Supported catalysts combine the advantages of homogeneous and heterogeneous catalysts. In fact, the use of a supported catalyst provides the selectivity typical of an homogeneous catalyst and, at the same time, the possibility to easily recover the catalyst, proper of an heterogeneous catalyst. In a supported catalyst, the active species is anchored to a support, that can be inert or catalytically active. The surface characterization is of crucial importance since it gives hints for improving the catalyst. Combinations of several methods of analysis are needed, all supported by theoretical modeling that rationalizes the experimental results. In this communication, we will illustrate this interplay between experimental characterization and modeling for two examples. The first one is inherent to the TiCl4/MgCl2/EtOH Ziegler-Natta pre-catalyst. Here we have combined the 1D and 2D 1H and 13C NMR spectra with the computation of the corresponding chemical shifts. The calculations were performed at the periodic Density Functional Theory (DFT) level. The match was perfect, and allowed us to fully assign the experimental spectra and characterize the pre-catalyst surface. It was found that TiCl4 underwent to alcholysis reactions, giving different TiClx(OEt)4-x species. For each composition, there are different structural isomers, differing from the relative position of the EtO groups with respect to the MgCl2 surface. The second example is a catalyst for the olefin metathesis reaction. Here, the periodic DFT method does not give a completely satisfactory description, since the support of the catalyst is amorphous SiO2. So we studied the catalyst using SiO2 clusters to model the support. Several SiO2 clusters were considered, in order to understand the effects of the conformation of the support on the grafting of the metal complex and on the reaction path. The catalyst was experimentally characterized with different spectroscopic techniques. We were able to relate the change of spectral features with the structural and chemical modifications of the active site. It is only when the power of in situ experiments and computational modeling are intimately connected and exploited together, that we gain a atomistically detailed understanding of the factors governing activity and selectivity of supported transition metal complexes.

Surface characterization of supported catalysts: The interplay between modeling and experiments

253rd National Meeting (2017)

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