Synthetic allophane-like particles: textural properties

Short-range ordered aluminosilicates particles were synthesized on the basis of a sol-gel process with initial solutions containing decimolar concentrations of AlCl3 salts and Si alkoxide (tetraethyl ortho silicate) with varying Al/Si molar and hydrolysis OH/Al ratios. Transmission electron microscopy micrographs revealed more or less condensed aggregates displaying homogeneous elemental compositions, with Al/Si molar ratios ranging from 0.2 to 1.7. Owing to the high number of samples, a correlation could be drawn between microanalysis data and intensities ratios of vibrational bands centered around 960, 560 and 1060 cm−1. On the basis of final Al/Si ratios determined from vibrational spectra, textural properties, evidenced by nitrogen adsorption volumetry, could be rendered as a function of spherules composition. Indeed, it appeared that as Al/Si increases, allophane-like materials develop successively, macropores, mesopores and micropores, until Al/Si = 1.3. For Al-rich materials, Al/Si > 1.3, low surface area is developed. On the basis of theoretical values calculated from reported size and shape and assuming the conservation of structure upon Al/Si increase, experimental data could be interpreted as a continuous increasing condensation of particles leading to supplementary microporosity for Al/Si molar ratios ranging from 0.8 to 1.3, and for higher molar ratios, condensation of particles prevents nitrogen access.