Molecular engineering of microporous crystals: (V) Investigation of the structure-directing ability of piperazine in forming two layered aluminophosphates

The crystallization process of the layered aluminophosphates AP2pip and AlPO-CJ9 from initial mixtures with compositions of Al2O3:P2O3:1.1 piperazine:277 H2O and Al2O3:P2O3:4.0 piperazine:277 H2O, respectively, was investigated using multiple techniques. During the crystallization of AP2pip, the formation and dissolution of highly polymerized but structurally disordered fragments containing four-coordinated P sites to Al-centered polyhedra were observed. 27Al → 31P CP/MAS HETCOR NMR analysis showed that the local P-site environment in the small building fragments formed in the initial mixture is same as that in the final crystal structure of AP2pip. The concentration of these small building fragments is very low. During the crystallization of AlPO-CJ9, the formation of small building fragments with the local P-site environment same or very similar to that observed in the final structure of AlPO-CJ9 were observed. The concentration of these small building fragments is high. These small fragments contained at least two complete coordination layers. The difference in the structure-directing role played by piperazine in the two synthesis systems was investigated in detail by designing several control experiments and analyzing the arrangements of the piperazine molecules in the interlayer region of AP2pip and AlPO-CJ9.

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► The nature of structure-directing ability of piperazine was investigated.
► The crystallization process of two layered aluminophosphates was investigated.
► The factors affecting the structure-directing ability of piperazine were studied.