Zeolite Imidazolate Framework Membranes on Polymeric Substrates

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Functional Inorganic Materials and Devices

Zeolite Imidazolate Framework Membranes on Polymeric Substrates Modified with Polyvinyl Alcohol and Alginate Composite Hydrogel Yang Li, Xu Zhang, Xuan Chen, Kaijie Tang, Qin Meng, Chong Shen, and Guoliang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20422 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Applied Materials & Interfaces

Zeolite Imidazolate Framework Membranes on Polymeric Substrates Modified with Polyvinyl Alcohol and Alginate Composite Hydrogel Yang Li,§a Xu Zhang, §a Xuan Chen,§a Kaijie Tang,a Qin Meng,*b Chong Shen b and Guoliang Zhang *a

a

Institute of Oceanic and Environmental Chemical Engineering, State Key Lab

Breeding Base of Green Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China b

College of Chemical and Biological Engineering, and State Key Laboratory of

Chemical Engineering, Zhejiang University, Yugu Road 38#, 310027 Hangzhou, P.R.China

KEYWORDS: metal organic framework membrane; polyvinyl alcohol (PVA); sodium alginate (SA); composite hydrogels; gas separation

ABSTRACT: Polyvinyl alcohol-sodium alginate (PVA-SA) composite hydrogels were first introduced to synthesize robust and well-intergrown ZIF/polymer

hollow

fiber

membranes.

Through

enough

adsorption

interaction with metal ions by chelation, sufficient nucleation sites for in situ 1

ACS Paragon Plus Environment

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MOF preparation are provided. By taking this method, we can not only easily prepare defect-free MOF membranes ignoring complex modification process and seed deposition, but also structurally fix crystalline MOF layers and greatly improve the stiffness and durability of MOF composite membranes. The strategy also gives appropriate level of generality for synthesis of versatile dense MOF membranes on a variety of polymeric supports. The fabricated ZIF-8/PES membrane presented remarkable gas separation performance with H2 permeance as large as 9.66×10-7 mol m-2 s-1 Pa-1 and high H2/CO2 separation factor up to 29.0. 1. INTRODUCTION With the development of the industries, the hydrogen economy has given rise to more and more attentions.1 Hydrogen (H2), which can be used in energy and fuel cells, is often separated from the gas mixtures mainly containing CO2,2 and thus an efficient H2 purification technology is in great demand to keep the cost down. Membrane separation has been regard as the most effective technology to separate gases because of the high efficiency, environmental friendliness, low energy consumption and convenient operation.3,4 However, traditional polymer membranes have suffered from their intrinsic trade-off limitation between permeability and selectivity.5 Metal-organic frameworks (MOFs), made up of metal ions/clusters and organic ligands, have been explored as an excellent type of membrane materials for gas separation such as separation of H2 from CO2 owing to their outstanding 2


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properties including unique pore size, versatile chemical properties and large surface areas.6,7 Zeolite imidazolate frameworks (ZIFs), as important subfamily of MOFs, have been widely used to fabricate ZIF-based membranes due to their facile production and considerable hydrothermal stability.8 Up to now, many kinds of ZIF membranes with different pore sizes, containing ZIF-8 (0.34 nm),9 ZIF-100 (0.335 nm),10 ZIF-9 (0.30 nm),11 ZIF-7 (0.30 nm),12 ZIF-95 (0.37 nm)13 and ZIF-90 (0.35 nm),14 have been explored for gas separation. Most ZIF-based membranes with continuous and well inter-grown crystal layers were constructed on inorganic substrates, such as alumina,15-17 zinc oxide18 and metal nets19. The inorganic substrates can endow the as-prepared membranes with enough mechanical strength, chemical stability and high temperature resistance. However, inorganic supports have tedious preparation process, higher prices, easy brittleness and difficulty to direct large-scale, restricting their wide applications. Comparatively, polymer membranes take the advantages of low cost, large membrane areas, high processing ability and benefit to industrial applications. Hence, the prospect of MOF/polymer membranes for gas separation is very promising.20-22 Recently, many useful progresses have been made on the synthesis of MOF/polymer composite membranes, but it is still a big challenge to form continuous MOFs layer on porous polymeric membranes mainly because of the poor adhesion between MOF and supports and the lack of adequate heterogeneous nucleation sites.23-29 To solve these problems, chemical 3



