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Oxidation of cellulose to favour its dissolution in alkaline aqueous solution before regeneration into a textile yarn

Martin Orpiszak

Supervisor and examiner: Minna Hakkarainen

May 21, 2021

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I – Sammanfattning Nyckelord : cellulosa, natriumperjodat oxidation, upplösning, regenerering, textilgarn

Dettae examensarbete är en del av CelluFil-projektet och syftar till att optimera förhållandena för upplösning av cellulosa i vattenlösning av natriumhydroxid följd av regenering i form av ett garn. Tidigare arbeten har visa tatt cellulosa kan lösas i vattenlösning vid -10°C, men sådana förhållanden är inte tillämpliga i industriell skala. Målet med detta projekt är att arbeta vid rumstemperatur.

För detta kommer karboxylgrupper att införas i cellulosakedjorna för att öka cellulosans lighet i ett alkaliskt vattenbaserat medium. Därefter kommer cellulosan att fällas ut igen i en sur lösning. Natriumperjodat gör det möjligt att oxidera alkoholgrupperna i cellulosakedjan på C2- och C3- positionerna genom att öppna glukosenheterna för att skapa två karbonylfunktioner (aldehyde) som sedan lätt kan oxideras till karboxylgrupper med överoxidering med natriumklorit.

Den första delen av rapporten är tillägnad litteraturöversikten om ämnet med focus på natriumperjodat oxidation. Därefter föreslås ett allmänt protokoll från natriumperjodat oxidation till regenering av cellulosa till garn i en svavelsyralösning. Flera förhållanden för natriumperjodat oxidationen testas vid olika temperaturer, med eller utan metallsalter och med olika oxidationsdoser.

Upplösningsutbytet är direkt kopplat till karboxylinnehållet infört i cellulosakedjorna. Periodatoxidationen leder också till minskningen av polymerisationsgraden men cellulosas DPv förblir tillräckligt hög med det riktade COOH-innehållet och för textilapplikationer. Den möjliga produktionen av hydroxylradikaler under natriumperjodat oxidationen har undersökts med EPR/Spin-trapping. Endast försök gjorda med UV-strålning visade OH°. Således förklaras fortfarande inte depolymerisationen av cellulosa under perjodat oxidation framställd under mörka förhållanden.

Eftersom, konsumtion av natriumperjodat är låg under oxidationen är dess återvinning en nyckelfråga för en industriell applikation. Det har visat sig att oxidationsfiltraten kan återanvändas flera gånger före total konsumtion av periodat. Kvaliteten på celluloser oxiderade med återvunna filtrat, särskilt deras upplösningsförmåga, bör kontrolleras.

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II – Abstract

Key words : Cellulose, periodate oxidation, dissolution, regeneration, textile yarn

This master thesis is part of the Cellufil Project and aims to optimize the conditions for dissolving cellulose in aqueous sodium hydroxide solution followed by regeneration it in the form of a yarn. Previous works have shown that cellulose could be dissolved in soda at -10°C, but such conditions are not applicable at industrial scale. The objective of the present project is to work at room temperature.

For that, carboxyl groups will be introduced in the cellulose chains in order to increase cellulose solubility in an alkaline aqueous medium, after this it is reprecipitated it in acidic solutions. Periodate makes it possible to oxidize the alcohol groups of cellulose on C2 and C3 positions by opening the glucose units to create two carbonyls functions (aldehyde) which can then be easily oxidized into carboxylic groups with overoxidation using sodium chlorite.

A first part of the report is dedicated to the literature review on the subject, focusing on periodate oxidation. Then, a general protocol is proposed from the periodate oxidation to the regeneration of cellulose into yarn in sulfuric acid solutions. Several conditions for the periodate oxidation are tested, at different temperatures, with or without metal salts and with different oxidant dosages.

The dissolution yield is directly linked to the carboxyl content introduced in the cellulose chains. The periodate oxidation also leads to the decrease of the cellulose degree of polymerization but cellulose DPv still remains sufficiently high in the case of the targeted COOH contents and for textile applications. The possible production of hydroxyl radicals during the periodate oxidation has been investigated by EPR/Spin-trapping. Only trials made with UV radiations showed OH°. Thus, cellulose depolymerization during periodate oxidation made in dark conditions is still not explained. Because the periodate consumption is low during the oxidation, its recycling is a key issue for an industrial application. It has been shown that the oxidation filtrates could be reused several times before total oxidant consumption. The quality of celluloses oxidized with recycled filtrates, especially their dissolving ability, should be checked.

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III – Purpose of the study

Cellulose is a renewable and natural polysaccharide that has been a research subject since polymer science was born. The raw material has high availability and no interfering interest in food supply, making cellulose a cheap material alternative and attractive as precursor for various derivatives.

Demand in textile increases and alternatives to petroleum based matters need to gain in importance. Process involving cellulose already exist but they use either non-green routes or expensive chemicals, that makes them difficult to industrialize.

An interesting process for making textile yarns with cellulose is based on its oxidation and its dissolution in sodium hydroxide before regeneration in an acidic solution. These include two oxidative steps with sodium periodate and sodium chlorite to favour cellulose dissolution while preserving its molecular weight, necessary for regeneration.

The objectives of the following project are (1) to optimize the sodium periodate oxidation conditions to reduce cellulose depolymerization for the production of a resistant yarn, (2) to investigate the relation between the cellulose polymerization degree and the amount of carboxyl groups introduced in the cellulose chains and (3) to improve the dissolution and regeneration conditions of the oxidized cellulose.

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Acknowledgments

First of all, I would like to thanks Nathalie and Gérard. Thank you both for being my supervisors, for your trust, advices and of course your time. It has been a very rewarding experience for me to participate in such an innovative project. Thank you also Dominique for your expertise.

Thanks to everyone in the lab, especially Karine and Killian for your help during my whole project. Thank you Claire and Hélène for your time for any questions I had. Generally, it was a pleasure to work in this environment for months, meet you all and share coffee breaks and lunch.

Thank you Lorette for your sharing and your opinion on the subject. I wish you a good continuation for your Ph thesis.

Thank you also Minna for being my supervisor in KTH.

Finally, I wish you all success and happiness in your projects.

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Table of Contents I – Sammanfattning ................................................................................................................................................ 3

II – Abstract ............................................................................................................................................................. 4

III – Purpose of the study ........................................................................................................................................ 5

IV – Introduction ..................................................................................................................................................... 8

V - Bibliography....................................................................................................................................................... 9

1. Generalities on cellulose ................................................................................................................................ 9

2. Dissolving pulp ............................................................................................................................................. 10

2.1. Prehydrolysis-kraft process ................................................................................................................... 11

2.1. Sulphite pulping process ....................................................................................................................... 11

3. Regenerated cellulose .................................................................................................................................. 12

3.1. Viscose .................................................................................................................................................. 12

3.2. Lyocell ................................................................................................................................................... 13

4. Mercerization ............................................................................................................................................... 13

5. Investigation of cellulose oxidation to favor cellulose dissolving ability ...................................................... 13

5.1. TEMPO Oxidation .................................................................................................................................. 14

5.2. Sodium periodate oxidation ................................................................................................................. 14

5.3. Sodium chlorite over oxidation ............................................................................................................. 16

6. Periodate recovery ....................................................................................................................................... 17

6.1. Periodate regeneration using ozone ..................................................................................................... 17

6.2. Periodate regeneration using sodium hypochlorite (NaClO) ................................................................ 17

VI – Material and methods ................................................................................................................................... 18

1. Material ........................................................................................................................................................ 18

2. Methods ....................................................................................................................................................... 18

2.1. Pulp oxidation ....................................................................................................................................... 18

2.2. Pulp and process analysis ..................................................................................................................... 20

VII – Results and discussion .................................................................................................................................. 26

1. Cellulose oxidation ....................................................................................................................................... 26

1.1. Optimisation of the cellulose oxidation by the periodate/chlorite system .......................................... 26

1.2 Investigation of radical production during periodate oxidation by EPR/Spin-trapping ......................... 34

2. Dissolution of the oxidized cellulose into an aqueous sodium hydroxide solution ..................................... 39

3. Regeneration of dissolved cellulose in sulfuric acid into yarn ..................................................................... 41

3.1. Yarn formation ...................................................................................................................................... 41

3.2. First analyses of the regenerated cellulosic yarns ................................................................................ 42

4. Recycling of the filtrates originating from cellulose periodate oxidation .................................................... 43

VIII – Conclusion and perspectives ....................................................................................................................... 47

IX – References ..................................................................................................................................................... 49

X – Appendix ......................................................................................................................................................... 52

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IV – Introduction

With the growth of the world population which is expected to reach 11 billion inhabitants at the end of the century, the textile industry is set to develop a lot with an increase in textile fibers’ demand. The raw materials most frequently used to make textiles are cotton fibers, synthetic fibers from petroleum ressources (nylon, acrylic, polyester) and artificial fibers made from cellulose (viscose, lyocell). Bio-sourced yarns are facing an increasing demand to limit the use of petro-ressources and because cotton planting requires space and lot of water, competes against food production and is sensitive to weather conditions and climate.

