Introduction

Porous silicas form a large family of solid state materials. According to the definition accepted by International Union of Pure and Applied Chemistry, they can be grouped into three classes based on their pore diameter (d): microporous (d < 2 nm), mesoporous (2 nm < d < 50 nm), and macro-porous (d > 50 nm) silicas.

Microporous zeolites, normally having pore diameters below 1 nm, are widely used as catalysts in industry. They have perfect crystalline framework. For example, zeolite LTA has a cubic unit cell with the cell parameter a = 1.2 nm, space group Pm3m. Twenty-four Si and Al atoms occupy the (24 k) sites (0.370, 0.183, 0) and 48 oxygen atoms occupy the sites of (24 m) (0.110, 0.110, 0.345), (12 h) (0, 0.220, 0.5), and (12 i) (0.289, 0.289, 0) respectively [1]. On the contrary, however, most mesoporous silicas synthesized so far have no ordering at the atomic level (i.e., the frameworks of these materials are amorphous). So-called ordered mesoporous silicas normally refer only the ordering of the mesopores.

Although mesopores can be formed during partial decomposition of zeolites [2] or via other methods, ordered meso-porous silicas did not become an active topic until 1992 when Mobil scientists reported their synthesis of the M41S series materials [3, 4]. Since then tremendous progress has been made in the synthesis of new mesoporous phases and in potential applications of these materials [5].

The most popular formation mechanism of these meso-porous silicas is so called liquid crystal templating (LCT) mechanism [4, 6]. According to this mechanism, the surfactant molecules self-organize into micelles which further form liquid crystals in the solution. Silicate species then deposit on the surface of the micelles and in the intermi-celle space to form a condensed silicate framework. When the surfactant is removed by calcination, an ordered meso-porous silica phase is produced with the same symmetry as the parent liquid crystal. The real mechanism seems to be much more complicated. For instance, during the formation of MCM-41, the most extensively studied mesoporous silica, pure liquid crystals may not exist in the solution. Interaction between individual micellar rods and the silicate species must take place and the presence of silicate layers on the surface of the micellar rods enhances the aggregation of these nanorods as indicated by an in-situ 14N nuclear magnetic resonance (NMR) study [7]. Furthermore, Stucky and co-workers believe that the dynamic interplay among ion-pair inorganic and organic species is crucial in the formation of the mesoporous phases [8-12]. Many synthesized meso-porous silicas have indeed structures without known liquid crystal analogs. In transmission electron microscopic (TEM) investigations of series specimens collected after different reaction times, it was proposed that the formation of the micelles and deposition of the silicate species may take place spontaneously [13-15]. Some of the mesoporous phases such as FSM-16 are formed via phase transformation of layered silicates and, therefore, must have a significantly different formation mechanism [16].

In this chapter, we are not going to deal with the arguments on the detailed formation mechanisms of the meso-porous silicas but instead discuss the structures of the final products. In comparison with microporous zeolites, a few important structural features may be expected from the mesoporous silicas. First, the pore structures of most meso-porous silicas retain the symmetries of the liquid crystals, while others do not have liquid crystal analogs but usually simple packed structures with high symmetries. Second, the shape and size of the pores in a mesoporous silica depend on the shape of the micelles and the length of the surfactant molecules. Third, the silica framework is amorphous and its density is variable. Consequently, the structures of the meso-porous silicas become highly tunable with almost an infinite variability.

Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (149-160)

In the following sections a general review of the structures of most typical mesoporous silicas and their derivatives will be given. Detailed synthetic conditions can be found in the original papers, previous review articles [17, 18], and a chapter by D. Zhao in this encyclopedia. They will not be mentioned in this chapter unless they are necessary in helping us to understand the structures.

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