Umwandlung Von Biomasse in Plattformchemikalien Mit Homogenen Polyoxometallat-Katalysatoren

Umwandlung von Biomasse in Plattformchemikalien mit homogenen Polyoxometallat-Katalysatoren

Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg

zur Erlangung des Grades

DOKTOR-INGENIEUR

vorgelegt von

Chiraphat Kumpidet

aus Ubon Ratchathani, Thailand

Transformation of biomass to platform chemicals using homogeneous polyoxometalate catalysts

Faculty of Engineering of the Friedrich-Alexander-Universität Erlangen-Nürnberg in partial fulfillment of the requirements

for the degree of

DOKTOR-ENGINEERING

presented by

Chiraphat Kumpidet

from Ubon Ratchathani, Thailand

Als Dissertation genehmigt von der Technischen Fakultät der

Friedrich-Alexander-Universität Erlangen-Nürnberg.

Tag der mündlichen Prüfung: 21.10.2020

Vorsitzende des Promotionsorgans: Prof. Dr.-Ing. Andreas Paul Fröba

Gutachter: Prof. Dr. Peter Wasserscheid

Gutachter: Prof. Dr. Anders Riisager

To my family

“I learned that courage was not the absence of fear, but the triumph over it. The brave man is not he who does not feel afraid, but he who conquers that fear”

Nelson Mandela

“If you hear a voice within you say “you cannot paint,” then by all means paint and that voice will be silenced”

Vincent Van Gogh

“Strive not to be a success, but rather to be of value”

Albert Einstein

“Your time is limited, so don’t waste it living someone else’s life”

Steve Jobs

Acknowledgements

First, I would like to thank my doctoral supervisor Prof. Dr. Peter Wasserscheid very much for the challenging task and the professional and personal support of this work. I would like to thank Prof. Dr. Anders Riisager for accepting the second correspondent. Furthermore, my thanks go to the other members of the exam, Prof. Dr.-Ing. Hannsjörg Freund and Prof. Dr. Thomas Drewello. Furthermore, I would like to thank my longtime group leader Prof. Dr. –Ing. Jakob Albert, who always stood by my side with advice and support and always had an open ear for all my questions. In addition, I would like to express my sincere thanks to all who contributed to the success of this work. Special thanks go to Dr. Amalie Modvig and Prof. Anders Riisager for WD-POM catalysts supporting and to Dr. Peter Schulz for all kind help about analysis. For the financing and very good supporting of my PhD study in Germany from June 2016 to May 2020, I thank the OCSC, TISTR and especially OEA Berlin. I thank Michael Schmacks, Sascha Jeschke, Julian Karl, Achim Mannke, and Sebastian Hoffmann for their great patience and energetic support in all technical matters. I would like to thank Alex Busch and Hendryk Partsch for a quick remedy for electronic and computer-related problems. I would also like to thank our Secretariat with Mrs Menuet and Mrs Singer for the pleasant cooperation. Furthermore, I thank all colleagues at the CRT for the nice working atmosphere and the constructive discussions, especially, Dorothea Voß, Anna Bukowski, Lisa Wagner, Vera Haagen, Stefan Dürr, Julian Kadar, Raman Narayanan and Sebastian Oshin, for all kind of help and suggestions. My biggest thanks go to my sister for her great support and patience especially taking care everything instead of me in Thailand and thank you to my family for all supporting.

Vorwort

Die vorliegende Doktorarbeit wurde unter der Anleitung von Universitätsprofessor

Dr. Peter Wasserscheid am Lehrstuhl für Chemische Reaktionstechnik der Universität

Erlangen-Nürnberg von Juni 2016 bis November 2020 durchgeführt.

Publications

Parts of this work have already been published in the following journals or as conference papers:

Patent: • J. Albert, C. Kumpidet, „Hochselektive Erzeugung von Ameisensäure aus Biomasse in Methanol“, invention message from 27.01.2020 (Az.: EP20153906)

Journals: • S. Maerten, C. Kumpidet, D. Voß, A. Bukowski, P. Wasserscheid, J. Albert; „Glucose oxidation to formic acid and methyl formate in perfect selectivity”. Green Chemistry 2020, 22, 4311. • A. Modvig, C. Kumpidet, A. Riisager, J. Albert; „Ru-Doped Wells–Dawson Polyoxometalate as Efficient Catalyst for Glycerol Hydrogenolysis to Propanediols “. Materials 2019, 12, 2175.

Conferences: • C. Kumpidet, A. Modvig, A. Riisager, P. Wasserscheid, J. Albert; „Production of Propanediols from Glycerol through Hydrogenolysis using Wells-Dawson Polyoxometalates (WD POMs)”; the Pure and Applied Chemistry International Conference PACCON-2020, 2020, Bangkok-Thailand. • C. Kumpidet, A. Modvig, A. Riisager, P. Wasserscheid, J. Albert; „Application of Transition Metal-Substituted Wells-Dawson Polyoxometalates (POMs) for Propanediols Production from By-product glycerol of Biodiesel industry”; 4th International Conference on New Energy and Applications ICNEA-2019, 2019, Yokohama-Japan.

III

• C. Kumpidet, A. Modvig, A. Riisager, J. Albert; „Bio-based 1,2 Propanediol Production from Glycerol via Hydrogenolysis using Transition Metal-substituted Wells-Dawson Polyoxometalates (POMs)“; 11th Samaggi Academic Conference and Careers Fair and the 8th Thai Student Academic Conference SACC-TSAC-2019, 2019, London-the United Kingdom. • C. Kumpidet, A. Modvig, A. Riisager, P. Wasserscheid, J. Albert; „Selective Hydrogenation of Glycerol to Propanediols over Transition Metal-Substituted Wells-Dawson Polyoxometalates“; 51.Jahrestreffen Deutscher Katalytiker, 2018, Weimar-Deutschland. • C. Kumpidet, P. Wasserscheid, J. Albert; „ Screening and selection of various solvents for selective oxidation of glucose to formic acid“; International Congress Engineering of Advanced Materials ICEAM-2017, 2017, Erlangen-Deutschland.

Table of Contents

Acknowledgements ...... I

Vorwort ...... II

Publications ...... III

Table of Contents ...... V

List of abbreviations and symbols ...... IX

1 Introduction ...... 1 2 Theoretical and technical background ...... 3 2.1 Biomass ...... 3

2.2 Suitable biogenic substrates for technical relevant processes ...... 7

2.2.1 Glucose ...... 7

2.2.1.1 Conventional manufacturing processes...... 8

2.2.1.2 Valuable products from Glucose ...... 8

2.2.1.3 Glucose oxidation (OxFA process) ...... 16

2.2.2 Glycerol ...... 18

2.2.2.1 Conventional manufacturing processes (as a by-product from biodiesel production) ...... 18

2.2.2.2 Valuable products from glycerol ...... 19

2.3 Polyoxometalates as catalysts ...... 27

2.3.1 Molecular Structure ...... 27 2.3.2 Classification ...... 28 2.4 Use of heteropolyacids in biomass oxidation ...... 29

2.5 Use of heteropolyacids in biomass hydrogenolysis ...... 31

3 Objective of this work ...... 33 4 Experimental section ...... 35 4.1 Materials and chemicals ...... 35

4.1.1 Biogenic materials as substrate ...... 35 4.1.2 Solvents and Gases ...... 35 4.1.3 Catalysts ...... 36 4.2 Catalyst Preparation ...... 36

4.2.1 Keggin-type POM (HPA-5) for glucose oxidation ...... 36 4.2.2 Wells-Dawson-type POMs for glycerol hydrogenolysis ...... 37 4.3 Experimental setup ...... 38

4.3.1 Experimental design of the two high-pressure autoclaves ...... 38

4.3.1.1 Hastelloy 10-fold reactor system for oxidation ...... 38

4.3.1.2 Stainless steel 10-fold reactor system for hydrogenation ...... 40

4.3.2 Principle of the batch experiments ...... 41

4.3.2.1 Procedure of glucose oxidation in organic solvents and post-reaction sampling …………………………………………………………..……………………………………………….41

4.3.2.2 Procedure of glycerol hydrogenolysis and post-reaction sampling ...... 42

4.4 Analytical Procedures for characterization ...... 42

4.4.1 pH measurement ...... 43 4.4.2 Karl Fischer (KF) Titration ...... 43 4.4.3 Fourier-Transform Infrared Spectroscopy (FT-IR) ...... 43 4.4.4 Inductively Coupled Plasma (ICP) ...... 43 4.4.5 Thermogravimetry (TG) ...... 43 4.4.6 X-ray diffractometry (XRD) ...... 44 4.4.7 Nuclear Magnetic Resonance Spectroscopy (NMR) ...... 44 4.4.8 Gas Chromatography (GC) ...... 45 4.4.9 Gas Chromatography/ Mass spectrometry (GC/MS) ...... 45 4.4.10 High-performance liquid chromatography (HPLC) ...... 46 4.5 Calculations...... 46

5 Results and discussions ...... 48 5.1 Influence of organic solvents on selective glucose oxidation ...... 48

5.1.1 Solvents Screening ...... 48 5.1.2 Oxidation of glucose in promising solvents ...... 52 5.1.3 Confirmation of the source of FA generation ...... 54 5.1.4 Investigation of MeOH oxidation ...... 56 VI

5.1.5 Anaerobic conversion of glucose in MeOH ...... 60 5.1.6 Mechanistic investigations of glucose oxidation in methanol ...... 64 5.1.7 Optimizing the reaction conditions for glucose oxidation in methanol ...... 69

5.1.7.1 Effect of reaction temperature...... 70

5.1.7.2 Effect of catalyst amount ...... 72

5.1.7.3 Effect of glucose concentration ...... 73

5.1.7.4 Effect of oxygen pressure ...... 74

5.1.7.5 Effect of stirring speed ...... 75

5.1.7.6 Effect of various substrates ...... 76

5.1.8 Recycling of the catalyst system ...... 79 5.2 Hydrogenation of glycerol using POM catalysts in aqueous phase ...... 81

5.2.1 Screening of WD-POM catalysts ...... 81 5.2.2 Comparison of metal-doped WD catalysts with benchmark systems ...... 82 5.2.3 Mechanistic investigation of glycerol hydrogenation ...... 85 5.2.4 Optimizing reaction conditions using Ru-WD POM ...... 87

5.2.4.1 Effect of glycerol (GL) concentration ...... 87

5.2.4.2 Effect of reaction time ...... 88

5.2.4.3 Effect of reaction temperature...... 89

5.2.4.4 Effect of catalyst amount ...... 90

5.2.4.5 Effect of hydrogen pressure ...... 91

5.2.4.6 Effect of stirring speed ...... 92

5.2.5 Catalyst recycling ...... 93 6 Summary and outlook ...... 95 7 Zusammenfassung...... 97 8 References ...... 100 9 Appendix ...... 117 9.1.1 List of figures ...... 117 9.1.2 List of tables ...... 122 9.1.3 List of schemes ...... 122

VII

9.1.4 Overview of chemicals substances and suppliers ...... 123 9.1.5 Calibration factors for quantifying the substrates and products from glucose oxidation ...... 125 9.1.6 Calibration factors for quantifying the products from GL hydrogenation ...... 126 9.1.7 Calibration data of gas chromatography ...... 126 9.1.8 Characterization of synthesized catalyst ...... 127 9.1.9 Illustration of liquid phase products ...... 129 9.1.10 Headspace GC-MS analysis of liquid products from 13C-label oxidation in MeOH solvent ...... 132

VIII

List of abbreviations and symbols

ρ density BASF SE Badische Anilin- & Soda-Fabrik Societas Europaea CRT Institute for Chemical Reaction Engineering (Germ. Lehrstuhl für Chemische Reaktionstechnik) °C degree Celsius DME dimethyl ether DMM dimethoxymethane DP degree of polymerization e.g. for example et al. et alii EtOAc ethyl acetate EtOH ethanol FA formic acid FID flame ionization detector FM formaldehyde g gram GC gas chromatography GL glycerol h hour(s) HPA heteropolyanion HPA-n heteropolyanion substitudes by n vanadium atoms

HPA-nox heteropolyanion substitudes by n vanadium atoms in its oxidized form

HPA-nred heteropolyanion substitudes by n vanadium atoms in its reduced form HPLC High-Performance Liquid Chromatography ICP-OES Inductively coupled plasma optical emission spectra IL ionic liquid IPA isopolyanion i-PrOH i-Propanol IUPAC International Union of Pure and Applied Chemistry K calibration factor kg kilogram L liter

m mass M molar mass MeOH methanol MF methylformate MHz megahertz min minute (s) mL mili liters mmol mili moles mol% mole percent MS mass spectrometry n degree of substitution NMR nuclear magnetic resonance spectroscopy OxFA Oxidative conversion of biomass to formic acid P&ID Piping and instrumentation diagram PD propanediol POM polyoxometalate ppm parts per million PTFE polytetrafluoroethylene r reaction rate rpm revolutions per minute RT room temperature s second (s) S selectivity T temperature TCD thermal conductivity detector THF Tetrahydrofuran Vol volume wt% weight percent XRD x-ray diffraction Y yield

1 Introduction

The dwindling of fossil resources and the effort to reduce the severity of global warming from greenhouse gas emissions have led to a change of the feedstock base towards a biorefinery approach. These issues have stimulated research into renewable resources such as biomass to replace petroleum derivatives. Biomass can provide renewable carbon sources and also has a capability for reducing fossil CO2 emissions. Biomass typically comprises organic materials such as grass, wood, algae, crops, agricultural residues, and wastes, as well as animal manure.[1] All of the biomass materials originally result from photosynthesis in a growing process under a condition of available atmospheric CO2, water, and sunlight. The CO2 released during the producing and using of bio-chemicals will be trapped by photosynthesis resulting in net-zero carbon emission (Figure 1).[2,3] Particularly, lignocellulosic biomass has the potential of becoming a promising alternative feedstock for the sustainable production of chemicals, as it is non-food-based and represents the most abundant biomass resource. Typically, lignocellulosic biomass consists of 40–50% cellulose, 25–30% hemicellulose, and 15–25% lignin.[3–5] Cellulose is the major component of lignocellulosic biomass. The organic carbon in the biosphere exists almost half in cellulose form. Consequently, the transformative technologies of cellulose to renewable bio-based chemicals have become a promising and highly attractive research target. Due to the complexity of the chemical composition of lignocellulosic biomass, technology development for its conversion to value-added chemicals remains a challenge, in particular to achieve target chemical products in high yield. A combination of chemical and biochemical processes, either performed in one-pot processes or separately, may be required. The critical reactions involved in biomass conversion are dehydration, hydrolysis, isomerization, reforming, aldol condensation, hydrogenation, hydrogenolysis, and oxidation.

Figure 1. The cycles of CO2 for petroleum- and biomass-derived chemicals (modified from Alonso et al.).[3] 1

Moreover, crude glycerol, which is the by-product of biodiesel production, is a promising feedstock for the bio-based economy of the future. The biodiesel industry generates a large amount of crude glycerol (GL) as the main by-product, corresponding to ca. 10 wt.% of the biodiesel production.[6] The global production of biodiesel reached 32.6 million liters in 2016 and is expected to increase in the coming years, with inevitable bio-GL generation.[7] The global market trend of glycerol is likely to become saturated owing to its defined application in many fields.[8] Therefore, it is urgently necessary to discover a new route of glycerol utilization for the sustainability of biodiesel manufacturing. Several catalytic technologies have been used for the transformation of GL to platform chemicals, such as steam reforming, dehydration, oxidation, esterification, etherification, carboxylation, acetylation, and chlorination. [9–11] Among GL valorization, propanediols (PDs) production via hydrogenolysis of GL is an interesting approach due to the extensive application of PDs in several different industries. [12,13]

Multifunctional catalysts have numerously been applied for improving biomass conversion. One interesting catalytic system is the employment of homogeneous polyoxometalates. Application of homogeneous polyoxometalates is an interesting catalytic system because of its structural architectures. Polyoxometalates (POMs) have excellent physicochemical properties, such as high solubility in various solvents, and high resistance vs. chemical and thermal disruption. They show the features of strong Brønsted acidity and high proton mobility as well as fast multi- electron transfer. [14–17] A promising technology using POMs is the oxidative conversion of biomass to formic acid (FA), using homogeneous polyoxometalate catalysts (POMs) developed by Albert’s group in the recent decade.[18–20] This method is called the OxFA-process. The main product, FA, is a simple chemical and an important raw material that is broadly used in the chemical, pharmaceutical, agricultural, textile, leather, and rubber industry.[21,22] The OxFA-process can provide high selectivity to FA in liquid phase, with full glucose conversion. The major by-product is carbon dioxide (CO2). In addition, application of POMs for bio-GL transformation is also attractive for PD production, especially 1,2-propanediol (1,2-PD). Metal-based catalysts, such as Pt, Ru, Ir, Rh, Pd, Ni, and Cu, have been typically been employed for hydrogenolysis of glycerol to generate 1,2-PD.[23–31] For noble metal and Ni-based catalysts it has also been reported that they have a considerable catalytic activity, resulting in further degradation of products to smaller-carbon chemical products, such as ethylene glycol, ethanol, methanol, and methane.[32]

The present work aimed to investigate and develop the application of POM-catalyst systems for two different reactions in a batch-type reactor. The first aim was to study the effect of various solvents on the selective oxidation of glucose as a model substrate for lignocellulosic biomass. The second aim was to investigate POM catalysts for hydrogenolysis of glycerol as a biomass derivative. The performance of the catalyst system should be optimized by the introduction of noble-metals such as Ru, Pt, and Pd into the POM molecular structure.

2 Theoretical and technical background

This chapter presents the principles of biomass, and its derivatives, including relevant processes and typical products in order to provide the basis for understanding the important theoretical fundamentals of this work. For this purpose, biogenic-material composition is exhibited. Subsequently, suitable biogenic substrates for different conversion methods are discussed. A promising possibility of creating chemical value from biogenic raw materials is the production of formic acid (FA) and methyl formate (MF) as a platform chemical and primary raw material for the chemical industry, including propanediols (PDs) which are also target products. The chemical and physical properties, as well as the conventional manufacturing methods, are also described. State of the art processes for the production from fossil raw material and novel concepts for the production from biomass by using polyoxometalate catalysts (POMs) are summarized and compared.

2.1 Biomass

The term "biomass" means all substances of organic origin containing carbon. These include in nature living phytomass and zoomass (plants and animals), residues, dead but not yet fossilized material. Phytomass and zoomass have been formed through a transformation process. It also includes plants and animals and their products, as well as organic household waste fractions, paper, and pulp, slaughterhouse waste, or vegetable oils. The boundary of fossil fuels begins with the peat, the secondary fossil product of rotting.[33] Furthermore, a subdivision of biomass can be done into primary and secondary outcomes. The direct photosynthetic utilization of solar energy produces the primary products, which include the entire plant mass as well as their products and residues. All outcome of zoomass, their excreta, and sewage sludge, count as secondary products, as they derive their energy only indirectly from the sun through the decomposition or conversion of organic substances in higher organisms.

Composition of biomass

As previously indicated, biomass has the potential to serve as a sustainable energy source and organic carbon source for our modern industrial society. For technical implementation, it is appropriate to start with the characterization of the individual components of the biomass. In general, biomass can be divided into three categories (Figure 2.) [34] :

Figure 2. Chemical Structure of Biomass Feedstocks (reproduced from Alonso et al.). [3]

A. Carbohydrates

Carbohydrates, which compose of sugar molecules and are synthesized in the primary metabolism of plants, are a large section of every year growing biomass. The carbohydrates sugar and starch consist of α-1,4 glucosidal linked monomer units and correspond to polysaccharides of variable chain length.[3] These sugars can be different depending on the carbon skeleton types. The pentose is a monosaccharide with five carbon atoms (ribose, arabinose, xylose, and lyxose). Hexoses with six carbon atoms can turn into the aldohexoses (glucose, allose, altrose, mannose, gulose, idose, galactose, and talose) as well as the ketohexoses (psicose, fructose, sorbose, and tagatose).[35] If the respective sugar is present as a monomer, it is called a monosaccharide. Disaccharides consist of two monosaccharides, and oligosaccharides comprise three to ten identical or different monosaccharides. Their types depending on the number of monosaccharide units present, such as di-, tri-, tetra-, pentasaccharides, etc., may be linked both linear (unbranched) and branched. Compounds of many monosaccharides are called polysaccharides.[36] Starch consists of 70 to 90% water-insoluble amylopectin and 10 to 30% of water-soluble amylose.[37]

Mostly independent of their chain length, carbohydrates can easily be decomposed by hydrolysis into subunits, which are outstandingly suitable for the production of 4

biogenic fuels. As raw material suppliers for the processing of carbohydrates, mainly sugar beet is cultivated in Central Europe, and sugar can is grown in tropical climatic regions for sugar production.[38] The most important sugar with an annual production volume of about 130 million tons worldwide is sucrose (consisting of the basic building blocks glucose and fructose), which is obtained from beets and sugar cane. In terms of energy, sugars represent a cost-effective, readily available, and above all renewable source of carbon with low storage and transport problems.[39] For the production of starch, mostly wood, maize, potatoes, and wheat are used. Starch-based products are traditionally used in large quantities for paper and corrugated board production, as well as in the textile industry and industrial biotechnology.[37] Despite their accessibility and low procurement prices, renewable carbohydrate-based feedstocks have so far been little used in the industry. The occurring problems are complex. Since there are usually several alcohol groups in the sugar molecule which have the same or similar reactivity, this over functionalization brings problems in selective conversion. The high hydrophilicity due to the high number of hydroxyl groups is another feature of carbohydrates. The resulting low solubility in classical organic solvents, which are nowadays used primarily in the chemical industry, hinders the use of sugars as a renewable raw material. The introduction of carbohydrates into industrial production, therefore, requires the development of cost-effective and selective production processes in the aqueous phase.[40]

B. Triglycerides

This group of lipids which can be represented as esters of the trihydroxy alcohol glycerol and higher monocarboxylic acids with more than eight carbon atoms in the molecule are referred to as fats or fatty oils.[2] For the most part, natural fats consist of triglycerides with three long-chain fatty acids, mostly comprising unbranched chains of 4 to 26, typically 12 to 22, carbon atoms.[41] If they are liquid at room temperature, they are also referred to as oils or, to distinguish them from mineral oils, fatty oils. Pure triglycerides of fatty acids are also known as neutral fats. Vegetable oils come from oil plants in which storage and reserve materials are stored mainly as oils in fruits and seeds.[38] In Germany, rape, and sunflower are the preferred crops. In a cold or warm pressing of the seeds, vegetable oil can be produced. The extraction meal is produced as by-products of oil-pressing and extraction. The vegetable oil, in turn, can be used as a starting material for paints and coatings, cosmetics, soaps, and as a biodegradable lubricant.[42] In order to use vegetable oil directly in diesel engines, it has to be broken up into glycerol and the monocarboxylic acid esters by means of transesterification with methanol. These vegetable oil methyl esters (PMEs) are also referred to as rapeseed oil and are used as a first-generation biogenic fuel as a biodiesel or diesel additive. [43]

C. Lignocellulose

When looking at possible natural raw materials, it becomes clear that lignocellulose represents a promising source of carbon for the production of liquid fuels and can be used as a raw material in the chemical industry.[44] With 170 billion tons of plant material produced annually worldwide, the proportion of lignocellulose is approximately 95%. The remainder is spread over fats, proteins, and minerals.[45] 5

Lignocellulose is the main constituent of plant material. It composes cellulose (40-50%), hemicellulose (25-30%) and lignin (15-25%), as shown in Figure 3.[46] The figure shows also structural sections of the three main components of lignocellulosic biomass.

Figure 3. Lignocellulose composition: cellulose, hemicellulose and lignin (reproduced from Alonso et al.). [3]

With a share of approx. 40%, cellulose is the main constituent of the entire biomass, making it the most common polysaccharide. Its molecular structure consists of β-1,4-glycosidically linked D-glucose molecules with β-cellobiose as the smallest repeating unit. The individual pyranose rings along the cellulose chain (so-called anhydroglucose units) are alternately rotated by 180° to each other and lead to a linear, unbranched molecular chain.[47] The coupling of the monomers takes place by a condensation reaction, in which two hydroxy groups form a water molecule, and the remaining oxygen atom connects the pyran ring of the two monomers.[48] In addition to this strong covalent bond, hydrogen bonds are formed between adjacent anhydroglucose units of the same molecule and lead to a rigid structure.[49,50] The linkage of single molecular strands is responsible for the highly ordered fibrillar structure of cellulose and determines important properties such as crystallinity and the associated low solubility in most polar solvents.[51] The average degree of polymerization (DP) gives information about the size of the whole molecule. It varies depending on the origin of the material between 1300-1700 for pulpwood and 800-10,000 for cotton or related plant fibers. In native celluloses, which occur as a builder in plant cell walls, even up to 15,000 monomer units can be achieved.[52] Cellulose and its derivatives are used as technical and textile pulp, as auxiliaries and materials in adhesives, emulsifiers, and as flow agents or thickeners in food and pharmaceutical products.[47]

Hemicellulose is an amorphous biopolymer. It mainly composes five different sugar monomers such as xylose, arabinose, galactose, glucose, and mannose. The general sugar monomer within the carbon skeleton dominates the classification. When xylose is contained as the major repeating unit, it is often referred to as xylan in the context of 6

hemicellulose.[53] The fibers of cellulose interwove via hydrogen bonds. Moreover, the hemicellulose is partially covalently bound to lignin via its side chains. Due to this cross-linking, it is assigned a crucial function in the structural assembly of plant cell walls.[54] The linkage of the main chain, which always consists only of β-D-xylopyranose units, is possibly either in β-1,3 or β-1,4-glycosidic form, the former occurring only in algae and seaweed. In addition, one or two branches per molecule can occur in deciduous and conifers. The solubility properties of hemicellulose and thus the availability of functional groups are influenced not only by the frequency and composition of the side chains but also by their substituents.[55] There are already some technically realized approaches to the use of hemicellulose for the production of purely petrochemical hydrocarbons. Examples include the recovery of short-chain alcohols (ethanol, propanols, butanols) via hydrolysis and fermentation followed by dehydration to olefins[56] and the acid-catalyzed hydrolysis of pentoses and hexoses to furfural and hydroxymethylfurfural (HMF).[57,58] One advantage of using hemicellulose compared to cellulose is the easy accessibility of the functional groups in the molecule due to its amorphous and branched structure.

In contrast to the other two building blocks of lignocellulosic biomass, lignin is a phenolic macromolecule whose monomeric units are linked by ether bridges composed of sinapyl alcohol, coniferyl alcohol, and paracoumaryl alcohol as the basic building blocks. These constituents give wood its strength, which is also referred to as lignification. The branched polycondensate of phenylpropane units is insoluble in water due to its low content of polar groups, similar to cellulose and hemicellulose.[59] The composition of lignin varies depending on the species of wood in the monomeric units. The softwood lignin, for example, pine or spruce, consists mainly of coniferyl units and contains only a few sinapyl and cumaryl units.[60] In contrast, deciduous lignin from beech or poplar comprises predominantly of sinapyl and very few coniferyl units.[61] So far, the lignin fraction from lignocellulose-containing raw materials has been used almost exclusively for the production of process heat.[62] In addition, lignin can be converted into a whole range of different aromatic compounds. Depolymerisation of lignin with subsequent hydrogenation can be perform to produce propyl aromatics and further on biofuels. It also leads to oxidative depolymerization to phenols or aromatic aldehydes.[63,64] These substances can then be used as flavors, agrochemicals, or basic chemicals for synthetic chemistry.[65]

2.2 Suitable biogenic substrates for technical relevant processes 2.2.1 Glucose

Figure 4. Chemical structure of Glucose

Glucose (Figure 4.) is a simple sugar which is the most common and abundant monosaccharide in nature. [66] It is found in several fruits, such as apples and berries, as well as vegetables, for example, onions and other parts of plants in its free state. Moreover, one of the main components of honey is unbonded glucose. The natural synthesis of glucose occurs through photosynthesis in plants and transforms solar energy with water and CO2. Glucose is the most important carbohydrate in biology due to its use by the cell as a source of energy and metabolic intermediate. Excess glucose is stored in plants in the form of starch and amylopectin, and, in animals as glycogen.[67] In energy metabolism, glucose is a crucial source of energy for all organisms. The energy is released from glucose and used to generate adenosine triphosphate (ATP) during cellular respiration.[68]

Glucose is also an important building block of the disaccharides, oligosaccharides, and polysaccharides. Disaccharides such as lactose and sucrose (cane or beet sugar) are formed from glucose with galactose and fructose via the glycosidic bond of β(1→4) and α(1→2)β, respectively. Oligosaccharides, for instance, raffinose, are a trisaccharide comprising of glucose, galactose, and fructose. Oligosaccharides exist in beans, broccoli, asparagus, and whole grains, including other vegetables. Polysaccharides can be found as starch and amylopectin, glycogen or cellulose.

