Redefining the Robeson upper bounds for CO2/CH4 and CO2/N2 separations using a series of ultrapermeable benzotriptycene-based polymers of intrinsic microporosity

Abstract

Membranes composed of Polymers of Intrinsic Microporosity (PIMs) have the potential for energy efficient industrial gas separations. Here we report the synthesis and gas permeability data of a series of ultrapermeable PIMs, of two-dimensional chain conformation and based on benzotriptycene structural units, that demonstrate remarkable ideal selectivity for most gas pairs of importance. In particular, the CO₂ ultrapermeability and high selectivity for CO₂ over CH₄, of key importance for the upgrading of natural gas and biogas, and for CO₂ over N₂, of importance for cost-effective carbon capture from power plants, exceed the performance of the current state-of-the-art polymers. All of the gas permeability data from this series of benzotriptycene-based PIMs are placed well above the current 2008 Robeson upper bounds for CO₂/CH₄ and CO₂/N₂. Indeed, the data for some of these polymers fall into a linear correlation on the benchmark Robeson plots [i.e. log(P_CO2/P_CH4) versus log P_CO2 and log(P_CO2/P_N2) versus log P_CO2], which are parallel to, but significantly above, that of the 2008 CO₂/CH₄ and CO₂/N₂ upper bounds, allowing their revision. The redefinition of these upper bounds sets new aspirational targets for polymer chemists to aim for and will result in more attractive parametric estimates of energy and cost efficiencies for carbon capture and natural/bio gas upgrading using state-of-the-art CO₂ separation membranes.