## 4 Catalytic Conversion of HMF on POPs
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Catalyst** & **Substrate** & **Solvent System** & **Time (min)** & **Temperature (\({}^{\circ}\)C)** & **X\({}_{\text{substrate}}\) (\%)** & **HMF Yield (\%)** & **Reference** \\ \hline TFP-DABA & Fructose & DMSO & 60 & 100 & \textgreater{}99 & 97 & [76] \\ \hline PVP8[SO3H]0,17-COF & Fructose & Tetrahydrofuran & 30 & 100 & \textgreater{}99.5 & 97.6 & [77] \\ \hline POPDS & Fructose & DMSO & 20 & 140 & 98 & 86 & [78] \\ \hline POPDS & Fructose & DMSO & 20 & 140 & 98 & 89 & [78] \\ \hline HO3-S-POP & Fructose & \
\begin{tabular}{c} Dioxane \\ aqueous \\ \end{tabular} & 15 & 140 & 100 & \textgreater{}70 & [81] \\ \hline STrzDBITH & D-Fructose & DMSO & 20 & 140 & 100 & 96.2 & [82] \\ \hline HCP-x & Fructose & DMSO & 30 & 139.85 & \textgreater{}99 & 96.7 & [83] \\ \hline SPPTPA-1 & Fructose & DMSO & 20 & 140 & 100 & 94.6 & [85] \\ \hline FeSPPTPA & Fructose & DMSO & 20 & 100 & 95 & 96.6 & [86] \\ \hline B-POP & Fructose &
\begin{tabular}{c} DMSO/ \\ Dioxane \\ \end{tabular} & 30 & 130 & 100 & 85 & [88] \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of reactive systems for the conversion of fructose into HMF using POPs as catalysts. FDCA is not the only important substance in the oxidation path of HMF. Others such as DFF, 5-hydroxymethyl-2-furan carboxylic acid (HMFCA), and 5-formyl-2-furan carboxylic acid (FFCA) can also be widely used in the preparation of fine chemicals, pharmaceuticals, chiral catalysis, and polyester industry [100; 101; 102]. Similarly, the oxidation route is not the only one worth discussing in the utilization of HMF. HMF, known as \"the sleeping giant\" [103], can activate C=O, C=C, and C-O itself for high-value conversion in green sustainable chemistry. 2,5-dihydroxyfuran (DHMF) is an important chemical in the production of artificial fibers and resins and can be produced by the hydrogenation of HMF [104]. Most studies have relied on the synergistic system of Cu\({}^{0}\) and Cu\({}^{+}\) to improve the catalytic activity in hydrogenation treatment [105]. However, due to the existence of reducing gases, the stability of the system needs to be strengthened [106]. Therefore, it has been very important to develop a stable structure of a Cu-supported catalyst in this direction. [PERSON] et al. [107] synthesized the catalyst Cu@C-POP by FC reaction with catechol and dimethoxymethane. Figure 8a shows the as-synthesized catechol-functionalized C-POP. Copper nanoparticles were then fixed to the polymer by the reduction deposition of two copper salts, respectively. The C-POP had a stable framework and formed a space structure that restricted the agglomeration and leaching of the copper nanoparticles. The system achieved 98% selectivity of DHMF at 150 \({}^{\circ}\)C and 20 bar H\({}_{2}\) for 10 h, and the catalyst could be reused seven times. Figure 8b shows the reaction energy distribution of the hydrogenation process of HMF conversion to DMHF. In addition to the conversion process at the location, we can also see the presence of Cu vacancy on the catalyst surface. The presence of vacancy increased the electron density of the CuO surface, thus improving the adsorption capacity of the substrate molecules. In particular, 5-ethoxymethylfurural (EMF), the etherification product of HMF not mentioned in the Figure 1, was identified as an important substance for the preparation of biodiesel and can produce a change from the currently used petroleum-based energy systems [108]. [PERSON] et al. used the soft template method to synthesize ordered mesoporous carbon (OMC) from resorcinol and pluronic F-127, and then sulfonated it to produce a catalyst, OMC-SO\({}_{3}\)H. The fructose was converted into EMF with a yield of 55.7% by a one-pot method [109].