# Highly Porous Organic Polymers for Hydrogen Fuel Storage
## 3 Highly Porous Organic Polymers for H\({}_{2}\) Storage
### Conjugated Microporous Polymers (CMPs)  
Since the first discovery of conjugated microporous polymers (CMPs) by [PERSON] et al. [85; 86], these polymers have been widely applied, including for H\({}_{2}\) gas storage. The CMPs are amorphous polymers that are made of rigid blocks linked via \(\pi\)-conjugated bonds, and possess three-dimensional (3D) network structures, as shown in Figure 9. The diversity of building blocks and a wide availability of different reaction types, enable chemists to fine tune the microporosity in CMPs. The basic synthetic route for building 3D CMPs is the cross coupling of two different monomers, at least one of which has more than two reactive sites, as illustrated in Figure 9.  
Figure 8: Schematic synthetic strategy for polymers of intrinsic microporosity (PIMs) produced through condensation under mild reaction conditions: K\({}_{2}\)CO\({}_{3}\), DMF, 60–120 \({}^{\circ}\)C (Reproduced with permission from [84]. Copyright © 2016 Elsevier). The pore size can be varied by modifying the length of a linker between the molecules with three or more branching sites, as seen for the phenylethynylene-based CMPs in Figure 10. It is interesting to note that the longer the linker, the smaller the total surface area; e.g., CMP-0 has a surface area of \(\sim\)1000 m\({}^{2}\)/g and CMP-1 and -2 have surface areas of \(\sim\) 34 and 634 m\({}^{2}\)/g, respectively. Different phenyl monomers were used to synthesize CMPs with different specific surface areas (SSA) and pore size. For instance, [PERSON] et al. used 1,3,5-triethynylbenzene to make HCMP-1 with \(S_{\text{BET}}\) = 842 m\({}^{2}\)/g and pore size = 1.1-1.6 nm, and 1,4-diethylynylbenzene to make HCMP-2 with \(S_{\text{BET}}\) = 827 m\({}^{2}\)/g and pore size = 0.9-1.6 nm [86]. [PERSON] et al. used the similar monomers, but a different metal catalyst, dicobalt carbonyl, to generate a serious of CMPs, POP-1, POP-2, POP-3, and POP-4, with smaller pore sizes (0.7-0.9 nm) but large surface areas (\(S_{\text{BET}}\) > 1000 m\({}^{2}\)/g) [87]. It is interesting to compare these series of CMPs in terms of H\({}_{2}\) at 77 K as shown in Table 4. POP-3 has the largest \(S_{\text{BET}}\), so its H\({}_{2}\) uptake is highest at this temperature even though the pore size is also largest, indicating that at low temperature it is surface area dominates the H\({}_{2}\) adsorption ability. On the other hand, POP-1 and POP-4 have the similar surface areas, but POP-1 has a higher H\({}_{2}\) uptake due to the smaller pore size, even though the micropore volume is also smaller indicating the number of pores is about the same for the two systems. Therefore, when at 77 K, the specific surface area, rather than the pore size, contributes more to the H\({}_{2}\) uptake. This is why most research groups have tried to make porous materials with as large surface area as possible. Unfortunately, there is no H\({}_{2}\) adsorption data for this group of CMPs at room temperature and therefore, no comparison of H\({}_{2}\) uptake for them under ambient conditions, when pore size would be expected to play a larger role. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**CMPs** & **S\({}_{\text{BET}}\) (m\({}^{2}\)/g)** & **Pore diamter (nm)** & **Micropore volume (cm\({}^{3}\)/g)** & **H\({}_{2}\) uptake** \\ \hline POP-1 & 1031 & 0.77 & 0.378 & 2.78 wt \% 77K/60 bar \\ \hline POP-2 & 1013 & 0.74 & 0.341 & 2.71 wt \% 77K/60 bar \\ \hline POP-3 & 1246 & 0.88 & 0.448 & 3.07 wt \% 77K/60 bar \\ \hline POP-4 & 1033 & 0.81 & 0.402 & 2.35 wt \% 77K/60 bar \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of specific surface area, pore size, and H\({}_{2}\) uptake in porous organic polymers (POPs) (Adapted from [87])  
Figure 10: CMPs with varied length linkers between branch points. Figure 9: Basic synthetic route for conjugated microporous polymer-1 (CMP-1) and post-synthesis treatment. (Polymerization is promoted by Pd/Cu coupling under N\({}_{2}\) atmosphere.)