Spatial heterogeneities within an individual catalyst particle during reaction as revealed by in-situ micro-spectroscopy

Abstract

Heterogeneous catalysts are solids, which are of fundamental importance in (petro-) chemical, pharmaceutical and environmental industries. The majority (> 85%) of all chemicals and transportation fuels have come into contact with at least one catalyst material during their manufacturing process. In order to design new and better catalysts, it is essential to obtain a thorough understanding of their working principles. Spectroscopic techniques are commonly employed as a key tool for the characterization of catalytic materials. Although interesting information can be obtained by conventional spectroscopic methods from examination of data recorded under static conditions, no insight into the chemical processes occurring under dynamic conditions is revealed. In addition, most spectroscopic techniques average information over the catalyst particle and therefore lack space-resolved information. As heterogeneous catalysts are found not to be spatially homogeneous, an increasing demand exists for the development of space-and time-resolved spectroscopic techniques. The research described in this PhD thesis explores the potential of in-situ micro-spectroscopic techniques to unravel spatial heterogeneities of individual catalytic solids with micrometer resolution. For this purpose, micrometer-sized zeolite crystals with the MFI-topology were used as model catalysts. The first part of the thesis deals with the morphology dependent intergrowth structure of MFI-type zeolite crystals. A more general consensus regarding the internal architecture is provided, revealing the presence of four intergrowth structure types and concurrent internal and external diffusion barriers. The catalytic activity of coffin- and boat-shaped zeolite ZSM-5 crystals, having the MFI-topology, was addressed by means of a combination of UV-Vis, confocal fluorescence, synchrotron-based IR, coherent anti-Stokes Raman scattering and X-ray absorption micro-spectroscopy during the acid-catalyzed conversion of styrene and thiophene derivatives. Multi-dimensional measurements to examine both reactant and reaction product distributions revealed spatial heterogeneities in catalytic activity and product formation, which were explained by the presence of a classic 90 rotational intergrowth structure. Generally, most reactant and more conjugated/elongated reaction products were found in the center of the crystals as compared to the crystal edges. A mechanistic model illustrating a more extensive pore blockage of the straight pore openings at the crystal edges was put forward. Elongated reaction products were found to be entrapped and aligned within the straight pores of the zeolite ZSM-5 crystals as revealed by polarization measurements. Finally, the effect of mesoporosity to enhance molecular diffusion on the catalytic activity was examined. Favored selectivity towards dimeric carbocation intermediates was found as well as boosted diffusion of reactant molecules inside the zeolite crystals. The latter was shown to lead to a more evenly distributed coloration of the individual ZSM-5 crystals. Polarization experiments substantiated that mesopores do not to participate to a large extent in the catalytic reaction, but serve mainly as highways to the active sites. The widely applicable micro-spectroscopic techniques to study individual catalyst particles described in this PhD thesis have proven to be a step forward to suffice the increasing demand of obtaining a fundamental understanding of catalytic reactions taking place down to the micron-scale, or even at the molecular level.