Synthesis and characterization of copper and aluminum salts of H3PMo12O40 for their use as catalysts in the eco-friendly synthesis of chromanes Original Research Article

Acid catalysts based on aluminum (or copper) salts of molybdophosphoric acid (H3PMo12O40) were prepared. They were synthesized from a heteropolyacid solution to which Al2(SO4)3, Al2O(CH3COO)4 or CuSO4 was added. The obtained salts were texturally characterized and a low specific surface area, between 1 and 9 m2/g, was observed. The analysis of the prepared salts by ICP-AES spectrometry indicated that the molar content of Mo and P was that corresponding to the anion [PMo12O40]3−. The presence of undegraded Keggin structure was confirmed by XRD, FT-IR, DRS and 31P MAS-NMR. The catalyst acidity was measured by means of potentiometric titration with a solution of n-butylamine in acetonitrile and by temperature-programmed desorption of pyridine. The AlPMo12O40 salt (from sulfate), which at 5 h of reaction showed the highest conversion of m-cresol (98%) and 75% and 25% selectivity to chromane 4 and 3, respectively, presented the highest total acidity and, simultaneously, acid sites with maximum acid strength very high. On the other hand, Cu0.5H2PMo12O40 catalysts at 5 h of reaction showed lower conversion of m-cresol than the former salt (90%) and also lower selectivity to chromane 4 and 3 (37% and 10%, respectively). In the latter case also 43% of open-chain compounds were observed. With the other salts, Al0.5H1.5PMo12O40, Cu3(PMo12O40)2 and AlPMo12O40 (from Al2O(CH3COO)4), only the reaction intermediaries, open-chain compounds, were obtained because these salts presented low total acidity and acid sites with maximum acid strength very low.