4940
Langmuir 1996, 12, 4940-4944
Nucleation Events during the Synthesis of Mesoporous Materials Using Liquid Crystalline Templating Oren Regev Department of Chemical Engineering, Ben-Gurion University of the Negev, Box 653, 84105 Beer-Sheva, Israel Received March 11, 1996. In Final Form: May 31, 1996X A novel family of silica-based mesoporous molecular sievesM41Sswas synthesized by one of the Mobil research groups. Surfactant molecules were mixed with a silica solution, resulting in a regular array of uniform mesopores, 2-10 nm in diameter, capable of acting as a potential catalyst. The final long-range ordering of the silica-surfactant biphase composite material was found to be, depending on the reaction conditions, hexagonal, cubic, or lamellar. In this paper, we present an insight into the intermediate structures present in the solution during the synthesis of the hexagonal derivativesMCM41sbefore longrange ordering is obtained. Cryo-transmission electron microscopy provides direct information on the primary building unitssclusters of elongated micellesswhich appear at the very early stage of the hexagonal mesophase formation, while small angle X-ray scattering is used for accurate structural analysis.
Introduction M41S is synthesized by hydrothermal treatment of silica and water with a surfactant (e.g. cetyltrimethylammonium chloride, CTAC) that is acting as a templating agent to an amorphous solution of silica. A sigmoid type curve is obtained when plotting the X-ray crystallinity of the reaction product against time of crystallization, preceded by an induction period. A typical induction period might take a few hours, and the final crystalline product is usually obtained after about 4 days. Following nucleation in the gel phase, numerous M41S nuclei appear at an early stage, and the crystalline product can be identified by small angle X-ray scattering (SAXS) after a few hours. Obtaining the large pore size of the mesoporous material (2-10 nm) in comparison to that of microporous material (
