Time-resolved in situ X-ray diffraction study of MCM-41 structure formation from a homogeneous environment

Formation of an ordered hexagonal mesoporous structure of siliceous materials in a homogeneous environment using Na2SiO3 as a silica source and cetyltrimethylammonium bromide as a structure directing agent has been studied by means of a time-resolved in situ X-ray diffraction technique. In this synthesis route the precipitation of solid phase is due to acidification of the reaction mixture, achieved by the hydrolysis of ethyl acetate. Before acidification, the reaction mixture contains disordered elongated rod-like surfactant micelles, whose surfaces are partly covered with silicate anions. Owing to the decrease in pH, silicate anions begin to polymerize; micelles are arranged into a hexagonal array through the intermicellar silicate condensation. Thus, the polycondensation of silicate species represents the driving force of the formation of solid particles. Due to the replacement of OH groups by Si-O-Si bonds in the course of polycondensation, the structure contracts; the increasing density of silica pore walls results in the increasing integral net intensity of the (100) reflection. Since the integral width of this reflection remains practically constant, the size of coherent regions does not change during the reaction mixture ageing from the size that has been achieved at the earlier stages of the synthesis.