Cost-effective fixation of CO2 into cyclic carbonates catalyzed by Zn bromide-based deep eutectic ionic liquid

Abstract

In this work, we designed a novel thermally stable Zn bromide-based deep eutectic ionic liquid (DEIL), synthesized from tetralkylphosphonium bromide (Bu4PBr) and ZnBr2 by a facile heating method. The structural changes and the physical properties of the catalyst were in-depth investigated. Among all the DEILs, ZnBr2·2Bu4PBr showed high activity, achieved 98.2% propylene carbonate (PC) yield at a low catalyst usage (0.15 mol%), and remained good stabilities for 8 cycles. Furthermore, based on experimental results and DFT calculations, we found that Bu4P+ cation of DEIL served as a frame charge-balanced species interacting with three possible anions of ZnBr42−, ZnBr3− and Br−. The interaction is accompanied by a reversible pre-equilibrium in which [Bu4P]2[ZnBr4] could dissociate and produce the true catalytically active species, [Bu4P][ZnBr3] and Bu4PBr. Meanwhile, a possible ring-opening mechanism was proposed, in which the synergetic effect of cations and anions in the catalytic system can promote ring-opening. The significance of this study is that it provides guidance for developing metal bromide-based DEILs for carbonate synthesis.