Pentasil-Type Zeolites: Radical Formation, Activity in the Olefin Oligomerization and Aromatization, Processes of Coke Deposition

The formation of radicals upon adsorption of different olefins and aromatic compounds on H-ZSM-5 has been studied. The sorption of C 3 H 6 on H-ZSM-5 is accompanied by the formation of oligomeric radicals even at −78°C. Adsorption of i-C 4 H 8 and 2,4,4-trimethylpentene-1 is accompanied by the appearance of identical ESR-signals from oligomeric particles with equivalent protons. In the case of the radical formation from branched olefins the outer surface of H-ZSM-5 crystals plays a noticeable role. The study of benzene and p-xylene sorption on H-ZSM-5, pre-treated by 2,4,6-trimethylpyrydine which blocked selectively the outer surface, shows that the majority of cation-radicals formed is located inside the channels and the process of their formation is not hindered by the diffusion. It is shown that catalytic low-temperature oligomerization of C 3 H 6 and i-C 4 H 8 takes place on the outer surface of H-ZSM-5 crystals whereas the high-temperature aromatic formation takes place inside the zeolite channels. In contrast to HM, the coke formed upon the olefin transformation on H-ZSM-5 is located on the outer surface of zeolite crystals.