Active Site Descriptors from ⁹⁵Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts

Abstract

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5− 15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV−vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5−2.0. Notably, solid-state 95 Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded 95 Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that 95 Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO−Mo(O) 2 −OSi) angles and low coordination numbers at Mo lead to highly deshielded 95 Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the 95 Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The 95 Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

Solid-State 183W NMR Spectroscopy as a High-Resolution Probe of Polyoxotungstate Structures and Dynamics

“…If computational predictions of 183 W chemical shifts can be improved, benchmarked, and validated by such experimental results, these methods offer the potential to link 183 W chemical shift tensors directly to electronic structure (frontier molecular orbitals) as previously explored for other nuclei . The spectroscopic signature ( 183 W chemical shifts) provided by 183 W NMR for each site in a polyoxotungstate should in principle provide a direct link among the local geometry, dynamics, and electronic structure, thereby offering experimental descriptors for site-specific reactivity, similar to what has recently been demonstrated for other challenging low-γ nuclei like 95 Mo. , Such approaches could be widely extensible to other polyoxotungstates as well as diverse other classes of W-containing materials.…”

“…32 The spectroscopic signature ( 183 W chemical shifts) provided by 183 W NMR for each site in a polyoxotungstate should in principle provide a direct link among the local geometry, dynamics, and electronic structure, thereby offering experimental descriptors for sitespecific reactivity, similar to what has recently been demonstrated for other challenging low-γ nuclei like 95 Mo. 33,34 Such approaches could be widely extensible to other polyoxotungstates as well as diverse other classes of Wcontaining materials.…”

Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WOx/SiO2 Catalyst: A Computational Perspective

“…Recent 95 Mo NMR studies of silica-supported molybdenum oxide catalysts indicated that high metathesis activity is related to strained tetra-coordinated dioxo Mo sites that are easily reduced. 64 Such Mo species are characterized by a SiO− Mo(O) 2 −OSi angle of approximately 90°. Our models of the dioxo Mo(VI) species, used in the previous work, 17 have strained 6-membered molybdasiloxane rings with the SiO− Mo(O) 2 −OSi angles in the range of 88−106°(Table S1).…”

Advancing Evidence-Based Data Interpretation in UV–Vis and Fluorescence Analysis for Nanomaterials: An Analytical Chemistry Perspective

“…The integrating spheres needed for ISARS and ISUV spectroscopic measurements are both commercially available from their respective vendors. Integrating spheres have been used in UV–vis-based spectroscopic acquisition for experimental quantification of the diffused UV–vis reflectance/absorption of the solid samples. − The key difference between the ISUV techniques and the integrating-sphere-based diffuse reflectance/absorption quantification is that the former detects forward propagated light. In contrast, the quantification of reflectance/absorption measures the light propagated backward to the integrating spheres when light is illuminated on solid samples.…”