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DEGREE PROJECT ENGINEERING CHEMISTRY, SECOND CYCLE, 30 CREDITS
STOCKHOLM SWEDEN 2020,
Algae Hydrocarbons
Designed for Bio-based Lubricants
ELIN SJÖHAG
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ENGINEERING SCIENCES IN CHEMISTRY, BIOTECHNOLOGY AND HEALTH
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Algae Hydrocarbons Designed for Bio-based Lubricants
Elin Sjöhag
June 2020
Degree Project in Fibre and Polymer Technology KF200X
In collaboration with Nynas AB
Supervisor (KTH) and examiner: Prof. Ulrica Edlund Supervisor (Nynas): Dr. Elena Minchak
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Abstract
Lubricants are a necessity for machineries in order to reduce friction and wear. However, over 95% of the lubricants used today are fossil-based. Vegetable oil-based lubricants are available but often possess lower oxidative stability and poorer low temperature properties than their fossil-based counterparts. Vegetable oils are therefore not a perfect solution to reduce crude oil usage. Another obstacle to overcome would be a replacement of commonly used thickeners in semisolid lubricants with bio-based alternative, which has proven to be a challenging task Marine biomass representatives such as micro-and macroalgae have the potential to be used in the future as renewable feedstock sources due to their chemical compositions as well as beneficial cultivation conditions such as usage of non-arable land and saltwater. Microalgae have a high lipid content, and in some species a high content of hydrocarbons similar to crude oil. Macroalgae have a high content of polysaccharides, some with gelling abilities. Even though microalgae contain promising hydrocarbons and lipids that can be used in lubricant applications, it is currently not economically feasible to use microalgae to produce low value products. Macroalgae are also too expensive to cultivate to be used in low price products. In order to use polysaccharides as additives in oil, they need to be more amphiphilic. In this study, alginate, derived from brown seaweed, was first oxidized in a ring opening reaction to later be reduced in a Schiff base formation and reduction to introduce more hydrophobic side groups.
The results revealed a severe degradation of the polysaccharide both in the oxidation and in the reduction reaction, from a starting molecular weight of 580 000 g/mol to ~ 10 000 g/mol.
Ethanol was proved to be a suitable solvent in the oxidation reaction which increased the possible alginate concentration. Both FTIR and 1H-NMR results indicated a successful oxidation and reduction. Future work involves incorporation of the hydrophobically modified alginate in a base oil and evaluation of the presence of the obtained bio-based base oil component on the oil properties, for example viscosity increase, oxidation stability and thickening behaviors.
Key words: Microalgae, macroalgae, biorefinery, alginate, hydrophobic, lubricant, thickener, grease, bio-based
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Sammanfattning
Smörjmedel är en nödvändighet för maskiner för att minska slitage och energiförbrukning.
Dock är över 95% av de använda smörjmedlen i dag fossilbaserade. Smörjmedel baserade på vegetabiliska oljor finns tillgängliga men har ofta en lägre oxidativ stabilitet och sämre låg- temperaturegenskaper än deras fossilbaserade motsvarigheter. Ett annat hinder att övervinna är att ersätta vanligt använda förtjockningsmedel i halvfast smörjmedel med biobaserat alternativ vilket också visat sig vara en utmanande uppgift. Representanter för marina biomassa som mikro- och makroalger har potential att användas i framtiden som förnybar råvara källor på grund av deras kemiska sammansättningar såväl som gynnsamma odlingsförhållanden.
Mikroalger har ett högt lipidinnehåll och vissa arter har ett högt innehåll av kolväten som liknar råolja. Makroalger har ett högt innehåll av polysackarider med en förtjockningsförmåga i vatten. Även om mikroalger innehåller lovande kolväten och lipider som kan användas i smörjmedelsapplikationer är det idag inte ekonomiskt möjligt att använda mikroalger för att producera produkter med lågt värde. Även makroalger är för kostsamma att kultivera för billiga produkter, För att kunna använda polysackarider som tillsatser i olja måste de vara mer hydrofoba. I denna studie oxiderades alginat först i en ringöppningsreaktion, för att senare reduceras i en Schiff-basformation och reduktion till en mer amfifilisk polysackarid. Resultaten visade en hög nedbrytning av polysackariden både i oxidationsreaktionen och i reduktionsreaktionen, från en startmolekylvikt av 580 000 g/mol till ~ 10 000 g/mol. Etanol kunde användas i oxidationsreaktionen för att öka den möjliga alginatkoncentrationen. Både FTIR- och 1H-NMR-resultaten indikerade en lyckad oxidation och reduktion. Framtida arbete involverar inblandning av det hydrofobt modifierade alginatet i en basolja och utvärdering av effekten av den erhållna biobaserade basoljekomponenten på oljeegenskaperna, till exempel ökning av viskositeten, oxidationsstabilitet och förtjockningsbeteenden.
Nyckelord: Mikroalger, makroalger, bioraffinaderi, smörjmedel, alginat, hydrofob, förtjockningsmedel, biobaserad
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List of Abbreviations
Ca2+ Calcium ion
COOH group Carboxylic group
D2O Deuterium oxide
EDC N-(3-Dimethylaminopropyl)-N'-ethylcarbodiimide
EPS Extracellular polysaccharides
EtOH Ethanol
FTIR Fourier transform infrared
G Guluronic acid
HCl Hydrochloric acid
M Mannuronic acid
MT Million tons (106 * 1000 kg)
MWCO Molecular weight cut-off
N2 Nitrogen gas
NaBH3CN Sodium cyanoborohydride
NaBH4 Sodium borohydride
NaOH Sodium hydroxide
NH2-C6 Hexylamine
NHS N-hydroxysuccinimide
NMR Nuclear magnetic resonance
O2 Oxygen gas
OH• Hydroxyl radical
OH group Hydroxyl group
PGA Propylene glycol alginate
SEC Size exclusion chromatography
TGA Thermal gravimetric analysis
VI Viscosity index
VM Viscosity modifier
%(v/v) Volume %
%(w/v) Weight/volume %
%(w/w) Weight %
v Sample Naming Summary
SA Sodium alginate
POA-50 Periodate oxidized alginate, 50% molar ratio of periodate POA-100 Periodate oxidized alginate, 100% molar ratio of periodate
POA-100-P Periodate oxidized alginate, 100% molar ratio of periodate, 10%(v/v) 2- propanol
POA-EtOH Periodate oxidized alginate, 100% molar ratio of periodate, 1:1 of water- ethanol solvent
Am(6)-100 Periodate oxidized alginate (POA-100), reductive aminated with hexylamine
Am(6)-EtOH Periodate oxidized alginate (POA-EtOH), reductive aminated with hexylamine
Am(6)-100-F Periodate oxidized alginate, reductive aminated with hexylamine (Am(6)-100) and Fischer esterified with butanol
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Table of Content
1 Introduction ... 1
1.1 Aim ... 1
1.2 Nynas AB ... 1
2 Background ... 2
2.1 The Role of Base Oils in Lubricants ... 2
2.1.1 Base Oil ... 2
2.1.2 Properties of Lubricants ... 3
