The role of small organic amine molecules and their aggregates in the crystallization of microporous materials

Abstract A series of 1,6-hexamethylenediamine (1,6-HDA) solutions with different concentrations were prepared and used as the structure directing agents for the crystallization of microporous materials. The surface tension of these solutions has been measured to inspect the assembly behavior of 1,6-HDA molecules. It has been found that the surface tension of these solutions decrease with the increase of 1,6-HDA concentration, which is similar to what in surfactant CTAB (Cetyl trimethyl ammonium bromide) solutions. When they were used to direct the crystallization of aluminophosphites, zincophosphates, and aluminophosphates, it was found that at low concentration, 1,6-HDA molecules cannot affect the crystallization process, and three-dimensional frameworks without 1,6-HDA in it were obtained. When 1,6-HDA concentration increased, the surface tension of these solution decreased sharply, and 1,6-HDA containing three-dimensional (3-D) frameworks were formed. 1,6-HDA molecules retained in these structure were not in assembling state, but in independent state. At high 1,6-HDA concentration, the surface tension of the solutions changed slowly accompanying with the assembly of 1,6-HDA molecules. It always resulted in the formation of layered structures with the inorganic layers interleaved by composite layers of diprotonated 1,6-HDA. Formation of large-micropore containing 3-D frameworks mainly resulted from the interaction between inorganic framework and organic amine at proper ratio, not from the assembly of 1,6-HDA molecules. Otherwise, the aggregates of 1,6-HDA molecules favors the formation of layered materials.