Chabazite

Abstract: Selective catalytic reduction (SCR) of NOx with ammonia using metal-exchanged molecular sieves with a chabazite structure has recently been commercialized on diesel vehicles. One of the commercialized catalysts, i.e., Cu-SSZ-13, has received much attention for both practical and fundamental studies. For the latter, the particularly well-defined structure of this zeolite is allowing long-standing issues of the catalytically active site for SCR in metal-exchanged zeolites to be addressed. In this review, recent progress is summarized with a focus on two areas. First, the technical significance of Cu-SSZ-13 as compared to other Cu ion-exchanged zeolites (e.g., Cu-ZSM-5 and Cu-beta) is highlighted. Specifically, the much enhanced hydrothermal stability for Cu-SSZ-13 compared to other zeolite catalysts is addressed via performance measurements and catalyst characterization using several techniques. The enhanced stability of Cu-SSZ-13 is rationalized in terms of the unique small pore structure of this zeolite catalyst. Second, the fundamentals of the catalytically active center; i.e., the chemical nature and locations within the SSZ-13 framework are presented with an emphasis on understanding structure–function relationships. For the SCR reaction, traditional kinetic studies are complicated by intra-crystalline diffusion limitations. However, a major side reaction, nonselective ammonia oxidation by oxygen, does not suffer from mass-transfer limitations at relatively low temperatures due to significantly lower reaction rates. This allows structure–function relationships that are rather well understood in terms of Cu ion locations and redox properties. Finally, some aspects of the SCR reaction mechanism are addressed on the basis of in situ spectroscopic studies.

Abstract: Synthetic methods are reported to prepare chabazite (CHA) zeolites (SSZ-13) at fixed elemental composition but with different arrangements of their framework Al atoms (Al–O(−Si–O)x–Al) between paired (x = 1, 2) and isolated (x ≥ 3) configurations. SSZ-13 zeolites (Si/Al = 15–30) that crystallized in hydroxide media and in the presence of only N,N,N-trimethyl-1-admantylammonium cations (TMAda+) contained only isolated framework Al, evident in their inability to exchange divalent Cu2+ or Co2+ cations that titrate paired Al sites. Addition of Na+ to the zeolite crystallization medium, with total cationic charge ((Na+ + TMAda+)/Al) and other synthesis parameters held constant, crystallized SSZ-13 zeolites at a fixed Si/Al ratio but with paired Al densities that increased linearly with the Na+ content incorporated into the crystalline solids. Crystallization media containing equimolar amounts of Na+ and TMAda+, but varying Al content, crystallized SSZ-13 at different compositions (Si/Al = 5, 15, 25) with the p...