University of Cambridge > > Electron Microscopy Group Seminars > Use of mesoporous materials to overcome reactant incompatibility and for homogeneous and heterogeneous catalysis

Use of mesoporous materials to overcome reactant incompatibility and for homogeneous and heterogeneous catalysis

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Microemulsions and other microheterogeneous media can be used to overcome incompatibility problems in organic synthesis. Reactant incompatibility is a well-known problem in preparative organic chemistry and a common approach to the problem is to use a two-phase system with added phase transfer catalyst. We have shown that the microemulsion concept can be seen not only as an alternative but also as a complement to phase transfer catalysis. Very high reaction rates were obtained when the reaction was performed in a microemulsion with added phase transfer catalyst. We have also shown that the microemulsion-based reaction medium need not be a one-phase system. Also Winsor systems, i.e., a microemulsion coexisting with an excess oil phase, an excess water phase or two excess phases, work well.

Liquid crystalline phases and dispersions of mesoporous materials are other types of microheterogeneous media useful for overcoming reactant incompatibility. Mesoporous materials can also be used as host for homogeneous catalysts. Such a catalyst, which may be a synthetic metal-organic compound or an enzyme, can be immersed into the aqueous pores of the mesoporous material. Catalyst-loaded fine particles of the mesoporous material are subsequently dispersed in an apolar phase that contains lipophilic reactants. The reaction occurs at the interface, i.e., the pore openings. The approach can also be reversed. Instead of using hydrophilic mesoporous oxide, such as silica, alumina or titania, a mesoporous material that is hydrophobic in character is used. It may be either mesoporous carbon or a mesoporous oxide that has been hydrophobized by a silanization procedure. A hydrophobic homogeneous catalyst is immersed in the pores and the catalyst-filled material is subsequently dispersed in an aqueous solution of hydrophilic reactants. Again the reaction occurs at the pore openings.

The approach of incorporating a homogeneous catalyst in the pores of a mesoporous material (“heterogenization”) combines the benefits of homogeneous catalysis with those of heterogeneous catalysis. The reaction process is that of homogeneous catalysis; yet, the work-up is as simple as for heterogeneous catalysis: the catalyst-loaded particles are simply filtered off and reused. We have also demonstrated that a column procedure can be used. The reactants are added to a column packed with the catalyst-containing particles and the products are recovered from the effluent.

This talk is part of the Electron Microscopy Group Seminars series.

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