# Polymeric and Crystalline Materials for Effective and Sustainable CO\({}_{2}\) Capture
## 2 Background
### MOFs, ZIFs, and Other Crystalline Materials for CO\({}_{2}\) Capture
The choice of synthesis parameters influences the resulting zeolite structure, composition, and proper  
Figure 1: Representation of different MOF structures. Reproduced with permission from the Royal Society of Chemistry from [92]. ties [98]. Natural zeolites occur in geological formations, while synthetic ones should be synthesized in the laboratory. Synthetic zeolites offer advantages such as uniform pore sizes, high purity, and tailored properties, making them preferred for many industrial applications [99]. A large segment of the current global economy (350 Billion United States dollars (USD)) is based on the use of crystalline microporous zeolites in various essential processes, such as petrochemical cracking [100], ion exchange for water softening and purification [101], and in the separation of gases [99]. Even though the zeolites generally exhibit higher thermal and chemical stability compared to ZIFs due to the strong covalent bonds between tetrahedral Si(Al)O\({}_{4}\) units, some ZIFs have been reported to exhibit exceptional stability under harsh conditions, including high temperatures, humidity, and chemical environments [102]. Such enhanced stability is attributed to the strong coordination bonds between metal ions and imidazolate ligands, as well as the rigid nature of the framework structure. As for their CO\({}_{2}\) adsorption capacity, ZIFs may offer advantages over zeolites in terms of enhanced surface areas and stronger chemical interactions with CO\({}_{2}\). [PERSON] and others performed a computational screening study to examine the families comprised of the experimentally known zeolites and their respective ZIF counterparts in the context of several environmental and industrial separations involving carbon dioxide, nitrogen, methane, oxygen, and argon [103]. In regard to pore size, ZIFs have demonstrated pore sizes approximately twice as large as those of their zeolite counterparts due to the larger size of the imidazole linkers (the average first neighbors Si-Si distance is 3.05 A in zeolites, compared with the average Zn-Zn distance of 6.00 A in ZIFs). Thus, while some zeolites can act as molecular sieves based on the relative sizes of the adsorbates under study, the larger apertures of the ZIFs make them lose the molecular size-sieving ability. Thus, zeolites are more likely to be used in the size exclusion separation of CO\({}_{2}\) from N\({}_{2}\), as in the case of CO\({}_{2}\) from CH\({}_{4}\) separation. In terms of the accessible surface area and accessible volume of the structures, the larger pore sizes of ZIFs in relationship to zeolites imply larger spaces to accommodate the guest molecules. This fact makes them especially useful for gas storage applications. Finally, based on interatomic potential-based calculations, ZIFs appear to be better candidates for gas separation processes involving mixtures of non-polar and quadrupolar molecules, such as the removal of carbon dioxide for natural gas purification. According to [PERSON] and coworkers, ZIFs also seem to surpass the performance of zeolites for the separation of carbon dioxide from blue gas. Finally, the work of [PERSON] et al. seems to prove the predictions of [PERSON]. Synthesized series of very stable ZIFs, termed ZIF-68, ZIF-69, and ZIF-70 exhibit unusual selectivity for CO\({}_{2}\) capture from CO\({}_{2}\)/CO mixtures and extraordinary capacity for storing CO\({}_{2}\): 1 liter of ZIF-69 can hold approximately 83 liters of CO\({}_{2}\) at 273 K under ambient pressure [104]. At last, covalent organic frameworks (COFs) are covalently linked to two- and three-dimensional (2D and 3D) organic crystalline structures obtained by employing reticular synthesis. These synthetic routes typically involve the self-assembly of organic building blocks under mild reaction conditions, leading to the formation of crystalline frameworks with predictable structures and properties [105].