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Chemistry on the inside 65

factant, but is typically of the order of 2–4nm. Interestingly, these dimen-
sions 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 mole-
cules 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 essen-
tially 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 (C ) template would have a pore diameter
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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
C , meaning that the smallest pores achievable using this method are
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around 1.8nm. Luckily, this is almost ideal in many ways, since the largest
zeolites have pore sizes of c. 1.3nm, 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 C trialkylammonium salt. This gives a
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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.