Carbocation Stability in H-ZSM5 at High Temperature

Glen A. Ferguson, Lei Cheng, Lintao Bu, Seonah Kim, David J. Robichaud, Mark R. Nimlos, Larry A. Curtiss, Gregg T. Beckham

Research output: Contribution to journalArticle

6 Citations (Scopus)

Abstract

Zeolites are common catalysts for multiple industrial applications, including alcohol dehydration to produce olefins, and given their commercial importance, reaction mechanisms in zeolites have long been proposed and studied. Some proposed reaction mechanisms for alcohol dehydration exhibit noncyclic carbocation intermediates or transition states that resemble carbocations, and several previous studies suggest that the tert-butyl cation is the only noncyclic cation more stable than the corresponding chemisorbed species with the hydrocarbon bound to the framework oxygen (i.e., an alkoxide). To determine if carbocations can exist at high temperatures in zeolites, where these catalysts are finding new applications for biomass vapor-phase upgrading (∼500°C), the stability of carbocations and the corresponding alkoxides were calculated with two ONIOM embedding methods (M06-2X/6-311G(d,p):M06-2X/3-21G) and (PBE-D3/6-311G(d,p):PBE-D3/3-21G) and plane-wave density functional theory (DFT) using the PBE functional corrected with entropic and Tkatchenko-Scheffler van der Waals corrections. The embedding methods tested are unreliable at finding minima for primary carbocations, and only secondary or higher carbocations can be described with embedding methods consistent with the periodic DFT results. The relative energy between the carbocations and alkoxides differs significantly between the embedding and the periodic DFT methods. The difference is between ∼0.23 and 14.30 kcal/mol depending on the molecule, the model, and the functional chosen for the embedding method. At high temperatures, the pw-DFT calculations predict that the allyl, isopropyl, and sec-butyl cations exhibit negligible populations while acetyl and tert-butyl cations exhibit significant populations (>10%). Moreover, the periodic DFT results indicate that mechanisms including secondary and tertiary carbocations intermediates or carbocations stabilized by adjacent oxygen or double bonds are possible at high temperatures relevant to some industrial uses of zeolite catalysts, although as the minority species in most cases.

Original languageEnglish (US)
Pages (from-to)11397-11405
Number of pages9
JournalJournal of Physical Chemistry A
Volume119
Issue number46
DOIs
StatePublished - Oct 26 2015
Externally publishedYes

Fingerprint

Zeolites
embedding
Density functional theory
Cations
density functional theory
alkoxides
zeolites
cations
Dehydration
catalysts
dehydration
Catalysts
alcohols
Temperature
Alcohols
Oxygen
upgrading
Alkenes
oxygen
minorities

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Ferguson, G. A., Cheng, L., Bu, L., Kim, S., Robichaud, D. J., Nimlos, M. R., ... Beckham, G. T. (2015). Carbocation Stability in H-ZSM5 at High Temperature. Journal of Physical Chemistry A, 119(46), 11397-11405. https://doi.org/10.1021/acs.jpca.5b07025

Carbocation Stability in H-ZSM5 at High Temperature. / Ferguson, Glen A.; Cheng, Lei; Bu, Lintao; Kim, Seonah; Robichaud, David J.; Nimlos, Mark R.; Curtiss, Larry A.; Beckham, Gregg T.

In: Journal of Physical Chemistry A, Vol. 119, No. 46, 26.10.2015, p. 11397-11405.

Research output: Contribution to journalArticle

Ferguson, GA, Cheng, L, Bu, L, Kim, S, Robichaud, DJ, Nimlos, MR, Curtiss, LA & Beckham, GT 2015, 'Carbocation Stability in H-ZSM5 at High Temperature', Journal of Physical Chemistry A, vol. 119, no. 46, pp. 11397-11405. https://doi.org/10.1021/acs.jpca.5b07025
Ferguson GA, Cheng L, Bu L, Kim S, Robichaud DJ, Nimlos MR et al. Carbocation Stability in H-ZSM5 at High Temperature. Journal of Physical Chemistry A. 2015 Oct 26;119(46):11397-11405. https://doi.org/10.1021/acs.jpca.5b07025
Ferguson, Glen A. ; Cheng, Lei ; Bu, Lintao ; Kim, Seonah ; Robichaud, David J. ; Nimlos, Mark R. ; Curtiss, Larry A. ; Beckham, Gregg T. / Carbocation Stability in H-ZSM5 at High Temperature. In: Journal of Physical Chemistry A. 2015 ; Vol. 119, No. 46. pp. 11397-11405.
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