Zeolitic Imidazolate Framework Membrane with Marked

Dec 17, 2017 - Best Model Fitting Parameter Values for Gas Permeance through the ZIF-7/MgO-Al2O3 Membrane ... favor low temperature) and the reaction ...
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Article Cite This: Chem. Mater. 2018, 30, 447−455

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Zeolitic Imidazolate Framework Membrane with Marked Thermochemical Stability for High-Temperature Catalytic Processes Seungju Lee,‡ Jaesung Kim,‡ Jieun Kim, and Doohwan Lee* Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering, University of Seoul, Siripdae-gil 13, Jeonnong-dong, Seoul 130-743, Republic of Korea S Supporting Information *

ABSTRACT: The thermochemical stability of metal organic framework (MOF) membranes is vital for the application in chemical-reaction and -separation processes, but understanding the stability of MOF membranes and structure−property relationships under antagonistic chemical atmosphere is still required. In this work, a supported zeolitic imidazolate framework (ZIF) membrane, ZIF-7/MgO-Al2O3, of unprecedented hydrothermal stability is obtained by a modulation of the acid−base chemistry at the membrane/support interface. The solid/solid interface acidity that has been overlooked in the fields turns out to have paramount inducing effects on the thermochemical stability of ZIF membranes, resulting in the catastrophic acid-catalyzed decomposition of ZIF frameworks at atomic level. The ZIF-7/MgO-Al2O3 of marked thermochemical stability permits the first significant application of MOF membranes for catalytic membrane reactor (MR) in severe and practical process conditions, performing water− gas shift reaction (CO + H2O ↔ CO2 + H2) at considerably high temperatures (473−673 K) and steam concentrations (20− 40%). The findings and results provide significant new insights on the property and stability of ZIF membranes with extensive opportunities for thermochemical processes that have been permitted only for the inorganic membranes such as zeolites, palladium, and metal oxides.

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INTRODUCTION Metal organic framework (MOF) membranesorganic− inorganic crystalline microporous solidshave significant potential to afford high molecular perm-selectivity and design flexibility varying their pore structure and functionality with diverse metal nodes and organic ligands.1 MOF membranes can provide greater thermal stability than polymer membranes and higher molecular permeability than inorganic counterparts attributed to the organic−inorganic hybrid compostion, structure, and framework flexibility for facile molecular diffusion. This is clear from Figure 1a which extensively survey the hydrogen permeability of various inorganic (palladium, 2−12 zeolites, 13−19 silica 20−27 ), polymer, 28−30 and MOF31−41 membranes reported in the literature (H2 selectivity of these membranes over CO2 and N2 is shown in Figure S1). Evidently, the current status and opportunity of MOF membranes lie within the mid-temperature regime where the organic and inorganic membranes have trade-off between permeability and thermal stability due to their intrinsic materials properties. However, further investigations on these MOF membranes under scrunity reveal that their properties have been reported under extremely limited thermochemical environment (Figure 1b), simply in the inert and near-dry atmosphere, whereas considerable hydrothermal stability has been known for some MOFs including ZIFs, MILs, UiOs, and pyrazolate-based MOFs in the microcrystalline powder forms.31−45 © 2017 American Chemical Society

Hydrothermal stability of MOFs is one of the critical factors for practical applications, because water vapor is extensively involved in a number of industrial processes as a reactant, product, medium, or common impurity. One may intuitively propose that the thermochemical stability of the supported MOF membranes reported in the literature or to be developed in the future can be fairly deduced from that of MOF crystal powders, but such the side-by-side translation largely fails, which will be shown and discussed shortly, as the membrane/support interface is not always inert. Thermochemical stability of MOFs has been studied usually with microcrystal powders in acidic, basic, and oxidation surroundings using liquid and gas medium; these are, therefore, solid/fluid interface conditions.42−45 The effects of solid/solid interface chemistry for thermochemical stability of MOF frameworks have received little attention to the best of our knowledge, which should be of significant importance for supported membranes, sensors, and any devices that have solid/ solid interfacial layers.46 Previously, the membrane/support (solid/solid) interface property and its modulation have been the focus dominantly for uniform film fabrication on mechanically supporting porous substrates to enhance the Received: October 20, 2017 Revised: December 11, 2017 Published: December 17, 2017 447

