ON THE POSSIBLE NATURE OF SITES RESPONSIBLE FOR THE ENHANCEMENT OF CRACKING ACTIVITY OF HZSM-5 ZEOLITES DEALUMINATED UNDER MILD STEAMING CONDITIONS

Acid sites in HZSM-5 zeolites dealuminated under mild hydrothermal conditions were studied using diffuse reflectance i.r. spectroscopy. It was shown that dealumination of the framework led to a decrease of the concentration of strongly acidic OH groups of the bridged type as well as to the appearance of new types of OH groups with weaker acidic properties. Using adsorption of probe molecules. (CH4, H2), Lewis acidic sites attributed to nonframework aluminium species were found that are capable of polarizing paraffin molecules. The conclusion has been drawn that these sites are responsible for a considerable enhancement of the catalytic activity in n-hexane cracking for HZSM-5 zeolites dealuminated under mild hydrothermal conditions. © 1990.