Selective Oxidations Catalyzed by Mesoporous Metal Silicates

Kholdeeva, Oxana A.

doi:10.1002/9781118356760.ch4

Cited by 21 publications

(10 citation statements)

References 477 publications

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“…58−60 The vibrational features (700 and 805 cm −1 ) that correspond to bulk WO 3 increase monotonically with W content (Figure S3), which shows that extra-framework WO x oligomers form at higher loadings. 61 Similarly, greater loadings tend to form oligomeric MoO x on Mo-BEA. Collectively, these Raman spectra show that all Ti-, Nb-, W-, and Mo-BEA activate H 2 O 2 to form M-(η 2 -O 2 ) species, while W-and Mo-BEA materials also possess distinguishable MO functions.…”

Section: Methodssupporting

confidence: 88%

Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

et al. 2020

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Rates and selectivities for alkene epoxidations depend sensitively on the identity of the active metal center for both heterogeneous and homogeneous catalysts. While group 6 metals (Mo, W) have greater electronegativities and the corresponding molecular complexes have greater rates for epoxidations than group 4 or 5 metals and molecular complexes, these relationships are not established for zeolite catalysts. Here, we combine complementary experimental methods to determine the effects of metal identity on the catalytic epoxidation of 1hexene with H 2 O 2 for active sites within the BEA framework. Postsynthetic methods were used to incorporate groups 4−6 transition-metal atoms (Ti, Nb, Mo, W) into the framework of zeolite BEA. In situ Raman and UV−vis spectroscopies show that H 2 O 2 activates to form peroxides (M-(η 2 -O 2 )) and hydroperoxides (M-OOH) on all M-BEA but also metal oxos (MO) on W-and Mo-BEAs, the latter of which leaches rapidly. Changes in turnover rates for epoxidation as functions of reactant concentrations and the conformation of cis-stilbene epoxidation products indicate that epoxide products form by kinetically relevant O-atom transfer from M-OOH or M-(η 2 -O 2 ) intermediates to the CC bond and show two distinct kinetic regimes where H 2 O 2 -derived intermediates or adsorbed epoxide molecules prevail on active sites. Ti-BEA catalyzes epoxidations with turnover rates 60 and 250 times greater than Nb-BEA and W-BEA, which reflect apparent activation enthalpies (ΔH ‡ ) for both epoxidation and H 2 O 2 decomposition that are lower for Ti-BEA than for Nb-and W-BEAs. Values of ΔH ‡ for epoxidation differ much more between metals than barriers for H 2 O 2 decomposition and give rise to large differences in 1hexene epoxidation selectivities that range from 93% on Ti-BEA to 20% on W-BEA. Values of ΔH ‡ for both pathways scale linearly with measured enthalpies for adsorption of 1,2-epoxyhexane from the solvent to active sites measured by isothermal titration calorimetry. These correlations confirm that linear free-energy relationships hold for these systems, despite differences in the coordination of active metal atoms to the BEA framework, the identity and number of pendant oxygen species, and the complicating presence of solvent molecules.

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Section: Methodssupporting

confidence: 88%

Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

et al. 2020

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“…The development of catalytic selective oxidation processes using green oxidants is a challenging goal of the modern chemical industry. − Dilute hydrogen peroxide is one of the most preferred oxidants in terms of both ecology and economics. − A range of catalysts containing transition metal ions incorporated into the framework or grafted onto the surface of solid carriers have been synthesized and evaluated for various H 2 O 2 -based oxidations. − So far, titanium- − and niobium-containing − molecular sieves have been widely recognized as catalysts for selective oxidations with H 2 O 2 . Meanwhile, Zr-containing silicates also demonstrated a pronounced catalytic activity in H 2 O 2 -based oxidations. − Epoxides and diols most often predominated in the oxidation of alkenes. ,,− However, in some cases, a significant amount of allylic oxidation products was also detected. , …”

Section: Introductionmentioning

confidence: 99%

Activation of H₂O₂ over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

Maksimchuk¹,

Evtushok²,

Zalomaeva³

et al. 2021

ACS Catal.

