Cs exchanged 12-tungstophosphoric acid supported on high-silica mesoporous Y zeolites for synthesis of ethyl lactate via catalytic esterification

Ethyl lactate is a promising green solvent, which can be produced via esterification of biomass derived lactic acid and ethanol. Herein, we report the development of highly active and stable polyoxometalates (POMs) supported on zeolite catalysts for synthesis of ethyl lactate. Repetitive stability tests were performed using the developed catalysts, and comparative characterization of the fresh and used catalysts was performed to understand the changes in the physicochemical properties of the catalysts. It was found that low-silica Y zeolites (silicon-to-aluminum ratio, SAR, of 2 and 5) are prone to be dealuminated (by lactic acid) under the reaction conditions, and thus being not suitable to be used as the catalyst support. Direct support of 12-tungstophosphoric acid (by impregnation) on high-silica Y zeolite (SAR40) led to deactivation during catalysis due to 12-tungstophosphoric acid leaching. This issue was solved by surface modification of the high-silica Y zeolite using cesium (Cs) ions before impregnation of 12-tungstophosphoric acid because the formed cesium phosphotungstate is insoluble in the reaction. As a result, a stable composite catalyst of Cs exchanged 12-tungstophosphoric acid supported on high-silica mesoporous Y was developed, which showed good reusability. In detail, after seven cyclic tests, the physiochemical properties of the catalyst and the catalytic performance remained relatively stable, e.g., the yield of ethyl lactate remained at around 29.6%. Findings of the study show that the stability of the zeolitic support and the support heteropolyacid are equally important for obtaining a stable catalyst for ethyl lactate synthesis via esterification.