Molecular engineering of microporous crystals: (III) the influence of water content on the crystallization of microporous aluminophosphate AlPO ₄-11

The influence of water content on the crystallization of the microporous aluminophosphate AlPO₄-11 was investigated by a combination of in situ UV Raman and ex situ XRD and NMR characterizations. Under high water content conditions (Al₂O₃:H₂O = 1:20-69), an AlPO₄-5 intermediated formed first and later co-existed with AlPO₄-11. In the last stage of crystallization, the crystalline AlPO₄-5 completely disappeared and well-crystallized AlPO ₄-11 was obtained. The position of the three signals observed in the ³¹P MAS NMR spectra of the isolated solid samples did not change during the hydrothermal treatment, suggesting that the fragments containing the P site in the junction of 6- and 4-membered rings of both AlPO₄-5 and AlPO₄-11 formed first and that the environments of the P sites were very similar to those in the final structure. A significant increase of Al concentration in liquid media was observed before the appearance of XRD detectable AlPO₄-11. Under low water content conditions (Al ₂O₃:H₂O = 1:15), the AlPO₄-11 was directly crystallized from the initial mixture. In situ Raman spectra showed that the 10-membered rings were complete by the time the transformation of octahedral Al to tetrahedral Al was finished and that the conformation of the protonated di-(i-propyl)amine changed little during the crystallization. The present study showed that the composition of the liquid phase is critical to the formation of a specific structure.