Zeolite structure effects on Cu active center, SCR performance and stability of Cu-zeolite catalysts

In this study, comparative studies over nine NH3-SCR Cu-zeolites with different topologies were conducted. A clear trend was observed that the NH3-SCR performance was gradually improved with the zeolite-framework structures changing from straight-channel (ZSM-5, Beta, Y) to cage-type (SSZ-13, SSZ-16, SSZ-17), bridged by hybrid-structures (OFF-ERI series: Offretite, ZSM-34, UZM-12). Characterizations of the catalysts with XRD, XRF, XPS, UV-vis, NH3-TPD, and H-2-TPR confirmed, under identical catalyst synthesis conditions, that the cage-type samples (Cu-SSZ-13, Cu-SSZ-16, and Cu-SSZ-17) possessed higher concentrations of strong acid sites and isolated Cu cations with respect to the straight-channel samples (Cu-ZSM-5, Cu-Beta, and Cu-Y). The cage-type catalysts showed superior SCR performance as compared to the straight-channel and hybrid types. This study corroborated the consensus from recent literature that a small cage aperture (similar to 3.8 angstrom) is critical to prevent zeolite dealumination. Moreover, a clear correlation was found between zeolite acid strength and the nature of exchanged Cu species. Cage type zeolites with stronger acidity led to stabilization of isolated Cu ion monomers while dimeric Cu ions form in straight-channel zeolites with lower acidity. Catalysts with dominant monomeric active sites display more desirable SCR activity and selectivity to N-2.