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

Abstract

Two series of V-containing BEA zeolite catalysts, V x SiBEA(I) and V x SiBEA(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 NH 4 VO 3 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 V x SiBEA(I) series and (2) in a rotating evaporator under vacuum via a membrane pump to obtain the V x SiBEA(II) series. The combined use of H 2-temperatureprogrammed reduction, diffuse reflectance (DR) UV−vis, Fourier transform infrared (FTIR), 51 V 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 V x SiBEA(I) and V x SiBEA(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 51 V MAS NMR in the V x SiBEA(I) series, vanadium was present mainly as mononuclear framework pseudo-tetrahedral V(V) species. In contrast, in the V x SiBEA(II) series, vanadium was present as mononuclear framework pseudo-tetrahedral and polynuclear extraframework 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 V x SiBEA(I) and V x SiBEA(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 V x SiBEA(I) series of catalysts with vanadium, present mainly as mononuclear framework pseudo-tetrahedral V(V) species, show lower turn over frequency values than the V x SiBEA(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.

Vanadium incorporation from aqueous NH4VO3 solution into siliceous Beta zeolite determined by NMR with formation of V-single site zeolite catalysts for application in SCR of NO

“…The obtained solids were then dried in air at 353 K for 24 h and labeled V 0.6 SiBeta(II) and V 1.6 SiBeta(II). V 0.1 SiBeta(I), V 0.5 SiBeta(I), V 1.0 SiBeta(I), V 1.4 SiBeta(I), V 0.6 SiBeta(II) and V 1.6 SiBeta(II) samples were all white, suggesting the presence of pseudo-tetrahedral V(V) species, in line with our earlier reports [30,31]. In contrast, V 7.5 SiBeta(I) and V 1.0 SiBeta(II) samples were pale yellow suggesting the presence of not only pseudo-tetrahedral but also pseudo-octahedral V(V) species, either as mononuclear and/or polynuclear V(V) species, in agreement with our earlier reports [30,31].…”

“…SiBeta(I) (Fig.6b, spectra a to d) and calcined V 0.6 SiBeta(II) (Fig.7b, spectrum a) exhibit two bands at 245 and 295 nm. The shift of these DR UV-vis charge-transfer bands to lower wavelengths upon calcination suggests a greater distortion of pseudo-tetrahedral V(V) species in calcined samples than that in as prepared ones, in agreement with earlier work[30][31]. In contrast, two main bands appear at 245 and 275 nm for calcined V 1.0 SiBeta(II) and at 245 and 295 nm for V 1.6 SiBeta(II) (Fig.7b, spectra b and c, respectively) attributed to oxygen-topseudo-tetrahedral V(V) charge transfer transitions, involving oxygen in bridging (V-O-Si) and terminal (V=O) positions, respectively with a shoulder at about 335 nm attributed to oxygen-to-pseudo-octahedral V(V) charge transfer transition.…”

The design, synthesis and catalytic performance of vanadium-incorporated mesoporous silica with 3D mesoporous structure for propene epoxidation

“…Analogous tendency has been reported in literature for V-containing catalysts. 38 Selectivity for acrolein (ACR) was affected neither by the reaction temperature nor by the catalyst structure, being about 10%, and only a slight decrease of ACR selectivity with increasing reaction temperature was observed. According to our earlier ndings, propene oxide and acetone are mostly primary products of propene oxidation, whereas acrolein is formed over nucleophilic forms of oxygen.…”

Nature of active phase of VO catalysts supported on SiBeta for direct dehydrogenation of propane to propylene

“…It is reported that SiBeta support can effectively disperse and stabilize the introduced VOx species due to the existence of vacant T-sites after removing lattice Al atoms [25,26]. The V-loaded SiBeta serving as effective heterogeneous catalysts have been applied in various catalytic reactions, such as methanol oxidation, NO reduction, conversion of carbohydrates into 2,5-diformylfuran and propene epoxidation [27][28][29][30]. Noticeably, it is confirmed that the catalytic performance of VSiBeta catalysts in some reactions depends on the type of VOx species, which is mainly controlled by V loading amount [21,26,31].…”