Microimaging of diffusion and reaction of guest molecules in MFI-type zeolite crystals

通过应用纳米多孔材料进行质量转化和分离而获得的增值产品永远不会比从材料内部到周围环境的运输速率所允许的更快。因此，除了与基础研究相关外，这些材料中的传质速率也是决定其技术性能的关键数字之一。几十年来，这种类型的信息只能从“宏观”测量中推断出来，基于对晶体床或颗粒的吸收和释放速率的测量。因此，任何关于物质传递基本过程的信息都必须基于模型假设，而这些假设往往是错误的。这些不一致的直接证据是通过扩散测量的显微技术的出现，即脉冲场梯度（PFG）NMR和红外显微成像和干涉显微镜，这往往揭示了数量级的差异，从而引发了我们对这种材料中的传质的理解范式转变。虽然微观扩散技术到目前为止主要应用于相当多样化的纳米多孔主客体系统，选择的主要重点是展示这些新技术的潜力相当一般，本项目的目的是提供这些技术的效率的证明，通过展示他们的证据的力量与协同应用MFI型沸石。它是沸石催化中的关键材料，鉴于其高度复杂的孔结构，它是深入研究传质的一个具有挑战性的系统。该项目将在很大程度上受益于这两种技术的互补性。这些是NMR的选择，用于揭示和分析化学反应过程中的反应物，中间体和产物，并记录（通过PFG NMR）其位移的概率分布（包括所有相关组分的短程和长程扩散率）以及显微成像记录瞬态浓度分布的能力，包括测量晶内通量。虽然在显微镜下操作，但通常观察1010个分子，两种技术都提供了高度统计相关性的数据。在传质领域所渴望的新型信息包括晶体内客体通量的测量和多组分吸附下它们的相互干扰的调查，通过介孔和外表面以及复合材料（混合基质膜）上的传输阻力的影响的传输增强的深入研究。在考虑转换现象，客人引起的主机变化（以下这种现象的迹象，在第一显微成像研究），正如主机引起的客人的变化应在我们的研究的重点，在这两种情况下，在寻找前所未有的信息的时空变化在个别MFI晶体。

The gain in value-added products by the application of nanoporous materials for mass conversion and separation can never be faster than allowed by the transportation rate from the material interior to the surroundings. The rate of mass transfer within such materials is therefore, besides its relevance for fundamental research, among the key numbers deciding about their technological performance. Over decades, this type of information could only be deduced from “macroscopic” measurements, based on measurement of uptake and release rates with beds of crystals or pellets. Any message about the elementary processes of mass transfer had therefore to be based on model assumptions, which often were found to be wrong. Direct evidence about these inconsistencies was attained by advent of microscopic techniques of diffusion measurement, namely pulsed field gradient (PFG) NMR and microimaging by IR and interference microscopy, which often revealed discrepancies of orders of magnitude and did thus initiate a paradigm shift in our understanding of mass transfer in such materials. While microscopic diffusion techniques have so far been mainly applied to quite diverse nanoporous host-guest systems, selected with the main focus on demonstrating the potentials of these novel techniques quite in general, the present project aims on providing the proof of efficiency of these techniques by demonstrating the power of their evidence with a concerted application to MFI-type zeolite. It is a key material in zeolite catalysis and – given its highly complex pore structure – a challenging system for the in-depth study of mass transfer. The project will largely benefit from the complementarity of the two techniques. These are the options of NMR for revealing and analyzing reactants, intermediates and products during chemical reactions and for recording (via PFG NMR) the probability distribution of their displacement (including the short- and long-range diffusivities of all involved components) and the ability of microimaging to record transient concentration profiles which includes the measurement of intracrystalline fluxes. Though operating microscopically, with observing typically 1010 molecules both techniques provide data of high statistical relevance. The novel type of information aspired in the field of mass transfer includes the measurement of intracrystalline guest fluxes and the investigation of their mutual interference under multi-component adsorption, the in-depth study of transport enhancement by mesopores and the impact of transport resistances on the external surface as well as in composite materials (mixed matrix membranes). On considering conversion phenomena, guest-induced host variation (following indications of this phenomenon in first microimaging studies) just as host-induced guest variation shall be in the focus of our studies, in both cases in search for unprecedented information about the spatio-temporal variation over the individual MFI crystal.