Constructing hierarchical structures of Pd catalysts to realize reaction pathway regulation of furfural hydroconversion

The technique of controlling nanoscale features precisely has been exploited to develop and improve bifunctional catalysts, which are important in the hydroconversion of furfural to high-value chemicals. Such bifunctional hydroconversion catalysts consist of metal sites and acid sites, and the intimacy extent of both active sites dictates the catalytic performance. In-depth exploration of the intimacy extent has been long limited by the lack of material synthesis and characterization method with nanometer precision. Herein, hierarchical catalytic structures with the spatial separation of Pd and acid sites were constructed by the multistep deposition strategy, wherein Pd was confined on defective CeO2 nanoclusters, which were dispersed on high-surface-area SiO2 with the strong acidity. The intimacy extent of Pd and acid sites in hierarchical catalytic structures of Pd/CeO2/SiO2 can be modulated by adjusting the CeO2 content, which leads to the aqueous-phase hydrogenation of furfural following a unique treble-site mechanism. The perpendicular adsorption of furfural on enriched oxygen vacancies of Pd/CeO2 contributes to the selective hydrogenation of the carbonyl group, and then the formed furfuryl alcohol undergoes the acid-catalyzed rearrangement of furan rings on SiO2 through the cooperative effect of acid sites and hydrogen spillover to give cyclopentanone. Meanwhile, the asynchronous catalysis effectively hinders the furan ring hydrogenation to tetrahydrofurfuryl alcohol by spatially separating Pd from the SiO2 surface. The superior catalytic performance is obtained on Pd/CeO2/SiO2-10 with a furfural conversion of 93% and cyclopentanone selectivity of 84%. We anticipate that the strategy of constructing hierarchical catalytic structures demonstrated here to spatially organize different active sites at the nanoscale will boost the further development of multifunctional catalysts.