In this report we reveal the presence of significant variations in Brønsted catalytic activity within and between individual H-ZSM-5 zeolite crystals. Fluorescence microscopy in combination with a fluorogenic probe was used to resolve the catalytic activity at the nanoscale. The observed variations in catalytic activity could be directly linked to structural parameters and crystal morphology observed in scanning electron microscopy and by specifically staining crystal defects. The obtained results are directly compared with ensemble averaged information from techniques such as pyridine IR spectroscopy and nitrogen physisorption, typically used to characterize acid zeolites. The inter- and intra-particle heterogeneities resolved by the employed fluorescence approach remain unaddressed by bulk characterization. Our experimental results relate the heterogeneous catalytic activity to variation in both the Si/Al ratio and mesoporosity induced during the zeolite synthesis.
Understanding the role of the hierarchical pore architecture of SSZ-13 zeolites on catalytic performance in the Methanol-to-Olefins (MTO) reaction is crucial for guide the design of better catalysts. We investigated the influence of the space velocity on the performance of a microporous SSZ-13 zeolite, and several hierarchically structured SSZ-13 zeolites. Single catalytic turnovers, as recorded with fluorescence microscopy verified that the hierarchical zeolites contain pores larger than the 0.38 nm apertures native to SSZ-13 zeolite. The amount of fluorescent events correlated well with the additional pore volume available due to hierarchical structuring of the zeolite. Positron Emission Tomography (PET) using 11C-labelled methanol was used to map the 2D spatial distribution of the deposits formed during the MTO reaction in the catalyst bed. PET imaging demonstrates that hierarchical structuring not only improves the utilization of the available microporous cages of SSZ-13 but also that the aromatic hydrocarbon pool species are involved in more turnovers before they condense into larger multi-ring structures that deactivate the catalyst.