Mesoporous niobium-silicates prepared by evaporation-induced self-assembly as catalysts for selective oxidations with aqueous H2O2

Abstract

Hydrothermally stable mesoporous niobium-silicates Nb-MMM-E have been first prepared following the convenient and versatile evaporation-induced self-assembly (EISA) methodology. The new materials have been characterized by elemental analysis, XRD, low-temperature N2 adsorption, SEM, XPS, DRS UV–vis, and Raman techniques. DR UV–vis spectroscopy enabled differentiation between isolated and oligomerized Nb centers on the silica surface. Use of niobium(V) ethoxide modified with acetylacetone (acac) as a Nb source favored the formation of dimeric and/or small oligomeric Nb centers on the surface of mesoporous silica while ammonium niobate(V) oxalate coupled with acac gave mostly site-isolated Nb species. The Nb-MMM-E materials catalyzed selectively oxidation of various unsaturated compounds of interest for fine and specialty chemistry using the green oxidant – aqueous hydrogen peroxide. Epoxidation of both electron rich and electron deficient CC bonds could be accomplished over Nb-MMM-E. While catalysts with isolated Nb centers were preferable for the selective formation of epoxides sensitive to ring opening and overoxidation, both single site and oligomerized Nb centers were equally effective for the production of relatively stable epoxides. The truly heterogeneous nature of the catalysis over Nb-MMM-E and absence of Nb leaching into solution have been proved by hot filtration tests coupled with elemental analysis. The catalysts could be easily recovered by filtration and reused several times without significant deterioration of the catalytic performance.