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

• Co-precipitation synthesis of robust niobium-containing silica catalyst.
• Comparison of niobium-containing catalysts in the liquid-phase epoxidation of limonene using mild oxidants.
• Niobium non-ordered mesoporous silicates: high pore volume and broad mesopore accessibility.
• Evaluation of the robustness of Nb-silica catalysts in repeated recovery and reuse tests.
• Good stability and performance of in-framework Nb-silica catalyst obtained by co-precipitation.

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.

The robustness of niobium-silica catalysts prepared via co-precipitation or grafting was tested in the batch-wise epoxidation of limonene with H2O2. Niobium(V) sites inserted by co-precipitation in silica were stable enough in repeated recycles. The nature of the Nb sites in the catalysts and the resistance to deactivation were also investigated.Figure optionsDownload as PowerPoint slide