Characterization of Allophanes Synthesized by the Use of Organic Sources and Organic Solvent

Allophanes, hollow spherical hydrous aluminosilicates with diameters of about 3 nm, have been synthesized using both organic sources and dry organic solvents. Water used as a reactant was added slowly via the gaseous phase. Tetraethylorthosilicate as Si source and aluminium triisopropylate as Al source have been employed with Al/Si ratios of 2 and 1, respectively. In all experiments dry n-hexane was used as the solvent. The allophane product was characterized by X-ray, IR, TG/DTA, ²⁹Si solid state NMR and transmission electron microscopy. Figure 1 shows a ²⁹Si high resolution solid state NMR spectrum. Whereas XRD, TEM, TG/DTA and NMR exhibit the characteristics commonly known for all natural allophanes such as indefinite broad reflections and hollow spherical shape of particle, IR shows certain features of the so-called stream deposit allophanes. This supports a structural model for the synthesized allophane where a tetrahedral sheet builds the outer wall of the sphere and also accounts for the size of only 3 nm.

Fig. 1. ²⁹Si CP/MAS NMR spectrum of the product with an Al/Si ratio of 2. "S" in the figure stands for spinning sidebands.