2.2.1.1 Conventional manufacturing processes

The production of glucose syrup (with more than 90% glucose in dry matter) mainly carried out by enzymatic hydrolysis.[69] Annual worldwide production of glucose syrup is expected to reach 30 Mtons by the end of 2022.[70] The enzyme amylases which is used for starch hydrolysis, generally originate from Bacillus licheniformis or Bacillus subtilis (strain MN-385), which show higher heat-tolerance than the formerly used enzymes. [71] Beginning in 1982, pullulanase from Aspergillus niger was used in the production of glucose syrup, whereby the yield of glucose could be increased.[72] The operating reaction is performed at a temperature of 55-60 °C and a pH of 4.6-5.2.[73] Corn syrup in the dry matter obtains glucose between 20% and 95%. [74] Mizuame, the Japanese sweetener of the glucose syrup, is commonly made by converting sweet potato or glutinous rice starch.[75]

Maize has been the leading source for conversion to sweeteners.[76] The world production of maize starch is around 70% converted to glucose-containing sweetener. In the United States, almost 100% are glucose-containing sweeteners. The production of glucose-based sweeteners in Japan is also 84% from maize. Although, other starch- bearing grains and roots can serve as the raw material to produce glucose-based syrups, such as rice, wheat, potato, cassava, barley,[69] sweet potato,[75] corn husk and sago.

2.2.1.2 Valuable products from Glucose

In recent years, conversion of glucose into various value-added chemicals and fuels (Figure 5), such as furans (e.g., 5-hydroxymethylfurfural—HMF and furfural),[77–79] sugars and polyols (e.g., sorbitol, glycerol, ethylene glycol, propylene glycol),[80,81] as 8

well as organic acids (e.g., levulinic, lactic, gluconic, acetic, formic, succinic acid) [82–85] has been reported widely. These chemicals can have a range of applications in existing markets.[86]

Figure 5. Platform chemicals and fuels obtained from glucose (reproduced from [87]).

Conversion of glucose into short-chain organic acids

Greener and more effective pathways for glucose and cellulose transformation into value-added chemicals in water have been investigated extensively, as detailed in Figure 6. Sugar alcohols such as sorbitol and mannitol can be produced via hydrogenation of glucose and cellulose. Besides, hydrogenation of glucose can generate diols, especially ethylene glycol and propylene glycol, which are important bulk chemicals. A lot of research has been presented for 5-Hydroxymethylfurfural (HMF) production from glucose and cellulose. Lactic acid, which is a commodity chemical used widely in pharmaceutical and food industries, can be formed through the conversion of glucose and cellulose. Glucose can be converted to levulinic acid that can be used for the preparation of polymers, resins, flavor substances, and high-value organic chemicals. This work focuses on products from glucose oxidation, such as gluconic acid, acetic acid, and formic acid, as details below.

Figure 6. Conversion of cellulose and glucose in water (reproduced from Song et al.). [88] 9

Gluconic acid

Gluconic acid is an essential chemical intermediate and is widely used in pharmaceutical, food, and petrochemical industries such as food additive, raw materials for medicines, biodegradable polymers, an ingredient for concrete, and cosmetic preparations.[89] In the present chemical industry, gluconic acid is generally produced by the enzymatic oxidation of glucose. For a direct transformation of cellulose or cellobiose into gluconic acid, which has been investigated in a limited number of studies so far, a consecutive reaction of hydrolysis and oxidation has been suggested. The design of a bifunctional catalyst was expected to play an important role in both hydrolysis and oxidation abilities.

Several studies have presented the production of gluconic acid by oxidation of glucose under the atmosphere of air or molecular oxygen with heterogeneous catalysts, i.e. supported noble metals, especially Au, Pt, and Pd.[90–92] The production of gluconic acid via catalytic oxidation of glucose can be accomplished in excellent selectivity (>99%). The selective oxidation of glucose to gluconic acid by using a Pd/C catalyst in water could achieve a high yield of 99.3% gluconic acid at 40 °C using air as an oxidant agent.[93] Bi was found to improve the stability, activity, and selectivity of the catalyst. However, the separation process for the Bi-base catalyst has to be considered thoroughly in particular for application in food, pharmaceutical, and cosmetic industries. In recent years, there was a strong research focus on using Au as a catalyst for aerobic oxidation of glucose under benign conditions.[94–96] The feature of the support and the particle size of Au played a crucial impact on the catalytic stability, activity, and selectivity. As reported by Biella et al.,[97] the gold colloids immobilized on carbon provided an excellent activity for glucose oxidation. To improve the stability of the catalysts, TiO2 was used as support of 0.45% Au[98] and 0.25% Au/Al2O3[99] under the atmosphere of oxygen at 40-60 °C and pH 9 in the aqueous system. The complete conversion of glucose and entire selectivity to gluconic acid was achieved over this catalyst in less than 4 h. The Au/Al2O3 catalyst tremendously enhanced activity with doping of basic metal oxides such as sodium oxide and calcium oxide.[91] Synthesized mesoporous carbon-confined Au catalysts with a positive fractional charge (Auδ+) displayed high activity with a TOF of 4.308 molglucose molAu−1 s−1 for selective glucose transformation into gluconic acid at 40 °C and pH 9. As a result of the incorporation of 3.3 nm Au nanoparticle in 5.4 nm mesopore channels, the contact between Au nanoparticles and glucose molecules could be promoted. Active oxygen species that appeared on the catalyst surface, supported the catalytic activity for glucose oxidation.[100] Owing to a low-efficiency of glucose oxidation to gluconic acid in the absence of alkali, bimetallic catalysts were applied to enrich the performance. Mono- and bimetallic catalysts (Au, Pt, Pd, and Rh) supported on carbon were applied for the aerobic oxidation of glucose. Bimetallic particles of Au–Pd affected a synergistic activity increase by a factor of 5.[101]

Acetic acid

Acetic acid is an organic acid which can be found naturally in many animals and plants. Its content in vinegar is around 4-12%. As known for more than 5000 years, acetic acid is produced via the process of wine fermentation.[102] In the modern industry, acetic 10

acid is produced from fossil feedstocks and can also be generated from biomass, and certain endeavors have been made to generate acetic acid by oxidation in super- and sub-critical water. Presently, the carbonylation of methanol is the primary process to produce acetic acid, which is an essential commodity chemical. China is the major producer accounting for 54% of world capacity, followed by the United States with 18%. There is no other country that produces more than 5%. The reason that China is overwhelming the production of acetic acid is linked to an abundance of methanol generation from coal, which is the crucial raw material for acetic acid production.[103] The major end-uses of acetic acid are the manufacturing of vinyl acetate, purified terephthalic acid (PTA), ethyl acetate, and acetic anhydride. Vinyl acetate monomer (VAM) is the largest end-use in polymer manufacturing for application in paper coatings, paints, textile treatment, and adhesives. PTA is used mostly for the production of polyethylene terephthalate (PET) solid-state (packaging) resins, films, and fibers. Acetic anhydride is used in the manufacture of cellulose flake production, cellulose plastics, and cigarette filter tow. Acetate esters are mainly used as solvents for paints, coatings, and inks. Other applications for acetic acids use produce monochloroacetic acid, butyl acetates, and ethanol.[102,103]

The most important synthetic ways to acetic acid are methanol carbonylation and liquid-phase oxidation of naphtha, butane, or acetaldehyde. Methanol carbonylation has been performed for the past 25 years[104] and probably remains the preferred pathway for industrial-scale production. Numerous new technologies for producing acetic acid have been investigated. The hydrothermal treatment of organic wastes with or without H2O2 by Quitain et al.[105] could produce acetic acid from 29 mg g−1 of glucose in the presence of H2O2 at 350 °C and 165 bar. The investigation of the hydrolysis and oxidation of several wastes to form organic acids in sub- and supercritical water revealed that under alkaline conditions the formation of acetic acid is enhanced. Such studies obtained 77% glucose conversion and 17% acetic acid yield in the presence of NaOH (55.6 wt% glucose) and 25% stoichiometric oxygen at 250 °C and 276 bar.[106] Jin et al. used the advantage of the acid-base catalysis of near-critical water for cellulose conversion to acetic acid in a two-step process (Figure 7). In the first step 2-hydroxymethyl furaldehyde (HMF) and lactic acid were generated in the absence of oxygen. Afterwards, HMF and lactic acid are oxidized to form acetic acid.

Figure 7. Mechanism of two-step process for the conversion of hexoses (mainly glucose) to acetic acid (reproduced from Jin et al.). [107] 11

Formic acid

Formic acid (FA) (Figure 8) is a colorless, pungent, corrosive liquid. Due to its polarity, it is completely miscible with water and other polar solvents such as ethanol and glycol. In addition, it is the strongest unsubstituted carboxylic acid with 3.74 pks.[108] Pure FA is highly hygroscopic. Their melting and boiling points are significantly higher than those of organic compounds with similar molar masses since melting and boiling also require hydrogen bonds between the individual molecules to be broken up.[109,110]

Figure 8. Structural formula of formic acid

The carboxyl group determines particularly strongly the properties of FA. FA can react as a carboxylic acid with alcohol to obtain formic acid ester.[111,112] The central carbon atom has a formal oxidation state of +2 and can, therefore, act as a proton transfer agent analogous to the carbonyl compounds. This is a typical reaction that occurs in contact with base metals such as iron, zinc, or sodium. The release of hydrogen produces the corresponding metal formate.[112] Due to its strong acidic nature, FA can also esterify many alcohols without the addition of mineral acids.[113] On the other hand, formic acid also sometimes acts like an aldehyde. The reducing action as hydroxyformaldehyde occurs, especially in aqueous solution. Thus, for example, when heating an ammoniacal silver nitrate solution, elemental silver can be deposited.[114] An overview of the most important reactions is given in Scheme 1.

Scheme 1. Typical reactions of formic acid. [115]

Formic acid (FA) is the simplest compound from the series of alkanoic acids and is referred to as methanoic acid, according to the international IUPAC nomenclature. In nature, it occurs mainly in ants, bees, other insects, and stinging nettles.[116] It was named after its discoverer, the English naturalist and chemist John Ray, who was the first to extract FA from red ants in 1671. He heated ants in a glass flask and caught the resulting distillate in a separate flask while cooling. This he called based on the Latin name "Formica" for ant as "formic acid."[117] The French chemist Joseph Louis Gay-Lussac was the first to synthesize FA from hydrogen cyanide in the 1820s. In addition, Marcellin Berthelot succeeded in 1855 to syntheses FA from carbon monoxide, a synthesis route, which is still widely used in industry today.[118] As formic acid occupies a special position among the carboxylic acids due to its particular chemical and physical properties.

a. Application of FA

Formic acid is widely used in nature and is used by many plant stingers and animal species (woodchucks, ants, jellyfish) as a constituent of poisonous mixtures for defense and attack purposes.[119] In the human organism, formic acid is formed in addition to formaldehyde in the metabolism of methanol. FA is readily biodegradable.[120] Due to their acidity, their aldehydic nature and their reducing properties, formic acid is used in a wide variety of industrial sectors, as shown in Table 1:

Table 1. Technical applications of formic acid. [115]

Application Proportion (%) silage 35 Textile and leather industry 25 Formats and others 20 Medicines, pesticides 10 Rubber industry 10

Formic acid is an important chemical that has been extensively used in chemical, pharmaceutical, rubber, leather, and agricultural industries. Moreover, formic acid has been considered to be a proper hydrogen storage compound. It can be simply and selectively decomposed into H2 and CO2 under mild conditions.[121,122] Heretofore, the production of formic acid has mostly relied on fossil raw materials. Many new innovative processes were proposed for formic acid production from biomass such as glucose or cellulose instead to eliminate in the future the dependence on petroleum- based sources.

b. Conventional production processes for formic acid

The world production of FA amounted to 720,000 tonnes in 2009. [115] The largest sales markets are currently in Asia (about 48% market share), Europe (about 30%), and Central and South America (about 17%). The annual growth rate for the period to 2018 is 3.7%, with the main growth markets seen in China, North America, and Africa. [115] The industrial production processes used for the production of formic acid can be divided into four groups:

• Oxidation of hydrocarbons • Hydrolysis of formamide • Production from formats • Hydrolysis of methyl formate

In the past, FA was produced as a by-product in acetic acid production from the oxidation of low-boiling petroleum fractions. This was carried out by oxidation of butane (USA) or naphtha (Europe) in the liquid phase.[123] In the work-up of the reaction mixture, first unreacted hydrocarbons, volatile neutral admixtures and water are separated from the oxidation product. In the subsequent column, the formic acid formed is separated by means of azeotropic distillation. The preferred entraining agents are benzene or chlorinated hydrocarbons. However, the construction of new 13

methanol carbonylation systems[115] has cannibalized this process. As a result, the share of global production of formic acid is expected to continue to decline along this route. The production of formic acid by the hydrolysis of formamide played also a decisive role in the past. In 1972, one-third of total formic acid production was covered in this way. For this purpose, methanol was first carbonylated to methyl formate at 45 bar and 80 °C in a first step and then converted by means of ammonolysis (40-60 bar, 80-100 °C) to formamide and methanol. In the last step, the formamide is hydrolyzed continuously with the aid of sulfuric acid at 80-100 °C to formic acid and ammonium sulfate. The high consumption of ammonia and sulfuric acid, as well as the inevitable production of large quantities of ammonium sulfate, made the process increasingly uneconomical compared to other process options.[112] As a further industrial production variant, the formation of free acid from its salts has established. For this purpose, sodium formate and calcium formate are preferably used.[115] These are by-products of the production of polyhydric alcohols such as pentaerythritol, trimethylolpropane, and 2,2-dimethyl-1,3-propanediol. The result is the sodium formate, from which the formic acid can be released by acidification with sulfuric acid and then distilled off.[112] The company Norsk Hydro has developed a process for recovering FA from calcium formate. By hydrolysis with sulfuric acid, formic acid and calcium sulfate are obtained.

Scheme 2. Formic acid production via direct hydrolysis of methyl formate. [115]

Today, the direct hydrolysis of methyl formate is the primary method of production since it does not produce any unwanted by-products. It is a two-step process (Scheme 2), with methanol initially reacting with carbon monoxide at 80 °C and 45 bar to form methyl formate. In the subsequent hydrolysis, this methyl formate is then cleaved to formic acid and methanol. The resulting methanol can be separated off and returned to the first process step, whereby a nearly complete conversion of the methanol by recycling in the process is possible. The two equilibrium reactions are shown in the following scheme for illustrative purposes.

c. Methyl formate as the important intermediate for FA production

Methyl formate (MF) or methyl methanoate, C2H4O2, is the methyl ester of formic acid. It is the simplest form of an ester, which is a colorless liquid, ethereal odor, low surface tension, and high vapor pressure.

In large manufacturing plants, most of MF is used as an intermediate in the production of formic acid and formamide. MF becomes an important and versatile intermediate due to increasing the production of synthesis gas from coal gasification. In addition, it is an essential precursor for several other compounds in commercial industries. It can also be used as a raw material for the production of high-purity carbon monoxide. A new application of MF is the production of foundry molds.[112]

MF can be generated by the condensation reaction of methanol and formic acid in the laboratory. Generally, industrial production is performed via the combination of carbon monoxide and methanol in a strong base catalyst, for instance, sodium methoxide, as shown in Figure 9. BASF, the leading chemical company for MF production, obtains 96% MF selectivity. The catalytic process is sensitive to water, which can come with the carbon monoxide feedstock. Typically, carbon monoxide derives from synthesis gas. Therefore, dry carbon monoxide is necessary for the process.[112]

Figure 9. The reaction of industrial methyl formate synthesis [112]

d. Oxidation of biogenic raw materials to formic acid

The recovery of formic acid from renewable raw materials dates back to 1983. McGinnis et al. presented at that time a concept for the oxidative conversion of biomass into different organic acids.[124] Various types of wood were partially oxidized in aqueous solution at 16-33 bar and 171-227 °C to a mixture of different carboxylic acids and methanol. The addition of metal salts catalyzed the conversion of lignocellulose into predominantly short-chain organic acids such as formic and acetic acid. The best results were achieved with iron sulfate in the processing of pinewood with an FA yield of almost 30% at 190 °C and 16.6 bar. A year later, the same research group published further studies on the oxidation of carbohydrates in aqueous solution.[125] The process of wet oxidation was extended to different model substrates (glucose, xylose, glucitol, dextran, and cellulose). Again, a positive effect could be noted by the addition of iron sulfate.

Another concept for the production of carboxylic acids from biogenic starting materials was described in1986 by Niemelä et al.[126] Cotton cellulose was treated with a sodium hydroxide solution at temperatures of 170-190 °C under a nitrogen atmosphere. The result was a highly inhomogeneous mixture of 65 different organic components, including formic acid, with a yield of 10.4%. Two years later, Niemelä's group again processed cotton cellulose, but this time under 2-4 bar oxygen.[127] In this case, conversion of 60-70% of the cellulose to short-chain carboxylic acids could be achieved, but with the same inhomogeneous result of about 60 different components. However, the presence of oxygen drastically changed the composition of the product mixture. At conditions of 190 °C and 4 bar O2 pressure with the addition of 1 molar sodium hydroxide solution, a maximum FA yield of 7.2% could be achieved. In 1990, this process was extended to the substrates xylan, starch, and chitin. Later this route was not pursued due to low yields of formic acid and insufficient selectivity.[128] The first method, which explicitly aimed at the production of formic acid from carbohydrates, was published in 2008 by Jin et al.[129] Hydrothermal oxidation was carried out using 30% hydrogen peroxide solution (H2O2) at temperatures between 200-300 °C starting from glucose as a model substrate. A maximum yield of formic acid

of 24% was obtained at a temperature of 250 °C and a reaction time of 60 seconds with a 240% excess of H2O2. The FA possibly formed via direct oxidation of glucose by α-scission rather than via oxalic acid by β-scission, as detailed in Figure 10. Caustic solutions were added to counteract the thermal and oxidative decomposition of formic acid under these extreme conditions. As a result of the formation of the alkali metal salts of formic acid, the maximum yield could be increased to 75%. This process was extended a year later by Jin and Enomoto to other substrates such as potato starch, cellulose, or rice husks.[130] However, the primary goal of this work was not the improvement of the catalytic system, but the design of a two-stage, continuous reactor system for acetic acid production.

Figure 10. The pathway for hydrothermal conversion of glucose into formic acid using H2O2 as the oxidant (reproduced from Jin et al.). [88,129]

Another important discovery for the thermal conversion of sugars was introduced by Taccardi et al. using ionic liquids.[131] The authors observed the thermal decomposition of glucose and cellulose over several de- or rehydration steps to levulinic acid and formic acid. Furthermore, Li and co-workers made experiments on the oxidation of biomass by various water-soluble vanadium salts in combination with strong mineral acids such as sulfuric acid.[132] Again, the model compound glucose was converted under hydrothermal conditions at 100-150 °C to formic acid.

2.2.1.3 Glucose oxidation (OxFA process)

In the Ph.D. thesis of Wölfel at the Department of Chemical Reaction Engineering at the University of Erlangen-Nuremberg,[133] a catalytically active system for the oxidative conversion of biogenic raw materials with oxygen to formic acid in good selectivity was found. The formation of FA by an oxidative C-C bond cleavage from biomass showed great potential even at low temperatures of less than 100 °C and avoided the formation of thermally induced by-products. As a stoichiometric oxidizing agent, only molecular oxygen was used in this work for reasons of economy and sustainability. On the one hand, atmospheric oxygen is the cheapest oxidizing agent. It also enables a completely closed and regenerative cycle for the use of biogenic raw materials.[134] Water has been identified as the best solvent due to its almost inexhaustible availability, low cost, non-toxicity, and non-flammability.[88] The most important criteria for catalyst selection were the solubility, stability, and activity in water.

Unlike other thermal biomass conversion processes, partial oxidation of biomass to formic acid is an exothermic reaction. Focusing on C1 molecules can also lead to the formation of thermal by-products. In principle, the OxFA-process represents a partial

catalytic oxidation of biogenic raw materials. By means of homogeneous catalysis, this transformation can already be carried out under mild conditions (90 °C, 30 bar) in aqueous solution, the main products being formic acid (HCOOH), carbon dioxide (CO2) and water. The basic idea is to "sacrifice" a portion of the primary energy stored in the biomass in favor of a selectively formed, better storable and transportable secondary energy source (formic acid). The resulting FA can then be cleaved either thermally to CO and H2O or catalytically to H2 and CO2. The resulting products can be used energetically and as cleaned building blocks. Consequently, this concept can be classified in the field of thermo-chemical conversion of biogenic raw materials. Since the process primarily aims to oxidize lignocellulose-containing biomass or biological waste, the resulting formic acid corresponds to a 2nd generation biogenic fuel. The basic idea of the OxFA process is summarized in the following energy diagram (Figure 11).

Figure 11. The energetic concept of the OxFA process (reproduced from Albert). [115]

The catalytic cycle for the oxidation of biomass to FA can be subdivided into the substep of substrate oxidation and catalyst reoxidation. As shown in Figure 12, the electron transfer to the substrate occurs from the oxidized catalyst, which itself is reduced. The reduced catalyst is then oxidized again by molecular oxygen, which can start a new cycle. The biogenic substrate used in this process is oxidized to FA (HCOOH), CO2, and water.

Figure 12. Reaction system of substrate oxidation with heteropolyanions (Figure reproduced from Albert). [20]

2.2.2 Glycerol

Figure 13. Chemical structure of glycerol

The enlarging production of biodiesel by transesterification of vegetable oil with methanol or ethanol results in a surplus production of glycerol (Figure 13). About 100kg glycerol is produced per ton of biodiesel. This led to a total supply of crude glycerol of 4.2 million tons in 2018.[135] This surplus impacts the price of the refined glycerol market. Within 10 years (in 2000-2010), the price reduced from around 4000 $/ton in EU and 2000 $/ton in USA to under 500 $/ton and 600 $/ton, respectively. The principal concern for operating biodiesel plants is its high production cost. Utilization of the glycerol co-product is a promising alternative for reducing the production cost. Glycerol is a versatile renewable raw material and is generally used in the chemical industry in over 2000 applications. Glycerol can be used in liquid detergents, pharmaceuticals, cosmetics, antifreeze, and as a solvent in foods and beverages. It is also used with sorbitol for toothpaste production and in food as a humectant and sweetener. In addition, glycerol could replace methanol in fuel cells.[136]

2.2.2.1 Conventional manufacturing processes (as a by-product from biodiesel production)

The industrial production of glycerol is operated by using different processes and feedstocks. In traditional manufacturing, glycerol is generated by the homogeneous transesterification reaction of vegetable oil with methanol, using a strong base as catalyst. Another pathway for producing glycerol is synthetic glycerol via propylene.[137] Glycerol can also be produced by fermentation with yeast such as Saccharomyces cerevisiae and Candida albicans, or with bacteria such as Bacillus subtilis, or with algae such as Dunaliella tertiolecta.[138]

Transesterification of Oil

The production of glycerol and fatty esters (or biodiesel) is presented in Figure 14, which shows a reaction of methyl-esters from triglyceride (oils) and methanol (alcohol).[139,140]

R O O 3 O O R1 Catalyst OH O O + 3 OH R HO OH + O O 2 O R2 R O 1 O O R3 Methanol Glycerol Triglyceride Fatty esters

Figure 14. Glycerol production through transesterification reaction (reproduced from Bagnato et al.) [141]

The biodiesel production can be performed either by homogeneous or heterogeneous catalysis. In the homogeneous process, the reaction of vegetable oils and methanol takes place by catalysis of sodium hydroxide or sodium methylate in the first step.[142,143] Afterwards, glycerol is separated from the product mixture at the settler unit. The rest flows to a catalyst removal unit by adding mineral acids to neutralize the basic catalyst. The stream is split into two sections; a glycerol recovery unit and an evaporator, which purifies the biodiesel from aqueous mixture. There are three output streams generated after the glycerol purification unit, the glycerol line, the recycle line which consist of water, dissolved salts, and unreacted methanol, as well as fatty acid line.[140]

2.2.2.2 Valuable products from glycerol

Meanwhile, the global market for refined glycerol is saturated as evidenced by a decreasing glycerol price at the world markets. Therefore, it is of great importance for scientists to find new applications for refined and crude glycerol. Several papers have been published on the utilization of glycerol derived from biodiesel production. Glycerol is regarded as one important biorefinery feedstock for pharmaceuticals, plastics, agricultural adjuvants, and transportation fuel sectors (Figure 15). The value- added products from the glycerol-based biorefinery consist of hydrogen, ethanol, 1,3-propanediol, 1,2-propanediol, lactic acid, propionic acid, epichlorohydrin, n-butanol, i-butanol, succinic acid, and poly-3-hydroxybutyrate.

Figure 15. A potentially crucial role of glycerol on biorefineries in future. [10]

A more detailed view on the target molecular accessible from glycerol is shown in Figure 16.

Figure 16. Glycerol as a platform for functional chemicals and fuels (reproduced from Pagliaro et al.). [10]

Fuel Oxygenates

As glycerol can polymerize and partially oxidize at high temperatures, it is unsuitable for adding directly into fuel. Thereby the produced polymer can clog the engine. On the contrary, oxygenated compounds, for instance, methyl tertiary butyl ether (MTBE), are applied as a fuel additive with the properties of octane improvement. Thereby it meets the requirements of American and European Standards (ASTM D6751 and EN 14214,

respectively) for diesel and biodiesel fuels. This new chemical is compatible with other biodiesel additives.[144] As a fuel additive, (2,2-dimethyl-1,3-dioxolan-4-yl) methyl acetate, could be generated from crude glycerol and used as a biodiesel additive. It has also been known as 1,2-isopropylidene glycerol acetate or “solketalacetin” due to its production from monoacetin and solketal (Figure 17).[145] It improves the viscosity of biodiesel, flash point, and oxidation stability. This new chemical can contend with other biodiesel additives.[144] Moreover, glycerol tertiary butyl ether (GTBE) is an outstanding additive, which has massive potential for diesel and biodiesel reformulation. Especially, incorporation of 1,3-di-, 1,2-di-, and 1,2,3-tri-tert-butyl glycerol in standard 30–40% aromatic-containing diesel fuel, leads to remarkably reduced emissions of hydrocarbons, carbon monoxide, particulate matter, and unregulated aldehydes.[146] The synthesis of such alkyl ethers is possible from the catalytic reaction of glycerol with isobutylene in the presence of an acid catalyst. The maximum yield can be achieved by operating in a two-phase system. The first phase of the system is a glycerol-rich polar phase containing the acidic catalyst. The other phase is an olefin-rich hydrocarbon phase consisting of the product ethers that can be easily isolated.[147]

Figure 17. Synthesis of (2,2-dimethyl-1,3-dioxolan-4-yl)methyl acetate (solketalacetin) starting from glycerol and acetic acid. [145]

Propanediols

The catalytic hydrogenolysis of glycerol is a chemical process that focusses on the cleavage of a chemical bond of C-C or C-O in the glycerol molecule. The addition of a hydrogen atom takes place simultaneously to generate a molecular fragment.[148] Glycerol conversion can lead to many valuable products such as 1,2-propanediol (1,2-PD) or propylene glycol, and 1,3-propanediol (1,3-PD), as shown in Figure 18. The different selectivity of 1,2-PD and 1,3-PD depends on the catalyst used and the reaction conditions.

Figure 18. Reaction routes of catalytic hydrogenolysis of glycerol using Cu-H4SiW12O10/SiO2 (Figure reproduced from Lee et al.). [149]

a. 1,2 propanediol

1,2 propanediol (1,2-PD) is an organic compound with the chemical formula CH3CH(OH)CH2OH. It is a colorless and viscous liquid, hardly odorless, and faded sweet taste. In addition, 1,2-PD shows the property as a strongly hygroscopic liquid. 1,2-PD is promptly miscible with water and other polar solvents such as acetone, alcohol, and chloroform. It can act as a solvent for polar organic substances, such as alcohols, phenols, natural products, dyes, and some resins. In nonpolar solvents, such as benzene, aromatic hydrocarbons, petroleum ethers, and carbon tetrachloride, 1,2-PD is insoluble. Moreover, 1,2-PD is non-irritating and shows low toxicity, as well as low volatility.[150] The industrialization of 1,2-PD was initiated in the 1930s from the transformation of propylene oxide.

