Identifying the Location of Real Active Sites in ZSM-5 Zeolites for Tetralin Conversion into Light Aromatics

Abstract

Light cycle oil (LCO) can no longer be used as fuel additives for diesel upgrading, and hence, the catalytic cracking of LCO into light aromatics has become an urgent job for its proper utilization. In this work, toward the rational design of catalysts for LCO cracking, various ZSM-5 zeolites were prepared and utilized in tetralin (as a presentative compound of LCO) cracking to determine the essential locations of active sites. Results show that tetralin could hardly penetrate into the micropores constituted by the 10-member-ring channels of ZSM-5 owing to the large molecular size. Instead, this reaction majorly proceeds via the acid sites residing on the external surfaces rather than those in the intracrystal mesopores. Meanwhile, the external acid sites also possess the advantage of rapid diffusion, which significantly decreases the formation of coke and promotes the catalytic lifetime. Consequently, the morphologies of ZSM-5 play a pivotal role in catalytic performances, among which the nanosized ZSM-5 catalyst was the most outstanding candidate, exhibiting a tetralin conversion of 87.0%, benzene/toluene/xylene total yields of 51.3%, and a lifetime of 22 h as a result of the most abundant external acid sites. These findings disclosed the locations of real active sites in ZSM-5 zeolites in the cracking of tetralin and developed methods to enhance the catalytic performances, which are helpful for the rational design of catalysts for tetralin cracking.