关键词: 二氧化碳加氢, 多相催化, C₂₊化合物

Abstract:

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 C₁ 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. "CH_x" 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.

Key words: CO₂ hydrogenation, Heterogeneous catalysis, C₂₊ species