Introduction
The chemical industry has been concerned for several decades with the search for highly porous materials capable of unprecedented gas capture. The porosity of materials is enabled by the presence of an ordered hypercrosslinked framework structure, which is why organic substances such as plastics, aromatic hydrocarbons, and highly ranked polymers have received particular attention in the context of the study (Tsyurupa and Davankov 193; Lu et al. 4589; MartĂn et al. 5475). In terms of applications, porous materials prove useful for gas capture, in the context of gas storage, detoxification, or separation, as well as for water filtration, as adsorbents, and as molecular sieves. A particular focus should be directed to the capture of carbon dioxide, allowing a qualitative reduction of industrial emissions and thus environmental protection for gas-intensive industries (US ES; Thirion et al. 1). Attention is also drawn to the possibility of porous materials being used as substrates for catalysts in catalytic reactions, allowing them to stimulate catalyst activity during the process as well as to prevent catalyst degradation. As it follows from the above, porous materials are of great interest in both academic and industrial discourse, so efforts have been made to develop the best possible materials. Ceramic monoliths, zeolites, and plastics are already in use by now, but existing substances are not universally helpful and may have drawbacks (Rozyyev et al. 3). This is why the critical challenge for industrial chemistry is to develop such highly porous materials that have unprecedented gas trapping, low synthesis costs, and thermal and chemical stability.
Definition of the Solution
The Friedel-Crafts reaction can be used to synthesize materials that primarily satisfy the requirements for low cost and high carbon dioxide capture efficiency. In this reaction, aromatic compounds interact with Lewis acids that have low production costs (Rozyyev et al. 3). These include organic alkyl halides, mostly polar solvents, or inorganic metal halides. The problem of fine synthesis, in this case, was the unexpected reactions of the interaction of organic solvents with metal halides, in particular AlCl3. The solution to this problem is to use an excess of organic solvent (CH3Cl, CH2Cl2, CHCl3) in combination with the use of anhydrous AlCl3: this synthesis has been shown to result in a moderately high yield of the organic product. In particular, as follows from Rozyyev et al., the use of 1,3,5-Triphenylbenzene as an organic substrate with CH2Cl2 and CH3Cl alkyl halides yields covalent organic polymers in high yields, namely COP-130 (92%) and COP-140 (87%), respectively (8).
Although, in reality, a vast number of COPs can be obtained from combinations of organic aromatic monomer and organic solvents in the presence of anhydrous aluminum chloride, only some of them have an unprecedented ability to capture carbon dioxide. By analyzing isotherms of argon absorption and desorption at 87 K, it was shown that it was COP-130 and COP-14 that had the maximum surface areas realized by the formation of a stacking hydrocarbon spherical polymer framework (Rozyyev et al. 12; Cui et al. 3; Tsyurupa and Davankov, 195; MartĂn et al. 5476). The largest surface area, in turn, is expected to correspond to the maximum amount of gas that can be absorbed by the sorbent (Lu et al. 4589). Thus, it was shown that the carbon dioxide capture capacity for COP-130 was 4.71 mmol g-1 and for COP-140 4.35 mmol g-1, which is on average 3-5 times higher than for other COPs obtained by the combined synthesis (Rozyyev et al. 13).
When discussing the binding mechanism, the heat of absorption, Qst, plays a significant role. More specifically, Qst corresponds to the strength with which the adsorbate binds to the adsorbent: the higher this value, the closer the interaction between the components exists (Ben et al. 3992). It is for this reason that Qst for a polymer must be determined for purity of analysis to be able to select the best material for carbon dioxide retention rates. The analysis demonstrated that coupling the organic polymer to CHCl3 showed higher Qst values (30 to 34 kJ mol-1), in contrast to using dichloromethane as a linker. To put it another way, despite the higher gas trapping capacity of COP-130, the retention rate of this substance in the organic framework was higher for COP-140, in which it was trichloromethane used as a linker (Rozyyev et al. 14). Rozyyev et al. attribute this phenomenon to approximately twice the oxygen content (7.92% vs. 3.02%) in the COP-140 framework, which allows for more efficient carbon dioxide retention. One of the mechanisms determining the higher Qst values for porous polymers is explained by Rabbani and Hani (1515). In particular, the interaction of polarizable CO2 molecules with hydrogen bonds within the polymer is pointed out; as it follows, the higher the oxygen content of the polymer, the higher the carbon dioxide retention rate.
Conclusion
The synthesis of highly porous materials that meet the requirements of low cost, high efficiency, and environmental safety is one of the critical challenges of industrial chemistry. Carbon dioxide sequestration is especially required in industries associated with the intensive emission of gases into the atmosphere. As a consequence, there is a need to find structures that have unprecedented effects and maximum efficiency as a sorbent. A solution to this problem was proposed based on the Friedel-Crafts reaction: dozens of combinations of organic monomer and linker solvent combined with anhydrous AlCl3 were studied. The analysis showed that COP-130 (1,3,5-Triphenylbenzene plus CH2Cl2) and COP-140 (1,3,5-Triphenylbenzene plus CHCl3) exhibited the best carbon dioxide capture, many times higher than other candidates. At the same time, the heat of adsorption was higher for COP-140 than for COP-130, which may indicate higher gas retention rates.
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