中文摘要：

采用平衡分子动力学和巨正则系综蒙特卡洛模拟方法对多孔沸石咪唑酯骨架材料（ZIF-8）中CH4和H2分子的吸附与扩散特性进行了比较研究.结果表明,采用柔性力场能够很好地复制ZIF-8在不同压力或温度条件下的晶体结构,也能准确地计算不同温度下CH4和H2分子在ZIF-8中的扩散系数,特别是高温下CH4分子因能够摆脱ZIF-8骨架笼口的空间限制而使其扩散系数出现大幅提升.同时,该力场也能粗略地模拟CH4和H2分子在ZIF-8中的等温吸附曲线,通过自编程序得到吸附和扩散平衡时CH4和H2分子在ZIF-8单元晶胞内的几率密度分布数据,并利用VMD软件可视化.结果显示CH4和H2分子在ZIF-8中的优先吸附位置均在大孔中心靠近咪唑环的区域,但CH4分子的优先吸附位置有两个不同层次,而H2分子的优先吸附位置只有一个层次,说明CH4和H2分子在ZIF-8中存在着不同的吸附机理.

英文摘要：

Metal-organic frameworks （MOFs） with flexible organic linkers and bridging tetrahedral metal ions were extensively investigated in order to improve the selectivity and storage properties of sorbents. The so-called zeolitic imidazolate frameworks （ZIFs） are a particularly interesting class of MOFs due to their exceptional chemical and thermal stability. Adsorption and diffusion characteristics of CH4 and H2 in porous zeolitic imidazolate framework-8 （ZIF-8） material were comparatively studied using the methods of equilibrium molecular dynamics （EMD） and the grand canonical Monte Carlo （GCMC） with the same force field. It was shown that using the force field of framework flexibility not only can reproduce the crystal structure of ZIF-8 at different temperatures and pressures well, but also can calculate the diffusion coefficients of CH4 and H2 in ZIF-8 at different temperatures accurately. Especially, adsorbed CH4 at high temperature can run away the space constraint at the entrance of framework cage in ZIF-8, which results in a sharp increase of its diffusion coefficient. At the same time, using the force field can also simulate the adsorption isotherms of CH4 and H2 molecules in ZIF-8 roughly. Thus, their probability density distribution data in the unit cell of ZIF-8 under the conditions of adsorption and diffusion equilibrium can be calculated via the self-compiled program and further visualized via the software of VMD in order to analyze the adsorption and diffusion behavior of the gases. The results indicate that the priority locations for adsorption of CH4 and H2 molecules in ZIF-8 are regions close to the imidazole rings in the center of big pores. But there are two different levels of the preferential adsorption sites for CH4 and only one level of that for H2, indicating that different adsorption mechanism of CH4 and H2 exists in ZIF-8. The difference of the preferential adsorption sites for CH4 and H2 in ZIF-8 is possibly resulting from the different size of the two molecules.