FAU topology is constructed from sodalite cages linked through hexagonal prisms, forming large supercages. HY zeolite possesses 3D interconnected 12-membered-ring pore channels with an aperture of approximately 0.74 nm, and the inner diameter of supercages reaches around 1.3 nm. The unit cell composition can be expressed as \(\ce{H_{x}Al_{x}Si_{192-x}O_{384}·yH_{2}O}\).
Commercial HY is prepared by ion exchange of NaY zeolite with ammonium salt followed by calcination. During ammonium exchange, \(\ce{Na+}\) cations in the NaY framework are replaced by \(\ce{NH4+}\); high-temperature calcination decomposes \(\ce{NH4+}\) and generates protonated H-form HY zeolite. Compared with medium-pore ZSM series zeolites (MFI, TON, FER), the large supercage structure of HY allows bulky heavy oil molecules to enter the internal pore space for catalytic conversion. However, conventional HY suffers from limited hydrothermal stability under high-temperature steam conditions.
Framework Al atoms are the origin of Brønsted acid sites in HY zeolite. The Si/Al ratio directly governs acid density and acid strength. Lower Si/Al HY contains high concentrations of Brønsted acid sites, which provide strong acid centers for C–C bond cleavage. Extra-framework aluminum species, formed during partial dealumination, contribute to Lewis acid sites.
The acid property of HY can be tuned via post-treatment. Steam dealumination removes partial framework Al, which is the core route to produce USY zeolite. As dealumination proceeds, the number of strong Brønsted acid sites decreases, hydrothermal stability improves, and framework shrinkage occurs. Excessively strong acid sites on unmodified HY tend to cause rapid coke deposition during heavy hydrocarbon cracking, shortening catalyst lifetime.
HY zeolite is the precursor of USY, the primary active component of FCC catalysts. Under FCC reaction conditions, large hydrocarbon molecules from vacuum gas oil or heavy feedstocks diffuse into HY supercages and undergo cracking to form gasoline, diesel and light olefins. The large pore structure is essential for processing bulky polycyclic hydrocarbon feedstocks that cannot access medium-pore zeolites such as ZSM‑5.
HY-based bifunctional catalysts are applied in hydrocracking processes. The acid sites of HY catalyze hydrocarbon cracking and isomerization, while supported noble metals perform hydrogenation–dehydrogenation functions. This combination converts heavy distillates into high-quality middle distillates and lube base oils.
HY zeolite is also used for aromatic disproportionation, alkylation, and removal of organic sulfur compounds. Its large pore structure is suitable for reactions involving bulky organic molecules. Nevertheless, raw HY exhibits poor hydrothermal stability and severe coking tendency, so modified HY or USY substitutes are commonly adopted in industrial units.
