Structural transformations of lamellar assembly of polysilsesquioxane nanosheets and arsenate adsorptions on transformed variants

Abstract

Various assembled structures of polysilsesquioxane nanosheets NH 2C 3H 6SiO 1.5 were provided by chemical transformations of lamellar polysilsesquioxane C 11H 23COOH–NH 2C 3H 6SiO 1.5 (LAS-NH 2C 3H 6SiO 1.5) in order to expose aminopropyl functions in the interlayer. The structure was analysed by TG, XRD, IR, NMR and XANES techniques and they provided evidence for the stability of polysilsesquioxane sheets in various chemical reactions during transformations of the mesostructure. The removal of interlayer surfactant and delamination occurs in LAS-NH 2C 3H 6SiO 1.5 after a room temperature reaction with Fe 3+ followed by hydrochloric acid. The further treatment of Fe 3+-NH 2C 3H 6SiO 1.5 with hydrochloric acid results in the formation of H +NH 2C 3H 6SiO 1.5 disordered assembly. On the other hand, the layer distance shrunk to d = 1.01 nm (2 θ = 8.72° by XRD) when LAS-NH 2C 3H 6SiO 1.5 was heated in an acetic acid–decane mixture. The TG weight loss of this solid agreed with the formula of CH 3COOH–NH 2C 3H 6SiO 1.5, the substitution product with acetic acid. The capacity of arsenate adsorption on Fe 3+ 0.56NH 2C 3H 6SiO 1.5 was 2.9 mmol/g, which was larger than those found in the functionalized mesoporous silica materials. We found no detectable adsorption on the assembly of H +NH 2C 3H 6SiO 1.5, the product of H +-exchange of Fe 3+ 0.56NH 2C 3H 6SiO 1.5, suggesting that the diffusion of arsenate in the interlayer space of assembly is difficult and the polysilsesquioxane layer is densely aggregated. In contrast, AcOH-NH 2C 3H 6SiO 1.5, a densely packed lamellar structure, provided the capacity of 0.76 mmol/g. However, ca. 45% of the adsorption capacity was reduced from the value observed for LAS-NH 2C 3H 6SiO 1.5, implying that the adsorption is also sensitive to the hydrophobicity of interlayer alkyl chains. The difference of adsorption properties revealed in this study is due to the mode of assembly of silsesquioxane nanosheets, suggesting the importance of how to expose the amine functions in the synthesis of adsorbents of oxyanions.