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Stability of Zeolites in Aqueous Phase Reactions

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journal contribution
posted on 14.08.2017, 00:00 by Sebastian Prodinger, Hui Shi, Sebastian Eckstein, Jian Zhi Hu, Mariefel V. Olarte, Donald M. Camaioni, Miroslaw A. Derewinski, Johannes A. Lercher
Strategies to understand and mitigate the corrosive interactions of zeolites in aqueous phase under reaction conditions have been explored using zeolite BEA as an example. The states of Si and Al atoms after chemical modification and during gradual degradation were followed by cross-polarization enhanced 29Si MAS NMR and 27Al MAS NMR as well as IR spectroscopy. The key to stabilizing a zeolite for aqueous phase catalysis is to reduce the pore concentration of water in the presence of reacting substrates. The concentration of tetrahedral aluminum, which is charge balanced by hydrated hydronium ions, is the most important parameter determining the concentration of water in the zeolite pores. Lower intraporous water concentrations, largely independent of ubiquitous defects, led to longer zeolite lifetimes during cyclohexanol dehydration. The concentration of intraporous water was directly related to the rate of hydrolysis of Si4+ from the zeolite lattice and its removal from the crystal. Dissolution of Si4+ led eventually to a loss of confinement of the catalytically active hydronium ions and decreased the catalytic activity. At low Brønsted acid site concentrations, water bound to lattice defects begins to exert a measurable influence on the stability under reaction conditions.

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