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

Article

Chemistry of Materials

Figure 1. (a) H2 permeance of various polymer, MOF, and inorganic membranes at various temperatures, (b) hydrothermal stability of MOF microcrystals (solid)42−45 and membranes (open) at various steam concentrations reported in the literature (membrane = Pd,2−12 zeolite,13−19 and silica,20−27 polymer,28−30 MOFs31−41). h. The Cu/Zn/Al2O3 based WGS catalyst (HiFUEL W230, 5.2 × 3.0 mm2 pellets) was purchased from Alfa-Aesar, crushed into smaller pellets using a mortar and pestle, sieved to the sizes range 600−850 μm, and utilized for the WGS reaction. Surface Modification of the α-Al2O3 Support. Aqueous solutions of Mg(NO3 )2 ·6H 2O were prepared at various Mg concentrations (0.2−0.4 M), and the α-Al2O3 support (L = 30 mm) was immersed in a 5 mL precursor solution under sonication for 10 min for impregnation. The sample was taken out, dried in a convection oven at 353 K for 1 h, and calcined in an electrical furnace at 823 K for 3 h (ramp = 10 K min−1). Preparation of Supported ZIF-7 and ZIF-8 Membranes. The supported ZIF membranes were prepared by the seeded secondarygrowth method. Two separate solutions of the metal ion precursor and the organic ligand were prepared by dissolving 1.36 g of Zn(NO3)2· 6H2O in 50 mL of DMF and 1.20 g of benzimidazole (bim) in 50 mL of methanol. These two solutions were mixed instantly and stirred in a flask for 30 min at 298 K. The resulting milky white solution was centrifuged at 6000 rpm for 20 min, and the collected ZIF-7 nanocrystals were washed thrice with copious amount of DMF. The ZIF-7 seeding solution was prepared by dispersing 0.4 g of ZIF-7 nanocrystals in 5 mL of DMF with an addition of 0.15 g of polyethylenimine. The outer surface of the tubular α-Al2O3 and the MgO-Al2O3 supports was sealed with Teflon tape to perform the seeding only onto the inner surface. The support was dip-coated with the seeding solution for 20 s and dried in a convection oven at 323 K for 12 h. The secondary growth of ZIF-7 membrane was conducted by immersing the seeded support in a precursor solution of 1.73 g of Zn(NO3)2·6H2O and 0.91 g of bim dissolved in 90 mL of DMF. The support was placed vertically on a specially designed Teflon holder in a round-bottom flask, allowing free access of the precursor solution to the inner side of the support. The synthesis temperature was steadily maintained using a heating mantle at 383 K by a PID-control, and the secondary growth of ZIF-7 membrane was conducted for 3 h. Finally, the resulting membrane was taken out, rinsed with methanol several times, and dried at 323 K for 72 h in a convection oven after removing the Teflon tape. The ZIF-8 membranes were prepared with the similar procedure. The precursor solutions for ZIF-8 nanocrystals were obtained by dissolving 1.17 g of Zn(NO3)2·6H2O in 8 mL of DI water and 22.7 g of 2-methylimidazole (mim) in 80 mL of DI water. These two solutions were mixed and stirred in a flask for 5 min at 298 K, centrifuged, and washed three times with copious amount of DI water. The secondary growth of the ZIF-8 membrane was conducted in a solution of 1.75 g of

