Wang Haiyan, Turner Elizabeth A, Huang Yining
Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7.
J Phys Chem B. 2006 Apr 27;110(16):8240-9. doi: 10.1021/jp060775f.
We have examined the adsorption of n-pentane in several representative zeolites such as silicalite (MFI), ferrierite (FER), zeolite L (LTL), and faujasite zeolites with FAU structure including siliceous Y (Si-Y) and Na-Y by using FT-Raman spectroscopy in combination with thermogravimetric analysis (TGA) with particular attention being paid to the conformational and dynamic behavior of the guest molecule. The results indicate that the framework topology mainly dictates the conformation of n-pentane in a zeolite. For the zeolites with channel systems such as silicalite, ferrierite, and zeolite L, the population of the all-trans conformer increases upon loading, given that the geometry of the isomer fits better in the channel. When n-pentane is adsorbed in zeolites with a large cavity, such as Si-Y and Na-Y, the distribution of the all-trans (TT) and trans-gauche (TG) conformers is similar to that of pure liquid, suggesting that the large supercage in the framework imposes minimal effect on the conformational equilibrium. The dynamics of the guest molecule is, however, influenced significantly by the existence of cations. Adsorption of n-pentane in a siliceous framework such as silicalite and Si-Y results in extensive molecular motion at room temperature, the degree of which decreases with decreasing temperature. In zeolites ferrierite, L, and Na-Y, the presence of cations in the framework markedly hinders the overall molecular motion. The cations clearly play a role in the observed static disorder of the guest molecule in zeolite L. Important information regarding the location of the n-pentane molecules within silicalite and ferrierite is also obtained.
我们使用傅里叶变换拉曼光谱结合热重分析（TGA），研究了正戊烷在几种代表性沸石中的吸附情况，这些沸石包括硅沸石（MFI）、镁碱沸石（FER）、L型沸石（LTL）以及具有FAU结构的八面沸石，如硅质Y沸石（Si-Y）和钠Y沸石（Na-Y），特别关注客体分子的构象和动态行为。结果表明，骨架拓扑结构主要决定了正戊烷在沸石中的构象。对于具有通道体系的沸石，如硅沸石、镁碱沸石和L型沸石，随着负载量增加，全反式构象体的比例增加，因为该异构体的几何形状更适合通道。当正戊烷吸附在具有大空腔的沸石中，如Si-Y和Na-Y时，全反式（TT）和顺反式（TG）构象体的分布与纯液体相似，这表明骨架中的大超笼对构象平衡的影响最小。然而，客体分子的动力学受到阳离子存在的显著影响。正戊烷吸附在硅质骨架如硅沸石和Si-Y中会导致室温下广泛的分子运动，其程度随温度降低而减小。在镁碱沸石、L型沸石和钠Y沸石中，骨架中阳离子的存在显著阻碍了整体分子运动。阳离子显然在L型沸石中观察到的客体分子静态无序中起作用。还获得了关于正戊烷分子在硅沸石和镁碱沸石中位置的重要信息。
