# Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions
## 1 Introduction
In addition, it was shown [27] that ferrous ions in the metal-exchanged MOF-5 disproportionated nitric oxide to nitrous oxide and the ferric nitrito complex. The degree of metal exchange was studied in Ni\({}^{2+}\)-exchanged MOF-5 using density functional theory, which showed two different reaction pathways for metal exchange [28]. In both pathways, the degree of metal exchange was controlled by the strength of the bond between the solvent and the exchanged metal. [PERSON] and [PERSON] [29] also showed that, in Ni\({}^{2+}\)-substituted MOF-5, the cation exchange process was endergonic and parameters such as cation character and solvent influenced the thermodynamics. [PERSON] and coworkers reported the redox chemistry of M-MOF-5 (where M = V\({}^{2+/3+}\), Ti\({}^{3+}\), Cr\({}^{2+/3+}\), Fe\({}^{2+}\), and Mn\({}^{2+}\)), where the introduced metal ions in the SBU are bound in a period of an uncommon trigonal all-oxygen ligand field available to both outer and inner spherical oxidants [30]. In their study, they observed that Cr\({}^{2+}\) converts into Cr\({}^{3+}\) in Cr-MOF-5 and Fe\({}^{2+}\) activates nitrogen oxide to yield a rare Fe-nitrosyl complex. The binding energies of hydrogen in primary and secondary hydrogen adsorption sites in Cd-MOF-5 and MOF-5 using DFT calculations indicates the same order of magnitude in both MOFs for all trapping sites but stronger binding for Cd\({}^{2+}\) in comparison with Zn\({}^{2+}\). The study shows that a binding energy of H\({}_{2}\) at the secondary adoption sites for Cd-MOF-5 improved by about 25% related to the MOF-5 [31]. [PERSON] et al. [32] synthesized a cobalt, partially exchanged mixed metal zeolitic imidazolate framework (ZIF-8). The mixed-metal MOF showed an improvement in the hydrogen uptake relative to monometallic parent MOF, which was due to the increase in pore volume and the higher affinity of the cobalt for hydrogen adsorption. [PERSON] et al. [33] studied the cobalt doping of MOF-5 in which various numbers of Co\({}^{2+}\) ions have been integrated through solvothermal techniques of crystallization into the framework of MOF-5. They also investigated the adsorption of gases including hydrogen on the prepared MOFs. The chemical composition revealed fractional isomorphic replacement of Zn in the MOF metal cluster with a limited quantity (up to 25%), suggesting that the substitution of more than one Co ion is extremely difficult. The hydrogen adsorption isotherm showed type I isotherm (according to IUPAC classifications), which was explained as a consequence of the host-guest interactions produced at subcritical temperature. Very little difference in hydrogen adsorption was observed between the prepared samples (about 1.2 wt.% for both MOF-5 and Co-MOF-5 at 1 bar). All previous studies indicate that MOF-5 is a practical system for investigating the exchange of metal using the PSE method for hydrogen adsorption. Co\({}^{2+}\)- and Ni\({}^{2+}\)-doped MOf-5 nanocrystals were reported using solvothermal methods [34; 35; 36]. These nanocrystals show enhanced hydrostaticities and adsorption properties. Here, we report the synthesis of mixed-metal organic framework (M-MOF-5, where the metal ions (M) are Ni\({}^{2+}\), Co\({}^{2+}\), and Fe\({}^{2+}\)) using the PSE technique. The merits of the M-MOF-5 as well as the as-synthesized MOF-5 are thoroughly characterized. Furthermore, the hydrogen adsorption isotherms are investigated for all MOFs to determine the hydrogen uptake capacities.