compatibility for the growth and adhesion of MOF membrane overlayer. Comprehensive advances have been achieved to this end via various synthesis and post-treatment strategies such as seeded growth,32,34 reactive seeding,47−50covalent ligandsupport linkage,31 and post functionalization,33,35,51,52 but the thermochemical stability of the membranes has been rarely discussed. In this work, we present (i) the marked importance of the membrane/support (solid/solid) acid−base interface chemistry for the thermochemical stability of ZIF membranes, elucidating the mechanism of the interface-induced acid-catalyzed decomposition of the ZIFs frameworks, (ii) a ZIF-7/MgO-Al2O3 membrane with unprecedented hydrothermal stability, which is achieved by the acid−base interface property modulation, and (iii) the first significant application study of MOF membranes for catalytic membrane reactor (MR) at markedly high temperatures (473−673 K) and hydrothermal atmosphere (20−40% H2O) performing industrially important catalytic water−gas shift reaction (WGS, CO + H2O ↔ CO2 + H2, ΔH298 = −41.4 kJ mol−1). The results privide significant new understanding on the thermochemical stability of supported MOF membranes and enlarge their window of opportunities for the thermochemical and catalytic processes that have been allowed only for the thermochemically stable inorganic membranes.

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EXPERIMENTAL SECTION

Materials and Reagents. Zinc nitrate hexahydrate (Zn(NO3)2· 6H2O, 98%, Sigma-Aldrich), benzimidazole (C7H6N2, 98%, Alfa), 2methylimidazole (C4H6N2, 99%, Sigma-Aldrich), sodium formate (HCO2Na, 99%, Alfa), N,N-dimethylformamide (HCON(CH3)2, 99.5%, Junsei Chem.), magnesium nitrate hexahydrate (Mg(NO3)2· 6H2O, 99%, Junsei), polyethylenimine (H(NHCH2CH2)nNH2, 50 w/v % in H2O, Sigma-Aldrich), and methanol (99.8%, Junsei Chem) were used without additional purification for the preparation of supported ZIF-7 and ZIF-8 membranes. A porous α-Al2O3 tube (OD = 10 mm, ID = 7 mm, L = 30 mm) of the nominal pore size of 0.2 μm (Pall corporation) was used for the membrane support. The cross-sectional ends of the support were sealed gas-tight with a glass glaze (IN 1001, Duncan) by thermal treatment at 1273 K for 5 min in an electric furnace. The support was washed thrice with D.I. water under sonication for 30 min and dried in a convection oven at 353 K for 12 448

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

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Chemistry of Materials

Figure 2. SEM micrographs of (a) α-Al2O3 supported ZIF-7 and (b) MgO-Al2O3 support ZIF-7 membranes treated at various hydrothermal conditions (scale bar = 2 μm; insets: surface atomic composition measured by XPS). Zn(NO3)2·6H2O and 0.97 g of mim dissolved in 80 mL of methanol. The synthesis was carried out at 393 K for 12 h, and the resulting membrane was rinsed with methanol several times and dried at 324 K for 24 h in a convection oven. Characterizations. The surface morphology and atomic composition of the ZIF membranes, α-Al2O3, and MgO-Al2O3 supports were characterized by field emission scanning electron microscopy (FESEM, SU-70, Hitachi) equipped with an energy-dispersive X-ray (EDX) detector. The surface composition of the samples was characterized by X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo Scientific). The X-ray diffraction (XRD, X’pert-MPD, Philips) patterns on the α-Al2O3 and the MgO-Al2O3 supports as well as the supported ZIF-7 membranes were obtained using monochromic Cu Kα radiation operated at 40 kV and 40 mA with a step size of 0.05° and a scan rate of 1 step s−1. The surface acidity of the supports was characterized by temperature-programmed desorption (TPD) of NH3 in a chemisorption analyzer (Autochem II 2920, Micromeritics). The samples were pretreated with flowing He (50 mL min−1) at 823 K for 1 h (ramp = 10 K min−1) and then treated with flowing diluted NH3 (10 mol % NH3/He-balance, 50 mL min−1) at 323 K for 0.5 h. The sample was purged with flowing He (50 mL min−1) at 373 K for 1 h, and then NH3−TPD was carried out by increasing the sample temperature from 373 to 823 K at 10 K min−1. The details in the permeance and selectivity measurement of the membranes are shown in Supporting Information (SI). Hydrothermal Stability Measurements. Hydrothermal stability of the ZIF-7 membranes was characterized at high temperatures (473− 673 K) and excessive H2O vapor concentrations (20−40 mol %) in the membrane evaluation system (Scheme S1) equipped with a microsyringe pump (KDS200, KD Scientific) and a heated vaporizer (523 K). Initially, the permeance of membrane for the mixed gas was measured in dry condition increasing temperature from 473 to 673 K in a stepwise manner (100 K interval, 10 h for each temperature, ramping rate = 50 K h−1). Subsequently, water vapor was introduced to the mixed gas stream at 20 or 40 mol % at various temperatures from 473 to 673 K, and the permeance and selectivity of the membrane were measured in time-onstream. The membrane was exposed to these hydrothermal conditions for an extended time of 180 h to investigate its structural stability. The samples were characterized further by SEM, EDX, XPS, and XRD analyses.

