A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.

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Protolytic cracking of ethane by zeolites has been studied using quantum-chemical techniques and a cluster model of the zeolite Broensted acid site. Previous computational studies have utilized small cluster models and have not accounted for the long-range effects of the zeolite lattice. These studies have found reaction barriers for cracking which are significantly higher than experimental values. In this work we used a larger zeolite cluster model containing five tetrahedral (Si, Al) atoms (denoted 5T) and searched for stationary points along one possible reaction path for cracking at the HF/6-31 G(d) level of theory. This path involves a multi-step cracking ... continued below

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12 p.

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Zygmunt, S. A. August 21, 1998.

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Protolytic cracking of ethane by zeolites has been studied using quantum-chemical techniques and a cluster model of the zeolite Broensted acid site. Previous computational studies have utilized small cluster models and have not accounted for the long-range effects of the zeolite lattice. These studies have found reaction barriers for cracking which are significantly higher than experimental values. In this work we used a larger zeolite cluster model containing five tetrahedral (Si, Al) atoms (denoted 5T) and searched for stationary points along one possible reaction path for cracking at the HF/6-31 G(d) level of theory. This path involves a multi-step cracking reaction, in which the proton is first transferred from the acid site to the adsorbed ethane molecule to form an ion-pair equilibrium complex. Subsequently the proton attacks the C-C bond to complete the cracking process. The activation barrier for cracking was calculated, including corrections for (i) vibrational energies at the experimental reaction temperature of 773 K; (ii) electron correlation and an extended basis set at the B3LYP/6-311+G(3df,2p) level; and (iii) the influence of the surrounding zeolite lattice in H-ZSM-5. The barrier we obtain, 53 {+-} 5 kcal/mol, is significantly smaller than previous theoretical results and is in good agreement with typical experimental values for small hydrocarbons. Work is currently in progress to extend this study by carrying out geometry optimization of these complexes using the B3LYP method of density functional theory.

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12 p.

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OSTI as DE00010930

Medium: P; Size: 12 pages

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  • 12th International Zeolite Conference, Baltimore, MD (US), 07/05/1998--07/10/1998

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  • Report No.: ANL/CHM/CP-97067
  • Grant Number: W-31109-ENG-38
  • Office of Scientific & Technical Information Report Number: 10930
  • Archival Resource Key: ark:/67531/metadc627760

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Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

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  • August 21, 1998

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  • June 16, 2015, 7:43 a.m.

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  • April 6, 2017, 7:57 p.m.

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Zygmunt, S. A. A computational study of ethane cracking in cluster models of zeolite H-ZSM-5., article, August 21, 1998; Illinois. (digital.library.unt.edu/ark:/67531/metadc627760/: accessed December 15, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.