface in a more or less intergrown and continuous fashion. The second mild technique developed for the facile preparation of MOF thin films on the substrates is the layer-by-layer method, also referred to as liquid phase epitaxy, which has been pioneered by ¨ll and co-workers.173,340,356,357 This technique relies on Fischer, Wo the sequential deposition of monolayers of metal salts and organic linkers on a functionalized substrate (Fig. 34). Unreacted or physisorbed components are removed between successive deposition steps by rinsing with an appropriate solvent. Because the two types of MOF building blocks are separated, the growth occurring in each cycle is self-limiting. In contrast to the first method, the LBL method permits the growth of smooth and homogeneous MOF ultrathin films with diameters in the nanometer range, dubbed SURMOFs. Additional distinct advantages are the good control over the thickness, crystallographic orientation and interpenetration of the MOF multilayers.353,358–361 The deposition of MOF thin films in both methods can be controlled by adjusting the surface chemistry.340,357 In particular, self-assembled monolayers (SAMs), which are functional organic thin films supported on a wide variety of substrates such as metals and oxides, have become an attractive interface for SURMOF growth. The exposed functional groups of the SAMs can be designed to specifically interact with the metal clusters, and therefore control the lateral structures as well as the crystallographic orientation of the SURMOFs.173,357,362 For example, oriented growth of the HKUST-1 thin films along the [100] and [111] crystallographic orientations was observed for the –CO2H and –OH terminated surfaces of Au-supported SAMs, respectively, as confirmed by X-ray diffraction.191 More recently, the layer-by-layer method coupled with the Langmuir-Blodgett (LB) technique has been developed by Makiura, Kitagawa and co-workers for preparing homogeneous,

Fig. 34 The layer-by-layer approach for the growth of the SURMOFs on a SAM-functionalized substrate. The approach involves repeated cycles of immersion in solutions of the metal precursor and solutions of an organic ligand. Between steps the material is rinsed with solvent. Reprinted with permission from ref. 357.

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Fig. 35 Schematic illustration of the fabrication of layer-based SURMOF on the substrate using a combined LB and LBL growth method. Reprinted with permission from ref. 363.

highly ordered and preferentially oriented MOF nanofilms onto unmodified silica and gold surfaces (Fig. 35).363,364 The MOF films consist of metalloporphyrin building units and the layers stack by weak interactions, such as p stacking. This film growth strategy is sufficiently versatile to open the way to obtain diverse types of well-ordered nanofilms with complete structural growth control in both the out-of-plane and in-plane orientations relative to the substrate.

10. MOF@MOF core–shell heterostructures The fabrication of MOF@MOF core–shell heterostructures is a promising way not only for modification of the porous properties but also for the addition of a new function to the MOF without changing the characteristic features of the MOF crystal itself.365 To construct the multifunctional core–shell heterostructures, two strategies have been put forward recently. Heteroepitaxial growth of a shell MOF crystal on the external surface of another seed MOF crystal could generate a composite crystal, in which the two coordination components with different metal centers and/or bridging linkers are segregated into different regions of the crystal.365 The success of this approach is based on a close crystal lattice match between the underlying MOF substrate and the deposited MOF. The surface of the MOF substrate is used as a particular SAM. Another strategy for preparing multifunctional MOFs with a core–shell heterostructure is the post-synthetic modification (PSM) including the selective reaction of the reactive residue of an organic linker and the controlled replacement of the framework metal ions (i.e., transmetalation) or ligands. This modification of the metal centers or the organic linkers of a MOF is selectively constrained to either the external surface or the internal core of the MOF crystals.


