(169d) Impact of Mesoporous Silica-Encapsulated Gold Core-Shell Nanoparticle Structure on Solvent-Free Aerobic Benzyl Alcohol Oxidation

In industrial settings, there is a consistent demand for heterogeneous catalysts that are both active and selective. Previous research has shown that encapsulation of gold nanoparticles within sufficiently thin mesoporous silica shells allowed for active, selective oxidation of benzyl alcohol. Importantly, it was found that mesoporous silica shells do not inhibit diffusion of reactants to the gold surface, due to the nanoscale diffusion path. External research has similarly reported a mesopore structure improving catalytic activity by promoting beneficial reactant orientation, despite obstructing the active surface. To demonstrate this effect is present in core-shell nanoparticles, a method for consistently modifying silica shell thickness without impacting other catalyst properties was developed.

Shell thickness was found to increase consistently as a function solution alkalinity. Increasing the concentration of sodium hydroxide also resulted in larger gold core diameters, making controlled studies difficult. To solve this problem, core diameter and shell thickness were decoupled by separating the synthesis into two phases. Surfactant-stabilized gold nanoparticles were first synthesized using a reverse microemulsion synthesis method, commonly used to produce monodisperse gold nanoparticles in the 7 to 10 nm range. These nanoparticles were then added to the synthesis reactor, in place of the unmixed reagents that originally formed the stabilized nanoparticles in situ.

Core-shell nanoparticles with increasing shell thicknesses were used to catalyze solvent-free aerobic benzyl alcohol oxidation. As predicted, nanoparticles with thicker silica shells demonstrated higher catalytic activity on a calculated surface atom basis. In order to compare selectivity, reaction times were extended such that all catalysts reached the same conversion benchmark. Catalysts of all shell thicknesses reached roughly equivalent selectivity towards benzaldehyde, suggesting that pore length is not directly responsible for catalyst selectivity.