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 materia ls in a homogeneous environment using Na2SiO3 as a silica source and cetylt rimethylammonium bromide as a structure directing agent has been studied by means of a time-resolved in situ X-ray diffraction technique. In this synt hesis route the precipitation of solid phase is due to acidification of the reaction mixture, achieved by the hydrolysis of ethyl acetate. Before acid ification, the reaction mixture contains disordered elongated rod-like surf actant micelles, whose surfaces are partly covered with silicate anions. Ow ing to the decrease in pH, silicate anions begin to polymerize; micelles ar e arranged into a hexagonal array through the intermicellar silicate conden sation. Thus, the polycondensation of silicate species represents the drivi ng 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 co ntracts; the increasing density of silica pore walls results in the increas ing integral net intensity of the (100) reflection. Since the integral widt h of this reflection remains practically constant, the size of coherent reg ions does not change during the reaction mixture ageing from the size that has been achieved at the earlier stages of the synthesis.