Among bio-sourced textile fibers, viscose fibers are the most common. The viscose process a been developed at the beginning of the 20th century in France. It consists in dissolving cellulose in carbon disulphide (CS2) in an alkaline medium to spin fibers after regeneration. However, the process is toxic and its use is forbidden in Europe. A more recent process, the Lyocell process developed in the 1990’s, uses an ionic liquid, the N-methylmorpholine-N-oxide (NMMO), to dissolve cellulose. But this solvent is difficult to recover and is very expensive which constitutes a limit to its industrialization. Novel alternatives are developing today, one of them consists in the improvement of cellulose dissolution into aqueous solvent, less toxic and less expensive, allowing controlled spinning.

In this context, this project (named CelluFil) follows the progress made by two other projects. The first one, RoCoCo, used an aqueous sodium hydroxide/urea (NaOH/Urea) solvent for the dissolution of cellulose at -10 °C. Dissolution yield was 80% at this temperature, with cellulose concentration of 1 or 2%, which is not sufficient for regeneration. Besides, as soon as the solution is placed at room temperature, it forms a gel. All these issues are limitations for industrialization.

DissoluCell, a 2nd project, focused on the modification of the cellulose structure to increase it dissolving ability in aqueous solution. The idea is to introduce carboxyl groups in the cellulosic chain, on carbon C6 with a TEMPO oxidation and on carbons C2 and C3 with a periodate oxidation. Results show that cellulose periodate oxidation enables to fully dissolve cellulose in a concentrated aqueous sodium hydroxide solution, at 5% cellulose concentration with a prior mercerization (in sodium hydroxide) before oxidation. Moreover, a first yarn was obtained after regeneration in acid conditions. This pre-oxidized cellulose is partially depolymerised by the treatment which may alter the quality of the textile yarn. On the other hand, the TEMPO oxidation has rather a limited interest since cellulose is highly depolymerised.

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V - Bibliography

1. Generalities on cellulose

Wood is mostly composed of 3 components: cellulose, hemicellulose and lignin. Cellulose is the most abundant polymer on earth and represents 50% of the plant biomass. It is the main constituent of all cell wall of plants and wood (up to 95% in cotton fibers and 40% in wood). In wood it acts as a reinforcing matrix surrounded by hemicelluloses and lignin. It gives to the wood fibers its properties of resistance and strength (tensile and tear) acting as a load bearing structure. Lignin has an important role for the compression strength and the load distribution within the fiber wall. The hemicellulose makes the link between the different components in the fibre wall. Cellulose is a linear chain made of D-glucose units linked by β-1,4 glycosidic bonds [1] whose structure is shown in Figure 1.

Figure 1. Structure of cellulose with its repeating unit and both extremities [2]

Its molecular formula is (C6H10O5)n where n represents the degree of polymerization which is the number of anydroglucose units (AGU) present in the chain. Its repeating pattern is cellobiose. On each AGU unit, three hydroxyl groups are present at position C2, C3 and C6. They allow to obtain cellulose derivatives because of their high reactivity. Besides, hydroxide groups allow to form intra and intermolecular links [3] within the polymer creating bigger structures called fibrils (Figure 2). These fibrils have a 3 to 5 nm section and are joined together to form aggregates with a 15 to 30 nm section.

Although cellulose is a polymer with a linear structure, it can develop interactions with other chains. Cellulose can form supramolecular interactions through hydrogen bonds and non-polar interactions. They are the source of the organization of cellulose chains linked by hydroxyl bonds.

Figure 2. Intra and intermolecular hydrogen bonds of cellulose [4]

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Due to the oriented structure of the polymer, both ends of the chain are different. One end is composed of a hemiacetal and is reducing. This reducing end (the unit on the right in figure 1) corresponds to the glucose unit whose hydroxyl in the anomeric position is free. The opposite end, called non-reducing end (the unit on the left in figure 1), has his hydroxyl group in anomeric position engaged in an osidic bond. Cellulose I is oriented with hemiacetals on the same side for each chain and it is the cellulose structure in the natural biomass. Cellulose II is thermodynamically more stable than cellulose I and, therefore, the transformation of cellulose I into cellulose II is irreversible.

Cellulose presents interesting characteristics as a density equal to 1.52 g/cm3, elongation at break 10 to 40%, Youngs’ modulus of 3 to 36 GPA and thermal conductivity of 0,06 W/m/K. It has been used as building material as wood, for clothing and as a source of energy [5].

The degree of polymerization can reach 10,000 glucose units in wood and 15,000 in cotton [6]. In dissolving pulp, the DP of cellulose is between 300 and 1700.

Due to the large number of hydrogen bonds [7], cellulose is insoluble in water and in many organic solvents which makes it very difficult to dissolve [8]. The cellulose swelling is a physical modification of fibers’ wall whose volume increases. Swelling can be an interesting property to further oxidize cellulose as it increases accessibility to reactive cellulose sites: C2, C3 and C6 [6]. It will be discussed more in the part about periodate oxidation of cellulose.

Cellulose has a semi-crystalline structure with a perfect and organized geometry in some parts (crystalline) and a random geometry and disorganized for other parts (amorphous) [9]. This alternation of amorphous and crystalline areas affects the reactivity of cellulose and the access of chemicals depends on this organization.

When dissolving cellulose, two types of swelling can occur [4]. One is inter-crystalline and corresponds to the penetration of the chemical into the amorphous parts which are the most accessible. The other one is intra-crystalline and corresponds to the penetration of the chemical into crystalline parts which are more resistant to solvent penetration due to their organization [11]. This second phenomenon is more tricky to implement because it needs specific characteristics of the solvent to attack and change cellulose chains structure on the hydroxyl groups and to split intermolecular hydrogen interactions. The chains separate from each other to form a complex with the solvent system. Therefore, crystallinity is a factor influencing the dissolution ability of cellulose but the molecular weight also plays a key role because the dissolution is more difficult when chains are longer [12].

2. Dissolving pulp

For some applications such as the production of regenerated cellulose, cellulose of high purity is required. For this, cotton or dissolving pulp is used. Dissolving pulps are cellulosic pulps intended not for paper or board making, but for manufacture of products like cellulose derivates and regenerated cellulose. They are obtained through wood delignification via well-known pulping processes, and the further purification is done during the bleaching sequence, where residual lignin and substantial amounts of the hemicelluloses are removed.

Dissolving pulps can be produced either by the prehydrolysis Kraft process (PHK) or by the acid sulphite pulping. In both processes, the conditions must be chosen in order that the remaining hemicelluloses in the fibers is reduced to a minimum. Furthermore, bleaching to high brightness is required in order to remove all lignin. The pulp yield in these processes is low, in the order of 35%, making dissolving pulp a rather expensive product [13]. The regeneration of cellulose into textile yarn can be an added value application for industries producing dissolving pulp. This represents an alternative to materials derived from petroleum resources [11].

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2.1. Prehydrolysis-kraft process

Kraft cooking is the dominant chemical pulping method globally. The cooking chemicals used are sodium hydroxide, and sodium sulphide. By leaving out the sodium sulphide and only use sodium hydroxide as the cooking chemical, the process is called soda cooking.

Firstly, a prehydrolysis removes the hemicelluloses, then the kraft cooking comes to remove lignin and finally a cold caustic extraction (CCE) removes residual hemicelluloses.

Before kraft cooking, water or acidic water pre-hydrolysis of the wood chips is necessary to dissolve and therefore eliminate a large fraction of the hemicelluloses. It is realized between 160 and 180°C at pH 4 in aqueous solution. Hemicelluloses, bearing acetyl functions, are partly depolymerized by acid hydrolysis and released acetic acid which will catalyze the hydrolysis reaction. This self-catalyzed reaction allows the dissolution of a large part of the hemicelluloses or contribute to their large depolymerization. Cellulose is more stable during this step thanks to its crystalline structure. A deprotonation occurs on the OH and then a nucleophilic attack is performed on the nearest carbon to form an epoxide. The deprotonated alcohol and the epoxide react with each other to transit reversibly.

However, for the hemicelluloses, the non-crystalline structure gives more freedom to the chains and the deprotonated alcohol does not react with the epoxide but it is protonated by water and perform an irreversible depolymerization.

After pre-hydrolysis and thus hemicelluloses removal, it is necessary to eliminate lignin through Kraft cooking. Wood chips are treated in a mixture containing sodium hydroxide (NaOH) and sodium sulphide (Na2S) at around 160°C for 2 to 5 hours. Delignification occurs and the liberated cellulose is partially depolymerised. During this step, dissolved lignin and sugars in the black liquor are burnt to give energy or separated, and used as a source for production of speciality chemicals.

Then a cold caustic extraction and several bleaching stages are added to remove last traces of hemicelluloses and residual lignin. After several washing the pulp obtained is almost pure cellulose and both hemicelluloses and lignin can be recovered for further valorisation.

