Solvent-free Crystallization of Zeolitic Imidazolate Framework

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Solvent-free Crystallization of Zeolitic Imidazolate Framework Membrane via Layer-by-Layer Deposition Pingping Yang, Zhan Li, Zhuangzhuang Gao, Mingqiu Song, Jinya Zhou, Qianrong Fang, Ming Xue, and Shilun Qiu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05764 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Sustainable Chemistry & Engineering

Solvent-free

Crystallization

of

Zeolitic

Imidazolate Framework Membrane via Layer-byLayer Deposition Pingping Yang, ‡ Zhan Li, ‡ Zhuangzhuang Gao, Mingqiu Song, Jinya Zhou, Qianrong Fang, Ming Xue,* Shilun Qiu †State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, 2699, Qianjin Street, Changchun, 130012, P. R. China. Email: [email protected] KEYWORDS: Zeolitic imidazolate framework, membrane, solvent-free, layer-by-layer, gas separation

ABSTRACT: Searching for the sustainable synthesis methods of MOF membranes has received huge attentions in both academia and industry. In this contribution, the compact ZIF-8 membrane with excellent gas separation performance was successfully prepared via an environment-friendly and simple method, in which the artful layer-by-layer deposition approach was used to achieve the solvent-free crystallization of well-intergrown and defect-free ZIF-8 membrane. The solvent-free process is a simple and generalized method

ACS Paragon Plus Environment

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for construction of MOF membranes, which can avoid using solvents and high autogenous pressure, and brings great benefits for the energy-saving, emissionreduction, safety as well as scalable manufacturing. In addition, the solvent-free crystallization of MOF membranes exhibits more unprecedented advantages to fill the common intercrystalline cracks, pinhole defects and grain boundary defects on the membrane. The resulting thin ZIF-8 membrane exhibited both competitive H2 permeability of 6.7×10-7 mol m-2 s-1 Pa-1 and high H2/CO2 selectivity of 12.3, simultaneously, which is one of the best ZIF-8 membranes ever reported.

INTRODUCTION Membrane separation has attracted more and more attention owing to its unprecedented features of lower cost, higher energy efficiency and easy operation compared to traditional separation technology like condensation, distillation and PSA.1-3 Metal–organic framework (MOF), which are consisted of metal ions or clusters bonding with organic linkers, have been extensively investigated in recent years due to their permanent porosity, diverse structure, tunable pore size and functionalization.4-7 Particularly, Zeolitic imidazolate framework (ZIF), a sub-class of MOF, which combine the unparalleled properties of both MOF and zeolite, for instance, preferential adsorption of gases as well as high chemical and thermal stability.8 These traits give ZIF materials the ability to


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manufacture advanced membrane materials exhibited excellent gas separation performance. Both permeability and selectivity are long-sought-after parameters to assess the performance of a membrane in mixed gas separation. However, nonnegligible drawbacks of commercially available polymeric membranes lie in trade-off between gas permeability and selectivity, which is known as Robeson’s upper bound.9 Most polymer membranes possess only high selectivity but low permeability, which signifies greater membrane areas needed for application. On the other hand, polymer membranes show excellent permeability but poor selectivity performance for their wide pore aperture range. As emerging microporous crystalline materials with well-defined pore structure, tunable pore size and functionalization, MOF membranes present a new promise and opportunity to surpass this limitation. 10-12 Generally, most of MOF membranes were constructed under solvothermal conditions in the presence of abundant solvents such as H2O, DMF, methanol, or other toxic organic solvents in sealed autoclaves under high pressure. Most of spaces are occupied by solvents in autoclaves, which largely reduces the space−time yields of MOFs.13-15 Moreover, polluting wastes and relatively high pressure due to the addition of solvents in the synthesis route naturally results in environmental and safety problems.16 Therefore, development of sustainable and easy scalability synthetic methods for MOF membranes are strongly desirable.


