Here’s a detailed analysis of ZSM-5 zeolite in nitrogen oxides (NOx) reduction, covering its mechanisms, modifications, applications, and challenges:

1. Role of ZSM-5 Zeolite in NOx Reduction

ZSM-5, a microporous aluminosilicate with a MFI framework structure (intersecting 10-membered ring channels), is widely used in Selective Catalytic Reduction (SCR) of NOx due to its:

• High surface area (300–450 m²/g) and tunable acidity (via Si/Al ratio adjustment), enabling efficient adsorption and activation of NOx and reductants (e.g., NH₃, hydrocarbons).
• Unique pore structure, which restricts the diffusion of bulky molecules, favoring the formation of light products (e.g., N₂, H₂O) over coke or secondary pollutants.
• Thermal stability (up to 800°C), making it suitable for high-temperature exhaust streams (e.g., diesel engines, industrial boilers).

2. Mechanisms of NOx Reduction over ZSM-5

A. Standard SCR (NH₃-SCR)

• Reaction Pathway:
  4NO+4NH3+O2→4N2+6H2O
  ZSM-5’s Brønsted acid sites (from Al-O-Si bridges) adsorb NH₃, while Lewis acid sites activate NOx. The confined pore structure enhances the selectivity for N₂ over N₂O (a potent greenhouse gas).
• Performance:
  • Low-temperature activity: Improved by metal doping (e.g., Fe, Cu). For example, Fe-ZSM-5 achieves >90% NOx conversion at 200–400°C.
  • Hydrothermal stability: Superior to amorphous supports (e.g., Al₂O₃), maintaining activity after repeated steam exposure.

B. Hydrocarbon-SCR (HC-SCR)

• Reaction Pathway:
  Using hydrocarbons (e.g., diesel fuel, ethanol) as reductants:
  2NO+C3H6+O2→N2+CO2+H2O
  ZSM-5’s acidity promotes the partial oxidation of hydrocarbons to reactive intermediates (e.g., acrolein), which reduce NOx.
• Performance:
  • Selectivity: Higher than ZSM-11 or beta zeolites due to its optimal pore size (0.55 nm), which limits the formation of byproducts (e.g., N₂O).
  • Coking resistance: Micropores suppress the formation of large coke precursors, extending catalyst lifespan.

• Previous: NAY zeolite for efficient gas separation
• Next: High-Performance Catalytic Cracking with SAPO-34