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Aqueous phase synthesis of ZIF-8 membrane with controllable location on an asymmetrically porous polymer substrate Ezzatollah Shamsaei, Xiaocheng Lin, Ze-Xian Low, Zahra Abbasi, Yaoxin Hu, Jefferson Zhe Liu, and Huanting Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12684 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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ACS Applied Materials & Interfaces

Aqueous phase synthesis of ZIF-8 membrane with controllable location on an asymmetrically porous polymer substrate Ezzatollah Shamsaei,† Xiaocheng Lin,† Ze-Xian Low,†‡ Zahra Abbasi,† Yaoxin Hu,† Jefferson Zhe Liu,§ and Huanting Wang*†. †

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia. ‡

§

Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria

3800, Australia. KEYWORDS: metal−organic framework membranes, ZIF-8, contra-diffusion, chemical vapor modification, gas separation.

ABSTRACT In this study, we have demonstrated a simple, scalable, and environmentally friendly route for controllable fabrication of continuous, well-intergrown ZIF-8 on a flexible polymer substrate via contra-diffusion method in conjunction with chemical vapor modification of the polymer surface. The combined chemical vapor modification and contra-diffusion method resulted in controlled formation of a thin, defect-free and robust ZIF-8 layer on one side of the support in aqueous solution at room temperature. The ZIF-8 membrane exhibited propylene permeance of 1.50×10-8 mol m-2 s-1 Pa and excellent selective permeation properties; after post heat-treatment, the

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membrane showed ideal selectivities of C3H6/C3H8 and H2/C3H8 as high as 27.8 and 2259, respectively. The new synthesis approach holds a promise for further development for the fabrication of high-quality polymer-supported ZIF membranes for practical separation applications.

1. INTRODUCTION Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are porous crystalline hybrid materials consisting of imidazolate ligands (Im) bridging tetrahedral metal ions (e.g., Zn, Co).1 They closely resemble the topologies of zeolites, due to the M-Im-M (M = Zn, Co) bond angle of 145°, which is close to the T-O-T (T = Al, Si, P) angle (140-170°) in zeolites.2-3 ZIFs show properties that combine the attractive features of both MOFs and zeolites such as tunable pore size and chemistry, large internal surface area and relatively good thermal and chemical stability.4-5 These properties make ZIFs excellent candidates for the fabrication of molecular sieving membranes for gas separation.6-8 ZIF-8 membranes, for example, have been reported to be capable of molecularly discriminating propylene (~4.0 Å) from propane (~ 4.3 Å) since the effective pore aperture size of ZIF-8 falls in the range of 4.0-4.2 Å (larger than its crystallographic value of 3.4 Å, owing to the swaying effect of the ligands).2, 9-11 ZIFs have been widely used to fabricate the so-called mixed matrix membranes (MMMs, consisting of pre-synthesized ZIF particles dispersed in a polymeric matrix) to afford a solution to go beyond the Robeson's upper-bound trade-off limit of the polymeric membranes.6,

12-15

While MMMs have been shown to enhance the permeation properties of polymeric membranes, further enhancements were made using in-situ synthesized ZIF membranes.16 For example, ZIF8 supported membranes prepared by Pan et al.10 showed superior propylene/propane


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permselectivity (permeability of propylene up to 200 barrers and a propylene/propane separation factor up to 50) compared to those of ZIF-8 MMMs, as for instance the ZIF-8/ 6FDA–DAM polyimide (a propylene permeability of 56.2 Barrer and propylene/propane ideal selectivity of 31.0) reported by Zhang et al.15To date, several synthesis methods have been reported for the formation of ZIF films on various substrates.6,

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In particular, polymer-supported ZIF

membranes are of great interest as they potentially combine the advantages of both polymer membranes (e.g. easy processing and low cost) and ZIFs (e.g. high selectivity). In principle, the growth of ZIF films on flexible polymeric substrates can be easily achieved due to favorable chemical interaction between the polymer and the organic ligand of ZIFs. Nagaraju et al.18 and Cacho-Bailo et al.19 grew ZIF-8 on a porous polysulfone using in situ (direct) growth. However, although the in situ synthesis is a simple method that allows for simultaneous nucleation, deposition and crystal growth, it is not very effective in preparing continuous ZIF membranes due to limited heterogeneous nucleation sites on the substrate.20 Alternatively, Ge at al.21 used secondary seeded growth to fabricate a continuous ZIF-8 film on an asymmetrically porous polyethersulfone substrate. Secondary seeded growth has been shown to effectively induce controlled ZIF growth on the polymer support, but the resulting ZIF layer often suffers from weak adhesion to the support, leading to membrane delamination.

