Titania-supported vanadia (VO x /TiO 2 ) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V 2 O 5 /15% TiO 2 /SiO 2 catalyst (consisting of submonolayer VO x species anchored onto a TiO x monolayer, which in turn is supported on SiO 2 ) during the oxidative dehydrogenation of ethanol. The VO x species in 5% V 2 O 5 /15% TiO 2 /SiO 2 show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO x species supported on SiO 2 with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V 2 O 5 /15% TiO 2 /SiO 2 is kinetically coupled to the formation of a V 4+ intermediate, while the formation of V 3+ is delayed and 10–70 times slower. The low-coordinated nature of various redox states of VO x species (V 5+ , V 4+ , and V 3+ ) in the 5% V 2 O 5 /15% TiO 2 /SiO 2 catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti 4+ /Ti 3+ couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiO x is rather complex. TiO x species might be involved in a fast electron transport between VO x species and might affect the electronic structure of VO x , thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.

A combined SOMC and DFT studies on the Union Carbide catalyst enable to propose that Cr(iii)-H, formed at high-loading, are active in ethylene polymerization. We therefore design a well-defined supported CpCr(iii)-R ethylene polymerization catalyst.

Ziegler-Natta (ZN) catalysts – typically formulated as TiCl4/MgCl2/AlR3 and possibly containing additional organic ligands – are essential to the production of polyethylene and polypropylene. Despite their industrial relevance and years of research on these materials, the role of each constituent (support, organic ligands, post-treatment with organic or inorganic modifiers…) on the structure of Ti surface sites responsible for polymerization remains poorly understood, partly because of the high complexity of such materials. Herein, we show how high-field 47/49Ti NMR can bring about new lights on the structures of the Ti surface sites in ZN pre-catalysts (prior to activation with alkyl aluminium) resulting from adsorption of TiCl4 on MgCl2 followed by a post-treatment with BCl3, an additive used to improve catalytic activity by increasing the amounts of active sites. The implementation of high-field NMR (900 MHz), low temperature (~100 K), magic angle spinning (10 kHz), CPMG echo train acquisition and DFT modelling, to study this material (TiCl4/MgCl2/BCl3) and molecular analogs, allows the detection of a 47/49Ti NMR signature and the development of a molecular level understanding of the NMR signature of Ti surface sites. The extracted 49Ti NMR parameters (δiso, exp = –170 ppm and CQ, exp = 9.3 MHz) from this signature analyzed by DFT modeling indicate the presence of one specific coordination sphere for Ti, namely a fully chlorinated hexacoordinated Ti site with a symmetric charge distribution, due to the post-treatment with BCl3 (that removes the alkoxide ligands) and the coordination environment provided by surface of an amorphous MgCl2.

X-ray absorption spectroscopy (XAS) is one of the most powerful characterization techniques, that has been intensively employed to study the Phillips polymerization catalyst (CrO3/SiO2). While Cr K-edge XAS signatures are used to evaluate the nature of surface (active) sites, they are highly sensitive to oxidation state, geometry and types of ligands, making interpretation challenging. In the specific case of CrO3/SiO2, CO has been particularly used both as a reductant to generate the expected low valent Cr sites and a probe to understand surface Cr sites. Considering the electronic properties of CO, a strong s-donor and pi-acceptor ligand, one may wonder the impact of the coordination of CO on Cr on its XAS signature. We herein built a molecular low-valent Cr library bearing isocyanide ligands, which mimic CO as its isoelectronic counterpart, as a model of low-valent Cr sites interacting with p-acceptor ligand. Cr K-edge XAS augmented with DFT calculations elucidated the profound effect of isocyanide ligand on both XANES and EXAFS regions giving a rise to characteristic features as well as the significant stabilization of low-spin Cr(II/III) species, which potentially alter the ease of interpretation of XAS spectra. Taking the herein demonstrated effect of p-acceptor ligand into account, experimental Cr K-edge spectra of CO-reduced Phillips catalyst at different temperatures were reproduced show-casing the effect of CO on Cr sites.

Since its emergence over 50 years ago, the structure of surface sites in Ziegler-Natta catalysts, which are responsible for a major fraction of the world’s supply of polyethylene (PE) and polypropylene (PP), has remained elusive. This is in part due to the complexity of the systems that involve multiple synthetic steps and components, namely the MgCl2 support, a transition-metal chloride (commonly TiCl4), and several organic modifiers, known as donors, that are used prior and in some instances during the activation step with alkyl aluminum. Due to the favorable NMR properties of V and its use in ZieglerNatta catalysts, we utilize 51V solid-state NMR spectroscopy to investigate the structure of V surface species resulting from the adsorption of VOCl3 on MgCl2(thf)1.5 as support. The resulting V-based ZieglerNatta catalyst shows similar ethylene polymerization activity upon activation with alkyl aluminum as its Ti homologs. Using DFT calculations, benchmarked on a library of molecular structural analogs, the experimentally obtained 51V NMR signature was analyzed to elucidate the structure of the surface sites. Using this approach, we demonstrate that the 51V NMR signature contains detailed information about the coordination environment around V, i.e. the type of ancillary and ligands as well as the effect of the morphology of the MgCl2 support on the geometry of the V surface sites, and corresponds to one species. Analysis of the NMR signatures shows that the adsorption of VOCl3 on MgCl2(thf)1.5 generates a welldefined hexacoordinated V-oxo species containing one alkoxy and four chloride ligands, whose local geometry results from the interaction of the V fragment with an amorphous MgCl2 surface. This study illustrates how NMR spectroscopy, which is highly sensitive to the local environment of the investigated nuclei, here V, enables us to identify the exact coordination sphere and to address the effect of support morphology on surface site structures.