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10.1

Heteroepitaxial MOF@MOF composites

Epitaxial growth of one porous MOF on the surface of another one can be attractive along with the functionalization of surfaces due to the fact that the control of interface and surface structure by heteroepitaxy can influence the properties of the MOF materials such as gas sorption and separation. In view of the requirement for the lattice match between the two MOF components, a variety of isoreticular frameworks with similar unit cell parameters are available for epitaxial growth where ligands and/or metals can be changed without disruption of the network topology. Early studies on the epitaxially grown crystals with core–shell heterostructures reported by MacDonald,366 Stang,367 Palmore,368 Hosseini369,370 and others were focused on molecular crystals, where a diffuse interface between the isostructural phases was clearly observed by TEM.371 Since the first example of heteroepitaxial growth of core– shell MOF single crystals reported by Furukawa et al.,365 much progress has been made in this area. By using the single crystals of a Zn-MOF, [Zn2(ndc)2(dabco)] (dabco = diazabicyclo[2.2.2]octane), as seeds for the heterometallic epitaxial growth of the isoreticular Cu-MOF, [Cu2(ndc)2(dabco)], the core–shell crystals with a Zn-MOF core and a Cu-MOF shell were obtained under solvothermal conditions (Fig. 36a). Optical microscopy images of the sliced single crystal clearly showed that the

Fig. 36 (a) Schematic illustration of the heteroepitaxial growth of the Cu-MOF on top of the Zn-core MOF. (b) Schematic model of the structural relationship between the core lattice and the shell lattice on the (001) surface. The red lines indicate the commensurate lattice between the core lattice and the shell lattice; the (5 5) structure of the core crystal or the pffiffiffiffiffi pffiffiffiffiffi 26 26 structure of the shell. Two Miller domains of the shell crystal are grown on the (001) surface of the core crystal while maintaining the pffiffiffiffiffi rotational angle (Djav = 11.6518), which corresponds to the 26 direction of the (001) surface. The inset shows the chemical structure of the (001) surface. Reprinted with permission from ref. 365.


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colorless core crystal was surrounded by the greenish shell crystal. Both the core and shell crystals had the same P4/mmm tetragonal space group, while there was a slight mismatch (B1%) in their unit cell parameters, which was sufficient to allow a detailed investigation of the structural correlation at the interface between the two crystalline components. The structural model for the (001) surface revealed an in-plane rotational epitaxial growth of the shell crystal with an average rotational angle of 11.71 with respect to the lattice of the core crystal (Fig. 36b). Considering the smaller lattice constant of the shell crystal, in-plane rotational epitaxial growth can compensate for the difference in the lattice constants. Such rotational crystal growth did not occur for the (100) surface because of the perfect lattice match between the two phases along this direction. Notably, by employing the stepwise LBL method, the heteroepitaxial growth of Zn-MOF on the [001] oriented thin film of Cu-MOF supported on Au SAMs has been achieved.372 More recently, Oh and co-workers have reported the precise and controllable preparation of MOF heterostructures through the selective isotropic and anisotropic nanoscale growth of MOFs (MIL-88B series, [M3O(bdc)3(NO3)], M = Fe, Ga and In).373 It was found that the seeds of MIL-88B(Fe) (A) nanorods directed the formation of analogous structures of MIL-88B(Ga) (B) and MIL-88B(In) (C) under consecutive solvothermal conditions, giving core–shell-type (A@B) and layer-type (C/A/C) heterocompositional nanorods, respectively. The increase in the length and width and the contrast of light and shade was clearly observed in the STEM images of A@B core–shell heterostructures. In contrast, an increase in length but not width noticeably revealed the formation of C/A/C layer-type hybrid nanorods, where the growth of MIL-88B(In) only occurred in the c-direction at both tips of the MIL-88B(Fe) seeds. Detailed studies on crystalline structures showed that the dissimilar growth behavior was caused by the size variation among the metal ions within the initial and new MOFs. The results may allow for a greater understanding of the growth of the heterostructured MOFs. In addition to the heterometallic epitaxial growth of MOF hybrid materials, the variation of the bridging linker in the epitaxy has also been explored. Koh et al. investigated the synthesis of layered MOF heterostructures using bdc and bdc-NH2 linkers, which was achieved by seeding in analogy to epitaxial growth of the core–shell MOFs.374 Core–shell structures were obtained by immersing the seed crystals of MOF-5 (L = bdc) in the synthesis solution of IRMOF-3 (L = bdc-NH2), and vice versa. The resultant composite crystals exhibited high spatial color contrast arising from colorless MOF-5 and orange IRMOF-3. The porosity of these heterostructures was retained, and their surface area values were between those of MOF-5 and IRMOF-3 depending on the composition of the products controlled by the feed ratio of the two linkers. Applying the seeded growth technique in the presence of core–shell MOFs, multi-layered crystals were also produced. Yoo et al. further prepared the hybrid films by heteroepitaxially growing IRMOF-3 films on the surface of the MOF-5 seed crystal layer on a porous alumina support.375

