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

Abstract

An ultraviolet (UV) Raman spectrometer was recently set up with the goal of improving Raman spectroscopy for catalysis and surface science studies. Using UV Raman spectroscopy, coke formation and oxidation in ZSM-5 and USY have been investigated. The coke species were generated by the reaction of propylene in the zeolites at temperatures from 297 to 773 K. The Raman bands of coke species can be clearly resolved since the fluorescence interference is successfully avoided in UV Raman spectra. There are three groups of strong Raman bands observed at 1360–1400, 1580–1640, and 2900–3100 cm−1. Various carbonaceous species like olefinic, polyolefinic, aromatic, polyaromatic and pregraphite species can be discriminated based on the positions and relative intensities of these Raman bands. At lower temperatures, olefinic and aromatic species are dominant for both zeolites, and these species desorb or partly convert into polyaromatic and pregraphite species at high temperatures. The coke species formed at high temperatures are quite different for the two zeolites: polyolefinic and aromatic species are predominant in ZSM-5, and polyaromatic and pregraphite species are the major species in USY.