Carbon dioxide utilization: A carbon-neutral energy cycle

Our reliance on fossil fuels results in CO₂ emissions that affect climate and the environment to such an extent that our recent history is referred to as the Anthropocene. The challenge in lowering these emissions can, in principle, be met by using energy technologies with a lower carbon footprint. Another approach — capturing atmospheric CO₂ and converting it to fuels — could also be effective in enabling a sustainable carbon cycle.

The urgent need for efficient CO₂ capture has led to the emergence of nanoporous metal–organic frameworks that show promising CO₂ uptake. But CO₂ sorption is only the first of many steps in processes that ideally would afford products such as CH₄, CH₃OH, CO and HCO₂H. The conversion of CO₂ into these single-carbon (C₁) molecules has been developed to a greater extent than have conversions into heavier products, underscoring the inertness of CO₂ and the high kinetic barriers associated with C–C bond formation. These shortcomings have been tackled head-on with a catalyst system that combines high-surface-area In₂O₃ with a hierarchical microporous and mesoporous zeolite (HZSM-5). Indeed, this bifunctional material effects the hydrogenation of CO₂ to gasoline products with carbon numbers that are predominantly in the range C₅–C₁₁, as a team led by Yuhan Sun and Liangshu Zhong describe in Nature Chemistry.

Gasoline is widely used in internal combustion engines for transportation and industry. One way to build carbon chains from CO and H₂ is the Fischer–Tropsch synthesis, an adaptation of which uses Fe-based catalysts to directly use CO₂ as a feedstock to give C₅₊ hydrocarbons. However, “the Anderson–Schulz–Flory distribution sets the limit on the C₅–C₁₁ hydrocarbon fraction to ∼48%, with an undesirable CH₄ fraction of ∼6%,” notes Zhong. The problem faced by many chemists, including the team of Sun and Zhong, is how to encourage chain growth while also suppressing CH₄ formation. These two points are addressed by the two active components of the present complementary catalyst. A defective In₂O₃ surface readily mediates CO₂ hydrogenation, and, in tandem, the zeolite offers pores in which the reduced products can meet each other and undergo C–C coupling to produce gasoline-range hydrocarbons. This methodology, at a CO₂ conversion of 13.1%, gives rise to selectivities for C₅₊ hydrocarbons as high as 78.6% while evolving CH₄ at only 1%. Moreover, the hybrid catalyst is incredibly robust, showing no signs of deactivation over 150 hours of use.

Credit: Lauren Robinson/Macmillan Publishers Limited

Uncovering the mechanism by which the catalyst operates called for computational studies, which indicate that surface oxygen vacancies on In₂O₃ are the reactive sites at which CO₂ is first chemisorbed before undergoing stepwise hydrogenation to give CH₃OH. The product then makes its way to acidic sites within the zeolite, where it is converted into hydrocarbons through aromatic hydrocarbon pool intermediates such as carbocations. The formation of CO — the key feedstock in Fischer–Tropsch chemistry — is suppressed by the In₂O₃ surface, which stabilizes key intermediates en route to CH₃OH. The system thus mediates direct conversion of CO₂ into liquid C₅₊ hydrocarbons, rather than a process in which CO₂ is reduced to CO, which then dissociates from one catalyst and undergoes further reactions elsewhere.

oxygen vacancies on In₂O₃ are key to the catalytic conversion of CO₂

Demonstrating the practical utility of their bifunctional system, the team reproduced their laboratory results when evaluating a pellet form of the catalyst in trials conducted under industrially relevant conditions. Nevertheless, the design of Sun, Zhong and co-workers certainly has room for improvement in terms of its control and stability. The obvious parameters to tune include the pellet shape, the surface structure of the oxide and the ratio between oxide and zeolite. Also, “the tight contact of bifunctional active sites significantly lowers the number of strongly acidic sites, deactivating HZSM-5 towards C₅₊ formation,” laments Sun. The team is now optimizing the catalyst, noting that oxygen vacancies on In₂O₃ are key to the catalytic conversion of CO₂. “We foresee that stabilizing these vacancies — for example, by strengthening dispersion interactions between In₂O₃ and the support — will greatly enhance the efficiency of this methodology,” they note.