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

Abstract

The Brønsted acid-catalyzed oligomerization of 4-fluorostyrene has been studied on a series of H-ZSM-5 zeolite powders, steamed under different conditions, with a combination of UV-Vis micro-spectroscopy and Scanning Transmission X-ray Microscopy (STXM). UV-Vis microspectroscopy and STXM have been used to monitor the relative formation of cyclic and linear dimeric carbocations as a function of the steaming post-treatment (i.e., parent vs. steaming at 600, 700 and 800 1C). It was found that the UV-Vis band intensity ratios of linear to cyclic dimeric species increase from 0.79 (parent H-ZSM-5) over 1.41 (H-ZSM-5 steamed at 600 1C) and 1.88 (H-ZSM-5 steamed at 700 1C) to 2.33 (H-ZSM-5 steamed at 800 1C). STXM confirms this trend in reaction product selectivity, as the relative intensities of the transitions attributed to the presence of the cyclic dimer in the carbon K-edge spectra decrease with increasing severity of the steaming post-treatment. Furthermore, STXM reveals spatial heterogeneities in reaction product formation within the H-ZSM-5 zeolite powders at the nanoscale. More specifically, a shrinking carbon core-shell distribution was detected within the zeolite aggregates, in which the relative amount of cyclic dimeric species is higher in the core relative to the shell of the zeolite aggregate and the relative amount of cyclic dimeric species in the zeolite core gradually decreases with increasing severity of the steaming post-treatment. These differences are rationalized in terms of spatial differences in Brønsted acidity within H-ZSM-5 zeolite powders as well as by changes in the formation process of linear and dimeric carbocations within H-ZSM-5 micro-and mesopores.

3D Raman Spectroscopy of Large Zeolite ZSM‐5 Crystals

“…[4] Therefore, steaming alters the overall activity of zeolites. [5][6][7][8][9][10][11][12][13][14] Af ine balance is required between the numbero fB rønsted acid sites (i.e.,t he amount of active sites) and the mesopore volume formed through steaming( i.e.,i mproving the accessibility towards the active sites).…”

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

“…have reported the UV/Vis absorption band at 490 nm originating from cyclic dimeric carbocations. The same species are found to be formed at the near‐surface acid sites and crystalline defects induced by steaming 49, 50…”

In Situ Microscopy and Spectroscopy Applied to Surfaces at Work

“…It should be noted that the use of resonant and non‐resonant photon energies allows chemically specific spatio‐temporal distribution of various different heteroatom adsorbates to be monitored. By this way, for example, catalytic site heterogeneity for zeolites could be visualized in relation to various acid (or base) site types (Brønsted vs. Lewis) or strengths (weak, medium, or strong) . As a future challenge, molecular recognition for gas separation membranes like metal–organic frameworks (MOFs) can be studied in terms of metal–adsorbate bonding using STXM in situ.…”