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modifications towards the surface of polymer substrates have been applied to form defect-free MOF membranes on polymeric substrates. For instance, Chen and co-workers successfully fabricated ultrathin and continuous ZIF-8 on PVDF hollow fiber support coated with APTES functionalized titania.30 In our previous studies, based on crosslinking and substitution reaction of PVDF molecules by ammoniation, high quality layers of CuBTC, ZIF-8, NH2-MIL-53 and ZIF-7 were suitably synthesized on the ammoniation-based PVDF hollow fiber membrane.31,32 In particular, metal-gels coating method was also promoted to construct different kinds of polycrystalline MOF membrane, and the composite membranes revealed excellent stiffness and gas separation performance.33,34 The sol-gel coating was considered as straightforward and convenient method to modify polymer membrane by improving surface properties, thus the synthesis process of MOF/polymer membranes can be greatly promoted due to more nucleation sites and better adsorption strength. Sodium alginate (SA), one kind of natural polymers extracted from brown algae, consists of β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit), arranged in M blocks and G blocks and alternating G and M blocks. SA is easily to form hydrogels through crosslinking with different multivalent metal ion in mild conditions and can act as abundant metal sites to synthesize MOF crystals on the substrates.35,36 However, the alginate hydrogel can not exhibit enough stretchability and limited in application by its mechanical behaviour. In this case, polyvinyl alcohol (PVA) has some unique characteristics, such as 4


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flexible chain structure, gas barrier property and strong adhesiveness. Besides, PVA can also chelate with various divalent and trivalent metal ions, such as Ca2+, Zn2+ and Al3+.37,38 To solve the above issues, we go to forward to think, if PVA is added into SA hydrogel to form composites, the density of chelated metal ions and the adhesiveness of hydrogel will be greatly increased. The problem of hydrogel shedding from the substrates can be effectively solved, and sufficient heterogeneous nucleation sites and strong polymer-polymer interaction can be provided for growing dense MOF membrane. In this work, metal ions cross-linked PVA and sodium alginate composite hydrogels were impregnated on/into different polymer hollow fibers and converted to stiff and dense MOF membrane through seed hydrothermal crystallization (Figure 1). We selected ZIF-8 as the target because of their intrinsic thermal and chemical stability. Our method may make the following features. First, PVA and sodium alginate both have enough adsorption interaction with metal ions by chelation, which can provide sufficient metal nucleation sites for in situ MOF preparation.39 Second, the sol-gel coating is straightforward to form functional interface on the porous hollow fiber and thus crystalline MOF membrane can be structured easily, ignoring some complex modification process and seed deposition. Meanwhile, the composite hydrogels are formed in aqueous solution instead of using organic solvents, which not only is friendly to the environment but also reduces the production cost in traditional synthesis. Third, different from the traditional methods by 5



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synthesizing the continuous MOF layers on polymer matrix, which possibly endow the poor structural stability of MOF dense layers because of the deformation of elastic polymer support under high pressure, the PVA-SA hybrid hydrogels can be fully impregnated on/into polymer matrix and contribute to promote the stiffness and durability of composite membranes. As far as we know, there is no report on using blending hydrogels as stable composite substrates to synthesize well inter-grown and continuous MOF membranes. Additionally, based on the mechanism of this strategy, different polymer hollow fibers such as polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) can be employed as substrates to prepare versatile dense MOF membranes. Further, the prepared hydrogel-based composite membranes displayed excellent gas separation behaviour. 2. EXPERIMENTAL SETUP 2.1. Materials. Zinc nitrate hexahydrate and zinc chloride were procured from

Changzheng

Chemical

Reagent

Co.,

China.

