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The Campanile at UC Berkeley. |
Jason Ryder
University of Alabama
B.S. Chemical Engineering, 1997
Ph.D. Chemical Engineering, UC Berkeley 2003
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Research Interests:
Molecular Modeling of Heterogeneous Catalysts
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| Background. Zeolites are crystalline aluminosilicates made up
of edge-sharing silicate and aluminate tetrahedra, which contain
channels of molecular dimensions (4-10 Angstroms in diameter) and hence
are often referred to as molecular sieves. The presence of trivalent
aluminum introduces negative charge within the crystal structure.
Counter ions, such as H+, adsorb to oxygen sites proximate to the
aluminum centers, balancing the charge. The resulting Brønsted acid
sites are widely accepted as the catalytic site for hydrocarbon
cracking, alkylation, and isomerization reactions in H-ZSM-5 zeolite.
These reactions are crucial elements in the multibillion dollar
petrochemical industry.
Significance of Research. The complete characterization of
local zeolite structure and activity remains an aim of heterogeneous
catalysis and reaction engineering research. Current experimental
methods provide an average picture of the zeolite; hence complete
information on local structure and reaction mechanisms within the
zeolite remains elusive. Computational methods have emerged as a
powerful complementary tool to experimental methods. Using electronic
structure calculations one can obtain local zeolite geometry and
potential energy surface information directly at the catalytically
active site. This lends insight into the development of rational
reaction schemes. In addition, these computations allow the estimation
of key quantities such as experimentally observable infrared
frequencies, thermodynamic information, and overall reaction rate
constants.
Computational Approach. Density functional calculations are
performed on a model zeolite system. We represent the zeolite using
clusters of between 30-50 atoms. Each cluster contains one or more Al
atoms surrounded by shells of O and Si atoms. Geometry optimization
calculations for minimum energy and transition-state structures are
performed using non-local, gradient-corrected density-functional theory
(DFT). Calculations are carried out to minimum for reactants, products,
and adsorbed structures and to a saddle point for transition-state
structures, indicated by one negative eigenvalue in the force constant
matrix. Vibrational modes of adsorbed complexes and transition-state
structures are computed for those atoms whose normal modes change most
during reaction, and therefore contribute to the zero-point energy
correction and prexponential factor. Overall reaction rate constants are
computed using standard statistical mechanics and absolute rate theory. |
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