22

The surface modification

has been commonly used to functionalize the support, thereby promoting heterogeneous nucleation and enhancing the ZIF-to-substrate adhesion strength.23-25 Very recently, we successfully developed a new strategy of vapor phase modification to introduce amine groups and reduce surface pore sizes of the polymer support; such surface modification enabled fast formation of a continuous ZIF-8 ultrathin layer in the presence of ammonium hydroxide (as a deprotonating agent) under sonication for only 3 minutes.23


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However, the sonication-induced crystallization offers limited control over the ZIF-8 crystal sizes and intergrowth, and thus membrane properties. Our group was one of the first groups to report contra-diffusion synthesis, which self-limits growth of ZIF films on porous substrate, and has great potential to offer better control in the membrane fabrication.11,

26-28

However, the

growth of ZIF films via contra diffusion method depends on the surface properties and porous structure of support; the formation of ZIF layer on both sides of support or within porous channels of support has been reported.

26-27

To achieve better control over the membrane

position, Brown et al. recently introduced an interfacial synthesis approach.29 The control over the membrane position relies on employing an oil/ water system, in which crystals grow at the interfaces between the two immiscible solvents. The resulting ZIF-8 membrane exhibited high gas separation performance with H2/C3H8 and C3H6/C3H8 separation factors as high as 370 and 12, respectively. In this work, we report a simple, effective and environmental friendly method for the fabrication of high-quality ZIF-8 membrane with controllable location on a polymer substrate in aqueous solution. Our synthesis method is based on contra-diffusion (CD) concept in conjunction with chemical vapor modification (hereafter chemical vapor modification-contra diffusion method). A flat sheet asymmetric30 bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) ultrafiltration membrane was prepared via phase inversion and employed as the support for growing a thin ZIF-8 layer via contra diffusion after modification. We have selected BPPO for ZIF-8 growth because of its outstanding membrane formation and mechanical properties as well as excellent hydrolytic stability.31 It can also be easily functionalized and crosslinked due to the abundant highly reactive –CH2Br groups. Using vapor-phase ethylenediamine (EDA), we have previously shown that amine functional groups can be


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covalently attached selectively on the top layer of the support without affecting the sublayer structure.23 The presence of the covalent link (amine groups) can be a driving factor for maintaining a high concentration of the metal ions selectively near the support surface. When combined with the slow diffusion of the ligand in contra diffusion process, the approach can lead to well-controlled crystal growth in the vicinity of the support surface. This results in the formation of thin, defect-free and robust ZIF-8 layer on one side of the support at room temperature without the addition of deprotonating agents, which has proven to be challenging when using other reported synthesis procedures.11, 26, 32-33

2. MATERIALS AND METHODS 2.1. Chemicals. BPPO was kindly supplied by Tianwei Membrane Co. Ltd., Shandong of China. Ethylenediamine (EDA, 99.5%),1-methyl-2-pyrrolidone (NMP, 99.5%), zinc acetate dihydrate (Zn(CH3COO)2.2H2O, 98%) and 2-methylimidazole (Hmim, C4H6N2, 99%) were purchased from Sigma-Aldrich, Australia and used as received. Methanol (absolute) was purchased from Merck, Australia. The water used for the experiments was purified with a water purification system (Milli-Q integral water purification system, Merck Millipore) with a resistivity of 18.2 MΩ/cm. Distilled water was obtained from a laboratory water distillation still (Labglass Aqua III). 2.2. Sample Preparation. BPPO support ultrafiltration membranes were prepared via nonsolvent induced phase separation at room temperature. The casting solution was prepared by dissolving 15 wt% of BPPO in NMP for 12 h with mechanical stirring at 200 rpm. The homogenous solution was left to degas for 10 h before use. Subsequently, the solution was cast on a clean glass plate using an adjustable micrometer film applicator (Paul N. Gardner Co., Inc.


