Direct Synthesis of Glycerol Carbonate from Glycerol and Carbon Dioxide by Brønsted Base Catalysis
Brønsted-Basen-katalysierte Synthese von Glycerincarbonat aus Glycerin und Kohlenstoffdioxid
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen University zur Erlangung des akademischen Grades einer Doktorin der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Karolin Schenk, M.Sc.
aus Hamburg
Berichter: Universitätsprofessor Dr. Walter Leitner
Universitätsprofessorin Dr. Iris Oppel
Tag der mündlichen Prüfung: 7. Mai 2018
Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek online verfügbar.
The present doctoral thesis was carried out at the Institut für Technische und Makromolekulare Chemie (ITMC) of RWTH Aachen University between February 2014 and August 2017 under the supervision of Prof. Dr. Walter Leitner.
Danksagung
Meinem Doktorvater Prof. Dr. Walter Leitner möchte ich besonders für das interessante und anspruchsvolle Thema danken. Die Möglichkeit in einem internationalen und interdisziplinären Projekt unter hervorragenden Laborbedingungen eigenständig zu arbeiten ist nicht selbstverständlich.
Meinen herzlichen Dank an Prof. Dr. Iris Oppel für die Übernahme des Zweitgutachtens sowie an Prof. Dr. Markus Albrecht und Prof. Dr. Paul Kögerler als weitere Mitglieder der Prüfungskommission.
Dr. Giancarlo Franciò danke ich sehr für die Aufnahme in seine Untergruppe. Vielen Dank für dein Vertrauen, die vielen hilfreichen Gespräche und die andauernde Unterstützung!
Dem EU-Projekt CyclicCO2R danke ich für die Finanzierung dieser Arbeit. Zusätzlich habe ich durch die Projekttreffen viele nette Menschen kennengelernt und in einem internationalen und interdisziplinären Team arbeiten können.
Vielen Dank an Ralf Thelen und das ganze Team der mechanischen Werkstatt für die Unterstützung bei der Arbeit mit den Autoklaven und dem Aufbau meiner Anlange im Technikum. Bei Thomas Müller und Stefan Aey möchte ich mich für die häufigen Notfalleinsätze in meiner Technikumsbox und die schnelle Reparatur von defekten Elektronikteilen bedanken. Günter Wirtz danke ich für die administrative Arbeit und die Hilfe bei „Verbindungsproblemen“ bei Telefonkonferenzen. Ines Bachmann danke ich für die Organisation der vielen NMR-Messungen und die Hilfe bei Hochdruck-NMR-Experimenten. Hannelore Eschmann, Elke Biener und Wolfgang Falter danke ich für die schnelle Hilfe bei GC-Problemen im Praktikum.
Daniel Geier bin ich sehr dankbar für die Unterstützung bei der Planung meiner Technikumsanlage und der Lab-View-Programmierung des Steuerungsprogramms.
Dr. Tobias Eifert und Andreas Ohligschläger danke ich sehr für die Hilfe und Unterstützung bei den in-situ IR-Messungen und der anschließenden Auswertung.
Danksagung
Herzlichen Dank an meine lieben Laborkollegen Dr. Kai Rohmann, Benjamin Schieweck und Akash Kaithal. Ich hatte eine schöne Zeit mit euch und mein musikalischer Horizont hat sich definitiv erweitert! Meiner Auszubildenden Julia Nowacki und meinem Forschungsstudenten Yannik Louven danke ich für ihren Beitrag an dieser Arbeit.
Celine Jung und Dr. Deven Estes danke ich herzlich für die Korrektur dieser Arbeit.
Mit meiner „Lunch-Group“ und der im letzten Jahr gegründeten Kochgruppe habe ich viele schöne Mittagspausen verbracht. Danke für die vielen Diskussionen zu den unterschiedlichsten Themen! Das war immer eine gute Ablenkung von der Chemie.
Vielen Dank an die gesamte Arbeitsgruppe Leitner und all die anderen Mitarbeiter des ITMCs für die nette Zeit! Die Karnevalspartys, Weihnachtsfeiern und Grillabende waren immer schöne Erlebnisse.
Meinen Eltern Kirsten und Olaf Schenk, meiner Schwester Klara Schenk, meiner restlichen Familie und meinen Freunden danke ich ganz herzlich für die Unterstützung auf meinem bisherigen Lebensweg!
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.
Contents
1 Introduction ...... 1
1.1 Motivation ...... 1 1.2 Organic carbonates ...... 1
1.2.1 Properties and application ...... 2 1.2.2 Synthesis ...... 4
1.2.3 Renewable organic carbonates from biomass and CO2 ...... 6
1.3 Direct carbonation of diols ...... 9
1.3.1 Reaction characteristics ...... 9 1.3.2 Reaction systems ...... 12
1.4 Aim of the thesis ...... 15
2 Synthesis of glycerol carbonate in the presence of acetonitrile ...... 16
2.1 Introduction ...... 16
2.1.1 State of the art ...... 16 2.1.2 Hydration of acetonitrile ...... 17 2.1.3 Aim of this section ...... 17
2.2 Analysis ...... 18
2.2.1 Quantitative analysis and reproducibility ...... 18 2.2.2 Phase observations ...... 20
2.3 Parameter variation of the carboxylation of glycerol ...... 21
2.3.1 Carbon dioxide pressure ...... 21 2.3.2 Amount of acetonitrile ...... 22 2.3.3 Base concentration ...... 23 2.3.4 Reaction time ...... 24 2.3.5 Temperature ...... 25 2.3.6 Catalyst ...... 25 2.3.7 Miscellaneous ...... 26
2.4 Reaction network ...... 27
2.4.1 Side product identification ...... 27 2.4.2 Water dependence ...... 30
Contents
2.4.3 Reactivity studies ...... 30
2.5 In-situ mIR spectroscopy ...... 31
2.5.1 Single component IR spectra ...... 31 2.5.2 In-situ investigations of the acetonitrile phase ...... 32 2.5.3 In-situ investigations of the glycerol phase...... 35
2.6 Reaction time profile ...... 37 2.7 Additional solvent ...... 39 2.8 Repetitive batch ...... 40 2.9 Conclusions ...... 41
3 Water stripping by carbon dioxide in the synthesis of glycerol carbonate ...... 43
3.1 Introduction ...... 43
3.1.1 Non-reactive dehydration systems ...... 43 3.1.2 Carbon dioxide for water extraction ...... 43 3.1.3 Aim of this section ...... 45
3.2 Preliminary studies...... 45 3.3 Reactor design ...... 46 3.4 Investigation of reaction conditions ...... 47
3.4.1 Temperature variation ...... 48 3.4.2 Catalyst amount ...... 48 3.4.3 Carbon dioxide pressure variation ...... 50 3.4.4 Carbon dioxide flow rate variation ...... 50 3.4.5 Reactor design ...... 51
3.5 Reaction network ...... 52
3.5.1 Characterisation of the reaction mixture ...... 53 3.5.2 Synthesis of hemi-carbonate ...... 54 3.5.3 In-situ investigations on hemi-carbonate ...... 55
3.6 Additives ...... 58 3.7 Downstream processing ...... 59 3.8 Substrate scope ...... 60
3.8.1 Glycols ...... 60 3.8.2 Phenol ...... 63 3.8.3 Sugars...... 63
II Contents
3.9 Conclusions ...... 68
4 Summary ...... 70 5 Experimental section ...... 71
5.1 General ...... 71
5.1.1 Chemicals ...... 71 5.1.2 NMR spectroscopy ...... 71 5.1.3 Single component IR spectroscopy ...... 71 5.1.4 In-situ IR spectroscopy ...... 71
5.2 High pressure reactions ...... 72
5.2.1 Apparatus specification ...... 72 5.2.2 Finger autoclave ...... 72 5.2.3 Window autoclave ...... 72 5.2.4 Window autoclave with ATR-mIR-probes ...... 73 5.2.5 Bubble column reactor...... 74
5.3 Phase behaviours ...... 75
5.3.1 General procedure for qualitative solvent testing ...... 75 5.3.2 General procedure for quantitative solvent testing ...... 75
5.4 Synthesis of glycerol carbonate ...... 75
5.4.1 General procedure for single batch reactions ...... 75 5.4.2 Repetitive batch catalysis ...... 75 5.4.3 Catalysis with slow acetonitrile addition ...... 76 5.4.4 General procedure for IR monitored catalysis ...... 76 5.4.5 General procedure for reactions in the bubble column reactor ...... 76 5.4.6 General procedure for water extraction experiments in a window autoclave ... 77
5.5 Synthesis ...... 78
5.5.1 Polyethylene glycol enfolded KBr[131] ...... 78 5.5.2 Magnesium methoxide[143] ...... 78 5.5.3 Glycerol monoacetates ...... 78 5.5.4 3-Acetamido-2-hydroxypropyl acetate[144] ...... 79 5.5.5 5-(Hydroxymethyl)oxazolidin-2-one[145] ...... 80 5.5.6 (2-Oxo-1,3-dioxolan-4-yl)methyl acetate ...... 80
6 References ...... 82
III Contents
Curriculum Vitae ...... 89 Eidesstattliche Versicherung ...... 91
IV
List of Abbreviations
atm Atmosphere ATR Attenuated total reflectance BPR Back pressure regulator br Broadened signal cm Centimeter cP Centipoise d Dublet DKR Dynamic kinetic resolution
N ET Normalised solvent polarity e.g. Exempli gratia (for example) equiv. Equivalent et al. et alii (and others) h Hours HMBC Heteronuclear multiple bond correlation HSQC Heteronuclear single-quantum correlation Hz Hertz IHM Indirect hard modeling IRE Internal reflection element ITMC Institut für Technische und Makromolekulare Chemie J Coupling constant m Multiplet MFC Mass flow controller min Minutes mIR Mid infrared NMR Nuclear magnetic resonance p Pressure [bar] ppm Part per million
-1 V̇ Volumetric flow rate [mLN min ]
Rf Retardation factor
rpm Rounds per minute RT Room temperature RWTH Rheinisch-Westfälische Technische Hochschule
List of Abbreviations
S Selectivity [%] s Singlet T Temperature [°C] t Time [s, min, h] t Triplet X Conversion [%] Y Yield [%] δ Chemical shift [ppm]
VI
List of Chemicals
DBU 1,8-Diazabicyclo[5.4.0]undec-7ene DMSO Dimethyl sulfoxide Gly Glycerol GlyAcet Glycerol monoacetate GlyCarb Glycerol carbonate [K(PEG)]Br Polyethylene glycol embedded potassium bromide LiTFSI Lithium bis(trifluoromethanesulfonyl)imide MEK Methyl ethyl ketone MIBC Methyl isobutyl carbinol (4-methyl-2-pentanol)
MMIM-CO2 1,3-Dimethylimidazolium-2-carboxylate 2-MTHF 2-Methyltetrahydrofuran PEG Polyethylene glycol PPNCl Bis(triphenylphosphine)iminium chloride
scCO2 Supercritical carbon dioxide
TBD 2,5,7-Triazabicyclo[4.4.0]dec-5-ene TFA Trifluoroacetic acid TFSI Bis(trifluoromethanesulfonyl)imide THF Tetrahydrofuran TsCl p-Toluenesulfonyl cloride
List of Figures
Figure 2-1: Different acids as 1H-NMR spectroscopy standard in an equimolar mixture of glycerol and glycerol carbonate (400 MHz, 25 °C, DMSO-d6)...... 19 Figure 2-2: 1H-NMR spectra of reaction mixture with and without maleic acid as standard. ... 20 Figure 2-3: Reaction mixtures after 16 h at different temperatures...... 21
Figure 2-4: Variation of CO2 pressure...... 22 Figure 2-5: Variation of MeCN amount...... 23 Figure 2-6: Variation of catalyst amount...... 24 Figure 2-7: Variation of reaction time...... 24 Figure 2-8:Variation of reaction temperature...... 25 Figure 2-9: Investigation of different Brønstedt basic catalysts...... 26 Figure 2-10: 1H-NMR sprectrum of the first column chromatographic fraction...... 29 Figure 2-11: 1H-NMR spectrum of the fourth column chromatographic fraction...... 29 Figure 2-12: Influence of water content on the glycerol carbonate yield...... 30 Figure 2-13: Superimposed single component ATR-spectra of reaction mixture compounds. ... 31 Figure 2-14: Stacked single component ATR-spectra of reaction mixture compounds...... 32 Figure 2-15: A: In-situ IR window autoclave; B: Schematic reaction mixture...... 33 Figure 2-16: Pictures of reaction mixture batch conditions...... 33 Figure 2-17: ATR-mIR spectra surface of the acetonitrile phase (lower IR-probe)...... 34 Figure 2-18: Peak integration of the acetonitrile phase spectra (lower IR-probe)...... 34 Figure 2-19: Schematic drawing of phase behaviour during the reaction course...... 35 Figure 2-20: Pictures of reaction mixture...... 35 Figure 2-21: ATR-mIR spectra surface of the reaction mixture (lower IR-probe)...... 36 Figure 2-22: Peak integration of IR spectra of the reaction mixture (lower IR-probe)...... 37 Figure 2-23: Reaction time profile for literature reaction conditions.[103] ...... 38 Figure 2-24: Reaction time profile for optimised reaction conditions...... 38 Figure 2-25: Direct carbonation of glycerol with additional solvent...... 40 Figure 2-26: Repetitive batch experiment...... 41
Figure 3-1: Investigation of phase behaviour of glycerol + CO2 and glycerol carbonate + CO2. 45
1 Figure 3-2: H-NMR spectra of a mixture of Gly, GlyCarb and H2O (400 MHz, 25 °C,
DMSO-d6)...... 46 Figure 3-3: Bubble column reactor...... 47 Figure 3-4: Direct synthesis of glycerol carbonate (8)...... 47 Figure 3-5: Reaction time profile at different temperatures...... 48
List of Figures
Figure 3-6: Reaction time profile for different K2CO3 amounts...... 49 Figure 3-7: Reaction time profile for different alkali carbonates...... 49 Figure 3-8: Reaction time profile for different pressures...... 50
Figure 3-9: Reaction time profile for different CO2 volumetric flow rates...... 51 Figure 3-10: Reaction time profile for different reactor designs...... 52 Figure 3-11: Comparison of single component and mixture ATR-IR spectra...... 53
1 13 Figure 3-12: H- C-HMBC NMR spectrum of mixture of glycerol, 10 mol% K2CO3 and CO2 at
1 atm. (600 MHz, RT, DMSO-d6)...... 54 Figure 3-13: Comparison of ATR-mIR spectra for different pressures at RT...... 56 Figure 3-14: Comparison of ATR-mIR spectra at different temperatures...... 56
Figure 3-15: Comparison of ATR-mIR spectra at different K2CO3 amounts...... 57 Figure 3-16: Comparison of hemi-carbonate mixture with and without addition of TFA...... 57
Figure 3-17: Reaction time profile for MMIM-CO2 and KI as additives...... 59
Figure 3-18: Reaction time profile for LiTFSI, [K(PEG)]Br and Nb2O5 as additives...... 59 Figure 3-19: Reaction time profile for the direct carbonation of different substrates...... 61 Figure 3-20: Reaction time profile for the synthesis of octane carbonate (26)...... 62
13 Figure 3-21: C-NMR spectrum of sorbitol reaction mixture (101 MHz, 25 °C, DMSO-d6)...... 64
Figure 3-22: Reaction progress of erythritol (31) (101 MHz, 25 °C, DMSO-d6)...... 65
13 Figure 3-23: C-NMR spectra of erythritol/K2CO3 mixture with different CO2 amounts
(101 MHz, 25 °C, DMSO-d6)...... 66
13 Figure 3-24: C-NMR spectra of threitol (33)/K2CO3 reaction mixture with or without CO2
(101 MHz, 25 °C, DMSO-d6)...... 67 Figure 5-1: Engineering drawing of a 20 mL finger autoclave...... 72 Figure 5-2: Engineering drawing of a 10 mL window autoclave...... 73 Figure 5-3: Engineering drawing of the 30 mL window autoclave with IR-probes...... 74 Figure 5-4: Engineering drawing of the bubble column reactor...... 74 Figure 5-5: Control panel of LabView program coded by Daniel Geier...... 77
IX
List of Schemes
Scheme 1-1: General structure of organic carbonates...... 2 Scheme 1-2: Reactions with cyclic carbonates.[24, 25, 26, 27] ...... 3 Scheme 1-3: Industrial production of organic carbonates.[9, 32b, 42-45] ...... 5 Scheme 1-4: Research on synthesis of organic carbonates.[8a, 8c, 41] ...... 6
Scheme 1-5: Synthesis of 6-membered cyclic carbonate (2) from protected D-mannose (1).[59] ...7 Scheme 1-6: Synthesis of terpene carbonates (4) on the example of limonene (3).[64] ...... 8 Scheme 1-7: Synthesis of oleochemical carbonates (6) on the example of methyl oleate (5)...... 8 Scheme 1-8: Direct synthesis of glycerol carbonate (8) from glycerol (7) and carbon dioxide. ... 9 Scheme 1-9: Equilibrium reaction of diol and carbon dioxide...... 10 Scheme 1-10: Presumed reaction path for the direct carbonation of diols by carbon dioxide.[51b]11 Scheme 1-11: Carbonyl substitution mechanism in the formation of ethylene carbonate...... 11 Scheme 1-12: Homogeneous catalyst systems.[87, 89, 93] ...... 13 Scheme 1-13: Heterogeneous catalyst systems.[85, 96a] ...... 14 Scheme 1-14: Stoichiometric systems.[100-102] ...... 15 Scheme 2-1: Equilibrium reaction in the synthesis of glycerol carbonate (8)...... 16 Scheme 2-2: Hydration of acetonitrile (9)...... 17 Scheme 2-3: Hydration of acetonitrile to acetamide and acetic acid...... 27 Scheme 2-4: Side reactions of glycerol...... 28 Scheme 2-5: Side reactions of glycerol carbonate...... 28 Scheme 3-1: Reaction mechanism for the base catalysed direct carbonation of glycerol...... 52
Scheme 3-2: Reaction of glycerol (7) and CO2 to glycerol hemi-carbonate (18)...... 55 Scheme 3-3: Equilibrium reaction to glycerol hemi-carbonate (18)...... 55 Scheme 3-4: Direct carbonation of ethylene glycol (19), propylene glycol (20) and 2,3-butanediol (21)...... 61 Scheme 3-5: Direct carbonation of 1,2-octanediol (25)...... 62 Scheme 3-6: Direct carbonation of phenol (27)...... 63
Scheme 3-7: Reaction of sorbitol (29) and CO2 to cyclic carbonate (30)...... 64
Scheme 3-8: Reaction of erythritol (31) and CO2 to cyclic carbonate (32)...... 64 Scheme 3-9: Reaction of erythritol (31) to threitol (33) and furans (34 and 35)...... 66 Scheme 3-10: Dynamic kinetic resolution of 1-phenylethanol (36)...... 67
List of Tables
Table 2-1: Catalytic systems for the synthesis of glycerol carbonate...... 17 Table 2-2: 1H-NMR spectroscopic standard validation...... 19 Table 2-3: Average conversion of Gly and yield of GlyCarb and deviation...... 20 Table 2-4: Miscibility of glycerol and glycerol carbonate with different solvents...... 39 Table 3-1: Extraction of GlyCarb from Gly/GlyCarb mixtures by different solvents ...... 60
1 Introduction
1.1 Motivation
Over the last two centuries, global energy demand has grown constantly due to the industralisation and assimilation of the living standard around the globe.[1] Since fossil resources haven been formed over thousands of years by transformation of biomass at high temperature and pressure, ongoing utilisation of these resources leads to their depletion.[2] Furthermore, the combustion of organic material produces carbon dioxide that is emitted to the atmosphere. The natural carbon cycle is not able to cope with the additional carbon dioxide. As a result, the carbon dioxide level in the atmosphere increases, which is the main cause for climate change and global warming.[3] Therefore, fossil resources must be replaced by alternative energy sources like electricity from sunlight or wind power and fuels from renewable biomass to reduce the levels of emitted carbon dioxide. Some efforts have already been made. In 2014, renewable energy sources accounted for 19 % of the global energy consumption. Different predictions for the year 2050 show renewable energy shares between 16 % and 92 %.[4]
The chemical industry also depends on fossil resources. The majority of commodity chemicals are derived from petrochemicals. Since the 1990’s efforts have been made to make the chemical industry more sustainable and ecologically benign. This brought about the principles of green chemistry focussing on the prevention of waste, design of safer chemicals and processes, and energy efficiency.[5] Switching to renewable feedstocks, like biomass and carbon dioxide, could lead to a circular economy - a “human carbon cycle”.[6] Chemicals directly produced from biogenic resources and CO2 consist of 100 % biologically or chemically fixated carbon that originally derives from carbon dioxide. So combustion of these materials closes the loop in the carbon cycle. By utilisation of fast growing biomass, the time scale of CO2 recycling is shortened. In this regard, organic carbonates are industrially rewarding compounds, due to their broad application range, growing market potential, and feasibly sustainable synthesis.
