Zeolitic Imidazolate Framework Membranes Supported on Macroporous Carbon Hollow Fibers by Fluidic Processing Techniques

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1 COMMUNICATION Separation Membranes Zeolitic Imidazolate Framework Membranes Supported on Macroporous Carbon Hollow Fibers by Fluidic Processing Techniques Kiwon Eum, Chen Ma, Dong-Yeun Koh, Fereshteh Rashidi, Zhong Li, Christopher W. Jones, Ryan P. Lively,* and Sankar Nair* This study addresses the challenge of generalizable fabrication of metalorganic framework (particularly zeolitic imidazolate frameworks (ZIF)) hollow fiber membranes that can allow a broader range of separations including hydrocarbon ( petrochemical ) as well as organics/water ( biorefining ) separations. We report a novel strategy that combines fluidic membrane processing with chemically inert carbon hollow fibers to produce robust ZIF membranes. Macroporous carbon hollow fibers are successfully fabricated by pyrolytic conversion of cross-linked polymer hollow fibers. This step allows the use of a wide range of relatively aggressive fluidic processing solvents and conditions. Using these inert fiber supports, the fabrication of ZIF-90 membranes is demonstrated and their butane isomer separations are investigated for the first time. Furthermore, ZIF membranes on carbon hollow fibers can be used in the separation of water/organic mixtures without the issue of fiber swelling or dissolution as seen in ZIF/polymer hollow fiber membranes. ZIF-8/carbon membranes show stable operation spanning several days for dehydration of furfural and ethanol, with high water permeances and separation factors. In all cases, the ZIF membranes are prepared without any seeding, support modification, or postsynthesis procedures, thereby simplifying the fabrication process and increasing the potential for larger-scale membrane fabrication. Metal-organic frameworks (MOFs) particularly the subclass known as zeolitic imidazolate frameworks (ZIFs) have recently created great interest for use in membrane-based separations [1,2] due to their advantageous properties including tunable pore characteristics as well as relatively good thermal Dr. K. Eum, C. Ma, Dr. D.-Y. Koh, Dr. F. Rashidi, Prof. C. W. Jones, Prof. R. P. Lively, Prof. S. Nair School of Chemical & Biomolecular Engineering Georgia Institute of Technology Atlanta, GA , USA ryan.lively@chbe.gatech.edu; sankar.nair@chbe.gatech.edu C. Ma, Prof. Z. Li School of Chemistry and Chemical Engineering South China University of Technology Guangzhou , China The ORCID identification number(s) for the author(s) of this article can be found under DOI: /admi and chemical stability. [3] ZIF-8 is the most extensively studied and accounts for the majority of published work on the topic of ZIF-containing membranes. [4] This is mainly due to the excellent properties of ZIF-8 for the challenging C 3 H 6 /C 3 H 8 separation, which is conventionally achieved via energy intensive cryogenic distillation processes. [5,6] Beginning from reports of ZIF-8 membrane fabrication on ceramic supports, [5,7] the literature on ZIF membranes is moving toward methodologies for membrane fabrication on scalable and low-cost supports such as polymeric hollow fibers. Recently, various hollow fiber membranes based upon MOFs such as ZIF-8, ZIF-90, ZIF-93, HKUST-1, and UiO-66 have been reported. [8] One of the hollow fiber-supported MOF membrane fabrication techniques is known as interfacial microfluidic membrane processing (IMMP), which has been used to successfully fabricate ZIF-8 membranes supported on poly(amide-imide) (PAI) hollow fiber supports. [9 12] However, considering the suitability of many other ZIF materials for use in molecular separations, it is important to develop more generalized IMMP (and related) techniques that result in robust membranes with a wider range of applications beyond C 3 H 6 /C 3 H 8 separation. In this regard, polymeric supports have certain disadvantages. For example, many polymers such as PAI and polyvinylidene fluoride (PVDF) suffer from chemical instability or swelling under exposure to strong solvents such as dimethylformamide (DMF) or N-methylpyrrolidone (NMP), which are required for the synthesis of many ZIF materials. [13] In