Understanding a catalytic system requires knowledge of the structure – activity relationship for the active species. For many heterogeneous catalysts, developing this relationship can be complicated by the heterogeneity of structural centers present on the catalyst surface. These challenges are well exemplified in the case of olefin metathesis, where it is typically reported that less than 5 % of the metal content of WOx, MoOx, or ReOx supported on silica or alumina is active.1 Despite a long history of industrial use, the nature of the active site is not known definitively, and limited progress has been made in the development of catalysts for large-scale applications. The low percentage of active metal content makes spectroscopic observation of the active site, and reaction intermediates, especially difficult. Another longstanding question is the mechanism for the formation of the proposed active site, an isolated metallocarbene species1, from the metal oxide of the as-synthesized catalyst.
The purpose of our work has been to define the structure of active centers for metathesis over silica-supported tungstena, identify their distinguishing characteristics from spectator sites, and elucidate the mechanism of their formation. We have employed a variety of synthesis strategies, including the use of high surface area mesopourous silica supports and well-controlled grafting techniques of W-compounds, to develop model catalyst samples that can aid our ability to probe these questions spectroscopically.
Through the combined application of UV-Vis, Raman, XAS, and 183W MAS NMR, we have been able to characterize the structure of WOx species dispersed on silica. Our recent work has focused on extending the use of these techniques towards studying how isolated tungstate species are converted into active sites upon exposure to the olefin reactant. By identifying and quantifying transient products formed at the onset of reaction, we have further worked to elucidate the mechanism for active site formation, and establish the number of W-sites that may be involved in the catalysis.
This work has also evaluated various pretreatment strategies that can increase the fraction of dispersed WOx sites that becomes active for metathesis, and reduce the induction time to achieve steady-state activity. By studying the influence of these pretreatments on the structure and oxidation state of tungsten surface species, we have further advanced our understanding of the precursor-sites to active tungsten centers, as well as insight to the active site formation process.
DFT calculations are being used to bolster our understanding of the relationship between the structure of tungsten oxide on silica and activity for olefin metathesis. The calculated results provide insight to the influence of the anchoring bond geometry of W-oxide to the silica surface on the energetic barriers for reduction and W-carbene formation. These results aid in explaining the chemistry behind the carbene formation process, as well as the low number of active metal centers in this catalytic system.
1. Lwin, S.; Wachs, I. ACS Catal. 4 (2014) 2505.