Figure 4.7. Possibilities for the synthesis of Vitamin K3. The small pore titanium zeolite TS-1 cannot fit the large naphthalene molecule into its pore system, and thus is effective in this transformation. The larger titanium MTS material is capable of interacting with the molecule, and the desired transformation can take place.
taining materials have been prepared by three different routes. These materials are capable of the fast and efficient catalysis of several reactions with excellent selectivity. Activity is greater than that found with the traditional amine-containing silicas, as is the stability of the catalyst, allowing more product to be prepared with a given amount of catalyst. The increased amount of amine groups which can be attached to the MTS materials gives them even more of an advantage over the traditional catalysts. Initial results on a catalyst with both amine and phenyl (non-polar) groups indicate a substantial rate increase over the simple amine-only material. The reasons for this are not yet understood, but may be due to improved transport of reagents and products onto and off the surface. Many important reactions can be carried out with such solid bases, and their uses in chemistry will increase. In particular, many reactions which either do not generate any side products or only generate water (condensation reactions) are amenable to catalysis using these materials. Early work on such systems indicates that the future for these materials is very rosy.
Sulphur-containing materials have been found to be excellent adsorbents for heavy metals. The sulphur atom is known to complex strongly to heavy metal ions, with gold and mercury being two particularly interesting examples. The higher amounts of sulphur which can be attached to the MTS materials means that their capacity for binding these metals, and removing them from e.g. drinking water, is much greater than that achieved with traditional materials.
Solid acids can also be prepared from these materials by transformation of the sulphur group to the sulphonic acid, very closely related to sulphuric acid, one of the most commonly used acids industrially. The material can be easily recovered and easily handled; since the acidity resides within pores, it cannot come into contact with living tissue. Important transformations, such as the formation of synthetic lubricants and intermediates for fragrances, have already been reported using these materials. The scope for such materials in future is enormous.
More sophisticated materials have been made by attachment of transition metal complexes to the surface. These materials are designed to enhance the fundamental activity of the metal ion, by providing it with an environment tailored to make it as active as possible, and to aid in its recovery afterwards. The heterogenisation of such (normally homogeneous) complexes has attracted a lot of attention, since the heterogeneous equivalents can be much more easily separated and recycled than the homogeneous ones, leading to much less waste being produced. These materials have been shown to be very active in a range of reactions, leading to many important product types. One particularly important area of chemistry is the selective preparation of one of a pair of mirror images of a compound. This so-called chiral (from Greek; chiros - hand) catalysis requires great control over the exact chemical environment of the catalytic site, and is one of the major challenges in synthetic chemistry. Many drugs and agrochemicals can exist as two forms which are mirror images of one another, only one of which is useful, the other being useless or even dangerous. It is therefore important to be able to prepare only the desired form. As an example of mesoporous materials containing chiral metal-centred catalysts, the group of Daniel Brunel in Montpellier has published work on transformations using zinc species. Selectivity to the desired form was good, approaching that achievable with conventional systems. Further refinement of these systems will lead to improvements in the design of the catalytic site, and its surrounds, and the prospects for this area of catalysis are exciting.
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