New mesoporous materials

In the past 20 years or so, the field of supramolecular chemistry has become enormously important, with Jean-Marie Lehn, Donald Cram and Charles Pedersen winning the Nobel Prize in 1987. The concept of supramolecular chemistry is that molecules can self-organise into definite structures, without forming covalent bonds, but rather through weaker interactions such as hydrogen bonding. A hydrogen bond is a special type of weak chemical bond, which holds water molecules together, giving water many unique properties - the same bond is critical to the formation of the double helix of DNA, and is often of extreme importance in biological systems. Hydrophobic interactions, also important in self-assembly, are interactions between oily molecules which minimise contact with water by causing the oily parts to huddle together. One example of the latter, although not at all new, is the ability of molecules containing a polar head group and a long non-polar hydrocarbon tail (surfactants) to form micelles in polar, aqueous environments. These micelles form because the water-repelling hydrocarbon tails gather together in the centre of a sphere, or sometimes a cylinder, to avoid contact with water. The polar head groups then form a layer on the surface of the sphere or cylinder, forming a barrier between the hydrocarbon tails and the water. The best-known example of these micelle-forming materials are detergents.

The diameter of the micelles depends on the exact nature of the sur factant, but is typically of the order of 2-4nm. Interestingly, these dimensions are exactly those required for the pores in a mesoporous catalyst. The high profile of supramolecular chemistry helped to highlight such systems, and chemists from Mobil were the first to realise that this chemistry could be applied to catalyst design. Whereas initial approaches to mesoporous zeolites relied on larger and larger individual template molecules, Mobil researchers found that they could use supramolecular assemblies of molecules as templates. They chose long chain quaternary ammonium salts as the micelle forming agent, and reacted Si and Al precursors around these using conditions similar to those for zeolite manufacture: removal of the template micelle, again by calcination, leaves a solid with pores, where the micelles were.

These materials, known as MTSs (Micelle Templated Silicas) can be prepared with a range of pore sizes (see Figure 4.4). As the pore size is essentially the diameter of the micelle template, it is easy to estimate the pore size obtained with a given template. For example, a MTS made with a dodecyl trialkylammonium (C12) template would have a pore diameter approximately twice the length of the dodecyl trialkylammonium species - roughly 2.2nm. As the chain length of the template molecules decreases, there comes a point where they do not form micelles. This happens around C8, meaning that the smallest pores achievable using this method are around 1.8 nm. Luckily, this is almost ideal in many ways, since the largest zeolites have pore sizes of c. 1.3 nm, almost seamlessly extending the range of pore sizes available to the chemist. At the other extreme, as the chain length increases, the ability of the quaternary salt to form micelles decreases, due to lack of solubility, and the largest template molecule which can easily be used is the C18 trialkylammonium salt. This gives a pore size of c. 3.7nm. This range of sizes is sufficiently broad to allow ingress and reaction of many large molecules, but the Mobil researchers managed to increase the pore dimensions even further by expanding the micelle. They did this by inserting hydrophobic mesitylene (trimethylben-zene) molecules into the interior of the micelle. The rationale is that the mesitylene molecules will preferentially exist in the hydrocarbon interior of the micelle, rather than in the aqueous environment outside the micelle, causing the micelle to expand (see Figure 4.5).

MTS materials grown using these expanded micelles have pore sizes from 4.0 to 10nm, depending on the quantity of mesitylene added during synthesis.

Figure 4.4. Preparation of MTS materials. The diagram shows self assembly of the surfactant into micelles followed by condensation of silica around the micelles. The micelles arrange themselves into an approximately hexagonal array. After the formation of the silica around the micelles, the micelles are burnt out, leaving pores where the micelles were. The pores are an accurate reflection of the size and shape of the micelles. This makes the pores uniformly sized and shaped.

Figure 4.4. Preparation of MTS materials. The diagram shows self assembly of the surfactant into micelles followed by condensation of silica around the micelles. The micelles arrange themselves into an approximately hexagonal array. After the formation of the silica around the micelles, the micelles are burnt out, leaving pores where the micelles were. The pores are an accurate reflection of the size and shape of the micelles. This makes the pores uniformly sized and shaped.

Figure 4.5. Expansion of a micelle by inclusion of a hydrophobic guest into the hydrophobic interior of the micelles. The guest is hydrophobic, and thus does not like being in water. The interior of the micelle is similarly water-repellent, and thus is a much more comfortable environment for the guest. The incorporation of the guest into the centre of the micelle causes an expansion, which in turn leads to larger pores in the resultant material.

Figure 4.5. Expansion of a micelle by inclusion of a hydrophobic guest into the hydrophobic interior of the micelles. The guest is hydrophobic, and thus does not like being in water. The interior of the micelle is similarly water-repellent, and thus is a much more comfortable environment for the guest. The incorporation of the guest into the centre of the micelle causes an expansion, which in turn leads to larger pores in the resultant material.

A further consideration in porous materials is the shape of the pores. Molecules have to diffuse through the pores to feel the effect of the catalytic groups which exist in the interior and, after reaction, the reaction products must diffuse out. These diffusion processes can often be the slowest step in the reaction sequence, and thus pores which allow rapid diffusion will provide the most active catalysts. It is another feature of the MTSs that they have quite straight, cylindrical pores - ideal for the rapid diffusion of molecules.

One final extension of the original methodology is that different templates can be used to structure the materials. Two of the most useful systems developed were discovered by Tom Pinnavaia of Michigan State University. These methods allow for the complete recovery of template, so that it can be reused, minimising waste in the preparation of the materials, and giving a much greater degree of flexibility to the preparation, allowing the incorporation of a great variety of other catalytic groups.

More recently, many workers have concentrated on controlling the size and shape of particles, with an eye on industrial applications, where such features must be well defined and controllable. Many shapes have been made, including fibres, spheres, plates, as well as membranes cast on surfaces. All these shapes could one day find application, not only in catalysis, but in adsorption of e.g. pollutants from water, molecular wires, and a host of other devices.

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