such circumstances, ZIF membranes formed by IMMP on solvent-sensitive polymer hollow fibers are highly prone to cracking or delamination due to swelling, plasticization, or shrinkage during the IMMP process and subsequent drying steps. In other cases, even though the polymer support may be sufficiently stable during membrane processing, it may be unstable under operational conditions, especially when exposed to oxygenated organic molecules like alcohols, furfural, furans, or aromatics. Here, we address these issues by creating carbon-supported ZIF membranes that are largely impervious to solvent-induced swelling and instabilities (1 of 6)

2 Figure 1. a) Schematic of the IMMP process and membrane structure at varying reaction time steps. b) Pore size distribution of crosslinked PVDF and carbon fiber. c) Case 1: cross-sectional SEM images of partially crumpled ZIF-90 membrane on crosslinked PVDF hollow fiber. d) XRD patterns of bare crosslinked PVDF, macroporous carbon fiber, and ZIF-90 carbon membrane, with a simulated XRD pattern of ZIF-90 shown for comparison. e) Case 2: cross sectional SEM images and EDX elemental map of zinc (orange) confirms the localization of the ZIF-90 layer at the inner surface of the macroporous carbon fiber. during operation. The morphological and molecular separation properties of ZIF-90 (for butane isomer separation) and ZIF-8 membranes (for dehydration of ethanol and furfural) were investigated in detail by a range of techniques. Previously, IMMP was utilized to fabricate ZIF-8 membranes on polymer supports. Deionized (DI) water was utilized as a solvent to deliver 2-methylimidazole linkers that comprise the organic component of ZIF-8 to the shell side of PAI hollow fibers during IMMP. [9 11] A schematic of the IMMP approach is shown in Figure 1a. However, synthesis of ZIF-90 membranes requires a different solvent (e.g., DMF), as the ZIF-90 linker (2-carboxyaldehyde imidazolate, OHC-Im) is only sparingly soluble in water. PAI fibers are soluble in DMF and other aprotic polar solvents (Figure S1a,b, Supporting Information), precluding their use as fiber supports for ZIF-90 membranes. On the other hand, crosslinked PVDF (c-pvdf) fibers (N 2 permeance: GPU, average pore size: 0.21 nm) exhibit improved tolerance toward solvents including DMF and NMP and are also insoluble in these solvents (Figure 1b). More details of the hollow fiber structure can be found in the Supporting Information. Figure 1c shows a cross-sectional SEM image of a ZIF-90 membrane grown on a c-pvdf fiber (Case 1). A ZIF-90 layer is clearly observed on the inner surface of the c-pvdf fiber, and X-ray diffraction (XRD) also confirms the presence of ZIF-90 (Figure 1d). However, the film is poorly attached to the hollow fiber support and moreover is partially crumpled, likely due to the swelling of the polymer support fiber. Indeed, the c-pvdf support, while more resistant to DMF than PAI, is still considerably swelled (10% increase in length upon immersion in DMF) during the membrane synthesis (Figure S1b, Supporting Information). Assuming an isotropic swelling of the support, this corresponds to a 10% increase in the inner diameter of the fiber during IMMP of ZIF-90 membranes. During the membrane drying process, the fiber shrinks back toward its original dimensions, creating an interfacial strain that results in detachment of the ZIF-90 membrane. Therefore, it is concluded that polymeric supports are of doubtful utility for synthesis of ZIF-90 membranes unless swelling can be substantially reduced. To overcome these limitations, here we investigate the use of macroporous carbon fibers in conjunction with IMMP to produce robust ZIF membrane structures. The fabrication methodology and solvent immersion tests for the macroporous carbon fibers are shown in Figure S1b of the Supporting Information. As seen in Figure 1e, a continuous and well-adhered ZIF-90 membrane (Case 2) was formed on the inner surface of the carbon fiber. The energy-dispersive X-ray (EDX) elemental map of zinc suggests that the location of the ZIF-90 layer is primarily on the inner surface of the fiber. The membrane thickness was estimated via scanning electron microscopy (SEM) analysis at multiple locations along the axial direction (Figure S3, Supporting Information) yielding a thickness of 3.1 ± 0.5 µm. Additionally, the generality of the above method was demonstrated (Case 3, Figure S4, Supporting Information) by fabricating a ZIF-8 membrane on the inner surface of the carbon fibers. Figure 2a compares the ideal molecular separation performance behavior of the ZIF-90/carbon membranes to (2 of 6)