Sodium

formate,

2-methylimidazole, polyvinyl alcohol (PVA) with a type of 1788 low viscosity and sodium alginate (SA) were provided by Hangzhou Bangyi Chemical Co., China. . The deionized water which used in this work was acquired from a home-made RO/NF integrated device, and its ion content was monitored by Metrohm 862 Compact IC and IRIS Intrepid ICP to attain σ≤0.5 μS cm-1. The hollow fibers such as polyethersulfone (PES) (1.8 mm o.d., 1.2 mm i.d., and molecular weight cut-off was MW-65000 Da), polyvinylideneuoride (PVDF) 6


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(1.4 mm o.d., 0.8 mm i.d., and molecular weight cut-off was MW-60000 Da) and poly-acrylonitrile (PAN) (1.3 mm o.d., 0.9 mm i.d., and molecular weight cut-off was MW-60000 Da) were fabricated by wet spinning method. All of the hollow fibers were then broken into the segments with length of 20 mm, and washed with deionized water and methanol for use. 2.2.

Synthesis

of

Zn2+

crosslinking

PVA-SA

composite

hydrogels/polymer hollow fibers. PVA was first added into water (10ml) and stirred at 50 °C to get 3 wt% PVA solution. After PVA powders were dissolved completely, SA with a designed concentration (1 wt%, 0.5 wt% and 0.25 wt%) was added into the above aqueous solution and stirred at 50 °C to dissolve. The composite hydrosols with the mass fraction ratio of 3:1 (or 6:1, 12:1) were obtained. After that, PES hollow fibers were immersed in the as-synthesized composite hydrosols. The modified PES hollow fibers were taken out and the redundant hydrsols were removed by nitrogen. The PVA-SA hydrogels/PES membranes were obtained by immersing hydrosols based hollow fibers into zinc nitrate hexahydrate (2 M, 10 mL) aqueous solution, then maintained in Zn2+ solution. The obtained PVA-SA hydrogels/PES hollow fibers were cleaned with DI water. In order to achieve a uniform coating on/into the PES hollow fibers, the hollow fibers were vertically settled on stainless wire and dried at 60 °C. The composite hydrogels modified PAN and PVDF membranes were prepared with the same method.

7



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2.3. Fabrication of ZIF-8/polymer composite membranes. The composite hydrogels modified polymer hollow fibers were beforehand soaked in 2-methylimidazole solution (0.5 M) at 50 °C to activate Zn (II) clusters and subsequently washed with methanol three times. After pouring into the synthetic solution in a Teflon lined stainless steel autoclave, the pre-treated composite membranes were settled vertically and heated at 80 °C for 24 h. The as-synthesized ZIF-8/polyer membranes on the outside of the hollow fibers were washed by pure methanol to remove loose ZIF-8 powders after hydrothermal synthesis process, and dried at room temperature. 2.4. Characterization. The chemical structures of the hollow fiber and composite

hydrogels

Fourier-transform

coating

infrared

hollow (FTIR)

fiber

were

spectroscopy

recorded

by

(Nicolet

using 6700,

Thermo-Scientific, USA). The structures and morphologies of the specimens were observed by scanning electron microscopy (SEM, TM-1000, Hitachi, Japan). Moreover, the X-ray diffraction (XRD) patterns were estimated by appling an X'Pert PRO diffractometer (PANalytical, Netherlands) with Cu Ka radiation (40 kV, 40 mA, λ=0.154056 nm) at room temperature. 2.5. Gas permeation experiments. As an important indicator for the performance evaluation of ZIF-8/PES membrane, gas permeability and selectivity were measured through single gas permeation tests. Firstly, the resulted ZIF-8/PES composite membranes were installed into a plexiglass tube by epoxy resins. Before carrying out gas permeation tests, the membrane 8