▪ Application of 1,2-PD

1,2-PD is a valuable commodity chemical. It is applied as biodegradable functional fluid. It has been used in substantial applications such as the production of unsaturated polyesters for thermoset composites, food processing equipment, food chemistry, agricultural adjuvants, the tobacco industry, pharmaceuticals, and cosmetics. 1,2-PD can tremendously decrease the freezing point of water. Therefore, the development of 1,2-PD replaces ethylene glycol as automotive antifreeze systems and aircraft de-icers.[27,150–152] The solidification temperatures and viscosities, including densities of propylene glycol solutions, depend on aqueous ratios. The hydroxyl groups in the molecule dominate the chemical properties of 1,2-PD. 1,2-PD can react with carboxylic acids to produce esters and water, well-known as "esterification reaction." It also reacts with chlorides and isocyanates to obtain esters and carbamates, respectively. Dipropylene glycol, tripropylene glycol, and polyether polyols are produced by the reaction of propylene oxide and 1,2-PD (Figure 19).[150]

Figure 19. Production of dipropylene glycol, tripropylene glycol, and polyether polyols produced by the reaction of propylene oxide and 1,2-PD. [150]

▪ Conventional production processes

Hydrolysis of Propylene Oxide. The direct hydrolysis of propylene oxide with water is the dominant process for 1,2-PD manufacture, as shown in Figure 20. The sequential addition of propylene oxide to 1,2-PD can also form dipropylene glycol and tripropylene glycol. All of these products are generated simultaneously, and isolated by distillation. The reaction of propylene oxide and water takes place in a molar ratio of 1:15 and at an initial temperature of 125 °C and a pressure of around 20 bar. After that, the reaction is heated up with its exothermic spontaneous response. [150] At the higher ratio of water to oxide, the product mixture can get a higher proportion of propylene glycol product (1,2-PD: dipropylene glycol:tripropylene glycol = 100:10:1). Nevertheless, such an increase in water to oxide ratio affects the throughput, raises the recycling rates, and increases the energy costs.

Figure 20. The hydrogenolysis of propylene oxide for the 1,2- propanediol production. [150]

Hydrogenolysis of Glycerol to 1,2-PD.[153,154] In recent years, bio-renewable glycerol has arisen as a promising starting material for 1,2-PD production. Glycerol hydrogenolysis to 1,2-PD,[155,156] provides an alternative pathway for the sustainable production of 1,2-PD. Hydrogenolysis of glycerol to 1,2-PD takes place in the presence of active metal catalysts and hydrogen supply. The hydrogenolysis process has been commercialized for the production of 1,2-PD (USP grade) (Figure 21).[157] The process operates in two stages via the intermediate acetol. In the first stage, acetol is generated from glycerol with promotion by a metallic catalyst. Next step, the hydrogenolysis of the acetol with a similar catalyst is performed to obtain 1,2-PD. The most efficient catalyst for the production with high 1,2-PD selectivity is copper chromite.[152] The yield of propylene glycol is 73% at a lower cost compared to using petroleum-based feedstock. The intermediate formed, acetol, is a vital monomer used in the industry of polyols production. Therefore, this process generates more potential utilisations and markets for biodiesel derived glycerol.

Figure 21. The hydrogenolysis process has been commercialized for the production of 1,2- propanediol. [150]

The studies of glycerol hydrogenolysis to 1,2-PD have focussed on various aspects, especially catalysts development,[27,152,158,159] the improvement of reaction systems,[152,160,161] and the kinetics of the reaction.[162] The catalyst investigations revealed that Ru, Cu, and Ni-based catalysts show the highest the conversion of glycerol. Cu-based catalyst give the highest 1,2-PD selectivity, which is contrary to the low selectivity of ethylene glycol. Cu-based catalyst is preferable to Ru and Ni catalysts as the ability of the C-C bond cleavage is lower; the reaction obtains less undesired cracking products. Whereas, Ru- and Pd-based catalysts show low selectivity of 1,2-PD because of the competition in the hydrogenolysis process.[163] Cu-Zn-based catalyst were shown to allow high selectivity to generate propylene glycol (100% selectivity) with ca. 20% conversion at 180 °C.[27] Stability improvement of the Cu-ZnO catalyst was found by the addition of Al2O3.[164] Dasari et al.[152] proposed that the hydrogenolysis using Cu–ZnO/Al2O3 as a catalyst takes place in two-stage through acetol as an intermediate compound. Dasari et al. also reported Cu–Cr catalysts for glycerol hydrogenolysis to propylene glycol.[152] They showed a high selectivity of 85% and obtained a conversion of ca. 55% at 200 °C, 14 bar H2 after 24 h reaction time. The catalytic reaction has to prefer breaking down a C-O bond over breaking C-C bonds to attain the required product. Otherwise, glycerol can convert to undesirable products like ethylene glycol, or even CH4.

1,3-Propanediol

1,3-Propanediol (1,3-PD) or trimethylene glycol is an organic compound with the formula CH2(CH2OH)2, a three-carbon diol. It is a viscous, clear, colorless, and odorless liquid which is miscible with water, alcohols, ethers, and formamide.

1,3-PD has chemical properties similar to 1,2-PD. Carboxylic acids can react with 1,3-PD to form esters via esterification at elevated temperature. Acid chlorides and isocyanates can also react with 1,3-PD to obtain esters and urethanes. 1,3-PD is a chemical that easily forms ethers. In continuous reflux of diol it can form 3,3´-Dihydroxydipropyl ether. In the presence of acidic catalysts, aldehydes and ketones 1,3-PD can react to generate 1,3-dioxanes.[150]

Decomposition of 1,3-PD can occur at aluminum oxide surfaces at temperatures higher than 250 °C, getting allyl alcohol, propanol, and other products. Diacids can react with 1,3-PD to yield polyesters such as polyesters with terephthalic acid, which has a crystalline melting point of 220 °C. Polyurethanes can also be produced from 1,3-PD.[150] 1,3-PD is reported to display a minor environmental impact due to it is biodegradable and to have low toxicity.

▪ Application of 1,3-PD

Recent developments in the use of 1,3-PD for the synthesis of new biodegradable polyesters lead to a strong market need for this emerging commodity chemical. 1,3-PD reacts with terephthalic acid to produce poly(trimethylene terephthalate), named 3GT (DuPont) or PTT (Shell), as shown in Figure 22.[27,165] The new polyester has superior properties than polyethylene terephthalate (PET) in several ways, for instance, good tensile elastic recovery, good dyeability.[165,166] Generally, it is used in carpet and textile fibers industries. Other applications of 1,3-PD as solvents, detergents, adhesives, resins, and cosmetics were also mentioned.[165]

catalyst COOH COOH + HO-CH2CH2CH2-OH Oligomer

Esterification

catalyst

OOC COOCH2CH2CH2-OH , vacuum n

Polycondensation

Figure 22. PTT synthesis by esterification method [167]

▪ Conventional production processes

Hydrogenation of 3-Hydroxypropanal. Industrial 1,3-PD production was based in the past on a two-step process. In the first step, acrolein is hydrated to provide 3-hydroxypropanal (3-hydroxypropionaldehyde) (Figure 23). The aldehyde is subsequently hydrogenated to produce 1,3-PD.[168]

Figure 23. Industrial production of 1,3-PD through a two-step process from acrolein. [150]

The hydrogenation of 3-hydroxypropanal has been investigated by using various catalysts and supports. The reaction can be carried out directly in the aqueous phase. Hydrogenation proceeds with a Raney nickel catalyst under pressure in the aqueous phase and the organic phase at a temperature of 110–150 °C under pressure of 20–40 bar with nickel-supported catalysts.[150] The generated 1,3-PD is isolated from the mixture of water and solvent by distillation. The desired product, 1,3-PD, is obtained in ca. 45% yield. The clogging of impurities in the feedstock on the active catalysts is a significant problem. Gallezot et al. found that a ruthenium catalyst, which was supported on macroporous titanium dioxide, performed suitably. The reduction of 3-hydroxypropanal to 1,3-PD allows yields of 98%. The same catalytic system with a different pore size of support (microporous or mesoporous) demonstrated deactivation.[169]

Degussa–DuPont has used this 1,3-propanediol production till the early 2000s. Afterward, it has been substituted by biotechnological production.

Hydroformylation of Ethylene Oxide. Shell has vastly studied an alternative process to synthesize 1,3-PD via hydroformylation of ethylene oxide (Figure 24).[170,171] The technique involves a two-stage process. In the first stage, ethylene oxide reacts with carbon monoxide to form 3-hydroxypropanal. Then, 3-hydroxypropanal is subsequently hydrogenated to give 1,3-PD with a yield of 92%.

Figure 24. 1,3-PD production via hydroformylation of ethylene oxide. [150]

The applied organometallic catalyst accelerates the ring-opening of ethylene oxide, and then carbon monoxide is inserted to generate the hydroxy aldehyde. Heterogeneous organometallic catalysts, such as copper chromite catalyst are used in the 3-hydroxypropanal reduction step with synthesis gas comprised of hydrogen and carbon monoxide as the hydrogen source.[172]

Alternatively, the two-stage process can be intergraded in one reaction, as illustrated in Figure 25. Shell Chemical Company claims this process using a combination of cobalt and ruthenium catalysts (1:1) with a 1,2-diphospholanoethane ligand.

Figure 25. 1,3-PD production by hydroformylation of ethylene oxide. [168]

The catalytic reaction of ethylene oxide occurs under the atmosphere of synthesis gas in methyl tert-butyl ether at elevated temperature and pressure in the presence of the catalyst. 1,3-PD is produced in a yield of ca. 90% in a one-pot reaction.[170]

Selective deoxygenation of glycerol. As mentioned before (see Section Hydrogenolysis of Glycerol to 1,2-Propanediol), propanediols (both 1,2-PD and 1,3-PD) can be generated from glycerol with heterogeneous or homogeneous catalysis (Figure 26). The product distribution, using an organometallic catalyst on a zirconium dioxide support, comprised of 1,3-PD (24% yield), 1,2-PD (12% yield), and 1-propanol (28% yield) under high pressure of hydrogen and temperature of 170 °C. The yield of 1,3-PD was lower when using alumina or silica supports.[173] A 1,3-PD yield of 72% was obtained when p-toluenesulfonic acid was applied as a catalyst.[174]

Figure 26. 1,3-PD production by deoxygenation of glycerol. [150]

Glycerol can be converted into 1,3-PD using organometallic catalysts through selective deoxygenation of a secondary alcohol. It was reported that this reaction might occur via the pathway of a cationic intermediate reduction or the route of a dehydration/ hydrogenation. Various works pointed to selective deoxygenation of glycerol. Drent and coworkers presented the deoxygenation of glycerol using heterogeneous palladium or platinum catalysts.[175] Even if the main product of the reaction was acrolein, 1,3-Pd could be obtained as a co-product.

2.3 Polyoxometalates as catalysts

Polyoxometalates (POMs) are complex compounds of oxygen and light transition metals such as vanadium (V), niobium (Nb), tantalum (Ta), molybdenum (Mo) or tungsten (W) in their highest oxidation state. These metal oxo anion clusters can also contain a variety of heteroatoms such as phosphorus (P), arsenic (As), silicon (Si) or germanium (Ge). POMs are a unique class of metal oxides regarding their structure, reactivity, and chemical and physical properties.[176,177] Their history dates back to the early 19th century. Berzelius succeeded in the first successful synthesis of the phosphomolybdate anion [PMo12O40]3- in 1826.[178] Forty years later, Marignac successfully produced silicotungstic acid for the first time.[179] The systematic investigation of properties and detailed characterization of POMs finally began at the beginning of the 20th century with the work of Rosenheim.[180] However, it took until 1991, before Pope and Müller in their paper "Chemistry of Polyoxometalates: Recent Variations on an Old Topic with Interdisciplinary References"[181] emphasized the importance of POMs for the development of tailor-made molecules and materials. This triggered a veritable boom in the exploration of this substance class.[16]

2.3.1 Molecular Structure

Generally, the polyoxometalates form a class of compounds based on metal oxide moieties of the formula [MOx]n, where M(addenda atom) is the transition metal described above (V, Nb, Ta, Mo, or W), and x can be values between 4 and 7.[176,181] These molecular clusters are usually anionic and can be complexed with heteroatom templates or cations as linkers. Furthermore, POMs can undergo defect connections with each other in which defects create vacant coordination sites that can be filled with linking atoms. Such clusters often contain highly symmetric, approximately spherical arrays of metal oxide units, and their structures are based on Archimedean and Platonic solids. The usual electron configuration of the metal atoms is d0.[16,182] Usually, complex structures, such as polyanions, are formed by protonation of an

oxometalate ion under suitable conditions (pH, concentration, temperature) with polycondensation of the [MO4]2- units. If the condensation takes place between two identical species, the reaction leads to an isopolyanion. Isoppolyanion ([MnO(4n-m)](2n-m)-) generally have a degree of condensation of n> 6.[177] Conversely, if the condensation of several oxoanions occurs around a central heteroatom X (X = Si, P, As, Ge) or another metal atom, the reaction leads to the formation of heteropolyanions. These have the general form [XoMnOm]y-. The formation of the various polyanions is illustrated in Scheme 3.

Scheme 3. Polycondensation of oxo anions (tetrahedral) to polyanions (octahedral). [177]

2.3.2 Classification

Isopolyoxometalates consist of an octahedral or square pyramidal metal framework [MOx]y and carry no further heteroatoms. Their physical cohesion is ensured either electrostatically, by hydrogen bonds or van der Waals forces. [183,184] This leads to reduced stability compared to the respective hetero analogues. The best-known representatives of such isopolyanions are of the so-called Lindqvist type with the general empirical formula [M6O19]n- shown in Figure 27.[183,185]

Figure 27. Lindqvist structure [M6O19]n- displaying terminal and bridging oxygen atoms. Left: balls and sticks; Right: polyhedral. Gray is M(addenda atom); red is oxygen. [186]

Heteropolyoxometalates consist of the described metal oxide clusters, which follow the same structural principles as isopolyoxometalates. However, they also contain heteroanions such as sulfate (SO42-) or phosphate (PO43-) and are by far the best-studied subgroup of polyoxometalates.[16] The most popular anions are representatives of the Keggin [XM12O40]n-, respectively Wells-Dawson-type [X2M18O62]n.[187] Other known anions include, for example, the Anderson [XM6O24]n- type, which is the only octahedral system.[188] In the Keggin and Wells-Dawson POMs, the tetrahedral coordinated heteroatoms (X) are spherically encapsulated by the metal oxide units, while the Anderson type forms a ring around the central heteroanion. The three structural types are illustrated in Figure 28-30.

Figure 28. Keggin structure [XM12O40]n-. Left: balls and sticks; right: polyhedral. Gray is M(addenda atom); red is oxygen; blue is X(heteroatom). [186]

Figure 29. Anderson structure [XM6O24]n‑. Left: balls and sticks; right: polyhedral. Gray is M(addenda atom); red is oxygen; blue is X(heteroatom). [186]

Figure 30. Wells−Dawson structure [X2M18O62]n‑. Left: balls and sticks; right: polyhedral. Common naming of regions within the molecule are shown. Gray, M(addenda atom); red, oxygen; blue, X(heteroatom). [186]

2.4 Use of heteropolyacids in biomass oxidation

Polyoxometalates (e.g. of the type of H3+n[PVnMo12-nO40]) dissociate to heteropolyacid (HPA) with the catalytically active heteropolyanion in aqueous solution means the degree of metal substitution into the HPA structure. These heteropolyoxometalates with acidic hydrogen are formed by condensation of more than two oxoanions and are well suited for the oxidative conversion of biogenic raw materials using molecular oxygen.[189,190] Crucial importance for the reversible-oxidative properties of these compounds, even at temperatures below 100 °C, has the degree of substitution by vanadium.[191] The two steps for the oxidative conversion of biomass in a catalytic process are first the substrate oxidation by the oxidized form of the catalyst under electron uptake and the subsequent reoxidation of the reduced HPA-n by molecular oxygen according to the following Scheme 4.

Scheme 4. Substrate oxidation by HPA-n and HPA reoxidation with O2. [191]

Another important criterion for the use of heteropolyacids in biomass oxidation is their hydrolytic stability. Again, the degree of substitution plays an important role. Substitution of the addenda atoms in the Keggin HPA structure [H3+nPVnMo12-nO40] have been reported to fulfil these requirements.[20] Thus, the substitution to HPA-1 brings a huge improvement in terms of stability in aqueous solutions compared to the V-free polyoxometalate. For higher degrees of substitution, however, the stability to water is discussed controversially. Thus, in work by Souchay et al., the authors talked about a decrease in stability with increasing degree of substitution from n> 1,[192] while work by Zhizhina et al. suggest that under certain temperature conditions it could behave in exactly the opposite way.[193] Finally, the work of Smith and Pope [194] concludes that at the same pH values, the stability of HPA-n decreases in solution with an increasing number of V atoms from a degree of substitution of n> 5. The substitution of MoVI by VV increases the basicity of the POM anion by a one-fold increase in negative partial charge. This has a direct effect on the acid strength of the corresponding heteropolyacid, which decreases with each additional electron absorbed.[189] However, a decreasing pH of the solution also affects the dissociation of differently substituted heteropolyanions. In general, the rule of thumb is that the more V atoms are included in the structure, the more complex the overall system becomes. Two fundamentally different active species of HPA-n structures in aqueous solution are described in the literature. Neumann and Khenkin, for example, assume that the oxidation of organic substrates by HPA-n with a low degree of substitution is a two-electron transfer from the oxidized POM molecule to the substrate with the reduction of the original, non-dissociated POM molecule. This is then reoxidized with the help of molecular oxygen back to the catalytically active species.[195,196] In this simultaneous electron transfer-oxygen transfer (ET-OT) mechanism, a transfer of an O atom from the HPA-n to the substrate takes place, which is oxidized (Scheme 5). The HPA-n absorbs the electrons emitted by the substrate and is in turn reduced. In the subsequent second step, this reduced species is then directly oxidized by oxygen. This Mars-van Krevelen-type reaction was first described in liquid homogeneous catalysis in 2001 in the oxidation of aromatics and alkylaromatics by HPA-2 in combination with oxygen.[197] It was later confirmed for the oxidation of anthracene and xanthene by HPA-2.[198]

Scheme 5. Partial steps of an HPA-n catalyzed substrate oxidation according to ET-OT mechanism. [198][189]

The mechanisms just described have already led in the past to the use of heteropolyacids in the implementation of bio-based compounds. Thus, in an article by Sonnen et al., the authors talked about the use of the water-soluble HPA-2-sodium salt in the delignification of pulps. In this wet oxidation, the lignin is separated from the remaining pulp fibers by HPA oxidation.[199] Furthermore, Evtuguin et al. described the oxidative delignification of wood by HPA-5/VO2+ in aqueous solution[200] by nearly complete oxidation of lignin to CO2 and low-molecular aromatics.[201] The group of Shatalov and Neto has also investigated the decomposition reactions of 30

cellulose in an acidic water-ethanol mixture under oxidative conditions.[202] These authors found that the acidity of the medium and the interaction of the heteropolyanions were crucial factors for the cleavage of the cellulose chains by solvolytic and oxidative degradation reactions. Other applications in the conversion of biogenic raw materials by heteropolyacids in conjunction with molecular oxygen can be found in the HPA-5 catalyzed ozonation of chemical pulps in organic solvents,[203] in the oxidation of natural ketones,[204] or the saccharification of natural lignocelluloses and polysaccharides in aqueous solutions.[205,206]

2.5 Use of heteropolyacids in biomass hydrogenolysis

There is limited research involving heteropoly acids (HPAs) in catalytic hydrogenation of biomass-derived compounds. The heteropolyacids have been applied as homogeneous catalysts for hydrogenolysis of biomass-derived materials consisting of noble and other metals (Ru, Rh, Pd, Pt, Cu, Ni,) substituted into heteropolyacids structure. The challenge of glycerol hydrogenolysis is the preferred fracture of C-O over C-C bonds. Some studies have found a promising catalytic system that could distinguish between those two bonds. Generally, heteropolyacid catalysts, including noble-metals, are more active than Cu- or Ni-based catalysts. Copper-based were shown to produce propanediols. Another problem for selective hydrogenolysis to obtain the desired diols is the selective cleavage of C-O bonds with respect to primary or secondary hydroxy group. The primary OH group in the glycerol molecule is cleaved easily because of a steric effect; consequently, high 1,2-propanediol (1,2-PD) selectivity is observed. However, some catalytic systems have shown high selectivity to produced 1,3 propanediol (1,3-PD). Dam et al. studied the effect of several tungsten-based additives over commercial catalysts (Pd/ SiO2, Pd/Al2O3, Pt/SiO2, and Pt/Al2O3) for glycerol hydrogenolysis in aqueous solution at 200 °C.[207] The tungsten-based additive studied was H2WO4 and silicotungstic acid (H4SiW12O40), as well as phosphotungstic acid (H3PW12O40). For comparison, hydrochloric acid was also checked. Moreover, the properties of polyoxometalates such as solubility, acidity, and redox potential can be adjusted by changing the counter cations and element components.[14,208] Acid-type polyoxometalates have been used as homogeneous catalysts because of good solubility in polar solvents, including water, whereas H2WO4 can be dissolved only in small amounts in an aqueous solution. The most typical polyoxotungstates with Keggin structure were H3PW12O40 and H4SiW12O40. Particularly, H4SiW12O40 is very stable in acidic solution. Tungsten-containing acidic additives and Pt-based catalysts could provide 1,3-PD with a selectivity of 20 to 40%. 1,2-PD was produced as the main product when using Pd active metal. The product distribution was affected insignificantly by different tungsten compounds and support types.[209] While the unification of tungsten and noble metal showed an effect on 1,3-PD generation from glycerol, there were some tungsten species reduced under the reaction conditions owing to electrostatic potential, as shown in Scheme 6.[189,210]

Scheme 6. Electrostatic potential of tungsten-based compounds and complexes. [189,210]

Alhanash et al. proved difunctional metal-acid catalysts such as Rh-doped acidic heteropolysalts Cs2.5H0.5[PW12O40] (TPA)[211] to be efficient for selective hydrogenolysis of glycerol to 1,2-PD under benign pressures of 3–14 bar of H2 and without external heating. Typical hydrogenolysis conditions of glycerol, with added noble metal as an active catalyst are temperatures of 160 to 190 °C and H2 pressure of 60 to 90 bar.

3 Objective of this work

The aim of this work was to develop the application of polyoxometalates (POMs) catalysts for the conversion of biomass to platform chemicals. For the present work, we focused on the investigation of two different reactions and systems. First, for the further development of the commercial OxFA process, [115,212] the effect of various solvents on glucose oxidation and product selectivity, especially formic acid (FA) selectivity, was investigated. Next, the hydrogenolysis of glycerol to produce propanediols (PDs) was studied in aqueous systems. The scope of these studies is illustrated in Figure 31.

Figure 31. POM catalyst application for biomass valorization as studied in this thesis.

The glucose oxidation in various solvents was expected to have an important effect on the production of bio-based chemicals such as FA. Several solvents were screened for testing stability and compatibility with the catalytic system of HPA-5, a Keggin-type POM. After that, the promising solvents were used for glucose oxidation to observe effect on product distributions. Mechanistic investigations regarding the reaction pathway from glucose to FA were also part of this study. Furthermore, the oxidation of promising solvents was studied for understanding the solvent influence on the oxidative system. Crucial factors such as residence time, reaction temperature, amount of catalyst, glucose concentration, oxygen pressure, and stirring speed were evaluated and optimized to obtain the best conditions for FA production. In addition, xylose and 3,4-dimethoxybenzyl alcohol (3,4-DMBA), which are model substrates for hemicellulose and lignin, respectively, were investigated to test the prospect of other main components in oxidative conversion. The recycling of the reaction media without product separation was studied in order to evaluate the industrial feasibility of the glucose oxidation in a given solvent.

For the glycerol hydrogenolysis, the application of Wells- Dawson-type POMs (WD-POM) as catalysts was studied. Various substituted WD POMs, such as Ru, Pt, and Pd metal-doped α2-WD POMs, were tested for selective hydrogenolysis of glycerol in aqueous solution. The doping of WD-POMs with noble metal that favor hydrogenation was expected to increase the conversion and selectivity of the desired-products, PDs. For a comparison of the noble-metal substituted WD POMs with the benchmark of the homogeneous precursors, the heterogeneous catalysts were operated with the equivalent mass of noble-metal to evaluate the catalytic system of the POMs for glycerol hydrogenolysis. A mechanistic explanation concerning the reaction pathway was proposed. A degradation mechanism could be used for the prediction and development of product selectivity. To improve the catalytic activity, parameters such as reaction time, hydrogenolysis temperature, catalyst amount, glycerol concentration, hydrogen pressure, as well as stirring speed, was studied to find out the optimum condition for the system. In addition, the study of long-term operation was a central task to test the robustness of the catalyst for the applied system.

4 Experimental section

The following chapter presents the technical framework and the procedures used for the individual test series. First, the chemicals used are listed. Subsequently, the experimental setups are explained, and the principle of the experiments is presented. This is followed by the description of the catalyst syntheses and of the stability investigations of all substances involved. The experiments for oxidizing and hydrogenating biomass-derived substrates and catalyst recycling are described and subsequently the associated analysis and the necessary calculation equations are presented.

4.1 Materials and chemicals 4.1.1 Biogenic materials as substrate

The model substrates used in this work were purchased commercially and were always used without further purification. A list of all the connections used and the corresponding source of supply can be found in Appendix 9.1.4. For oxidation, D(+)-glucose, D(+)-xylose (≥98.0%), glyoxal (40% sol. in water), and erythrose from Merck KGaA, as well as 3,4-dimethoxybenzyl alcohol from Acros and glycolaldehyde dimer from Aldrich were used. The 13C-glucose for the 13C-labelling test was provided by Eurisotop. For the investigations of the product mixture a simulated product solution with formic acid (99.5%) from VWR, methyl formate (97%) and dimethoxymethane from Alfa Aesar, and acetic acid (99.8%) from Merck were used. The substrate glycerol for hydrogenolysis (double distilled, 99.5%) was purchased from VWR.

4.1.2 Solvents and Gases

A list of all the used solvents can be found in Appendix 9.1.4. The water used for the homogeneously catalyzed conversion of biomass was demineralized domestic water. Methanol (98.0%), ethyl acetate, and ethyl lactate were all purchased from Sigma Aldrich. Ethanol (0.05% H2O), n-butanol, n-pentanol, n-hexanol, dimethyl sulfoxide, tetrahydrofuran, polyethylene carbonate, trimethyl phosphate, tri-n-butyl phosphate, tris(2-butoxyethyl)phosphate, triethylene glycol monomethyl ether, diethylene glycol monomethyl ether, and tetraethylene glycol dimethyl ether were all supplied by Merck KGaA. n-Heptanol (99%), limonene, butyl acetate (≥99%), and polyethylene glycol 200, were purchased from Alfa Aesar. Acetone and n-propanol were supplied by VWR. γ-Valerolactone was purchased from Acros. The 13C-methanol was provided by Deutero.

All gases used were purchased from Linde AG. As oxidizing gas, oxygen 5.0 was used. Hydrogen gas 4.5 was used for glycerol hydrogenation. Helium 4.6, and nitrogen 5.0

were used for the inert test of the autoclave. Argon 4.6 and hydrogen 4.5 were additionally used as carrier or fuel gases for the GC analysis.

4.1.3 Catalysts

For the synthesis of the catalysts (see Section 4.2), commercially available chemicals were used. The precursors of the synthesized catalysts such as vanadium(V)oxide and molybdenum(VI)oxide (99.5%) were purchased from Alfa Aesar. Hydrogen peroxide (30%), which is an oxidizing agent, was supplied by AppliChem. Phosphoric acid (25%) was purchased from Merck.

4.2 Catalyst Preparation 4.2.1 Keggin-type POM (HPA-5) for glucose oxidation

The synthesis of Keggin heteropolyoxometalates was based on a method reported by Odyakov and Zhizhina [213] and was optimized for the synthesis of HPA-5 by Albert [20]. This procedure is composed of a two-step synthesis, with the first step producing the stable vanadium precursor H9[PV14O42]. Based on this, HPA-5 is obtained in the second stage by the synthesis of the molybdenum precursor (HPA-0) and subsequent addition of the vanadium precursor. The amount of used precursors and chemicals is shown in Table 2.

Table 2. Compounds for HPA-5 preparation.

Chemicals MW(g/mol) Mass (g) Volume (ml) n (mmol) 1st step

V2O5 181.88 20.02 - 110 H2O2 (30%) 34.02 - 150 - H3PO4 (25%) 98.07 2.95 - 7.6 2nd step MoO3 143.93 44.33 - 310 H3PO4 (25%) 98.07 16.85 - 43

1st Step: Synthesis of the vanadium precursor

For the synthesis of the vanadium precursor, V2O5 was first suspended in about 8 °C cold water with constant stirring. Slow dropwise addition of 30% hydrogen peroxide solution at lower than 10 °C resulted in the dark red coloration of the vanadium (V) peroxo species formed. The mixture solution was taken out from the cooling bath. The temperature of the mixture increased spontaneously to 30-35 °C and the excess O2 was released. The color of the solution changed to the brown-orange of the unstable H6[V10O28] form. After the evolution of the gas subsided, the stable dark brown H9[PV14O42] vanadium precursor was formed by the slow addition of 25% phosphoric acid.

