Stable Dimerized Alkoxy Species of 2-Methylpropene on Mordenite Zeolite Studied by FT-IR

Ishikawa, Hiroyuki; Yoda, Eisuke; Kondo, Junko N.; Wakabayashi, Fumitaka; Domen, Kazunari

doi:10.1021/jp990444l

Cited by 63 publications

(77 citation statements)

References 32 publications

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“…Isobutene formed on zeolite acid sites is unstable, suggesting that the π-bonded isobutene on the acid sites of zeolite is dimerized, resulting in 2,4,4-trimethyl-2-pentoxy formed via 2,2,4-trimethyl-1-pentene and 2,2,4-trimethyl-2-pentene 35) . Therefore, dimerization and oligomerization of isobutene tend to proceed over zeolites with higher acidity.…”

Section: Formation Of Carbonaceous Species On H-beta Zeolitesmentioning

confidence: 99%

Decomposition of <i>t</i>-Butanethiol into Hydrogen Sulfide without Hydrogen Addition over H-Y and H-beta Zeolites

昂

光男

慎太郎

et al. 2015

J. Jpn. Petrol. Inst.

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Catalytic direct decomposition of t-butanethiol (TBT) into hydrogen sulfide over zeolites without hydrogen addition was examined as a new desulfurization process for fuel cell systems. TBT is a widely used odorant in pipeline natural gas, and was easily decomposed into hydrogen sulfide and isobutene over H-Y and H-beta at low temperatures of 25-150 . However, catalyst deactivation of H-beta was observed at 60 and oligomerized products of isobutene were observed on the catalyst surface after long reaction times. The deactivation rate of TBT decomposition over H-beta increased with higher acid amounts of H-beta. The amount of oligomerized products deposited on the catalyst increased with lower TBT conversion in the initial stage of reaction. The deposition of oligomerized products and catalyst deactivation decreased after several hours. The amount of the oligomerized products deposited on the catalyst reached approximately 6 wt% after 8 h and remained constant after 125 h over H-beta (Si/Al 92.5) at 150 . The initial TBT conversion was constant during 125 h.

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Section: Formation Of Carbonaceous Species On H-beta Zeolitesmentioning

confidence: 99%

Decomposition of <i>t</i>-Butanethiol into Hydrogen Sulfide without Hydrogen Addition over H-Y and H-beta Zeolites

昂

光男

慎太郎

et al. 2015

J. Jpn. Petrol. Inst.

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“…It is, however, about 26 kJ mol À1 less stable than the adsorption p complex of isobutene with the Brønsted site. Nevertheless, all experimental attempts [13,14] to produce evidence for either the tertbutyl cation or an alkoxide have been unsuccessful so far, and the existence of tert-butyl cations in zeolites remains controversial.Herein we report density functional theory (DFT) calculations for the reaction of isobutene with H-ferrierite (H-FER, 1) with formation of the p complex 2 (Scheme 1). We applied periodic boundary conditions to a large simulation cell of dimensions 1870 1417 1496 pm 3 .…”

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confidence: 99%

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confidence: 99%

Protonated Isobutene in Zeolites: tert‐Butyl Cation or Alkoxide?

