Surface characteristics and activity in selective oxidation of o-xylene of supported V 2O 5 catalysts prepared by standard impregnation and atomic layer deposition

Abstract

Alumina-, silica-, and titania-supported vanadium oxide systems with V 2O 5 loadings ranging from 3 to 12 wt.%, corresponding to 0.02–0.09 V/(Al,Si,Ti) atomic ratios, were prepared by atomic layer deposition (ALD) and compared with the corresponding impregnated catalysts. The surface acidic properties of the supports and catalysts were investigated using ammonia adsorption microcalorimetry to determine the number and strength of the surface acid sites. Deposition of V 2O 5 on alumina and titania supports gave rise to catalysts with lower amounts of acid sites than the respective supports, while for the samples prepared on silica, an increase of the number of acid sites was observed after V 2O 5 deposition. As a common trend, the surface acid strength was greater for the ALD catalysts than for the impregnated ones, suggesting a stronger interaction of the VO species with the support centers, which act as electron attractor centers creating Lewis-like vanadium species. Redox cycles were performed, involving temperature programmed reduction (TPR) analyses separated by an oxidation treatment (TPO). The results evidenced the good reversibility of the redox behavior of the vanadium centers in every case, while significant differences were observed when comparing the temperatures of reduction ( T max). Lower T max values were observed for the better dispersed vanadia catalysts. After reduction, the V centers had a final formal average oxidation state corresponding to +3 or less (+2.5 to +2). The reactivity of the vanadia systems was examined by measuring their performance for the oxidation of o-xylene to phthalic anhydride. Activity tests indicated the superior selectivity of the V 2O 5 systems based on the more acidic supports (Al 2O 3 and TiO 2). The nature of the support governed the activity, and the more concentrated catalysts gave rise to improved selectivity to phthalic anhydride.