Efficient conversion of cellulose to 5-hydroxymethylfurfural using a bifunctional hydrophobic SBA-15 catalyst: The effects of hydrophobicity, morphology and acidity

Owing to the negative impact of water on catalyst activity, and the consequent side reactions, the hydrophobicity and acidity of the catalyst are important factors in the conversion of cellulose to 5-hydroxymethylfurfural in an aqueous solvent. In the present study, we prepared SBA-15 catalysts comprising hydrophobic methoxy groups, phosphotungstic acid as Lewis acid groups, and -SO3H Bronsted acid groups. The introduction of methoxy groups increased the contact angle of the catalyst from 57.42 to 87.80 degrees and changed its morphology from stubby short rods to long curved sticks. The maximum 5-hydroxymethylfurfural yield (61.24%) was obtained using 2SBA-15SO(3)H, which comprised 237.31 mu mol center dot g(-1) of the Bronsted acid and 14.44 mu mol center dot g(-1) of the Lewis acid and had a contact angle of 75.93 degrees The low yields of levulinic acid and formic acid (below 10%) confirmed the resistance of such SBA-15 catalysts to water. Furthermore, adequate hydrophobicity facilitated glucose isomerization, whereas the catalyst morphology, which tended to comprise short rods, improved the fructose dehydration rate. Density functional theory analysis of the acidity sites route revealed a 3-DG pathway for 1,4,5,6-tetrahydroxyhexan-2-one generation with Lewis acid sites, and a free energy of 0.13 eV, paralleled by glucose isomerization to fructose through a 1,2-enediol.