MOF-Based Membranes for Gas Separations Qian, Qihui; Asinger, Patrick A; Lee, Moon Joo ...
Chemical reviews,
08/2020, Letnik:
120, Številka:
16
Journal Article
Recenzirano
Metal–organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features ...have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.
Membrane‐based separations offer energy‐efficient solutions for various applications, but commercial polymer membranes show limited performance and stability. Mixed‐matrix membranes (MMMs), ...incorporating nanoporous inorganic materials in polymer matrices, have been of great interest to circumvent these polymer‐specific issues. However, reaching the percolation threshold is crucial to leverage high‐performing inorganic phases fully, yet the traditional sphere‐like nanofillers require high loadings that easily result in agglomerations and non‐selective defects. Here, a branch‐shaped zeolitic imidazole framework‐8 (ZIF‐8) nanoparticle is synthesized where its unique morphology automatically interconnects, readily forming percolated networks within the polymer matrix at loadings as low as 20 wt.%. Because of the high surface‐area‐to‐volume ratios of branched ZIF‐8 (BZ), strong polymer–particle interactions suppress polymer chain dynamics and the rotation of the ZIF‐8 ligand. This interphase confinement results in enhanced membrane stability and a smaller diffusion cut‐off than traditional ZIF‐8. With pre‐connected diffusion pathways and confined ZIF pores, BZ MMMs significantly outperformed MMMs with sphere‐like ZIF‐8 for H2‐based separations. Overall, the findings provide a novel approach to enhance filler effects in MMMs even at low loadings without any alignment, which can enable the development of advanced membranes in fields where percolation is desired, including separations, sensors, conductors, and batteries.
Branch‐shaped ZIF‐8 (BZ) is synthesized for the first time. Compared to the typical sphere‐like ZIF‐8 nanoparticles, BZ readily forms percolated networks when incorporated into mixed‐matrix membranes. Moreover, strong BZ–polymer interactions confine metal–organic framework ligand rotation and reduce polymer chain mobility resulting in more selective hydrogen‐based separations and exceptional plasticization resistance.
An assortment of hydrothermal reactions of chromic(III) nitrate and 2-aminoterephthalic acid was systematically studied to yield high-quality amine-functionalized MIL-101(Cr) nanoparticles ...(MIL-101(Cr)-NH2). A comprehensive understanding of the role that synthesis conditions and basic modulators have on the formation of MIL-101(Cr)-NH2 in aqueous media was extracted and reported herein. With the aid of a NaOH modulator at optimized concentration, sub-20 nm MIL-101(Cr)-NH2 nanoparticles were prepared with good yield, minimized particle agglomeration, and a high Brunauer–Emmett–Teller (BET) surface area of 2800 ± 200 m2/g. To the best of our knowledge, these are the smallest particle sizes and the highest surface areas reported for directly synthesized MIL-101(Cr)-NH2. Owing to their superior surface area and Lewis basic amine functionality, the MIL-101(Cr)-NH2 nanoparticles exhibit a high CO2 adsorption of up to 3.4 mmol/g at 288 K and 1 bar and a superior CO2/N2 selectivity of 26.5 at 308 K and 0.1 bar. A high isosteric heat of −54.6 kJ/mol for CO2 adsorption implies the strong interaction between CO2 and the amine-functionalized framework. The successful synthesis of sub-20 nm amine-functionalized MIL-101(Cr) nanoparticles offers a great opportunity to engineer advanced MIL-101(Cr)-based functional adsorbents and membranes for CO2 capture and separation.
Gas‐separation polymer membranes display a characteristic permeability–selectivity trade‐off that has limited their industrial use. The most comprehensive approach to improving performance is to ...devise strategies that simultaneously increase fractional free volume, narrow free volume distribution, and enhance sorption selectivity, but generalizable methods for such approaches are exceedingly rare. Here, we present an in situ crosslinking and solid‐state deprotection method to access previously inaccessible sorption and diffusion characteristics in amine‐functionalized polymers of intrinsic microporosity. Free volume element (FVE) size can be increased while preserving a narrow FVE distribution, enabling below‐upper bound polymers to surpass the H2/N2, H2/CH4, and O2/N2 upper bounds and improving CO2‐based selectivities by 200 %. This approach can transform polymers into chemical analogues with improved performance, thereby overcoming traditional permeability–selectivity trade‐offs.
