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Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H‐ZSM‐5 Crystals by Time‐ and Space‐Resolved Optical and Fluorescence Microspectroscopy
Author(s) -
Stavitski Eli,
Kox Marianne H. F.,
Weckhuysen Bert M.
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700568
Subject(s) - chemistry , steric effects , reactivity (psychology) , photochemistry , selectivity , fluorescence , catalysis , reaction rate , molecule , styrene , chemical reaction , zeolite , organic chemistry , polymer , copolymer , optics , medicine , physics , alternative medicine , pathology
A combination of in‐situ optical and fluorescence microspectroscopy has been employed to investigate the oligomerization of styrene derivatives occurring in the micropores of coffin‐shaped H‐ZSM‐5 zeolite crystals in a space‐ and time‐resolved manner. The carbocationic intermediates in this reaction act as reporter molecules for catalytic activity, since they exhibit strong optical absorption and fluorescence. In this way, reactant selectivity and restricted transition‐state selectivity for 14 substituted styrene molecules can be visualized and quantified. Based on a thorough analysis of the time‐ and space‐resolved UV/Vis spectra, it has been revealed that two main parameters affect the reaction rates, namely, the carbocation stabilization effect and the diffusion hindrance. The stabilization effect was tested by comparison of the reaction rates for 4‐methoxystyrene versus 4‐methylstyrene and in the series 4‐bromo‐, 4‐chloro and 4‐fluorostyrene; in both cases less electronegative substituents were found to accelerate the reaction. As to the steric effect, bulkier chemical groups bring down the reaction rate, as evident from the observation that 4‐methoxystyrene is more reactive than 4‐ethoxystyrene due to differences in their diffusivity, while heavily substituted styrenes, such as 3,4‐dichlorostyrene and 2,3,4,5,6‐pentafluorostyrene, cannot enter the zeolite pore system and therefore do not display any reactivity. Furthermore, β‐methoxystyrene and trans ‐β‐methylstyrene show limited reactivity as well as restricted reaction‐product formation due to steric constraints imposed by the H‐ZSM‐5 channel system. Finally, polarized‐light optical microspectroscopy and fluorescence microscopy demonstrate that dimeric styrene compounds are predominantly formed and aligned within the straight channels at the edges of the crystals, whereas a large fraction of trimeric carbocations along with dimeric compounds are present in the straight channels of the main body of the H‐ZSM‐5 crystals. Our results reinforce the observation of a non‐uniform catalytic behavior within zeolite crystals, with specific parts of the zeolite grains being less accessible and reactive towards reactant molecules. The prospects and potential of this combined in‐situ approach for studying large zeolite crystals in the act will be discussed.