Coke formation during the methanol conversion to olefins in zeolites studied by UV Raman spectroscopy

Coke formation on/in ZSM-5, USY and SAPO-34 zeolites was investigated durin g the methanol conversion to olefins at temperatures from 298 to 773 K usin g ultraviolet (UV) Raman spectroscopy. The fluorescence interference that u sually obscures the Raman spectra of zeolites in the conventional Raman spe ctroscopy, particularly for coked catalysts, can be successfully avoided in the UV Raman spectroscopy. Raman spectra are almost the same for adsorbed methanol on the three zeolites at room temperature. However, the Raman spec tra of the surface species formed at elevated temperatures are quite differ ent for the three zeolites. Coke species formed in/on SAPO-34 are mainly po lyolefinic species, and in/on ZSM-5 are some aromatic species, but polyarom atic or substituted aromatic species are predominant in USY at high tempera tures. Most of the coke species can be removed after a treatment with O-2 a t 773 K, while some small amount of coke species always remains in these ze olites, particularly for USY. The main reason for the different behavior of coke formation in the three zeolites could be attributed to the different pore structures of the zeolites. (C) 2000 Elsevier Science B.V. All rights reserved.