1.2 Organic carbonates
Organic carbonates can be linear (I), cyclic (II) or polymeric (III) as shown in Scheme 1-1. Depending on the substituents these three groups can further be divided into subgroups like aliphatic or aromatic carbonates.
Introduction
Scheme 1-1: General structure of organic carbonates. I: linear carbonate; II: cyclic carbonate; III: polycarbonate.
1.2.1 Properties and application Organic carbonates are either colourless liquids or solids depending on their structure. Densities range from 1.07 g cm-3 for dimethyl carbonate to 1.33 g cm-3 for ethylene carbonate, all of which exceed the density of water. Viscosities of short-chained dialkyl carbonates are lower than the viscosity of water (0.89 cP), with 0.59 cP for dimethyl carbonate and 0.75 cP for diethyl carbonate, while viscosities of cyclic carbonates are considerably higher (2.5-3.1 cP for ethylene, propylene and butylene carbonate). Generally, organic carbonates possess high dielectric constants and high dipole moments. Depending on their structure, organic
N [7] carbonates have ET values from 0.185 (diethyl carbonate) to 0.552 (ethylene carbonate). They possess high boiling and flash points and low pour points, leading to low odour levels and evaporation rates. Their low toxicities and high biodegradabilities make them favourable for all applications.[8] Pure organic carbonates are stable under ambient conditions and are not affected by air or moisture. At elevated temperatures, thermal decomposition could occur, especially in the presence of acidic or basic impurities.[9]
Organic carbonates find applications in a variety of industrial sectors. These applications can be divided into the utilisation as inert or reactive chemical compound, respectively.[8] The main use of organic carbonates is as solvents. Due to their high polarity, carbonates can be used for electrochemical purposes, e.g. lithium batteries[10] or electropolymerisation.[11] In the oil processing industry, organic carbonates are used to separate carbon dioxide from natural
[12] [13] gas streams by selective physical absorption of CO2 or for the recovery of oil. Carbonates can also be added to lubricants and hydraulic fluids.[14] Due to their ecologically benign behaviour and low toxicity, carbonates are attractive additives in degreasing,[15] paint stripping[16] and cleaning formulations.[17] Even in cosmetic and medical applications, organic carbonates are used as co-solvents.[18] Chemical transformations like oxidation,[19] hydrogenation,[20] metathesis[21] and carbonylation[22] reactions have been successfully performed in linear and cyclic carbonates as solvents.
2 Introduction
Several organic carbonates serve as reactive intermediates in the chemical industry (see Scheme 1-2). For example, dimethyl carbonate was shown to be a safer and cleaner alternative to phosgene and dimethyl sulphate for carbonylation and methylation reactions, producing
[23] either methanol or CO2 as waste instead of inorganic salts. More generally, organic carbonates are (hydroxy)alkylation agents for aromatic amines, alcohols, and thiols (I).[24] Reactions of aliphatic amines and carbonates lead to carbamate products (II).[25] With carboxylic acids, cyclic carbonates react to give hydroxyl alkyl esters or diesters (III).[26] In the presence of aliphatic alcohols, carbonates act as transesterification agents leading to an exchange of the carbonyl moiety (IV).[27] These strategies can be used for the synthesis of polymers[28] or compounds with agricultural[29] or medical applications.[30]
Scheme 1-2: Reactions with cyclic carbonates.[24, 25, 26, 27] I) Hydroxyalkylation, X = NH, O, or S; II) synthesis of carbamates; III) synthesis of hydroxyl alkyl esters or diesters; IV) transesterification reaction.
Under suitable reaction conditions, cyclic carbonates undergo ring-opening polymerisation to polycarbonates that possess a broad range of applications.[31] Due to their excellent impact and heat resistance as well as transparency, bisphenol-based polycarbonates are applied in data and image storage, electronics, optical components, and as construction materials.[32] The
3 Introduction
industrial application of aliphatic polycarbonates is mostly limited to the production of macromonomers for the synthesis of polyurethanes and other copolymers, although they have been explored for utilisation as thermoplastics, binders, surfactants, foams, and others.[33] Due to their biocompatibility and bioresorbability, aliphatic polycarbonates are studied for biomedical applications, like tissue engineering scaffolds, hydrogels and drug delivery carriers.[33-34]
Short-chained dialkyl carbonates exhibit suitable properties for applications as fuel additives.[35] They blend well with gasoline and diesel, have low toxicity and biodegradability and have high oxygen contents.[36] The addition of dimethyl carbonate or diethyl carbonate to diesel reduces the emissions of its combustion.[37] The market potential of organic carbonates is growing. In 2015, global market size for dimethyl carbonate was valued over 410 million USD and its compound general growth rate is expected to exceed 5 % by 2024.[38] Similar growth rates are estimated for the global polycarbonate market with a size of 15 billion USD in 2015.[39]
1.2.2 Synthesis Organic carbonates were originally produced industrially by the reaction of phosgene and alcohols.[40] Due to the high toxicity and hazardousness of phosgene and the co-production of hydrogen chloride or salt waste respectively, alternative synthesis routes were developed (see Scheme 1-3).[41] The companies EniChem and UBE developed a catalytic oxidative carbonylation of methanol with carbon monoxide and oxygen to obtain dimethyl carbonate. While EniChem used CuCl as catalyst[42], UBE’s system was based on Pd-nitric oxide.[43] For the synthesis of acyclic (poly)carbonates, both Texaco and Asahi Kasei designed a multi-step process, where ethylene carbonate is first produced from ethylene oxide and carbon dioxide followed by subsequent transesterification giving the dialkyl carbonate. In this process, ethylene glycol is produced as a by-product.[32b, 44] Huntsman commercialised the catalytic
[9, 45] carboxylation of alkyl oxides with CO2 to produce several cyclic carbonates.
4 Introduction
Scheme 1-3: Industrial production of organic carbonates.[9, 32b, 42-45]
Although, these industrial processes are advantageous to produce organic carbonates compared to the phosgene process, they still have various drawbacks such as harsh reaction conditions or toxic starting materials.[8c] Therefore, recent research focuses on improving existing processes or on the development of new synthesis routes. Scheme 1-4 shows the main synthesis pathways investigated. The transesterification of alcohols with organic carbonates leads to cyclic or acyclic carbonates depending on the starting material. A broad variety of organic carbonates is accessible at mild to moderate reaction conditions via this carbonate interchange reaction.[8a, 46] Instead of an organic carbonate, urea can be used as carboxylation agent.[47] However, to make the carbon interchange reactions competitive, the carbon transfer agent must be easily recyclable from its decarboxylated form, preferentially by reaction with
CO2. Otherwise, stoichiometric amounts of waste are produced. However, the conversion of carbon dioxide usually requires reagents with high energy content like H2 or epoxides. Thus, addition of CO2 to oxygen-containing heterocycles like epoxides or oxetanes is an atom- efficient synthesis pathway to 5- or 6-membered cyclic carbonates. Many systems have been developed that produce cyclic carbonates in high yields under mild conditions.[41a, 48] However, epoxides and oxetanes are hazardous chemicals and need special handling. Furthermore, they have to be produced by oxidation of alkenes or cyclisation of 3-chloro-1-hydroxyl compounds.
The synthesis of organic carbonates from alcohols and CO2 directly is the most sustainable process, since alcohols and CO2 are non-toxic compounds and readily available, the only by- product is water and only a single reaction step is necessary.[8c, 49] A detailed discussion of this synthesis route will be given in 1.3. Various other synthetic pathways are also under investigation.[8a, 50]
5 Introduction
Scheme 1-4: Research on synthesis of organic carbonates.[8a, 8c, 41]
1.2.3 Renewable organic carbonates from biomass and CO2 The use of renewable carbon feedstocks for the chemical industry is highly desirable mainly due to the depletion of fossil resources. Much research focusses on the transformation of biomass and carbon dioxide into valuable chemicals and fuel compounds.[51] The synthesis of organic carbonates offers the possibility to combine biogenic substrates and carbon dioxide, providing products consisting of more than 99 % renewable carbon by chemical and biological
CO2 fixation. A variety of biogenic substrates is available through biorefining, since plants consist of different structural elements like proteins, carbohydrates and fats.[52] The percentages of these components within the biomass differ depending on the plant. E.g. wood has a higher ratio of cellulose, hemicellulose and lignin, while seeds have a higher ratio of fats.
Regarding CO2 utilisation, 12 principles were introduced inspired by the 12 principles of green chemistry.[53] These principles include not only scientific considerations like catalysis, sustainability, and innovation that are crucial for successful CO2 utilisation, but also address economic and societal issues. Furthermore, life-cycle assessments containing carbon footprint estimations are valuable tools to evaluate CO2 utilisation processes, because just the
[54] incorporation of CO2 does not make a process more sustainable by default. The following sections give examples of the synthesis of organic carbonates from renewable substrates and carbon dioxide.
6 Introduction
1.2.3.1 Sugar carbonates Saccharides are carbohydrate compounds with high contents of oxygen groups like ketone, aldehyde and hydroxyl functions. They are readily available from plants, non-toxic and hold stereochemical information. Apart from their biocompatibility and biodegradability, polymers consisting of monosaccharides are easily functionalised due to the high content of hydroxyl groups.[55] Cyclic carbonates and polycarbonates have been reported starting from D-xylose,[56]
D-glucose,[57] and isosorbide[58]. However, these synthesis routes require phosgene or other carbonylation agents for the transformation of the diol to the carbonate. The group of Buchard reported a method that utilises carbon dioxide for the carbonation of D-mannose (1)[59] and
2-desoxy-D-ribose[60] to their corresponding carbonates using 1,8-diazabicyclo[5.4.0]undec-7- ene (DBU), p-toluenesulfonyl cloride (TsCl) and triethylamine as reagents (see Scheme 1-5).
Scheme 1-5: Synthesis of 6-membered cyclic carbonate (2) from protected D-mannose (1).[59]
1.2.3.2 Terpene carbonates Terpenes are versatile acyclic and cyclic unsaturated compounds that consist of multiples of isoprene units. Naturally they occur in essential oils of leaves, flowers, and fruits of many plants. That makes terpenes major components in the flavour and fragrance industry. Additionally, they are used for medical formulations. Applications of terpenes as renewable substrates for the chemical industry are under consideration.[61]
Limonene (3) or rather limonene oxide has found application as a substrate for polymers. Limonene oxide polymerises with carbon dioxide to give a polycarbonate with excellent thermal resistance, hardness and transparency.[62] Limonene dicarbonate can be used as a substrate for non-isocyanate polyhydroxyurethanes.[63] Kleij and coworkers synthesised cyclic carbonates from various terpene scaffolds by initial oxidation of a double bond to the corresponding epoxide and subsequent cycloaddition of carbon dioxide in the presence of an AlIII aminotriphenolate catalyst and bis(triphenylphosphine)iminium chloride (PPNCl) (see Scheme 1-6).[64]
7 Introduction
Scheme 1-6: Synthesis of terpene carbonates (4) on the example of limonene (3).[64]
1.2.3.3 Oleochemical carbonates Oleochemical compounds are derived from plant or animal oils and fats. Cleavage of triglycerides yields fatty acids and their derivatives. Depending on the primary source, fatty acids are unbranched saturated or unsaturated carbonic acids with an even chain length between 4 and 28 carbon atoms. Traditionally, oleochemicals have been used for the production of soaps and detergents due to their amphiphilic properties. Fatty acid methyl esters, referred to as biodiesel because of their similarity to long chained petrochemicals, are an alternative to fossil based fuels.[65]
Carbonated oleochemicals (6) possess promising applications as fuel additives, industrial lubricants and starting materials for non-isocyanate polyurethanes and polyesters.[66] According to carbon footprint estimations, carbonated fatty acid methyl esters show great
CO2-saving potential compared to currently used diisononyl phthalate in the plasticiser market.[67] In general, carbonated fatty acid esters (6) are prepared in two steps (see Scheme 1-7). Catalytic epoxidation of one or more double bonds is followed by cycloaddition of carbon dioxide to the carbonate. Catalytic systems for the carboxylation are ammonium salts,[68] also in combination with acidic co-catalysts[69], organic phosphonium salts,[66] switchable ionic liquids,[70] or phase-transfer catalysts.[67]
Scheme 1-7: Synthesis of oleochemical carbonates (6) on the example of methyl oleate (5).
8 Introduction
1.2.3.4 Glycerol carbonate Glycerol (Gly, 7) is a trivalent alcohol that is easily obtained from biogenic fats. It is a by- product from different biomass conversion processes especially of the biodiesel production. Apart from direct glycerol utilisation in the pharmaceutical, cosmetic, and food industry, glycerol can be transformed into more valuable derivatives, e.g. esters, ether, acetals and
[71] ketals, or other C3-chemicals by hydrogenation and hydrogenolysis.
Glycerol’s corresponding five-membered cyclic carbonate possesses two different functional groups, a hydroxyl group and a 2-oxa-1,3-dioxolane group, leading to a wide reactivity.[72] Therefore, glycerol carbonate (GlyCarb, 8) can be a starting material for a variety of reactions, including transcarbonation, esterification, and N-alkylation.[73] Industrially, glycerol carbonate is synthesised by the transesterification of glycerol with ethylene or propylene carbonate.[71a, 74] Utilisation of urea instead of carbonates is reported in literature, since urea is a cheap bulk
[75] chemical produced from CO2. The cycloaddition of carbon dioxide to glycidol is an atom efficient alternative.[76] However, for the synthesis of glycidol, glycerol carbonate (8) itself can be used as intermediate, which makes this route less attractive.[77] Direct coupling of glycerol and carbon dioxide under elimination of water (see Scheme 1-8) is advantageous and reaction systems will be discussed in 1.3.2, 2.1 and 3.1.
Scheme 1-8: Direct synthesis of glycerol carbonate (8) from glycerol (7) and carbon dioxide.
1.3 Direct carbonation of diols
1.3.1 Reaction characteristics
1.3.1.1 Reactivity of substrates Carbon dioxide is the product of all combustion processes of organic material. The carbon atom reaches its highest oxidation state with a hypothetical charge of +IV in CO2. Therefore, all reactions that reduce carbon dioxide to lower oxidation states require energy in the form of heat, electrons, or radiation.[78] Usually high energy substrates, like hydrogen, alkenes or
9 Introduction
amines, and suitable catalysts have to be used to make the overall energy balance favourable.[51a] Only transformations into compounds that possess +IV-oxidized carbon like organic and inorganic carbonates are quasi-neutral or even slightly exothermic processes.[51b] These reactions are suitable also for the activation of carbon dioxide by organocatalysts[79] or metal complexes.[80]
Alcohols are frequently used as starting materials. However, they are quite unreactive both as nucleophile or electrophile. In nucleophilic substitution reactions, hydroxyl groups often fail to be replaced by another functional group, because OH- is a poor leaving group. Activation can
- occur by protonating the alcohol transferring OH into H2O as leaving group, but the acidic environment might also deactivate the nucleophile.[81] In the reverse case, alcohols have to be deprotonated to represent effective nucleophiles for substitution reactions.
1.3.1.2 Equilibrium reaction The direct carbonation of alcohols with carbon dioxide (see Scheme 1-9 for diols) implies four elementary reactions for thermodynamic considerations. Two O-H-bonds are broken in the alcohol, two O-H-bonds are formed in water, two C-O-bonds are broken in carbon dioxide, and two C-O bonds are formed in the carbonate. The enthalpy change is mainly influenced by the OH-bond energies in the alcohols and the OR-bond energies in the formed carbonates.
Carbon dioxide mainly drives the entropic content of the reaction. CO2 goes from the gas phase into a liquid or solid product. This results in positive Gibbs energies for several alcohols and thus very low equilibrium yields of around 1 % for the carbonate.[51b] For the synthesis of glycerol carbonate from glycerol and carbon dioxide (R = CH2OH in Scheme 1-9), an
-5 equilibrium constant of 6.41 x 10 is calculated at standard conditions. Increasing the CO2 pressure to 50 bar increases the equilibrium concentration by almost two orders of magnitude to 3.27 x 10-3. Higher temperatures result in a decrease of the equilibrium constant to 1.506 x 10-3 at 180 °C.[82] Overall, even at higher pressures the reaction is very limited by thermodynamics.
Scheme 1-9: Equilibrium reaction of diol and carbon dioxide.
