On the nature of the sites of dihydrogen molecular and dissociative adsorption in ZnHZSM-5. II. Effects of sulfidation

The nature of zinc ions in ZnHZSM-5 zeolite prepared by incipient wetness impregnation was studied by DRIFT spectroscopy using dihydrogen adsorbed at low temperature as a molecular probe. The results obtained indicate the appearance of low-coordinated zinc ions following dehydroxylation at high temperature. It is concluded that the ions that most strongly perturb adsorbed hydrogen are localized on the surface of nanometric ZnO clusters that are formed in the channels of the zeolite framework upon high temperature pretreatment. This was demonstrated by the inhibition of dihydrogen adsorption by high temperature sulfidation of the zeolite with H2S. The latter presumably converts ZnO particles into ZnS clusters. This was also confirmed by UV diffuse reflectance spectra of the sulfided samples. By contrast, sulfidation did not influence DRIFT spectra of hydroxyl groups in either HZSM-5 or HY zeolites, indicating that substitution of framework oxygen by sulfur does not occur in these materials.