Date of Award
Master of Science
This paper presents the culmination of an investigation on carbon nanotubes as catalysts for the hydrogenation of carbon monoxide. Carbon nanotubes (CNTs) have been found to have extraordinary physical properties and the potential for use in a variety of applications. They have been utilized as catalyst supports in many reactions, including the conversion of syngas to ethanol. The specific role played by CNTs in these reactions, aside from that of a support structure, has not been evaluated, however. Presented here are parametric studies on Fischer-Tropsch Synthesis with carbon nanotubes as active catalysts. The use of as-produced CNTs (containing trace amounts of iron from the synthesis process) resulted in a 100-fold increase in carbon monoxide conversion per unit mass of catalyst over a traditional Fe-Zn-K/γ-alumina catalyst. This value (CO conversion per unit mass of catalyst) was raised to nearly 1500 times as high as for Fe-Zn-K/γ-alumina when purified CNTs were used in the same FT synthesis. Because iron is a primary catalyst in the FT synthesis, it can be argued that the iron in the CNTs was responsible for the catalytic behavior. However, the iron content in the MWNTs (0.014 g, ≈ 5 mass%) and SWNTs (0.04 g, ≈ 27 mass%) compared to that of the traditional iron-loaded alumina support (2.5 g, ≈ 12.5 mass%), strongly suggests that iron alone cannot be responsible for the catalysis. Although single-walled nanotube (SWNT) catalysts provided high CO conversion, methane represented the bulk of the products. Conversely, multi-walled nanotubes (MWNTs) produced mostly liquid hydrocarbons and oxygenates, indicating that the CNT structure is an important factor in the hydrogenation of CO. The parametric experiments show that temperature, pressure and the syngas composition all play key roles in the distribution of liquid products. In general, an increase in temperature correlated to an increase in hydrocarbon chain length and the formation of more alcohols; above a certain temperature, the distribution shifted to 100% alcohols. Likewise, lower pressures resulted in hydrocarbons of shorter carbon chain length and at higher pressures, more alcohols were formed. Studies were also conducted on the effect of syngas composition and the effect of different types of CNTs. Syngas with 1:1 ratio (H2:CO) produced longer hydrocarbon chains than those produced by 3:1 syngas. The type of CNTs used in FT also affected the products but no clear relationships could be discerned.
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