Structural Stability and Brønsted Acidity of Thermally Treated AlPW₁₂O₄₀in Comparison with H₃PW₁₂O₄₀

The thermal stability and hydroxyl coverage of aluminum salt of 12-tungstophosphoric acid, AlPW12O40, was studied by X-ray diffraction and solid-state NMR spectroscopy in comparison with the pure H3PW12O40 heteropoly acid. The most important differences of these materials consist in the higher thermal stability of the local structure and the stronger temperature dependence of the concentration of acidic protons of AlPW12O40H3PW12O40 showed shrinkage of the unit cell in the X-ray patterns and changes of the local structure in the P-31 MAS NMR spectra already upon dehydration at 473 K, while no structural modifications were found for AlPW12O40 UP to 673 K. On AlPW12O40, acidic protons and AlOH groups are formed by dissociation of water molecules at the multivalent aluminum cations upon dehydration at temperatures of 373-423 K. After dehydration at temperatures higher than 423 K, the hydroxyl groups of AlPW12O40 dehydroxylate and nonhydroxylated Al3+ cations are formed. The accessibility of acidic protons as studied by adsorption of pyridine was found to be similar for dehydrated H3PW12O40 and AlPW12O40, that is, a lower accessibility occurs with increasing dehydration temperature. The adsorption of probe molecules for studying the acid strength indicated that AlPW12O40 dehydrated at 373-523 K is as superacidic as dehydrated H3PW12O40.