SSZ-13 (CHA): structure, Cu-SSZ-13 and uses

SSZ-13 is an aluminosilicate member of the zeolites with the chabazite framework, IZA code CHA. The framework is built by stacking double six-membered rings (D6R) in an ABC sequence; between the prisms open large CHA cages, and the cages connect through 8-ring windows about 3.8 × 3.8 Å (Shan et al. 2021). That makes SSZ-13 a small-pore zeolite: big cages, narrow doors.

The framework is simple. It has only one crystallographically distinct T site (Shan et al. 2021), which is why Cu-SSZ-13 is used as the model system for working out how Cu-exchanged zeolites function (Shan et al. 2021). Al in place of Si gives the framework its exchange sites and Brønsted acid sites, and the Si/Al ratio of SSZ-13 is typically 10–54 (Li J. et al. 2022; Shan et al. 2021).

Diesel engines run lean, with oxygen in excess, so the three-way catalyst of a petrol car cannot remove their NOₓ. Instead ammonia reduces NOₓ to N₂ over a catalyst downstream of the oxidation catalyst and particulate filter: selective catalytic reduction with ammonia (NH₃-SCR) (Shan et al. 2021). Cu-SSZ-13 has become the commercial catalyst for this step thanks to its wide working temperature window and its hydrothermal stability (Li P. et al. 2022), and it gives very high NOₓ conversion and N₂ selectivity (Beale et al. 2015). The duty is harsh: 200–600 °C in normal driving, below 200 °C at cold start, and above 650 °C when the upstream filter is regenerated (Shan et al. 2021).

The small pores explain much of its success (Shan et al. 2021):

• Stability. The 3.8 Å windows trap the Al(OH)₃ released by hydrolysis, about 5 Å in kinetic diameter, so it re-inserts rather than leaving, which slows dealumination during hydrothermal ageing.
• Poison resistance. Long-chain hydrocarbons in exhaust are too big to enter the 8-rings.
• Mobile copper. Under reaction conditions Cu ions solvated by ammonia move within about 9 Å of their anchoring Al and pass through the 8-ring windows to pair up in the CHA cages.

Those mobile pairs are part of the mechanism: Paolucci and co-workers showed that mobilised Cu ions travel through zeolite windows and form transient ion pairs that carry out an O₂-mediated redox step essential to SCR, with electrostatic tethering to framework Al limiting how far each ion can roam (Paolucci et al. 2017).

1. Hydrothermal synthesis with TMAda⁺. SSZ-13 is conventionally crystallised with N,N,N-trimethyl-1-adamantammonium hydroxide (TMAdaOH) as the organic structure-directing agent, at 160 °C for 90–120 h (Shan et al. 2021).
2. Cheaper or faster routes. Because TMAdaOH is costly, SSZ-13 has also been made with N,N-dimethylethylcyclohexylammonium bromide (solvent-free) or choline chloride, by seed-assisted transformation of FAU zeolite in about a day, and in only 1.5 h at 240 °C under solvent-free conditions (Shan et al. 2021). Dry-gel conversion raises the Si/Al ratio up to 182 (Li J. et al. 2022).
3. Copper loading. Cu-SSZ-13 is usually made by ion exchange of the SSZ-13 zeolite, or in one pot with a Cu–tetraethylenepentamine complex as the template (Shan et al. 2021).

The CHA framework is not only aluminosilicate. SAPO-34 is a silicoaluminophosphate with the same CHA structure (Yang et al. 2021), and Cu-SAPO-34 is isostructural with Cu-SSZ-13 as an SCR catalyst (Beale et al. 2015). SAPO-34 is the commercial catalyst for methanol to olefins (MTO): more than 20 MTO units in China, with an olefin capacity above 10 Mt a year, use it, and lower-olefin yields above 85% are reached (Yang et al. 2021). More generally, the pore and cavity size of the zeolite or zeotype catalyst steers methanol conversion toward olefins or gasoline (Olsbye et al. 2012). High-silica SSZ-13 itself (Si/Al above 30) has also performed well in MTO (Li J. et al. 2022).

For a paper, slide or poster, draw SSZ-13 as layers of double six-rings stacked ABC, with one CHA cage outlined and an 8-ring window marked 3.8 Å; for an SCR figure, put a Cu ion beside an Al site in the cage and NH₃ and NO molecules at the window. The LabCanvas figure editor Library has a zeolite object that draws a ring framework (choose 6-rings, or 4- and 8-rings) with Si/Al and O atoms and cations in the pores, which you can recolour and label. The LabCanvas 3D Studio has zeolite models you can rotate and export as high-resolution images.