A protection/deprotection strategy was developed to enhance the separation performance of amine‐functionalized PIM‐1. Thermal deprotection yielded a synergistic boost in size‐sieving and CO2 diffusion, ultimately surpassing the H2/CH4, O2/N2 and H2/N2 upper bounds and boosting CO2 permeability by 200 %.
Physical aging is a slow structural relaxation process characteristic of glassy polymers that results in reduced membrane permeabilities. In this study, PIM-1, the archetypal polymer of intrinsic ...microporosity (PIM), was post-synthetically modified to introduce components that are known to influence physical aging, such as hydrogen bonds and crosslinks. The effects of physical aging were monitored by permeation and sorption experiments, and structural changes were examined by positron annihilation lifetime spectroscopy (PALS) and other characterization techniques. The results suggest that higher initial fractional free volume is the primary factor contributing to higher rates of physical aging and that the introduction of hydrogen bonds and crosslinks reduces the initial free volume of PIM-1. In contrast, structural factors such as hydrogen bonds and crosslinks were the key factors in determining how permselectivity changed with physical aging. This study provides useful structure-property correlations and design principles related to free volume, hydrogen bonds, and crosslinks on physical aging behavior of microporous polymer membranes.
Physical aging rates strongly correlate with the initial free volume of microporous polymers. Introducing hydrogen bonds and crosslinks can reduce the initial free volume and significantly impact gas separation selectivity over time.
Microporous polymers have transformed the field of membrane-based gas separations over the past two decades. However, applying sorbent materials as membranes presents a unique challenge in ...quantifying emergent phenomena, such as how strongly sorbing penetrants influence transport of co-permeating species. These mechanistic features are of urgent concern for many current and emerging applications, yet they remain significantly understudied. In previous work, amine-functionalized PIM-1 (PIM-NH2) has proven to be an exemplar of these effects, exhibiting CO2/CH4 mixed-gas selectivities that are 2.6 times higher than selectivities calculated from pure-gas measurements. Here, we investigate the generalizability of this sorption-induced emergent phenomena through the synthesis of a novel amine-functional microporous poly(arylene ether) (PAE-NH2). The effects of amine functionalization on gas transport were analyzed through variable-temperature pure-gas sorption tests, ternary mixed-gas sorption modeling, and dual-mode sorption analysis for N2, CH4, CO2, and H2S. Compared to its nitrile-functional counterpart (PAE-CN), pure-gas sorption for PAE-NH2 was 69 % higher for CO2 and 26 % higher for H2S at 1 atm and 35 °C, suggesting increased affinity to both CO2 and H2S. However, the higher total sorption for H2S resulted in strong competitive sorption effects, decreasing permeability of both CO2 and CH4 for mixture experiments, which was reported in our complementary study on the mixed-gas separation performance of the same materials in this work. The strength of gas–polymer interactions were quantified by evaluating experimental isosteric heats of sorption for the acid gases in PIM-1, PIM-NH2, PAE-CN, and PAE-NH2. Amine-functionalized samples showed highly exothermic interactions, with minima in isosteric heats of approximately −44 kJ mol−1 for CO2 and −34 kJ mol−1 for H2S. Leveraging microporosity and amine-functionality in membranes are general approaches to access competitive sorption for many industrially relevant gas separations.
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•Amine-functionalization of microporous poly(arylene ether) (PAE-CN) into PAE-NH2 was successful in solid-state.•High-pressure H2S, CO2, CH4, and N2 sorption isotherms were collected.•High-precision dual-mode sorption (DMS) modeling was used to fit isotherms and calculate energetics of sorption.•Amine functionality enhanced H2S and CO2 uptake relative to nitrile-functional materials.
