Role of acid sites and selectivity correlation in solvent free liquid phase dehydration of sorbitol to isosorbide

A number of Bronsted acids (methanesulfonic acid, p-toluene sulfonic acid, triflic acid, sulfamic acid, citric acid, NaHSO4, and boric acid) and Lewis acids (metal sulfate/triflates) were employed in solvent free dehydration of sorbitol and their influence on anhydroalcohols selectivity has been investigated. The outcome indicated that all the acid catalysts produced first mono-dehydrated product sorbitan followed by second dehydration of 1,4-sorbitan to isosorbide. However, the formation and yield isosorbide were found to depend on the nature of acid sites and their acidic strength. The Bronsted acids are more efficient to convert sorbitol to isosorbide than Lewis acids. The Bronsted acids having lower pK(a) value (i.e. strong acid) exhibited high catalytic activity as well as yield of isosorbide. In the case of Lewis acids, the catalytic activity and selectivity were radically depended on which metal used and their stability during the reaction. The water formed during reaction induced Bronsted acidity on Lewis acid metal site. The Lewis-assisted Bronsted acid site enabled high yield of isosorbide up to 70% at moderate temperature (160 degrees C). (C) 2014 Elsevier B.V. All rights reserved.