Reactivity in the confined spaces of zeolites: the interplay between spectroscopy and theory to develop structure–activity relationships for catalysis

The Lewis acid properties of a series of isolated and well-defined Sn centers in different micro- and mesoporous materials have been investigated by means of DFT calculations and IR spectroscopy of probe molecules, and the results have been related to the experimentally measured catalytic activity of these materials for different reactions. Different types of Sn centers have been detected and modelled: inactive fully coordinated Sn atoms in framework positions that weakly interact with Lewis bases, and highly active strong Lewis acid sites associated with Sn centers with one or two hydrolyzed Sn–O–Si bridges. The relative amount of strong and weak sites depends on the zeolite structure, and can be modified by the catalyst pre-treatment conditions. On the other hand, it has been established that Sn-containing mesoporous materials obtained either by direct synthesis or by post-synthesis incorporation of Sn, show a similar distribution of centers. Finally, the influence of solvent and free space available around the active site on the diastereoselectivity of the cyclization of citronellal to isopulegol were investigated, and some hints were obtained on how to improve the catalyst performance.