Determination of Number of Activated Sites Present during Olefin Metathesis by Supported ReOx/Al2O3 Catalysts

Lwin, Soe; Wachs, Israel E.

doi:10.1021/acscatal.5b01597

Cited by 11 publications

(9 citation statements)

References 10 publications

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“…Re 2 O 7 /Al 2 O 3 is a unique industrial heterogeneous catalyst because it operates at low temperature (25–80 °C), ,,,, and is compatible with functionalized substrates when activated with Me 4 Sn. , The structure of the active site is still a matter of debate, though the Lewis acidic sites in Al 2 O 3 appear to play a key role in metathesis activity (Scheme b). , Detailed spectroscopic studies on Re 2 O 7 /Al 2 O 3 showed that tetrahedral ReO 4 – coordinated to a pair of Al III and Al IV atoms on the alumina surface are active site precursors. Inactive species are associated with ReO 4 – bound to two Al IV and one Al III atoms in a very distorted geometry where extra oxygen from the surface coordinates to the Re center. ,, …”

Section: Introductionmentioning

confidence: 99%

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

et al. 2016

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Re2O7 supported on γ-alumina is an alkene metathesis catalyst active at room temperature, compatible with functional groups, but the exact structures of the active sites are unknown. Using CH3ReO3/Al2O3 as a model for Re2O7/Al2O3, we show through a combination of reactivity studies, in situ solid-state NMR, and an extensive series of DFT calculations, that μ-methylene structures (Al-CH2-ReO3-Al) containing a Re═O bound to a tricoordinated Al (AlIII) and CH2 bound to a four-coordinated Al (AlIVb) are the precursors of the most active sites for olefin metathesis. The resting state of CH3ReO3/Al2O3 is a distribution of μ-methylene species formed by the activation of the C-H bond of CH3ReO3 on different surface Al-O sites. In situ reaction with ethylene results in the formation of Re metallacycle intermediates, which were studied in detail through a combination of solid-state NMR experiments, using labeled ethylene, and DFT calculations. In particular, we were able to distinguish between metallacycles in TBP (trigonal-bipyramidal) and SP (square-pyramidal) geometry, the latter being inactive and detrimental to catalytic activity. The SP sites are more likely to be formed on other Al sites (AlIVa/AlIVa). Experimentally, the activity of CH3ReO3/Al2O3 depends on the activation temperature of alumina; catalysts activated at or above 500 °C contain more active sites than those activated at 300 °C. We show that the dependence of catalytic activity on the Al2O3 activation temperature is related to the quantity of available AlIII-defect sites and adsorbed H2O.

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Section: Introductionmentioning

confidence: 99%

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

et al. 2016

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“…This implies that ReO x /USY has the lowest density of Re species. However, chemical titration has shown that, at low Re loading, only a small portion (1–2%) of Re species over Re 2 O 7 /γ-Al 2 O 3 are active for olefin metathesis reactions. , In the detailed SXRD and ND data analysis of ReO x /USY in the next section, it is shown that all Re species in ReO x /USY could participate in the reaction. The classical titration method was not used to quantify the number of active sites in ReO x /USY because of its large discrepancies, as pointed out in the recent literature .…”

Section: Results and Discussionmentioning

confidence: 97%

Rational Design of Synergistic Active Sites for Catalytic Ethene/2-Butene Cross-Metathesis in a Rhenium-Doped Y Zeolite Catalyst

Zhao

et al. 2021

ACS Catal.

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Synthesizing atomically dispersed synergistic active pairs is crucial yet challenging in developing highly active heterogeneous catalysts for various industrially important reactions. Here, a single molecular Re species is immobilized on the inner surface of a Y zeolite with Brønsted acid sites (BASs) within atomic proximity to form Re OMS–BAS active pairs for the efficient catalysis of olefin metathesis reactions (OMS: olefin metathesis site). The synergy within the active pairs is revealed by studying the coadsorption geometry of the olefin substrates over the active pairs by synchrotron X-ray and neutron powder diffraction. It is shown that the BAS not only facilitates olefin adsorption but also aligns the olefin molecule to the Re OMS for efficient intermediate formation. Consequently, for the cross-metathesis of ethene and trans-2-butene to propene, this catalyst shows high activity under mild reaction conditions without observable deactivation. The catalyst outperforms not only traditional ReO x -based catalysts but also the best industrially applicable WO x -based catalyst thus far that we discovered previously. The concept of using two isolated active sites of different functionalities within atomic proximity in a confined cavity can provide opportunities for designing synergistically catalytic materials.

