Introduction to CO₂ utilisation

The concentration of carbon dioxide (CO₂), a well-known greenhouse gas, has been increasing in the atmosphere for hundreds of years, which is related to the frequent occurrence of extreme weather.¹ According to the Global Energy & CO₂ Status Report 2019, the emissions of CO₂ into the atmosphere have reached a worrisome level of 33.1 Gt each year.² The amount of CO₂ emission has been rising dramatically, endangering the balance of the natural ecosystem and threatening the survival of human beings. Therefore, the pursuit of capturing, storing and converting CO₂ to suppress the amount of CO₂ emission is urgent.

In the last decades, there have been considerable advancements in CO₂ chemistry and industry. The process of carbon dioxide capture and storage (CCS) removes CO₂ that would otherwise be emitted from fossil fuel power stations and other chemical plants through industrial processes and then allows it to be transported for permanent underground storage.³ This process is effective, however, the process of carbon dioxide capture and utilization (CCU) is even more attractive due to the sustainability, nontoxicity and easy availability of CO₂, and its potential to be transformed into diverse high value-added chemicals.⁴ Although the thermodynamic stability and kinetic inertness of CO₂ make it difficult for it to be utilized efficiently, many kinds of strategies have been developed to construct important hydrocarbon fuels, fine chemicals and pharmaceuticals from CO₂.⁵ Not only can the utilization of CO₂ help reduce the carbon content in the atmosphere, but it can also provide clean energy and value-added products for the future. Therefore, the process of CCU has received increasing attention from all over the world and great effort from science, industry and government agencies has been made to develop this process.⁶ In terms of the CCU process, it covers a wide range of scientific problems including CO₂ hydrogenation, biological carbon fixation, CO₂ reduction and fine chemical production from CO₂.

Carbon dioxide can act as an ideal carbon source to synthesize hydrocarbon fuels via photo-, transition metal, and electro-catalysis. One of the goals of CO₂ utilization is to acquire valuable products, such as CO, HCOOH and CH₃OH, using homogeneous catalysts. In this respect, the latest advances in the field of the production of commonly used organic solvents from CO₂ are summarized and discussed by Wu and co-workers (DOI: 10.1039/d0gc03280h). Moreover, the Das group (DOI: 10.1039/D0GC04040A) provide an overview of the photochemical reductions of CO₂ to formic acid. A primary advantage of CO₂ reduction via photo-catalysis is that the selective reduction of CO₂ can be achieved by photocatalysts with reducing capabilities. The relevant works in this realm are demonstrated well in a review from Dong, Lan and co-workers (DOI: 10.1039/d0gc01497d) on the selective reduction of CO₂ to HCOOH in H₂O. Meanwhile, the He group (DOI: 10.1039/d0gc03111a) and the Li and Lu group (DOI: 10.1039/d0gc02836c) have realized the reduction of CO₂ to CO with a rhenium catalyst with bifunctional pyrene groups and a Z-scheme heterojunction of Co₁-C₃N₄@α-Fe₂O₃, respectively.

In CCU processes, transition metal catalysis is expected to play an important role. This was exemplified in the use of an active Pt/In₂O₃ catalyst to convert CO₂ to methanol, which was reported by Liu and co-workers (DOI: 10.1039/d0gc01597k). The catalyst loading methods, catalyst loading volume, loading ratios of the iridium/titanium oxide (Ir/TiO₂) catalyst and even the pre-treatment temperature are all important for this CO₂ hydrogenation, as demonstrated in the work by Su, Huang and co-workers (DOI: 10.1039/d0gc02302g). A new avenue allowing precise control over the catalytic activity of metal catalysts is the stabilization of catalysts via the metal-oxide interface, as detailed in the article by Jung, Sun and co-workers (DOI: 10.1039/d0gc02279a). The controlled assembly of catalytic centers allows the tuning of the coordination number of metal catalysts with ligands, achieving the efficient reduction of CO₂ to CO, which is detailed in the article by Geng, Zeng and co-workers (DOI: 10.1039/d0gc02689a). Zhong, Jin and co-workers (DOI: 10.1039/d0gc02785e) have achieved an efficient green reduction of bicarbonate, which is produced by the capture of CO₂ under an alkaline environment.

