Pre-combustion CO2 capture from the water-gas-shift reaction exhausts for hydrogen fuel production demands efficient H2/CO2 separations at elevated temperatures (100-300 °C). Compared to conventional CO2 capture technologies such as pressure swing adsorption (PSA), H2-selective membrane-based separation technologies are highly attractive due to much higher energy efficiency. Polybenzimidazole (i.e., m-PBI) is the leading polymer membrane material for H2/CO2 separations due to its excellent thermal stability and the highest H2/CO2 selectivity among glassy polymers at high temperatures, which is a result of highly tight chain packing in m-PBI promoted by intra- and inter-molecular hydrogen bonding. However, for the same reason, m-PBI suffers from miserably low H2 permeability (e.g., 2-3 Barrer at 35 °C), which demands fundamentally new strategies of macromolecular design to maximize the potential of PBI-based membranes for energy-efficient pre-combustion CO2 capture at elevated temperatures. In this talk, several new PBI designs will be introduced and discussed with an emphasis on the exploration of the fundamental structure-property relationship to achieve high H2 permeability and high H2/CO2 selectivity simultaneously. Specifically, the first strategy involves the design of semi-interpenetrating polymer network (s-IPN) structures, where linear m-PBI chains penetrate through specifically designed crosslinked model networks enabling high permeability (e.g., H2 permeability 10 times that of m-PBI) with maintained selectivity. The second strategy is to incorporate shape-persistent iptycene units (i.e., triptycene and pentiptycene) into PBI structure to introduce configurational free volume that allows for much enhanced H2 permeability due to structure bulkiness along with excellent size sieving properties. Further microporosity tuning is enabled by acid doping in these iptycene-based PBIs. Several of these acid-doped iptycene-PBI membranes show ultrahigh H2/CO2 selectivity at temperature up to 200 °C. Discussions in this talk will emphasize on new macromolecular design concepts and the understanding of fundamental structure-property relationships for these new PBI membranes for energy-efficient pre-combustion CO2 capture.

Microporous polybenzimidazole (PBI)-based membranes for re-combustion CO2 capture

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