Propylene Metathesis over Molybdenum Silicate Microspheres with Dispersed Active Sites

Skoda, David; Zhu, Ran; Hanulikova, Barbora; Styskalik, Ales; Vykoukal, Vit; Machac, Petr; Simonikova, Lucie; Kuritka, Ivo; Poleunis, Claude; Debecker, Damien P.; Román-Leshkov, Yuriy

doi:10.1021/acscatal.3c02045

Cited by 5 publications

(4 citation statements)

References 69 publications

(195 reference statements)

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“…The in situ diffuse reflectance FTIR (DRIFTS) spectra of the M 1.5 -KIT-6, Mo 2 /KIT-6, and Mo 2 /M 1.5 -KIT-6 catalysts obtained during temperature-programmed desorption (TPD) of adsorbed pyridine in flowing He (40 sccm) between 150 and 300 °C and ∼0.1 kPa pyridine are shown in Figures and S22. Both the support and the Mo-doped catalyst contain predominantly Lewis acid sites, as inferred from the strong peak at ∼1448 and 1608 cm –1 , respectively, with one additional band at 1595 cm –1 . , Interestingly, this latter band was more prominent and well resolved in the bare transition supports (M 1.5 -KIT-6) and its intensity decreased upon Mo incorporation with a simultaneous increase in the intensity of the vibrational band at 1608 cm –1 (Figure S22). Such an interaction could possibly be due to the presence of different strengths of electrophilic Lewis acid sites that are characteristic of transition metals (Groups IV and V).…”

Section: Resultsmentioning

confidence: 98%

“…The higher activity was attributed to an increased population of the isolated dioxo species (OMoO) in the precatalysts. Recently, Skoda et al reported that porous molybdenum silicate nanospheres show 2-fold greater propylene metathesis activity compared to supported MoO x prepared by incipient wetness technique . The higher activity is attributed to increased dispersion of MoO x species.…”

Section: Introductionmentioning

confidence: 99%

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Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Uchagawkar,

Ramanathan,

Zhu

et al. 2024

ACS Catal.

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We show that the electronic environment around active Mo centers supported on mesoporous silicates can be tuned by the addition of transition metals creating highly dispersed bimetallic catalysts that display enhanced activity for ethylene + 2-butene metathesis to propylene. The bimetallic catalysts are prepared by incorporating electrophilic Lewis acid metals (M) such as Nb, Ta, Zr, or Hf as dopant promoters into mesoporous KIT-6 supports using a one-pot sol–gel technique followed by impregnation of the Mo species. All the bimetallic Mo/M-KIT-6 catalysts display better activity than monometallic Mo/KIT-6 catalyst (28.7 ± 1.1 mmol (molMo s)−1), with (Mo/Nb-KIT-6) catalysts exhibiting maximum propylene formation rates (54.2 ± 0.5 mmol (molMo s)−1) at an identical Mo loading. Comprehensive catalyst characterization results (deploying high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), diffuse reflectance ultraviolet–visible (DR UV–vis), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS)) qualitatively show an increased population of the four-coordinated Mo sites in the promoted Mo catalysts, while Raman spectroscopy reveals the presence of Mo dioxo species (OMoO) on both the monometallic and promoted bimetallic catalysts. These results suggest that the addition of transition metals alters Mo coordination, yielding isolated bimetallic precursors [(O)2Mo(−O–M)(−O–Si)] in addition to the conventional ((O)2Mo(−O–Si)2) species. Further, the intrinsic propylene formation rates on the bimetallic formulations follow a linear correlation with the Lewis acid strengths exhibited by the Mo dioxo species (OMoO) revealing that catalyst activity can be enhanced by incorporating a second metal with increasing electrophilic character. Complementary 15N-pyridine solid-state NMR spectra display different chemical shifts associated with the metal centers suggesting that four-coordinated dioxo MoO x species with varying geometric and electronic configurations exist, depending on the added metal. A similar linear correlation was also observed between the average chemical shifts of the adsorbed 15N-pyridine and the Lewis acid strengths (ΔH ads,pyridine), providing informative descriptors regarding the molecular origins of the electronic effects influencing olefin metathesis on Mo-based catalysts. Our results demonstrate that simple catalyst synthesis methods can be harnessed for tuning the electronic environment around metal centers in heterogeneous catalysts for enhancing activity and selectivity.

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Section: Resultsmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Uchagawkar,

Ramanathan,

Zhu

et al. 2024

ACS Catal.

