Loading Dependence of Self‐Diffusivities of Gases in Zeolites
Loading Dependence of Self‐Diffusivities of Gases in Zeolites
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DOI：
10.1002/ceat.200700127
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发表时间：
2007-09
影响因子：
2.1
通讯作者：
R. Krishna;J. Baten
中科院分区：
文献类型：
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作者：
R. Krishna;J. Baten
不同气体（ch4、n2、Kr、c2h6和c3h8）在LTA、FAU和MFI分子筛中的自扩散率（di,self）的实验数据显示，其对摩尔负载q i的依赖性不同。在LTA中，除n2外，所有分子的self都随着qi的增加而增加。在FAU和MFI中，D i，self随q i的增加而急剧下降。为了深入了解负载依赖性背后的原因，进行了分子动力学（MD）模拟，以确定七种气体（ch4, n2, Kr, c2h6, c3h8， Ar和Ne）在六种不同的全硅分子筛结构（MFI， AFI， FAU， CHA， DDR和LTA）中的自扩散率。模拟结果表明，d1、self随qi的变化是由分子筛内分子大小、分子形状和约束程度等多种因素决定的。对于一维通道（AFI）和交叉通道结构（MFI），其自密度随气的增加而减小。对于由窗口（FAU， CHA， DDR， LTA）分隔的笼状分子筛结构，窗口的大小是一个重要的决定因素。当窗口较宽（FAU）时，所有分子的di都随qi的减小而减小。如果窗口较窄（CHA， DDR和LTA），则d1，self通常随着q i急剧增加，在饱和时达到最大值并降低到接近零的值。d1的锐度随qi的增加而增加，是由窗口处的约束度决定的。弱约束的分子，如Ne，不表现出d1的增加，而随q1的增加而增加。
Experimental data on the self-diffusivities, D i,self , of a variety of gases (CH 4 , N 2 , Kr, C 2 H 6 , and C 3 H 8 ) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q i . In LTA, D i,self appears to increase with q i for all molecules except N 2 . In FAU and in MFI the D i,self shows a sharp decrease with increasing q i . In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self-diffusivities of seven gases (CH 4 , N 2 , Kr, C 2 H 6, C 3 H 8, Ar, and Ne) in six different all-silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D i,self with q i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one-dimensional channels (AFI) and intersecting channel structures (MFI), the D i,self invariably decreases with increasing qi. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D i,self decreases with q i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D i,self often exhibits a sharp increase with q i , reaches a maximum and reduces to near-zero values at saturation. The sharpness with which D i,self increases with q i , is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D i,self with q i .