Dehydration of sugars to 5-hydroxymethylfurfural and non-stoichiometric formic and levulinic acids over mesoporous Ta and Ta-W oxide solid acid catalysts

Abstract

A series of mesoporous Ta and Ta-W oxides have been prepared and employed as solid acid catalysts for the dehydration of fructose and glucose to 5-hydroxymethylfurfural (HMF). Solid state 31P MAS NMR spectroscopic results using trimethylphosphine (TMP) as a probe molecule show that the acid strength and the ratio of Brönsted to Lewis acid sites increase gradually with the addition of tungsten in tantalum oxide. It is found that high sugar conversion and HMF selectivity are achieved over catalyst with relatively high ratios of Brönsted to Lewis acid sites. Unexpected stoichiometric excess of formic acid relative to levulinic acid can be observed mainly because of direct decomposition of fructose over Lewis acid sites. The addition of 2-butanol leads to the increase of sugars conversion and the HMF selectivity, especially for the catalyst with high ratio of Brönsted to Lewis acid sites. Among them, Ta7W3 oxide catalyst shows 54% HMF selectivity and good reusability with the addition of 2-butanol by extracting HMF from aqueous phase and removing humins deposed on the surface of the catalyst.