The dependence of the structure-directing effect of piperazine and the crystallization pathways of open-framework aluminophosphates on the local environment of the initial mixture

Abstract

In the synthetic system of Al2O3–x P2O5–6.4 piperazine–227 H2O, three open-framework aluminophosphates (2D AlPO-CJ9, 3D AlPO-JU88, and 2D AP2pip) were obtained by varying the Al2O3/P2O5 ratio. The crystallization processes of AlPO-CJ9, AlPO-JU88, and AP2pip were investigated using solid state NMR in combination with XRD and other characterization techniques. The results showed that a small change in the Al2O3/P2O5 ratio can significantly affect the type and distribution of the small fragments formed during the crystallization process. The structure-directing effect of piperazine was influenced and modified by the Al2O3/P2O5 ratio. For compounds crystallized from the same synthetic system but separate synthetic batches, i.e., AlPO-CJ9, AlPO-JU88, and AP2pip, we found differences in the crystallization pathways of the small fragments, the types and distributions of the small fragments, the evolutions of the concentrations of Al and P and the pH values of the liquid phases. These differences were studied by comparing the crystallization processes of AlPO-CJ9, AlPO-JU88, and AP2pip from different synthetic batches and showed that the structural details of the same compounds obtained from different synthetic batches were different. Therefore, the topological structure-directing effect is more appropriate than the structure-directing effect in describing the role that the added agents played in the crystallization of open-framework materials. The structure-directing effect of the added agents and the crystallization pathways of the open-framework materials are dependent on the local environment, i.e., the types and distributions of the small fragments, the relative ratio of the species.