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

Chrysotile-type Mg₃Si₂O₅·(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 °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 °C) were between 105 and 202 m²/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 °C. Sodium silicate obtained nanotubes were less thermally stable (below 500 °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 μmol CO₂/g cat, and their catalytic activity was assessed on aldol condensation of acetone reaction at 300–350 °C, where the main products obtained were mesityl oxide and isophorone, which are valuable products for fine-chemistry industry.