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.
Highlights
Production of chemicals from biomass resources is a promising strategy for solving the problem of the depletion of fossil fuels
The Zr amounts in Zr(SO4)2/SiO2 and Zr(SO4)2/activated carbon (AC) catalysts were detected by inductive coupled plasma optical emission spectroscopy (ICP-OES) analysis, and the active species by inductive coupled plasma optical emission spectroscopy (ICP-OES) analysis, and the active
)2/AC and Zr(SO4)2/SiO2 catalysts, resulting in the decrease in electron density oxygen and the increase in electron density on Zr, which reduces the degree of H delocalization from crystal water and decreases the B acid strength
Summary
Production of chemicals from biomass resources is a promising strategy for solving the problem of the depletion of fossil fuels. Converting renewable biomass-derived malic acid to various malic acid esters has attracted considerable attention because they are widely used in the production of spices, mosquito-repellent incenses, and intermediates of medicines [1,2,3]. Homogenous (liquid) acids show excellent catalytic activity for malic acid esterification, they present two main drawbacks: (1) These liquid catalysts are difficult to separate from the reaction mediums, and inevitably suffer from the problem of non-recyclability and environmental unfriendliness; (2) excess high-acid strength will cause undesirable side reactions, among which mainly include intramolecular dehydration and intermolecular dehydration of alcohols. Heterogeneous (solid) acid catalysts can overcome the un-recyclability of liquid acid and are, strongly recommended
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