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Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (Bu 4 N) 2 [W 5 O 18 Zr(H 2 O) 3 ] (1) and (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze the epoxidation of CC bonds in unfunctionalized alkenes and α,β-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the interaction of Zr-POM and H 2 O 2 . The interaction of 1 and 2 with H 2 O 2 and the resulting peroxo derivatives have been investigated by UV−vis, FTIR, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICPatomic emission spectroscopy techniques. The interaction between an 17 O-enriched dimer, (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OCH 3 )} 2 ] (2′), and H 2 O 2 was also analyzed by 17 O NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(μ-η 2 :η 2 -O 2 ){ZrW 5 O 18 } 2 ] 6− as well as monomeric Zr-hydroperoxo [W 5 O 18 Zr(η 2 -OOH)] 3− and Zr-peroxo [HW 5 O 18 Zr(η 2 - O 2 )] 3− species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing CC bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding α-oxygen transfer to the substrate as the rate-determining step. The calculations also showed that protonation of Zr-POM significantly reduces the free-energy barrier of the key oxygen-transfer step because of the greater electrophilicity of the catalyst and that dimeric species hampers the approach of alkene substrates due to steric repulsions reducing its reactivity. The improved performance of the Zr(IV) catalyst relative to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and a low tendency to form energy deep-well and low-reactive Zr-peroxo intermediates.

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“…Acetylacetone has been widely employed as a hydrolysis-retarding dopant in sol-gel and template syntheses of mesoporous metal-silicates [20,52]. Use of this chelating agent in the synthesis of Ti-MMM-E catalysts by EISA not only prevented TiO 2 precipitation but also assisted in the generation of specific di(oligo)meric Ti centers within the silica matrix [48].…”

Section: Catalyst Synthesis and Characterizationmentioning

confidence: 99%

“…However, the leaching degree could be reduced to 17-20 ppm by using 50% hydrogen peroxide instead of 30%, or by using catalysts with a lower tungsten content (Table 2), which is in agreement with the literature [39] and our results for the catalyst's stability to aqueous H2O2 and boiled water (see Section 2.2). It is quite typical of mesoporous metal-silicates that metal leaching is caused by cooperative action of H2O2 and water (a combination of hydrolysis and complexing processes) [20]. Thus, it can be reduced by decreasing the water content in the reaction mixture, i.e., by using more concentrated H2O2.…”

Section: Catalytic Studiesmentioning

confidence: 99%

“…However, incorporation of tungsten into mesoporous silicates is not a trivial task since tungsten is considerably larger than silicon, with an ionic radius of 0.42 Å, compared to 0.26 Å [19]. Thus, tungsten-containing mesoporous molecular sieves usually have a serious drawback of the active metal leaching under turnover conditions of liquid-phase oxidation [20][21][22][23][24]. Numerous efforts have been focused on synthesizing materials in which the active tungsten species are well isolated and anchored on the support through W-O-Si covalent bonds.…”

Section: Introductionmentioning

confidence: 99%

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Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

et al. 2018

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Mesoporous tungsten-silicates, W-MMM-E, have been prepared following evaporation-induced self-assembly methodology and characterized by elemental analysis, XRD, N 2 adsorption, STEM-HAADF (high angle annular dark field in scanning-TEM mode), DRS UV-vis, and Raman techniques. DRS UV-vis showed the presence of two types of tungsten oxo-species in W-MMM-E samples: isolated tetrahedrally and oligomeric octahedrally coordinated ones, with the ratio depending on the content of tungsten in the catalyst. Materials with lower W loading have a higher contribution from isolated species, regardless of the preparation method. W-MMM-E catalyzes selectively oxidize of a range of alkenes and organic sulfides, including bulky terpene or thianthrene molecules, using green aqueous H 2 O 2 . The selectivity of corresponding epoxides reached 85-99% in up to 80% alkene conversions, while sulfoxides formed with 84-90% selectivity in almost complete sulfide conversions and a 90-100% H 2 O 2 utilization efficiency. The true heterogeneity of catalysis over W-MMM-E was proved by hot filtration tests. Leaching of inactive W species depended on the reaction conditions and initial W loading in the catalyst. After optimization of the catalyst system, it did not exceed 20 ppm and 3 ppm for epoxidation and sulfoxidation reactions, respectively. Elaborated catalysts could be easily retrieved by filtration and reused several times with maintenance of the catalytic behavior.

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Selective Oxidations Catalyzed by Mesoporous Metal Silicates

Cited by 21 publications

References 477 publications

Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Activation of H₂O₂ over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

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Selective Oxidations Catalyzed by Mesoporous Metal Silicates

Cited by 21 publications

References 477 publications

Alkene Epoxidations with H2O2 over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Alkene Epoxidations with H2O2 over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Activation of H2O2 over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates

Tungsten-Based Mesoporous Silicates W-MMM-E as Heterogeneous Catalysts for Liquid-Phase Oxidations with Aqueous H2O2

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Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Alkene Epoxidations with H₂O₂ over Groups 4–6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Activation of H₂O₂ over Zr(IV). Insights from Model Studies on Zr-Monosubstituted Lindqvist Tungstates