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 l oading of 0.8 wt% revealed a high thermal stability of bridging OH groups t hat was practically the same as in the pure hydrogen forms. It was conclude d that the incipient wetness impregnation of NH(4)ZSM-5 zeolite with zinc n itrate and the subsequent high-temperature treatment results only in a mino r amount of ion exchange. A considerable part of the modifying zinc forms n anometric ZnO clusters inside the channels of the zeolite. The use of the l ow-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 L ewis sites, with an H-H stretching frequency of adsorbed molecular hydrogen of 3940 cm(-1), dissociatively adsorbed hydrogen, methane and propane at b oth room and elevated temperatures. These sites are represented either by Z n2+ ions on the walls of the main channels of the zeolite (alpha sites acco rding to Mole et al.) or by Lewis-base pairs on the surface of nanometric c lusters of zinc oxide.