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

晶内扩散率是决定纳米多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的质量分离和多相催化的质量转化中。在混合物中的扩散是特别相关的，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，在过去的十年中，随着微成像技术的引入，特别是干涉显微镜和红外微光谱学的引入，在非平衡条件下测量晶内扩散成为可能，并因此提供了记录瞬态晶内浓度谱的选择。目前，微成像技术主要应用于单组分吸附的研究。混合吸附现象一般只以积分模式来考虑，即作为所考虑的晶体上各组分浓度的平均值。在目前的项目中，我们将克服这一限制，将注意力集中在对不同混合成分分布的选择性测量以及由此记录的剖面的时间演变上。到目前为止，这类信息是无法通过直接观察获得的。克服这一限制使我们有可能通过实验确定扩散矩阵的各个元素，在多组分吸附条件下，扩散系数作为标量的位置。

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.