Investigation of structural order and morphology of MCM-41 mesoporous silica using an experimental design methodology

A 2(4)supercript stop factorial experiment program was carried out to study the main and interaction effects of four factors (mixture CTMABr/SiO2, H2O/SiO2, and EthAc/SiO2, and reaction time) on pore ordering, hexagonal unit cell parameter a(0), and morphology of MCM-41. The MCM-41 was synthesized from a sodium silicate solution using cetyltrimethylammonium bromide (CTMABr) surfactant and ethyl acetate (EthAc) pH modifier. None of the factors acted independently to determine pore ordering, in contrast to earlier limited literature data, which suggested a higher CTMABr/SiO2 disturbs the assembly of the MCM-41 structure. However, there is no contradiction between these results considering that the poorly ordered product was obtained previously from a reaction mixture with the higher EthAc/SiO2 and lower H2O/SiO2, which are shown to hinder pore ordering. A combination of these factors, resulting in a higher concentration of acetic acid (hydrolysis of EthAc), and thus, in a lower mixture alkalinity, implies that the pH affects pore ordering in MCM-41. This is consistent with extensive literature data on this mesoporous material. A small (up to similar to 5%) variation of a(0) due to the reaction composition and time variation was insignificant compared to the reported doubling of a(0) caused by the effects of varying the surfactant alkyl chain length, addition of swelling organic compounds, or hydrothermal restructuring. Particle morphology (hexagonal platelets, gyroids, and crescent-like or worm-shaped particles) depended on the combination of mixture CTMABr/SiO2, H2O/SiO2, and EthAc/SiO2. This is consistent with the literature evidence that morphogenesis of hexagonally ordered silica is a complex phenomenon involving a variety of reaction variables.