Preparation and Characterization of a Hydrophobic ... - ACS Publications

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Preparation and Characterization of a Hydrophobic Metal−Organic Framework Membrane Supported on a Thin Porous Metal Sheet Jian Liu, Nathan Canfield, and Wei Liu* Energy and Environment Technology Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: A hydrophobic metal−organic framework (MOF) UiO-66-CH3 is prepared and its solvothermal stability is investigated in comparison to UiO-66. It is confirmed that the MOF stability is enhanced by introduction of the two methyl groups, while the water adsorption is reduced. Given its hydrophobicity and stability, UiO-66-CH3 is proposed as an attractive membrane material for gas separation under moisture conditions. The UiO66-CH3 membrane is prepared on a 50 μm thin porous Ni support sheet for the first time by use of a secondary growth method. It is found that uniform seed coating on the support is necessary to form a continuous membrane. In addition to growth time and temperature, the presence of a modulator in the growth solution is found to be useful for controlling hydrothermal membrane growth on the seeded support. A dense, intergrown membrane layer is formed by 24 h growth over a temperature range from 120 to 160 °C. The membrane surface comprises 500 nm octahedral crystals, which are supposed to grow out of the original 100 nm spherical seeding crystals. The separation characteristics of resulting membranes are tested with pure CO2, air, CO2/air mixture, and humid CO2/air mixture. CO2 permeance as high as 1.9 × 10−6 mol/(m2 s Pa) at 31 °C is obtained. Unlike the hydrophilic zeolite membranes, CO2 permeation through this membrane is not blocked by the presence of water vapor in the feed gas. The results suggest that this MOF is a promising membrane material worth further investigation for separation of CO2 and other small molecules from humid gas mixtures.

1. INTRODUCTION Metal−organic frameworks (MOFs) are a relatively new class of 3D porous materials with well-defined pore structures comparable to zeolites.1−3 A great number of possible combinations of different metal ions and organic ligands provide an expectation to tailor the pore structures and surface chemistry over a wide range of choices. Membrane-based gas separation is an attractive alternative to adsorption processes, because a thin-film membrane uses a small amount of the functional material and the membrane separation does not need frequent regeneration. In addition, membrane separation does not require a high adsorption capacity, which provides latitude to tailor the material structures and improve the selectivity for a given separation problem.4 Novel membranes have been actively sought by the research community for applications to postcombustion CO 2 capture (CO 2 /N 2 separation) and natural gas upgrade (CO2/CH4 separation).5 In addition to the mixed-matrix membrane composed of MOFs in a conventional polymer matrix,6 a MOF membrane may be fabricated in an asymmetric form in which a thin active surface layer is supported by a porous sublayer to minimize the mass transport resistance. It is reported that MOF membranes on the porous support have potential of high permeance and selectivity for separation of various gas mixtures.7−22 However, membrane hydrothermal stability is the other important performance characteristic. Information on hydrothermal stability of the MOF membrane material and effects of water © 2016 American Chemical Society

vapor on separation performances has been very limited. Ideally, a membrane can work in humid flue gas without requiring an additional gas drying process. In addition to the membrane material, the membrane support is one critical factor for practical applications. The MOF crystals are mostly grown in organic solvent at elevated temperatures, which presents a big challenge for preparation of a pure MOF membranes on polymeric supports. Using porous ceramic supports present scale-up and cost concerns, which have been perceived for practical applications of other inorganic membrane products such as zeolites. In this work, we attempt to identify a hydrophobic MOF framework that provides good molecular separation functions such as CO2/N2 and has adequate hydrothermal stability for applications to humid gas mixtures. We also aim to demonstrate the fundamental feasibility of making such a membrane on a thin (50 μm), robust porous metal support sheet that can be scaled up and produced at low costs. The thin metal sheet support provides a unique combination of desirable performance attributes that could not be obtained with other support materials, such as high membrane area packing density, uniform pore size at the submicrometer level, chemical stability, Received: Revised: Accepted: Published: 3823

December 15, 2015 February 16, 2016 February 29, 2016 February 29, 2016 DOI: 10.1021/acs.iecr.5b04739 Ind. Eng. Chem. Res. 2016, 55, 3823−3832

