Efficient storage of hydrogen is an important aspect in utilizing hydrogen as a pollution free energy source. The adsorptive storage of molecular hydrogen on microporous materials is widely studied and Metal-organic frameworks (MOFs) are among the most promising hydrogen adsorbents. They exhibit a highly developed microporosity, yielding large surface areas and high hydrogen excess adsorption uptakes. Isoreticular Metal-organic frameworks (IRMOF) materials which consist of zinc oxide clusters connected by organic linker molecules have nanoscale pores and large surface area, providing many hydrogen molecule binding sites.
The objective of this study is to model adsorption of hydrogen in IRMOF-1, IRMOF-2, IRMOF-3, IRMOF-7, IRMOF-8, IRMOF-10, IRMOF-10 with amine groups at two positions and IRMOF-10 with Bromine at two positions by Path Integral Grand Canonical Monte Carlo (PI-GCMC) simulations using standard force fields. From the simulations, hydrogen adsorption isotherms at 300 K and 77 K were generated for low pressure conditions (up to 10 bar). The energy of adsorption of hydrogen was calculated for each IRMOF. The relationships of hydrogen adsorption with surface area, free volume and energy between adsorbate and framework were analyzed. Free volume and Surface area were calculated by geometrical methods using the structure of each IRMOF. Density distribution was developed and analyzed. Classical MD simulations were performed to calculate the self-diffusivity of hydrogen in IRMOF structures and activation energy was calculated for each IRMOF.
For the IRMOFs studied, the maximum average hydrogen adsorbed is less than 6 wt% at 77 K and 10 bar. Analysis of density distributions shows that hydrogen adsorption within all cages is preferred at vertices at both low and high temperature in entire pressure range. The presence of functional groups in the framework provides more surface area and creates favorable hydrogen adsorption sites. But when they are present near to vertices - the main adsorption sites, they hinder the hydrogen adsorption. Even functional groups that enhance hydrogen adsorption in terms of the number of hydrogen adsorbed per cage, may actually hinder adsorption on a gravimetric basis (i.e., the functional groups aren't worth their weight in enhancement of hydrogen capacity).
Numerous previous studies have analyzed what are the dominant factors for hydrogen adsorption in IRMOFs in different pressure ranges. For example, it has been suggested that energetic effects are the dominant factor at low loading, surface area is identified as the dominant factor in medium pressures and free volume in high pressures. However, energy of adsorption, surface area and free volume are generally not independent in these materials. Therefore one goal of this study is to present a careful view of the relationship between the structural properties of IRMOFs and their hydrogen adsorption capability using statistical analysis of correlations within the systems.
Keywords: hydrogen adsorption, GCMC simulation, statistical mechanics, adsorption isotherm, self-diffusivity, activation energy, MOF, IRMOF
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