In situ NMR and XRD studies of the growth mechanism of SBA-1

In situ 17O, 14N and 29Si nuclear magnetic resonance (NMR) coupled with in situ energy dispersive X-ray diffraction (EDXRD) have been used to investigate the growth of the siliceous mesoporous material, SBA-1, synthesised under acidic conditions from a micellar solution of the surfactant hexadecyltriethylammonium bromide (HTEAB) and tetraethylorthosilicate (TEOS). For the last decade, the mechanism of growth of such materials has been thought to be driven by electrostatic interactions described as a co-assembly process between the silica species (I+) and the micelles (S(+)X(-)). However, this postulated model referred to as the "charge density matching model" has never been fully supported by experimental data for the acidic syntheses. We have carried out a detailed in situ study which challenges the so-called S(+)X(-)I(+) pathway and instead suggests that a salting-out effect coupled with a drastic change in the water activity are responsible for the composite I(1)3 (SBA-1 space group Pm3n) mesophase precipitation. Substantial reorganisation of the precipitated phase then results in the final structure.