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module should be swept with the target gas for removing residual air. After that, the volumetric flow rates of H2, N2 and CO2 were tested by a soap-film flowmeter, and the pressures of feed gas varied from 0.05 to 0.20 MPa, which was kept at room temperature. The ideal gas separation factor was calculated by the ratio of permeated flow rates of different gas. The selectivities for H2/CO2 and H2/N2 mixtures were also evaluated. 3. RESULTS AND DISCUSSION PVA and SA are water-soluble organic molecules (Figure S1), their aqueous solution has a normal fluid state and is easy to be coated on polymer substrates. We chose PVA/SA mixture aqueous as coating solution to functionalize the PES hollow fiber. In order to prove the PVA and SA crosslinked by metal ions (Zn2+), chemical relationship among PVA, SA and Zn2+ was characterized by FTIR (Figure 2a). The original PES hollow fiber individually immersed in a PVA and SA mixed solution was regarded as control to identify that the network was formed between PVA and SA molecules induced by metal ions. Compared with the original PES hollow fiber, the bands at 3374 cm-1 have an extreme broad peak owing to the characteristic of hydrogen bond and hydroxyl groups (–OH) stretching vibration between PVA and SA.37 Additionally, the bands at 1730 cm-1 represent unhydrolyzed vinyl acetate stretching in PVA. The bands at 1656 and 1409 cm−1 are attributed to asymmetric stretching of -COO- and symmetric stretching of carboxylate salts groups in SA, 9



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respectively.40 It is obvious that different bonds at 2933 cm-1 for -CH2 groups are confirmed as a feature groups for chemical relationship of PVA-SA blend hydrogels.38 These results prove that composite hydrogels has been coated on the surface of hollow fibers. To further confirm the above results, the XPS analysis was also used. In Figure 2b, the composition of S2p decreased to 0.25 % after coating, which manifests that the original PES hollow fiber was enveloped by the PVA-SA hydrogel networks. To explain the PVA-SA networks, the changes of electron binding energy of C-O were characterized.41 PVA and SA have two principal peaks at 532.2 eV and 531.0 eV (Figure 2c) because of the C=O and C-O groups, respectively. However, in Figure 2d, the binding energy of the C=O and C-O group shifted to lower values (531.2 and 529.9 eV, respectively), indicating that electrons moved from PVA and SA to the metals. It demonstrates that the electronic cloud density around the oxygen atoms was distinctly reduced by Zn2+ in the coordination process with the PVA-SA/metal networks forming on the polymeric substrate surface. Therefore, the oxygen-containing functional groups of PVA and SA were affected, leading to a variation of binding energy and correlation fraction. The SEM was exploited to characterize of original PES hollow fiber and PVA-SA composite hydrogels modified PES hollow fiber. From Figure 3, a continuous and irregular structure covers onto the surface of PES hollow fiber, and the thickness of this layer is about 200 nm. Moreover, compared with the 10


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pure PES hollow fiber (Figure S2), the PES hollow fiber coated with PVA-SA composite hydrogels has smaller pores structures. It is clear that PVA-SA composite hydrogels with a porous three-dimensional (3D) network structure was successfully impregnated in/on the support and the pore size of PES hollow fiber was effectively reduced. After PVA-SA hydrogels modified PES hollow fibers were fabricated, the ZIF-8 membrane was synthesized. The XRD was applied to analyze the crystalline peak of the as-synthesized ZIF-8/PES composite membrane. Figure 4 shows that the peak positions of ZIF-8/PES composite membrane well matched with simulation ZIF-8, indicating highly crystalline of ZIF-8 structure on the composite hydrogels modified PES hollow fiber. Moreover, there is a weaker peak at around 2θ=20°, which is attributed to the crystalline region of PVA-SA.40,42,43 This phenomenon indicates that the PVA-SA hybrid hydrogels did not disappear in the ZIF-8 mother solution and presented good stability. For the surface morphology of ZIF-8/PES composite membrane, it was found from the SEM image (Figure. 3c) that the ZIF-8 film was well inter-grown by octahedral ZIF-8 crystals without obvious pinholes, cracks, or other defects. Moreover, the ZIF-8 layer with the thickness about 27 μm connected with PES hollow fiber tightly and equably, and no ZIF-8 crystals occurred inside the substrate which illustrated that the PVA/SA composite hydrogels with huge network structures could prevent the MOF mother solution from diffusing into the spongy pore of PES hollow fiber (Figure 3d). In order to 11



