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Computational exploration of the gas adsorption on the iron tetracarboxylate metal-organic framework MIL-102 a
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D. Damasceno Borges , M. Prakash , N.A. Ramsahye , P.L. Llewellyn , S. Surblé , P. d
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Horcajada , C. Serre & G. Maurin a
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS UM ENSCM, Université Montpellier, Place E. Bataillon, Montpellier Cedex 05, 34095 France b
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS UM ENSCM, Ecole Nationale Supérieure de Chimie Montpellier, 8 rue de l'Ecole Normale, Montpellier Cedex 05, 34296 France
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Laboratoire MADIREL, UMR 7246, CNRS, Centre de Saint Jérôme, Aix-Marseille Université, Marseille Cedex 2, 13397 France d
Institut Lavoisier, UMR 8180 CNRS, Université de Versailles St Quentin en Yvelines, 45 Avenue des Etats-Unis, Versailles 78035, France Published online: 15 May 2015.
To cite this article: D. Damasceno Borges, M. Prakash, N.A. Ramsahye, P.L. Llewellyn, S. Surblé, P. Horcajada, C. Serre & G. Maurin (2015): Computational exploration of the gas adsorption on the iron tetracarboxylate metal-organic framework MIL-102, Molecular Simulation, DOI: 10.1080/08927022.2015.1030645 To link to this article: http://dx.doi.org/10.1080/08927022.2015.1030645
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Molecular Simulation, 2015 http://dx.doi.org/10.1080/08927022.2015.1030645
SIMULATION OF FRAMEWORK MATERIALS Computational exploration of the gas adsorption on the iron tetracarboxylate metal-organic framework MIL-102 D. Damasceno Borgesa, M. Prakasha, N.A. Ramsahyeb, P.L. Llewellync, S. Surble´d, P. Horcajadad, C. Serred and G. Maurina*
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Institut Charles Gerhardt Montpellier, UMR 5253 CNRS UM ENSCM, Universite´ Montpellier, Place E. Bataillon, Montpellier Cedex 05, 34095 France; bInstitut Charles Gerhardt Montpellier, UMR 5253 CNRS UM ENSCM, Ecole Nationale Supe´rieure de Chimie Montpellier, 8 rue de l’Ecole Normale, Montpellier Cedex 05, 34296 France; cLaboratoire MADIREL, UMR 7246, CNRS, Centre de Saint Je´roˆme, Aix-Marseille Universite´, Marseille Cedex 2, 13397 France; dInstitut Lavoisier, UMR 8180 CNRS, Universite´ de Versailles St Quentin en Yvelines, 45 Avenue des Etats-Unis, Versailles 78035, France (Received 22 January 2015; final version received 12 March 2015) Density functional theory calculations have been combined with forcefield-based grand canonical Monte Carlo simulations to explore the adsorption of CO2, N2, CH4 and H2 on the small one-dimensional channel MIL-102, a naphthalene tetracarboxylate-based metal-organic framework (MOF) built up from a connection of trimers of trivalent iron. A detailed analysis is provided on the preferential arrangement of the confined adsorbates as well as the energetics of the host/guest interactions. The co-adsorption properties of this solid for the elimination of CO2 from hydrogen, natural and flue gases are then revealed. The so-predicted performances are further compared with those reported so far for a diverse series of MOFs. Keywords: MOF; carbon dioxide capture; molecular simulation; density functional theory; Monte Carlo
1.