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Herein, a facile strategy was successfully developed to grow continuous MOF membranes on porous support under solvent-free and low-pressure conditions. To illustrate the feasibility of the solvent-free transformation, ZIF-8, a prototypical of MOF materials with commercial prospects, was employed to fabricate compact and well-intergrown membranes.8,17 ZIF-8 has a sodalite (SOD) zeolite type topology with extraordinary thermal and hydrothermal stability, and a relative narrow window of about 0.34 nm, which is identified as a suitable material for effective separation of H2 from a mixture of H2 and CO2 (kinetic diameters of 0.29 and 0.33 nm, respectively). Nowadays steam-methanereforming still takes dominant in large-scale hydrogen production, in which water-gas shift (WGS) reaction is coupled. H2 and CO2 mixtures yielded from WGS reaction need effective separation technology to obtain high-purity hydrogen while sequestrating carbon dioxide. ZIF-8 membranes were expected to purify H2 from the mixtures of H2 and CO2 with high efficiency. H2 with smaller diameter displays the rapid inter-cage hopping across the windows of ZIF-8, while CO2 with relatively larger size predominantly hops in intra-cage. Thus, H2 exhibits higher permeation rate than CO2 through ZIF-8 membrane. In addition, Hmim linkers in the ZIF-8 structure possess strong coordination sites for CO2 gas, so the permeability of CO2 further decreases and the H2/CO2 separation performance enhances greatly.18 To date, various protocols have been developed to synthesis continuous ZIF-8 membranes for gas separation application. Caro et al. firstly synthesized ZIF-8 membrane


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by microwave-assisted solvothermal approach.19 Eddaoudi et al. prepared ZIF-8 membrane through liquid phase epitaxy approach for single and mixed gas permeance research.20 An in-situ counter-diffusion method was developed by Jeong et al. in which porous support was firstly immersed in zinc source solution followed by solvothermal transformation in a Hmim solution.21 An electrospinning technique was also applied to grow continuous ZIF-8 membrane by our group.22 Yang et al. adopted an ionothermal synthesis strategy to construct ZIF-8 membrane.23 Li and Zhang et al. explored a gel-vapor deposition way to synthesis ZIF-8 membrane.24 Very recently, Tsapatsis et al. successfully synthesized a ZIF-8 nanocomposite membrane via an all-vapor-phase processing route. 25

In the present work, high-quality and defect-free ZIF-8 membrane was successfully fabricated on porous Al2O3 supports by means of a solvent-free crystallization method based on a layer-by-layer deposition approach followed by in-situ heat treatment (Scheme 1). The resulting thin ZIF-8 membranes exhibited both high permeability and high selectivity for the purification of H2 from H2/CO2 gas mixtures. EXPERIMENTAL SECTION Materials. All the chemicals in this work were commercially available and used without further purification. 2-methylimidazole (Hmim, 98%, Sigma-Aldrich), ZnO powder (99.9%,


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Sigma-Aldrich), ZnO dispersing solution (≤ 50nm, 50wt.%, in water, Aladdin), alumina powder (200 nm, 99.99%, Aladdin). Preparation of ZIF-8 powder. A mixture of ZnO powder (3.0 mmol, 244 mg) and Hmim (6.0 mmol, 492 mg) was hand-grinded slowly to obtain a uniform mixture and then transferred in a Teflon lined autoclave. The white powder was obtained under 180°C for 10h. Preparation of α-Al2O3 supports. Porous α-alumina disks with thickness of 2 mm and diameter of 25 mm were used as substrates, which were made by pressing high quality alumina powder in a stainless-steel mold under 12 MPa. After sintering at 800°C for 12h, the total porosity of alumina disks was around 47%. One side of the disks was polished by sandpapers (grit 1200). Preparation of ZnO and Hmim coated α-Al2O3 supports. The high concentration ZnO nanoparticles dispersing solution was diluted into 4 wt% homogeneous dispersions as dip-coating precursors. The withdrawn speed of Al2O3 support was 50 mm/min for the deposition, and then left in an oven at 40°C for 3h to dry. The procedures should be repeated for 3 times to get a uniform ZnO layer containing about 20mg ZnO. Sequently, 50mg Hmim uniformly crystallized on the surface of the ZnO-coated porous Al2O3 substrate by casting a 1mL Hmim aqueous solution and dried naturally.


