Ermittlung kinetischer Details der C1- und C2-Chemie mit in-situ-Spektroskopie

• Determination of kinetic details in C1- and C2-chemistry with in-situ-spectroscopy

Ohligschläger, Andreas; Liauw, Marcel (Thesis advisor); Herres-Pawlis, Sonja (Thesis advisor)

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

Dissertation, RWTH Aachen University, 2021

Abstract

Chemical transformations of C1- and C2-substances are often performed in highly concentrated solutions or neat conditions to simplify the purification of the product. Substrates and products may behave as very different solvents so that e.g. the polarity of the reaction mixture changes with ongoing reaction. This influences the reaction kinetics, since the solvation enthalpies of substrates and transition states change at the same moment. Thus, the apparent rate coefficient depends on conversion and time. This makes time-resolved measurement of the concentrations necessary to accomplish accurate reaction kinetics. In-situ-IR- and in-situ-Raman-spectroscopy are used for this task. Three reaction systems of C1- and C2-chemistry are investigated that are based on methanol and exhibit a distinct feature: 1. The ionic liquid synthesis of dimethyl carbonate and nucleophilic nitrogen-bases is affected by a strong increase of solvent polarity, since two neutral molecules form an ionic product. The apparent rate coefficient increases with ongoing reaction and follows the model of a kinetic salt effect. The progress of the apparent rate coefficient shows bends that can be assigned to spontaneous changes in the solvent environment. 2. The esterification of acetic acid and ethanol is investigated in a biphasic liquid/liquid reactor. The produced ester is extracted to the unipolar phase. The partition coefficients, mass transfer coefficients and reaction kinetics are determined in a single reactor. The combination of the latter two results shows that the reaction proceeds under kinetic limitation. 3. The Guerbet reaction from ethanol to n-butanol can be described by a network with de-sired and undesired single-steps. The reaction network is divided into redox-catalyzed and base-dependent subsystems whose kinetics are determined in separated experiments. A model of the whole reaction network is assembled and reactions are simulated with varied starting parameters. The plot n-butanol yield versus temperature and hydrogen pressure follows a saddle as either the first reaction step is suppressed (high pressure) or the side-reactions are accelerated disproportionately high (high temperature). Lastly, a high activation barrier of the dehydrogenation of acetaldehyde/1-ethoxyethanol can be identified as a good feature of a Guerbet redox-catalyst.