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Ab initio calculations in heterogeneous and homogeneous catalysis : I. Methanol to gasoline with ZSM-5. II. Carbonyl ligand effects on metal-metal bonds

Citation

Miller, Ann Elizabeth (1993) Ab initio calculations in heterogeneous and homogeneous catalysis : I. Methanol to gasoline with ZSM-5. II. Carbonyl ligand effects on metal-metal bonds. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-09042007-152645

Abstract

This thesis is composed of two studies in catalysis. The first is an exploration, using computational techniques, of the mechanism for the first carbon-carbon bond formation in the Methanol to Gasoline (MTG) reaction. The second is a study of the factors important to the understanding of ligand effects on metal-metal bonds, and in particular, to metal clusters. Three possibilities were considered as ways to activate a carbon in the MTG process prior to formation of C2 or higher hydrocarbons. These were a free radical mechanism, a surface ylide mechanism, and a possible defect site which might lead to steric crowding of CH2 groups. Although the free radical mechanism was found to be thermodynamically within the parameters of the MTG process, it contained a high transition state. Consideration of the molecules available prior to hydrocarbon build-up and their specific electronic structure, led to the view that the available carbon atoms (methanol, dimethyl ether, etc.) were unlikely to be activated by a free radical intermediate. The surface-stabilized ylide which has been proposed as an intermediate by many was studied to determine if in fact the ylide was stabilized. The total energy of the ylide was compared to that of the naked site on the zeolite and free methylene. Free methylene ranged, depending on the geometry of the ylide, between 50 and 80 kcal more stable. These numbers are qualitatively correct, but more electron correlation would have to be incorporated in the calculation to get an accurate value for the destabilization. Starting from a defect site, two CH2 groups were each attached to two oxygen atoms. It was thought that two CH2 groups would take up considerably more space than either the original Al atom or the four hydrogens. Molecular Mechanics calculations showed the zeolite to be sufficiently flexible to prevent crowding of the CH2's. The second study involved determining the effects of colinear carbonyl ligands on osmium-osmium bonds. Calculations at the Dissociation Consistent Cl level showed that the ligands were weakening the metal-metal [pi]-bonds through back-donation.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Goddard, William A., III
Thesis Committee:
  • Goddard, William A., III (chair)
Defense Date:1 March 1993
Record Number:CaltechETD:etd-09042007-152645
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-09042007-152645
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3327
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:11 Sep 2007
Last Modified:26 Dec 2012 02:59

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