2.1. Sulphite pulping process

The sulphite process uses aqueous sulphur dioxide (SO2) and the active component during the process are HSO3

- or SO32- . It is done between 130 and 160°C overpressure at pH 1.5 – 4. Lignin is removed

from wood by hydrophilization of lignin. Lignin-carbohydrate linkages have been destroyed by acidic conditions of the cook and the pulp is less difficult to bleach than for kraft pulping

But there are ethers that are broken (figure 3). Acid depolymerization of lignin is slow but contributes to the delignification. The depolymerization of cellulose gives relatively weak pulps. Also hemicelluloses are depolymerized and sugars are released into the pulping liquor.

In the sulphite cooking process, sulphurous acid (H2SO3) and bisulphite ions (H2SO3- ) are the active

chemicals to degrade and dissolve lignin. Sulphite pulping can be performed at a pH ranging from 1-2 in acid sulphite pulping to 7-9 in neutral sulphite pulping.

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Figure 3. sulphite pulping delignification

The acid sulphite process degrades cellulose more than the kraft process, which leads to have weakest fibers. Generally, the average degree of polymerization of dissolving pulp is around 1000.

3. Regenerated cellulose

A generated cellulose fiber is an artificial textile fiber. It is obtained from natural biomass (wood, hemp, cotton, etc) and transformed by chemical and physical operations into cellulose that can be used for textile applications. To get regenerated cellulose, dissolution in a suitable solvent is a prereq-uisite. However, cellulose is insoluble in water due to its strong intra- and intermolecular bonding. Some solvents overcome this problem and the processes described below allow to obtain regenerated cellulose. These bio-based products are gaining in importance because of their ability to replace cotton and other textile fibres from fossil origin, such as nylon and polyester.

3.1. Viscose

The viscose process is the most industrialized and consists in dissolving cellulose fibres in carbon disulphide CS2 to manufacture viscose after regeneration in sulfuric acid for the textile industry. Hydroxyl groups in cellulose are derivatized by CS2 to form a cellulose xanthate. This derivative is soluble in alkaline aqueous solution and cellulose is then after regenerated in aqueous acid solution. However, the main disadvantage of this process is the use of a very toxic solvent.

The dissolving pulp is first mechanically shredded and then lets to react with caustic soda (NaOH) in order to favour cellulose swelling: this is mercerization. After controlled cellulose depolymerization with a catalyst (DP around 250) carbon disulphide is added to make cellulose soluble. The yellow prod-uct formed is called cellulose xanthate. The solution is then washed to remove impurities and pumped into a bath of dilute sulfuric acid where the cellulose will be regenerated in the form of filaments that will be stretched: these filaments are called rayon. Remaining products are removed and the yarns undergo finishing treatments [15].

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Viscose fibres have some advantages over cotton fibres. Dissolving pulps are worldwide available in large quantities and they are less expensive. However, this process remains very polluting because it uses CS2 which is highly toxic and not recoverable. This viscose process is currently forbidden in Europe.

3.2. Lyocell

An alternative to the viscose process is a direct dissolution of the cellulose in NMMO (N-methyl-morpholine-N-oxide), an ionic liquid, and subsequent precipitation of the cellulose filaments in a NMMO-water mixture. These fibers are named Lyocell fibers and like the rayon fibers, their major uses are in the textile industry. NMMO dissolves directly cellulose without any chemical modification. The dissolution is made at high temperature (90 – 100 °C). NMMO spreads between the cellulose chains, destroying the hydrogen bonds. The viscous solution obtained is filtered and extruded through a spinneret in an aqueous spinning bath. The solvent is non-toxic and recoverable. The water is recovered by evaporation from the spinning and washing bath. The solvent is recovered up to 97% but it is a very expensive recovery process [17].

Lyocell fibers have interesting mechanical strength but the NMMO solvent is expensive to recovered which contributes to limit the industrialization of the process.

4. Mercerization

Cellulose mercerization consists in soaking the pulp in a 20% concentrated aqueous sodium hydroxide solution. This operation allows the swelling of the fibers walls improving the accessibility of chemicals on the surface of the fibers. In the previous project Dissolucell, mercerization was done before the cellulose periodate oxidation, in order to increases carboxyl introduction.

Mercerization (in sodium hydroxide) changes cellulose from cellulose I to cellulose II and activates cellulose hydroxyl groups. The dissolving ability of cellulose depends on the degree of polymerization and also on its crystallinity.

When soaking in sodium hydroxide, the fibers walls swell and the accessibility of the oxidant on the surface of fibers is improved because they are separated. If a washing treatment is conducted at this step, cellulose structure will rearrange itself so that hemiacetals are found opposite from one chain to another. This new structure confers to cellulose stronger mechanical properties with more hydrogen bonds. A microscopic observation would make it possible to observe a fusion of the wall.

5. Investigation of cellulose oxidation to favor cellulose dissolving ability

Cellulose -OH groups are sensitive to oxidizing agents. The oxidation products of cellulose are usually called “oxycelluloses”. The type of oxidant and the oxidation conditions influence their structure. In particular, carboxyl functions could be introduced in the cellulose chains, which may render cellulose more soluble in aqueous solution. This has been investigated in previous projects conducted at LGP2 in collaboration with CTP (Centre Technique du Papier, Grenoble). TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) oxidation and periodate oxidation have been studied.

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5.1. TEMPO Oxidation

The cellulose oxidation by the TEMPO system has been found to promote efficiently cellulose nano-fibrillation into individual nanofibrils with widths of 3 to 4 nm [18].

The hydroxyl groups in C6 position of the cellulose chain are selectively converted into carboxylic groups [6] (Figure 4). As a result, the fibrils of the fibers separate from each other due to the repulsive forces which are superior to the hydrogen bonds holding them together.

Figure 4. TEMPO oxidation of cellulose [6]

In previous studies, Saito et al [19] have also shown that the TEMPO oxidation leads to cellulose depolymerization, probably due to the intermediate formation of aldehyde groups which undergo beta-elimination in alkaline medium.

This has been also observed in the Dissolucell project where the TEMPO system has been investigated. Applied on a dissolving pulp with an initial DPv around 900, the TEMPO oxidation decreased the DPv till to 276 although the carboxyl content has been increased (from 6.5x10-3 COOH/100 AGU to 0.2 COOH/100 AGU after oxidation). The TEMPO system is thus too aggressive and another system has been studied, periodate oxidation followed by chlorite over-oxidation.

5.2. Sodium periodate oxidation

Sodium periodate (NaIO4) is well-known to react with vicinal hydroxyl compounds and oxidizes cellulose on C2 and C3 carbons of AGUs units. Hydroxyl groups are converted into aldehydes resulting in a breaking of the glucose ring between carbon 2 and 3. The periodate oxidation of an inner and a terminal sugar unit in a polysaccharide structure is illustrated in Figure 5.

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Figure 5. Periodate oxidation of an inner and a terminal sugar unit in a polysaccharide structure

During this oxidation, periodate is partially consumed so a certain amount remains in the solution. Besides, the reaction also produces its reduced form, iodate IO3

-.

The carbonyl groups formed are very reactive and can be selectively oxidized into carboxyls by sodium chlorite (Figure 6).

Figure 6. Periodate oxidation (step 1) and chlorite oxidation (step 2) of cellulose [18]

Used at room temperature, periodate is usually applied in large excess during long reaction times in order to maximise the oxidation .

Moreover, in their work, Jonas et al. [20] studied the periodate oxidation and its influence on cellulose crystallinity. They prepared a solution at cellulose concentration 13.3 g/L and added 5 moles of periodate for 1 mole of anhydroglucose units, thus with a large excess of periodate and they studied the influence of the pH on dialdehyde formation, in buffered solutions at pH 5.5 and 4.5 and unbuffered solution at pH < 4.5, changing the reaction time 24, 48, 72, 96, 168, and 240 h. Results showed that cellulose becomes more compact when the degree of oxidation increased but the crystallinity decreases with lower pH and when the reaction time increases. The pH doesn’t influence the degree of oxidation. Contrary to Kim and Kuga [21] they achieved more than 80% oxidation yield.

Isogai et al [22] showed the influence of aldehyde content in relation with the morphology of the nanocellulose obtained. The dispersion is more homogeneous when the aldehyde content increases due to repulsive forces. Henrikki et al [18] talked about high oxidized cellulose when the carboxyl content is between 1.20 and 1.75 mmol/g.

This feature indicates that the oxidant’s attack on crystalline cellulose proceeds highly heterogeneously. It suggests that the oxidation of crystalline cellulose is a self-accelerating process; i.e., when a glucopyranose ring on the surface is converted to dialdehyde, the neighbouring groups become more susceptible to oxidant because of the local loss of crystalline order.

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Numerous studies already investigated the cellulose oxidation by periodate. In particular one publica-tion relates the addition of metal salt [24] to increase the conversion of hydroxyl into carbonyls, vary-ing also the temperature of the reaction. This could be a way to improve the carboxylation of cellulose. The wall of cellulose fibers is full of nanopores that avoid chemical to penetrate the structure and decrease the reaction kinetic as well as the reaction yield. Salt (NaCl in the case of Nur Alam et al [25]) is supposed to swell the fibers and thus increases pores size involving a better accessibility within the structure by increasing the ionic strength. This higher ions concentration in pores than in the bulk does not create depletion attraction that closes the pores and it leads to a raise in oxidation rate in the periodate oxidation. Besides, the strong intermolecular bonding inside the cellulose structure is broken due to the formation of chloride ions which makes a stronger bonding with hydroxyl groups [26].

