Investigation of Surface Acidity using a Range of Probe Molecules

Vimont, Alexandré; Daturi, Marco; Winfield, John M.

doi:10.1002/9780470660768.ch4

Cited by 4 publications

(10 citation statements)

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“…Previous studies of TMPO-loaded solid acid catalysts have identified a clear separation of the 31 P NMR chemical shift ranges of Brønsted and Lewis acid sites interacting with TMPO, respectively, 60–86 ppm and 55–60 ppm , Although a linear correlation between TMPO 31 P NMR chemical shifts and Brønsted acid site strength suggested the presence of heterogeneous acid strengths in zeolites, it is widely accepted that zeolites with high Si/Al ratios, without defects such as extraframework Al species (Figure S1), contain acid sites with homogeneous strength, regardless of the acid site siting and the zeolite framework type. , Although Al pairing, in the form of Al–O–Si–O–Al moieties, has been identified as the major factor influencing zeolite acid site strength, 29 Si MAS NMR shows that such Al pairs are not present (or are too dilute to be detected) in the Si-rich (Si/Al = 15) sample used, as Q 4 (2Al) Si resonances are not observed (Figure S2).…”

Section: Resultsmentioning

confidence: 93%

“…Unfortunately, providing an accurate and complete characterization of the acid properties of zeolites, namely, acid site density, nature, strength, and accessibility, remains a major challenge. Spectroscopists have addressed this issue using several probe molecules that interact specifically with Brønsted and Lewis acid sites . Certain protocols mix diverse probe molecules that selectively populate the different inner zeolite cavities and elucidate the nature of the acid sites. , …”

Section: Introductionmentioning

confidence: 99%

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What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

et al. 2021

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Elucidating the nature, strength, and siting of acid sites in zeolites is fundamental to fathom their reactivity and catalytic behavior. Despite decades of research, this endeavor remains a major challenge. Trimethylphosphine oxide (TMPO) has been proposed as a reliable probe molecule to study the acid properties of solid acid catalysts, allowing the identification of distinct Brønsted and Lewis acid sites and the assessment of Brønsted acid strengths. Recently, doubts have been raised regarding the assignment of the 31P NMR resonances of TMPO-loaded zeolites. Here, it is shown that a judicious control of TMPO loading combined with two-dimensional 1H–31P HETCOR solid-state NMR, DFT, and ab initio molecular dynamics (AIMD)-based computational modeling provides an unprecedented atomistic description of the host–guest and guest–guest interactions of TMPO molecules confined within HZSM-5 molecular-sized voids. 31P NMR resonances usually assigned to TMPO molecules interacting with Brønsted sites of different acid strength arise instead from both changes in the probe molecule confinement effects at ZSM-5 channel system and the formation of protonated TMPO dimers. Moreover, DFT/AIMD shows that the 1H and 31P NMR chemical shifts strongly depend on the siting of the framework aluminum atoms. This work overhauls the current interpretation of NMR spectra, raising important concerns about the widely accepted use of probe molecules for studying acid sites in zeolites.

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Section: Resultsmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

et al. 2021

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“…Studies using CO at low temperatures can be particularly powerful, since different types of surface sites can be detected and compared. The aluminum fluorides mentioned above and various oxofluorides of different structural types have been studied in this way and the results compared with nanoscale aluminum fluorides; accounts of the methodology and some illustrative results are to be found in [ 10 , 11 ].…”

Section: Aluminum Fluorides and Halogenated Aluminasmentioning

confidence: 99%

“…The experimental procedures for quantifying β − activity from the long-lived chlorine-36 isotope by Geiger-Müller counting have been well documented in the literature [ 10 , 11 , 12 ] and are not described here. Although the behavior observed when [ 36 Cl]-Bu t Cl is exposed to SF 4 -fluorinated γ-alumina, β-AlF 3 and aluminum chlorofluoride (ACF) is similar, there are differences in the details [ 12 ].…”

Section: Anhydrous Hydrogen Chloride As a Surface Probementioning

confidence: 99%

Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

Lennon

Winfield

2017

Molecules

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Aspects of the chemistry of selected metal fluorides, which are pertinent to their real or potential use as Lewis acidic, heterogeneous catalysts, are reviewed. Particular attention is paid to β-aluminum trifluoride, aluminum chlorofluoride and aluminas γ and η, whose surfaces become partially fluorinated or chlorinated, through pre-treatment with halogenating reagents or during a catalytic reaction. In these cases, direct comparisons with nanostructured metal fluorides are possible. In the second part of the review, attention is directed to iron(III) and copper(II) metal chlorides, whose Lewis acidity and potential redox function have had important catalytic implications in large-scale chlorohydrocarbons chemistry. Recent work, which highlights the complexity of reactions that can occur in the presence of supported copper(II) chloride as an oxychlorination catalyst, is featured. Although direct comparisons with nanostructured fluorides are not currently possible, the work could be relevant to possible future catalytic developments in nanostructured materials.

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“…For FTIR spectroscopy studies, the powdered catalyst samples were pressed into disks of ∼10 mg/cm 2 and activated in situ in the IR quartz cell equipped with KBr windows and attached to a high vacuum system. 77 Activation consisted of heating the catalyst at a rate of 10 o C/min under 13 kPa of O2 from room temperature (RT) to 350 o C. The sample was held at 350 o C for 45-60 mins followed by cell evacuation at the same temperature for 30 mins. The sample disk was moved from the furnace part of the cell to the optical section.…”

Section: In Situ Ftir Spectroscopy and Probe Molecules/reactionsmentioning

confidence: 99%

Dynamics of CrO₃–Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

et al. 2016

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A series of supported CrO3/Fe2O3 catalysts were investigated for the high temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman and XAS spectroscopy during the high temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O=)2Cr +6 O2 species and a bulk Fe2O3 phase containing some Cr 3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high temperature WGS/RWGS reactions show that the catalyst transforms during reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 and surface dioxo (O=)2Cr +6 O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium-iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe o and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C 16 O2/H2 → C 18 O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* from the surface region of the chromium-iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites were quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly an order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect.

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Investigation of Surface Acidity using a Range of Probe Molecules

Cited by 4 publications

References 64 publications

What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

Dynamics of CrO₃–Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

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Investigation of Surface Acidity using a Range of Probe Molecules

Cited by 4 publications

References 64 publications

What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

What Is Being Measured with P-Bearing NMR Probe Molecules Adsorbed on Zeolites?

Metal Fluorides, Metal Chlorides and Halogenated Metal Oxides as Lewis Acidic Heterogeneous Catalysts. Providing Some Context for Nanostructured Metal Fluorides

Dynamics of CrO3–Fe2O3 Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

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Product

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Dynamics of CrO₃–Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity