Controlled Deposition of Silica on Titania-Silica to Alter the Active Site Surroundings on Epoxidation Catalysts

Ardagh, M. Alexander; Bregante, Daniel T.; Flaherty, David W.; Notestein, Justin M.

doi:10.1021/acscatal.0c02937

Cited by 14 publications

(9 citation statements)

References 48 publications

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“…We have shown that ∆𝐺 ‡ does not depend on the size of the confining pore 18,39,40,61 or the density of (SiOH) x , 5 because ∆𝐺 ‡ corresponds to the free energy required to exchange electrons between Ti-OOH species and the C=C following all changes to the configuration of the alkene and solvent. Alkenes preferentially associate with hydrophobic, siloxane regions of the pores upon adsorption within the zeolite; therefore, values of ∆𝐺 primarily reflect differences in the characteristic pore size and do not depend significantly on (SiOH) x density.…”

Section: Influence Of H 2 O Structure and Reorganization On Liquid-phase Epoxidation Catalysismentioning

confidence: 97%

The Shape of Water in Zeolites and Its Impact on Epoxidation Catalysis

Bregante

Chan²,

Tan³

et al. 2020

Preprint

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Solvent structures that surround active sites reorganize during catalysis and influence the stability of surface intermediates. Within the pores of a zeolite, H2O molecules form hydrogen-bonded structures that differ significantly from bulk H2O. Spectroscopic measurements and molecular dynamics simulations show that H2O molecules form bulk-like three-dimensional structures within 1.3 nm cages, while H2O molecules coalesce into oligomeric one-dimensional chains distributed throughout zeolite frameworks when the pore diameter is smaller than 0.65 nm. The differences between the motifs of these solvent structures provide opportunities to manipulate enthalpy-entropy compensation relationships and significantly increase rates of catalytic turnover events. Here, we explain how the reorganization of these pore size-dependent H2O structures during alkene epoxidation catalysis gives rise to entropy gains that increase turnover rates by up to 400-fold. Collectively, this work shows how solvent molecules form discrete structures with highly correlated motion within microporous environments, and that the reorganization of these structures may be controlled to confer stability to reactive intermediates.

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Section: Influence Of H 2 O Structure and Reorganization On Liquid-phase Epoxidation Catalysismentioning

confidence: 97%

The Shape of Water in Zeolites and Its Impact on Epoxidation Catalysis

Bregante

Chan²,

Tan³

et al. 2020

Preprint

Self Cite

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“…As an alternative to silylation strategies, Notestein et al . proposed in 2020 to apply an overcoating treatment with silica on Ti/SiO 2 materials in order to modify the local environment around grafted Ti species without altering the active sites or creating diffusion limitations in the solid [126] . As a result of a higher heat of adsorption of the olefin (limonene) on the SiO 2 ‐overcoated catalyst, the latter outperformed Ti‐Beta zeolite used as benchmark in terms of epoxide selectivity at 0 °C.…”

Section: Strategies To Improve Ti‐based Heterogeneous Epoxidation Catalystsmentioning

