STRUCTURAL STABILITY AND BRØNSTED ACTIVITY OF H3PW12O40 AND ITS ALUMINIUM SALT AlPW12O40 UPON HEATING

Heteropoly acids and their salts, due to the more reactivity than conventional inorganic and organic acids, have found applications in various solution reactions and as industrial catalysts [1]. The great importance for application in heterogeneous catalysis at elevated temperatures is their thermal stability. In the present work, AlPW12O40 was studied upon thermal treatment in comparison with the pure H3PW12O40. A treatment of the sample materials under vacuum at 373-673 K led to a loss of the water spacers. H3PW12O40 showed shrinkage of the unit cell in the X-ray patterns and changes of the local structure in the P MAS NMR spectra already upon dehydration at 473 K. In comparison with pristine H3PW12O40, the AlPW12O40 aluminum salt showed higher thermal stability up to 673 K. Upon dehydration of H3PW12O40 at the 373 K the H MAS NMR spectra are dominated by strong signal of a high number of acidic protons, H, at about 9.1 ppm [2, 3]. An increase of temperature cause its intensity decreases. In contrast, the H MAS NMR spectra of AlPW12O40 dehydrated at 373-523 K consist of up to four signals due to strongly physisorbed water and different hydroxyl groups. On AlPW12O40, acidic protons are formed due to the dehydration at moderate temperatures (373-423 K) by dissociation of water molecules at the multivalent aluminum cations. In addition to the acidic protons occurring in the H MAS NMR spectrum at 9.1 ppm, AlOH groups causing H MAS NMR signals at 4.2 and 5.6 ppm are formed on dehydrated AlPW12O40 (eq. 1, 2). Al(H2O)n → Al(OH)2 + 2H (n-2)H2O (1) Al(H2O)n → Al(OH) + 1H (n-1)H2O (2) Upon dehydration at temperatures higher than 423 K, the hydroxyl groups (acidic protons and AlOH groups) of AlPW12O40 dehydroxylate again and nonhydroxylated Al cations are formed (eq. 3). Al(OH)n + nH → Al + nH2O (3) These nonhydroxylated Al cations compensate the negative charges of the dehydrated Keggin units and stabilize the structure of AlPW12O40 at higher thermal treatments. 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 [4]. The adsorption of probe molecules for studying the acid strength indicated that AlPW12O40 dehydrated at 373-523 K is as superacidic as dehydrated H3PW12O40. References: [1] Kozhevnikov, I.V., “Catalysts for fine chemical synthesis. Vol. 2. Catalysis by polyoxometalates” Wiley & Sons, Chichester, England, 2002. [2] Baba, T.; Ono, Y. Appl. Catal., A 1999, 181, 227–238. [3] Uchida, S.; Inumaru, K.; Misono, M. J. Phys. Chem. B 2000, 104, 8108–8115. [4] Filek, U.; Bressel, A.; Sulikowski, B.; Hunger, M. J. Phys. Chem. C – in print.