2.1.3 Additives in Lubricants ... 4
2.1.4 Viscosity Modifiers ... 4
2.2 Bio-based Lubricants... 5
2.2.1 Properties of Vegetable oils ... 5
2.2.2 Chemical Modification of Vegetable Oil ... 6
2.2.3 Bio-based Hydrocarbons as Lubricant Base Oil... 7
2.3 Greases and Thickener ... 8
2.3.1 Bio-based Grease ... 8
2.3.2 Cellulose as Thickener... 9
2.3.3 Chitin as Thickener ... 10
2.3.4 Isocyanide-functionalized Thickener ... 10
2.4 Marine Biorefinery ... 11
2.5 Microalgae ... 11
2.5.1 Microalgae Applications ... 12
2.5.2 Algae Lipid ... 12
2.5.3 Terpenes... 14
2.5.4 Alkenones ... 15
2.5.5 Insight on Economic Feasibility ... 16
2.6 Macroalgae ... 16
2.6.1 Macroalgae Applications ... 17
2.6.2 Insight on Economic Feasibility ... 18
2.6.3 Red Seaweed... 18
2.6.4 Green Seaweed and Ulvan ... 20
2.6.5 Starch in Lubricant Applications ... 21
2.7 Brown Algae and Alginate ... 21
2.7.1 Brown Algae ... 21
2.7.2 Seafarm ... 21
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2.7.3 Fucoidan ... 21
2.7.4 Laminarin... 22
2.7.5 Alginate ... 22
2.7.6 Applications of Alginate ... 23
2.7.7 Molecular Weight of Alginate ... 24
2.8 Chemical Modification of Alginate... 24
2.8.1 Degradation ... 24
2.8.2 Modification of Hydroxyl Group ... 25
2.8.3 Modification of Carboxylic Group ... 26
2.8.4 Ring Opening Oxidation ... 27
2.8.5 Schiff Base Formation and Reduction ... 29
3 Experimental ... 30
3.1 Materials ... 30
3.2 Periodate-mediated Oxidation – Protocol 1 ... 30
3.3 Periodate-mediated Oxidation – Protocol 2 ... 31
3.4 Reductive Amination... 31
3.5 Fischer Esterification... 32
3.6 Characterization of Alginate and Modified Alginate ... 32
3.6.1 Fourier Transform Infrared (FTIR) Spectroscopy ... 32
3.6.2 Nuclear Magnetic Resonance (NMR) ... 32
3.6.3 Size Exclusion Chromatography (SEC) ... 33
3.6.4 Thermogravimetric Analysis (TGA) ... 33
3.6.5 pH Measurement... 33
4 Results and Discussion ... 33
4.1 Periodate Oxidation ... 33
4.1.1 Fourier Transform Infrared Spectroscopy ... 34
4.1.2 Nuclear Magnetic Resonance ... 35
4.1.3 Size Exclusion Chromatography ... 37
4.2 Reductive Amination... 37
4.2.1 Fourier Transform Infrared Spectroscopy ... 38
4.2.2 Nuclear Magnetic Resonance ... 39
4.2.3 Size Exclusion Chromatography ... 40
4.3 Fisher Esterification ... 40
4.3.1 Fourier Transform Infrared Spectroscopy ... 40
4.4 Thermogravimetric Analysis ... 41
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5 Conclusion ... 43
5.1 Literature Study ... 43
5.2 Laboratory Work ... 43
6 Future Work ... 43
7 Acknowledgements ... 44
8 References ... 44
9 Appendix ... 53
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1 Introduction
Replacing fossil-based feedstock with a bio-based alternative is an ongoing trend with many new products on the market. Therefore, there is an increasing demand of research in this area.
Numerous different types of feedstock have been investigated, where macro- and microalgae seems to be promising alternatives 1. Lubricating oils are necessary for machineries in order to extend the service life and decrease energy consumption 2. There are bio-based alternatives for lubricating oils available, such as vegetable oil. However, over 95% of the used lubricants are based on petroleum because fossil-based lubricant oils perform better for a low price. There is therefore an interest in finding new alternatives to replace or improve the already existing bio- based lubricants 3.
Marine algae (e.g., microalgae and macroalgae) include several photosynthetic organisms. A few of the benefits of using algae as a renewable feedstock are that they are fast growing and can fix CO2 at a high rate. There is also no competition of agricultural land since it grows in water. Microalgae can be seen as a promising resource for biofuel but also for lubricants, due to the high lipid content and in some species high content of hydrocarbons 4. Macroalgae contain high amounts of carbohydrates 5, which are additionally a potential source for thickener in lubricants.
One interesting macroalgae is brown alga. It grows in nearshore costal water, with alginate as the main carbohydrate with dry content up to 40%(w/w) 1. Alginate contains carboxyl groups and hydroxyl groups in the backbone, which makes the polysaccharide water soluble but also a great candidate for chemical modifications 6. Other marine biorefinery products have been used for lubricants, but not alginate. Moreover, other polysaccharide derivatives such as ethyl cellulose and chitosan have previously been investigated as biodegradable thickeners in semisolid lubricants, which have shown good lubricity but low mechanical stability 7.
1.1 Aim
The aim for this project was first to perform an extensive analysis off the available literature on micro- and macroalgae to summarize the information on different components to be evaluated as a base oil source or additive. In particular, algae oil components from microalgae was approached as possible base oil components while carbohydrates from macroalgae as possible thickeners in lubricating greases. Moreover, it was desired to hydrophobically modify alginate, extracted from brown algae, through different synthesis routes to improve the mixability with oils.
1.2 Nynas AB
Nynas is a different kind of oil company which wants to use oil in different applications, not burn it. With around 1000 employees, production facilities in Europe and offices in over 30 countries, Nynas is dedicated to researching, producing, and supplying specialty naphthenic oils and bitumen for a growing global market. Nynas products bind, cool, isolate, lubricate, soften, and have long service lives, wholly recyclable in some applications. Research and Development is conducted by the Technology Center based at the Nynäshamn refinery. Nynas’
commitment to contributing to sustainable development is supported by involvement in
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numerous activities including investigating possibilities of using renewable feedstock for current applications.
2 Background
2.1 The Role of Base Oils in Lubricants
When two surfaces move relative to each other friction and wear will occur. Lubricants are therefore needed to reduce friction and wear 2. Based on different types of lubricants, they can be divided into liquids, semisolids (greases), and solids. Liquids are the most common type and consist of a mixture of a base oil and additives. Solid lubricants can be used as a fine powder coating or as an additive in liquid lubricants and greases. Lubricants are necessary for extending the service life of machineries and engines. In 2014, 41 million tons (MT) of lubricants were produced and the market is growing 8. The purpose of lubricants depends on the application and can serve numerous of important roles. For example, to separate moving plates, transfer heat, reduce friction, transfer powder, prevent corrosion, etc. The separation of the moving plates is essential, since it reduces both friction and wear 2.
