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Catalytic valorization of biobased phenols and isohexides through amination and oxidation
Book - Dissertation
Aromatic and aliphatic (di)amines are unambiguous industrial precursors for polymers, active pharmaceutical ingredients, dyes and agrochemicals; produced in enormous amounts each year. As a result, chemical processes towards these molecules have a non-negligible impact on the environment. Classic procedures rely almost solely on the exploitation of (preactivated) finite fossil feedstocks. Moreover, some of these processes display significant shortcoming such as an inefficient atom- and step economy, generate huge amounts of hazardous chemical waste or even fail to efficiently yield the desired product. In the context of a progressively more eco-conscious society, the shift towards a more sustainable production of fuels, chemicals and energy becomes increasingly important. Therefore, this doctoral manuscript focusses on the sustainable catalytic transformation of two promising groups of molecules, namely phenols and isohexides, which can be obtained from renewable sources (e.g., lignocellulosic biomass).In the first part of this thesis, direct phenol-to-aniline amination with ammonia in the liquid phase is presented, which at the time was the first report of its kind. Using a supported noble metal, phenol is partially reduced in the presence of H2 to cyclohexanone, which quickly reacts with ammonia to cyclohexanimine through a nucleophilic addition and dehydration sequence. This reactive imine intermediate can then be dehydrogenated to aniline under the right reaction conditions. Since water is formed as the only by-product, this transformation is defined by a high atom economy and no hazardous waste is generated which is in sharp contrast to classic aniline production processes. Moreover, since H2 is both consumed and produced throughout the reaction cycle there is no net need of a reductant. Pd/C was shown as a very performant and reusable catalyst for this reaction since it allows for a relatively fast phenol hydrogenation under a low H2 partial pressure and, at the same time, is able to catalyze the dehydrogenation towards aniline. The influence of H2 and NH3 partial pressure and the choice of solvent were investigated, resulting in a very high aniline yield up to 95% under optimized conditions by maximizing phenol conversion while limiting secondary amine formation. The more sterically hindered cresols were converted into its corresponding anilines, albeit under slightly more reducing conditions.In the second part, supported Ni was investigated as an alternative to catalysts based on expensive noble metals such as Pd and Rh for the reductive amination of phenol. Different Ni catalysts were screened, and Ni/Al2O3 was identified as the most performant catalyst for this transformation. The influence of key parameters was investigated, including the acid-base properties of the supporting material by means of FTIR spectroscopy using cyclohexylamine as the probing molecule. It was found that the moderate activity of rather acidic materials could be explained by strong adsorption of the basic cyclohexylamine, or the slightly less basic NH3, resulting in a catalyst inhibition effect. Al2O3 only possesses a moderate acidity, enough to catalyze some reaction steps, but N-containing molecules do not interact very strongly with the support. Both NH3 and H2 partial pressure drastically affected phenol conversion, as well as the choice of solvent; a cyclohexylamine yield up to 93% was obtained at the optimized conditions. Both substrate and reactant scope were expanded by successfully converting various phenolic and phenol-derived substrates; but also by using amines instead of ammonia. Particularly the synthesis of the industrially relevant H12MDA (93% yield from bisphenol F) is of significant interest.In the final part, the transformation of fully biobased isosorbide into the isohexide-derived diketone was explored as a potential pathway towards novel diamines for the polymer industry. Classic approaches are unsatisfactory in terms of the principles of green chemistry, or even fail to produce any diketone due to the unreactive exo-OH group. Therefore, a nitroxyl radical mediator was used in combination with bromides under electrochemical conditions. Here, the active oxoammonium salt of TEMPO is responsible for the oxidation of both -OH groups, and is regenerated by the transport of electrons to the terminal oxidant, the anode. The bromide conveniently serves as the electrolyte, but also promotes aforementioned electron transport through in situ formed elemental bromine or the hypobromite. Under mild reaction conditions in water or a water-ACN mixture, a diketone yield up to 85% was achieved using this double-mediated system. Heterogenized TEMPO was also found to be an effective mediator for this oxidation (76% diketone yield), allowing for a simple separation and thus easier processing of the diketone product. As an example, the diketone product was subjected to a two-step amination-reduction sequence in methanol using commercially available catalysts, generating isohexide-diamine isomers with a yield of 69% with respect to the initial isosorbide content.