New insights into the di-n-propylamine (DPA) molecule as an organic structural directing agent (OSDA) in the crystallization of AlPO4-11 molecular sieve

Abstract

Advanced solid-state NMR techniques, including PT-J-HMQC ²⁷Al/³¹P double-resonance NMR, 2D ³¹P and ¹H DQ-SQ MAS NMR experiments, have been used to study the structure-directing effect in the crystallization of the molecular sieve AlPO₄-11 synthesized by a solvent-free method. Our studies clearly demonstrate the mechanism of crystallization of the molecular sieve AlPO₄-11 for the first time as follows: (i) the organic template of di-n-propylamine (DPA) molecules can effectively induce transformation from the 4- and/or 6-MR chains to a 2D-layered structure through H-bond interactions; (ii) the formation of a twin molecule (two-DPA) drives the transformation from a 2D-layered structure to 3D crystals. Theoretical calculations further confirm that the formation of the twin DPA molecule is thermodynamically stable in the channels of AlPO₄-11. Deep understanding of the role of OSDAs in crystallization is potentially important for the design and synthesis of aluminophosphate-based molecular sieves with novel structures and new routes.