Vanadium-Substituted Phosphomolybdic Acid: Efficient Catalyst for Producing Bioadditives from Biomass-Derived Furfural

In this work, the activity of vanadium-doped and undoped phosphomolybdic acids with general formulae H3+nPMo12−nVnO40 (n = 0, 1, 2, and 3) was evaluated in the acetalization of furfural with alkyl alcohols. The main focus was to assess how vanadium charge affects the catalytic activity of phosphomolybdic acid and to link these effects to changes in structural properties. The main reaction parameters, such as charge and concentration of the catalyst, temperature, time, type of alcohol and aldehyde, and charges of vanadium and of H+ ions, were studied. Various Brønsted acids (sulfuric, p-toluenesulfonic, undoped, and doped phosphomolybdic acids) were evaluated on the condensation reactions of furfural with methyl alcohol. Notably, H4PMo11VO40 was the most active and selective catalyst for the formation of methyl acetal furfural. Water has a leveling effect on the strength of these acids. Nonetheless, under reaction conditions, the presence of vanadium affected their acidity strength, and it was possible to verify that the vanadium-monosubstituted phosphomolybdic acid was the strongest. The superior performance of H4PMo11VO40 was attributed to its additional acidity, resulting from the presence of very strong Brønsted acid sites (H+) and Lewis acid sites, due to the inclusion of V5+ ions in its structure. The novelty of this work is the assessment of vanadium-doped phosphomolybdic acids in the homogeneous phase in the condensation reactions of furfural with various alcohols and of methyl alcohol with various aldehydes.