A crystalline AlPO₄-5 intermediate: designed synthesis, structure, and phase transformation.

Area of Science:

• Materials Science
• Crystallography
• Chemical Engineering

Background:

• AlPO4-5 is a key aluminophosphate molecular sieve extensively studied for its crystallization mechanisms.
• Understanding intermediates is crucial for controlling the synthesis of AlPO4-5.
• 4-dimethylaminopyridine (DMAP) is a common organic structure-directing agent (OSDA) in AlPO4-5 synthesis.

Purpose of the Study:

• To rationally prepare a crystalline AlPO4-5 intermediate.
• To investigate the role of DMAP in the transformation of the intermediate to the final AlPO4-5 framework.
• To gain insights into the crystallization mechanism of aluminophosphate molecular sieves.

Main Methods:

• Synthesis of a novel crystalline AlPO4-5 intermediate ((C7H11N2)2(H3O)(Al3P4O16)·(H2O)2.5) by adjusting orthophosphoric acid and DMAP concentrations.
• Characterization of the intermediate and the final AlPO4-5 product.
• Transformation studies using thermal treatment and steam-assisted conversion (SAC).
• Analysis of DMAP's supramolecular assembly templating (SAT) effects via fluorescence spectroscopy.

Main Results:

• A crystalline AlPO4-5 intermediate with alternating organic DMAP and inorganic [Al3P4O16]3- layers was successfully synthesized.
• The intermediate transforms into the AlPO4-5 framework upon heating or via SAC.
• π-π stacking interactions of DMAP and hydrogen bonds between layers are critical for the phase transformation.
• The SAT effects of DMAP were confirmed, highlighting its templating role.

Conclusions:

• The study successfully prepared a crystalline AlPO4-5 intermediate, offering a new pathway to study molecular sieve crystallization.
• The findings elucidate the importance of DMAP's supramolecular interactions in directing the formation of the AlPO4-5 structure.
• This crystalline intermediate provides a valuable tool for advancing the understanding of aluminophosphate molecular sieve crystallization mechanisms.