2.1.1 Base Oil
There are three main types of base oils used in liquid and semisolid lubricants: Mineral, synthetic, and vegetable. Mineral oils are produced by refining crude oil and are without doubt the largest group of base oils used in lubricants. Mineral oil can further be divided into paraffinic and naphthenic base oil 9. The composition of crude oil varies and is mainly based on hydrocarbons and other organic compounds 10. The hydrocarbons present in crude oil are paraffinic (linear), naphthenic (cyclic), and aromatic, as seen in Figure 1. The hydrocarbons in the oil will affect the properties of the base oil and the formulated lubricant product 2. Mineral base oil contains different hydrocarbons with molecular weight between 300 and 600 g/mol.
The linear paraffinic hydrocarbons can be 15-30 carbons and the naphthenic hydrocarbons consist of 5-6 membered rings with side chains up to 20 carbons long 11.
Figure 1. Different hydrocarbon structures in crude oil, a) iso-paraffin, b) naphthene, c) aromatic. Redrawn from M. Torbacke et.al., 2.
Synthetic oils are obtained by means of chemical synthesis and are usually applied when special performance is required, for example very high temperature resistance. Common
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synthetic lubricants are synthetic esters, polyalkyleneglycols, polyalphaolefins, and silicones 9. Vegetable oils are derived from a variety of seeds, nuts, fruits, and cereal grains 12. Typical oils used in lubrication products are rapeseed, sunflower, castor, and soybean. Vegetable oils are a preferred option when there is a high risk of lubricant leakage out into the environment, hence a more environmentally friendly lubricant is required 9.
Lubricants are a mixture of a base oil and additives, and usually consist of mostly base oil.
Thus, the properties of the base oil will serve as a foundation for the properties of the lubricant.
Among the properties central for lubricants solvency, polarity, and oxidative stability should be specifically mentioned. Solvency refers to the ability to dissolve solids, liquids, or gases.
The solvency will affect the stability of the final lubricant product. Polarity of the base oil will affect the ability to interact with additives and the surface in contact with the oil. The chemical structure will affect the oxidative stability, where example double bonds are more easily oxidized. Different types of additives can be used to enhance or improve certain properties of the final lubricant product 2.
2.1.2 Properties of Lubricants
The properties of lubricants can be divided into performance, long life, and environmental properties. Performance properties include viscosity, thermal properties, low and high temperature properties, and air and water contamination sensitivity. These properties are central from the start of usage of the lubricant. The long-life properties will affect the service life of the lubricant itself and the material in contact with the lubricant. These properties include oxidation stability, hydrolytic stability, and corrosion inhibition. Environmental properties are important, and involves toxicity, biodegradability, long time effect to the environment, and acute influence on peoples or the environment 2.
The most significant property of lubricants is viscosity, which determines the thickness of the lubrication film and therefore the performance of the lubricant. The viscosity will change depending on the temperature, pressure, and shear rate. The dynamic viscosity refers to the proportionality factor between shear rate and shear stress. A rheometer can measure the dynamic viscosity. More commonly, kinematic viscosity is used to define lubricants, one reason being it is easier to measure. A capillary viscometer is used to measure the kinematic viscosity. Kinematic viscosity is defined as dynamic viscosity divided by the density.
Generally, the viscosity decreases with an increase of temperature and the lubricant gets therefore thinner with elevated temperatures 2. The viscosity index (VI) is defined as the relationship between viscosity and temperature, it is a dimensionless number that is often used to compare different oils. An oil with high VI number has a viscosity that is less affected by a temperature change. When oils are cooled down the viscosity increases since movements of the molecules change and become slower, until the oil is solidified. The pour point defines the lowest temperature at which a liquid can pour with gravity alone. At high temperatures flash point is important to be aware of for the safety reasons. It is the lowest temperature where the vapor auto-ignites over a heated oil 10. Lubricants will also evaporate when heated, and the volatility is defined as the lubricants ability to vaporize. It depends on the vapor pressure and a lower molecular weight base oil will vaporize more easily. Air and water will always come in contact with the lubricant and can in some cases cause problems. For example, air can cause foaming which can cause loss of lubricants and water contaminants can cause corrosion. When lubricants are used as cooling oils, thermal properties as for example heat capacity are important properties as well 2.
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The oxidative stability is vital for long life service and depends on the composition of the base oil. Oxidation is enhanced with contact of oxygen or metals at elevated temperatures. The oxidation process is a chain reaction, where hydrocarbons in the lubricant reacts with oxygen to form oxidation products, for example carboxylic acids. The carboxylic acids will later cleave and form radicals which will cause degradation of the lubricant. An antioxidant for example, can be used to stop the chain reaction. Another way that lubricants can degrade is by hydrolytic degradation, where lubricants containing ester bonds react with water to form alcohols and acids. The hydrolytic reaction rate increases with elevating temperatures and is catalyzed by metals. A lubricant with low hydrolytic stability should therefore not be used in environments where water is present. Corrosion inhibition properties are important for long service life, since corrosion not only causes damage to metal surfaces but also increases the rate of lubricant oxidation 2.
2.1.3 Additives in Lubricants
To obtain certain desired properties, several different additives are used in lubricants. The desired properties depend on the application of the lubricant. The additive can be bulk-active or surface-active, where it can have chemical or physical actions. The surface-active additives can protect the materials in contact from corrosion, wear, and reduce friction. They can also be used as extreme pressure additives. Corrosion inhibitor additives most commonly form a film on the surface of the metal, protecting against corrosion. There are also surface-active additives that act on liquid-liquid and liquid-gas interfaces, example defoamers that decreases the foam formation or emulsifiers that can be used when water is added for cooling purposes 2.
The bulk active additives are usually larger molecules. Some react physically with the lubricant, such as viscosity modifiers and pour point depressant. The other type interacts chemically with other compounds in the lubricant, examples are antioxidants and detergents.
Antioxidants slow down the rate of oxidation, which otherwise can cause premature degradation. All additives are synthesized to be soluble in the lubricant, however some have lower solubility in order to be active on the surface 2.
2.1.4 Viscosity Modifiers
Usually, the viscosity will at high temperature decrease rapidly which limits the temperature usage range for lubricating oils. One option to increase the viscosity is by using a lubricating oil of a higher viscosity. However, it is not always a better choice as it might affect low temperature properties of the final product. Therefore, viscosity modifiers (VM) are used, with the purpose of thickening the lubricant at higher temperatures but not increasing the viscosity too much at lower temperatures 13. Viscosity modifiers are polymer bulk additives that change the viscosity of the base fluid 2. The three vital features of VMs function are thickening efficiency, viscosity-temperature relationship, and shear thinning. There are generally two categories of VM, namely thickeners and viscosity index improvers. A variety of polymers have been used as VMs, including polyalkyl methacrylate, olefin copolymer, polyisobutylene, and hydrogenated styrene–diene 13. The molecular weight of the polymer can range from 10 000 – 150 000 g/mol, where larger polymer chain will increase the viscosity more 2. The concentration of added polymer affects the result and a higher concentration increases the viscosity more due to entanglement, aggregation, or micelle formation. A low concentration of polymers is desired, since polymers are more expensive than base oil 13. The choice of viscosity modifier depends on the application, type of base oil, required shear stability and needed viscosity grade 2.
