Silica support modifications to enhance Pd-catalyzed deoxygenation of stearic acid

The catalytic deoxygenation of fatty acids has recently received significant attention as a low-hydrogen approach to biomass feedstock deoxygenation for production of hydrocarbon fuels with superior properties to biodiesel. Unfortunately, it is a challenging reaction to push to high yields. Of typical catalysts, Pd/C is typically reported to give the best performance, while most oxide supports are inferior, with exceptions for very specific preparation and pre-treatment protocols. Here, we investigate the role of organosilane-modified silicas as supports for the Pd-catalyzed deoxygenation of stearic acid at 300 °C under inert atmosphere. Comparing aminopropylsilane-modified, phenylsilane-modified, and unmodified silica supports with Pd incorporated by several methods, it is first shown that changes in dispersion alone do not account for improvements in deoxygenation yields. Capping silanols with phenylsilane is also ineffective on its own in improving deoxygenation yields. The most effective treatment is shown to be a co-deposition of phenylsilane and aminopropylsilane before Pd incipient wetness impregnation, followed by direct reduction of the catalyst, which gives heptadecane yields >85%, exceeding even the productivity of Pd/C. These results demonstrate that basic, aromatic-rich surfaces are accessible through organosilane grafting and that these surfaces can control Pd particle sizes and the adsorption of stearic acid and products. This work improves our understanding of support effects for biomass feedstock deoxygenation catalysts and could help design new catalysts that take advantage of modified inorganic supports.