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

Handzlik, Jarosław

doi:10.1002/qua.21397

Cited by 21 publications

(26 citation statements)

References 52 publications

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“…In these reports, both materials were characterized by XAS. The fitted MoO bond lengths (1.70–1.71 Å, Table S2) are consistent with values reported for molecular dioxomolybdates and DFT simulations, ,, as well as the value found in this work (1.68 Å). However, the Mo–O distance reported for the grafted molybdate derived from Mo(O) 2 [OSi(O t Bu) 3 ] 2 is longer (1.92 Å) than the corresponding Mo–O paths in silica-supported materials made from Mo(O)(O t Bu) 4 and MoO 2 Cl 2 (1.87 and 1.88 Å, respectively).…”

Section: Resultssupporting

confidence: 90%

X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

Scott

2021

J. Phys. Chem. C

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Dispersed Mo oxides associated with an oxide support catalyze a wide variety of hydrocarbon transformations. The supported catalysts typically contain several types of molybdate species. Because a small fraction is catalytically active, characterization of the precise nature of these active sites and their activation mechanisms is challenging. In this work, monomolybdate sites with uniform first coordination spheres were synthesized by a direct solid−solid reaction between MoO 2 Cl 2 and silica. The Mo(O) 2 (OSi) 2 sites were characterized by X-ray absorption spectroscopy, enabled by comparison to several well-defined model compounds. In molecular MoO 2 (OSiPh 3 ) 2 , a prominent feature at ca. 3 Å in the Fourier-transform extended X-ray absorption fine structure arises due to constructive interference between single-, double-, and triple-scattering paths involving the near-linear silanolate ligands. Coordination of additional ligands causes the Mo−O−Si angle to decrease, weakening this feature. Even stronger attenuation is characteristic of Mo(O) 2 (OSi) 2 sites, which are inferred to have smaller Mo−O−Si angles and a range of Mo−Si distances imposed by the nature of their attachment to the silica surface as molybdasiloxane rings; smaller rings with lower Mo−O−Si angles contribute little intensity. While Mo(O) 2 (OSi) 2 sites may be useful in mechanistic studies involving supported molybdate catalysts, they are not single-site materials. The impact of the intrinsic heterogeneity of their attachment to the support on their reactivity will be important to consider in structure-property studies.

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Section: Resultssupporting

confidence: 90%

X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

Scott

2021

J. Phys. Chem. C

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“…An inner sphere centered at the metal atom and with a radius of ∼6 Å was allowed to relax while keeping the surrounding silica fixed to maintain its amorphous character. Handzlik et al showed that an inner sphere of this size is sufficient to describe the metal site properly. , The calculations were performed with the B3LYP , functional and the D3 dispersion correction . The basis sets used for the various elements were as follows: Def2TZVP and Stuttgart’s pseudopotentials for the metals; 6-31++G(d,p) for the hydroxyls within the inner sphere and the ammonia molecules; 6-31G(d) for the remaining atoms in the inner sphere; and 3-21G for the rest.…”

Section: Methodsmentioning

confidence: 99%

“…This limits the quality of the levels of theory that can be employed (e.g., hybrid or double-hybrid functionals are not feasible), so that the accuracy of the energies computed in a mechanistic study may be unreliable. On the other hand, a cluster model of similar size of a PBC simulation cell can be fairly easily treated with better density functionals and larger basis sets, and even higher-level wave function methods through hybrid approaches . Physically, the use of a cluster model is justified because silica is an insulator, and long-range interactions between repeated cells are negligible.…”

Section: Introductionmentioning

confidence: 99%

“…On the other hand, a cluster model of similar size of a PBC simulation cell can be fairly easily treated with better density functionals and larger basis sets, and even higher-level wave function methods through hybrid approaches. 25 Physically, the use of a cluster model is justified because silica is an insulator, and long-range interactions between repeated cells are negligible. Additionally, it is simpler to create many copies of the same site with different amorphous environment, so that the properties of the site can be averaged over multiple configurations and more directly compared to experimental measurements.…”

Section: Introductionmentioning

confidence: 99%

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Cluster Model Simulations of Metal-Doped Amorphous Silicates for Heterogeneous Catalysis

Caricato

2021

J. Phys. Chem. C

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This Feature Article discusses how cluster models can be effectively used for quantum chemistry simulations of metal-doped amorphous silicates. These materials have been successfully used as heterogeneous catalysts for a variety of reactions, including olefin metathesis and polymerization, and alcohol dehydration. The amorphous surface provides a large surface area and distorted metal sites that are very reactive. However, the disordered microscopic nature of these silicates makes them hard to characterize, and progress toward better catalysts relies on a trial-and-error approach. Simulations can in principle provide insights on structure–property relations that may lead to a rational design approach, but again the disordered structure of the active sites makes the formulation of reliable models difficult. Using cluster models, it is possible to create multiple replicas of the same site with different distorted structures and silica environments, such that one can obtain a more realistic picture of the behavior of the material. Simulations based on the METal-doped Amorphous SIlicate Library (METASIL), which includes 70 cluster structures for single-site Zr-, Nb-, and W-doped silica, show very good agreement with a variety of experimental characterization techniques. These calculations validate the models and, more importantly, they allow to identify important structural descriptors of the active sites as well as a number of potential issues in the interpretation of experimental data. The Feature Article also discusses what challenges are still open in the simulation of amorphous materials and how these may be addressed in combination with new approaches such as machine learning algorithms.

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“…Dangling bonds were saturated with H [33,34]. Cr(II) species were attached to the (1 0 0) crystal face by replacing two of these H atoms.…”

Section: Methodsmentioning

confidence: 99%

Spectroscopic and structural characterization of Cr(II)/SiO2 active site precursors in model Phillips polymerization catalysts

Zhong

Lee

Liu

et al. 2012

Journal of Catalysis

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

Cited by 21 publications

References 52 publications

X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

X-ray Absorption Spectroscopy Investigation into the Origins of Heterogeneity in Silica-Supported Dioxomonomolybdates

Cluster Model Simulations of Metal-Doped Amorphous Silicates for Heterogeneous Catalysis

Spectroscopic and structural characterization of Cr(II)/SiO2 active site precursors in model Phillips polymerization catalysts

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