Introduction
Porous hybrid materials, including metal-organic frameworks (MOFs), have attracted great attention during the past two decades as their physical and chemical properties open them to a wide range of potential applications in several domains, including energy, environment and health.[1,2] In particular, the design of novel MOFs for CO2 capture is still the subject of a large research effort, since some of these hybrid materials have already been shown to be promising for this purpose [2 – 5] with a performance that either equals, or sometimes surpasses that of other porous solids (e.g. zeolites,[6] mesoporous silica, [7] carbons,[8]). This is particularly true for the selective adsorption of CO2 from CO2/CH4 and CO2/N2 gas mixtures, which is of prime importance for natural gas purification and flue gas treatment, respectively.[9] The search for optimal MOFs for such applications has up to now been primarily driven by experimental trial-and-error methods and/or the computational high-throughput screening of thousands of existing and hypothetical materials.[10] A number of strategies have been used to tailor-design new MOFs specifically targeted for CO2 capture. One of the most exploited routes consists of engineering MOFs with specific chemical features on either the metal nodes or the organic linkers with the aim of enhancing the affinity of their surfaces for CO 2. Many MOFs possessing metal centres with uncoordinated unsaturated sites, such
*Corresponding author. Email:
[email protected] q 2015 Taylor & Francis
as CPO-27 (Ni),[11] STA-12 (Ni) [12] and MIL-100(Fe, Cr) [13] have been shown to interact with CO2 more strongly than less polar or apolar gases. The functionalisation of the organic linkers of existing MOFs with amino-, nitro-, acid- and sulfonic- groups has been proposed as a second route to boost the interactions between the pore wall and CO2, although this strategy does not greatly affect the strength of the interactions with N2 or CH4. This method has already been used on a series of MOFs, including UiO-66 (Zr),[7,14 –16] CAU-10(Al),[17] MIL-125(Ti) [18] and Zeolitic Imidazolate Frameworks (ZIF)s [19] among others. The specific sites where these molecules are most likely to adsorb are often the uncoordinated unsaturated metal sites and/or the functionalised groups. However, their concentration is usually relatively low, not exceeding a few mmol g21. Therefore, this only leads to an improvement in selectivity at low coverage, which often does not meet the requirements of the relevant applications. Another approach envisaged is the use of ultra-small ˚ ) to pore MOFs (pore diameter within the range 3.0– 3.5 A favour the selective adsorption of CO2 viaa size exclusion effect. A typical illustration is the work reported by Nugent et al. [20], where a series of Cu and Zn SIFSIX MOF-type materials, whose porosities considerably restrict the adsorption of CH4 and N2 at low pressure as compared with the adsorption of CO2, achieve a high selectivity for the latter gas. This can be explained by the
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larger kinetic diameters of CH4 and N2with respect to that of CO2. Here, we propose an alternative route by considering a MOF with (i) small one-dimensional channels, which are expected to favour a selective CO2 adsorption, mainly driven by the confinement effect, (ii) a polar environment created by terminal water molecules coordinated to metal sites, thus boosting the affinity of the solid for CO2, as already shown in Cu-BTC [21] and MIL-102(Fe) [22] and (iii) water stability,[23] which is a crucial pre-requisite for a further use in real post-combustion conditions. Therefore, we selected the iron (III) naphthalene tetracarboxylate-based MIL-102 material (MIL stands for Materials of the Institut Lavoisier), which is a derivative of the small one-dimensional channel type Cr solid previously synthesised under hydrothermal conditions.[24] The choice of this metal with respect to Cr is motivated by its non-toxicity and biocompatibility, which is of prime importance for further applications. As a first step, a model of MIL-102(Fe) was constructed starting with the experimental structure of the Cr analogue. This model was subsequently optimised using density functional theory (DFT) calculations, relaxing both the unit cell parameters and the atomic positions of the constituents. The cell dimension of the optimised structure was compared with that experimentally elucidated from the X-ray diffraction data collected on a MIL-102(Fe) synthesised sample. This structure was also used to calculate the partial charges for all the atoms of the MOF. Using this model, forcefield-based grand canonical Monte Carlo (GCMC) simulations were applied to explore the adsorption properties of CO2, CH4, N2 and H2 in this solid, allowing us to elucidate the preferential arrangement of the confined adsorbates, as well as to obtain information on the energetics of the host/guest interactions. Finally, the selective CO2 adsorption performance of MIL-102(Fe) from three different mixtures were predicted and further compared with those already reported for a series of existing
MOFs. The mixtures considered were CO2/CH4, CO2/N2 and CO2/H2, of prime importance for societally relevant CO2 elimination applications.
2.
