<b>Carbonium Ions. XIII. Ultraviolet Spectra and Thermodynamic Stabilities of Cycloalkenyl and Linear Alkenyl Cations</b>

Deno, N. C.; Bollinger, J. Martin; Friedman, Norman R.; Hafer, Karen.; Hodge, J. D.; Houser, John J.

doi:10.1021/ja00902a027

Cited by 62 publications

(44 citation statements)

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“…At 275°C, that is, without any methanol conversion(Figure 3), the dominant 1 H MAS NMR signals at 3.6 and 3.0 ppm, characteristic for Brønsted acid sites35 and methoxy groups (CH 3 O−),…”

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confidence: 99%

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34

et al. 2014

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Little is known on the early stages of the methanol-to-olefin (MTO) conversion over H-SAPO-34, before the steady-state with highly active polymethylbenzenium cations as most important intermediates is reached. In this work, the formation and evolution of carbenium ions during the early stages of the MTO conversion on a H-SAPO-34 model catalyst were clarified via 1 H MAS NMR and 13 C MAS NMR. Several initial species (i.e., three-ring compounds, dienes, polymethylcyclopentenyl, and polymethylcyclohexenyl cations) were, for the first time, directly verified during the MTO conversion. Their detailed evolution network was established from theoretical calculations. On the basis of these results, an olefin-based catalytic cycle is proposed to be the primary reaction pathway during the early stages of the MTO reaction over H-SAPO-34. After that, an aromatic-based cycle may be involved in the MTO conversion for long times on stream.

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confidence: 99%

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34

et al. 2014

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“…---k2(D) k2 ' Although one can accurately obtain the k-,/k-, ratio, it is possible only to obtain an order of magnitude value for k-, or k-,. Deno et al (5) have reported a value of -1.9 for the pK, of 1,3-dimethylcyclopentenyl cation. Equilibrium measurements in this laboratory have shown that a 2-methyl substituent is stabilizing (over 2-H) by a factor of at least 100 (6).…”

Section: Resultsmentioning

confidence: 98%

Mechanism of a base-catalyzed carbonium ion rearrangement

Sorensen¹,

Miller²,

Urness³

1970

Can. J. Chem.

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The rearrangement of cis-1,2,3,4,5-pentamethylcyclopentenyl cation to the trans isomer in aqueous sulfuric acids has been found to follow Ho with unit slope (d log k,,,,,. = dHo). This rearrangement involves the intermediacy of a neutral compound, 1,2,3,4,5-pentamethylcyclopentadiene, which on reprotonation is only partly efficient in leading to the trans ion product. Under both steady-state intermediate and direct protonation conditions, this diene protonates to give about 55 % of the cis ion and only 45 % of the thermodynamically more stable trans isomer.

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“…Assuming no changes to the sulfated zirconia, the difference spectrum suggests the presence of a band at bands at 1580 cm -1 and at 310 nm (original spectrum, measured in reflectance), which were ascribed to a monoenic allylic cation. Their assignment was based on literature data of ions on zeolite surfaces (21) and in solution (19,20) and the fact that the band at 310 nm partially disappeared upon exposure to NH 3 , which is presumed to neutralize positive charges. Accordingly, the pronounced band at 310 nm that forms during n-butane isomerization 9 (Figures 3-5) in our experiments may also be attributed to the π-π * transition of allylic cations.…”

Section: In Situ Uv-vis Spectroscopymentioning

confidence: 99%

“…Specifically, the bands at 370 and 430 nm are identified as dienic and trienic allylic cations (21,22); for such ions, a chain length of at least 5 or 7 carbon atoms, respectively, is required. The assignment is deduced from the work of Pazé (22) (19); and as with polyenes, the coefficients should increase with conjugation (up to a factor of 2 per additional double band (34)). The spectra could thus be deceiving with respect to the amount of dienic and trienic allylic cations on the surface.…”

Section: In Situ Uv-vis Spectroscopymentioning

confidence: 99%

“…Chen et al (9) assigned a band at 292 nm to allylic cations; Spielbauer et al (8) attributed three bands at 306, 366, and 400 nm to allylic cations, polyenylic cations, and polycyclic aromatic compounds, respectively. Unsaturated species are obviously formed at some point during the interaction of n-butane or its reaction products with the surface; and the UV-vis spectra of a number of cationic hydrocarbon species in solution (19,20) or on zeolite surfaces (21,22) are available in the literature and can be used for interpretation.…”

Section: Introductionmentioning

confidence: 99%

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Isomerization of n-butane and of n-pentane in the presence of sulfated zirconia: formation of surface deposits investigated by in situ UV–vis diffuse reflectance spectroscopy

Ahmad

Melsheimer

Jentoft

et al. 2003

Journal of Catalysis

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Catalytic performance and formation of carbonaceous deposits were studied simultaneously during alkane isomerization over sulfated zirconia in a fixed bed flow reactor with an optical window for in situ UV-vis diffuse reflectance spectroscopy. The reactions of n-butane (5 kPa) at 358 and 378 K and of n-pentane (0.25 kPa) at 298 and 308 K passed within 5 h or less through an induction period, a conversion maximum, and a period of deactivation; a steady activity of 41 and 47 µmol g -1 h -1 (isobutane formation) and ≈2.5 µmol g -1 h -1 (isopentane, both temperatures) remained. UV-vis spectra indicate the formation of unsaturated surface deposits; the band positions at 310 nm (n-butane reaction) and 330 nm (npentane) are within the range of monoenic allylic cations. More highly conjugated allylic cations (bands at 370 and 430 nm) became evident during n-butane reaction at 523 K. The chronology of events suggests that the surface deposits are (i) a result only of the bimolecular and not the monomolecular reaction mechanism, and (ii) are formed in a competitive reaction to the alkane products.

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Carbonium Ions. XIII. Ultraviolet Spectra and Thermodynamic Stabilities of Cycloalkenyl and Linear Alkenyl Cations

Cited by 62 publications

References 0 publications

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34

Understanding the Early Stages of the Methanol-to-Olefin Conversion on H-SAPO-34

Mechanism of a base-catalyzed carbonium ion rearrangement

Isomerization of n-butane and of n-pentane in the presence of sulfated zirconia: formation of surface deposits investigated by in situ UV–vis diffuse reflectance spectroscopy

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