Heterogeneously catalysed amination and isomerisation of isohexides

• Heterogen Katalysierte Aminierung und Isomerisierung von Isohexiden

Engel, Rebecca Veronika; Palkovits, Regina (Thesis advisor); Bolm, Carsten (Thesis advisor)

Aachen (2016)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen, 2016

Abstract

A renewable alternative to the depleting fossil resources is biomass. An important part in the utilisation of biomass is the conversion and functionalisation of platform chemicals such as isohexides. Therefore, this work concentrates on the catalytic valorisation of isohexides, more precisely isosorbide and isomannide, comprising the amination and isomerisation. The main products, isohexide amines and isoidide, respectively, are promising monomers in the production of biogenic polymers.The direct catalytic amination of the isohexides using solid catalysts was investigated, and ruthenium supported on activated carbon was identified as a suitable catalyst for this transformation. Despite the hydrothermal and highly basic conditions, the catalyst showed no significant leaching. Reversible catalyst deactivation through product adsorption was detected. Furthermore, a significant variation in the product distribution depending on the stereochemistry of the substrates was observed, indicating a preferential amination of endo-configured hydroxyl groups. Additionally, the role of hydrogen was investigated in order to clarify, whether the reaction follows a hydrogen autotransfer mechanism. Using a heterogeneous ruthenium catalyst, molecular hydrogen was required, although in low amounts. This hydrogen is most likely needed for the activation of the catalyst under reaction conditions and to saturate its surface. The isohexide isomerisation was investigated in detail, to obtain a deeper understanding of this reaction. The reaction mechanism was studied by means of deuterium labelling experiments with isomannide and dimethyl isosorbide as substrates. An alternative to the in the literature discussed dehydrogenation/re-hydrogenation mechanism was observed: a direct C-H activation. However, it occurs only at elevated temperatures. Further insights were gained through kinetic investigations. In this context, concentration-time profiles were collected at different temperatures. Interestingly, in the beginning of the reaction course, an unknown fourth compound was observed. This substance was produced mainly during the heating phase of the reaction. First statements about the potential structure of this compound were made. Additionally, the kinetic data was modelled. Besides the rate constants, also the Arrhenius parameters were obtained. The applied solid catalyst is easy to separate and recyclable. Furthermore, the amination reaction is conducted at low hydrogen pressures and in aqueous solution. Therefore, the reported reaction is a benign route towards biogenic isohexide amines. These new insights into the isohexide isomerisation reaction provide a foundation for further process development for the production of isoidide. Altogether, first steps towards the production of these biomass-derived alternative monomers were taken.