Interest in liquid-phase reactions over heterogeneous catalysts is growing rapidly, partially because of the desire to find efficient methods for biomass conversion to renewable fuels and chemicals. The presence of a solvent can affect reactions at surfaces by competing with reactants and products for adsorption sites and solvating adsorbed species. Mass transport limitations can also have a pronounced effect on liquid-phase reaction rates. Because many heterogeneous catalysts were designed to be stable under gas-phase reaction conditions, their operation in liquid reaction media at moderately elevated temperatures can result in unexpected structural changes. In some cases, components derived from the evolving catalyst contribute significantly to the catalytic activity. Solvents, as well as byproducts from biomass feedstocks, can also act as homogeneous catalysts to alter the intrinsic reactivity of the heterogeneous catalyst. In this contribution, we discuss each of these phenomena and provide illustrative examples.

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.