Determination of phase composition of MCM-48/lamellar phase mixtures using nitrogen adsorption and thermogravimetry

It is shown that gas adsorption and thermogravimetry are suitable for the determination of the phase composition of MCM-48/lamellar phase mixtures, which often are formed at different stages of the MCM-48 synthesis. The occurrence of the lamellar phase can be easily detected by analyzing as-synthesized materials by X-ray diffraction (XRD). However, it is not convenient to use this technique to quantify the composition of MCM-48/lamellar mixtures because of the overlap of the characteristic XRD peaks of these two phases and an inherent difficulty in applying the quantitative analysis for materials with amorphous frameworks presenting solely low-angle XRD peaks due to the long-range ordering of the pore arrays. On the other hand, the quantification of the lamellar phase in MCM-48/lamellar mixtures can be readily achieved by fitting the nitrogen adsorption isotherm for the calcined mixed phase with a linear combination of nitrogen adsorption isotherms for pure calcined MCM48 and pure calcined (collapsed) lamellar phases. When the pure phases are properly selected, a good fit is often achieved in the entire relative pressure range and the sum of fitting coefficients is close to 1. In such a case, the fitting coefficients can be regarded as weight fractions of the pure components in the mixture. A semiquantitative evaluation of the content of the lamellar phase in MCM-48/lamellar mixtures can also be made on the basis of thermogravimetric (TGA) weight change patterns of as-synthesized samples because of a dramatically different weight loss behavior for as-synthesized MCM-48 and lamellar phase. Phase composition estimates from gas adsorption and TGA data were found to be similar. Because of the fact that adsorption and TGA data are often acquired during a routine analysis of surfactant-templated materials, the methods reported in this study can readily be used to assess the phase purity of MCM-48.