Formation of 5-(Hydroxymethyl)furfural by Stepwise Dehydration over TiO₂ with Water-Tolerant Lewis Acid Sites

Abstract

The reaction mechanism for the formation of 5-(hydroxymethyl)furfural (HMF) from glucose in water over TiO2 and phosphate-immobilized TiO2 (phosphate/TiO2) with water-tolerant Lewis acid sites was studied using isotopically labeled molecules and 13C nuclear magnetic resonance measurements for glucose adsorbed on TiO2. Scandium trifluoromethanesulfonate (Sc(OTf)3), a highly active homogeneous Lewis acid catalyst workable in water, converts glucose into HMF through aldose–ketose isomerization between glucose and fructose involving a hydrogen transfer step and subsequent dehydration of fructose. In contrast to Sc(OTf)3, Lewis acid sites on bare TiO2 and phosphate/TiO2 do not form HMF through the isomerization–dehydration route but through the stepwise dehydration of glucose via 3-deoxyglucosone as an intermediate. Continuous extraction of the evolved HMF with 2-sec-butylphenol results in the increase in the HMF selectivity for phosphate/TiO2, even in highly concentrated glucose solution. These results suggest that limiting the reactions between HMF and the surface intermediates improves the efficiency of HMF production.