Thermal stability and catalytic activity of Wells-Dawson tungsten heteropoly salts

Potassium-tungsten Dawson type salts, pure and monosubstituted with vanadium, are presented in this study. These compounds were thoroughly characterized by P-31-NMR, P-31 MAS-NMR, scanning electron microscopy, EDX microprobe, X-ray diffraction, infrared spectroscopy, X-ray- photoelectron spectroscopy and thermal analysis. Both Dawson salts are thermally stable up to 400 degrees C. Above that temperature they recrystallize in a potassium-tungsten Keggin type salt and P2O5-WO3(-K2O) compounds. Vanadium that was initially part of the tungsten framework is expelled out of the structure and covers the surface of the new phases. Dawson salts showed activity in methanol selective oxidation. Formaldehyde is the only product detected during methanal reaction on intact Dawson salts, which indicates the presence of redox sites. After decomposition, dimethyl ether along with formaldehyde is produced, due to the exposure of acidic phosphorous sites. The selectivity of the decomposed vanadium containing Dawson salt towards dimethyl ether is lower than the pure potassium-tungsten salt, since vanadium is covering acid sites. (C) 2000 Elsevier Science B.V. All rights reserved.