Article

Industrial & Engineering Chemistry Research

A vacuum filtration technique was used to lay down the MOF seeding crystals onto the porous metal support. The coating solution with a concentration of 0.2 mg/mL was used to obtain a uniform coating. In the vacuum filtration, a porous Ni sheet disk of about 4 cm diameter was cut and mounted on a glass funnel filter. For a given coating solution, the seed crystal loading on the support can be adjusted by the amount of coating solution to be filtered. The wetted sheet was dried under ambient air conditions and weighed to determine the seed loading. Material Structure. The powder and membrane samples were characterized by XRD (Bruker, D8 Advance) to identify crystal phases. The MOF seed crystals were slightly pressed into discs with 25.4 mm diameter and 3 mm thickness in a powder sample holder. The MOF membranes were attached on the surface of a sample holder with a clip. The measurement was conducted with Cu Kα radiation from 5° to 80°. The membrane surface and fractured cross-section were first analyzed by using scanning electronic microscope (SEM, JEOL JSM-5900). A conductive coating of carbon was applied to the sample using ion beam induced dissociation of a carbon precursor at settings of 30 kV and 1 nA. Adsorption Test. CO2 and N2 adsorption experiments were conducted on a volumetric gas adsorption system (HPVA100, Micromeritics, Georgia, USA). The sample was activated at 150 °C for 12 h under dynamic vacuum before any gas adsorption test. The water vapor adsorption isotherms were obtained using a water vapor adsorption analyzer (VTI-SA+, TA Instrument, Florida, USA). The sample was regenerated at 150 °C with dry N2 flow for 12 h before the measurements. Membrane Separation Test. Separation characteristics of the membrane were tested with air, pure CO2, CO2/air mixture, and humid CO2/air mixture. The testing apparatus and method were reported previously.26 Gas flow from respective gas cylinders was introduced into the feed side of the membrane test cell by use of mass flow controllers. In the tests with humid gas, deionized water was delivered by a syringe pump and prevaporized prior to mixing with the feed gas stream. The feed side of the test cell was typically maintained under atmospheric pressure, while the permeate side was pulled to vacuum. The permeated water vapor was scrubbed by use of a 3A molecular sieve adsorbent bed. The amount of the permeated water vapor was determined by the weigh change of the adsorbent bed. The residual gas discharged by the vacuum pump was swept by a helium gas stream and analyzed by an online mass spectrometer (Accu Quad, Kurt J. Lesker Co.). The gas permeation rate was determined based on the residual gas composition and the sweep gas flow rate. Permeance of individual molecule and selectivity are calculated based on the following equations:

thermal stability, mechanical strength, and high permeability. The resulting membrane performances will be characterized with dry and humid gases to understand impacts of water vapor on permeance and selectivity. Optimization of the membrane preparation and structures is not the objective of this work.

2. MATERIALS AND EXPERIMENTAL MOF Powder Preparation. All chemicals were obtained from Sigma-Aldrich and Fisher Scientific and used as-received. The UiO-66 was synthesized using the procedure reported in the literature.23 ZrCl4 (0.106 g, 0.45 mmol) and 1,4benzenedicarboxylic acid (H2BDC) (0.068 g, 0.45 mmol) were added into 50 mL N,N-dimethylformamide (DMF) at room temperature. Then, the mixture was sealed and placed in a preheated oven at 120 °C for 24 h. Crystallization was carried out under static conditions. After cooling in air to room temperature, the resulting solid was filtered, repeatedly washed with methanol, and dried at room temperature. UiO-66-CH3 powder was synthesized by modifying the above procedure.24 ZrCl4 (0.35 g, 1.5 mmol) and acetic acid (2.57 mL, 45 mmol) were first dissolved in 50 mL DMF under ultrasound. 2,5-Dimethylterephthalic acid (0.29 g, 1.5 mmol) was then added to the solution under ultrasound. The reaction solution was heated at 120 °C for 24 h. The resulting solid was isolated by centrifugation and dried at ambient temperature. Subsequently, the as-synthesized sample was soaked in chloroform for three 18-h periods at room temperature to remove DMF and the organic linker, and dried at room temperature again. The UiO-66-CH3 powder as seeding crystals for membrane growth was prepared by reducing the reaction time from 24 to 4 and 8 h. The seed crystals obtained with 4 h reaction were well dispersed in the solvent as a homogeneous sol, while the seed crystals obtained with the longer growth time were large enough to cause some precipitation with time. Material Stability. Solvent stability tests were conducted by immersing the MOF powder in a solvent and soaking for different times. Water and hexane were used to represent the respective aqueous and organic solutions. The material was soaked at room temperature and boiling point to assess its hydrothermal stability. Typically, 20 mg of a MOF material and 10 mL of solvent were put into an autoclave reactor and mixed well, and then, the reactor was heated in water to 100 °C for 24 to 168 h. The stability tests in hexane were conducted at room temperature for 168 h. The MOF stability was evaluated based on the crystallinity information obtained from X-ray powder diffraction (XRD) experiments of the same sample before and after the solvent soaking. To assess thermal stability of the MOF powder prepared in this work, thermal-gravimetric analysis (TGA) was performed on the TG (TG-DSC 111, SETARAM Instrumentation) with a temperature profile from 22 to 800 °C at a heating rate of 5 °C/min. The test sample was blanketed under dry N2 flow during the run. MOF Membrane Preparation. For membrane preparation, a 50 μm-thick porous Ni sheet prepared in-house was used as the support.25 The metal sheet has uniform pore structures on the exterior surface and throughout its thickness. The mean pore size is about 0.7 μm, and the porosity is about 40% (by volume). Such surface pore structures enable direct deposition of a thin-layer molecular sieve membrane.26 The porous Ni alloy sheet is also stable in organic solvents at elevated temperatures, which are necessary conditions to grow MOF membranes.