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The key to success in this heterostructure system is to choose excellent candidates for epitaxial growth. The modular [M2(L)2P] (L = dicarboxylate linker, P = bidentate nitrogen pillar ligand) structure (Fig. 36a) is so versatile that its three components can be varied without changing the original tetragonal topology. Through sequential ligand functionalization, Furukawa, Kitagawa and co-workers fabricated multifunctional core–shell heterostructures with spatially separated porous networks of different pore sizes.376 The core–shell structure can be used as a single-crystal extractor exhibiting size selectivity where the core crystal is the storage container and the shell crystal is the size separation filter. The crystal extractor was prepared from [Zn2(L)2(dabco)] where L = bdc was used for the core crystal and L = 9,10-anthracene dicarboxylate (adc) for the shell crystal, resulting in significant differences in pore sizes and surface areas between the core and shell crystals (pore sizes for core: 7.5 7.5 Å2 and 5.3 3.2 Å2; for shell: 4.5 2.7 Å2 and 1.7 1.7 Å2). CLSM images at different points indicated that the shell crystal of [Zn2(adc)2(dabco)] covered all the surfaces of the core crystal of [Zn2(bdc)2(dabco)], which was confirmed by the Raman mapping measurement. The core–shell crystals exhibited the capability of selective adsorption and accumulation of cetane over the branched isomer isocetane from mixtures as determined by GC-MS, even when the cetane/isocetane ratio was as low as 1 : 100 (Fig. 37a). In contrast, the core crystal alone showed no selective adsorption, while the cetane storage capacity of the shell crystal was much lower than that of the

Fig. 37 (a) The selective adsorption of cetane over isocetane from mixtures with a cetane/isocetane ratio of 1 : 100 as determined by GC-MS. (b) The amount of cetane stored in core only, shell only, and core–shell crystals when using a 1 : 100 mixture of cetane/isocetane. Reprinted with permission from ref. 376.

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Fig. 38 Schematic illustration of (a) the series of frameworks [M2(L)2P] and (b) the anisotropic epitaxial growth of the second framework, [Zn2(ndc)2(dpndi)] (crystal B), on the core framework, [Zn2(ndc)2(dabco)] (crystal A). (c) The optical microscopic image of the heterocrystal. (d) The infrared spectra by microscopic attenuated total reflection (ATR) measurements of the orange part (crystal B) and the colorless part (crystal A) of the heterocrystal shown in (c) and the single crystal samples of pure A and B. Reprinted with permission from ref. 377.

