Innovations in catalysis

Immobilised homogeneous catalysts prepared from fibrous polymer supports are set give better separation and to improve production yields, says Stephanie Buckley from Johnson Matthey, manufacturer of FibreCat catalysts

Catalysts are commonly grouped as either homogeneous or heterogeneous, with both types demonstrating advantages and disadvantages. Homogeneous catalysts offer superior selectivity but have been held back by the practical difficulties of separating them from the reaction matrix.

But with FibreCat the user can now achieve activities and selectivities commonly associated with homogeneous catalysts while being able to easily filter the catalyst from solution, leaving a clean product.

Using a fibrous polymer support to anchor the homogeneous catalyst allows a wide flexibility in both the preparation of the catalyst and use during the reaction. Fibres can be processed into various forms to suit reactor type and also have good mechanical properties so that no degradation is seen during agitation. The fibres readily swell in a wide range of solvents to allow easy accessibility to the active sites and greater durability.

ligand loading

Inert polymer fibres are functionalised using graft copolymerisation, with careful control of the degree of functionalisation to achieve the ligand loading required. In addition, the process is designed to locate the functionalised groups at the edge of the fibre so that they are easily accessible. Also, since the ligand is contained within the fibre no extra ligand needs be added during the reaction. A range of precious metal precursors can be deposited onto the functionalised fibre to produce the active catalyst.

In general the homogeneous catalyst formed is covalently linked to the fibrous polymer, resulting in a strong covalent bond between the homogeneous catalyst and the support.

A range of catalysts is now available for coupling reactions, including hydrogenations, selective oxidations and dihydroxylation reactions.

Levels of selectivity have been compared with those achievable with conventional homogeneous catalysts in a variety of reactions. In general the FibreCat versions show comparable activities to the non-anchored versions but may take longer to reach the equivalent levels. Once the reaction is complete, the catalysts are easily removed by filtration, leaving a clean product that does not require any further separation. Figures 1 and 2 show FibreCat in some common reactions.

There are many benefits achievable when using fibrous catalysts to replace conventional homogeneous type catalysts.

reaction conditions

The fibres are compatible with a wide range of solvents, from aqueous/organics to non-polar hydrocarbons, and swell readily, allowing easy access to the active sites. Furthermore, the standard fibres are stable up to a temperature of 120ºC. Dependent on reaction conditions and the way in which the fibres have been functionalised, they have been used up to 150ºC.

FibreCat catalysts offer a range of benefits to the customer over both conventional homogeneous and heterogeneous catalysts. Better separation of the catalysts improves product yields and speeds up total process time, while covalent bonding ensures low leaching and a pure end product.