# Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions
## 3 Results  
### Characterization of the Prepared MOFs  
MOF-5 crystals were obtained from solvothermal method [37] in pure crystalline form after washing with DMF several times, and their crystallinity was tested using PXRD analysis, as shown in Figure 2. Several preparation factors (including the inert atmosphere, temperature, crystallization time and the quality of the chemical) affect the crystalline structure and pore properties of the prepared MOF that give rise to dissimilar adsorption properties. The PXRD pattern of the as-synthesized MOF-5 is comparable with the simulated pattern of MOF-5 crystal. A slight different in the peak intensities were observed around 10\({}^{\circ}\) and 15\({}^{\circ}\), which may be due to the orientation of crystal faces or textural effects. The PXRD patterns indicate the possibility of forming interpenetrated phases and the presence of Zn(OH)\({}_{2}\) species in the cavities [38]. This kind of difference in PXRD was also observed by other researchers [38] due to the formation of interpenetrated phases. The PXRD patterns of Ni-MOF-5 and Co-MOF-5 obtained from 0.5 M concentrations demonstrate that the M-MOFs are isostructural to MOF-5, as shown in Figure 3. The observed variation between the patterns of MOF-5 and Fe-MOF-5 may due to the occurred phase change, which leads to minor collapse of the framework during metal exchange. The PXRD of Ni-MOF-5 and Co-MOF-5 obtained from 1 M and 0.25 M concentrations are also isostructural to MOF-5 (Figure S1). This indicates that the MOF framework is stable after metal exchange with the additional molar concentration of metal ions. The PXRD patterns indicate that crystallinity of the M-MOFs retained their structures. In addition, the PXRD patterns of MOF-5, Co-MOF-5, and Ni-MOF-5 exchanged in DCM (Figure 4) and obtained from 0.5 M concentration demonstrate their stability after the solvent exchange. As a prototype, further experiments were carried out only on metal-exchanged MOFs in 0.5 M concentration of metal ions. The ICP-MS analysis of Ni-MOF-5, Co-MOF-5, and Fe-MOF-5 confirmed the incorporation of metal ions and revealed that the metal exchange with reference to Zn\({}^{2+}\) was 18%, 15%, and 99%, respectively. The ICP-MS analysis of Fe-MOF-5 indicates that almost complete replacement of Zn\({}^{2+}\) took place, which leads to either collapse of the framework or a phase change to an amorphous solid. As expected, the SEM micrographs (Figure 5) show the generation of crystalline MOF-5 and excluded the likelihood of growth or contamination with another phase in the course of the metal exchange. The micrographs also show that the parent MOF-5 and metal-exchanged M-MOF-5 have homogenous polyhedral crystals or flakes with sizes ranging from 2 to 20 micro meters and the absence of other major morphologies. However, the morphologies of Fe-MOF-5 has been changed from flakes to needles. The exchange of metal ions were also quantified by Energy-Dispersive X-ray analysis (EDX). The distribution of metal ions was assessed by mapping the data of Ni-MOF-5, as shown in Figure S2. The EDX mapping of the Ni element shows that the Ni\({}^{2+}\) ions are distributed in the area of crystalline particles. This clearly proves the existence of exchanged metals in the microcrystalline sample. The EDX elemental map of Ni-Co-5 verified that, indeed, they consist of Ni\({}^{2+}\) ion (about 16%) with reference to Zn\({}^{2+}\) ions (Figure S3). The mapping data of Co-MOF-5 are shown in Figure S4. The EDX elemental map of Co-MOF-5 also proved that they certainly contain about 15% Co\({}^{2+}\) ion with reference to Zn\({}^{2+}\) ions (Figure S5). Figure 2: PXRD patterns of the as-synthesized (red) and simulated (black) MOF-5. Using Fourier transform infrared (FTIR) analysis, the chemical integrity, successful formation, and compositional similarity of Co-MOF-5 and Ni-MOF-5 were further confirmed (Figures S6-S8).