Monitoring MCM-41 synthesis by X-ray mesostructure analysis

• X-ray diffraction technique was applied to the MCM-41 mesostructure monitoring.
• The approach was able to differentiate a great variety mesostructure states.
• Mesostructure alteration happens due to irregular polycondensation.
• The polycondensation distribution is responsible for the variety of pore geometry.
• The pore geometry correlates with material properties and hydrothermal stability.

The electron density maps calculated from X-ray diffraction patterns of the mesoporous silica material МСМ-41 present averaged mesostructure images which in contrast to transmission electron microscopy (TEM) images are exceptionally repeating and represent the whole sample. It was shown that the averaged mesostructure parameters such as a unit cell parameter, a pore diameter, a wall width, a pore shape estimated from the X-ray powder diffraction data in combination with the continuous electron density function approach allow monitoring continuous set of the silica framework states at different stages of the material synthesis.The mentioned technique supplemented by N2-adsorption measurements and transmission electron microscopy was applied for consideration of the МСМ-41 hydrothermal stability. Attention has been given to the variations of the synthesis conditions affecting the hydrothermal stability, in particular the maintaining the basicity of the synthesis solution as well the substitution of the synthesis solution with water or a salt solution at hydrothermal treatment. The averaged pore shape was observed to be changed from cylindrical to hexagonal-prismatic form. The observed wall thickness was in the range from 0.75 to 1.25 nm. The competition of silica polycondensation and surface hydrolysis was shown to be responsible for the variety of framework geometry and hydrothermal stability. It has been established that the pore diameter increases generally due to the osmotic pressure of water. If the pores acquire the average prismatic hexagonal shape, the sample has low hydrothermal stability. Under the conditions favorable for the polycondensation the pores have averaged cylindrical shape and material demonstrates higher hydrothermal stability. Cooperative mechanism of mesostructure destruction under hydrothermal conditions was observed using TEM data and was discussed in connection with irregular polycondensation.

There are a lot of TEM images convincing in the regularity of ordered mesoporous silica structure. Simultaneously, a detailed examination of TEM images detects individuality of almost every pore. It makes a difficulty in interpretation of substance properties, especially, hydrothermal stability. The X-ray powder diffraction pattern in contrast to TEM images is exceptionally repeating and presents the averaging picture for the whole sample. The electron density map, calculated from X-ray diffraction data, intends to give averaged structure which is different from every specific pore. The paper makes an effort to show whether the E-map is able to reflect fine quantitative changes on the X-ray powder diffraction pattern, converting them into information suitable for considering the substance properties on example of MCM-41 and its hydrothermal stability.Figure optionsDownload as PowerPoint slide