Towards a molecular understanding of shape selectivity

Zeolites are crystalline materials with ordered pore structures that are widely used as industrial catalysts. They are particularly important in oil refining, where the outcome of chemical transformations is strongly influenced by the pore topology of the zeolite catalyst. In a review, Berend Smit and Theo Maesen argue that this so-called shape selectivity can be rationalized using a straightforward thermodynamic analysis of how pore topology affects the free energies of formation of the reactants, intermediates and products. Despite the need for drastic simplifications, the approach can explain experimental observations and even guide the search for zeolite structures optimized for specific catalytic applications. The 'shape selectivity' of zeolites can be rationalized using a straightforward thermodynamic analysis of how pore topology affects the free energies of formation of the reactants, intermediates and products involved in the chemical transformations for oil refining catalysed by the zeolite. It is shown that despite some drastic simplifications, the approach can even guide the search for zeolite structures that are particularly suitable for desired catalytic applications. Shape selectivity is a simple concept: the transformation of reactants into products depends on how the processed molecules fit the active site of the catalyst. Nature makes abundant use of this concept, in that enzymes usually process only very few molecules, which fit their active sites. Industry has also exploited shape selectivity in zeolite catalysis for almost 50 years, yet our mechanistic understanding remains rather limited. Here we review shape selectivity in zeolite catalysis, and argue that a simple thermodynamic analysis of the molecules adsorbed inside the zeolite pores can explain which products form and guide the identification of zeolite structures that are particularly suitable for desired catalytic applications.