Adsorption Studies on Cu-ZSM-5: Characterization of the Unique Properties of Ion-Exchanged Cu

Abstract

Abstract Simultaneous temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements of isopropylamine have been used to characterize a series of Cu(H)-ZSM-5. At the Bronsted acid sites in H-ZSM-5, isopropylamine decomposes to propene and ammonia between 575 and 650 K. The addition of Cu decreases the concentration of these Bronsted acid sites and leads to a new propene-ammonia decomposition feature between 650 and 800 K in TPD-TGA. This high-temperature feature is not observed when Cu is incorporated into a silicalite sample. implying that it is associated with Cu at the Al sites. Furthermore, the sum of Bronsted acid sites and high-temperature Cu sites is approximately constant and close to the Al content for H(Cu)-ZSM-5 series. Propene also interacts strongly with the Cu that is present at the Al sites in a stoichiometry close to 1/Cu at room temperature. Infrared measurements indicate that the Cu sites are neither Bronsted acid nor Lewis acid sites. All of the samples examined, including ion-exchanged silicalite, contained significant quantities of other Cu which do not appear to be associated with Al sites. Finally, ion exchange of Cu into a Na-ZSM-5 sample is shown to result in Bronsted acid sites, showing that protonic sites are easily incorporated into the zeolite during ion exchange, even when this is not done intentionally. The implications of these results for the characterization of Cu-exchanged zeolites are discussed.