Phosphonic Acid Modifiers for Enhancing Selective Hydrodeoxygenation Over Pt Catalysts: The Role of the Catalyst Support

Bifunctional catalysts comprised of both noble metal and Brønsted acid sites are of growing interest for many reactions, such as the hydrodeoxygenation (HDO) of oxygenates produced by the deconstruction of lignocellulosic biomass. One method of preparing a bifunctional metal-acid catalyst is to modify the supporting material of a metal catalyst with acid-containing ligands, such as phosphonic acids (PAs), which provide tunable Brønsted acid sites at the metal-support interface. Here, we explore the potential for PA modification to improve HDO rates on Al₂O₃, TiO₂, CeO₂, and SiO₂-Al₂O₃ supports. PAs containing either alkyl or carboxylic acid (CA) tails were used to modify Pt catalysts on each support. PA modification improved HDO rates for the model compound benzyl alcohol when applied to Pt supported by Al₂O₃, TiO₂, and CeO₂, but had a negative effect on the HDO rate of Pt/SiO₂-Al₂O₃. Measurements of the relative strength of Brønsted acid sites present on the modified and unmodified catalysts suggested that PAs improved HDO when they provided new or stronger Brønsted acid sites. Additionally, the strength of the Brønsted acid sites provided by PA modifiers were tunable by altering the tail functionality, which affected the rates of HDO. These results suggest that PAs can be used to modify a variety of supports to prepare bifunctional acid-metal catalysts.