Selective Oxidation of Methyl Lactate over Carbon-Supported Noble Metal Catalysts under Base-Free Conditions

The selective oxidation of biomass-derived alcohol-containing functional compounds with molecular oxygen over solid catalysts is a promising approach to renewably produce value-added ketones, aldehydes, and/or carboxylic acids. Activated carbon (AC) supported Pt, Pd, Rh, Ru, and Au nanoparticles at comparable sizes (average: 1.4–2.9 nm) were evaluated in the liquid-phase oxidative dehydrogenation of methyl lactate (ML) to methyl pyruvate (MP) with oxygen under base-free conditions. Ru/AC gives not only much higher MP selectivity (ca. 90%) but also an order of magnitude in the initial turnover frequency (TOF) of ML higher than other noble-metal catalysts for this ML-to-MP reaction. Metallic Ru (Ru/AC: 84.3% Ru⁰ and 15.7% RuO_x) is also found to be superior to other Ru species (RuCl₃, RuO₂, and Ru­(OH)₃) supported on AC in terms of higher activity and selectivity. The oxidative dehydrogenation of ML is always accompanied by the hydrolysis reactions of MP and ML and consecutive secondary reactions (decarboxylation, decarbonylation, oxidation, etc.) producing C1–C3 byproducts, which would be promoted by the surface acidic and basic sites. The effects of reaction variables (including ML concentration, O₂ pressure, reaction temperature, and solvent) on ML oxidation were further examined on Ru/AC. A kinetic rate based on the Langmuir–Hinshelwood model was proposed, indicating that the rate-determining step of ML oxidation over the Ru/AC catalyst would be the surface reaction between independently adsorbed ML and oxygen species on active sites of different nature. γ-Valerolactone is identified as an efficient solvent for ML oxidation, producing high and stable activity and MP selectivity (87–94%) throughout 5 reaction runs.