Aluminophosphates

Aluminophosphates are a class of microporous crystalline materials built from aluminum and phosphorus oxo-tetrahedra (AlO4 and PO4) linked by shared oxygen atoms. The resulting frameworks form channels and cages similar to zeolites. In their pure form, aluminophosphates are typically neutral; framework substitution with silicon yields silicoaluminophosphates (SAPOs), introducing acidity and expanding applicability.

Structure and synthesis: The topology depends on synthesis conditions and structure-directing agents used during hydrothermal crystallization. Aluminum and phosphorus occupy tetrahedral sites in a three-dimensional network; pore sizes range from microporous to mesoporous in some variants. SAPOs incorporate silicon to create Brønsted acid sites; metal doping can add Lewis acidity or redox functionality.

Applications: Aluminophosphates and SAPOs serve as catalysts and adsorbents in petrochemical processing. SAPOs, such as SAPO-34, are prominent in methanol-to-olefins (MTO) catalysis, while others are used for isomerization, cracking, and selective adsorption of small hydrocarbons. They offer high thermal stability and adjustable acidity and porosity through composition and synthesis.

Characterization and synthesis considerations: Characterization typically involves X-ray diffraction to determine framework, solid-state NMR and FTIR for local environment and acidity, and gas adsorption for porosity. Synthesis is accomplished via hydrothermal methods using structure-directing agents, with post-synthesis modification (ion exchange, silicon incorporation) to tune properties.