3 Figure 2. a) Unary gas permeabilities of ZIF-90/carbon fiber and ZIF-8/PAI hollow fiber membranes at 25 C. The data on ZIF-8 membranes are taken from ref. [10]. b) Binary equimolar n-c 4 H 8 /i-c 4 H 8 gas mixture separation properties of ZIF-90/carbon fiber membranes as a function of temperature at a feed pressure of 1 bar. Error bars were obtained from three independently fabricated ZIF-8/torlon and ZIF-90/carbon membrane. ZIF-8/PAI fiber membranes that have been reported previously. [11] In general, ZIF-8/PAI fiber membranes exhibit three key trends: (i) permeability decreases with an increasing kinetic diameter of the guest molecule, (ii) small departures from this general trend are observed for light hydrocarbons (CH 4, C 2 H 4, and C 2 H 6 ) due to the contribution of strong adsorption, [11] and (iii) a sharp permeability cut-off between C 3 H 6 and C 3 H 8, indicating an effective aperture size of 4.2 Å for ZIF-8. [14] ZIF-90 membranes show similar molecular separation performance to ZIF-8, and the trends described in (i) and (ii) also hold. The nominal (crystallographic) pore sizes of ZIF-8 and ZIF-90 are nearly identical at 3.4 and 3.5 Å, respectively. However, the significant permeability drop associated with molecular sieving is observed at much larger kinetic diameters of 5.0 Å for ZIF-90 corresponding to the n-c 4 H 10 /i-c 4 H 10 pair. Interestingly, size exclusion of guest molecules beyond the effective aperture diameters of ZIF-8 and ZIF-90 is not absolute, based on the low but measureable permeabilities of large guest molecules such as SF 6. It is likely that these observed membrane permeabilities are mainly contributed by transport through defects in the ZIF membrane. Based on previous measurements of guest molecule sorption and diffusion in ZIF-90, we estimated the permeabilities of a 3.1 µm ZIF-90 membrane utilizing the Maxwell Stefan approach (Equations S6 and S7, Supporting Information, with parameters in Table 1). The n-c 4 H 10 permeability is estimated to be 206 Barrer (1 Barrer = mol m 2 s 1 Pa 1 ), which is quite comparable to the measured 192 Barrer through the ZIF-90 membrane. However, the i-c 4 H 10 permeability is estimated to be 0.03 Barrer, Table 1. Maxwell Stefan diffusivities, Langmuir model parameters, theoretical and experimental permeabilities of n-c 4 H 10 and i-c 4 H 10 on ZIF-90 at 35 C. M-S diffusivity [cm 2 s 1 ] C s [mmol g 1 ] B [bar 1 ] Ideal permeability [Barrer] which is much smaller than the measured 16 Barrer through the membrane, indicating the presence of small concentrations of defects in the ZIF-90 membrane. Previous work on ZIF-8 membranes indicated a lower concentration of defects than the ZIF-90 membranes discussed here. This difference may be explained by the stability of the solvent interfaces utilized in the IMMP technique to fabricate the ZIF membranes. We used the DI-water/1-octanol solvent pair for ZIF-8 membrane fabrication. These solvents are immiscible, leading to the creation of a sharp and stable interface. However, the DMF/1-octanol solvent pair for ZIF-90 membrane fabrication is somewhat miscible and likely leads to a relatively unstable interface. Further work is needed to characterize the defects in more detail, investigate other possible immiscible solvent pairs, or heal the defects using several techniques reported in the literature. [15] Figure 2b shows the performance of ZIF-90/carbon membranes for the separation of an equimolar binary mixture of n-c 4 H 10 and i-c 4 H 10 mixture at 1 bar feed pressure as a function of temperature. Molecular sieving effects are clearly observed, with a n-c 4 H 10 /i-c 4 H 10 separation factor of 12 being maintained in the temperature range C. The separation factor is substantial but still much lower than that expected from previous diffusivity data. [16] The n-c 4 H 10 permeance increases with temperature, which