An eye on the inside of zeolite crystals in the act: Studying Brønsted acidity with in-situ micro-spectroscopy

A combination of in-situ optical, fluorescence and synchrotron-based infrared microscopy has proven to be a very valuable tool to study zeolitic crystals during catalytic reaction. Using this approach, large coffin-shaped H-ZSM-5 crystals have been studied in a space- and time-resolved manner during the acid-catalyzed oligomerization of styrene. During the liquid-phase oligomerization, carbocationic intermediates are formed that act as reporter molecules for catalytic activity. These molecules exhibit strong absorption, fluorescence as well as infrared activity. Optical microscopy allows revealing a space- and time-resolved product formation, demonstrating a non-uniform catalytic behaviour. Moreover, polarized light measurements shed more light into to product molecule alignment, showing that the product molecules are aligned within the straight pores of the zeolite. Complementary confocal fluorescence measurements visualized the three-dimensional product distribution within one zeolite crystal. By using synchrotron-based infrared microscopy the chemical nature of these reaction products as well as their alignment within the zeolite pores could be unravelled. The combined optical and fluorescence microscopic approach was employed to study the effect of mesoporosity, introduced via post-synthesis desilication, on the catalytic activity. Apart from the visualization of the complex intergrowth structure of boat-shaped ZSM-5 crystals, it has been demonstrated that the introduction of intracrystalline mesoporosity facilitates the transport of styrene molecules inside the zeolite volume and limits the product formation to dimeric carbocation intermediates. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, using various styrene compounds, differing in their reactivity, a non-homogenously distributed Brønsted acidity over the crystals' volume has been demonstrated. More reactive styrene compounds react on both strong and weak Bronsted acid sites, giving rise to a uniform fluorescence pattern, whereas weaker acid sites are not capable of protonating the less reactive styrene compounds, therefore leading to fluorescence signals merely at locations of strong Bronsted acid sites. Keywords: ZSM-5, in-situ micro-spectroscopy, styrene oligomerization.