Differences in the Location of Guest Molecules within Zeolite Pores As Revealed by Multilaser Excitation Confocal Fluorescence Microscopy: Which Molecule Is Where?

Abstract

A detailed and systematic polarized confocal fluorescence microscopy investigation is presented on three batches of large coffin-shaped ZSM-5 crystals (i.e., parent, steamed at 500 °C, and steamed at 700 °C). In total, six laser lines of different wavelength in the visible region are employed on two crystal positions and three orientations with respect to the polarization plane of the excitation laser light. A fluorescent probe molecule is generated inside the zeolite pores, originating from the acid-catalyzed oligomerization of 4-fluorostyrene. A thorough analysis of the polarization plane of emitting fluorescent light reveals insight into the orientation of the fluorescent probe molecule restricted by the highly ordered zeolite channel framework, thereby visualizing pore accessibility and clearly distinguishing the occupation of straight and sinusoidal channels by the probe molecule. Spectral features are, furthermore, observed to tell apart molecules situated in one or the other pore. Special focus was given on the rim and tip regions of the zeolite ZSM-5 crystals. On the basis of the confocal approach of the investigation, the aforementioned features are evaluated in three dimensions, while the degradation of the zeolite framework upon postsynthesis steam treatment could be visualized by occupation of the sinusoidal pores.

Single Molecule Nanospectroscopy Visualizes Proton-Transfer Processes within a Zeolite Crystal

“…19−21 Hence, selective staining of zeolite acid sites 22 is becoming a popular strategy to study Brønsted reactivity of zeolite particles where fluorescent species are generated after an acid catalyzed conversion of molecules, such as furfuryl alcohol, 23−26 thiophene, 27 and styrene. 28−30 …”

X‐ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal: Crystallography and Molecular Spectroscopy in One

“…The latter two bands appeared to be red‐shifted (up to 20 nm) as compared to the fluorescence microscopy spectra (600 and 650 nm) of the same species (Supporting Information, Figure S3). These two emission bands have been previously attributed to linear dimeric and trimeric species that are confined along the straight pores of ZSM‐5 20, 32, 47. The higher‐energy XEOF band at 530 nm is assigned to cyclic dimeric species.…”

“…Upon the protonation of 4‐methoxystyrene on zeolite ZSM‐5, oligomeric carbocations are formed, revealing the location of accessible Brønsted acid sites 20, 32. If excited by X‐rays, these molecules undergo photoemission in the optical region (UV/Vis), a phenomenon that is generally known as X‐ray excited optical luminescence,33, 34, 35 here referred to as XEOF.…”

X‐ray Excited Optical Fluorescence and Diffraction Imaging of Reactivity and Crystallinity in a Zeolite Crystal: Crystallography and Molecular Spectroscopy in One

“…The emission maximum was about 615 nm for the highly reactive domains (spectrum 2, Figure 3 a), and shifted towards higher energies (600 nm) for the domains with lower XEOF intensity (spectrum 1, Figure 3 a). We attribute the observed shift to intermolecular interactions of the closely packed oligomeric carbocations 32, 33…”

“…For this study we used large zeolite ZSM‐5 crystals18, 30, 31, 32 and a Brønsted acid‐catalyzed probe reaction based on the oligomerization of 4‐methoxystyrene. Upon the protonation of 4‐methoxystyrene on zeolite ZSM‐5, oligomeric carbocations are formed, revealing the location of accessible Brønsted acid sites 20, 32.…”