5 2.2 Bio-based Lubricants
Replacing fossil-based lubricants with bio-based alternatives has received a growing interest, to manage the environmental problems linked to the usage of petroleum-based products. With escalating costs of cured oil and environmental concerns, there is an increasing need to develop renewable oil 14. Today, over 95% of lubricant oils are based of petroleum. It is estimated that a big portion of the used lubricants ends up into the environment which is a problem since most petroleum-based lubricants are not biodegradable. A bio-based lubricant is derived from a bio- based raw material, often vegetable oils but also animal fat. They are usually environmentally friendly, known to have good lubricity, high flashpoint, high resistance to shear, and high viscosity index. However, there are problems connected with replacing mineral oils with bio- based, which is why the usage of bio-based oils today is still limited. Bio-based oils often have poor low temperature properties and low oxidative stability. This problem can however be solved by suitable chemical modifications and addition of appropriate additives. Today, bio- based lubricants are a preferred choice in the areas where the lubricant is released into the environment during its service life 3. Castor oil is a good choice for lubricants due to its high viscosity and good performance at lower temperatures 15.
2.2.1 Properties of Vegetable oils
All vegetable oils consist of molecules of similar structures, triglyceride molecules, which comprises of two parts. The glycerol part comprises of three hydroxyl group linked together and the fatty acid part which is connected with a carbonyl group. The fatty acids range from 4 to 36 carbons, and can be saturated, monounsaturated, or polyunsaturated. Double bonds present in unsaturated fatty acids are reactive and research is focusing on modifying these double bonds 3. See Figure 2 for an example of a triglyceride molecule. As mentioned previously, vegetable oils have some limitations. Mainly low oxidative stability, hydrolytic instability, and poor low temperature fluidity. This can sometimes be solved either by using additives or by chemical modifications 16.
Figure 2. Chemical structure of a triglyceride with two saturated lauric fatty acids and one unsaturated oleic fatty acid. Redrawn from J. C. J. Bart, et.al., 14.
The most important structural features of triglycerides which affect the properties of biolubricants are the molecular weight, degree of unsaturation, and the presence of functional groups (e.g., ester linkage, carboxyl-, and hydroxyl groups). Increasing the molecular weight of biolubricants increases the VI, lubricity, and flashpoint. High molecular weight can also result in worse cold temperature properties and higher viscosity. Functional groups will affect the lubricity, mainly due to polarity effects. The order of influence of different functional
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groups on lubricity can be represented as followed:
COOH > CHO>OH>COOCH3>CO>COC. Hydroxyl groups can also act as free radical scavengers and therefore increase the oxidative stability 17.
The hydrolytic instability depends on the presence of ester linkages. The oxidative stability depends on several factors, such as, structure, extraction condition, contaminants (water, trace metals, radicals, and peroxides), storage condition, and presence of natural antioxidants. The carbon-carbon double bonds will cause rapid oxidation 14, which is the major cause of low oxidative stability in vegetable oils. However, saturation of too many double bonds will result in poor low-temperature properties. The low-temperature fluidity can be altered by other chemical modification, example branching. Linear molecules form more easily monolayers, hence, too much branching can also decrease the lubricity 17.
2.2.2 Chemical Modification of Vegetable Oil
There are three main positions in triglycerides that can be easily subjected to chemical modifications. Ester groups, β-hydrogen positions, and double bonds. There are numerous possible chemical reactions of vegetable oil. Examples of reactions of the ester moieties are hydrolysis, esterification, reduction, and amidation. Double bonds can react in a polymerization, oxidation, addition reaction, etc., 18.
The oxidative stability of vegetable oil is low and by reducing the amount of unsaturation the stability can increase. One way to reduce saturation is by estolides formation 18. Estolides are being produced from the reaction based on an unsaturation in one fatty acid molecule and a carboxylic group in another fatty acid. Estolides have improved thermal oxidative stability and low-temperature properties compared to vegetable oil. One disadvantage with vegetable oils is the narrow viscosity range, compared to mineral oil-based lubricants with a wide range of viscosities. Estolides can be an alternative, with a variety of viscosities depending on chemical structure, polymerization degree, etc., 19. See Scheme 1 for an example of an estolide formation from oleic acids.
Scheme 1. Formation of estolides from oleic acid, redrawn from S. C, Cermak et.al., 20. The hydrogen atoms on the β-carbon position on the alcohol fragment in ester molecules are well known to lead to poor oxidative and thermal stability. The thermal decomposition of these β-hydrogens will produce products that can undergo polymerization, which will increase the viscosity and result in precipitates. Additives can only partially reduce this problem. A solution to the problem is to replace the glycerol with an alcohol that does not contain β-hydrogen atoms. In Scheme 2 the thermal decomposition of esters with and without β-hydrogen atoms
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is compared. The esters without β-hydrogen atoms can also decompose at high temperature, but has radical character and will therefore be a slower reaction 18.
Scheme 2. Thermal decomposition of esters with and without β-hydrogen, redrawn from J. C.
J. Bart 18.
High degree of branching will give a lubricant with excellent low-temperature properties and a more linear will have poor low-temperature properties. A high saturation will have good oxidative stability but poor low-temperature stability. Hence, the desired chemical structure of the base oil will depend on the desired properties of the lubricant. When modifying vegetable oil, the end product can perform better. However, the added processing cost required for modification will likely limit the commercial use modified vegetable oil 18.
Synthetic ester formation can also be seen as an opportunity for improvements of bio-based lubricants. Synthetic esters have many technical advantages as lubricants compared to mineral base oils. Bio-based carboxylic acids and alcohols can also be used to produce a broad variety of bio-based synthetic ester oils. Ester lubricants have good thermal and oxidative stability and have increased biodegradability compared to mineral oil. However, preparation of esters is more complicated in comparison to the synthetic lubricant polyalphaolefin 18.
2.2.3 Bio-based Hydrocarbons as Lubricant Base Oil
Renewable hydrocarbon-based biolubricants have many attractive qualities such as very good cold-temperature properties and both oxidative and hydrolytic stability. The hydrocarbon- based biolubricant is also similar to commonly used lubricant products. Iso-alkanes in the C24–
C50 range are examples of bio-based hydrocarbons used as lubricants. Iso-alkanes lack unsaturation sites and ester linkage and have therefore excellent oxidative and hydrolytic stability. The alkyl branches in the molecular structure will provide good cold-flow properties.
Iso-alkanes also lack polar groups and are therefore compatible with mineral oil-based systems.