core–shell crystal (Fig. 37b). Thus, the improvement in the selectivity and capacity of the cetane storage arose from the large pore volume of the core framework and the small pores of the defect-free shell crystal. The [Zn2(L)2P] system with diamine pillar ligands (P) of different length scales has also been used by Sakata, Kitagawa and co-workers to fabricate sandwich-type block MOF heterocrystals by face-selective epitaxial growth, where only two surfaces of a rectangular prism crystal were used selectively as substrates for epitaxial growth of the second MOF crystal (Fig. 38).377 The anisotropic hybridization of two frameworks was achieved due to the tetragonal nature of the structure. In the tetragonal structure, four (100) surfaces have rectangular lattices comprising both carboxylate layer ligands and pillar ligands whereas the remaining two (001) surfaces with square lattices only contain layer ligands. Because of choosing the same layer ligand in both crystals, there is no requirement for the lattice distance in the [001] direction to match, thus allowing free usage of the pillar ligand. As a result, the reaction of [Zn2(ndc)2(dabco)] with the second much longer pillar diamine, N,N0 -di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide (dpndi), led to only epitaxial growth at the (001) surfaces. The epitaxial relationship of the heterocrystals was characterized by XRD. This strategy will open the way for fabrication of a multifunctional MOF where several porous properties are integrated into a single crystal. 10.2

MOF@MOF composites formed through PSM

Post-synthetic modification of MOFs is a frequently employed strategy for obtaining modulated and functionalized MOFs.378,379 The complete replacement of structural organic linkers or


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framework metal ions involved in the network connectivity as well as the covalent PSM of the reactive residues of organic linkers has been reported.380,381 As a strategy for preparing multifunctional MOFs with core–shell heterostructures, spatially controlled PSM of the metals or linkers of a MOF to produce a core–shell composite crystal has been recently developed.382 However, it is challengeable to selectively constrain this modification to the external surface or the internal core of the MOF crystal. Hupp, Nguyen and co-workers reported the selective functionalization of a MOF surface in a core–shell fashion via covalent post-synthetic modification of a MOF, [Zn2(ndc)2(TMS-dpe)] (14, TMS-dpe = 3-[(trimethylsilyl)ethynyl]-4-[2-(4-pyridinyl)ethenyl]pyridine).383 Here, the dipyridyl ligand TMS-dpe acts as a pillar linker possessing silyl-protected acetylenes that could be deprotected easily using tetrabutylammonium fluoride (NBu4F, TBAF). They proposed that given the bulk of the NBu4+ counterion, the fluoride-based deprotection would only occur on the outermost layers of the MOF crystals. To verify this, they reacted the surface-deprotected MOF with ethidium bromide monoazide in a ‘‘click’’ reaction, the CuI-catalyzed Huisgen cycloaddition of azides to terminal alkynes. The resultant MOF was fluorescent only on the external surface, confirming that the functionalization occurred exclusively on the outer surface. UV-Vis adsorption measurements indicated that less than 0.8% of the dipyridyl ligands had undergone cycloaddition. They also demonstrated that the modification of the MOF with the PEG chains could render the MOF surface hydrophilic. Such surface functionalities are important in catalysis owing to their gating properties for controlling access to MOF channels. In an extension of these studies, they also investigated similar chemistry with [Zn2(tcpb)(TMS-dpe)] (15, tcpb = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene), which has a non-interpenetrated framework with larger pores.384 The ‘‘click’’ chemistry was used to functionalize the interior and exterior surfaces of the MOF crystals selectively and independently with different organic molecules (Fig. 39). The surface deprotection was carried out in the aqueous solution of KF by taking advantage of the poor solubility of KF in chloroform. When the MOF was filled with chloroform, the aqueous solution of KF would not enter the pores of the MOF. Subsequent reaction with ethidium bromide monoazide converted the terminal alkyne groups into triazoles. The interior modification was achieved by treating the

Fig. 39 Schematic illustration of the strategy for functionalizing the internal and external surfaces of a MOF (15) crystal independently with two different azides. Reprinted with permission from ref. 384.


