New supported metal oxides : toward understanding of industrial catalysts for olefin metathesis through a combined synthetic, 17O NMR and DFT approach. – OXOCAT

The selected materials will be in a first stage characterized by “classical” techniques, mostly using vibrational and routine MAS NMR spectroscopies in order to check their structure. The variety of materials prepared in the synthetic part of the project will allow structure-activity relationships to be drawn. In parallel, 17O NMR/DFT data of selected metal oxo molecular systems (Mo and W) will be obtained to provide an extensive set of NMR parameters, which accurately describe the local oxygen structure. Our main purpose in this proposal is to follow the evolution of NMR parameters for those oxygen environments (i) after grafting on surface and (ii) in catalysis-related states: initiation into active species and deactivation.

Our first results have been obtained on the grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C. It was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined pre-catalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. 17O labelling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding 17O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the 17O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and interactions.

A clear correlation between the type of grafted elements and the 17O NMR/DFT parameters is expected for a deep characterisation of surface species in supported metal oxo catalysts. In the final stage of the project, mechanistic studies using not only classical methods but also our 17O NMR/DFT approach will be carried out, using 17O enriched samples selected for their catalytic performance. We anticipate from our acquired expertise, that our method will help to identify surface oxo species involved in catalysis, resulting from initiation or deactivation. This proposal is the first use of 17O solid-state NMR to molecularly defined supported catalysts, in the view of shedding some light on shady aspects of industrially relevant catalytic materials.

Oral communication in an international conference: Well-defined silica-supported tungsten oxo alkyl derivatives as models of WO3/SiO2 olefin metathesis catalysts, M. Taoufik, L. Delevoye, R. M. Gauvin, ACS 245th National Meeting, Division of Catalysis Science and Technology, 7-12 april 2013, New Orleans, USA.

Owing to the strategic importance of olefins as building blocks for the world chemical industry, development of efficient alkene metathesis processes is of utmost relevance. The main drawbacks of these heterogeneous systems are 1) the low concentration of isolated active species, which complicates their characterization and 2) the difficulty to activate during the initial contacting of freshly catalysts with olefin, which complicates both their development and their understanding. In this context, tailored heterogeneous catalysts with known structure-activity relationship may improve lifetimes and activity by increasing the numbers of active sites. We propose to prepare well-defined supported metal oxo species of molybdenum, and tungsten , containing either carbenic moieties, alkyls fragments amenable to carbene formation, or solely oxo ligands. These materials are targeted as they may be (precursors to) models of the active species in industrially-relevant olefin metathesis catalysts. They will be in a first stage characterized by “classical” techniques, mostly using vibrational and routine MAS NMR spectroscopies. Major efforts will be put at selective 17O enrichment of the oxo moiety within selected surface species, for characterization by 17O solid-state NMR, the other major aspect of the project. The well-defined metal oxo species will be probed in normal and functionalized olefin metathesis reactions, both in batch and in dynamics reactors. The variety of materials prepared in the synthetic part of the project will allow structure-activity relationships to be drawn. In parallel, 17O NMR/DFT data of selected metal oxo molecular systems (Mo and W) will be obtained to provide an extensive set of NMR parameters, which accurately describe the local oxygen structure. Our main purpose in this proposal is to follow the evolution of NMR parameters for those oxygen environments (i) after grafting on surface and (ii) in catalysis-related states: initiation into active species and deactivation. It will require surface models for alumina and silica to be refined using a theoretical chemistry approach combining DFT and experimental 17O NMR results. A clear correlation between the type of grafted elements and the 17O NMR/DFT parameters is expected for a deep characterisation of surface species in supported metal oxo catalysts. In the final stage of the project, mechanistic studies using not only classical methods but also our 17O NMR/DFT approach will be carried out, using 17O enriched samples selected for their catalytic performance. We anticipate from our acquired expertise, that our method will help to identify surface oxo species involved in catalysis, resulting from initiation or deactivation. This proposal is the first use of 17O solid-state NMR to molecularly defined supported catalysts, in the view of shedding some light on shady aspects of industrially relevant catalytic materials.