Higher molecular weight will provide higher VI, lubricity, and flashpoint due to a decrease of volatility. However, poor cold-flow properties are linked to higher molecular weight 17. Biolubricants have many advantages, low content of contaminants and a variety of chemical structures that can be altered. The currently used biolubricants are mostly represented by esters, however renewable hydrocarbons are superior due to improved stability, good compatibility with additives, etc. Several methods are being analyzed in order to produce alkanes from vegetable oils. Among where hydrodeoxygenation is evolving to be the main process. The process involves first saturating and decomposition of triglycerides, which are after
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decarboxylated and possible decarbonylated to form saturated hydrocarbons with a chain length of C11 – C20 17.
2.3 Greases and Thickener
Greases belongs to semisolid products, where thickener agents are dispersed in liquid lubricants. The choice of thickeners is important for the final products and there are several different types of thickeners used during grease production. Common greases contain 80-85%
of base oil, 10-15% thickener, and 5-10% additives. The thickener can be represented by soaps, polymers, or clay. Non-soap bases thickeners are not common, with exception of polymer- based greases, where polyurea-based thickeners are most common. Advantages with greases compared to liquid lubricants are that they can act as a sealant to prevent lubricant leakage, prevent entrance of contaminants, and they can also hold solids in suspension much better than liquid lubricants 9. The properties of greases are determined by the base oil properties along with the used thickener and additives 21.
2.3.1 Bio-based Grease
Common greases contain thickeners like polyurea and metallic soaps, which are not normally biodegradable. Therefore, a biodegradable grease will not only need biodegradable oil as the base oil but also biodegradable thickener 3. A potential for a bio-based alternative to grease is oleogels. Usually, oleogels consist of an organic hydrophobic solvent and an amphiphilic molecule, which can form a network that immobilizes the medium 22.
Oleogels are defined as solid-like gels showing the dynamic rheological properties which otherwise fits the rheological characteristics of traditional greases used for lubrication 23. The main applications for oleogels are in cosmetics and pharmaceuticals 22. Both low and high molecular weight oil gelators are used in oleogels. Low molecular weight gelators are small molecules that self-assembles to form a stable crystal network which stabilizes the oil. The self-assemble process is driven by physical interactions such as van der Waals, hydrogen bonds, and hydrophobic interactions. The interactions are controlled by temperature. High molecular weight oil gelators are molecules such as proteins and polysaccharides, which forms three dimensional networks in the oil phase through physical interactions. The molecular weight, conformation, and concentration will strongly affect the oleogel due to the polymeric nature of the high molecular weight gelators. The hydroxyl groups and other side groups on the polymers can affect the ability of the molecule to form inter- and intramolecular interactions. The interactions will lead to higher ordered structures such as helical or crystal organizations 24.
Generally, low-molecular weight amphiphilic molecules are responsible for the gelling behavior of oleogels. However, biopolymers, natural waxes and resins can also cause gelling in oil and have therefore gained special attention. Both cellulose derivatives and lignocellulosic materials have been used to yield a gel-like dispersion in vegetable oil. The produced oleogel has potential of being used as environmentally friendly lubricating grease 7. It is common in the food industry to use polysaccharides as stabilizers and thickening agents for water-based systems. Yet, using polysaccharide for gelation of oil is not straightforward, due to the hydrophilic characteristic of most polysaccharides 24.
Replacing the traditional used thickener in greases, such as lithium, calcium, and aluminum soaps, with thickeners from renewable feedstock is a challenging task. Metallic soaps exhibit
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very good technical performance and provides the grease the desired rheological and thermal properties. Previous studies on using different gelators, example cellulose derivatives, in vegetable oil have reported good lubrication properties but a low mechanical and physical stability. A significant improvement has been shown when the cellulosic derivatives were functionalized with isocyanate groups, which will chemically interact with the hydroxyl groups on the vegetable oil 25.
2.3.2 Cellulose as Thickener
Cellulose is one of the most important molecules obtained from renewable resources and could be considered for non-renewable polymeric compounds and thickeners. Cellulose derivatives have successfully been used as thickener in food, cosmetics, paints, pharmaceuticals, etc., 26. It contains hydroxyl groups, which are reactive and several different cellulose derivatives have therefore been synthesized. The cellulose derivatives that are widely used in the cosmetic and pharmaceutical industries are methyl-, ethyl-, and hydroxylpropylmethyl cellulose. Methyl cellulose is more water soluble above a certain substitution level, compared with ethyl cellulose that is more hydrophobic at a degree of substitution of 2.4 – 2.5 and can be dissolved in organic solvents 24.
Ethyl cellulose has been studied as a thickener for renewable grease lubricants as well as viscosity modifier in vegetable oil lubricants. Methyl cellulose has also been successfully used as a thickener 27. See Figure 3 for repeating unit of cellulose and ethyl cellulose. R. Sánchez et. al. has performed several studies 23,26,28,29 in using cellulose and chitin derivatives as thickener in bio-based grease. Cellulose and its derivatives have been proven to form gel-like dispersions in castor oil and can therefore be used as biodegradable thickeners. Especially ethyl cellulose in combination with α-cellulose or methyl cellulose provided the castor oil appropriate thermal and rheological behaviors. Certain ratio of ethyl cellulose and methyl cellulose derivatives have proven to provide a grease with good rheological behavior and mechanical stability 29.
Currently, ethyl cellulose is the only polysaccharide that has been found to directly gel liquid oil. Using other polysaccharides for gelling oil has mainly been achieved using indirect methods such as solvent exchange, drying, emulsion template, etc. One example is the macroalgae polysaccharide carrageenan that was formulated to a hydrogel to later be converted to an alcohol gel. The alcohol gel was then dried with supercritical CO2 to yield a porous aerogel, which was absorbed with oil to form an oleogel. The direct method to obtain an oleogel from ethyl cellulose involves heating the mixture of polysaccharide and oil to the glass transition temperature of ethyl cellulose (~140oC). The oil is gelled by polymer dissolution above the glass transition, followed by hydrogen bond formation between the polymer chain upon cooling. Studies have shown that a higher molecular weight of the polysaccharide will provide an increase of gel hardness 24.
Cellulose laurate ester has also been explored as a potential additive in lubricants. The synthesis of cellulose laurate through an esterification reaction between cellulose and lauroyl chloride was achieved in a study. Results showed an increase in lubricity with an increasing concentration of cellulose laurate ester in n-butyl palmitate/stearate 27.
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Figure 3. Repeating unit of cellulose and ethyl cellulose, redrawn from J. Rostami et. al, 30 and V. V. Myasoedova et.al., 31.
2.3.3 Chitin as Thickener
The bio-based compounds chitin, chitosan and acetylated derivatives have also been investigated to be used as thickener in vegetable oil 17. Chitin is one of the most abundant organic materials and is an important component in exoskeleton in animals 7. Chitin is a linear polysaccharide with β-(1-4)-N-acetyl-D-glucosamine residues, it is insoluble in all common solvent. Therefore, in order to use chitin it has to be modified to a more soluble derivative.