Besides photocatalysis and transition metal catalysis, other catalytic modes can also reduce CO₂ effectively Recently, Wu, Wu, Han and co-workers (DOI: 10.1039/d0gc03051a) have demonstrated an electrodeposited Cu–Pd bimetallic catalyst for the selective electro-reduction of CO₂. Unlike molecular catalysts, Chen, Yan and co-workers (DOI: 10.1039/d0gc03506h) have made great efforts towards the selective reduction and subsequent valorization of the reduced forms of CO₂via heterogeneous catalysis. Wang, Song and co-workers (DOI: 10.1039/d0gc03779f) have described another efficient process, where plasma-enabled catalysis is used for the hydrogenation of CO₂ to generate higher hydrocarbons. Wang and co-workers (DOI: 10.1039/d0gc03510f) have developed a highly efficient catalytic system used for the synthesis of alkylidene cyclic carbonates from CO₂ using hydroxyl and azolate ionic liquids. CO₂ reduction on graphdiyne is at an earlier stage of development for applications, therefore, density functional theory calculations can help further understand the mechanism of these reactions, as demonstrated by Wang, Wang and co-workers (DOI: 10.1039/d0gc03742g).

CO₂ has proved its value as a renewable C₁ resource to construct fine chemicals through different mechanisms. Compared to the widely investigated carbonylation from a series of carbonyl sources (e.g., CO, COS or DMF), the transformations of CO₂ show the advantages of being green and economical, due to its abundance, availability, sustainability and nontoxicity. Such examples have been illustrated in the articles by D'Elia, Kleij and co-workers (DOI: 10.1039/d0gc03824e) and Guo, Lamb and co-workers (DOI: 10.1039/d0gc03465g). In a work by Chen, Xi and co-workers (DOI: 10.1039/d0gc02254c), the carboxylation of styrenes with amines and CO₂ has been described as a method for the synthesis of γ-aminobutyric acids via visible-light photoredox catalysis. In a related work, Li and co-workers (DOI: 10.1039/d0gc02667k) studied the Rh-catalyzed regioselective arylcarboxylation of acrylamides with aryl boronic acids and CO₂. Industrial ore calcination production always discharges unused sulfur as waste and so it can serve as a rich sulfur source for chemical synthesis. Therefore, Zhang, Yu and co-workers (DOI: 10.1039/d0gc03723k) have used both CO₂ and sulfur to synthesize sulfur-containing carbonyl compounds. Sodium trihydroxyaryl borates as robust tetracoordinate organoboron catalysts for reductive formylation of amines with CO₂ have been discussed in the work of Zhao, Wang, Li and co-workers (DOI: 10.1039/d0gc01741h). The transformation of low-concentration CO₂ to high value-added chemicals has become a major target in the capture of (waste) CO₂, as demonstrated by Zhou, Lu and co-workers (DOI: 10.1039/d0gc03009k) through the capture of low-concentration CO₂ by super-basic guanidines, yielding important oxazolidine-2-ones.

In addition to the potency of CO₂ as C₁ source, it can also act in other roles in organic synthesis. Cui, Shi and co-workers (DOI: 10.1039/d0gc03705b) have reported the oxidative dehydrogenation of light alkanes with CO₂, in which CO₂ plays the role of a weak oxidant. Liu, Zhang and co-workers (DOI: 10.1039/d0gc03333b) have demonstrated the application of CO₂ in multicolored light-emitting diodes. The neutral waterborne cationic polyurethane from CO₂-polyol could act as a water-dispersible binder to overcome the bottleneck in heavy metal-free anti-corrosion coatings, as demonstrated by Wang and co-workers (DOI: 10.1039/d0gc02592e). Moreover, polyurethane-urea adducts could be synthesized from CO₂ and furfuryl amines, as demonstrated by Cheng, Zhao and co-workers (DOI: 10.1039/d0gc03695a). CO₂ is taken up by organisms through carbon fixing enzymes and generates important intermediate metabolites for cell growth, and a thermodynamic view of biological carbon fixation is detailed in the report by Li, Zhang and co-workers (DOI: 10.1039/d0gc03493b).

In summary, we have committed to ensuring that CO₂ utilisation contributes to its full potential in tackling the major global energy challenge. It is clear that the highlighted papers in this themed collection have demonstrated the urgency of reducing greenhouse gas emissions in response to climate change and the energy dilemma. Beyond any doubt, there is significant work left to be done in the field of CCU. These works not only provide hope for turning CO₂ into valuable hydrocarbon fuels and fine chemicals, but also provide some guidelines on CCU to contribute to a more sustainable future for human beings.

Prof. Da-Gang Yu

Sichuan University

Prof. Liang-Nian He

Nankai University

Acknowledgements

We thank the National Natural Science Foundation of China (21822108) and National Key Research and Development Program of China (2016YFA0602900) for financial support.

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