Justin Notestein, Northwestern University
January 21, 2010
11:00 AM - 12:00 PM
Supported Metal Oxide Catalysts using Grafted Multi-dentate Complexes
Abstract:
Supported metal oxide catalysts are ubiquitous materials covering the range of acid, base, and redox chemistry, but they can exhibit complex relationships between reactivity and surface coverage (loading) due to the prevalence of structure-sensitive catalyst mechanisms. In general, structures tend to evolve from isolated cations at low surface coverage to polymeric surface species at intermediate loadings, and crystallites with properties akin to the bulk material at higher loadings. However, even within these broad classifications, it is challenging to control the precise nature of the atomic connectivity to the surface and within the metal oxide domain. A further challenge with supported metal oxide catalysts is the absence of universal methods for evaluating the number of surface sites, such as via scanning microscopy or volumetric and spectroscopic chemisorption measurements for supported metal nanoparticles. Thus, some of the significant design goals in the area of metal oxide catalyst synthesis include: 1) development of methods for creating uniform, isolated surface species, 2) development of methods that specifically form the intermediate, polymeric structures, and 3) creation of synthetic handles beyond metal content and support type, which will allow for fine-tuning of catalyst structure and function and a priori knowledge of the number of surface sites. Developing atomically-precise supported metal oxides, in particular using earth-abundant oxides, promises to usher in new forms of selective catalysis. Three research foci will be discussed that tackle the synthesis of supported metal oxide catalysts for selective oxidation of aromatics to phenolics and the epoxidation / dihydroxylation / allylic oxidation of alkenes. All areas share the use of organic ligands to control the structure of the final supported oxide catalyst. In the first area, supported iron oxides are synthesized from highly chelating precursor complexes. In particular, a chelating acid ligand allows the resulting supported metal oxide to retain the characteristics of an isolated cation at much higher surface densities than observed for more common metal halides. In the second area, small manganese oxide clusters are pre-formed in solution with cyclic amine ligands and subsequently supported using traditional exchange or impregnation techniques or via the formation of new clusters on suitably-modified hybrid solids. These clusters allow direct access to the intermediate, polymeric oxide structures and possess unusual reactivity in epoxidation / dihydroxylation. Finally, cyclic phenol ligands are used to create site-isolated Ta cations on various supports. The structure of the ligand predictably controls the selectivity of the catalyst towards C=C oxidation or allylic CH oxidation. All catalytic oxidations are carried out in the l iquid phase with suspended solid catalysts and aqueous H2O2 as the oxidant. Tests for heterogeneity of the catalysis are included as this is often a concern in l iquid phase oxidations. Surface structures are probed using temperature programmed techniques, diffuse reflectance UV-visible-NIR spectroscopy, solid state 13C NMR spectroscopy, and X-ray absorption spectroscopy where applicable.
Date posted
Jun 17, 2019
Date updated
Jun 17, 2019