## 2 Synthesis and Structure of POPs  
A POP is a kind of organic porous functional material with a microporous and mesoporous structure. As shown in Figure 2, according to their structural characteristics, they can be divided into amorphous categories (representing PAFs, CMPs, and HCPs) and crystalline categories (representing COFs and covalent triazine frameworks (CTFs) [42]). POPs can be classified into different categories and achieve various functions based on different monomers and synthesis routes, such as Suzuki coupling [43], the Friedel-Crafts alkylation reaction [44], Schiff-base condensation [45], and Hagihara cross-coupling [46], etc. Their unique physical and chemical properties have enabled POPs to achieve good results in adsorption, catalysis, sensing, battery production, energy storage, biomedicine, film material, and other fields [47; 48; 49; 50; 51]. COFs are emerging crystalline porous molecular materials composed of organic molecules linked by strong covalent bonds (such as B-O, C=N, C-N, C=C, etc.) and are composed of two main components: the covalent bond skeleton and the modifiable functional group. COFs were first synthesized and reported by [PERSON] et al. in 2005 [52]. On account of the large specific surface area, periodicity, and regular and adjustable channel structure, the functional surface and the inner wall of the channel can be used for catalysis after loading the active component. From the perspective of topological structure, COFs can be divided into two categories. Two-dimensional COFs have a unique, layered morphology, namely conjugate planes and \(\pi\)-\(\pi\) structures stacked between layers, which form regular channels in the longitudinal direction. Three-dimensional COFs are connected by covalent bonds to form a network, so they are highlighted by many open sites and low density [53]. Both structures can ensure the stability of COF crystallinity by placing active sites into their own structures, which is also very representative of heterogeneous catalysis. The polymerization of COFs is generally carried out according to the principles of dynamic covalent chemistry and reticular chemistry, and the final product is determined by the thermodynamic stability of the substrate. However, due to their dependence on dynamic reversible reaction synthesis, their stability is lower than that of other POP subclasses, and due to the limitations of reversible reaction types, their synthesis scheme and structural diversification need to be developed. Unlike COFs, PAFs are usually formed through irreversible coupling reactions, such as Ullmann coupling. The term PAFs was first proposed in 2009 [54]. A diamond-like structure was formed by a rigid benzene ring, as shown in Figure 2. Interestingly, in addition to a large internal surface area, a very strong skeleton formed by a bottom-up C-C bond between aromatic structural units enabled excellent structural stability even  
Figure 2: Partial classification of POPs. under some extreme conditions [55]. In summary, there are three important aspects that determine the synthetic diversity and performance of PAF materials, which are symmetrical structure-containing monomers, efficient coupling reactions, and the topological design of the framework. By modifying the functional group of the framework unit, the specific function of a certain substance can be realized. For example, the capture of CO\({}_{2}\) can be realized through the amino group on the aromatic structure [56]. At the same time, its adjustable physical and chemical properties and large specific surface area can also be well used in biomass catalysis. Porous materials with good structural properties often have a higher preparation cost, but HCPs differ from other types of POPs in that they can be directly synthesized by post-modification easily through Friedel-Crafts (FC) reactions in mild conditions [57].