Theoretical Investigation of the Mechanism, Performance and Kinetic Experimental Phenomena on Brønsted Acidic Ionic Liquids Catalyzed Dehydration of Sorbitol to Isosorbide

Abstract

The dehydration of sorbitol to isosorbide catalyzed by Brønsted acidic ionic liquids (BILs) have been investigated in detail by performing density functional theory (DFT) calculations. It is found that the double dehydration processes involve an intrinsically consistent molecular mechanism: protonation followed by cyclization, and that throughout most of elementary steps, the sulfonic group in the cation of BIL functions as a Brønsted acid/base catalyst, and the anion participates in the reaction as a nucleophile. For 1‐propylsulfonic acid‐3‐methylimidazolium trifluoromethanesulfonate ([C3SO3Hmim]CF3SO3)‐catalyzed reaction, the calculated free energy barrier of the first dehydration step is higher than that of the second dehydration step, rationalizes well the experimentally observed kinetics that the latter has a larger rate constant than the former. In addition, the association energy of [C3SO3Hmim]CF3SO3 with sorbitol is larger than that with either 1,4‐sorbitan or isosorbide, which is consistent with observed association equilibrium constants. BIL carrying a methyl group at the C2 position of imidazolium ring, [C3SO3Hdmim]CF3SO3, shows much better catalytic performance than other tested BILs. The effectiveness of the catalytic system is attributed to the widely distribution of hydrogen‐bonding interactions in transition states and the superior nucleophilicity of the CF3SO3 anion.