Iron Catalyzed Hydrogenation of Heavy Crude Oil Model Compounds
Creators: Antwi Peprah, Benjamin
Publisher: name: University of Alberta Library
Year: 2023

<p>The development of economically viable processes for processing and transporting of heavy crude oil and bitumen are critical for continued development in the Canadian oil sands. Current technologies for preparing such crudes for transport to market include full upgrading to synthetic crude oil, or dilution before transport. An emerging family of technologies collectively referred to as partial upgrading can convert low-API gravity heavy crude oil into a refinery- and pipeline-grade product, with lower capital and operating expenses relative to current processes such as full upgrading or dilution. The processing goal of these technologies is to reduce both viscosity and density to enhance bitumen transportability while significantly reducing or avoiding the use of diluents. One key process in partial upgrading is the selective hydrogenation of the complex polycyclic aromatic molecules found in the asphaltene fraction. This approach alters the heavy oil structure, and thus reduces the viscosity and density of the oil, potentially resulting in a pipeline-ready product. <br>
Developing efficient, practical, and economically viable catalytic processes for partial hydrogenation of heavy crude oils is an important research priority. Iron-based catalysts systems are ideal in this regard. Iron is earth-abundant, non-toxic, and inexpensive, making its utilization in heavy crude oil upgrading more attractive. Iron catalysts are usually less active than cobalt, nickel, or platinum-group transition metals for hydrogenation of aromatic molecules, but this is an advantage in partial upgrading. Selective hydrogenation and limited defunctionalization are preferred to more complete reactions mediated by commercially available hydroprocessing catalysts. <br>
In this thesis, I explored partial hydrogenation and limited desulfurization reactions mediated by known air-stable, petroleum-soluble, and well-defined diiron sulfido complex, Fe2S2(CO)6. Several polycyclic aromatic and heteroaromatic compounds were hydrogenated and desulfurized using Fe2S2(CO)6 as a precatalyst in a batch microreactors coupled with an agitator. The behavior of the catalyst system was evaluated over a wide range of temperatures, gas pressures, solvent systems, catalyst supports and metal loadings. The active phase, consisting of iron sulfide nanoparticles, was prepared from purified Fe2S2(CO)6 in toluene; no sulfur additive is required. The precatalyst is simple to prepare and recyclable, while the behavior of the system is highly reproducible. The results demonstrate that Fe2S2(CO)6 precatalyst leads to partial hydrogenation of polycondensed aromatics under moderate conditions using a carbon support. The reactivity trends show self-consistent substrate dependence, varying with the resonance energy stabilization of the starting compounds and partially saturated intermediates coupled with the surface adsorption enthalpy of the aromatic ring system.<br>
Selective hydrogenation and defunctionalization of carbocyclic and heterocyclic aromatic heavy oil model compounds were also explored using unsupported Fe2S2(CO)6 exclusively. The reactivity of the anthracene series is governed by stereoelectronic effects imposed by the phenyl substituents, decreasing in the order anthracene &gt; 9-phenylanthracene &gt; 9,10-diphenylanthracene. Nitrogen- and sulfur-containing heterocycles are partially hydrogenated, with limited heteroatom removal for benzothiophene, showing that the unsupported iron sulfide catalyst is well-suited for the selective partial hydrogenation and limited defunctionalization, which are key for catalytic partial upgrading of heavy petroleum. The unsupported iron sulfide also catalyzes hydrogenation of mixed “feeds” comprising combinations of model compounds, demonstrating that carbocyclic and heterocyclic molecules can be processed simultaneously with little self-inhibition in overall activity, a key requirement for compositionally challenging bitumen feeds.<br>
Hydrogen donor solvents can be substituted for hydrogen gas in these partial upgrading reactions. Unsupported iron sulfide catalysts derived from Fe2S2(CO)6 mediate transfer hydrogenation of carbocyclic- and N-heterocyclic aromatic compounds for the first time. Readily available hydrogen donors, including tetralin, 2-propanol, indane, indoline, and tetrahydroquinoline were tested. Using tetralin, indane or 2-propanol resulted in only limited hydrogenation of the target molecules, while indoline and tetrahydroquinoline hydrogen donors afforded near quantitative conversions. The thermodynamically favorable adsorption of N-heterocycles to the catalyst surface leads to facile dehydrogenation and hydrogen atom transfer to the substrate. The results demonstrate that a dual hydrogen donor system outperforms single donor systems to permit efficient transfer hydrogenation of various polycyclic aromatic and heteroaromatic compounds under moderate reaction conditions with good tolerance for alkyl and aryl substituents. Partially hydrogenated products are obtained in high yields with no “oversaturated” side products formed. This approach provides a cost-effective and viable catalytic protocol for partial upgrading of the challenging asphaltene fraction present in heavy crude oil, which requires partial saturation but limited heteroatom removal. </p>