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Issue 26, 2010
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Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study

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Abstract

A series of H-ZSM-5 crystallites with different framework Si/Al ratios was studied by analyzing the kinetics and reaction mechanism of the oligomerization of 4-fluorostyrene as molecular probe reaction for Brønsted acidity. The formation of carbocationic species was followed by UV-Vis spectroscopy. Three carbocationic products were observed, namely a cyclic dimer, a conjugated linear dimer and a larger, more conjugated carbocation. Rate constants for the formation of all three products show a maximum at a Si/Al ratio of 25. Oligomerization of 4-fluorostyrene within the larger supercages of zeolite H-Y leads solely to cyclic dimers. The experimental observations were rationalized by DFT calculations, which show that the selectivity of the styrene oligomerization is controlled by the steric properties of the intrazeolite micropore voids. Two reaction pathways were considered for the formation of the conjugated linear carbocation. The conventional mechanism involves a hydride transfer between two dimeric hydrocarbons (HCs) in the zeolite pores. We propose an alternative monomolecular path, in which the hydride transfer takes place between a hydrogen atom of a dimeric HC and a zeolitic proton, yielding a conjugated carbocation and molecular H2. Computed free energies indicate that the preference for a particular reaction mechanism is determined by the local shape of the zeolite micropores.

Graphical abstract: Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study

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Publication details

The article was received on 04 Feb 2010, accepted on 12 Apr 2010 and first published on 14 May 2010


Article type: Paper
DOI: 10.1039/C002442B
Citation: Phys. Chem. Chem. Phys., 2010,12, 7032-7040

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    Styrene oligomerization as a molecular probe reaction for zeolite acidity: a UV-Vis spectroscopy and DFT study

    I. L. C. Buurmans, E. A. Pidko, J. M. de Groot, E. Stavitski, R. A. van Santen and B. M. Weckhuysen, Phys. Chem. Chem. Phys., 2010, 12, 7032
    DOI: 10.1039/C002442B

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