Computational study of hydrocarbon adsorption in metal-organic framework Ni2(dhtp).

Enhancing the efficiency of the Rankine cycle, which is utilized for multiple renewable energy sources, requires the use of a working fluid with a high latent heat of vaporization. To further enhance its latent heat, a working fluid can be placed in a metal organic heat carrier (MOHC) with a high heat of adsorption. One such material is NiDOBDC, in which linear alkanes have a higher heat of adsorption than cyclic alkanes. We carried out molecular dynamics simulations to investigate the structural, diffusive, and adsorption properties of n-hexane and cyclohexane in NiDOBDC. The strong binding for both n-hexane and cyclohexane with NiDOBDC is attributed to the increase of the heat of adsorption observed in experiments. Our structural results indicate the organic linkers in NiDOBDC are the primary binding sites for both n-hexane and cyclohexane molecules. However, at all temperatures and loadings examined in present work, n-hexane clearly showed stronger binding with NiDOBDC than cyclohexane. This was found to be the result of the ability of n-hexane to reconfigure its structure to a greater degree than cyclohexane to gain more contacts between adsorbates and adsorbents. The geometry and flexibility of guest molecules were also related to their diffusivity in NiDOBDC, with higher diffusion for flexible molecules. Because of the large pore sizes in NiDOBDC, energetic effects were the dominant force for alkane adsorption and selectivity.