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

Abstract

The influence of water content on the crystallization of the microporous aluminophosphate AlPO 4-11 was investigated by a combination of in situ UV Raman and ex situ XRD and NMR characterizations. Under high water content conditions (Al 2O 3:H 2O = 1:20–69), an AlPO 4-5 intermediated formed first and later co-existed with AlPO 4-11. In the last stage of crystallization, the crystalline AlPO 4-5 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 AlPO 4-5 and AlPO 4-11 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 AlPO 4-11. Under low water content conditions (Al 2O 3:H 2O = 1:15), the AlPO 4-11 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.