Influence of the Preparation Procedure of Vanadium-Containing SiBEA Zeolites on Their Catalytic Activity in Propene Epoxidation

Two series of V-containing BEA zeolite catalysts, VₓSiBEA(I) and VₓSiBEA(II), were prepared by a two-step postsynthesis preparation procedure which consists, in the first step, in the dealumination of TEABEA zeolites by a treatment with nitric acid solution to obtain SiBEA zeolites with a Si/Al atomic ratio of 1000 and then, in the second step, in bringing SiBEA into contact with an aqueous NH₄VO₃ solution with different concentrations at a pH of 2.7. After 3 days, the solids were recuperated from the reaction mixtures: (1) on a sinter funnel and washed several times with distilled water to obtain the VₓSiBEA(I) series and (2) in a rotating evaporator under vacuum via a membrane pump to obtain the VₓSiBEA(II) series. The combined use of H₂-temperature-programmed reduction, diffuse reflectance (DR) UV–vis, Fourier transform infrared (FTIR), ⁵¹V magic-angle spinning (MAS) NMR, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy allows determining the nature and environment of vanadium in both series of V-containing SiBEA zeolites after different treatments. The characterization of VₓSiBEA(I) and VₓSiBEA(II) series reveals the formation of various forms of vanadium species depending on the V content and conditions applied upon preparation of each series of V-containing SiBEA zeolites. As evidenced by DR UV–vis and ⁵¹V MAS NMR in the VₓSiBEA(I) series, vanadium was present mainly as mononuclear framework pseudo-tetrahedral V(V) species. In contrast, in the VₓSiBEA(II) series, vanadium was present as mononuclear framework pseudo-tetrahedral and polynuclear extra-framework pseudo-octahedral V(V) species. As shown by EPR, the oxidation state of V species easily changes upon calcinations in oxygen, outgassing under vacuum at 773 K and treatment with hydrogen at a high temperature (873 K). The presence of Brønsted and Lewis acidic centers was evidenced in both VₓSiBEA(I) and VₓSiBEA(II) series by FTIR spectroscopy with pyridine used as a probe molecule. The catalytic activity tests in propene epoxidation revealed that the highly dispersed mononuclear framework pseudo-tetrahedral V(V) species are responsible for high selectivity to propene oxide, whereas polynuclear extra-framework pseudo-octahedral V(V) species catalyzed mainly total oxidation. The VₓSiBEA(I) series of catalysts with vanadium, present mainly as mononuclear framework pseudo-tetrahedral V(V) species, show lower turn over frequency values than the VₓSiBEA(II) series in which vanadium is present as both mononuclear framework pseudotetrahedral V(V) species and polynuclear extra-framework pseudo-octahedral V(V). It thus demonstrates that apart from highly dispersed isolated vanadium species, the availability of vanadium species to reagents also plays an important role in the gas-phase propene epoxidation.