Carboxyl-functionalized ionic liquids enable green preparation of chitosan-based ionic gel membranes for H2S separation

Abstract

With the advantages of non-toxicity, biodegradability, renewability, excellent gas affinity and film-forming properties, chitosan (CS), a derivative of the natural biopolymer chitin, has been regarded as an environmentally friendly candidate for fabricating gas separation membranes. However, the extremely strong hydrogen bonding within/between CS molecules results in poor solution processability in neutral solvents, and high membrane crystallinity that is unfavorable for gas permeation. Herein, three carboxyl-functionalized ionic liquids (CFILs) were employed to prepare dense CS-based ionic gel membranes for H₂S separation. These CFILs play the following roles: (1) endowing CS with water dissolubility via carboxyl-amine interactions and thus water processability, (2) facilitating gas diffusion by plasticizing the CS membrane, and (3) affording moderate affinity to H₂S and CO₂. Notably, the optimal H₂S and CO₂ permeability reaches 642 and 287 Barrer, respectively, with the desired H₂S/CH₄ selectivity of 29.1 and (H₂S + CO₂)/CH₄ selectivity of 42.1 achieved. These values rise by 869%, 1048%, 164% and 177%, respectively, compared to those in the CS/acetic acid membrane. Meanwhile, the CS/CFIL membrane can stably and efficiently separate the H₂S/CO₂/CH₄ ternary mixed gas for at least 100 hours. Moreover, the used gel membrane can be recycled via simple dissolution using water, and the re-casted gel membrane is found to almost retain gas separation performance relative to the fresh one. Summarily, this work presents a green method for fabricating recyclable CS-based ionic gel membranes with good separation performance for natural gas purification.