Hydrothermally stable heterogeneous catalysts for conversion of biorenewables
The catalytic conversion of biomass-derived molecules to fuels and chemicals involves reactions carried out in the aqueous phase. The corrosive effects of the reactive environment can cause degradation of heterogeneous catalysts, but the detrimental effects depend on the state of water. For example, water vapor, superheated steam and sub- and supercritical liquid water can behave very different from each other. In this review, we focus on the hydrothermal stability of the heterogeneous catalysts in order of the increasing severity of the reaction medium: superheated steam, liquid water at temperatures below 200 °C, liquid water at temperatures above 200 °C and supercritical water. This review addresses changes in the physical structure of heterogeneous catalysts used for biomass conversion reactions. These physical changes influence the catalytic performance, but other causes for deactivation include sintering of the metal phase or coking or carbon deposition on catalysts. The latter phenomena are not the primary focus of this review. We also describe recent approaches designed to improve the hydrothermal stability of heterogeneous catalysts in biomass conversion reactions.