Evolution of mesoporous materials during the calcination process: structural and chemical behavior

We describe herein the study of the temporal evolution of hexagonal mesophases of silica, titania and zirconia as a function of temperature. Detailed in situ X-ray diffraction (XRD) studies with a high temperature XRD chamber system have been conducted in conjunction with thermogravimetric–differential thermal analysis coupled with mass spectrometry (TG–DTA/MS) to better understand the processes related with template removal from MCM-41 type mesophases. The thermal behavior of the cationic surfactants in the mesostructured systems has been analyzed, and the processes involved have been elucidated. In the case of Si-MCM-41, an initial change occurs up to 250°C with an increase in intensity of all reflections, with the (1 1 0) and (2 0 0) reflections increasing later and at a higher rate than the (1 0 0) reflection. After 300°C, changes are less pronounced and the intensities remain unchanged while the sample is kept at 550°C. The TG–DTA/MS data show that the decomposition mechanism in air involves three steps. An initial endothermic step is assigned to Hofmann elimination of trimethylamine, leading to a hydrocarbon chain. The second step is exothermic and results from a carbon chain fragmentation. Finally, oxidation occurring at 320°C converts the remaining organic components to carbon dioxide. Template removal appears to be completely different for the transition metal based materials: a single step complete oxidation of the surfactant is observed around 300°C in TG–DTA/MS. This is accompanied with the drastic decrease in d-spacing and initial sharp increase in reflection intensity in the XRD pattern, which generally leads to the loss of the well ordered hexagonal structure