A comparative and multitechnical approach to the acid character of templated and non-templated ZSM-5 zeolites

Abstract

ZSM-5 zeolites with varying SiO 2/Al 2O 3 ratios (28, 50, 75, 100, 150) were synthesized with and without the aid of a template. The molar oxide compositions of the reactants were xSiO 2:Al 2O 3: yNa 2O:1500 H 2O with x=28–150 and y=6.7. The samples were characterized by XRD, IR, SEM, and BET surface area measurements. The zeolites synthesized with template were found to be pure. Non-templated zeolites with SiO 2/Al 2O 3 greater than 50, however, contained α-quartz along with ZSM-5. The acidity was measured by stepwise temperature-programmed desorption (STPD) of ammonia and calorimetric heats of adsorption of ammonia. The number of acid sites decreased almost linearly with the SiO 2/Al 2O 3 ratio of the zeolites. It was found that the acidity of zeolites measured both by STPD of ammonia and calorimetry was in fairly good agreement. Aniline alkylation and cumene cracking reactions were carried out over the two series of zeolites to evaluate the catalytic properties involving acidity. Cumene-cracking was correlated with the number of strong acid sites in the zeolite as determined by microcalorimetry. In both series of zeolites, certainly Brönsted and Lewis acid sites were present. The possible presence of more Lewis sites in non-templated zeolites than in templated zeolites might be responsible for high aniline alkylation and low cumene-cracking activity.