Valorisation of xylose to lactic acid on morphology-controlled ZnO catalysts

Abstract

The aqueous-phase catalytic conversion of xylose was firstly investigated on amphoteric oxide catalysts (ZnO, TiO₂, SnO₂, ZrO₂, Y₂O₃ and Al₂O₃) in a batch reactor. It was found that the global activity is related to the catalyst acidity, and the stronger acid sites are responsible for pentose dehydration to furfural while the weaker ones are involved in sugar retro-aldol condensation to lower carbon chemicals. The distinguished performance of the ZnO catalyst in the conversion of xylose to lactic acid motivated the exploitation of its acid/base properties as dictated by its morphology. Nanoparticles, polyhedra, rods and dandelion-like ZnO catalysts were synthesized and distinguishable physico-chemical properties were defined, as assessed by XRD, FE-SEM and XPS. The results disclosed that the polyhedral morphology provided the most active catalyst whereas the product distribution only changed slightly according to the catalyst morphology. It was seen that the activity of ZnO catalysts is profoundly affected by the strength of the basic sites: the higher the concentration of strong O²⁻ basic sites, the higher the xylose conversion. A bifunctional acid–base reaction mechanism is suggested at which the O²⁻ centres are particularly involved in the retro-aldol condensation reaction.