Promotion effect of molten salt hydrate on co-esterification of biomass-derived levulinic and formic acids

Efficient esterification of biomass-derived levulinic acid (LA) to levulinate was investigated in molten salt hydrates (MSH) for the first time. A series of molten salt hydrates (LiBr·3H2O, LiCl·3H2O, ZnCl2·3H2O, CaCl2·3H2O and CaBr2·3H2O) were examined in terms of their catalytic activity and Hammett acidity (H0). Among them, CaBr2·3H2O, ZnBr2·3H2O and ZnCl2·3H2O exhibited excellent activity. It was found that formic acid (FA), a compound that is cogenerated with LA in converting cellulose and hemicellulose as well as their derived sugars, can significantly enhance esterification of LA when reacting in MSHs. Mechanism analysis demonstrated that the promotion effect of FA on the formation of levulinate was resulted from transesterification of LA with butyl formate (BF). Under same conditions, as high as 99.8 % of butyl levulinate (BL) yield was obtained in CaCl2·3H2O, while it was merely 19.6% in the absence of CaCl2·3H2O. Hammett acidity measurement confirmed that CaCl2·3H2O is able to promote the protonation of LA and FA, and thus accelerating the esterification and transesterification reactions. Besides, reacting in CaCl2·3H2O can spontaneously form a biphasic reaction system: an organic phase that mainly consists of BL and n-butanol, and an aqueous phase composed of CaCl2, acids and water. Such a biphasic system help boost the esterification equilibrium and the subsequent separation and purification of the synthesized esters.