# Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions
## 1 Introduction  
As a clean alternative fuel with zero-CO2 releases and high energy density, hydrogen characterizes a possible source of energy. However, its dedicated storage is challenging owing to the low temperature or high pressure necessary to attain enough energy density [1, 2]. A wide range of materials have been used for the purpose of H2 storage including chemisorption and physisorption materials [2]. However, there is no storage material that has fulfilled the recent U.S. Department of Energy capability objectives of 4.5 wt.% or 30 g/L at pressures under 100 atm and a temperature of \(-\)40 to 60 \({}^{\circ}\)C [3]. In this regard, metal-organic frameworks (MOFs) are good candidates as physical adsorbents for hydrogen storage [4, 5, 6, 7, 8]. These materials are assembled from metal ions (or clusters) linked together via the coordination of organic bridging ligands. The features of the MOFs (such as stable porosities and large internal surface area) make them good candidates for H2 adsorption. Recently, zirconium-based MOF showed outstanding total adsorption capacity(65.7 mmol/g) at 100 bar and 77 K and recorded a volumetric working capacity of 37.2 g/L between 100 and 5 bar at 77 K [9]. In order to upgrade the hydrogen uptake in MOFs, cation exchange in secondary building units (SBUs) or the incorporation of additional metal in the SBUs is an alternative [10; 11; 12; 13]. Such an incorporation of bimetallic or mixed metal MOFs (M-MOFs) found many applications in gas adsorption and catalysis [10; 11]. M-MOFs generate defects (that contain multiple metal ions, vacancies, and hierarchical pore sizes) in the materials and synergistic effects between the metals, which improves its intrinsic properties [14]. The defects lead to high porosity and high interactive adsorption sites in the MOFs that might increase the uptake capacity. As a consequence of multiple metal atoms, mixed metal MOFs can display improved performance in different fields in comparison with single metal MOFs including gas separation and adsorption, sensing, catalysis, and photoactive materials [15; 16]. In principle, there are two broad approaches to synthesizing M-MOFs, namely post-synthesis exchange or one-pot synthesis. The one-pot strategy hinges on the similarity between the metal ions used in terms of ionic radius, chemical behavior, and columbic charge [17]. Post-synthetic exchange is an ideal strategy for the preparation of M-MOFs [18]. We recently showed that, using the post-synthetic exchange method, the H\({}_{2}\) uptake in M-MOFs of Cu-BTC (BTC = benzene-1,3,5-tricarboxylate) was enhanced because of the higher charge density of metal that polarizes the hydrogen [19]. MOF-5 was reported in 1999 [20] and consists of Zn\({}_{4}\)O units connected by linear BDC (BDC = 1,4-benzenedicarboxylic acid) ligands to form a cubic network (Figure 1). It is one of the highest studied MOFs due to its high robustness and reversible hydrogen adsorption kinetics. MOF-5 was the first example to show hydrogen storage in MOF reported by [PERSON] and coworkers [21; 22]. The highest Brunauer-Emmett-Teller (BET) surface area reported for MOF-5 under optimized synthesis conditions is about 3800 m\({}^{2}\)/g [23]. The reported total gravimetric capacity of MOF-5 is 11.5 wt.% at a high pressure of 170 bar and at 77 K [23]. This indicates that it is one of the best candidates to modify the material towards further improvement in hydrogen adsorption [24; 25], which can be accomplished by introducing active functions using either ligand or metal exchange in MOFs [15; 18]. Figure 1: A view of MOF-5 along the \(z\)-axis, where the central large sphere represents the cavity size in MOF-5. Selective catalytic epoxidation of olefin has been reported by a MOF-5 derivative obtained by partial substitution of Zn\({}^{2+}\) with Mn\({}^{2+}\)[25], in which the SBU of MOF-5 played a major role in the catalytic reactions. The formation of metal-stable Co-MOF-5 enabled the dynamic behavior of MOF-5 [26].