Coking, aging, and regeneration of zeolites: VII. Electron microscopy and EELS studies of external coke deposits on USHY, H-OFF, and H-ZSM-5 zeolites

Abstract External coke deposits produced by n -heptane cracking on USHY, H-OFF, and H-ZSM-5 zeolites have been located by a combination of transmission electron microscopic observations and electron energy loss spectroscopic (EELS) measurements performed with a field-emission gun scanning transmission electron microscope equipped with an electron spectrometer. The local structure of coke is determined from the fine structure at the high-energy side of the C K EELS peak, in comparison with reference compounds such as graphite, coronene, and pentacene. In H-ZSM-5 and H-OFF the coke forms an external envelope around the zeolite crystal and stands as an empty mold after zeolite extraction. Its structure is similar to that of coronene (polyaromatic-pregraphitic). In USHY part of the coke is in the form of 1-nm-large carbon filaments protruding from the zeolite mesopores and micropores. Its structure is more like that of pentacene (linear polyaromatic).