Incorporating Microporous Zn3 and Zn2Cd MOFs into Pebax/PVDF Mixed Matrix Membranes for Improved Carbon Dioxide Separation Performance

A pair of related metal–organic frameworks (Zn3 and Zn2Cd) developed in our group were incorporated into Pebax 30R51 and PVDF Kynar 761 polymers to fabricate mixed matrix membranes (MMMs). These MOFs were chosen due to the carbon dioxide molecular sieving ability of Zn3, and the slightly larger pore aperture of Zn2Cd that allows carbon dioxide and larger gases to enter the pores. For Pebax-based MMMs, this work demonstrated an over two-fold and four-and-a-half-fold increase in carbon dioxide permeability for Zn3- (15 wt %) and Zn2Cd-containing (10 wt %) MMMs over the pristine polymer. Separation selectivity (CO2:N2) of 4.21 and 7.33 were observed for Zn3 and Zn2Cd (10 wt %). For PVDF-based MMMs, the incorporation of Zn3 and Zn2Cd (10 wt %) increased the carbon dioxide permeability approximately two- and three-fold. The CO2/N2 selectivity of the PVDF membranes increased 73% (1.01 to 1.86) and 68% (1.01 to 1.68) when 15 wt % Zn3 and Zn2Cd were incorporated into PVDF. The improved performance of Pebax over PVDF based MMMs is attributed to matching the permeability of the polymer bulk phase (Pebax over PVDF) and the dispersed phase (Zn3 and Zn2Cd). The lower permeability allows the MOF, which has slow kinetics associated with molecular sieving, to participate in the permeation process better. With regards to Zn3 vs Zn2Cd, while Zn3 acts as a molecular sieve and Zn2Cd does not, we hypothesize that the faster diffusion of carbon dioxide gas in Zn2Cd can outcompete the lower nitrogen gas permeability and molecular sieving properties of Zn3. However, we expect that further increasing the pore aperture would increase the permeabilities of nitrogen gas such that differences in diffusion kinetics due to molecular size would be unimportant.