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demonstrate the role of PVA-SA hybrid hydrogels, we also tried to form ZIF-8 dense layer on the original PES support. From Figure S3, we can clearly recognize that the ZIF-8 crystals were fragmented and non-continuous by lack of heterogeneous nucleation sites on the PES hollow fibers, which is in accordance with our previous report.27 Specific information about the effect of PVA and SA on prepared ZIF-8 membrane was provided in Figure S4 and Figure S5. To study the universal applicability of our strategy, ZIF-8/polymer composite membranes were fabricated on some other polymer supports, such as PAN and PVDF hollow fiber. As displayed in Figure S6, it can be seen that a dense and well inter-grown crystal ZIF-8 film formed on both PAN and PVDF hollow fibers. It manifests that the composite hydrogels is feasible as versatile precursor to fabricate dense MOF membrane, and can be used for more extensive applications. The stiffness is a vital property for MOF membranes in practical application. Despite the fact that MOF crystals occupy good hardness, the stability of continuous MOF layers may be still low because of the deformation discrepancy between MOF and polymer. Therefore, the prepared MOF-polymer membrane should hold good hardness to hinder the deformation of MOF layer. To investigate the stiffness of the resulted MOF membrane, it was installed between the sheets and then exerted a constant external pressure which was displayed in our previous report.44 The changes of ZIF-8/PES composite 12


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membrane diameter with time were shown in Figure 5 to characterize the stiffness of the resulted membrane at the pressure of 50 g. Compared with the pure PES hollow fiber and ZIF-8 formed on the unmodified PES hollow fiber, it is evident that the ZIF-8/PES composite membrane showed much smaller diameter variations. The result demonstrates that the PVA-SA hydrogels interlayer effectively restricted the deformation of MOF layer and improved the compression strength and stiffness of the composite membrane to a considerable extent. Based on the above characterization, we went forward to investigate the permeation behaviour of prepared MOF/polymer composite membrane (Figure S7). The permeation test was carried out under different pressures to further verify the mechanical stability of the prepared ZIF-8/PES membranes. The membranes synthesized with a concentration of SA about 1.0 wt% was set as an experimental sample. As shown in Figure S8, with the H2 and CO2 partial pressures increasing from 0.05 MPa to 0.20 MPa, the permeation rate of H2 decreased and CO2 increased slightly, leading to a decrease in separation factors under increased pressure. The reason could be that ZIF-8 crystalline cages were filled and occupied by infiltrated hydrogen and stronger diffusion resistance formed under high pressure.35 In spite of that, the composite membrane still maintained favorable gas separation performance at 0.2 MPa, H2 permeance was as large as 6.64×10-7 mol m-2 s-1 Pa-1 with relatively high

13



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H2/CO2 separation factor of 12.26. This demonstrates that the composite membrane possesses excellent mechanical stability. To improve the gas separation performance of ZIF-8/PES membrane, the effect of SA on composite membrane was investigated. The composite membranes were fabricated with different SA concentrations in hybrid hydrogels (Figure S9 and Figure S10). As shown in Figure S9, when the PES hollow fiber was coated with PVA only, the resulted ZIF-8 membrane displayed poor separation factor. With the concentrations of SA increasing from 0.25 wt% to 1.0 wt%, H2 permeances of ZIF-8/PES membranes increased obviously from 3.29×10-7 mol m-2 s-1 Pa-1 to 9.66×10-7 mol m-2 s-1 Pa-1 at 0.05 MPa. Meanwhile, the selectivity for H2/CO2 also showed an upward trend and increased from 11.52 to 29.0. The higher concentrations of SA would cause the greater degrees of crosslinking, and the porous networks had more polar oxygen groups and large pore structures, thus promoting H2/CO2 separation factor and H2 permeance to increase. However, when the concentration of SA further increased to 1.5wt%, the PVA-SA mixture hydrosols might become too viscous to prepare a functional coating impregnated in/on polymer hollow fiber. Figure 6 shows the gas separation results of the composite membrane as a function of kinetic diameter of gas molecules. Although the kinetic diameter of CO2 is smaller than N2, the resulted ZIF-8/PES membrane presented a little larger N2 permeance than CO2. It is likely the composite membrane can adsorb and limit CO2 molecule permeate through the membrane, according to the 14


