The stability of niobium-silica catalysts in repeated liquid-phase epoxidation tests: A comparative evaluation of in-framework and grafted mixed oxides

Two types of niobium-containing silica catalysts, (i) in-framework mixed oxide, obtained by co-precipitation type synthesis, and (ii) grafted niobium sites on silica supports, obtained via post-synthesis deposition from niobocene dichloride, were prepared. Both solids were tested in a series of five repeated batch-wise liquid-phase epoxidation tests of limonene with 50% aqueous hydrogen peroxide. Nb/SiO₂ prepared by co-precipitation synthesis kept fully its epoxidation activity during the first two recycles, with a gradual decrease in activity in the following runs. Conversely, Nb/SiO₂ prepared by deposition suffered from a more marked loss of activity since the first reuses. Most of the isolated Nb sites were maintained and no large Nb₂O₅ domains were formed during the recovery and regeneration tests. The distribution, geometry and coordination state of the Nb sites was therefore preserved, especially for the Nb/SiO₂ sample prepared via co-precipitation. The presence of small Nb₂O₅ domains, even formed during repeated recycles, was not detrimental for the epoxidation reaction and, in general, Nb(V) sites inserted in a silica matrix proved to be a rather robust epoxidation catalyst for alkenes in the presence of aqueous hydrogen peroxide.