Results and discussion
2.1 Structure description and characterisation The DFT-optimised MIL-102(Fe) structure shows cell ˚ , b ¼ 12.47 A ˚ , c ¼ 9.588 A ˚, parameters, a ¼ 12.37 A ˚ a ¼ 90.058, g ¼ 89.878, b ¼ 120.118, V ¼ 1279.4 A3, similar to the ones experimentally refined for the hydrated form of the synthesised MIL-102(Fe) sample (a ¼ 12.702 ˚ and c ¼ 9.720(1) A ˚ , a ¼ g ¼ 908, b ¼ 1208, (1) A 3 ˚ V ¼ 1358.1(2) A – see ‘Experimental’ section). It contains trimers of iron(III) octahedra, interlinked via tetracarboxylate anions. Each Fe atom is bonded to four oxygen atoms – two O atoms from two different bidentate ˚ ), one m3carboxylate ligands (mean d (Fe – O) ¼ 1.934 A ˚ oxo atom (mean d (Fe –O) ¼ 1.847 A) and one terminal O ˚ ; d (Fe– OH2) ¼ 2.106 A ˚ ). site (d (Fe – OH) ¼ 1.850 A In contrast to the Cr version, MIL-102(Fe) is synthesised in the absence of fluorine-containing media (see the ‘Experimental’ section for the details of the synthesis protocol). This explains why the structure was DFToptimised with the three terminal sites of each trimer saturated with one-OH group and two terminal water molecules pointing towards the centre of the channels. The corresponding chemical formula is thus FeIII 6 O2(H2O)4 (OH)2{C10H4(CO2)4}3, leading to a density of 1.71 g/cm3. The distribution of the – OH group in each trimer was randomly chosen. A view along the z-axis of MIL-102(Fe) reveals a one-dimensional hexagonal channel described by six interconnected trimer chains. The pore wall of this channel contains two hydroxyl groups and four water molecules for every hexagon outlined by the trimer units, as illustrated in Figure 1. The pore volume of this structure was then estimated using the thermodynamic method developed by Myers and
Figure 1. (Colour online) (a) A view along the z-axis of the MIL-102(Fe) structure with a 1D-type cylindrical channels decorated with – OH group and H2O molecules, (b) perspective view of two trimer chains connected by naphthalene linkers. Each naphthalene ligand ensures the connection of four trimer blocks.
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Molecular Simulation
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treatment above 500 K for the removal of the terminal water molecules would induce an additional cost to the process.
Monson [25], which considers a helium probe molecule. In this calculation, He is modelled as a Lennard-Jones (LJ) ˚ and 1/ fluid with the corresponding parameters s ¼ 2.58 A kB ¼ 10.22 K,[26] and MOF atoms were treated using the LJ parameters taken from the universal force field (UFF).[27] The calculation yielded a value of 0.12 cm3/g, regardless of the distribution of the –OH group over the terminal sites. This value is lower than the one reported for the Cr analogue [32] (0.158 cm3/g) calculated from a structure with the absence of terminal water molecules leaving two uncoordinated metal centres on each trimer. The same scenario considered for MIL-102(Fe) led to a pore volume of 0.17 cm3/g, which is closer to the value previously reported for the Cr version.[32] Figure 2 reports the pore size distribution (PSD) calculated using the geometric methodology reported by Gelb and Gubbins [28] (where the UFF LJ parameters were used for all of the framework atoms) for the MIL-102 (Fe) structure either in the presence or in the absence of the terminal water molecules in each trimer. Two plausible OH terminal group distributions were tested. These are referred to as P1 and P2 in Figure 2, and correspond to two – OH groups located at opposite sides of the pore wall (P1), and two located on adjacent sites (P2). One can observe that all of the MIL-102(Fe) structures considered here ˚ . Such a exhibit a similar channel diameter of 5.6 A dimension is expected to allow the kinds of small gas molecules considered in the most common gas capture/ separation applications to enter the channels. In the following section, we describe an investigation of the adsorption properties of the MIL-102(Fe) containing two terminal water molecules in each trimer. This corresponds to the structural form whose performance requires characterisation, taking into account that a thermal
2.2 Sorption properties We have previously emphasised that using the geometric method [29] to calculate the accessible surface area, where a nitrogen-sized probe molecule is rolled over the framework MOF surface is not always appropriate to accurately capture the Brunauer– Emmett –Teller (BET) area of MOFs with small or intermediate pore sizes.[30] Indeed, the theoretical BET area was estimated according to the criteria proposed by Rouquerol et al. [31] from the adsorption isotherm calculated by GCMC simulations for N2 at 77 K (see the computational section for details).The resulting low BET area of , 70 m2/g (Figure 3) is within the same range of the Langmuir surface area previously reported for MIL-102(Cr).[32] The single component adsorption behaviour of apolar and polar gas molecules (H2, CH4, N2 and CO2) was subsequently predicted using the MIL-102(Fe) model at ambient temperature (the parameters used are described in the computational section). The adsorption isotherms for N2 and CO2 were first calculated using the structural models coupled with either electrostatic potential (ESP)derived partial charges [32] or Mulliken [33] partial charges, for all atoms of the MOF. Figure 4 clearly shows that there is very little difference between the adsorption isotherms obtained using the two charge calculation schemes, and thus the partial charges do neither greatly influence the shape of the adsorption isotherms, nor does affect the amount adsorbed in the whole range of pressure up to 40 bar. However, some small differences are evident in the low pressure domain for CO2, with the
Figure 2. (Colour online) PSD for the MIL-102(Fe) structures with the presence or the absence of the terminal water molecules in each trimer calculated for two different configurations of the terminal – OH groups (P1 and P2 illustrated as insets).
