CO2-Selective mixed matrix membranes of bimetallic Zn/Co-ZIF vs. ZIF-8 and ZIF-67

Abstract

Bimetallic metal–organic frameworks (MOF) have received growing attention in several fields for gas separation/adsorption, energy storage and catalysis because of their synergic effect and enhanced properties compared to their parent structures. Here, we first report on a facile and low cost direct synthesis of solid solution of bimetallic Zn/Co-ZIF and its monometallic counterparts, ZIF-8 and ZIF-67 at room temperature. Zn/Co-ZIF showed higher pore volume and BET surface area than their parents, homogeneous distribution of metals within the framework and a slightly higher Zn-rich external surface. Next, mixed matrix membranes (MMM) of Zn/Co-ZIF, ZIF-8 and ZIF-67 based 6FDA-ODA polyimide were fabricated with different content (3–10 %wt) via a wet approach using undried MOF for CO2/CH4 single and mixed gas (10:90 vol%) separation application to determine the effect of the metal center on the gas transport properties. While Zn/Co-ZIF (10 wt%)/6FDA-ODA had the highest CO2/CH4 selectivity (39.8) and CO2 permeability (45.8 Barrer) compared to the MMM of ZIF-8 (PCO2 = 41.6 Barrer, SCO2/CH4 = 36.1) and ZIF-67 (PCO2 = 26.2 Barrer, SCO2/CH4 = 26.3) at the same concentration, the best performance was observed for Zn/Co (7.5 wt%)/6FDA-ODA MMM with 104% and 41% increase in CO2 permeability up to 44.2 Barrer and CO2/CH4 ideal selectivity to 45.1, compared to the neat polymer, respectively.