Sustainable synthesis of vanillin through base-free selective oxidation using synergistic AgPd nanoparticles loaded on ZrO2

Abstract

The synergistic effect between bimetallic nanoparticles, as well as the support effect, is particularly important for supported catalysts in heterogeneous catalysis. In this work, an AgPd/ZrO₂ bimetallic catalyst was reported for the base-free aerobic oxidation of vanillyl alcohol to vanillin, a challenging transformation of an aromatic bio-alcohol into a valuable fine chemical. The crystalline structure and textural properties of ZrO₂ were modulated by using different zirconium precursors and precipitants during the precipitation process. The ZrO₂ obtained from zirconium nitrate and trimethylamine was found to be the most suitable support. It displayed abundant reactive oxygen species and the largest specific surface area. Then, an Ag promoter was added to the Pd/ZrO₂ catalyst and the synergy between Ag and Pd was tuned by varying their ratios. The Ag_0.5Pd₁/ZrO₂ catalyst showed an enhanced performance, i.e., 100% conversion of vanillyl alcohol and 95% selectivity to vanillin at 120 °C and 3 bar O₂ within 4 h, in comparison with the Pd/ZrO₂ catalyst (69% and 87%, respectively). Moreover, the vanillin productivity reached 133.0 mol mol⁻¹ h⁻¹, the best result reported in the literature to date. Various characterization methods demonstrated that the addition of Ag can reduce the mean size of Pd nanoparticles (4.9 nm vs. 3.8 nm), and Ag⁺ species were beneficial to stabilizing Pd⁰ species in a high fraction due to electron transfer. The reactive Pd⁰ sites closely interacting with the Ag promoter allowed lowering of the apparent activation energy of the reaction (27.3 vs. 37.5 kJ mol⁻¹). In addition, the Ag_0.5Pd₁/ZrO₂ catalyst exhibited superior stability during consecutive recycling uses owing to the strong metal–support interactions.