Synthesis of mesoporous calcium-containing silica particles by pseudomorphic transformation

Pseudomorphic transformation is introduced as a post-synthetic route to convert preformed spherical non-porous SiO₂-CaO particles into mesoporous particles while retaining control over the particle size and morphology. The transformation was carried out with cetyltrimethylammonium bromide (CTAB) as a structure-directing agent, and the NaOH concentration was varied to tune the balance between silica dissolution and reprecipitation. Three distinct transformation regimes were observed. At low NaOH concentration, the size and shape of the parent particles were retained while mesoporosity was introduced. At intermediate NaOH concentration, the particles lost their spherical morphology and formed agglomerated structures, although maximum pore volume and surface area were obtained. At higher NaOH concentration, spherical particles were recovered, but with pronounced surface roughness and reduced porosity. The Ca/Si ratio increased slightly with increasing NaOH concentration, indicating preferential silica dissolution, while X-ray diffraction confirmed that the transformed particles remained amorphous and that Ca²⁺ was incorporated as a network-modifying ion. The amount of calcium released in acidic medium followed the specific surface area of the samples, indicating that release is mainly controlled by the accessibility of calcium-containing sites rather than by the bulk calcium content. Pyrene moieties co-condensed into the parent particles further revealed molecular-scale redistribution during transformation, as excimer emission disappeared despite pyrene enrichment. Pseudomorphic transformation thus provides a route to mesoporous SiO₂-CaO particles in which morphology, pore accessibility, and composition can be tuned through the NaOH-controlled dissolution-reprecipitation balance.