Molecular dynamics simulation of diffusion and separation of CO2/CH4/N2 on MER zeolites
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摘要: 采用巨正则蒙特卡洛(GCMC)模拟方法研究了CO2、CH4和N2在MER型沸石中的吸附性能，模拟结果与实验结果吻合证明模型和力场是可靠的。在此基础上，以纯硅MER型沸石作为对照，采用分子动力学(MD)模拟方法研究了CO2、CH4和N2在K-MER型沸石中的扩散和分离性能。结果表明，CO2、CH4和N2在MER型沸石中存在亚扩散现象，扩散方式为构型扩散，在沸石三维通道中的扩散存在各向异性。沸石与气体之间的作用力和沸石骨架外阳离子均影响气体分子的扩散能力，而沸石骨架外阳离子是影响气体分子扩散能力的主要因素。CO2和N2的自扩散系数随吸附浓度的增加而减小；CH4的自扩散系数随吸附浓度的增大先增加后减小。CO2、CH4和N2的自扩散系数随温度的升高均增加，扩散活化能大小顺序为N2 (16.51 kJ/mol) > CH4 (8.39 kJ/mol) > CO2 (4.38 kJ/mol)。K-MER型沸石膜对CO2/CH4、CO2/N2和N2/CH4分离体系均有良好的分离选择性。气体分子的渗透率~104 GPU(1 GPU = 3.35×10−10 mol/(s·m2·Pa))。Abstract: The adsorption behaviors of CO2, CH4 and N2 on MER zeolites were investigated by Grand Canonical Monte Carlo (GCMC) simulation method. The calculated pure gas uptake agreed well with the experimental data, which proved that the current simulation model and COMPASS force field are reliable. On this basis, molecular dynamics (MD) simulations were carried out for diffusion and separation of CO2, CH4 and N2 on K-MER zeolites, with the silicon MER zeolite as the reference. The results show that mean squared displacement (MSD) versus simulation time is sublinear. In MER zeolites, the configuration diffusion regime is dominant resulted from the tight fit of the gas molecules and the zeolite pore size. The diffusion of CO2, CH4 and N2 in MER zeolites with three-dimensional cage structures is anisotropic. The gas molecules diffuse preferentially along with the direction of x axis in K-MER zeolites. Extra-framework cations in zeolite plays important influence on the gas diffusion. In K-MER zeolites, the self-diffusion coefficients of CO2 and N2 are negative correlated with loading, whereas for the self-diffusion coefficient of CH4, it firstly increases and then decreases with the increase of loading. All of the self-diffusion coefficients of CO2, CH4 and N2 increase with the elevation of temperature. The order of diffusion activation energy is N2 (16.51 kJ/mol)﹥CH4 (8.39 kJ/mol)﹥CO2 (4.38 kJ/mol). K-MER zeolite membrane has good separation selectivity for gas mixture system of CO2/CH4, CO2/N2 and N2/CH4. The permeance of CO2 and N2 through K-MER zeolite membrane is as high as 104 GPU (1 GPU= 3.35×10−10 mol/(s·m2·Pa)).
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Key words:
- carbon dioxide /
- MER zeolite /
- diffusion /
- separation /
- molecular dynamics simulation
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图 2 不同模拟时间下N2在纯硅MER型沸石中的MSD与时间的关系
Figure 2 MSD of N2 on silicon MER zeolite versus time at different simulated time at 298 K
图 3 CO2、CH4和N2在K-MER与纯硅MER型沸石中的吸附等温线
Figure 3 Adsorption isotherms for CO2, CH4 and N2 on K-MER and silicon MER zeolites at 298 K
图 4 CO2、CH4和N2在K-MER与纯硅MER型沸石中的等量吸附热
Figure 4 Adsorption heat for CO2, CH4 and N2 on K-MER and silicon MER zeolites at 298 K
图 5 CO2、CH4和N2在纯硅MER(a)与K-MER(b)型沸石中的均方位移与时间的关系
Figure 5 MSD of CO2, CH4 and N2 on silicon MER (a) and K-MER (b) zeolites versus time at 298 K
图 7 K-MER型沸石骨架原子与骨架外K+之间的径向分布函数
Figure 7 Radial distribution function between the framework atoms and extra-framework K+ of K-MER zeolite
图 8 CO2、CH4和N2在K-MER型沸石中自扩散系数与浓度的关系
Figure 8 Loading dependence of self-diffusion coefficients for CO2, CH4 and N2 on K-MER zeolites at 298 K
图 9 CO2、CH4和N2在K-MER型沸石中自扩散系数与温度的关系
Figure 9 Temperature dependence of self-diffusion coefficients for CO2, CH4 and N2 on K-MER zeolites
图 11 MD模拟5 ns后混合气体穿过沸石膜的快照
Figure 11 Snapshot of mixed gas permeating through zeolite membrane after a MD simulation time of 5 ns
表 1 CO2、CH4和N2在MER型沸石中的自扩散系数
Table 1 Self-diffusion coefficient of CO2, CH4 and N2 on MER zeolites at 298 K
Adsorbate DSi-MER/(10−12 m2·s−1) DK-MER/(10−12 m2·s−1) total x y z total x y z CO2 132.18 22.45 3.40 106.18 12.37 6.10 1.78 4.48 CH4 170.92 0 0 170.92 2.15 2.15 0 0 N2 218.18 28.28 18.70 171.18 5.90 5.75 0 0.10 
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