Improving mixed-matrix membrane performance via PMMA grafting from functionalized NH2–UiO-66†
The major obstacles in gas separation by mixed-matrix membranes (MMMs) are poor dispersion and poor affinity between polymers and fillers. The present study demonstrates that these challenges can be overcome appropriately by utilizing a series of synthesized stand-alone MMMs. The matrix used was polymethyl methacrylate (PMMA) and the MMMs were synthesized by in situ polymerization of methyl methacrylate (MMA) in the presence of UiO-66, NH2–UiO-66 and vinyl group attached UiO-66. In situ polymerization of MMA in the presence of vinyl attached UiO-66 resulted in PMMA grafted UiO-66 with a high degree of grafting. Microscopic analysis by field emission scanning electron microscopy (FESEM) revealed that desirable nanoscale dispersion of UiO-66, suitable distribution across the membrane thickness as well as strong interface were achieved by using UiO-66 functionalized with amino groups and in particular with PMMA grafting. The permeability of pure He, CO2, N2, and CH4 gases was determined using a constant volume/variable pressure method at 25 °C and 4 bar. The results demonstrated that the permeability of the aforementioned gases as well as the ideal selectivity in He/CH4, He/N2, CO2/N2, and CO2/CH4 increased by increasing the amount of MOF loading. Particularly, the PMMA membrane containing PMMA grafted UiO-66 (g-MMM) exhibited the highest selectivity for He/CH4 and He/N2 and the best permeation for helium gas, which was remarkably well-positioned on Robeson's upper bound curves. The results demonstrated that the g-MMM could be a promising candidate for practical helium separation.