Direct synthesis of glycerol carbonate from glycerol and carbon dioxide by Brønsted base catalysis

• Brønsted-Basen-katalysierte Synthese von Glycerincarbonat aus Glycerin und Kohlenstoffdioxid

Schenk, Karolin; Leitner, Walter (Thesis advisor); Oppel, Iris Marga (Thesis advisor)

Aachen (2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

The utilisation of carbon dioxide and renewable resources as feedstocks for the chemical industry is highly desirable due to fossil fuel depletion and climate change. Chemicals directly produced from these substrates could lead to a circular economy. In this regard, the synthesis of organic carbonates directly from an alcohol and CO2 is a sustainable target transformation since alcohols are readily available from renewable resources. Organic carbonates then can be used as greener alternatives for existing high-boiling polar solvents, carbonation sources or monomers for polycarbonates. The present thesis investigated the direct synthesis of cyclic carbonates and, in particular, glycerol carbonate from glycerol and carbon dioxide by Brønstedt base catalysis. Glycerol is a trivalent alcohol that is a by-product from different biomass conversion processes especially of the biodiesel production and is currently considered as a waste product. The synthesis of the corresponding five-membered cyclic carbonate leads to a value-added compound as it possesses two different functional groups, a hydroxyl group and a 2-oxa-1,3-dioxolane group, leading to a wide reactivity and a broad range of industrial applications. The aim of this thesis is to provide a better understanding of the direct synthesis of glycerol carbonate from glycerol and CO2. The reaction produces water as by-product which should be removed to shift the equilibrium towards the product side. Two different approaches to remove H2O were investigated: I) the reactive dehydration by acetonitrile, and II) the water extraction by a continuous carbon dioxide flow. For both water removal methods, the influence of reaction parameters such as temperature, catalyst, CO2 pressure or additives on the product yield was studied. Furthermore, the reaction mechanism and the formation of side products were investigated. In the case of reactive water removal with acetonitrile as dehydration agent, glycerol carbonate was synthesised in 17 % yield from glycerol and carbon dioxide with potassium carbonate as catalyst. It was found that the glycerol carbonate yield reaches a maximum depending on the reaction conditions due to a complex reaction network leading to side and decomposition reactions. For the non-reactive water removal in the synthesis of glycerol carbonate, a new semi-continuous process was developed with CO2 acting both as reagent and stripping gas. Glycerol carbonate was synthesised in 13 % yield. Investigations on the reaction mechanism identified glycerol hemi-carbonate as intermediate and the subsequent cyclisation to the cyclic carbonate as rate-determining step.