A computational study of ethane cracking in cluster models of zeolite H-ZSM-5. Page: 4 of 12
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06/-26-''98 15:3: 5 2 12t9 464 34899
A COMPUTATIONAL STUDY OF ETHANE CRACKING IN CLUSTER MODELS OF
S. A. ZYGMUNT*, L. A. CURTISS4 and L. E. ITON
Valparaiso University, Valparaiso, IN, USA; email@example.com
"Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL, USA
Protolytic cracking of ethane by zeolites has been studied using quantum-chemical techniques
and a cluster model of the zeolite Brensted 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/6-3 1G(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 optimizations of these complexes using the B3LYP method of density functional theory.
The transfer of a proton from the Bronsted acid site to an adsorbed molecule is an important
step in acid catalysis by zeolites. However, this process is not yet fully understood at an atomic
level. In particular, protonated intermediate species have not yet been well characterized. In the
case of hydrocarbon cracking, does proton transfer lead to a stable intermediate carbonium ion, or
a short-lived transition-state? Such a question is difficult to answer using spectroscopic
techniques, but can be addressed through the use of ab initio quantum chemistry. As a first step
toward future investigations of cracking reactions involving larger hydrocarbons, we have carried
out a detailed theoretical study of the potential surface of ethane interacting with the Bransted
acid site of a cluster model of a zeolite.
--5 ANL MS3 /CHM P.02
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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/m1/4/: accessed November 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.