Combined IR and catalytic studies of the role of Lewis acid sites in creating acid sites of enhanced catalytic activity in steamed HZSM-5

摘要

The properties of Lewis acid sites in mildly and severely steamed HZSM-5 were followed by low temperature IR studies of CO adsorption. We performed also combined IR and catalytic studies in which butene―1 isomerisation was followed in steamed HZSM-5 in which Lewis sites were blocked by CO (CO adsorption was controlled by IR). The information on relative acid strength of Lewis acid sites was obtained in IR experiments of ammonia desorption followed by CO adsorption. All the results suggest that the less electroacceptor Lewis sites, characterized by IR band of adsorbed CO 2190 cm~(-1), are responsible for the formation of strongly acidic and highly active Bronsted sites. Their amount increases upon mild steaming and decreases upon severe steaming parallel to the catalytic activity of zeolite. Our IR results agree with ~(27)A1 MAS NMR results and with EPR data of NO adsorption.