The search for optimal framework polymers meeting high gas absorption capacity, especially carbon dioxide, has been a pressing issue for academic discourse in recent decades. The increased interest in these materials is no coincidence: hypercrosslinked polymer scaffolds are ordered structures with the ability to trap and retain gases. Modern industries are associated with copious amounts of atmospheric emissions, including carbon dioxide, destructively affecting the environmental safety of nature and communities (US ES, n.d.). As a consequence, there is a great need to develop compounds that are low cost and have a high affinity for carbon dioxide, allowing not only absorption but also effective retention of the gaseous substance. The present literature review aims to explore the state of the current scientific literature with respect to framework polymers and to identify the key parameters that determine the effectiveness of such absorption.
Composition of the Framework Matrix
It is paramount to recognize that the authors of all of the scientific papers used next tend to recognize the value of the applications of framework polymers, specifically with respect to their adsorption capabilities. Nevertheless, there is disagreement as to which frameworks should be used. One of the fundamental works in this area is the work of Rozyyev et al. (2021), who conducted studies for dozens of combinations of organic polymers and solvents used as linkers. The experiment of Rozyyev et al. was based on the alkylation reaction, in which aromatic monomers interacted with alkyl halides as solvents and anhydrous aluminum chloride. Thus, the framework polymers proposed by the authors contained only C, H, and O and in different fractional ratios. Rozyyev et al. (2021) were able to demonstrate that increasing the amount of oxygen in the framework composition is a predictor for better carbon dioxide sequestration due to oxygen’s ability to form hydrogen bonds. Remarkably, this is one of the few studies that choose to reject the presence of other inclusions in the composition of framework polymers.
On the contrary, most of the research work is aimed at trying to introduce additional atoms, mostly non-metallic, into the framework matrix. A large part of the body of such works are works studying the efficiency of halogen inclusions. For example, Thirion et al. (2016) studied perfluorinated covalent organic polymers (COPs), especially COP-175. The choice of fluorine as a matrix component was due to its known hydrophobic properties, which allowed for increasing the preservation period for such COPs. The use of chlorine atoms as components of the polymer framework was also shown in Martín et al. (2011); the presence of chlorine allowed a reduction in the total carbon fraction of the polymer, resulting in less condensation, thereby improving the degradation resistance of the polymers. Another work in which halogen inclusions were used is that of Tsyurupa & Davankov (2002). In this study, the authors suggested the use of bromine atoms in polystyrene (in particular polysulfone) chains, explaining this choice in the same way as the choice of chlorine motivated Martín et al.
A large fraction of the research also includes studies of polymers with non-metallic elements Si, Ge, and N as part of their scaffolds. In particular, Ben et al. (2011) aimed to evaluate the effectiveness of spherical frameworks based on aromatic monomers, as was Rozyyev et al. (2021) with the difference in the use of silicon and germanium as the central quaternary components of the matrix: in this case, four aromatic rings were attached to the central atom PAF-3 or PAF-4, depending on which nonmetal was used. This is partly similar to the inclusion of metallic atoms in the MOF matrix, which also have fluorine atoms that stimulate the hydrolytic properties of the polymer (Bhatt et al., 2016). Nitrogen is also a common element in the composition of the framework matrix: Rabbani and El-Kaderi (2012) showed that nitrogen allows the formation of hydrogen bonds, which increases the affinity for polarized carbon dioxide molecules and thus generally increases the ability to increase its capture. This ability appears to echo the results of Rozyyev et al. (2021), who used high oxygen content as a predictor of hydrogen bonding. However, Rozyyev et al., in contrast to Rabbani and El-Kaderi, made the synthesis considerably cheaper in their decision to abandon third-party inclusions in the framework. Nitrogen in the functional end amino groups was also used by Cui et al. (2015), who reported the same reasons for choosing an atom with moderate electronegativity as Rozyyev et al. and Rabbani and El-Kaderi. Lyu et al. (2022) also used nitrogen but as part of extended aliphatic amino chains in the COF-609 framework, indicating an enhanced absorption capacity. Thus, to date, much of the research has focused on the presence of foreign atoms within the framework matrix of the porous polymer, while Rozyyev et al. operate only on the products of alkylation reactions.
