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Molecular Sieve Materials: ZeoliteThis theoretical research program seeks to better understand chemical reactions arising from acid catalysis in a family of catalytic materials called zeolites. Zeolites are aluminosilicates which have a very porous structure consisting of cavities and channels through which molecules of the right size and shape may readily diffuse. Below is a wireframe representation of the structure of zeolite ZSM-5, where the tetrahedral (silicon or aluminum) atoms sit at the vertices and the red wires represent Si-O-Si or Si-O-Al linkages. The dotted blue lines show the boundaries of the unit cells.
The unique and useful catalytic properties of zeolites result from the presence of Bronsted acid sites in the interior. Where an aluminum atom replaces a silicon atom in the zeolite framework, a charge-balancing cation is required to preserve overall charge neutrality. When the cation is a proton, the zeolite can be a proton donor, or Bronsted acid, and can catalyze a wide range of industrially useful chemical reactions. Our research involves the use of high-level computational quantum mechanics to calculate the stable equilibrium structures of complexes formed when small molecules adsorb at the acid site in zeolites. We also seek to locate the unstable equilibium structures resulting from the transfer of the proton from the zeolite framework to the adsorbed molecule. The process of proton transfer is a key step in all acid-catalyzed reactions, and yet it is poorly understood at an atomic level. A knowledge of the energies of these various structures yields predictions of the potential energy barriers for the reactions, and this in turn gives information about the reaction rates.
Protolytic cracking of ethane by zeolites is investigated using
quantum-chemical techniques and a cluster model of the zeolite acid
site. In this work we have used a zeolite cluster model containing
five tetrahedral (Si, Al) atoms and have located stationary points
along a reaction path for cracking at the HF/6-31G(d) level of theory.
The cracking transition state is shown in the figure below. The
HF/6-31G(d) activation barrier for cracking was corrected by using
higher levels of theory and a more realistic model of the zeolite
framework. This was done by including (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 largest effect is found from the long-range electrostatic effects
of the surrounding zeolite that decrease the barrier by 14 kcal/mol.
The final barrier we obtain, 52 ± 5 kcal/mol, is significantly
smaller than previous theoretical results and is in reasonable agreement
with typical experimental values for small hydrocarbons. More info: |
For
example, the structures at the right show two possible complexes
arising from the interaction of a water molecule with the acid site
in the zeolite H-ZSM-5. On the right, the water is hydrogen-bonded
to the acid site and the adjacent oxygen atom, while on the left
the acidic proton has been transferred to the adsorbed water, forming
a hydronium ion (H3O+).. Our calculations
show that the complex on the right is the true stable equilibrium
structure, while the one on the left represents an unstable equilibrium,
or transition state structure.
Cracking
Transition State