IMAGING THE PORE STRUCTURE AND POLYTYPIC INTERGROWTHS IN MESOPOROUS SILICA

Mesoporous silica, SBA-2, has been investigated by using high-resoluti on transmission electron microscopy supported by computer image simula tions. The complete pore structure connecting the discrete supercages of which the silica is composed has been determined. In addition, unex pected well-defined cubic-hexagonal (polytypic) intergrowths have been uncovered, involving a hitherto unknown mesoporous structure that we designate STAC-1, It was found that both SBA-2 and STAC-1 contain a tw o-dimensional pore system and that the symmetry of the SBA-2 structure must be lower than that (space group P6(3)/mmc) determined previously on the basis of X-ray powder diffraction methods. Polytypic irregular ities in these mesoporous materials indicate that, similar to micropor ous zeolitic systems, a range of solids with structures intermediate b etween end members SBA-2 and STAC-1 might be prepared.