Abstract
As it is known, the high surface area, permanent porosity, tunable pore sizes, rigidity, structural flexibility and thermal stability make MOFs and especially Zn-MOF-5 as promising materials. But the small enthalpy change (heat of adsorption) and its low desorption temperature hinders its use. Heat of adsorption varies with the type of metal, framework topology, surface area, and pore size, and hence the extent of hydrogen adsorption in MOF materials can be influenced by manipulating these parameters. Therefore, a substitution approach is presented in the present work to improve the thermodynamic properties of MOF-5. DFT method is used to investigate the effect of the substitution of Zn by Cd and Mg on structural, thermodynamic and hydrogen storage properties of Zn-MOF-5-connector. Results of calculation show that the formation energy for Zn-connector is greater than that of Cd and Mg connectors which enhances its stability and therefore increases the decomposition temperature to 191.79 and 175.81 K for Mg and Cd connector, respectively. In addition, the adsorption and diffusion of H2 are investigated and discussed. Furthermore, a significant increase in the gravimetric and volumetric capacities from 1.57 wt% and 12.26 g L-1. H2 for Zn based connector to 6.37 wt% and 40.16 g L-1. H2 for Mg-connector is observed.
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