Methanol-to-Olefin Conversion over Zeolite Catalysts: Active Intermediates and Deactivation

Methanol-to-olefin (MTO) conversion over various zeolites with different topologies, Si/Al molar ratios, and crystallite sizes were investigated to verify the effects of pore shape and size, acidity, and external surface area on the catalytic activity, product selectivity, and deactivation. The IR and electron spin resonance (ESR) study of zeolite catalysts used in MTO also proceeded to deduce the active intermediates formed in their cages or pores. The zeolites with 8 membered-ring (MR) pore entrances such as CHA, ERI, LTA, and UFI commonly exhibited high selectivity to lower olefins due to their small entrances, but the CHA catalyst with the smallest cage maintained its activity longer than other 8MR zeolites. The slow condensation of polymethylbenzene (PolyMB) to polyaromatic hydrocarbons (PAH) on MOR zeolite with a high Si/Al molar ratio due to its low concentration of strong acid sites resulted in a slow deactivation. The extremely small crystallites of H-SAPO-34 and H-ZSM-5 less than 100 nm showed an adverse effect in MTO; while the large crystallites above 1,000 nm also exhibited poor catalytic performance because of their small external surface. The study of IR regarding the adsorbed and occluded materials on zeolites demonstrated the effect of pore shape and size on the active intermediates: the zeolites with larger pores and cages allowed the formation of alkylbenzenes with long alkyl groups which preferred to be condensated to PAH. The well-resolved hyperfine splitting of ESR spectra observed on H-SAPO-34 used in MTO clearly illustrated the presence of hexamethylbenzenium radical cations. The small intersections of phosphorous-modified H-ZSM-5 allowed the formation of tetramethylbenzenium radical cations in MTO. The formation of PolyMB radical cations, their role as active intermediates and the effect of topology, acidity, and crystallite size of zeolites on their deactivation were discussed.