In their work, Sirvio et al [24] proposed an optimization of the quantity of periodate introduced to oxidize cellulose. They worked with a mass of dry cellulose of 0.5 g and the concentration of the sus-pension during the oxidation is 10 g/L. A ratio (moles of periodate/moles of AGU) equal to 0.621 is favoured. In theory, the maximum oxidation rate would be 62.1% and as 1 mole of NaIO4 reacts with 2 moles of carbonyls, the maximum formation of carbonyls should be 1.2 mol per AGU unit or 7.65 mmol CHO/g of cellulose. In practice, 0.432 mmol of aldehyde per gram of cellulose is obtained at room temperature. They obtained a maximum amount of aldehyde (2.842 mmol CHO/g) after 3 hours reaction at 75°C, by oxidizing 37.1% of the AGUs. Sirvio et al [24] also catalysed the reaction with lithium chloride (LiCl). In case of the molar ratio LiCl / AGU equal to 7 (21 mmoles LiCl for 0.5 g cellulose), 1.226 mmol of aldehyde/g of cellulose are obtained after 1 hour reaction at 55°C. The maximum aldehyde content obtained is 3.785 mmol/g (75°C – 3h), which represents approximately 50% oxidation. In comparison, the same amount of aldehyde is found for the non-assisted oxidation after 2 hours at 75°C. However, cellulose DPv has not be controlled. That is however an important parameter.

5.3. Sodium chlorite over oxidation

Over oxidation using chlorite allows to convert the carbonyls created by the periodate oxidation into carboxyls.

In their study, Henrikki et al [18] oxidized 6 g cellulose with sodium periodate at 55°C, followed by chlorite overoxidation at room temperature for 48 hours. The periodate oxidation temperature has been varied and aldehyde ranging from 0.36 to 1.68 mmol/g have been obtained after periodate oxidation. However, oxidation conditions (periodate and chlorite content), except temperature, are unknown. Chlorite has been introduced in excess in order to convert all aldehydes into carboxylic acids because the presence of residual aldehydes after chlorite oxidation may cause depolymerization of cellulose by β-elimination due to hydrolysis at glycosidic bond level [27].

Chlorite oxidation has also been used after hydrogen peroxide oxidation [28]. The formation of aldehyde is noticed to be limited below 85°C while processing chlorite oxidation. This is due to the creation of ketones instead of aldehyde during the hydrogen peroxide oxidation. In that way, it is confirmed that sodium chlorite only reacts with carbonyls.

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6. Periodate recovery

Periodate is expensive (lab price = 200 to 300 €/kg) and harmful so its recovery is necessary for industrial process. Besides the oxidation of cellulose requires a large amount of periodate. Some recent researches investigated its regeneration using sodium hypochlorite or ozone. The objective is to reuse the IO4

- that has not been consumed and reoxidized the IO3- in IO4

-.

6.1. Periodate regeneration using ozone

Elisabeth Höglund [29] studied both regeneration with sodium hypochlorite and ozone. Ozone was mixed with the filtrate solution recovered after cellulose periodate oxidation. The reaction was per-formed at room temperature for 17h or at 50°C for 2 to 5h, varying the pH. IO4

- regeneration was obtained when the pH is alkaline. According to this work, the experiment can be conducted at room temperature for 17h but it is not noticed how this recovered periodate is efficient.

Jonas Lindh et al [20], realised several oxidations of cellulose with periodate with long reaction time up to 240 h. The effluent after oxidation was collected and treated with ozone. In a first approach they manage to recover 70% of the periodate. The same regeneration procedure was employed by Slavica Koprivica et al [30] who recovered up to 100% of periodate. The amount of recovered periodate depends on the iodate concentration, thus at pH 13 periodate was fully recovered from a 100 mM iodate solution at room temperature. They performed a cyclic process 3 times and obtain 90% periodate regeneration after the third one.

In the process, the pH is first adjusted to 13 with a sodium hydroxide aqueous solution, then the filtrate is treated by ozone (whose concentration was between 0.1–151.3 mg.L-1) to regenerate IO4

- and finally the pH is brought back to pH 4 with diluted HCl solution, before starting another cycle of cellulose oxidation.

6.2. Periodate regeneration using sodium hypochlorite (NaClO)

NaClO is a secondary oxidant that has been used in the process of regeneration of an aqueous solution of periodate. The IO4

- regeneration from IO3- is the following:

NaClO + NaIO3 + 2NaOH → NaCl + Na3H2IO6

Elisabeth Höglund [29] showed that a high amount of hypochlorite (3.3 times the amount of initial periodate) is required for a 2 hour reaction time at 100°C in order to achieve 95% recovery. In more recent work, Slavica Koprivica et al [30] obtained 81% recovery with a ratio of 1.52 and a reaction time of 4 h. With a lower ratio of 0.95 and a reaction time of 2.5 h the regeneration was almost as high (77%).

18

VI – Material and methods

1. Material

The pulp used in this study is the dissolving pulp Tembec Biofloc 94 (Tartas mill, France) obtained by sulphite process. The DPv is around 900.

The chemicals used are the following:

From Carl Roth: sodium periodate (99,8%), sodium hydroxide (soda) (≥98%), sulfuric acid (96%), so-dium chlorite (80%), acetic acid (99,5%), sodium sulphite (98%), potassium iodide (99,5%), sodium thiosulfate, methylene blue, hydrochloric acid (37%).

From Sigma-Aldrich: cupriethylene-diamine, Hydroxylamine hydrochloride (99%), sodium borohy-dride (CMR) (98%), sodium carbonate (99,8%).

From Fluka: barbital (5,5-diéthylbarbituric acid or veronal) is from Fluka.

2. Methods

2.1. Pulp oxidation

The pulp oxidation protocol has been already proposed in the previous projects Rococo and Dissolu-cell. The general protocol is presented in figure 7.

The diagram in appendix 1 illustrates the experimental manipulations described. On the left, from top to bottom, the general protocol.

1. Market dissolving pulp re-suspension / pulping in water

2. Periodate oxidation NaIO4 (2h – Room temperature / 50°C – in the dark – with or without additive : LiCl)

3. Over oxidation with an excess of chlorite NaClO2 (16h – Room temperature)

4. Cellulose dissolution in aqueous sodium hydroxide (Room temperature)

Figure 7. General protocol for the oxidative sequence NaIO4/NaClO2 before dissolution of cellulose and its regeneration

19

2.1.1. Pulping

The pulp in the form of sheets is pulped, fluffed by hand and then its average dryness is measured to form samples of 20 g of pulp in dry equivalent as each periodate oxidation will be done on this amount of pulp. The average dryness is around 41% (two different pulping batches were prepared during the project).

2.1.2. Periodate oxidation

This reaction is carried out on 20 g of pulp in dry equivalent. Reaction proceeds in an Erlenmeyer under magnetic stirring away from light. The amount of periodate introduced depends on the ratio (moles of periodate/ moles of anhydroglucose unit) which is chosen equal to 0.3, 0.621, 1 in a first time. The pulp is concentrated at 1% (10 g/L), so reaction occurs in a 2L solution. After periodate is completely dissolved, the pH is measured. The pulp is introduced little by little then the pH is measured again. After 2 hours, the final pH is controlled. The periodate solution is titrated before the reaction with cellulose and after the reaction after filtration to determine the periodate consumption (see titration section). The pulp is properly washed and stored in the fridge for the chlorite over-oxidation.

2.1.3 Chlorite over oxidation

The hydroxyl groups present in C2 and C3 of the AGUs units have been oxidized previously with sodium periodate to form carbonyl groups. The chlorite over oxidation will form carboxyl. Chlorite is introduced in excess to oxidize all the aldehydes to prevent the cellulose from being unstable during dissolution in concentrated sodium hydroxide because of the presence of unwanted carbonyls. The reaction conditions are thus the same for all samples. It is carried out at room temperature, in the dark without any stirring. 25 g of chlorite is weighed and distilled water is added up to 500 g. 10 g of pulp are introduced when the chlorite is dissolved. Then 5 ml of acetic acid are added. The whole is let to react 16h at room temperature. The pulp is finally filtered, properly washed and stored in the fridge.

2.1.4. Cellulose dissolution in sodium hydroxide (10%)

The pulp is then after dissolved in a sodium hydroxide solution prepared at 10% concentration. The dissolving ability of the oxidised pulp in this solvent will be determined by gravimetry after centrifugation and filtration of the undissolved fraction. The dissolved fraction will be used to make yarn in a regeneration solution. This will be fully described in this section “material and methods”.

