Bottom-up synthesis of aluminophosphate nanosheets by hydrothermal process

Abstract

Hydrothermal treatment of aqueous mixtures of boehmite (AlOOH), phosphoric acid, and tetramethylammonium hydroxide provided two types of layered aluminophosphates having tetramethylammonium ion (TMA+) as an interlayer cation, despite the fact that TMA+ ion acts as a structure-directing agent of microporous AlPO4 materials and less likely leads to a lamellar structure. The layered aluminophosphates were formed in amorphous gels, so that they were obtained as precipitates. Upon dispersing the precipitates to water under agitation, the layered aluminophosphates were transferred to the aqueous phase, resulting in transparent aqueous sols. Because they had bulky interlayer cation TMA+, it is likely that in the sols, layered aluminophosphates were exfoliated, providing aluminophosphate nanosheets. Moreover, their characterization results suggest that the layered compound formed at 170 °C consisted of aluminophosphate layers with kanemite-like structure. Furthermore, the prolonged hydrothermal treatment at 170 °C led to the formation of microporous ATT-type AlPO4 crystals. Under the synthesis condition employed in this study, layered aluminophosphates were formed at early stage, and then structurally converted to microporous AlPO4 crystals.