Role of Filler Porosity and Filler/Polymer Interface Volume in Metal–Organic Framework/Polymer Mixed-Matrix Membranes for Gas Separation

Abstract

Metal-organic frameworks (MOFs) and inorganic fillers are frequently incorporated into mixed-matrix membranes (MMMs) to overcome the traditional trade-off in permeability ( P) and selectivity for pure organic polymer membranes. Therefore, it is of great interest to examine the influence of porous and nonporous fillers in MMMs with respect to the possible role of the polymer-filler interface, that is, the void volume. In this work, we compare the same MOF filler in a porous and nonporous state, so that artifacts from a different polymer-filler interface are excluded. MMMs with the porous MOF aluminum fumarate (Al-fum) and with a nonporous dimethyl sulfoxide solvent-filled aluminum fumarate (Al-fum(DMSO)), both with Matrimid as polymer, were prepared. Filler contents ranged from 4 to 24 wt %. Gas separation performances of both MMMs were studied by mixed gas measurements using a binary mixture of CO/CH with gas permeation following the theoretical prediction by the Maxwell model for both porous and nonporous dispersed phase (filler). MMMs with the porous Al-fum filler showed increased CO and CH permeability with a moderate rise in selectivity upon increasing filler fraction. The MMMs with the nonporous Al-fum(DMSO) filler displayed a reduction in permeability while maintaining the selectivity of the neat polymer. A linear dependence of log P versus the reciprocal specific free fractional volume (sFFV) rules out a significant contribution from a void volume. The sFFV includes the free volume of the polymer and the MOF, but not the polymer-filler interface volume (so-called void volume). The sFFV for the MMM was calculated between 0.23 cm/g for a 24 wt % Al-fum/Matrimid MMM and 0.12 cm/g for a 24 wt % Al-fum(DMSO)/Matrimid MMM. The negligible effect of an interface volume is supported by a good matching of theoretical and experimental density of the Al-fum and Al-fum/(DMSO) MMMs which gave a specific void volume below 0.02 cm/g, often even below 0.01 cm/g.

Potential of adsorbents from agricultural wastes as alternative fillers in mixed matrix membrane for gas separation: A review

“…Mainly, the adsorption process occurs in the MMM comes from the dispersed filler particles. The fillers incorporated into the polymer matrix can be classified into porous (zeolite [65], graphene oxide [66], carbon molecular sieve (CMS) [67], metal organic framework (MOF) [68] and carbon nanotubes (CNTs) [69]) and nonporous (silica [70] and titanium oxide (TiO 2 ) [71]) fillers [72]. For porous filler, the transportation of gas molecules through the filler follows the effects of molecular sieve and surface diffusion [73].…”

Encapsulation of a Porous Organic Cage into the Pores of a Metal–Organic Framework for Enhanced CO2 Separation

“…The reduction in permeability can be attributed to the almost non‐existent pore volume of CB6. Hence, CB6 alone can be considered a nonporous filler with high CO 2 affinity, which leads to the increased selectivity . MIL‐101/Matrimid MMMs showed the expected enhanced CO 2 (from 7 Barrer to 16 Barrer) and CH 4 (from 0.2 Barrer to 0.4 Barrer) permeability but no increase in selectivity.…”

Encapsulation of a Porous Organic Cage into the Pores of a Metal–Organic Framework for Enhanced CO2 Separation

“…Hence,C B6 alone can be considered an onporous filler with high CO 2 affinity,w hich leads to the increased selectivity. [31] MIL-101/Matrimid MMMs showed the expected enhanced CO 2 (from 7Barrer to 16 Barrer) and CH 4 (from 0.2 Barrer to 0.4 Barrer) permeability but no increase in selectivity.T he distinct enhancement in permeability can be assigned to the high pore volume of MIL-101. However,the large pores of MIL-101 and mediocre affinity to Angewandte Chemie Forschungsartikel CO 2 over CH 4 prevent afavorable adsorption of CO 2 and thus an increase in selectivity.F or CB6@MIL-101-36/Matrimid MMMs,a ne nhancement of the CO 2 permeability to 15 Barrer up from 7Barrer for the neat polymer membrane was observed.…”

“…To simulate flue gas conditions,wecalculated the CO 2 /N 2 selectivities for CB6, 29,36) from the N 2 and CO 2 adsorption isotherms using ideal adsorbed solution theory (IAST) for aC O 2 /N 2 mixture (15:85) at 293 K ( Figure 3d and Figure S23). Forp ostcombustion CO 2 capture,p orous sorbents should have both high CO 2 uptake capacities (in the low P/P 0 region) and high CO 2 /N 2 selectivity.T he IAST values for MIL-101, CB6, and CB6@MIL-101-W (W = 19, 29, 36) were determined to be 19,70,31,58, and 108 at 293 K, respectively.The encapsulation of CB6 into MIL-101 allowed us to introduce CO 2 selectively because of the intrinsic micropores of CB6 and the retained micropores of MIL-101, while losing some of the uptake capacity of the non-selective mesopores of MIL-101. Thus,the combined properties of CB6 and MIL-101 showed aC O 2 /N 2 selectivity increase from 31 to 108, upon loading CB6 into MIL-101.…”