Enhanced gas separation performance of 6FDA-DAM based mixed matrix membranes by incorporating MOF UiO-66 and its derivatives

Functionalization and post-synthetic modification (PSM) of metal-organic frameworks (MOFs) are two important routes to obtain MOFs with full potential in mixed matrix membrane (MMM) fabrication. We synthesized UiO-66 and two derivatives UiO-66-NH₂ and UiO-66-NH-COCH₃ with less than 50 nm particle size. The CO₂ uptakes at 10 bar in the two functionalized UiO-66s were improved by 44% and 58%, respectively, with respect to the pristine solid. The MOF nanoparticles were incorporated into the highly permeable polymer 6FDA-DAM, making MMMs with 5–24 wt% particle loadings. All fillers and membranes were characterized accordingly, and their gas separation performances were evaluated by feeding CO₂/CH₄ equimolar mixtures at 2 bar pressure difference at 35 °C. CO₂ permeability (P_CO2) of pristine 6FDA-DAM (P_CO2 = 997 ± 48 Barrer, α_CO2/CH4 = 29 ± 3) increased by 92% with 20 wt% UiO-66 loading, while maintaining the CO₂/CH₄ selectivity. Improvements of 23% and 27% were observed for P_CO2 with the same 20 wt% loading of UiO-66-NH₂ and UiO-66-NH-COCH₃, respectively. The α_CO2/CH4 was improved up to 16% using both functionalized UiO-66 type MOFs. The best separation performance in this work was obtained with 14 wt% UiO-66 MMM (P_CO2 = 1912 ± 115 Barrer, α_CO2/CH4 = 31 ± 1), 16 wt% UiO-66-NH₂ MMM (P_CO2 = 1223 ± 23 Barrer, α_CO2/CH4 = 30 ± 1) and 16 wt% UiO-66-NH-COCH₃ MMM (P_CO2 = 1263 ± 42 Barrer, α_CO2/CH4 = 33 ± 1) at 2 bar feed pressure difference. The measurement was also conducted with various binary compositions (CO₂ = 10 – 90%), both at low and high pressures up to 40 bar at 35 °C, showing no pressure-related CO₂-induced plasticization. The atomistic modelling for the MOF/polymer interface was consistent with a moderate MOF surface coverage by 6FDA-DAM which did not play a detrimental role in the membrane performance.