shapeselective

Shape-selective refers to a property of certain catalytic systems in which the selectivity for reactants, intermediates, or products is governed largely by the three-dimensional shape and size of molecules relative to the pores or cavities of the catalyst. In porous solids such as zeolites and metal-organic frameworks, as well as in some enzyme active sites, the geometry of the internal framework imposes steric constraints that favor only certain molecular orientations and sizes. As a result, molecules that cannot access the active sites or cannot adopt favorable transition-state geometries are less likely to react, while those that fit are converted more readily, producing product distributions that differ from those predicted solely by thermodynamics.

In practice, shape-selective catalysis takes advantage of pore size, shape, and connectivity to steer reactions. Zeolites with well-defined pore openings and cages can suppress formation of bulky byproducts and favor specific isomers—for example, certain ZSM-5 catalysts promote the formation of para-xylene over ortho- and meta-xylene. Metal-organic frameworks and hierarchical materials with tuned pore architectures are used to extend shape selectivity to larger molecules. Enzymes also exhibit a form of shape selectivity through active-site geometry and substrate binding.

Design strategies include tailoring pore dimensions, implementing constricted channels, and introducing hierarchical porosity to balance diffusion with selectivity. Limitations include diffusion limitations, catalyst deactivation, and trade-offs between activity and selectivity.