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surface-modified MOF with TBAF in DMF to deprotect the interior alkynes, and then with benzyl azide. In this way, all the silyl-protected alkyne groups were converted into triazoles, but with different substituents on the interior and exterior surfaces of the crystals. The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra of successive pyridine dissolutions of the final MOF material confirmed its ‘‘core–shell’’ composition. By taking advantage of the equilibrium state of coordination bonds, Furukawa, Kitagawa and co-workers prepared coordinatively immobilized monolayers of benzoate functionalized boron dipyrromethene (BODIPY) dyes on the carboxylate-terminated surfaces of [Zn2(L)2(dabco)] (L = bdc or ndc) and HKUST-1.385 The formation of an organic monolayer was achieved by a simple ligand-exchange reaction involving coordination bonds on the crystal surface alone. The tetragonal [Zn2(L)2(dabco)] crystal has only four (100) surfaces terminated by zinc–carboxylate bonds, where the ligand-exchange reaction took place to produce a fluorescent dye monolayer as confirmed by CLSM (Fig. 40), face index analysis and AFM. The two (001) surfaces terminated by zinc-amine bonds remained unchanged. A similar study was made for the formation of an organic monolayer on the crystal surface of HKUST-1. The crystal structure of HKUST-1 exhibits regular octahedral morphology, and all the surfaces denoted as the (111) faces are terminated by carboxylate groups, which hence can support dye monolayers. Therefore, the assembly of the organic monolayer is most likely determined by the lattice structures of MOF substrates. As an alternative to the modification of the organic ligands, selective replacement of framework metal ions (i.e., transmetalations) in isoreticular MOFs to obtain core–shell heterostructures has also been explored recently by Lah and co-workers.386 The porous isoreticular MOFs, [M6(btb)4(dipy)3]

Fig. 40 Representations of surface-modified crystals (left), CLSM images (middle) and transmission images (right) of the crystals at z = A and z = B in (a) the c axis orientation and (b) the a axis orientation. Scale bars: A, 180 mm; B, 190 mm; C, 130 mm; D, 130 mm. Reprinted with permission from ref. 385.

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(where M = Zn, Co, Cu and Ni; and dipy = 4,4 0 -dipyridyl), have different framework stabilities depending on the framework metal ions. The transmetalations from thermodynamically less stable Zn-MOF and Co-MOF to more stable Cu-MOF and Ni-MOF can occur by soaking single crystals of Zn-MOF and Co-MOF in the DMF solution of Cu(II) and Ni(II) ions, respectively, and no reverse transmetalation was observed. By simply controlling the soaking time, selectively transmetalated Co(II)/ Cu(II)- and Co(II)/Ni(II)-core–shell heterostructures were formed via kinetically controlled replacement that was mainly restricted to the external shell region of the crystals.

11. MOF–enzyme composites Enzymes as natural catalysts feature high reactivity, selectivity, and specificity under mild conditions, contributing to green and sustainable processes in chemical, pharmaceutical, and food industries.387 Unfortunately, the use of enzymes for industrial applications has been hindered by their low operational stability, difficult recovery, and loss of activity under operational conditions. Immobilization of enzymes on a solid support can enhance their stability as well as facilitate ease of separation and recovery for reuse while maintaining activity and selectivity.388,389 Great efforts have been made to search suitable supports for enzyme immobilization, and silica materials offering high surface areas and mesopores have attracted the most attention.390–392 However, the leaching of the adsorbed enzyme is still often observed during the reaction process due to the weak interactions between enzyme molecules and silica supports. The tunable but uniform pore sizes and functionalizable pore walls of porous MOFs may make them appealing to accommodate enzymes for catalytic applications. Nevertheless, the micropore size of most MOFs precludes the entry of large-sized enzymes and could result in only external surface attachment with low enzyme loading via adsorption and/or covalent bonding reaction.393–396 Recent advances in mesoporous MOFs (mesoMOFs)397 provide opportunities for enzymatic catalytic applications and some reports dealing with the immobilization of enzymes inside the nanopores of MOFs have been published.48,398,399 The channel-type mesoMOF, [Cu(bpdc)(ted)0.5]400 (16), was first employed to encapsulate the microperoxidase-11 (MP-11), which is the product of proteolytic degradation of cytochrome c.398 MP-11 is a relatively small protein with molecular dimensions of about 1.1 1.7 3.3 nm such that the protein with a specific orientation could enter the large channels (2.2 nm) of 16. After stirring in a solution of MP-11 at room temperature, the loading amount of MP-11 in the MOF was calculated to be 30 mmol g1 based on the UV-Vis spectra. The catalytic activity of the immobilized MP-11 was tested with the oxidation of methylene blue and a conversion of approximately 63% was achieved, while the free MP-11 gave only less than 10% conversion. Ma and co-workers recently selected the robust Tb-mesoMOF, [Tb16(TATB)16],401 as an excellent platform for investigating the interaction between enzyme molecules and MOF pores.48,402,403

