Hydrogen adsorption in microporous materials is considered an interesting alternative for hydrogen storage. Thus, novel porous anionic Zeolite-like Metal-Organic Framework materials (ZMOF) with RHO and SOD topologies are promising adsorbents due to their high surface area and, more importantly, to the negative charge of their structure, since a change of the affinity for hydrogen can potentially be produced by ion exchange to increase hydrogen adsorption.
Both RHO- and SOD-ZMOF materials were prepared basically following the procedure published elsewhere (Y.L. Liu et al., Chem. Commun. 2006), but modifying the nucleation and crystallization times. Ion exchange was carried out with a solution of metal nitrate (Li+, Na+, K+, Cs+, Ca2+ Mg2+, Cu2+) in a mixture of ethanol and water.
Powder X-ray Diffraction (XRD) and Scanning Electronic Microscopy (SEM) studies made clear that SOD-ZMOF shows in general a higher metal load capacity than RHO-ZMOF. Depending on of the exchangeable ion, RHO-ZMOF can be converted into an amorphous phase, whereas SOD-ZMOF is always stable. In this sense, thermogravimetric analysis and thermal treatments combined with in-situ XRD measurements have evidenced a significantly higher thermal stability of both as-prepared and ion-exchanged SOD-ZMOF materials in comparison to their RHO-structured homologues.
The porosity of these materials was measured by nitrogen adsorption at 77 K and the affinity for hydrogen was tested by hydrogen adsorption, also at 77 K. The differences observed in hydrogen affinity can be explained as a function of the size and charge of the exchanged ion and the ion exchange degree in the ZMOF materials.