Heteropolyacid supported on the composite of bentonite and ionic liquid containing acidic polymer: A highly selective catalyst for glycerol acetalization to solketal

Abstract

Acetalization of glycerol that is the by-product of biodiesel to fuel additives has received considerable attention. However, formation of a mixture of products in this acid-catalysed process is a challenging issue. In this research, a highly selective catalyst is devised that can selectively acetalize glycerol to solketal. To prepare the catalyst, bentonite clay was first decorated with an acidic polymer containing sulfonic acid and carboxylic acid functional groups. Then, the carboxylic acid groups of the polymer were changed to ionic liquids through reaction with triethanolamine. Finally, phosphotungstic acid was immobilized on the bentonite-polymer composite and the resultant catalyst was characterized. To achieve the highest catalytic activity, the catalyst amount and phosphotungstic acid loading have been optimized. It was found that 10 wt.% catalyst with 20 wt.% phosphotungstic acid loading led to 100% conversion and 99% yield to solketal under solvent-free condition at 55 °C. Moreover, the catalyst could be recycled up to four reaction runs without significant loss of its activity. Comparative studies with ionic liquid-free control catalyst approved that the presence of ionic liquid in the structure of the catalyst played an important role in the selectivity of the catalyst and ionic liquid-free counterpart led to less selectivity and formation of 2,2-dimethyl-1,3-dioxane-5-ol as by-product.