2nd Step: Synthesis of the HPA-5 complex

To prepare the molybdenum precursor (HPA-0), MoO3 was first suspended in water at room temperature with constant stirring. The subsequent addition of 25 % H3PO4 and heating to the boiling point at about 120 °C led to the formation of the yellow H3[PMo12O40] precursor (HPA-0). Thereafter, the reaction mixture was further boiled while adding portions of the H9[PV14O42] precursor from the first stage. Depending on the stoichiometric ratio of the starting materials, the finished polyoxometalate with the corresponding degree of substitution n was formed. The reaction mixture was then concentrated by evaporation and cooled to room temperature. This process was followed by a dead-end filtration of the concentrated mixture to remove unreacted starting materials (V2O5 or MoO3) or other impurities. The remaining filtrate was then dried by rotary evaporation at 55 °C in the water bath using a vacuum system to obtain a crystalline solid.

4.2.2 Wells-Dawson-type POMs for glycerol hydrogenolysis

The synthesis procedure to produce the WD-POMs was performed by Dr. Amalie Modvig from DTU.[214,215] The protocol consisted of a three-step process. First, α-WD (α-K6P2W18O62) was synthesized according to the method of Nadjo et al., modified by Graham and Finke.[216,217] The next step, α2-WD (α2-K10P2W17O61) was generated as the potassium salt following the Finke protocol.[218] The last step, metal- doped M-WD (α2-KXP2MW17O61) was synthesized with the doping of hetero addenda metals (M=Ru, Pd, and Pt) by modification of previously published procedures.[219,220]

At first, α2-K10P2W17061·XH2O (2.00 g, 0.42 mmol) was dissolved in 8 mL of 90 °C distilled water. A metal precursor (0.45 mmol, 1.1 equivalents) was dissolved in approx. 5 mL water at room temperature and was added to the α2-WD solution slowly under vigorous stirring. The solution was kept stirring for 1 h at 90 °C. Afterward, KCl (1.19 g, 0.02 mmol) was added to the reaction solution, and the reaction solution was allowed to cool to room temperature under continuous stirring for 1 h. The precipitate was gathered by filtration and washed with cold millipore water. The purification of the received precipitate was carried out by recrystallization in boiling water to give the α2-KxP2MW17O61 (M-WD).

4.3 Experimental setup 4.3.1 Experimental design of the two high-pressure autoclaves

Two 10-fold screening devices with slightly different configurations and vessel materials were separately used for the catalytic oxidation and hydrogenation of biogenic raw materials under pressure. The set-up of both systems is described below.

4.3.1.1 Hastelloy 10-fold reactor system for oxidation

The first system consisted of 20 ml high-pressure autoclaves (R-1 to R-10) made of corrosion-resistant Hastelloy C276 from Parr Instruments (see Figure 32-1). It was applied for the oxidative reactions. All valves, fittings, and pipes were made of stainless steel 1.4571 and oil-free components to prevent fires and explosions. Between head and vessel, the reactor was sealed by means of a Teflon round ring (PTFE). The oxygen supply was provided from an oxygen cylinder (see Figure 32-2) by the connection of individual autoclaves with oxygen line couplings. Oxygen can be allowed to pass through the connection by opening ball valves (V-2 and V-3). The pressure was manually adjustable by a needle valve (V-9 to V-18). For an anaerobic test of biogenic raw materials, nitrogen was applied through a pressure regulator (V-7) and a three-way valve (V-6). The off-gas can be released from the autoclaves by using the three-way valve (V-4). The internal reactor pressure was detected both analogously via a pressure gauge (PI-1 to PI-10) at the head of the reactor (see Figure 32-3) and electronically through a KELLER pressure sensor type PA-23SY/160 bar (PIR-1 to PIR-10) (see Figure 32-4). The heating plate (see Figure 32-5), transferred the heat through the inside contact in the hole of the heating plate to the reactors. The heating plate was operated of a heating power via a control unit from the HORST model HT MC-1 (see Figure 32-6). Moreover, a stirring system was embedded with the heating plate, which was controlled by a rotary controller (see Figure 32-7). In order to protect excessive pressure in the reactor, a rupture disk made of Hastelloy C276 with a nominal burst pressure of 90 bar was assembled to the head of the reactors. All signals of sensors were converted into temperature and pressure values in an 8/16-channel universal input transmitter of Nokeval. All values can also be monitored via a connected computer. Figures 32 and 33 show the illustrations of essential plant components and P&ID of the reactor system.

2 4

1 7

Figure 32. Illustration of the applied Hastelloy 10-fold batch reactor system: (1) Hastelloy reactor, (2) oxygen supply, (3) manometer, (4) pressure sensor, (5) heating plate with magnetic stirrer, (6) temperature controller, (7) magnetic stirring controller.

Figure 33. P&ID of Hastelloy 10-fold batch reactor system.

4.3.1.2 Stainless steel 10-fold reactor system for hydrogenation

The experiments of glycerol hydrogenolysis were carried out in a 10-fold high-pressure reactor system. Each reactor unit consisted of a 20 ml stainless steel 1.4571 vessel (R-1 to R-10) (see Figure 34-1) from Parr Instruments, which was sealed with a Teflon round ring. All components, such as valve, fitting, and pipe, were made of stainless steel 1.4571. The central monitoring unit (see Figure 34-3) controlled the temperature of the heating plate (see Figure 34-2). The heating plate was also equipped with an adjustable magnetic stirring system (see Figure 34-4). To measure the internal pressure of the reactor, digital pressure transducers (PIR-1 to PIR-10) (see Figure 34-5) and pressure gauges (PI-1 to PI-10) (see Figure 34-6) were used. The supply of the reaction gas hydrogen was led into the system through three-way valves (V-3 and V-4) and adjusted manually to the desired pressure by using a needle valve (V-5). For safety reasons, to prevent overpressure of the reactors, a rupture disk made of stainless steel 1.4571 with a nominal burst pressure of 81 bar was attached to the reactor head. The feeding of the inert gases nitrogen or helium was released by the three-way valve (V-26) for reactor purging or leaking test, respectively. Figures 34 and 35 show the P&I flow diagram of the reactor system and illustrations of significant plant components.

1 4

Figure 34. Illustration of stainless steel 10-fold batch reactor system: (1) Stainless steel reactor, (2) heating plate with magnetic stirrer, (3) temperature controller, (4) magnetic stirring controller, (5) pressure sensor, (6) manometer.

Figure 35. P&ID of the applied stainless steel 10-fold batch reactor system.

4.3.2 Principle of the batch experiments

Regardless of the type of autoclave used, the batch experiments and sampling after the reaction were operated similarly. The operation of all batch experiments is explained in more detail below.

4.3.2.1 Procedure of glucose oxidation in organic solvents and post- reaction sampling

The investigation of various solvent effects on glucose oxidation was performed in the 10-fold screening plant containing a magnetic stirrer. The parallel reactor system consists of ten 20-mL batch reactors made of Hastelloy (HC 276). In a typical procedure, 1 mmol of substrate glucose and 0.1 mmol of HPA-5, including a magnetic bead, were introduced into the respective solvents (table 3) in the autoclave. The reactor was then assembled using the PTFE gasket ring as a sealing. All assembled reactors were put in the heating blocks and connected to the couplings of the gas supply lines. The system was purged with oxygen (10 bar) 3 times to displace residual air from the reactor and to ensure a pure oxygen atmosphere. After that, the oxygen was charged to the desired initial pressure, 20 bars. The stirrer was set 300 rpm to increase heat distribution into the liquid phase. The system was heated up to the reaction temperature of 90 °C. Afterwards, the stirrer speed was raised to 1000 rpm to improve oxygen transfer into the solution. At the required temperature, the oxidation was held for 24 h. The temperature and the pressure of the system were monitored using a thermocouple and pressure transducer. The temperature, pressure, and reaction time during the operation were recorded.

After the reaction was finished, the reactor was cooled down by setting to room temperature, and the stirrer speed was decreased to 300 rpm. After the reactor reached RT (<30 °C), the gas phases of each reactor were sampled into a gas bag and then analyzed with a Varian GC 450-TCD-FID. The remaining liquid was filtered by using a 0.2-micron filter (made of polyester, Altmann Analytik) before further analysis. The liquid phase was analyzed using nuclear magnetic resonance (NMR) for the evaluation of catalyst stability. The 31P and 51V NMR spectra were compared before and after oxidative reaction to identify the stability of the used HPA-5 catalyst under the applied conditions. For catalytic oxidation of glucose, the studies with the most promising solvents were carried out under the same conditions of the stability test. The liquid phase was analyzed by using 1H NMR and 13C NMR for the qualification of liquid products. Moreover, high-performance liquid chromatography (HPLC) was used to analyze the remaining glucose after the oxidation process for calculation of glucose conversion and quantification of liquid product distribution.

4.3.2.2 Procedure of glycerol hydrogenolysis and post-reaction sampling

All catalytic activity tests were performed in the 10-fold screening plant, a parallel autoclave reactor system consisting of ten 20-mL stainless steel vessels. The aqueous mixtures were prepared by dissolution of 10 wt.% glycerol in distilled water placed into the reactor. Typically, 0.1 g of POM catalysts were introduced into the aqueous mixtures in the autoclave. The system was purged with hydrogen (10 bar) 3 times, respectively, and then, the initial hydrogen pressure was adjusted to 35 bar. The stirrer was set to 300 rpm, and heating was switched on. When the reaction temperature of 200 °C was reached, the stirrer was set to 1000 rpm. The systematic pressure was increased to 50 bar due to the vapor pressure of the solvent. After 24 h reaction time, the reactor was cooled down to room temperature (<30 °C), and the gas phase was collected in a gas bag and analyzed with a Varian GC 450-TCD-FID. The remained liquids were analyzed by using a JEOL NMR ECX-400 MHz. The calculation of conversion and selectivity was carried out based on the carbon atoms in the substrate. The mass balance of the transformation could be closed by using NMR- and GC-measurements results.

4.4 Analytical Procedures for characterization

For the qualitative and quantitative description of the series of experiments conducted in Chapter 5, a large number of analytical methods were used in this work. These are described in detail in the following section.

4.4.1 pH measurement

Measurements of the pH were always carried out at room temperature using the Lab 860 from Schott Instruments. For this purpose, the electrode was calibrated using buffer sets for calibration of Merck1*(4.000, 7.000, 9.000 pH values) at 20 °C (* It is a brand name or trade name). For the measurement, the electrode was immersed in a sample until the pH value was obtained.

4.4.2 Karl Fischer (KF) Titration

Water contents in the organic solvents were measured using a 756 KF Coulometer from Metrohm. By electrochemical means, iodine is generated directly in the iodide-containing electrolyte. The generator electrode without diaphragm was used for producing iodine at the anode. Karl Fischer reagent was supplied by Merck KGaA, with a type of “CombiCoulomat fritless” for coulometric water. With the limitation of the method and the Karl Fischer reagent used, it is suitable for a range of water content between 0.1 and 20 wt.%.

4.4.3 Fourier-Transform Infrared Spectroscopy (FT-IR)

FT-IR spectroscopy of POM catalysts was carried out with a Jasco FT/IR-4600 spectrometer to clarify the structure. The measurements were performed by using solid potassium bromide (KBr) pellets. In a first step, a disc of 13 mm diameter was produced in a manually operated hydraulic laboratory press MP 150 of Perkin-Elmer at a pressure of 7.63 t cm-2. The mass ratio of sample to KBr was 2 mg sample to 200 mg KBr, which corresponds to a content of 1 wt.%. In the second step, the absorbance of the sample with 64 scans in a measuring range of 400-4000 cm-1 was measured with a DTGS broadband detector, resulting in a resolution of 1 cm-1.

4.4.4 Inductively Coupled Plasma (ICP)

To determine the elemental, composition of the synthesized catalysts, inductively coupled plasma optical emission spectroscopy (ICP-OES) was used. The instrument was operated by Dr. Nicola Taccardi (CRT, FAU Erlangen). The measurements were carried out on a Perkin Elmer Plasma 400 by dissolving ca. 20 mg of the HPA-5 catalyst in 100 ml of double-distilled water. Water-insoluble samples were mixed before the measurement with a mixture of concentrated nitric and hydrochloric acid in a mixing ratio of three to one. The sample was then treated in a microwave. To calibrate the elements, ICP standard solutions (1000 μg.ml-1) were used.

4.4.5 Thermogravimetry (TG)

The thermogravimetric method was used for measuring the water and moisture content in the synthesized catalysts. It was carried out by using a SETSYS 1750 CS Evolution and the commercial software Calisto Data Acquisition from the SETARAM

Instrumentation company. For this purpose, 80 mg of the substance was weighed into a quartz crucible and examined in a three-stage temperature program. First, the sample was preheated for 60 min at a constant temperature of 20 °C. After that, the sample was heated at a constant heating rate of 10 K min-1 up to 350 °C and kept there isothermally for 30 min. Finally, the temperature was lowered to 20 °C at 10 K min-1 and again kept isothermally for 15 min. Helium was used as a purge gas. At each of the three temperature levels, the mass decrease was measured as a function of the temperature. The surface water and water of crystallization evaporate during the course of the temperature program, and therefore, the hydrate water content can be determined by gravimetric detection of the weight loss.

4.4.6 X-ray diffractometry (XRD)

For the characterization and fine structure analysis of the synthesized powdery crystalline polyoxometalates in solid form, X-ray powder diffraction (XRPD) was used. The X-ray powder diffractometer (Phillips) contains a Cu anode as a radiation source, from whose emission spectrum only the Cu-Kα radiation (λ Cu-K α = 0.154 nm) was used for the measurement. The recording of the diffractograms was always carried out continuously at a constant angular rate. In the measurement method used, the diffractogram was recorded in the diffraction angle range of 2θ = 2°-50° with a sampling rate of 0.01° (2θ) s-1. The evaluation of the diffractograms was carried out by means of the High Score software provided by the manufacturer.

4.4.7 Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR was used to characterize the synthesized catalysts, to determine their stability for various solvents, and to qualify liquid products for oxidation and hydrogenolysis, including quantitative analysis of liquid products after hydrogenolysis. The measurements were carried out on a Jeol ECX-400 MHz spectrometer (9.4 Tesla) at 20 °C.

The stability of synthesized POM catalysts before and after the oxidation in various solvents was monitored via 31P- and 51V-NMR analysis. The 31P-NMR spectra were recorded using a 512 scans method within a restricted range of -7 to +5 ppm at an excitation frequency of 161.98 MHz. The resulting resolution was 0.24 Hz. The external standard used was 1%wt phosphoric acid solutions in deuterium oxide, which were likewise sealed in inlets. The investigation of vanadium-containing catalysts was carried out without using a standard by means of 51V NMR with 2024 scans at an excitation frequency of 105.25 MHz. The chemical shift range was limited to -580 to -460 ppm, resulting in a resolution of 0.77 Hz.

The qualification analysis of oxidation products and quantification of hydrogenolysis products were performed with 64 scans at an excitation frequency of 400.13 MHz for 1H-NMR, resulting in a resolution of 97.61 MHz. The range of chemical shifts was scanned from 0 to +12 ppm for the oxidized liquid products by using an external reference of mixed benzene and benzene-d6 (1:12). Qualitative determinations by means of 13C-NMR were carried out with 1000 scans. The range of chemical shifts was 44

limited to 0-200 ppm. The excitation frequency set was 100.61 MHz. The resulting resolution was 0.77 Hz. Inlets were used in the same type of 1H-NMR as the external reference.

In order to obtain a reference peak in the NMR spectra for better comparability of different samples from glycerol hydrogenolysis, inlets with DMSO-d6 were used as the external standard for quantitative analysis by 1H-NMR. The external standards consisting of DMSO and DMSO-d6 in a gravimetric ratio of 1:4 were used. These standard mixtures were previously sealed in glass tubes (inlets) and calibrated with solutions of 0.3 wt.%, 3 wt.%, and 8 wt.% hydrogenolysis products (1,2-propanediol, 1,3-propanediol, n-propanol, i-propanol, ethanol, methanol, acetol, ethyl acetate, and acetone in distilled water. The slope of the calibration curve was obtained from plotting the component concentration and the signal ratio of the component to DMSO. The corresponding calibration factors and the calculation of all component’s concentration are provided in Appendix 9.1.6.

4.4.8 Gas Chromatography (GC)

The composition of the gas phase in the autoclave was determined quantitatively by means of a Varian GC gas chromatograph (GC). This was calibrated for the permanent gases hydrogen, carbon monoxide, carbon dioxide, and oxygen as well as the hydrocarbons methane, ethane, ethene, propane, and propene in order to quantify the volumetric fraction of the respective gas. Calibration data can be found in Appendix 9.1.7. The GC was equipped with a separate column of the type ShinCarbon ST100/120 (2 m x 0.75 mm ID) for the separation and metrological detection of the respective gases. The injection of the sample into the separation column was carried out via a 250 μl test loop, which was depressurized for 600 ms via a sample valve against atmospheric pressure immediately before injection. As a result, the formation of an overpressure in the test loop was avoided by the manual injection of the sample. The carrier gas used was argon with a column pressure of 4.82 bar. At the exit of the column, a thermal conductivity detector (TCD) was installed as a front analysis unit for the detection of the permanent gases and a downstream flame ionization detector (FID) for the determination of any resulting hydrocarbons. The signal recording and its quantitative evaluation took place with the help of the commercial software galaxy. Two different temperature programs were used to determine the respective gas compositions. For the use of previously unknown raw materials or unknown catalysts, a method using TCD (300 °C) and FID (200 °C) was used, as it could detect the formation of previously unobserved gaseous hydrocarbons. For this purpose, the temperature was first kept isothermally for 2.5 min at 40 °C and then heated linearly to 250 °C within the following 20 °C min-1. This temperature was kept constant for 12 minutes in order to clean the separation column of possibly applied high-boiling compounds.

4.4.9 Gas Chromatography/ Mass spectrometry (GC/MS)

In order to confirm the origin of FA under the reaction conditions using MeOH as the solvent, 13C-substances were used to identify the carbon source of FA. The 13C-label 45

experiments were investigated using a GC-MS Agilent 7890B GC/ 5977A MSD-system with HP-5ms Ultra Inert column (30 m x 250 µm x 0.25 µm) and a single quadrupole MS. The analysis was carried out by Ms. Birgit Brunner at CVT, Universität Bayreuth. For double-checking, the 13C-label measurement was analyzed by headspace GC-MS using a Shimadzu QC 2010/ QP2010 SE GC-MS system equipped with CTC combi PAL headspace and with a CP-Sil PONA CB column (50 m x 210 µm x 0.5 µm) using the liquid injection method. The chromatograms and spectra were recorded by Dr.Peter Schulz at CRT, Friedrich-Alexander-Universität Erlangen-Nürnberg.

4.4.10 High-performance liquid chromatography (HPLC)

The liquid samples were measured for the quantitative determination of glucose and organic acids in high-performance liquid chromatography (HPLC). A HPLC instrument from Jasco was used in this work. The separation column used was a SH1011 (300 mm × 8 mm inner diameter) from Shodex. The outer material of the column is made of stainless-steel type SUS-316, the base material in the column of styrene, divinylbenzene, and copolymer. The isolation took place via a cation exchange resin from the Shodex HPLC Columns. The eluent used was 0.005 M sulfuric acid at a constant flow rate of 1 mL min-1. The separation column was held at 50 °C during measurements in Jasec JEATSTREAM2-PLUS. To analyze the samples, a refractive index (RI) detector Series 200 from Perkin Elmer was used. The measurement methodology lasted 16 minutes for all samples, except those in which 3,4-dimethoxybenzyl alcohol was used as the substrate. Samples with this model substrate required a 70-minute measurement due to the slow elution. All samples were filtered prior through a disposable filter with a 0.2 μm pore diameter polyester membrane before injection into the HPLC instrument. These filters of the type Micropur PET were obtained from Altmann Analytik. The separation in this column is based on size exclusion chromatography for the sugars and on ion exclusion chromatography for the carboxylic acids. For this purpose, the stationary phase contains ionic groups such as -NR3+ or -SO3-, which more or less strongly bind the molecules to be analyzed according to their ionic interaction. The quantification of the main products such as formic acid and acetic acid, as well as the substrates glucose, xylose, and 3,4-dimethoxybenzyl alcohol were realized using calibrations from three to six different concentrations. The calibration factors are shown in Appendix 9.1.5.

4.5 Calculations

HPA-5 Catalyst was synthesized for the glucose. The yield of the synthesized HPA-5 catalyst was calculated considering the amount of vanadium and molybdenum used according to Equation 15 and 16. Here n represents the number of moles.

푛(푣푎푛푎푑푖푢푚 푖푛 퐻푃퐴−5) 푌(푉) = (15) 푛(푣푎푛푎푑푖푢푚 푖푛 푉2푂5)

푛(푚표푙푦푏푑푒푛푢푚 푖푛 퐻푃퐴−5) 푌(푀표) = (16) 푛(푚표푙푦푏푑푒푛푢푚 푖푛 푀표푂3)

The remaining water content in the synthesized HPA-5 structure was analyzed via thermogravimetric (TG) analysis, as shown in Equation 17.

푚 −푚 %푚 = 푖 푓 × 100 (17) 푚푓

Where %m is water content by weight in HPA-5, mi is an initial mass of HPA-5, and mf is the final mass of HPA-5.

The following equations provide the basis for the results presented in Chapter 5. The substrates conversion was calculated by the different amount of substrate before and after the reaction, following Equation 18.

n (C base of consumed substrate) Substrate conversion (%) = × 100 (18) n (C base of substrate,t=0)

The percent yield of products was calculated using Equation 19. The total yield is calculated according to equation 20 and 21 for glucose oxidation in MeOH and glycerol (GL) hydrogenolysis from the sum of the partial yields.

푛 (퐶 푏푎푠푒 표푓 푝푟표푑푢푐푡 퐴) 푌 = (19) 푝푟표푑푢푐푡 퐴 푛(퐶 푏푎푠푒 표푓 푠푢푏푠푡푟푎푡푒,푡=0)

푌푡표푡푎푙 푦푖푒푙푑 표푓 푔푙푢푐표푠푒 표푥푖푑푎푡푖표푛 = 푌퐹퐴/푀퐹 + 푌퐺퐴퐷 + 푌퐺퐿푂 + 푌퐸푅푇 + 푌퐶푂2 + 푌퐶푂 (20)

푌푡표푡푎푙 푦푖푒푙푑 표푓 퐺퐿 ℎ푦푑푟표푔푒푛표푙푦푠푖푠 = 푌1,2푃퐷 + 푌1,3푃퐷 + 푌푃푟푂퐻푠 + 푌푀푒푂퐻 + 푌퐸푡푂퐻 + 푌퐴푐푒푡표푙 +

푌퐴푐푒푡표푛푒 + 푌퐸푡푂퐴푐 + 푌퐶퐻4 + 푌퐶2퐻6 + 푌퐶3퐻8 (21)

For the calculation of the selectivities of the individual products, the equations 22 was used.

푛 (퐶 푏푎푠푒 표푓 푝푟표푑푢푐푡 퐴) 푆 = × 100 (22) 푝푟표푑푢푐푡 퐴 ∑ 푛(퐶 푏푎푠푒 표푓 푎푙푙 푝푟표푑푢푐푡푠 푎푓푡푒푟 푟푒푎푐푡푖표푛)

5 Results and discussions

The section of results and discussions is divided into two parts according to the two different reactions investigated. The first part involves the investigation of the effect of various solvents for oxidative glucose conversion using the Keggin-type POM catalyst (HPA-5). Another section deals with application Well-Dawson-type POM catalyst for selective glycerol hydrogenolysis.

5.1 Influence of organic solvents on selective glucose oxidation

The selective oxidation of glucose has already been commercialized with POM catalyst in aqueous solution (OxFA GmbH, Scheßlitz). However, FA selectivity is limited to less than 60% with primary glucose as substrate without an in-situ extracting agent. Therefore, the next step in the development of the OxFA-process was to improve the liquid product selectivity in order to make the process economically and technically feasible. Different solvent systems were tested for suitability in the process as well as the stability of the active catalyst. Therefore, all selected solvents were tested for compatibility with the HPA-5 catalyst. Subsequently, the effect of a model substrate glucose in all promising solvents was considered. The origin of FA formation was confirmed by using 13C-labelled glucose and the respective solvent. Moreover, optimization of vital parameters in the most promising solvent was carried out.

5.1.1 Solvents Screening

The as-synthesized HPA-5 catalyst was thoroughly characterized by ICP-OES (metal composition), XRD (structure), 31P and 51V NMR, FT-IR, and TGA (water content). The results are shown in Appendix 9.1.8.

The selection of all solvents for the investigation depended on the critical criteria for selective glucose oxidation. For this purpose, the HPA-5 catalyst was used as a model system to find out a suitable solvent. For principal suitability the following criteria were found essential;

• no azeotrope formation with formic acid (FA) or any liquid products; • significant boiling point difference between solvent and FA; • good solubility for substrate, catalyst, and oxygen; • stability under the process conditions.

Various solvents were chosen, ranging from polar protic solvents, such as methanol, ethanol, and propanol, to high-boiling, polar aprotic solvents such as DMSO. The system with pure water served as a benchmark for the other systems.

The comparison of the vapor pressure curves of the pure solvents under investigation, such as methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), n-butanol (n-BuOH), dimethyl sulfoxide (DMSO), are shown in Figure 36. All the lower and higher boiling solvents do not form a binary azeotrope with formic acid, unlike water. The solvents can be separated from formic acid by simple distillation.

Vapor pressure curves of various solvents and formic acid 3,8 LIQUID PL WATER 3,6 LIQUID PL METHANOL 3,4 LIQUID PL ETHANOL 3,2 LIQUID PL PROPANOL 3,0 LIQUID PL BUTANOL LIQUID PL PENTANOL 2,8 LIQUID PL HEXANOL 2,6 LIQUID PL DMSO 2,4 LIQUID PL FFORMI-01ORMIC ACID 2,2 2,0 1,8 1,6 1,4

Vapor pressure (bar) 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Temperature (°C)

Figure 36. Comparison of the vapor pressure among various solvents and formic acid (simulated using the UNIQUAC group contribution method in Aspen Plus).

First, the catalyst solubility was tested by adding 0.1 mmol HPA-5 in 10 mL of each solvent. For a stability test, the mixtures were processed by using the 10-fold batch reactor Hastelloy C276, high-pressure vessel autoclave at 90 °C under 20 bar oxygen with 1000 rpm for 24 h. After this treatment, the solutions were analyzed by using 31P-NMR, and 51V-NMR for investigation of oxidation stability. The analysis results of all solvents after oxidation are summarized in Table 3

The color of the solution gives a first indication of the oxidation state of the vanadium-based catalyst.[177] In the completely oxidized form, the color of the HPA-5 solution is orange, the oxidation state is +5. The oxidized HPA-5 can be transformed to be reduced form, it produces dissociated VO2+,[200] then the vanadium is in its +4 oxidation state. The color of the reduced HPA-5 form is dark blue. For the combination of oxidized and reduced HPA-5 in the reaction mixture, the color is green.

51V-and 31P-NMR spectroscopies were carried out to determine the structural stability of the HPA-5 catalyst in these stability tests. The assignment of vanadium resonances based on the previous work of Pettersson et al. [221]as well as the report of the Evtuguin group.[200,202] HPA-1, HPA-2, and HPA-3 were assigned to the main peak at -532, -542, and -546 ppm, respectively (see Figure 37). The different isomers of HPA-2 cause the overlapping peaks in the range of -510 and -530 ppm. The vanadium resonances at chemical shifts higher than -550 ppm were attributed to HPAs containing vanadium atom number higher than three. A group of overlapping peaks between -570 ppm to -600 ppm was assigned to the different positional isomers of HPA-4 and HPA-5. It is complicated to interpret overlapping spectra of the individual HPA isomer resonances around -545 due to a very broad peak of hydrated VO+.[221]

1.0 CK-GC-WAS-7R__51V-1.jdf HPA-1 HPA-2 0.9 HPA-4 and 0.8 HPA-5

0.7

0.6

0.5 HPA-3 0.4

Normalized Intensity 0.3

0.2

0.1

-0.1

-380 -400 -420 -440 -460 -480 -500 -520 -540 -560 -580 -600 -620 -640 -660 Chemical Shift (ppm)

Figure 37. 51V NMR spectrum for the synthesized HPA-5.

Figure 38 depicts the 31P NMR spectrum of different HPA-n (n=1-5), providing the information of the HPAs composition. The series of investigations[222] determined 31P NMR spectra of PVxMo12-xO40, which could be identified separately according to x: HPA [PVMo11O40]4– results in line 1, as shown in Figure 38, 2 represents the group of lines for HPA [PV2Mo10O40]5–, 3 the group of lines for HPA [PV3Mo9O40]6–, 4 the group of lines for HPA [PV4Mo8O40]7-, and 5 the group of lines for HPA [PV5Mo7O40]8-.

