Micro-Imaging of Transient Concentration Profiles of Guest Mixtures in Zeolite Crystals

晶内扩散系数是决定纳米孔材料技术应用效果的关键参数之一，特别是在选择性吸附质量分离和多相催化质量转化方面。混合物中的扩散特别重要，因为在所有这些应用中，传质发生在多组分吸附的条件下。对于平衡测量，我们使用脉冲场梯度(PFG)核磁共振方法，这是一种强大的、成熟的技术。然而，直到最近十年，随着显微成像技术的引入，特别是干涉显微镜和红外显微光谱技术的引入，测量非平衡条件下的晶内扩散才成为可能，从而提供了记录瞬时晶内浓度分布的选择。到目前为止，显微成像方法主要应用于单组分吸附的研究。混合物吸附现象一般只考虑一个整体模式，即被考虑的晶体上的单个组分的浓度的平均值。通过本项目，我们将把注意力集中在选择性测量不同混合物组分的分布和由此记录的轮廓的时间演变上，从而克服这一限制。到目前为止，这种类型的信息无法通过直接观察获得。克服这一限制使我们有可能在实验上确定扩散矩阵的各种元素，在多组分吸附的条件下，这些元素假定扩散系数的位置为标量。

The intracrystalline diffusivity is among the key parameters deciding about the effectiveness of the technological application of nanoporous materials, notably in mass separation by selective adsorption and mass conversion by heterogeneous catalysis. Diffusion in mixtures is of particular relevance since, in all these applications, mass transfer occurs under the conditions of multicomponent adsorption. For equilibrium measurements we dispose, with the pulsed field gradient (PFG) NMR method, of a powerful, well-established technique. The measurement of intracrystalline diffusion under non-equilibrium conditions, however, has become possible in only the last decade with the introduction of the techniques of micro-imaging, notably of interference microscopy and IR micro-spectroscopy, and the thus provided option of recording transient intracrystalline concentration profiles. So far, the methods of micro-imaging were mainly applied for the investigation of single-component adsorption. Phenomena of mixture adsorption were, generally, considered in only an integral mode, namely as the average of the concentrations of the individual components taken over the considered crystal. With the present project we are going to overcome this limitation by directing our attention on the selective measurement of the distribution of the different mixture components and the temporal evolution of the thus recorded profiles. So far, this type of information was inaccessible by direct observation. Overcoming this limitation provides us with the possibility to experimentally determine the various elements of the diffusion matrix which, under the conditions of multicomponent adsorption, assume the position of the diffusion coefficient as a scalar quantity.