Ultraviolet Raman spectroscopy characterization of coke formation in zeolites

Ultraviolet (UV) Raman spectroscopy has been used to characterize coke formation in ZSM-5 and USY zeolites under propene at temperatures from 300K to 773K. The strong fluorescence background always present with normal Raman spectra is completely avoided in UV Raman spectra. Three groups of UV Raman bands near similar to 1390, similar to 1600 and similar to 3000 cm(-1) regions were detected for the two zeolites, and these bands varied significantly at different stages of coke formation. At room temperature, adsorbed propene was formed in the two zeolites and showed similar spectra. At elevated temperatures, the coke formation behavior in the two zeolites is quite different. For example, at 773K the coke species in ZSM-5 are mainly polyolefinic and aromatic species, but polyaromatic and pregraphite species are predominant in USY. The major portion of coke species formed in ZSM-5 can be removed even by He purging at 773K while the coke species in USY are very stable and can only be removed in O-2 flow at temperatures above 773K. The difference in coke formation in ZSM-5 and USY is likely due to the different pore structure and acidity of the two zeolites.