Deep eutectit solvents : properties and biocatalytic applications

• Stark eutektische Lösungsmittel : Eigenschaften und biokatalytische Anwendungen

Maugeri, Zaira; Leitner, Walter (Thesis advisor); Pohl, Martina (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2015)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2014

Abstract

Deep eutectic solvents (DES) have emerged over the last decade as a novel class of ionic liquids (ILs). In its broadest sense, DESs are usually formed by mixing a quaternary ammonium salt (typically choline chloride and derivatives) with hydrogen bond donor molecules such as amines, amides, alcohols, carboxylic acids, sugars or polyols. The mixing of these two components upon gentle heating and in a specific molar ratio leads to a depression of the melting point, resulting in most of the cases in a liquid at room temperature. DESs share several properties with conventional ILs (e.g. similar conductivities, polarities, viscosities, densities, surface tensions, refractive indexes, chemical inertness, etc.) and they are usually cheap due to the relatively inexpensive components and their simple synthesis, where no waste is produced and no further purification steps are needed. Furthermore, DESs’ components are often biodegradable and non-toxic. The first part of this PhD Thesis focused on the synthesis of novel bio-based DESs using choline chloride and, as hydrogen bond donors (HBDs), several carboxylic acids and some saccharide-derived polyols. The novel DESs were characterized by measuring melting points, pH, water contents and viscosities. Likewise, kinetic studies on water absorption were conducted and solubilities in protic and aprotic organic solvents assessed. DESs were able to solve protic compounds (e.g. alcohols) whereas a second phase was formed with aprotic compounds (e.g. ketones, esters). Based on this property a novel procedure to separate mixtures of alcohol-ester, alcohol-ketone and amine-ketone was established, leading in most of the studied cases to 70-90% of efficiency and 90-99% of purity of the separated compounds. The second part of this PhD Thesis focused on various biocatalytic applications in these solvents. Several reactions were assessed (such as transesterifications, enantioselective epoxidations, peptide syntheses, carboligations and reductions) by using isolated enzymes (Candida antarctica lipase B, alpha-chymotrypsin from bovine pancreas, benzaldehyde lyase from Pseudomonas fluorescens) and whole cells (baker’s yeast). In most of the reported cases DESs were compatible with those biocatalysts, even despite the denaturing behavior of strong hydrogen bond donors towards proteins (e.g. urea). The amount of water present in the reaction mixture resulted to be critical, analogous to observations done in other non conventional media. For each reaction and enzyme the most suitable DES was identified.