Brønsted acid strength in US-Y: FTIR study of CO adsorption

Макарова, М. А.; Al-Ghefaili, Khalid M.; Dwyer, John

doi:10.1039/ft9949000383

Cited by 66 publications

(46 citation statements)

References 6 publications

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“…However, the activity of the H-USY in the hexane isomerization and cracking was in conflict with this finding. 24,50 The present study seems to resolve this contradiction. The ∆ν OH shift, induced by N 2 adsorption, was smaller for H-USY than for H-, which is in agreement with the hexane conversion activities (Table 3).…”

Section: Discussioncontrasting

confidence: 69%

Hexane Isomerization and Cracking Activity and Intrinsic Acidity of H-Zeolites and Sulfated Zirconia−Titania

2006

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Adsorption of N2 was studied on zeolite H-Y, ultrastabilized H-Y (H-USY), H-mordenite, H-ZSM-5, H-beta, and on sulfated zirconia-titania (SZT) mixed oxide by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) at 298 K and at N2 pressures up to 9 bar. The adsorption-induced DeltanuOH red-shift of the nuOH bands was used as a measure of the intrinsic acid strength of the Brnønsted acid sites. The intrinsic acid strength of the solids follows the order of H-ZSM-5 approximately H-mordenite approximately H-beta > H-USY > SZT approximately H-Y. The solids were characterized by their hexane conversion activities at 553 K and 6.1 kPa hexane partial pressure. The reaction was shown to proceed predominantly by a bimolecular mechanism, while the reaction was first order in hexane and zero order in alkenes. The site-specific apparent rate constant of the bimolecular hexane conversion was shown to parallel the intrinsic acid strength of the samples, suggesting that the ratio of the apparent and the intrinsic activity, that is, the KA' equilibrium constant of alkane adsorption on the hydrocarbon-covered sorption sites, is hardly dependent on the catalyst structure.

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Section: Discussioncontrasting

confidence: 69%

Hexane Isomerization and Cracking Activity and Intrinsic Acidity of H-Zeolites and Sulfated Zirconia−Titania

2006

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“…∆S°(T) ϭ ∆S* Ϫ γ R ln(T/T*) (6) ∆H°(T) ϭ ∆H* Ϫ γ R (T Ϫ T*) (7) T* is a reference temperature, e.g. the ambient one, and ∆H* and ∆S* are the standard changes in enthalpy and entropy at that temperature.…”

Section: A/a M ϭ θ ϭ K(t)p/[1 ϩ K(t)p] (1)mentioning

confidence: 99%

Thermodynamics of Hydrogen Bonding between CO and the Supercage Brønsted Acid Sites of the H-Y Zeolite − Studies from Variable Temperature IR Spectrometry

Areán

Manoilova

Цыганенко

et al. 2001

Eur. J. Inorg. Chem.

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A procedure was developed for studying the thermodynamics involved in the interaction of carbon monoxide with Brønsted acid sites in zeolites, without the need for microcalorimetric measurements. The observables are the integrated infrared intensity of the C−O stretching mode of the OH···CO adducts, temperature, and equilibrium pressure. These quantities were measured by means of variable-temperature IR spectroscopy, using a home made IR cell equipped with a

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“…43 In Figure 3b the difference spectra between spectra 2-11 and spectrum 1 (Figure 3a) are reported. 23,27,29,30 We can notice (i) the OH band with maximum at 3621 cm -1 is clearly asymmetric on the low-frequency side and has a remarkable half-width (fwhm ) 35 cm -1 ), (ii) an isosbestic point is clearly observed at 3540 cm -1 , indicating that a stoichiometric reaction is taking place; (iii) a new IR absorption centered at 3342 cm -1 proportionally develops on dosing with CO; this new absorption band is quite complex, as on the high-frequency side of the main peak a shoulder at about 3425 cm -1 can be clearly seen, conferring to it a pronounced asymmetric profile (which seems to partially mirror the shape of the unperturbed OH band). 23,27,29,30 We can notice (i) the OH band with maximum at 3621 cm -1 is clearly asymmetric on the low-frequency side and has a remarkable half-width (fwhm ) 35 cm -1 ), (ii) an isosbestic point is clearly observed at 3540 cm -1 , indicating that a stoichiometric reaction is taking place; (iii) a new IR absorption centered at 3342 cm -1 proportionally develops on dosing with CO; this new absorption band is quite complex, as on the high-frequency side of the main peak a shoulder at about 3425 cm -1 can be clearly seen, conferring to it a pronounced asymmetric profile (which seems to partially mirror the shape of the unperturbed OH band).…”

Section: Spectra Of Co and N 2 Adsorbed On H-gazsm-5(25) Treated At Imentioning

confidence: 99%

Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules

Areán¹,

Palomino²,

Zecchina³

et al. 1996

J. Phys. Chem.

105

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Three MFI-type, H-GaZSM-5 gallosilicates were synthesized with Si/Ga ratios of 25, 50, and 75, respectively. Incorporation of gallium into the zeolite framework was checked by powder X-ray diffraction, which showed an approximately linear variation of d spacings with Si/Ga ratio. Fourier transform IR spectroscopy of CO and N 2 , adsorbed at liquid nitrogen temperature, was used to characterize these gallosilicates after activation in vacuo at temperatures from 673 to 973 K. It was found that both probe molecules formed stable adducts with polarizing centers having either Brönsted or Lewis acid character. On samples activated at a temperature smaller than 700 K Brönsted sites were found to be predominant; apart from regular Si(OH)Ga sites, adsorbed carbon monoxide also revealed the presence of a small fraction of Brönsted acid groups associated with nonregular sites. These could be located either at the mouth of small cavities (presumably associated with defective regions of the zeolite framework where gallium was segregated) or on amorphous regions. On the sample with Si/Ga ) 25 activated at temperatures higher than 700 K, gallium showed a marked tendency to go into either partial or total extraframework positions, thus generating coordinatively unsaturated Ga 3+ centers with strong Lewis acid character. These centers were revealed by formation of CO and N 2 adducts where stretching frequencies of the adsorbed molecules showed a strong hypsochromic shift, ∆ν. Values of ∆ν as high as 82 and 25 cm -1 were found for CO and N 2 , respectively. In the case of dinitrogen, interaction with Lewis acid centers also brings about a pronounced increase of IR band intensity, and this effect can advantageously be used to monitor extraframework species.

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Brønsted acid strength in US-Y: FTIR study of CO adsorption

Cited by 66 publications

References 6 publications

Hexane Isomerization and Cracking Activity and Intrinsic Acidity of H-Zeolites and Sulfated Zirconia−Titania

Hexane Isomerization and Cracking Activity and Intrinsic Acidity of H-Zeolites and Sulfated Zirconia−Titania

Thermodynamics of Hydrogen Bonding between CO and the Supercage Brønsted Acid Sites of the H-Y Zeolite − Studies from Variable Temperature IR Spectrometry

Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules

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