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USA) with a gap of 200 µm at room temperature (22 ± 2 °C) and immediately immersed in a coagulation bath of deionized water. After peeling off from the glass plate, the membranes were removed from the bath, washed and kept in fresh deionized water (DI) for at least one day to thoroughly remove the residual solvents. The vapor-phase EDA modification was conducted in a custom-made container according to our previous work.23 In brief, 20 mL of EDA was allowed to vaporize, and stabilized for 1 h. The support membranes were quickly placed inside the containment with the top layer exposed and suspended above the EDA solution. After surface modification at room temperature for 16 h, the surface modified membranes were removed from the containment and immediately washed with pure water to completely remove the residual EDA. The resultant membrane were denoted as BPPO-EDA. For preparation of the BPPO supported ZIF-8 membranes, the modified BPPO supporting membrane was cut into 32 mm diameter discs, which were then mounted on a home-made setup (Figure S1, Supporting Information), where the zinc acetate solution and Hmim solution were separated by the supporting membrane. Zinc acetate solution was prepared by dissolving 0.09 g of Zn (CH3COO)2.2H2O (0.5 mmol, Sigma–Aldrich) in 20 mL of deionized water, and Hmim solution was prepared by adding 0.649 g of Hmim (8 mmol, Sigma–Aldrich) in 20 mL of deionized water. The designed Hmim: Zn2+ molar ratios in the system was 16 and was kept constant in our study. After crystallization at room temperature (22 ± 2 °C) for 60–120 min, the membrane samples were taken out and rinsed with DI water several times. Finally, the composite membranes were dried in ambient conditions for 24 h, followed by heating at 120-200 °C for 2 h before tests. The resulting samples were denoted as ZIF-8-BPPO-EDA-t-T, where “t” and “T” denote the crystallization time and heat treatment temperature, respectively.


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2.3. Characterization. Fourier Transform Infrared (FTIR) spectra of the membranes were recorded using an attenuated total reflectance (ATR) FTIR (Perkin Elmer, USA) in the range of 400-4000 cm-1 at an average of 20 scans with a resolution of 4 cm-1. Scanning electron microscopy (SEM; FEI Nova NanoSEM 450) with an X-ray detector (Bruker Nano GmbH, Germany) was used for imaging the surface and cross-sectional morphologies of membranes. Energy-dispersive X-ray spectroscopy (EDS) line-scan analysis of the membrane samples was conducted using EDX equipped in Nova NanoSEM 450 (Quantax 400 X-ray analysis system, Bruker, USA). The membranes were fractured in liquid nitrogen, fixed on stubs with doublesided carbon tape and then sputter coated with roughly 2 nm iridium (Ir) layer to ensure good electrical conductivity. The images were recorded at an accelerating voltage of 5 kV with different magnifications. Thermogravimetric analyses (TGA) were carried out on a SETARAM (TGA 92) device from 30 to 800 °C at a heating rate of 10 °C min-1 under air flow. Powder Xray diffraction (XRD) patterns were measured using a Miniflex 600 diffractometer (Rigaku, Japan) with Cu Kα radiation (15 mA and 40 kV) at a scan rate of 2° min-1 with a step size of 0.02°. The XRD studies were carried out at room temperature. The single gas permeation of composite membranes was measured using the pressure rise method.34 The schematic of the single gas permeation setup is shown in Figure S2. To measure the gas permeation flux, the composite membrane (16 mm diameter disc) was attached to a porous stainless steel holder (pore size ~200 nm) using epoxy resin (Torr seal, Varian), and then placed inside a larger Pyrex tube and connected to a sensitive pressure transducer (MKS 628B Baratron) and a vacuum line. The effective remaining membrane area was 1 cm2. For each single gas measurement, the pure single gas was fed to one side (feed) of the membrane while the other side (permeate) of the membrane was under vacuum. Since the feed side was at ambient


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pressure, a pressure difference of 1 atm was maintained between the permeate side and the feed side during permeation measurements. After allowing enough time to achieve a steady state conditions, the permeate side was shut off from vacuum and the pressure buildup of the permeating gas was measured by the pressure transducer and continuously recorded in a computer. To accomplish a single test, the pressure was allowed to reach a few Torr. To repeat an analysis, the permeate side was evacuated again and then shut off from vacuum so as to record the pressure rise. All the gas permeation tests were performed at room temperature. The molar flow rate of the permeating gas was calculated based on the recorded pressure. The permeance, Pi, of each gas was calculated according to the following equation, Pi= (V/RTA∆p) (dp/dt) where, V is the volume of the permeate side that was obtained by calibration using a bubble flowmeter (m3) , R is the ideal gas constant (m3 Pa K−1 mol−1), T is the temperature (K), ∆p is the pressure difference across the membrane (Pa), A is the effective membrane area (m2), and dp/dt is the rate of pressure rise in the permeate side (Pa/s). The ideal selectivity Sij is defined as the ratio of the two permeances Pi and Pj. Permeation data are average values recorded from at least three samples, which were prepared from different batches.

3. RESULTS AND DISCUSSION Figure 1 illustrates the synthesis of dense and defect-free polymer supported ZIF-8 membrane using chemical vapor modification-contra diffusion method. As illustrated in the figure (step (1)), the surface chemistry and pore size of the top layer of the BPPO are modified by using EDA-vapor. Substitution of bromide functional group with amine groups during EDA-vapor modification was confirmed by FTIR (Figure 2). Upon modification, the peak at 586 cm-1 and


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633 cm-1 attributed to the benzyl bromide (–CH2Br) groups (C–Br stretching) almost disappear and a new broad band in the range of 3100–3600 cm-1 emerges, which is attributed to the N–H stretching and confirms the amination of BPPO. TGA results (Figure S3, Supporting Information) also show that when the temperature is below 150 ºC, BPPO-EDA have more weight loss (~4%) than BPPO due to higher moisture residual in the membrane as a result of the introduction of hydrophilic amine groups. Additionally, the decomposition of EDA initiates at ~ 180 °C, which is higher than the boiling point (117 °C) of the EDA, indicating that there is an interaction between the EDA molecules and BPPO. Furthermore, SEM images (Figure 3 a, b) show an obvious decrease in the size of the nanopores at the top surface of the membrane after EDA vapor-phase modification. The reduction in the pore size of the support (from 25.5 to 15 nm) after its modification was further confirmed by TOC analysis (see measurements of the support pore size in the Supporting Information). The changes in the surface microstructure can be attributed to the partial cross-linking effect of the EDA, since the crosslinking causes tightening of the polymer network which reduces the pore size of the BPPO substrate. In addition, the final decomposition of the BPPO-EDA in the TGA results (Figure S3) is much slower than BPPO, which indicates a higher thermal stability due to partial crosslinking of the BPPO substrate. Note that partial crosslinking reduces the flexibility of the polymer support, which is favorable for avoiding ZIF layer cracking. The ZIF-8 membrane is formed on the pre-treated support by applying contra diffusion synthesis, in which the metal precursor solution and ligand (Hmim) solution are separated by the modified BPPO substrate (step (2) in Figure 1), at room temperature for various crystallization times. As demonstrated in our previous study,23 the direct heterogeneous nucleation and growth of a dense ZIF-8 layer on untreated BPPO surface was unsuccessful (Figure S4, Supporting


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Information). In fact, due to the fast diffusion of zinc ions through the pores of the unmodified support, crystallization occurs constantly inside the support channels, where there can be a high concentration of the reactant solutions, until the entire path through the ZIF-8 layer becomes “plugged”. A similar phenomenon was observed by Hara et al. in preparing copper-benzene tricarboxylate (Cu-BTC) or ZIF-8 membranes using porous α-alumina capillary substrate by applying a typical contra diffusion method.28, 33

Figure 1. Schematic diagram of the preparation of a BPPO polymer supported ZIF-8 membrane using chemical vapor modification and subsequent contra diffusion synthesis.

Figure 2. FTIR ATR spectra of the untreated BPPO support, BPPO modified with EDA-vapor (BPPO-EDA), BPPO-EDA supported ZIF-8 layer (ZIF-8-BPPO-EDA), and ZIF-8 powder.


