Catalytic and structural properties of co-precipitated Mg–Zr mixed oxides for furfural valorization via aqueous aldol condensation with acetone

Abstract

A series of MgO–ZrO2 mixed oxides with different nominal Mg/(Mg+Zr) atomic ratios (0, 0.1, 0.3, 0.5, 0.75, 0.85 0.92) were prepared by co-precipitation and calcined at 873K. The catalytic activity in aldol condensation of furfural with acetone was tested in aqueous medium. These solids were systematically characterized by XRD, TEM, XPS, N2 adsorption–desorption isotherms and thermal analysis.MgxZr1−xO2−x solid solution with cubic ZrO2 structure was the only phase observed for the mixed solids with Mg/(Mg+Zr) atomic ratio ≤0.3, which had very low activity. Two different phases were observed in solids with a nominal Mg/(Mg+Zr) atomic ratio ≥0.5: cubic MgO and c-MgxZr1−xO2−x. Moreover, the relative concentration of the latter phase decreases as the Mg concentration increases. Zr4+ cations are present at the surface of the c-MgO, the concentration of which is lower in the Mg-richer samples. No significant differences were found between the overall catalytic activity of these samples. Therefore, three different active surface sites can be found in these solids: Mg–O–Mg sites co-existing with Mg–O–Zr sites at the surface of c-MgO, and Mg–O–Zr sites at the surface of c-MgxZr1−xO2−x. The Mg–O–Zr sites on c-MgO are much more active than the other two sites, which show similar intrinsic activity. The overall activity of each catalyst depends on the amount of each of these surface sites and on their intrinsic activity. Reutilization tests and characterization of the used catalysts suggests that the most probable causes of deactivation are fouling and/or poisoning of the surface sites by furfural derived heavy compounds, leaching of Mg2+ and Zr4+ cations, or hydration of the Mg oxide.