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Preparation of ZIF-8 membrane under solvent-free condition. The ZnO and Hmim coated porous Al2O3 substrates were sealed in Teflon containers to synthesis the ZIF-8 membranes by solvent-free method at 180°C for 10h. Then, the containers were cooled to room temperature naturally and the prepared ZIF-8 membranes were dried at 80°C for 3h before further analyzed. Characterization. Powder X-ray diffraction (PXRD) analyses were calculated on a PANalytical B.V. Empyrean powder diffractometer using a Cu Kα source (λ = 1.5418 Å). Microscopic images were characterized by scanning electron microscopy (SEM, JEOL JSM6510A) at an acceleration voltage of 15 kV. Elemental mappings were determined using Energy-dispersive X-ray spectroscopy (EDS) on a JEOL JSM-6510A microscopy. The macroscopic images of ZnO layer and Hmim crystalline layer were performed on an Optical microscope (Leica DM 2500P). The sorption isotherm for N2 was measured by using a Quantachrome Autosorb-IQ2 analyzer at 77 K in a liquid nitrogen bath. CO2 adsorption isotherms were characterized at 273 and 298 K in a low pressure range of 1 bar. Esoteric heat of adsorption (Qst) for the ZIF samples was calculated from the CO2 sorption isotherms tested at 273 and 298 K according to the Clausius-Clapeyron equation using the ASiQwin software installed in Quantachrome Autosorb-IQ2 instruments. Clausius-Clapeyron equation is in the form:


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𝑝2

ln(𝑝1) =

𝑄𝑠𝑡 1 𝑅 (𝑇 1

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1

― 𝑇2)

(

1

)

where Qst is the isosteric heats of adsorption, Ti represents a temperature at which an isotherm i is measured, pi represents a pressure at which a specific equilibrium adsorption amount reached at Ti, R is the universal gas constant (8.314 JK-1 mol-1). Gas Permeation. The schematic diagram of the experimental gas-separation device is presented in Figure S1. For the permeation measurements of the single gas and their binary mixtures, the fabricated ZIF-8 membrane was sealed in a permeation module using O-rings at room temperature. Pressures were hold at 1 bar at both feed side and permeate side. The permeate side was swept by gas Ar to maintain the concentration of the permeating gas at a low level so as to provide a driving force to permeate. For the single gas permeation measurement, a volumetric flow rate of 50 ml min-1 gas controlled by flow meter was kept on the feed side. In the binary mixed gas permeation test, a total flow rate was recorded at 100 ml min-1 (each gas of 50 ml min-1). The concentrations of both single gases and mixed gases on the permeate side were measured by a calibrated gas chromatograph (SHIMADZU GC-2014C). The membrane permeance, Pi ( mol m-2 s-1 Pa-1), is defined as: 𝑁𝑖

𝑃𝑖 = ∆𝑝𝑖𝐴 (

2


) 8

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where Ni is the permeation rate of component i (mol s-1), A is the tested membrane area tested (m2) and pi is the trans-membrane pressure of i (Pa). The ideal separation factor is calculated by the quotient of permeance Pi and Pj: 𝑝𝑖

𝑎𝑖,𝑗 = 𝑝𝑗 (

3

)

The separation factor ai,j of a binary mixed gas is calculated as the quotient of the molar ratios of the components (i, j) in the permeate side (X), divided by the quotient of the molar ratios of the components (i, j) in the feed side (Y): 𝑋𝑖

𝑎𝑖,𝑗 = 𝑌

𝑋𝑗

𝑖 𝑌 𝑗

(

4

)

RESULTS AND DISCUSSION Porosity of ZIF-8 crystals by solvent-free method. PXRD diffraction peaks of the solvent-free prepared ZIF-8 powder was in good agreement with the simulated pattern of ZIF-8 (Figure S2), indicating that pure ZIF-8 phase was capable of being formed in the absence of any solvents. In the reaction process, the acid-base neutralization reaction between amphoteric ZnO and acidic Hmim ligand was energetically favorable due to the


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Zn–O bonds replaced by the Zn–N bonds, and a small amount of water formed as the only byproduct. Furthermore, the nitrogen sorption isotherm exhibited that the BrunauerEmmett-Teller (BET) surface area of the sample was 1878 m2 g-1. It is worth of mention it exceeded the most reported values of products synthesized by solvent-based strategy2632

and highly closed to the theoretical calculation value of 1947 m2g-1 (Figure S3).8 The

result implied that the solvent-free resulting ZIF-8 crystals had higher effective porosity and fewer defects. All these features will be propitious to achieve both high permeability and high selectivity for the separation application.

Scheme 1. Schematic diagram of fabrication of ZIF-8 membrane.

