(329e) Improving the Stability of Pt/Al2O3 Catalysts for Biomass Conversion in Liquid Water

The limited hydrothermal stability of many supported metal catalysts is a major concern for the development of heterogeneously catalyzed processes for biomass conversion. For example, alumina supports are converted into boehmite when exposed to hot liquid water. This transformation results in losses of surface area, acid site concentration, and metal dispersion. The formation of boehmite starts by the reaction of water molecules with the hydroxyl groups of alumina.

This transformation can be prevented when the hydroxyl groups are capped. For example, silylation with tetraethyl orthosilicate followed by calcination provided a silica coating. Although some of the deposited silicon is leached in an aqueous environment, boehmite formation was not observed. ²⁹Si MAS NMR spectroscopy was used to confirm the formation of stable Al-O-Si domains. In addition to the stabilizing effect, the silica species also increased the H₂ yield in aqueous phase reforming. 

A stabilizing effect was also observed when polyols were present in the aqueous medium. These compounds chemisorb to surface hydroxyl groups of alumina. The resulting layer of carbonaceous deposits inhibits hydrolytic attack while high surface area of the catalyst is retained. The stabilizing effect becomes more pronounced for larger polyols (e.g. sorbitol) because these compounds can form additional bonds with the surface. Although some blockage of metal sites is observed, the sufficient fraction of the metal surface area remains accessible to convert reactants.