Broadly, in the first stage of such a process, CO₂ and H₂ are reacted over an iron-based catalyst to produce light olefins (C₂ - C₆). These light olefins can then be further reacted in a second, sequential reactor by oligomerizing to higher linear olefins. These higher hydrocarbons can be direct substitutes for crude-oil-distilled middle-distillate fuels.

The NRL team according focused on two main objectives within the study: first, to develop an optimized catalyst for CO₂ hydrogenation to produce the initial light olefins; and second, to use pure ethylene as a model olefin for the development of new and improved ASA-supported nickel catalysts for the direct and selective oligomerization to the higher molecular weight olefins.

The mechanism of the first stage first produces CO and water. The CO is carried forward in an exothermic FT synthesis step, producing predominantly mono-unsaturated hydrocarbons. Carbon dioxide is also hydrogenated directly to methane, in a widely cited thermodynamically favorable and highly competitive side reaction.

The water formed in the primary reactions negatively influences catalyst activity and product selectivity.

One objective of this study is to modify the catalyst surface to prevent the water vapor produced in the reaction sequence from negatively influencing catalyst activity by reoxidation or accelerated crystallization of the metal oxide surface. By decreasing the effects of water vapor on the catalyst surface, the equilibrium at the catalyst surface should favorably shift to the production of desired intermediates such as olefins over intermediates such as CO or methane.

—Drab et al.

The team investigated γ-alumina-supported iron-based catalysts modified with manganese and potassium promoters and a silica-stabilized coating under fixed-bed reactor conditions. The stabilizer is introduced by impregnating the K/Mn/Fe on Al₂O₃ catalyst with tetraethylorthosilicate (TEOS) to minimize the deactivating effects of water on catalyst activity in CO₂ hydrogenation. The K/Mn/Fe on Al2O3 catalyst modified with the TEOS and reduced in CO produced a lighter end fraction of olefins compared to the catalyst reduced in H₂.

Towards the second objective, they found that ethylene oligomerization over pelletized amorphous silica−alumina (ASA)-supported Ni catalysts demonstrated high conversion and selectivity toward the jet fuel fraction (C₈−C₁₆) at a very low mass hourly space velocity (MHSV).

These results indicate promising directions for conditions and catalyst compositions to use in the scale-up demonstration of CO2-to-fuel conversion, where there is ongoing work to improve reactor design, catalyst compositions, and reaction and process conditions (e.g., CO₂ hydrogenation in a single step to liquid olefins).

—Drab et al.

Resources

• David M. Drab, Heather D. Willauer, Matthew T. Olsen, Ramagopal Ananth, George W. Mushrush, Jeffrey W. Baldwin, Dennis R. Hardy, and Frederick W. Williams (2013) Hydrocarbon Synthesis from Carbon Dioxide and Hydrogen: A Two-Step Process. Energy & Fuels doi: 10.1021/ef4011115