Coke formation in zeolites studied by a new technique: Ultraviolet resonance Raman spectroscopy

A common problem with zeolite catalysts used in hydrocarbon conversion is deactivation by coke deposition. Using UV Raman spectroscopy, coke formation and oxidation in ZSM-5 and USY were evaluated. The coke species were generated through the reaction of propylene in the zeolites at 297K-777K. Three groups of strong Raman bands were noted at 1360-1400, 1580-1640, and 2900-3100/cm. Various carbonaceous species, e.g., olefinic, polyolefinic, aromatic, polyaromatic, and pregraphite species were discriminated based on the position and relative intensities of these Raman bands. At lower temperatures, olefinic and aromatic species were dominant for both zeolites, and these species desorbed or partially converted into polyaromatic and pregraphite species at high temperatures. The formed coke species at high temperatures were very different for the two zeolites in that polyolefinic and aromatic species were predominant in ZSM-6, while polyaromatic and pregraphite species were the major species in USY.