SSZ-13 Structure: CHA Framework

SSZ-13 belongs to the CHA (chabazite) framework type — the same topology as SAPO-34. However, SSZ-13 is an aluminosilicate, not a silicoaluminophosphate, and this chemical difference fundamentally changes its catalytic role.

Pore and Cage Architecture

The CHA structure provides two features essential for SCR: the chabazite cages isolate Cu ions at specific cation positions (primarily the six-membered ring site), preventing migration and clustering during high-temperature exposure; and the 8-MR windows restrict access to small molecules such as NO, NH3, and O2 while excluding larger hydrocarbons that would compete for active sites.

Aluminosilicate Chemistry

Unlike SAPO-34, which has limited ion-exchange capacity, SSZ-13 is a true aluminosilicate. Framework Al sites create negative charges that can be balanced by Cu2+ or Fe3+ ions through aqueous ion exchange. This gives SSZ-13 the high metal loading (2 to 4 wt percent Cu) needed for SCR activity.

Hydrothermal Stability

The high-silica CHA framework (Si/Al 12 to 30 in SCR grades) provides exceptional hydrothermal stability. Cu-SSZ-13 retains CHA crystallinity to 850 C dry and 750 C in 10 percent steam — stability that Cu-ZSM-5 and Cu-Beta cannot approach. The structural basis: Cu ions in chabazite cages lack the mobility to migrate and form inactive CuO clusters.

See SSZ-13 Properties and the SSZ-13 product hub.