Characterization of La2O3-TiO2 and V2O5/La2O3-TiO2 catalysts and their activity for synthesis of 2,6-dimethylphenol

The La2O3-TiO2 (1:5 molar ratio) mixed oxide was prepared by a co-precipitation method with in situ generated ammonia and was impregnated with various amounts of vanadia (4-12 wt.%). The La2O3-TiO2 and the V2O5/La2O3-TiO2 catalysts were subjected to thermal treatments from 773 to 1073 K and were investigated by X-ray diffraction, FT-infrared, BET surface area, and O-2 chemisorption methods to establish the effects of vanadia loading and thermal treatments on the surface structure of the dispersed vanadium oxide species and the temperature stability of these catalysts. Conversion of cyclohexanol to cyclohexanone/cyclohexene was investigated as a model reaction to assess the acid-base properties of these materials. The catalytic property was evaluated for a single step synthesis of 2,6-dimethylphenol from cyclohexanone and methanol in the vapor phase at normal atmospheric pressure. Characterization results suggest that the co-precipitated La2O3-TiO2 when calcined at 773 K is in X-ray amorphous state and exhibits reasonably high specific surface area. The amorphous La2O3-TiO2 is converted into crystalline compounds La2Ti2O7 and La4Ti9O24 at 873 and 1073 K, respectively. The La2O3-TiO2 also accommodates a monolayer equivalent of V2O5 (12 wt.%) in a highly dispersed state on its surface. The V2O5/La2O3-TiO2 catalyst is thermally quite stable up to 873 K calcination temperature. When subjected to thermal treatments beyond 873 K, the dispersed vanadium oxide selectively interacts with La2O3 portion of the mixed oxide and forms a LaVO4 compound. The remaining TiO2 appears in the form of anatase phase. The La2O3-TiO2 and the V2O5/La2O3-TiO2 differ in terms of acid-base properties and the 12% V2O5/La2O3-TiO2 catalyst provided a maximum yield of 2,6-dimethylphenol among various catalysts investigated. (C) 2001 Elsevier Science B.V. All rights reserved.