Kinetic H2S/CO2 Selectivity in an Exceptionally Sterically Hindered Amine Membrane

Facilitated transport membranes (FTMs) show great promise for H2S/CO2 separation, an industrially important yet challenging process. Herein, we report FTMs with excellent H2S/CO2 separation performance and investigate how contradictory thermodynamic and kinetic reaction preferences affects FTM selectivity. For membranes based on an extremely sterically hindered di-tert-butylamine carrier, CO2 transport occurs exclusively via a slow bicarbonate pathway. Reducing the membrane thickness shifts the reaction preference from the thermodynamically favored bicarbonate pathway to the kinetically favored amine-H2S reaction, leading to a 10-fold improvement in H2S/CO2 selectivity. This unusual trend of increasing selectivity with decreasing thickness, the opposite of typical FTMs, enables simultaneous improvements in membrane permeance and selectivity. This translates to an exceptional H2S/CO2 permselectivity of 20, and an overall separation performance surpassing the H2S/CO2 upper bounds.