Chemical and Biomolecular Engineering
Faculty Advisor: Jason Hicks
Plasma-Assisted Catalysis for Upgrading Ethane to Valuable Liquid Products through Carbon-Nitrogen Coupling
Annually, over 140 billion m3 of valuable natural gas is released from wells during oil production . Due to a lack of efficient storage methods, much of this natural gas is flared as a means of removal, releasing more than 350 metric tons of CO2 to the environment . Since this flared gas contains light hydrocarbons, an alternative is to upgrade the gas to higher value products directly at the site of generation. Plasma-assisted catalysis is a relatively new technology that has promising results for this application. Plasma is an electrically generated source of reactive ions, radicals, and electrons. Applying these reactive plasma species to a catalytic reaction has shown improvements over traditional catalytic reactions . Specifically, improved reaction kinetics at low temperatures and pressures have been observed, allowing for reduction of capital and operating costs . Additionally, because renewable sources can provide the electricity needed to generate a plasma, on-site and mobile plasma units are a great opportunity for utilization of alternative energy. Natural gas upgrading with plasma has been shown to be possible, with one such reaction coupling methane, the main component of natural gas, with nitrogen. This reaction can produce nitrogen containing aromatic species, such as pyridine, which is a valuable liquid used in the pharmaceutical industry [4,5]. The challenge for this reaction is that it is not selective under plasma conditions, and often forms upwards of 10 different byproducts.
Catalysts such as ZSM-5 provide shape selectivity, meaning the size of the pores allow for the formation of products of a certain size. ZSM-5 is often used in the production of benzene, and therefore could assist in the formation of nitrogen containing aromatics when coupled with a plasma. Preliminary results show that using this catalyst and incorporating nitrogen into the hydrocarbon feed under plasma conditions leads to the production of pyridine, a chemical with a similar structure to benzene with the inclusion of a nitrogen in the ring. The combination of the plasma with the catalyst allows for pyridine production at temperatures of ~200 °C, which is lower than the current industrial temperature of 400 °C. Previous studies in the group have focused on the plasma-only reaction between ethane and nitrogen, investigating the impact of flow rate, feed composition, and power input in order to identify regimes that favor formation of nitrogen containing hydrocarbons. Ethane, the second largest component of natural gas, was chosen to study this reaction since it is easier to activate than methane; the knowledge gained will be helpful for later studies on methane, which is more difficult to couple with nitrogen. Guided by the knowledge gained from these previous non-catalytic experiments, this work will focus on incorporating a catalyst into the plasma reaction in order shift the selectivity towards pyridine. ZSM-5 is a tunable catalyst and can be synthesized with different acidities, as well as with different metals incorporated into the catalyst. Understanding how these factors affect the selectivity of the reaction will allow for improved performance of a reaction that can be used for on-site upgrading of hydrocarbons to valuable liquid products.
Objective 1: Understanding the role of the acidity of ZSM-5 will be important for producing the desired products. High acidity generally leads to formation of aromatics, but rapid deactivation of the catalyst is often seen. Lower acidity tends to form less aromatics, but the catalyst often has a longer life. Understanding this trade off can help determine a suitable catalyst.
Objective 2: After understanding the effect of ZSM-5 on the reaction, various metals will be incorporated into the catalyst. The metal as well as the plasma will both activate the ethane, and the shape of the ZSM-5 will help form the aromatic compounds. This metal supported ZSM-5 could help to increase yields by converting more ethane to the aromatic precursors than the plasma alone, while the plasma continues to activate the nitrogen. Metals, such as platinum and platinum-tin, which are commonly used for ethane conversion to ethylene (an aromatic precursor) will be tested. Molybdenum and gallium, which are often used with ZSM-5 for methane aromatization, will also be investigated.
Objective 3: Previous studies in the group have shown that under plasma conditions, a simulated natural gas stream of methane, ethane, and propane combined with nitrogen forms similar products to the ethane-nitrogen reaction. After identifying a suitable catalyst for the ethane-nitrogen reaction, a simulated natural gas stream will be used to gain an understanding of how this reaction differs with a mixed stream. Being able to use a mixed feed eliminates having to purify the gas stream before upgrading.
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