Acid properties of ZSM-20-type zeolite

Abstract

The acid properties of an HZSM-20 Zeolite have been characterized, in comparison to an Hy Zeolite, by IR spectroscopy, temperature-programmed desorption (TPD) of ammonia, time-resolved temperature-programmed FTIR of ammonia desorption and by microcalorimetry. Also, catalytic properties were tested for n-hexane conversion. These studies show that for HZSM-20 there is a shift of the bridging OH band to lower wavenumbers, an increase in the ammounia desorption temperature and a higher initial heat of ammonia chemisorption compared to HY. Thus, HZSM-20 contains medium–strong Brønsted sites, ranging in strength between zeilites HZSM-5 and HY. Obviously, the higher Si: Al ratio of HZSM-20 results in an enhanced number of strong acid sites with no second-neighbour aluminium atoms. This leads to a change in the distribution of Brønsted-acid sites. Superacid Brønsted sites could not be detected. Furthermore, the IR spectra of adsorbed pyridine and ammonia revealed the presence of at least two kinds of Lewis-acid sites. Some Lewis sites of HZSM-20 exhibit a weak acid strength and the strong lewis sites are weaker than those of HY. The different acidic properties of both zeolites are reflected in their catalytic activity and selectivity in n-hexane conversion.