Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites

Suzuki, Katsuki; Sastre, Germán; Katada, Naonobu; Niwa, Miki

doi:10.1039/b711961e

Cited by 53 publications

(48 citation statements)

References 34 publications

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“…This shift corresponds to some calculations which also used plane wave approach 19,26 . Note however that we are not using the customary notation of scaling factor for computed frequencies, as a scaling factor is typically based on a full range (or a very limited number of sub-ranges) of vibrational frequencies of a target system 67 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 28 , although their computed HO3 vibration was significantly higher, 3538 cm -1 , than ours. For better visibility the computed HO3 bands were omitted from Fig.…”

Section: Resultsmentioning

confidence: 99%

Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

et al. 2015

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We observed surprising difference in the FTIR (Fourier transform infrared) hydroxyl spectra of the structurally isomorphous, proton exchanged H-CHA and H-SAPO-34 molecular sieves when measured by transmission (TR) or diffuse reflectance (DRIFT) techniques. Experimental and density functional theory (DFT) based model evidence is presented in this paper to prove that the essential reason for this spectral difference is that DRIFT emphasizes the vibrations of surface hydroxyl sites. Vibrations of the bulk Brønsted acidic hydroxyls shift to higher frequencies when they become surface species, the IR beam is reflected from approximately the top ~15 to 20 Å thick layer of the particles, hence the proportion of surface related IR bands becomes significant compared to the bulk related ones in the DRIFT spectra while the opposite is valid for the TR spectra. We demonstrate that the surface hydroxyls are Brønsted acidic both on the H-CHA and the H-SAPO-34 particles and the upshifted vibrations noticed primarily in the DRIFT spectra are Al-OH vibrations on the surface even of H-SAPO-34, not P-OH groups as most researchers believe. We also show that the bulk Brønsted sites might involve HO1, HO2 and HO4 type hydroxyls associated with the known geometrically different oxygen positions on both molecular sieves, but only HO1 surface hydroxyls are associated with the red-shifted vibration intensified in the DRIFT spectra. Moreover, a single surface model cannot account for every vibration observed in DRIFT spectra. From the combination of IR vibrations of three adequate surface models one can as properly match the experimental DRIFT spectra as the TR spectra from the combination of the calculated bulk HO1…HO4 vibrations of these molsieve crystals.

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Section: Resultsmentioning

confidence: 99%

Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

et al. 2015

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“…Very precise analysis be comes possible by obtaining not only the experimental observation but also the theoretical calculation. Our study using the DFT calculation was first applied to chabazite 14) . Chabazite has a simple structure with only one kind of T-site like Y zeolite, and therefore the structure unit consists of relatively small number of atoms.…”

Section: Dft Calculation To Support the Experimental Observationmentioning

confidence: 99%

Combined Method of Ammonia IRMS-TPD Experiment and DFT Calculation to Characterize Zeolite Acidity

Niwa

Katada

2009

J. Jpn. Petrol. Inst.

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Our recently developed potential characterization method for studying the zeolite acidity is reviewed. In this method, ammonia TPD is a principle technique and first the principle of the method was explained. In order to overcome the drawback existed in the usual TPD, IRMS-TPD method was developed, in which IR and MS were measured simultaneously to monitor the thermal behavior of ammonia molecules on the surface and in the gas phase. The developed method made it possible not only to measure the Brønsted and Lewis acid sites individu ally but also to clarify the distribution of Brønsted acid sites. In addition, DFT calculations were carried out to confirm the obtained information of the Brønsted acidity. Based on these studies, a new technique was devel oped by combining the IRMS-TPD experiment and DFT calculation to precisely characterize the zeolite acidity. Using the developed method, the modified Y zeolites active for the cracking of hydrocarbons were studied.

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“…[29,30] FTIR spectroscopy with probe molecules has been used in the determination of the number of acid sites for cation modified ZSM-5. [33][34][35] Niwa and co-workers reported that IR combined with NH 3 -TPD simultaneously provides information on the strength and quantity of acid sites of zeolites [23,24,36,37] and ZrO 2 . [38] Studies that explore how the cations affect the acidity of ion-exchanged zeolites are rare.…”

Section: Introductionmentioning

confidence: 99%

Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

Wu¹,

Weitz

2014

Applied Surface Science

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A study of the acid sites in M n+ -ZSM-5 zeolites (M n+ = H + , Al 3+ , Ca 2+ , and Ba 2+ ), synthesized by ion-exchange from the NH 4 + form, has been carried out using X-ray diffraction, 27 Al MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. 27 Al MAS NMR spectroscopy confirms that Lewis acid sites in all of the M n+ -ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H + -ZSM-5 and Al 3+ -ZSM-5. Brønsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in M n+ -ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Brønsted acid sites in M n+ -ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in M n+ -ZSM-5 increases in the same order as the acidity of cations with Ba 2+ < Ca 2+ < NH 4 + < Al 3+ < H + . Though the strength of both Lewis and Brønsted acid sites is virtually identical for all M n+ -ZSM-5 zeolites, the ratio of Brønsted to Lewis acid sites varies as a result of the loaded cation. In addition, modification of acid sites in M n+ -ZMS-5 by loading different cations is discussed in terms of the accessibility of the surface of the zeolite channels and the reactivity of the cations with Brønsted acid hydroxyls. Weconclude that larger cations can effectively constrict the zeolite channels and impede the ability of pyridine to access acid sites on the surface of the channels.

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Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites

Cited by 53 publications

References 34 publications

Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

Combined Method of Ammonia IRMS-TPD Experiment and DFT Calculation to Characterize Zeolite Acidity

Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

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