Enhanced Hydrothermal Stability of γ-Al₂O₃ Catalyst Supports with Alkyl Phosphonate Coatings

In this study, monolayers formed from organophosphonic acids were employed to stabilize porous γ-Al₂O₃, both as a single component and as a support for Pt nanoparticle catalysts, during exposure to hydrothermal conditions. To provide a baseline, structural changes of uncoated γ-Al₂O₃ catalysts under model aqueous phase reforming conditions (liquid water at 200 °C and autogenic pressure) were examined over the course of 20 h. These changes were characterized by X-ray diffraction, NMR spectroscopy, N₂ physisorption, and IR spectroscopy. It was demonstrated that γ-alumina was rapidly converted into a hydrated boehmite (AlOOH) phase with significantly decreased surface area. Deposition of alkyl phosphonate groups on γ-alumina drastically inhibited the formation of boehmite, thereby maintaining its high specific surface area over 20 h of treatment. ²⁷Al MAS NMR spectra demonstrated that hydrothermal stability increased with alkyl tail length despite lower P coverages. Although the inhibition of boehmite formation by the phosphonic acids was attributed primarily to the formation of Al₂O₃-PO _x bonds, it was found that use of longer-chain octadecylphosphonic acids led to the most pronounced effect. Phosphonate coatings on Pt/γ-Al₂O₃ improved stability without adversely affecting the rate of a model reaction, catalytic hydrogenation of 1-hexene.