DNP Enhanced Solid-State NMR Spectroscopy of Heterogeneous Catalysts

Abstract

International audienceHeterogeneous catalysts are key to efficient processes in the chemical industry. However, they are difficult to improve because of the lack of access to their active-site structures. Although solid-state NMR is the method of choice to describe at atomic level the structure of catalysts, it is plagued by its intrinsic low sensitivity. This limitation is further exacerbated by the small fraction of active sites on the materials and by their often disordered and multi-site nature. Recently it has been demonstrated that this limit can be overcome by using dynamic nuclear polarization (DNP) which allows enhancement factors of up to 250 in solutions, at a magnetic field of 9.4 T and sample temperatures of ca. 100 K. Key to transposing the high enhancement factors observed for bulk frozen solutions to materials is the use of incipient wetness impregnation. In this approach, the materials are wetted by a minimal amount of radical solution. If the first proof of concepts was reported on model mesoporous silica materials, recent applications by our group and others concern a diverse range of chemical systems such as nanoparticles, mixed oxides, cementitious materials or microcrystalline solids. Here we will present new applications of DNP SENS in heterogeneous catalysis. We will show that DNP SENS allows to directly measure structural information of surface reaction intermediates in alkene metathesis catalysts, namely by obtaining C-C connectivities and bond distances. We will also show how the gain in sensitivity provided by DNP allows us to determine the full three-dimensional atomic-scale structure of a catalytically relevant organometallic complex anchored on a silica surface. This is done through a series of multi-dimensional and multi-nuclear NMR experiments producing several inter-nuclear distance constraints and the implementation of sophisticated NMR structure determination protocols