5% of never dried oxidised cellulose (w) are introduced very gradually in the 10% sodium hydroxide aqueous solution during 2 hours under stirring at room temperature (5g cellulose into 100g of the solution). At the end of the dissolution, the solution is centrifuged at 4000 rpm during 30 min. The supernatant (dissolved fraction) is recovered for yarn production. The undissolved fraction is abundantly washed to remove soda, and weight after filtration and dehydration, to calculate the dissolution yield as follows:

ƞ(%) = (1 −𝑚𝑎𝑠𝑠 𝑜𝑓 𝑛𝑜𝑛 − 𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒

𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑒𝑙𝑙𝑢𝑙𝑜𝑠𝑒 ) ∗ 100

20

The dissolved fraction is regenerated into sulfuric acid 4N with a needle (Figure 8).

Figure 8. Regeneration in sulfuric acid 4N

2.2. Pulp and process analysis

2.2.1 Cellulose viscosimetric degree of polymerization

Cellulose degree of polymerisation has been measured by viscosimetry: DPv. A framing of desired DP value can be established as a function of the properties of the regenerated yarn.

The protocol follows the AFNOR NHT 12-005 & ISO 5351/1-1981 standard. 50 mg of pulp, knowing its dryness, are introduced into a flask with 50 ml of 0.5 M cupriethylenediamine (CUED). The mix is purged with nitrogen, covered with aluminium (no light) and placed under stirring for 2h. The DPv measurement is then made in a thermostatic bath at 25°C using a capillary viscosimeter (calculations in appendix 2)

2.2.2. Reduction of carbonyl functions prior to cellulose viscosimetric degree of polymerization

The aim is to reduce the carbonyl groups present on cellulose prior to its dissolution into CUED for DPv measurement. The reduced carbonyl groups no longer cause depolymerization once in the CUED (alkaline medium). It is therefore interesting to use sodium borohydride in the case of oxidized pulp containing carbonyl groups on carbohydrates.

5 g (in dry equivalent) of never dried pulp are introduced in a beaker containing 250 ml of distilled water. 50 mg of sodium carbonate are introduced. Then 50 mg of NaBH4 are added in this alkaline solution (pH between 8 and 9). After 2 hours, the pulp is filtered and dried at room temperature for the DPv measurement.

2.2.3. Periodate titration and consumption

By iodometry Two iodometric titrations are realised with sodium thiosulfate. One on the fresh periodate solution after complete dissolution before the reaction with cellulose and one on the filtrate recovered after 2 hours, at the end of the periodate oxidation.

21

The redox couples involved during this dosage are:

𝐼𝑂4− ∕ 𝐼−

𝐼2 ∕ 𝐼− (yellow brown /colourless)

𝑠2𝑂32− ∕ 𝑠4𝑂6

2− (incolore/incolore)

During the iodometric dosage, the oxidant (𝐼𝑂4−) reacts with the reducing agent (𝐼−) which is

introduced in excess.

Iodine formed 𝐼2 (oxidant) is then titrated by thiosulfate (reducing agent) and the solution will change colour from yellow-brown to colourless.

½ Redox reactions occurring:

(1) 𝐼𝑂4− + 8𝑒− + 8𝐻+ → 𝐼− + 4𝐻2𝑂

(2) 2𝐼− → 𝐼2 + 2𝑒−

(3) 2𝑆2𝑂32− → 𝑆4𝑂6

2− + 2𝑒−

Titration reactions equilibrium :

(4) 𝐼𝑂4− + 8𝐻+ + 8𝐼− → 𝐼− + 4𝐻2𝑂 + 4𝐼2

(5) 𝐼2 + 2𝑆2𝑂32− → 2𝐼− + 𝑆4𝑂6

2−

Thus, 1 mole of 𝐼𝑂4− gives 4 moles of I2 and 1 mole of I2 is titrated by 2 moles of thiosulfate. So, at the

equivalent point: 𝑛𝐼𝑂4− =1

8∗ 𝑛𝑡ℎ𝑖𝑜𝑠𝑢𝑙𝑓𝑎𝑡𝑒

The protocol of the titration is the following: in a beaker, add 20 ml sulfuric acid 4N, then 20 ml of a 10% KI solution and 10 ml of the solution to be titrated. The equivalent volume of titrated thiosulfate is read when the solution becomes colourless.

During the cellulose periodate oxidation, iodate (IO3-) is created. Therefore, the two reactions below

are those which occur simultaneously during the final assay of the reaction medium, after reaction with cellulose. Indeed, the reaction medium contains a mixture of iodate (reaction product) and periodate (the amount that doesn’t react with cellulose).

IO4- + 8 e- + 8 H+ → I- +4 H20

IO3- + 6 e- + 6 H+ → I- +3 H20

In the fresh periodate solution, there is no iodate, as a consequence, iodometry could be used. However, the filtrate after oxidation contains a mixture of periodate and iodate. To distinguish both oxidants, another titration method has been used, UV spectroscopy.

By UV spectroscopy

The figure 9 shows the absorption spectrum of iodate and periodate. It is clearly seen that the absorption spectra are different.

22

Figure 9. Absorption spectrum of periodate and iodate between 200 and 350 nm.

A calibration curve (figure 10) has been made at 290 nm on two solutions, one containing only periodate, and the other one only iodate. For that, NaIO4 and KIO3 solutions have been prepared at 4.10-3 M. Initial solutions have been diluted with various dilution factors 1/10, 3/10, 5/10, 7/10. Then the absorbance has been measured for each solution. Results shown that iodate does not absorb at 290 nm, the absorption did not vary with the concentration. To complete this analysis, it has been verified that a solution containing 50% of each initial solution at 4.10-3 M, led to the right concentration of periodate (2. 10-3 M) (grey point in the calibration curves).

Figure 10. Calibration curves for periodate and iodate at 290 nm

23

UV analysis of the filtrates at 290 nm would allow to measure the concentration of periodate alone thanks to Beer Lambert’s law:

𝐴𝐵𝑆 = ɛ ∗ 𝑙 ∗ 𝐶

(ɛ in L.mol-1.cm-1 equal to 0.224 thanks to the calibration curve, l = 1 cm, C in mol/L)

Thanks to initial titration with sodium thiosulfate and UV spectroscopy of the solution after the oxidation, a calculation of the amount of periodate consumed can be deduced:

2.2.4. Carboxyl groups quantification on cellulose

Quantification using the methylene blue method The methylene blue method is the most common for small amounts of carboxyl in cellulose. The cationic charge of this basic dye allows it to have good adsorption on cellulose fibers and good interaction with its anionic charges [31]. The method will be described in the material and methods part.

The aim of this experiment is to determine the quantity of carboxyl groups generated after a double oxidation of cellulose (with periodate then with chlorite). These groups make it possible to increase the solubility of cellulose in an alkaline aqueous medium. The first method employed to measure COOH rate uses methylene blue.

50 mg of pulp is let to react without stirring overnight with a solution of methylene blue – 0.1 M of methylene blue and 2.5 ml of 0.06 M barbital solution – then filtered. The absorbance of the filtrate is measured at 664 nm.

Let C be the initial concentration of methylene blue, and C’ the concentration of methylene blue after the reaction, M the exact dry mass of cellulose that reacts with methylene blue overnight, E is the mass of water introduced in addition, the number of milliequivalents of carboxylic groups per 100 g of cellulose is given by the formula:

[𝐶𝑂𝑂𝐻] =𝐶. 0,05 − 𝐶′(0,05 + 𝐸. 10−3)

𝑀. 100

The concentration of methylene blue in the solution after reaction must be higher than half of the initial concentration. The mass of cellulose will be adjusted accordingly.

Quantification made by conductimetric monitoring The aim is also to measure the level of carboxyl groups in oxidized cellulose. An excess of hydrochloric acid is added on the oxidized cellulose aqueous suspension. Then, the excess of HCl and the amount of RCOOH are titrated by a sodium hydroxide solution of a known concentration. The titration is followed by conductometry. In a similar protocol, 300 mg of dry oxidized pulp are added in a 50 ml solution containing 1 mM NaCl under stirring. 15 ml of 0.01 M HCl are added bringing the pH around 2.5 – 3. A slight amount of 0.01 M sodium hydroxide is added under conductimetric and pH monitoring up to a pH around 11.

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2.2.5. Carbonyl’s quantification

The measure of carbonyls content after the periodate oxidation can allow to establish a relation between carboxyl’s content and carbonyls. During the chlorite oxidation, it is supposed that all carbonyls will be converted in carboxyl as the chlorite is present in excess. In his thesis, Julien Leguy [32] proposed 2 methods based on Green’s publication (1960). The amount of aldehydes is deduced from a reaction with hydroxylamine hydrochloride. The method involves a reaction between the aldehydes of cellulose dialdehydes and a hydroxylamine salt, NH2OH.HCl, under mild acidic conditions, to form an oxime upon a release of hydrochloric acid. The quantity of hydrochloric acid released is determined by reverse titration by adding excess sodium hydroxide, which is titrated by a solution of chloric acid of known concentration. The principle of this reaction is represented in the Figure 11.