10 Introduction
1.3.1.3 Reaction mechanism The synthesis of cyclic carbonates directly from diols and carbon dioxide presumably proceeds via the reaction steps shown in Scheme 1-10. First, one alcohol moiety is deprotonated by a base (I). Then, the alkoxide attacks CO2 nucleophilically resulting in a hemi-carbonate (II). The other hydroxyl group is removed by an acid leading to a zwitterionic species (III). By following ring-closure, the cyclic carbonate is obtained (IV). A neutralisation reaction regains acid and base catalysts by releasing water (V).[51b]
Scheme 1-10: Presumed reaction path for the direct carbonation of diols by carbon dioxide.[51b] - I: Deprotonation of one hydroxyl group by base; II: addition of CO2; III: acidic removal of OH group; IV: ring- closure to cyclic carbonate; V: acid base reaction to afford water.
Ma et al. investigated the uncatalysed carbonation of ethylene glycol by computational methods. They calculated two pathways of hydroxyl dehydration mechanisms in line with Scheme 1-10 and two carbonyl substitution mechanisms (one is shown in Scheme 1-11), which include a second nucleophilic attack on the carbon dioxide moiety by the other hydroxyl group. In conclusion, they stated that a carbonyl substitution mechanism is more favourable than the substitution of the hydroxyl group in absence of a catalyst. However, for all pathways the intramolecular ring-closure is the rate-determining step.[83]
Scheme 1-11: Carbonyl substitution mechanism in the formation of ethylene carbonate.
11 Introduction
1.3.2 Reaction systems The direct carbonation of alcohols with carbon dioxide is very challenging due to the low reactivity of the substrates and the thermodynamic limitations. Therefore, developments for efficient catalytic systems and ways of influencing the equilibrium are crucial.
1.3.2.1 Dehydration systems
Organic carbonates and water are the products of the reaction of alcohols and CO2 (see Scheme 1-9). However, the equilibrium yield is only around 1 %. Apart from increasing the carbon dioxide pressure, water removal is a suitable option to shift the equilibrium to higher carbonate yields. There are two types of dehydration agents: reactive and non-reactive systems.[49a] Non-reactive dehydration agents remove water through stripping by inert gas, physisorption or selective separation. In the synthesis of organic carbonates, gas phase, inorganic absorbent and membrane separation systems have been applied. Although, they are favoured due to their inertness and recyclability, they often suffer from poor performances at reaction conditions. For example, the highest yield of dimethyl carbonate, which has been obtained using a non-reactive dehydration system, is 9.1 % in a gas phase reaction.[84] In contrast, reactive dehydration systems bind water chemically by reaction into hydration products of the water removal agents. Examples are orthoesters, acetals and nitriles.[41b] Yields as high as 94 % for dimethyl carbonate and 99 % for propylene carbonate can be obtained with
[85] 2-cyanopyridine as dehydration agent and CeO2 as catalyst. Drawbacks are the additional costs for supply of fresh water removal agent and downstreaming processes to remove hydration products and recycling of unreacted compounds decreasing the atom economy and the sustainability of the process. Furthermore, the catalyst has to be active in both the organic carbonate formation and the hydration of the water removal agent or a second catalyst must be added.
1.3.2.2 Catalytic systems The choice of suitable combinations of catalyst and dehydration agent is crucial to achieving feasible yields in the synthesis of organic carbonates from alcohols and carbon dioxide. Usually quiet harsh conditions are necessary with temperatures ranging from 120 °C to 180 °C and pressures varying from 30 bar to 150 bar. Different homogenous and heterogeneous systems are reported and listed below.
For homogeneous catalyst systems, Sakakura et al. reported a dimethyl carbonate yield of 14 % in a system with dibutyltin oxide as catalyst and 1,1-dimethoxyphenylmethane as dehydration
12 Introduction agent.[86] Addition of ammonium triflates increased the yield of dimethyl carbonate to 40 % with 2,2-dimethoxypropane as water removal agent (see Scheme 1-12).[87] Podila et al. showed that dibutyltin oxides also catalyse the synthesis of glycerol carbonate resulting in yields of 7 % in presence of acetonitrile.[88] If 2-cyanopyridine is used as dehydration agent and promoter a glycerol carbonate yield of 18.7 % is obtained without any further additives as reported by Su et al.[89] Huang et al. studied different alkali carbonates and organic bases in the synthesis of propylene carbonate. For Cs2CO3 and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as catalyst, they obtained yields of 15.6 % and 22.5 % respectively.[90] Da Silva et al. obtained 20 % yield of propylene carbonate with K2CO3 as catalyst and benzonitrile as dehydration agent. With adiponitrile, the yield was increased to 27 %.[91] Huang et al. reported 24.2 % of propylene
[92] carbonate yield in acetonitrile with Zn(OAc)2 as catalyst. Following up on these findings, Castro-Osma et al. tested 81 different metal salts via high-throughput screening for their application as catalysts in the synthesis of propylene carbonate. They identified Zn(OTf)2 as best catalyst with an overall propylene carbonate yield of greater than 50 % in acetonitrile.[93]
Scheme 1-12: Homogeneous catalyst systems.[87, 89, 93]
In heterogeneous catalysis, Zhang et al. reported a dimethyl carbonate yield of 10 % using
[94] Ce0.5Zr0.5O2 as catalyst and trimethoxymethane as dehydration agent. Honda et al. combined CeO2 and 2-cyanopyridine as catalytic system resulting in yields of up to 94 % for dimethyl carbonate and 99 % for several cyclic carbonates (see Scheme 1-13).[85] For the synthesis of glycerol carbonate from glycerol and carbon dioxide in the presence of acetonitrile, La2O3 in combination with ZnO or Cu resulted in yields of up to 15 % as reported
13 Introduction
from the groups of He, Zhao and Wei.[95] Li et al. obtained glycerol carbonate yields of up to 18 % using hydrotalcites based on Zn, Al and La.[96]
Scheme 1-13: Heterogeneous catalyst systems.[85, 96a]
1.3.2.3 Stoichiometric systems
Catalytic systems usually need harsh reaction conditions with high CO2 pressures, high temperatures and dehydration agents. As alternative, some stoichiometric reaction systems were proposed, operating at room temperature and atmospheric pressure of carbon dioxide. Hoffman reported a Mitsunobo-type reaction system for the synthesis of dialkyl carbonates. He synthesised dipentyl carbonate in 81 % isolated yield with triphenyl phosphine and diethyl azodicarboxylate as reagents.[97] Kadokawa et al. obtained dibenzyl carbonate in 90.7 % yield in the presence of tributyl phosphine, carbon tetrabromide and 2-cyclohexyl-1,1,3,3- tetramethylguanide.[98] For the synthesis of unsymmetrical dialkyl carbonates, Bratt and Taylor developed a method with DBU, methanesulfonic anhydride and pyridine as reagents and obtained e.g. n-propyl phenyl carbonate in 35 % isolated yield.[99] Yamazaki et al. prepared aromatic and allylic linear carbonates in yields up to 99 % with Cs2CO3 and dichloromethane in 1-methyl-2-pyrrolidinone.[100] Lim et al. gained cyclic and linear carbonates in a reaction system with DBU, [BMIM]PF6 as ionic liquid and dibromomethane as solvent in up to 86 % yield.[101] For the synthesis of 6-membered cyclic carbonates, Gregory et al. used DBU, p-toluenesulfonyl chloride and triethylamine and achieved yields of up to 70 %.[102]
14 Introduction
Scheme 1-14: Stoichiometric systems.[100-102]
1.4 Aim of the thesis
The present thesis investigates the direct synthesis of glycerol carbonate from glycerol and carbon dioxide by Brønstedt base catalysis. The aim is to provide a better understanding of the reaction mechanism and to pave the way for further sustainable and renewable chemical products. The synthesis of glycerol carbonate proceeds via an unfavourable equilibrium reaction where water is formed as by-product. Two different approaches to remove the water are studied: I) the reactive dehydration by acetonitrile, and II) the water extraction by a continuous carbon dioxide flow.
15
2 Synthesis of glycerol carbonate in the presence of acetonitrile
2.1 Introduction
2.1.1 State of the art Due to its low toxicity, high biodegradability and physical properties, glycerol carbonate (GlyCarb, 8) offers direct applications in many different industrial areas.[73a] Glycerol carbonate can be a chemical intermediate for a variety of reactions in the chemical industry thanks to its two different functional groups and the wide reactivity (see 1.2.3.4). An advantage is also that GlyCarb can derive from fully renewable resources with an overall atom economy of 87 % with water as only by-product, if synthesised directly from glycerol and carbon dioxide (see Scheme 2-1).
Scheme 2-1: Equilibrium reaction in the synthesis of glycerol carbonate (8).
For the direct synthesis of glycerol carbonate, different heterogeneous and homogeneous catalyst systems in the presence of acetonitrile have been proposed (see Table 2-1). In this case, acetonitrile serves as sacrificial dehydration agent to shift the unfavourable equilibrium to the product side (vide infra). The groups of He, Zhao and Wei reported that La2O3, in combination with ZnO or Cu as catalyst results in glycerol carbonate yields of up to 15 %.[95] Hydrotalcites based on Zn, Al, and La yielded 16 % GlyCarb (18 % after a prolonged reaction time) according to Li et al.[96] Podila et al. showed that homogeneous dibutyltin oxides also catalyse the synthesis of glycerol carbonate resulting in yields of 7 %.[88] Brønsted bases were also identified as suitable catalysts for the synthesis of glycerol carbonate by Huang et al. Alkali carbonates like K2CO3 and Cs2CO3 achieved 8 % and 10 % yield of GlyCarb respectively. Organic bases like 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) or 1,8-diazabicyclo[5.4.0]undec-7-en (DBU) yielded up to 12 % GlyCarb at the same reaction conditions.[103] Selectivity to GlyCarb is in all cases
Synthesis of glycerol carbonate in the presence of acetonitrile around 60 % due to the acetalization reaction of glycerol and the hydration products of acetonitrile.
Table 2-1: Catalytic systems for the synthesis of glycerol carbonate.
Catalyst T [°C] p(CO2) [bar] GlyCarb yield Ref. La2O3 + ZnO/Cu 170 40 15 % [95] heterogeneous Zn, Al, La hydrotalcites 170 60 18 % [96] Bu2SnO 150 140 7 % [88] K2CO3 175 80 8 % [103] homogeneous Cs2CO3 175 80 10 % [103] DBU 175 80 12 % [103]
2.1.2 Hydration of acetonitrile In the reaction with water, nitriles hydrolyse to either amides or carboxylic acids (see Scheme 2-2). For acetonitrile (MeCN, 9), the first addition of water to the cyanogen group results in the formation of acetamide (10). In a secondary hydrolysis, acetamide can react to acetic acid (11) and ammonia. Usually, the rate constant of amide hydrolysis is much higher than the nitrile hydrolysis, since nitrile groups are not very reactive. In contrast, amide C-N bonds are more labile.[104] Therefore, the carboxylic acid is the most commonly observed product for alkaline hydrolysis of nitriles. In acidic media, amide formation can occur preferentially due to concentration dependency of the reaction rates.[105] Conventional hydration of nitriles requires high amounts of strong acids or bases or an excess of water in catalytic reactions.[106] At harsh conditions in high-temperature water, autocatalysed hydrolysis of acetonitrile can occur.[107]
Scheme 2-2: Hydration of acetonitrile (9).
2.1.3 Aim of this section This section addresses the synthesis of glycerol carbonate from glycerol and carbon dioxide in presence of the reactive dehydration reagent acetonitrile. An accurate analysis method is developed and verified to obtain reliable experimental results. The influence of important reaction parameters such as CO2 pressure, reaction temperature, amounts of catalyst and dehydration agent is investigated. The complex reaction network of the glycerol carbonate synthesis in presence of acetonitrile is further disclosed and several side products are
17 Synthesis of glycerol carbonate in the presence of acetonitrile identified. The reaction progress and the phase behaviour are monitored over time by in-situ IR spectroscopy and single experiments stopped after certain reaction times. Other approaches to improve the reaction yields such as the addition of a second solvent or a repetitive batch experiment were studied.
2.2 Analysis
2.2.1 Quantitative analysis and reproducibility The valid quantification of experimental results is of high importance. Therefore, an analytic method has to be developed that detects glycerol, glycerol carbonate and possible by-products like acetates in a reliable manner. Gaschromatographic analysis was tested but found inconsistent due to decomposition of glycerol carbonate during measurements at high temperatures. 1H-NMR spectroscopic analysis in deuterated dimethyl sulfoxide proved to be an appropriate method for the characterisation of reaction samples. For quantitative results, a standard has to be found which neither possesses signals that overlap with signals from the reaction mixture nor reacts with compounds from the reaction mixture. Since the reaction mixture is likely to contain water and its resonance overlaps with the glycerol signals, different acids were tested as standard. Acids possess exchangeable protons at chemical shifts > 10 ppm and due to proton exchange the signals of water and alcohol functions are shifted to higher chemical shifts. This facilitates the analysis of the non-exchangeable protons in the reaction mixture. Different organic acids and a combination of butanone and trifluoroacetic acid (TFA) were tested (Figure 2-1). Maleic acid was chosen as standard because it is a non-volatile solid and its resonance does not interfere with any glycerol or glycerol carbonate signal.
18 Synthesis of glycerol carbonate in the presence of acetonitrile
Figure 2-1: Different acids as 1H-NMR spectroscopy standard in an equimolar mixture of glycerol and glycerol carbonate (400 MHz, 25 °C, DMSO-d6).
Table 2-2 shows the detection of both glycerol and glycerol carbonate by 1H-NMR spectroscopic analysis in self-made mixtures with maleic acid as standard. With an average detection of 97 % and 100 % with a deviation of 4 % and 3.5 % for glycerol and glycerol carbonate respectively, this analysis method was applied for the determination of conversion and yield in experiments. For comparison, glycerol carbonate was isolated by column chromatography.
Table 2-2: 1H-NMR spectroscopic standard validation.
Glycerol Glycerol carbonate Reference detected Deviation Reference detected Deviation [mmol] [mmol] [%] [mmol] [mmol] [%] 0.141 0.128 9.8 0.122 0.128 5.3 0.100 0.101 0.6 0.122 0.122 0 0.104 0.104 0 0.123 0.120 2.8 0.120 0.118 1.6 0.123 0.120 2.8 0.250 0.240 3.9 0.199 0.202 1.2
Figure 2-2 shows an example of the 1H-NMR spectra of the crude reaction mixture with and without maleic acid. Without the acid standard, glycerol signals are overlapped by water and glycerol carbonate signals are broadened by resonances of the OH functionalities. With addition of maleic acid, the baseline becomes flat and the spectrum can be analysed reliably.
19 Synthesis of glycerol carbonate in the presence of acetonitrile
Figure 2-2: 1H-NMR spectra of reaction mixture with and without maleic acid as standard.
The analysis of multiple experiments at the standard reaction conditions resulted in an average 1H-NMR yield of GlyCarb of 16 % with a deviation of 2.5 %. The average Gly conversion determined by 1H-NMR spectroscopy was 33 % with a deviation of 5.5 % (see Table 2-3). Thus, the experiments are fairly reproducible.
Table 2-3: Average conversion of Gly and yield of GlyCarb and deviation. Reaction conditions: 0.45 g glycerol (5 mmol), 2 mL MeCN, 5 mol% catalyst, T = 155 °C, p(CO2) = 80 bar, t = 16 h.
Average Deviation 1 2 3 4 5 [%] [%] Conversion of Gly [%] 34 39 29 37 26 33 5 Yield of GlyCarb [%] 14 13 17 19 16 16 2
2.2.2 Phase observations Glycerol and acetonitrile are immiscible at room temperature. Therefore, the reaction mixture is biphasic in the beginning. After the reaction, the observable phase behaviour depends on the reaction temperature (see Figure 2-3). Below 185 °C, the reaction mixture remains biphasic. At 185 °C, the glycerol and acetonitrile phases become miscible. This indicates a complex phase behaviour and reaction network which will be discussed in 2.4 and 2.5. Additionally, the glycerol phase darkens from yellow to brown with increasing temperature.
20 Synthesis of glycerol carbonate in the presence of acetonitrile
125 °C 145 °C 155 °C 185 °C
Figure 2-3: Reaction mixtures after 16 h at different temperatures.
2.3 Parameter variation of the carboxylation of glycerol
[103] Adapting the reaction conditions from Huang et al., K2CO3 was chosen as initial catalyst for the direct synthesis of glycerol carbonate (GlyCarb) from glycerol and carbon dioxide. Building on results obtained from project partners within “CyclicCO2R”, a temperature of 145 °C and
80 bar of CO2 pressure were used as benchmark conditions. With a K2CO3 amount of 5 mol% and 8 equiv. of acetonitrile relative to glycerol a GlyCarb yield of 13 % (10 % isolated) was obtained. This was already an improvement to the 8 % yield reported in literature at harsher reaction conditions.[103] Accordingly, different reaction parameters were varied and their influence on the reaction outcome was investigated.
2.3.1 Carbon dioxide pressure Since the reaction of glycerol and carbon dioxide to glycerol carbonate is equilibrium controlled (see 1.3.1.2), it is expected that the GlyCarb yield increases with higher CO2 pressures. Therefore, the pressure of CO2 was varied between 30 bar and 100 bar measured at reaction temperature of 145 °C. Figure 2-4 shows the glycerol carbonate yield depending on the
CO2 pressure varying between 13 % and 15 %. This is within the margin of error showing that the carbon dioxide pressure has no detectable influence on the GlyCarb yield under these reaction conditions. The solubility of carbon dioxide in both glycerol and acetonitrile does not significantly increase by the pressure variation. For further experiments a pressure of 80 bar at reaction temperature was chosen.
21 Synthesis of glycerol carbonate in the presence of acetonitrile
20
15 15 15 14 13
10
NMR yield ofGlyCarb -
H 5
in the crudein mixture [%] 1
0 30 (1.05) 60 (2.06) 80 (3.49) 100 (3.91)
CO2 pressure [bar] (m(CO2) [g])
Figure 2-4: Variation of CO2 pressure. Reaction conditions: 0.45 g glycerol (5 mmol), 2 mL MeCN (8 equiv.), 5 mol% K2CO3, T = 145 °C, t = 16 h. Yields were determined by 1H-NMR spectroscopy with MTBE as standard.
2.3.2 Amount of acetonitrile Acetonitrile is used as dehydrating agent to remove water from the reaction, as described in 1.3.1.2 and 2.1.2. Figure 2-5 shows the results of varying the amount of acetonitrile between 1 equiv. to 8 equiv. relative to glycerol. The GlyCarb yield varied between 10 % and 14 % without a tendency to increase or decrease with added equivalents of acetonitrile. In all cases, the reaction mixture was still biphasic when the reaction was stopped. At 145 °C the yield seems to be independent of the added amount of acetonitrile. Therefore, eight equivalents of acetonitrile were chosen for further experiments.
