Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation |
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Authors: | VB Kazansky VYu Borovkov AI Serikh RA van Santen BG Anderson |
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Affiliation: | (1) Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prospect 47, Moscow, 117334, Russia;(2) Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands |
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Abstract: | A DRIFT study of ZnHZSM-5 zeolites with Si/Al ratios of 15 or 41 and a Zn loading of 0.8 wt% revealed a high thermal stability
of bridging OH groups that was practically the same as in the pure hydrogen forms. It was concluded that the incipient wetness
impregnation of NH4ZSM-5 zeolite with zinc nitrate and the subsequent high-temperature treatment results only in a minor amount of ion exchange.
A considerable part of the modifying zinc forms nanometric ZnO clusters inside the channels of the zeolite. The use of the
low-temperature adsorption of dihydrogen as a probe indicated the appearance, after high-temperature vacuum pretreatment,
of three different Lewis acid sites connected with coordinatively-unsaturated Zn2+ ions. The strongest Lewis sites, with an H–H stretching frequency of adsorbed molecular hydrogen of 3940 cm−1, dissociatively adsorbed hydrogen, methane and propane at both room and elevated temperatures. These sites are represented
either by Zn2+ ions on the walls of the main channels of the zeolite (α sites according to Mole et al.) or by Lewis-base pairs on the surface
of nanometric clusters of zinc oxide.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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Keywords: | ZnHZSM-5 zeolites DRIFT spectra of adsorbed dihydrogen dissociative adsorption nanometric ZnO clusters |
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