Ri SmΔpi

(1)

Δpi = pF xi − pP yi

(2)

Pi =

αij =

Pi Pj

(3)

3. RESULTS AND DISCUSSIONS 3.1. Stability and Gas Adsorption Properties. Solvothermal stability is the most important factor for a MOF material to be considered for membrane preparation, because 3824

DOI: 10.1021/acs.iecr.5b04739 Ind. Eng. Chem. Res. 2016, 55, 3823−3832

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Industrial & Engineering Chemistry Research the thin-film membrane needs to be stable over a long period of operation time under working conditions for practical applications. Different from adsorption processes with bulk materials, a little degradation in the membrane could result in drastic failures of a membrane separation process. Stability of the MOF materials selected for the present membrane research in solutions was assessed with water and hexane as a model solvent. The crystallinity is a good indicator of the framework stability, because it can show if the material can retain its porous crystal structure that is essential for gas separation purposes. UiO-66 is a stable Zr-based MOF as recently reported in the literature and the preparation can be modified to produce several derivatives.23 UiO-66-CH3 is a derivative MOF from the UiO-66 with introduction of two methyl groups into the aromatic rings. Its framework structure is illustrated in Figure 1.

Figure 2. XRD patterns for the UiO-66 after different conditioning experiments.

Figure 1. Crystal structure of the UiO-66-CH3. Zr atom green; C atom gray; O atom red. Hydrogen atoms are omitted for clarity.

Due to the addition of the methyl groups, the UiO-66-CH3 has a smaller pore size (0.42 nm) than the UiO-66. Enhancement of the stability is expected with the addition of two hydrophobic methyl groups similar to results reported in literature.24,27,28 The powder was immersed in hexane at room temperature for 7 days and in boiling water for 24 h (100 °C). XRD patterns for the UiO-66 and the UiO-66-CH3 after different conditioning are shown in Figures 2 and 3, respectively. The XRD patterns after treatment look the same as the fresh sample for both frameworks. The results confirm excellent stability of this type of framework. Its stability may result from the eight strong coordination bonds between Zr and O atoms. Additional tests showed that the UiO-66-CH3 can retain its crystal structure even after being heated in boiling water for a week and its hydrothermal stability is definitely outstanding among known MOFs. Better thermal stability of the UiO-66-CH3 is also revealed by TGA studies. Figure 4 shows that the UiO-66 is stable up to about 450 °C, while the UiO-66-CH3 does not decompose until about 500 °C. The experimental observation is consistent with theoretical expectation that the UiO-66-CH3 has an enhanced thermal stability. The weight loss for the UiO-66 during 150−300 °C and the weight loss around 150 °C for the UiO-66-CH3 could be due to the evaporation of DMF molecules in the pores.

Figure 3. XRD patterns for the UiO-66-CH3 after different conditioning experiments.

The less water affinity of the UiO-66-CH3 is clearly displayed by the water vapor adsorption isotherms in Figure 5. The adsorption isotherm for a hydrophilic MOF-Ni/DOBDC is included in the plot for comparison. Both the UiO-66 and UiO66-CH3 are hydrophobic MOFs, having limited water adsorption capacities at relative humidity less than 20%. Beyond this humidity level, a steep increase in the water uptake is shown by both the UiO-66 and UiO-66-CH3. At the same humidity level, however, the UiO-66 consistently adsorbs more water than the UiO-66-CH3. The results confirm that the extra hydrophobicity is generated by the introduction of two methyl groups into the pores of the crystal structure. The shape of the water adsorption isotherms for these two MOFs is similar to that of the hydrophobic carbon materials. Another feature worthy of note is that water adsorption is reversible. There is little hysteresis for the UiO-66-CH3. By comparison, the hysteresis is more pronounced for the UiO-66. This suggests that the adsorbed water can be more easily desorbed 3825


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Figure 4. TGA results for the UiO-66 and the UiO-66-CH3 (heating rate of 5 °C/min).

Figure 6. CO2 and N2 adsorption isotherms at 25 °C for the UiO-66CH3. Solid symbols represent adsorption data points and empty symbols represent desorption data points.

Sads =

qCO2 qN2 0.86 0.098 / / = = 49.7 pCO2 pN2 15 85

(4)

The adsorption properties suggest that the UiO-66-CH3 can be a promising membrane material for CO2/N2 separation. Because of its hydrophobicity, the UiO-66-CH3 membrane may not be blocked by adsorbed water in humid gas. In other words, the UiO-66-CH3 has potential to separate CO2 from humid flue gas. 3.2. Membrane Preparation and Characterization. Secondary growth is a commonly adopted strategy for preparation of zeolite membranes. We think that the MOF crystal growth process bears some similarity to zeolite crystal growth, and the secondary growth may be applicable to formation of a MOF membrane. In the secondary growth, it is critical to obtain a good seed coating layer on the support. Different volumes of the coating solution were passed through the porous Ni support sheet to obtain seed coatings of different thickness. The texture and composition of seeded surfaces was analyzed by SEM/EDS. The seed coated surface images are shown in Figure 7 along with the parent seed crystals. The elemental compositions are listed in Table 1. The seeding crystal (Figure 7a) exists as spherical particles with an estimated size of 100 nm. A 5 mL portion of the coating solution was found to be insufficient to fully cover the Ni support of 7.1 cm2 (Figure 7b). Presence of small holes and/or cavities (Figure 7c) was observed on the support sheet seeded using 10 mL of the solution. The seed coating obtained using 20 mL of the solution (Figure 7d) turned out to be the most uniform, without any visible defect. Thus, this seeded support was chosen for membrane growth in the next step. The solution used to grow a MOF membrane on the seeded support was the same as the one used for preparation of the parent MOF powder, which was described previously. It is important to use anhydrous solvent because the amount of water in the growth solution for membrane synthesis is critical to the nucleation and intergrowth of UiO-66-CH3 crystals.29 Different growth temperatures and times were evaluated for obtaining a uniform, dense membrane. A control experiment was run using the bare Ni support sheet. It was confirmed that

