One-pot hydrothermal synthesis of ultrafine Pd clusters within Beta zeolite for selective oxidation of alcohols†
Highly dispersive and ultrafine Pd clusters were straightforwardly encapsulated within the crystals of Beta zeolite. This series of Pd@Beta material was hydrothermally synthesized by performing a unique acid co-hydrolysis of silica and Pd salt precursors in the early gelation stage, by virtue of which the interaction between Si and Pd species was strengthened to inhibit the potential phase separation in the following high pH alkali crystallization. The ultrafine Pd clusters within the wall of the zeolite framework enabled the catalysis to occur in the microchannels, resulting in a high and recyclable yield of above 95% and a large turnover number of ∼50 000 in the oxidation of benzyl alcohol under an O2 atmosphere in water, which can be extended to other microchannel-diffusive aromatic alcohols. Moreover, the catalyst also showed high activity in the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA). The excellent activity is attributable to the synergistic effect of Pd clusters and preferential substrate adsorption behavior. Shape-selective catalysis and stable reusability were achieved. This result provides a novel straightforward synthesis route for fabricating zeolite encapsulated noble metal clusters that effectively and stably catalyze aerobic organic oxidation reactions with shape-selectivity.