Chitosan is a deacylated derivative which is soluble in acidic solutions, see Figure 4 for the repeating untis of chitin and chitosan 32. Chitosan is both biocompatible and biodegradable 7. The results of utilizing chitosan as a thickener showed that using chitosan with a degree of acetylation of 0.3 gave similar rheological properties as commonly used lithium grease.
However, the formed bio-based greases presented poor mechanical stability. The study compared low and medium molecular weight chitosan (390 000 and 560 000 g/mol). The results indicated that higher concentration of low molecular weight chitosan was required to produce stable gels 28. Another study also managed to use acylated chitosan pared with castor oil, and the results indicated that the product had good friction properties 3.
Figure 4. Repeating unit of chitin and chitosan, redrawn from A. Usman, et.al., 32. 2.3.4 Isocyanide-functionalized Thickener
Vegetable oils have been successfully used as bio-based lubricants. However, substituting traditional thickening additives with others derived from renewable sources is a much more challenging task, as mentioned previously. In cellulosic derivative-based gel-like dispersions, the physical and mechanical stability and the thermo-rheological behavior are limited. This is pointed out as a consequence of chemical incompatibility between the polysaccharide and the
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base oil. This problem can be solved by introducing the formation of chemical gels, for example by isocyanide functionalize methylcellulose and dispersing it in vegetable oil. Isocyanate- functionalized chitin and chitosan can also be used to chemically gel oil. In a study, chitosan and chitin were functionalized with 1,6-hexamethylene diisocyanate and dispersed in castor oil. The purpose was to promote chemical reaction between the isocyanate group on the isocyanate functionalized biopolymer with the hydroxyl group in the castor oil. Thus, yielding oleogels which in some cases presented a suitable thermal resistance and rheological characteristic 7.
This method can also be applied to the biopolymer lignin. In pulp and paper industry, a large amount of residual lignin is produced. It is considered to be the main renewable aromatic resource, with potential to be an excellent feedstock for synthesis of chemicals and polymers.
Therefore, lignin has been investigated as a possible thickener for bio-based greases as well.
For that reason, the residual Kraft lignin was modified with hexamethylene diisocyanate and dispersed in castor oil. The obtained friction coefficient was similar to commercially used greases 25.
2.4 Marine Biorefinery
In biorefineries, similar to crude oil refineries, biomass is used as raw material to produce several different products. The biomass can be used to produce fuel, materials, chemicals, power and heat, food, etc. Both marine micro- and macroalgae are used as biomasses for biorefinery 33. Marine algae (e.g., microalgae and macroalgae) include several photosynthetic organisms, they are a diverse group of plants ranging from the microscopic algae to complex seaweed kelps. They are found nearly everywhere with over 200 000 species 4. Microalgae have a high content of lipids (>60% in some species) and macroalgae have a high carbohydrate content, up to 75%. There is an interest in using carbohydrates derived from macroalgae and lipids from microalgae in production of biofuels. However, the cost of production and harvesting macro- and microalgae is too high which prevents the general marine biorefinery to be economically feasible today 1.
2.5 Microalgae
Microalgae are unicellular plants that live aggregated, in filaments, or individually. They are a major part of the photosynthetic productivity at earth and produce approximately 50% of the atmospheric oxygen. Of over 50 000 described microalgae, only a few are being cultivated.
The major usages of microalgae are food, fertilizer, biofuel and extraction of lipids, sugars, antioxidants, proteins, and pigments. The lipid content in microalgae is usually high (30-50%) and the most abundant polysaccharides are cellulose and starch 1. Generally, microalgae lipids are esters of glycerol with C14 to C22 chain-length fatty acids that can be saturated and unsaturated. The hydrocarbon amount is normally less than 5%. One exception is the green alga B. braunii that instead contains up to 61% of hydrocarbons 34. Microalgae use a large amount of water in production, about 1 L of microalgae biodiesel requires 3000 L of water.
Water recycling is therefore needed to reduce environmental and economic costs 35. However, microalgae can also grow in brackish or salt water, which secures freshwater resources to humans 36.
12 2.5.1 Microalgae Applications
As mentioned, microalgae are unicellular organisms that are a major part of the production of oxygen. Microalgae have gained attention the past years, since they contain 50% essential amino acids, 65% protein, up to 60% carbohydrates and up to 70% of lipids. They offer a broad range of products, example are protein supplements, antiaging creams, and biopigments 4. Microalgae are also used to produce several different fuels, such as biodiesel, biohydrogen, bioethanol, biogas and bio-oils 37. They can double in mass in less than a day and contain high triglyceride oil content, which make them a perfect feedstock for biofuels 38. Transesterification is a promising method to produce biodiesel from algae oil. High oil content species of microalgae can yield 19 000 – 57 000 liters of microalgae oil/acre/year 39. It is estimated that a 30 million acres of microalgae cultivation could replace the imported oil of the USA supply 4. Biodiesel belongs to renewable fuels that consists of monoalkyl fatty acid esters. Biodiesel can be produced from vegetable oil, animal fats, or algae oil by esterification and transesterification reactions. It is more of a challenge to produce biodiesel from algae oil compared to terrestrial vegetable oils. The reason is because algae oil comprises of polar fatty acids as well as non- polar triglycerides and free fatty acids. Fatty acid alkyl esters can additionally be used in production of renewable chemicals 40.
2.5.2 Algae Lipid
Microalgae contain unusual fatty acids and lipids, which makes microalgae noticeable different compared to higher plants. Due to environmental adaptation, microalgae lipid content differs significantly. Five industrial important microalgae, Chlamydomonas reinhardtii (Chlamy), Chlorella vulgaria (Chlorella), Nannochloropsis sp. (Nanno), Scenedesmus sp. (Scene), and Schizochytrium limacinum (Schizo), have been used in a study to characterize the lipid content.
The result showed a mixture of neutral lipids (triglycerols and free fatty acids), polar lipids, unsaponifiable matters, chlorophyllides, and others, with different compositions 41.
As mentioned before, microalgae are a promising feedstock for lipid type chemical products.
The lipid content can be higher than 50%, and the cultivation medium can control the lipid content. One example is nitrogen starvation, which will lead to a higher lipid content for some microalgae. One important obstacle to overcome is to find an efficient method to extract the lipid, in order to reduce the cost of production. A method that has gained special interest is extraction with supercritical fluid, particularly supercritical carbon dioxide. The method is environmentally friendly and very efficient 36.
In order to have a functional production of biolubricant from microalgae, it is important to cultivate microalgae with a high biomass productivity with a high lipid content at a minimum cost. The green microalgae Chlorella vulgaris was successfully grown in a cheap medium. The lubricant oil from the microalga had high viscosity, viscosity index of 185, high flash point at 185 oC, and a low pour point at -6 oC. The extracted lipid from Chlorella vulgaris appeared to have low content of polyunsaturated fatty acids which indicates a good oxidative stability 42. Regarding using renewable feedstock in products such as biofuels, the food versus fuel debate is an important issue to notice 43. Given that when food is used in production of fuel an ethical debate is raised, where the use of grains and oilseeds for production of biofuels production will compete with the animal and human food production 44. Therefore, using non-edible oils is a good choice because it does not compete with food crops and can be grown on wastelands.
