Salt effect on the formation of surfactant-silica mesophase

Abstract

Recently, a new synthetic approach to obtain mesoporous molecular sieves has been discovered by scientists at the Mobil corporation, using the formation of surfactant-silica mesostructures. The surfactant-silica mesostructures consist of a periodic array of the silica frameworks which are templated by the surfactant micelles. The mesostructures are obtained by self organization of the silica frameworks surrounding the surfactant micelles during the hydrothermal reaction in an aqueous solution of surfactant and silica sources. MCM-41, one of the mesoporous molecular sieves thus obtained, has uniform hexagonal arrays of cylindrical channels with the cross-sectional diameter controllable in the range of 2 - 10 nm. The MCM-41 material has attracted much attentions as adsorbents, catalysts, and hosts for large molecules due to its large pore diameter compared with conventional zeolites. However, practical applications of the MCM-41 have been severely inhibited by its weak hydrothermal stability compared with zeolites. The present work has been performed in order to improve the weak hydrothermal stability of MCM-41. Various inorganic and organic salts such as LiCl, NaCl, KCl, $CH_3COONa$, $Na_4EDTA$, $K_4EDTA, $Na_2SO_4$ and $NaNO_3$ were added to the synthetic mixture after permitting the hydrothermal reaction to proceed. The effects of the salts were investigated with X-ray diffraction and magic angle spinning (MAS) $^29Si$ NMR spectroscopy. MCM-41 materials obtained by using proper amount of salts during the synthesis process showed no sign of structural losses in XRD after heating in boiling water for 12 h, while the structure of MCM-41 synthesized without adding salts was completely lost. Thus, hydrothermal stability of MCM-41 was improved by salt effect. In view of the salt effect on the formation of the surfactant-silica mesostructure, it is reasonable that the anions constituting the salt can moderate electrostatic interaction between the positively charge...