Figure 11. Principle of the titration of carbonyls by oximation

1 mole of carbonyl undergoes the oximation reaction with 1 mole of hydroxylamine releasing 1 mole of H+. The H+ are titrated thus indicating indirectly the carbonyls present (it is assumed that all the carbonyls react).

In a typical experiment, 100 mg of cellulose react with 25 mL of an aqueous 0.25 M hydroxylamine hydrochloride solution at room temperature for 2 hours. The initial pH is around 3.2. Then a certain volume of HCl is added and the pH decreases. After the 2 h reaction, the sample is titrated with a NaOH solution (0.01 M) back to pH 3.2. However with this protocol strong acidic conditions are met so another method is proposed. Instead of adding HCl, NaOH 0.12 M is added to pH 5.2, then it is titrated by HCl to the initial pH 3.2.

However, hydroxylamine is present in high concentration in comparison with the low aldehydes content. Even if both experiments are done with a blank without cellulose, the too high hydroxylamine and sodium hydroxide content make the interpretations of results difficult and approximative and it is necessary to look at other methods in literature.

In their work, Calvini et al. [33] compare the conventional method based on hydroxylamine without titration (oximation with a buffer pH of 4.4 in acetic buffer), with a high hydroxylamine/AGU ratio equal to 32.4, then determination of the nitrogen by gas chromatography (with device EA 110 CHNS-O) with a direct analysis with a new method based on a direct titration of oxidised cellulose by sodium hydroxide (without the use of hydroxylamine).

Results are relatively similar in both methods. In the direct titration, oxidation rates of AGU vary between 2% and 46% (which represents 20 mmol to 460 mmol dialdehyde or 40 to 920 mmol CHO per mole of AGU) while the method using hydroxylamine seems to give smaller oxidation rates for high oxidation values. Calvini et al explained this with the heterogenous kinetic limitation of the oximation at the high degree of oxidation of AGUs.

Experiments use pure cellulose (Whatman paper n°1) and room temperature periodate oxidations that last 1 h, 24 h, 48 h, 120 h and 264 h. The cellulose concentration is 1 g/100 ml, the periodate concentration is 0.1 M and the ratio periodate/AGU is 1.62. The initial pH is 4.2 and final pH (respectively to the previous oxidation times) are 4.2, 4.10, 4.08, 3.94 and 3.68. Cellulose needs to be properly washed with distilled water after periodate oxidation and then dried at room temperature.

25

The method consists of 30 to 100 mg of oxidised dried cellulose immerged in an exact 100 ml volume of sodium hydroxide 0.01 M. The reaction occurs between 1 and 2 hours then 10 ml of HCl 0.01 M are added to the solution and the sodium hydroxide consumption is measured by titration of the excess HCl with 0.01 M sodium hydroxide. During the 1 to 2 hours reaction in soda, the solution tends to turn yellow and become slightly opalescent. The weight loss in sodium hydroxide after the reaction was also measured by filtering the sample through a filter crucible at the end of the analysis.

Jürgen et al [34] proposed a method based on fluorescence. A Fluorophore is attached to an amine (from hydroxylamine) thanks to a spacer. Then the fluorophore is analysed by fluorescence at a proper wavelength. The spacer enables to have a homogenous separation between amine and the fluorophore. Then GPC (Gel Permeation Chromatography) is processed to get a repartition of aldehyde and thus a homogeneous value of carbonyls. This method is efficient but requires a specific material.

26

VII – Results and discussion

The study has been conducted on a dissolving pulp substrate of 900 DPv. This is a commercial pulp used for the production of regenerated cellulose by a conventional process.

The objectives of the project are to fully dissolve this pulp in an aqueous sodium hydroxide solution in a sufficient concentration (objective 1) to regenerate the dissolved cellulose into yarn (objective 2). Additionally, the possibility of periodate recycling/regeneration to make the process economically viable, will be studied.

To render the dissolving pulp soluble in sodium hydroxide solutions, the pulp will be oxidized by the periodate/chlorite system to introduce hydrophilic carboxyl groups on the cellulose chains.

1. Cellulose oxidation

1.1. Optimisation of the cellulose oxidation by the periodate/chlorite system

The periodate oxidation conditions applied are the following: 2 hours, without light, room temperature and variable dosages of periodate, expressed in mole periodate / mole of AGU in cellulose (r value in the table 1).

To introduce a large quantity of carboxyl groups, addition au salts as done by Sirvio in his work [24] will be also tested. LiCl has been selected. 36 g LiCl has be added (moles of LiCl/moles of AGU = 7). We assume that around 5 COOH units/100 AGU are required to improve the cellulose solubility in our solvent. This quantity is our objective.

First, the pH before and after the periodate oxidation have been measured. Results are given in the following paragraph.

1.1.1. pH measurements

The pH is controlled during the periodate oxidation. This value is important since acidic conditions will favour the cellulose depolymerisation. The table 1 presents the initial pH of the periodate solution before the introduction of pulp. Then the pH is measured just after the pulp introduction and finally after the 2 hours reaction.

Conditions r Initial pH pH just after adding cellulose

pH end of the reaction

Room T°C 2h

1 0.621

0.3

3.9 4.2 5.1

4.0 3.6 4.2

3.2 3.4 3.3

Room T°C 2h – LiCl

1 0.621

0.3

3.9 4.6 5.4

3.7 4.2 5.2

2.7 2.9 2.8

50 °C 2h – LiCl

1 0.621

0.3

4.1 4.2 5.3

3.1 3.4 4.7

1.4 1.7 2.1

Table 1. pH monitoring for 3 experiments

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First during the periodate oxidation, pH is decreasing whatever the operating conditions. The pH starts acidic and decreases to 1.5 to 3.5 depending on the conditions applied. pH decrease is more important with LiCl addition and when temperature is higher. In these strong acidic conditions, the cellulose aspect after the overoxidation with chlorite totally changed, the cellulose is completely gelled and strongly depolymerised (Figure ).

Figure 12. Visual aspect of the pulp in the case of a periodate oxidation heated at 50°C with LiCl

Cellulose depolymerisation can be the result of the pH and/or the temperature effect. To investigate these effects, a periodate oxidation was conducted in an acetate buffer 0.1 M to keep constant the pH during the reaction (pH around 3.5 – 4). For this experiment, the stochiometric ratio perio-date/AGU was 1 and three different conditions have been tested. The first one without LiCl and ace-tate buffer, the 2nd one without LiCl but with an addition of an acetate buffer and the 3rd one with both LiCl and the acetate buffer. The three reactions are carried out at 50°C during 2h. Results are given in table 2.

Conditions (r=1) No LiCl – No buffer With a buffer – No LiCl

With a buffer and LiCl

pH before pulp introduction

5.6 3.9 3.6

pH after pulp introduction

5.3 3.8 3.4

pH after 2h 3.0 3.7 3.3

Table 2. pH monitoring for 3 experiments heated at 50°C with/without buffer, with/without LiCl

With the buffer, the pH remains almost constant in all cases but the oxidized cellulose is still like a gel, depolymerisation again occurs. It can be concluded that the temperature elevation to 50°C clearly degrades cellulose. At this temperature it is possible that more aldehydes are created and thus carboxyl groups but the cellulose also undergoes significant depolymerization which is not wished for the regeneration (Figure 13). In order to preserve cellulose, it has been decided to pursue the experimentations at room temperature in the further work.

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Figure 13. Visual aspect of pulps obtained after 2 hours reaction at 50°C. No LiCl and no buffer, no LiCl with a buffer and with LiCl + buffer, respectively.

1.1.2. Periodate consumption during cellulose oxidation

Periodate consumption is calculated after residual periodate titration in the oxidation filtrate. UV spectroscopy method has been used (see material and methods section).

Results are presented in table 3.

Conditions r NaIO4 consumed

(%)

n(COH) formed (mmol)

COH units for 100 AGU

% oxidized AGU

Room T°C 2h

1 0.621

0.3

2.6 11.5 9.4

6.3 17.7 7.0

5.1 14.4 5.6

2.6 7.2 2.8

Room T°C 2h LiCl

1 0.621

0.3

2.6 4.5 5.8

6.5 7.0 4.3

5.3 5.6 3.5

2.6 2.8 1.7

50 T°C 2h LiCl

1 0.621

0.3

32.9 32.7 42.4

81.1 50.2 31.5

65.8 40.7 25.5

32.9 20.3 12.8

Table 3. Periodate consumption and carbonyls content

When the oxidation is processed at 50°C, more periodate is consumed. However periodate can also degrade itself, without participating directly to cellulose oxidation.

Thanks to the amount of periodate consumed, the carbonyl content could be calculated, making the hypothesis that all consumed periodate reacts to form carbonyls. That means that 1 mole of periodate produces 2 moles of carbonyls (CHO) per AGU.

𝑁𝐶𝑂𝐻 =𝑛𝐶𝑂𝐻

𝑛𝐴𝐺𝑈∗ 100

With 𝑁𝐶𝑂𝐻 the number of COH units formed for 100 AGU units. The percentage of oxidized AGU is then obtained by dividing 𝑁𝐶𝑂𝐻 by 2, because there are, at most, 2 COH units in one AGU unit.

