Polyelectrolyte membranes with tunable hollow CO2-philic clusters via sacrificial template for biogas upgrading

Abstract

High-performance membrane materials are urgently demanded for efficient membrane-based biogas upgrading. Polyelectrolytes with convertible ionic groups, spontaneously forming interconnected facilitated transport pathways for gas molecules via microphase separation, exhibit promising potential in membrane-based separation. However, there is a great challenge to further enhance the gas permeability and selectivity simultaneously due to the limited microphase separation and low free volume generated by the strong interaction between ionic groups. Herein, we demonstrate a novel strategy to construct the tunable hollow quaternary ammonium (-N+(CH2CH3)3) clusters with good CO2 affinity via sacrificial template method in quaternized polysulfone (QAPSf) membranes for achieving the fast transport of CO2. The templates (POSS) induce the formation of –N+(CH2CH3)3 clusters through forced microphase separation in the membranes and then are etched away to generate hollow CO2-philic clusters to increase the fractional free volume in membranes and thus improve CO2 diffusion. After that, the B(OH)4- as counterions by ion exchange endow the hollow CO2-philic clusters with high CO2-facilitating ability, which simultaneously enhance the CO2 permeability and CO2/CH4 selectivity. The resultant QAPSf-B(POSS-2) membrane exhibits superior separation performance with an optimal permeability of 1113 Barrer and a CO2/CH4 gas selectivity of ~45, i.e. 334% and 73% higher than the pristine QAPSf membrane, respectively, breaking the trade-off between permeability and selectivity and surpassing the Robeson 2008 upper-bound.