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 degreesC 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 1 0) reflection. After 300 degreesC, changes are less pronounced and the intensities remain unchanged while the sample is kept at 550 degreesC. 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 degreesC 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 degreesC 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. (C) 2001 Elsevier Science B.V. All rights reserved.