Diffusion of alkane mixtures in zeolites: Validating the Maxwell-Stefan formulation using MD simulations
Diffusion of alkane mixtures in zeolites: Validating the Maxwell-Stefan formulation using MD simulations
复制标题
DOI：
10.1021/jp044257l
复制
发表时间：
2005-04-07
影响因子：
3.3
通讯作者：
van Baten, JM
中科院分区：
文献类型：
--
作者：
Krishna, R;van Baten, JM
分子动力学（MD）模拟了FAU沸石中含有甲烷、乙烷、丙烷和正丁烷的纯组分、二元、三元和四元混合物，温度为300 K，分子负载范围为Theta，接近饱和极限。由(N) = -p[Delta](Gamma)（del Theta）定义的N维Maxwell-Stefan （M-S）扩散系数[Delta]矩阵与自扩散系数D-i，D-self一起确定。此外，采用构型偏置蒙特卡罗（CBMC）模拟得到了纯组分吸附等温线和饱和容量Theta（i,sat）。根据Delta（ij）、D-i、D-self和Theta（i,sat）的信息，确定了不同的M-S扩散系数：(1)组分-D-i，反映了物种i与沸石的相互作用，自交换-D-ii，(2)二元交换- D-ij。所获得的数据强调了M-S公式的主要优点，即在给定的占用率下，theta = Sigma(n)(i)(=1) theta (i)/ theta （i,sat）在沸石中，无论物种i是单独存在还是与其他物种混合存在，-Di对物种i几乎具有相同的值。同样的优势也适用于自我交换-Dii；给定占用值为0时，无论从纯组分、二元或三元混合物数据确定，其值都是相同的。对于所研究的所有二元和三元混合物，验证了二元交换系数D-ij可以使用先前开发的插值公式的推广，从自交换参数D-ii和D-jj的相应值插值（Skoulidas et al., Langmuir, 2003,19,7977）。我们还证明，如果对纯组分参数-Di和Dii的占用依赖性进行适当建模，则该信息足以在整个负载范围内为含有2,3或4组分的混合物提供非常好的矩阵[Delta]估计。烷烃在MFI和LTA中的混合扩散模拟证实了上述M-S配方的优点也适用于这些沸石拓扑结构。
Molecular dynamics (MD) simulations have been carried out for pure components, binary, ternary, and quaternary mixtures containing methane, ethane, propane, and n-butane in FAU zeolite at 300 K for a range of molecular loadings Theta, approaching saturation limits. The n-dimensional matrix of Maxwell-Stefan (M-S) diffusivities [Delta], defined by (N) = -p[Delta](Gamma)(del Theta), was determined along with the self-diffusivities, D-i,D-self. Additionally, configurational-bias Monte Carlo (CBMC) simulations were carried out to obtain the pure component sorption isotherms and the saturation capacities Theta(i,sat). From the information on Delta(ij), D-i,D-self, and Theta(i,sat),the various M-S diffusivities were determined: (1) component -D-i, reflecting the interactions of the species i with the zeolite, self-exchange -D-ii, and (2) binary exchange D-ij. The obtained data underline the major advantage of the M-S formulation that at a given occupancy, theta = Sigma(n)(i)(=1)Theta(i)/Theta(i,sat) Within the zeolite, the -Di has nearly the same value for species i whether this species is present on its own or in a mixture with other species. The same advantage holds, too, for the self-exchange -Dii; the value at a given occupancy, 0, is the same whether determined from pure component, binary, or ternary mixture data. For all binary and ternary mixtures studied, it was verified that the binary exchange coefficient D-ij can be interpolated from the corresponding values of the self-exchange parameters D-ii and D-jj using a generalization of the interpolation formula developed earlier (Skoulidas et al., Langmuir, 2003, 19, 7977). We also demonstrate that if the occupancy dependence of the pure component parameters -Di and Dii are modeled properly, this information is sufficient to provide very good estimates of the matrix [Delta] for mixtures with 2, 3, or 4 components over the entire range of loadings. Simulations of mixture diffusion of alkanes in MFI and LTA confirm that the above-mentioned advantages of the M-S formulation also hold for these zeolite topologies.