Catalytic Water−Gas Shift Reaction in MR and PBR. The water−gas shift (WGS) reaction was carried out in a membrane reactor (MR) and a packed bed reactor (PBR) under the same reaction conditions, and the results were compared. The configuration of the MR is the same with the membrane evaluation reactor module as described before; however, the Cu/Zn/Al2O3 catalyst (2.26 g) was packed in the shell side at the membrane section (30 mm length) fixing the catalyst bed using quartz wool (details are in Scheme S2). The reactor temperature was raised to 473 K (ramp = 10 K h−1) in Ar flow (50 mL min−1), and the catalyst was reduced in 20 mol % H2 flow (Ar balance, 30 mL min−1) at 473 K for 2 h. The WGS reaction was conducted feeding a mixture of CO (7%), CO2 (26%), H2 (46%), and H2O (21%) to the shell side (retentate) at the gas hourly space velocity (GHSV) of 800 and 1300 h−1 (catalyst bed density = ∼ 1 g cm−1). The feed composition was chosen simulating the typical product composition of the methane steam reforming. To the tube side (permeate), Ar was fed as a sweep gas at 50 mL min−1. The reaction was carried out at various temperatures between 423 and 573 K, and the effluents from the retentate and the permeate side were analyzed online using a gas chromatograph (7890A, Agilent) equipped with a thermal conductivity detector and a packed-bed column (Hayesep-D, 100/120 mesh, Valco Instruments). The CO conversion and H2 recovery in the MR were calculated by ij FCO,perm + FCO,retn yzz zz × 100 CO conversion (%) = jjjj1 − zz j FCO,in k { FH2,perm × 100 H 2 recovery (%) = FH2,perm + FH2,retn

where Fin, Fperm, and Fretn represent the inlet molar flow rate and the outlet molar flow rate of the permeate and retentate, respectively. The configuration of the PBR was the same as the MR, but the membrane tube was replaced with a dense ceramic tube of the same size. The catalyst loading amount and reaction condition were identical to those applied for the PBR. 449

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

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Chemistry of Materials

Scheme 1. Mechanism of the Framework Decomposition of ZIF-7 Membrane Supported on α-Al2O3 via Acid-Catalyzed Hydrothermal Reactions: Acid−Base Neutrality at the Membrane/Support (Solid/Solid) Interface Introduced by the MgOAl2O3 Intermediate Layer Radically Enhances the Hydrothermal Stability of ZIF Membrane

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RESULTS AND DISCUSSION The ZIF-7 (Zn(bim)2, bim = benzimidazole) studied here for the membrane material is known to have high thermal and hydrothermal stability in powder forms.32,53,54 However, the ZIF-7 fabricated as a densely intergrown polycrystalline membrane overlayer on the porous α-Al2O3, the most common porous ceramic substrate being utilized for microfiltration and membrane support, underwent unexpected and drastic decomposition upon an exposure to hydrothermal atmosphere (473 K, 20% H2O, Figure 3a). As shown, the as-prepared ZIF-7 membrane (after activation) displayed an uniform ZIF-7 polycrystalline membrane overlayer grown on the support without noticeable defects or pinholes (Figure 3a1). The characteristic XRD spectra (Figure 2a) and the atomic composition (Figure 3a, inset) obtained by XPS analysis confirmed the formation of ZIF-7 membrane with high crystallinity. However, an exposure of the membrane to a hydrothermal atmosphere (20 mol % H2O, 573 K, 24 h) was catastrophic, resulting in a complete decomposition of its crystalline structure from the grain boundaries throughout the bulk microcrystals with numerous voids and defects. A complete detachment of the ZIF-7 layer from the support was also observed (Figure 3a2). Furthermore, the characteristic XRD pattern of ZIF-7 totally vanished (Figure 2a), and the atomic composition varied largely indicating the formation of zinc hydroxides with a disappearance of the bim ligands from the structure (Figure 3a2, inset). The results collectively confirm the total decomposition of the crystalline ZIF-7 framework with breakage of the covalent Zn− N bonds in the atomic level. A decomposition mechanism of the ZIF-7 framework accounting the experimental results can be proposed based on the solid/solid interface-induced acid-