22 Synthesis of glycerol carbonate in the presence of acetonitrile
20
15 14 13 13
10
10
ield ield ofGlyCarb
NMR y -
H 5
in the crudein mixture[%] 1
0 0.25 (1 eq.) 0.5 (2 eq.) 1 (4 eq.) 2 (8 eq.) Volume of MeCN [mL] (equivalents based on glycerol)
Figure 2-5: Variation of MeCN amount. Reaction conditions: 0.45 g glycerol (5 mmol), 5 mol% K2CO3, T = 145 °C, p(CO2) = 80 bar, t = 16 h. Yields were determined by 1H-NMR spectroscopy with MTBE as standard.
2.3.3 Base concentration The reaction mechanism (see 1.3.1.3) implies that a higher base concentration would promote the addition of CO2 to glycerol to form a hemi-carbonate. To investigate the role of the base concentration, K2CO3 was added in amounts from 2.5 mol% to 25 mol% (see Figure 2-6). For 2.5 mol% and 5 mol% there was no significant difference in the isolated GlyCarb yield with 9 % and 10 % respectively. For 10 mol% K2CO3 the yield decreased to 7 % and to 3 % for 25 mol%, which represents the solubility limit of K2CO3 in glycerol at room temperature. These results indicate that higher base concentrations do not favour the product formation but hamper it.
Furthermore, these results imply that K2CO3 is indeed a Brønsted base catalyst because even
2.5 mol % K2CO3 gave an isolated GlyCarb yield of 9 %. Therefore, CO2 has to be involved in the reaction. For further experiments 5 mol% of K2CO3 was used as the standard amount.
23 Synthesis of glycerol carbonate in the presence of acetonitrile
12 10 10 9
8 7
6
4 3
2 Isolated yield Isolatedyield ofGlyCarb [%]
0 2,5 5 10 25
Amount of K2CO3 [mol%]
Figure 2-6: Variation of catalyst amount. Reaction conditions: 0.45 g glycerol (5 mmol), 1 mL MeCN (4 equiv.), T = 145 °C, p(CO2) = 80 bar, t = 16 h. GlyCarb was isolated by column chromatography.
2.3.4 Reaction time After 16 h of reaction time, GlyCarb was isolated in yields of 10 %. Therefore, the reaction time was prolonged to investigate whether the yield could be increased (Figure 2-7). After a reaction time of 88 h (3.5 days) the isolated yield of GlyCarb was doubled to 23 %. By prolonging the reaction time to one week, a yield of 31 % was achieved. This implies that the yield constantly increases, although not linearly, because the rate of conversion decreases.
40
31 30 23
20
10
10 Isolated yield Isolatedyield ofGlyCarb [%]
0 16 88 168 Time [h]
Figure 2-7: Variation of reaction time. Reaction conditions: 0.45 g glycerol (5 mmol), 5 mol% K2CO3, 2 mL MeCN, T = 145 °C, p(CO2) = 80 bar. GlyCarb was isolated by column chromatography.
24 Synthesis of glycerol carbonate in the presence of acetonitrile
2.3.5 Temperature Next, the influence of the reaction temperature was investigated (see Figure 2-8). At a temperature of 125 °C, GlyCarb was only detected in small quantities by 1H-NMR spectroscopy. As expected, the conversion of Gly increased with increasing temperature. At 155 °C, GlyCarb was isolated in 14 % yield after overnight reaction. Increasing the temperature to 175 °C, however, gave a lower GlyCarb yield of 9 %. In this case it is important to notice, that the conversion of glycerol increases steadily with higher temperature according to the 1H-NMR spectrum. At 175 °C, the Gly conversion was seven times higher than the GlyCarb yield. This indicates that more side products were generated (for detailed discussion see 2.4.1). This was confirmed by the more intense side product signals in both the 1H-NMR spectra and a larger number of spots in the thin-layer chromatography. Furthermore, the reaction mixture generally became darker with increasing temperature, changing from yellow to brown, indicating the formation of more side products (for more details see 2.4.1).
80 67 70 60 50 39 40 Y(GlyCarb) 30 20 X(Gly) 20 Conversion or yield [%] 7 10 4 12 14 9 0 125 145 155 175 Temperature [°C]
Figure 2-8:Variation of reaction temperature. Reaction conditions: 0.45 g glycerol (5 mmol), 5 mol% K2CO3, 2 mL MeCN, p(CO2) = 80 bar, t = 16 h. Conversions were determined by 1H-NMR spectroscopy with MTBE as standard, GlyCarb was isolated by column chromatography.
2.3.6 Catalyst Different inorganic and organic Brønstedt bases were tested as catalysts in the reaction of glycerol and carbon dioxide to glycerol carbonate. The alkali carbonates gave very similar
GlyCarb yields of around 14 %. Other potassium bases like KOH and KHCO3 resulted in slightly lower yields of 12 % and 11 % respectively. The organic anion tert-butoxide gave the
25 Synthesis of glycerol carbonate in the presence of acetonitrile
same yield as K2CO3. Organic bases like TBD and DBU and N-heterocyclic carbene carboxylate,
[108] known for their potential to activate CO2, resulted in higher yields of around 18 %. Several imidazolium-based ionic liquids with amine functionalities in one side-chain were investigated but no GlyCarb yield was detected. Since K2CO3 is an inexpensive, non-toxic and readily available compound, it was decided to continue with this base.
25
20 18 18 17 14 14 14 15 13 12 11 10
Yield Yield ofGlyCarb [%] 5
0
Figure 2-9: Investigation of different Brønstedt basic catalysts. Reaction conditions: 0.45 g glycerol (5 mmol), 2 mL MeCN, 5 mol% catalyst, T = 155 °C, p(CO2) = 80 bar, t = 16 h. 1 GlyCarb was isolated by column chromatography. For MMIM-CO2 as catalyst, yield was determined by H-NMR spectroscopy with maleic acid as standard.
2.3.7 Miscellaneous Instead of acetonitrile, other nitriles like benzonitrile or 2-cyanopyridine were applied as dehydration agents at the same reaction conditions. However, no GlyCarb yield was detected. The same was true for 2,2-dimethoxypropane. So, it can be concluded, that these dehydration agents do not efficiently remove water under the investigated reaction conditions.
A variety of compounds was tested as additives to promote the reaction. Since Castro-Osma et al. obtained good yields for the carbonation of propylene glycol using zinc triflate,[93] different triflate sources like triflic acid, lithium triflate, and ethylmethylimidazolium triflate were used as catalysts both with or without the addition of K2CO3. However, in the synthesis of glycerol carbonate, the addition of the triflate anion did not result in higher yields. Lewis acids were also added to promote the elimination of water as shown in 1.3.1.3, but aluminium, cerium and indium salts as additives resulted in low yields. Methanol and ethanol were added
26 Synthesis of glycerol carbonate in the presence of acetonitrile as sacrificial alcohols for esterification reactions to bind acetamide and acetic acid and thus, lower the formation of glycerol acetate, but the formation of methanol or ethanol acetate was not detectable. Ammonium carbonate was tested as additive to prevent the formation of glycerol acetate due its influence on the equilibrium of the hydration reaction.[90a] Under the applied reaction conditions, the selectivity of glycerol carbonate remained unchanged.
2.4 Reaction network
2.4.1 Side product identification The synthesis of glycerol carbonate from the trialcohol glycerol and carbon dioxide is an equilibrium reaction far on the substrate site (see 1.3.1.2). Water has to be removed to increase the product yield. Acetonitrile can serve as reactive water removal agent (see 2.1.2). MeCN (9) reacts with one equivalent of water to acetamide (10) and by the reaction with another equivalent of water acetic acid (11) and ammonia can be the resulting products (see Scheme 2-3).
Scheme 2-3: Hydration of acetonitrile to acetamide and acetic acid.
However, the hydration products of acetonitrile undergo side reactions with glycerol or glycerol carbonate (see Scheme 2-4 and Scheme 2-5). Esterification with an alcohol moiety from either glycerol or glycerol carbonate is one possibility resulting in the corresponding acetates (12 and 16, reactions I). Glycerol monoacetate (12) is the main side product and is formed in up to stoichiometric amounts with respect to glycerol carbonate. At high reaction temperatures or longer reaction times other side products can be detected in smaller amounts. A mixture of these side-products was obtained by column chromatography and could not be further purified to the isolated compounds. By comparison of the NMR signals (see Figure 2-10 and Figure 2-11) with literature and authentic samples, all side products were identified. In addition to the monoacetates, di- and triacetates are formed. Furthermore, disubstituted glycerol with an acetate and an amide function is found (13, reaction II, Scheme 2-4). Hydroxymethyl-2-oxazolidinones (14 and 15) are found as side products (reaction III, Scheme 2-4 and reaction II, Scheme 2-5). According to literature these heterocycles can be formed by the reaction of glycerol or glycerol carbonate and urea.[109] Since there was no evidence for the
27 Synthesis of glycerol carbonate in the presence of acetonitrile formation of urea under the applied reaction conditions, another reaction mechanism might be active. In addition, the change in colour suggests the formation of side products with higher molecular weight, e.g. humins.
Scheme 2-4: Side reactions of glycerol. I: Actetalisation of glycerol with acetamide or acetic acid; II: Reaction of glycerol to an aminoacetate; III: Reaction of glycerol to oxazolidinones.
Scheme 2-5: Side reactions of glycerol carbonate. I: Esterification of glycerol carbonate with acetamide or acetic acid; II: Reaction of glycerol carbonate to oxazolidinones.
28 Synthesis of glycerol carbonate in the presence of acetonitrile
Figure 2-10: 1H-NMR sprectrum of the first column chromatographic fraction. Acetates in accordance to literature reference.[110]
Figure 2-11: 1H-NMR spectrum of the fourth column chromatographic fraction. 4-(hydroxymethyl)oxazolidin-2-one in accordance to literature reference.[111]
29 Synthesis of glycerol carbonate in the presence of acetonitrile
2.4.2 Water dependence
Since the reaction of glycerol and CO2 is an equilibrium reaction, the influence of the water content, present at the beginning of the reaction, on the product yield was investigated (see Figure 2-12). With dried glycerol (96 ppm of water) glycerol carbonate is obtained in 14 % yield at optimized reaction conditions. If 0.05 equivalents of water in regard to glycerol are added, the yield stays the same with 14 %. Only after adding 0.1 or more equivalents of water a decrease of the glycerol carbonate yield is obvious with 8 % (0.1 equiv.) and only minor GlyCarb signals were detected in the NMR after the addition of one equivalent of water.
20
15 14 14
10 8
5
NMR yield ofGlyCarb -
H 5
in the crudein mixture [%] 1 2 0 0 none 0.05 0.1 0.25 0.5 1 Added water [equivalents based on glycerol]
Figure 2-12: Influence of water content on the glycerol carbonate yield. Reaction conditions: 0.45 g glycerol (5 mmol), 2 mL MeCN, 5 mol% K2CO3, T = 155 °C, p(CO2) = 80 bar, t = 16 h. Yields were determined by 1H-NMR spectroscopy with maleic acid as standard.
2.4.3 Reactivity studies To investigate the reaction network in greater detail, some control experiments were performed. Under standard reaction conditions but without glycerol, acetonitrile is only hydrated to acetamide in 1.6 % if 0.5 equivalents of water are added. This result leads to the conclusion that acetonitrile is involved in the reaction mechanism, that facilitates the hydration. Addition of 1 equiv. of acetamide to the reaction mixture results in 11 % yield of glycerol carbonate and 15 % yield of glycerol monoacetate. Therefore, acetamide does not hamper the reaction to a large extent. If acetic acid is added, only glycerol acetates are formed in 40 % yield. Glycerol acetate was also used as substrate resulting in minor glycerol carbonate formation of 4 % yield and hydrolysis to glycerol. Glycerol carbonate itself decomposes slightly
30 Synthesis of glycerol carbonate in the presence of acetonitrile under standard reaction conditions. Without acetonitrile, basic decomposition to glycerol takes place.
2.5 In-situ mIR spectroscopy
2.5.1 Single component IR spectra Several reactions take place in the reaction mixture as discussed before (see 2.4.1). In the synthesis of glycerol carbonate from glycerol and urea, FTIR spectroscopy was successfully applied to monitor the reaction progress and analyse the reaction mechanism.[112] Since carbonyl functionalities absorb IR radiation in high molar absorptivities, the different components were measured to assess whether the reaction progress could be monitored also in this case. Single component IR spectra of the different reaction components show that the desired product glycerol carbonate can be distinguished from the side products (see Figure 2-13 and Figure 2-14). Glycerol carbonate has a CO absorption band at 1760 cm-1, while glycerol acetate and the oxazolidinones absorb at 1713 cm-1. Glycerol adsorbs with highest intensity at a wavenumber of 1028 cm-1.
1
0,9 1713 1760 0,8 Glycerol 1028 0,7 Glycerol 0,6 carbonate Glycerol 0,5 acetate Oxazolidinones
Absorption 0,4 Aminoacetate
0,3 MeCN 0,2
0,1
0 2000 1800 1600 1400 1200 1000 800 600 Wavenumber [cm-1]
Figure 2-13: Superimposed single component ATR-spectra of reaction mixture compounds.
31 Synthesis of glycerol carbonate in the presence of acetonitrile
MeCN
Aminoacetate
1713 Oxazolidinones
Glycerol Absorption 1717 acetate 1760 Glycerol carbonate
Glycerol
2000 1800 1600 1400 1200 1000 800 Wavenumber [cm-1]
Figure 2-14: Stacked single component ATR-spectra of reaction mixture compounds. Glycerol, glycerol carbonate, glycerol acetate, oxazolidinoes, aminoacetate, acetonitrile.
2.5.2 In-situ investigations of the acetonitrile phase Since the major components are distinguishable by IR, the reaction progress was monitored in-situ in collaboration with T. Eifert from the Liauw group at the RWTH Aachen University. The reactor is a window autoclave equipped with two IR-probes, one at the bottom and one at the top of the autoclave (see Figure 2-15). In this manner, it is possible to follow the reaction progress at two different locations of the autoclave which is useful for multiphase systems. The optimised reaction conditions were adjusted to comply with the changed autoclave volume (see Figure 2-15B for a schematic drawing of the phase ratios). In comparison to the standard reaction conditions only the ratio of the gas phase volume was changed.
32 Synthesis of glycerol carbonate in the presence of acetonitrile
A B
Figure 2-15: A: In-situ IR window autoclave; B: Schematic reaction mixture.
Pictures of the reaction mixture at different reaction times are shown in Figure 2-16. In the beginning, the acetonitrile phase fills half of the autoclave. With proceeding reaction time, the reaction mixture darkens and the level of the liquid decreases. After 45 h the liquid level is hardly observable through the windows and covers only the bottom of the autoclave. After releasing the pressure, the liquid level increases again. This leads to the conclusion, that there is a complicated phase behaviour in addition to the complex reaction network.
t = 15 min t = 21 h t = 45 h t = 51 h
155 °C, 93 bar 155 °C, 87 bar 154 °C, 76 bar RT, 0 bar
Figure 2-16: Pictures of reaction mixture batch conditions.
Figure 2-17 shows the measurement of the acetonitrile phase (lower IR-probe) and illustrates the change in the IR spectra during the reaction. The bands of glycerol carbonate and glycerol monoacetate increase over the reaction time. However, the rate of the glycerol carbonate formation changes after 25 h. The peak integrations of the glycerol carbonate, glycerol acetate and acetamide bands show this trend more clearly (see Figure 2-18). A constant increase of the peak area is visible for glycerol monoacetate and acetamide. The peak area of the glycerol carbonate signal increases first, then reaches a plateau after 25 h reaction time. The constant intensity of the carbonyl band of glycerol carbonate indicates that the GlyCarb absorption
33 Synthesis of glycerol carbonate in the presence of acetonitrile remains unchanged. Since the volume of the liquid phase decreases, this might even be a sign for a decrease of the GlyCarb amount in the reaction mixture, since the IR-absorption depends on the concentration. Thus, for an exact quantification of the reaction components by IR-spectroscopy, the volume variation of the MeCN phase must be taken into account.
Figure 2-17: ATR-mIR spectra surface of the acetonitrile phase (lower IR-probe).
2,5
2
1,5 glycerol carbonate 1 glycerol
Peakintegration acetate 0,5 acetamide
0 0 20 40 Time [h]
Figure 2-18: Peak integration of the acetonitrile phase spectra (lower IR-probe).
34 Synthesis of glycerol carbonate in the presence of acetonitrile
2.5.3 In-situ investigations of the glycerol phase To monitor both the acetonitrile and the glycerol rich phase, conditions were chosen in which the lower IR probe measures the glycerol rich phase and the upper IR probe measures the acetonitrile rich phase (see Figure 2-19A for schematic drawing). In Figure 2-20 pictures of the reaction mixture after different reaction times are shown. In the beginning, glycerol and acetonitrile are distinct phases and almost fill the whole autoclave. After one hour under reaction conditions the reaction mixture is already slightly yellow. With proceeding reaction time, the reaction mixture gets darker, the glycerol level sinks under the level of the lower IR probe and the overall liquid level decreases (see schematic drawing in Figure 2-19B). Furthermore, the overall pressure of the reaction system drops.
A B
Figure 2-19: Schematic drawing of phase behaviour during the reaction course. A: Phase ratios at the start of the reaction; B: phase ratios over the reaction progress.
t = 0 h t = 1 h t = 5 h t = 22 h
RT, 0 bar 155 °C, 55 bar 153 °C, 49 bar 155 °C, 40 bar
Figure 2-20: Pictures of reaction mixture.
Figure 2-21 shows the results of the measurement of the lower IR-probe and illustrates the change in the IR spectra during the reaction. The glycerol band decreases, while the bands of glycerol carbonate and glycerol monoacetate increase. After ca. 5 hours a clear decrease in band intensities occurs in line with the observation that the IR probe measures the acetonitrile-rich phase from now on. Figure 2-22 shows the peak integration of the important signals. In the beginning, the peak area of the glycerol signal decreases. After 5 hours, when the IR probe measures the acetonitrile phase, the signal intensity stays about the same.
35 Synthesis of glycerol carbonate in the presence of acetonitrile
Presumably, the maximum solubility of glycerol in acetonitrile is reached and therefore no changes are observed. The trend is similar for the area of overlapping peaks of glycerol hemi- carbonate and acetamide. In the first 5 hours, the intensity decreases. Then there is a sudden decrease explained by the change of the measured phase. In the acetonitrile phase these signals increase after this point. In contrast, the graphs of the peak areas of glycerol carbonate and glycerol monoacetate are quite steady even after 5 hours. So the concentrations of both components seem to be similar in the glycerol and acetonitrile phase. As expected a constant increase in the peak area is visible for glycerol monoacetate. The peak area of the glycerol carbonate signal increases within the first 8-9 hours. Afterwards, it decreases again. These results indicate that glycerol carbonate reaches a maximum yield during the reaction which cannot be precisely identified via the IR measurements.
Figure 2-21: ATR-mIR spectra surface of the reaction mixture (lower IR-probe).
