SAPO-34 Structure: CHA Framework

SAPO-34 belongs to the CHA (chabazite) framework type. Its small-pore 8-membered ring windows and internal chabazite cages create the confined environment responsible for SAPO-34’s unmatched MTO selectivity.

Pore and Cage Architecture

The CHA structure consists of double six-ring (D6R) units arranged to form chabazite cages. Each cage connects to six neighbouring cages through 8-MR windows. Molecules must pass through these narrow windows to enter or exit a cage.

Silicoaluminophosphate Composition

Unlike aluminosilicate zeolites, SAPO-34 is a silicoaluminophosphate — silicon substitutes for phosphorus in the AlPO4-34 framework. Si content (5 to 15 mol percent in the framework) creates the charge imbalance that generates acid sites. This is fundamentally different from SSZ-13, which is an aluminosilicate with framework Al creating charge-balancing sites.

Key Structural Features

• 8-MR windows (3.8 A) — only small molecules can enter or exit
• Chabazite cages trap reaction intermediates, creating shape selectivity
• Rhombohedral crystal system
• Mild acidity from Si substitution in AlPO framework

See SAPO-34 vs SSZ-13 for CHA chemistry comparison and the SAPO-34 product hub.