Ethanol Dehydration in HZSM-5 Studied by Density Functional Theory: Evidence for a Concerted Process: Evidence for a concerted process

Seon Ah Kim, David Robichaud, Gregg Beckham, Mark Nimlos, Robert Paton

Research output: Contribution to journalArticlepeer-review

44 Scopus Citations

Abstract

Dehydration over acidic zeolites is an important reaction class for the upgrading of biomass pyrolysis vapors to hydrocarbon fuels or to precursors for myriad chemical products. Here, we examine the dehydration of ethanol at a Brønsted acid site, T12, found in HZSM-5 using density functional theory (DFT). The geometries of both cluster and mixed quantum mechanics/molecular mechanics (QM:MM) models are prepared from the ZSM-5 crystal structure. Comparisons between these models and different DFT methods are conducted to show similar results among the models and methods used. Inclusion of the full catalyst cavity through a QM:MM approach is found to be important, since activation barriers are computed on average as 7 kcal mol-1 lower than those obtained with a smaller cluster model. Two different pathways, concerted and stepwise, have been considered when examining dehydration and deprotonation steps. The current study shows that a concerted dehydration process is possible with a lower (4-5 kcal mol-1) activation barrier while previous literature studies have focused on a stepwise mechanism. Overall, this work demonstrates that fairly high activation energies (∼50 kcal mol-1) are required for ethanol dehydration. A concerted mechanism is favored over a stepwise mechanism because charge separation in the transition state is minimized. QM:MM approaches appear to provide superior results to cluster calculations due to a more accurate representation of charges on framework oxygen atoms.

Original languageAmerican English
Pages (from-to)3604-3614
Number of pages11
JournalJournal of Physical Chemistry A
Volume119
Issue number15
DOIs
StatePublished - 2015

Bibliographical note

Publisher Copyright:
© 2015 American Chemical Society.

NREL Publication Number

  • NREL/JA-5100-63785

Keywords

  • biomass pyrolysis
  • Bronsted acid site
  • dehydration
  • density functional theory (df)
  • ethanol
  • ethylene
  • zeolite

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