36 Synthesis of glycerol carbonate in the presence of acetonitrile
60
50 Glycerol carbonate 40 Glycerol 30 hemicarbonate/ acetamide
20 Glycerol acetate Peakintegration
10 Glycerol 0 0 10 20 Time [h]
Figure 2-22: Peak integration of IR spectra of the reaction mixture (lower IR-probe).
2.6 Reaction time profile
The in-situ IR experiments indicate that the glycerol carbonate yield decreases during the reaction. To verify these findings, batch experiments were conducted in which reactions were stopped after certain reaction times. Four different conditions are used with different ratios between glycerol and acetonitrile and at different temperatures. Figure 2-23 shows the results for a molar ratio of glycerol and acetonitrile of 1:3. At 175 °C these conditions match literature conditions from Huang et al.[103] They usually stop their reactions after 12 hours and achieve a yield of 8 % of glycerol carbonate. This agrees with the results within this study. However, stopping the reaction after 4 hours resulted in a higher yield of glycerol carbonate (11 %). Furthermore, a change in the reaction phase behaviour was observed. Starting with two phases
(glycerol + K2CO3 and acetonitrile), the reaction mixture merged into one phase after 8 hours. By lowering the reaction temperature to 155 °C a higher maximum glycerol carbonate yield of 14 % was achieved after 16 hours. In this case the phases merge later between 16 and 42 hours of reaction time. In both cases the glycerol carbonate yield reached a maximum.
37 Synthesis of glycerol carbonate in the presence of acetonitrile
20
15
10 155 °C 175 °C
5
NMR yield ofGlyCarb
-
H
in the crudein mixture [%] 1 0 0 20 40 60 Time [h]
Figure 2-23: Reaction time profile for literature reaction conditions.[103] Filled symbols represent experiments with biphasic reaction mixture after the given reaction time; hollow symbols represent experiments with monophasic reaction mixtures after the given reaction time. Reaction conditions: 1.4 g glycerol (15 mmol), 1.4 mL MeCN (Gly:MeCN = 1:3), 2.5 mol% K2CO3, p(CO2) = 80 bar. Yields were determined by 1H-NMR spectroscopy with maleic acid as standard.
Reaction conditions with a higher amount of acetonitrile relative to glycerol (8 equiv.) were identified as good reaction conditions in 2.3. In this case, the glycerol carbonate yield increased to 17 % at both 155 °C and 175 °C (see Figure 2-24). While this yield is already achieved after 8 hours at 175 °C, it takes twice as long (16 hours) at 155 °C. Furthermore, the yield is constant between 8 h and 16 h at 175 °C but decreases again at prolonged reaction times of 42 h. At these reaction conditions, the reaction remains biphasic over the entire reaction time. In conclusion, the glycerol carbonate yield depends on temperature and the acetonitrile/glycerol ratio. A maximum yield can be achieved by choosing the right reaction time. At higher temperature the decomposition reactions were more pronounced, especially with a lower acetonitrile amount.
20
15
10 155 °C 175 °C
5
NMR yield ofGlyCarb
-
H
in the crudein mixture [%] 1 0 0 20 40 60 Time [h]
Figure 2-24: Reaction time profile for optimised reaction conditions. Reaction conditions: 0.45 g glycerol (5 mmol), 2 mL MeCN (Gly:MeCN = 1:8), 5 mol% K2CO3, p(CO2) = 80 bar. Yields were determined by 1H-NMR spectroscopy with maleic acid as standard.
38 Synthesis of glycerol carbonate in the presence of acetonitrile
2.7 Additional solvent
The reaction of glycerol and carbon dioxide to glycerol carbonate in presence of acetonitrile exhibits complex phase behaviour. At temperatures above 150 °C and low acetonitrile amounts, the GlyCarb yield decreases after having reached a maximum yield. This seems to correlate with the merging of the phases, since the decrease is not as pronounced at higher acetonitrile ratios, where the phases do usually not merge. Therefore, additional solvents were tested to maintain the biphasic system. Table 2-4 shows the qualitative miscibility of glycerol or glycerol carbonate with some solvents of different polarities at room temperature. It is evident that glycerol is only soluble in very polar solvents like water or short-chained alcohols. Glycerol carbonate is less polar than glycerol and therefore miscible with a broader range of solvents.
Table 2-4: Miscibility of glycerol and glycerol carbonate with different solvents. [7] Solvents are ordered according to decreasing ET(30) values.
Solvent Glycerol Glycerol carbonate Water ✓ ✓ Methanol ✓ ✓ Ethanol ✓ ✓ 2-Propanol ✓ ✓ Acetonitrile ✓ Acetone ✓ 4-Methyl-2-pentanol ✓ 4-Methyl-2-pentanon ✓ Ethyl acetate ✓ Tetrahydrofuran ✓ 2-Methyltetrahydrofuran ✓ 1,4-Dioxane ✓ Methyl tert-butyl ether Diethyl ether Methyl cyclopentyl ether Toluene Pentane
Some of the tested solvents were applied as co-solvent in the direct carbonation of glycerol with CO2 (see Figure 2-25). Except for methanol, which is miscible with all reaction components, all solvents were expected to maintain the biphasic reaction system because of the poor solubility of glycerol. However, at the given reaction conditions all reaction mixtures were monophasic and dark brown at the end of the reaction time. Furthermore, the GlyCarb yield was even lower in most cases than without additional solvent.
39 Synthesis of glycerol carbonate in the presence of acetonitrile
10
8 8 8
6 6 6 6 5 5 5 4
4
NMR yield ofGlyCarb
-
H in the crudein mixture [%] 1 2
0 None MeOH MIBC EtOAc THF 2-MTHF Et2O n-BuOEt Toluene
Figure 2-25: Direct carbonation of glycerol with additional solvent. Reaction conditions: 1.4 g glycerol (15 mmol), 2.5 mol% K2CO3, 1 mL MeCN, 0.5 mL additional solvent, T = 175 °C, p = 80 bar, t = 16 h. Yields were determined by 1H-NMR spectroscopy with maleic acid as standard.
2.8 Repetitive batch
A repetitive batch concept was tested to improve the glycerol carbonate yield. In this case, the reaction mixture was heated for a certain time. Subsequently the acetonitrile phase was removed, and fresh acetonitrile was added for further reaction progress. Figure 2-26 shows the results. In the first cycle, 3.5 % yield of glycerol carbonate was obtained. This is lower than the result achieved in 2.6, but only the acetonitrile phase was analysed. The second cycle resulted in 6.5 % GlyCarb yield. This corresponds to a total yield of 11 % after 7 h only in the acetonitrile phase. For subsequent overnight reaction, the temperature was lowered to 155 °C. Additional 5 % yield of GlyCarb were obtained. The overall yield after 23 h is comparable to the yield obtained in a single batch experiment (see 2.6). Still, this is a possible set-up for a continuous stirred tank reactor with product separation.
40 Synthesis of glycerol carbonate in the presence of acetonitrile
20
0,5 15 4,5 3. cycle - glycerol phase 10 3. cycle - MeCN phase yield yield ofGlyCarb 6,5 6,5
2. cycle - MeCN phase NMR - 5
H 1. cycle - MeCN phase
in the crudein mixture [%] 1 4,5 4,5 4,5 0 3,5 7 23 Reaction time [h]
Figure 2-26: Repetitive batch experiment. Reaction conditions: 1.54 g glycerol (16.7 mmol), 2 mL MeCN (added for each cycle), 5 mol% K2CO3, T = 175 °C 1 (155°C overnight), p(CO2) = 80 bar. Yields were determined by H-NMR spectroscopy with maleic acid as standard.
An experiment with slow continuous addition of acetonitrile was also conducted. It was expected that the formation of by-products would decrease because acetonitrile would be available only in small quantities. However, this approach has not been as successful, since similar yields to standard batch experiments were obtained.
2.9 Conclusions
The direct synthesis of glycerol carbonate from glycerol and carbon dioxide in presence of acetonitrile as reactive dehydration system was studied. Starting from a reaction temperature of 145 °C, a CO2 pressure of 80 bar, and acetonitrile amount of 8 equiv. and K2CO3 as catalyst with 5 mol% concentration resulting in 10 % glycerol carbonate yield, the influence of various reaction parameters was investigated.
A change of the CO2 pressure and acetonitrile amount did not affect the glycerol carbonate yield at 145 °C. Also, the K2CO3 concentration within the solubility limits had a minor impact on the reaction outcome. Several Brønstedt bases were suitable catalysts, while organic bases gave higher yields (18 %) than alkali carbonates (14 %). Increasing the reaction temperature from 125 °C to 175 °C resulted in higher yields with a maximum at 155 °C (14 % isolated yield of glycerol carbonate), although the glycerol conversion constantly increased.
41 Synthesis of glycerol carbonate in the presence of acetonitrile
For the reaction network, several side and consecutive products were identified apart from glycerol monoacetate. The formation of di- and triacetates, oxazolidinones and an aminoacetate was favoured at higher temperatures or prolonged reaction times. Investigation of the reaction progress by in-situ IR spectroscopy or single experiments stopped at different reaction times revealed a maximum yield of glycerol carbonate depending on the reaction conditions. In general, at a reaction temperature of 175 °C the glycerol carbonate yield decreased more rapidly than at 155 °C.
Furthermore, complex phase behaviour was observed for the reaction. Starting from a biphasic mixture, phases merged at lower acetonitrile amounts which presumably favoured glycerol carbonate decomposition. Addition of an inert solvent for phase separation did not improve the yield at high reaction temperatures. Also, repetitive removal of the acetonitrile phase and addition of new MeCN resulted in comparable yields to batch experiments.
In conclusion, glycerol carbonate was synthesised in 17 % yield from glycerol and carbon dioxide in presence of acetonitrile as dehydration agent and potassium carbonate as catalyst. Compared to literature, this is the highest yield reported so far for a reaction system with very inexpensive and readily available reagents. Furthermore, the reaction network was further disclosed.
42
3 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.1 Introduction
3.1.1 Non-reactive dehydration systems The application of non-reactive dehydration systems in the synthesis of organic carbonates from alcohols and carbon dioxide is highly desirable due to their inertness and recyclability compared to reactive water removal agents. However, there are only limited examples of the successful application of non-reactive water removal systems.
The group of Sakakura developed a circulating system for the synthesis of dimethyl carbonate from methanol and carbon dioxide with molecular sieves as dehydration agent. The reaction set-up consists of two parts, a reactor and a dehydration column. The reaction mixture is continuously circulated between both parts, while it is cooled before passing the dehydration column to improve the water removal performance of the molecular sieves. In this manner, they achieved the synthesis of dimethyl carbonate in 45 % yield.[113] Dibenedetto et al. connected a membrane to their reactor to remove water from the reaction mixture. In the synthesis of diethyl carbonate from ethanol and CO2 they obtained a yield of 2.3 % after circulating several times.[114] The group of Wohlrab reported a diethyl carbonate yield of
-1 -1 0.06 % at a productivity of 47 mmol L h for the reaction of ethanol and CO2 in a continuous plug flow reactor where a membrane has been applied.[115] Ozorio et al. reported yields of 5.8 % glycerol carbonate with metal-impregnated zeolites in neat glycerol. In this case, the zeolite acts as both catalyst and water removal agent.[116]
3.1.2 Carbon dioxide for water extraction
3.1.2.1 Carbon dioxide as extraction medium Since carbon dioxide is readily available, inexpensive, non-flammable, chemically inert under many conditions and non-toxic, its application as processing medium is highly desirable. Since most organic transformations take place in liquid phase, it is beneficial, that CO2 liquefies at pressures of 65 bar at 25 °C. In this state, CO2 behaves like any other solvent, however with a low density of 0.47 g cm-1. In addition, supercritical carbon dioxide is obtained at relatively mild conditions with 31 °C and 74 bar. scCO2 combines gaseous properties with high densities
Water stripping by carbon dioxide in the synthesis of glycerol carbonate
-1 up to 0.9 g cm , when highly compressed. Furthermore, physical properties of scCO2, like density, diffusivity, viscosity and surface tension, are tuneable by varying pressure and temperature. In general, carbon dioxide both in liquid or supercritical state behaves similar to hydrocarbon solvents with very low polarizability.[117] Utilisation of carbon dioxide instead of classical organic solvents is advantageous in terms of mass transport, product recovery and environmental considerations. However, energy consumption of high pressure equipment has to be considered.[118]
For decades, carbon dioxide has been a well-known extraction agent in the food and nutrition industry. The most prominent example is the mild and selective extraction of caffeine from
[119] coffee beans. Furthermore, scCO2 has found industrial application in some hydrogenation reactions,[118b, 120] polymer processing,[121] and cleaning or drying processes.[118a] The potential of supercritical fluids in continuous multiphase reaction systems was investigated by several groups.[122]
3.1.2.2 Solubility of water in carbon dioxide Water dissolves in carbon dioxide, although only in low mole fractions. In general, the solubility of water in CO2 increases with temperature. With increasing pressure starting from one atmosphere, the solubility of water decreases. However, from pressures above 50 bar it increases again.[123] At 100 °C and 100 bar the mole fraction of dissolved water in carbon dioxide is 0.02, and 0.3 at 200 °C and the same pressure.[123d] King et al. stated that in extraction operations in addition to the desired compound, water may be extracted by carbon dioxide despite the low solubility.[123b]
Andanson et al. investigated the purification of ionic liquids by supercritical carbon dioxide. They reported that even very hygroscopic and water-soluble ionic liquids were efficiently dried
[124] by extraction with a scCO2 flow at 40 °C and 100 bar. For this reason, it was concluded, that
CO2 could be applied as stripping agent for water in the direct carbonation of glycerol.
3.1.2.3 Mutual solubility of glycerol and carbon dioxide Carbon dioxide and glycerol possess only a low mutual solubility. As in many binary gas-liquid systems, the solubility of carbon dioxide in glycerol increases with higher pressures and decreases with elevated temperatures.[125] In the range of 100 °C to 200 °C, the mole fraction of
CO2 dissolved in glycerol decreases from 0.06 to 0.02 at 100 bar, which corresponds to a solubility decrease from 0.92 mol L-1 to 0.28 mol L-1.[125b] On the other hand, the solubility of glycerol in carbon dioxide increases both with higher pressures and temperatures.[125b, 126] An
44 Water stripping by carbon dioxide in the synthesis of glycerol carbonate increase of the molar fraction of glycerol dissolved in carbon dioxide from 2.4 ∙ 10-5 to 6.9 ∙ 10-5 is measured at 100 bar as the temperature increases from 100 °C to 140 °C. Still glycerol is
-4 -1 barely soluble in CO2 (2.4 ∙ 10 mol L at 140 °C and 100 bar), which opens applications for biphasic systems with glycerol as stationary phase. Furthermore, no glycerol swelling was observed during carbon dioxide solubilisation.[125b]
3.1.3 Aim of this section This section shows the development of a semi-continuous process for the direct synthesis of cyclic carbonates from glycols and carbon dioxide. In this case, carbon dioxide serves both as reagent and water stripping agent. A proof of concept for glycerol carbonate is shown and the influence of the reactor design, the reaction temperature, catalyst amount, CO2 pressure and flow rate on the glycerol carbonate yield is discussed. The reaction mechanism is investigated, and an intermediate is identified. The impact of additives on the reaction outcome is studied. The applicability of this process to other alcohol substrates is tested including ethylene and propylene glycol, 2,3-butanediol, 1,2-octanediol, phenol, and the sugars sorbitol and erythritol.
3.2 Preliminary studies
The phase behaviour of the binary systems glycerol/scCO2 and glycerol carbonate/scCO2 was investigated qualitatively. Figure 3-1 shows pictures of both systems at two different conditions. As glycerol and carbon dioxide are barely miscible (see 3.1.2.3), no phase transitions were expected and have not been observed in the experiment. Since glycerol carbonate is less polar than glycerol, it might be more soluble in carbon dioxide. However, also for the GlyCarb/CO2 system no change in the phases was observable with the naked eye. Therefore, a selective stripping of water from a reaction mixture is likely.
Gly GlyCarb
40 °C, 100 bar CO2 100 °C, 280 bar CO2 40 °C, 90 bar CO2 100 °C, 180 bar CO2
Figure 3-1: Investigation of phase behaviour of glycerol + CO2 and glycerol carbonate + CO2. Left: Glycerol and CO2 at T = 40 °C, p(CO2) = 100 bar and T = 100 °C, p(CO2) = 280 bar in a window autoclave. Right: Glycerol carbonate and CO2 at T = 40 °C, p(CO2) = 90 bar and T = 100 °C, p(CO2) = 180 bar in a window autoclave.
45 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
A mixture of equimolar amounts of glycerol, glycerol carbonate and water was prepared, mimicking a reaction mixture at 50 % conversion given that the reaction proceeds with 100 % selectivity. A constant CO2 flow was introduced to the autoclave to extract water from the reaction mixture. Figure 3-2 shows 1H-NMR spectra of the mixture before and after stripping with carbon dioxide. The integrals of Gly and GlyCarb represent a mixture of approximately 1.25:1 for both components independent from the extraction time. Although the water signal is superimposed with either glycerol (t = 0h) or glycerol carbonate (t = 2 h), the share of the water protons in the total proton number decreases considerably after 2 h of stripping with
CO2. Therefore, CO2 can be applied for the removal of water from a glycerol/glycerol carbonate mixture without stripping substrate or product.
1 Figure 3-2: H-NMR spectra of a mixture of Gly, GlyCarb and H2O (400 MHz, 25 °C, DMSO-d6). -1 (A) Before and (B) after stripping with CO2 for 2 h at T = 100 °C, p = 100 bar and V̇ = 0.12 L min .
3.3 Reactor design
For the direct synthesis of glycerol carbonate from glycerol and carbon dioxide with continuous water removal by CO2 flow, the mixing of carbon dioxide and glycerol is crucial, as
CO2 acts both as reagent and stripping agent. A bubble column reactor was chosen because it facilitates gas liquid exchange due to a turbulent stream (see Figure 3-3A). The gas is inserted from the bottom and rises through the liquid. The reactor was built at the mechanical
46 Water stripping by carbon dioxide in the synthesis of glycerol carbonate workshop at ITMC, RWTH Aachen University and is shown in Figure 3-3B. It withstands temperatures of up to 200 °C and pressures up to 120 bar. The reactor is heated by four individual heating sleeves. A sampling unit was integrated to monitor the reaction progress over time. A B
Figure 3-3: Bubble column reactor. A: Schematic drawing of bubble column reactor with (1) inlet, (2) outlet, (3) frit. B: Picture of reactor with (1) inlet, (2) outlet, (3) frit, (4) sampling unit, (5) heating.
