Surfactant templated metal oxide

Brinker CJ Dunphy DR, Morphological control of surfactant-templated metal oxide films, Curr. Opin. Colloid Interface Set, 2006, 11, 126-132. 

Czuryszkiewicz T, Rosenholm J, Kleitz F, Linden M (2002) Synthesis and characterization of mesoscopically ordered surfactant/cosurfactant templated metal oxides. Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium, Book Series Studies in Surface Science and Catalysis, Pts A and B 142 1117-1124... 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

An excellent example that shows the potential of combining various bottom-up techniques is the joint work by Whitesides and Stucky [4]. Hierarchical metallic oxides were produced by combining (i) sol-gel self-assembly of neutral surfactants, (ii) spherical polystyrene templates, and (iii) molds with micrometric cavities (micromolding). Figure 3.12 shows how the described materials are hierarchically organized at several scales ranging from a few nanometers to hundreds of micrometers. 

Complexation of metal ions and subsequent incorporation of the resulting metal complex into the oxide matrix during surfactant-templated synthesis prevents aggregation and leads to a homogeneous distribution of metal centers in the mesostructure. Copper- and vanadium-substituted mesoporous silicas were prepared in this way [107,108], Such materials have great potential in the field of catalysis. 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

In addition to the one-dimensional templated structure of the MCM-41 materials, two- and three-dimensional systems have also been prepared. A number of papers have used the lamellar structures of amphiphile assemblies to prepare flat, striated metal oxide materials [72,73]. These materials often exhibit enhanced properties over materials that have uncontrolled three-dimensional growth. Vesicles have also been used to engineer spherical imprints into silicates [74,75]. Even more elaborate supramolecular surfactant systems, that yield toroidal and other unusually shaped metal oxides, have also been reported [76,77]. 

Fig. 8.16. Supramolecular assembly of surfactants to create organised rod-like micelles can template the formation of metal oxides with well defined mesoporous structure, i.e. MCM-41 materials. Adapted from [66].

Compared with metal oxides, less attention has been paid to the synthesis of mesostructured metal sulfides [61,205]. The only systematic work was reported by Anderson and Newcomer [205]. The liquid-crystal templating approach was applied to metal sulfides, such as Mo, W, Co, Fe, Zn, Ga, Sn and Sb sulfides. All of the products were lamellar and consisted of bilayers or interdigitated layers of surfactant molecules sandwiched between metal sulfide layers. 

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