Preparation, characterization, and catalytic activities of silica-supported tantalum oxide for the vapor phase decomposition of methyl tert-butyl ether

Abstract

Silica-supported tantalum oxide catalysts [Ta oxide/SiO 2 (ALK)] were prepared by chemical reaction between tantalum alkoxide and surface hydroxyl groups of SiO 2. The structure and physicochemical properties of Ta oxide/SiO 2 (ALK) were compared with those of Ta oxide/SiO 2 (IMP) prepared by impregnation on SiO 2 with acidic TaCl 5 solution, and of bulk hydrated tantalum oxide. Highly dispersed tantalum oxide species were formed on the surface by controlling the preparation conditions, such as type of alkoxy group of tantalum alkoxide, concentration of alkoxide in the solution, and impregnation temperature, as confirmed by Fourier transformation of the Ta L III-EXAFS data. Tantalum oxide of Ta oxide/SiO 2 (ALK) did not crystallize up to temperatures as high as 1500 K, while bulk hydrated tantalum oxide and tantalum oxide of Ta oxide/SiO 2 (IMP) crystallized at 970 K. The acid strength of Ta oxide/SiO 2 (ALK) was moderate, while hydrated tantalum oxide and Ta oxide/SiO 2 (IMP) were strongly acidic. Ta oxide/SiO 2 (ALK) displayed mainly a Lewis acid character. Ta oxide/SiO 2 (ALK) was initially less active in the decomposition of methyl tert-butyl ether, but the selectivities for 2-methylpropene-1 and methanol were higher, as compared with hydrated tantalum oxide and Ta oxide/SiO 2 (IMP). Furthermore, the activity and selectivity of Ta oxide/SiO 2 (ALK) were preserved for ten hours and not inhibited upon addition of water.