Hydrophobic microporous and mesoporous oxides as Brønsted and Lewis acid catalysts for biomass conversion in liquid water

Abstract

The use of heterogeneous catalysts in liquid water, even at the moderate temperatures (<523 K) typical of most condensed-phase biomass conversion processes, is often fraught with issues related to structural instability and to active site inhibition caused by deactivation mechanisms that differ from those prevalent in the gas phase at higher temperatures. For porous silica-based oxides, one strategy to address these issues is to design or functionalize oxide surfaces with hydrophobic moieties or domains. Hydrophobic moieties can be present either at external crystallite surfaces or within the internal porous voids where most active sites typically reside. Both extracrystalline and intracrystalline hydrophobic environments can prevent the condensation of bulk water within internal void spaces and thus alleviate any transport restrictions its presence may cause, while only intracrystalline environments can influence the kinetic effects of molecular water at active sites. As a result, hydrophobic environments at both external and internal crystallite surfaces can have fundamentally different consequences for reactivity, in spite of the phenomenological similarities of their effects on observed reaction rates. The conceptual distinction between these two forms of hydrophobicity, together with accurate assessments of transport and kinetic contributions to measured reaction rates, can inform the placement of hydrophobic domains at appropriate locations in porous solids to cause predictable changes in reactivity. This mini-review discusses these concepts within the context of recent studies that have used hydrophobic Brønsted and Lewis acidic microporous and mesoporous oxides in catalytic reactions of biomass-derived molecules in liquid water and biphasic water–organic mixtures.