Origin of the Low Olefin Production over HZSM-22 and HZSM-23 Zeolites: External Acid Sites and Pore Mouth Catalysis

The methanol-to-olefin (MTO) reaction was studied over two one-dimensional 10-ring zeolites with similar pore structures: ZSM-22 (TON) and ZSM-23 (MTT). It was found that the initial low but detectable production of olefins over both zeolites was catalyzed by external and/or pore mouth acid sites through hydrocarbon pool mechanism. Evidence is listed as follows: The uncalcined HZSM-22 had similar MTO activity as the calcined catalyst. The HZSM-22 zeolite with smaller crystal size had higher MTO activity. Both zeolites treated by HNO3 to selectively leach the external acid sites showed significantly reduced initial MTO activity. Both zeolites coked through catalytic cracking of 1,3,5-triisopropylbenzene showed significantly lower initial MTO activity. This conclusion may also be suitable for other one-dimensional zeolites with pore size below 5.7 Å in the MTO reaction.