is indicative of a positive activation energy of permeation for n-c4h10 in ZIF-90. The measured permeances of n-c 4 H 10 and i-c 4 H 10 are 60 and 5 GPU, respectively at 25 C (1 GPU = mol m 2 s 1 Pa 1 ). With the measured membrane thickness of 3.1 µm, we estimate the expected permeances to be 66 and 0.01 GPU, respectively. While the measured n-c 4 H 10 permeance is quite comparable Measured permeability [Barrer] n-c 4 H ± ± ± i-c 4 H ± ± ± to that predicted from the M-S equations, it appears that nearly all the observed i-c 4 H 10 permeance arises from defects in the ZIF-90 membranes. Optimization of the ZIF-90 IMMP on carbon fibers could focus on reduction of defect permeance by examining improvements in the surface and porosity characteristics of the support, other solvent (3 of 6)

4 Figure 3. Binary vapor permeation properties of ZIF-8/carbon fiber membranes in (a,c) furfural dehydration, and (b,d) ethanol dehydration. The separation factors are calculated based upon the liquid composition (a,b) and composition of the vapor mixture in contact with the membrane (c,d). Error bars were obtained from three independently fabricated ZIF-8/carbon membrane. pairs for a more stable interface, IMMP variations, or postsynthesis treatments. Finally, we investigate the potential use of ZIF-8/carbon fiber membranes for dehydration of organics via vapor permeation. We choose water/furfural and water/ethanol mixtures as examples, given the relevance of furfural and ethanol in the production of useful chemicals from renewable biomass sources and the large amounts of energy required for these separations by conventional means such as distillation. Although water adsorbs weakly in ZIF-8, its diffusivity is expected to be very high due to the small kinetic diameter ( 2.6 Å). [17] Additionally, the small pores of ZIF-8 would limit permeation of furfural (5.7 Å) and ethanol (4.5 Å). Figure 3a shows the results of water/furfural separation performance using ZIF-8 carbon fiber membranes, with a vaporized liquid feed of 7 mol% water/93 mol% furfural that is typical of the current industrial furfural dehydration process. [18] We observe a gradual decrease in water separation factor from 1040 to 570 over the first 5 d, which does not indicate defect formation since the water permeance decreases during this unsteadystate period. Rather, it is mainly due to the very slow uptake and low diffusivity of furfural in ZIF-8 followed by competitive permeation of furfural with water. After the transient period, we observe stable steady-state operation over an additional 4 d of measurement with a high water permeance (2350 GPU) and separation factor (570). Due to the replacement of polymeric hollow fibers with the carbon support, there are no apparent issues of swelling or plasticization upon contact with furfural. To further illustrate this key issue, we also performed vapor pervaporation experiments with a ZIF-8/PAI hollow fiber membrane (Figure S5, Supporting Information). Rapid increases in permeance of both water and furfural are observed due to the swelling/plasticization of the PAI fibers. The water separation factor dropped dramatically from 620 to 180. After 140 h of operation, the fibers lost mechanical integrity and broke. This is further supported by Figure S1c (Supporting Information), wherein we observe severe swelling of the PAI fiber upon saturation with furfural for 10 min. Figure 3b shows the results of water/ethanol separation performance, with a vaporized liquid feed of 13 mol% water/ 87 mol% ethanol that is close to the azeotropic composition. The observed performance trends are similar to those in furfural dehydration. A stable water permeance of 2700 GPU and separation factor of 20 are obtained over 7 d of continuous operation. Given the same liquid feed composition, pervaporation and vapor permeation experiments are thermodynamically identical. The only difference is that the membrane is immersed in the liquid (pervaporation) or suspended above the liquid surface (vapor permeation). In this work we have adopted the latter approach, since we find that the integrity of the specific epoxy material used for mounting the membranes is unsatisfactory under exposure to liquid furfural and ethanol, but stable under vapor exposure over the time period (7 10 d) of our measurements. Other available mounting materials for hollow fiber membranes in organic solvent separations may not have this issue, so we do not believe this is a critical issue with the materials and processes discussed here. [19] As in most membrane separations based upon pervaporation and vapor permeation, the observed overall separation factor is influenced by both the membrane selectivity and the vapor-liquid equilibrium (VLE) characteristics. [20 23] Although the overall separation factor (Figure 3a,b) is the quantity of ultimate interest, we can also (4 of 6)

