PTFE supported gold nanoparticles as photocatalysts for oxidative esterification of aldehydes

Homogeneous, small gold nanoparticles (d = 1.87 nm) have been prepared by photochemical reduction of HAuCl4 in the presence of Irgacure® 2959 under high power UV irradiation at 365 nm produced by a LED source. These particles have been deposited on PTFE microbeads (d = 200 μm) and evaluated for the catalysis of the oxidative esterification of aldehydes in the presence of H2O2. Under green light (λ ≈ 530 nm) and repeated addition of H2O2, the catalytic system is accelerated and achieves complete conversions in minutes. Furthermore the catalyst can be recycled ten times consecutively without any activity loss by adding a step for AuNP recycling. The mechanism of the reaction follows a zero rate order, and Hammett free energy relationships for substituted benzaldehydes afforded a positive ρ value (ρ = 2.35) demonstrating that the initial hemiacetalisation equilibrium (ρ = 2.31) is the rate determining step. Kinetics data are in agreement with an Eley-Rideal type mechanism and permits proposing a reaction mechanism.