A comprehensive thermal study of the state of tetrapropylammonium (TPA) phase in ZSM-5

Kassem, M. E.; Sayed, Moein B.; Arafa, W.; El-Samman, H.; Al-Emadi, Ismail M.

doi:10.1016/0040-6031(92)85025-q

Cited by 6 publications

(4 citation statements)

References 13 publications

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“…[47] The coupled defect species in this case is the electron hole defect, which is either trapped on oxygen next to an aluminum substitutional, V 1 (X 1 ) center, or in the pure siliceous environment, V 2 (X 2 ) center, the result of the Brùnsted site dehydrogenation. [29±31] The trapping of a hole by an aluminum impurity is a plausible process due to the localization at the dehydrogenated site [30,31,48,49] and was confirmed by our calculations. [24] In contrast, we could not find atomic configurations that would stabilize a hole exclusively on one siloxane bridge.…”

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confidence: 69%

Computational Investigation into the Origins of Lewis Acidity in Zeolites

et al. 2000

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The catalytic abilities of zeolites are attributed largely to their acidic properties, in both the Brønsted (proton donating) and Lewis (electron accepting) senses, although the origin and mechanism of the Lewis acidity remain unclear. Here is presented a computational approach to identifying Lewis acid sites in zeolite frameworks by analyzing the major candidate structures, such as the Al hydroperoxide shown in the Figure.

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confidence: 69%

Computational Investigation into the Origins of Lewis Acidity in Zeolites

et al. 2000

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“…Appearance of more than a decomposition band indicates inclusion of TPA in more than a phase. The 768 K-band is of higher temperature than that (753 K) previously reported for pure ZSM-5, which assigns stronger interaction to the more anionic framework Al of the present zeolite . The lower temperature 708 K-band assigns yet weaker interaction of TPA + with the modifier element than that (738 K) shown for Ga in GaZSM-5 .…”

Section: Resultscontrasting

confidence: 55%

“…Correlation Thermal TGA Analysis of Pure and Modified TPA-ZSM-5. TPA + decomposes respectively at 550, 653, 738, and 753 K for pure TPABr, physically occluded in silicalite, associated with Ga in GaZSM-5, and associated with Al in pure ZSM-5 . Gradual rise in TPA + decomposition temperature explains stronger interaction with more anionic framework .…”

Section: Resultsmentioning

confidence: 95%

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IR Spectral Investigation of the Multistage Single Catalytic Process of CH₄ Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element

Sayed

2001

Energy Fuels

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The acidic zeolite HZSM-5 is modified by partial isomorphous substitution of the framework aluminum (Al) with an oxidative element during the zeolite synthesis. This is to develop an oxidative-acidic mediated catalyst suitable for the direct conversion of methane (or natural gas) into higher hydrocarbons in a multistage single catalytic process. The pure and modified ZSM-5 materials are highly crystalline of high morphology, which is shown by XRD and SEM. Pure ZSM-5 shows a single decomposition TGA band for the TPA+ cations associated with the anionic framework Al. Besides, modified ZSM-5 shows a lower temperature band revealing weaker interaction of TPA+ cations with the modifier element. The infrared spectrum of modified ZSM-5 shows a high frequency 3685 cm-1 band assigned to Bronsted sites weakly interacting with the modifier in addition and compared to the well-established more acidic Al-associated Bronsted sites absorbing at 3610 cm-1. Thus, XRD, TGA, and IR show evidence of incorporating the modifier element in the framework. The modifier site catalyzes the CH4 oxidation into H2CO using molecular O2 near 1 bar and 373 K. This is explicit in the spectral appearance of υC  O at 1711 cm-1 and δC - H at 1420 and 1375 cm-1. This stage of the catalytic conversion is so peculiar to modified ZSM-5 that can never be catalyzed at any circumstance over ordinary ZSM-5. Bronsted sites associated with Al and modifier element interact strongly with H2CO yielding unsaturated species defined by the weak υC  C at 1675 cm-1. Evidence of such interaction is shown as a downward shift of 23 cm-1 in the υC  O of the gaseous H2CO at 1734 cm-1 to 1711 cm-1 on the surface and absence of the Bronsted sites absorbing at 3610 and 3685 cm-1. In effect, the latter Bronsted bands recover at the end of the catalytic process. The unsaturated species are higher aldehydes equilibrated in a keto−enol tautomeric structure. They eventually decompose in the temperature 473−573 K range into a mixture of hydrocarbons identified by υC - H at 2960, 2935, 2913, and 2868 cm-1 and δC - H at 1469 and 1385 cm-1 (aliphatics) and by υring at 1600 and 1510 cm-1 (aromatics). The IR spectral profile of the products is similar to that of Mobil methanol-to-gasoline MTG process. The mechanistic implications of this process are proposed on the basis of the IR spectral findings observed in the reaction gas phase and on the surface that shows no evidence for the MTG route, however.

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Structural interpretation of the ambiguous loopy effect of gamma irradiation on the phase transition thermodynamics of US-HY zeolite

Sayed

1993

Thermochimica Acta

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A comprehensive thermal study of the state of tetrapropylammonium (TPA) phase in ZSM-5

Cited by 6 publications

References 13 publications

Computational Investigation into the Origins of Lewis Acidity in Zeolites

Computational Investigation into the Origins of Lewis Acidity in Zeolites

IR Spectral Investigation of the Multistage Single Catalytic Process of CH₄ Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element

Structural interpretation of the ambiguous loopy effect of gamma irradiation on the phase transition thermodynamics of US-HY zeolite

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A comprehensive thermal study of the state of tetrapropylammonium (TPA) phase in ZSM-5

Cited by 6 publications

References 13 publications

Computational Investigation into the Origins of Lewis Acidity in Zeolites

Computational Investigation into the Origins of Lewis Acidity in Zeolites

IR Spectral Investigation of the Multistage Single Catalytic Process of CH4 Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element

Structural interpretation of the ambiguous loopy effect of gamma irradiation on the phase transition thermodynamics of US-HY zeolite

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IR Spectral Investigation of the Multistage Single Catalytic Process of CH₄ Conversion into Higher Hydrocarbons over HZSM-5 Structurally Modified with an Oxidative Element