3.4 Investigation of reaction conditions
The direct synthesis of glycerol carbonate from glycerol and carbon dioxide with continuous water stripping by CO2 was investigated. By the steady removal of the water by-product, the yield of glycerol carbonate is expected to increase compared to equilibrium conditions. Similar starting conditions compared to the acetonitrile system were chosen with a temperature of
-1 140 °C, CO2 pressure of 80 bar and a CO2 flow of 100 mLN mL .
Figure 3-4: Direct synthesis of glycerol carbonate (8).
47 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.4.1 Temperature variation The reaction was conducted at temperatures of 140 °C, 150 °C and 170 °C (see Figure 3-5). At a temperature of 140 °C a steady increase of the glycerol carbonate yield to 5 % is observable within the first 50 h. Additional reaction time does not lead to a further increase of the GlyCarb yield. It seems that after 50 h equilibrium conditions are reached. At this temperature the reaction mixture turns slightly yellowish with increasing reaction time. If the temperature is elevated to 150 °C or higher, a brownish and highly viscous reaction mixture is obtained with minor production of glycerol carbonate between 1 % and 2 %.
10
5 140 °C 150 °C
NMR yield ofGlyCarb 170 °C
-
H
in the crudein mixture [%] 1
0 0 20 40 60 80 Time [h]
Figure 3-5: Reaction time profile at different temperatures. Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C, 150 °C and 170 °C; p = 80 bar; -1 V̇ = 100 mLN min ; 5 mol% K2CO3.
3.4.2 Catalyst amount A higher base concentration is expected to increase the yield of glycerol carbonate, due to more favourable deprotonation of glycerol and thus, formation of the glycerol hemi-carbonate.
Figure 3-6 shows the results for K2CO3 amounts of 0 mol%, 5 mol% and 25 mol% relative to glycerol. The absence of base resulted in the equilibrium yield of 1 %. Base concentrations exceeding the solubility limit of K2CO3 in glycerol decreased the GlyCarb yield to 2 % after two days. This can be attributed to the higher viscosity and inhomogeneity of the reaction mixture.
48 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
10 ofGlyCarb 5 5 mol% 0 mol%
NMR yield 25 mol%
-
H
in the crudein mixture [%] 1
0 0 10 20 30 40 50 60 Time [h]
Figure 3-6: Reaction time profile for different K2CO3 amounts. Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C (170 °C at 0 mol% K2CO3); p = 80 bar; -1 V̇ = 100 mLN min ; 0 mol%, 5 mol% and 25 mol% K2CO3.
A comparison between the carbonates of potassium and caesium is shown in Figure 3-7. In general, caesium carbonate dissolves better in organic solvents than potassium carbonate
[103] which explains the improved yields Huang et al. reported using Cs2CO3. However, glycerol is very polar and therefore, both K2CO3 and Cs2CO3 give similar reaction progress and yields.
Since K2CO3 is inexpensive and non-toxic, it was chosen as catalyst for further investigations.
10 ofGlyCarb 5 K2CO3
Cs2CO3
NMR yield
-
H
in the crudein mixture [%] 1
0 0 20 40 60 80 Time [h]
Figure 3-7: Reaction time profile for different alkali carbonates. -1 Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C; p = 80 bar; V̇ = 100 mLN min .1 (150 mLN min for Cs2CO3); 5 mol% of K2CO3 or Cs2CO3.
49 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.4.3 Carbon dioxide pressure variation
The CO2 pressure can influence the equilibrium yield of the direct synthesis of glycerol carbonate from glycerol and carbon dioxide due to the increasing solubility of water in scCO2. Variation of the carbon dioxide pressure between 20 bar and 120 bar shows that indeed higher pressures result in higher GlyCarb yields (see Figure 3-8). While a pressure of 20 bar yields 1 % of GlyCarb after 45 h, pressures of more than 40 bar result in 3 % to 4 % yields. With prolonged reaction times of 70 h, a pressure of 120 bar results in a GlyCarb yield of 7 % in comparison to an unchanged yield of 5 % for CO2 pressure of 80 bar.
10
ofGlyCarb 80 bar 5 20 bar
40 bar
NMR yield -
H 120 bar
in the crudein mixture [%] 1
0 0 20 40 60 80 Time [h]
Figure 3-8: Reaction time profile for different pressures. Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C; p = 20 bar, 40 bar, 80 bar and 120 bar; -1 -1 -1 V̇ = 100 mLN min (150 mLN min for 120 bar, 200 mLN min for 20 and 40 bar); 5 mol% K2CO3.
3.4.4 Carbon dioxide flow rate variation
Since the water is removed by bubbling CO2 through the reaction mixture, the influence of the carbon dioxide flow was investigated. With higher CO2 flow it is expected that the reaction system is better mixed and therefore, more water is stripped by carbon dioxide. Figure 3-9 shows the reaction progress for different flow rates. If the reactor is pressurised but no CO2 is passed through the reaction mixture, no GlyGarb formation was detected. As expected, the water removal by the constant CO2 flow is crucial. Variation of the flow rate from
-1 -1 100 mLN min to 300 mLN min had only a minor impact on the reaction rate at short reaction times. The same overall yield of 5 % GlyCarb was obtained. However, increasing the flow rate
-1 to 500 mLN min almost triples the glycerol carbonate yield to 14 % after 52 h. At prolonged
50 Water stripping by carbon dioxide in the synthesis of glycerol carbonate reaction times no further increase of the yield is observable, suggesting that an equilibrium is reached due to incomplete water removal.
15
10
ofGlyCarb 100 mL/min 0 mL/min
5 300 ml/min
NMR yield -
H 500 mL/min
in the crudein mixture [%] 1
0 0 20 40 60 80 Time [h]
Figure 3-9: Reaction time profile for different CO2 volumetric flow rates. -1 -1 Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C; p = 80 bar; V̇ = 0 mLN min , 100 mLN min , -1 -1 300 mLN min , 500 mLN min ; 5 mol% K2CO3.
3.4.5 Reactor design Since the reactor design exhibits a strong influence on the phase boundary surface and the mixing of the glycerol and carbon dioxide phase, two different reactor set-ups were compared (see Figure 3-10). The bubble column reactor (described in 3.3) gives 5 % GlyCarb yield in 30 g-scale. In an autoclave set-up, 1 g-scale is investigated, and the reaction mixture is stirred by a magnetic stirring bar. Furthermore, the CO2 is channelled over a glycerol film and not through the glycerol phase. It was expected, that the autoclave gives poorer results due to insufficient mixing. However, a GlyCarb yield of 8 % was obtained in the autoclave set-up in comparison to 4 % yield in the bubble column reactor. The larger head space of CO2 and the liquid film instead of a bulk phase probably have effects on the reaction yield. In conclusion, the GlyCarb yield is improved by reactor design. With a better mixing and gas transport, the GlyCarb yield could be increased.
51 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
10 ofGlyCarb 5 Reactor
Autoclave
NMR yield
-
H
in the crudein mixture [%] 1
0 0 10 20 30 40 50 Time [h]
Figure 3-10: Reaction time profile for different reactor designs. Reaction conditions: glycerol (7); bubble column reactor or window autoclave; T = 140 °C; p = 80 bar; -1 V̇ = 100 mLN min ; 5 mol% K2CO3.
3.5 Reaction network
A plausible reaction mechanism for the direct synthesis of cyclic carbonates is described in 1.3.1.3. Scheme 3-1 shows the catalytic cycle for the base catalysed reaction of glycerol and carbon dioxide to glycerol carbonate. After deprotonation of glycerol, the alkoxide (17) attacks carbon dioxide nucleophilically. Following ring closure and elimination of water gives glycerol carbonate and regenerates the basic catalyst. This section addresses the investigation of the reaction intermediate glycerol hemi-carbonate (18).
Scheme 3-1: Reaction mechanism for the base catalysed direct carbonation of glycerol. - I: Deprotonation of glycerol (7) by the base B ; II: nucleophilic attack of the glycerol alkoxide (17) on CO2; III: ring closure to glycerol carbonate (8) by elimination of water.
52 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.5.1 Characterisation of the reaction mixture The reaction mixture of the synthesis of glycerol carbonate from glycerol and carbon dioxide was analysed by IR and NMR spectroscopy after different reaction times. It was observed that unidentified signals evolve directly after the system is pressurised with CO2. Next, these new signals are further investigated.
Figure 3-11 shows the IR spectra of glycerol, glycerol carbonate, K2CO3 and mixtures of glycerol and K2CO3 both with and without the presence of carbon dioxide. Unidentified bands at 1643 cm-1 and 1392 cm-1 are visible, that do not match with any of the major components.
Furthermore, these bands can only be detected if the solution of glycerol and K2CO3 has been
-1 exposed to CO2. The position of the band at 1643 cm points to the presence of a presumably non-cyclic carbonyl function. Therefore, the absorption bands could be assigned to the glycerol hemi-carbonate.
1
1760
Glycerol 0,5 Gly+K2CO3
Gly+K2CO3+CO2 Absorption 1392 Glycerol carbonate
1643 K2CO3
0 2000 1800 1600 1400 1200 1000 800 Wavenumber [cm-1]
Figure 3-11: Comparison of single component and mixture ATR-IR spectra. Glycerol, K2CO3 dissolved in glycerol with and without added CO2, glycerol carbonate and K2CO3.
Also in the proton and carbon NMR spectra of the mixture of glycerol, K2CO3 and CO2 new signals were detected. In the 1H-NMR spectrum signals at 3.5 ppm and 3.7 ppm appeared. In the 13C-NMR spectrum a set of four signals arose, one of them in the carbonyl region at 157 ppm. The 1H-13C-HMBC spectrum confirms that the signal sets correlate with each other (see Figure 3-12). In the 1H-13C-HSQC spectrum no cross-correlation signal of the carbonyl carbon and any proton signal can be observed. This proves the assumption that a hemi-
53 Water stripping by carbon dioxide in the synthesis of glycerol carbonate carbonate of glycerol is formed. Apart from this species no other compounds were detected by both IR and NMR spectroscopy at temperatures up to 140 °C.
1 13 Figure 3-12: H- C-HMBC NMR spectrum of mixture of glycerol, 10 mol% K2CO3 and CO2 at 1 atm. (600 MHz, RT, DMSO-d6).
3.5.2 Synthesis of hemi-carbonate IR and NMR spectroscopic measurements of the reaction solution indicate the formation of a glycerol hemi-carbonate as intermediate in the synthesis of glycerol carbonate from glycerol and carbon dioxide. To confirm these findings the isolation of the hemi-carbonate of glycerol would be favourable.
The synthesis of alkali monoalkyl carbonates from methanol, ethanol, or i-propanol was reported previously.[127] These hemi-carbonates were formed by deprotonation of the alcohol by alkali metal and subsequent reaction with carbon dioxide. The hemi-carbonate salts precipitated from the parent alcohol and were obtained by filtration. For the synthesis of hemi- carbonates from diols like ethylene or propylene glycol, Xu et al. isolated the alkoxides and suspended them in dry acetonitrile for carbonisation.[128] This strategy was adopted for the formation of glycerol hemi-carbonate (18) (see Scheme 3-2). The alkoxide was synthesised by reaction of glycerol and KOH. After the removal of water in vacuo, a colourless solid was obtained. The solid was then suspended in acetonitrile under CO2 atmosphere. The resulting solid was insoluble in polar solvents except for water. However, protonation of the carbonate moiety occurs in water and, thus, only glycerol signals were detected in a solution of deuterated water.[127a]
54 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
Scheme 3-2: Reaction of glycerol (7) and CO2 to glycerol hemi-carbonate (18).
Several Brønstedt bases were used for the synthesis of the hemi-carbonate in glycerol. Alkali hydroxides and carbonates effectively deprotonated glycerol and the hemi-carbonate was formed at atmospheric pressures of carbon dioxide. The presence of base is essential, as experiments with Zn(OTf)2 and p-toluenesulfonic acid yielded no hemi-carbonate.
3.5.3 In-situ investigations on hemi-carbonate Since the reaction of glycerol (7) and carbon dioxide to glycerol carbonate (8) consists of two distinct reaction steps, investigations on the equilibrium of the formation of the glycerol hemi- carbonate (18, see Scheme 3-3) were conducted. The influence of carbon dioxide pressure, reaction temperature and base concentration were surveyed in-situ by IR spectroscopy in collaboration with Andreas Ohligschläger from the working group of Prof. Dr. M. Liauw (RWTH Aachen University).
Scheme 3-3: Equilibrium reaction to glycerol hemi-carbonate (18).
Figure 3-13 shows IR spectra of the reaction mixture at different CO2 pressures ranging from 0 bar to 35 bar. In general, the absorption bands at 1643 cm-1 and 1392 cm-1 increased with higher carbon dioxide pressures. The changes from 0 bar to 5 bar and afterwards to 20 bar led to strong increases in the intensity of the carbonyl absorption. This is assigned to an increase of the concentration of the hemi-carbonate of glycerol. Further increase of the pressure to 35 bar results in only a negligible change of the absorption intensity.
55 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
2 0,5
1,5 0,25
RT, 0 bar 1 0 RT, 5.8 bar Absorption 1800 1600 1400 1200 RT, 19.3 bar 0,5 RT, 35 bar
0 1800 1600 1400 1200 1000 800 Wavenumber [cm-1]
Figure 3-13: Comparison of ATR-mIR spectra for different pressures at RT. Conditions: glycerol with 2.5 mol% K2CO3; T = 22 °C; p = 0 bar, 5.8 bar, 19.3 bar, 35 bar.
The temperature had a minor influence on the position of the equilibrium (see Figure 3-14). The absorption intensity did not change upon increasing the temperature from 20 °C to 100 °C. At 150 °C an increase of the absorption intensity occured. However, the whole spectrum shifts to higher intensities due to the temperature. Therefore, it is likely that the concentration of glycerol hemi-carbonate is not affected.
2 0,5
1,5 0,25 RT, 35 bar 1 50 °C, 49.4 bar 0 Absorption 100 °C, 41.3 bar 1800 1600 1400 1200 0,5 150 °C, 45.8 bar
0 1800 1600 1400 1200 1000 800 Wavenumber [cm-1]
Figure 3-14: Comparison of ATR-mIR spectra at different temperatures. Conditions: glycerol with 2.5 mol% K2CO3; T = 22 °C, 50 °C, 100 °C, 150 °C; p = 35 bar to 50 bar.
56 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
Figure 3-15 compares the IR spectra of two solutions of K2CO3 in glycerol with different concentrations. As expected, the absorption intensity for the carbonyl function increases considerably upon changing the base concentration from 2.5 mol% to 10 mol%.
2 0,75
1,5 0,5
0,25
1 0 50 °C, 49.4 bar, 1800 1600 1400 1200
Absorption 5 mol% K2CO3 50 °C, 57 bar, 0,5 10 mol% K2CO3
0 1800 1600 1400 1200 1000 800 -1 Wavenumber [cm ]
Figure 3-15: Comparison of ATR-mIR spectra at different K2CO3 amounts. Conditions: glycerol with 2.5 mol% or 10 mol% K2CO3; T = 50 °C; p = 50 bar or 57 bar.
The stability of the glycerol hemi-carbonate in presence of acids was tested by the addition of trifluoroacetic acid (TFA). Figure 3-16 displays the 1H- and 13C-NMR spectra before and after the addition of TFA. The signals of the hemi-carbonate assigned by the circles vanish upon addition. This implies that the hemi-carbonate decomposes within acidic environments.
A B
Figure 3-16: Comparison of hemi-carbonate mixture with and without addition of TFA. 1 13 A: H-NMR (400 MHz, 25 °C, DMSO-d6,) and B: C-NMR (101 MHz, 25 °C, DMSO-d6) of glycerol, K2CO3 and CO2 mixture, bottom: without addition of TFA.
57 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.6 Additives
The investigations on the reaction mechanism revealed that the glycerol hemi-carbonate (18) is easily formed at room temperature and atmospheric pressure of carbon dioxide if a Brønstedt base is present. Therefore, the cyclisation and elimination of water seems to be the rate-determining step in the direct synthesis of glycerol carbonate from glycerol and CO2. To promote this reaction step, different additives were tested in the reaction (see Figure 3-17 and Figure 3-18).
The carboxylate of an imidazolium based carbene (MMIM-CO2) was tested as it represents an activated form of CO2 and proved to be an efficient catalyst in the transcarbonation reaction of dimethyl carbonate and glycerol to glycerol carbonate.[129] However, in the direct synthesis of glycerol carbonate no beneficial effect of the addition of MMIM-CO2 was observed (see Figure 3-17). KI was added to introduce a halide source into the reaction mixture. It was expected that a hydroxyl/halide exchange would take place in-situ facilitating the ring closure to the cyclic carbonate due to the better leaving group properties of halides. As Figure 3-17 shows, the yield of glycerol carbonate is not influenced by the addition of KI. Kumar and Jain reported improved yields for the synthesis of dimethyl carbonate from methanol and carbon dioxide
[130] with polyethylene glycol wrapped KBr ([K(PEG)]Br) as promoter in addition to K2CO3. They proposed that the hydroxyl/halide exchange increases the reaction rate and polyethylene glycol forms a crown ether-like complex with the potassium cation which increases the mobility of the bromide anion.[131] Therefore, [K(PEG)]Br was tested as additive in the reaction of glycerol and carbon dioxide to glycerol carbonate. However, the addition of [K(PEG)]Br showed no improvement (see Figure 3-18). Furthermore, the addition of LiTFSI and Nb2O5, both Lewis acidic compounds, was investigated. Lithium salts have been applied as additives
[132] [133] for CO2 hydrogenation reactions and Nb2O5 as catalyst in dehydration reactions. As Figure 3-18 shows, both compounds do not significantly enhance the glycerol carbonate yield under the applied reaction conditions.
58 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
10 ofGlyCarb 5 None MMIM-CO2
NMR yield KI
-
H
in the crudein mixture [%] 1
0 0 20 40 60 Time [h]
Figure 3-17: Reaction time profile for MMIM-CO2 and KI as additives. -1 Reaction conditions: glycerol (7); bubble column reactor; T = 140 °C; p = 80 bar; V̇ = 100 mLN min ; 5 mol% K2CO3; 5 mol% MMIM-CO2 or KI.
15
10
ofGlyCarb None LiTFSI
5 [K(PEG)]Br
NMR yield -
H Nb2O5
in the crudein mixture [%] 1
0 0 10 20 30 40 50 Time [%]
Figure 3-18: Reaction time profile for LiTFSI, [K(PEG)]Br and Nb2O5 as additives. -1 -1 Reaction conditions: glycerol (7); window autoclave; T = 140 °C; p = 80 bar; V̇ = 100 mLN min (200 mLN min for [K(PEG)]Br and Nb2O5); 5 mol% K2CO3; 5 mol% LiTFSI, [K(PEG)]Br or Nb2O5.
