Tandem Reductive Catalytic Upgrading of Orange Peel Waste Derived Methyl Levulinate and Limonene into Γ-Valerolactone and P-Cymene Promoted by Pd/ZrO2 and ZrO2 Catalysts

Orange peel waste (OPW), a significant biomass byproduct derived from the juice processing industry, can be used as valuable resource to produce various chemicals, including methyl levulinate (ML) and limonene (LIM). This study introduces a tandem catalytic process involving the dehydrogenation of LIM into p-cymene and the (transfer) hydrogenation of ML into gamma-valerolactone (GVL), unlocking the full potential of OPW chemo-catalytic valorisation in the spirit of circular economy. This process is promoted by the heterogeneous Pd/ZrO2 and t-ZrO2 catalysts in the presence of ethanol as hydrogen-donor solvent. Under batch conditions, Pd/ZrO2 not only shows superior performance in the transfer hydrogenation of ML compared to t-ZrO2, but also promotes the conversion of LIM into p-cymene primarily through the hydrogenation/dehydrogenation route. Most important, Pd/ZrO2 exhibits good activity in the simultaneous upgrading of both ML and LIM across various ML:LIM ratios. Continuous gas-flow conditions result in improved outcomes in terms of ML and LIM conversion, as well as GVL and CYM selectivity. Notably, a strong correlation between CYM and H-2 yields has been established providing compelling evidence for the LIM isomerization-dehydrogenation pathway. However, the simultaneous upgrading of LIM and ML was found to be not efficient in the gas-phase due to side oligomerization reactions.