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 HNO 3 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.