Restructuring of mesoporous silica: high quality large crystal MCM-41 via a seeded recrystallisation route

Abstract

Post-synthesis recrystallisation of calcined small crystallite MCM-41 has been used to prepare highly ordered large crystallite MCM-41 materials in a two step synthesis route. The calcined small crystallite `primary' MCM-41, obtained from the first step, is used as the `silica source' in an otherwise normal secondary synthesis or recrystallisation procedure. During the secondary synthesis the calcined primary MCM-41 crystallites are preserved and act as seeds for further crystal growth; the real source of the silicate ions required for crystal enlargement is amorphous silica which is part of the primary MCM-41 yield. In addition to having larger crystallites, the resulting `secondary' MCM-41 exhibits much better long range structural ordering as observed by powder XRD and TEM. The recrystallisation causes a modest increase in basal (d₁₀₀) spacing and a decrease in pore size resulting in thicker pore walls and a slight decrease in surface area and pore volume. These structural changes are accompanied by a morphological transformation from spherical particles to larger sheet or plate like domains. TGA, ²⁹Si NMR, SEM and powder XRD studies provide strong evidence for a seeding mechanism.