Synthesis and catalytic activity of chrysotile-type magnesium silicate nanotubes using various silicate sources

Chrysotile-type Mg3Si2O5 center dot(OH)(4) nanotubes (MgSi-NTs) were obtained (by a hydrothermal method) using different SiO2 sources. Sources of silica, as raw material, were varied from sodium silicate (typical preparation) to colloidal silica of 12-22 nm colloid size. Around 13 g of MgSi-NTs were prepared by using 7.8 L Teflon-lined autoclave at 240 degrees C. In all synthesis attempts, XRD was used to identify chrysotile crystal phases, which were later confirmed by TEM, showing bundles of hollow cylindrical particles of MgSi-nanotubes from 60 to 218 nm length and internal diameters from 5 to 8 nm. Specific surface areas obtained by nitrogen adsorption of MgSi-NTs samples (annealed at 600 degrees C) were between 105 and 202 m(2)/g with average pore diameter from 7 to 32 nm, depending on silica precursor. Thermal stability of MgSi-NTs was examined by TGA-DTA, XRD and TEM. From TEM images, it was clear that nanotubes from colloidal silicas can hold up below 600 degrees C. Sodium silicate obtained nanotubes were less thermally stable (below 500 degrees C), though. The use of colloidal silica, instead of sodium silicate, upgraded specific surface area and pore volume values of resulting MgSi-NTs. Number of basic sites and properties were measured by CO2-TPD ranging between 66 and 89 mu mol CO2/g cat, and their catalytic activity was assessed on aldol condensation of acetone reaction at 300-350 degrees C, where the main products obtained were mesityl oxide and isophorone, which are valuable products for fine-chemistry industry.