γ-Al2O3 supported Ni-Pd catalysts with different Ni:Pd ratios were studied in the hydrogenation of two industrially-relevant platform molecules derived from biomass, namely levulinic acid and hydroxymethylfurfural. The bimetallic catalysts showed better performances in both processes in comparison to the monometallic counterparts, for which a too strong interaction with the alumina support reduced the activity. The behavior of the bimetallic catalysts was dependent on the Ni:Pd ratio, and interestingly also on the targeted hydrogenation reaction. The Pd-modified Ni-rich system behaves like pure Ni catalyst, but with a strongly boosted activity due to a higher number of Ni active sites available, Pd being considered as a spectator. This high activity was manifested in the levulinic acid hydrogenation with formic acid used as an internal hydrogen source. This behavior differs from the case of the Pd-rich system modified by Ni, which displayed a much higher Pd dispersion on the support compared to the monometallic Pd catalyst. The higher availability of the Pd active sites while maintaining a high surface acidity allows the catalyst to push the HMF hydrodeoxygenation reaction forward towards the green biopolymer precursor 2,5-bis(hydroxymethyl)-tetrahydrofuran, and in consequence to strongly modify the selectivity of the reaction. In that case, residual chlorine was proposed to play a significant role, while Ni was considered as a spectator.
Levulinic acid (LA) is a platform molecule, and its valorization toward biofuel additives like γ-valerolactone or tetrahydrofuran is considered as an important step in planning future biorefinery schemes. In this study, various Ni based catalysts were studied for the LA hydrogenation with formic acid (FA) used as a hydrogen source. Two different ways of catalytic activity improvement are discussed (nickel loading vs addition of dopants). The influence of Ni doping by small amount of noble metals (Pt, Pd, Ru, Rh) showed that Ni−Pd is the most active catalyst. Its high catalytic performance is attributed to the synergic effect between two metals and interaction with chlorine. The effect of chlorine on catalytic performance and properties of the catalysts was evaluated by variety of surface and bulk-sensitive characterization methods. It was shown that addition of chlorine is one of the key factors required for high catalytic performance. Chlorine influences distribution of metals on the surface of the catalyst, their interaction with support and facilitates the formation of small crystallites, which is beneficial for reaching high catalytic activity.
The influence of the nature of carbon materials used as a support for Ru/C catalysts on levulinic acid hydrogenation with formic acid as a hydrogen source toward gamma-valerolactone was investigated. It has been shown that the physicochemical properties of carbon strongly affect the catalytic activity of Ru catalysts. The relationship between the hydrogen mobility, strength of hydrogen adsorption, and catalytic performance was established. The catalyst possessing the highest number of defects, stimulating metal support interaction, exhibited the highest activity. The effect of the catalyst grain size was also studied. It was shown that the decrease in the grain size resulted in the formation of smaller Ru crystallites on the catalyst surface, which facilitates the activity.
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