Anna Lubańska

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 2 O 3 , TiO 2 , kaolin and γ-Al 2 O 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 4 SiW 12 O 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 2 O 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 4 SiW 12 O 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.

The formation of MTBE on supported and unsupported H4SiW12O40

Abstract Sulphonated resins used in the industry as the catalysts in the synthesis of methyl tert -butyl ether (MTBE) until now, the most important anti-knocking additive to the gasoline, are not satisfactory because of environmental reasons and catalyst’s not adequate stability. This stimulates the search for new catalysts for this process. Among others heteropoly acids (HPA) are considered to be prospective ones. The present paper is the review of the authors’ investigations on physical chemistry of the MTBE synthesis in gas phase on dodecatungstosilicic acid H 4 SiW 12 O 40 (HSiW). It has been preceded by the study of the interaction of both substrates, methanol and iso -butylene and the product MTBE, with crystalline HSiW comprising classical sorption experiments, FTIR and calorimetric measurements. Together with reaction kinetic measurements it enabled to propose the mechanism of the catalytic reaction according to which iso -butylene not penetrating the bulk of HSiW crystallites gets protonated at the surface with the loosely bonded protons (protons forming hydrogen bond between HSiW anions) and subsequently reacts with methanol from next-to-surface layer or from the gas phase. Methanol penetrating into the bulk is forming protonated clusters thus diminishing the concentration of loosely bonded protons. At high enough methanol concentration this leads to the diminishing of the formation rate of carbocation. In the result reaction order positive at the initial state of catalytic process (low bulk concentration of methanol and high concentration of loose protons) turns into negative at the reaction steady state. A set of kinetic equations has been proposed describing all these processes. A series of oxide supports: SiO 2 , TiO 2 , SiO 2 -Al 2 O 3 , γ-Al 2 O 3 and AlPO 4 with increasing basicity (characterised by the net oxygen negative charge calculated according to Sanderson’s theorem) was used for the preparation of HSiW supported catalysts. It has been shown that the activity decreases with the increasing basicity as a result of strong bonding protons by the surface of the support.