HOLLOW NANOSPHERES, ALLOPHANES - ALL-ORGANIC SYNTHESIS AND CHARACTERIZATION

Allophanes, hollow spherical aluminosilicates with diameters of 3-4 nm , have been synthesized by the 'all-organic' synthesis method. The mai n wall of these spheres consists of a tetrahedral silicon oxide/hydrox ide sheet with partial replacement of Si by Al. Octahedrally coordinat ed aluminium species (Al-hydroxide) act as counter-ions on the inner s urface of the spheres, Pore openings of about 0.35 nm allow descriptio n as isolated zeolite-like supercages capable of an exchange of water and small hydrocarbons. The novel synthesis method uses both organic e ducts (aluminium trialkylates and tetraalkyl orthosilicates) as well a s dry organic solvents (alkanes). Slow addition of water as a reactant through diffusion via the gaseous phase, followed by hydrolysis and p olycondensation reactions, leads to the formation of the allophanes. H ereby the educts and solvents may be varied and differently combined w ithout significant change in the final allophane product, displaying t he universality of the method. The reaction takes place at room temper ature and is completed after one to two days. The formation of these a llophanes can be explained by means of hydrophilic/hydrophobic interac tions of the polar hydrolysis products and the non-polar reaction envi ronment caused by the organic solvent. The products were characterized by XRD, TG/DTA, transmission electron microscopy, IR and Al-27/Si-29 solid state NMR, the latter two allowing for the modelling of the stru cture as well as of the reaction mechanism. Structural analogy to the so-called type TI allophanes (stream deposited allophanes) is confirme d. (C) 1998 Elsevier Science B.V. All rights reserved.