L. Baraket et A. Ghorbel, Characterisation and catalytic properties of vanadium oxide supported on alumina prepared by sol-gel method, J SOL-GEL S, 19(1-3), 2000, pp. 89-94
Vanadia-alumina xerogels were prepared by sol-gel method using organic prec
ursors of metal and acetic acid as hydrolysis source. The purpose of this w
ork is to study the change in the structure of dispersed vanadia as a funct
ion of preparation parameters, mainly the hydrolysis ratio k = [CH3COOH]/[A
l(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 spectrosco
py 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 tre
nd 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 indu
ces a fall in partial oxidation selectivity.