(521bi) Modeling Active Site Formation of Atomically Dispersed Tungsten Oxide on Silica during Olefin Metathesis

The growing demand for poly(propene) derived from light olefins has led to the development of olefin metathesis catalysts that exhibit high activity and selectivity toward propene. Atomically dispersed tungsten oxide on amorphous silica (WO_x/SiO₂) is a promising catalyst to produce propene from ethene and 2-butene. However, the formation mechanism of the active sites of WO_x/SiO₂ is debated due to the low abundance of active sites and the amorphous nature of silica. Herein, we investigate the role of grafting site heterogeneity in active site formation on WO_x/SiO₂. The study examines how the dihedral angle of grafted WO_x/SiO₂ and local ring strain of silica affect the formation of the W-carbene active site for ethene and 2-butene metathesis to propene. Cluster models of WO_x/SiO₂ are used to study the effect of the vicinal silanol dihedral angle interaction on the formation of the W-carbene species. We vary the size of the supporting siloxane ring structure and model the reaction pathway for W-carbene active site formation using ethene or 2-butene. Density functional theory (DFT) and microkinetic modeling are used to quantify the effect of the WO_x grafting site heterogeneity on SiO₂ for the active site formation. We also predict that 2-butene promotes formation of W-carbene via the pseudo-Wittig reaction faster than ethene. The transferability of qualitative trends predicted using WO_x/SiO₂ cluster models to periodic slab models of amorphous silica is also investigated. In summary, this study provides molecular-level insight into propene metathesis on the industrially relevant WO_x/SiO₂ catalyst.

References:

(1) Howell, J. G.; Li, Y.-P.; Bell, A. T. Propene Metathesis over Supported Tungsten Oxide Catalysts: A Study of Active Site Formation. ACS Catal. 2016, 6 (11), 7728–7738. https://doi.org/10.1021/acscatal.6b01842.