Highly effective esterification of lignin to produce a pharmaceutical intermediate using novel silica mesoporous molecular sieves as catalysts

Abstract

The use of lignin to prepare fine chemicals is a hot research topic. Here, we propose a strategy for esterification of the syringyl group, which is abundant in lignin, to produce 3,4-dimethoxyphenyl acetic acid ester. Lignin reacts with alcohols over acidic pure silica catalyst, L-MCM-41, to generate 3,4-dimethoxyphenyl acetic acid esters. The esterification yield from alkali lignin (Indulin AT) reached more than 60%. Furthermore, L-MCM-41 was easily and quickly prepared using hydroxyl free radicals. The obtained L-MCM-41 exhibited enhanced Lewis acidity and stability compared to silica MCM-41 derived from a traditional hydrothermal system. The enhanced Lewis acidity of the surface silicon atoms of L-MCM-41 can be attributed to the electron-withdrawing effect of abundant geminal hydroxyl groups that they bear. Based on spectroscopic data and the concentration of hydroxyl free radicals, the mechanism underpinning the enhanced acidity and stability of the mesoporous silica molecular sieves is proposed. This work enables production of fine chemicals using lignin as feedstock and provides a reliable and simple method for the synthesis of strongly acidic mesoporous silica molecular sieves.