In situ FTIR study of the formation of MCM-41

Abstract

The formation of both siliceous and aluminium-containing MCM-41 has been followed insitu using attenuated total reflectance Fourier-transform infrared (ATR FTIR) spectroscopy together with a range of complementary techniques. Key stages in the reaction are reflected by changes in the intensity of IR bands at 1030 and 1105 cm⁻¹, which are assigned to the internal Si–O vibrations of Si(OSi)₃ and Si(OSi)₄ groupings (Q₃ and Q₄, respectively). Assignments were confirmed using solid state NMR. Under the conditions of these experiments, the reaction rate is limited by the relatively slow dissolution of the silica source. The resulting silicate oligomers then react more rapidly with surfactant cations in a co-operative assembly process to give the embryonic MCM-41 structure. Slower condensation reactions subsequently increase the product Q₄/Q₃ ratio as further crosslinking develops. From an analysis of both the CH₃ head group and CH₂ in-chain IR vibrations, the surfactant appears to be complexed or micellised at all times during the synthesis.