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Microalgae are therefore a potentially good oil producer for biofuel production, as it contains high amount of mostly neutral lipids with low degree of saturation and does not compete with food production. Microalgae can produce 100 000 L oil/ha/year compared to sunflower that produces 952 L/ha/year 43.
2.5.2.1 Algae Lipid Lubricants
Another application for microalgae lipids is as bio-based lubricant. Since microalgae lipids have shown satisfactory lubricating properties and could a potential feedstock for lubricant production. To increase the productivity of microalgae cultivation, different methods have been used for modeling and optimization, to reach the highest biomass productivity and lipid content
45. The oil from the microalga Chlorella sp. has been proven to be a possible substituent for mineral oil. A study combined Chlorella sp. oil with mineral oil to analyze the lubrication properties. To make algae oil ideal for lubricating applications, inorganic alteration or transesterification is necessary. The altered algae oil from Chlorella sp. in combination with mineral oil revealed enhancement of properties such as wear rate, coefficient of friction, compared to mineral oil. An algae oil concentration of 20% gave a lubricant with good boundary lubricant film 46. A study on high temperature algae lubricants produced a deuterium substituted hydrocarbon base oil by growing microalgae in heavy water (D2O). By using the deuterium fatty acids from the grown algae, a biolubricant with higher thermal stability could be obtained 47. The length of the fatty acids in algae oil will affect the product, where fatty acids with long chain (>C18) are more viscous 34.
An additional organism similar to microalgae that has a potential of being a future source for lubricants is the cyanobacteria, known as blue-green algae. It is a diverse group of photosynthetic gram-negative prokaryotes with a potential source of useful products, example extracellular polysaccharides (EPS). In a study on C. epiphytica, the composition of the polysaccharides was found to be arabinose, xylose, glucose, galactose, and mannose. It also contained deoxyhexose fucose sugars which were sulfated. The polysaccharides in the cyanobacteria have been suggested as a potential lubricant to be used in oil recovery and drilling process, where the polymer adsorbs to the pipe surface to increase the lubricity property of drilling fluid. C. epiphytica contain EPS that are found to be good hydrophobic dispersants and emulsifiers. Analysis has shown that the EPS have potential as biolubricants with some characteristics that is better than conventional grease. The emulsifier activity of EPS was recorded to be higher compared to the well-known emulsifier xanthan gum 48.
2.5.2.2 Polymers from Algae Lipids
Algae fatty acids can also be converted into difunctional molecules, which can be used in preparation of polyesters. In a study, crude extract from a microalga were obtained containing various unsaturated fatty acids. Carbon monoxide, methanol, and a catalyst was used to convert the multicomponent mixture to linear diesters. Some diesters were reduced to linear diols, and were used in a polycondensation reaction to produce polyesters with high melting and crystallization temperature 49. Another bio-based polymer group that algae oil can be converted to are different polyurethanes. This is performed by preparing polyols from algae via oxidation followed up by epoxy ring opening. In a study, the synthesized polyols were reacted with diphenyl diisocynate using cyclopentane as a blowing agent to generate a rigid polyurethane foam 50.
14 2.5.3 Terpenes
As stated, microalgae can be used to produce first generation biofuels but they also have the potential to produce a number of metabolites that resembles petroleum fuel more 51. The class of compounds terpenes are derived from the five carbon molecules isopentenyl diphosphate and its double-bond isomer dimethylallyl diphosphate. Terpenes represent one of the most diverse class of compounds found in nature. They comprise of over 50 000 different molecules which include metabolites such as sterols and carotenoids as well as secondary metabolites that is commonly used for medical purposes. Terpenes are commonly classified depending on the number of carbons; hemi- (C5), mono- (C10), sesqui- (C15), di- (C20), tri- (C30), tetra- (C40) and polyterpenes (>C50). The terpenes produced by plants are high value to humans, they are used in the medical industry, cosmetics, as chemical feedstock, and in agriculture. They are however chemically complex and large-scale production of terpenes is therefore often costly and complicated. Microalgae will likewise to plants fix CO2 trough photosynthesis to finally produce numerous of carbon compounds, such as terpenes. Cultivated algae can produce a large amount of terpenes that are used in renewable fuel, pharmaceutical, and in nutritional supplements 52. Terpenes are the most promising metabolites in algae that can offer a possible new fuel source. Both monoterpenes (C10) and sesquiterpenes (C15) are reported as potential fuel candidates 51. A study demonstrated a bioconversion of algae carbohydrate and protein into terpene compound, which could make the production of algae biofuel more economically feasible. The production of biodiesel from algae has primary focused on transesterification of algae lipids 53.
2.5.3.1 Squalene
Squalene is an unsaturated triterpene compound (C30H50) 54 and is used in the cosmetic, medical, and food industry. It is extracted from various naturally occurring raw materials.
Examples are shark liver oil and vegetable oils such as olive, palm, sunflower, soybean, and corn oil. However, supply from these sources are today limited and other sources are being investigated. Microalgae can be a good option 51. Some of the green microalgae Botryococcus braunii can produce the triterpene hydrocarbons squalene and botrycoccene 51. The long chain hydrocarbons (i.e. squalene) in B. Braunii are a potential source of liquid biofuels, however they differ from mineral oil and will need treatment. In a study, saponite supported catalyst was used to upgrade squalene, where a major product was the saturated squalane, as seen in Scheme 3 54. Squalane is resistant to oxidation and does not require preservatives, that is otherwise used in the cosmetic industry 55. The microalgae Schizochytrium mangrovei has also proven to contain a high amount of squalene 56.
Scheme 3. Hydrogenation of the unsaturated squalene to the saturated squalane. Redrawn from L. O. Garciano, et.al., 54.
Radical polymerization is a method to polymerize monomers with carbon-carbon double bonds. However, monomers containing multiple double bond can yield polymers with cross-
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linked structures, and steric hindered double bond will obstruct polymerization 57, and is therefore probably not a good method to increase the molecular weight of squalene.
2.5.3.2 Botryococcus braunii
A microalga that has attracted special interest is Botryococcus braunii, as mentioned above.
The algae oil in B. braunii consists mostly of hydrocarbons from isoprene compared to most algae that contain glycerol esters 54. The hydrocarbons are mostly located outside the cells.
Different types of B. braunii yield different hydrocarbons, ranging from C23to C40 34. An outdoor pilot-scale (0.4 m3) production of B. braunii managed to have a biomass productivity of 0.02 g L-1 day-158. As most raw materials, B. braunii oil needs refining to improve the quality of the oil. The composition of the oil from B. braunii differs from petroleum, terrestrial plants, and algae oil. The oil will therefore need a different refining process. To be economically feasible, the process will need to be cheap. The reason for the need of oil upgrading is based on the presence of unsaturated double bonds in the crude oil, which can lead to chemical reactions 54. The hydrocarbons in B. braunii can be processed similar way as petroleum to produce biodiesel or lubricants by fractionation 58.
