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-NH 2 and UiO-66-NH-COCH 3 with less than 50 nm particle size. The CO 2 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 CO 2 /CH 4 equimolar mixtures at 2 bar pressure difference at 35°C. CO 2 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 CO 2 /CH 4 selectivity. Improvements of 23% and 27% were observed for P CO2 with the same 20 wt% loading of UiO-66-NH 2 and UiO-66-NH-COCH 3 , 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-NH 2 MMM (P CO2 = 1223 ± 23 Barrer, α CO2/CH4 = 30 ± 1) and 16 wt% UiO-66-NH-COCH 3 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 (CO 2 = 10-90%), both at low and high pressures up to 40 bar at 35°C, showing no pressure-related CO 2-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. with improved permeation and selectivity have been described extensively, including the very promising approach of mixed matrix membranes (MMMs) [5]. MMM technology exploits the distinct and complementary properties of both polymer and inorganic materials with different physicochemical properties, selectivity and permeation flux for their selective separation. Various materials, generally porous, such as carbon molecular sieves (CMS) [6,7], zeolites and silicas [7,8], metal oxides [9], carbon nanotubes (CNTs) [10], metal organic frameworks (MOFs) [11-14], graphene [15,16], etc. have been embedded in continuous polymer