catalyzed Zn−N bond breaking (Scheme 1). The decomposition can be initiated by protonation of the N atoms of the bim linkers at the membrane/support interface due to some Brønsted acid sites existing on the surface of the porous α-Al2O3 support. The framework decomposition can proceeds further via hydrolysis of the Zn−N covalent bonds and its propagation throughout the entire ZIF membrane film. The acidity and the acid strength on the porous α-Al2O3 support was very low as characterized by NH3-TPD (Table 1), thus the material is often Table 1. Acidity of the Supports Measured by NH3-TPD support

acidity (mmol-H+ g−1)

α-Al2O3 MgO-Al2O3 SiO2-Al2O3

0.08 0.00 3.20

treated as inert for membrane support. However, the presence of slight acidity at the membrane/support (solid/solid) interface turns out to be fatal, causing the total and rapid decomposition of the entire ZIF-7 framework and the overall membrane structure. In good agreement, the decomposition was much more significant when silica−alumina solid acid was used as the supporting substrate of ZIF-7 membrane (Table 1, Figure S2). Intermolecular proton transfer in the gas phase is generally unfavorable, but the high dielectric constant (ε = 78.3) and significant concentrations of H2O molecules in the inner-pore and the surface of the ZIF-7 framework in this work seem to greatly facilitate the propagation of the acid-catalyzed hydrolysis of the Zn−N bonds throughout the membrane. It is likely that the solid/solid interface acidity induced thermochemical instability of ZIF frameworks also occurs to some extent in the 450

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supported on the MgO-Al2O3 showed no structural degradation (Figure S4). It is our conjecture that other families of MOF membranes including carboxylate-, imidazolate-, and pyrazolatebased MOFs may also be susceptible to the interface acidity induced hydrolysis to some extent, as their framework decomposition in acidic or basic solutions has been also reported.42−45,57,58 The property and stability of the ZIF-7/MgO-Al 2 O 3 membrane were further investigated by measuring the gas perm-selectivity with an equimolar mixture of major industrial gases (H2, CO, CH4, and CO2) over severe thermal and hydrothermal conditions (473−673 K, 0−40% H2O) for an extended time (∼100 h). The results shown in Figure 4a confirm the exceptionally high thermal and hydrothermal stability of the ZIF-7/MgO-Al2O3 against these hostile reaction conditions. Selective permeation of H2 (kinetic diameter = 0.29 nm) through the ZIF-7 membrane (pore size = 0.3 nm) over CH4 (0.38 nm), CO (0.378 nm), and CO2 (0.33 nm) was clearly observed. Conversely, the perm-selectivity of the ZIF-7/Al2O3 membrane (Figure 4b) deteriorated rapidly upon exposure to even a relatively mild hydrothermal atmosphere (20 mol % H2O, 573 K), reflecting the fatal decomposition of the ZIF-7 framework. Figure 5 displays the permeance and selectivity of the ZIF-7/ MgO-Al2O3 for the equimolar mixture gases as a function of temperature. The membrane had high H2 selectivity over other gases: H2/CO (5.9), H2/CH4 (6.8), H2/CO2 (10.0), and H2/ H2O (15.7) at 298 K. The permeation of these gases was activated displaying an increase with temperature. Gas diffusion through microporous materials such as zeolites and MOFs can be described by the gas translational mechanism in which adsorbed molecules are considered to move between the sorption sites (cages) in a translation mode by overcoming the obstruction formed by the small channels that connects adjacent sorption sites.20,59 At elevated temperatures with weak adsorption affinity, the gas permeance can be given by