2.5.4 Alkenones
Alkenones are long hydrocarbon chains (i.e., 36-40 carbons), which is approximately twice as long compared to fatty acids, and have a high melting point of (~70 °C), see Figure 5 for a methyl alkenones compared to a fatty acid. Alkenones are unsaturated, with trans double bonds, and contain a methyl or ethyl ketone. The long hydrocarbon has a waxy texture at room temperature and is therefore a possible alternative for commonly used waxes in cosmetic products, example lip balms 59.
Figure 5. Chemical structure of 37:3 methyl alkenone compared to a 18:2 fatty acid. Redrawn from G. W. O’Neil et.al., 60.
Isochrysis is a microalga that contains alkenones and is produced industrially for shellfish feed production. Alkenones is therefore an abundant and renewable hydrocarbon, available from a microalga that is already produced industrially. It is a possible source for thickener in skin creams that can replace the commonly used candellilla wax, which there has been a shortage of the past two decades. In a study where alkenones was used as thickener, alkenones presented similar thickening ability as candelilla wax 59. A method presented in a study to isolate pure alkenones from the microalga Isochrysis yielded 3.5%(w/w). Alkenones can also be converted to smaller hydrocarbons to be used as fuel. It can be processed to hydrocarbons via catalytic hydroprocessing 61. The current price of Isochrysis is around $400/kg, and only sold by a few sellers and is highly dependent by the shellfish feed market. A study calculated that the
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Isochrysis lipid-derived fuel would have a price of $2 642/L, which is anything but cost- completive with fossil-based fuel 62. See Table 1 for a comparison of fatty acids, alkenones, and the triterpene hydrocarbon botrycoccene, derived from different microalgae sources.
Table 1. Comparison of the three algae-derived molecules; fatty acid, alkenone, and botrycoccene 62.
Fatty acids Alkenones Botrycoccenes
Algae source Various E. hux., G. Oceanica, Isochrysis
B. braunii
Carbon number
Linear, 14-22 Linear, 35-41 Branched, 30-34
Double bonds 0-6
methylene-interupted cis-disubstituted alkenes
1-3 trans disubstituted separated by 5
methylenes
6 mono-, di- and trisubstituted seperated by 2-3 carbons
Heteroatoms Carboxylic acid Methyl or ethyl ketone
None
2.5.5 Insight on Economic Feasibility
A very important factor with utilizing renewable feedstock is the economic feasibility.
Microalgae can produce significant amounts of biomass and oil and are therefore a potential feedstock for several different products. Bioethanol, biodiesel, biogas, and bioplastic are a few examples. More high value chemical products can also be produced from algae, they can be used in the medicine and cosmetic industry. Though, the production cost of algae cultivation will need to decrease significantly, to one tenth of the current cost. This can be achieved by improved reactor designs and by using more efficient and genetically modified microalgae strains. Furthermore, a sufficient extraction method is needed to produce higher value products.
It is also difficult to estimate the production cost because of the variation between algae strains, lack of publications and wide variation of technologies 37. Today, due to the high cost of algae cultivation has only been achievable to commercialize high value products. The lower valued product of microalgae, such as lipids for biofuels will need more efficient and lower cost production 34. Algae can comprise up to 80% of carbohydrates and proteins, and therefore a bioconversion of carbohydrates and proteins to fuel and value-added bioproducts should improve the economic feasibility 53.
2.6 Macroalgae
Macroalgae are commonly termed seaweed and are divided into red, green, and brown algae.
They have many advantages for biorefinery applications. For example fast growing, high photosynthetic efficiency, low lignin content, and high carbohydrate content, which can be used to produce several liquid and gas fuels 5. It is also cheap to cultivate macroalgae and can the algae can fix CO2 much faster compared to terrestrial plants 63. Commercially used carbohydrates from seaweed are alginate, agar, and carrageenan 64. Other carbohydrates are laminarin and fucoidan 65. The east Asian countries China, Korea, Japan, Indonesia, and the Philippines accounted for 95% of the world’s supply of macroalgae in 2010 66. In Asian
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countries, seaweed is mainly used directly as food compared to in European countries where seaweed is mainly used for extraction of food hydrocolloids, such as carrageenan, agar, and alginate 63. Currently, about 15 million metric tons of algae products are obtained annually.
Only a small fraction of the products is used for obtaining extracts, around 25 000 metric tons
67.
Macroalgae contain carbohydrates, both glucose polysaccharides such as cellulose, but also other complex polysaccharides such as ulvan, alginate, agar, and carrageenan. The polysaccharides are divided into energy storage and structural polysaccharides. The major monosaccharide in the storage polysaccharides is glucose, whilst the structural polysaccharides contain several different monosaccharides 1, see Table 2 for a summary of the polysaccharides in macroalgae. Polysaccharides are carbohydrate polymers, with long chains of monosaccharides units bound together by glycosidic bonds. They will break down into oligosaccharides or monosaccharides upon hydrolysis. The molecules can be both highly branched or linear 32. They are natural molecules from the carbon-capture process photosynthesis, followed by additional biosynthetic modifications. Some are produced in very large quantities. Most polysaccharides are insoluble in water and organic solvents, however charged polysaccharides such as alginate and chitosan have different solubility properties 68. Seaweeds does not need the same rigidity as terrestrial plants and therefore have low lignin content or a complete absence of lignin. The low lignin content simplifies biorefinery, and macroalgae are therefore a good feedstock for carbohydrate extraction. The water content of macroalgae is much higher than land living plants, about 70-90%(w/w). The lipid content is much lower than microalgae, around 1-5 dry %(w/w) 1.
Table 2. The major polysaccharides in red, green, and brown macroalgae 1. Algae Storage polysaccharides Structural polysaccharides Red Floridean starch Agar, carrageenan, cellulose
Green Starch Ulvan
Brown Laminarin Alginate, fucoidan, cellulose
2.6.1 Macroalgae Applications
The major use of macroalgae is human food with 83-90% of the market. A common food usage is algae hydrocolloids, where the key polysaccharides used are agar, alginate, and carrageenan.
In addition of food uses, macroalgae are used for production of several different biomaterials and bioproducts. Several polysaccharides derived from macroalgae are bioactive and can be used for therapeutic applications. It is expected that the macroalgae-based biorefinery will dramatically develop in the future as a result of both the environmental and economic benefits
66, and the global production of macroalgae has indicated that macroalgae can be mass cultivated with the currently existing farming technology 67. The benefits with using macroalgae as a source of renewable feedstock is that the growth rate is faster compared to terrestrial biomass production, there is no competition for agricultural land, high carbohydrate content, and a low lignin content 1.
