'Green' synthesis plant wins RSC award

New awards for innovation in teamwork were presented by The Royal Society of Chemistry at a ceremony which took place in Burlington House on Thursday January 16, 2003.

A family of versatile, easy-to-handle catalysts for manufacturing optically-active materials, under development for five years, wins the award for Huddersfield-based Avecia. Thomas Swan, the County Durham-based speciality producer, won for its multi-purpose synthesis process which uses supercritical carbon dioxide as a solvent.

The Avecia team won its award for a series of catalysts which are already being used within the pharmaceutical industry. The Catalytic Asymmetric Transfer Hydrogenation (CATHy) catalysts are used primarily to make chiral molecules from non-chiral starting materials using cheap, plentiful reagents.

Developed by a team lead by John Blacker, the CATHy catalysts are asymmetric organometallic complexes comprising a precious metal, such as rhenium, a chiral ligand, and a cyclopentadiene group. This works by shifting a hydrogen atom from a reagent such as isopropanol to one face of a substrate molecule. If this is a ketone, the product will be a chiral alcohol; for an imine, the result is a chiral amine.

The catalysts are simple to make, as their building blocks are commercially available, and equally easy to handle. They are neither air- nor water-sensitive and do not need hydrogen gas to work.

Thomas Swan's award, meanwhile, was for its groundbreaking process using supercritical carbon dioxide as a solvent for organic synthesis. Hailed as 'the first green chemical plant', the Swan-SCF process was developed jointly by the Consett-based company and researchers at the University of Nottingham, led by Martyn Poliakoff.

According to Thomas Swan's general manager, Dai Hayward, the Swan-SCF process 'is remarkable for its accuracy, processing only what needs to be targeted and so eliminating waste and problem by-products.'

Replacing the organic solvents usually used for synthesis processes both simplifies the process and delivers environmental improvements. Simply reducing the pressure returns the CO2 to a gaseous state, removing the need to strip away solvents by distillation. And, of course, there is no problem with waste solvent, which would need to be treated and disposed of in a conventional process. Moreover, the CO2 performs better as a solvent than its conventional counterparts: for hydrogentation reactions, for example, it is highly efficient at dissolving hydrogen, allowing the process to be performed continuously.

The process can be used to carry out industrially-significant reactions such as hydroformations, etherification and Friedel-Crafts alkylations and acylations. The latter two are attracting particular interest from other process operators, according to Thomas Swan; Friedel-Crafts reactions are highly efficient in supercritical media, with conversion rates some 60-70 per cent higher than in conventional processes. What's more, they produce no by-products or chlorinated wastes.

The efficiency gains can also be seen in other reactions, the company claims, with 100 per cent selectivity and 100 per cent conversion in a single pass for 'a large number of reactions'. Says Hayward, 'the selectivity is remarkable in many reactions, targeting specific reactive sites of molecules to optimise the synthetic outcomes and eliminate waste.'

The low viscosity, high diffusivity environment provided by supercritical carbon dioxide, which combines the properties of a liquid and a gas, maximises mass transport; moreover, it allows temperature, pressure, flow-rate and residence times to be adjusted independently. Developing the process took five years, and the first full-scale plant - a continuous multi-purpose synthesis unit - started up last July. The plant was built by Swedish company Chematur, which already had experience in building plants using supercritical materials.

Currently, it is producing an acetone derivative, trimethylcyclohexanone, which is used in styrene products. This, however, is only a basic trial product, primarily to calibrate the equipment.

Badge detects styrene levels

Also highly commended at the RSC Awards, was research to monitor personal exposure to styrene monomer. Commonly used in many areas of the plastics industry, styrene can cause respiratory problems, drowsiness and memory loss if inhaled. Exposure is widespread, especially in the production of glass fibre-reinforced plastics.

Steve Ross from PiezOptic in Ashford, Kent, led a six-person team to develop the sensor, which is mounted on a badge. Based on piezo-optics, the sensor incorporates a solid-phase tribromide reagent which is dispersed in a polymer resin and 'spotted' onto a piezoelectric film.

The reagent changes colour from yellow to white on exposure to styrene. An LED shines light up through the film and the reagent spot, generating heat which causes the film to expand and a charge to form on its surface. As the amount of heat generated depends on the colour of the spot - which in turn depends on the amount of styrene in the atmosphere - the size of the charge also gives a measure of styrene concentration. The system can differentiate styrene levels between 0-70ppm.