Layer-by-layer deposition of ZnO and Hmim. It is well known the layer-by-layer method was inherently more capable of growing homogeneous, compact and thin MOF membranes.33-35 Firstly, the smooth and homogeneous ZnO nanoparticles layer was tightly coated on the porous Al2O3 support (Figures 1b and 2a). Deposition of uniform ZnO layers also can enhance the membrane-support interaction, which avoided a problem of growing MOF crystals directly on the rough surface of support with many voids (Figure


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1a). PXRD patterns exhibited the typical peak positions of the ZnO at 31.7o, 34.4o and 36.2o (Figure S4). Furthermore, EDS map indicated there was a sharp transition between Al2O3 support (Al signal) and ZnO layer (Zn signal) with an average thickness of 3 μm (Figure 2b). Then, a polycrystalline of Hmim layered on the surface of the ZnO layer using a process similar to recrystallization (Figure 1c). Leica Optic micrographs demonstrated that both ZnO layer and Hmim layer are continuous and uniform (Figure S5). This layer-bylayer deposition approach plays a supporting role in the solvent-free crystallization.


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Figure 1. The top-section SEM images of (a) bare porous Al2O3 support, (b) ZnO layer on the support, (c) polycrystalline of Hmim layer on the ZnO layer, (d, e) the as-synthesized continuous ZIF-8 membrane and (f) a cross-section SEM image of the ZIF-8 membrane. ZIF-8 membrane by solvent-free crystallization. As shown in Figure 1d, a continuous and extremely compact ZIF-8 membrane was fabricated under solvent-free and low pressure. The XRD pattern of the as-prepared ZIF-8 membrane showed a pure ZIF-8 phase,


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which was consistent with the standard pattern of ZIF-8 (Figure S4). In addition, the absence of ZnO peaks indicated the high transformation rate in this reaction. Different magnification SEM images showed that the ZIF-8 membrane was consisted of densely packed micro-crystallites with no visible pinholes and cracks (Figures 1d, e). As shown in Figure 1f, the precursory layers of ZnO and Hmim were completely transformed into an extraordinarily dense ZIF-8 polycrystalline layer, which was firmly bonded with the support. The mechanism on solvent-free crystallization of ZIF-8 membrane is that Hmim ligand melted and progressed deeper into the interlayer of ZnO layer, which promoted the full contact of reactants and thoroughly etched ZnO nanoparticles. Then, a small amount of water continued to form as the intermediate liquid phase and retained at the reaction interface, which rendered precursors sufficiently mobile to facilitate crystals crystallization and protonate the organic linker even in the absence of solvent. The morphological of ZIF-8 crystals started to evolve and the grains grew larger, consequently the intercrystalline gaps gradually shrunk and defect-free ZIF-8 membranes were such successfully prepared without adequate solvents and high pressure normally employed in the solvent-based process.


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Figure 2. (a, b) Cross-view SEM image and corresponding EDS mapping of the ZnO layer; (c, d) cross-view SEM image and corresponding EDS mapping of the ZIF-8 membrane. Green represents ZnO (Zn), and red represents Al2O3 support (Al). It is obvious that the thickness of ZIF-8 membrane increased relative to ZnO layer after solvent-free transformation (Figure 2). This volume expansion phenomenon was caused by the physical density of porous ZIF-8 is less than that of raw ZnO. The volume expansion (VE) during the transformation of ZnO to ZIF-8 crystals can be properly quantified as:

VE =

𝑀𝑍𝐼𝐹 ― 8 𝑀𝑍𝑛𝑂

(5)

