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

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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.