(509cl) Tuning the Activity of Benzyl Alcohol Hydrodeoxygenation Using Organic Ligands

Hydrodeoxygenation (HDO) is a crucial reaction in biomass upgrading to transform bio-oil into usable fuels and value-added chemicals. Recently, phosphonic acid (PA) self-assembled monolayers (SAMs) have been shown to increase the selectivity for HDO of aromatic oxygenates through the addition of Brønsted acidity at the interfacial sites on supported metal catalysts. However, the traditional method of SAM deposition blocks active sites on metals like Pd, thereby limiting overall catalytic performance. In this work, a novel SAM deposition technique, referred to as “reverse deposition,” was investigated to improve activity by freeing active sites on the surface, while maintaining selectivity benefits. Furthermore, the study of different methods of surface modification can provide insights in the structure-activity relations in HDO catalysis.

Catalysts were prepared via reverse deposition (Fig 1) by precoating the metal oxide support with PA SAMs prior to performing incipient wetness impregnation to deposit metal nanoparticles. Reactor studies (Fig 2) showed that Pd/Al₂O₃ catalysts modified with methyl phosphonic acid (MPA), chloromethyl phosphonic acid (ClMPA), and aminomethyl phosphonic acid (NH₂MPA) increased benzyl alcohol HDO activity and selectivity over unmodified 5%Pd/Al₂O₃ and 1%Pd/Al₂O₃. The improvements were attributed to Brønsted acid sites introduced in the PA ligands. Additionally, for all SAMs tested, catalysts prepared by reverse deposition outperformed traditionally modified catalysts with respect to the normalized rate of production per mass of Pd. The increase in activity for reverse deposition is attributed to greater access to Pd active sites. CO chemisorption results show that traditional deposition significantly decreased available Pd surface area; however, the surface area from reverse deposition was similar to that from unmodified Pd/Al₂O₃, providing evidence that active sites are no longer being blocked. Analysis of infrared spectra after CO adsorption provides additional insight into the effect PA ligands and deposition techniques have on the particle size distribution and metal site availability.