Structure–Activity–Kinetics Relationship of Ruthenium Species in Transfer Hydrogenation of Biomass-derived Levulinic Acid to γ-Valerolactone

Catalytic transfer hydrogenation (CTH) of levulinic acid (LA) as a biomass-derived platform to γ-valerolactone (GVL) is a sustainable process towards value added fuel additives and green solvents without the need to use external molecular H2. The structure-activity-kinetics relationship of Ru species on ZrO2 is systematically explored in this study on the liquid-phase CTH of LA through the use of isopropanol as a hydrogen donor. A series of Ru/ZrO2-AD catalysts with variable Ru-loading and different chemical states such as metallic Ru, oxidized RuOz, mixed Ru/RuOz, and hydroxylated RuOxHy species have been synthesized and comprehensively characterized using physicochemical and spectroscopic techniques. Catalytic results prove that the LA conversion and GVL selectivity cannot be depended on the Ru loading alone, but the dispersion of Ru, oxidation state, surface acidity, and metal-support interactions. Hydroxylated RuOxHy/ZrO2-AD catalyst is the most active and selective catalyst of the studied catalysts with respect to mild reaction conditions, which is due to abundant Ru-OH and Ru-O-Zr interfacial sites that support alcohol dehydrogenation and hydrogen transfer. Kinetic analysis shows a pseudo-first-order behavior, indicating a surface controlled hydrogen transfer mechanism. The apparent activation energy values are significantly lower in catalysts of hydroxylated and mixed-valence Ru species [RuOxHy/ZrO2-AD (36.3 kj/mol) < Ru/RuOz/ZrO2-AD (38.7 kj/mol) < Ru/ZrO2-AD (41.9 kj/mol) < RuOz/ZrO2-AD (44.2 kj/mol)] and prove their contribution to the reduction of the kinetic barrier of CTH reaction. This relationship between structure and activity is further demonstrated by the fact that the medium-strength acidic sites and the highly dispersed Ru species have a synergistic role in the catalysis of the esterification with subsequent hydrogenation of the product, where alkyl levulinates are identified as key intermediates, confirming a tandem esterification-hydrogenation pathway. Thus, this work gives a well-defined structure-activity-kinetics correlation of Ru based catalysts and gives mechanistic data towards the rational design of highly efficient CTH catalysts in biomass upgrading and sustainable production of fuels and chemicals.