CO2 Hydrogenation Catalysis. Группа авторов
[2]. The anthropogenic emission of CO2 is associated with energy consumption, i.e. the combustion of carbon‐based fossil fuels, which currently account for around 85% of the world's energy.
According to the Paris Agreement of the United Nations, an overall limit on total cumulative CO2 emissions is crucial for our future development [3, 4]. According to the 2 °C scenario, further cumulative emissions should be limited to below one trillion ton of CO2. The spread of renewable energy (35%), advances in energy conservation (40%), and carbon capture and sequestration (CCS) technologies (14%) are sure to contribute to addressing the problem (Figure 1.2) [3]. However, it is clear that these methods will not completely solve the issues arising from the vast quantities of emitted CO2. In 2017, the International Energy Agency (IEA) presented the Energy Technology Perspectives (Beyond 2 °C Scenario: B2DS), which placed a much greater emphasis on the role of CO2 utilization for reducing emissions [3]. Indeed, in the next decade, we will still rely on carbon‐based products for fuels, polymers, commodity chemicals, cosmetics, detergents, and fabrics in modern life. If these chemicals were to be derived from CO2 instead of fossil oils, a sustainable carbon cycle will be possible.
1.1 Direct Use of CO2
Apart from chemical applications, already today, CO2 is used directly in enhanced oil recovery (EOR), beverage carbonation, food processing (e.g. coffee decaffeination and drinking water abstraction), welding, as a cleaning agent for textiles, and as a solvent in the electronics industry [5]. These approaches are commercially viable. In particular, 70–80 Mton of CO2 is consumed for EOR in the oil sector. Although such direct utilization of CO2 addresses a significant amount of CO2 emissions, these topics are beyond the scope of this book.
Figure 1.1 Atmospheric CO2 concentration at Mauna Loa Observatory.
Source: Data from National Oceanic and Atmospheric Administration, Global Monitoring Laboratory [2].
Figure 1.2 IEA 2 °C Scenario (2DS) in Energy Technology Perspectives 2017.
Source: Data from Market‐driven future potential of Bio‐CC(U)S [3].
1.2 Chemicals from CO2 as a Feedstock
CO2 has been recognized as an inexpensive and abundant industrial C1 carbon source. The various chemicals that can be produced by CO2 conversion are shown in Table 1.1 [6]. The largest chemical use of CO2 is in the production of urea from ammonia. However, since a huge amount of CO2 is emitted during methane steam reforming to supply H2, urea production does not contribute to carbon sequestration at present.
Table 1.1 Chemicals produced commercially from CO2.
Source: From Omae [6]. © 2012 Elsevier.
| Chemical | Scale of production/ton |
|---|---|
| Anthropogenic CO2 emissions (2018) | 33 100 000 000 |
| Urea [7] | 181 000 000 |
| Diphenyl carbonate (Asahi Kasei Process) [8] | 1 070 000 |
| Salicylic acid | 90 000 |
| Cyclic carbonate | 80 000 |
| Polypropylene carbonate | 76 000 |
| Acetylsalicylic acid | 16 000 |
| Methanol (CRI process) [9] | 4000 |
The catalytic copolymerization of CO2 with epoxides, which provides a thermodynamic driving force due to the strained three‐membered ring, is the most prominent example of the synthesis of CO2‐based polymers without formal reduction of the carbon oxidation state. Another example, the manufacture of diphenyl carbonate from ethylene oxide, bisphenol A, and CO2 instead of phosgene was developed beginning in 1977 by Asahi Kasei Chemical Corporation to address environmental and safety issues. The first commercial facility started operation in 2002 [8]. This process produces high‐quality polycarbonate and high‐purity monoethylene glycol in high yields without waste or wastewater. In addition, the phosgene‐free process emits approximately 2.32 ton/tonPC less CO2 than the phosgene process according to life‐cycle assessment (LCA). Diphenyl carbonate has a large market (3.6 Mton in 2016) for use in automotive parts and accessories, glazing, and medical devices. The phosgene‐free technology has already been licensed to Taiwan, South Korea, Saudi Arabia, China, and Russia.
Since 2011, in Iceland, carbon recycling international (CRI) operated the first commercial plant for methanol production from CO2 via syngas by the reverse water‐gas shift (rWGS) reaction (George Olah Renewable Methanol Plant) [9]. At present, more than five million liters of methanol per year is produced using low‐cost electricity and high‐concentration CO2 in the flue gas from an adjacent geothermal power plant. It should be noted that this technology is at present only viable in Iceland; however, if there is a surplus of green electricity in the future from an excess of renewable energy, then this process will be attractive at other places, too.
Notably, the amount of CO2 utilized by all these approaches, including urea and carbonate production, is very small compared with the magnitude of anthropogenic emissions. Therefore, CO2 conversion into chemicals is unlikely to significantly reduce emissions. Comparatively, it should be noted that fuels are produced and consumed on a much larger scale than these chemicals.
1.3 Application and Market Studies of CO2 Hydrogenation Products
Hydrogenation of CO2 could be an efficient option for developing more environment‐friendly products as alternatives to fossil‐based ones. In terms of practicality, the distribution infrastructure of carbon‐based chemicals is well established. However, their manufacturing is currently several times more expensive than their conventionally produced counterparts, mainly due to the costs associated with H2 production. Some of the key features of CO2 hydrogenation products and conventional fuels are given in Table 1.2.
Table 1.2 Characteristics of various energy vectors.
| Compound | Energy density (GJ m−3) | Approx. price per energy (US$/GJ) | Boiling point/melting point (°C) | Ignition point/flash point (°C) | Vapor pressure at 25 °C (kPa) |
|---|---|---|---|---|---|