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

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.

Acid catalysts based on aluminum (or copper) salts of molybdophosphoric acid were synthesized from a heteropolyacid solution to which Al2(SO4)3, Al2O(CH3COO)4 or CuSO4 was added. The salts were characterized by different physicochemical techniques. The catalyst acidity was measured by potentiometric titration and by temperature-programmed desorption of pyridine. The catalysts were used in the reaction between m-cresol and geraniol for the synthesis of chromanes.Figure optionsDownload high-quality image (41 K)Download as PowerPoint slide