Theoretical Investigations of Isolated Mo(VI) and Mo(IV) Centers of a Molybdena−Silica Catalyst for Olefin Metathesis

Abstract

A molybdena−silica catalyst for olefin metathesis is theoretically studied using the two-layer ONIOM method and the conventional cluster approach. Various possible structures and locations of Mo methylidene sites are examined. Both the fully hydroxylated and partially dehydroxylated silica surface is considered. It is predicted that four-coordinate Mo(VI) alkylidene species are active in olefin metathesis, while five-coordinate Mo(VI) alkylidene centers as well as surface Mo(IV) alkylidene complexes are not the metathesis active sites. The theoretical wavenumbers of the C−H stretching vibrations for the Mo methylidene, Mo ethylidene, and molybdacyclobutane centers are compared with the reported IR data. It is concluded that the four-coordinate Mo(VI) alkylidene species and the corresponding square pyramidal molybdacyclobutane forms are most likely detected on the real catalyst by the IR measurements. The theoretical frequencies for the five-coordinate Mo(VI) methylidene species and the related molybdacyclobutane structure are also satisfactorily consistent with the experimental data. The comparison of the calculated energies of ethene adsorption on the Mo(IV) oxide precursors with the reported experimental value suggests that mono-oxo Mo(IV) species are present in the reduced catalyst.

Computational Insights into Active Site Formation during Alkene Metathesis over a MoOx/SiO2 Catalyst: The Role of Surface Silanols

“…In the case of the MoO x /SiO 2 metathesis catalyst, no computational works on the initiation routes have been reported, except for thermochemical studies of the mechanism involving monografted hydroxy dioxo Mo­(VI) species, which are rather unlikely. , Only the propagation mechanism of ethene metathesis was studied for this system. ,− Attempts were made to determine structure–reactivity relationships and to account for heterogeneity of the active sites. ,− It was shown that activity of digrafted mono-oxo Mo­(VI) methylidene species is strongly affected by its geometry, depending on the local structure of silica, while tetragrafted Mo­(VI) methylidene species and digrafted Mo­(IV) methylidene species should be inactive …”

Well-Defined Molybdenum Oxo Alkyl Complex Supported on Silica by Surface Organometallic Chemistry: A Highly Active Olefin Metathesis Precatalyst

“…Supported MoO 3 catalysts feature different types of surface metallic species (monomeric, oligomeric, and clustered molybdates), leading to the proposal of several activation pathways from monopodal or bipodal surface species. − Trunschke and co-workers suggested, on the basis of IR and Raman spectroscopy and microcalorimetry studies of the activation of dispersed MoO 3 on SBA-15, that the active species arises from a monopodal bis­(oxo) hydroxide species that in the presence of propylene gives a monopodal isopropoxyl molybdenum oxo carbene with concomitant release of an acetone molecule (Scheme a) . However, the actual active species has also been stated to be an isolated bipodal molybdenum oxo carbene surface species. ,, Recent works from Stair and co-workers bring more experimental evidence regarding the role of the isolated bipodal bis­(oxo) molybdenum surface species that can be transformed into molybdenum carbene species by pseudo-Wittig reaction with release of aldehydes (Scheme b) . Importantly, such catalysts can be easily regenerated by treatment at high temperature under a flow of inert gas.…”

“…4 However, the actual active species has also been stated to be an isolated bipodal molybdenum oxo carbene surface species. 3,5,7 Recent works from Stair and co-workers bring more experimental evidence regarding the role of the isolated bipodal bis(oxo) molybdenum surface species that can be transformed into molybdenum carbene species by pseudo-Wittig reaction with release of aldehydes (Scheme 1b). 6 Importantly, such catalysts can be easily regenerated by treatment at high temperature under a flow of inert gas.…”

SBA-15 as a Support for Effective Olefin Metathesis Catalysts

“…The experimental work in this field was accompanied by a computational approach to the study with the aim to describe the surface process on a molecular level and to help in understanding the role of support [79]. In computational studies of Mo oxide species on silica surface DFT methods were usually used and silica support was approximated by a structure similar to β-cristobalite [80][81][82].…”