# Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions
## 3 Results
### Surface Area and Hydrogen Adsorption
Since a physical adsorption phenomenon is strongly related the surface area, it is expected to see a sharp decrease in the adsorbed amount of hydrogen for the Fe-MOF-5 sample relative to the parent MOF-5 and other M-MOFs. The hydrogen adsorption measurements at 77 K are shown in Figure 8. Co-MOF-5 showed slightly higher affinity for hydrogen adsorption than that of the parent MOF-5, which is in agreement with the results of other researchers [33]. The hydrogen adsorption isotherm of Fe-MOF-5 was significantly lower than that of the parent MOF-5, which was expected, as mentioned earlier, since the preparation of this MOF was accompanied by a sharp drop in the surface and micropore areas. The slopes of all adsorption isotherms at the low-pressure region are steeper than that of the Fe-MOF-5, which also indicates stronger interactions. The gravimetric uptake of H\({}_{2}\) at 110 kPa for all prepared MOFs is given in Table 3. The gravimetric hydrogen adsorptions of MOF-5, Co-MOF-5, Ni-MOF-5, and Fe-MOF-5 were 1.46, 1.53, 1.53, and 0.99 wt%, respectively. Both Ni-MOF-5 and Co-MOF-5 hydrogen uptakes are slightly higher than what is reported by [PERSON] et al. [33] and show about a 5% increase in hydrogen uptake compared with that of the parent MOF-5. The \begin{table}
\begin{tabular}{c c c} \hline
**BET Surface Area (m\({}^{2}\)/g)** & **Hydrogen Adsorption (wt.\%)** & **Reference** \\ \hline
3362 & 1.32 & [39] \\ \hline
572 & 1.3 & [33] \\ \hline
501–840 & 1.75 & [37] \\ \hline
2050 & 1.2 & [40] \\ \hline
2449 & 1.46 & [41] \\ \hline
572 & 1.3 & [42] \\ \hline
2800 & 1.25 & [43] \\ \hline
261–350 & 0.21–0.5 & [24] \\ \hline
2023–3210 & 0.76–1.24 & [44] \\ \hline
520 & 0.97 & [45] \\ \hline \end{tabular}
\end{table} 
Table 2: BET surface area and hydrogen adsorption capacity (at 77 K and 1 bar) of MOF-5. hydrogen adsorption isotherms were modeled using the Sips model, which is used to define adsorption in microporous materials [47]:  
\[\frac{C}{C_{0}}=\frac{\left(aP\right)^{1/n}}{1+\left(aP\right)^{1/n}}\]  
where \(C\) is the amount adsorbed (wt%), \(C_{0}\) is the saturated adsorption constant (wt%), \(a\) is the affinity constant, \(P\) is the pressure in bar, \(n\) is a constant that indicates the heterogeneity of the adsorbate-adsorbent system (dimensionless). This model can describe the Freundlich and Langmuir types if the isotherms are at low and high pressures. The parameters of the Sips model were calculated by fitting the model to the measured adsorption data using the least square method. The calculated parameters along with the correlation coefficient (\(R^{2}\)) are listed in Table 3. Excellent agreement was observed between the experimental data and the Sips model, as can be seen from Figure 8. In addition, the correlation coefficient was very close to 1.0, which indicates a perfect fit. Table S1 indicates the gravimetric hydrogen adsorption in mixed-metal MOFs. A Co/Ni mixed-metal expanded IRMOF-74 shows enhanced hydrogen adsorption properties after metal exchange [48]. In this reported case, about 15% of enhancement is observed for mixed-metal MOFs in hydrogen adsorption. A mixed-metal MOF of Cu-BTC displays higher uptake in comparison with the parent MOFs [19]. These reported MOFs show about 60% enhancement for M-Cu-BTC (M = Ni\({}^{2+}\), Zn\({}^{2+}\) and Fe\({}^{2+}\)). However, the current study shows only 5% enhancement in comparison with the parent MOF. Figure 8: Hydrogen adsorption isotherms at 77K of MOF-5 and M-MOF-5 (experimental circles) and the Sip model (continuous line). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Sips Parameters**} \\ \hline
**Material** & **Amount of H\({}_{2}\) Adsorbed (wt.\%)** & \(C_{0}\) & \(a\) & \(n\) & \(R^{2}\) \\ \hline MOF-5 & 1.46 & 1.87299 & 0.05005 & 1.34862 & 0.999962 \\ \hline Co-MOF-5 & 1.53 & 1.98935 & 0.04594 & 1.35045 & 0.999959 \\ \hline Ni-MOF-5 & 1.53 & 2.12790 & 0.03329 & 1.41521 & 0.999994 \\ \hline Fe-MOF-5 & 0.99 & 4.64781 & 0.00097 & 1.66597 & 0.999365 \\ \hline \hline \end{tabular}
\end{table}