Date of Award

January 2012

Degree Type

Open Access Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

First Advisor

Buchang Shi

Department Affiliation

Chemistry

Abstract

Since the Fischer-Tropsch (FT) catalytic polymerization reaction has been discovered in 1920s, many kinds of possible mechanisms have been reported. However, there is still debate because there is a lacking of experimental evidence to discriminate the validity of each mechanism. To define the mechanism of Fischer-Tropsch-Synthesis (FTS), we must define the C-C formation pathways as well as the structure of initiator species in the process, and finally provide an explanation in mechanistic detail as to how branched hydrocarbon compounds are formed in the reaction sequence.

In order to understand the cobalt catalyzed mechanism of FTS, we conducted H2/D2 switching and competition experiments, and found that there is an inverse isotope effect during FTS and there is deuterium enrichment in the hydrocarbon products. The presence of the inverse isotope effect was indicated from the H₂/D₂ switch experiments by an increase in CO conversion by changing the syngas reagent from hydrogen to deuterium during the cobalt catalyzed FTS. The inverse isotope effect was again confirmed by performing the H₂/D₂ competition experiments, and we also observed deuterium was enriched with the increasing carbon number in the FT hydrocarbon products. The findings of inverse isotope effect and deuterium enrichment, led us to propose a modified alkylidene mechanism for cobalt catalyzed FT reactions.

In order to understand structure of the C₂, C₃ species, we performed the deuterium tracer experiments, and found that the structure of C2 and C3 resembles ethene and propene in the Co/SiO₂ catalyzed FTS, but not alcohol.

According to the modified alkylidene mechanism, the mono-methyl branched hydrocarbons should be formed but not ethyl or dimethyl branched hydrocarbons. Branched hydrocarbons with identification of carbon number 8, 10, 11 confirmed this conclusion.

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