(461g) Development of Catalysts for CO2 Hydrogenation to Olefins

The Naval Research Laboratory (NRL) is investigating a catalytic process to produce synthetic jet fuel from CO₂ and H₂.  The current method consists of extracting CO₂ from seawater, then combining the extracted CO₂ with H₂ produced through water electrolysis.  The CO₂ and H₂ are then reacted over catalysts in two steps to first produce olefins (C₂-C₄), which are oligomerized into the liquid hydrocarbon fraction (C₆-C₁₇) for synthetic jet fuel.  Although there are several promising catalysts for both reaction steps, problems exist with water removal, which prevents high conversion of CO₂ and high selectivity towards liquid hydrocarbons.  Future directions require carefully controlled synthesis of new catalysts that are water tolerant and improve the activity and selectivity of CO₂ hydrogenation to olefins for liquid hydrocarbon synthesis.

One of the most difficult aspects of chemistry involving CO₂ is not the catalytic activation, but the thermodynamic limitation of utilizing the highly stable molecule.^[1]  By coupling the endothermic reverse water-gas shift (RWGS) reaction with the slightly exothermic Fischer-Tropsch (FT) process, FT from CO₂ and H₂ (CO₂-FT) becomes more favorable as higher chain compounds are formed.^[1-2]  However, high conversion of CO₂ can only be achieved if the FT step is fast enough to overcome the thermodynamic limitation of RWGS, which is the main challenge for reactions involving CO₂.  Therefore, to effectively synthesize jet fuel from CO₂ and H₂, new catalysts must be identified which are active for both RWGS and FT. 

Because the cost of catalysts for CO₂ hydrogenation is also important, catalysts must be synthesized from low-cost materials.  Mo₂C/γ-Al₂O₃ is a promising catalyst because of the ease of synthesis, low-cost, high activity, and relatively low oxygen binding energy (OBE), which allows it to freely exchange oxygen with CO₂.  A previous study showed that Mo₂C was active and selective for CO₂ reduction into CO.^[3]  Mo₂C also has a much lower OBE than other transition metal carbides (TMCs), which contributes to its high CO₂ hydrogenation activity.  By doping Mo₂C with Fe, it is possible to further enhance the activity for FT, as Fe-based catalysts are highly active for olefin production through FT and CO₂-FT.^[4]

References

[1] K. Müller, L. Mokrushina, W. Arlt, Chemie Ingenieur Technik 2014, 86, 497-503.

[2] U. Rodemerck, M. Holeňa, E. Wagner, Q. Smejkal, A. Barkschat, M. Baerns, ChemCatChem 2013, 5, 1948-1955.

[3] M. D. Porosoff, X. Yang, J. A. Boscoboinik, J. G. Chen, Angewandte Chemie International Edition 2014, 53, 6705-6709.

[4] H. D. Willauer, R. Ananth, M. T. Olsen, D. M. Drab, D. R. Hardy, F. W. Williams, Journal of CO2 Utilization 2013, 3–4, 56-64.