3.7 Downstream processing
Around 10 % yield of glycerol carbonate was obtained in the direct carbonation of glycerol with carbon dioxide when water was stripped by a continuous CO2 flow. Under the conditions investigated, this seemed to be the maximum yield due to thermodynamic restrictions (see 3.4.4). Therefore, a process to separate glycerol and glycerol carbonate to recycle glycerol and thus increase the overall yield is highly desirable. Since the boiling point of glycerol carbonate is rather high, different solvents were tested to remove GlyCarb from the reaction mixture by
59 Water stripping by carbon dioxide in the synthesis of glycerol carbonate extraction. In 2.7 the miscibility of glycerol and glycerol carbonate with different solvents was discussed. For extraction experiments, solvents were used, which are miscible with GlyCarb but immiscible with Gly. Table 3-1 shows the results of the experiments conducted with an equimolar mixture of Gly and GlyCarb and close to the boiling point of the solvent. Acetone and 1,4-dioxane merged with the glycerol phase, thus extraction with these solvents was not feasible. For acetonitrile, 2-methyltetrahydrofuran and 4-methyl-2-pentanol, both glycerol and glycerol carbonate were found in similar ratios in the solvents. Therefore, these solvents are not suitable for the selective extraction of GlyCarb from glycerol. Methyl-tert-butylether (MTBE) and 4-methyl-2-pentanone showed a more selective extraction for glycerol carbonate than for glycerol. However, MTBE extracted only 17 % of the present GlyCarb and 4-methyl-2-pentanone dissolved 36 % of glycerol in addition to 89 % of GlyCarb. These results indicate that other solvents and conditions should be tested for an efficient extraction process.
Table 3-1: Extraction of GlyCarb from Gly/GlyCarb mixtures by different solvents Conditions: equimolar mixture of glycerol and glycerol carbonate in a solvent (1 mL) was stirred for 1 h at [a] 50 °C, [b] 100 °C, [c] 80 °C, [d] 75 °C, [e] 130 °C, [f] 110 °C; values determined by 1H-NMR spectroscopy.
phase c(Gly) 풏(Glysolv) c(GlyCarb) 풏(GlyCarbsolv) solvent -1 [%] -1 [%] separation [mol L ] 풏(Glyges) [mol L ] 풏(GlyCarbges) Acetone[a] - - - - 1,4-Dioxane[b] - - - - Methyl-tert-butylether[a] ✓ 0.05 4 0.16 17 Acetonitrile[c] ✓ 0.85 92 1.53 93 2-Methyltetrahydrofuran[d] ✓ 0.53 78 1.23 93 4-Methyl-2-pentanol[e] ✓ 0.96 69 0.79 62 4-Methyl-2-pentanone[f] ✓ 0.17 36 1.02 89
Another downstream process is the separation of water and carbon dioxide to recycle CO2.
Possibilities to remove water from the CO2 stream are condensation, decompression, and absorption or membrane systems.[134] Each separation technique varies in their investment and operating costs. Therefore, the most economic feasible separation technique is depending on the process scale.
3.8 Substrate scope
3.8.1 Glycols
3.8.1.1 Ethylene glycol, propylene glycol and 2,3-butanediol The direct synthesis of cyclic carbonates derived from ethylene glycol (19), propylene glycol (20), and 2,3-butanediol (21) was investigated to evaluate the substrate scope for the process involving water extraction by CO2 (see Scheme 3-4 and Figure 3-19). The compounds differ in
60 Water stripping by carbon dioxide in the synthesis of glycerol carbonate their number of primary and secondary hydroxyl groups. While ethylene glycol possesses two primary OH-groups, 2,3-butanediol features two secondary hydroxyl groups. Propylene glycol is similar to glycerol, but has only one primary and one secondary OH-group, lacking the third one (see Scheme 3-4). Therefore, slightly different reactivity is expected for the three glycols.
Scheme 3-4: Direct carbonation of ethylene glycol (19), propylene glycol (20) and 2,3-butanediol (21).
Figure 3-19 shows the reaction progress for the direct synthesis of ethylene carbonate (22), propylene carbonate (23) and 2,3-butylene carbonate (24). For all compounds the yield does not exceed 3 % after 20 h of reaction time. For achieving prolonged reaction times, a modification of the reaction set-up would be necessary since these glycols are easily stripped by CO2 under the applied reaction conditions due to their higher volatilities and higher miscibilities with carbon dioxide in comparison to glycerol.[135] Therefore, a recycling unit should be implemented to increase the contact time of glycol, carbon dioxide and catalyst at reaction temperature and pressure.
5
2,5 Ethylene glycol of cyclic carbonate ofcyclic Propylene glycol
2,3-Butanediol
in the crudein mixture [%]
NMR yield
-
H 1 0 0 5 10 15 20 25 Time [h]
Figure 3-19: Reaction time profile for the direct carbonation of different substrates. Reaction conditions: ethylene glycol (19), propylene glycol (20) or 2,3-butanediol (21); bubble column reactor; -1 T = 140 °C; p = 80 bar; V̇ = 400 mLN min ; 5 mol% K2CO3 (5 mol% Cs2CO3 for 2,3-butanediol).
61 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.8.1.2 1,2-Octanediol 1,2-Octanediol (25) was also tested as substrate for the direct carbonation to 1,2-octane
carbonate (26) with water removal by continuous CO2 flow (see Scheme 3-5). Since this compound consists of a longer alkyl chain which lowers the miscibility with water, improved efficiency of the dehydration by carbon dioxide is expected.
Scheme 3-5: Direct carbonation of 1,2-octanediol (25).
For the reaction of 1,2-octanediol, a steady increase of the yield was observable (see Figure 3-20). After 20 h a yield of 12 % was detected. In comparison to glycerol and other glycols this is a very high yield. However, both 1,2-octanediol and 1,2-octane carbonate possess a high mobility in the CO2 phase as most of the compounds accumulated in the cooling trap after the pressure release of CO2. This mixture was recycled into the autoclave and after 45 h a total yield of 20 % (13 % isolated yield) was obtained. Using modified process set-up with a recycling unit for the CO2 phase, an even higher 1,2-octane carbonate yield is expected.
25
20
15
ofoctane carbonate 10
in the crudein mixture[%] 5
NMR yield
-
H 1 0 0 10 20 30 40 50 Time [h]
Figure 3-20: Reaction time profile for the synthesis of octane carbonate (26). -1 Reaction conditions: 1,2-octanediol (25); window autoclave; T = 140 °C; p = 80 bar; V̇ = 100 mLN min ; 5 mol% Cs2CO3. After 29 h the autoclave is refilled with the content of the cooling trap.
62 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.8.2 Phenol Diphenyl carbonate (28) is one of the most important linear carbonates in the chemical industry due to its application in polycarbonate synthesis. Therefore, the developed process was tested in the synthesis of diphenyl carbonate (see Scheme 3-6). Under the reaction conditions optimised for glycerol carbonate, no diphenyl carbonate was detected. Only decomposition products of phenol (27) were visible in NMR spectra of the reaction mixture.
Scheme 3-6: Direct carbonation of phenol (27).
3.8.3 Sugars
The use of sugar alcohols for the synthesis of cyclic carbonates with CO2 is appealing due to the low toxicity and the high carbon content from biologically and chemically captured carbon dioxide (see 1.2.3.1). Furthermore, carbonate functionalities are potential protecting groups in sugar modification reactions and are usually synthesised by reaction with dimethyl carbonate or ethylene carbonate.[136]
3.8.3.1 Sorbitol Sorbitol (29) is obtained by reduction of glucose and has applications in the food and personal care industries. The direct carbonation with CO2 to cyclic carbonates (30 and isomers) was investigated (see Scheme 3-7). Figure 3-21 shows the 13C-NMR spectrum of the reaction mixture. The signals with the highest intensities belong to sorbitol (29). For a selective synthesis of the cyclic carbonate seven signals were expected, six in the range of 60 ppm to 80 ppm and one between 150 ppm and 160 ppm. However, several sets of signals were detected from 60 ppm to 90 ppm. At chemical shifts above 150 ppm no signals were observed. Since this region is characteristic for signals of carbonate functions, it was concluded that the carbonate was not formed under the applied reaction conditions. Instead, decomposition or polymerisation reactions occurred under the basic reaction conditions that have not been further investigated.
63 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
Scheme 3-7: Reaction of sorbitol (29) and CO2 to cyclic carbonate (30).
13 Figure 3-21: C-NMR spectrum of sorbitol reaction mixture (101 MHz, 25 °C, DMSO-d6). -1 Reaction conditions: sorbitol (29); window autoclave; 5 mol% K2CO3; T = 140 °C; p = 80 bar; V̇ = 100 mLN min ; t = 24 h.
3.8.3.2 Erythritol The polyol erythritol (31) is used in the food industry. Although it occurs naturally in some fruits, industrially it is obtained by fermentation of glycose. Erythritol was tested in the direct carbonation with carbon dioxide to synthesise a sugar-derived cyclic carbonate (32 or isomers) (see Scheme 3-8).
Scheme 3-8: Reaction of erythritol (31) and CO2 to cyclic carbonate (32).
64 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
Figure 3-22 shows the 13C-NMR spectra at different reaction times for the reaction of erythritol (31) and carbon dioxide in the range of 60 ppm to 160 ppm and an excerpt from 60 ppm to 80 ppm. After 2 h, a low concentration of a carbonate compound was observable, shown by one signal in the range of 150 ppm to 160 ppm and four signals between 60 ppm and 75 ppm. With prolonged reaction times, the carbonate signals vanished and three sets of signals consisting of two singlets each were detected in addition to the erythritol (31) signals.
Figure 3-22: Reaction progress of erythritol (31) (101 MHz, 25 °C, DMSO-d6). -1 Reaction conditions: erythritol (31); window autoclave; 5 mol% K2CO3; T = 140 °C; p = 80 bar; V̇ = 100 mLN min . Signals assigned: C: carbonate, ⚫ erythritol (31), threitol (33), X 1,4-anhydrothreitol (34), 1,4-anhydroerythritol (35).
The emerging signals were assigned to the formation of threitol (33), 1,4-anhydrothreitol (34) and 1,4-anhydroerythritol (35) according to Scheme 3-9. The 13C-NMR spectra in Figure 3-22 after different reaction times show, that erythritol (31) was racemized to threitol (33) and after 19 h nearly a 1:1 mixture of the diastereomers was obtained. At 26 h reaction time, the ratio of 31 and 33 remained almost equimolar. 1,4-anhydroerythritol (35) was obtained in 20 % and 1,4-anhydrothreitol (34) in 10 % yield. After 96 h reaction time, the yields of the 2,3-dihydroxytetrahydrofuranes 35 and 34 increased and equalised to 37 % each. 13 % of both erythritol (31) and threitol (33) has not been converted. Several groups reported the synthesis of 1,4-anhydroerythritol (35) from erythritol (31) either by acid catalysis,[137] with pyridinium chloride,[138] or by conversion of 31 into the cyclic carbonate with dimethyl carbonate and subsequent intramolecular etherification.[136b] However, racemisation during the dehydration
65 Water stripping by carbon dioxide in the synthesis of glycerol carbonate reaction has not been reported before. To investigate the influence of carbon dioxide on the dehydration reaction, the reaction was conducted at different CO2 pressures.
Scheme 3-9: Reaction of erythritol (31) to threitol (33) and furans (34 and 35).
13 Figure 3-23 shows the C-NMR spectra of an erythritol/K2CO3 mixture stirred at 140 °C at different carbon dioxide pressures. In the absence of CO2, 1,4-anhydroerythritol (35) was obtained in 15 % yield. In a carbon dioxide atmosphere, erythritol was converted to 35 in 12 % yield and racemised to threitol (33) in 4 % yield. At a CO2 pressure of 80 bar, an equimolar mixture of erythritol and threitol was obtained and the corresponding dehydrated compounds were observed in 16 % and 6 % yield for 35 and 34 respectively. The yield of the 2,3-dihydroxytetrahydrofurans was increased to an overall yield of 30 % in almost half of the reaction time, if CO2 was continuously fed into the autoclave. These results reveal that continuous water removal by a carbon dioxide flow increases the yield of the dehydration products of erythritol and that CO2 could be employed to racemise alcohols in the presence of a Brønstedt base.
13 Figure 3-23: C-NMR spectra of erythritol/K2CO3 mixture with different CO2 amounts (101 MHz, 25 °C, DMSO-d6). Reaction conditions: flask or autoclave; erythritol (31); 5 mol% K2CO3; T = 140 °C; p = 0 bar, 1 bar or 80 bar; t = 26 h or 48 h. Signals assigned: ⚫ erythritol (31), threitol (33), X 1,4-anhydrothreitol (34), 1,4-anhydroerythritol (35).
66 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
To verify these findings, the experiments were repeated with threitol (33) as substrate (see Figure 3-24). Without carbon dioxide, approximately 10 % of 33 was converted into
1,4-anhydrothreitol (34). At a CO2 pressure of 80 bar, 39 % erythritol (31), 6 % 34 and 2 % 1,4-anhydroerythritol (35) were obtained. Also in this case, the alcohol is racemised in the presence of carbon dioxide.
13 Figure 3-24: C-NMR spectra of threitol (33)/K2CO3 reaction mixture with or without CO2 (101 MHz, 25 °C, DMSO-d6).
Due to the potential of facile racemisation of secondary alcohols in carbon dioxide by base catalysis, this combination was tested for an enzymatic dynamic kinetic resolution (DKR) of (rac)-1-phenylethanol (36) (see Scheme 3-10). Several groups reported the compatibility of
[139] enzymes and CO2 and achieved kinetic resolution in high stereoselectivities. Dynamic kinetic resolution with high yields and enantioselectivities was achieved with combinations of enzymes and metal catalysts,[140] zeolites,[141] or basic resins.[142] Preliminary tests for the DKR with Brønsted base and carbon dioxide showed that no racemisation of (S)-1-phenylethanol
(S-36) by K2CO3 in liquid or supercritical CO2 was observed and 1-phenylethanol acetate (37) was formed from 1-phenylethanol and vinyl acetate in presence of K2CO3. Therefore, this approach was not further investigated.
Scheme 3-10: Dynamic kinetic resolution of 1-phenylethanol (36).
67 Water stripping by carbon dioxide in the synthesis of glycerol carbonate
3.9 Conclusions
The direct synthesis of glycerol carbonate from glycerol and carbon dioxide was investigated while the water that is formed during the reaction was extracted by continuous CO2 flow. Based on the reaction conditions used for the acetonitrile batch experiments, a reaction temperature of 140 °C and a K2CO3 concentration of 5 mol% were used as starting conditions
-1 to study the influence of different reaction parameters. With a CO2 flow rate of 100 mLN min a glycerol carbonate yield of 5 % was obtained.
Temperatures above 140 °C resulted in brown viscous reaction mixtures without noteworthy glycerol carbonate yield. Regarding the catalyst, an increase of the concentration to 25 mol% decreased the yield and the use of Cs2CO3 did not change the reaction profile. With increasing
CO2 pressure, the yield of glycerol carbonate increased. At 120 bar, 7 % of glycerol carbonate was obtained after 70 h of reaction time. The CO2 flow rate had a minor influence up to the
-1 -1 value of 300 mLN min . However, with a flow rate of 500 mLN min a glycerol carbonate yield of 13 % was observed. In all experiments, the yield stagnated after a certain reaction time leading to the conclusion that a shifted reaction equilibrium is reached. Therefore, the extraction of glycerol carbonate from the glycerol phase was investigated giving MTBE and 4-methyl-2-pentanon as potential solvents for a downstream process.
Investigations on the reaction mechanism showed that glycerol hemi-carbonate is formed as intermediate under basic conditions. The reaction of deprotonated glycerol and CO2 occurs already at mild conditions, so it was concluded that the cyclisation reaction to glycerol carbonate is the rate-determining step.
The substrate scope of the process was tested. Low molecular weight glycols were too volatile for the reaction set-up applied in this thesis. However, with a recycling unit to increase the reaction time at high temperature and pressure a higher yield is expected. 1,2-octanediol was successfully converted to octane carbonate in 13 % isolated yield (20 % NMR yield). Phenol decomposed at reaction conditions. The sugar compound erythritol underwent a dehydration reaction to 1,4-anhydroerythritol and racemisation to threitol and 1,4-anhydrothreitol in presence of K2CO3 and carbon dioxide.
In conclusion, glycerol carbonate was synthesised in 13 % yield from glycerol and carbon dioxide by water stripping with a continuous CO2 flow. This is the highest glycerol carbonate
68 Water stripping by carbon dioxide in the synthesis of glycerol carbonate yield with a non-reactive dehydration system reported so far. The hemi-carbonate was identified as intermediate. With adjustments in the process set-up, this method is expected to be suitable for the synthesis of other cyclic carbonates.
69
4 Summary
Glycerol carbonate has a high potential as renewable compound due to its biogenic origin and diverse reactivity. Therefore, the direct synthesis of glycerol carbonate from glycerol and carbon dioxide with water removal was studied. Two approaches were investigated, the reactive dehydration by acetonitrile (Chapter 2) and the non-reactive water extraction with supercritical carbon dioxide (Chapter 3).
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 homogeneous catalyst. This is the highest glycerol carbonate yield reported for this reaction system until now and compares to other mainly heterogeneous catalytic systems. It was found that the glycerol carbonate yield reaches a maximum at different reaction times depending on the reaction conditions due to a complex reaction network leading to side and decomposition reactions. For example, at 175 °C a maximum glycerol carbonate yield of 11 % is obtained after 4 h when 3 eq. of acetonitrile are added, while 17 % glycerol carbonate yield is reached within 8 h to 16 h reaction time with 8 eq. of acetonitrile added.
For the non-reactive water removal in the synthesis of glycerol carbonate, a novel semi- continuous process was developed with CO2 as reagent and stripping gas. Glycerol carbonate was synthesised in 13 % yield from glycerol and carbon dioxide. This is the highest reported yield of a cyclic carbonate derived from a diol with a non-reactive dehydration system. Investigations on the reaction mechanism identified the glycerol hemi-carbonate as intermediate and the subsequent cyclisation to the cyclic carbonate as rate-determining step.
Comparison of both water removal strategies shows that higher yields can be achieved with the reactive dehydration system. However, depending on the costs of the substrate, the dehydration agent and the separation process to obtain the pure product the synthesis of cyclic carbonates by water removal through stripping might be a feasible alternative if efficient recycling of glycerol and CO2 is developed.
Overall, this thesis demonstrates that scCO2 stripping is a viable alternative for the synthesis of industrially interesting cyclic carbonates from renewable substrates.
5 Experimental section
5.1 General
5.1.1 Chemicals All chemicals were purchased from commercial sources and used as received, unless otherwise indicated. Pre-dried acetonitrile (Carl Roth) was stored over molecular sieves (3 Å) and under argon. Glycerol was dried at 120 °C under vacuum and stored over molecular sieves (3 Å).
K2CO3 was stored under Argon.