𝜌𝑍𝐼𝐹 ― 8 / 𝜌𝑍𝑛𝑂


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where M represents molecular weight, ρ represents physical density. The physical densities of ZnO and ZIF-8 are 5.61 g cm-3 and 0.95 g cm-3, respectively.36 According to theoretical calculation, the volume change is equal to 16.5 times of ZnO layer. The thickness variation is estimated to be approximate 2.5 times of ZnO based on an assumption of isotropic volume expansion. Actually, the membrane thickness was about 7 μm, which was 2.3 times of raw ZnO layer. The smaller practical expansion due to the ZnO layer had some empty spaces between ZnO nanoparticles, which were reserved for ZIF-8 growth in the in-plane direction. During the volume expansion process, the pinhole defects, grain boundary defects and intercrystalline cracks were greatly healed, consequently, the well-intergrown and defect-free ZIF-8 membranes were successfully fabricated.37-38 Gas permeation and separation. Gas permeation performances for single gases of H2, CO2, N2, and CH4 as well as equal molar binary mixtures of H2/CO2, H2/N2 and H2/CH4 were systematically tested to examine gas separation properties of the supported ZIF-8 membrane. The permeation tests of these gases with different diameters were carried out (H2: 2.9 Å; CO2: 3.3 Å; N2: 3.6 Å; CH4: 3.8 Å). The single gas permeability decreased with increasing kinetic diameters, H2 with the smallest kinetic diameter had highest permeance of 8.7×10-7 mol m-2 s-1 Pa-1 among the tested gases, the permeation behaviors of N2 and CH4 mainly followed the molecular sieving mechanism, which exhibited ideal separation factors of H2/N2 = 7.5 and H2/CH4 = 8.3, respectively, both values surpassed their corresponding Knudsen constant of 3.7 and 2.8 (Figure 3a). N2 and CH4 with larger kinetic


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diameter could also pass slowly through the membrane, on account of the flexibility of the ZIF-8 framework.39 It was worth mentioning that CO2 had the lower permeance which was similar with other reported ZIF-8 membranes,40 due to the strong interaction between CO2 and ZIF-8 framework. The CO2 adsorption performance of the solvent-free method resulting ZIF-8 powders were studied and displayed CO2 capacities of 53.0 and 30.3 mg g-1 at 273 K and 298K, respectively, which were comparable with the best reported CO2 uptake values of ZIF-8 (Figure 4a).41-43 To further investigate the affinity of the ZIF-8 for CO2, the Qst of CO2 was calculated from the CO2 sorption data at 273K and 298 K under ambient pressure. The Qst value slightly decreased at low CO2 loading and then leveled off, the initial Qst value for the ZIF-8 sample was 18.87 kJ mol-1 (Figure 4b), which was larger than the best initial Qst values of ZIF-8 (13–18 kJ mol-1).44-45 The results of the CO2 adsorption test demonstrated that the structure of ZIF-8 synthesized by solventfree method had a special adsorption for CO2.


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Figure 3. (a) Single gas (H2, CO2, N2, CH4) permeances through the prepared ZIF-8 membrane at normal temperature and pressure vs molecular kinetic diameters. (b) Permeances of H2, CO2 and corresponding H2/CO2 separation factor over time.


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Figure 4. (a) CO2 sorption capacities of the ZIF-8 powders at 273 K (black square) and 298 K (red circular). (b) Qst of the sample calculated from the CO2 sorption data at 273 and 298K using the Clausius–Clapeyron Equation under ambient pressure. Furthermore, mixtures gas separation investigation of H2 and other gases with a volume ratio of 1:1 were studied on the ZIF-8 membranes. The calculated gas permeation fluxes and separation factors of the mixtures H2/CO2, H2/N2, and H2/CH4 surely exceeded their


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corresponding Knudsen separation factors (Table S1), which revealed that the ZIF-8 membrane synthesized by this way is appropriate for hydrogen purification. Particularly, the separation factor for H2/CO2 gas pair stabilized to 12.3 (Figure 3b). In addition, gas separation properties of three ZIF-8 membranes obtained from different synthesis batches were also tested to investigate the reproducibility of ZIF-8 membranes, and all H2 permeances and separation factor of H2/CO2 were similar (Table S2), which indicated the reliable manufacturing of this solvent-free fabrication method. To further study the recyclability of the ZIF-8 membrane, the separation performance of H2/CO2 mixture was recorded for 180 hours in total, including three cycle assessments. The permeance and selectivity remained stable, showing excellent recyclability of ZIF-8 membrane (Figure S6). In practical H2/CO2 separation, the pressure is usually high, thus the effect of feed pressure on separation performance was evaluated (Figure S7). Both H2/CO2 selectivity and H2 permeance slightly decreased under elevated feed pressure, while CO2 permeance showed subtle increase. The slightly decreased selectivity but increased CO2 permeance can be explained by gate opening phenomenon of ZIF-8 structure under high pressure, since enlarged pore structure reduced size-selective effect of ZIF-8 while favored faster diffusion of CO2. Regarding H2 permeance, increase in flux did not respond linearly to the increase in feed pressure due to more H2 molecules were adsorbed into pores of ZIF-8 under elevated pressure, thus permeance of H2 which is normalized by feed pressure decreased. Although subtle loss in separation efficiency occurred under high pressure,


