Micro-Structural Optimization of Polybenzimidazole-Based Membranes for H2/CO2 Separation At Elevated Temperatures

There is compelling need to develop novel separation methods to improve the energy efficiency of synthesis (syn) gas processing operations including H₂ and H₂/CO production to meet power, chemicals, and fuel producer needs, as well as carbon capture and removal of other undesirable syngas impurities. To be technically and economically viable, a successful separation method must be applicable to industrially relevant gas streams at realistic process conditions and compatible with large gas volumes. H₂ selective membrane technology is a promising method for syngas separations at elevated temperatures (>150 °C) that could be positioned upstream or downstream of one or more of the water-gas-shift reactors (WGSRs) or integrated with a WGSR depending on application specific syngas processing. Polybenzimidazole (PBI)-based polymer chemistries are exceptional candidates for H₂/CO₂ separations at elevated temperatures. In general, these materials possess excellent chemical resistance, very high glass transition temperatures (> 400 °C), good mechanical properties, and an appropriate level of processability.  Although commercially available PBI polymers have demonstrated commercially attractive H₂/CO₂ selectivity, their H₂ permeability is low.  Our team is employing structural and chemical manipulations to tailor the polymer free-volume architecture with the ultimate goal of enhancing H₂ permselectivity while retaining the inherent thermochemical stability characteristics of PBI. We will discuss our synthetic approaches and their influences on the gas transport behavior of these PBI-based materials.  In general, a decrease in H₂/CO₂ selectivity was observed with an increase in H₂ permeability.  H₂ permeability and H₂/CO₂ selectivity at 250 °C ranged from 50 to 1000 barrer and 5 to 45, respectively.