Deuterium Tracer Studies Of The Mechanism Of Cobalt Catalyzed Fischer-Tropsch Synthesis

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

Development of Novel Mass Spectrometric Methodology for Mixture Analysis: Studies of p-Benzynes\u27 Reactivity in Solution and Gas-Phase

Mass spectrometry has evolved to be an indispensable analytical tool for identification of unknowns and molecular analysis of complex mixtures. The integration of high resolution mass spectrometry, gas-phase ion-molecule reactions, and collision-activated dissociation (CAD) techniques to mass spectrometers play vital role in structural elucidation of unknown molecules. In addition, the coupling of separation techniques, such as high performance liquid chromatography (HPLC) and gas chromatography (GC), to the mass spectrometers enabled trace level analytes in complex mixtures. This dissertation focuses on the development of a positive mode atmospheric pressure chemical ionization (APCI) mass spectrometry method for analysis of lubricant base oils, and development and validation of a liquid chromatography-negative mode electrospray ionization-mass spectrometry (LC-(-)ESI-MS) method for analysis of glycerol in renewable diesel feedstock. In addition, the reactivity of a para-benzyne analog, 5,8-didehydroquinoline, was explored in solution and gas-phase, and the reaction products were characterized utilizing tandem mass spectrometry techniques

Polar Effects Control the Gas-Phase Reactivity of Para-Benzyne Analogs

We report herein a gas-phase reactivity study on a para-benzyne cation and its three cyano-substituted, isomeric derivatives performed using a dual-linear quadrupole ion trap mass spectrometer. All four biradicals were found to undergo primary and secondary radical reactions analogous to those observed for the related monoradicals, indicating the presence of two reactive radical sites. The reactivity of all biradicals is substantially lower than that of the related monoradicals, as expected based on the singlet ground states of the biradicals. The cyano-substituted biradicals show substantially greater reactivity than the analogous unsubstituted biradical. The greater reactivity is rationalized by the substantially greater (calculated) electron affinity of the radical sites of the cyano-substituted biradicals, which results in stabilization of their transition states through polar effects. This finding is in contrast to the long-standing thinking that the magnitude of the singlet-triplet splitting controls the reactivity of para-benzynes

Comparison of Atmospheric Pressure Chemical Ionization and Field Ionization Mass Spectrometry for the Analysis of Large Saturated Hydrocarbons

Direct infusion atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was compared to field ionization mass spectrometry (FI-MS) for the determination of hydrocarbon class distributions in lubricant base oils. When positive ion mode APCI with oxygen as the ion source gas was employed to ionize saturated hydrocarbon model compounds (M) in hexane, only stable [M – H]⁺ ions were produced. Ion–molecule reaction studies performed in a linear quadrupole ion trap suggested that fragment ions of ionized hexane can ionize saturated hydrocarbons via hydride abstraction with minimal fragmentation. Hence, APCI-MS shows potential as an alternative of FI-MS in lubricant base oil analysis. Indeed, the APCI-MS method gave similar average molecular weights and hydrocarbon class distributions as FI-MS for three lubricant base oils. However, the reproducibility of APCI-MS method was found to be substantially better than for FI-MS. The paraffinic content determined using the APCI-MS and FI-MS methods for the base oils was similar. The average number of carbons in paraffinic chains followed the same increasing trend from low viscosity to high viscosity base oils for the two methods