Combined thermometric analysis of a series of highly dealuminated USY zeolites

Abstract

A transient reactor model was used to consolidate and quantify the data of ammonia adsorption microcalorimetry and single ammonia TPD runs, using several members of the commercial CBV series of dealuminated HY zeolites. The model accounts for the competitive re-adsorption of molecules desorbing from multiple sites, using the microcalorimetric data to constrain the desorption energy parameters to physically meaningful values. All TPD profiles were well represented by models consisting of only two or three distinct site types, even though the differential heat plots imply a continuous distribution of sites in the 160-80 kJ/mol range for all catalysts. Increasing dealumination severity is evidenced by a decrease in total site density for sites 100 kJ/mol. While there was no evidence of an enhancement of adsorption strength within the series studied, the range of site strengths available remained the same. There is correspondence between the density of adsites = 100 kJ/mol and the framework Al3+ concentration, although the ratio of these is less than one, even for Si/Al80. The inherent limits in using ammonia adsorption as an acidity probe are briefly mentioned.