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Fig. 41 (a) Molecular structure of MP-11 and the nanoscopic cages in Tb-mesoMOF. (b) Optical images of Tb-mesoMOF and MP-11@TbmesoMOF. (c) Normalized single-crystal absorbance spectrum derived from specular reflectance for MP-11@Tb-mesoMOF (red) and solution optical spectrum for free MP-11 in buffer solution (black). Kinetic traces for the oxidation of DTBC by (d) free MP-11 in buffer (0.6 mM); (e) MP-11@TbmesoMOF (2.0 mg), Tb-mesoMOF (2.0 mg), and MP-11@MCM-41 (2.0 mg) in methanol with H2O2. Reprinted with permission from ref. 48.

Two nanoscopic cages of 3.9 and 4.7 nm in diameter are connected through pentagonal and hexagonal windows with diameters of 1.5 and 1.7 nm, respectively, which are large enough to allow the entry of MP-11 molecules and accommodate them in the mesopores (Fig. 41). Immersing freshly synthesized Tb-mesoMOF crystals in MP-11 solution for 50 h at 37 1C led to a saturated loading of 19.1 mmol g1. The encapsulation of MP-11 was proven by the single crystal optical adsorption spectroscopy and the significant decrease of the BET surface area of the MOF. A bathochromic shift of the encapsulated MP-11 in Tb-mesoMOF was observed, indicative of the interactions between the trapped MP-11 molecules and the porous substrate. For comparison, a mesoporous silica material, MCM-41, was selected to immobilize MP-11. However, the loading capacity of MCM-41 was 3.4 mmol g1, much lower than that of Tb-mesoMOF due to its lower surface area. The catalytic activities of MP-11@Tb-mesoMOF and MP-11@MCM-41 were assessed by monitoring the oxidation reaction of 3,5-dit-butyl-catechol (DTBC) to the corresponding o-quinone. MP-11@Tb-mesoMOF demonstrated a high initial reaction rate for the reaction, which was more than twice that for MP-11@MCM-41. Moreover, although the free MP-11 had the highest initial reaction rate, the fast deactivation due to the aggregation of MP-11 resulted in a low final conversion of only 12.3% after 25 h. The severe leaching of MP-11 incorporated in


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MCM-41 also gave a low conversion of 17.0% for MP-11@MCM-41, much lower than that of 48.7% for MP-11@Tb-mesoMOF. It should be noted that MP-11@Tb-mesoMOF had a good recyclability, indicating the strong interactions between the MOF pores and MP-11, and the confinement effect of the MOF host matrix.48 Continuous efforts have also been made to immobilize other enzymes including the heme protein cytochrome c (Cyt c) and myoglobin (Mb) into the Tb-mesoMOF, despite the larger molecular dimensions of the proteins (2.6 3.2 3.3 nm and 2.1 3.5 4.4 nm for Cyt c and Mb, respectively) compared to the access pore sizes.402,403 Mechanistic studies assumed that the enzyme molecule possesses a dynamic and flexible structure and can undergo a significant conformational change during translocation into the MOF interior through the relatively small nanopores.

