Doping Pd/SiO₂ with Na⁺: Changing the reductive etherification of CO to furan ring hydrogenation of furfural in ethanol

The production of biofuels and chemicals by hydrogenation of furfural has attracted much attention recently. Herein the effect of Na⁺ doping on the catalytic performance of Pd/SiO₂ in hydrogenation and reductive-etherification of furfural in ethanol was systematically studied. Two Pd/SiO₂ catalysts with and without the modification by Na⁺ were prepared by impregnation and calcination. Their catalytic properties were compared for the hydrogenation of furfural and furfural diethyl acetal under mild conditions. The silanol groups on Pd/SiO₂ catalysed the acetalization of furfural and alcohol and the resulted acetal underwent hydrogenolysis on Pd nanoparticles (NPs) with an average particle size of 8 nm, leading to a moderate yield (∼58%) of furfuryl ethyl ether. Doping Na⁺ on Pd/SiO₂ led to the diminishing of silanol groups as well as strong interaction between Na⁺ and Pd NPs. No acetalization occurred on Na⁺ modified Pd/SiO₂ due to the exchange of H⁺ of Si-OH with Na⁺, thus the reductive etherification of CO group in furfural was completely inhibited. Meanwhile the hydrogenation of furan-ring over Na⁺ coordinated Pd NPs could proceed with very high selectivity (>90%) forming tetrahydrofurfural in high yield. Kinetics study on the hydrogenation of furfural diethyl acetal over Pd/SiO₂ and Na⁺ doped Pd/SiO₂ suggested that the Na⁺ greatly impeded the hydrogenolysis of C-O-C bond of acetal, while the hydrogenation of the furan ring took place selectively.