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reports elsewhere.10 On the other hand, the PVA-SA hydrogels containing abundant oxygen-containing polar groups may have certain adsorption capacities for CO2. Thus, when CO2 passed through the ZIF-8/PES membrane, it can be preferentially adsorbed by PVA-SA hydrogels layer, resulting in the lower gas permeance compared with other gases and the higher H2/CO2 selectivity. From Figure 6, it can be found that the ZIF-8/PES membrane displayed considerably large H2 permeance of 9.66×10-7 mol m-2 s-1 Pa-1 as well as the ideal separation factors of H2/CO2 and H2/N2 reached 29.0 and 14.25, respectively, which were much higher than their Knudsen diffusion selectivity (4.69 and 3.74). Moreover, the separation selectivities for equimolar mixtures H2/CO2 and H2/N2 were about 26.54 and 12.89, respectively. It indicated that the as-synthesized ZIF-8/PES membrane was compacted and defect-free. Furthermore, compared with the other MOF/polymer membranes reported in the literature (Figure 7 and Table S1), the PVA-SA mixture hydrogels based ZIF-8/PES composite membrane in the present study exhibits very competitive gas separation performance. In view of application, the reproducibility and durability of the ZIF-8/PES composite membrane were also tested. The gas permeation experiment was carried out for 64 hours (Figure 8). Both the gas permeances and separation factors only showed slight change and remained almost constant in long term operation, which testify the well durability of the composite membrane. In addition, four membranes were fabricated in the same operation condition to 15



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testify the reproducibility our method (Table S2). All of the resulted membranes displayed similar performance and the average value of H2 permeance was 9.66×10-7 mol m-2 s-1 Pa-1, which exhibited good reproducibility of the new synthesis strategy. 4. CONCLUSIONS In summary, a stiff, integrated and reproducible MOF/polymer composite membrane was successfully fabricated on PVA-SA hybrid hydrogels modified polymer hollow fibers. By using this new method, high-quality crystalline MOF membrane can be structured more easily, ignoring the complex modification process and seed deposition. Meanwhile, the composite hydrogels can be formed in aqueous solution instead of using organic solvents, which not only is friendly to the environment but also cuts down the production cost in synthesis. Most importantly, different from the traditional ways by developing the MOF layers simply on the surface of polymer matrix, which possibly give the poor stability of MOF layers owing to the curvature of elastic matrix under high pressure, the PVA-SA hybrid hydrogels can be infixed on/into polymer matrix adequately and contribute to the structural stability of composite membrane. By combining the PVA-SA hybrid hydrogels with the MOF layer on the surface, the stiffness and durability of composite membranes can be greatly increased. The H2 permeance can reach as large as 9.66×10-7 mol m-2 s-1 Pa-1 with competitively high H2/CO2 and H2/N2 separation factor of 29.0 and 16


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14.25, respectively. In addition, our strategy exhibits good feasibility and can be employed to fabricate versatile MOF membranes on different substrates, which may provide a

low cost and easy processing platform for in situ

construction of robust and high-quality MOF membranes in industry. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Additional SEM, experimental simple setup, and gas separation performance of the ZIF-8/PES composite membrane. AUTHOR INFORMATION Corresponding Author *Address: Institute of Oceanic and Environmental Chemical Engineering, State Key Lab Breeding Base of Green Chemical Synthesis Technology, Zhejiang University of Technology, Hangzhou 310014, China. Email: [email protected]. Phone/Fax: +86-571-8832 0863 Co-corresponding Author *Address: Department of Chemical and Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang, 310027, PR China. E-mail address: [email protected]. Tel: 86-571-87953193; Fax: 86-571-87951227 17



