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A series of fluorescent probe molecules based on the commercially available trans-4-(4-(diethylamino)styryl)-N-methylpyridinium iodide (DAMPI) scaffold has been developed. The dynamic radii of these DAMPI-type probes covered a range of 5.8 to 10.1 Å and could be changed by varying the alkyl substituents on the amine donor group, with limited effect on the electronic properties. These probe molecules allow for the direct evaluation of the molecular accessibility into confined spaces, more specifically the micropore architecture of zeolite materials. Evaluation of industrially relevant zeolite materials with 8- (CHA), 10- (MFI) and 12-membered ring pores (FAU) showed that steric bulk influences the rate of adsorption, the amount of probe molecule taken up by the zeolite as well as the interaction of the probe molecule with the zeolite material. Furthermore, a positive linear correlation is found between the pore-probe size difference and total probe uptake by the zeolite. The absorption spectra of each probe molecule within the zeolites show that this DAMPI-type compound is chemically bound to the zeolite's acid sites. The new approach shows the general principle of determining size-accessibility relationships in microporous solids with a series of fluorescent probes of systematically tunable size.

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Optical absorption and confocal fluorescence micro-spectroscopy were applied to investigate Brønsted acidity in millimetre-sized extrudates of Na(H)-ZSM-5 and SiO2 with varying ZSM-5 content. Partially (residual Na present) and fully proton-exchanged extrudates were employed, using thiophene oligomerization as a probe reaction. Time-resolved in situ optical absorption spectra and time dependent DFT calculations revealed several initial reaction pathways during the oligomerization reaction. In particular, it was found that protonated thiophene monomers reacted by either oligomerization (via reaction with un-reacted thiophene monomers) or ring-opening, depending on the Brønsted acid site density in each sample. Moreover, fully-exchanged extrudates not only have significantly higher reactivity than partially-exchanged samples, but they also favour the formation of ring-opening products, that are not formed on the partially-exchanged samples. Confocal fluorescence microscopy was employed to visualise non-invasively in 3D, the heterogeneity and homogeneity of thiophene oligomers on partially- and fully-exchanged extrudates, respectively. Furthermore, it was observed that extrudates with high binder content produce a higher relative amount of conjugated species, related with a higher quantity of available monomer in the binder, which is able to react further with intermediates adsorbed on active sites. Moreover, these conjugated species appear to form near the external surface of ZSM-5 crystals/agglomerates.

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