Active, Selective, and Recyclable Zr(SO4)2/SiO2 and Zr(SO4)2/Activated Carbon Solid Acid Catalysts for Esterification of Malic Acid to Dimethyl Malate

Abstract

The esterification of malic acid using traditional homogenous catalysts suffers from the difficulty in reuse of the catalyst and undesirable side reactions. In this work, Zr(SO4)2/SiO2 and Zr(SO4)2/activated carbon (AC) as solid acid catalysts were prepared for malic acid esterification with methanol. The conversion of malic acid over these two catalysts is comparable to that over H2SO4 and unsupported Zr(SO4)2∙4H2O catalysts; however; a 99% selectivity of dimethyl malate can be realized on these two supported catalysts, which is much higher than that of conventional H2SO4 (75%) and unsupported Zr(SO4)2∙4H2O (80%) catalysts, highlighting the critical role of AC and SiO2 supports in tuning the selectivity. We suggest that the surface hydroxyls of AC or lattice O2− ions from SiO2 donate electrons to Zr4+ in Zr(SO4)2/AC and Zr(SO4)2/SiO2 catalysts, which results in the increase in electron density on Zr4+. The enhanced electron density on Zr4+ reduces the degree of H delocalization from crystal water and then decreases the Brønsted acid strength. Consequently, the reduced Brønsted acid strength of Zr(SO4)2/AC and Zr(SO4)2/SiO2 catalysts suppresses the intermolecular dehydration side reaction. In addition, these two supported catalysts can be easily separated from the reaction system by simple filtration with almost no loss of activity.