Molecular engineering of microporous crystals: (VII) the molar ratio dependence of the structure-directing ability of piperazine in the crystallization of four aluminophosphates with open-frameworks

The crystallization fields of initial mixtures with compositions of Al ₂O₃:3.0 P₂O₅:x pip:277 H ₂O, where x = 2.4-12 and pip = piperazine, were investigated. Pure and highly crystalline three-dimensional (3D) AlPO-CJB2, two-dimensional (2D) AP2pip, 3D AlPO-JU88, and 2D AlPO-CJ9 were obtained when x = 3, 3.4, 6.5, and 12, respectively. Along with the increase in the ratio of piperazine to Al ₂O₃ in the initial mixture, the density of piperazine in the final structure as well as the non-bonding interaction between the inorganic framework and the protonated piperazine increased. The crystallization processes of AlPO-CJB2, AP2pip, and AlPO-JU88 were investigated using multiple techniques. In the crystallization of AlPO-CJB2, a pure and highly crystalline AP2pip intermediate phase was first formed and then dissolved. A species that may promote the dissolution of AP2pip and the formation of AlPO-CJB2 was identified. Under the synthetic condition of AP2pip, however, this species was not formed; thus, the transformation of AP2pip to AlPO-CJB2 was not observed even for significantly prolonged crystallization times. Additionally, the crystallization rate of AP2pip was significantly accelerated by an increased molar ratio of the reactants with respect to the aluminum source. The results indicate that the type and concentration of the species in the liquid phase were the governing factors in determining the final products and that the formation of particular structures is energy-directed.