Postdoctoral Researcher, University of Kansas
Develop novel synthesis methods to improve W dispersion and accessibility to maximize propene yield
Propene is an important raw material, which can be used to synthesize tire, cloth, detergent, carpet and so on. The current industrial process uses the WO3/SiO2 catalyst. However, the traditional WO3/SiO2 catalyst faces the problem of low utilization of W species (dispersion and accessibility). Our goal is to develop novel catalyst synthesis methods to improve W dispersion and accessibility to maximize propene yield.
Abstract: The 13C NMR signatures of metallacyclobutanes provide insight into their frontier molecular orbitals and their activity in olefin metathesis.Metallacyclobutanes are an important class of organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M–Cα/α′ and M···Cβ distances, long Cα/α′–Cβ bond length, and isotropic 13C chemical shifts for both early d0 and late d4 transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have 13C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d0 systems, with upfield shifts to ca. −30 ppm for the α-carbon of metallacycles with higher dn electron counts (n = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (δ11 ≥ δ22 ≥ δ33) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local π*(M–Cα/α′) character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates.
Pub.: 14 Jun '17, Pinned: 28 Jun '17
Abstract: Molybdenum-incorporated mesoporous silicates with different Si/Mo ratios were synthesized via the Evaporation Induced Self-Assembly (EISA) method. SAXS and nitrogen physisorption results confirm the ordered mesoporosity. The BET surface area, pore volume and pore size distribution generally decrease with Mo loading while preserving the mesoporosity. Highly dispersed monomeric and oligomeric MoOx species (Mo6+ oxidation state) were evident from diffuse reflectance UV–Vis and XPS spectra. H2-TPR results confirm the existence of polymeric MoOx and bulk crystalline MoO3 species. At lower loadings, Mo exits in both tetrahedral and octahedral symmetry as established by Mo L3-edge XANES studies. Mo-EISA catalysts with highly dispersed isolated MoOx sites display stable activity for 2-butene + ethylene metathesis that is comparable to those reported for WO3/SiO2 catalysts.
Pub.: 02 Mar '17, Pinned: 28 Jun '17
Abstract: Tungsten-incorporated 3D mesoporous silicates, W-KIT-6, W-KIT-5, and W-SBA-16 catalysts, outperform supported tungsten oxide catalysts (WO3/SiO2 and WO3/KIT-6) for the metathesis of ethylene and 2-butene to propene at 450 °C. All catalysts exhibit steady activity and stability during 7 h runs in a fixed-bed reactor. Furthermore, on all mesoporous catalysts, a maximum propylene yield was obtained at an optimum W loading at which the catalyst acidity is also maximum. Slightly delayed addition of the W source during the one-pot synthesis protocol markedly improves the propene yield on W-KIT-6 catalyst. XPS results conclusively show that this enhancement is due to preferential enrichment of active W species on the catalyst surface. Extended runs lasting five days reveal very little loss of activity even which is easily recovered by calcination in air.
Pub.: 06 Oct '16, Pinned: 28 Jun '17
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