Influence of synthesis parameters on the physico-chemical characteristics of SBA-15 type ordered mesoporous silica

Abstract The identification of parameters governing structural, textural properties and hydrophilic/hydrophobic balance of SBA-15 type ordered mesoporous silica (OMS) has been investigated by the study of the influence of the temperature and the stirring at the ripening step (first synthesis step), the duration of the two principal synthesis steps (ripening and aging), and the heating process at the aging step. The synthesis medium has been analyzed by dynamic light scattering (DLS), both the as-synthesized and calcined SBA-15 ordered mesoporous solids have been characterized by small angle X-ray diffraction (SAXRD), nitrogen sorption experiments, scanning and transmission electron microscopy (SEM/TEM) and 29 Si magic-angle spinning (MAS) NMR spectroscopy. The results show that, the morphology of elementary particles and aggregates is designed during the ripening step and is governed by the temperature and the stirring conditions. The elementary particles were formed exclusively at the early stage of the ripening step, while the morphology of the aggregates results from the assembling of elementary particles that is a slower process occurring during the ripening step. Furthermore, the silanol content changes a little with the ripening temperature but strongly increases with the aging duration. However, calcination seems to suppress any differences between the as-synthesized samples. If only porous characteristics and mesostructure are concern, a mesoporous SBA-15 type silica can be prepared from a freshly precipitated solution (i.e. 0.5 h of ripening) followed by a 24 h aging step at 90 °C. The influence of the heating process during the aging step has been also investigated. Compared to conventional oven, microwave oven allows an increase of the specific surface area, pore diameter, total pore volume and unit cell parameter, whatever the duration of the ripening. However, a minimum ripening duration of 2 h is required before aging in microwave oven to obtain a well ordered material with high textural characteristics.