*2.2. CO2 Processing—Examples*

The annual conference, "Carbon Dioxide as Feedstock for Fuels, Chemistry and Polymers" (previously known as "CO2 as Feedstock for Chemistry and Polymers"), in Germany is one of project sources devoted to the employment of CO2. A few recently proposed strategies for CO2 use, presented below, originate from there.

The Power-to-Gas (P2G) strategy [28] is a method for carbon dioxide managemen<sup>t</sup> with good prospects. It consists in using the renewable energy or an energy surplus originating from power plants to produce chemical energy carriers. Figure 2 presents this schematically. Countries in which the power industry is to a large extent based on renewable energy sources (e.g., wind or solar) encounter problems with the energy surplus storage or managemen<sup>t</sup> [28]. According to the P2G strategy this problem may be resolved by the use of this surplus for water electrolysis, resulting in the origination of hydrogen, which in turn in a reaction with carbon dioxide forms methane or methanol, which are compounds which may be stored and used as an energy source in industry and in the power sector [29,30]. Such processes still require optimizing, increasing the overall productivity, and minimizing the costs. Nevertheless, since 2011 we have been observing a growth of such projects in countries like Germany, Denmark, Switzerland, or Spain [28].

The utilization of CO2 to produce polymers and chemical compounds is another opportunity for its use [31]. In this way, for example, polyhydroxy alcohols (polyols), polypropylene carbonate (PPC), and cyclic carbonates are obtained. Propylene carbonate and ethylene carbonate is mainly synthesized by catalytic CO2 cyclization to epoxides [32]. The use of non-toxic and freely available CO2 not only allows the achievement of compounds with higher added value, but also makes the reaction an example of a green process. Moreover, the reaction is thermodynamically favorable as it uses the high free energy of the epoxides to balance the high thermodynamic stability of the carbon dioxide. However, the differentiation in the rate of the CO2 cycloaddition reaction depending on the starting substrates, and thus competition with the reaction yielding polycarbonate by-products, requires selective catalysts. Active sites on the catalyst surface are Lewis acids. Therefore, Kelly et al. grafted ZrCl4·(OEt2)2 on the surface of dehydroxylated silica at 700 ◦C (SiO2-700) and 200 ◦C (SiO2-200) by surface organometal chemistry (SOMC), and tested in the cycloaddition of CO2 (also from CO2 from cement factory flue gas) with propylene oxide [33]. As reported by the authors, despite a certain degree of leaching of weakly bound or absorbed zirconium complexes during the first catalysis, the catalyst was active, recov-

erable, and suitable for reuse in further catalytic cycles. Then, Sodpiban et al. described the heterogeneous catalysts consisting of metal halides (ZnCl2, SnCl4) as active precursors immobilized on the surface silica with ionic liquids that were based on functionalized quaternary ammonium halide salts [34]. The best catalytic systems (ZnCl2(1.99)-IL-I and SnCl4(0.66)-IL-Br) allowed for the practically quantitative conversion of terminal epoxides to the corresponding carbonates under relatively mild conditions (25–40 ◦C, 1 bar). The catalysts were also tested in a stream of dilute gases of CO2 (50% CO2/N2 mixture) and CO2 from contaminated sources (20% CH4 in CO2 with H2S as the catalyst poison), obtaining quantitative conversion for the above-described catalysts. The catalysts were deactivated only by the loss of the silica matrix and dehalogenation of quaternary ammonium halide groups with simultaneous poisoning of the active metal centers. Metal-organic frameworks (MOFs) or porous organic polymers (POPs) are new trends in the search for CO2 cycloaddition catalysts [32]. MOFs are porous crystalline materials with a defined structure and high development of the specific surface area—SSA. On the other hand, POP can be an ideal structure for porphyrin metal (Mg or Al) complexes, giving highly active and selective catalysts under mild conditions [35]. The research carried out in this area allows us to render the financial benefits on market principles. For example, there are already commercial plants producing ethylene carbonate in the reaction of epoxide with carbon dioxide (Asahi Kasei Corporation, Japan). In turn, Novomer, Bayer, or BASF are carrying out investments aimed at implementation of such projects. Breakthrough innovations are expected there [36]. For example, polypropylene carbonate produced with the use of carbon dioxide contains 43 wt% of CO2. It is biodegradable, stable at high temperatures, flexible, transparent, and features a shape memory effect. This interesting profile of practical properties translates into a wide range of applications. PPC is used in production of packaging foils; foams; softeners; and dispersants for brittle plastics, in particular for originally brittle bioplastics, e.g., polylactic acids (PLA) or polyhydroxyalkanoates (PHA). PPC is frequently used in the production of new materials. PPC combination with PLA or PHA results in obtaining biodegradable, semi-transparent, and easy to process plastics, replacing the widely used acrylonitrile butadiene styrene (ABS). Polyethylene carbonate (PEC) is an equally often studied polymer which employs CO2. PEC is used as a substitute or additive to traditional plastics made from petroleum. PEC contains 50 wt% of CO2. Its most interesting practical property consists in the resistance to oxygen transport (permeation), which makes it an interesting packaging material for food. Polyurethane blocks made from polyols, obtained from carbon dioxide, are another example. Such products are used as mattress foams and insulating materials.

