Despite their potential for the scalable production of mixed-matrix membranes (MMMs), the MMMs prepared by the polymer-modification-enabled in situ metal–organic framework formation (PMMOF) process showed a considerable reduction in gas permeability as the filler loading increased. It was hypothesized that a correlation existed between the decrease in permeability and the change in the properties of the polymer, such as free volume and chain flexibility, upon in situ MOF formation. Herein, we aim to address the permeability reduction by using a cross-linked polyimide (6FDA-DAM:DABA (3:2)). It was found the degree of cross-linking affected not only the properties of the polymer, but also the in situ formation of the ZIF-8 filler particles in the cross-linked polymer. The proper degree of cross-linking resulted in suppressing C3H6 permeability reduction, suggesting a possible strategy to overcome the issue of PMMOF. The swelling of the polymer followed by chain rearrangement during the PMMOF, as well as the structural rigidity of the polymer, were found to be critical in mitigating permeability reduction.
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Polyimide/ZIF-7 mixed-matrix membranes: understanding the in situ confined formation of the ZIF-7 phases inside a polymer and their effects on gas separations
Polymer-modification-enabled in situ metal–organic framework formation (PMMOF) is potentially a paradigm-shifting preparation method for polymer/MOF mixed-matrix membranes (MMMs). However, the actual reaction conditions of the in situ formation of MOFs in a confined polymer free volume are expected to be quite different from that in a bulk solution. ZIF-7 is an interesting filler material not only due to its use in selective light gas separations but also for its three different crystal phases. Herein, we carried out systematic investigations on the in situ confined formation of ZIF-7 phases inside a polymer (6FDA-DAM) by PMMOF. The reaction conditions of ZIF-7 in the polymer free volume were deduced based on a bulk-phase ZIF-7 phase diagram constructed by varying the ZIF-7 precursor concentrations and ratios. Based on the understanding of the reaction conditions, the ZIF-7 crystal phases formed inside the polymer during the PMMOF process were controlled, yielding 6FDA-DAM/ZIF-7 MMMs with three different crystal phases. The ZIF-7 phases had significant effects on the gas transport of MMMs with layered ZIF-7-III fillers exhibiting the highest performance enhancement for H 2 /CO 2 separation ( i.e. , H 2 permeability of ∼1630 Barrer and H 2 /CO 2 selectivity of ∼3.8) among other phases. Furthermore, the MMMs obtained by the PMMOF process showed enhanced H 2 /CO 2 separation performance, surpassing the upper bound.
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- Award ID(s):
- 1929596
- NSF-PAR ID:
- 10201963
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 8
- Issue:
- 22
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 11210 to 11217
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Polyimides (PI) synthesized from 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamines have been frequently studied as gas separation membranes. The use of 6FDA in polyimides creates a bent structure than can increase fractional free volume (FFV) and gas permeability. Here, we demonstrate that 6FDA is also a useful building block for PI‐ionene materials, which contain cations directly within the polymer backbone. These new 6FDA‐containing PI‐ionenes were combined with several different imidazolium ionic liquids (ILs) to form thin membranes. The thermal properties of all the derivatives were investigated to determine the relationship between regiochemistry and degradation as well as the intermolecular forces that are present within these structures. The gas separation properties of these 6FDA‐containing PI‐ionene + IL materials were investigated, showing modest CO2permeabilities similar to other polyimide‐ionenes and CO2/CH4and CO2/N2permselectivities that were relatively higher than other polyimide‐ionenes.
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Abstract Vapor phase ligand treatment (VPLT) of 2‐aminobenzimidazole (2abIm) for 2‐methylimidazole (2mIm) in ZIF‐8 membranes prepared by two different methods (LIPS: ligand induced permselectivation and RTD: rapid thermal deposition) results in a notable shift of the molecular level cut‐off to smaller molecules establishing selectivity improvements from ca. 1.8 to 5 for O2/N2; 2.2 to 32 for CO2/CH4; 2.4 to 24 for CO2/N2; 4.8 to 140 for H2/CH4and 5.2 to 126 for H2/N2. Stable (based on a one‐week test) oxygen‐selective air separation performance at ambient temperature, 7 bar(a) feed, and 1 bar(a) sweep‐free permeate with a mixture separation factor of 4.5 and oxygen flux of 2.6×10−3 mol m−2 s−1is established. LIPS and RTD membranes exhibit fast and gradual evolution upon a 2abIm‐VPLT, respectively, reflecting differences in their thickness and microstructure. Functional reversibility is demonstrated by showing that the original permeation properties of the VPLT‐LIPS membranes can be recovered upon 2mIm‐VPLT.
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Abstract Vapor phase ligand treatment (VPLT) of 2‐aminobenzimidazole (2abIm) for 2‐methylimidazole (2mIm) in ZIF‐8 membranes prepared by two different methods (LIPS: ligand induced permselectivation and RTD: rapid thermal deposition) results in a notable shift of the molecular level cut‐off to smaller molecules establishing selectivity improvements from ca. 1.8 to 5 for O2/N2; 2.2 to 32 for CO2/CH4; 2.4 to 24 for CO2/N2; 4.8 to 140 for H2/CH4and 5.2 to 126 for H2/N2. Stable (based on a one‐week test) oxygen‐selective air separation performance at ambient temperature, 7 bar(a) feed, and 1 bar(a) sweep‐free permeate with a mixture separation factor of 4.5 and oxygen flux of 2.6×10−3 mol m−2 s−1is established. LIPS and RTD membranes exhibit fast and gradual evolution upon a 2abIm‐VPLT, respectively, reflecting differences in their thickness and microstructure. Functional reversibility is demonstrated by showing that the original permeation properties of the VPLT‐LIPS membranes can be recovered upon 2mIm‐VPLT.
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Abstract Mixed matrix membranes (MMMs) comprising size‐sieving fillers dispersed in polymers exhibit diffusivity selectivity and may surpass the upper bound for gas separation, but their performance is limited by defects at the polymer/filler interface. Herein, a fundamentally different approach employing a highly sorptive filler that is inherently less sensitive to interfacial defects is reported. Palladium nanoparticles with extremely high H2sorption are dispersed in polybenzimidazole at loadings near the percolation threshold, which increases both H2permeability and H2/CO2selectivity. Performance of these MMMs surpasses the state‐of‐the‐art upper bound for H2/CO2separation with polymer‐based membranes. The success of these sorption‐enhanced MMMs for H2/CO2separation may launch a new research paradigm that taps the enormous knowledge of affinities between gases and nanomaterials to design MMMs for a wide variety of gas separations.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0ta02761h