Figure 5. Water vapor adsorption isotherm at 25 °C for three MOFs, Ni/DOBDC, UiO-66, and UiO-66-CH3. Solid symbols represent adsorption data points and empty symbols represent desorption data points.

from the UiO-66-CH3 by reducing partial pressure of H2O, which is a desirable property for pressure-driven separation processes. The above characterization results indicate that the UiO-66CH3 can be an excellent candidate material to make membranes. Its pore size around 0.42 nm is appropriate for molecular or gas separation applications. Molecular separation functions of the UiO-66-CH3 are exemplified by selective CO2 adsorption over N2. The pure gas adsorption isotherms of the UiO-66-CH3 measured at room temperature are shown in Figure 6. CO2 adsorption capacity is much higher than N2 over the range of partial pressure measured, from zero to 95 kPa. The adsorption loading increases with partial pressure. At pCO2 = 95 kPa, the CO2 adsorption capacity is about 2.5 mol/kg. By contrast, N2 adsorption capacity at the same partial pressure is less than 0.1 mol/kg. The CO2/N2 selectivity projected based on the adsorption equilibrium constants can be 49.7 for a mixture of 15% CO2 and 85% N2 under atmospheric pressure (eq 4). 3826


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Figure 7. SEM images for the UiO-66-CH3 seed crystals and seed coatings obtained with a coating solution of a solid loading 0.2 mg/ mL. (a) MOF powder used as seed crystals; (b) 5 mL of coating solution; (c) 10 mL; (d) 20 mL. The scale bars are shown in the images.

preseeding of the support was necessary to form a MOF membrane layer. Impacts of growth conditions on the membrane formation are shown by SEM images and EDS spectra in Figure 8. No MOF crystal growth was observed with the membrane grown at 120 °C for 4 h (Figure 8a). The membrane surface texture looks similar to the seeded one, comprised of agglomerates of MOF nanoparticles with obvious pores. When the growth time was increased to 24 h at the same temperature (120 °C), a membrane layer with intergrown MOF crystals is formed (Figure 8c). However, the membrane surface does not look dense. It seems that a higher temperature is needed to have more vigorous UiO-66-CH3 crystal growth to seal the intercrystal voids on the surface. As shown in Figure 8e, the membrane grown at 140 °C indeed has larger octahedral crystals (about 500 nm) rather than the spherical particle agglomerates of the seed crystal. Intergrowth between those octahedrons is clear evidence to the formation of a UiO-66CH3 membrane, which is confirmed by XRD analysis later. When the growth temperature was further increased to 160 °C, the resulting MOF membrane (Figure 8g) has more isolated octahedral crystals than the membrane grown at 140 °C, which suggests some overgrowth. The membrane preparation reproducibility was checked by making two duplicate samples in one batch of growth. The resulting membrane samples looked same from appearance and SEM analysis. We believe that an optimum growth temperature would be determined by

Figure 8. SEM images and EDS spectra for the UiO-66-CH3 membranes grown under different growth conditions. (a) 120 °C, 4 h; (c) 120 °C, 24 h; (e) 140 °C, 24 h; (g) 160 °C, 24 h. (b, d, f, h) Corresponding EDS spectrum for the four membranes. (i and j) Cross sections for membranes obtained under 160 °C, 24 h. The scale bars are shown in the images.

the seed coating and growth solution, which are certainly a subject for future studies. The elemental composition of the membrane was analyzed by use of EDS. The results are shown in Figures 8b, d, f, h, and j

Table 1. Surface Compositions of Seed-Coated and Grown UiO-66-CH3 Membrane Samples sample name

material

MOFMem_120 MOFMem_140 MOFMem_160 MOFMem_160LM

seed/Nia seed/Nia seed/Nia membrane/Nib membrane/Nib membrane/Nib membrane/Nib membrane/Nic

element content, atom % preparation condition 5 mL 10 mL 20 mL 120 °C 120 °C 140 °C 160 °C 160 °C

4h 24 h 24 h 24 h 24 h, less modulator

Ni

Zr

C

O

C/Zr

33.70 19.73 10.23 12.20 5.61 2.15 5.31 2.39

1.85 2.36 4.18 3.29 6.41 6.89 6.56 5.70

58.50 69.01 72.82 71.99 72.04 73.25 73.95 74.48

5.75 8.70 12.33 12.49 14.85 16.17 14.14 17.38

31.6 29.2 17.4 21.9 11.2 10.6 11.3 13.1

a

Different volumes of the coating solution used to lay down the seeding crystals on a Ni support surface of 7.1 cm2. bGrowth with standard growth solution. cAcetic acid used as a modulator was 25% of the amount used in the standard solution. 3827


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Industrial & Engineering Chemistry Research and 9b for different samples. Peaks in these plots are labeled manually, since the characters automatically generated by the

Figure 9. UiO-66-CH3 membrane grown with 25% modulator under 140 °C for 24 h. (a) Membrane surface, (b) EDS spectrum of the membrane. The scale bar is shown in the image.

