On the synthesis of a hierarchically-structured ZSM-5 zeolite and the effect of its physicochemical properties with Cu impregnation on cold-start hydrocarbon trap performance

A hierarchically structured zeolite (self-pillared pentasil; SPP) comprised of MFI nanosheets or lamellae has been synthesized in various Si/Al ratios and mesoporosities. It turns out that a simple removal of ethanol in a synthesis sol resulted in increased mesoporosity, while the additional reduction of water further increased mesoporosity. In addition, we could synthesize the SPP particle with the actual Si/Al ratio as low as ∼23 with a modest mesoporosity. With these hierarchically structured SPP particles, we further conducted copper impregnation on them in order to use as a hydrocarbon (HC) trap. The resulting Cu-impregnated SPPs could not only adsorb HCs in the exit gas stream including water vapor, but also serve as an active oxidizer of HCs. Specifically, Cu-impregnated SPP with an actual Si/Al ratio of ∼22 and medium mesoporosity exhibited very high performance in cold-start trap tests; desirably adsorbing propene and toluene even in the presence of 10 vol% steam, holding them up to higher temperatures (90 °C for propene and 190 °C for toluene), and furthermore, oxidizing the hydrocarbons. The preferred adsorption can be attributed to the larger amount of exchanged Cu²⁺ ions in SPP particles with a lower Si/Al ratio, while the additional oxidation was due to the CuO particles dispersed on the SPP surface. However, the hydrothermal stability test revealed that the zeolite structure in the Cu-impregnated SPPs was collapsed and transformed into another undesired phase, thus losing the above-mentioned adsorption ability. Nevertheless, the corresponding oxidation performance was well maintained, indicating the robust, active role of the CuO particles.