The role of Ru and RuO₂ in the catalytic transfer hydrogenation of 5-hydroxymethylfurfural for the production of 2,5-dimethylfuran

We have previously shown that 2,5-dimethylfuran (DMF) can be produced selectively from 5-hydroxymethylfurfural in up to 80 % yield via catalytic transfer hydrogenation with 2-propanol as a hydrogen donor and Ru/C as a catalyst. Herein, we investigate the active phase of the Ru/C catalyst by using extended X-ray absorption fine structure, X-ray photoelectron spectroscopy, and high-resolution TEM analyses. The results reveal that RuO₂ is the dominant phase in the fresh (active) catalyst and is reduced to metallic Ru during the reaction with the hydrogen produced insitu from 2-propanol. The deactivation of the catalyst is correlated with the reduction of the surface of RuO₂. Reactivity studies of individual phases (bulk RuO₂ and reduced Ru/C catalysts) indicate that RuO₂ mainly catalyzes the Meerwein-Ponndorf-Verley reaction of 5-hydroxymethylfurfural that produces 2,5-bis(hydroxymethyl)furan and the etherification of 2,5-bis(hydroxymethyl) furan or other intermediates with 2-propanol and that the reduced Ru/C catalyst has moderate hydrogenolysis activity for the production of DMF (30 % selectivity) and other intermediates (20 %). In contrast, a physical mixture of the two phases increases the DMF selectivity up to 70 %, which suggests that both metallic Ru and RuO₂ are active phases for the selective production of DMF. The oxidation of the reduced Ru/C catalyst at different temperatures and the insitu hydrogen titration of the oxidized Ru/C catalysts were performed to quantify the bifunctional role of Ru and RuO₂ phases. The mild oxidation treatment of the Ru/C catalyst at 403K could activate the catalyst for the selective production of DMF in up to 72 % yield by generating a partially oxidized Ru catalyst. Double trouble: A selective hydrodeoxygenation of 5-hydroxymethylfurfural to 2,5-dimethylfuran is achieved with a partially oxidized Ru/C as a catalyst and 2-propanol as a hydrogen donor. The oxidized Ru/C catalyst demonstrates bifunctional behavior, in which Ru catalyzes the dehydrogenation of 2-propanol and the hydrogenation-hydrogenolysis of 5-hydroxymethylfurfural and RuO₂ promotes dimethylfuran production via hydrogenolysis.