Figure 10. Possible mechanism for secondary crystal growth of the UiO-66-CH3. The inserted picture shows a coupon with seed crystals before and after MOF membrane growth at 140 °C for 24 h.

software were too small to read. The original images are provided in the Supporting Information as Figures S1−S5, which correspond to Figures 8b, d, f, h, and j, respectively. The EDS spectra in Figure 8 confirm that Zr and Ni are the two dominating metal elements for all the membranes. Zr and Ni are attributed to the MOF and support material, respectively. Presence of Ni indicates that the MOF membrane layer was thin enough for the EDS probe to penetrate through the thickness and into the support sheet. The elemental compositions of the membrane samples are summarized in Table 1. A trend of decreasing Ni/Zr with increasing growth time and temperature can be seen, which suggests that the membrane thickness increased with growth time and temperature. A cross-sectional view of the membrane grown at 140 °C for 24 h was obtained by fracturing the membrane sheet. A distinctive MOF membrane layer is shown on the 50 μm-thick metal support (Figure 8i). The apparent thickness of the membrane is about 5 μm which is similar to the seed coating thickness. The higher magnification (Figure 8j) reveals that the membrane layer has a much denser texture than the support. The pores in the support are intact. Acetic acid was used as the modulator to facilitate MOF crystal growth. The importance of the modulator to secondary growth of the UiO-66-CH3 membrane is a new discovery out of this work. Figure 9 shows the surface texture of a membrane grown under 160 °C for 24 h but using only 25% of the acetic acid as used in the previous growth (Figure 8). By comparing Figure 9a to Figure 8g, one can see that growth of large crystals on the membrane surface was absent with the growth solution of less acetic acid. The membrane surface remains porous with lower acetic acid content. The results confirm that secondary growth of the MOF crystal can be tuned by adjusting the amount of acetic acid in the growth solution, which provides a controllable parameter in addition to growth conditions. The presence of the acetic acid modulator may help to prevent hydrolysis of ZrCl4 and maintain the formation of Zr4+ ions for MOF crystal growth. A possible crystal growth mechanism proposed in this work is depicted in Figure 10. The 100-nm spherical seeding crystals provide reaction sites for Zr4+ and the organic linker. Reaction time, temperature and modulator concentration can be adjusted to control growth of octahedral crystals. When the three process parameters exceed certain threshold values, the spherical crystals can grow larger in some preferred orientation to form the final octahedral crystals.13 Figure 10 gives a visual comparison for the seed coated metal substrate and a membrane sample, reflecting the

surface crystallinity. The membrane comprised of a dense, intergrown MOF crystal layer looks smoother and darker than the seeded substrate’s porous coating layer of smaller MOF crystals. Surface compositions of the three-seeded supports and five membrane samples are summarized in Table 1. The Ni atom percent for the seeded sheets decreases with increasing volume of the seeding solution used, which suggests increased thickness of the seeding layer. The membranes grown above 120 °C show substantially less Ni atom percent than the corresponding seeded sheet (20 mL of the solution). The reduced Ni atom percent after growth is expected. One sharp contrast between the seeded and grown membrane is C/Zr atomic ratio. The ratios for the seeded sheets are far greater than the stoichiometric value of 10 for the UiO-66-CH3. This difference suggests that the seed coating layer contains a lot of materials other than the UiO-66-CH3 crystal. The C/Zr ratios after secondary growth above 120 °C get close to the stoichiometric value, which suggest growth of pure UiO-66-CH3 crystals. Growth time of 4 h at 120 °C was not long enough to grow pure crystals from the seeding layer. To confirm crystallinity of the membranes, four membrane samples were analyzed by XRD. The results are plotted in Figure 11 in comparison to the original seed powder and the calculated XRD pattern using Mercury software. The sample names in Figure 11, MOFMem_120, MOFMem_140, MOFMem_160, and MOFMem_160LM, represent the membranes grown at 120 °C for 24 h, 140 °C for 24 h, 160 °C for 24 h, and 160 °C for 24 h with 25% modulator, respectively. The XRD patterns of the UiO-66-CH3 seeding crystal match the calculated pattern. All four of the MOF membranes show the characteristic peaks of UiO-66-CH3 in their patterns. In addition, the XRD pattern of the UiO-66-CH3 obtained in this study in which acetic acid was used as a modulator show better crystallinity as compared with that the UiO-66-CH3 obtained without the modulator. The strong peaks in their XRD patterns after 45° belong to the porous Ni substrate. The XRD and SEM analyses clearly show that UiO-66-CH3 membranes are successfully prepared on top of a porous Ni metal substrate. 3.3. Gas Separation Test and Results. The gas separation characteristics of this MOF membrane are elucidated by separation tests with different gas mixtures containing CO2 under various conditions. The membrane was first tested with air and pure CO2 gas to assess single gas permeance. Then, a gas mixture comprising 16.6% CO2 in air was used to compare 3828