1.0 CK-GC-WAS-7R__31P_REOX-1.jdf

[PO4]3- 0.9

0.8

0.7

0.6 1

0.5 2

0.4 Normalized Intensity 0.3 3

0.2 4 5

0.1

2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0 -5.5 -6.0 Chemical Shift (ppm)

Figure 38. 31P NMR spectrum for the synthesized HPA-5. The peak marked [PO4]3- ions is due to the phosphoric acid inlets. The peaks in the shaded region belong to the [PV5Mo7O40]8- from the synthesized catalyst.

As indicated in table 3, the solvent systems were categorized into three groups. In the first group of solvents, HPA-5 was insoluble or partially soluble. This group includes n-heptanol, tetrahydrofuran, limonene, ethyl acetate, ethyl lactate, polyethylene glycol 200, tris(2-butoxyethyl)phosphate, diethyleneglycol-monomethyl ether, and tetraethyleneglycol-dimethyl ether, as shown in entry 16-24. These solvents cannot dissolve HPA-5 completely at the applied condition, and are not considered and discussed in the following. The second group includes solvents that dissolve HPA-5 completely and the color of the oxidative solutions was green. 31P-NMR and 51V-NMR results were different from the reference peak of the aqueous solvent. The solvents in this group include acetone, gamma-Valerolactone, Butyl acetate, Polyethylene carbonate, Trimethylphosphate, Tri-n-butyl phosphate, and Triethyleneglycol- monomethyl ether (entry 9-15). The last group showed promising features as solvent for the oxidation with the HPA-5 catalyst. The HPA-5 was excellent soluble in these solvents, and the color of the solution was orange or yellow. In addition, the 31P and 51V NMR spectrum showed no change when compared with the reference peak of water. The solvents in this group were methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, and dimethylsulfoxide (entry 2-8). Consequently, the further study of oxidation with a glucose substrate was performed using the solvents of this last group.

Table 3. The results of the oxidation stability tests of various organic solvents.

Entry Solvent Physical properties NMR Analysis solubility Color of 31P 51V of HPA-5 solution 1 Distilled Water + orange red reference 2 Methanol + orange red + + 3 Ethanol + orange yellow + + 4 n-Propanol + yellow + + 5 n-Butanol + orange yellow + + 6 n-Pentanol + orange yellow + + 7 n-Hexanol + orange red + + 8 Dimethylsulfoxide + red orange + + 9 Acetone + dark green - - 10 gamma-Valerolactone + dark green - - 11 Butyl acetate + dark green - - 12 Polyethylene carbonate + dark green - - 13 Trimethylphosphate + dark green - - 14 Tri-n-butylphosphat + green - - 15 Triethyleneglycol-monomethyl ether + dark green - - 16 n-Heptanol - orange 17 Tetrahydrofuran - blue green 18 Limonene - dark green 19 Ethyl acetate - dark orange 20 Ethyl lactate - dark green n/a 21 Polyethylene glycol 200 - dark green 22 Tris(2-butoxyethyl)phosphate - green 23 Diethyleneglycol-monomethyl ether - dark blue 24 Tetraethyleneglycol-dimethyl ether - dark blue

Reaction conditions: 0.1 mmol HPA-5 catalyst dissolved in 10 mL different solvents, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. Note: The representation of (+) to be good for using as oxidative solvent, (-) show bad effect

5.1.2 Oxidation of glucose in promising solvents

The catalytic oxidation of glucose in the promising solvents (methanol, ethanol, n-propanol, n-butanol, n-pentanol, n-hexanol, and dimethylsulfoxide) was performed in the 20 mL Hastelloy C276 10-fold batch reactor using 0.1 mmol HPA-5 catalyst and 1 mmol glucose in 10 mL of solvents. The system was purged with O2, then pressurized with O2 (20 bar) and preheated to 90 °C. After the reaction was completed (after 24 h), the reactor was cooled down to below 30 °C. The gas in the system was analyzed by using GC with TCD and FID. Then, liquid samples from each experiment were measured by 31P-, and 51V-NMR for investigating the oxidative stability.

Table 4. The results of glucose oxidation in promising organic solvents

Physical properties NMR Analysis Entry Solvent solubility Color of 31P 51V of glucose solution 1 Distilled Water + red orange + + 2 Methanol + red orange + + 3 Ethanol + red orange + + 4 n-Propanol + orange brawn + + 5 n-Butanol + orange brawn + + 6 n-Pentanol - orange brawn + + 7 n-Hexanol - orange brawn + + 8 Dimethylsulfoxide + yellow + + Reaction conditions: 1 mmol glucose and 0.1 mmol HPA-5 catalyst dissolved in 10 mL different solvents, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. Note: The representation of (+) to be good for using as oxidative solvent, (-) show bad effect

After completion of the reaction time, some residue of glucose could be observed in n-pentanol and n-hexanol (entry 6-7 in Table 4) due to a limited solubility. Therefore, these solvents were ignored for the following quantitative analysis. The 51V- and 31P-NMR were measured to confirm the stability of the HPA-5 catalyst after glucose addition as the substrate of the oxidation reaction. The addition of glucose into the solvent affected the re-oxidation of HPA-5 in the promising solvents. However, the rate of reduced HPA-5 transformation to oxidized HPA-5 form was higher than electrons transferring of oxidized HPA-5 to glucose. Therefore, all solvents still showed red/orange/yellow color after the glucose oxidation.

The quantitative analysis of liquid products and glucose conversion was carried out by using HPLC. The results of the glucose oxidation in different solvents are summarized in Figure 39. The results show that the use of alternative solvent systems leads to very interesting results. Methanol showed the best selectivity to formic acid (FA) and formic acid ester products of higher than 90 % with almost undetectable gas by-products. Also dimethylsulfoxide (DMSO) enabled a higher selectivity to FA than water (the benchmark solvent) with 54 %. However, this solvent still produced high amounts of CO2 and CO, which a caused a carbon loss instead of generating the desired product FA. For other alcohols such as ethanol, n-propanol, and n-butanol, lower FA and FA ester selectivity were found. Acetic acid was detected in the liquid phase as a by-product, including CO2 and CO in the gas phase. The most promising solvent system based on the experiments was clearly methanol. Therefore, in-depth studies were performed only with methanol solvent henceforth.

100 FormicFA* acid/methyl formate* 90 AceticAA** acid/methyl acetate** 80 COCO2 70 2 CO 60 total gas 50 total liquid 40 conversion (%mol) 30

20 Conversion/Selectivity Conversion/Selectivity (%) 10 0 water methanol ethanol n-propanol n-butanol DMSO Solvents

Figure 39. Quantitative analysis of liquid oxidation products from glucose by using different solvents. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL solvent, 20 bar O2, 90 °C, 24 h, 1000 rpm. *Formic acid partly transforms into methyl formate, ethyl formate, propyl formate, and butyl formate by using methanol, ethanol, n-propanol, and n-butanol solvents, respectively. **Acetic acid could transform into methyl acetate, ethyl acetate, propyl acetate, and butyl acetate by using methanol, ethanol, n-propanol, and n-butanol solvents, respectively.

Due to using alcohol as the solvent for the oxidative reaction, produced FA rapidly underwent esterification to form methyl formate (MF). Similar behavior was observed in all cases with other alcohol solvents such as ethanol, n-propanol, and n-butanol. They formed obtained ethyl formate, propyl formate, and butyl formate, respectively. In the case of produced acetic acid, this acid could also react with alcohol solvents via esterification to produce the esters such as methyl acetate, ethyl acetate, propyl acetate and butyl acetate, by using MeOH, ethanol, n-propanol, and n-butanol, respectively. More explanation of the subsequent esterification is provided in the next section (chapter 5.1.3). However, MF can be converted to FA in downstream processes. In addition, MF has high value for chemical industry (not lower than FA). Therefore, FA and MF are target products of glucose oxidation in this study.

5.1.3 Confirmation of the source of FA generation

The FA formation was expected to occur from glucose oxidation only. Nevertheless, FA could theoretically also be generated from methanol (MeOH) oxidation. In order to confirm the origin of FA under the reaction condition with MeOH as the solvent, 13C-glucose was used as feedstock to identify the carbon source of FA. In our investigation, three experiments were performed at the same conditions of 90 °C for 24 h (initial O2 pressure of 20 bar, 1000 rpm). Different substrate combinations of labeled and unlabeled compounds were used. A comparison of 13C-glucose as the substrate with 12C-glucose was performed. Next, a mixture of 13C-glucose and 12C-glucose (1:1) was investigated, and the last experiment studied the mixed solvent of 13C-MeOH and 12C-MeOH (1:9). These experiments distinctly proved that FA is 54

formed from glucose only with evidence of 13C-labelled results by GC-MS analysis (Figure 40). Methyl formate (MF) was immediately produced via the esterification as a continual consecutive product of FA given that FA was synthesized in the MeOH solvent and the presence of the HPA-5 catalyst. MF was the main product that could be observed clearly in the liquid product (see Figure 40).

1) GC chromatogram

2.A) MS-Oxi-Glu-MeOH-90°C-24h

2.B) MS-Oxi-Glu-(13:12MeOH=1:9)-90°C-24h

2.C) MS-Oxi-(13:12Glu=1:1)-MeOH-90°C-24h

2.D) MS-Oxi-13Glu-MeOH-90°C-24h

Figure 40. Chromatogram and mass spectrum of 13C-labelled oxidative liquid products by GC-MS analysis; 1) Chromatogram, 2) Mass spectrum of A) benchmark oxidative glucose in MeOH, B) oxidative glucose in 13C-MeOH:MeOH (1:9), C) oxidative 13C-glucose:glucose (1:1) in MeOH, D) oxidative 13C-glucose in MeOH. Reaction conditions: 1 mmol 13C-glucose/glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH/13C-MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. 55

Typical esterification involves a carboxylic acid, which in this work is FA with an alcohol such as our MeOH solvent in the presence of an acid catalyst (HPA-5). The equation for the reaction between FA and MeOH is shown in Scheme 7.

O O O + + HO O OH H H Formic acid Methanol Methyl formate Water

Scheme 7. Esterification of FA and MeOH

Figure 41. Methyl formate derived from MeOH (blue) and the acyl group (orange) derived from FA

The mass spectra showed the results of glucose oxidation; the benchmark 12-glucose oxidation presented m/z=60 (Figure 40.2A), while 13C-glucose oxidation obtained m/z=61(Figure 40.2D). One m/z increasing, is to be expected from the acyl group of synthesized 13C-FA, which originated from 13C-glucose transformation (the orange part in Figure 41). Moreover, the spectrum of mixed 13C-glucose and natural 12C-glucose (1:1) (Figure 40.2C) shows the FA generated from glucose with the m/z intensity ratio of 61:60 at about 1:1 ratio, as expected. Consistent with the results of glucose oxidation in 13C-MeOH:MeOH (1:9), it was evident that FA is not derived from MeOH as insignificant m/z=62 increasing was observed. This was double-checked by using headspace GC/MS (see in Appendix 9.1.10). Again, these spectra confirmed that MeOH is not a source for FA generation under the applied oxidative conditions.

However, produced MF can be hydrolyzed with water under the acid reaction conditions (see Scheme 8). In this case, acidic hydrolysis of MF to FA occurs owning to the presence of the strong acid catalyst HPA-5. For this reason, both MF and FA could be detected in the liquid phase as products glucose oxidation.

H+ O O + O + OH H H HO O Methyl formate Water Formic acid Methanol

Scheme 8. Acidic hydrolysis of MF to form FA.

5.1.4 Investigation of MeOH oxidation

While it was shown that methanol oxidation does not form MF under the applied conditions, it is known from the literature that via oxidation or dehydration, MeOH can 56

be transformed into formaldehyde (FM), dimethoxymethane (DMM), dimethyl ether (DME), methyl formate (MF), etc., as can be seen from Figure 42. [223] MeOH oxidation involves diversified parallel and consecutive reactions. The exact reaction routes depend on the properties of the used reaction system and the applied reaction conditions.

MeOH O O OH + H H MeOH Route A DME water

Route B O2 MeOH O O O O + H H Route B.1 FM DMM water

Route B.2 O2 MeOH O O O HO O + H H FA Route B.2.1 MF water

Route B.2.2 O O2 +O C- + H H carbonmonoxide water O + O C O H H carbondioxide water

Figure 42. Reaction routes concerned in MeOH oxidation.

Therefore, MeOH oxidation was carried out to find out possible side product which could be generated from solvent transformation under the applied oxidation conditions. For this purpose, 0.1 mmol HPA-5 and 10 mL MeOH were processed for 24 h at 90 °C and 110 °C, initial 20 bar oxygen pressure, and 1000 rpm in a batch experiment. The operation procedure in the 10-fold reactor system was carried out according to chapter 4.3.2.1. Samples were taken from both gas and liquid phases. The gas phase was determined by GC measurement, as shown in Figure 43. In addition, the liquid phase was checked for changes in the 1H-NMR and 13C-NMR spectra after the oxidation, according to Figure 44 and 45, respectively. The superimposed spectra of the anaerobic reaction at 110 °C are also compared.

3.A) GC-gas: without glucose at 110°C in O2 atmosphere

3.B) GC-gas: without glucose at 110°C in N2 atmosphere

Figure 43. Chromatogram of MeOH oxidation without glucose by GC-gas analysis; A) under O2 atmosphere, B) under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar gas atmosphere, 110 °C, 24 h, 1000 rpm.

As shown in Figure 43, DME and formaldehyde (FM) were detected in the gas phase at 110 °C (both with and without oxygen). The intensity of the DME signal under O2 atmosphere showed a higher almost doubled value compare to the reaction under N2. The progress of MeOH oxidation is consistent with route A and route B in Figure 42. Dehydration of two MeOH molecules takes place to generate DME. MeOH is also oxidized to formaldehyde (FM), which reacts with another MeOH molecule via sequential condensation to generate DMM according to route B.1.

Liquid phase analysis by 1H-NMR and 13C-NMR spectroscopy in Figure 44 and 45 indicates DMM and DME formation. At the same time, FA and CO2, which are products of a consecutive reaction, (route B, and B 2.2) were undetectable in both gas and liquid phases. It is noticed from the results of the analysis that MF and CO could not be obtained (route B2.1 and route B2.3, respectively). In the N2 atmosphere, DMM could not be produced even at a high temperature of 110 °C, as shown by 1H-NMR and 13C-NMR spectra in Figures 44 and 45. By raising the reaction temperature under O2 atmosphere, DMM formation obviously increased.

CK-Ox-BL-MeOH-O2-90C-B43_1H-1.jdf 0.60 water MeOH

0.55

0.50

0.45

0.40

0.35 Benzene-d6 C) At 110°C, 24h, N 0.30 2

0.25 Normalized Intensity

0.20

0.15 B) At 110°C, 24h, O2

0.10

0.05 A) At 90°C, 24h, O2

9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm)

Figure 44. Comparison of 1H-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 110 °C under O2 atmosphere, C) at 110 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90/110 °C, 24 h, 1000 rpm.

CK-Ox-BL-MeOH-O2-90C-B43_13C-1.jdf 0.8

0.7 MeOH

0.6

0.5

Benzene-d6 0.4

C) At 110°C, 24h, N2 NormalizedIntensity 0.3

0.2 B) At 110°C, 24h, O2

0.1

A) At 90°C, 24h, O 0 2

160 150 140 130 120 110 100 90 80 70 60 50 40 Chemical Shift (ppm)

Figure 45. Comparison of 13C-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 110 °C under O2 atmosphere, C) at 110 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90/110 °C, 24 h, 1000 rpm. 59

In this system, MeOH can be converted into high-value oxygenated chemicals such as DMM and DME. The upgrade of the product mix in the downstream process required low effort and energy consumption for product separation. Due to the advantage of a different boiling point of all products and solvent, purification steps can mitigate the complexity of the process in the downstream. As an environmentally friendly and nontoxic MeOH downstream product, DMM or methylal is extensively used in various industrial fields, for instance, pharmaceuticals, cosmetics, rubber, and paints.[224] It is used for blending into diesel fuel (DMM-blend). Moreover, DMM is the simplest form of the OME family, well-known as OME1, and was reported as a vital platform chemical reacting with formaldehyde for providing high yields of OME3-5. In the past few years, research involving OME3-5 synthesis has gained wide interest with a great number of patents and research. As a promising alternative fuel, OME3-5 can solve the problem of diesel exhaust. It shows physical-properties such as viscosity, cetane number, and vapor pressure, similar to standard diesel fuel.[225] Nowadays, commercial production of DMM is carried out via a two-step process, which depends on fossil feedstock. The first step involves the MeOH oxidation to formaldehyde (FM) on redox sites, followed by condensation of FM with methanol to yield DMM over acidic catalysts.[224,226,227] Future work should, therefore, develop this process for direct synthesis of DMM from the selective oxidation of methanol by using the HPA-5 catalyst.

5.1.5 Anaerobic conversion of glucose in MeOH

After confirmation of the origin of FA, and the detailed investigation of MeOH oxidation, a further experiment was the study of glucose conversion under anaerobic conditions. For this purpose, a pure nitrogen atmosphere was applied instead of oxygen. The substrate used for this set of experiments was water-soluble glucose. For the anaerobic conversion of glucose with the exclusion of oxygen, 10 mL of MeOH, 0.16 g/0.1 mmol HPA-5, and 0.18 g/1 mmol glucose were used. The reactions took place in the 10-fold Hastelloy high-pressure autoclave under a pure inert gas atmosphere (20 bar nitrogen) at 90 °C and 1000 rpm for 24 h. The reactor was flushed three-times with 10 bar of nitrogen to remove all oxygen from the reaction space. After the reaction, gas and liquid samples were taken and examined by GC, HPLC analysis and NMR spectroscopy (Figure 46 and 47).

CK-Ox-Glu-MeOH-N2-90C-B17_1H-1.jdf water MeOH 0.13 A) glucose oxidation at 90°C under O2

0.12

0.11

0.10

0.09

0.08 8.05 0.07 12.6HO O 0.06 FA

Normalized Intensity Benzene-d6 0.05

0.04

0.03 B) glucose oxidation at 90°C under N2 0.02

0.01

9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm)

Figure 46. Comparison of 1H-NMR spectra after glucose conversion; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 1mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90 °C, 24 h, 1000 rpm.

CK-Ox-Glu-MeOH-N2-90C-B17_13C-1.jdf MeOH A) glucose oxidation at 90°C under O2

0.30

O O 0.25 160.7 49.9 methyl formate

0.20

0.15 Benzene-d6

Normalized Intensity

0.10

0.05 B) glucose oxidation at 90°C under N2

170 160 150 140 130 120 110 100 90 80 70 60 50 40 Chemical Shift (ppm) Figure 47. Comparison of 13C-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 1mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90 °C, 24 h, 1000 rpm.

The results in Figures 46 and 47 show that the oxidation of glucose to MF is possible even without the external addition of oxygen into the system. Accordingly, the oxygen transferred to the substrate originates exclusively from the catalyst itself under anaerobic conditions. Only a stoichiometric reaction is possible since only one oxygen atom per cycle can be transferred from each HPA-5 molecule. Consequently, only traces of oxygenates are formed.

The observed reactivity can be explained through the the well-known Mars-van- Krevelen mechanism.[228] This Mars-van-Krevelen mechanism was first described in 2001 for the oxidation of aromatics and alkyl aromatics by HPA-2 in combination with oxygen[197] and could be confirmed for the oxidation of anthracene and xanthene by HPA-2.[198] The two-stage mechanism involves, first, an oxygen atom transfers from the HPA-n to the substrate, which is then oxidized. The HPA-n receives the electrons emitted by the substrate and is reduced in return. In the subsequent second step, this reduced species is then directly oxidized by molecular oxygen to the catalytically active species as shown in Equation (23) and (24). [197] Comparing with the oxidation of glucose in this work, it could be concluded that the reaction starts with an oxygen transfer from HPA-5 (POMox) to glucose (S) and then HPA-5 takes electrons discharged from glucose to yield oxidized products (SO) and reduced HPA-5 (POMred). Afterward, the reduced HPA-5 (POMred) is reoxidized by reaction with molecular oxygen. The reoxidation under the condition of adequate molecular oxygen is very fast and not the rate-determining step. As can be seen from the orange solution, it contains the oxidized HPA-5 form after glucose oxidation.

푆 + 푃푂푀표푥 → 푆푂 + 푃푂푀푟푒푑 (23)

1 푃푂푀 + 푂 → 푃푂푀 (24) 푟푒푑 2 2 표푥

In the case of a lack of molecular oxygen or in an anaerobic process, the mechanism of POM reduction and glucose oxidation is similar to the aerobic system, but the regeneration step is different. Owing to the lack of O2 the green color of the liquid solution indicates that HPA-5 acted in reduced form. This affects glucose conversion and product distribution.

Comparing the conversion in presence and absence of O2 condition (Figure 48), shows that conversion declined below 60%. Remaining glucose includes of undissolved glucose. Because of the limitation of glucose solubility in MeOH, a large amount of glucose is still observed in solid form. Other products that were detected, include glyoxal, glycolaldehyde, formic acid/methyl formate, as well as acetic acid/methyl acetate.

100 Formic acid/Methyl formate* 90 Acetic acid/Methyl acetate** 80 Glyoxal 70 Erythrose 60 Glycolaldehyde 50 COCO2 40 2 CO 30 total gas 20 total liquid

Conversion/Selectivity Conversion/Selectivity (%) 10 conversion (%mol) 0 Oxygen Nitrogen Gas

Figure 48. Glucose conversion and product distribution in presence and absence of oxygen. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2/N2 pressure, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH. ** Esterification/hydrolysis between acetic acid (AA) and methyl acetate (MeOAc) in MeOH.

In the absence of molecular oxygen (O2), the activity of oxidation was decreased not only for glucose oxidation but also for MeOH oxidation, according to Figure 46-47, and 49. Figure 49 illustrates the analysis result of the gas product, which exhibits only a few oxidized MeOH products such as DME and FM. The water content was lower than 1% in the anaerobic system, even if it was carried out at high temperature (110 °C), as evident from Table 5. At 110 °C, oxidation without glucose could yield 5% water consistent with the investigated result of MeOH oxidation in section 5.1.4. The results corresponded to the mechanism of MeOH oxidation, as illustrated by Figure 42. The increasing water content could occur from MeOH oxidation and FA esterification.

Table 5. Water content in liquid phase samples under anaerobic/aerobic conditions at various conditions (w/wo substrate, temperature, and reaction cycle)

Entry Substrate Temperature (°C) Gas Reaction cycle Water Content (wt%) 1 glucose 90 O2 1 5.4 2 - 110 O2 1 4.7 3 - 110 N2 1 0.9 4 glucose 110 N2 1 0.7 5 glucose 90 O2 3 12.4 Reaction conditions: 1 mmol/without glucose and 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar O2/N2, 90 °C, 24h, 1000 rpm.

A: GC-gas analysis: glucose oxidation at 90°C under O2 O

dimethy lether

B: GC-gas analysis: glucose oxidation at 90°C under N2

Figure 49. Chromatogram of MeOH oxidation without glucose by GC-gas analysis; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2 pressure, 90 °C, 24 h, 1000 rpm.

5.1.6 Mechanistic investigations of glucose oxidation in methanol

Mechanistic studies of glucose conversion are important to understand the transformation pathway of intermediates before eventually breaking down to FA. Therefore, glucose oxidation was monitored over time. The experiments were carried out in the batch process under 20 bar initial O2 pressure, using HPA-5 as the catalyst, at 90 °C for 1-48 h. After the end of the experiment and cooling of the reactor to room temperature, a gas sample was taken and analyzed by GC. The liquid phase was then analyzed by NMR for identifying the components. The quantitative analysis of oxidative products was carried out by HPLC measurement. The results of these experiments are summarized in Figure 50. Excellent glucose conversion was achieved already after 1 h reaction time. The formation of FA had a tendency to increase over the period of reaction. Moreover, erythrose, glyoxal, and glycolaldehyde contents, which are reaction intermediates, decreased to nearly zero after 24 h for glyoxal and 48 h for erythrose and glycolaldehyde.

100 Formic acid/Methyl formate* 90 Glyoxal 80 70 Erythrose 60 Glycolaldehyde

50 COCO22 40 CO 30 total gas 20 total liquid 10 conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 1 3 6 12 24 48 Reaction time (h)

Figure 50. Glucose conversion and product selectivities at different reaction times. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 1-48 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in methanol

Excellent selectivity with complete conversion could be achieved for glycose oxidation. This confirms the assumption that the initial C-C bond cleavage is catalyzed by higher-substituted heteropolyacids. It was a diol cleavage while it did not cleave between aldehyde and alcohol, as suspected in previous work. Generally, in homogeneous aerobic oxidation of the liquid phase, molecular oxygen reacts through a route of metal-catalyzed free radicals.[229] This includes the activation of oxygen molecules by cleavage of oxygen-oxygen bonds[230], the consumption of O2 for reoxidation of redox-active metals, or the use of reducing agents. The product distribution depends on the catalyst, the solvent, and the applied reaction conditions. As well-known, C-C bonds in compounds are less active than C-H bonds for oxidation reaction, even though bond disassociation energies of C-C bonds are lower. This reactivity differences can be explained by steric constraints and unfavorable frontier orbitals when active intermediate species interact with C-C bonds in molecules.[231] The C-C bond cleavage is an important step to understand glucose degradation. Especially, phosphovanadomolybdates of the Keggin structure, [PVxMo12-xO40](3+x)- for x = 5 (HPA-5) as catalysts, show oxidative dehydrogenation via an electron-transfer oxidation from the substrate to the polyoxometalate (POM). After that, HPA-5 is reoxidized by oxygen. These electron and oxygen transfer (ET-OT) typically involves a transfer of the electron from the substrate to the POM, followed by a transfer of oxygen from the reduced polyoxometalate to the substrate[232] which is known as Mars-van Krevelen reaction [228] under gas-phase oxidation at high temperature.

In the following, the assumed mechanism for FA formation is shown in three steps. In the first step, the C-C bond of the first diol in the glucose molecule is cleaved under acidic conditions (see Scheme 9). Two intermediates, namely glyoxal and erythrose are produced through dehydration.[115]

OH OH OH HPA-5 +1 +1 0 + O OH 0 O + 1/2 O + H +1 + HO 2 O +1 -H2O O OH OH OH Glucose Glyoxal Erythrose Scheme 9. Initial C-C bond cleavage in the glucose molecule to glyoxal and erythrose[233]

In the next step, the formed erythrose (2,3,4-trihydroxybutanal) is oxidized by another oxygen atom from another POM molecule to glyoxal and glycolaldehyde (2-hydroxyethanal). Again, water is split off. This reaction is shown in Scheme 10. In the final reaction step (see Scheme 11), the three intermediates are converted by further three oxygen molecules to FA. In the MeOH solvent, FA can be esterified immediately to form methyl formate (MF). Therefore, the MF/FA selectivity was almost 100%, according to this glucose degradation mechanism.

OH +1 HPA-5 +1 -1 0 O +1 OH + 1/2 O O +1 + O 2 -H2O O OH OH Erythrose Glyoxal Glycolaldehyde Scheme 10. Oxidation of the intermediate erythrose to glyoxal and glycolaldehyde[233]

+1 O +1 O Glyoxal +

+1 HPA-5 O O O +1 + 6/2 O2 6 O 6 MeOH Glyoxal Methyl formate + -1 O +1 OH Glycolaldehyde

Scheme 11. Oxidation of the C2 intermediates in selective glucose degradation to formic acid and consecutive esterification to form methyl formate in methanol. [233]

As mentioned in chapter 5.1.3, the chance for MF hydrolysis can be increased by generated water and an acidic catalytic system. The further FA transformation to MF was also investigated, see Table 6. The MF hydrolysis was found to be a function of the reaction time and the water content with no FA detectable after the first hour, while after 3 h, FA was found. With increasing reaction time, water content from oxidative reaction increased (from 1 h to 48 h reaction time, the water content increased from 2.64 to 6.23%, respectively) and this affected the degree of hydrolysis up to 56% after 48 h. Thus, FA, which is the product of MF hydrolysis, occurred due to prolonged oxidation time.

Table 6. Degree of hydrolysis of methyl MF with methanol in different temperatures

Entry Time (h) Conversion FA/MF Hydrolysis of MF Water content (%) selectivity (%) to FA (%) (wt%) 1 1 100 49 0 2.6 2 3 100 68 10 2.9 3 6 100 82 17 3.5 4 12 100 91 19 4.6 5 24 100 96 54 5.4 6 48 100 98 56 6.2 0 Reaction conditions: 1 mmol glucose and 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 C, 1-48h, 1000 rpm.

