A reaction within a porous solid can be controlled by opening and closing a chemical gate across the ends of the pores.

Masahiro Fujiwara and colleagues at the National Institute of Advanced Industrial Science and Technology, Japan, have grafted disulfide bridges across the pore outlets of a mesoporous solid which has acidic sites in the structure.  A mesoporous material contains pores with diameters between 2 and 50 nanometres.  

The acidic positions in Fujiwara's mesoporous solid are catalytic sites for the reaction being monitored - the dimerisation of -methylstyrene.  

When the disulfide bridge is intact there is no evidence of dimerisation, as the reaction relies on the reagent reaching the catalytic sites within the porous solid.  Adding a reducing agent breaks the disulfide link, opening the gate and allowing the reagent into the pores so the reaction can take place.  If the gate is closed again, by adding an oxidising agent, the catalytic reaction stops.  

This method for controlling a reaction is more attractive than established methods, explained Fujiwara. 'In general, cumbersome treatments are required for stopping a reaction,' said Fujiwara, 'for example, stopping the reactants, decreasing temperature or removing the catalyst.'

'However, this system uses a simple method to control the catalytic reaction by just addition of a molecule,' he added.

'As there is large scope for functionalising mesoporous materials, other gating systems could be used to control the reaction rate and selectivity of catalytic reactions,' said Fujiwara.

Alison Stoddart