Porous Polymer Efficiency
Each of the papers shown earlier reported the high abilities of the polymers they developed with respect to carbon dioxide sequestration. In this context, it is worth clarifying that in addition to the hydrogen bonds already discussed, which form a strong interaction with polarized gas molecules, adsorption heat and the surface area also play a role in retention. The surface area, as a critical indicator of polymerization geometry, has been shown to be a fundamental predictor of the trapping ability: the higher this parameter is when the polymer is compactly packed, the more carbon dioxide can be absorbed (Lu et al., 2014). Another parameter is the heat of adsorption, which can be defined as the index of the bond strength between the adsorbate and the adsorbent in exothermic absorption reactions (Builes et al., 2013). Accordingly, the higher the index of this energy, the better the binding of the substance by the framework polymer. Overview Table 1 below shows a comparison of the main polymers that have been named by the authors of the articles as the most effective in terms of carbon dioxide sequestration.
Table 1: Overview data on the performance of various framework polymers
As the overview table shows, the carbon dioxide capture performance was maximum for BILP-4 in Rabbani & El-Kaderi. However, a combination of factors, including optimal micropore size, largest surface area, and enthalpy of adsorption, shows that samples COP-130 and COP-140 were the most advantageous in terms of maximum effect. Including the fact that no additional substances were used to make these polymers and the organic synthesis was based on a simple alkylation reaction, COP-130 and COP-140 appear to be the most promising polymers for industrial carbon dioxide capture. Thus, the literature review was able to demonstrate the diversity of matrix polymers and the differential opportunities for their synthesis; a review table based on a meta-analysis of sources reported unique characteristics for each of these polymers. As a critical conclusion of the literature review, COP-130 and COP-140 can indeed be described as the most optimal and winning substances for targeting purposes.
References
Ben, T., Pei, C., Zhang, D., Xu, J., Deng, F., Jing, X., & Qiu, S. (2011). Gas storage in porous aromatic frameworks (PAFs). Energy & Environmental Science, 4(10), 3991-3999. Web.
Bhatt, P. M., Belmabkhout, Y., Cadiau, A., Adil, K., Shekhah, O., Shkurenko, A., & Eddaoudi, M. (2016). A fine-tuned fluorinated MOF addresses the needs for trace CO2 removal and air capture using physisorption. Journal of the American Chemical Society, 138(29), 9301-9307. Web.
Builes, S., Sandler, S. I., & Xiong, R. (2013). Isosteric heats of gas and liquid adsorption. Langmuir, 29(33), 10416-10422. Web.
Cui, Y., Zhao, Y., Wang, T., & Han, B. (2015). Benzimidazole‐linked porous polymers: Synthesis and gas sorption properties. Chinese Journal of Chemistry, 33(1), 131-136. Web.
Lu, W., Wei, Z., Yuan, D., Tian, J., Fordham, S., & Zhou, H. C. (2014). Rational design and synthesis of porous polymer networks: toward high surface area. Chemistry of Materials, 26(15), 4589-4597. Web.
Lyu, H., Li, H., Hanikel, N., Wang, K., & Yaghi, O. M. (2022). Covalent Organic Frameworks for Carbon Dioxide Capture from Air. Journal of the American Chemical Society, 144(28), 12989-12995. Web.
Martín, C. F., Stöckel, E., Clowes, R., Adams, D. J., Cooper, A. I., Pis, J. J., & Pevida, C. (2011). Hypercrosslinked organic polymer networks as potential adsorbents for pre-combustion CO 2 capture. Journal of Materials Chemistry, 21(14), 5475-5483. Web.
Rabbani, M. G., & El-Kaderi, H. M. (2012). Synthesis and characterization of porous benzimidazole-linked polymers and their performance in small gas storage and selective uptake. Chemistry of Materials, 24(8), 1511-1517. Web.
Rozyyev, V., Hong, Y., Yavuz, M. S., Thirion, D., & Yavuz, C. T. (2021). Extensive screening of solvent‐linked porous polymers through Friedel–Crafts reaction for gas adsorption. Advanced Energy and Sustainability Research, 2(10), 1-26. Web.
Thirion, D., Kwon, Y., Rozyyev, V., Byun, J., & Yavuz, C. T. (2016). Synthesis and easy functionalization of highly porous networks through exchangeable fluorines for target specific applications. Chemistry of Materials, 28(16), 5592-5595. Web.
Tsyurupa, M. P., & Davankov, V. A. (2002). Hypercrosslinked polymers: basic principle of preparing the new class of polymeric materials. Reactive and Functional Polymers, 53(2-3), 193-203. Web.
US ES. (n.d.). Everything you need to know about carbon capture. U.S. Embassy & Consulates in Italy. Web.