Metal nanoparticles immobilized on molecularly modified supports as multifunctional catalysts for the selective hydrogenation of aromatic substrates

• Metallnanopartikeln immobilisiert auf molekular modifizierten Trägern als multifunktionelle Katalysatoren für die selektive Hydrierung aromatischer Substrate

El Sayed, Sami; Leitner, Walter (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021

Abstract

The catalytic selective hydrogenation of unsaturated moieties in aromatic substrates (e.g. aromatic ring, C=C, C=O, etc.) has been widely applied for several decades in all fields of the chemical industry (petrochemical, fine chemical, pharmaceutical, agrochemical, etc.). However, to cope with the increasing complexity of aromatic substrates, for example coming from biomass, the perpetual development of efficient, selective, and tunable catalytic systems is essential. A state-of-the-art is provided in chapter 1, summarizing the importance of the selective hydrogenation of various functionalities in aromatic substrates as well as the main catalyst families that are currently available. On this basis, the motivation and objectives of this work are described. In the following two chapters, we report the design, synthesis and characterization of multifunctional catalysts consisting of ruthenium nanoparticles (Ru NPs) immobilized on various molecularly modified supports as well as their application to the hydrogenation of substituted aromatic substrates in batch conditions and in continuous flow processes. Chapter 2 discusses the selective hydrogenation of benzofurans to dihydrobenzofuran derivatives using a multifunctional catalyst composed of Ru NPs immobilized on a Lewis Acid-functionalized supported ionic liquid phase (Ru@SILP-LA). Using a molecular approach, the individual components (metal NPs, ionic liquid with chlorozincate anions as Lewis acid, silica as support) of the catalytic system were assembled to bring the metal and acid sites in intimate contact on the support material. The resulting Ru@SILP-LA catalyst allows the hydrogenation of O-containing heteroaromatic rings while keeping the aromaticity of C6-rings untouched. [ZnCl4]2- anions were identified to be the predominant chlorozincate species using X-ray Photoelectron Spectroscopy, and were found to be in close interaction with the metal NPs according to STEM-HAADF-EDX. The Ru@SILP-[ZnCl4]2- catalyst was found to be highly active, selective, and stable for the selective catalytic hydrogenation of various benzofuran derivatives in batch and continuous flow conditions, delivering easy access to biologically relevant dihydrobenzofuran motifs. The concept of multifunctional catalysts was extended in Chapter 3 with the development of a NPs-based catalytic system with switchable reactivity, meaning that its reactivity can be changed at will during a reaction through the modification of the catalyst’s environment or the application of an external stimulus. In particular, a catalytic system composed of Ru NPs immobilized on an amine-functionalized polymer-grafted silica (Ru@PGS) has been designed to respond adaptively to the feed gas composition used in catalytic hydrogenation. The resulting Ru@PGS catalyst was found to be active and stable for the hydrogenation of biomass-derived furfuralacetone. In pure hydrogen (H2), the substrate’s unsaturations (furan ring, C=C, C=O) were fully hydrogenated forming the corresponding saturated alcohol. However, using a mixture of hydrogen and carbon dioxide (H2/CO2) the C=O hydrogenation step was selectively switched off, producing the saturated ketone in excellent yield and selectivity. This selectivity switch is fully reversible allowing switching back and forth between H2 and CO2/H2 in continuous flow reactor to produce in high yields either one product or the other. The change in selectivity is attributed to the reversible formation of an alkylammonium formate species coming from the Ru-catalysed hydrogenation of CO2 assisted by the amine-functionalized support. These studies highlight the great potential of NPs on molecularly modified supports for the production of multifunctional catalytic systems with tailor-made reactivity.