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

Abstract

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 withmore » 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.« less

Authors:
Publication Date:
Research Org.:
Argonne National Lab., IL (US)
Sponsoring Org.:
US Department of Energy (US)
OSTI Identifier:
10930
Report Number(s):
ANL/CHM/CP-97067
TRN: AH200127%%499
DOE Contract Number:  
W-31109-ENG-38
Resource Type:
Conference
Resource Relation:
Conference: 12th International Zeolite Conference, Baltimore, MD (US), 07/05/1998--07/10/1998; Other Information: PBD: 21 Aug 1998
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; 10 SYNTHETIC FUELS; BROENSTED ACIDS; ELECTRON CORRELATION; ETHANE; CRACKING; ZEOLITES; CATALYTIC EFFECTS; MATHEMATICAL MODELS

Citation Formats

Zygmunt, S A. A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.. United States: N. p., 1998. Web.
Zygmunt, S A. A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.. United States.
Zygmunt, S A. Fri . "A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.". United States. https://www.osti.gov/servlets/purl/10930.
@article{osti_10930,
title = {A computational study of ethane cracking in cluster models of zeolite H-ZSM-5.},
author = {Zygmunt, S A},
abstractNote = {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.},
doi = {},
url = {https://www.osti.gov/biblio/10930}, journal = {},
number = ,
volume = ,
place = {United States},
year = {1998},
month = {8}
}

Conference:
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