Carbon nanotubes - a boon for chiral catalysts

Researchers in China have created a new catalyst that could help in the production of chiral molecules for medical drugs. The catalyst, which consists of platinum nanoparticles encapsulated in carbon nanotubes, is the most active of its type ever reported, they say.

A chiral molecule is asymmetric - not symmetrical - in such a way that its mirror image cannot be super-imposed on itself. Although a chiral molecule’s two manifestations, known as enantiomers, (the molecule and its mirror opposite) are often produced in reactions in equal quantities, they can have different properties. In medicine, one enantiomer can be more effective than the other, and researchers are keen to find ways to encourage production of single enantiomers, rather than mixtures.

An asymmetric catalyst manipulates a reaction to favour the production of only one enantiomer. Can Li and colleagues at the Chinese Academy of Sciences in Dalian, have developed a heterogeneous asymmetric catalyst that could be one of the most effective yet. They submerged carbon nanotubes in an aqueous solution of H₂PtCl₆. Heating and slow drying introduced the platinum into the nanotube channels, and after reduction using sodium formate, the platinum nanoparticles were encapsulated inside the nanotubes.

In the past, most asymmetric catalysts have been homogeneous, where they are dissolved in a solvent along with the reactants. However, separation of the products can be difficult. Heterogeneous catalysts, which are in a different phase to the reactants, are better in this respect. When the team used their catalyst in a standard hydrogenation reaction, they found that 96 per cent of the product was just one enantiomer, produced at a rate or ’turnover frequency’ above 1 x 10⁵ per hour, which is superior to that of the well-known Pt/Al₂O₃ catalyst under the same reaction conditions. 

The carbon nanotubes create an environment where the reactants and catalyst are in high concentration and in close proximity. ’This catalyst has the potential application in industry for the production of chiral compounds, which can be used as chiral drugs such as [those] for treating high blood pressure,’ says Qihua Yang, a member of Li’s group. 

Alfons Baiker, an expert in asymmetric catalysts at the Swiss Federal Institute of Technology in Zurich, Switzerland, thinks the discovery is ’scientifically interesting but not really exciting.’ The trouble, he says, is that the rate is good, but the 96 per cent selectivity is not good enough. In medicine, selectivity is crucial because the wrong enantiomer could pose health risks.  

Li and his team believe that their work will open up opportunities for developing more novel, highly active and enantioselective heterogeneous chiral catalysts in the future. 

Jon Cartwright