Mark Davis at California Institute of Technology, Pasadena, US, explains why catalysts need to learn to cooperate in order to compete with nature  

An enzyme's catalytic activity largely occurs within a relatively small region known as the active site. Here the side chains of multiple amino acids interact both with each other and with reacting molecules, with the joint effects of the different interactions having a large influence on the enzyme's catalytic activity and selectivity. The way these interactions work together is characteristic of enzymes, and is known as cooperative catalysis. An ability to mimic this behaviour using synthetic catalyst materials is highly desirable.

Numerous research groups around the world are attempting to harness the power of cooperative catalysis, to make improved solid catalysts that can combine high catalytic activity and selectivity with the easy separation and recycling offered by solid catalysts. These catalysts typically consist of small organic molecules attached to the surface of a solid support such as silica gel, which fixes the location of the catalytic species much in the same way amino acids are spatially localised within enzymes. A short tether (usually consisting of a few carbon atoms) gives local flexibility and motion that can be important for catalysis to occur.

While enzymes have a considerable head start (thanks to million of years of evolution), researchers are beginning to be able to make highly active catalysts in which multiple different functional groups play an important role. Materials functionalised with both acid and thiol groups, for instance, are excellent catalysts for the synthesis of bisphenol A, a key building block to polycarbonate plastics. These solid catalysts exhibit high activity and selectivity without the stench for which thiol-containing molecules are notorious. They can also replace the use of mineral acids, leading to greener processes.  

Synthetic catalysts containing carboxylate, amine and imidazole groups can mimic protease enzymes, which use these three functionalities in the catalytic triad. In these catalysts, all three functionalities are necessary for good catalytic results.  

Other catalysts containing both acidic and basic groups benefit from the immobilisation of the two otherwise mutually-destructive agents. In solution acids and bases rapidly neutralise, but when both are attached to a solid surface neutralisation can be avoided, and cooperativity between the two types of molecules leads to catalytic activity greater than that achievable through acid or base catalysis alone. These catalysts are shown to be synthetic mimics of aldolase enzymes, which catalyse carbon-carbon bond forming reactions.

Rapid advances have also been made in the engineering of polyfunctional surfaces with particular spatial arrangements. The two cooperating functional groups have been arranged in discrete pairs, resulting in synthetic active sites not unlike those in enzymes, and the distance between the two groups can be tuned for a reaction of interest. The next steps will likely involve arranging three (or more) different groups in a single active site, and further refinements of the synthetic methods necessary to position them accurately. The ultimate goal is to engineer an active site on a solid support where the distance between each functional group is optimised for the desired catalysed reaction. While natural enzymes can exhibit very high activity and selectivity in catalysing chemical reactions, synthetic heterogeneous catalysts can offer other advantages such as solvent versatility, thermal stability, high volumetric productivity and recyclability. By designing future catalysts to utilise cooperativity among multiple functional groups the utility and scope of heterogeneous catalysts will likely be greatly increased.

Read Eric Margelefsky, Ryan Zeidan and Mark Davis' Tutorial Review on 'Cooperative catalysis by silica-supported organic functional groups' in issue 6, 2008 of Chemical Society Reviews