Correlation between Acidity and Catalytic Performance of Mesoporous Zirconium Oxophosphate in Phenylglyoxal Conversion

Abstract

The one-step conversion of α-keto aldehydes to α-hydroxy carboxylic acids and their derivatives using solid acids is of great appeal in biomass utilization. In this work, ordered mesoporous zirconium oxophosphate (ZrPO) catalysts with tunable acidity were prepared and their catalytic performance was evaluated using phenylglyoxal (PG) conversion to ethyl mandelate (EM). The roles of Lewis acid sites (LAS) and Brønsted acid sites (BAS) of ZrPO were investigated by kinetic studies of PG conversion combined with temperature-programmed desorption of ammonia (NH3-TPD) and solid-state NMR characterizations. It is found that the ratio of LAS to BAS on the ZrPO plays a dominant role in this reaction. ZrPO-0.75-500, with a LAS/BAS ratio of 2.1, was found to be the best catalyst. The reaction pathways, i.e., the direct isomerization of PG to EM by LAS and via the formation of an intermediate hemiacetal by BAS, are therefore proposed. Moreover, the effect of water on the catalytic activity was studied. A moderate amount of water induced either by catalyst pretreatment at the proper temperature or deliberately dosed on the ZrPO materials achieves a maximum catalytic activity. The highest catalytic activity, i.e., 82% of PG conversion and 92% of EM selectivity, was obtained on ZrPO-0.75-500 that was dosed by 15 μmol water per 50 mg of the catalyst. It is concluded that ZrPO with a suitable combination of LAS and BAS is required to efficiently and selectively catalyze the conversion of α-keto aldehydes to α-hydroxy carboxylic acid derivatives. It is feasible, from a practical point of view, to tune the density of LAS and BAS on the catalysts to achieve a better catalytic performance.