In the calcination step, the as-synthesized mesoporous aluminosilicates undergo dehydration (the removal of physically adsorbed water) and dehydroxylation (the removal of silanol groups from the silica surface) [52,53]. In
the rehydroxylation step, the hydroxyl groups are restored on the silica surface under ambient conditions. As mentioned in Section 3.1, different calcination heating rates can change the atomic ordering of the SiO2 structure. This implies that the surface area of a specific oriented face of the SiO-2 nanoparticles may also be changed during synthesis [54].