Another idea consists in the use of CO2 as a source of carbon for industrial biotechnologies. In this strategy carbon dioxide is used as food for algae or bacteria [37–40]. In the first case CO2 feeds cultures of microalgae in special photo-bioreactors or in open ponds. In this case algae may be genetically modified to increase its effectiveness. Biomass is the end product. This method is willingly used to produce various chemicals, in particular in the production of biodiesel and aircraft fuel. The second strategy assumes the use of genetically modified bacteria, which use CO2 as a source of metabolic carbon, and at the same time as the skeleton to produce specially designed molecules. Modern biotechnology offers already a possibility to "reprogram" bacteria towards synthesis of specified targets. Intensive work continues on modern bacteria strains capable of carbon dioxide consumption and its conversion into specified chemical products [39,40]. An interesting example is the recent research on carbonic anhydrases (CA), enzymes found in algae, archaea, eubacteria, vertebrates, and plants that can convert CO2 into bicarbonate ions [41]. CA catalyzes the hydration of CO2, which can finally lead to CaCO3 in the presence of Ca2+. In turn, CaCO3 is already a raw material, e.g., for cement or ceramics. The main advantages of CA include the economically viable sequestration of carbon dioxide and its carbonation at low concentration. However, despite the high catalysis rate, the stability of CA is a significant challenge for its industrial applications. However, these difficulties have been partially

overcome by strapping CA on appropriate surfaces, e.g., biochar, alginate, polyurethane foam, or nanostructured materials.

**Figure 2.** Power-to-Gas strategy. Energy from renewable energy sources is used in electrolysis to produce hydrogen. Hydrogen with carbon dioxide is converted into methane in the CO2 methanation process. The methane is then stored, released into the gas grid, or used in cogeneration and in gas turbines to produce energy.

Preparing an environmentally friendly solvent and agen<sup>t</sup> with specific properties can also involve carbon dioxide. The subject matter is a supercritical fluid of CO2 [42]. Such a fluid behaves like gas and liquid at the same time. It is gas-like because it is inviscid and expands to fill a container and liquid-like in terms of density, high heat capacity and conductivity, and solubility. It is non-toxic, non-explosive, thermally stable, and widely available. It is mainly used as a solvent or working fluid. Supercritical CO2 is an effective solvent for complicated extractions, e.g., nonpolar organic compounds. It does not cause the toxic residual solvent problem, and it is easy to separate/remove from the system. Due to its low critical point, it is an ideal liquid for extracting volatile compounds, compounds with high molecular weight, and compounds with a low degradation temperature. Supercritical CO2 has proved helpful in the following areas:


The critical temperature and pressure of carbon dioxide (*T*cr = 31.1 ◦C and *p*cr = 73.8 bar) are roughly similar to the ambient conditions. Supercritical CO2 reduces the compression work significantly in the closed-loop compression cycle. Heat dissipation to approximately ambient temperature is observed. Therefore, it is also an attractive working fluid in energy generation technologies and systems, as amply summarized in [53].

However, the use of carbon dioxide would not be possible without an appropriate method of its capture. The equipment of Climeworks company offers an interesting

solution [54], which sucks the air containing CO2 or exhaust gas, and with the involvement of special filters made of porous granulate modified with amines, binds CO2. After the filter saturation with carbon dioxide it is heated to approx. 100 ◦C, using low-quality heat as the source of energy. CO2 is released from the filter and gathered in the form of pure gas, which may be used as a substrate. The air free of carbon dioxide is released into the atmosphere. The cycle is repeated and the applied filters may be used many times, even in a few thousand cycles. This technology may be an important element in the aforementioned concepts, but it is important first of all as an industrial "generator" of clean air. Moreover, the topic of separating CO2 from gases is being intensively developed even with computational modeling. For example, Ghiasi et al. report that the calculated permeation barrier, selectivity, and thermodynamic functions for CH4, H2S, N2, and CO2 passing through finite porosity graphene doped with nitrogen atoms indicate a highly efficient and selective material for carbon dioxide separation [55]. In turn, Shaikh et al. describe the reaction mechanism of CO2 absorption by the amino-acid ionic liquid [56]. They reveal the reaction pathway employing DFT calculations. Using the MD method, they report the cation–anion interaction for two different glycinate-based ionic liquids with structurally similar cations with different alkyl chain lengths. Since the gases for CO2 recovery are approximately 10% water, the authors also provide simulations with its participation. They note that the interaction between the cation and anion is reduced in the presence of water by reducing the diffusion coefficient of the cation, thus reducing carbon dioxide uptake. Nevertheless, ionic liquid is a promising agen<sup>t</sup> for CO2 capture, due to the high CO2 solubility, recycling (almost zero vapor pressure), and fine-tuning dependence on the task.

#### **3. Carbon Dioxide Methanation and Nanocatalysis—The Focal Point in CO2 Conversion**

Catalysis is one of important elements of smart CO2 management. In particular, many papers have been devoted to catalytic conversion of carbon dioxide to methane. Figure 3 shows an increasing number of papers.

**Figure 3.** Quantity of publications on catalytic CO2 methanation from 2010 to 2019. Data from the ISI Web of Science (Thomason Reuters) database. Query conducted for: "catalytic CO2 methanation".

Catalytic methanation is a central issue of the Power-to-Gas concept [28]. According to statistics, in 2011 the share of papers on CO2 methanation in all Power-to-X projects (where X is: Gas, Power, Chemicals) was already 27% (Figure 4). The share of catalysis among various CO2 methanation strategies was 44% and that was the second largest contribution, immediately next to biological methods.

**Figure 4.** Share of further processing of hydrogen in Power-to-X (X: Gas, Power, Chemicals, Fuels). Data extracted from [28].