At room temperature, between 3.5 to 15 CHO have been created per 100 AGU, whereas, values are significantly higher (>25) at 50°C. The highest values found at 50°C are not explained.

29

1.1.3. Determination of carboxyl content in cellulose after periodate/chlorite oxidation

After periodate oxidation, an overoxidation has been performed with chlorite, with an excess of oxidant, to convert carbonyls into carboxyls. The operating conditions of the chlorite oxidation are given in the material and methods section. Carboxyl groups are determined by the methylene blue method, also explained in the material and methods section.

The figure 14 shows the number of COOH for 100 AGU after the periodate oxidation and after the overoxidation using chlorite, depending on the conditions used in the periodate oxidation of cellulose.

Figure 14. Number of carboxyl after methylene blue quantification for samples oxidized 2 hours at room temperature.

Before any oxidation, the COOH quantity in the pulp is 0.87 / 100 AGU. After periodate oxidation, the carboxyl content is around 3.4/100 AGU whatever the quantity of periodate applied. As expected, overoxidation increased the carboxyl content and the quantity of carboxyl is improved when the dose of periodate was augmented. The target of 5% AGUs oxidized is not so far with the explored conditions since in our trials, more than 5 COOH/AGU are obtained, and 5 COOH/AGU is equal to 2.5% AGU oxidized as there are 2 COOH groups in one AGU unit.

For samples oxidized with addition of LiCl, and samples oxidized at 50°C, the results are shown in appendix 3 as they are not satisfactory: no conclusion or trend can be drawn. Indeed, for the oxidation using LiCl, it can be hypothesized that LiCl modifies the reaction and more chemical parameters need to be studied to understand the mechanism, but it doesn’t improve the COOH rate either the cellulose DPv. Results are cleaner and repeatable at room temperature without any salt addition.

The highest COOH content is obtained for the periodate/AGU molar ratio equal to 1. A comparison can be made with the results obtained in the Dissolucell project (table 4).

2h – Room T°C 2h – 50 °C – LiCl Dissolucell

r 1 0.621 0.3 0.621 0.3 1

COOH / 100 AGU 5.5 4.7 4.2 5.8 4.1 8,2 Table 4. Number of COOH units for 100 AGU for the samples oxidized at 50°C and room temperature compared with the

rate obtained in Dissolucell

In the Dissolucell project, a higher COOH content has been found, using similar operating condition in the periodate/chlorite oxidation system. The main difference is the presence of a mercerization step before the oxidation that probably allows a better swelling of the fibers, which may improve the oxidation. In our case, mercerisation is not made to reduce the cost in sodium hydroxide.

30

To confirm the COOH results, a conductometric titration has been also carried out. Both methods, methylene blue and conductometry, gave similar results. The figures in appendix 4 present the conductivity monitoring as well as pH. The degree of oxidation calculated thanks to the equations written in the method is 3.8 COOH/100 AGU. Appendix 5 presents the same 2 figures but the reactional medium is composed of 50% distilled water and 50% ethanol, the precision is considered similar as the previous method.

The magnitude is similar with both methods and precision is not higher for one method from another. For practical reason, next measures will be done with the protocol using methylene blue.

1.1.4. Cellulose degree of polymerisation

Calculated by viscometry, all values are presented in the table 5. It gathers values obtained with and without sodium borohydride reduction. Borohydride reduction is made to convert residual carbonyls into hydroxyls to avoid the cellulose depolymerisation during dissolution in the solvent CUED. In fact, CUED is slightly alkaline and β-elimination may occur during dissolution if cellulose exhibits carbonyls. This will distort the DPv results.

Sample designation r DPv

Without NaBH4

reduction

Overoxidized by chlorite

0.3 0.621

1

623 531 446

With NaBH4 reduction

Overoxidized by chlorite

0.3 0.621

1

652 568 512

Oxidized by periodate

0.3 0.621

1

679 578 439

Table 5. DPv for pulp oxidized by periodate at room temperature

Cellulose oxidation affects the DPv since the initial DPv was around 850. Cellulose DPv after periodate and after the full oxidation (periodate followed by chlorite) are similar. Cellulose depolymerisation is mainly due to the periodate oxidation and not due to chlorite oxidation. Chlorite oxidation is known to not affect the cellulose DPv.

A slight difference is observable between overoxidized samples if they are or aren’t reduced. The difference is not significant, since it is comprised in the accuracy of the DPv measurement (± 50). Another explanation is that some of COH created remain in cellulose after overoxidation and the theoretical complete transformation of COH into COOH is not true despite the excess of chlorite during the overoxidation. Besides, the DPv measured for the initial dissolving pulp is also slightly different if the reduction is applied (DPv = 879) or not (DPv = 838) before DPv measurement. Again, the difference is low, close to the measurement error, or it is possible that some carbonyl groups are already present in the initial pulp.

Values of DPv for samples oxidized with LiCl as additive are presented in appendix 6. Same observation has been made: periodate oxidation is responsible for cellulose depolymerisation.

31

The figures 14 presents the evolution of the DPv as a function of the COOH units / 100 AGU. This trend was expected: the more COOH are created, the more cellulose is depolymerised. Both figures are issued from 2 different oxidative reactions with a month difference. The leading coefficient is nearly the same, so a general law can be estimated. From left to right, the points represent, in both figures, the initial dissolving pulp, an oxidized pulp with a ratio equal to 0.3, then 0.621 and finally a ratio equal to 1.

Figure 14. DPv evolution as a function of carboxyl creation for over oxidized samples oxidised in January and November.

Results show that there is a relation between cellulose DPv and COOH groups introduction. When the periodate oxidation proceeds, cellulose depolymerisation is increased.

Therefore, it could be possible to decrease the DPv by increasing the periodate content during the oxidation in order to increase the COOH rate. The DPv is still at a reasonable value for r=1 (DPv higher than 400), so there is a flexibility on this parameter.

1.1.5. Determination of carbonyl content in cellulose after periodate oxidation The theoretical carbonyl quantification is issued from the periodate consumption. To verify the hypothesis, carbonyls have been titrated after cellulose oximation by hydroxylamine. In this method, 100 mg of cellulose react with 25 mL of an aqueous 0.25 M hydroxylamine hydrochloride solution at room temperature for 2 hours. The initial pH is around 3.2. Then a certain volume of NaOH (0.12 M) is added and the pH increases till 5.2. After the 2 h reaction, the sample is titrated with a HCl solution (0.01 M) back to pH 3.2. A blank solution without cellulose is made with the same conditions. The issue is that hydroxylamine is a massive molecule that cannot react with all the carbonyls. So the degree of oxidation calculated is underestimated. Besides, when the COH content is deduced from the COOH content measured after overoxidation, the amount of COH is also underestimated as chlorite do not react with all COH groups. New experiments inspired by the literature (Calvini) using sodium hydroxide should allow to obtain a better framing to estimate the COH content.

32

Because, the hydroxylamine method is not adapted for the quantification of low amount of carbonyls present in cellulose, the COOH groups created are compared with theoretical COH groups, already presented before (Table 6. Comparison between carbonyl and carboxyl content

Conditions r Calculated COH units/100 AGU

COOH units/100 AGU

Room T°C 1 0.621

0.3

5.1 14.4 5.6

5.5 4.7 4.3

Table 6. Comparison between carbonyl and carboxyl content for the oxidation proceeded at room temperature.

In the case of conventional periodate oxidation, CHO seems to be converted totally into COOH, except for the molar ratio 0,62. The reason is not known.

However, when the solution is heated, the periodate consumption leads to an increase in carbonyls formed, without correlation with COOH formed after overoxidation (see appendix 7). Therefore, it is highly possible that periodate is degraded when it is heated at this temperature and a certain content does not participate to cellulose oxidation.

1.1.6. Optimisation of the periodate oxidation conditions To increase the carboxyl content, new periodate/AGU molar ratios have been tested, 1.5 and 2. Indeed, the cellulose DPv measured after the first trials, at lower periodate dosages, is at least superior than 350 in the case of overoxidized pulp (r=1) without prior NaBH4 reduction before DPv measurement. Cellulose for textile application can withstand more depolymerisation, since in viscose process the DPv is around 200-250. Moreover, higher COOH rate may favour cellulose dissolution into alkaline solutions. In this part, same experiments are done on the dissolving pulp: oxidation with periodate and over oxidation with chlorite, followed by pH monitoring, COOH rate determination and cellulose DPv measurement. Operating conditions of chlorite oxidation are unchanged and periodate oxidation is made at room temperature, during 2 hours as before. Only the dose of periodate applied is varied.

Table 7 shows the results of pH as well as periodate consumption. The periodate concentration is, in this experiment, calculated by UV spectroscopy before and after the reaction.

r Initial pH pH just after adding cellulose

pH end of the reaction

IO4- consumed

(%)

1 1.5 2

5.1 5.0 4.6

5.1 4.7 4.5

5.0 4.9 4.8

3.1 7.9 3.7

Table 7. pH monitoring and periodate consumed

pH values are similar to those obtained with lower amounts of periodate and the oxidized cellulose presents a correct visual aspect (no gel aspect). Even for higher ratio, the consumption of periodate is still low. For the ratio equal to 1.5, the consumption is twice bigger than for the others, but this result is difficult to explain, even considering the IO4

- titration accuracy. As a consequence, it is imperative to think about periodate recovery and reuse.

