Nanoscale intimacy in hierarchical catalytic structures for mediating furfural hydroconversion over Pd/CoxOy/SiAlOz

Precisely controlling nanoscale metal-acid intimacy of bifunctional catalysts is increasingly being exploited to improve the performance toward the tandem reactions. It remains a critical challenge to rationally design the spatial organization of dual-functional sites due to the lack of effective synthesis and material characterization methods at the nanoscale. Here we impose hierarchical structures into bifunctional catalysts, wherein Pd particles are controllably deposited on cobalt oxide (CoxOy) nanoglues, which themselves are dispersed on high-surface-area and acid silica-alumina (SiAlOz). The hydroconversion of furfural demonstrates that the catalytic performance is optimized by tailoring the intimacy between Pd and acid sites, that is, with an appropriate nanoscale rather than the closest intimacy. The conversion from furfural to cyclopentanone on hierarchical catalytic structures in Pd/CoxOy/SiAlOz undergoes the treble-site catalytic mechanism, comprising the dissociation of H2 on Pd, the preferential adsorption and hydrogenation of the carbonyl on CoxOy, followed by the hydrogenative ring-rearrangement of furfuryl alcohol intermediate on SiAlOz via the synergistic effect of acid sites and hydrogen spillover. Besides, the asynchronous catalysis resulting from Pd ensemble isolation from SiAlOz surface profoundly restrains the generation of tetrahydrofurfuryl alcohol by elongating the distance of hydrogen spillover, weakening the hydrogenation of the furan ring flat adsorbed on SiAlOz. Resultant Pd/CoxOy/SiAlOz-40 with an optimal Pd-acid intimacy delivers a conversion of 96% and selectivity toward cyclopentanone of 88%, outperforming most state-of-the-art noble metal catalysts. We anticipate that such a concept and synthesis strategy of hierarchical catalytic structures will extend the understanding of the metal-acid intimacy and provide a feasible approach for delicately regulating the location of functional sites in bifunctional catalysts, the results of which may enable the properties of multifunctional catalysts to be modulated to specific target processes.