Research in

The Program --

This 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 the 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 activation energy barriers for the reactions, which in turn allows predictions of the reaction rates.

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. The transition state is a short-lived complex intermediate between two separate stable equilibrium structures. In this case the reaction simply involves the exchange of the proton between two adjacent oxygen atoms in the zeolite framework. According to our calculations, the activation barrier for this proton exchange reaction is less than 5 kcal/mol, which is consistent with experimental evidence that such a reaction takes place readily in zeolites even at room temperature. We are currently examining the process of proton transfer between the zeolite and adsorbed hydrocarbon molecules. This is the first step in the "cracking" of hydrocarbons into two or more smaller molecules, and is very important in the refining of petroleum.

A very simple example of a hydrocarbon cracking reaction is illustrated below, with structures determined from high-level quantum-mechanical calculations. First, propane (C₃H₈) is weakly adsorbed at the acid site in the zeolite. The cracking reaction takes place when the acidic proton "attacks" the C-C bond, leading to the transition-state complex with the proton partially inserted into the C-C bond. Thereupon the propane molecule breaks apart, producing methane (CH₄) and an ethyl cation (C₂H₅⁺) bound to an oxygen atom on the zeolite framework. In the last step of the catalytic process, which is not shown here, a proton is transferred from the C₂H₅⁺ species back to one of the oxygen atoms of the zeolite, producing a new acidic site and an ethene (C₂H₄) molecule.

Our calculations, which also take into account the influence of the rest of the zeolite lattice not shown in these figures, yield an activation energy barrier of approximately 42 kcal/mol for the propane cracking reaction in the zeolite ZSM-5. This is in reasonable agreement with the experimental value of 47 kcal/mol, and illustrates the ability of computational quantum mechanics to give quantitatively reliable results for hydrocarbon reactions in zeolites.

This work is carried out by Prof. Stan Zygmunt, in collaboration with Dr. Larry Curtiss and Dr. Lennox Iton at Argonne National Laboratory, and involves undergraduate students at Valparaiso University.

The Students --

In the summer of 1995, 1996, 1998, and 2000 students worked with Prof. Zygmunt both at Valparaiso and at Argonne National Laboratory to carry out the theoretical calculations described above. Students used DEC and IBM workstations, Linux PCs, and a CRAY supercomputer to perform this demanding, state-of-the-art computational work. Some of this work has been featured in technical papers which have been published in major scientific journals (see below, with students' names in bold). In addition, these students gave oral presentations of their work to staff members at Argonne and to faculty and students at Valparaiso. Two more students will be working with Prof. Zygmunt in the summer of 2001 to study cracking, dehydrogenation and H/D exchange reactions of alkanes in zeolites.

An Assessment of Density Functional Methods for Studying Molecular Adsorption in Cluster Models of Zeolites, S. A. Zygmunt, R. M. Mueller, L. A. Curtiss, and L. E. Iton, J. Mol. Struct. (Theochem) 430 9 (1998).

Evidence for Dimeric and Tetrameric Water Clusters in HZSM-5, D. H. Olson, S. A. Zygmunt, M. K. Erhardt, L. A. Curtiss, and L. E. Iton, Zeolites 18 347 (1997).

Computational Studies of Water Adsorption in the Zeolite H-ZSM-5, S. A. Zygmunt, L. A. Curtiss, L. E. Iton, and M. K. Erhardt, J. Phys. Chem. 100 6663 (1996).

The Funding --

Funding is supplied by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences, under contract No. W-31-109-ENG-38. Funding is also supplied by a grant from the donors of the Petroleum Research Fund, administered by the American Chemical Society.