§ These

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authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank for financial support the National Natural Science Foundation of China (Grant Nos. 21736009 and 21476206), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY18B060010) and the Minjiang Scholarship from Fujian Provincial Government. REFERENCES (1) Zhang, F.; Zou, X.; Gao, X.; Fan, S.; Sun, F.; Ren, H.; Zhu, G. Hydrogen Selective NH2-MIL-53(Al) MOF Membranes with High Permeability. Adv. Funct. Mater. 2012, 22, 3583-3590. (2) Cacho-Bailo, F.; Etxeberria-Benavides, M.; David, O.; Tellez, C.; Coronas, J. Structural Contraction of Zeolitic Imidazolate Frameworks: Membrane Application on Porous Metallic Hollow Fibers for Gas Separation. ACS Appl. Mater. Interfaces 2017, 9, 20787-20796. (3) Cao, L.; He, X.; Jiang, Z.; Li, X.; Li, Y.; Ren, Y.; Yang, L.; Wu, H. Channel-facilitated Molecule and Ion Transport Across Polymer Composite Membranes. Chem. Soc. Rev. 2017, 46, 6725-6745. (4) Low, Z. X.; Budd, P. M.; McKeown, N. B.; Patterson, D. A. Gas Permeation Properties, Physical Aging, and Its Mitigation in High Free Volume Glassy Polymers. Chem. Rev. 2018, 118, 5871-5911. (5) Park, H. B.; Kamcev, J.; Robeson, L. M.; Elimelech, M.; Freeman, B. D. Maximizing the Right Stuff: The Trade-Off between Membrane Permeability and Selectivity. Science 2017, 356, eaab0530. 18


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Figure 1. Scheme of (a) preparation of ZIF-8/polymeric membrane by PVA-SA composite hydrogels, (b) formation of PVA-SA mixture hydrogels, (c) “Egg-box” model of Zn2+ cross-linked sodium alginate.

24


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Figure 2. (a) FTIR spectra and (b) X-ray photoelectron spectra (XPS) of PES, PES/PVA-SA and PES/PVA-SA@Zn2+ hollow fiber s. High resolution XPS spectra of O1s in (c) PVA-SA and (d) PVA-SA@Zn2+ coated composite hollow fibers.

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Figure 3. SEM images of PVA-SA hydrogels coated PES hollow fibers (a,b), ZIF-8/PES composite membrane (c,d).

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Figure 4. XRD patterns of simulation ZIF-8 crystal, Zn2+ based PVA-SA hydrogels/PES composite hollow fiber and ZIF-8/PES composite membrane.

27



0.0

ZIF-8/PES composite membrane 50g ZIF-8/Original PES hollow fiber 50g Original PES hollow fiber 50g

-0.5

Compression (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1.0 -1.5 -2.0 -2.5 -3.0 0

10

20

30

40

50

60

Time (min) Figure 5. Change of the compression of original PES hollow fiber, ZIF-8/original PES hollow fiber and ZIF-8/PES composite membrane vs. time.

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Figure 6. Single and mixture gas permeances of the ZIF-8/PES composite membrane as a function of their kinetic diameter at 25 °C and 0.05 MPa.

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Figure 7. Comparison gas separation performance of hydrogels-based ZIF-8/PES membrane with other polymer supported MOF membranes in literature (Detailed data is presented at Table S1).

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Figure 8. Permeance and separation factor of prepared ZIF-8/PES composite membrane as a function of operating time at 25 ℃ and 0.05 MPa.

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Table of Contents

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Zeolite Imidazolate Framework Membranes on Polymeric Substrates

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