Catalysis: Catalytic trees improve selectivity

Chemists at the University of St Andrews, UK, have synthesised a range of tree-like catalysts which, to the researchers’ surprise, show improved selectivity for certain reactions depending on the number of branches in the tree.

The researchers were originally investigating the use of dendrimers as a way to aid the recovery of small catalysts participating in homogeneous reactions. ’The idea was that if you attach small catalytic units to the branches of large dendrimers, the catalysts can take part in homogeneous reactions but, being part of a larger entity, are more easily recoverable,’ says Russell Morris, the group’s leader.

However, the researchers noticed that the dendrimer systems showed markedly improved selectivity for the reaction that the team was studying - phosphine-mediated hydroformylation of olefins to make aldehydes - depending on the size of dendrimer. The reaction produces both linear and branched aldehydes, and certain sizes of dendrimer produced a significantly higher proportion of the linear configuration, which is the form that is more useful to the chemical industry.

Morris’s team base their dendrimers around cubic polyhedral silsesquioxanes. Phosphine-based catalysts are grafted to the end branches.

’We discovered that if the dendrimer has a small number of outer branches, say eight, we typically obtained ratios of linear to branched aldehyde product of three, four or five to one,’ said Morris. ’However, when we increased the size of the dendrimer to 16 or 24 end branches, we obtained ratios of 13, 14 or 15 to one.’

Further increasing the size of the dendrimer, however, resulted in a decreased efficiency of selectivity, suggesting that there is an optimal size of tree.

’It would seem that the optimal size of dendrimer creates the most efficient thermodynamic environment for the selective formation of linear aldehyde, providing the specific angles and distances between the bonds of the catalyst that are needed for the optimal selectivity,’ Morris says.

He and his team have now begun to carry out detailed molecular modelling of the system to unravel the structures and energies of the various transition states during the reaction.

Simon Hadlington