Reactivity of a Silica-Supported Mo Alkylidene Catalyst toward Alkanes: A DFT Study on the Metathesis of Propane

Abstract

The metathesis of alkanes is a process in which a given alkane is transformed into higher and lower homologues. Here, we carried out DFT calculations in order to get insights into the most favorable reaction pathway for the metathesis of propane into mainly ethane and butane catalyzed by a silica-supported molybdenum alkylidene bearing an imido ligand at 150 °C. The overall catalytic process is divided into two stages, precursor activation and catalytic cycle, and both of them consist of the same types of reactions, (i) ligand exchange, (ii) proton transfer between two α-carbons, and (iii) ligand rearrangement, which in turn consists of several steps, such as β-H elimination, alkene cross-metathesis, and alkene insertion. Our results suggest that the formal ligand exchange reaction with propane proceeds through a dissociative mechanism with the formation of a high-energy molybdenum alkylidyne species. The calculated energetics at 150 °C indicates that the active species is a molybdenum propylidene species that is formed with an overall Gibbs activation barrier of 39.4 kcal mol −1 . The catalytic cycle to the main products (ethane and butane) has an energy span of 43 kcal mol −1 , whereas the cycle for the production of minor products (methane and pentane) has a much higher energy span, in agreement with experiments. These data suggest that the catalytic cycle is the rate-determining stage in the whole process and thus the precursor activation should be faster. The results obtained here help to rationalize the chemical reactivity of supported molybdenum alkylidene catalysts toward alkanes.

Metal Alkyls with Alkylidynic Metal‐Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

“…Hence, according to DFT calculations, the most favorable alkyl exchange mechanism during alkane metathesis with silica‐supported WMe 6 likely involves metal alkylidyne species as key intermediates, which are capable of activating the C−H bonds of alkanes. This finding is consistent with previously reported calculations on Mo‐based systems …”

“…The stabilization of alkylidynes in such low‐coordinated compounds hence requires the presence of ancillary ligands, such as phosphines or, in the case of silica‐supported species, siloxane‐bridges. Similarly, ligand fields that can stabilize alkylidynes are also present in typical alkyl‐alkylidene catalysts, which are known to catalyze the alkane metathesis reaction . This also holds true for Ti‐species in certain ligand fields, such as these involving PNP (PNP=N[ 2 ‐P(CHMe 2 ) 2 ‐4‐methylphenyl] 2 ) pincer‐type ligands.…”

“…Other well‐defined alkane metathesis catalysts are based on silica‐supported metal alkyl‐alkylidene d 0 surface species. They, along with metal alkylidynes, are proposed key intermediates for this transformation; the latter are observed in a few cases …”

Synthesis of High Metal Loading Single Atom Catalysts and Exploration of the Active Center Structure

“…Ligands play a critical role in the catalytic process. For example, metal hydrides are necessary for C−H bond activation, as well as for chain walking, [209] whereas a metal carbene is essential for olefin metathesis [210] . In some reactions, the real active center comprises single metal atoms with functional ligands, as well as well‐defined surfaces [211–212] .…”

“…For example, metal hydrides are necessary for CÀ H bond activation, as well as for chain walking, [209] whereas a metal carbene is essential for olefin metathesis. [210] In some reactions, the real active center comprises single metal atoms with functional ligands, as well as well-defined surfaces. [211][212] The ligand may control the oxidation state, geometry, and d n electron configuration of the metal center, [212] contributing significantly to the activity, selectivity, and stability of catalysts.…”