Structure Determination of Surface Sites in Heterogeneous Olefin Metathesis Catalysts from 95Mo NMR Signatures

Heterogeneous olefin metathesis catalysts composed of highly dispersed Mo sites on support carriers are used to produce key olefin building blocks for higher value-added products, e.g. the Shell Higher Olefin Process (SHOP). However, understanding the nature of Mo sites in catalysts, i.e. molybdenum oxides supported on amorphous supports like silica, MoOx/SiO2, is hampered by a low fraction of surface metal sites (low loading), the possible distribution of surface species and the lack of very sensitive characterization methods to distinguish subtle changes in the coordination environment of Mo sites on amorphous supports. Here, we show how 95Mo NMR spectroscopy enables identifying various Mo sites in olefin metathesis catalysts, pointing out the presence of isolated tetrahedral Mo-dioxo sites along with MoOx clusters, whose quantities depend on the preparation methods and Mo-loadings. Most notable is the use of NMR signatures as a structural refinement tool. Following a validation with molecular mono- and di-oxo Mo(VI) complexes characterized by single-crystal XRD measurements, we are able to determine the exact coordination environment, geometry and structural parameters of the dominant Mo sites, a highly dispersed dioxo Mo bis-grafted species in a tetrahedral environment with d(Mo=O) = 1.71 ± 0.07 Å, d(Mo-OSi) = 1.91 ± 0.15 Å, ∠(O-Mo-O) = 107 ± 7° and ∠(Mo-O-Si) = 140 ± 40°. This structure closely resembles this of the [MoO2(OSi(OtBu)3)2] molecular analogue. Additionally, this signature indicates a narrow distribution of Mo sites, highlighting the well-defined nature of these sites. Noteworthily, while well-defined, the large promotional effect observed upon co-feeding 2,3-dimethyl-1-butene further supports the role of nearby protons to initiate metathesis, rather than the sole nature of the isolated Mo sites themselves, hence the importance of promoters and the small fraction of active sites in their absence.