Oxide supports for 12-tungstosilicic acid catalysts in gas phase synthesis of MTBE

A series of simple and mixed oxides: SiO2, AlPO4, SiO2-Al2O3, TiO2, kaolin and gamma-Al2O3 differing by their basicities as characterised by effective negative charge on oxygen calculated according to Sanderson's theorem was used for the preparation of supported H4SiW12O40 catalysts. Using gas phase synthesis of MTBE as the catalytic test reaction it has been shown that on the supports of highest basicity, i.e. gamma-Al2O3 and kaolin the heteropolyacid was decomposed resulting in the catalyst which exhibited only low activity. On the other hand, the least basic oxide SiO2 gave the most active catalysts. However, the sequence of activities differed from that of basicities by the fact that the activity of TiO2-supported catalyst was at the coverage with H4SiW12O40 Theta = 0.25 nearly as high as SiO2 supported one and at Theta = 1.0 TiO2-supported catalyst was the best. This indicates that the basicity of the support is only one of the factors determining the activity of supported heteropolyacid catalysts in acid-base type reactions. On the other hand good correlation was obtained between the catalytic activity and the neutralisation heat of the acid sites on the surface of catalyst which was determined using thermometric titration with n-butylamine solution in toluene. (C) 2002 Elsevier Science B.V. All rights reserved.