Synchrotron X-ray diffraction-diffusion studies of the preparation of SBA-15 materials

The growth of SBA-15 periodically ordered structure in diluted reaction gels has been studied using low angle X-ray diffraction-diffusion with a synchrotron radiation source. The formation of the hexagonal phase has been observed at the very early stages of the reaction. As the synthesis proceeds, the unit cell size of the formed SBA-15 material is gradually decreasing following an exponential decay function for a given synthesis temperature. This contraction of the hexagonal unit cell, which is apparent from the shift of all the observed reflections to lower d-values, coincides with an increase in the intensity ratio of (110) and (200) peaks. Both effects can be explained by the condensation reactions taking place within the silicate walls of SBA-15 leading to the formation of a well ordered material with relatively dense structure and hexagonally shaped pores. This work demonstrates that valuable structural information can be obtained for dilute reaction mixtures containing periodic phases with d-spacing values up to 25 nm. (C) 2003 Elsevier Inc. All rights reserved.