# Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions
## 3 Results
### Surface Area and Hydrogen Adsorption  
The nitrogen adsorption isotherms at 77 K are shown in Figure 7. A sharp increase in the amount of adsorbed nitrogen was witnessed at a low relative pressure while a marginal increase was noticed as the pressure increases further to reach a plateau after a relative pressure of 0.2, which indicates that the material is microporous, that the adsorption is monolayer, and that equilibrium was reached. According to the IUPAC classification, this behavior is classified as Type I and noticed for all MOFs with the exception of Fe-MOF-5. Ni-MOF-5 and Co-MOF-5 showed comparable adsorption capacities for nitrogen; however, a significant loss in the adsorption capacity was evident in Fe-MOF-5, which may be due to the small atomic size of the Fe ion (0.126 nm) compared with the average pore size of the parent MOF-5 (1.66 nm), which makes it easy to access and maybe block some of the pores. The reduced surface area of the sample was compared with the reported surface area mainly due to the formation of interpenetrated phases and the presence of Zn(OH)\({}_{2}\) species in the cavities [38]. The obtained structural parameters of MOF-5 and M-MOF-5 are shown in Table 1. The calculated BET surface area of the parent MOF-5 was 921 m\({}^{2}\)/g while the Langmuir area for the same sample was 1043 m\({}^{2}\)/g, which was confirmed in repeated trials with an activation temperature of 150 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**MOFs**} & **BET Surface Area (m\({}^{2}\)/g)** & \begin{tabular}{c} **Langmuir** \\ **Surface Area** \\ **(m\({}^{2}\)/g)** \\ \end{tabular} & \begin{tabular}{c} **Micropore** \\ **Area (m\({}^{2}\)/g)** \\ \end{tabular} & \begin{tabular}{c} **Micropore** \\ **Volume** \\ **(cm\({}^{3}\)/g)** \\ \end{tabular} & \
\begin{tabular}{c} **Average Pore** \\ **Diameter (nm)** \\ \end{tabular} &
\begin{tabular}{c} **Total Pore** \\ **Volume** \\ **(cm\({}^{3}\)/g)** \\ \end{tabular} \\ \hline MOF-5 & 921 & 1043 & 892 & 0.347 & 1.66 & 0.383 \\ \hline Co-MOF-5 & 853 & 934 & 840 & 0.325 & 1.60 & 0.342 \\ \hline Ni-MOF-5 & 802 & 897 & 780 & 0.303 & 1.76 & 0.353 \\ \hline Fe-MOF-5 & 325 & 525 & 242 & 0.106 & 2.53 & 0.223 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Surface area and pore parameters of MOF-5 and M-MOF-5 samples. Figure 7: Nitrogen adsorption isotherms at 77K of MOF-5 and M-MOF5. The quality and surface area of MOF-5 depends on many factors including the synthesis route (solvothermal, direct mixing at room temperature, sonochemical, and microwave-assisted), precursor concentration, polarity of the solvent, and temperature. This may explain the wide range of surface areas [38] reported in the literature, as shown in Table 2.  
The BET surface area of our MOF-5 is within that range; however, it is lower than the area reported by [PERSON] [21] because our preparation conditions were not optimized. Such a reduction in surface area has been reported before [38]. The reduction in the apparent surface area may also be due to catenation or partially occupied cavities by Zn (OH)\({}_{2}\) species that make the hosting cavities inaccessible for probe molecules [38,46]. This observation also supports the behavior of early peaks with lower intensity that was observed in the PXRD patterns of synthesized MOF-5. After the metal exchange, Co-MOF-5 and Ni-MOF-5 showed slightly lower surface areas compared with that of the parent MOF-5. However, a large reduction in the surface area was observed for the Fe-MOF-5 sample (BET and Langmuir areas are 325 and 525 m\({}^{2}\)/g, respectively). This may be due to the significant presence of extra framework metal species that decrease the surface area proportional to its concentration [33]. This is also evidenced by the drastic drop in the micropore area and volume, which went down to 242 m\({}^{2}\)/g and 0.106 cm\({}^{3}\)/g, respectively. It is also noted that the micropore volume of Fe-MOF-5 occupies only 74% of the total pore volume while it is around 97% for all other M-MOFs.