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

Abstract

A series of simple and mixed oxides: SiO 2, AlPO 4, SiO 2-Al 2O 3, TiO 2, kaolin and γ-Al 2O 3 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 H 4SiW 12O 40 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. γ-Al 2O 3 and kaolin the heteropolyacid was decomposed resulting in the catalyst which exhibited only low activity. On the other hand, the least basic oxide SiO 2 gave the most active catalysts. However, the sequence of activities differed from that of basicities by the fact that the activity of TiO 2-supported catalyst was at the coverage with H 4SiW 12O 40 Θ=0.25 nearly as high as SiO 2 supported one and at Θ=1.0 TiO 2-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.