Selective aldol condensation of biomass-derived levulinic acid and furfural in aqueous-phase over MgO and ZnO†
The aldol condensation of furfural with levulinic acid in the aqueous phase was investigated over a series of solid catalysts, including oxides (MgO, ZnO, TiO2, ZrO2, MgO–Al2O3, CeO2, Nb2O5, SnO2, and WO3) and acidic zeolites (HY, Hβ, HZSM-5, H-MOR, and SAPO-34). Two isomeric condensation products, β- and δ-furfurylidenelevulinic acids (β- and δ-FDLA), were produced after acidification. MgO and ZnO were evaluated as active and selective catalysts with respect to aqueous phase aldol condensation. MgO gave a high selectivity towards δ-FDLA due to the water-tolerant basicity. In contrast, ZnO was highly selective towards β-FDLA, indicative of an acid-catalyzed aldol reaction mechanism. Nano-ZnO with a relatively high surface area gave rise to enhanced yields of β-FDLA. In-depth investigations revealed that the exposed surface hydroxyl groups on ZnO worked as active sites for the aldol reaction.