Simultaneous Enhancement of the Propylene Permeability and Selectivity in Mixed-Matrix Membranes with Ag+-Modified Covalent Organic Frameworks

Covalent organic frameworks (COFs) are promising fillers for mixed-matrix membranes (MMMs) in gas separation due to their purely organic nature and tunable porosity. However, the inherently large pore sizes of most COFs limit propylene/propane selectivity solely on size sieving. Therefore, we employed a postsynthetic modification strategy to introduce Ag+ ions to the β-ketoenamine linkage carboxylic acid COF (TpDAT) framework to tailor the pore environment and facilitate propylene transport. The resulting TpDAT-Ag/6FDA-TMPDA MMMs achieved a synchronous improvement in the C3H6 permeability and C3H6/C3H8 ideal selectivity compared to the pure 6FDA-TMPDA membrane. The best performance of MMMs (10 wt % loading) demonstrated a C3H6 permeability of 142 Barrer and a C3H6/C3H8 ideal selectivity of 15, outperforming the performance of most polymer-based membranes. Density functional theory calculations further confirmed the favorable interaction between the C3H6 molecules and TpDAT-Ag. The outperforming separation performance together with the long-term stability renders this Ag+-modified COF a promising candidate in MMMs for C3H6/C3H8 and other gas separations.