Effective Control of Particle Size and Electron Density of Pd/C and Sn-Pd/C Nanocatalysts for Vanillin Production via Base-Free Oxidation

The effective control of particle size and electron density of metal active sites is challenging yet important for supported nanoparticles, as size effects and promoter effects play vital roles in heterogeneous catalysis on the nanoscale. In this work, we report Pd/C and Sn-Pd/C nanocatalysts for the base-free aerobic oxidation of vanillyl alcohol to vanillin, a challenging reaction not only in the fundamental research of selective oxidation of alcohols but also for the practical transformation of bio-based alcohols to value-added chemicals. We effectively tuned the mean size of Pd nanoparticles from 1.8 to 6.7 nm by varying the temperature used for catalyst preparation and further modified the electron density of the Pd/C catalyst by adding a SnO2 promoter with different loadings. TEM, HAADF-STEM, XPS, CO chemisorption, and in situ DRIFT-IR of CO adsorption characterizations allowed us to get insight into the unique catalytic properties of Pd nanocatalysts. It was conjectured that the base-free aerobic oxidation of vanillyl alcohol to vanillin over the Pd/C catalyst can be a structure-sensitive reaction and the Pd particle size was decisive for the dispersion of Pd, the proportion of catalytically active Pd0 sites, and the intrinsic turnover frequency (iTOF). The 1 wt % Pd/C (1.8 nm) catalyst showed an iTOF value of 268 h–1 and 100% yield to vanillin at 120 °C, 5 bar of O2, and 20 mg of the catalyst within 9 h. We further demonstrated that Sn4+ ions in SnO2 as an electronic promoter can promote Pd/C activity by the formation of highly active, electron-sufficient Pd0 sites which significantly lowered the apparent activation energy of reaction. The 0.1Sn-Pd/C catalyst showed a higher iTOF value of 458 h–1 and a yield of 100% to vanillin at 120 °C, 3 bar of O2 and 15 mg of the catalyst within 6 h. Moreover, we verified a satisfying reusability and an adequate substrate scope over the 0.1Sn-Pd/C catalyst.