Promoting active site renewal in heterogeneous olefin metathesis catalysts

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

“…This trend approximately mirrors what is observed for the initial 1nonene metathesis rates in the liquid phase at 70 °C, albeit the gas phase reaction rates are much lower despite the higher temperatures, possibly pointing to significantly lower amounts of active sites under these activation conditions. In the presence of 1 mol % of a mixture of organic promoter species (85% 2,3-dimethyl-2-butene and 15% 2,3-dimethyl-1-butene; denoted 4MEs), 11 the steady-state propene metathesis rates increase by 1−2 orders of magnitude for all catalysts (Figure 2b, orange) to 7.8 × 10 −2 , 3.6 × 10 −2 , 3.7 × 10 −2 , 0.74 × 10 −2 , and 0.062 × 10 −2 s −1 for MoSOMC 2.3 , MoIWI 2.5 , MoIWI 4.8 , MoIWI 7.8 , and MoIWI 15.6 , respectively, corresponding to enhancement factors of ca. 120, 90, 25, 22, and 80.…”

“…These enhancements in catalyst activity are similar in magnitude to values recently obtained for silica-supported W (×26 at 250 °C) and Mo (×30 at 200 °C) oxo catalysts and arise from the ability of the branched promoter olefins to facilitate proton transfers involved in the activation and renewal of catalytic sites. 11 The substantial increase in reaction rates and the very different enhancement factors suggest differences in the numbers of active sites formed as a function of the Mo loading and synthesis method. Repeated catalytic tests of MoSOMC 2.3 and MoIWI 2.5 under both unpromoted and promoted conditions show that the rates and promotional effects are consistent and reproducible (Table S3.5).…”

“…Surface Brønsted and Lewis Acid Sites. Both Brønsted and Lewis acid sites are proposed to be important for olefin metathesis catalytic activity 11,30 with Brønsted acid sites required for the formation of active sites and Lewis acid sites associated with coordinatively unsaturated Mo centers. Therefore, we examined the presence and nature of such acid sites by low-temperature solid-state 15 N{ 1 H} cross-polarization (CP) magic angle spinning (MAS) NMR analysis of the fully oxidized (pre)catalysts contacted with 15 N-pyridine and desorbed under high vacuum at room temperature.…”

“…For example, the presence of Brønsted acid sites, evidenced here by solid-state 15 N{ 1 H} CPMAS NMR of pyridine-adsorbed samples, has been proposed to be involved in generating the active high-valent metal(VI) alkylidenes from low-valent M(IV) species. 10,30,65,66 As recently shown for both Mo and W-based heterogeneous catalysts, this process is dynamic and can be accelerated by the presence of organic mediator species such as branched olefins, which act as proton shuttles, 11 yielding almost 2 orders of magnitude in the increase in catalytic activity for MoSOMC 2.3 . The different propene metathesis enhancement factors observed on the addition of branched promoter olefins for the SOMC (×120) and classical catalysts (×22−90) show how the dynamic equilibrium of active and inactive Mo sites, which can be altered by the composition of the gas feed, also depends on the strain and reducibility of surface sites, which depend in turn on the synthetic approach.…”

“…However, the small fractions of active sites suggest that specific (coordination) environments are required for their generation. The presence of proximate Brønsted acid sites, for instance, has been proposed to assist catalyst initiation. , 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 readily transformed for conversion into active sites . However, little is known of the structures and associated spectroscopic signatures of these sites and the structural differences between active, dormant, or inactive sites.…”