Mesostructured composite materials, with features ranging from 20 to 500 A in size, are obtained by the kinetically controlled competitive assembly of organic and inorganic species into nanostructured domains. Short-range order is limited, and long-range order is determined by weak forces such as van der Waals or hydrogen-bonding. Three-dimensional mesoporous materials obtained by removing the organic phase are of particular interest for applications such as catalysis and chemical sensing or separation, for which structural features such as cavity shape, connectivity and ordered bimodal porosity are critical. But atomic-scale structural characterization by the usual diffraction techniques is challenging for these partially ordered materials because of the difficulty in obtaining large (> 10 microm) single crystals, and because large repeat spacings cause diffraction intensities to fall off rapidly with scattering angle so that only limited small-angle data are available. Here we present a general approach for the direct determination of three-dimensional mesoporous structures by electron microscopy. The structure solutions are obtained uniquely without pre-assumed models or parametrization. We report high-resolution details of cage and pore structures of periodically ordered mesoporous materials, which reveal a highly ordered dual micro- and mesoscale pore structure.

Recently, we have developed a new electron crystallography (EC) method for study of three dimensional (3D) structures of silica-mesoporous materials, and the 3D-structural solutions of MCM-48 and SBA-1, -6, and -16 were briefly reported. The method gives a unique structure solution through the Fourier sum of the 3D-structure factors, both amplitudes and phases, which are obtained from Fourier analyses of a set of highresolution electron microscope (HREM) images. The method was fully described in an application for structure analyses of two MCM-48 crystals with different crystal morphologies. Little structural difference was observed between the two crystals, although small differences in the structure factors were observed. The space group of MCM-48 was determined to be Ia3 hd, and the wall surface of the two crystals followed exactly the periodic minimal surface of gyroid (G). The wall separated two interpenetrating and noninterconnecting channel systems with different chiralities. After structural analysis of MCM-48, the structures of two different carbon networks, CMK-1 and CMK-4, which were synthesized within the channels of MCM-48 from different carbon sources, were studied by electron microscopy (EM). It was observed that in both cases carbon networks were equally formed in the two channels of MCM-48 without changing the space-group symmetry and that the symmetry of Ia3 hd was retained after the dissolution of silica mesoporous MCM-48 for CMK-4 but changed to I4 1 /a for CMK-1. The simplest model for structure change in CMK-1 was proposed on the basis of the observations of extra reflections in ED patterns and domain structures in HREM images as that the carbon networks equally formed in two noninterconnecting channels of MCM-48 were displaced during the dissolution relative to each other without rotation along the [001] axis by keeping each network rigidly. It is stressed that the method must be extended further for structural study of new materials with orders in two different lengths scales, atomic and mesoscopic scales.

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The presence of various counteranions at the interfacial region of the silicate-surfactant mesophase introduces opportunities for manipulation of the phase structure. Well-ordered 3D-hexagonal P63/mmc, cubic Pmn, 2D-hexagonal p6mm, and cubic Iad mesoporous materials have been synthesized with the same surfactant, cetyltriethylammonium bromide, in the presence of various acids. The counteranions of acidic media have resulted in increasing the surfactant packing parameter g in the order SO42- < Cl- < Br- < NO3-, which leads to the different time course of formation of mesostructures. The effect of counteranions on the formation of mesostructures is explained in terms of not only the adsorption strength on the headgroups of the surfactant micelle but also the rate of silica condensation affecting the charge density matching between the surfactant and silica. It has been found that the mesophase is always transformed from the larger g parameter into the smaller one. The distinct morphologies of the 3D-hexagonal P63/mmc mesophases have been rationally explained by supposing this particular mesostructure. The cubic Iad phase has been first synthesized under acidic conditions.

The layered polysilicate kanemite can be used to synthesize the novel mesoporous silica KSW‐2 (see picture), which contains square channels. The individual silicate sheets of kanemite are bent during the gradual leaching of hexadecyltrimethylammonium (C16TMA) surfactants from a layered C16TMA–kanemite complex.