Phosphonic acid modifiers for enhancing selective hydrodeoxygenation over Pt catalysts: The role of the catalyst support

Bifunctional catalysts comprised of both noble metal and Bronsted 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 Bronsted 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 Bronsted acid sites present on the modified and unmodified catalysts suggested that PAs improved HDO when they provided new or stronger Bronsted acid sites. Additionally, the strength of the Bronsted 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. (C) 2019 Elsevier Inc. All rights reserved.