Total Hydrogenation of Furfural over Pd/Al₂O₃ and Ru/ZrO₂ Mixture under Mild Conditions: Essential Role of Tetrahydrofurfural as an Intermediate and Support Effect

The total hydrogenation of furfural (FAL) in aqueous solution was investigated under mild conditions over a series of a physical mixture of Pd- and Ru-supported catalysts. Monometallic catalysts showed unfulfilled selectivity toward total hydrogenation: supported Pd catalysts tend to give a mixture of tetrahydrofufural (THFAL), furfuryl alcohol (FA), and tetrahydrofurfuryl alcohol (THFA) while supported Ru catalysts are apt to hydrogenate the C=O bond. When supported Pd and Ru catalysts were mixed, the yield of THFA was significantly improved. A support-dependent promotion effect was noticed; in particular, Pd/Al₂O₃ and Ru/ZrO₂ mixture gave 99% yield of THFA at 30 °C and 0.5 MPa H₂. Detailed kinetics studies suggest that the reaction pathway with THFAL as the intermediate could be involved in this process. The apparent activation energy of furan hydrogenation over Pd/Al₂O₃ and the subsequent C=O bond hydrogenation over Ru/ZrO₂ is about 21 and 30 kJ/mol, respectively. The low activation energies may well explain the observed activity of total hydrogenation of FAL over bimetallic mixtures. This two-step reaction process was also investigated in a fixed-bed reactor with a two-catalyst bed, which gave stable THFA yield and shows promising industrial application.