Heterogeneous Catalysis of CO2 Hydrogenation to C2+ Products

The increasing anthropogenic emission of CO2 leads to global warming, to address which three strategies can be considered: (1) decrease fossil fuel consumption through increased utilization efficiency and lower per capita consumption; (2) replace fossil fuels with renewable energy sources like wind, tidal, solar, and biomass energies; (3) utilize CO2 efficiently. Despite efforts to reduce energy use and increase the use of carbon-neutral biofuels, it seems that fossil fuels will continue to be a major energy source for the next few decades. Tremendous effort is therefore being focused on developing effective technologies for CO2 capture and transformation. In particular, the transformation of CO2 into fuels and chemicals via reduction with renewable hydrogen is a promising strategy for mitigating global warming and energy supply problems. The hydrogenation of CO2, especially to C2+ hydrocarbons and oxygenates, has sparked growing interest. The C2+ species can be used as entry platform chemicals for existing value chains, thus providing more advantages than C-1 compounds. However, optimizing catalyst design by integrating multifunctionalities for both CO2 activation and C-C coupling remains an ongoing challenge. Here, we provide a timely review on the recent progress that has been made in the hydrogenation of CO2 to higher-order alkanes, olefins, and alcohols by various heterogeneous catalysts. The thermodynamics and kinetics, as well as possible reaction pathways for CO2 hydrogenation, are discussed. The hydrogenation of CO2 to hydrocarbons usually involves the initial generation of CO via a reverse water-gas shift (RWGS) reaction followed by hydrogenation of the CO intermediate. The RWGS reaction proceeds through a redox route and an associative pathway. "CHx" insertion (carbide-type) and "CO" insertion are two proposed mechanisms for this Fischer-Tropsch-like synthesis. Fe-or Co-based catalysts have been widely used to catalyze the hydrogenation of CO2 to C2+ hydrocarbons via the CO intermediate. C2+ hydrocarbons can also be obtained by combining CH3OH synthesis with the methanol-to-hydrocarbon process (MTH). This reaction pathway has been realized over bifunctional systems comprising a CH3OH synthesis catalyst and an MTH catalyst. Alternatively, CO2 hydrogenation can occur via a RWGS reaction to the CO intermediate, and subsequent formation of higher alcohols from syngas. Higher alcohols (mostly CH3CH2OH) have been produced by using a hybrid tandem catalyst. Understanding of the activation mechanism, precise C-C coupling, and synergy control between the two active components requires further research. In the final part, we describe the future challenges and opportunities in heterogeneous catalysis of CO2 hydrogenation. The combination of calculations (precise theoretical models) and experiments (in-situ spectroscopic techniques) will facilitate the design of advanced catalysts to achieve both high CO2 conversion and C2+ product selectivity.