Effect of pore size and acidity on the coke formation during ethylbenzene conversion on zeolite catalysts

The present work provides solid-state 13C NMR spectroscopic evidence that the zeolites acidity and the pore size strongly affect the catalytic behavior of ethylbenzene disproportionation and coke formation. The medium-pore zeolite H-ZSM-5 (ca. 0.56 nm) and the large-pore zeolite H-Y (ca. 0.74 nm) used in this study have exclusively Brønsted acid sites, but with different acid strength. Due to the transition state shape selectivity of ethylbenzene disproportionation, ethylbenzene transalkylation on H-Y takes place at low reaction temperature without side-reactions. On H-ZSM-5, dealkylation/realkylation was observed and generation of alkylcarbenium ions resulted in secondary reactions. These alkylcarbenium ions initiate coke formation on zeolite H-ZSM-5 via oligomerization, six-ring closure, and aromatization of alicyclic hydrocarbons. Ethylbenzene disproportionation on large-pore zeolites Y is an attractive reaction due to its low reaction temperature, high selectivity without side-reactions, and low coke formation. Medium-pore zeolite H-ZSM-5 showed higher reactivity for dealkylation of ethylbenzene and protolytic cracking of light alkanes due to its narrow channels and stronger Brønsted acid sites.

The acidity and pore size of zeolites strongly affect their catalytic behavior. In situ MAS NMR studies indicate that the transition state shape selectivity in the ethylbenzene disproportionation and the formation of alkylcarbenium ions are responsible for oligomerization and coke formation.Figure optionsDownload high-quality image (27 K)Download as PowerPoint slide