金属-有机骨架材料中吸附气体的扩散速率

采用分子动力学方法,以甲烷为探针分子研究了不同压力条件下气体在具有不同孔道结构的金属-有机骨架材料(MOFs)中的扩散速率.通过计算气体在八种材料中的自扩散系数,并结合气体分子在材料中的质心分布图等,讨论了气体扩散速率与孔道结构之间的关系.研究结果表明:对于同时含有孔笼(pocket)和三维正交孔道(channel)结构的MOF材料(P-C材料),低压时甲烷气体吸附在孔笼结构中,随着压力的升高,气体分子开始进入正交孔道,同时其自扩散系数增加;而对于只含有三维立方孔道结构的IRMOF(isoreticular MOF)系列材料,在中低压范围内,气体分子在其中的自扩散系数随压力变化较小.当压力进一步升高时,气体分子在材料孔道中的吸附逐渐接近饱和,其自扩散系数均降低.因此,在不同MOF材料中气体分子扩散速率的差异主要取决于孔道结构的不同.对P-C材料,中低压下通过控制压力可以控制气体在其中的扩散速率,从而为MOF材料在气体存储、分离等方面的实际应用提供参考信息.

Diffusion Rates of Gas Molecules Adsorbed in Metal-Organic Frameworks

A systematic equilibriummolecular dynamics study was performed to investigate the diffusion rates of gas molecules as a function of the pressure in metal-organic frameworks (MOFs) with different structures. Methane was chosen as the probe molecule. The self-diffusion coefficients in eight typical MOFs were calculated at roomtemperature. Combined self-diffusion coefficients with the contour plots of the center of mass (COM) probability densities of methane, the relationship between the diffusion rates of gas molecules and the structure of the pores in the MOFs is discussed. Results show that methane tends to adsorb in pockets in MOFs with pocket and channel pores (P-C materials) at low pressure. With an increase in pressure, the gas molecules move to the channel and the self-diffusion coefficient increases. However, the diffusion coefficient of methane changes a little in the low and middle pressure range in the IRMOFs (isoreticularMOFs) with only one kind of pore.With a further increase in pressure, the self-diffusion coefficient of methane decreases in all the studied MOFs. Therefore, the difference in diffusion rates of methane in different MOFs may be mainly attributed to the pore structures of the materials. In addition, diffusion rates of the gas molecules in the P-C materials could be controlled in a wide range by varying the pressure, providing useful information for the application of MOFs in gas storage and separation.