confidence: 99%

Titanosilicate Epoxidation Catalysts: A Review of Challenges and Opportunities

2021

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Epoxidation reactions are tremendously important for modern chemistry, as they lead to series of highly useful bulk and fine chemicals, monomers, and intermediates for organic synthesis. Progress in epoxidation processes goes hand in hand with the advancement made in catalysis science. In this context, heterogeneous catalysts, and particularly Ti‐based formulations, are playing a central role and have seen tremendous developments over the past two decades, leveraging on advanced materials science. The aim of this review is to illustrate the various strategies of titanosilicate catalysts preparation that can lead to more versatile, more performant, and greener epoxidation processes. We successively cover (i) supported catalysts, obtained by the grafting of Ti species onto preformed silica supports, (ii) microporous crystalline titanosilicates (zeolites), and (iii) amorphous titanosilicates obtained by sol‐gel chemistry. For each category, with an emphasis on catalyst preparation, the challenges that have to be tackled to boost catalyst performance are highlighted. From that point, we present a critical review of the different approaches that have been proposed in the primary literature to tailor the properties that govern catalysts performance (activity, selectivity, stability, ease of handling). This is done by better controlling the nature of the active surface species, adapting particles size and shape, optimizing texture, modifying surface chemistry, etc. These lines of attack encompass molecular approaches for the grafting of well‐defined species, top‐down and bottom‐up synthesis of hierarchically porous zeolites, advanced sol‐gel routes potentially performed in non‐conventional media or coupled with original processing, preparation of self‐standing monoliths, etc. Future research directions are discussed with emphasis on the application scope of new catalytic materials and possible approaches to increase catalyst performance.

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“…Compared to homogeneous catalysts, heterogeneous counterparts are preferable due to their easy separation and subsequent reuse . Conventional supports such as metal oxides and various forms of supported metals often feature not well-defined structures, − and unambiguous structural determination of the anchored catalytic species remains challenging …”

Section: Introductionmentioning

confidence: 99%

Insights into the Structure–Activity Relationship in Aerobic Alcohol Oxidation over a Metal–Organic-Framework-Supported Molybdenum(VI) Catalyst

et al. 2021

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The understanding of structure–activity relationships at the atomic level has played a profound role in heterogeneous catalysis, providing valuable insights into designing suitable heterogeneous catalysts. However, uncovering the detailed roles of how such active species’ structures affect their catalytic performance remains a challenge owing to the lack of direct structural information on a specific active species. Herein, we deposited molybdenum(VI), an active species in oxidation reactions, on the Zr6 node of a mesoporous zirconium-based metal–organic framework (MOF) NU-1200, using solvothermal deposition in MOFs (SIM). Due to the high crystallinity of the NU-1200 support, the precise structure of the resulting molybdenum catalyst, Mo-NU-1200, was characterized through single-crystal X-ray diffraction (SCXRD). Two distinct anchoring modes of the molybdenum species were observed: one mode (Mo1), displaying an octahedral geometry, coordinated to the node through one terminal oxygen atom and the other mode (Mo2) coordinated to two adjacent Zr6 node oxygen atoms in a tetrahedral geometry. To investigate the role of base in the catalytic activity of these Mo centers, we assessed the activity of Mo-NU-1200 for the aerobic oxidation of 4-methoxybenzyl alcohol as a model reaction. The results revealed that Mo-NU-1200 exhibited remarkably higher catalytic reactivity under base-free conditions, while the presence of base inhibited the catalytic reactivity of this species. SCXRD studies revealed that the molybdenum binding motifs (structures of the supported metal on the Zr6 node in the MOF) changed over the course of the reactions. Following the oxidation without base, both pristine coordination modes (Mo1 and Mo2) evolved into a new coordination mode (Mo3), in which the molybdenum atom coordinated to two adjacent oxygen atoms from the Zr6 node in an octahedral geometry, while in the presence of base, the pristine Mo1 coordination mode evolved entirely into the pristine Mo2. This study demonstrates the direct observation of an active species’ structural evolution from metal installation to subsequent catalytic reaction. As a result, these subtle structural changes in catalyst binding motifs led to distinct differences in catalytic activities, providing a compelling strategy for elucidating structure–activity relationships.

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Controlled Deposition of Silica on Titania-Silica to Alter the Active Site Surroundings on Epoxidation Catalysts

Cited by 14 publications

References 48 publications

The Shape of Water in Zeolites and Its Impact on Epoxidation Catalysis

The Shape of Water in Zeolites and Its Impact on Epoxidation Catalysis

Titanosilicate Epoxidation Catalysts: A Review of Challenges and Opportunities

Insights into the Structure–Activity Relationship in Aerobic Alcohol Oxidation over a Metal–Organic-Framework-Supported Molybdenum(VI) Catalyst

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