Scott, Valerie J. (2011) Investigations of C-H activation and the conversion of methanol to triptane. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:02272011-102614437
Broadly speaking, this thesis represents research towards understanding the mechanisms and important species related to small molecule conversion, namely methane to methanol and methanol to higher hydrocarbons. The first section is on understanding the catalytic formation of methanol from methane, with specific interest in using gold (Au). While this transformation is known to occur catalytically, very little is understood about how it happens. To study this reaction, well-defined Au-complexes were synthesized and reactions relevant to the possible catalytic cycles were examined. In doing so, the first simple Au(III)-monoalkyl complex was generated and characterized: (Idipp)AuI2Me, where Idipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Kinetics experiments demonstrated that the complex reductively eliminates methyl iodide, which is relevant to the functionalization step in CH activation. At low concentrations of iodide, the reductive elimination happens faster, from an unobserved 3-coordinate intermediate. However, at high iodide concentrations, the pathway is still consistent with reductive elimination, but from a 5-coordinate intermediate. This is in contrast to the related platinum-system, as well as to density functional theory calculations done on the Au-system. The second section studies the C-H activation step alone by close examination of the microscopic reverse: protonation of a metal-alkyl. It had previously been noted that the observed kinetic isotope effects (KIEs) were unusually high for the protonolysis of a few Pd complexes and one Pt complex. It was hypothesized that these high KIEs and involvement of quantum mechanical tunneling may indicate a change in the mechanism of the protonolysis reaction, from protonation at the metal center and reductive coupling to direct protonation of the M-Me bond. The experiments described here were designed to explicitly test this theory and demonstrated that no correlation can be made between mechanism and tunneling. The third section is focused on the study of the conversion of methanol to highly branched alkanes that make good fuel additives, namely 2,2,3-trimethylbutane (triptane), amidst other alkanes, olefins, and aromatics. Catalyzed by ZnI2 or InI3 at high temperatures, the reaction is hydrogen deficient: aromatics are formed as unsaturated by-products necessary for alkane generation. While the product distributions are somewhat different for the two different catalysts, the general mechanism is the same. While typical InI3 reactions generate more alkanes, more aromatics, and fewer olefins than ZnI2 reactions, longer reaction times and higher temperatures make the ZnI2 reaction look like the InI3 profile. Furthermore, InI3 can activate alkanes; it was found that InI3 can “upgrade” other alkanes with methanol. Notably, a 1:1 mixture of 2,3-dimethylbutane and methanol can be converted into triptane with good selectivity and little aromatic formation; ZnI2 can carry out similar chemistry at higher temperatures. Quantification of the iodine-containing products in each reaction mixture was attempted because of its relevance to the system’s industrial viability and found that these concentrations were significantly higher than would be acceptable in an industrial setting.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||C-H activation, methanol conversion, methane, Au, kinetic isotope effect, triptane|
|Degree Grantor:||California Institute of Technology|
|Division:||Chemistry and Chemical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||14 February 2011|
|Author Email:||valerie.j.scott (AT) gmail.com|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Valerie Scott|
|Deposited On:||28 Feb 2011 20:08|
|Last Modified:||26 Dec 2012 04:33|
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