Kinetic study of MCM-41 synthesis

doi:10.1016/S1466-6049(00)00043-X

International Journal of Inorganic Materials

Volume 3, Issue 1, January 2001, Pages 75-86

https://doi.org/10.1016/S1466-6049(00)00043-X Get rights and content

Abstract

A kinetic study of MCM-41-type materials formation has been made to optimize the synthesis conditions in particular the synthesis time and temperature. The changes in morphology and textural properties of materials as a function of hydrothermal synthesis time at four different temperatures have been followed. From the characterization results, the synthesis mechanism is postulated. The present work shows clearly that the thickness of the wall separating two adjacent pores increases with hydrothermal synthesis time and temperature while pore size remains constant. The increase in the wall thickness, indicating the enhancement of polycondensation of silica around the micelles of surfactant, should be very important for strengthening the thermal stability of MCM-41 materials.

Introduction

In 1992, Mobil scientists [1], [2] have reported the synthesis of new mesostuctured materials called MCM ( $M$ obil $C$ rystalline $M$ aterials). Hexagonal (MCM-41), cubic (MCM-48) and lamellar (MCM-50) structures have been identified. Among these phases, MCM-41 is the most studied. The synthesis of pure silica mesoporous molecular sieves consists of the condensation and polymerization of an inorganic source of silicium around the micelles of surfactant.

According to Mobil scientists, MCM-41 is formed via a LCT ( $L$ iquid $C$ rystal $T$ emplating) mechanism [1], whereas for Monnier et al. [3], MCM-41 is obtained from the transformation of the lamellar MCM-50 phase. Kind of surfactants, silicium/surfactant ratio, the alkyl chain length of surfactant, pH value, hydrothermal synthesis time and temperature are known to affect the pore diameter, the wall thickness and the structure of the final compound. Generally, quaternary ammonium salts, C_nH_2n+1(CH₃)₃NBr, are used as surfactants. Beck and coworkers [1] have investigated the effect of the surfactant chain length variation and the silicium/surfactant ratio on the MCM-41 synthesis. They concluded that MCM-41 is synthesized when n varied from 6 to 16 and the pore diameter is proportional to the alkyl chain length (+2.5 Å per carbon), with a CTMABr/silicium molar ratio less than one. Elder et al. [4] showed that the optimum pH value is about 9–10. However, in literature few studies concerning the influence of physical parameters such as synthesis time and temperature of the micellar gel has been made. Cheng et al. [5] have studied the effect of temperature and time on the synthesis, they have shown the results contrary to those made by Monnier et al. [3] that hexagonal MCM-41 phase is already present at the beginning of the synthesis made at 100 and 150°C, and the yield is maximum after 48–72 h synthesis. At 150°C the hexagonal phase is transformed into the lamellar or the amorphous phase after 96 h. For longer reaction times, the amorphous phase is the major product.

This work deals with a systematic study on the effect of heating time and temperature on the MCM-41 synthesis, on the pore size and thickness of walls separating two pores of materials obtained.

Section snippets

Synthesis

Cetyltrimethylammonium bromide was dissolved in water at 40°C to obtain a clear micellar solution. Then sodium silicate was added to this solution and the pH value was adjusted with sulfuric acid. pH value and surfactant/silicium molar ratio were fixed at 10 and 0.62 according to literature [6]. After stirring for several hours at room temperature, the homogenous gel was sealed in Teflon autoclaves and heated. The synthesis temperature and time vary respectively from 80°C to 140°C and from few

X-ray diffraction analysis

Fig. 1 reports the XRD patterns of materials obtained at 80°C (Fig. 1A), 100°C (Fig. 1B), 120°C (Fig. 1C) and 140°C (Fig. 1D) at different synthesis times. It has been reported [8] that X-ray diffractogramms of powder MCM-41 material exhibit a typical four peaks pattern with a very strong feature at a low angle (100 reflection line) and three another weaker peaks at a higher angle (110, 200 and 210 reflection lines). The absence of the last three peaks suggests the disordered structure of

Conclusion

An optimization of MCM-41 synthesis conditions led us to propose the mechanism. Different synthesis steps, which are detected in the synthesis of zeolites, have been clearly observed. Referred to the different steps of zeolites synthesis, we describe these steps in the formation of MCM-41 materials with the following terms, step I: hydrolysis and condensation of silica source, step II: polycondensation of silica source around the cylindrical micelles, step III: the increase in the wall

Acknowledgements

This work has been performed within the framework of PAI/IUAP 4-10. Gontran Herrier thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship.

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^☆: Paper presented at the First International Conference of Inorganic Materials, Versailles, France 16–19 September, 1998.

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Kinetic study of MCM-41 synthesis☆

Abstract

Introduction

Section snippets

Synthesis

X-ray diffraction analysis

Conclusion

Acknowledgements

J. Am. Chem. Soc.

Nature

Science

Chem. Mater.

J. Chem. Faraday Trans.

Chem. Mater.

J. Am. Chem. Soc.