(741h) Mesoporous Au@SiO2 Core-Shell Nanoparticles for Highly Active Solvent-Free Benzyl Alcohol Oxidation

Mesoporous silica-encapsulated gold core-shell nanoparticles (Au@SiO₂) were synthesized via a bottom-up synthesis to catalyze the selective, solvent-free aerobic oxidation of benzyl alcohol. The pore size distribution, structure, and gold composition of Au@SiO₂ CSNPs were evaluated using N₂ gas adsorption, transmission electron microscopy, inductively-coupled plasma mass spectroscopy. The nanoparticles exhibit a pore size distribution with average diameter of 25.5 Å, a size which enhances selectivity of the smaller, desired product (i.e. benzaldehyde) relative to larger, undesired products (i.e. benzoic acid/benzyl benzoate).

The addition of potassium carbonate during the solvent-free oxidation of benzyl alcohol increased conversion from 17.3% to 60.4% while only decreasing selectivity from 98.7% to 75.0%. The activity on CSNPs far exceeded both a silica-supported and a ceria-supported bare gold nanoparticle control catalyst with similar nanoparticle size and equivalent gold loading (which jumped from negligible activity without potassium carbonate to 10.3% and 4.7% respectively). When allowed to react, silica-supported NPs took 6 times as long in the presence of potassium carbonate to reach comparable conversion to CSNPs, while only achieving 49.4% selectivity. With the only major difference between the catalysts being silica geometry, these results suggest that the pore size distribution within the inert silica shell of CSNPs physically inhibits the formation of undesired products, despite a basic environment which would drastically reduce selectivity under typical conditions. The ceria-supported bare gold nanoparticle control catalyst, known to improve benzyl alcohol oxidation selectivity via strong metal-support interactions, again demonstrated far lower activity to CSNPs, taking 16 times as long to reach comparable conversions. As with selectivity, the unusually high activity of the CSNPs can also be attributed to the mesopore structure, preventing orientations which lead to competitive adsorption. As such, these CSNP particles are a promising platform for analysis of the impact of functionalization on mass transport and surface chemistry discretely.