Faujasite (zeolite X and Y): structure, USY and FCC

Faujasite is one of the zeolites, and its framework type, FAU, is shared by the synthetic zeolites X and Y. It is built from two units: sodalite cages and double six-membered rings (D6R), also called hexagonal prisms. Every sodalite cage is joined through D6Rs to its four nearest sodalite cages, the way each carbon atom in diamond is joined to four others (Ooe et al. 2023). The space left between them forms the supercages, 12–13 Å across (Ooe et al. 2023; Vogt and Weckhuysen 2015).

Neighbouring supercages share twelve-membered rings (Pirngruber et al. 2010), giving a three-dimensional pore system about 7.3 Å wide (Vogt and Weckhuysen 2015), much more open than the 4.1 Å eight-rings of zeolite A (Chen et al. 2025). Each supercage also opens through a six-ring window into a sodalite cage, and exchangeable cations sit in these windows (Pirngruber et al. 2010). The cell is cubic with a = 24.345 Å (Ooe et al. 2023).

FAU can be described as a cubic stacking of layers called faujasite sheets. Stacked hexagonally, the same sheets give the polymorph EMT, and where the cubic stacking reverses at a twin boundary the local structure resembles EMT (Ooe et al. 2023). In zeolite X, mirror-twin stacking faults on (111) planes act as barriers to the diffusion of molecules through the crystals (Feldhoff et al. 2009).

X and Y have the same FAU framework; X differs only by its lower Si/Al ratio (Feldhoff et al. 2009). In one study of the sodium forms, low-silica X (LSX) had Si/Al close to 1, X about 1.25 and Y 2.5–2.8; the commercial X was sold as Molecular Sieve 13X (Liguori et al. 2019).

More aluminium means more framework charge and more exchangeable cations, so LSX and X have a higher cation exchange capacity than Y (Liguori et al. 2019), and X has more and stronger basic sites than Y (Barthomeuf 2005). Zeolite 13X is a commercial high-performance CO₂ adsorbent, used as the benchmark for new ones (Li et al. 2026). The more siliceous Y, in its acid form, is the one used as a cracking catalyst.

Fluid catalytic cracking is one of the main conversion processes in oil refining and produces most of the world’s gasoline; at the start of 2014 it ran in over 300 of 646 refineries (Vogt and Weckhuysen 2015). Zeolite Y, synthetic faujasite invented by Breck at Union Carbide, has been the main cracking component of FCC catalysts since 1964 (Vogt and Weckhuysen 2015).

• Why Y works. Framework aluminium creates Brønsted acid sites, and the supercages leave room for the bimolecular reactions of carbenium-ion cracking. Y is also easy to make, needing no organic structure-directing agent or autoclave (Vogt and Weckhuysen 2015).
• Why it must be stabilised. As made, Y is not very stable to hot steam. In the unit the catalyst cycles between about 500 °C and 760 °C, and steam in the regenerator pulls aluminium out of the framework (dealumination), removing acid sites and damaging the lattice (Vogt and Weckhuysen 2015).
• Ultrastable Y (USY). Controlled steaming and washing/leaching cycles make the so-called ultrastable Y (Vogt and Weckhuysen 2015).
• Rare-earth Y. The first FCC zeolite was Mg-stabilised; rare-earth-stabilised Y followed and is used today. Rare-earth ions are thought to sit in the hexagonal prisms (site S-I), holding framework aluminium in place, with excess rare earth in the supercage (site S-II) (Vogt and Weckhuysen 2015).
• In the catalyst particle. The zeolite sits in a matrix of alumina, silica and clay that pre-cracks large molecules and binds the particle; ZSM-5 is added to raise the propylene yield (Vogt and Weckhuysen 2015).

For a paper, slide or poster, show the two building units, a sodalite cage and a double six-ring, then one supercage with its tetrahedral arrangement of sodalite cages, and finally the cubic cell viewed along <110>, where the 12-ring pores line up. Draw the framework as T–T sticks or as SiO₄/AlO₄ tetrahedra, and mark the cation sites in the hexagonal prisms and supercage when acidity or ion exchange matters. In the LabCanvas figure editor the Library has a zeolite object and an FAU crystal file that draws the framework as a high-resolution SVG; the LabCanvas 3D Studio has an FAU model you can turn and export as an image.