Hydrogen-bond acceptance's role in designing room temperature ionic liquid (RTIL) membranes for gas separations: Part II, β-parameter and relative humidity impacts on membrane stability

Abstract

RTIL H-bond acceptance ability (β-parameter) and the porous support's polymer are important factors for RTIL-membrane stability. Under dry conditions, the stability of RTILs supported in porous polyethersulfone decreased with increasing β-parameters. Under humid conditions, the CH4-permeance vs. rH relationship for [emim][TfO] changed when the porous support's polymer changed. With proper selection of the porous support's chemistry, it is possible to create RTIL-membranes that are stable over a range of rHs for both low and high β-parameter RTILs. However, in some RTIL/polymer supported membranes, a reversible and repeatable dual mode CH4-permeance behavior occurs with significantly lower CH4-permeances at elevated rHs. This dual mode is the result of RTIL/polymer interaction and not the result of changes in RTIL/transporting gas physical chemistry. RTIL capillary rise vs. rH measurements are consistent with, but insufficient to state that, the dual mode results from changes in the RTIL/polymer interfacial capillary forces. Increasing gas rH leads to increased gas permeability with greater permeability enhancement for CH4 transport. This leads to decreased CO2/CH4 selectivity with increasing rH. Mixed-gas CO2/CH4 separation data for three benchmark RTIL-membranes, verified to be stable from 0%-rH to >90%-rH, confirmed this permeance increase and selectivity decrease statement.