5-Hydroxymethylfurfural production from hexose sugars using adjustable acid- and base-functionalized mesoporous SBA-15 catalysts in aqueous media

The conversion of glucose and fructose to 5-hydroxymethylfurfural (HMF) was investigated using a series of functionalized SBA-15 catalysts in a glass tube reactor. The synthesized SBA-15 as a support was grafted with various organosilanes in order to acquire the acid-modified SBA-15 (SBA-A), acid- and hydrophobicity-modified SBA-15 (SBA-AC), and base-modified SBA-15 (SBA-N) catalysts. Effects of the reaction temperature and time, reactant concentration, and catalyst loading level on the catalyst performances in this reaction, in terms of the hexose conversion level, product yield, and selectivity, were evaluated. The prepared catalysts were extensively characterized by X-ray diffractometry, nitrogen adsorption–desorption, Fourier transform infrared spectrometry, CHN/S elemental analysis, solid-state nuclear magnetic resonance spectroscopy, contact angle measurement, and acid–base titration. The hexose-to-HMF reaction was greatly enhanced by increasing the reaction temperature, whereas the other factors had only a minor effect. In the case of the glucose-to-HMF production, the SBA-N catalyst exhibited reasonably high levels of glucose conversion and HMF yield, but its catalytic activity was pretty low for the fructose-to-HMF reaction. Herein, fructose was identified as an intermediate substance for further dehydration to HMF. The different catalyst types provided different functions, where isomerization required a basic property, whereas dehydration was optimal over an acidic one. Suppression of the levulinic acid formation was obtained via an increased hydrophobicity of the SBA-AC catalyst.