Enhancing the properties and gas separation performance of PBI-polyimides blend carbon molecular sieve membranes via optimization of the pyrolysis process

The evolution of desirable physico-chemical structure and properties in high performance gas separation membranes involve steps that must carefully be designed, controlled and optimized. This study investigates the role of key parameters in the fabrication and performance analysis of carbon molecular sieve (CMS) membranes prepared through blending of poly (benzimidazole) (PBI) and three polyimides containing different dianhydride moieties in their chemical structure. Results indicate that the chemical structure of the blend components, microstructure of the precursor, blend composition and the pyrolysis conditions play important roles in the transport properties of the resulting membranes. The influence of the type of polyimide used in the blend on the permeability of the carbon membranes followed a trend. Using a higher pyrolysis temperature resulted in membranes with a lower permeability but higher selectivity. In addition, a higher degree of vacuum in the pyrolysis chamber increased the selectivity of the membranes by as much as 40% at the expense of permeability. The highest gas pair selectivity for O2/N2, CO2/CH4 and CO2/N2 could be obtained from PBI-Kapton carbonized at 10−7 Torr and 800 °C. These results also suggest that CMS membranes derived from PBI-Kapton blend precursors are exceptional candidates for the CO2/CH4 separation, offering enhanced selectivity in the range of up to 204.5 depending on the pyrolysis protocol. The results of this study suggest that high performance gas separation membranes can be obtained by adopting a judicious combination of blending technique and optimized pyrolysis conditions.