Abstract
Sodium-type Y zeolite (NaY) is a typical low-silica faujasite (FAU) topological microporous aluminosilicate crystal, which serves as the most critical precursor zeolite for petroleum refining catalysts. NaY possesses an integrated three-dimensional large-cage pore system assembled by sodalite β-cages and double six-ring connectors. With a standard 12-membered-ring pore window of 0.74 nm and internal supercage diameter of 1.2 nm, NaY exhibits ultrahigh specific surface area, excellent cation exchange capacity and strong hydrophilicity. The intrinsic framework Si/Al molar ratio of industrial NaY ranges from 5.0 to 5.5, with abundant exchangeable Na⁺ cations balancing skeleton negative charge. This paper elaborates the basic unit construction, complete pore structure, physicochemical characteristics, modification evolution and mainstream petrochemical applications of NaY zeolite. The conclusion proves that NaY is an irreplaceable raw material for preparing HY, USY and REY modified zeolites, widely used in fluid catalytic cracking, hydrocracking and gas adsorption separation industries.
Keywords: NaY zeolite; FAU topology; crystal structure; supercage; ion exchange; petroleum catalysis
1. Introduction
Microporous faujasite zeolites are divided into X-type and Y-type according to different silicon-aluminum ratios. NaY zeolite refers to sodium-form Y-faujasite with low Si/Al ratio, which has been industrialized and mass-produced earliest among large-pore zeolite materials. Different from medium-pore MFI zeolite and small-pore CHA zeolite, NaY owns interconnected oversized cage structure, which allows free diffusion and reaction of polycyclic aromatic hydrocarbons and heavy hydrocarbon macromolecules.
Pure NaY has almost no catalytic acidity, but its skeleton Na⁺ can be easily replaced by H⁺, rare earth ions and metal ions via liquid ion exchange, so as to prepare acidic modified zeolite catalysts. At present, more than 80% of commercial FCC catalyst active components are derived from modified NaY zeolite, making NaY the most fundamental zeolite material in modern oil refining engineering.
2. Complete Crystal Structure of NaY Zeolite
2.1 Basic Skeleton Building Units
The whole NaY crystal belongs to cubic Fd-3m space group, and the framework is composed of three secondary structural units:
(1) Tetrahedral basic unit: SiO₄ and AlO₄ tetrahedrons, the minimum composition unit of FAU skeleton;
(2) β-cage (sodalite cage): Closed spherical cage constructed by 6 four-membered rings and 8 six-membered rings, inner cavity size 0.66 nm;
(3) D6R double six-membered ring: Special connecting unit linking adjacent β-cages stably.
2.2 Assembly Mode and Pore Structure
A complete NaY unit cell consists of 8 independent β-cages arranged in diamond lattice. Every two β-cages are connected through one double six-membered ring, forming a large open α-supercage inside the crystal.
Core pore structure parameters of industrial NaY:
① Pore window: 12-membered ring aperture, 0.74 nm;
② Supercage inner diameter: 1.2 nm;
③ Channel feature: Omnidirectional three-dimensional interconnected pore channels, no dead-end pores;
④ Unit cell formula: Na₅₆[(AlO₂)₅₆(SiO₂)₁₃₆]·264H₂O.
2.3 Cation Distribution Feature
Due to low Si/Al ratio, NaY carries high negative framework charge. Exchangeable Na⁺ cations are distributed in three fixed sites: inside β-cages, on D6R sites and supercage surface. These movable Na⁺ endow NaY with ultrahigh ion exchange performance, which is the core structural basis for subsequent acidification and metal modification.
3. Fundamental Physicochemical Properties
(1) Silicon-aluminum characteristic: Industrial commercial NaY, SiO₂/Al₂O₃ = 5.0–5.5, low-silicon hydrophilic skeleton;
(2) Sodium content: Raw Na₂O mass fraction 12.0%–13.0%, neutral alkalinity, no Brønsted acid sites;
(3) Textural property: BET specific surface area ≥660 m²/g, micropore volume ≥0.35 cm³/g, high adsorption capacity;
(4) Thermal stability: Structural stable below 600 ℃; skeleton collapse occurs above 700 ℃ under hydrothermal condition;
(5) Surface property: Strong hydrophilicity, high saturated water adsorption capacity ≥32 wt%, suitable for water-containing gas purification.