presence of other protic chemicals such as alcohols, although it would largely depend on the interface acidity, proton affinity of the framework, and the inner-pore concentration of the chemicals.55,56 In marked contrast, the ZIF-7 membrane fabricated on the surface-modified α-Al2O3 by the Mg impregnation and calcination treatment (ZIF-7/MgO-Al2O3) exhibited no structural degradation (Figure 2b) even after a much harsher hydrothermal treatment (40 mol %, 673 K, 24 h). It should be noted that this markedly high thermal and hydrothermal stability is unprecedented among ZIF and other MOF membranes reported so far in the literature (Figure 1a,b) and confirms the criticality of the acid−base interface chemistry of the supported MOF membranes for hydrothermal stability. It is evident from the results that the enhanced membrane stability is attributed to the interface modification that provides the acid− base neutrality at the membrane/support interface by forming an MgO-Al2O3 overlayer onto the acidic α-Al2O3 support. The formation of a uniform MgO-Al2O3 layer was collectively confirmed by NH3-TPD (Table 1), SEM, XPS, and energydispersive spectroscopy (EDS) as shown in Figure S3. The SEM micrographs (Figure 2b) revealed that the ZIF-7 membrane film remained intact strongly attached onto the MgO-Al2O3 support (Figure 2b3). The typical XRD pattern (Figure 3b) and the

P=A

i −E y 8 expjjj a zzz πMRT k RT {

where P is the permeance (mol m−2 s−1 Pa−1), A is the structural parameter, R is the gas constant, M is the molecular weight, T is temperature, and Ea is the activation energy for permeation. The model fitting with the experimental permeance data (Figure 5) nicely elucidate the temperature and molecular-weight dependence of the permeation of these small gases through the ZIF-7 membrane. The best fitting model parameters are shown in Table 2. The applicability of ZIF-7/MgO-Al2O3 membrane for thermochemical process was further investigated by employing it for a catalytic membrane reactor (MR). The MR is an advanced reactor system that concurrently performs chemical reaction and product separation affording high product yields and process efficiency. The WGS reaction was chosen for the study considering its industrial significance for hydrogen and syngas production. The reaction was carried out at high temperatures (473−573 K) to investigate the performance, stability, and opportunity of the ZIF-MR. The WGS reaction is endothermic and its conversion is limited by thermodynamic equilibrium at elevated tempeatures, therefore a concurrent separation of hydrogen product employing a H2 separation MR can lead to significance enhancement in the product yields. The geometry of MR was tubular with a concentrically installed ZIF-

Figure 3. XRD patterns of the ZIF-7/Al2O3 and ZIF-7/MgO-Al2O3 membranes: the activated and the hydrothermally treated samples are compared.

atomic composition (Figure 2b2, inset) persisted against the antagonistic hydrothermal treatment confirming that the crystalline ZIF-7 framework was unharmed. It should be noted that this structure−property relationship of the supported ZIF membranes is not limited to ZIF-7 but also applies to other ZIF families, as the nature of Zn−N bonding in their frameworks remains to be similar at the atomic level. For example, ZIF-8 has been extensively recognized for its high thermal and hydrothermal stability, but the ZIF-8 membrane supported on the porous α-Al2O3 also displayed substantial decomposition by a hydrothermal treatment (473 K, 20% H2O, 24 h), whereas that 451

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

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Chemistry of Materials

Figure 4. Gas permeance and stability of (a) ZIF-7/MgO-Al2O3 and (b) ZIF-7/α-Al2O3 membranes under various thermal (dry) and hydrothermal conditions in time-on-stream.

its steady performance in the MR under the WGS reaction for extended time (573 K, 100 h, Figure S5). Figure 6 displays the theoretical equilibrium conversion and the experimental results obtained using the MR. For

Figure 5. Gas permeance and selectivity through the MgO-Al2O3 supported ZIF-7 membrane as a function of temperature.

