Finely tuning the microporosity in phosphoric acid doped triptycene-containing polybenzimidazole membranes for highly permselective helium and hydrogen recovery

Abstract

High-performance polymer membranes with well-defined microporosity and size-sieving ability are especially attractive for helium and hydrogen recovery. Here we report novel macromolecular engineering of polybenzimidazole (PBI) membranes that integrate hierarchical triptycene units for high permeability and polyprotic acid doping for size sieving via controllable manipulation of microporous architecture. The triptycene moieties disrupt chain packing and introduce additional configurational free volume, leading to significantly boosted He and H2 permeabilities compared to previously reported PBI membranes. The acid doping resulted in crosslinked PBI membranes via hydrogen bonding and proton transfer with dramatically enhanced gas selectivities. Via adjusting the H3PO4-doping level, triptycene-based polybenzimidazole (TPBI) composite membranes (TPBI-(H3PO4)x) exhibit the highest gas selectivities for He enrichment (i.e., α(He/CH4) = 7052 ± 156) and H2 purification (i.e., α(H2/CH4) = 5128 ± 110) among existing polymeric gas separation membranes. Additionally, under mixed-gas conditions at 150 °C, the TPBI-(H3PO4)0.98 membrane displays a H2 permeability of 46.7 Barrer and a H2/CO2 selectivity of 16, far beyond the Robeson's 2008 upper bound for H2/CO2 separation. The facile and diverse tunability and excellent gas separation performance make TPBI-(H3PO4)x membranes highly attractive for helium and hydrogen separation.