| Autor: |
Berkson ZJ; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Zhu R; Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States., Ehinger C; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Lätsch L; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Schmid SP; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Nater D; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Pollitt S; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland.; PSI, CH-5232 Villigen, Switzerland., Safonova OV; PSI, CH-5232 Villigen, Switzerland., Björgvinsdóttir S; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Barnes AB; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland., Román-Leshkov Y; Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States., Price GA; Applied Sciences, bp Innovation & Engineering, BP plc, Saltend, Hull HU12 8DS, U.K., Sunley GJ; Applied Sciences, bp Innovation & Engineering, BP plc, Saltend, Hull HU12 8DS, U.K., Copéret C; Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich CH-8093, Switzerland. |
| Abstrakt: |
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. |
