Effects of structural and chemical characteristics of zeolites on the properties of their bridging hydroxyl groups

Pel'menshchikov, A. G.; Pavlov, V. I.; Zhidomirov, G. M.; Beran, S.

doi:10.1021/j100296a043

Cited by 29 publications

(11 citation statements)

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“…The outlined experimentally observed correlation between the acid strength of BA sites, n OH and Al content is also supported by numerous quantum chemical calculations employing (i) cluster models, 40,44,[80][81][82] (ii) the embedded schemes of Sierka et al 51 (H-FAU with Si/Al of 47, 23 and 3) and Shor et al 49 (H-FAU with Si/Al of 47, 23 and 11) which report only a weak correlation between acidity and n OH , but they confirm that the main decrease in the acidity is related to an increase of n in the Si(nAl) descriptor, (iii) the periodic DFT study of Ugliengo et al 58 (H-CHA samples with Si/Al of 11 : 1, 5 : 1, 3 : 1 and 1 : 1) reporting a linear correlation between formation energy (per aluminium atom) of CHA and corresponding n OH .…”

Section: Discussionsupporting

confidence: 54%

“…Along with the Si/Al ratio, the + SiOAl angle is often considered to be a major factor determining n OH . A n OH /+ SiOAl correlation was observed in a number of studies, including cluster models, [39][40]82 atomistic simulation techniques 83 and periodic DFT investigations. 54,57 In addition, Demuth et al reported a larger flexibility of the H-MOR framework compared to that of Na-MOR.…”

Section: Discussionmentioning

confidence: 94%

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Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER

Grajciar

Areán

Pulido

et al. 2010

Phys. Chem. Chem. Phys.

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Periodic DFT calculations were performed on H-FER models having Si/Al ratios of 71 : 1, 35 : 1 and 8 : 1, in order to investigate the effect of aluminium content on the properties of the zeolite Brønsted acid sites. Relative stability of these sites was found to be dependent on Si/Al ratio, which is the main factor dictating the relative concentration of Brønsted acid sites having different types of local configuration, to the point that some types of acid site are formed only when the aluminium content of the zeolite is relatively high. The number of AlO(4) tetrahedra sharing an oxygen with the SiO(4) tetrahedron involved in the Brønsted acid site determines the Si-O(H)-Al angle, O-H stretching frequency and deprotonation energy (and hence acid strength). For Brønsted acid protons not involved in intra-zeolite H-bonding, a correlation was found between Si-O(H)-Al angle and O-H stretching frequency.

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Section: Discussionsupporting

confidence: 54%

Section: Discussionmentioning

confidence: 94%

Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER

Grajciar

Areán

Pulido

et al. 2010

Phys. Chem. Chem. Phys.

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“…As to the Bronsted sites, at low coverage the adsorption occurs mainly on the bridging OH groups adsorbing at 3610 and 3630 cm-1 in HZSM-5 and HY zeolites, respectively (spectrum 4 in Figure 3a and spectrum 3 in Figure 3b). The so-called (A, B, C) trio of bands appears at ~2890, ~2400, and ~1700 cm-1, typical for medium-strong and strong H complexes in vapors, liquids, and solids.313*34 According to experimental and theoretical 1 O-H bond lengths and TOH angles (T = Si, Al) of the terminal OH groups not involved in the modeled interactions are equal to those optimized for Si(OH)417 (Du point group symmetry) and 20• 1(0 )318 molecules.6 R(SiO) are equal to those for Si(OH)4.17'02, 03, 04, and O5 atoms in lb are fixed to model steric constraint of the Si-(OH)-Al fragment in the lattice (see refs 18 and 19 for more details); their positions correspond to those of (HO)3SiOHAl(OH)3 molecule (see the table), shifted from the optimal positions by AR(SiO,) = AR(A10,) = 0.05 A and A/SiO,A1 = 5°for lb' and by AR(SiO,) = AR(A10,) = -0.05 A and A/SiO,AI = -5" for lb". d For H20 molecule, its gas-phase geometry was adopted: ( ) = 0.96 A and zHOH = 104°.…”

Section: Resultsmentioning

confidence: 99%

Acetonitrile-d3 as a probe of Lewis and Broensted acidity of zeolites

Pelmenschikov¹,

Santen²,

Jänchen³

et al. 1993

J. Phys. Chem.

252

191

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DOI to the publisher's website. • The final author version and the galley proof are versions of the publication after peer review. • The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal. If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the "Taverne" license above, please follow below link for the End User Agreement:

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“…Both the broadness and asymmetry of the IR absorption band corresponding to bridged hydroxyls provide clear evidence for some sort of heterogeneity of these Brönsted acid sites. In principle, this heterogeneity could be ascribed to a range of O−H force constants stemming from local variations of bond lengths and bond angles in the zeolite framework. − However, isolated OH groups having a range of force constants should be equally well probed by CO and by N 2 . The spectroscopic evidence shown here proves that this is not the case.…”

Section: Resultsmentioning

confidence: 99%

“…[66][67][68][69][70] However, isolated OH groups having a range of force constants should be equally well probed by CO and by N 2 . In principle, this heterogeneity could be ascribed to a range of O-H force constants stemming from local variations of bond lengths and bond angles in the zeolite framework.…”

Section: Scheme 2 Schemementioning

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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Effects of structural and chemical characteristics of zeolites on the properties of their bridging hydroxyl groups

Cited by 29 publications

References 0 publications

Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER

Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER

Acetonitrile-d3 as a probe of Lewis and Broensted acidity of zeolites

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

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