Figure 3. (Colour online) Calculation of the BET area for MIL102(Fe) using the GCMC simulated isotherm for N2 at 77 K. The inset reports a plot of Vexcess (1 –P/P0) versus P/P0 with a linear plot in a low domain of pressure that satisfies the second consistency criterion as defined by Rouquerol et al. [31].
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N2 Uptake (molecule/u.c.)
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Figure 4. Single component adsorption isotherms calculated at 303 K on MIL-102(Fe) for CO2 (left) and N2 (right). Comparison between ESP (square solid symbol) and Mulliken (circle empty symbol) charges used for describing the atoms of the framework.
uptake being slightly higher in the case of the model using the ESP charges. This is consistent with a higher adsorption enthalpy calculated at low coverage; a value of 2 27 kJ/mol was obtained with the charges from the ESP model, and 2 25 kJ/mol for Mulliken charge model. This means that there is a slightly stronger host/guest electrostatic interaction as the ESP charges carried by the atoms of MIL-102(Fe) are larger than those calculated by the Mulliken partitioning scheme. At higher pressures, the adsorption isotherm is mainly governed by intermolecular interactions between the guest molecules, and this explains a similar profile in both cases. Figure 5 reports the comparison between the simulated adsorption isotherms for CO2, N2 (in both cases using ESP charges for the atoms of the MOF), as well as CH4 and H2 on MIL-102(Fe), calculated at 303 K. One should note that as we selected microscopic uncharged single LJ site models to describe both CH4 and H2, there is indeed no host/guest electrostatic interaction, and the resulting adsorption isotherm is thus invariant of the partial charges considered for the MOF. The majority of the adsorption of
CO2 and CH4 occurs essentially at a pressure of less than 10 bar, with the presence of a plateau corresponding to 2.6 and 1.8 molecules per unit cell, respectively. The adsorption process of N2 spans a larger domain of pressure up, to 40 bar, while the amount of H2 adsorbed increases almost linearly with increasing pressure, with the saturation having not been attained at a pressure of 40 bar. The affinity of MIL-102(Fe) for the different gases can be defined by the slope of the adsorption isotherm in the low pressure domain and corresponds to the following sequence: CO2 . CH4 . N2 . H2. This is consistent with the order of the adsorption enthalpies calculated for these gases, which are summarised in Table 1. A relatively flat enthalpy profile with increasing gas loading can also be seen from Figure 5. This behaviour, which has been already observed for a series of porous solids,[34 –39] suggests the absence of strong adsorption sites at the MIL102(Fe) surface. This is confirmed by the calculated CO2 adsorption enthalpy of 2 27 kJ/mol, which is significantly smaller than the values previously reported in other porous solids that have been revealed to lead to strong CO2/host
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Adsorption isotherms (left) and enthalpy profiles (right) for CO2, CH4, N2 and H2 on MIL-102(Fe) simulated at 303 K.
Molecular Simulation
30 bar. The maximum H2 uptake (, 4.7 mmol/g) is relatively low, which agrees with what has been previously reported for the Cr analogue.[32] A microscopic picture of the adsorption mechanism was further obtained for all gases by a careful analysis of the GCMC simulations. We first evidenced that the terminal water molecules represent the preferential interaction sites for CO2. At the initial stage of adsorption, the CO2 molecules mainly interact via their oxygen atoms with the hydrogen atoms on the water molecules, with a ˚ , as mean characteristic OCO2 –HH2 O distance of about 2 A indicated by the corresponding radial distribution function (RDF) plotted in Figure 7. Such an interaction is illustrated in the snapshot provided in Figure 8(a). This specific interaction can be seen from our GCMC simulations over the whole range of pressure (Figure 7), and this supports the relatively flat profile for the CO2 adsorption enthalpy (Figure 5). One observes that when the Mulliken charges are considered, the peak present in the RDF for the OCO2 – HH2 O pair is much broader (Figure 7), which suggests that CO2 is less localised around the water molecules when using this model. This is consistent with weaker CO2 – H2O electrostatic interactions because the Mulliken charges centred on these atoms are around 15% smaller than their corresponding ESP charges. Figure 9 shows the RDFs calculated between CH4 and the different possible interaction sites involving hydrogen atoms from the –OH groups, H2O molecules and organic linkers present at the surface of MIL-102(Fe). This plot evidences that CH4 only weakly interacts with the pore wall, ˚ , which emphasises with characteristic distances of over 4 A that CH4 is predominantly localised in the centre of the pore as illustrated by the snapshot provided in Figure 8(b). The same behaviour was also observed for H2 and N2. As a further step, the GCMC simulations were performed to predict the separation performances of MIL-102(Fe) at 303 K for the three binary mixtures of
Table 1. Adsorption enthalpies simulated at low coverage and 303 K for all gases on MIL-102(Fe). Adsorption enthalpy (in kJ/mol) ESP charges
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interactions such as the small pore solids, including MOFs (Sc-BDC, MIL-91, STA-12) [35] and zeolites (LTA, Chabazite, NaA, SSZ-13) [36 –39] among others.[39] The adsorption isotherm for H2 was also calculated at 77 K, and is reported in Figure 6. One observes that this gas is rapidly adsorbed at low pressure, although the saturation plateau is only reached at a pressure greater than
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Figure 6. Adsorption isotherms for H2 on MIL-102(Fe) simulated at 77 K (square solid symbol) and at 303 K (circle empty symbol).
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Figure 7. (Colour online) Radial distribution functions (g (r)) calculated at 303 K for the OCO2 – HH2 O pair at different loadings (left) and for a fixed loading (1.1 mmol/g) using the ESP (black) and the Mulliken (red) partial charges for the atoms of the MOF framework (right).
D. Damasceno Borges et al.
Figure 8.
(Colour online) GCMC simulated preferential arrangements of CO2 (a) and CH4 (b) at low loading and 303 K in MIL-102(Fe).
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Figure 9. (Colour online) Radial distribution functions (g (r)) calculated at 303 K for different CH4 – H pairs (black: HOH, red: HH2 O , green: Horganic linker) for a fixed loading of 0.45 mmol/g.
interest, more specifically CO2/CH4 (50:50), CO2/N2 (15:85) and CO2/H2 (50:50). Figure 10 reports the corresponding co-adsorption isotherms. These plots show that MIL-102(Fe) selectively adsorbs CO2 over the other gases as expected from the affinity sequence evidenced from the single component adsorption. The adsorption enthalpy differences between CO2 and CH4, N2 or H2, i.e. D(DH(CO2)–DH(CH4)) ,8.5 kJ mol21, D(DH(CO2)–DH (N2)) , 11.5 kJ mol21 and D(DH(CO2) – DH(H2)) ,20.0 kJ mol21 are relatively moderate compared with those previously reported for certain MOFs with small or intermediate pore sizes in dry states including MIL-91(Al),[40] Sc-BDC [35] and STA-12(Ni).[41] However, MIL-102(Fe) shows the advantage of maintaining an almost constant value of these adsorption enthalpy differences in a wide range of pressure up to 20 bar, as evidenced by Figure 5. The so-obtained S(CO2/CH4) selectivity, which spans from 5 to 7 in the pressure domain [1–10 bar] is within the same range of values than those we and others previously reported for a few non-functionalised MOFs under similar conditions, including MIL-53(Al) (4–7),[42] MIL-47(V) (3.5–4),[43] Cu-BTC (5–10),[44]
MOF-177 (4.4),[45] MIL-100(Cr) (4.5),[46] MIL-101(Cr) (4),[47] UiO-66(Zr) (4–5),[14] MIL-125(Ti)-NH2 (4.5–7), MIL-68(Al) (5–9) [48] and ZIF-8 (4–7).[49] The S(CO2/ N2) selectivity of ,26 at 1 bar, which corresponds at the operating pressure for the separation of flue gas from power plants, is similar to the performance of other MOFs such as UiO-66(Zr), ZIF-68, HKUST-1 [50] and others [24]. This latter performance is however lower than the one shown by some small pore MOFs such as UiO-66(Zr)2CO2H (60-85),[7] SIFSIX-2-Cu-i [29] (72), Bio-MOF-11 (75),[51] Cu-TDPAT (79) [52] and others. The MIL-102 (Fe) material is also highly selective towards CO2 over H2 at 1 bar with S(CO2/H2) ,340, which is a selectivity value that exceeds the performance of several different of MOFs, including MOF-5 (,10) [53] and MOF-177 (,10),[54] materials from the IRMOFs series (from 20 to 80),[55] HKUST-1 (,100) [61] and some ZIFs (ZIF-3 (,100) and ZIF-10 (, 10)),[56] whereas it remains below the separation ability of the best MOFs reported so far, i.e. soc-MOF (600),[57] Mg2 (dobbc) (800) [62] and some ZMOFs such as rht-ZMOF (550) [58] and rho-ZMOF (almost infinite).[59]
3.