12. MOF-other molecular species composites Molecular materials such as organic dyes,404,405 organometallic compounds,406,407 metalloporphyrins,408–410 biomolecules411,412 and other functional molecules413 have also been composited with MOFs for various applications. The use of MOFs as molecular encapsulators takes advantage of the powerful confinement effect of MOFs, which is proposed as an efficient way to protect molecules from aggregation, heterogeneous distribution and leaching. Impregnation procedures are mostly used to encapsulate these molecular materials. Alternatively, the self-assembly of MOFs in the presence of molecular moieties in the MOF precursor solutions could lead to irreversible in situ encapsulation. Dyes are widely used as colorants, photographic sensitizers, markers and fluorescent probes. The encapsulation of various dyes in porous MOFs has been extensively investigated.414–416 Up to now, the adsorption of dye molecules has been established as a method to prove pore accessibility and to remove the dye pollutants,101,417 but the functional applications of the resultant dye@MOF composites have been seldom studied. For instance, the encapsulation of solvatochromic dyes in the MOF was demonstrated to be an elegant way to create a multifunctional material for visual detection.418 By encapsulating the cationic pyridinium hemicyanine dye in an anionic bio-MOF-1, [Zn8O(ad)4(bpdc)62Me2NH2] (ad = adeninate), Qian, Chen and co-workers have presented a new two-photon-pumped microlaser based on the resultant dye@bio-MOF-1 composite, which exhibited two-photon fluorescence owing to the large absorption cross-section and the encapsulation-enhanced luminescence efficiency of the dye (Fig. 42).405 To develop porous MOFs as platforms for biomimetic catalysis, in addition to the immobilization of enzymes in MOFs, one can also encapsulate the active centers of enzymes in the cavities of MOFs.408–410 This strategy is particularly appealing to mimic heme proteins because their active centers are iron porphyrin. Consequently, the encapsulation of catalytically active metalloporphyrins (MPs) in MOFs to obtain the critical


Fig. 42 (a) Schematic illustration of encapsulating hemicyanine cationic dye DMASM in bio-MOF-1. (b) Single-photon- and two-photon-excited emission spectra of DMASM@bio-MOF-1. (c) Two-photon-pumped lasing spectra of DMASM@bio-MOF-1 under different pumped pulse energy. Inset: microscopy image of a DMASM@bio-MOF-1 single crystal excited at 1064 nm (left) and the power dependence profile of the fluorescence intensity (right). Reprinted with permission from ref. 405.

catalytic features associated with enzymes has gained a lot of attention. Eddaoudi and co-workers selected an anionic zeolite-like MOF to in situ encapsulate a cationic porphyrin via self-assembly.408 After loading different transition metal ions like Mn, Cu, Zn or Co into the encapsulated porphyrin, the resultant MP@MOF composites can serve as catalysts for the oxidation of cyclohexane. Larsen et al., following a similar approach, encapsulated Fe3+- and Mn3+-tetrakis(4-sulphonatophenyl)porphyrin irreversibly in the nanocages of HKUST-1 to mimic heme enzymes in terms of both structure and reactivity.409 The MP@MOF composites were also prepared by wet infiltration of Fe- and Ru-based phthalocyanine complexes into MIL-101(Cr) and exhibited excellent catalytic performance in the selective oxidation reactions.410,419

13. Conclusions The combination of the highly porous structure, chemical versatility and structural tailorability of MOFs, along with the low cost and availability for mass production, has provided many opportunities for the preparation of MOF composites. The design and syntheses of MOF composites are a promising and low-cost route to achieve novel materials bearing a set of combined properties superior to those of the individual components. MOFs can be engineered with a wide variety of functional materials, such as metals, oxides, polymers, POMs, carbon, and so on, as outlined in this review, enriching the porous structures with additional functionalities. Driven by the constant demand for optimization of composite properties, the particular efforts are directed towards the design and formation of specially constructed architectures rather than the random mixtures. Generally, MOF composite systems introduced in this review can be classified into three

