Study on intra-zeolite diffusional motion by fluorescence correlation spectroscopy

分子筛中的扩散在沸石作为吸附剂和催化剂的大部分应用中起着重要的作用。一组微孔晶体中的扩散简明图示如下：被吸附的物种除了通过相互连接的窗口跳到相邻的笼子之外，还在笼子内的几个吸附位置之间迁移，偶尔从一个沸石晶体移动到另一个沸石晶体。各种测量吸附物种晶内扩散系数的方法包括核磁共振自旋-晶格驰豫、脉冲场梯度(PFG)核磁共振、准弹性中子散射(QENS)和三重态-三重态能量转移技术。并利用分子动力学和蒙特卡罗模拟方法对实验结果进行了理论分析。尽管有这些努力，但在精确测量比苯大的分子的扩散系数方面，它们仍然是一个困难。我们的想法是应用一种相对较新的技术，f…更多的荧光相关光谱在测量溶液和生物体系(膜和细胞)中的扩散系数方面已经非常成功，但到目前为止还没有应用于沸石等非均相体系。我们搭建了一个以Ar离子激光器为激发光源、以硅雪崩光电二极管为探测器、以相关板为分析系统的显微镜实验装置。我们确认，我们的设置对于溶液体系工作正常，但是当我们应用于沸石体系时，出现了困难。由于在沸石中扩散太慢，我们不能测量适当的关联，因此我们被迫修改检测系统。因此，我们将单分子光谱学应用于单个沸石晶体内的扩散问题。我们使用了表观照明技术，并结合了冷却的CCD摄像机的图像采集。我们跟踪了沸石晶体上单分子荧光图像的时间进程，并分析了20-30个分子的轨迹。这样，我们就可以得到吸附物种的扩散系数。我们可以建立各种分子在沸石中扩散系数的测量方法，但由于时间限制，我们无法获得许多分子的数据。因此，我们对结果没有确切的看法，这将是我们未来的工作。较少

Diffusion in zeolites plays an important role in most of the applications of zeolites as adsorbents and catalysts. The concise picture of the diffusion in a group of these microporous crystals is drawn as in the following : adsorbed species migrate among a few adsorption sites within a cage besides a jump to neighboring cages through interconnecting windows, and occasionally travel from one zeolite crystal to another. Various methods have been developed for the measurement of intracrysalline diffusivity of adsorbed species including the NMR spin-lattice relaxation, pulsed field-gradient (PFG) NMR, quasielastic neutron scattering (QENS) and triplet-triplet energy transfer techniques. The results were also discussed on the theoretical basis utilizing molecular dynamics and Monte Carlo simulation methods. In spite of these efforts, their still remains a difficulty in the precise measurement of diffusivity of molecules larger than benzene. Our idea is to apply a relatively new technique, f … More luorescence correlation spectroscopy which has been very successful in the measurement of diffusion coefficients in solution and biological systems (membranes and cells) but so far not been applied to heterogeneous systems such as zeolites. We have constructed a setup based on microscope with an Ar-ion laser as a excitation light source, a Si-avalanche photodiode as a detector and a correlation board as an analyzing system. We confirmed that our setup works properly for solution systems, however, difficulty arises when we applied to the zeolite systems. Because of too slow diffusion in zeolites we could not measure proper correlation, thus we are forced to modify the detection system. We, therefore, applied a single molecule spectroscopy to the problem of diffusion within a single zeolite crystal. We used epi-illumination technique coupled with the image capture by cooled CCD camera. We followed the time course of the fluorescence image of single molecules on a zeolite crystals and analyzed the trajectories of 20-30 molecules. By doing so, we could obtain the diffusion coefficient of adsorbed species. We could establish the methodology for the measurements of diffusivity within zeolites for various molecules, we could not obtain data for many molecules because of time limitation. As a result, we have no solid view for the outcome and this will be our future works. Less