The results of 1H and 13C NMR analysis in Figure 51.A and 51.B, show the relationship between the MF hydrolysis and MeOH oxidation as a function of time. The orange area shows the chemical shifts of FA and MF. FA was first detected after 3 h from hydrolysis, while the progress of MeOH oxidation went along the reaction time, as can be seen from increasing DMM in the blue area (Figure 51). These NMR spectra are consistent with the results of quantitative analysis by HPLC and our MF hydrolysis studies (see Table 6).

To compare the C1 selectivity theoretically expected according to the proposed three-stage decomposition mechanism with those experimentally determined, the following series of experiments were carried out. 10 mL of MeOH and 0.16 g/0.1 mmol HPA-5 were weighed into the 10-fold high-pressure autoclave. Either 0.14 g /1.2 mmol of erythrose (70% solution) 0.04g/0.3 mmol of glycolaldehyde dimer, or 0.14 g/2.4 mmol of glyoxal (40% solution) were added. In each case, a reaction temperature of 90 °C, an initial oxygen pressure of 20 bar and a stirrer speed of 1000 rpm for 24 h was applied. The yields and selectivities achieved are summarized in Figure 52.

Time variation.esp 1 water O O 0.15 A) H NMR Analysis O O dimethoxymethane MeOH 0.14 HO O Methyl formate 0.13 F o r mi c ac i d 0.12 Benzene

0.11 (external standard) 48 h 0.10

0.09 0.08 24 h 0.07

Normalized Intensity 0.06

0.05 12 h

0.04

0.03 6 h

0.02 3 h 0.01 1 h

9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm)

Time variation-13C NMR.esp 13 MeOH 0.45 B) C NMR Analysis Benzene (external standard) 0.40

0.35 48 h 0.30

0.25 24 h

0.20 Normalized Intensity

0.15 12 h

0.10 6 h

0.05 3 h 1 h

170 160 150 140 130 120 110 100 90 80 70 60 50 40 Chemical Shift (ppm)

Figure 51. Investigation of the effect of MeOH oxidation to hydrolysis of MF; A) by 1H NMR analysis B) by 13C NMR analysis.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 60 Glycolaldehyde 50 COCO22 40 CO 30 total gas 20 total liquid

10 conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 Glycolaldehyde Erythrose Glyoxal Glucose Substrates

Figure 52. Glucose conversions and product selectivities with different intermediates as a substrate. Reaction conditions: 0.3/1.2/2.4 mmol glycolaldehyde/erythrose/glyoxal for pure substrate and 0.1:0.4:0.4 mmol for mixed substrate, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

The C2 body of glyoxal and C4 body of erythrose were completely converted under these conditions to the C1 compounds FA, while the C2 compound glycolaldehyde could not achieve full conversion in 24 h, as shown in Figure 52. The time to obtain full conversion of glycolaldehyde as a substrate was different from the others tested compounds despite of the same reaction conditions. It is likely that glycolaldehyde has a lower decomposing rate than glucose, erythrose, and glycolaldehyde. Therefore, it is conceivable that all intermediates could be converted to FA via the proposed mechanism, as explained previously. The cleavage of the diols is faster than glycolaldehyde oxidation corresponding to the result of time variation in Figure 50. The results show a higher amount of glycolaldehyde than erythrose and glyoxal contents at 6 and 12 h. In addition, the figure shows that all intermediates seem to yield FA.

5.1.7 Optimizing the reaction conditions for glucose oxidation in methanol

The optimization of the glucose oxidation in MeOH focused on maximizing FA/MF yield and minimizing the investment and operation costs. In order to identity optimal conditions in this study, parameters such as reaction temperature, catalyst amount, glucose concentration, oxygen pressure, and stirring speed were investigated.

5.1.7.1 Effect of reaction temperature

In the conversion of glucose, different intermediates are formed before the final decomposing to FA. Therefore, it is possible that there are different degradation paths. The temperature variation can influence the reaction path and, thus, also the yield. In a temperature range of 50-110 °C (50, 70, 90, and 110 °C), the effect of temperature on glucose conversion was investigated. As shown in Figure 53, almost all the temperatures showed a complete conversion of glucose within 24 h. The intermediates such as glyoxal, erythrose, and glycolaldehyde were detected most at 50 °C. From the results, it becomes clear that the temperature has an essential effect on the FA/MF yield. The intermediates were detected less at higher temperature while FA/MF selectivity was increasing.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 Glycolaldehyde 60 COCO2 50 2 CO 40 total gas 30 total liquid 20

10 conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 50 70 90 110 Temperature (°C)

Figure 53. Glucose conversion and product selectivities with different temperatures. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 50-110 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

A concern about using higher temperatures for glucose oxidation with HPA-5 is the degradation of the products, especially at the highest applied temperature, 110 °C. However, there was no significant decomposition of the products. FA decomposition to CO2 and CO did not occur, as shown by the result of GC analysis in Figure 54A. CO2 and CO were not detected in the gaseous product. Moreover, the analysis of the liquid phase by 1H NMR also assured that FA was stable under the used oxidative condition as displayed in Figure 54B.

As discussed previously, the generation of MF is a result of subsequent esterification of the FA product in MeOH after FA production. At the same time, water could occur during the MeOH oxidation, simultaneous DME and DMM formation, according to Route A and B1, as shown in Figure 42. There is a definite possibility that MF is hydrolyzed to FA by the appeared water in the presence of a strong-acid catalyst such an HPA-5. The high degree of MeOH oxidation influenced the detectable FA, as illustrated by Figure 54.

0.11 Temp variation-1H NMR.esp 1 A) H-NMR Analysis MeOH

0.10

0.09

0.08

0.07 Benzene 110 °C 0.06 (external standard)

0.05

Normalized Intensity 0.04 90 °C 0.03

0.02

0.01 70 °C

8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Chemical Shift (ppm) TempB) variation-13C13C-NMR NMR.esp Analysis MeOH 0.25

Benzene (external standard) 0.20

110 °C 0.15

Normalized Intensity 0.10

90 °C 0.05

70 °C 0

170 160 150 140 130 120 110 100 90 80 70 60 50 40 Chemical Shift (ppm)

Figure 54. Investigation of product decomposition at various temperatures; A) by 1H-NMR analysis and B) by 13C-NMR analysis.

The hydrolysis of MF at various of temperatures of 50, 70, 90, and 110 °C is listed in Table 7. It is apparent that raising temperature accelerates the activity of acidic hydrolysis. For example, increasing the temperature from 90 °C to 110 °C, water content rose from 5.4% to 11.8%, corresponding to the degree of hydrolysis from 54% to 62%. Higher hydrolysis from 33% to 62% was observed at higher temperatures from 50 °C to 110 °C.

Moreover, DMM formation was also dependent on the adjusted temperature. A raise in temperature from 50 to 110 °C increased DMM formation. This shows an attractive pathway for further development of the DMM or OME1 production as biofuels.

Table 7. Hydrolysis activity of MF to FA with water and water content of liquid phase product in different temperatures.

Entry Temperature Conversion FA/MF Hydrolysis of MF Water (°C) (%) selectivity (%) to FA (%) content (%) 1 50 100 60 33 3.6 2 70 100 82 46 4.9 3 90 100 96 54 5.4 4 110 100 99 62 11.8 0 Reaction conditions: 1 mmol glucose and 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 50-110 C, 24 h, 1000 rpm.

Apparently, using high temperature results in excellent yield to the target products FA and MF. Consequently, the temperature of 90 °C was chosen as the standard temperature for further optimization experiments.

5.1.7.2 Effect of catalyst amount

In this section, it was investigated whether a change in the catalyst concentration affects the product selectivity. For this purpose, five experiments were carried out with various amounts of the catalyst HPA-5. Other factors, such as substrate, and solvent amount, temperature, and pressure, were kept constant. 1 mmol glucose was used and 10 mL MeOH was added. The examined quantities of the catalyst were 0.005 mmol, 0.01 mmol, 0.05 mmol, 0.1 mmol, and 0.15 mmol. The temperature was defined 90 °C and 20 bar initial O2 pressure. From each test batch, an HPLC analysis was quantitatively evaluated for liquid-phase products, while the gas phase was analyzed by GC. Glucose conversion and product selectivity obtained with various catalyst amounts are shown in Figure 55.

It appears that the selectivity of FA/MF increased with a higher catalyst amount. The catalyst amount, therefore, has a significant influence on the selectivity. The FA and MF selectivity were almost doubled from 45 to 85% when the catalyst amount was increased from 0.01 to 0.05 mmol. Increasing catalyst content up to 0.1 mmol, can achieve FA/MF selectivity higher than 95%. It was unnecessary to increase the amount of catalyst higher than 0.1 mmol as this did not show improvement of FA/MF yield or

selectivity. Therefore, the optimum of the catalyst amount for glucose oxidation for further experiments was 0.1 mmol.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 Glycolaldehyde 60 50 COCO22 40 CO 30 total gas 20 total liquid 10 conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 0.005 0.01 0.05 0.1 0.15 HPA-5 amount (mmol)

Figure 55. Glucose conversions and product selectivities with different catalyst amounts. Reaction conditions: 1 mmol glucose, 0.005-0.15 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

5.1.7.3 Effect of glucose concentration

Various glucose concentrations should also be tested to obtain the highest performance for glucose oxidation. Five different glucose amounts were tested to examine the effect of relative glucose concentration in the system, such as 0.5 mmol, 1 mmol, 2 mmol, 4 mmol, and 6 mmol.

As shown in Figure 56, the range of glucose amount from 0.5 to 2 mmol, shows full glucose conversion under the applied conditions. Further increasing glucose to 4 and 6 mmol was tested too. The results were drastically changed as the O2 gas, which was applied at the beginning at 20 bar pressure, was almost fully consumed. It is noted that glucose conversion and product selectivity change when the O2 gas is insufficiently supplied for the oxidative reaction. As a result, the experiments which used 4 and 6 mmol were duplicated with a higher O2 amount (35 bar) to reinvestigate the efficiency of the process. The conversion of 4 and 6 mmol glucose could then achieve complete transformation and similar product selectivity compared to low glucose amounts (0.5-2 mmol). With these results, it is apparent that the oxidative system can be applied with a higher amount of glucose without the change of product selectivity if enough O2 is supplied.

100 Formic acid/Methyl formate* 90 80 Glyoxal 70 Erythrose 60 Glycolaldehyde 50 COCO22 40 CO 30 total gas 20 total liquid 10 conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 0.5 1 2 4** 6** Glucose amount (mmol)

Figure 56. Glucose conversions and product selectivities with different glucose amounts Reaction conditions: 0.5-6 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2 (**35 bar initial O2), 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

5.1.7.4 Effect of oxygen pressure

In the here-propose liquid phase process, all compounds have to dissolve in MeOH including O2. O2 pressure can promote the solubility of O2 in MeOH. The higher solubility of oxygen in MeOH was a factor that was considered for explaining the very high rate of oxidation in this solvent. Report of Battino et al.[234] and Quaranta et al.[235] demonstrate that the oxygen solubility of MeOH is 2.15 mM at 20 °C and 101.3 kPa partial pressure of oxygen, while water dissolves only 0.26 mM at identical conditions. Therefore, the oxygen solubility in the MeOH is eight times higher than in water (as a benchmark solvent).

The solubility of any gas component increases with its partial pressure, according to the Henry law.

퐻푖,푗푥푖 = 푝푖 (25)

From Equation 25, it follows that the system pressure must be increased to increase the O2 solubility in the methanol phase.

To study the reaction behavior as a function of O2 pressure, the standard approaches were run with oxygen pressures of 3, 5, 8, 10, 15, 20, and 25 bar. All experiments with the model substrate glucose were carried out in a 10-fold batch plant at a temperature of 90 ° C, and a stirring speed of 1000 rpm for 24 h. In each case, 0.1 mmol of catalyst and 1 mmol of substrate were dissolved in 10 mL of MeOH.

The results of the experiments under various oxygen pressures were compared to the standard experiment at 20 bar O2 pressure displayed in Figure 57.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 60 Glycolaldehyde 50 COCO22 40 CO 30 total gas 20 total liquid 10 conversion (%mol)

Conversion/Selectivity Conversion/Selectivity (%) 0 3 5 8 10 15 20 25 O Pressure (bar) 2

Figure 57. Glucose conversions and product selectivities with different oxygen pressures. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 5-25 bar O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

As evidenced by the results in Figure 56, the product selectivity is unaffected by O2 over a wide pressure range. The HPLC analysis of samples revealed complete conversion of glucose in the range of pressure 3 to 25 bar, and very high FA/MF yield in the liquid phase. It means that the pressure of O2 has an insignificant effect on glucose oxidation in MeOH using HPA-5. Nevertheless, at low O2 pressure, particularly below 8 bar, the product selectivity changed due to incomplete conversion. The target product selectivity dropped to 75% (at 3 bar) from 95% (at 8 bar). FA/MF yield was lower according to the decreasing O2 pressure. It is reasonable to assume that the system lacked O2 for driving further oxidation of intermediates.

The results seem to indicate that the lowest O2 pressure, which is adequate for the oxidation condition, is 8 bar. As mentioned in the earlier part, the system should be optimized towards the possibility of using atmospheric pressure with continuous O2 feeding to decrease investment and operation costs. To assure that all the experiments have sufficient O2 the further tests have been carried out at 20 bar pressure.

5.1.7.5 Effect of stirring speed

An option of increasing the oxygen transfer into MeOH is to increase the speed of the impeller stirrer. Increasing the stirrer speed leads to a better distribution of the O2 in the liquid phase. By increasing the interfacial surface, the diffusional flow of the O2 into the liquid phase can be improved. The stirring speed on the other side determines the hydrodynamic shear at the substrate surface. Increasing the agitation speed raises the contact area of the two phases, and eliminates the interfacial mass transfer resistance.[236] For this purpose, experiments were carried out, to vary the stirrer speed 200 and 500 up to 1,000 rpm. Figure 58 presents the results of this series of experiments.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 Glycolaldehyde 60 CO2 50 CO2 40 CO 30 total liquid 20 total gas 10 conversiton (%mol) Conversion/Selectivity Conversion/Selectivity (%) 0 200 500 1000 Stirring speed (rpm)

Figure 58. Glucose conversion and product selectivities with different stirring speeds. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 200-1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

The results of these experiments show that O2 transport limitation can be excluded under these reaction conditions. The total glucose conversion was quite constant after 24 h at all stirring speeds tested. The yield of FA/MF increased slightly as stirring speed increased from 200 rpm to 500 rpm, and the increasing rate was constant between 500 and 1,000 rpm. This means that it is unnecessary to increase the stirring speed higher than 500 rpm for the applied conditions.

5.1.7.6 Effect of various substrates

The representatives of biogenic material were studied by using model substrates of the three main components from lignocellulosic biomass. Test molecules were chosen as closely as possible to the structures of the three constituents in biomass. As described in section 2.1.2, the major components of lignocellulosic biomass are cellulose, hemicellulose, and lignin. The well-defined molecular structures of the applied test molecules enable a better understanding of the HPA-5 catalyzed reaction in MeOH on the gradual degradation of the carbon structure.[237] Another requirement for the model substrates was that they should be soluble in MeOH. Therefore, no additive had to be added for the experiments. Since cellulose is composed of linked glucose units, glucose was chosen as a model substrate for cellulose. Hemicellulose composes mainly of the five sugar monomers xylose, arabinose, galactose, glucose, and mannose; whereby xylose is the most abundant sugar. Therefore, xylose was selected as a model substrate for hemicellulose.[238] The structures of the main constituent’s cellulose and hemicellulose, as well as the selected model substrates, are shown in Figure 59.[239] The third major component, the phenolic macromolecule lignin, consists of the three major repeating structures sinapyl alcohol, coniferyl alcohol, and coumaryl alcohol. Possible model substrates are 4-hydroxy-3-methylbenzoic acid (4H3MB) or 3,4-dimethoxybenzyl alcohol (3,4-DMBA). 4H3MB is very similar in structure to the building blocks of lignin, sinapyl alcohol, coniferyl alcohol, and coumaryl alcohol. It was also used in the course of previous work as model substrate 76

for lignin. [237,240] The structures of lignin and these possible model substrates are illustrated in Figure 60.

Cellulose Glucose

Hemicellulose (Three typical hemicellulose structure) Xylose

Figure 59. Used model substrates of cellulose and hemicellulose in comparison with the main components of lignocellulosic biomass [238,239,241,242]

4-hydroxy-3-methylbenzoic acid (4H3MB)

Lignin

3,4-dimethoxybenzyl alcohol (3,4-DMBA)

Figure 60. Used model substrates of lignin in comparison with the main components of lignocellulosic biomass. [243–245]

In order to assess the conversion of different functional groups in MeOH-soluble sugars in the catalytic oxidation to FA, experiments with C5, C6, and C9 sugars or their derivatives were performed in the high-pressure autoclave. The investigation of the possible products was studied in MeOH at 90 °C. The experiments were performed with the model substrates glucose for cellulose, xylose for hemicellulose, and 3,4-DMBA for lignin.

For the conversion of glucose, xylose, and 3,4-DMBA in each case, 10 mL of MeOH, 0.1 mmol of HPA-5 as the catalyst, and 1 mmol of the respective substrates were tested. The reaction of the C6 and C5 compounds took place over a period of 24 h at 90 °C, 20 bar O2 pressure, and a stirrer speed of 1000 rpm.

First, the color of the reaction solutions after the oxidation was visually examined. As shown in Figure 5.0x, in Appendix 9.1.9, all oxidative solutions of various substrates were orange. The orange color shows the oxidized HPA-5 state. This indicates that all used model substrates were oxidized by using the HPA-5 catalyst in MeOH without preventing reoxidation of the catalyst.

As already mentioned, the investigation of the possible intermediates and degradation paths is based on the results of HPLC chromatograms. Figure 61 shows the evaluation of the experiment with the substrate xylose and the catalyst HPA-5 in MeOH. No other products could be detected apart from FA and MF. Over a reaction time of 24 h, it was possible to convert all model substrates completely except 3,4-DMBA. It was interesting to observe that the selectivity to FA/MF of all model substrates was above 95%.

The conversion of these substrates generated glyoxal, erythrose, and glycolaldehyde as intermediates as well as FA and MF as final products. The reaction pathway was shown in the three-stage decomposition process reported by Albert[115], as discussed previously in this work (section 5.1.6). According to the theory described there, glyoxal and erythrose were formed first in the degradation of glucose, which are further split, giving rise to glycolaldehyde and another glyoxal molecule. Glyoxal and glycolaldehyde are C2 molecules that can be further converted into FA and sequentially convert to MF. The results of the present work support this theory by the detection of glycolaldehyde, glyoxal, and erythrose, which were also tested in the following HPLC analysis.

The results of the series of experiments are shown in Figure 61. The series of experiments show that all tested substrates can be selectively reacted since no other products could be detected apart from FA/MF at full conversion. In the experiments over a reaction time of 24 h, it was possible to completely convert all C6 and C5 model substrates, independently of their functional groups, with almost the same FA/MF selectivity of above 95% to the desired C1 products.

100 Formic acid/Methyl formate* 90 Glyoxal 80 Erythrose 70 Glycolaldehyde 60 COCO2 50 2 40 CO 30 total gas 20 total liquid conversion (%mol) Conversion/Selectivity Conversion/Selectivity (%) 10 0 glucose xylose 3,4-dimethoxybenzyl alcohol Substrates

Figure 61. Conversions and product selectivities with different substrates. Reaction conditions: 1 mmol glucose/xylose/3,4-dimethoxybenzyl alcohol, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

5.1.8 Recycling of the catalyst system

In order to transfer the laboratory experiments into a technical process for FA production from biomass, catalyst recyclability was investigated in further series of experiments. To study the recyclability of HPA-5, the following series of experiments were carried out with glucose as the substrate in the 10-fold reactor. For the recycling of the catalyst in a batch reactor with glucose, 0.18 g/1 mmol glucose and 0.16 g/0.1 mmol HPA-5 were dissolved in 10 mL MeOH. This solution was then stirred for 24 h at 90 °C, 20 bar initial O2 pressure and 1000 rpm. After each reaction, the product mixture was checked for its composition. The liquid phase was analyzed by HPLC, and a sample of the gas phase was taken and examined by GC. Subsequently, the rest of the reaction mixture was added to the reactor with 0.18 g/1 mmol fresh glucose as well as fresh O2 and processed for 24 h under identical conditions. This procedure was repeated three times, and the product solution was quantitatively analyzed after each reaction. The FA and MF formed in each glucose oxidation were accumulated in the oxidative solution.

The results of these recycles of experiments of the HPA-5 catalyst with the biogenic raw materials glucose are summarized in Figure 62. From the results, it can be seen that the activity of the catalyst does not appreciably decrease over three cycles. The overall conversion of glucose remains almost constant at 100%; it only reduces gradually to 99% for the second run, and 98% for the third cycle. Considering the selectivities of FA/MF, these were also stable at 98-99% from run 1 to 3. The oxidative activity is consistent with the water content, as shown in Table 5. The water content of the third cycle for glucose oxidation was increased from 5.4 to 12.4 due to the accumulation of water content from three recycles. Overall, in the reaction with 79

glucose, no noticeable decrease in the activity and selectivity of the HPA-5 catalyst can be observed over several cycles. Besides the FA/MF product and CO2, no further products could be observed under these reaction conditions, which was advantageous for comparing the individual recycling steps. The reaction solution was yellow-orange after all steps, which confirms that under the chosen reaction conditions, a complete reoxidation of HPA-5 took place.

100 Formic acid/Methyl formate* 90 80 Glyoxal 70 Erythrose 60 Glycolaldehyde

50 COCO22 40 CO 30 total gas 20 total liquid 10 Conversion/Selectivity Conversion/Selectivity (%) conversion (%mol) 0 Cycle-1 Cycle-2 Cycle-3 Oxidative reuses

Figure 62. Glucose conversions and product selectivities with different substrates. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH.

5.2 Hydrogenation of glycerol using POM catalysts in aqueous phase

Application of polyoxometalate (POM) catalysts for hydrogenolysis of pure glycerol (GL) as a model of crude glycerol from biodiesel production was investigated in a second part of this thesis. Glycerol hydrogenolysis involves a two-step process, as mentioned in section 2.2.2.4; GL is dehydrated, providing a double bond at the beginning, following by the selective hydrogenation to yield the desired PDs. [6,163] The acidity of the catalyst dominates the position of the remaining hydroxyl group in the PD molecules. [6,163] Due to the strong acidity of WD POMs, they are suitable catalysts for the hydrogenolysis of GL. In addition, WD POMs can adjust the chemical properties by selective replacement of addenda atoms affording an active site. Substituted POMs with addenda atoms show advantageous properties such as high thermal stability and noticeable redox properties.[158,246] The screening of WD POMs with/without substitution of a noble metal such as Ru, Pt, and Pd was studied for GL hydrogenolysis. The framework metal in this study was tungsten owing to its high ability and stability, to promote the performance of hydrogenation reactions. Optimization of the conditions using the most promising WD POM catalyst was performed to find out the highest GL conversion and PD selectivity. The influence of GL concentration was studied. Then, the effect of reaction time was tested for the appropriate period. Investigation of temperature was also performed, including catalyst amount, hydrogen pressure, and stirring speed. The recycling of the catalyst will be also discussed in this chapter.

5.2.1 Screening of WD-POM catalysts

This study focuses on applying WD POMs, with tungsten as framework metal for GL hydrogenolysis. For this purpose, metal-doped α2-WD POMs were selected with different noble-metal doping such as Ru, Pd, and Pt as well as without doping for the comparison. The α2-WD POMs were synthesized by a procedure involving three steps. First-stage, isomeric pure α-phospho-tungstic-WD(α-WD) POM was synthesized in the form of its potassium salt α-K6P2W18O62, according to Nadjo et al. and an adjusted procedure by Graham and Finke.[216,247] Second-step, the monolacunary α2-phospho-tungstic-WD (α2-WD) POM was built as the potassium salt α2-K10P2W17O61 by the following method of Finke.[218] For obtaining M-WD (α2-KXP2MW17O61) in the last step, the hetero addenda metals (M=Ru, Pd, and Pt) were added into α2-WD structure according to the adaptation from previous protocols.[219,220,248,249] All catalysts such as α-WD, α2-WD, Ru-WD, Pd-WD, and Pt-WD were used for the studies of GL hydrogenolysis in a 10-fold screening plant with 20 mL stainless-steel high-pressure vessels equipped with magnetic stirring. The conditions for a typical reaction were 1.0 g GL, 0.10 g WD POM catalyst (with/without active metal 0.02 mmol), and 10 mL water. All components were charged into the reaction vessel. After the assembled system, it was purged with N2 and H2 gas (each gas three times) before compression to 50 bar as an initial pressure for the reaction. In the beginning, the stirring speed was set at 300 rpm, and then the heating system was turned on. The stirring speed was adjusted again to 1000 rpm when the desired temperature (200 °C) 81

was reached. After 24 h, the system was cooled down to room temperature, and the liquid- and gas-phase was sampled for analysis.

70 n-PrOH i-PrOH 60 1,2-PD 1,3-PD 50 Acetol 40 Acetone EtOAc 30 EtOH MeOH

20 CHCH44 CC2H62H6

Conversion/Selectivity Conversion/Selectivity (%) 10 CC3H83H8 other product 0 Total gas α-WD α2-WD Ru-WD Pd-WD Pt-WD Total liquid WD-catalyst Conversion (%)

Figure 63. Catalytic performance of WD-based catalysts in selective GL hydrogenolysis. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g WD, 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm.

The results of the preliminary catalyst screening are illustrated in Figure 63. The unsubstituted-metal WD-POMs (α-WD and α2-WD), were almost inactive for GL conversion. The presence of Pt and Pd in WD catalysts resulted in a low activity to transform GL with only 1-3% conversion while the highest activity of 26% was achieved by using the Ru-WD catalyst. However, all metal-substituted catalysts (Ru-, Pd-, and Pt-WD) obtained the highest selectivity for 1,2-propanediol (1,2-PD). In contrast, the main product was detected to be ethyl acetate and acetal for α-WD and α2-WD, respectively, without detectable 1,3-propanediol (1,3-PD) and 1,2-PD content. Among these catalyst systems, Ru-WD was the most promising catalyst for GL hydrogenolysis to produce PDs, with the highest selectivity for 1,2-PD (42%) and 1,3-PD (9%). In addition, the catalytic process by using Ru-WD catalyst also produced n-propanol (n-PrOH) in the liquid phase and methane (CH4) in the gas phase. Therefore, the hydrogenolysis of GL by using various Ru-based catalysts, was studied in more detail.

5.2.2 Comparison of metal-doped WD catalysts with benchmark systems

Further catalyst screening experiments were carried out. In the homogeneous system, the water-soluble metal precursors of WD-POMs like RuCl3, PdCl2, and H2PtCl6 were used. Moreover, commercial-supported catalysts such as Ru/C, Pd/C, and Pt/C were also tested as heterogeneous catalysts. The same noble metal amount (0.2 mmol) was used in each series of experiments in order to permit a direct comparison of the efficiency of different catalyst activities. The experiments were carried out similarly to

the previous study. Aqueous reaction mixtures were prepared by the dissolution of GL and catalyst in water. The hydrogenolysis of glycerol was performed in a 10-fold reactor, a parallel autoclave reactor system comprising of ten 20 mL vessels. The appropriate amount of catalyst (0.02 mmol active metal, 1.0 g glycerol) was added to 10 mL of the aqueous reaction mixture. The reactor system was purged three times with N2 (10 bar) and three times with H2 (20 bar). Then, the system was pressurized to the 50 bar initial H2 pressure, stirred at 300 rpm, and heated up to the reaction temperature of 200 °C. Stirring was increased to 1000 rpm when reaching the desired temperature. After the indicated reaction period of 24 h, the reactors were allowed to cool down to room temperature. The gas products were sampled and analyzed by the GC instrument, while 1H and 13C NMR analysis determined the liquid products.

Figure 64 illustrates the comparison of the product contribution by using noble metal-doped WD-POM with the benchmark catalysts. The results showed that Pt- and Pd-containing WD catalysts, including the metal precursors and the carbon-supported heterogeneous catalysis, resulted in inferior GL conversions (1%–7%). Pd/C showed the highest selectivity to 1,2-PD, whereas precursor PdCl2 obtained prominently n-PrOH in the liquid-phase product. For the Pt-based catalysts, the 1,2-PD was the primary product for using Pt-WD and Pt/C catalyst, with the homogeneous precursor H2PtCl6, n-PrOH was obtained mainly. The highest catalytic activities were received by using Ru-based catalysts. The tendency of these results provided a good agreement with earlier studies of Dasari and co-workers.[152] In comparison, RuCl3 and Ru/C both provided lower GL conversion (17%) and preferred the formation of the undesired CH4 gas product (44%). These results of using RuCl3 and Ru/C catalyst provide compelling evidence that both catalytic systems catalyze the undesired methanation reaction, possibly corresponding to the reaction route suggested by ten Dam and Hanefeld (Scheme 12).[6] The dehydration is a factor in thermal induction to generate acrolein. Afterwards, the decomposition to carbon monoxide (CO) and short-chain alkanes such as ethane (C2H6), propane (C3H8) follows. These products were detected in trace amounts after the reaction. Generated CO reacts with H2 over the active sites of the Ru catalyst to produce methane.

