The Second Rideal Lecture. What makes a catalyst selective ?

Abstract

Selectivity, i.e. the ability to catalyse preferentially one of several possible chemical reactions, is likely to become the most highly valued property of modern heterogeneous catalysts in industry. Once the reaction path of a catalysed reaction is known in every atomic detail, one can define the conditions which the catalyst must fulfil in order to enable the reaction to proceed. Whenever these conditions differ for different reaction paths, the catalyst which meets the requirements of only one of these paths will be selective for this particular reaction. The subject of catalyst selectivity can therefore be discussed in terms of the catalyst requirements for defined reaction mechanisms. Using this approach and modern results, an attempt is made to arrive at a systematic classification of these catalyst requirements.

For reactions catalysed by isolated metal atoms in the surface of the catalyst, selectivity can be due to the different numbers of vacant ligand sites required, as appears to be the case with sulphide catalysts. For reactions requiring ensembles of contiguous metal atoms, different ensemble requirements control the selectivity of alloy and metal-organic complex catalysts. The stereochemistry of ligands attached to a catalytic site and acting as “templates” controls stereoselectivity, including enantioselectivity.