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structural types: (1) MOFs used as supports/matrices to host/ disperse nanoscale objects (nano-objects); (2) MOF core–shell structures where MOF crystals are used as either cores or shells; and (3) MOFs grown/deposited onto two- or three-dimensional solid substrates. In the first type, MOFs act as porous supports to accommodate nano-objects of diverse compositions. Using MOFs as supports for nano-objects provides many advantages. The confinement effect of MOFs having different pore/channel sizes can prevent aggregation and diffusion, and generate uniform distribution of the nano-objects. In particular, the MOFs with cage-like pores possessing large cavity dimensions and relatively reduced openings are more suitable supports to preserve leaching. The crystalline porous structures of MOFs limit the migration and aggregation of small metal clusters/ NPs, making MOFs highly potential as host matrices for MNPs. Some ultrafine nano-objects, such as silica and polymer nanoparticles with diameters of several nanometers, which are unattainable through conventional processing methods, can be obtained by using nanoporous MOF supports. In addition to the discrete nanoparticles, 1-D and 2-D nanomaterials such as CNTs and graphenes can also be dispersed into the frameworks of MOFs to harvest the enhanced properties. For the second type, MOFs have been widely used as porous shells to take the advantages of the size selectivity of MOF pores in applications related to separation, catalysis and sensing. On the other hand, to improve the stability, water dispersibility and biocompatibility of NMOFs for bio-applications such as imaging and drug delivery, surface modification by coating NMOFs with silica and organic polymers is a commonly employed strategy. Both two- or threedimensional substrates can be used to fabricate the third type of MOF composites. Depositing MOFs onto two-dimensional substrates provides a common way to prepare MOF-based thin films. When three-dimensional substrates, like the meso-/macro-porous polymers and silica configured as monoliths and beads, are used to disperse the MOF particles, hierarchically porous structures could be obtained, which is important for the applications in dynamic systems. The investigation of MOFs as platforms for preparing composites is still in its infancy; nevertheless, very rapid progress is being made. The structures and properties of the parent MOFs are the primary considerations when designing the composites. It should be noted that the current studies are mostly focused on a small number of ‘‘star’’ MOFs such as MOF-5 and extended IRMOF series, HKUST-1, MIL series and ZIF family. More alternative chemically and thermally stable porous MOFs and practical large-scale synthetic methods are urgently required. Meanwhile, it is clear that the nature of the functional materials added to MOFs is extremely critical not only to construct the composite structures but also to influence the composite properties. For example, the surface functionalities on the substrates have demonstrated the importance of the SAMs in nucleation, morphology and properties. The nature of the interface between the MOF and the secondary component also has a significant impact on the composite performance. Therefore, an understanding of the interactions present at the interface is highly desirable in order to examine

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the structure–property relationships and design the next generation of high-performance MOF composites. Furthermore, the composite studies should be extended to a wider range of functional species. The combination of different guest moieties within one MOF will open up new avenues for developing multifunctional materials. It should be pointed out that the structures and structuredependent properties depend strongly on the different preparation methods. To date, some successful approaches have been explored to achieve precise control over the morphology of composites. For instance, the double solvent method displays distinct superiority in the encapsulation of ultrafine metal and oxide NPs in MOFs. More versatile and facile fabrication strategies are expected to be developed in future for the continuous advancement of functional MOF composite materials. Though many challenges still exist, the rapid development of MOF composites in recent years has predicted well for the bright future of this new type of functional materials. With sustained research efforts towards this exciting field, their practical applications in different fields may eventually be achieved.

Acknowledgements We are grateful to the editors for kind invitation. The authors are pleased to acknowledge the fine work of the talented and dedicated graduate students, postdoctoral fellows, and colleagues who have worked with us in this area and whose names can be found in the references. The authors would like to thank AIST and METI for financial support. Q.L.Z. thanks JSPS for postdoctoral fellowship.

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