5.1.2 NMR spectroscopy The NMR analyses were carried out with either a Bruker AV-300 (1H: 300 MHz, 13C: 75 MHz) or Bruker AV-400 (1H: 400 MHz, 13C: 101 MHz) spectrometer at room temperature. The samples were diluted in a deuterated solvent (deuterated dimethyl sulfoxide, chloroform) and transferred into a 5 mm NMR-tube. The chemical shifts were given in ppm and the coupling constants in Hz. The internal references for 1H and 13C-NMR were the signals of the residual protons of the deuterated solvents.
5.1.3 Single component IR spectroscopy Single component IR spectra were recorded at room temperature using a Bruker-Alpha-FT-IR device equipped with a diamond-ATR apparatus.
5.1.4 In-situ IR spectroscopy In-situ IR spectra were measured and recorded using a fibre-optic mIR probe (Infrared Fibre Sensors, Germany), which is a custom-made gold coated Hastelloy shaft with a diamond ATR- prism as internal reflection element (IRE) resisting pressures of up to 200 bar and temperatures of up to 170 °C. The IR probe is connected to a Matrix-MF multiplex spectrometer (Bruker Optik GmbH, Germany). All IR spectra were recorded from 650 to 4000 cm-1 with 32 scans and a resolution of 4 cm−1 with the program Opus 6.5. Spectra under pressure were measured in a specially equipped 30 mL window autoclave constructed by the mechanical workshop at the Institut für Technische und Makromolekulare Chemie.
Experimental section
5.2 High pressure reactions
High pressure experiments with compressed gases are potentially dangerous and must be carried out with suitable equipment. Proper precautions have to be taken, including but not limited to the use of blast shields and pressure relief mechanisms, to minimise the risk of personal injury.
5.2.1 Apparatus specification The autoclaves used during this work were constructed out of stainless steel (steel grade
1.45.71, V4A) by the mechanical workshop at the Institut für Technische und Makromolekulare Chemie (ITMC). All accessories, like fittings, tubing, valves and pressure gauges were purchased from commercial sources. The norm-fittings of the companies Hoke and Hylok were used and are compatible with each other.
5.2.2 Finger autoclave The catalysis reactions were performed in 20 mL finger autoclaves, as shown in Figure 5-1. The autoclave is equipped with an analogue pressure gauge and a needle valve. The needle valve enables the evacuation and filling with gas. To avoid blind activity, the steel autoclaves were equipped with glass reaction inlets and magnetic stir bars. Pressures of up to 100 bar and temperatures of 200 °C can be applied. The autoclaves were heated in aluminium cones.
Figure 5-1: Engineering drawing of a 20 mL finger autoclave. 1) Reaction chamber, 2) inlet, 3) pressure gauge and 4) needle valve.
5.2.3 Window autoclave Some catalysis reactions were performed in 10 mL window autoclaves, as shown in Figure 5-2. The autoclave is equipped with two windows made from conic thick borosilicate glass
72 Experimental section embedded in a metal ring opposing each other, an electronic pressure gauge, thermocouple and a needle valve. The latter enables the evacuation and filling with gas. An additional inlet enables the charging of liquids and solids. The autoclave is heated from the bottom on a heating plate. The autoclave can handle pressures up to 400 bar and temperatures up to 150 °C.
Figure 5-2: Engineering drawing of a 10 mL window autoclave. 1) Reaction chamber, 2) pressure gauge, 3) inlet, 4) needle valve and 5) conical high pressure windows.
5.2.4 Window autoclave with ATR-mIR-probes For ATR-mIR-measurements under pressure, a 30 mL window autoclave with ATR-mIR-probes was used, as shown in Figure 5-3. The main body is equipped with two borosilicate glass windows opposing each other, an electronic pressure gauge, thermocouple, and two inlets, one for gases and the other for liquids and solids. An outlet at the bottom enables a simple discharging and cleaning of the reactor. Stirring is conducted via a shaft drive stirrer with agitator speed control (PREMEX). The autoclave is heated from the bottom on a heating plate. The autoclave with its accessories can handle pressures up to 200 bar and temperatures up to 150 °C. Two process compatible immersion probes for ATR-mIR spectroscopy are installed at two different positions in the reactor. One enters from the top and can monitor the upper half of the reactor content, while a second one is introduced horizontally at the bottom and can monitor the lower half of the reactor content. This autoclave was used in collaboration with the group of Prof. Liauw (ITMC, RWTH Aachen University).
73 Experimental section
Figure 5-3: Engineering drawing of the 30 mL window autoclave with IR-probes. 1) Reaction chamber, 2) ATR-mIR probe, 3) outlet, 4) mechanical stirrer, 5) conical high pressure windows and 6) inlet.
5.2.5 Bubble column reactor The reactions under continuous water stripping were performed in a bubble column reactor (70 mL volume) as shown in Figure 5-4. The reactor is equipped gas inlet and outlet and a sampling unit at the bottom. The reaction is heated by four separate electrical heating sleeves. Gas turbulences are achieved by a porous metal disc (average size 150 µm). Pressures of up to 100 bar and temperatures of 200 °C can be applied. The reaction set-up for water extraction experiments further includes a mass flow controller (MFC, Bronkhorst, Liquiflow L) linked with an electrically heated and pneumatically actuated needle valve (Sitec, Mikro 7110.3014) installed before the reactor. A second valve serves as back pressure regulator (BPR, Sitec, Micro 7110.3014) after the reactor. Two pressure gauges (Wika, ECO-1) were installed before and after the reactor to monitor the pressure. The set-up is controlled via LabView.
Figure 5-4: Engineering drawing of the bubble column reactor. 1) Gas inlet, 2) gas outlet, 3) heating unit, 4) sample valve, 5) rising pipe, 6) porous metal disc.
74 Experimental section
5.3 Phase behaviours
5.3.1 General procedure for qualitative solvent testing Glycerol or glycerol carbonate (~0.1 mL) were mixed with a solvent (1 mL). Phase behaviour was observed.
5.3.2 General procedure for quantitative solvent testing Glycerol (~0.15 g) and glycerol carbonate (~0.2 g) were mixed in a test tube. The desired solvent was added (1 mL) and the test tube closed with a septum. The mixture was stirred for 1 h at a temperature close to the boiling point of the solvent. After cooling down, the two phases were separated, and both analysed by NMR spectroscopy with propylene carbonate as standard and DMSO-d6 as solvent.
5.4 Synthesis of glycerol carbonate
5.4.1 General procedure for single batch reactions Under air, the desired amounts of glycerol, catalyst, additive and acetonitrile were filled into a glass reaction inlet equipped with a magnetic stirring bar and inserted into a 20 mL steel autoclave. The desired amount of CO2 was weighed in the autoclave at a pressure of ca. 50 bar and the autoclave was heated in an aluminium cone. After the reaction time, the autoclave was cooled down in an ice bath and vented. The reaction mixture was transferred into a flask using methanol to rinse the autoclave and homogenize the phases. Solvents were evaporated, and the residue was analysed by NMR spectroscopy with maleic acid as standard and DMSO-d6 as solvent. The crude product mixture was subjected to column chromatography (eluent: dichloromethane : acetone = 1:1, Rf = 0.47).
5.4.2 Repetitive batch catalysis
Glycerol (1.54 g, 16.7 mmol), K2CO3 (0.11 g, 0.84 mmol, 5 mol%) and acetonitrile (3 mL) were filled into a glass inlet equipped with a magnetic stirring bar and put into a 20 mL steel autoclave. CO2 was pressurised (3.2 g, equates to 80 bar at 175 °C). The autoclave was heated in an aluminium cone. After 3.5 h reaction time, the autoclave was cooled down in an ice bath and vented. The upper phase was removed by a pipette and analysed. Acetonitrile (2 mL) was added to the remaining mixture and CO2 was pressurised (3.0 g). The reaction procedure was repeated for another 3.5 h at 175 °C and 16 h at 155 °C. After the end of the reaction time, the
75 Experimental section phases were separated. The solvents were evaporated, and the mixtures analysed by NMR with maleic acid as standard and DMSO-d6 as solvent.
5.4.3 Catalysis with slow acetonitrile addition
Glycerol (1.4 g, 15 mmol), K2CO3 (52 mg, 0.38, 2.5 mol%) and acetonitrile (0.2 mL) were filled into a 20 mL steel autoclave equipped with a second valve to dose liquid and a magnetic stirring bar. The autoclave was pressurised with CO2 (3.0 g, 80 bar reaction pressure) and heated to 175 °C in an aluminium cone. Acetonitrile (4 µL min-1, total volume 1.9 mL) was continuously dosed to the reaction mixture by HPLC pump (Knauer Smartline). After a reaction time of 8 h, the acetonitrile flow was stopped, and the autoclave was cooled down in an ice bath and vented. The reaction mixture was transferred into a flask using methanol as rinsing solvent. Solvents were evaporated, and the residue was analysed by NMR spectroscopy with maleic acid as standard and DMSO-d6 as solvent.
5.4.4 General procedure for IR monitored catalysis
A solution of K2CO3 (5 mol%) in glycerol (~1.7 g or ~9 g respectively) was introduced into the autoclave. Acetonitrile (~8 mL or 10 mL respectively) was added. The autoclave was closed and pressurised at room temperature with ~30 bar of CO2 (~5-8 g). The reaction mixture was heated to 155 °C. Spectra were recorded every 10 minutes. After the reaction time, the heating was turned off and the autoclave cooled down. The reaction mixture was transferred into a flask with methanol. Solvents were evaporated, and the residue was analysed by NMR spectroscopy with maleic acid as standard and DMSO-d6 as solvent.
5.4.5 General procedure for reactions in the bubble column reactor
A solution of catalyst (K2CO3 or Cs2CO3 in desired concentration) and additive (where applicable) in glycerol or any other glycol (~ 20-30 g) was introduced into the bubble column reactor. The reactor was installed in a high pressure box and all connections were tightened carefully. The temperature, pressure and flow rate were controlled via a LabView program. The control panel is shown in Figure 5-5. A temperature of 50 °C was applied and CO2 was pressurised to ca. 40 bar. The pressure was monitored to detect leaking. The temperature was increased in 20 °C increments to the desired temperature and the pressure and flow rate were adjusted. During the reaction, samples were taken through the sample unit by carefully open the valves. The sample unit at the bottom of the reactor was purged two times before the sample was taken. After the reaction time, the heating was turned off and the pressure was released. Samples were analysed by NMR spectroscopy with DMSO-d6 as solvent.
76 Experimental section
Figure 5-5: Control panel of LabView program coded by Daniel Geier.
5.4.6 General procedure for water extraction experiments in a window autoclave
A solution of K2CO3 (desired concentration) and additive (where applicable) in glycerol (~ 2-3 g) or any other substrate was introduced into a window autoclave. The autoclave was either installed in a high pressure box or in a fume hood connected to a high pressure station. In both cases, all connections were tightened carefully. The autoclave was heated by a heating plate. In the high pressure box, pressure and flow rate were controlled by a LabView program (see 5.4.5). For experiments that were conducted in a fume hood, the pressure and flow rate were regulated manually and constantly checked by monitoring the pressure gauge and gas meter. During the reaction, samples were taken by stopping the CO2 flow and venting the autoclave carefully. After the reaction time, the heating was turned off and the pressure was released.
77 Experimental section
5.5 Synthesis
5.5.1 Polyethylene glycol enfolded KBr[131] Polyethylene glycol 400 (3.2 g, 8 mmol) was dissolved in water (30 mL). KBr (0.95 g, 8 mmol) was added. The mixture was stirred for 72 h. The water phase was extracted three times with dichloromethane (30 mL). The combined organic phases were evaporated, and the product obtained as highly viscous liquid. Yield = 1.05 g (2 mmol, 25 %)
5.5.2 Magnesium methoxide[143] Magnesium (1 g, 41 mmol) was stirred at room temperature for 1.5 h in methanol (dried and degassed, 30 mL). Afterwards, the mixture was refluxed for 4 h. The mixture was cooled to 0 °C and a white solid precipitated. Methanol was removed by filtration and the solid was washed with 10 mL methanol. The solid was dried in vacuo at 70 °C.
Yield = 0.81 g (9.7 mmol, 24 %)
5.5.3 Glycerol monoacetates
Glycerol (4 g, 43 mmol) and acetic acid (2.5 mL, 43 mmol, 1 equiv.) were charged into an autoclave (20 mL) and CO2 was added (30 bar). The autoclave was heated at 155 °C for 18 h and then cooled down in an ice bath and vented. The reaction mixture was neutralised by
NaHCO3 and concentrated by evaporation. The residue was washed with acetone and filtrated.
Column chromatography was performed (eluent: dichloromethane : acetone = 1:1, Rf = 0.71) to obtain a colourless liquid.
Yield = 2.2 g (16 mmol, 37 %; 88 % 1-glycerol monoacetate, 12 % 2-glycerol monoacetate)
78 Experimental section
1-Glycerol monoacetate
1 H-NMR (400 MHz, 25 °C, DMSO-d6): δ = 4.89 (d, 1H, OH), 4.64 (t, 1H, OH), 3.85 - 4.05 (m,
2H, CH2), 3.63 (m, 1H, CH), 3.34 (m, 2H, CH2), 2.01 (s, 3H, CH3) ppm.
13 C-NMR (101 MHz, 25 °C, DMSO-d6): δ = 170.5, 69.3, 65.8, 62.7, 20.8 ppm.
2-Glycerol monoacetate
1 H-NMR (400 MHz, 25 °C, DMSO-d6): δ = 4.67 – 4.75 (m, 3H, OH, CH), 3.40 – 3.52 (m, 4H,
CH2), 2.00 (s, 3H, CH3) ppm.
13 C-NMR (101 MHz, 25 °C, DMSO-d6): δ = 170.2, 75.7, 59.8, 21.11 ppm.
5.5.4 3-Acetamido-2-hydroxypropyl acetate[144]
3-Amino-1,2-propandiol (0.45 g, 4.9 mmol, 1 equiv.) was dissolved in pyridine (0.6 mL). Acetic anhydride (0.95 mL, 10 mmol, 2 equiv.) was added. The mixture was stirred at room temperature for 3 h. Volatile compounds were removed in vacuo at 75 °C. The residue was purified by column chromatography on silica (volume ratio ethyl acetate : methanol = 5:1,
Rf = 0.41) to obtain a slightly yellow liquid.
Yield = 0.604 g (3.5 mmol, 70 %)
1 H-NMR (400 MHz, 25 °C, DMSO-d6): δ = 7.87 (t, 1H, NH), 5.11 (d, 1H, OH), 3.82-3.95 (m,
2H), 3.68 (m, 1H), 2.99-3.15 (m, 2H), 2.01 (s, 3H, CH3), 1.80 (s, 3H, CH3) ppm.
13 C-NMR (101 MHz, 25 °C, DMSO-d6): δ = 170.4, 169.5, 67.3, 66.2, 41.9, 22.6, 20.7 ppm.
79 Experimental section
5.5.5 5-(Hydroxymethyl)oxazolidin-2-one[145]
3-Amino-1,2-propandiol (0.5 g, 5.6 mmol, 1 equiv.) and diethylcarbonate (0.67 g, 5.6 mmol, 1 equiv.) were added to a two-neck flask equipped with reflux condenser and stirred at 110 °C for 2.5 h. Bis(2-methoxyethyl) ether (7.5 mL) and magnesium methoxide (0.5 g, 5.8 mmol, 1 equiv.) were added and the mixture was heated to 150 °C overnight. The solid was filtered off and washed with methanol. The solvents were removed in vacuo at 120 °C and a yellowish residue was obtained. The residue was purified by column chromatography on silica (eluent: ethyl acetate : methanol = 5:1, Rf = 0.33) to obtain a colourless liquid.
Yield = 0.285 g (2.4 mmol, 43 %)
1 H-NMR (400 MHz, 25 °C, DMSO-d6): δ = 7.37 (s, 1H, NH), 5.08 (t, 1H, OH), 4.52 (m, 1H), 3.53, (m, 1H), 3.45 (m, 2H), 3.22 (m, 1H) ppm.
13 C-NMR (101 MHz, 25 °C, DMSO-d6): δ = 159.2, 76.2, 62.1, 41.4 ppm.
5.5.6 (2-Oxo-1,3-dioxolan-4-yl)methyl acetate
Glycerol carbonate (0.5 g, 4.2 mmol, 1 equiv.) was dissolved in pyridine (0.25 mL). Acetic anhydride (0.4 mL, 4.2 mmol, 1 equiv.) was added. The reaction was stirred at room temperature for 24 h. Volatile compounds were removed in vacuo at 75 °C. The residue was purified by column chromatography on silica (eluent: dichloromethane : acetone = 1:1,
Rf = 0.85) to obtain a colourless liquid.
Yield = 0.51 g (3.2 mmol, 76 %)
80 Experimental section
1 H-NMR (400 MHz, 25 °C, DMSO-d6): δ = 5.03 (m, 1H), 4.56 (t, 1H), 4.18-4.33 (m, 3H), 2.05 (s, 3H, CH3) ppm.
13 C-NMR (101 MHz, 25 °C, DMSO-d6): δ = 170.1, 154.7, 74.3, 66.0, 63.4, 20.5 ppm.
81
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Curriculum Vitae
Name: Karolin Schenk Date of birth: 20.01.1990 Place of birth: Hamburg (Germany)
Education Postgraduate studies 02/2014-05/2018 RWTH Aachen University (D) Doctoral studies under supervision of Prof Dr Walter Leitner: “Direct Synthesis of Glycerol Carbonate from Glycerol and Carbon Dioxide by Brønsted Base Catalysis”
Undergraduate studies 10/2008-11/2013 RWTH Aachen University (D) Chemistry studies; graduation: Master of Science
05/2013-11/2013 Master thesis at RWTH Aachen University under supervision of Prof Dr Walter Leitner: “Synthesis of chiral pyrene-tagged ligands for the application in asymmetric Hydrogenation”
01/2013-03/2013 IDEA-League exchange at Imperial College London (GB) under supervision of Prof Dr James Durrant
04/2011-06/2011 Bachelor thesis at RWTH Aachen University under supervision of Prof Dr Iris Oppel: “Use of porphyrins in supramolecular coordination chemistry”
Secondary education 07/2004-06/2008 Albert-Schweizer-Gymnasium, Hamburg (D), graduation: Allgemeine Hochschulreife
89
Eidesstattliche Versicherung
Ich versichere hiermit an Eides Statt, dass ich die vorliegende Doktorarbeit selbstständig und ohne unzulässige fremde Hilfe erbracht habe. Ich habe keine anderen als die angegebenen Quellen und Hilfsmittel benutzt. Ich erkläre, dass die schriftliche und die elektronische Form vollständig übereinstimmen. Die Arbeit hat in gleicher oder ähnlicher Form noch keiner Prüfungsbehörde vorgelegen.
______Ort, Datum Unterschrift
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