Crystal facet engineering of metal oxides for upgrading biomass-derived oxygenates: a perspective

Abstract

Coordinatively unsaturated metal cations and oxygen anions on the surface of amphoteric metal oxides serve as acid and base centers, respectively, enabling a number of acid/base-catalyzed reactions. In particular, the O atom of oxygenates can readily coordinate with surface metal cations and can therefore initiate various reactions that are important for upgrading biomass-derived oxygenates. Metal oxide catalysts prepared via conventional methods expose various facets with distinct properties, making it difficult to elucidate the reaction chemistry and mechanism mediated on metal oxides at an atomic level. Advances in synthesis of nanocrystals of metal oxides with predominantly exposing a certain facet allow one to explore the reaction chemistry on well-defined metal oxide facets under real reaction conditions and thus may provide a powerful approach to investigate the facet-dependent structure–activity relationships of metal oxide-mediated reactions. In this perspective, we first elaborated on the fine structure and properties of various facets of TiO₂ and CeO₂ and then surveyed the dehydration/dehydrogenation of alcohols, ketonization of carboxylic acids, and aldol condensation of ketones and aldehydes mediated by metal oxides. In particular, we discussed the relationship between catalytic performances and the facet-dependent acid–base property and fine surface geometrical structure. Finally, we described challenges and opportunities associated with future research on metal oxide-mediated reactions via crystal facet engineering.