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After modification of the BPPO with EDA-vapor before contra diffusion synthesis, a compact ZIF-8 layer was selectively formed on only one side (pre-treated side) of the support. Figure 3 and 4 show the SEM images and XRD patterns of the ZIF-8 membranes grown for different lengths of time. As shown in Figure 3c and Figure 4, a large amount of ZIF-8 crystals with clear facets is observed on the modified support even after 60 min of the contra diffusion synthesis at room temperature. Nanopores of the skin layer of the support are still observable through intercrystalline gaps between the ZIF-8 crystals in the high magnification image (inset in Figure 3c). With increasing the reaction time to 90 min, a dense and continuous ZIF-8 layer is formed on the modified BPPO skin layer, as shown in Figure 3d. Eventually, after 120 min of reaction, a layer of well intergrown ZIF-8 crystals with rhombic dodecahedron morphology and a thickness of about 2 µm fully covered the support surface without any visible defects such as pinholes or cracks (Figure 3e, f and Figure S5 in Supporting Information).There are only few studies reporting such a thin continuous polycrystalline ZIF-8 film23-24, 35 and most of the membranes prepared by the conventional in situ methods are too thick (in the range of tens of micrometers), showing lower gas flux through the membranes.11,

36

The continuous thin ZIF-8 membranes

remained unchanged even with further growth, demonstrating the self-limiting crystal growth, in which the crystals continue to grow only if the metal ions and the ligand molecules are in contact. Another important observation is that unlike the conventional contra diffusion method in which the crystals grow along the whole thickness of the support, ZIF-8 crystals can be observed only at the very outermost section of the EDA-vapor-modified BPPO support (Figure 3f). Energy-dispersive X-ray spectroscopy (EDS) line-scan analysis (Figure S6 in the Supporting Information) further confirmed the presence of ZIF-8 within the support sublayer as zinc was detected up to about 1 µm underneath the support surface. This means that the heterogeneous


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nucleation and crystal growth occur on the skin layer of the pre-treated support, which is contrary to what was observed for the untreated BPPO, where the nucleation and crystal growth happened all along the support channels with a non-continuous film, if any, on the surface. As explained in our previous work, EDA can simultaneously create amino groups, which can coordinate to the free zinc ions and provides a large number of nucleation sites; and reduce the substrate pore size induced by its crosslinking effect.23 In the present work, therefore, the reduction in the surface pore size and the coordination interaction between the support surface and zinc ions can lead to a decrease of the diffusion rate of Zn2+ and provide a relatively high precursor concentrations at the support surface and restricting the reaction zone in this vicinity (Figure 1). This high precursor concentration and the large number of previously formed nucleation sites result in the faster and thinner crystal growth in the vicinity of the support skin layer as compared to the slow and undirected crystal growth along the channels of untreated BPPO.

Figure 3. SEM images of untreated BPPO (a), vapor-phase-EDA-modified BPPO (BPPO-EDA) (b), ZIF-8@BPPO-EDA grown for 60 min (c), 90 min (d), and 120 min (e, f).


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Figure 4. XRD patterns of ZIF-8@BPPO-EDA membranes as a function of growth time. Figure 5 presents ZIF-8 membranes synthesized by contra diffusion (at the reaction time of 2h) after modification of the BPPO via immersion of the polymer in 60% EDA aqueous solution at room temperature for 3h. It is obvious that the crystals are grown along the whole thickness of the polymer. As shown, ZIF-8 crystals not only block the polymer micro-channels but also grow within the whole porous structure of the polymer as the interface between the ZIF-8 and the polymer matrix is hardly distinguishable. This is because the immersion of the polymeric support in the nucleophilic diamine solution can result in an extremely high degree of modification (a large number of nucleation sites) within the bulk polymer and the subsequent formation of ZIF-8 crystals when applying contra diffusion synthesis. This shows that crystal formation within the whole polymer support is unavoidable when modifying the BPPO via solution immersion method. Instead, as already shown, contra-diffusion method in conjunction with vapor phase modification of the support offers more degrees of freedom in directing the formation of ZIF-8 membranes. It is worth mentioning that growing MOFs inside the pores of the support can be very attractive for molecular separations and such membranes have been shown to outperform mixed matrix membranes (MMM) for organic solvent nanofiltration (OSN) applications, as


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demonstrated by Livingston et al.16, 36-37 Although the resulting ZIF-8 membrane (Figure 5) was too brittle to allow permeation experiments, this work demonstrates a new methodology for in situ growth of ZIFs predominantly inside the supports, which needs to be improved further for practical applicability.

