Theoretical calculations of silica supported Mo2(η3-C3H5)4 species

2007

J. Phys. Chem. C

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.

Section: Models Of Silica Surfacementioning

confidence: 99%

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

2007

J. Phys. Chem. C

“…The structure of amorphous silica resembles that of ␤-cristobalite, and the kinds and concentration of the surface hydroxyl groups on silica and on ␤-cristobalite are also similar [31][32][33][34]. Therefore, in many theoretical studies involving amorphous silica, the ␤-cristobalite structure is employed [35][36][37][38][39].…”

Section: Computational Detailsmentioning

confidence: 99%

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

Int J of Quantum Chemistry

2007

ABSTRACT:The two-layer ONIOM method is applied for description of Momethylidene center on silica, which is the active site of olefin metathesis. Two clusters of different size are employed to model the silica surface. For the larger one, two differently defined inner layers (the model systems) are tested. In all the calculations, the B3LYP functional is adopted for the inner layer, while the Hartree-Fock method or the local density approximation (SVWN5 functional) is used for the treatment of the real system. Based on the reference results obtained for the real system from the B3LYP calculations, it is concluded that both B3LYP:HF and B3LYP:SVWN5 schemes are suitable for proper description of the geometry of the Mo center and its activity in ethene metathesis. The former combination is a little better choice, however. It is also shown that the inner layer, including the third coordination sphere of molybdenum, is large enough to obtain satisfactory results. On the other hand, the relative energies and some geometrical parameters of the active site are dependent on the size of the entire system studied. The sensitivity of the geometrical parameters and reaction energies to the changes of the scale factor g, involved in the ONIOM scheme, is studied as well. It is concluded that the link atom distance is not crucial for obtaining correct results.

“…The high surface area silica is an amorphous material, and modeling its surface is a difficult task. On the basis of 29 Si NMR, 1 H NMR, and IR results, both geminal and single as well as hydrogen-bonded and isolated silanols are distinguished on the silica surface. − It was reported that the structure of amorphous silica resembles that of β-cristobalite, and the kinds and concentration of the surface hydroxyl groups on silica and on β-cristobalite are also similar. ,,, Therefore, many theoretical simulations of amorphous silica are based on the β-cristobalite structure. − On the fully hydroxylated (100) plane of β-cristobalite, all the silanols are of geminal type. Hydroxyl groups of adjacent geminal pairs can form hydrogen bonds, which facilitate partial dehydroxylation at elevated temperatures, leading to siloxane linkages and isolated hydroxyls, so-called vicinal silanols. ,,,,− , On the other hand, only isolated single hydroxyl groups can be present on the (111) surface of β-cristobalite. ,,,,, It is proposed that real silica surface contains segments resembling both (100) and (111) faces of β-cristobalite. ,, …”

Section: Models Of Silica Surfacementioning

confidence: 99%

“…50,51,54,55 Therefore, many theoretical simulations of amorphous silica are based on the β-cristobalite structure. [59][60][61][62][63][64][65][66][67][68][69][70][71] On the fully hydroxylated (100) plane of β-cristobalite, all the silanols are of geminal type. Hydroxyl groups of adjacent geminal pairs can form hydrogen bonds, which facilitate partial dehydroxylation at elevated temperatures, leading to siloxane linkages and isolated hydroxyls, so-called vicinal silanols.…”

Section: Introductionmentioning

confidence: 99%

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

2005

J. Phys. Chem. B

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.