Total Hydrogenation of Furfural and 5-Hydroxymethylfurfural over Supported Pd–Ir Alloy Catalyst

Hydrogenation of aqueous furfural was conducted with SiO2-suported palladium-based bimetallic catalysts. The combination of palladium and iridium gave the best performance for the total hydrogenation to tetrahydrofurfuryl alcohol. Higher H2 pressure and lower reaction temperature were advantageous to suppress side reactions. The synergy between Pd and Ir in the hydrogenation catalysis is most remarkable for substituted furans as substrates. Furfural was first converted into furfuryl alcohol, which was further converted to tetrahydrofurfuryl alcohol. A small amount of tetrahydrofurfural was formed in the first step (∼20% selectivity), and the subsequent hydrogenation of tetrahydrofurfural was much slower. The combined yield of tetrahydrofurfuryl alcohol and tetrahydrofurfural reached 98%. The yield of tetrahydrofurfuryl alcohol reached 94% with larger amount of catalyst. Total hydrogenation of 5-hydroxymethylfurfural was also possible using Pd–Ir/SiO2 catalyst. The performance of Pd–Ir/SiO2 catalyst was slightly changed by repeated uses, and the used catalyst can be regenerated by calcination and reduction at 573 K. Characterization results showed that Pd–Ir alloy particles with size of ≤4 nm were formed on the catalyst. Addition of Ir much increased the TOF values as compared with Pd/SiO2 with similar particle size, especially for C═O hydrogenation. One factor of higher activity of Pd–Ir/SiO2 than Pd/SiO2 can be the change of adsorption mode: Ir atom on the surface promotes the adsorption at C═O site, whereas the Pd surface strongly interacts with furan ring.