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“…Turek et al 38 and Segawa et al 51 found from pyridine adsorption FT-IR measurements that (i) at low rhenia loading, only surface Lewis acid sites are present; and (ii) at medium and high rhenia loading, the number of surface Lewis acid sites decreases and the number of surface Brønsted acid sites increases with increasing rhenia loading. Comparison with propylene metathesis activity 9,15,52 suggests that the olefin metathesis activity correlates with the number of Brønsted acid sites. The pyridine FT-IR findings of Xiaoding et al, however, found that both the number of surface Lewis and Brønsted acid sites increase with rhenia loading.…”

Section: Molecular Structures Of Dehydrated Surface Reo Xmentioning

confidence: 99%

Identifying the Catalytic Active Site for Propylene Metathesis by Supported ReO_x Catalysts

Zhang

Wachs

2021

ACS Catal.

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A series of supported ReO x catalysts were investigated that allowed identifying the unique surface anchoring sites on oxide supports responsible for activating the surface ReO4 sites for propylene metathesis (the catalytic active site). The catalysts were synthesized by incipient-wetness impregnation of aqueous HReO4 onto the oxide supports (Al2O3, ZrO2, TiO2, SiO2, and CeO2), characterized under dehydrated and propylene metathesis reaction conditions with in situ spectroscopy (Raman, DRIFTS, UV–vis and NAP-XPS), and chemically probed (CH3CHCH2–TPSR, CH2CH2/CH3CHCHCH3 titration and steady-state self-metathesis of propylene to ethylene and 2-butene). The initially calcined supported rhenia species anchor as isolated surface Re7+O4 sites on the oxide supports by reacting with the surface hydroxyls (terminal S–OH, bridged S–OH–S, and tricoordinated S3–OH) of the oxide supports. The specific oxide support was found to control the number of activated sites (Al2O3 ≫ ZrO2 > CeO2 > TiO2 > SiO2) and propylene metathesis activity (Al2O3 ≫ ZrO2 ≫ TiO2 ∼ CeO2 ∼ SiO2), revealing that the oxide support action is a potent ligand for the surface ReO x sites. The activation and specific activity of the surface ReO x sites depend on several factors (nature of surface hydroxyls (S3–OH > S–OH–S > S–OH), coordination of the oxide support surface cation (ZrO7, AlO6, CeO4) and electronegativity of the oxide support cation (SiO2 > Al2O3 > TiO2 > ZrO2 > CeO2). No relationships exist between olefin metathesis activity and acid strength of surface Lewis and Brønsted sites. Prior studies primarily focused on supported ReO x /Al2O3, and the lack of examination of non-Al2O3 supported rhenia catalysts precluded comparison between efficient and inefficient olefin metathesis catalysts, which prevented identifying the catalytic active site for olefin metathesis by supported ReO x catalysts.

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Determination of Number of Activated Sites Present during Olefin Metathesis by Supported ReO_x/Al₂O₃ Catalysts

Cited by 11 publications

References 10 publications

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

Rational Design of Synergistic Active Sites for Catalytic Ethene/2-Butene Cross-Metathesis in a Rhenium-Doped Y Zeolite Catalyst

Identifying the Catalytic Active Site for Propylene Metathesis by Supported ReO_x Catalysts

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Determination of Number of Activated Sites Present during Olefin Metathesis by Supported ReOx/Al2O3 Catalysts

Cited by 11 publications

References 10 publications

Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts

Role of Tricoordinate Al Sites in CH3ReO3/Al2O3 Olefin Metathesis Catalysts

Rational Design of Synergistic Active Sites for Catalytic Ethene/2-Butene Cross-Metathesis in a Rhenium-Doped Y Zeolite Catalyst

Identifying the Catalytic Active Site for Propylene Metathesis by Supported ReOx Catalysts

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Determination of Number of Activated Sites Present during Olefin Metathesis by Supported ReO_x/Al₂O₃ Catalysts

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

Role of Tricoordinate Al Sites in CH₃ReO₃/Al₂O₃ Olefin Metathesis Catalysts

Identifying the Catalytic Active Site for Propylene Metathesis by Supported ReO_x Catalysts