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ZIF-8 membrane still showed high H2/CO2 selectivity of 10.2 and considerable H2 permeance of 5.6 ×10-7 mol m-2 s-1 Pa-1 at 5 bars. It is worth noting that the plot of the H2/CO2 separation factor and H2 permeance far beyond the Robeson upper bound which meant that the ZIF-8 membrane has both high gas permeability and selectivity (Figure 5). And the ZIF-8 membrane made by solvent-free crystallization revealed better performance for H2/CO2 mixture separation in comparison with most ZIF-8 membranes made by solvent-based method, underlining the practicability of such alternativeo route (Table S3). 19-20,40,46-56

Figure 5. H2/CO2 separation factor vs H2 permeability for the ZIF-8 membrane prepared by solvent-free method compared with ZIF-8 membranes made by popular solvent-based method. The upper bound lines for polymer membranes are drawn according to ref 9.


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CONCLUSIONS One of the most important challenges of MOF membranes is reliable and scalable preparation. In this contribution, we demonstrated the fabrication of ZIF-8 membrane by means of solvent-free method, which avoided using solvents and high-pressure synthesis conditions, and brought great benefits for MOF membranes manufacturing. This strategy is inherently more capable of growing well-intergrown, denser and defect-free ZIF-8 membranes with high crystallinity and effective porosity, resulting in high permeability and selectivity simultaneously. Furthermore, the H2/CO2 gas mixture separations of these ZIF-8 membranes were investigated in three cycle assessments about two months, exhibiting the good regeneration performance. The promising result of this study inspired the extension of this work, which offer MOF membranes great potential for gas separation applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. PXRD, N2 sorption isotherm of ZIF-8 powders; PXRD and SEM images of membranes; Leica Optic microscope analysis; Gas separation analysis. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (M.X). Author Contributions ‡ P. Y. and Z. L. contributed equally to this work. Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21571076, 21390394, 21571079), and “111” project (B07016). REFERENCES

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The well-intergrown and defect-free ZIF membrane with excellent gas separation performance was fabricated via a sustainable and environment-friendly method.


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The well-intergrown and defect-free ZIF membrane with excellent gas separation performance was fabricated via a sustainable and environment-friendly method. 84x47mm (300 x 300 DPI)



Scheme 1. Schematic diagram of fabrication of ZIF-8 membrane. 119x29mm (300 x 300 DPI)


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Figure 1. The top-section SEM images of (a) bare porous Al2O3 support, (b) ZnO layer on the support, (c) polycrystalline of Hmim layer on the ZnO layer, (d, e) the as-synthesized continuous ZIF-8 membrane and (f) a cross-section SEM image of the ZIF-8 membrane. 80x91mm (300 x 300 DPI)



Figure 2. (a, b) Cross-view SEM image and corresponding EDS mapping of the ZnO layer; (c, d) cross-view SEM image and corresponding EDS mapping of the ZIF-8 membrane. Green represents ZnO (Zn), and red represents Al2O3 support (Al).


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Figure 3. (a) Single gas (H2, CO2, N2, CH4) permeances through the prepared ZIF-8 membrane at normal temperature and pressure vs molecular kinetic diameters. (b) Permeances of H2, CO2 and corresponding H2/CO2 separation factor over time. 39x57mm (300 x 300 DPI)



Figure 4. (a) CO2 sorption capacities of the ZIF-8 powders at 273 K (black square) and 298 K (red circular). (b) Qst of the sample calculated from the CO2 sorption data at 273 and 298K using the Clausius–Clapeyron Equation under ambient pressure. 40x60mm (300 x 300 DPI)


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Figure 5. H2/CO2 separation factor vs H2 permeability for the ZIF-8 membrane prepared by solvent-free method compared with ZIF-8 membranes made by popular solvent-based method. The upper bound lines for polymer membranes are drawn according to ref 9. 39x29mm (300 x 300 DPI)


Solvent-free Crystallization of Zeolitic Imidazolate Framework

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