Table 2. Best Model Fitting Parameter Values for Gas Permeance through the ZIF-7/MgO-Al2O3 Membrane gas

A

Ea (kJ/mol)

χ2

H2 CO CH4 CO2 H2O

0.6 0.4 0.9 0.4 0.2

3.4 5.7 7.1 7.4 11.7

0.97 0.96 0.93 0.97 0.97

Figure 6. Performance of MR and PBR for the WGS reaction at various temperatures (dot line = equilibrium CO conversion; inset = enhancement in the CO conversion in the MR over the PBR, and the H2 recovery to the permeate side): Feed composition = CO (7%), CO2 (26%), H2 (46%), and H2O (21%); GHSV = 800 h−1).

7 membrane inside, and Cu/Zn/Al2O3 (HiFUEL W230, Alfa) catalysts (size = 600−850 μm) were packed in the annulus between the reactor housing and the membrane tube (reactants feed = CO (7%), CO2 (26%), H2 (46%), and H2O (21%); GHSV = 800 and 1300 h−1): the details are in the experimental and Scheme S2. The exceptional thermochemical stability of the ZIF-7/MgO-Al2O3 membrane could be confirmed again from

comparison, the results obtained with a conventional packed bed reactor (PBR) at the same condition are also displayed. The H2 permeation rate through the ZIF-7/MgO-Al2O3 membrane (∼10−6 mol cm−2 s−1) in this work was greater than the areal time yield of the MR (∼10−7 mol cm−2 s−1) which was 452

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

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determined by multiplying the surface to volume ratio of the reactor, namely d/4, with the space time yield.60−62 Therefore, considerable enhancements in the product yields are expected owing to the rapid and concomitant separation of the H2 product at relavent rates using the ZIF-MR. As shown, the CO conversion in the PBR exhibited a volcano-type profile with temperature, because of a trade-off between the equilibrium conversion (thermodynamics favor low temperature) and the reaction rates (kinetics favor high temperature). Therefore, the CO conversions in the PBR are limited. Notably, the CO conversion in the ZIF-MR was significantly greater (7−25%) than that of the PBR due to the concomitant H2 separation, displaying the values even above the equilibrium limitation level by the Le Chatelier’s principle. The product yields can further increase with increasing reaction pressure, because H 2 separation is driven simply by the cross-membrane partial pressure difference. The H2 recovery to the permeate side was ∼50−60% with a very low CO and CO2 concentration in the stream (CO = 0.2%, CO2 = 4.2% on the dry basis), which was one order magnitude lower than those obtained in the PBR (CO = 2.5%, CO2 = 36.6%). This CO concentration in the permeate falls under the tolerable threshold (∼0.5%) for CO poisoning of Pt electrodes in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs), suggesting a potential opportunity of the membrane process overcoming the limitations of the current multistep fuel processing for clean hydrogen production.63,64 The new findings and results presented in this work provide significant understanding on the structure−property relationship of the supported MOF membranes, enlarging their opportunities for thermochemical processes that have been allowed only for the inorganic membranes such as zeolites, Pd, and metal oxides.

Doohwan Lee: 0000-0003-2016-8890 Author Contributions ‡

(S.L., J.K.) These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from Korea Research Foundation (Grant NRF-2017R1D1A1A09000524).

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CONCLUSIONS The membrane/support acid−base interface chemistry has paramount effects on the hydrothermal stability of the supported ZIF membranes, as even slight acidity at the solid/ solid interface can greatly induce a catastrophic decomposition of the ZIF frameworks by the acid-catalyzed breakage of Zn−N covalent bonding. Markedly high hydrothermal stability of the ZIF-7/MgO-Al2O3 membrane could be achived via the acid− base interface property modulation, and this afforded the first significant application of the ZIF membrane for a hightemperature catalytic membrane reactor under severe and practical reaction conditions. The results allow significant potential of crystalline organic−inorganic hybrid ZIF membranes for the applications in high-temperature thermochemcial processes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04409.

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REFERENCES

Experimental details on the gas permeance and selectivity measurement, Schemes S1 and S2, and Figures S1−S5 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 453

DOI: 10.1021/acs.chemmater.7b04409 Chem. Mater. 2018, 30, 447−455

Article

Chemistry of Materials

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