Existence of a microporous corona around the mesopores of silica-based SBA-15 materials templated by triblock copolymers

Materials showing long-range two-dimensional hexagonal order (called SBA-15 in the literature) were produced by templating a silica precursor (TEOS) w ith two Pluronic copolymers, EOxPOyEOx, of nearly the same x/y (0.3) ratio but different y values (x 18, y = 60 and x 20, y = 70, respectively); These materials were hydrothermally treated to increase the condensation of sili cate species around the Pluronic aggregates and calcined to liberate the he xagonal array of mesopores. All materials, i.e., before and after hydrother mal treatment and calcination, were investigated by X-ray diffraction (XRD) , and all calcined samples were further characterized by transmission elect ron microscopy and N-2 sorption experiments. The large number and narrow wi dth of the XRD powder diffraction lines demonstrate the good crystallograph ic quality of the materials. This allows us to quantitatively exploit the X RD reflection intensities and to show that simple structural models of the silica lattice cannot account for them. This means that SBA-15 materials ca nnot be regarded as an ideal hexagonal lattice of pores imbedded in a unifo rm silica matrix. The structure of the silica walls is more complex and sho ws a "corona" region of lower density around the cylindrical organic aggreg ates.:This corona becomes microporous upon calcination, and we suggest that it arises from the partial occlusion of the PEO chains in the silica matri x. Modeling the XRD intensities allows us, for all the solids of this serie s::to derive estimates of pore diameters, corona, and wall thicknesses and to examine the influence on these structural parameters of the hydrothermal treatment.