100 n-PrOH 90 i-PrOH 80 1,2-PD 70 1,3-PD 60 Acetol 50 Acetone 40 EtOAc 30 EtOH 20 MeOH

10 CHCH44

Conversion/Selectivity Conversion/Selectivity (%) 0 CC2H62H6

CC3H83H8 other product Total gas Total liquid Catalysts Conversion (%)

Figure 64. Catalytic performance of WD-based catalysts and other metal-based catalysts in selective GL hydrogenolysis. Reaction conditions: 1 g GL (11 mmol GL), 0.02 mmol active metal, 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm.

Scheme 12. Undesired reaction pathway using non-acidic Ru catalysts (Figure reproduced from Tendam et al.). [6]

On the contrary, the Ru-WD catalyst gave the highest selectivity to 1,2-PD (42%) with the highest GL conversion (26%) among the various tested catalytic systems. The Ru-WD catalyst predominantly produced the desired 1,2-PD as the primary reaction product, as earlier found by Huang et al. (Scheme 13).[250] Several acidic sites at the Ru-WD-POM promote the protonation process followed by acid- and thermally induced consecutive dehydration. Keto-enol tautomerization subsequently occurred, leading to form acetol as the reaction intermediate. Afterward, 1,2-PD was produced via selective hydrogenation. This reaction series is commonly considered as kinetically controlled as acetol is a relatively constant intermediate.[6,163,250]

Scheme 13. Desired reaction pathway using acidic Ru catalysts (Figure reproduced from Huang et al.). [250]

Table 8. Catalytic performance of WD-based catalysts and other metal-based catalysts in glycerol (GL) hydrogenolysis a.

Entry Catalyst Conversion pH c Product selectivity (%) b (%) 1,2-PD b 1,3-PD b CH4 d 1 Ru-WD 26 5.6 42 9 10 2 RuCl3 17 3.1 17 4 44 3 Ru/C e 17 4.1 31 1 44 4 Pd-WD 1 5.9 60 0 1 5 PdCl2 3 2.7 16 19 0 6 Pd/C e 2 3.4 74 0 0 7 Pt-WD 3 6.0 63 15 0 8 H2PtCl6 7 2.5 13 18 6 9 Pt/C e 6 4.1 78 6 0 10 α-WD 1 6.4 8 0 1 11 α2-WD 0 6.3 18 0 1 12 none 0 7.0 0 0 0 a 0 Reaction conditions: 1 mmol GL and 0.02 mmol active metal of catalyst dissolved in 10 mL water, 50 bar initial H2, 200 C, 24 h, 1000 rpm; other products of n-PrOH, EtOAc, Acetone, and Acetol were also observed in the experiments including small amount of i-PrOH, EtOH, MeOH, propane, and ethane. b GL conversion and liquid-phase selectivities were determined by 1H-NMR. c pH value of solution after the reaction. d Gas-phase selectivities were determined by GC. e 5 wt.% metal on activated carbon.

It was evident that the catalyst structure affected GL conversion and product selectivity. Interestingly, the moderate acidity (pH=5.6) of the reaction solution using the Ru-WD catalyst resulted in the highest activity of the GL conversion and product selectivity into PDs (see Table 8). As expected, the RuCl3 precursor showed the highest acidity of all used Ru catalysts resulting in a pH value of 3.1. Moreover, also Ru/C showed a higher acidity (pH = 4.1) than the Ru-WD catalyst, which obviously negatively affected both activity and selectivity.

From these results, it is apparent that the Ru-WD catalyst is a very interesting catalyst for GL hydrogenolysis. Therefore, the investigation of the optimization condition for GL hydrogenolysis was performed by using the Ru-WD catalyst.

5.2.3 Mechanistic investigation of glycerol hydrogenation

As outlined in the theoretical and technical background (section 2.2.2.4), glycerol (GL) hydrogenolysis is a chemical process that cleaves the chemical bond of C-C or C-O in the glycerol molecule, in particular, C-O bond breaking leads to desired products generation such as 1,2-propanediol (1,2-PD) and 1,3-propanediol (1,3-PD).[148] The catalytic GL hydrogenolysis involves a two-stage reaction process that compose of GL dehydration over the acid sites and hydrogenation into 1,2-PD or 1,3-PD. The initial dehydration pattern of glycerol is promoted by the acid contained in the catalytic systems; it is the vital stage in imposing the selective production of the catalyst. 1,2-PD will be generated if the GL molecule gets rid of one of the two primary hydroxyl groups. If the secondary alcohol is eliminated, GL hydrogenolysis can form 1,3-PD, as illustrated by Scheme 14.

Scheme 14. Reaction pathways for the generation of PDs using non-acidic Ru catalysts (Figure reproduced from Dam et al.). [207]

The first step of GL hydrogenolysis starts with dehydration to generate acetol or to form 3-hydroxypropanal or 3-hydroxypropionaldehyde (3-HPA), whereby this step is a thermodynamically preferent pathway (Figure 65).[251] In the second step, the intermediate products such as acetol and 3-HPA are hydrogenated to 1,2-PD and 1,3-PD, respectively.

Figure 65. Reaction energies for conversion of glycerol to 1,2-PD and 1,3-PD, and their intermediates and degradation products (Figure reproduced from Dam et al.). [207]

From the experimental results of the previous section, the main products detected were C2 and C3 alcohols such as 1,2-PD, 1,3-PD, 1-propanol (n-PrOH), 2-propanol (i-PrOH) and ethanol (EtOH), in accompany with the corresponding gaseous alkanes such as methane (CH4). Regardless of a catalytic system, 1,2-PD is the prevailing product, while 1,3-PD is detected only in low amount.[173,211] The formation of 1,2-PD is favored under thermodynamic control.

5.2.4 Optimizing reaction conditions using Ru-WD POM

According to the preliminary screening results, Ru-WD was found to show the highest performance for GL hydrogenolysis along with high selectivity to PDs. Hence, the Ru-WD catalyst was selected for further studies regarding the influence of various reaction parameters on the performance of the catalytic system. First, the effect of GL concentration in aqueous solution was investigated.

5.2.4.1 Effect of glycerol (GL) concentration

The hydrogenolysis of glycerol was performed in a 20-mL 10-fold stainless steel reactor equipped with a magnetic stirrer. The reaction mixture contained 10 mL of water and 1 g of GL as well as 0.10 g Ru-WD catalyst. The reaction system was purged with H2 and pressurized to 50 bar H2. The conditions for the experiments were at a temperature of 200 °C, stirring speed of 1000 rpm, and 24 h reaction time. After the required reaction time, the reactor was cooled down to room temperature. The products in the gas phase and the liquid phase were sampled and measured by GC and NMR analysis, respectively. The effect of GL concentration on GL conversion and product selectivity is displayed in Figure 66.

100 n-PrOH 90 i-PrOH 80 1,2-PD 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH MeOH 30 CHCH44 20

CC2H62H6 Conversion/Selectivity Conversion/Selectivity (%) 10 CC3H83H8 0 other product 5 10 20 50 Total gas GL concentration (wt%) Total liquid Conversion (%)

Figure 66. GL conversion and product selectivities with different GL concentration. Reaction conditions: 0.5-5.0 g GL (6-55 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm.

The conversion of GL decreases with increasing GL concentration in the aqueous solution. Higher dilution led to higher total GL conversion; using 5 wt.% GL in water achieved the highest GL conversion of 33%. Figure 66 Furthermore, at a low concentration of 5 wt.%, the main product was ethyl acetate (EtOAc). On the contrary, the 1,2-PD selectivity increased with raising the GL concentration from 5 to 50%, up to 65% at GL concentration of 50%. Noticeably, there were only two liquid by-products (n-PrOH and 1,3-PD) with selectivities above 10%. Only traces of all other by-products 87

such as i-PrOH, Acetol, EtOAc, EtOH, and MeOH were detected. Acetol was almost undetectable. Obviously, acetol favors transforming to 1,2-PD in the presence of the Ru-WD catalyst. Regarding the gas phase, methane selectivity increased with increasing conversion. These observations brought about the presumption that distinct reaction routes are favored at divergent GL concentrations, as previously reported by Dasari et al. for similar systems.[152]

For combining the highest yield of PD products and GL transformation, the concentration of 10% GL was selected to perform further studies as an optimized GL concentration for the GL hydrogenolysis.

5.2.4.2 Effect of reaction time

The kinetics of the reaction system were examined under the aforementioned optimized conditions. Six individual experiments were performed with reaction times between 6–120 h at 200 °C with 10 wt.% GL solution. The reaction of glycerol hydrogenolysis was performed in a 20-mL 10-fold stainless steel reactor, using a 10 mL aqueous solution of GL. The standard reaction was carried out under the following conditions: 200 °C reaction temperature, 50 bar initial hydrogen pressure, 10 wt.% aqueous glycerol solution, 100 mg of Ru-WD catalyst, and various reaction times of 6, 12, 24, 48, 72, and 120 h.

The results indicated an overall trend of rising conversion with increasing reaction time, providing a GL conversion of 50% after 120 h, as shown in Figure 67. The maximum 1,2-PD selectivity was 51% for the reaction period of 72 h. However, 1,2-PD selectivity is relatively stable for all reaction times. However, after 120 h, the selectivity to methane increased rapidly, reaching a maximum of 28%, while the 1,2-PD selectivity slightly decreased to 43%, implying that the consecutive hydrogenolysis of GL to methane was preferred at prolong reaction times. Interestingly, 1,3-PD, which is another PD product, showed also a constant selectivity in the range of 8-11% for 6-72 h. Except after 120 h, 1,3-PD selectivity decreased to 2% in an aqueous mixture.

100 n-PrOH 90 i-PrOH 1,2-PD 80 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH 30 MeOH CHCH4 20 4 CC2H62H6

Conversion/Selectivity Conversion/Selectivity (%) 10 CC3H83H8 0 other product 6 12 24 48 72 120 Total gas Time (h) Total liquid Conversion (%)

Figure 67. GL conversion and product selectivities with different reaction times. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 6-120 h, 1000 rpm.

Consequently, all further investigations were performed at an optimized reaction period of 24 h.

5.2.4.3 Effect of reaction temperature

The temperature range of 150-250 °C (150, 200, 225, and 250 °C) was varied to study the effect of temperature on GL hydrogenolysis. Due to the limitation of a high-temperature operation by using the 10-fold reactor, the 300 mL PARR reactor was used for the experiments at 225 and 250 °C.

Figure 68 shows that the conversion of GL rose with increasing temperatures (as expected) from 7% at 150 °C up to 33% at 250 °C. The selectivity towards 1,2-PD increased from 28% to 47% from 150 to 225 °C and then drastically declined to 10% at 250 °C. The selectivity shifted drastically towards n-PrOH as the main product (59%) instead. It is conceivable that due to excessive hydrogenolysis, a high degree of dehydroxylation product was formed, especially n-PrOH at the high reaction temperature. In addition, it appears that the selectivity also changed depending on the reaction temperature. At 150 °C, the main liquid products were 1,2-PD (28%), EtOAc (17%), n-PrOH (13%), and i-PrOH (12%), and the major gaseous product was methane (11%). All other products were observed in selectivities below 10%. The only other product formed at 225 °C was n-PrOH (18% selectivity). As shown in Figure 67, 225 °C was found an optimum temperature for the reaction system, giving the best compromise between GL conversion and 1,2-PD yield.

100 n-PrOH 90 i-PrOH 1,2-PD 80 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH MeOH 30 CHCH44 20

C2H6C2H6 Conversion/Selectivity Conversion/Selectivity (%) 10 C3H8C3H8 0 other product 150 200 225 250 Total gas Temperature (°C) Total liquid Conversion (%)

Figure 68. GL conversion and product selectivities with different temperatures. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 150-250 °C, 24 h, 1000 rpm.

As our 10-fold reactor cannot be used at a temperature above 200 °C due to the limitation of the system, we decided to investigate all further reaction at 200 °C which proved to be a very suitable temperature.

5.2.4.4 Effect of catalyst amount

In these experiments, the catalyst amounts were altered in order to validate if the GL conversion could be increased by higher catalyst loading. The effect of catalyst loading on the GL hydrogenolysis was studied by using a different weight of the catalyst (0.05, 0.1, and 0.2 g of Ru-WD) under otherwise identical reaction conditions. The experiments were performed at 200 °C, 1000 rpm under 50 bar initial H2 pressure, for 24 h using 1 g of glycerol dissolved in 10 mL water. The effect of catalyst amount on GL conversion and PDs selectivity is shown in Figure 69.

100 n-PrOH 90 i-PrOH 80 1,2-PD 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH MeOH 30 CH4CH4 20 Conversion/Selectivity Conversion/Selectivity (%) C2H6C2H6 10 CC3H83H8 0 other product 0.05 0.1 0.2 Total gas Catalyst amount (g) Total liquid Conversion (%)

Figure 69. GL conversion and product selectivities with different catalyst amounts. Reaction conditions: 1 g GL (11 mmol GL), 0.05-0.2 g Ru-WD (0.01-0.04 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm.

The higher catalyst addition indeed resulted in higher GL conversion. Nevertheless, the effect was not directly proportional to the added catalyst dosage as a 100% increase, was only achieved with a fourfold catalyst amount (0.05 to 0.2 g Ru-WD). It can be assumed that not all Ru in the Ru-WD catalyst was catalytically active. Another interesting observation was that increasing GL conversion seemed to be the non-acidic route (Scheme 12), causing higher undesired methane and ethane selectivity. Different from the acidic reaction route (Scheme 13), it can obtain high selectivity to PD products.

5.2.4.5 Effect of hydrogen pressure

The glycerol hydrogenolysis reaction generally takes place at elevated hydrogen pressures, hydrogen pressure is an essential parameter in the reaction. The pressure effect was investigated at 5, 25, and 50 bar total initial hydrogen pressures, respectively. The reaction was carried out under the following conditions: 200 °C reaction temperature, 24 h reaction time, 1000 rpm stirring speed, 10 wt.% aqueous glycerol solution, with 0.1 g Ru-WD catalyst added. The influence of the initial hydrogen pressure on GL conversion and product distributions in the hydrogenolysis of glycerol is shown in Figure 70.

100 n-PrOH 90 i-PrOH 80 1,2-PD 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH MeOH 30 CHCH44 20 CC2H62H6

Conversion/Selectivity Conversion/Selectivity (%) 10 C3H8C3H8 0 other product 5 25 50 Total gas Total liquid H2 Pressure (bar) Conversion (%)

Figure 70. GL conversion and product selectivities with different hydrogen pressures. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 5-50 bar initial H2, 200 °C, 24 h, 1000 rpm.

As expected, the GL conversion increased with increasing hydrogen pressure up to 50 bar. The selectivity to 1,2-PD was slightly higher at low hydrogen pressures of 5 and 25 bar. With the lowest hydrogen pressure, methanol, acetone and CO2 formed as main side products. In contrast, n-PrOH and 1,3-PD were the prevailing side products at higher H2 pressure, indicating that the desired acidic reaction pathway (Scheme 15) was preferred under higher hydrogen pressures.

5.2.4.6 Effect of stirring speed

The effect of stirring speed in the hydrogenolysis of GL was studied. The investigation was performed with three stirring rates: 200, 500, and 1000 rpm and otherwise typical reaction conditions: reaction temperature of 200 °C, 50 bar initial H2 pressure, 24 h, 1 g of glycerol, and 0.1 of Ru-WD in 10 mL water.

The results plotted in Figure 71 show that increased GL conversion and thus increased reaction rate (as a constant reaction time was applied) was observed with increasing stirring rate. This strongly indicates that lower stirring speeds (<500 rpm) led to diffusion-limited regime where the transfer of H2 into the liquid phase is a limiting step. Notably, 1,2-PD was the main product at all variations giving selectivities up to 50% with n-PrOH as the only major by-product in the liquid phase. The two products can be easily separated by distillation, as the boiling point of n-PrOH (97 °C) is far below that of 1,2-PD (188 °C). Consequently, downstream processing is potentially easy for upscaling the process to a technical scale.

100 n-PrOH 90 i-PrOH 80 1,2-PD 1,3-PD 70 Acetol 60 Acetone 50 EtOAc 40 EtOH MeOH 30 CH4CH4 20

C2H6C2H6 Conversion/Selectivity Conversion/Selectivity (%) 10 C3H8C3H8 0 other product 200 500 1000 Total gas Stirring speed (rpm) Total liquid Conversion (%)

Figure 71. GL conversion and product selectivities with different stirring speeds. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 200-1000 rpm.

5.2.5 Catalyst recycling

Catalyst reusability was investigated in order to assess of catalyst stability and the potential for implementation in industry. The catalyst reusability was studied in three cycles, without any separation process. Each sample was taken after reaction for analysis and then fresh GL was added to the previous batch. No make-up amount of fresh catalyst was added. The typical operation was carried out at similar conditions for all cycles namely at 200 °C, 50 bar initial H2 pressure, for 24 h.

The reusability of the Ru-WD catalyst was examined using reaction conditions of 50 bar H2 and 200 °C. The results shown in Figure 72 and Table 9 confirmed that the catalyst largely maintained its initial selectivity over the three recycling runs, whereas lower total GL conversion for each subsequent reaction run (from 26% to 10%) suggests decreasing catalyst activity. However, the initial GL concentration in each experiment increased gradually due to incomplete conversion with substrate successively added. Therefore, conversion alone does not give a fair comparison. Thus, for the second reaction run where 1.0 g GL substrate was added to the remaining solution from the first run containing 74% unconverted GL (i.e., 0.74 g), the initial substrate concentration was increased from 10 to almost 18 wt.%. Similarly, the initial GL concentration in the third reaction was around 25 wt.% after the addition of another 1.0 g of fresh GL to the solution after the second run. Taking the difference in GL concentrations into account and reporting the total amount of converted GL for each run revealed that the same quantity of GL was converted in each run, demonstrating that the catalyst activity remained constant. Hence, overall, the results found in the recycling experiments are well in line with the results obtained at the same GL concentration (Figure 66), and 10% GL conversion in the third recycling run does not imply catalyst deactivation.

100 n-PrOH 90 i-PrOH 1,2-PD 80 1,3-PD 70 Acetol 60 Acetone 50 EtOAc EtOH 40 MeOH 30 CHCH44

20 CC2H62H6 Conversion/Selectivity Conversion/Selectivity (%) 10 CC3H83H8 other product 0 Total gas 1 2 3 Total liquid Catalyst recycle (round) Conversion (%)

Figure 72. Effect of catalyst recycle to GL conversion and product selectivities. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm. Procedure of recycling experiments: addition of 1 g of fresh GL were charged into the apparatus after 24 h of the 1st and 2nd round.

Table 9. Recycling of Ru-WD catalyst in GL hydrogenolysis a.

Entry Conversion (%) b Converted GL (g) Product Selectivity in Phases (%)

Liquid b Gas c

PD PD

- -

PrOH

CH4

EtOH

C2H6 C3H8

EtOAc

MeOH

Acetol

1,2 1,3

n Acetone 1 26 0.26 42 9 17 1 0 1 7 1 10 3 7 2 d 14 0.24 49 11 20 2 0 0 6 1 4 1 4 3 e 10 0.25 54 7 14 0 0 0 8 3 6 2 5 a Reaction conditions: Glycerol (1.0 g), catalyst (0.02 mmol active metal), water (10 mL), 50 bar H2, 200 °C, 24 h, 1000 rpm; small amounts of i-PrOH, EtOH, MeOH, propane and ethane were also observed in most of the experiments. b GL conversion and liquid-phase selectivities were determined by 1H-NMR; c Gas-phase selectivities were determined by GC. d Second reaction run. e Third reaction run.

6 Summary and outlook

In order to develop biomass conversion to platform chemicals, polyoxometalates (POMs) were applied and studied as catalysts in homogeneous reaction systems. The investigations in this thesis were separated into two parts.

In the first part, the studies focused on the optimization of the OxFA process to increase formic acid selectivity to generate value-added products instead of CO2. For this purpose, a solvent variation was carried out. From the 24 solvents studied, methanol was found the most promising one for glucose oxidation in the presence of H8[PV5Mo7O40] (HPA-5). Under these conditions, complete glucose conversion and excellent formic acid (FA) and methyl formate (MF) selectivity were achieved. The FA product generated in methanol solvent was esterified subsequently to MF. In parallel, the presence and formation of water could cause MF hydrolysis to FA. Therefore, both FA and MF were detected and accounted as the main products of glucose oxidation. The results obtained with 13C-glucose and GC-MS analysis confirmed the glucose substrate as the only source for FA formation (and not the methanol solvent). In the independent investigation of methanol oxidation, we found that methanol oxidation can produce dimethoxymethane (DMM) and dimethyl ether (DME). The mechanism of glucose oxidation could be elucidated to proceed through the conversion of glucose to glyoxal and erythrose. In a second step, erythrose could be oxidized to generate glyoxal and glycolaldehyde. The glyoxal and glycolaldehyde produced were converted subsequently to FA/MF.

For the optimization of the catalyst system for the selective oxidation of glucose to FA/MF in MeOH, crucial factors were investigated to obtain both high glucose conversion and FA/MF selectivity. For the studied oxidation, a temperature of 90 °C and a residence time of 24 h provided excellent glucose conversion (100%) and FA/MF selectivity (96%). We found that complete conversion and high selectivity can be achieved in the presence of sufficient O2 (above 5 bar). The stirring speed of 500 rpm was sufficient to obtain full conversion and high selectivity for FA/MF (95%). Xylose and 3,4-dimethoxybenzyl alcohol as model substrates for hemicellulose and lignin, respectively, could also produce FA/MF in excellent yield in the catalytic oxidation. Moreover, the oxidation system showed the technically necessary recyclability, with stable glucose conversion and FA/MF selectivity over three consecutive catalytic cycles.

In the second part, the application of Well-Dawson-type POM (WD POM) catalysts for the selective hydrogenolysis of glycerol (GL) to propanediols (1,2-propanediol and 1,3-propanediol; PDs) was investigated. Catalyst screening of the three noble-metal- based catalysts Ru-WD, Pd-WD, and Pt-WD POM showed that Ru-WD POM was most selective for PDs (52%) and most reactive for GL hydrogenolysis (26% conversion). Comparison with the benchmark experiment using water-soluble metal precursors such as RuCl3, PdCl2, and H2PtCl6, and commercial-supported catalysts such as Ru/C, Pd/C, and Pt/C, Ru-WD POM showed the superiority of Ru-WD POM with regard to both activity and yield of PDs.

The optimization of GL hydrogenolysis using Ru-WD POM was performed to identify conditions for the highest GL conversion and selectivity for PDs. The GL concentration variation showed a noticeable effect on GL hydrogenolysis with increasing GL concentration lower GL conversion, but higher PD selectivity was observed as expected. The optimum GL concentration was found to be 10 wt.%. Increasing the residence time resulted in elevated hydrogenolysis activity, with rather steady PDs selectivity, except for longer than 120 h. The optimum residence time was 24 h based on energy consumption vs. outcome. Increasing the temperature gave rise to higher GL conversion, and slightly higher 1,2-PD selectivity. The optimum temperature for the applied GL hydrogenolysis was 200 °C. Increasing the catalyst amount did not affect the catalysis proportionally. The proper amount was 0.1 g Ru-WD to obtain reasonable GL conversion and PDs yield. H2 pressure and stirring speed also play a crucial role for both the product selectivity and the conversion. The process seems to be controlled by mass transfer. With the highest level of stirring speed 1000 rpm, and an initial H2 pressure of 50 bar, the highest GL conversion (26%) could be attained, although the PDs selectivity slightly dropped from 55% to 43%. Importantly, the Ru-WD POM proved reusable in three consecutive reactions, without exhibiting any apparent deactivation.

In summary, the application of POM catalysts for biomass conversion is a promising pathway for both glucose oxidation and glycerol hydrogenolysis. Glucose oxidation should be validated in a continuous process to improve the process economics and efficiency, an important aspect towards industrial implementation. As another finding, the DMM production via methanol oxidation by HPA-5, is promising and should be explored further. For glycerol hydrogenolysis, further improvement in reaction rates and product yields is necessary.

7 Zusammenfassung

Gegenstand der vorliegenden Dissertation waren Untersuchungen zur Anwendung von Polyoxometallaten zur Umwandlung von Biomasse in Wertprodukte. Reichlich vorhandene Lignocellulose-Biomasse und Glycerin als Nebenprodukt der Biodieselherstellung sind attraktive Ausgangsstoffe für die Herstellung von Biomass- Plattform-Chemikalien. Die Untersuchungen gliederten sich in zwei Abschnitte.

Im ersten Teil konzentrierte sich die Untersuchung auf die Verbesserung der Ameisensäure-Selektivität als Zielprodukt der homogen-katalysierten Glucoseoxidation unter Verwendung von H8[PV5Mo7O40] (HPA-5), einem Polyoxometallat vom Keggin-Typ. In dem etablierten OxFA-Prozess ist die Ausbeute an Ameisensäure bei der Umwandlung von Glucose als Cellulosemodellverbindung ohne Einsatz eines in situ-Extraktionsmittels auf weniger als 60% begrenzt. Die Wirkung verschiedener Lösungsmittel wurde untersucht, um die Ameisensäureausbeute in einem einphasigen Reaktionsgemisch zu erhöhen. Das vielversprechendste Ergebnis wurde mit Methanol als Lösungsmittel erzielt. Ein herausragendes Ergebnis der durchgeführten Studien zu Variation des Lösungsmittels zur Oxidation von Biomasse war, dass eine hervorragende FA/MF-Produktselektivität von 96% und eine vollständige Glucoseumwandlung in Methanol als Lösungsmittel erzielt werden konnten. Die Ergebnisse der Umsetzung von 13C-markierter Glucose bestätigten Glucose als Quelle der FA-Bildung. Entsprechend konnte gezeigt werden, dass das Lösungsmittel Methanol nicht zu Ameisensäure umgesetzt wurde. Der Mechanismus der Glucoseoxidation konnte mit der Umwandlung von Glucose zu Glyoxal und Erythrose erklärt werden. Die gebildete Erythrose wird anschließend zu Glyoxal und Glykolaldehyd oxidiert, welche wiederum in FA/MF umgewandelt werden.

Um optimale Bedingungen für die oxidative Reaktion in Methanol als Lösungsmittel zu finden, wurden zusätzlich die Auswirkungen von Verweilzeit, Temperatur, Sauerstoffdruck, Katalysatormenge, Substratkonzentration, Rührgeschwindigkeit sowie verschiedener Modellsubstrate untersucht. Darüber hinaus wurde das Katalysatorrecycling bei der Glucoseoxidation in Methanol untersucht, um die Überführung von Laborversuchen in ein technisches Verfahren für die industrielle Produktion zu ermöglichen. Für die untersuchte Oxidation konnten bei einer Temperatur von 90 °C und einer Verweilzeit von 24 Stunden ausgezeichnete Glucose-Umsätze und FA/MF-Selektivitäten erhalten werden. Wir fanden heraus, dass in Gegenwart von ausreichend O2 (oberhalb von 5 bar O2-Partialdruck) eine vollständige Umwandlung und eine hohe Selektivität (≥95%) erreicht werden kann. Eine Rührerdrehzahl von 500 Umdrehungen pro Minute führte zu einem vollständigen Umsatz sowie einer hohen FA/MF-Selektivität von 95%. Xylose und 3,4-Dimethoxybenzylalkohol als Modellsubstrate für Hemicellulose bzw. Lignin konnten bei der katalytischen Oxidation ebenfalls zu FA/MF umgesetzt werden. Darüber hinaus zeigte das Oxidationssystem die technische Recyclingfähigkeit durch stabile Glucose-Umsätze und FA/MF-Selektivitäten über drei Katalysezyklen. Alles in allem wurde ein vielversprechender Prozess für eine zukünftige industrielle Umsetzung erarbeitet.

Die erzeugte Ameisensäure kann zum einen mit Methanol unter Bildung von Methylformiat verestern, zum anderen kann das Methylformiat jedoch in wässriger Lösung und unter sauren Bedingungen auch hydrolysiert werden, um eine höhere FA-Ausbeute zu erhalten. Die Eigenschaften des FA- und MF-Gemisches, wie etwa das Siedeverhalten, deuten darauf hin, dass eine Produktreinigung unkompliziert ist. Letztere könnte mit geringen Investitionskosten als nachgeschalteter Prozess betrieben werden. Die separate Untersuchung der Methanoloxidation führte zu DMM und DME als Produkte, welche potenzielle Plattformchemikalien für die Produktion von alternativen Biokraftstoffen für die Zukunft sind. Die vorliegenden Untersuchungen zeigen auch eine zusätzliche interessante Einsatzmöglichkeit von Methanol - neben seiner Verwendung als Lösungsmittel zur Steigerung der Aktivität der Glucoseumwandlung und zur Erzielung einer außergewöhnlichen FA-Selektivität- auf: Mit DMM und DME wurden hochattraktive Nebenprodukte erhalten.

