Solid state NMR of immobilized catalysts and nanocatalysts

In the last decade solid state NMR techniques have evolved into a major tool for the characterization of immobilized homogeneous catalysts, tethered to a solid support material. Of particular interest are supports consisting of porous materials, which are periodically structured on the mesoscopic length scale, and have very large specific surfaces and pore volumes. Combining one- and two-dimensional 13C-, 29Si- and 31P-solid state MAS-NMR techniques, it is now possible to elucidate details of the binding of the catalyst molecule to the surface on the molecular level. These solid state NMR techniques do not require crystalline samples or special clean well defined surfaces like the conventional surface science techniques, but can work with typical ill-defined real-world systems. This review discusses, after a short introduction, the salient features of these materials and the applied NMR experiments to give the reader a basic knowledge of the systems and the experiments. The rest of the review focuses on recent examples, which demonstrate the power of the method. First the structural properties of the Wilkinson's catalyst, bound to the surface of SBA-3 are analyzed in detail. It is shown that two of the three phosphine ligands of the catalyst are replaced by a linker. Then a short overview about solid state NMR characterization of surface reactions and surface species of metallic nanocatalysts is given. The review is closed with an outlook about very recent developments in solid state NMR methodology. It is reported that fast-MAS, indirect detection and hyperpolarization solid state NMR techniques lead to astonishing sensitivity improvements, which bring the structural characterization of technical catalysts with low specific surface areas into reach.