Ag/ZSM-5 traps for C2H4 and C7H8 adsorption under cold-start conditions

Abstract

A series of Ag ion-exchanged ZSM-5 (Si/Al = 11.5) zeolites with different Ag loadings (0.5, 0.8, 1.6, 3.0, 4.6 wt%) were investigated for their ability to adsorb and desorb C2H4 and C7H8 under simulated diesel exhaust conditions. Ion-exchanging ZSM-5 zeolites with Ag (Ag(0.8)/ZSM-5) led to an increase in the adsorption capacities of C2H4 and C7H8 compared to HZSM-5 in the absence of H2O. The C2H4 and C7H8 adsorption capacities of Ag(0.8)/ZSM-5 slightly decreased in the presence of H2O, indicating that H2O competes with both C2H4 and C7H8 for adsorption sites. Minor competitive adsorption between C2H4 and C7H8 was observed. Density functional theory calculations suggested that C2H4 is preferentially adsorbed in the sinusoidal channel of ZSM-5, while C2H4 and C7H8 are competing at the straight channels and the intersection of the sinusoidal and straight channels. With an increase in Ag loading from 0.5 to 4.6 wt% both C2H4 and C7H8 adsorption capacities increased from 38 to 163 and from 127 to 304 μmol/g, respectively. Temperature programmed desorption experiments indicated that an increase in Ag loading can lead to an increase in C2H4 and C7H8 desorption temperatures. During desorption the adsorbed C7H8 reacted to form CO, CH2O and C6H6, while C2H4 desorbed as is. Reductive pretreatments at 200 and 600 °C over Ag(4.6)/ZSM-5 resulted to a 33 and 58% decrease in C2H4 adsorption capacity, respectively, compared to its oxidized form. The C2H4 adsorption capacity can be mostly recovered after oxidation treatments at 600 °C, indicating that Ag+ reduction is reversible in ZSM-5 zeolites. Reductive pretreatments did not alter the C7H8 adsorption capacity.