CHARACTERIZATION OF HEXAGONAL AND LAMELLAR MESOPOROUS SILICAS, ALUMINOSILICATES AND GALLOSILICATES BY SMALL-ANGLE X-RAY-SCATTERING

Various mesoporous silicas and the corresponding aluminosilicates or g allosilicates have been synthesized using a series of literature or 'h ome-made' recipes. The efficiency of Al or Ga incorporation into the s iliceous walls of these materials depends markedly on the trivalent so urce and the evolution (ageing) of the so-formed Si-M-III gel-type pha ses at different starting pH values and temperatures, after adding the surfactant-structuring compounds. Crystallization at low temperature (e.g. <373 K) yielded mesoporous compounds with hexagonal topology (MC M-41 type), involving double-layered Si walls possibly partly substitu ted by Al or Ga. Such structures remain stable after calcination in ai r at 873 K. When the same gels are crystallized at 423 K for 2 d, lame llar frameworks (MCM-50 type) are preferentially stabilized. They read ily collapse on heating. The ultra-small-angle X-ray scattering (USAXS ) data and the first part of the SAXS data show a power behaviour that indicates a fractal interface before calcination. After calcination, in the case of MCM-50 type materials, the fractal dimension significan tly increases, the fractality region being larger than in the precurso r. By contrast, in the case of the MCM-41 type materials, the fractal region tends to disappear after calcination. The second part of the SA XS curve reflects the hexagonal or lamellar structure. Some precursors simultaneously exhibit both structures. The hexagonal parameter range s from 4.6 to 5.8 nm, while the thickness of the wall is estimated to be of the order of 0.7 nm from observations of the satellite peaks in the vicinity of the successive peaks characterizing the hexagonal arra y. The successive peaks relative to the lamellar structure are consist ent with the superposition of two or three layers, the thicknesses of which are of the order of 3.3, 2.85 and 2.5 nm. Predominant hexagonal structures are maintained after calcination while lamellar structures collapse during calcination. In the case of hexagonal structure, the h exagonal array is slightly contracted.