Abstract
SSZ-13 is a classic CHA-type microporous zeolite with cubic cage structure and 0.38 nm eight-membered ring pore opening, which has become the core catalyst for diesel vehicle NH₃-SCR denitrification and methanol-to-olefins (MTO) reaction. Hydrothermal stability is the key performance restricting the long-cycle service life of SSZ-13 under high-temperature steam exhaust conditions. Conventional low-silicon SSZ-13 suffers from serious framework dealumination, structural collapse and active metal shedding after hydrothermal aging above 750 °C. High hydrothermal stability SSZ-13 is optimized via high-silicon skeleton regulation, Al-site distribution control, rare earth cation modification and post-synthetic defect repair. The optimized SSZ-13 zeolite can maintain complete crystal framework after 850 °C severe hydrothermal aging, retaining excellent NOx removal efficiency and catalytic activity. This paper concludes structural advantages, anti-hydrothermal failure mechanism, mainstream modification strategies and typical industrial scenarios of high-stability SSZ-13, and compares the performance difference between SSZ-13 and SAPO-34 under high-temperature humid working conditions.
Keywords: High hydrothermal stability; SSZ-13; CHA zeolite; framework dealumination; NH₃-SCR; diesel exhaust purification
1. Introduction
Vehicle exhaust emission standards have been upgraded globally, putting forward stricter requirements for high-temperature water resistance of molecular sieve denitration catalysts. Traditional SAPO-34 chabazite presents poor skeleton stability, easy crystal dissolution and rapid activity attenuation under long-term high-temperature hydrothermal conditions, which cannot adapt to National VI diesel engine exhaust working conditions (600–800 °C, 5%–10% water vapor atmosphere).
As silicon-rich CHA zeolite, SSZ-13 possesses rigid Si-O-Si skeleton, adjustable silicon-aluminum ratio and controllable acid sites. Compared with SAPO-34, original SSZ-13 has intrinsic better hydrothermal resistance. By optimizing synthesis gel proportion and ion modification, high hydrothermal stability SSZ-13 can resist super-high temperature steam erosion above 850 °C, which is the mainstream commercial denitrification zeolite at present. Meanwhile, high-stability SSZ-13 also shows outstanding application potential in MTO reaction, CO₂ selective adsorption and industrial flue gas denitrification.
2. Structural Characteristics of High Hydrothermal Stability SSZ-13
2.1 Basic Crystal Structure
High hydrothermal stability SSZ-13 belongs to CHA topological structure, consisting of stacked double six-membered rings and elliptical cage cavities. Key structural parameters are as follows: eight-membered ring pore size 0.38 nm, inner cage diameter 1.10 nm, three-dimensional interconnected microporous channel; industrial optimized SiO₂/Al₂O₃ ratio ranges from 18 to 30. High silicon framework reduces polar Al-O bonds, decreases water molecule adsorption affinity, and fundamentally improves hydrothermal corrosion resistance.
2.2 Anti-hydrothermal Failure Mechanism
The main deactivation path of common SSZ-13 under hydrothermal condition is framework dealumination: high-temperature steam breaks Si-OH-Al bonds, leading to skeleton defect expansion, cage structure collapse and Cu active site migration. High hydrothermal stability SSZ-13 solves the above problems via two core mechanisms:
(1) Optimized Al pairs distribution: Al atoms disperse uniformly to reduce adjacent Al-rich domains, inhibiting selective steam dealumination;
(2) Low-defect rigid skeleton: fewer hydroxyl defects strengthen Si-O-Al bond energy, improving high-temperature steam corrosion resistance.
3. Modification Technologies for Improving Hydrothermal Stability
Four industrial mature modification methods for high-stability SSZ-13 are summarized below:
(1) Silicon-rich hydrothermal synthesis: Increase raw material silica content, control Si/Al ratio at 20–30, reduce framework aluminum density, weaken skeleton hydrophilicity;
(2) Alkaline earth/rare earth ion modification: Introduce La³⁺, Ce³⁺, Mg²⁺ cations to anchor framework Al sites, suppress Al detachment during high-temperature hydrothermal treatment;
(3) Post-synthetic defect repair: Secondary silicon supplementation repairs lattice defects, reduces terminal hydroxyl groups that easily combine with water vapor;
(4) Gradient ammonium exchange: Reduce residual sodium ions in crystals, avoid Na⁺ induced skeleton fragmentation under humid high-temperature environment.
