Continuous flow hydrogenation of methyl and ethyl levulinate: an alternative route to γ-valerolactone production

Heterogeneous continuous transformation of methyl levulinate (ML) and ethyl levulinate (EL) to γ-valerolactone (GVL), as a promising C₅-platform molecule was studied at 100°C. It was proved that the H-Cube^® continuous hydrogenation system equipped with 5% Ru/C CatCart^® is suitable for the reduction of both levulinate esters. While excellent conversion rates (greater than 99.9%) of ML and EL could be achieved in water and corresponding alcohols, the selectivities of GVL were primarily affected by the solvent used. In water, 100% conversion and ca 50% selectivity that represent ca 0.45 mol_GVL g_metal ^-1 h^-1 productivity towards GVL, were obtained under 100 bar of total system pressure. The application of alcohols as a solvent, which maintained high conversion rates up to 1 ml min^-1 flow rate, resulted in lower productivities (less than 0.2 mol_GVL g_metal ^-1 h^-1) of GVL. Therefore, from a synthesis point of view, the corresponding 4-hydroxyvalerate esters could be obtained even at a higher reaction rate. The addition of sulfonated triphenylphosphine ligand (TPPTS) allowed reduction of the system pressure and resulted in the higher selectivity towards GVL.