Fig. 1 Schematic diagram of this work
Experimental section
Materials
2-aminobenzimidazole (C7H7N3, 99%, Heowns) and 2-methylimidazole (C4H6N2, 98%, Heowns) were employed as organic linker sources of NH2-ZIF-8 nanocrystals. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 97%, Meryer) was employed as the zinc source. Sodium formate (CHO2Na, 98%, TCl) was employed for the deprotonation of imidazoles. Ethanol (C2H5OH, 99.5%, Jiangtian Chemical Technology), n-heptane (GC, Aladdin), dimethylacetamide (CH3CON(CH3)2, 99.5%, Meryer, hereafter DMAc), tetrahydrofuran (C4H8O, GC, Meryer, hereafter THF), N, N-Dimethylformamide (C3H7NO, 99%, Aladdin, hereafter DMF) and deionized (DI) water were employed as solvents. Polydimethylsiloxane (PDMS, part A and B, SILGARDTM184) was employed for surface modification of porous substrates. N-bromosuccinimide (NBS, 99%, Aladdin) and azodiisobutyronitrile (AIBN, 99%, Aladdin) were utilized as bromo-functionalized reagents. Triethylamine ((C2H5)3N, 99.5%, TCl) and acetic anhydride (C4H6O3, 99.5%, TCl) were employed as the catalyst and dehydrating reagent in the synthesis process of polyimide, respectively. Polyacrylonitrile (here-after PAN, mean pore diameter of ~20 nm) was used as the porous substrates. All the above reagents and materials were used without further purification. Before polymerization, the monomers 6FDA (C19H6F6O6, 99.5%, Sigma-Aldrich) DAM (C9H14N2, 99.5%, Sigma-Aldrich) were purified by vacuum sublimation at 215°C and 75°C, respectively.
Preparation of PDMS modified substrates (mPANs)
Firstly, we prepared 0.3 wt% PDMS solution: 0.5 g PDMS part A and 0.05 g part B were added to 109.45 g n-heptane, and the solution was stirred for 4 h. Secondly, the PAN substrates were fixed on the surface of the glass sheet by Kapton tapes, and we spread the PDMS solution to cover the surface of the substrates and started spin-coating at the spinning speed of 500, 1000, 2000 and 3000 rpm for 1min, respectively. After spin-coating process, the substrates were placed in a fume hood and dried for 12 h. We denote the modified substrates as mPANs.
Synthesis of polymers
The illustration of the synthesis of bromo-functionalized polyimide (PI-Br) are shown in Fig. S1.35,36 Firstly, we synthesis the pristine 6FDA-DAM via step growth polymerization.35 Before the reaction, purified 6FDA and DAM were dried under vacuum at 120 °C and 40 °C for 8 h. The monomer solution was prepared by adding 10 mmol 6FDA and 10 mmol DAM in a 100 mL bottle with 25.5 mL DMAc. The ratio of the monomers in the solution is 20 wt%. The solution was kept in 0°C, stirring by the mechanical stirrer at 800 rpm under Ar atmosphere for 24 h. Then the yellow colloidal solution was stirring at 25°C under Ar purge for another 24 h by adding acetic anhydride and triethylamine. After reaction, we precipitated and washed the dark yellow solution in methanol and placed the white polymer powder in a vacuum oven at 180 °C overnight to remove the absorbed water and solvent residues.
Secondly, we dissolved 6mmol self-made 6FDA-DAM powder in 50 ml CH2Cl2, stirring under N2 purge. Then we added 6mmol NBS and 0.04 mmol AIBN to the mixture, and performed the reaction at 80°C for 4 h under reflux condensation. Following this, we precipitated and washed the product mixture in cold methanol and placed the light-yellow polymer powder under vacuum at 180 °C overnight to remove the absorbed water and solvent residues. The 1HNMR results are shown in Fig. S2 and indicate the successful synthesis of PI-Br.
Synthesis of MOF nanocrystals
In this work, we synthesized amino-functionalized ZIF-8 nanocrystals (NH2-ZIF-8) via an improved mixed-linker strategy by modulating the reaction temperature.15,37-39 First, we dissolved 34 mmol 2-methylimidazole, 6 mmol 2-aminobenzimidazole and 10 mmol sodium formate in 100 ml deionized water, under 70°C and stirred for 2 h. Second, we dissolved 10 mmol Zn(NO3)2·6H2O in 100 ml DMF and stirring for 5 min. Then we mixed the above solutions and stirred for 1 h. After reaction, the MOF crystals were collected by centrifugation, washed several times in methanol and placed under vacuum at 85 °C overnight.
