Amine adsorption in H-ZSM-5

Abstract We have examined the adsorption of simple amines, including methylamine, ethylamine, n-propylamine, isopropylamine, t-butylamine and pyridine on H-ZSM-5 using temperature-programmed desorption (TPD), thermogravimetric analysis (TGA) and transmission infrared spectroscopy. For each adsorbate, a well-defined adsorption state corresponding to a coverage of one molecule per aluminum atom could be easily identified in TPD-TGA. With the exceptions of methylamine and pyridine, the 1:1 complexes formed by each amine reacted completely to desorb as ammonia and alkene products. Amine molecules adsorbed in excess of one per aluminum atom desorbed unreacted at much lower temperatures. Approximately half of the 1:1 species formed by methylamine also reacted to form a complex mixture of products, while the 1:1 species formed by pyridine desorbed unreacted above 700 K. Infrared spectroscopy was used to demonstrate that the stoichiometric complexes for each amine were protonated and associated with the hydroxyls at the aluminum sites in the zeolite. These results imply that the acid sites in H-ZSM-5 are simple Bronsted sites and that TPD-TGA measurements of amine adsorption can be used to quantify the acid site density.