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an impression that there is a tradeoff between the permeance and selectivity, i.e., the higher selectivity is associated with a lower permeance. However, the membrane grown with the solution of less modulator breaks up this tradeoff. Although both of the membranes were grown at 160 °C for 24 h, the membrane grown with a solution containing less modulator has both higher CO2 permeance and CO2/N2 selectivity than the one grown with the standard solution. Relative ordering of the membrane selectivity seems to be contrary to the previous surface textures shown by SEM analysis. It is noted that SEM analysis mostly reveals the outer surface texture, while membrane separation performances are determined by whole structures across the membrane thickness. Membrane MOFMem_120, which was grown at 120 °C for 24 h, may have a denser membrane texture inside the layer than the other membrane samples. The slow growth could minimize generation of membrane defects inside the membrane layer. Rapid growth at 140 and 160 °C leads formation of large crystals that may leave more chances for formation of mesodefects. The present result suggests that the pores other than the MOF framework channels need to be minimized to obtain high CO2/N2 selectivity. CO2 and N2 permeance with different CO2/air mixtures are plotted in Figure 12 to elucidate the separation mechanism.

Figure 11. XRD patterns for the UiO-66-CH3 seed and membranes on the thin porous Ni support (the theoretical XRD pattern was calculated for the UiO-66-CH3 using Mercury).

separation results of a gas mixture to single gas. Lastly, 3.3% H2O was introduced to the gas mixture to assess impacts of water vapor on separation performances. It took a certain period of time for the membrane to reach a stable working state under each new set of testing conditions. The longer time was needed when humid gas was introduced. The permeate gas composition was continuously monitored by online mass spec analysis. Two or 3 measurements under the steady state were conducted to ensure the consistency. When the membrane was in a stable working state, the results from duplicate measurements were highly reproducible and relative variation was less than 5%. A few examples on reproducibility of membrane permeance measurements are provided in the Supporting Information as Table S1. The testing results for four MOF membrane samples are compared in Table 2. In general, CO2 permeance with pure CO2 is close to the value obtained with the CO2/air mixture. The CO2 permeance for the first two membranes grown at 160 °C is fairly high (1.7−1.9 × 10−6 mol/(m2 s Pa)) at 31 °C. All the four membranes show CO2/N2 selectivity either based on single-gas or mixed gas measurements. There is some variation in the selectivity value between the single and mixed gas measurement. However, relative ordering of the membrane samples for selectivity remains the same. The membrane grown for 24 h at 120 °C shows the highest CO2/N2 selectivity. A comparison of the three membrane samples grown at different temperatures for the same time gives

Figure 12. Impact of CO2 molar fraction in feed gas (dry) on membrane separation performances (MOFMem_160LM membrane after testing in humid gas; separation temperature 31.7 °C; feed pressure of 1 bar; permeate pressure of 11 mbar).

CO2 permeance decreases with increasing CO2 molar fraction in the feed gas, while N2 permeance increases. In other words, CO2 permeance decreases with increasing pCO2, and N2 permeance also decreases with increasing pN2. Such a transport

Table 2. Gas Separation Testing Results of As-Prepared UiO-66-CH3 Membranesa CO2 permeance, mol/(m2 s Pa) membrane name MOFMem_160LM MOFMem_160 MOFMem_140 MOFMem_120

pure gas 1.6 1.4 5.3 1.7

× × × ×

−6

10 10−6 10−7 10−7

dry mix 1.9 1.7 5.1 2.3

× × × ×

CO2/N2 selectivity wet mix

−6

10 10−6 10−7 10−7

8.6 9.6 2.8 3.8

× × × ×

−7

10 10−7 10−7 10−8

pure gas 1.4 1.2 3.1 5.2

b

dry mixc

wet mixd

2.2 1.3 2.4 5.9

1.7 1.1 1.9 2.0

1 bar feed pressure, 10 mbar of permeate side pressure, 31 °C. bPure gas represents CO2 and air. cDry mix represents the feed gas containing 16.7 vol % CO2 in air. dWet mix represents the feed gas containing 16.7 vol % CO2 and 3.3 vol % H2O in air. a

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reducing the effective separation layer thickness of the membrane. Water vapor is ubiquitous in flue gas. Understanding impacts of water vapor on the membrane separation performance is helpful for assessing potential flue gas CO2 capture applications. The results in Table 2 show that introduction of 3.3% H2O into the CO2/air mixture significantly lowered CO2 permeance for all the membranes. Corresponding CO2/N2 selectivity was also decreased. The decreased CO2 permeance by H2O likely results from competitive adsorption of H2O on active sites for CO2. The previous adsorption isotherms of H2O suggest that less than 30% RH needs to be controlled to minimize H2O adsorption. 3.3% H2O at 31 °C, which was used in the present tests, is equivalent to about 80% RH. We propose that the adsorption site for CO2 and H2O is close to Zr-ligand clusters, and the adsorption site for H2O can be close to metal atoms and ligand bridge. There could be some competitive adsorption between CO2 and H2O on the same site. But, there are separate sites for H2O adsorption. It is worth noting that this MOF membrane remained highly permeable, even though it was significantly reduced compared to dry gas. By contrast, gas permeation of hydrophilic microporous membranes (zeolite, carbon) can be substantially blocked by water vapor at such a high humidity level. Although hydrophobic all-silica zeolites such as silicalites exist, preparation of the silicalite membrane often requires removal of the organic template by calcination and is more complicated than the present MOF membrane preparation process. Furthermore, the present MOF frameworks have a smaller channel size than the silicalite membrane. Impacts of water vapor on the membrane performance are elucidated by changing the separation temperature at a constant pH2O (Figure 13). As expected, CO2 permeance increases