Figure 5. SEM images of cross-section of ZIF-8 membrane synthesized by contra-diffusion (at the reaction time of 2 h) after modification of the BPPO via immersion of the polymer in 60% EDA aqueous solution at room temperature for 3 h. To further evaluate the quality of the obtained ZIF-8 membranes, single-component gas permeation experiments were conducted, and the results are summarized in Figure 6 and Table S1 (Supporting Information). In comparison, single gas permeation of the untreated BPPO and its vapor phase modified counterpart were tested. Due to their large pores (pore size of 25.5 and 15 for untreated and treated supports, respectively), none of these membranes showed any obvious gas selectivity. However, upon modification, H2 permeance was decreased by more than half when compared to the untreated support. This is attributed to the reduced pore size of the support induced by partial crosslinking effect of the EDA modification, which increases the support dimensional stability and surface tightness. This also lessens the flexibility of the


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polymeric substrate, which is beneficial for reducing ZIF layer cracking.24-25 Membranes started to display molecular sieve performance, favoring the smaller molecules, with a moderate H2 permselectivity after 90 min of the crystal growth (H2/C3H8 ideal selectivity of 32.9 compared to Knudsen diffusion selectivity of 4.7). However, no clear C3H6/C3H8 ideal separation selectivity was observed at this point, indicating the presence of grain boundary micro-defects. As crystallization time is extended, more pronounced molecular sieving behavior with an obvious increase in the propylene/ propane ideal selectivity can be observed which reaches as high as 16 after 120 min reaction time. It is worth mentioning here that except for a few membranes,11, 29, 33, 38-39

majority of ZIF-8 membranes reported so far have not shown any decent C3H6/ C3H8

selectivity.40 Figure 7 depicts C3H6/ C3H8 separation performance of ZIF-8 membranes developed in this study in comparison to those reported in the literatures. As can be seen, our membranes not only overtake polymeric and ZIF-8 mixed-matrix membranes in terms of C3H6/C3H8 selectivity and C3H6 permeance they are also amongst the best ZIF-8 membranes previously reported (Table S2, Supporting Information). For instance, the high quality ZIF-8 membranes made in water-octanol system by interfacial microfluidic membrane processing (IMMP) method could achieve H2/C3H8 ideal selectivity of more than 600 and the permeance of H2 around 50 ×10-8 mol m-2 s-1 Pa-1.29 While in the current study, the H2/C3H8 selectivity and H2 permeance are considerably enhanced, with H2/C3H8 selectivity of 833.3 and H2 permeance of 75×10-8 mol m-2 s-1 Pa-1. The enhancement in permeance in this study is in agreement with the reduction in thickness of the ZIF-8 layer (~2 versus ~9 µm in 29) and also the highly porous and asymmetric structure of the support, which minimizes the overall hydraulic resistance of the permeate flow through the membrane structure; whereas the enhanced selectivity is mainly due to the improved membrane quality, such as the well-structured grain boundary and absence of


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pinholes or defects. Highly improved grain boundary structure, on the other hand, can be related to the confinement effects of the crystals within the porous support as the confinement can increase the compactness of the grain boundary structure.41 Another possible reason for the high quality grain boundary structure can be the well-intergrown ZIF-8 crystals with no preferred orientation as a result of the aqueous synthesis. It was found that water, being less acidic, in comparison with organic solvents can more easily deprotonate the organic ligand on the growing surface, leading to growth occurring in more directions, resulting in a better crystals intergrowth and formation of denser ZIF membranes.38, 42 Attributed to the formation of high heterogeneous nucleation density in the vicinity of the support surface, the EDA vapor modification is helpful for the controlled synthesis of thin, defect-free and reproducible ZIF-8 membranes. In summary, the enhanced gas permeation properties strongly suggest that the chemical vapor modificationcontra diffusion method provides a new route for preparing high quality ZIF-8 membranes having superior grain boundary structure as compared to those prepared by other methods.

Figure 6. Single gas permeances (a) and ideal selectivities (b) as a function of kinetic diameter of gas molecules of the ZIF-8 membranes grown for 120 min and activated at 120 °C (ZIF8@BPPO-EDA-120-120) and at 150 °C (ZIF8@BPPO-EDA-120-150).


