SBA-15 as a support for palladium in the direct synthesis of H2O2 from H2 and O2

The behavior of a catalyst prepared by deposition of Pd nanoparticles over a mesoporous SBA-15 support is compared with that of Pd supported over commercial silica in the direct synthesis of H2O2 from H2 and O2. The activity was studied in a batch and semi-batch autoclave operating at r.t. and at pressures of 6.5 bar using methanol as the solvent in the presence or absence of CO2. The use of CO2-expanded methanol leads to a significant increase in the rate of H2O2 synthesis of both Pd–SiO2 and Pd–SBA-15 samples, especially in the latter. The effect derives from an increased solubility of H2 and O2 which increases the reaction rate, but also determines a relocation of Pd particles which migrate inside the mesopore channels of SBA-15. For the longer times of reaction, elongated Pd particles form which probably partially inhibit the reactant diffusion. This determines a decrease of the productivity which, however, at the maximum is about 2.5 times higher than that of the analogous Pd–SiO2 sample in semi-batch continuous tests. The use of CO2-expanded methanol leads also to an enhanced rate of H2O2 decomposition, particularly in Pd–SBA-15 sample where together with the H2O2 hydrogenolysis an additional path due to H2O2 decomposition catalyzed by weak acid groups (silanols) is present. The addition of pyridine to the solution strongly inhibits both reactions, but also the synthesis of H2O2 due probably to the strong chemisorption over the Pd particles. The grafting with Ti of the inner walls of SBA-15 channels to decrease the amount of silanol groups has a positive effect to control the decomposition of H2O2 catalyzed by these groups.