Abstract
Glycerol carbonate esters (GCEs), which are valuable biomass-deriv. compds., have been prepd. through the direct esterification of glycerol carbonate and long org. acids with different chain lengths, in the absence of solvent, and with heterogeneous catalysts, including acidic-org. resins, zeolites, and hybrid org.-inorg. acids. The best results, in terms of activity and selectivity towards GCEs, were obtained using a Nafion-silica composite. A full reaction scheme has been established, and it has been demonstrated that an undesired competing reaction results in the generation of glycerol and esters derived from a secondary hydrolysis of the endocyclic ester group, which is attributed to water formed during the esterification reaction. The influence of temp., substrate ratio, catalyst-to-substrate ratio, and the use of solvent has been studied and, under optimized reaction conditions and with the adequate catalyst, it was possible to achieve 95 % selectivity for the desired product at 98 % conversion. It was demonstrated that the reaction rate decreased as the no. of carbon atoms in the linear alkyl chain of the carboxylic acid increased for both p-toluenesulfonic acid and Nafion-silica nanocomposite (Nafion SAC-13) catalysts. After fitting the exptl. data to a mechanistically based kinetic model, the reaction kinetic parameters for Nafion SAC-13 catalysis were detd. and compared for reactions involving different carboxylic acids. A kinetic study showed that the reduced reactivity of carboxylic acids with increasing chain lengths could be explained by inductive as well as steric effects.