Preparation of MMCMs
We employed spin-coating method for the preparation of MMCMs.40,41 In this study, we utilize nano-sized NH2-ZIF-8 crystals as MOF fillers, distributed in 6FDA-DAM or PI-Br, wherein the corresponding MMMs are denoted as MMCM-A and MMCM-B, respectively. To further reduce the thickness of the thin films, we increased the spin-coating speed and employed NH2-ZIF-8 with smaller size. Firstly, for MMCM-A, we placed the 6FDA-DAM powder under vacuum at 180 °C overnight. Moreover, we employed MOF/THF suspension rather than MOF powder to facilitate dispersing the MOF nanocrystals in the polymer solution. To prepare MMCMs with a certain filler composition, we dropped the MOF/THF suspension into a certain volume of 6FDA-DAM/THF solution (4 wt% of 6FDA-DAM) wherein MOF crystals accounts for 10-40 wt% to the total amount of MOF and 6FDA-DAM. Then we stirred the resulting solutions overnight and sonicated them for 3 h to eliminate the bubbles.
Secondly, we fixed the mPANs on the glass plates by Kapton tapes and placed the plates on the spin-coating machine. Then we spread the solution onto the whole area of mPANs and started spin-coating for 50 s at the speed of 1000 rpm. We employed NH2-ZIF-8 nanocrystals (mean diameter of ~133 nm), with increasing the spinning speed to 2000 rpm to obtain MMCM-A with a thinner selective layer. Following this, we placed the membranes under vacuum at 40°C overnight.
Fabrication of MMCM-B was conducted by a similar protocol. The prepared NH2-ZIF-8/THF suspension was distributed into a dilute PI-Br/THF solution (4 wt% of PI-Br). In spin-coating process, we coated the solution onto the whole area of mPANs and started spin-coating for 50 s at the speed of 2000 rpm. Similar with MMCM-A, we employed NH2-ZIF-8 with mean size of ~82 nm and increased the spinning speed to 3000 rpm to obtain MMCM-B with thinner selective layers. After the spin-coating process, we placed the membranes under vacuum at 40°C overnight.
Characterization
The surface area and pore structure of the synthesized NH2-ZIF-8 samples was measured by Micromeritics instrument (ASAP 2460) under N2 sorption at 77 K. The morphologies of the NH2-ZIF-8 nanocrystals and the membranes were characterized by Scanning electron microscopy (SEM) (Regulus R-8100). The X-ray diffraction (XRD) spectrum of NH2-ZIF-8 were characterized on Rigaku D/max 2500v/pc using Cu Kα radiation. Before analyzing the surface area, we degassed NH2-ZIF-8 nanocrystals at 120°C overnight. The interfacial interactions in MMCMs were characterized by Fourier Transform Infrared (FT-IR, Bruker Vertex 70). The cross-sections of the MMCMs were prepared under freezing conditions wherein the membranes were fractured after being immersed in liquid N2 for 120 s. Before characterization, we coated the SEM samples with Platinum to increase the conductivity.
Results and Discussion
Characterization of NH2-ZIF-8 nanocrystals
In this study, we synthesized three types of NH2-ZIF-8 nanocrystals with different sizes.37-39 As shown in Fig.2a, the XRD patterns of synthesized NH2-ZIF-8 nanoparticles are all consistent with the simulated pattern, confirming the successful synthesis. The SEM images of NH2-ZIF-8 nanocrystals show that the three types of synthesized NH2-ZIF-8 crystals have size of 82±18 nm (Fig. 2b), 133±15 nm (Fig. 2c) and 245±70 nm (Fig. 2d), respectively. We employed N2 adsorption measurement to examine the pore structures of the NH2-ZIF-8 samples. According to the results in Fig. 2e, we calculate the BET surface areas of the NH2-ZIF-8 nanocrystals are 985.4 cm2 g-1, 625.2 cm2g-1 and 460.7 cm2g-1, respectively, indicating a high gas adsorption capacity which facilitates the low-resistance molecular diffusion. As shown in Fig. 2f, it can be found that new infrared absorption peaks exist at 3183 and 3381cm-1, corresponding to the stretching vibration of -N-H, confirming the successful incorporation of amino groups in ZIF-8 crystals.