Polymer Rigidity Improves Microporous Membranes

Abstract

Gas separation with membranes has been commercialized for more than 30 years, and includes processes such as the production of nitrogen (N2) from air and the removal of carbon dioxide (CO2) from natural gas. Commercial membranes have been largely derived from polymers with moderately rigid chains that pack closely to create small intermolecular spaces (or "free volume") that impart moderate to high gas selectivity. However, their relatively low gas permeability slows down the separation processes. Microporous organic polymers (MOPs) (1?3) offer higher permeability, but the polymer chains must be made sufficiently rigid to maintain good selectivity. On page 303 of this issue, Carta et al. (4) describe a soluble, highly rigid MOP, from which a highly permeable membrane with good selectivity was fabricated. For example, oxygen (O2) and N2 have only a 5% difference in kinetic diameters (which are related to the smallest effective dimensions of the gases), but the gas throughput of the smaller O2 molecule is very much higher through their membrane.