Universal promotion of heterogeneous olefin metathesis catalysts by controlling dynamic active site renewal

Abstract

Olefin metathesis is a versatile strategy for large-scale olefin interconversion, yet mechanistic details over industrial heterogeneous catalysts have remained ambiguous for decades. Here, from rigorous kinetic measurements, spectroscopic studies, and computational modeling of propylene metathesis over model and industrial WOx/SiO2 catalysts, we identify a hitherto unknown site renewal and decay cycle, mediated by proton transfers involving proximal Brønsted acidic OH groups, which operates concurrently with the classical Chauvin cycle. We show how this cycle can be manipulated using small quantities of promoter olefins to drastically increase steady-state propylene metathesis rates by up to 30-fold at 250oC with negligible promoter consumption. The increase in activity and considerable reduction of operating temperature requirements were also observed on MoOx/SiO2 catalysts, showing that this strategy is general and can address major roadblocks associated with industrial metathesis processes.

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

“…For instance, interactions of functionalized olefins containing ester groups and surface silanol groups were recently found to enrich the near-surface concentration of olefins and influence product selectivities for ring-closing metathesis reactions catalyzed by well-defined cationic Mo alkylidenes supported on mesoporous silicas . The silica-supported Mo oxo system is known to possess strong Brønsted acid sites that could act as adsorption sites, , and even on bare hydroxylated silica, the interaction energies of hydrocarbons are known to increase as a function of the chain length and are greater for alkenes than alkanes . The catalytic reaction tests and spectroscopic analyses discussed above show that substrate-silica interactions are non-negligible for long-chain olefinic hydrocarbons and indeed have significant effects on catalytic reaction properties at low reaction temperatures (<100 °C).…”

“…Decades of research have yielded highly active and selective olefin metathesis catalysts based on molecular Mo-, W-, and Ru-alkylidenes that are highly active at low temperatures (e.g., room temperature to 100 °C) and tolerant to functional groups in many instances, enabling broad applications in organic and polymer syntheses (Figure a). − By comparison, heterogeneous olefin metathesis catalysts, mostly based on supported Mo or W oxides, are industrially used for the upgrading of light olefins but require high-temperature activation and/or reaction conditions (>150 °C and even 400 °C for W-based catalysts) . They are composed of ill-defined surface structures with low (<5%) quantities of active sites and are proposed to require complex initiation processes at high temperatures, involving, for instance, surface OH groups. − These shortcomings have limited their broader adoption.…”

Olefin-surface interactions: a key activity parameter in silica-supported olefin metathesis catalysts

“…For instance, interactions of functionalized olefins containing ester groups and surface silanol groups were recently found to enrich the near-surface concentration of olefins and influence product selectivities for ring-closing metathesis reactions catalyzed by well-defined cationic Mo alkylidenes supported on mesoporous silicas. 54 The silica-supported Mo oxo system is known to possess strong Brønsted acid sites that could act as adsorption sites, 11,55 and even on bare hydroxylated silica the interaction energies of hydrocarbons are known to increase as a function of chain length and are greater for alkenes than alkanes. 56 The catalytic reaction tests and spectroscopic analyses discussed above show that substratesilica interactions are non-negligible for long-chain olefinic hydrocarbons and indeed have significant effects on catalytic reaction properties at low reaction temperatures (<100 °C).…”

“…8 They are composed of ill-defined surface structures with low (<5%) quantities of active sites, and are proposed to involve complex initiation processes at high temperatures, involving for instance surface OH groups. [9][10][11] These shortcomings have limited their broader adoption.…”

Active Site Descriptors from 95Mo NMR Signatures of Silica-supported Mo-based Olefin Metathesis Catalysts

“…The presence of proximate Brønsted acid sites, for instance, has been proposed to assist catalyst initiation. 10,11 Furthermore, it has been suggested that the active sites originate from strained and correspondingly high-energy configurations of the tetrahedral Mo dioxo unit, which are more reactive and amiable for conversion into active sites. 9 Yet, little is known about the structures and associated spectroscopic signatures of these sites and the structural differences between active, dormant, or inactive sites.…”