Résumé
Nb
2O
5–SiO
2 catalysts were synthesized by a new sol–gel route and their activity was tested in the epoxidation of cyclooctene with H
2O
2. The mechanism involves the reaction of a cyclooctene molecule with a Nb
O
O
H group formed by reaction of H
2O
2 with the Brønsted and/or Lewis acid sites of the catalysts.
▪
Niobium–silicon mixed oxide nanocomposites, containing 7–37
wt.% Nb, were synthesized by a new sol–gel route and characterized for textural and acid properties by N
2 adsorption, NH
3 TPD and FTIR of adsorbed acetonitrile. The materials were tested as catalysts for epoxidation of cyclooctene with H
2O
2. High surface areas (>150
m
2
g
−1), decreasing with Nb content, were found for materials containing up to 23.0
wt.% Nb. FTIR and TPD measurements evidenced the presence of both Brønsted and Lewis acid sites with different strength. The amount of strong acid sites, relative to those of low or moderate strength, increased with Nb content.
The materials containing up to 23.0
wt.% Nb were active and stable catalysts for the epoxidation of cyclooctene. The highest activity and H
2O
2 selectivity was found for the catalyst with the lowest Nb content, that showed also high stability in reuse. Catalytic properties were related to acid sites and to the presence of NbO
x
species with different coordination.