Constructing ether-rich and carboxylate hydrogen bonding sites in protic ionic liquids for efficient and simultaneous membrane separation of H2S and CO2 from CH4

Abstract

Removing H2S and CO2 is of great significance for natural gas purification. With excellent gas affinity and tunable structure, ionic liquids (ILs) have been regarded as nontrivial candidates for fabricating polymer-based membranes. Herein, we firstly reported the incorporation of protic ILs (PILs) having ether-rich and carboxylate sites (ECPILs) into poly(ether-block-amide) (Pebax) matrix for efficient separation H2S and CO2 from CH4. Notably, the optimal permeability of H2S reaches up to 4310 Barrer (40 °C, 0.50 bar) in Pebax/ECPIL membranes, along with H2S CH4-1 and (H2S + CO2) CH4-1 selectivity of 97.7 and 112.3, respectively. These values are increased by 1125%, 160.8% and 145.9% compared to those in neat Pebax membrane. Additionally, the solubility and diffusion coefficients of the gases were measured, demonstrating that ECPIL can simultaneously strengthen the dissolution and diffusion of H2S and CO2, thus elevating the permeability and permselectivity. By using quantum chemical calculations and FT-IR spectroscopy, the highly reversible multi-site hydrogen bonding interaction between ECPILs and H2S was revealed, which is responsible for the fast permeation of H2S and good selectivity. Furthermore, H2S CO2-1 CH4-1 (3/3/94 mol%) ternary mixed gas can be efficiently and stably separated by Pebax/ECPIL membrane for at least 100 h. Overall, this work not only illustrates that PILs with ether-rich and carboxylate hydrogen bonding sites are outstanding materials for simultaneous removal of H2S and CO2, but may also provide a novel insight into the design of membrane materials for natural gas upgrading.