behavior suggests that adsorptive diffusion plays a major role in molecular transport across the membrane. If vapor-phase diffusion dominates, the permeance would be nearly constant at different partial pressures. In addition, N2 would have a higher permeance than CO2 due to its higher Knudsen diffusivity if vapor-phase diffusion dominates. Thus, molecular sieving effects due to selective adsorption are clearly shown by the prepared MOF membrane. Then, a question arises why the measured membrane selectivity is so much less than what is predicted based on the adsorption isotherms? Possible answers to the question are discussed by aid of the following transport equations. Molecular transport across the membrane proceeds with adsorption/surface diffusion and gas diffusion through unhindered membrane pores as described by these two mechanisms: s JCO2 = Deff,CO2

ΔCads,CO2 δm

g + (1 − θads)Deff,CO2

ΔCg,CO2 δm (5)

s JN2 = Deff,N2

ΔCads,N2 δm

g + (1 − θads)Deff.N2

ΔCg,N2 δm

(6)

Assume ideal gas, p = CRT J /ΔpCO2 PCO2 = CO2 PN2 JN2 /ΔpN2 s Deff,CO2

=

ΔCads,CO2 ΔpCO2

s Deff,N2

ΔCads,N2 ΔpN2

1

g + (1 − θads)Deff,CO2 RT 1

g + (1 − θads)Deff,N2 RT

(7)

Assume permeate side pressure is negligible relative to feed side pressure C

1

g ads,CO2 s Deff,CO2 + (1 − θads)Deff,CO2 pCO2 RT PCO2 = Cads,N2 1 g s PN2 Deff,N2 p + (1 − θads)Deff,N2 RT N2

(8)

If adsorption dominates the transport, C

ads,CO2 s s Deff,CO2 Deff,CO2 pCO2 PCO2 = = Sads s Cads,N2 s PN2 Deff,N2 Deff,N2 p N2

(9)

For a MOF material of high adsorption selectivity (Sads), its low membrane selectivity can be attributed to two possible causes. The first cause is explained by use of eq 8 where gas diffusion through the unhindered membrane pores still plays a significant role. The unhindered membrane pores consist of defects and MOF channels unoccupied by adsorbed molecules. To enhance the membrane selectivity, all the membrane defects (non MOF pores) need to be eliminated so that adsorptive diffusion becomes a dominating transport mechanism. The second cause is explained by use of eq 9 where effective diffusivity of adsorbed CO2 is much less than that of adsorbed N2. Unfortunately, there is no information available about effective diffusivity of CO2 and N2 molecules in this MOF framework. We postulate that CO2/N2 selectivity should be significantly increased by making the adsorptive diffusion be dominating transport mechanism. This may be realized by eliminating membrane defects in the preparation and/or by

Figure 13. Membrane performance with humid feed gas (MOFMem_120; feed gas 3.3% H2O and 16.6% CO2 in air; feed pressure of 1 bar; permeate pressure of 8 mbar).

rapidly with temperature. This can be explained by decreased adsorption of H2O in the MOF pores. CO2/N2 selectivity did not change much, because both CO2 and N2 permeance increased with temperature. Thus, CO2/N2 selectivity is mainly determined by the membrane structure and surface chemistry. The CO2 permeance and CO2/N2 selectivity obtained with the present membrane sample MOFMem_160LM are compared to the other MOF membranes in Table 3. Most MOF membranes in the literature were prepared on porous α3830


Article


isotherms show that under the flue gas conditions, the UiO-66CH3 has CO2/N2 adsorption selectivity as high as 50. This is a promising membrane material for molecular or gas separation applications. The feasibility to directly deposit a UiO-66-CH3 membrane on the thin-porous metal sheet support is shown by use of secondary growth technique. The resulting membrane shows good adhesion and no cracks after tests under various conditions. Molecular sieving functions of the membrane are demonstrated through gas permeation and separation tests with CO2, air, CO2 and air mixture, and humid CO2 mixture. CO2 permeance as high as 1.90 × 10−6 mol/(m2 s Pa) is obtained with CO2/N2 selectivity. In particular, this membrane is tolerant to the presence of water vapor in the feed gas. It is found that given the same MOF framework, membrane textures and separation characteristics can be significantly affected by the membrane preparation conditions and procedures. The results suggest that the membrane performance may be improved by optimizing the membrane preparation and structures in the future. Using functional groups other than CH3 is proposed to tailor MOF adsorption chemistry and channel structures for specific separation applications.