Conclusion
The small one-dimensional channel MIL-102(Fe) combines a relatively high degree of confinement and a polar environment with the presence of terminal water molecules on the trimers of trivalent iron, features that are potentially exploitable for gas adsorption/separation purposes. The Fe-form of MIL-102 was successfully synthesised showing a relatively high thermal stability up to 543 K. This motivated a computational investigation coupling quantum and forcefield-based simulations with the aim (i) to gain insight into the microscopic adsorption mechanism of CH4, CO2, N2 and H2 and (ii) to further reveal the separation performance of this hybrid porous solid for the CO2 elimination from hydrogen, natural and flue gases. MIL-102(Fe) was thus predicted to be CO2
Molecular Simulation B 1.6
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Figure 10. (a) CO2/N2 (15:85), (b) CO2/CH4 (50:50) and (c) CO2/H2 (50:50) co-adsorption isotherm simulated at 303 K in MIL-102(Fe) using the ESP partial charges for the MOF.
Figure 11. (Colour online) (a) and (b) shows the selected cluster models for representing the organic and the inorganic nodes of MIL-102(Fe), respectively, (c) horizontal view of the inorganic cluster. The atoms represented as ball in both the inorganic and organic clusters are those considered for the extraction of the ESP charges.
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selective over the three other species associated with separation capabilities that are complementary or sometimes surpass those of some MOFs explored so far for such applications. The improvement of these separation properties might be achieved by considering a preliminary activation of the sample, which would allow a strengthening of the CO2/pore wall interactions via the uncoordinated unsaturated sites. This would be however obtained at the expense of an additional cost for the process. This work will initiate a screening approach combining highthroughput experimental and computational tools with the aim to build a database collecting the adsorption properties of a large series of small-pore MOFs to confirm the interests of such sub-classes of MOFs for CO2 capture. 4.
Simulation
4.1 DFT calculations The MIL-102(Fe) structure was first built from the atomic coordinates of the structural model of MIL-102(Cr) previously reported,[3] substituting Cr by Fe, and by including one – OH and two –H2O terminal groups for each iron trimer. The geometry optimisation of this structure was performed at the DFT level with the Quickstep module available as part of the CP2K code.[60] During the geometry optimisation, both the positions of each atom of the MIL-102(Fe) framework and the cell parameters
Table 2.
were relaxed. The Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE GGA) functional [61] was used along with a combined Gaussian basis set and plane wave pseudopotential strategy as implemented in the code. A triple zeta basis set (TZVP-MOLOPT) was considered for all atoms,[62] except for the metal atoms, where a double zeta basis set was used (DZVP-MOLOPT).[74] The pseudopotentials used for all of the atoms were those derived by Goedecker et al. [63]. The van der Waals interactions were taken into account via the use of the semi-empirical dispersion corrections as implemented in the Grimme DFT-D3 method.[64] The Dmol3 module as implemented in the Materials Studio software [65] was subsequently employed to calculate the charges of each atom of the MIL-102(Fe) framework using the PBE GGA functional combined with the double numerical basis set containing polarisation functions (DNP) [66] on all atoms. We have carried out two different charge analyses: (i) a Mulliken [35] partitioning approach, which was performed from the DFT-optimised structure; (ii) an ESP fitting strategy,[40] which was applied to two individual clusters cleaved from the optimised unit cell of MIL-102(Fe) to model both the organic and inorganic nodes of the structure, which are represented in Figure 11. These clusters include building units (e.g. metaloxide corner and linker) representative of their respective unit cells. All the terminations were saturated by methyl groups by considering standard sp3 geometry (Table 2).
Mulliken and ESP partial charges considered for each atom of the MIL-102(Fe) framework. Charges (a.u.) Atom types
Mulliken
ESP
Fe1
1.1960
1.3400
Fe2
1.1960
1.5300
C1
0.6140
0.7926
C2
2 0.0532
20.4138
C3
2 0.0812
20.0362
C4
0.0543
0.2815
O1
2 0.5430
20.5511
O2
2 0.6330
20.7231
O3
2 0.6760
20.7670
O4
2 0.8090
21.2950
H1
0.1600
0.1422
H2
0.3415
0.4000
H3
0.3450
0.4030
Molecular Simulation Table 3.
Crystal data and structure refinement parameters for MIL-102(Fe) containing occluded water molecules into the pore.
