Researchers at the University of Oxford have developed a method to convert CO₂ directly into aviation fuel using a novel, inexpensive iron-based catalyst. The catalyst shows a carbon dioxide conversion through hydrogenation to hydrocarbons in the aviation jet fuel range of 38.2%, with a yield of 17.2%, and a selectivity of 47.8%, and with an attendant low carbon monoxide (5.6%) and methane selectivity (10.4%).

The conversion reaction also produces light olefins—ethylene, propylene, and butenes—totalling a yield of 8.7%. These are important raw materials for the petrochemical industry and are presently also only obtained from fossil crude oil. An open-access paper on their work is published in the journal Nature Communications.

There are two ways to convert CO₂ to liquid hydrocarbons; an indirect route, which converts CO₂ to CO or methanol and subsequently into liquid hydrocarbons, or the direct CO₂ hydrogenation route, which is usually described as a combination of the reduction of CCO₂O2 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 then be obtained from the products after industrially recognized treatments such as distillation or hydro-isomerization. The second, direct route is generally recognized as being more economical and environmentally acceptable as it involves fewer chemical process steps, and the overall energy consumption for the entire process is lower.

… there are few reports of the direct catalytic conversion of CO₂ to jet fuel range hydrocarbons. The key to advancing this process is to search for a highly efficient inexpensive catalyst, that can preferentially synthesise the target hydrocarbon range of interest.

… In this investigation, we report the preparation of iron-based catalysts using the Organic Combustion Method (OCM) and determined their catalytic performance for the direct and efficient conversion of CO₂ to jet fuel range hydrocarbons. In brief, the Fe–Mn–K catalyst shows a CO₂ conversion of 38.2% and selectivity to C₈–C₁₆ hydrocarbons of 47.8% with a correspondingly low selectivity for CH₄ and CO. In addition, the process also shows a high molar production ratio of olefin-to-paraffin for C₂–C₄ hydrocarbons.

—Yao et al.

The Organic-Combustion Method (OCM), also known as the Solution Combustion Method, was developed to prepare highly active metal catalysts for a variety of processes. The method combines a solution of the required metal precursors as an oxidizing agent and a suitable organic fuel as a reducing agent. This redox mixture ignites upon heating to initiate a self-propagating exothermic reaction that sustains high temperatures for a period of time sufficient to decompose all the organic material. The final product is usually a crystallized material.

The CO₂ hydrogenation to jet fuel range hydrocarbons process through a Tandem Mechanism in which the Reverse-Water Gas Shift reaction (RWGS) and Fischer-Tropsch synthesis (FTS) reaction are catalyzed by Fe₃O₄ and χ-Fe₅C₂ respectively. Yao et al.

The Oxford team first activated the Fe-Mn-K catalyst with syngas. The Fe₃O₄ catalyst precursor is fully carburized to χ-Fe₅C₂ during the catalyst activation (reduction) process, while Fe₃O₄ is partially regenerated from the oxidation of χ-Fe₅C₂ by CO₂/water in the first hours of the catalytic reaction.

Jet fuel synthesis via CO₂ hydrogenation initially takes place by the RWGS reaction (CO₂ + H₂ → CO + H₂) on the catalytically active sites of Fe₃O₄, and subsequently by the FTS reactions (CO + H₂ → C_nH_m + H₂O) on catalytically active sites on χ-Fe₅C₂. The researchers thus refer to the process as a “Tandem Mechanism”.

Resources

• Yao, B., Xiao, T., Makgae, O.A. et al. (2020) “Transforming carbon dioxide into jet fuel using an organic combustion-synthesized Fe-Mn-K catalyst.” Nat Commun 11, 6395 doi: 10.1038/s41467-020-20214-z