Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

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 NH(4)ZSM-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 (alpha sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxid

Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

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 NH(4)ZSM-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 (alpha sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxid

On the role of zinc oxide nanometric clusters in preparation of ZnNaY zeolite by ion exchange

Dehydration of Zn2+ exchanged NaY zeolite was studied by means of DRIFT spectra of adsorbed dihydrogen and carbon monoxide as molecular probes. Unlike alkaline earth forms of faujasites dehydration of ZnY zeolite at moderately high temperatures results in the formation of bridging protons and nanometric ZnO or mixed hydroxylated ZnO microclusters instead of bridging hydroxy groups and MeOH+ cations. After subsequent dehydration at above 573 K the zinc oxide clusters interact with acidic protons resulting in Zn2+ ions localized at SII sites. These sites adsorb hydrogen in its molecular form. Heterolytic dissociative adsorption does not take place even at elevated temperatures