Characterisation and catalytic properties of vanadium oxide supported on alumina prepared by sol-gel method

Vanadia-alumina xerogels were prepared by sol-gel method using organic precursors of metal and acetic acid as hydrolysis source. The purpose of this work is to study the change in the structure of dispersed vanadia as a function of preparation parameters, mainly the hydrolysis ratio k = [CH3COOH]/[Al(O-s-Bu)(3)] and the vanadium content V/V + Al (%). The amount of acetic acid affects considerably the solid texture, thus the pore size increases with the k ratio. In the same time, V-51 NMR spectroscopy shows a decrease in the vanadium coordination from six-fold to fourfold. The investigation by temperature programmed reduction (TPR) shows that the reducibility of catalysts is determined in the order: K = 3 > K = 2 > K = 1. Furthermore, the catalytic activity in paraxylen oxidation follows a trend similar to that observed in TPR. The increase of vanadium content from 5 to 10%, favours the association of vanadium monomeric species, resulting in a new surface species. Hence, the coordination of vanadium changes from tetrahedral to octahedral, which induces a fall in partial oxidation selectivity.