## 3 Catalytic Conversion of Fructose on POPs
For example, regarding amorphous PAFs and HCPs, the former has an extremely high surface area, while the latter has economical and mild synthesis conditions, both of which have achieved extremely high conversion rates in biomass conversion, especially fructose. Periodic COFs, with their unique crystal structure and synthesis methods, have not only been applied in traditional thermal catalysis but also have the possibility of realizing efficient biomass conversion through electrocatalysis, and hence, its development was worth our deep thinking. Due to the fact that most COFs are synthesized by reversible reactions, they are not only less stable than other POP types but also limited to the symmetry of the structural monomers. Therefore, they need to be developed to improve their synthesis scheme and  
Figure 5: Conversion mechanism of fructose in preparation of HMF [74]. topological structure to improve their stability [75]. [PERSON] et al. prepared irreversible end-ketone intervariant structures with significant chemical stability by an alkali condensation reaction and treated with \(1,3,5\)-triformyl-phloroglucinol (TFP) and \(2\), \(5\)-diaminobenzene sulfonic acid (DABA) to obtain a solid acid catalyst, called TFP-DABA [76]. Two-dimensional COFs enhanced the degree of sulfonation and increased the number of Bronsted acid sites by virtue of their pore structure formed by the accumulation of layered structures. With this characteristic, the catalytic system was reacted in dimethyl sulfoxide (DMSO) at \(100\)\({}^{\circ}\)C for \(12\) h to obtain \(97\%\) HMF from fructose and \(65\%\) of its subsequent oxidation product \(2\), \(5\)-formylfuran (DFF). No significant decrease in activity was observed after three cycles. Most fructose catalytic conversion systems with POPs as a catalyst could not get rid of their dependence on DMSO, which stabilizes HMF and inhibits the occurrence of side reactions but is also a very harmful solvent to the environment. By exploring the influence of non-covalent interactions on catalytic performance, [PERSON] et al. modified the environment around the active site of the catalyst by using the movable and aggregable properties of the groups on a highly flexible linear polymer, and even achieved better conversion efficiency without DMSO. As shown in Figure 6, they prepared a novel COF with large mesoporous channels by the condensation reaction of \(1\),\(3\),\(5\)-tris(\(4\)-aminophenyl)-benzene (TPB) and \(2\),\(5\)-dimethoxyerephthalaldehyde (DMTA) [77]. We can clearly see that the perfluoralkyl chain with sulfonic acid groups was confined to the pore through the reaction of the sulfone ring with the hydroxyl group on the COF framework, and then through the hydrogen bond interaction to create the desired solvent environment. When PVP@[SO\({}_{3}\)H]\({}_{0.17}\)-COF was inserted into the highly flexible polymer synthesis catalyst, the fructose conversion rate was greater than \(99.5\%\) and the HMF yield was as high as \(99.1\%\) at \(100\)\({}^{\circ}\)C for only \(30\) min. The catalyst could still maintain the fructose conversion rate after five cycles, and the HMF yield only decreased by \(2\%\). The studies carried out by [PERSON] et al. were not limited to monocatalytic centers [78]; they used \(2\),\(2^{\prime}\)-benzidinedisulfonic acid and \(4\),\(4^{\prime}\)-diaminostilbene-\(2\),\(2^{\prime}\)-disulfonic acid with cyanuric chloride to synthesize catalysts (POPDS and POPSDS) containing sulfonic acid and secondary amine, which became excellent catalysts to prepare HMF. The reaction at \(130\)\({}^{\circ}\)C for \(20\) min achieved a \(98\%\) fructose conversion and \(86\%\) and \(89\%\) HMF yields, and the catalyst maintained six cycles of experiments. Figure 7 shows the mechanism of the conversion of glucose and fructose into HMF. It can be seen in the figure that the two groups had different functions. The amino group was mainly responsible for the isomerization transformation, while the sulfonic acid group played a role in the subsequent transformation.