An evaluation by t.d./m.s. of the use of weak bases (ammonia, primary amines and pyridine) as probes for the study of zeolite acid sites

Abstract

The use of weak bases as probe molecules, to study ZSM-5 acid sites, was evaluated by thermogravimetric analysis and thermal desorption/mass spectrometry. Ammonia and pyridine were desorbed intact from the acid sites whereas the primary amines (methylamine, ethylamine, n-propylamine and n-butylamine) decomposed to give alkene and ammonia. Therefore ammonia and pyridine are more appropriate for use in studying acid site strengths. The primary amines sorbed on ZSM-5 react to give dialkylamines; in the case of methylamine, trimethylamine was also formed. The extent of reaction depends upon the amine, conditions of sorption and thermal analysis, and can result in inaccurate determinations of the number of acid sites per unit cell of HZSM-5. The use of anhydrous ammonia resulted in errors due to the overlap of the physisorbed and protonated desorption peaks, whereas the number of acid sites per unit cell determined in ammonium ion exchanged ZSM-5 was consistent with the chemical analysis for aluminium. Results from the sorption of pyridine indicated a smaller number of acid sites, presumably due to steric hindrance.