# Polymeric and Crystalline Materials for Effective and Sustainable CO\({}_{2}\) Capture
## 2 Background
### MOFs, ZIFs, and Other Crystalline Materials for CO\({}_{2}\) Capture  
For efficient CO\({}_{2}\) capture and utilization, various classes of porous solid materials were brought to the table as a potential effective solution: metal-organic frameworks (MOFs) [61], carbon-based materials [62], zeolites [63], zeolitic imidazolate frameworks (ZIFs) [64], nanoporous silica materials [65], porous aromatic frameworks (PAFs) [66], covalent organic frameworks (COFs) [67], hybrid ultramicroporous materials (HUMs) [68], and others. Moreover, when selecting adsorbent materials for effective CO\({}_{2}\) capture applications, it is essential to consider critical parameters such as adsorption capacity, selectivity, kinetics, regenerability, stability, mechanical robustness, cost, and energy consumption [69]. MOFs belong to a class of crystalline materials that consist of coordination bonds between metal or metal oxide connected by organic linkers (referred to as secondary building units [SBUs]) via a self-assembly process forming a three-dimensional structure [70]. In this regard, metal-organic frameworks (MOFs) are very promising candidates for COadsorption [71]. They are perhaps the most thoroughly studied class of porous adsorbents that offer tremendous potential due to (1) their high versatility (tunable composition and structure), offering an important CO\({}_{2}\) adsorption capacity, (2) high CO\({}_{2}\) affinity, (3) large surface areas (greater than 7000 m\({}^{2}\)g\({}^{-1}\)), and (4) high pore volumes [72]. They can be readily self-assembled from metal ions with an organic linker through metal-organic bonds. The following are the most reported groups of MOF materials. An elegant classification was proposed by the researchers from the Scientific Research National Center of Paris, based on the nature of an organic linker and a metal centre [73]. 1. polycarboxylates of trivalent cations (Fe\({}^{3+}\), Cr\({}^{3+}\) and Al\({}^{3+}\)) associated with inorganic units in the form of oxo-trimers (MIL-88, MIL-100, MIL-101, MIL-127) or the chains of octahedrons MO6 (MIL-53, MIL-160, NOTT-300 (NOTT states for Nottingham University)), CAU-23 (CAU states for Christian Albrechts University). Among them, the MOFs carrying Cr\({}^{3+}\) ions are generally the most chemically stable due to the strong bond between the cation and a ligand. 2. carboxylates of tetravalent cations (Zr\({}^{4+}\) and Ti\({}^{4+}\)) have experienced considerable growth since the discovery of MOF Ui-66. They are generally constructed based on oxo-clusters Zr6, such as NU-1000 (NU stands for Northwestern University), MIP-200 (MIP stands for Materiaux Institut Paris), but also Zr12 (MIP-206) and sometimes ZrO8 polyhedral chains (MIL-140). 3. azolates of divalent cations, such as zeolitic imidazolate frameworks or those based on Ni12 oxo-clusters (fcu-Ni(DP), where FCU stands for face-centred cubic) and PCN-601 (PCN stands for porous coordination network). The properties of both metal ions and organic linkers, as well as synthetic conditions (the solvent system, pH, metal-ligand ratio, and temperature), determine the physical, structural, and morphological features of MOF networks (e.g., porosity, pore size, and pore surface) [74]. Figure 1 illustrates common MOFs structures. Some of the most well-studied MOF structures include MOF-5 (also known as IRMOF-1) [75], one of the earliest and most extensively studied MOF.