Pacific Northwest National Laboratory, Richland, Washington 99352, USA.
J Phys Chem B. 2011 Mar 31;115(12):2842-9. doi: 10.1021/jp1115299. Epub 2011 Mar 8.
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 Ni\DOBDC, 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 Ni\DOBDC. The strong binding for both n-hexane and cyclohexane with Ni\DOBDC is attributed to the increase of the heat of adsorption observed in experiments. Our structural results indicate the organic linkers in Ni\DOBDC 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 Ni\DOBDC 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 Ni\DOBDC, with higher diffusion for flexible molecules. Because of the large pore sizes in Ni\DOBDC, energetic effects were the dominant force for alkane adsorption and selectivity.
提高朗肯循环的效率，该循环可用于多种可再生能源，需要使用具有高热潜的工作流体。为了进一步提高其潜热，可以将工作流体置于具有高热吸附的金属有机热载体（MOHC）中。一种这样的材料是 Ni\DOBDC，其中线性烷烃的吸附热高于环状烷烃。我们进行了分子动力学模拟，以研究 n-己烷和环己烷在 Ni\DOBDC 中的结构、扩散和吸附特性。n-己烷和环己烷与 Ni\DOBDC 的强结合归因于实验中观察到的吸附热增加。我们的结构结果表明，Ni\DOBDC 中的有机链接是 n-己烷和环己烷分子的主要结合位点。然而，在目前工作中研究的所有温度和负载下，n-己烷显然与 Ni\DOBDC 的结合比环己烷更强。这是因为 n-己烷能够在更大程度上重新配置其结构，从而在吸附质和吸附剂之间获得更多的接触，而环己烷则不能。客体分子的几何形状和灵活性也与其在 Ni\DOBDC 中的扩散性有关，具有更高灵活性的分子扩散性更高。由于 Ni\DOBDC 中的孔径较大，能量效应是烷烃吸附和选择性的主要作用力。