Formula
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9
Chemical formula weight (g mol21) Calculated density (g cm23) Crystal system Space group ˚) a (A ˚) c (A ˚ 3) V (A Z ˚ ): Ka1) Radiation (lCu (A 2u range (8) No. reflections No. atoms No. intensity-dependent parameters No. profile parameters RP RB
FeIII 6 O2{H2O}4·(OH)2.{C10H4 – (CO2)4}3 ·12H2O 1600 1.94 Hexagonal P-6 (n8169) 12.702 (1) 9.720 (1) 1358.1 (2) 1 1.5406 6 – 80 308 20 51 10 0.137 0.107
˚ 2. Note: Overall thermal parameter 1.1 A
4.2
GCMC simulations
GCMC simulations were performed at 303 K to probe the adsorption of the single components H2, CO2, CH4 and N2 and their binary mixtures using the Complex Adsorption and Diffusion Simulation Suite (CADSS) code. Atomistic models were employed for the adsorbate molecules, and the MIL-102(Fe) framework was described by the structural model geometry optimised using DFT, as outlined above.The interactions between certain adsorbates (CO2, N2) and the surface of MIL-102(Fe) were described by a combination of site to site LJ parameters
and Coulombic potentials, whereas the interactions between the other adsorbates (CH4, H2) and the MIL-102(Fe) framework were only treated using LJ potentials. The LJ parameters for each atom of the MOF were taken from the DREIDING forcefield [67] for the organic linker, the UFF [35] forcefield for the inorganic node and the TIP3P model [68] for the water molecules coordinated to the iron centres. The partial charges for all atoms of the MOF, including the terminal water molecules, were extracted from the DFT calculations as described later. A single uncharged LJ interaction site model was used to describe a CH4 molecule with potential
Figure 12. (Colour online) Rietveld plot of MIL-102(Fe)-Cu radiation. XRPD patterns were collected in a SIEMENS D5000 ˚ ) from 68 to 808 (2u) using a step size of 0.028 and 12 s per step in diffractometer (u – 2u) using Cu Ka1 radiation (l ¼ 1.54056 A continuous mode.
10
D. Damasceno Borges et al.
Atom
x/a
y/b
z/c
Fe1 Fe2 m3-O1 m3-O2 O3 O4 O5 O6 O7 O8 C1 C2 C3 C4 C5 C6 C7 Ow1 Ow2 Ow3
0.04521 0.53802 0 2/3 0.50548 2 0.06472 0.12879 0.38280 2 0.19430 0.46403 2 0.15214 2 0.16177 2 0.20199 2 0.15322 2 0.12340 0.45053 2 0.09584 0.47733 2 0.49558 0.13635
0.16429 0.38347 0 1/3 0.09473 0.16993 0.34487 0.39135 2 0.02289 0.24408 0.08005 0.08360 2 0.02595 0.18449 0.29491 0.14928 0.39934 0.73931 2 0.17440 0.43845
0 0 0 0 0.83005 0.85729 0 0 0.85300 0.15342 0.79015 0.64519 0.57116 0.57463 0.64742 0.78601 0.57127 0.23875 1/2 0
parameters taken from the TraPPE forcefield.[69] The CO2 molecule was represented by the conventional rigid linear triatomic model, with three charged LJ interaction ˚ ) located on each atom sites (C 2 O bond length of 1.149 A as previously derived by Harris and Yung [70]. The N2 molecule was described by a three charged sites model taken from the TraPPE forcefield,[71] whereas a singlesite LJ model was used to represent H2.[72] All of the LJ cross interaction parameters including adsorbate/adsorbate
and adsorbate/MOF were determined by the Lorentz – Berthelot mixing rule. The simulation box comprised 36 (3 £ 3 £ 4) unit cells of the DFT optimised structure of MIL-102(Fe). For the simulations of pure components, four types of trial moves were considered: attempts (i) to displace a molecule (translation or rotation), (ii) to regrow a molecule at a random position, (iii) to create a new molecule and (iv) to delete an existing molecule. For the simulations of the mixtures, a molecular identity exchange move was introduced as an additional type of trial move, to speed up the time to reach equilibrium and reduce the statistical errors. Short-range dispersion forces described by LJ ˚, potentials were truncated at a cut-off radius of 12 A whereas the long-range electrostatic interactions were handled using the Ewald summation technique. The fugacities for each adsorbed species at a given thermodynamic condition were computed with the Peng –Robinson equation of state.[73] For each state point, 5 £ 107 Monte Carlo steps were used for both equilibration and production runs. The adsorption enthalpy at low coverage (DH) for each single gas was calculated using the Widom’s revised test particle insertion method. [74] Finally, in order to gain insight into the adsorption mechanism of the gas species in MIL-102(Fe), some additional data were calculated at different pressure, including the RDFs between the guests and the host, as well as the spatial distribution for all of the guests. The selectivities have been calculated through the following expression SA=B ¼ ðxA =xB Þ=ðyA =yB Þ w where xA and xB are for the mole fractions of species A and B in the adsorbed phase, and yA and yB are the mole fractions of the same species in the gas phase. In this study, A is fixed to CO2,
100
80 Weight loss (%)
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Table 4. Atomic coordinates for the experimental hydrated MIL-102(Fe) structure.