Der zweite Teil der Arbeit befasst sich mit der Anwendung von Wells-Dawson (WD) POM-Katalysatoren für die selektive Hydrogenolyse von Glycerin (das als Biodiesel-Nebenprodukt in Großen Mangen zur Verfügung steht) zu Propandiolen (1,2-Propandiol und 1,3-Propandiol; PDs). Das Katalysatorscreening der drei Edelmetallkatalysatoren Ru-WD, Pd-WD und Pt-WD POM ergab, dass das Ru-WD POM für die Glycerin-Hydrogenolyse am selektivsten (52%) und reaktivsten war (26% Umwandlung). Der Vergleich mit wasserlöslichen Metallsalzen wie RuCl3, PdCl2 und H2PtCl6 und kommerziell erhältlichen Trägerkatalysatoren wie Ru/C, Pd/C und Pt/C ergab, dass Ru-WD POM sowohl hinsichtlich der Aktivität, als auch der Selektivität überlegen ist.

Die Optimierung der Glycerin-Hydrogenolyse unter Verwendung des Ru-WD POM wurde durchgeführt, um die besten Bedingungen bezüglich eines hohen Glycerin-Umsatzes sowie hoher Selektivitäten von PDs zu identifizieren. Die Glycerin-Konzentration zeigte einen merklichen Effekt auf die Glycerin-Hydrogenolyse, wobei eine Erhöhung der Glycerin-Konzentration einen geringeren Umsatz ergab, wohingegen eine höhere PD-Selektivität auftrat. Die optimale Glycerin-Konzentration wurde mit 10% gefunden. Wie erwartet, konnte eine Verlängerung der Reaktionsszeit eine erhöhte Hydrogenolyse-Aktivität bei eher gleichbleibender PDs-Selektivität (mit Ausnahme von einer Reaktionszeit von mehr als 120 Stunden) bewirken. Daher erwies sich der Zeitraum von 24 Stunden als Optimum im Vergleich zwischen Ergebnis und Zeit, einschließlich des Energieverbrauchs. Durch Erhöhen der Temperatur konnte der Glycerin-Umsatz und die 1,2-PD-Selektivität leicht gesteigert werden. Daher sollte die optimale Temperatur für die angewendete Glycerin-Hydrogenolyse bei 200 °C liegen. Die zunehmende Katalysatormenge beeinflusste die Katalyse nicht direkt proportional. Die beste Katalysator-Menge betrug 0,1 g Ru-WD, um einen angemessenen Glycerin-Umsatz sowie PD-Ausbeuten zu erhalten. Der Einfluss von H2-Druck und Rührgeschwindigkeit spielte auch eine entscheidende Rolle, sowohl für die Produktselektivität, als auch für den Umsatz, da der Prozess durch den Stoffübergang kontrolliert wird. Mit der höchsten Rührgeschwindigkeit von 1.000 Umdrehungen pro Minute und dem anfänglichen H2-Druck von 50 bar konnte der höchste Glycerin-Umsatz (26%) erreicht werden, selbst wenn die PD-Selektivität leicht abfiel (von 55% auf 43%). Wichtig ist, dass sich das Ru-WD-POM in drei aufeinanderfolgenden Reaktionen unter optimierten 98

Bedingungen als wiederverwendbar zeigte, ohne dass eine nennenswerte Deaktivierung auftrat. Zusammenfassend erwiesen sich die hier gezeigten Anwendungen von POM-Katalysatoren als sehr vielversprechend für die Herstellung von Biomass-stämmigen Plattform-Chemikalien. Die Glucoseoxidation, die Glycerinhydrogenolyse, wie auch die DMM-Produktion durch Oxidation unter Einsatz von HPA-5, bieten neue alternative Wege für die künftige Entwicklung attraktiver Bioraffinerie-Konzepte.

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9 Appendix 9.1.1 List of figures

Figure 1. The cycles of CO2 for petroleum- and biomass-derived chemicals (modified from Alonso et al.).[3] ...... 1

Figure 2. Chemical Structure of Biomass Feedstocks (reproduced from Alonso et al.). [3] ...... 4

Figure 3. Lignocellulose composition: cellulose, hemicellulose and lignin (reproduced from Alonso et al.). [3] ...... 6

Figure 4. Chemical structure of Glucose ...... 7

Figure 5. Platform chemicals and fuels obtained from glucose (reproduced from [87])...... 9

Figure 6. Conversion of cellulose and glucose in water (reproduced from Song et al.). [88] ...... 9

Figure 7. Mechanism of two-step process for the conversion of hexoses (mainly glucose) to acetic acid (reproduced from Jin et al.). [107] ...... 11

Figure 8. Structural formula of formic acid ...... 12

Figure 9. The reaction of industrial methyl formate synthesis [112] ...... 15

Figure 10. The pathway for hydrothermal conversion of glucose into formic acid using H2O2 as the oxidant (reproduced from Jin et al.). [88,129] ...... 16

Figure 11. The Energetic concept of the OxFA process (reproduced from Albert). [115] ...... 17

Figure 12. Reaction system of substrate oxidation with heteropolyanions (Figure reproduced from Albert) [20]...... 17

Figure 13. Chemical structure of Glycerol ...... 18

Figure 14. Glycerol production through transesterification reaction (reproduced from Bagnato et al.) [141] ...... 19

Figure 15. A potentially crucial role of glycerol on biorefineries in future. [10] ...... 20

Figure 16. Glycerol as a platform for functional chemicals and fuels (reproduced from Pagliaro et al.). [10] ...... 20

Figure 17. Reaction routes of catalytic hydrogenolysis of glycerol using Cu-H4SiW12O10/SiO2 (Figure reproduced from Lee et al.). [148] ...... 22

Figure 18. Production of dipropylene glycol, tripropylene glycol, and polyether polyols produced by the reaction of propylene oxide and 1,2-PD. [149] ...... 23

117

Figure 19. The hydrogenolysis of propylene oxide for the 1,2- propanediol production. [149] ...... 23

Figure 20. The hydrogenolysis process has commercialized for the production of 1,2- propanediol. [149]...... 24

Figure 21. PTT synthesis by esterification method [166] ...... 25

Figure 22. 1,3-PD industrial production through the two-step process of hydration of acrolein. [149] ...... 25

Figure 23. 1,3-PD production via hydroformylation of ethylene oxide. [149] ...... 26

Figure 24. 1,3-PD production by hydroformylation of ethylene oxide. [167] ...... 26

Figure 25. 1,3-PD production by deoxygenation of glycerol. [149] ...... 27

Figure 26. Lindqvist structure [M6O19]n- displaying addenda and kind of oxygen atoms. Left: balls and sticks; Right: polyhedral. Gray is addenda; red is oxygen. [185] ...... 28

Figure 27. Keggin structure [XM12O40]n-. Left: balls and sticks; right: polyhedral. Gray is addenda; red is oxygen; blue is heteroatom. [185]...... 29

Figure 28. Anderson structure [XM6O24]n‑. Left: balls and sticks; right: polyhedral. Gray is addenda; red is oxygen; blue is heteroatom. [185]...... 29

Figure 29. Wells−Dawson structure [X2M18O62]n‑. Left: balls and sticks; right: polyhedral. Common naming of regions within the molecule are shown. Gray, addenda; red, oxygen; blue, heteroatom. [185] ...... 29

Figure 30. Scope of application POM catalysts for 2 types of biogenic feedstock ...... 33

Figure 31. Illustration of Hastelloy 10-fold batch reactor system: (1) Hastelloy reactor, (2) oxygen supply, (3) manometer, (4) pressure sensor, (5) heating plate with magnetic stirrer, (6) temperature controller, (7) magnetic stirring controller...... 39

Figure 32. P&ID of Hastelloy 10-fold batch reactor system...... 39

Figure 33. Illustration of Stainless steel 10-fold batch reactor system: (1) Stainless steel reactor, (2) heating plate with magnetic stirrer, (3) temperature controller, (4) pressure sensor, (5) manometer, (6) magnetic stirring controller...... 40

Figure 34. P&ID of Stainless steel 10-fold batch reactor system...... 41

Figure 35. Comparison of the vapor pressure among various solvents and formic acid (simulated using the UNIQUAC group contribution method in Aspen Plus)...... 49

Figure 36. 51V NMR spectrum for the synthesized HPA-5...... 50

118

Figure 37. 31P NMR spectrum for the synthesized HPA-5. The peak marked PO43- ions that came from the phosphoric acid inlets. The peaks in the shaded region belong to the [PV5Mo7O40]8- from the synthesized catalyst...... 51

Figure 38. Quantitative analysis of oxidative liquid products by using promising solvents Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL various solvents, 20 bar O2, 90 °C, 24 h, 1000 rpm. *formic acid could transform into methyl formate, ethyl formate, propyl formate, and butyl formate by using methanol, ethanol, n-propanol, and n-butanol solvents, respectively. **acetic acid could transform into methyl acetate, ethyl acetate, propyl acetate, and butyl acetate by using methanol, ethanol, n-propanol, and n-butanol solvents, respectively...... 54

Figure 39. Chromatogram and mass spectrum of 13C-labelled oxidative liquid products by GC- MS analysis; 1) Chromatogram, 2) Mass spectrum of A) benchmark oxidative glucose in MeOH, B) oxidative glucose in 13C-MeOH:MeOH (1:9), C) oxidative 13C-glucose:glucose (1:1) in MeOH, D) oxidative 13C-glucose in MeOH. Reaction conditions: 1 mmol 13C-glucose/glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH/13C-MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm...... 55

Figure 40. Methyl formate derived from MeOH (blue) and acyl group (orange) derived from FA ...... 56

Figure 41. Reaction routes concerned in MeOH oxidation...... 57

Figure 42. Chromatogram of MeOH oxidation without glucose by GC-gas analysis; A) under O2 atmosphere, B) under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2/N2, 110 °C, 24 h, 1000 rpm...... 58

Figure 43. Comparison of 1H-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 110 °C under O2 atmosphere, C) at 110 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90/110 °C, 24 h, 1000 rpm...... 59

Figure 44. Comparison of 13C-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 110 °C under O2 atmosphere, C) at 110 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90/110 °C, 24 h, 1000 rpm...... 59

Figure 45. Comparison of 1H-NMR spectra after glucose oxidation; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 1mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90 °C, 24 h, 1000 rpm...... 61

Figure 46. Comparison of 13C-NMR spectra after MeOH oxidation; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 1mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2/N2, 90 °C, 24 h, 1000 rpm...... 61

Figure 47. Glucose conversion and product distribution in presence and absence of oxygen. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2/N2 pressure, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH. ** Esterification/hydrolysis between acetic acid (AA) and methyl acetate (MeOAc) in MeOH...... 63

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Figure 48. Chromatogram of MeOH oxidation without glucose by GC-gas analysis; A) at 90 °C under O2 atmosphere, B) at 90 °C under N2 atmosphere. Reaction conditions: 0.1 mmol HPA-5 catalyst without glucose dissolved in 10 mL MeOH, 20 bar initial O2 pressure, 90 °C, 24 h, 1000 rpm...... 64

Figure 49. Glucose conversion and product selectivities at different reaction times. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 1-48 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in methanol ...... 65

Figure 50. Investigation of the effect of MeOH oxidation to hydrolysis of MF; A) by 1H NMR analysis B) by 13C NMR analysis...... 68

Figure 51. Glucose conversions and product selectivities with different intermediates as a substrate. Reaction conditions: 0.3/1.2/2.4 mmol glycolaldehyde/erythrose/glyoxal for pure substrate and 0.1:0.4:0.4 mmol for mixed substrate, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 69

Figure 52. Glucose conversion and product selectivities with different temperatures. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 50-110 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 70

Figure 53. Investigation of product decomposition in various temperatures; A) by 1H-NMR analysis and B) by 13C-NMR analysis...... 71

Figure 54. Glucose conversions and product selectivities with different catalyst amounts. Reaction conditions: 1 mmol glucose, 0.005-0.15 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 73

Figure 55. Glucose conversions and product selectivities with different glucose amounts Reaction conditions: 0.5-6 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2 (**35 bar initial O2), 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 74

Figure 56. Glucose conversions and product selectivities with different oxygen pressures. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 5-25 bar O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 75

Figure 57. Glucose conversion and product selectivities with different stirring speeds. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL methanol, 20 bar initial O2, 90 °C, 24 h, 200-1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 76

Figure 58. Used model substrates of cellulose and hemicellulose in comparison with the main components of lignocellulosic biomass [237,238,240,241] ...... 77

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Figure 59. Used model substrates of lignin in comparison with the main components of lignocellulosic biomass. [242–244] ...... 77

Figure 60. Conversions and product selectivities with different substrates. Reaction conditions: 1 mmol glucose/xylose/3,4-dimethoxybenzyl alcohol, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 79

Figure 61. Glucose conversions and product selectivities with different substrates. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm. *Esterification/hydrolysis between formic acid (FA) and methyl formate (MF) in MeOH...... 80

Figure 62. Catalytic performance of WD-based catalysts in selective GL hydrogenolysis. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g WD, 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm...... 82

Figure 63. Catalytic performance of WD-based catalysts and other metal-based catalysts in selective GL hydrogenolysis. Reaction conditions: 1 g GL (11 mmol GL), 0.02 mmol active metal, 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm...... 84

Figure 64. Reaction energies for conversion of glycerol to 1,2-PD and 1,3-PD, and their intermediates and degradation products (Figure reproduced from Dam et al.). [206] ...... 86

Figure 65. GL conversion and product selectivities with different GL concentration. Reaction conditions: 0.5-5.0 g GL (6-55 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm...... 87

Figure 66. GL conversion and product selectivities with different reaction times. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 6-120 h, 1000 rpm...... 89

Figure 67. GL conversion and product selectivities with different temperatures. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 150-250 °C, 24 h, 1000 rpm...... 90

Figure 68. GL conversion and product selectivities with different catalyst amounts. Reaction conditions: 1 g GL (11 mmol GL), 0.05-0.2 g Ru-WD (0.01-0.04 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 1000 rpm...... 91

Figure 69. GL conversion and product selectivities with different hydrogen pressures. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 5-50 bar initial H2, 200 °C, 24 h, 1000 rpm...... 92

Figure 70. GL conversion and product selectivities with different stirring speeds. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2, 200 °C, 24 h, 200-1000 rpm...... 93

Figure 71. Effect of catalyst recycle to GL conversion and product selectivities. Reaction conditions: 1 g GL (11 mmol GL), 0.1 g Ru-WD (0.02 mmol), 10 mL water, 50 bar initial H2,

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200 °C, 24 h, 1000 rpm. Procedure of recycling experiments: addition of 1 g of fresh GL were charged into the apparatus after 24 h of the 1st and 2nd round...... 94

9.1.2 List of tables

Table 1. Technical applications of formic acid. [115] ...... 13

Table 2. The information of chemical amounts for HPA-5 preparation...... 36

Table 3. The results of the oxidation stability test ...... 52

Table 4. The results of glucose oxidation in promising solvents ...... 53

Table 5. Water content in liquid phase samples under anaerobic/aerobic system with various conditions (w/wo substrate, temperature, reaction cycle) ...... 63

Table 6. Degree of hydrolysis of methyl MF with methanol in different temperatures ...... 67

Table 7. Hydrolysis activity of MF to FA with water and water content of liquid phase product in different temperatures...... 72

Table 8. Catalytic performance of WD-based catalysts and other metal-based catalysts in glycerol (GL) hydrogenolysis a...... 85

Table 9. Recycling of Ru-WD catalyst in GL hydrogenolysis a...... 94

9.1.3 List of schemes

Scheme 1. Typical reactions of formic acid. [115] ...... 12

Scheme 2. Formic acid production via direct hydrolysis of methyl formate. [115] ...... 14

Scheme 3. Polycondensation of oxo anions (tetrahedral) to polyanions (octahedral). [176] .. 28

Scheme 4. Substrate oxidation by HPA-n and HPA reoxidation with O2. [190] ...... 29

Scheme 5. Partial steps of an HPA-n catalyzed substrate oxidation according to ET-OT mechanism. [197][188] ...... 30

Scheme 6. Electrostatic potential of tungsten-based forms. [188,209] ...... 32

Scheme 7. Esterification of FA and MeOH ...... 56

Scheme 8. Acidic hydrolysis of MF to form FA...... 56

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Scheme 9. Initial C-C bond cleavage in the glucose molecule to glyoxal and erythrose[232] .. 66

Scheme 10. Oxidation of the intermediate erythrose to glyoxal and glycolaldehyde[232] ...... 66

Scheme 11. Oxidation of the C2 intermediates in selective glucose degradation to formic acid and consecutive esterification to form methyl formate in methanol. [232] ...... 66

Scheme 12. Undesired reaction pathway using non-acidic Ru catalysts (Figure reproduced from Tendam et al.). [6] ...... 84

Scheme 13. Desired reaction pathway using acidic Ru catalysts (Figure reproduced from Huang et al.). [249] ...... 84

Scheme 14. Undesired reaction pathway using non-acidic Ru catalysts (Figure reproduced from Dam et al.). [206] ...... 86

9.1.4 Overview of chemicals substances and suppliers

Model (biogenic) substrates, intermediates and products

Name of substrate Physical state at RT Supplier Test reaction D(+)-Glucose Solid Merck Oxidation D(+)-Glucose U-13C6 (99% 13C) Solid Eurisotop Oxidation D(+)-Xylose (≥98.0%) Solid Merck Oxidation 3,4dimethoxybenzyl alcohol Liquid ACROS Oxidation Glyoxal (40% sol. in water) Liquid Merck Oxidation Glycolaldehyde dimer Solid Aldrich Oxidation Erythrose Liquid Merck Oxidation Formic acid (99.5%) Liquid VWR Oxidation chemicals Methyl formate (97%) Liquid Alfa Aesar Oxidation Dimethoxymethane (98%) Liquid Alfa Aesar Oxidation Acetic acid (99.8%) Liquid Merck Oxidation Glycerol bidistilled (99.5%) Liquid Hydrogenation 1,3-Propanediol (99%) Liquid Alfa Aesar Hydrogenation i-Propanol (99.9%) Liquid Merck Hydrogenation n-Propanol (99.6%) Liquid VWR Hydrogenation chemicals Acetone (100.0%) liquid VWR Hydrogenation chemicals Propionic acid (≥99.5%) Liquid Sigma- Hydrogenation Aldrich Ethanol (0.05% H2O) liquid Merck Hydrogenation Methanol, 99.8% liquid Sigma- Hydrogenation Aldrich Acetaldehyde (≥99.5%) Liquid Merck Hydrogenation

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Solvents

Name of substance Physical state at RT Supplier Test reaction Methanol, 99.8% liquid Sigma- Oxidation Aldrich Methanol-13C, d3 (99%) liquid Deutero Oxidation Ethanol (0.05% H2O) liquid Merck Oxidation n-Propanol liquid VWR Oxidation chemicals n-Butanol liquid Merck Oxidation iso-butanol liquid Sigma- Oxidation Aldrich n-Pentanol liquid Merck Oxidation n-Hexanol liquid Merck Oxidation n-Heptanol, 99% liquid Alfa Aesar Oxidation Dimethylsulfoxide liquid Merck Oxidation Tetrahydrofuran liquid Merck Oxidation Acetone liquid VWR Oxidation chemicals Name of substance Physical state at RT Supplier Test reaction gamma-Valerolactone liquid ACROS Oxidation Limonene liquid Alfa Aesar Oxidation Butyl acetate, 99+% liquid Alfa Aesar Oxidation Ethyl acetate liquid Sigma- Oxidation Aldrich Ethyl lactate liquid Sigma- Oxidation Aldrich Polyethylene glycol 200 liquid Alfa Aesar Oxidation Polyethylene carbonate liquid Merck Oxidation Trimethylphosphate liquid Merck Oxidation Tri-n-butylphosphat liquid Merck Oxidation Tris(2-butoxyethyl)phosphate liquid Merck Oxidation Triethyleneglycol-monomethyl liquid Merck Oxidation ether Diethyleneglycol-monomethyl liquid Merck Oxidation ether Tetraethyleneglycol-dimethyl liquid Merck Oxidation ether Distilled Water liquid in-house Oxidation/ Hydrogenation

Chemicals for POMs synthesis

Name of substance Physical state at RT Supplier Test reaction Vanadium(V)oxide solid Alfa Aesar Oxidation Molybdenum(VI)oxide solid Alfa Aesar Oxidation (99.5%) Hydrogen peroxide (30%) liquid AppliChem Oxidation Phosphoric acid (25%) liquid Merck Oxidation Distilled Water liquid In-house Oxidation/ Hydrogenation 124

Solvent for analysis

Name of substance Physical state at RT Supplier Analysis Deuterated water liquid Merck NMR Deuterated DMSO liquid Merck NMR

Gases

Name of substance Physical state at RT Supplier Test reaction Oxygen gas Linde Oxidation Hydrogen gas Linde Hydrogenation Nitrogen gas Linde Oxidation/ Hydrogenation

Standard gas for calibration

Name of substance Physical state at RT Supplier Test reaction Methane gas Linde Oxidation Ethane gas Linde Hydrogenation Propane gas Linde Oxidation/ Hydrogenation

9.1.5 Calibration factors for quantifying the substrates and products from glucose oxidation

퐶(푠푢푏푠푡푎푛푐푒 푖) = 퐾. 퐼푛푡푒푛푠푖푡푦푠푢푏푠푡푎푛푐푒 푖

Substance Calibration factor K Calibration point Glucose 0.0846 5 Xylose 0.0895 5 3,4 Dmethoxy benzylalcohol 0.007 3 Erythrose 0.1874 5 Glyoxal 0.0698 5 Glycolaldehyde-dimer 0.0945 5 Formic acid 0.2156 6 Acetic acid 0.1784 5 Methyl formate 0.301 5

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9.1.6 Calibration factors for quantifying the products from GL hydrogenation

퐼푛푡푒푛푠푖푡푦푝푟표푑푢푐푡 푖 퐶(푝푟표푑푢푐푡 푖) = 퐾. 퐼푛푡푒푛푠푖푡푦퐷푀푆푂

Calibration factor K Tube 1,2- 1,3- No./point n-PrOH i-PrOH EtOH MeOH AA Acetol Ace GL PD PD DM-5/3 0.081 0.137 0.074 0.039 0.058 0.046 0.072 0.101 0.042 0.311 DM-8/3 0.082 0.142 0.077 0.044 0.058 0.048 0.076 0.103 0.042 0.337 DM-9/3 0.101 0.181 0.098 0.063 0.075 0.060 0.102 0.135 0.055 0.407 DM-10/3 0.014 0.023 0.013 0.007 0.009 0.009 0.013 0.018 0.007 0.055 DM-11/3 0.012 0.021 0.013 0.007 0.009 0.008 0.012 0.016 0.007 0.055 DM-12/3 0.014 0.022 0.012 0.006 0.008 0.007 0.012 0.016 0.006 0.055 DM-15/3 0.012 0.021 0.012 0.006 0.009 0.008 0.012 0.016 0.007 0.049 DM-16/3 0.014 0.023 0.013 0.006 0.009 0.008 0.012 0.017 0.007 0.053 DM-18/3 0.013 0.022 0.013 0.007 0.009 0.008 0.014 0.019 0.008 0.049 DM-19/3 0.011 0.020 0.012 0.006 0.008 0.008 0.013 0.017 0.007 0.052

9.1.7 Calibration data of gas chromatography

퐶(푠푢푏푠푡푎푛푐푒 푖) = 퐾. 퐼푛푡푒푛푠푖푡푦푠푢푏푠푡푎푛푐푒 푖

Substance Calibration factor K Calibration point Carbon dioxide 0.0001 5 Carbon monoxide 0.0002 5 Methane 0.00001 6 Ethane 0.000006 4 Propane 0.000004 4

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9.1.8 Characterization of synthesized catalyst

Information of measurement

Analysis Value Y(V), % 99 Y(Mo), % 99 P: V: Mo 1: 5: 7 Hydrate water, wt.% 16

Molecular formula H8(PV5Mo7O40) * 14 H2O

1000 synthesized HPA-5 900 HPA-5 (reference) 800

700

600

500

Counts 400

300

200

100

0 2.000 12.000 22.000 32.000 42.000 Position 2Ɵ Cu

Figure 1.C. XRD measurement of synthesized HPA-5 compared with HPA-5 reference.

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31 1.2 JA_KL_HPA-5_C6__51V-3.jdfP : Synthesized HPA5 1.1 31P : HPA5 (reference) 1.0

0.9

0.8

0.7

0.6

0.5

0.4 NormalizedIntensity

0.3

0.2

0.1

-0.1

-380 -400 -420 -440 -460 -480 -500 -520 -540 -560 -580 -600 -620 -640 -660 Chemical Shift (ppm)

Figure 2.C. 31P-NMR analysis of synthesized HPA-5 compared with HPA-5 reference.

CK-WAS-S-3R__31P_REOX-1.jdf51 1.2 V : Synthesized HPA5 51 1.1 V : HPA5 (reference)

1.0

0.9

0.8

0.7

0.6

0.5

NormalizedIntensity 0.4

0.3

0.2

0.1

8 6 4 2 0 -2 -4 -6 -8 Chemical Shift (ppm)

Figure 3.C. 51V-NMR analysis of synthesized HPA-5 compared with HPA-5 reference

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100

80 HPA-5 1054 70 884

60 781

50 T/% 40 948 30

νas Mo=O ν P-O ν Mo-O -Mo 20 as νas Mo-Ob-Mo as c

0 1100 1000 900 800 700 600 Wavenumbers/cm-1

Figure 4.C. FTIR analysis of synthesized HPA-5

9.1.9 Illustration of liquid phase products

Figure 1.Ox. Stability test of various solvents oxidation using HPA-5 catalyst under reaction conditions: without glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL various solvents (tested 24 solvents following the list of Table 1.), 20 bar initial O2, 90 °C, 24 h, 1000 rpm

Figure 2.Ox. Liquid product of glucose oxidation in promising solvents (water, DMSO, MeOH, EtOH, n-ProOH, n-BuOH, n-PeOH, n-HexOH) using HPA-5 catalyst under reaction conditions: without glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL promising solvents, 20 bar initial O2, 90 °C, 24 h, 1000 rpm

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Figure 3.Ox. Liquid product of glucose oxidation with various retention time under reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 1-48 h, 1000 rpm

Figure 4.Ox. Liquid product of glucose oxidation with various temperatures under reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 70-100 °C, 24 h, 1000 rpm

Figure 5.Ox. Liquid product of glucose oxidation with various HPA-5 amounts reaction conditions: 1 mmol glucose, 0.005-0.15 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm

Figure 5.Ox. Liquid product of glucose oxidation with various glucose amounts under reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 200-1000 rpm

Figure 5.Ox. Liquid product of glucose oxidation with various oxygen pressures reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 5-25 bar initial O2, 90 °C, 24 h, 1000 rpm

130

Figure 5.Ox. Liquid product of glucose oxidation with various stirring speeds under reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 100-1000 rpm

Figure 5.Ox. Liquid product of glucose oxidation with various substrates (glucose, xylose, and 3,4-dimethoxybenzyl alcohol) under reaction conditions: 1 mmol various substrates, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm

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9.1.10 Headspace GC-MS analysis of liquid products from 13C- label oxidation in MeOH solvent

The analysis results of headspace GC-MS analysis show spectra of the experiments using different 13C-label as substrate or solvent with application of HPA-5 catalyst. The liquid samples after oxidation were analysed by headspace GC-MS a Shimadzu QC 2010/ QP2010 SE GCMS-system equipped with CTC combi PAL headspace and separated with CP-Sil PONA CB column (50 m x 210 µm x 0.5 µm) and with liquid injection method.

OH Meth an ol

O O O O

Methyl formate dimethoxymethane

dimethyl ether

Figure 6.Ox. Chromatogram of 13C-glucose:glucose (1:1) oxidative liquid products by headspace GC-MS analysis; reaction conditions: 0.5 mmol 13C-glucose and 0.5 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm.

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Figure 7.Ox. Mass spectrum of 13C-labelled oxidative liquid products by GC-MS analysis;) Mass spectrum of A. benchmark oxidative glucose in MeOH, B. oxidative glucose in 13C-MeOH:MeOH (1:9), C. oxidative 13C-glucose in MeOH, D. oxidative 13C-glucose:glucose (1:1) in MeOH. Reaction conditions: 1 mmol glucose, 0.1 mmol HPA-5 catalyst dissolved in 10 mL MeOH, 20 bar initial O2, 90 °C, 24 h, 1000 rpm.

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