Gas-phase selective oxidation of chloro- and methoxy-substituted toluenes on TiO2-Sepiolite supported vanadium oxides

Catalytic behaviour of vanadium oxide systems (5-20 wt.% V2O5) supported on TiO2-Sepiolite (with titania loading around the theoretical monolayer, 12 wt.%, 12Ti-Sep) and, as reference, on Sepiolite calcined (Sepc) in the p-substituted toluene derivatives selective oxidation was studied. In all the catalysts studied p-chloro (p-CIBA) and p-methoxybenzaidehyde (p-MeOBA) were the main products. The yields to benzaldehydes obtained with %V/12Ti-Sep are comparable with some of those reported in the literature under similar kinetically controlled experimental conditions (Sp-CIBA = 70% and X = 19% were obtained at 653 K on 20 V/12Ti-Sep). The activity for benzaldehyde derivatives formation of the %V/12Ti-Sep systems was superior to that exhibited by the %V/Sepc systems and in both series it notably increased when vanadium loading did. Furthermore, both p-substituents favourably influenced the partial oxidation of the methyl group to the corresponding aldehyde, whose formation at isoconversion is as follows: p-CIBA > p-MeOBA > BA. A concerted reaction mechanism in the oxidation of the toluenes to benzaldehydes on polymeric vanadia species has been proposed which is able to explain the toluene substituent effects as well as the activity of the catalysts. (C) 2008 Elsevier B.V. All rights reserved.