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

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 + (CH 2 CH 3 ) 3 ) clusters with good CO 2 affinity via sacrificial template method in quaternized polysulfone (QAPSf) membranes for achieving the fast transport of CO 2 . The templates (POSS) induce the formation of –N + (CH 2 CH 3 ) 3 clusters through forced microphase separation in the membranes and then are etched away to generate hollow CO 2 -philic clusters to increase the fractional free volume in membranes and thus improve CO 2 diffusion. After that, the B(OH) 4 - as counterions by ion exchange endow the hollow CO 2 -philic clusters with high CO 2 -facilitating ability, which simultaneously enhance the CO 2 permeability and CO 2 /CH 4 selectivity. The resultant QAPSf-B(POSS-2) membrane exhibits superior separation performance with an optimal permeability of 1113 Barrer and a CO 2 /CH 4 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.