Visualizing the Crystal Structure and Locating the Catalytic Activity of Micro‐ and Mesoporous ZSM‐5 Zeolite Crystals by Using In Situ Optical and Fluorescence Microscopy

Abstract

A combination of optical and fluorescence microscopy was used to study the morphology of micro‐ and mesoporous H‐ZSM‐5 zeolite crystals (17×4×4 μm) and to evaluate, in a spatially resolved manner, the effect of mesoporosity, introduced via desilication, on catalytic performance. For this purpose, the oligomerization of various styrene molecules was used as a model reaction, in which the carbocation intermediates formed in the zeolite pores act as reporter molecules. In situ confocal fluorescence measurements after the template removal process showed that the crystals generally consist of three different subunits that have pyramidal boundaries with each other. Examination of these crystals during styrene oligomerization revealed differences in the catalytic activity between the purely microporous and the combined micro‐ and mesoporous crystals. The introduction of intracrystalline mesoporosity limits the formation to dimeric carbocation intermediates and facilitates the transport of styrene molecules inside the zeolite volume. This leads to a more uniform coloration and fluorescence pattern of the crystals. Moreover, the oligomerization of various styrene compounds, which differ in their reactivity, provides a good way of estimating the Brønsted acid strength in a spatially resolved manner, showing a nonhomogeneously distributed Brønsted acidity over the volume of the crystals. More detailed information on the structure of the ZSM‐5 crystals was revealed for mesoporous crystals during the oligomerization of 4‐methoxystyrene. This reaction induced an “explosion” of the crystal leading to the formation of a complex system with at least eight different subunits. Finally, polarized‐light microscopy was used to unravel the pore geometry in these individual building blocks. The observed differences in catalytic behavior between micro‐ and mesoporous ZSM‐5 crystals are strengthened by the microspectroscopic techniques employed, which show that upon desilication the crystal morphology is affected, the product distribution is changed towards less conjugated carbocation intermediates, and that a gradient in Brønsted acid strength appears to be present.

Styrene oligomerization as a molecular probe reaction for Brønsted acidity at the nanoscale

“…More specifically, a band and a shoulder located at 610 nm and 640 nm, assigned to more conjugated linear carbocations. 19,21,28 When the oligomerization reaction is performed on H-ZSM-5-700, significant differences are observed in the UV-Vis absorption spectra, as shown in Fig. 1(c), where the intensities of the absorption bands at 610 nm and 640 nm drastically increase.…”

Unified Internal Architecture and Surface Barriers for Molecular Diffusion of Microporous Crystalline Aluminophosphates

“…[16][17][18][19][20][21][22] Interestingly, the arrangement of these subunits, known as the internal architecture of a crystal, was found to be dependent not on the crystal framework type, but on the external morphology. For example, in the case of large MFI crystals (i.e., ZSM-5 and silicalite molecular sieves) it was found that four related but distinct intergrowth structures exist depending on the length/width (L/W) aspect ratios of the crystals.…”

In Situ Synchrotron‐Based IR Microspectroscopy To Study Catalytic Reactions in Zeolite Crystals

“…[1][2][3][4][5][6][7][8] These spectroscopic methods have proven to be very successful in elucidating valuable structure-function relationships for acid-base catalytic reactions. In the work of Roeffaers et al fluorescence microscopy has been applied to demonstrate the crystal-face-dependent catalysis on layered double hydroxide (LDH) materials.…”