While significant advancements have been made in the synthesis of microporous polymers for gas separations in the last two decades, little is known regarding structure–property relationships under ...industrially relevant conditions involving highly condensable gases such as H2S. Recent work on the mixed-gas transport in an amine-functionalized microporous PIM (PIM-NH2) has demonstrated benefits of simultaneous plasticization resistance and competitive sorption for increased permselectivity in binary CO2/CH4 mixtures. In this work, we elucidate the effects of analogous competitive sorption relationships through pure- and mixed-gas permeation in an amine-functional microporous poly(aryl ether) (PAE-NH2) and provide comparisons with PIM-NH2 and the corresponding nitrile-functional counterparts, PAE-CN and PIM-1. In binary mixed-gas tests, PAE-NH2 show a 2.5- and 2.4-fold increase in CO2/CH4 and CO2/N2 mixed-gas permselectivities, respectively, compared to the pure-gas case. In 20/20/60 H2S/CO2/CH4 mixtures, amine-functionalized derivatives retain increases in CO2/CH4 selectivities compared to the nitrile-functional analogues. Additionally, PIM-NH2, PAE-CN, and PAE-NH2 have excellent plasticization resistance in 50/50 CO2/CH4 binary mixed-gas tests up to 26 atm.
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•PIM-1, PAE-CN, and amine-functional (PIM-NH2, PAE-NH2) polymers were measured in CO2/CH4, CO2/N2, H2S/CO2/CH4 mixtures.•Competitive sorption was observed in all four materials, but effects were most apparent in the amine-functional membranes.•CO2- and H2S-based permselectivities increased from pure- to mixed-gas tests due to exclusion of N2/CH4 from the polymer.•The amine-functional materials demonstrated excellent plasticization resistance up to 26 atm in a 50/50 CO2/CH4 mixture.
Fractional free volume (FFV) is a commonly used metric for the development of structure–property relationships for polymer membranes. The most common method to calculate FFV uses Bondi's group ...contribution method, first introduced in 1964. While updated in 1997, there has not been a significant compilation of new structural motifs since the advent of linear microporous polymers. In this study, we critically examined the assumptions in Bondi's original method and provide four recommendations to streamline and improve the accuracy of calculating van der Waals volume (VW) for any group. Using these recommendations, we created an updated list of VW values for structural groups commonly present in microporous polymers. The VW and FFV values were then calculated for a database of 123 microporous and high free volume polymers from the literature, showing an average 7% decrease in VW and corresponding increase in FFV by a factor of 24% when compared to prior group contribution correlations in the literature. The significant apparent increase in estimated FFV provides a new perspective to understand and interpret the role of free volume on the separation performance of linear microporous polymers. Additionally, standardization of the group contribution method allows for the direct comparison of FFV values across studies.
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•Bondi's group contribution method to calculate fractional free volume (FFV) was refined.•An updated procedure to calculate van der Waals volume for any group is presented.•New FFV values were calculated for 123 polymers, including microporous polymers.•On average, new FFV values were greater than reported literature FFV by a factor of 24%.
An optimized acid hydrolysis method was developed to yield carboxylic acid-functionalized PIM-1 (PIM-COOH) with >89% conversion in 48 h using a postpolymerization reaction of PIM-1. Physical ...characterization of PIM-1 and PIM-COOH revealed that the average size of free volume elements in PIM-COOH decreased relative to that in PIM-1. Compared to PIM-1, PIM-COOH showed a significant increase in CO2- and H2-based selectivities with a corresponding decrease in permeabilities and sorption capacities for all gases considered. The dual-mode sorption model, time-lag method, and sorption–diffusion model were applied to glean molecular-level insights into diffusion and sorption in these polymers. Results indicate that improvements in selectivities for CO2-based gas pairs for PIM-COOH are primarily driven by diffusion selectivity and that PIM-COOH displays transport behavior consistent with the sorption–diffusion model. To better understand performance under more realistic conditions, pure- and mixed-gas permeation values for CO2/CH4 are reported for a 330 day aged PIM-COOH sample.
The directed self-assembly (DSA) of block copolymers (BCPs) has shown promise in fabricating customized two-dimensional (2D) geometries at the nano- and meso-scale. Here, we discover spontaneous ...symmetry breaking and superlattice formation in DSA of BCP. We observe the emergence of low symmetry phases in high symmetry templates for BCPs that would otherwise not exhibit these phases in the bulk or thin films. The emergence phenomena are found to be a general behavior of BCP in various template layouts with square local geometry, such as 4
and 3
434 Archimedean tilings and octagonal quasicrystals. To elucidate the origin of this phenomenon and confirm the stability of the emergent phases, we implement self-consistent field theory (SCFT) simulations and a strong-stretching theory (SST)-based analytical model. Our work demonstrates an emergent behavior of soft matter and draws an intriguing connection between 2-dimensional soft matter self-assembly at the mesoscale and inorganic epitaxy at the atomic scale.
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