How the valence orbitals and the Lewis acidity of trivalent metal cations influence the catalytic activity of phosphotungstic acid salts in the furfural acetalization reactions with alkyl alcohols

In this work, the protons of phosphotungstic acid were exchanged with metal cations, generating efficient catalysts which were evaluated in furfural acetalization reactions with alkyl alcohol. The goal was to understand how the valence orbitals of the metal cations impact their activity. Trivalent metal cations of the Boro Group (Group XIII) and metals of the first transition series were used to prepare salts with general formulae MPW 12 O 40 (M 3+ = Ga, Al, In, Fe, and Al). All the metal phosphotungstates were characterized by FT-IR, TG/DSC, XRD, SEM-EDS, and potentiometric titration with n-butylamine. The main reaction parameters, such as the catalyst load, temperature, time, and type of alcohol, were investigated. Iron(III) phosphotungstate was the most active and selective catalyst among the metal salts investigated. Their highest activity was attributed to their highest Lewis acidity strength, a consequence of semi-filled “d” valence orbitals with lower energy, which favored the formation of hydronium cations that can also catalyze the acetalization reaction. Special attention was devoted to correlating the properties of catalysts with their activity. A reaction mechanism was also proposed based on experimental results.