Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain alpha-olefins at low temperatures (<100 degrees C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the alpha-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si-OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Automated summary

This study compares the reactivity of different types of Mo-based metathesis catalysts for the metathesis of long-chain alpha-olefins at low temperatures. It is found that catalysts based on supported Mo oxo species exhibit strong activity dependence on the alpha-olefin chain length, while molecular and supported Mo alkylidenes do not show such dependence. Furthermore, the decrease in activity is attributed to the strong adsorption of internal olefin metathesis products on supported Mo oxo catalysts. This study highlights the importance of support and active site composition in catalytic activity and emphasizes the need to consider adsorption in catalyst design.