5 isolate the membrane effects by plotting the vapor-phase separation factor (Figure 3c,d) in which the VLE effects are not present (Equation S5, Supporting Information). The vapor-phase separation factors of the ZIF-8 membrane are quite similar for dehydration of both ethanol [22] and furfural. [18] The overall water separation factor (i.e., including the VLE factors) for furfural dehydration is much higher, owing to the lower volatility of furfural than water and its higher activity coefficient, whereas in the case of ethanol a combination of higher volatility and activity coefficient for ethanol leads to similar overall and vapor-phase separation factors. In conclusion, we have demonstrated significant advances in the engineering of IMMP techniques for fabrication of functional ZIF membranes on hollow fiber supports. To allow more generalized application of ZIF membranes for gas and vapor/liquid separations, we have reported a strategy involving the pyrolytic conversion of crosslinked polymer precursors to macroporous carbon hollow fibers. This step allows the use of a wide range of relatively aggressive IMMP solvents such as DMF. By control of the IMMP conditions, we have demonstrated the fabrication of selective ZIF-90/carbon fiber membranes for butane isomer separation. Furthermore, ZIF-8 and ZIF-90 membranes on carbon hollow fibers can be used in the separation of water/organic mixtures without the issues of fiber swelling or dissolution as seen in the case of ZIF/polymer hollow fiber membranes. The ZIF/carbon membranes show stable operation during vapor permeation measurements spanning several days, with high water permeances and separation factors for dehydration of furfural and ethanol. In all cases reported here, the ZIF membranes are prepared without the use of any seeding, support modification, or post - synthesis procedures, thereby simplifying the fabrication process and increasing the potential for larger-scale ZIF membrane fabrication. Experimental Section Experimental details including materials, ZIF/c-PVDF and ZIF/carbon membrane fabrication, characterization of fibers and membranes, gas/ vapor permeation measurements, Maxwell Stefan equations, and instrumentation are given in the Supporting Information. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements K.E., C.M., and D.-Y.K. contributed equally to this work. K.E. and S.N. acknowledge partial support from the National Science Foundation (CBET ). C.M. and Z.L. acknowledge support from the Key Program of National Natural Science Foundation of China (No ). R.P.L. and D.-Y.K. acknowledge support from the American Chemical Society Petroleum Research Fund. Conflict of Interest The authors declare no conflict of interest. Keywords carbon, gas separation, hollow fibers, liquid separation, metal-organic frameworks Received: January 20, 2017 Revised: March 3, 2017 Published online: April 20, 2017 [1] H.-C. Zhou, J. R. Long, O. M. Yaghi, Chem. Rev. 2012, 112, 673. [2] Y. Pan, Z. Lai, Chem. Commun. 2011, 47, [3] M. Shah, M. C. McCarthy, S. Sachdeva, A. K. Lee, H.-K. Jeong, Ind. Eng. Chem. Res. 2011, 51, [4] a) C. Zhang, W. J. Koros, J. Phys. Chem. Lett. 2015, 6, 3841; b) B. R. Pimentel, A. Parulkar, E.-K. Zhou, N. A. Brunelli, R. P. Lively, ChemSusChem 2014, 7, 3202; c) Y. Zhang, X. Feng, S. Yuan, J. Zhou, B. Wang, Inorg. Chem. Front. 2016, 3, 896; d) H. Zhu, H. Liu, I. Zhitomirsky, S. Zhu, Mater. Lett. 2015, 142, 19. [5] a) H. T. Kwon, H.-K. Jeong, A. S. Lee, H. S. An, J. S. Lee, J. Am. Chem. Soc. 2015, 137, 12304; b) J. Yu, Y. Pan, C. Wang, Z. 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