Gewinnung von biomassebasierten polaren Verbindungen aus Wasser durch Adsorption

• Separation of polar biomass-derived compounds from aqueous phase by adsorption

Schroer, Guido; Palkovits, Regina (Thesis advisor); Pich, Andrij (Thesis advisor)

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

Dissertation, RWTH Aachen University, 2021

Abstract

This work focuses on the development of a process for the separation of polar molecules, which can be obtained via biomass valorization and contain a vicinal diol motif, by adsorption on boronic-acid containing polymers. The separation is based on the principle of "molecular recognition". Under basic conditions, boronic acids and diols can reversibly form boronate-diol esters, which can be cleaved upon a pH switch to acidic medium.Boronic acid containing polymers with high boron contents were synthesized via an optimized polymerization of p-vinylphenylboronic acid (p-VPBA). Polymers with different content and nature of cross-linkers were obtained in yields of over 90% by a free-radical polymerization. The polymer properties and composition were explored by elemental analysis, IR spectroscopy, nitrogen physisorption, water vapor adsorption, analysis of the swelling behavior and solid-state NMR.The adsorption conditions were optimized using D-fructose as a representative substrate. Thereby, a compromise between maximum polymer loading and polymer stability under sorption conditions against dissolution was identified. The polymers containing 20mol% cross-linker exhibited high stability accompanied with a high adsorption capacity. The adsorption of 22 diols, polyols and saccharides was investigated. Based on the results, a model to describe the sorption behavior was proposed. A significant influence of the swelling ability of the polymer was concluded and the adsorption was described as a self-sustaining swelling- and adsorption-induced process. Substrates with a high molecular complexation constant formed more negatively charged boronate-diol esters at the beginning of the adsorption leading to a repulsion of the polymer chains and a facilitated swelling of the polymer making more inner binding-sites available for adsorption.Studies of the adsorption kinetics supported the proposed model. The adsorption process could be divided in three steps: (1) a fast initial adsorption on the polymer surface, which was dependent of the molecular complexation constants, (2) an adsorption-induced swelling of the polymer and adsorption in more inner domains of the polymer particle, and (3) a slow, diffusion-limited adsorption in the polymer core.Variation of the desorption conditions demonstrated the potential for efficient desorption of diols, such as ethylene glycol (EG) or 2,3-butane diol (2,3-BDO), into a neutral ethanol-water mixture. In addition, an efficient desorption of monosaccharides, for example D-fructose or D-galactose, into aqueous solution applying CO2 was observed. Consequently, the substrates could be desorbed into solutions, which were not contaminated with molecular acids. First applications of the polymers for the separation of diols from reactions mixtures were also promising. It was possible to increase the D-fructose yield by an adsorption-assisted D-glucose isomerization procedure from 29% to 56% and to separate 2,3-BDO from a fermentation broth solution obtaining high polymer loadings of 250 mg(2,3-BDO) per g(polymer).