60
40
0
100
200
300
400
500
Temperature (˚C)
Figure 13.
TGA of MIL-102(Fe) (theoretical and experimental wt% Fe2O3 considering the dry solid ¼ 36.9 and 38.0, respectively).
Molecular Simulation
11
MIL-102 DMF&EtOH MIL-102 DMF
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MIL-102 as
4000
3500
3000
2500
2000
1500
1000
500
Wavelength (cm–1)
Figure 14. (Colour online) FTIR spectrum of the as-synthesised MIL-102 (MIL-102 as; black), MIL-102(Fe) exchanged with DMF (MIL-102(Fe) DMF; blue) and MIL-102(Fe)_DMF exchanged with EtOH (MIL-102 DMF and EtOH; red).
Figure 15. (Colour online) X-ray thermodiffractogram of MIL-102(Fe) performed under air. X-ray thermodiffraction was performed using a u-u Bruker D8 Advance diffractometer equipped with a HTK-1200N (Anton Parr) high temperature chamber and a LynxEye XE detector (Cu radiation). Diagrams were collected every 10 8C between 30 and 400 8C.
while for B we have considered CH4, N2 and H2. The simulations have been performed with the following molar fractions in the gas phase (50 – 50 for CO2/CH4 and CO2/ H2; 15– 85 for CO2/N2) at 303 K.
5. 5.1
Experimental Synthesis
In 50 ml of distilled water, 2.70 g (10 mmol) of FeCl36H2O and 3.04 g (10 mmol) of 1,4,5,8-naphthalenetetracarboxylic acid were dissolved , placed in a 125 ml-teflonlined autoclave and heated at 1008C for 15 h. The resulting solid was recovered by filtration and washed with distilled water and acetone. The as-synthesised product was suspended in 50 ml of dimethylformamide (DMF) under stirring for 1 h, and then re-dispersed in 50 ml of under stirring for 30 min. The solid was recovered by filtration and dried under air.
5.2
Structure solution
Since no suitable single crystals were obtained, a structure determination from powder diffraction was carried out. A structure-less whole pattern profile refinement by the Le Bail method using Fullprof [75] and the Winplotr program [76] starting from the cell parameters of the chromium analogue MIL-102(Cr).[32] Structure determination was performed using the corresponding atomic coordinates of the parent chromium structure as a starting model in the Rietveld refinement. The pseudo-Voigt function was used to describe the individual line profile. Soft distance and angular constraints were applied to regularise the starting structural model. An overall thermal factor was applied throughout the refinement. The final Rietveld refinement was carried out over the angular range of 6 –808 (2u) by using 352 reflections. Details of the refinement are summarised in Table 3. The final Rietveld plot is given in Figure 12 (Table 4).
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12
D. Damasceno Borges et al.
Five to ten mg of MIL-102(Fe) was used for thermogravimetry analysis (TGA) measurements. MIL-102(Fe) was analysed under an oxygen flow (20 ml min21) using a Perkin Elmer Diamond TGA/DTA STA 6000 (Waltham, MA, USA) running from room temperature to 6008C with a scan rate of 58C min21. The MIL-102(Fe) solid shows two different weight losses. The first weight loss, between 30 and 1508C, is due to the departure of solvent and the one from 150 to 4508C corresponds to the linker departure, so the degradation of the hybrid solid (Figure 13). A small amount of solids was analysed by a thermo Nicolet Fourrier-transformed infra-red (FTIR) spectrometer (Thermo, USA). The spectrum was recorded from 4000 to 400 cm21. After the DMF and EtOH exchange, the absence of the n (CZO) bands at around 1710 and 1650 cm21 corresponding to the free acid and the DMF is in agreement with the purity of the MIL-102(Fe) solid (Figures 14 and 15). Disclosure statement No potential conflict of interest was reported by the authors.
Funding The research leading to these results has received funding from the European Community Seventh Framework Program (FP7/2007-2013) [grant agreement number 608490] (project M4CO2) and from the ANR ‘CHESDENS’. G.M. thanks the Institut Universitaire de France for its support.
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