Table 3. CO2 and N2 Permeance and Selectivity of Different MOF Membranes permeance, 10−8 mol/ (m2 s Pa) membrane MOF-5/αAl2O3 MOF-5/αAl2O3 MIL-53/αAl2O3 ZIF-7/αAl2O3 ZIF-7/αAl2O3 ZIF-8/ TiO2 ZIF-8/αAl2O3 ZIF-22/ TiO2 ZIF-90/αAl2O3 HKUST1/Cu mesh HKUST1/αAl2O3 CAU-1/αAl2O3 UiO-66CH3/Ni sheet

selectivity

measurement condition

CO2

dry single gas

120

150

0.8

14

dry single gas

100

125

0.8

15

dry single gas

16

18

0.9

16

N2

CO2/N2

ref

dry single gas

1.2

1.2

1.0

10

dry single gas

0.35

0.22

1.6

11

dry single gas

1.6

0.60

2.7

17

dry single gas

4.5

2.0

2.3

18

dry single gas

2.2

2.5

0.9

13

dry single gas

3.0

2.5

1.2

19

dry single gas

27.7

27.2

1.0

20

dry single gas dry gas mixturea dry single gas dry gas mixtureb Wet gas mixturec

50

50

50−130

1.0 18−23

190

130

1.4

190

86

2.2

86

50

1.7

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ASSOCIATED CONTENT

S Supporting Information *

21

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04739. Original EDS results for MOF membranes grown under difference conditions, and repeated gas permeation measurement results (PDF)

22 this workd this workd this workd

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AUTHOR INFORMATION

Corresponding Author

1 to 8% CO2 in N2 at 22 °C bDry gas mixture consisted of 16.6 vol% CO2 in air. cWet gas mixture consisted of 3.3 vol% H2O and 16.7 vol % CO2 in air. dMeasurements conducted at 31.7 °C. a

*E-mail: [email protected]. Phone: 509-375-2524. Notes

The authors declare no competing financial interest.

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alumina supports and the resulting membranes were characterized with single gas permeation measurements. The best separation performances were obtained recently by Yin et al.22 with α-alumina-supported CAU-1the amino-decorated 12connected [Al4(OH)2(OCH3)4(H2N-BDC)3]·xH2O framework with triangular aperture of 0.3−0.4 nm. It would be interesting to know stability of this MOF material in hot water and organic solvent, and membrane separation performances in the presence of water vapor. The other membranes showed either low CO2 permeance or poor to no CO2/N2 selectivity. For example, a MOF membrane prepared on the Cu mesh was reported with low CO2 permeance and no CO2/N2 selectivity. The currently reported MOF membrane prepared on thin (50 μm) metal sheet supports has strong potential for low-cost manufacturing in the future. Excellent CO2 permeance and resistance to water vapor are shown by the present MOF membrane. The results are promising for future studies.

ACKNOWLEDGMENTS This work was partially supported by US Department of Energy, ARPA-E, under contract number DE-AR0000138. The authors would like to thank Ms. Sokhom B Seka student internfor doing membrane separation measurements and our colleaguesDrs. Praveen Thallapally and Pete McGrailfor discussions around MOF materials.

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4. CONCLUSIONS It is found in this work that both hydrothermal stability and hydrophobicity of UiO-66 framework can be enhanced by incorporating methyl groups into the organic ligand, i.e., forming the UiO-66-CH3 framework. The UiO-66-CH3 shows excellent stability in boiling water and organic solvent. Its thermal stability in gas is up to 475 °C. The adsorption 3831

NOMENCLATURE δm = membrane thickness, m θads = fraction of the total pore occupied by adsorbed molecules αij = membrane selectivity toward molecule i over molecule j ΔPi = partial pressure differential of molecule i across the membrane, Pa C = gas concentration, mol/m3 Cads,CO2 = concentration of adsorbent CO2 in the MOF membrane, mol/m3 Cads,N2 = concentration of adsorbent N2 in the MOF membrane, mol/m3 Cg,N2 = concentration of N2 in gas phase, mol/m3 Cg,CO2 = concentration of CO2 in gas phase, mol/m3 Dseff,CO2 = effective diffusivity of adsorbed CO2 in the MOF pore, m2/s DOI: 10.1021/acs.iecr.5b04739 Ind. Eng. Chem. Res. 2016, 55, 3823−3832

Article

Industrial & Engineering Chemistry Research Dgeff,CO2 = effective diffusivity of CO2 in gas phase inside membrane pores, m2/s Dseff,N2 = effective diffusivity of adsorbed N2 in the MOF pore, m2/s Dgeff,N2 = effective diffusivity of N2 in gas phase inside membrane pores, m2/s JCO2 = CO2 flux, mol/(m2 s) JN2 = N2 flux, mol/(m2 s) p = partial pressure, bar PCO2 = CO2 permeance, mol/(m2 s Pa) Pi = permeance of molecule i, mol/(m2 s Pa) PN2 = N2 permeance, mol/(m2 s Pa) pN2 = partial pressure of N2, Pa pCO2 = partial pressure of CO2, Pa PF = feed side pressure of the membrane cell, Pa PP = permeate side pressure of the membrane cell, Pa R = gas constant Ri = permeation rate of molecule i through the membrane, mol/s Sads = CO2/N2 selectivity based on adsorption Sm = membrane area exposed to feed gas, m2 T = temperature, K xi = molar fraction of molecule i in the feed side yi = molar fraction of molecule i in the permeate side

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