Characterization of La 2O 3-TiO 2 and V 2O 5/La 2O 3-TiO 2 catalysts and their activity for synthesis of 2,6-dimethylphenol

Abstract

The La 2O 3-TiO 2 (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 La 2O 3-TiO 2 and the V 2O 5/La 2O 3-TiO 2 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 La 2O 3-TiO 2 when calcined at 773 K is in X-ray amorphous state and exhibits reasonably high specific surface area. The amorphous La 2O 3-TiO 2 is converted into crystalline compounds La 2Ti 2O 7 and La 4Ti 9O 24 at 873 and 1073 K, respectively. The La 2O 3-TiO 2 also accommodates a monolayer equivalent of V 2O 5 (12 wt.%) in a highly dispersed state on its surface. The V 2O 5/La 2O 3-TiO 2 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 La 2O 3 portion of the mixed oxide and forms a LaVO 4 compound. The remaining TiO 2 appears in the form of anatase phase. The La 2O 3-TiO 2 and the V 2O 5/La 2O 3-TiO 2 differ in terms of acid–base properties and the 12% V 2O 5/La 2O 3-TiO 2 catalyst provided a maximum yield of 2,6-dimethylphenol among various catalysts investigated.