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“…This is consistent with the fact that a higher temperature is required to generate the metathesis active sites in the WO x /SiO 2 system, compared to the MoO x /SiO 2 catalyst. 4,7,8,14,15,21,22,76,77 The only exception is silanol-assisted activation of the M(IV) species (M = Mo, W) through the M(VI) hydride species. This mechanism is more favored for the tungsten system since W hydrides are more stable than Mo hydrides.…”

Section: Formation Of W(vi)mentioning

confidence: 99%

Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WO_x/SiO₂ Catalyst: A Computational Perspective

Handzlik,

Gierada,

Kurleto

2024

J. Phys. Chem. C

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A WO x /SiO 2 system is the major industrial catalyst for olefin metathesis, but the mechanism of the active site formation from the surface tungsten oxide species is not yet fully understood. In this work, detailed density functional theory (DFT) studies of the initiation mechanisms for olefin metathesis over the WO x /SiO 2 catalyst have been carried out. It is shown that dioxo W(VI) species can be reduced to monooxo W(IV) species by alkenes or activated without reduction to form W(VI) alkylidene species. A surface silanol group interacting with a W species plays a key role in these transformations. The first step, protonation of the alkene to generate W(VI) alkoxy species, is common to both pathways. The monooxo W(IV) species can be oxidized to form finally a W(VI) alkylidene site through an alkene complex or W(VI) hydride species. All of these mechanisms are competitive with each other, and the kinetic preferences depend on the geometry of the W sites and the local structure of the silica support. The structure−reactivity relationship is complex. The proposed routes of silanol-assisted reduction and activation are kinetically preferred over the non-silanol-assisted formation of the active sites, i.e., the pseudo-Wittig mechanism, vinylic C−H activation, allylic C−H activation, and oxidative coupling-1,4-hydrogen shift mechanism.

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“…This catalyst, comprising mono-molybdates, poly-molybdates, and MoO3 crystalline clusters, represents typical industrial supported catalysts, as reported in prior literature. 19 The kinetics were measured under differential conditions, free from mass and heat transport limitations (Supplementary Eq. S1-S4).…”

Section: Kinetics Of Self-and Cross-metathesis On Moox-somcmentioning

confidence: 99%

Insights into the catalytic promotion of propylene self-metathesis over silica-supported molybdenum oxide using substituted olefins

Zhu,

Adamji,

Berkson

et al. 2024

Preprint

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Olefin metathesis is a versatile strategy for functional group interconversion around C=C bonds. Here, we investigate in detail a recently discovered promotional effect, where co-feeding 2,3-dimethyl-2-butene isomers (4MEs) increases propylene self-metathesis rates on silica-supported Mo and W oxides by orders of magnitude. Through detailed kinetic measurements on MoOx/SiO2, we validate a dynamic site renewal and decay cycle, analogous to WOx/SiO2, which operates in tandem with the Chauvin cycle and can be effectively modulated by co-feeding 4MEs. Active site titrations indicate that the promotional effect results from an increased density of active sites rather than enhanced per-site catalytic activity. Spectroscopic analyses reveal that the renewal and decay of Mo alkylidene active sites involve proton-transfer mediated by proximal acidic Si-OH groups. Additionally, the co-fed promoters not only reduce Mo (VI) to Mo (IV), thereby increasing the number of pre-active sites, but also act as proton relays, enhancing proton-transfer steps. This dual functionality elucidates the mechanism underlying the enhanced metathesis activity observed with promoter addition.

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Propylene Metathesis over Molybdenum Silicate Microspheres with Dispersed Active Sites

Cited by 5 publications

References 69 publications

Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WO_x/SiO₂ Catalyst: A Computational Perspective

Insights into the catalytic promotion of propylene self-metathesis over silica-supported molybdenum oxide using substituted olefins

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Propylene Metathesis over Molybdenum Silicate Microspheres with Dispersed Active Sites

Cited by 5 publications

References 69 publications

Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Insights into Dopant-Mediated Tuning of Silica-Supported Mo Metal Centers for Enhanced Olefin Metathesis

Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WOx/SiO2 Catalyst: A Computational Perspective

Insights into the catalytic promotion of propylene self-metathesis over silica-supported molybdenum oxide using substituted olefins

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Role of Surface Silanols in Active Site Formation during Olefin Metathesis over a WO_x/SiO₂ Catalyst: A Computational Perspective