Carbon Neutral Jet Fuel From CO2

Carbon Neutral Jet Fuel From CO₂

Since the Industrial Revolution, our economy has revolved around fossil fuels. Coal, oil, and gas are called fossil fuels because they are mostly made of the fossil remains of long-ago beings. Chemical energy within them is a kind of stored sunlight originally accumulated by ancient plants. Our civilization runs by burning the remains of humble creatures who inhabited the Earth hundreds of millions of years before humans came on the scene. 

However, burning fossil fuels is not sustainable. Sooner or later, humans will have to switch to sustainable energy. The use of fossil fuels continues to grow with an expected annual increase of 1.3%, continually exacerbating this problem in the form of climate change. The aviation sector causes 11% of transportation-related emissions in the United States, and virtually all comes from jet fuel. Given these recognized environmental concerns, developing clean, energy-efficient technologies for producing sustainable or renewable aviation fuels is imperative. 

Jet fuel, the generic name for aviation fuels, is used in aircraft powered by gas turbine engines. It is colorless to straw-colored in appearance. Jet fuel is a mixture of a variety of hydrocarbons.

In the past years, researchers around the world have shown great interest in using CO2 as fuel because it would not only help mitigate greenhouse gas emissions but also produce valuable chemical commodities. Therefore, CO2 conversion and utilization should be necessary for greenhouse gas control and sustainable development.

However, the activation of CO₂ is a formidable challenge. Carbon dioxide is a fully oxidized, thermodynamically stable, and chemically inert molecule. Also, hydrocarbon synthesis by hydrogenation of CO₂ usually favors the formation of short-chain rather than long-chain hydrocarbons, which we want. Therefore, most of the research in this area has focused on the selective hydrogenation of CO2 to CH4 and other short-chain hydrocarbons. Limited research has been done on producing liquid hydrocarbons of molecularity C5+. 

There are two ways to convert CO₂ to liquid hydrocarbons: the indirect route, which converts CO2 to CO or methanol and subsequently into liquid hydrocarbons, and the direct CO2 hydrogenation route. The direct route involves reducing CO2 to CO via the reverse water gas shift (RWGS) reaction and the subsequent hydrogenation of CO to long-chain hydrocarbons via Fischer-Tropsch synthesis (FTS). Jet fuel can be obtained from the products after industrial treatments such as distillation or hydro-isomerization. 

The direct route is more economical and environmentally acceptable, involving fewer chemical process steps and lowering overall energy consumption. 

However, researchers have mostly neglected the catalytic conversion of CO2. The key to advancing this process is to search for a highly efficient, inexpensive catalyst. Iron-based catalysts, widely used in RWGS and FTS reactions, are typically prepared by chemical co-precipitation routes, which unfortunately consume significant amounts of water. 

In this study, researchers at the University of Oxford reported the preparation of iron-based catalysts using the Organic Combustion Method (OCM). They determined their catalytic performance for the direct conversion of CO2 in the material testing lab. The catalyst shows a CO2 conversion of 38.2% and selectivity to C8–C16 hydrocarbons of 47.8%, with correspondingly low selectivity for CH4 and CO, with an attendant low carbon monoxide (5.6%) and methane selectivity (10.4%). 

The process also produces light olefins ethylene, propylene, and butenes, totaling a yield of 8.7%. These are critical raw materials for the petrochemical industry and are only obtained from crude oil. As this carbon dioxide is extracted from air and re-emitted from jet fuels when combusted in flight, the overall effect is a carbon-neutral fuel. 

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