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Figure 7. Comparison of C3H6 permeability and C3H6/C3H8 selectivity of the membranes in the present work with previously reported membranes. Closed and open symbols indicate separation data obtained from single and binary gas permeation analysis, respectively. Hexagon: inorganic supported ZIF-8 membranes;10 pentagon: ZIF-8 mixed matrix membranes;15 triangle: polymer membranes;43 circle: carbon membranes;44 star: polymer supported ZIF-8 membranes in this study. Finally, we investigated the effects of activation temperature on the morphology and performance of ZIF-8 membranes. Membranes (grown for 120 min) were further exposed to 150 and 200 ºC for 2 h under oxidative conditions (air). For the membranes activated at 150 ºC, a compact well-intergrown ZIF-8 grains of rhombic dodecahedral shape with no defects (i.e. pinholes or cracks) in the entire membrane surface can be observed (Figure S7 a,b, and c). A very intimate contact between ZIF-8 and the support is also observed in the cross section view of the membranes as the interface between ZIF-8 and the support is hardly distinguishable (Figure S7 c). Figure 8 shows the room-temperature C3H6/C3H8 permeation properties of ZIF-8 membranes activated at different temperatures. As shown in the figure, by increasing the activation temperature from 120 to 150 ºC the C3H6/C3H8 ideal selectivity is significantly


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increased, with minimal effect on the permeance. This unique behavior indicates that the ZIF8@BPPO-EDA membrane becomes even denser with probably more compact grain boundary structure when activated at a higher temperature. This positive result is likely attributed to the fact that the BPPO and BPPO-EDA form crosslinking structure by heating (Scheme S1),45 which can further increase the stability and tightness of the support. This was further supported by the 20 (±4) % reduction observed in the hydrogen permeability of the BPPO-EDA substrate upon heating at 150 °C for 2 h. Since a fraction of ZIF-8 is formed within the support porous structure, it may subsequently enhance the interfacial interaction between the ZIF-8 and the support and also increase the compactness of the grain boundary structure, thereby minimizing the non-selective intercrystalline diffusion and leading to improved separation performance. Thermally induced cross-linking reaction was verified by FTIR result (Figure S8, Supporting Information), where bands attributed to the benzyl bromide (CH2Br) and amine groups for BPPO and BPPO-EDA, respectively, disappeared upon their thermal treatment at 150 ºC. Another possible reason for the observed improvement in the membrane selectivity activated at a higher temperature can be the complete removal of residual solvent molecules from the ZIF-8 layer.46 However, further analysis is required to fully understand the effect of annealing temperature on the grain boundary and subsequent gas performance. Further increasing the activation temperature up to 200 ºC results in a significant increase in the propylene permeance (more than one order of magnitude) with a drop in C3H6/C3H8 ideal selectivity from 27.8 to 4.5, indicating the grain boundary structure of the membranes was compromised. While the XRD patterns in Figure S9 indicate that the ZIF-8-BPPO-EDA samples did not undergo obvious structural alterations in the studied temperature range compared to the simulated ZIF-8 pattern, the FTIR shows a drop in the intensity of the Zn-N peak for the membrane activated at 200 °C. The


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activation process at the elevated temperature caused some degradation of the ZIF-8 layer (Figure S7 d, e, f). However, as can be seen from the cross sectional view (Figure S7 f), the degradation was apparently restricted to the membrane surface, resulting in still reasonable C3H6/C3H8 ideal selectivity of 4.5 (compared to the Knudsen propylene/propane selectivity of ∼1.02). Similar degradation behavior of ZIF-8 membranes upon activation process at high temperatures were recently reported and was correlated to the corrosion action of water or methanol on ZIF-8 grains.41,

47

These results indicate that the activation temperature plays a

critical role in determining the gas permeation properties of the ZIF-8 membranes. However, an elaborate choice of activation conditions (temperature, duration, and environment) is required in order to achieve ZIF-8 membranes with the best performance.

