On the extended recrystallisation of mesoporous silica: characterisation of restructured pure silica MCM-41

Mesoporous silica MCM-41 may be restructured by performing secondary synthesis (i.e., recrystallisation) in which calcined MCM-41 is used as silica source. The influence of a number of factors on the recrystallisation has been investigated. The factors investigated are, ( 1) the time allowed for hydrothermal recrystallisation at 150degreesC, (2) the amount of amorphous silica available during recrystallisation and ( 3) the use of large pore MCM-41 as silica source. Increasing the time allowed for recrystallisation ( from 48 to 212 hours) has little effect on the basal (d(100)) spacing but results in a gradual and considerable increase in the pore wall thickness. The pore size therefore decreases slightly while both pore volume and surface area undergo more significant reductions. These textural changes are accompanied by restructuring of the particle morphology from the small sphere shaped particles of the original MCM-41 to much larger particles that are both elongated and sheet (plate)-like. Restructuring to the larger particles ( for 48 hours recrystallisation) is most effective when there are only limited amounts of amorphous silica available in the recrystallisation gel. By using large pore MCM-41 materials as silica source it is possible to show that the structural and morphological transformations observed are consistent with a recrystallisation mechanism in which a significant proportion of the original MCM-41 particles are preserved and act as seeds for the restructuring.