2005

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Acid zeolite catalysts are industrially used for a variety of hydrocarbon transformation processes. Initially it was assumed that these reactions follow mechanisms known from chemistry in superacidic media, and involve carbocations as intermediates formed upon protonation of hydrocarbons by Brønsted acid sites.[1] However, NMR spectroscopic studies failed to find simple carbenium ions as intermediates, and instead produced evidence for surface alkoxides. [2,3] Around the same time, quantum chemical calculations employing small cluster models showed that alkoxides are minima on the potential energy surface (PES) and, hence, intermediates, while carbenium ions represent saddle points on the PES and are only present as extremely short-lived transition states.[4] So far, evidence for persistent carbenium ions has only been produced for cyclic alkenyl or aromatic carbenium ions by NMR, [5] UV/Vis, [6] and IR spectroscopy [7] or computational techniques.[8]Among the carbenium ions derived from small alkenes, the tert-butyl cation has attracted much interest because it is more stable than primary or secondary carbenium ions; the competing formation of tert-butoxide may be sterically hindered. Previous computational studies have shown that, depending on the framework and the position at the zeolite wall to which they are bound, tert-butoxides may be as unstable as the tert-butyl cation. [9][10][11][12] Only the embedded cluster study by Boronat et al. [12] reports a local minimum on the PES for the tert-butyl cation in mordenite. It is, however, about 26 kJ mol À1 less stable than the adsorption p complex of isobutene with the Brønsted site. Nevertheless, all experimental attempts [13,14] to produce evidence for either the tertbutyl cation or an alkoxide have been unsuccessful so far, and the existence of tert-butyl cations in zeolites remains controversial.Herein we report density functional theory (DFT) calculations for the reaction of isobutene with H-ferrierite (H-FER, 1) with formation of the p complex 2 (Scheme 1). We applied periodic boundary conditions to a large simulation cell of dimensions 1870 1417 1496 pm 3 . [15] We show that the complex with the tert-butyl cation (4)-one possible structure formed upon proton transfer to isobutene in 2-is a local minimum on the potential energy surface. For the first time we have evaluated entropy contributions to assess the stability of 4 relative to the adsorption p complex 2 and other possible proton transfer products, that is, surface alkoxides (3, 5), at finite temperatures.Our simulation cell of H-FER (1) has the composition HAlSi 71 O 144 . After aluminum substitution at the crystallographic position T2, [16] the proton is most stable at O7. [15] We used the Perdew-Burke-Ernzerhof (PBE) density functional [17] with a plane wave basis set. [18] Stationary points obtained by relaxation of the positions of all atoms in the cell are characterized by harmonic frequencies, from which zeropoint vibrational energies, finite temperature energy, and entropy contributions are ...

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“…In the first sight, this is in accordance with the fact that to date no free carbenium ions are experimentally observed on the surface of acidic zeolites. [26][27][28] Moreover, Brouwer 22 found that double-bond shift occurs and follows a concerted reaction mechanism; and in situ spectroscopic studies (solid-state NMR and IR) of the interaction of Brønsted acid sites with hexene indicated that protonation of adsorbed olefins results in the formation of covalent alkoxide species.…”

Section: Introductionmentioning

confidence: 99%

Zeolite-catalyzed Isomerization of 1-Hexene to trans-2-Hexene: An ONIOM Study

Li¹,

Zhu²,

Liu³

et al. 2011

Bulletin of the Korean Chemical Society

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Details of the double-bond isomerization of 1-hexene over H-ZSM-5 were clarified using density functional theory. It is found that the reaction proceeds by a mechanism which involves the Brønsted acid part of the zeolite solely. According to this mechanism, 1-hexene is first physically adsorbed on the acidic site, and then, the acidic proton transfers to one carbon atom of the double bond, while the other carbon atom of the double bond bonds with the Brønsted host oxygen, yielding a stable alkoxy intermediate. Thereafter, the Brønsted host oxygen abstracts a hydrogen atom from the C 6 H 13 fragment and the C-O bond is broken, restoring the acidic site and yielding trans-2-hexene. The calculated activation barrier is 12.65 kcal/mol, which is in good agreement with the experimental value. These results well explain the energetic aspects during the course of double-bond isomerization and extend the understanding of the nature of the zeolite active sites.

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Stable Dimerized Alkoxy Species of 2-Methylpropene on Mordenite Zeolite Studied by FT-IR

Cited by 63 publications

References 32 publications

Decomposition of <i>t</i>-Butanethiol into Hydrogen Sulfide without Hydrogen Addition over H-Y and H-beta Zeolites

Decomposition of <i>t</i>-Butanethiol into Hydrogen Sulfide without Hydrogen Addition over H-Y and H-beta Zeolites

Protonated Isobutene in Zeolites: tert‐Butyl Cation or Alkoxide?

Zeolite-catalyzed Isomerization of 1-Hexene to trans-2-Hexene: An ONIOM Study

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