Metathesis Activity and Properties of Mo−Alkylidene Sites Differently Located on Silica. A Density Functional Theory Study

Abstract

Ethene metathesis proceeding on Mo-methylidene centers on silica is investigated with density functional theory, applying the cluster approach. Three different locations of the active sites are considered, in which the Mo center replaces a pair of geminal silanols, two silanols from adjacent geminal pairs and two single silanols, respectively. It is shown that metathesis activity of the Mo-methylidene sites strongly depends on their location on silica. Different reactivity of the centers toward alkene is explained by differences in their geometrical and electronic structure parameters. The calculated C-H stretching vibrations of the proposed Mo-methylidene, Mo-ethylidene, and molybdacyclobutane surface complexes are well consistent with the reported IR spectra for the corresponding species generated on real molybdena-silica catalysts. On the basis of the obtained results it is proposed that among the studied cases, the Mo centers replacing two silanols from adjacent geminal pairs of silica surface are the most adequate models of the real active sites.

Application of the ONIOM (QM/QM) method in the study of molybdena–silica system active in olefin metathesis

“…The first structure (1) was previously shown to be a reasonable model of the active site, on the basis of the comparison between the theoretical and experimental results [29]. The present model 1 and the corresponding structure from the Ref.…”

“…The present model 1 and the corresponding structure from the Ref. 29 differ from each other in the orientation of the hydroxyl group bonded to the Si2 atom. In the models 1-4, the Mo-methylidene center is attached to the silica cluster by replacing a pair of geminal silanols.…”

“…As was previously shown for the Mo-methylidene center 1, a molybdacyclobutane intermediate having trigonal bipyramidal geometry (TBP) is the initial product of the ethene addition [29]. Then, it can split into another ethene molecule and a new Mo-methylidene site or rearrange to an isomeric molybdacyclobutane complex with square pyramidal geometry (SP).…”

“…properties and metathesis activity of the Mo-alkylidene sites located on silica [29]. In the present work, the applicability of the two-layer ONIOM method for this system is tested.…”

Structure of molybdenum oxide supported on silica SBA-15 studied by Raman, UV–Vis and X-ray absorption spectroscopy

“…Supported molybdenum oxide catalysts have been studied for numerous reactions like partial oxidation reactions of alcohols [1,2], ethers [3], alkanes and olefins [4][5][6], metathesis reactions [7,8] and the selective catalytic reduction (SCR) of nitric oxide [9,10]. Previous studies have shown that the catalytic properties may depend on the molybdenum oxide structure as well as the choice of the support material [3,[11][12][13].…”