Figure 8. Room-temperature propylene/propane permeation properties of ZIF-8 membranes grown for 120 min (ZIF-8@BPPO-EDA-120) as a function of activation temperatures. It should be noted that the propylene/propane selectivity obtained in this study is the best ever obtained for ZIF-8 membrane on polymeric supports, but it is still lower than those obtained with inorganic-supported ones. For example, alumina-supported ZIF-8 membrane prepared by an in situ counter-diffusion method11 exhibited both higher C3H6/C3H8 selectivity (∼50) and propylene permeability (∼200 ×10-10 mol m-2 s-1 Pa-1) than the best membrane obtained in this work with the C3H6/C3H8 selectivity and propylene permeability of 27.8 and 75×10-10 mol m-2 s-1 Pa-1,


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respectively. However, since the contra diffusion method is dependent on the surface properties and porous structure of support, the performance of the membranes could be further improved by investigating the effect of the modification reaction condition. Optimizing activation conditions could also further improve the performance of the ZIF-8 membranes. In addition, it is worth mentioning that the synthesis procedure developed here offers a number of advantages over those reported in the literature. First of all, almost all of previously reported ZIF membranes were made by using organic solvents (e.g. methanol, dimethyl formamide, octanol) and/or alkaline additives (e.g. sodium formate, ammonia) under non-ambient conditions (i.e. high temperature, high pressure).11, 27, 48-49 In many other cases the use of seed crystals and a long reaction time were unavoidable.26, 38, 50-51 The high quality ZIF-8 membranes in this study were made in aqueous solution under ambient conditions (room temperature, atmospheric pressure) in a relatively short period of time (less than 2 h), without any additives or seed crystals. The synthesis process requires significantly smaller amounts of metal salt and organic ligand reagents. For example, ∼ 41-90% savings in the usage of the reagents (per cm2 of permeable area) could be achieved compared to ZIF-8 membranes fabricated by the microfluidic experimental approach.35

4. CONCLUSIONS In summary, we reported a novel strategy, contra-diffusion based synthesis in conjunction with vapor modification, for room temperature synthesis of high-quality ZIF-8 membranes on an asymmetric polymeric substrate in aqueous solution. The ZIF-8 membranes have shown excellent gas permeation properties (e.g. propylene/propane ideal selectivity of 16 with propylene permeance of 150×10-10 mol m-2 s-1 Pa-1), intensely indicating impressively enhanced


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membrane microstructure (in particular enhanced grain boundary structure). More importantly, by increasing the activation temperature from 120 to 150 ºC, the propylene/propane selectivity was further increased (almost two-fold), without compromising the high permeance of propylene, indicating the important role of thermal activation conditions (in particular activation temperature) in microstructures of ZIF-8 membranes. With efficient synthesis conditions, the strategy developed here provides an effective and environmentally friendly route for preparing high-quality ZIF membranes on the surface of polymeric support.

ASSOCIATED CONTENT Supporting Information Measurements of the support pore size, photo of a home-made contra-diffusion cell, schematic diagram of gas permeation set-up, TGA thermograms of BPPO and BPPO-EDA, SEM images of ZIF-8 membranes grown by conventional CD method using untreated BPPO, SEM images of ZIF-8@BPPO-EDA at different magnifications, EDS line scan across ZIF-8@BPPO-EDA, SEM images of ZIF-8@BPPO membranes activated at different temperatures, FTIR spectra of the BPPO, BPPO-EDA support after being heated at 150 °C, FTIR spectra and XRD patterns of ZIF8@BPPO-EDA membranes as a function of activation temperature (°C), Heat-induced crosslinking reaction of BPPO substrate, Single gas permeances and ideal selectivities for the ZIF8@BPPO-EDA, Comparison of gas permeation properties of the ZIF-8@BPPO-EDA composite membrane in this work with other ZIF-8 membranes in the literature. . This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]; Tel: +61 39 9053449 Present Addresses ‡

Centre for Advanced Separations Engineering, Department of Chemical Engineering,

University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom. Funding Sources This work was supported by the Australian Research Council (Project No: DP140101591, and FT100100192)

ACKNOWLEDGMENT The authors acknowledge use of the facilities and the assistance of Kathryn Waldron at the Monash Center for Electron Microscopy. Ezzatollah Shamsaei thanks Monash University for MGS and FEIPRS scholarships.

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Table of Contents and Synopsis

This study provides a simple, effective and environmental friendly route for the selective fabrication of ZIF-8 membrane on a flexible polymer substrate for enhanced gas separation. The ZIF-8 membranes exhibited high propylene over propane selectivity.


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Aqueous Phase Synthesis of ZIF-8 Membrane with ... - ACS Publications

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