Improved Catalytic Transfer Hydrogenation of Levulinate Esters with Alcohols over ZrO₂ Catalyst

Levulinic acid and its esters (LE) are polyfunctional molecules that can be obtained from lignocellulosic biomass. Herein, the catalytic conversion of methyl and ethyl levulinates into γ-valerolactone (GVL) via catalytic transfer hydrogenation (CTH) by using methanol, ethanol, and 2-propanol as the H-donor/solvent, was investigated under both batch and gas-flow conditions. In particular, high-surface-area, tetragonal zirconia has proven to be a suitable catalyst for this reaction. Under batch conditions, 2-propanol was found to be the best H-donor, with ethyl levulinate giving the highest yield in GVL. However, high autogenic pressures are needed in order to work in the liquid phase at high temperature with light alcohols. The reactions occurring under continuous gas-flow conditions, at atmospheric pressure, were found to be much more efficient, also showing excellent GVL yields when EtOH was used as the reducing agent (GVL yield of around 70% under optimized conditions). However, the deposition of heavy carbonaceous compounds over the catalytic surface, in this way blocking the active Lewis acid sites, led to a progressive change in the chemo-selectivity, promoting the alcoholysis of angelica lactones back to LE. Nevertheless, the in situ regeneration of the catalyst, performed by feeding air at 400 °C for 2 h, allows an almost complete recovery of the initial catalytic behaviour, proving that the deactivation is reversible. The reaction has been tested also using a true bioethanol, derived from agricultural waste. These results represent the very first examples of the CTH of LE under continuous gas-flow conditions reported in literature.