One-dimensional zeolites as hydrocarbon traps

Abstract

Several zeolites with varying pore dimensionality have been investigated as hydrocarbon (HC) traps by studying the temperature programmed desorption of propane and toluene mixtures. In one-dimensional (1-D) zeolites, the diffusive motion of propane is blocked by the more-strongly adsorbed toluene thus raising the desorption temperature of propane above its single-component desorption temperature. In the 1-D zeolite EUO, propane is trapped within the pores by the toluene and is only able to desorb to the gas phase once toluene begins to desorb. This leads to an increase in the propane desorption temperature of 35 °C over the single-component value. In the 1-D zeolite mordenite, propane desorbs in the presence of toluene at an intermediate temperature that is 100 °C larger than the single component case. Toluene then desorbs at an even higher temperature. It is suggested that propane undergoes an activated process of passing toluene within the 1-D pores that requires the intermediate temperatures. This HC trapping is not observed in the three-dimensional pore zeolites ZSM-5 or Y. Propane in the binary mixture with toluene in these zeolites desorbs at the same temperature as in the single component cases, suggesting that propane is able to diffuse around the more-strongly adsorbed toluene due to the connectivity of the pores. Therefore, it is the one-dimensionality of the zeolites that allows the trapping of propane to higher temperatures in the presence of the more-strongly adsorbing toluene. This phenomenon may find application in reducing cold start HC emissions from automotive exhaust.