Influence of the molecular-oriented structure of ionic liquids on the crystallinity of aluminum hydroxide prepared by a sol–gel process in ionic liquids

Abstract

The influence of the structure of ionic liquids on the crystallinity of aluminum hydroxide (Al(OH)₃) prepared by a sol–gel process with aluminum isopropoxide (Al(OPrⁱ)₃) in imidazolium-based ionic liquids was investigated. When Al(OH)₃ was prepared in ionic liquids having long alkyl chains, such as 1-butyl-3-methylimidazolium salts and 1-methyl-3-octylimidazolium salts, highly crystalline products were obtained. In contrast, Al(OH)₃ obtained using the 1-ethyl-3-methylimidazolium salt was an amorphous material, indicating that hydrophobic interaction of the alkyl tail of the imidazolium cation of the ionic liquid strongly affects the crystallinity of sol–gel products and the local structure of the ionic liquid. Moreover, the crystallinity of Al(OH)₃ prepared in ionic liquids increased relative to the amount of additional water (ionic liquid/water = 1.28/2.0–3.5/0.2, w/w). In the case of addition of a small amount of water (ionic liquid/water = 3.5/0.2, w/w), the product was amorphous. These results implied that the presence of an ionic liquid and a sufficient amount of water was crucial for the successful synthesis of sol–gel products with high crystallinity. ¹H NMR analyses revealed a shift of the peak associated with the imidazolium cation upon addition of water, which suggested that the molecular orientation of the ionic liquid was similar to that of a micelle.