Nano-sized MIL-101(Cr) MOF for CO2 separation: Crystal downsizing effects on adsorption and mixed matrix membranes

In this paper, nano-sized MIL-101(Cr) metal-organic framework (MOF) and the micro-sized one were synthesized. To reduce the average crystal size, a surfactant namely cetyltrimethylammonium bromide (CTAB) in its optimum amount (Cr3+:CTAB = 1:0.3) along with microwave heating (600 W, 30 min) were applied. The prepared MOFs were characterized by FESEM, EDS, PXRD, BET, FTIR, and TGA analyses. Moreover, CO2 and CH4 adsorption isotherms and kinetics were investigated at 298 K and equilibrium pressures up to 1 bar. The results revealed that the CO2/CH4 ideal adsorption selectivity was improved by 27.15 % for the downsized MOF, socalled MIL-101(Cr)/CTAB, compared to the micro-sized one. Furthermore, by incorporation of the MOFs into the Ultem (R) 1000 polymer, mixed matrix membranes (MMMs) were fabricated. The morphological analysis of the prepared membranes displayed proper dispersion and compatibility between the polymer and MOFs. The best separation performance was obtained for the MMM containing 20 wt% of the mesoporous MIL-101(Cr)/CTAB. For this MMM, the CO2 permeability and CO2/CH4 ideal perm-selectivity values were enhanced by 136 % and 18 %, respectively at 298 K and 4 bar, compared to the neat membrane.