Efficient separation of ethylene/ethane by incorporation of silver salts into protic imidazole ionic liquids

Efficient purification of ethylene (C2H4) from ethane (C2H6) is one of the seven chemical separations which can change the world. Chemical absorption is considered as the most promising method for C2H4/C2H6 separation. However, the two critical separation properties of absorbent, the solubility and selectivity, are always trade-offs, which are still needed to both promote and balance in the viewpoint of industrial application. Herein, a series of novel silver-containing protic imidazolium ionic liquids (SPILs), with high chemical activity for efficient C2H4 absorption, have been designed and synthesized. The physical properties of the prepared silver-containing ionic liquids (SILs) were first characterized. Then the separation performances of C2H4/C2H6 by SPILs and silver -containing aprotic ionic liquids (SAILs) were carefully compared. The C2H4/C2H6 selectivity of SPILs (16.73) was eight times higher than the value of the latter. Then, the C2H4 absorption mechanism was confirmed by density functional theory (DFT) calculations. The charge density of Ag+ was greater and the Gibbs free energy (Delta G) of C2H4 absorption was more negative in [Bim]n[Ag(NTf2)n+1]. The proposed SPILs, [Bim]1.5[Ag(NTf2)2.5], had a high C2H4/C2H6 selectivity of 41.2 and C2H4 solubility of 1.86 mol C2H4/mol Ag+, which were the highest value of ionic liquids (ILs) absorption reported so far. The [Bim]1.5[Ag(NTf2)2.5] exhibited excellent reversibility after five absorption-desorption cycles. An absorbent designed strategy of SPILs was proposed with enhanced solubility and selectivity for C2H4/C2H6 separation.