Surface acidic properties of a HMCM-22 zeolite: Collidine poisoning and hydrocarbon adsorption studies

The total and external surface acidic properties of a HMCM-22 (SiO2/Al2O3 = 33) zeolite have been studied by temperature-programmed ammonia desorption (TPAD) and temperature-programmed collidine desorption (TPCD), respectively. Ammonia TPD results indicate that the Bronsted acid sites in HMCM-22(33) have a relatively uniform acid strength, and most of them are strong. Collidine TPD shows that the Bronsted acid sites on the external surface accessible to collidine accounts for 12% of the total Bronsted acid sites. Strong Bronsted acid sites are present on the zeolite surface as indicated by the collidine desorption at high temperature. The two distinct desorption regions in the low- and high-temperature ranges shown in the collidine TPD curve are ascribed to collidine physisorption and chemisorption, respectively. Chemisorbed collidine on the zeolite surface does not influence the sorption rate of 3-methylpentane, indicating that it does not block 10-ring pore mouth openings. This means that collidine can selectively poison the Bronsted acid sites on the MCM-22 surface without influencing the access to intracrystalline acid sites. It is also found that collidine poisoning on the MCM-22 surface does not influence ethylbenzene sorption rate, indicating that the very low activity of collidine-poisoned HMCM-22 zeolite, in the benzene alkylation with ethylene reaction, is not caused by zeolite pore mouth blocking. This supports the external surface reaction mechanism proposed by Cheng et al.