Adsorption of Toluene on Alkali Metal Ion-Exchanged ZSM-5 and β-Zeolites under Humid Conditions

Alkali metal ion-exchanged ZSM-5 and beta-zeolites were tested as hydrocarbon traps for reducing cold-start emissions. The hydrocarbon trap performances were investigated by temperature-programmed desorption employing toluene as a probe molecule both in the presence and absence of water. Toluene adsorption was significantly inhibited in the presence of water. The extent of this inhibitory effect was strongly dependent on the nature of the alkali metal cation. The Cs cation was the most effective in suppressing the inhibition. The suppression of the inhibition is likely due to an enhancement of the interaction between the toluene molecules and zeolite-framework oxygens bearing high negative charge. Having investigated hydrocarbon trap performances of alkali metal ion-exchanged Y and niordenite zeolites as well as ZSM-5 and beta, it was concluded that both the type of alkali metal cation employed and the Si/Al ratio of zeolite were the major factors that affected the hydrocarbon trap performance rather than the nature of framework structure of zeolite. On the other hand, the durability was influenced by the type of framework structure used. The stability decreased in the order: Cs/ZSM-5 > Cs/beta > Cs/Y approximate to Cs/mordenite. The adsorption capacity of Cs/ZSM-5 was preserved even after thermal treatment at 900 degrees C.