33

The amount of carboxyl groups is also determined and results are presented in Figure 15.

Figure 15. COOH rate determination (oxidation conditions: 2h – room temperature)

Focus is made on the COOH content after chlorite over oxidation. It can be observed that COOH amount increases with the periodate/AGU molar ratio 1 to 1.5, but increasing this ratio till to 2, does not change the COOH content. A maximum of 5.2 COOH / 100 AGU is obtained. Moreover, even if the COOH content does not increase from periodate/AGU molar ratio 1.5 to 2, cellulose DPv still decreases (figure 16).

Figure 16. DPv for experiments at room temperature for 2 hours with molar ratio equal to 0.3, 0.621, 1, 1.5 and 2 from left to right

0 0,5 1 1,5 2 2,5

200

250

300

350

400

450

500

550

600

650

700

r

DP

v

Last measures Previsous measures

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Even if the cellulose DPv measurement, repeated twice for the periodate/AGU molar ratio 1 (r=1), varies from around 100 units for r=1, globally cellulose DPv exhibits a constant decrease with the increase of the quantity of periodate applied. Cellulose DPv values are all still over 250 which was the value obtained in the Dissolucell project. The absence of mercerisation before oxidation in our case may explain this difference.

The difference between samples can further be observed by X-ray diffraction to observe crystalline regions and maybe difference in the crystallinity structure. It can be observed via optical microscopy as shown in a following part “Optical microscopy and FT-IT analysis”. X-ray diffraction can be interesting to characterize the structure and to have a better understanding of effect of the cellulose oxidation by the periodate/chlorite system.

1.2 Investigation of radical production during periodate oxidation by EPR/Spin-trapping

1.2.1. Theory

During periodate oxidation, cellulose DPv is decreased. This part focuses on the analysis of periodate oxidation mechanism possible side reactions during its reaction. To study the mechanism of action of sodium periodate on cellulose, the analytical method of electronic paramagnetic resonance (EPR) analysis is used to detect single electron containing species that are called paramagnetic molecules, in particular radicals. This technique also allows to obtain information on the molecular environment of studied radicals [38].

Following the application of a magnetic field, this spectroscopic technique detects transitions of un-paired electrons in a molecular species. As a proton, an electron has a spin which gives it a magnetic property called “magnetic moment” [40]. An external magnetic field it applied and allows electrons to orient in a parallel or antiparallel direction compared to the direction of the applied magnetic field. To these two directions correspond two energy levels E1 and E2. The EPR measures the transitions of unpaired electrons between these two levels.

At the beginning, electrons are located in the lowest energy level and superior levels are unpopulated. To move the electrons from the lower level to the higher level, a fixed irradiation frequency is used. The resonance condition is obtained when the external magnetic field and the applied frequency allow the transition of electrons between energy levels. The resonance condition is written hν = 𝑔𝑒 ∗ 𝐵0 ∗ β, where “hν” corresponds to an extra energy brought by the irradiation frequency which is able to switch spins from lower energy levels to higher energy levels, β is the Bohr magneton, 𝐵0 is the mag-netic field and 𝑔𝑒 is a constant of proportionality [38]. The spectrum (absorption curve) represents the absorbed energy as a function of the external magnetic field. Because of device sensitivity, the registered curve is the 1st derivative of the absorption curve [40].

In this study, EPR is used to detect hydroxyl radicals, suspected to take part in sodium periodate reac-tions on cellulose. As the lifetime of hydroxyl radicals is short, they can’t be analysed by simple EPR. The spin-trapping method is used to detect them thanks to a “spin-trap” that catches radicals and form an adduct, another radical species more stable (Figure 17). The spectrum obtained is character-istic of the stable species, and can be studied.

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Figure 17. General scheme of a radical scavenging experiment followed by an EPR Analysis [translated from French to English, 41]. With R the radical, P the trap, (PR) the spin adduct.

Usually, the trap used to detect OH° radicals is 5,5-diméthylsulfoxide-1-pyrroline N-oxyde, DMPO. It can be oxidized and gives DMPO-Ox° with an associated characteristic spectrum with 6 lines. It can also be modified by an addition reaction and form DMPO-OH° with a different spectrum, a 4 lines spectrum with hyperfine splitting constants aN = aH = 14.9 G.

The spacing between the observed lines corresponds to an isotropic hyperfine coupling constant, aH.

Sodium formate is another spin-trap. It is used as a scavenger molecule to differentiate DMPO-OH° resulting from a nucleophilic addition of water on DMPO, and DMPO-OH° resulting from the presence of hydroxyl radicals. Formate scavenger leads to the DMPO-COO- adduct in the presence of hydroxyl raicals.

1.2.2. EPR/Spin-trapping trials

The experimental work has been done in the Molecular Chemistry Department – UGA in Grenoble.

The table 8 presents the chemicals molar content in a tube designed for the analysis. The 7 lines describe the composition of the samples. Many combinations need to be realised in order to compare all spectrums and identify the presence or the absence of specific radicals.

Chemicals (µmol)

N° of experiment

Composition NaIO4 DMPO Formate Model

compound Total volume (µL)

1 NaIO4 only 0.3 300

2 NaIO4 + DMPO 0.3 30 300

3 NaIO4 + DMPO +

Formiate 0.3 30 1500 300

4 NaIO4 + DMPO +

glucose 0,3 30 1.125 300

5 NaIO4 + DMPO +

glucose + formate 0.3 30 1500 1.125 300

6 NaIO4 + DMPO + methyl glucose

0.3 30 1.125 300

7 NaIO4 + DMPO + methyl glucose +

formate

0.3 30 1500 1.125 300

Table 8. Composition of analysed samples and chemicals content.

36

The introduction of all chemicals in a tube is made in the following order : distilled water, model compound (glucose or methyl glucose), the spin-trap (formate), the other spin-trap (DMPO), then the oxidant (sodium periodate) at the end.

For the model compound, the concentration is deduced from the molar ratio :

periodate : modele compound = 1 : 1.875

The chemicals content in table 9 are calculated for 1L solutions.

Preparation Concentration

NaIO4 Mass : 0.214 g for 1L solution M = 213.89 g/mol

2.0.10-2 M

Glucose Mass : 0.676 g for 1L solution M = 180.156 g/mol

3.75.10-3 M

Methyl glucose Mass : 0.728 g for 1L solution M = 194.182 g/mol

3.75.10-3 M

DMPO Mass : 11.32 g for 1L solution M = 113.16 g/mol

0.1 M

Sodium formate Mass : 230.15 g for 1L solution M = 46.03 g/mol

5 M

Table 9. Preparation of each chemical solutions for EPR/Spin-trapping study.

The kinetic of the possible degradation of periodate may be followed by spectrum acquisition after 30 min, 1h, 1h30 reaction time. The following paragraphs present the results. For each combination, the tube is analysed at room light and/or UV light exposure.

1.2.2.1. Trials made on visible light exposure For the first analysis, the behaviour of periodate in water only is investigated (experiment 1) (Figure 18). If periodate is exposed at visible light, the signal obtained is only noise and does not correspond to any radical species.

Figure 18. X-band EPR spectra of NaIO4 under room light exposure.

This observation allows to conclude that periodate is not a radical.

37

Moreover, no signal (only noise) appeared whatever the trials (experiment 2 to 7) if the tube is exposed at the visible light. No radical could be detected even in the presence of a spin-trap.

Next analysis presented are all made under UV light exposure with addition of spin-trap (DMPO) to capture possible radicals.

1.2.2.2. NaIO4 + DMPO with UV light exposure With a UV light exposure, in the trail mixing periodate and DMPO, the creation of radicals is fast and intense as shown in the figure 19. Besides, the intensity of the signal increased during time until t=1297 sec, then it decreased, due to the decrease in radical content.

Figure 19. X-band EPR spectra of NaIO4 + DMPO under UV light exposure

The spectrum observed is specific of the DMPO-OH° adduct so the presence of hydroxyl radicals is highly possible. However, as seen in the theory, the DMPO-OH° adduct can also result from the nucleophilic addition of water on DMPO. In order to discriminate this nucleophilic addition, sodium formate is added.

1.2.2.3. NaIO4 + DMPO + Formate with UV light exposure

The adduct that is observed with the addition of formate is DMPO-COO- and on the figure 20 both adduct, DMPO-OH° and DMPO-COO- are visible. The blue curve presents the experimental acquisition while the orange one is the simulation made thanks to experimental data.

38

Figure 20. X-band EPR spectra of NaIO4 + DMPO + Formate under UV light exposure

It allows to have a clean curve and to estimate the quantify of each adduct (table 10).

Adduct Content (%) aN (G) aH (G) DMPO – NaIO4 –

Formate DMPO-OH° DMPO-COO-

28 72

15.17 15.91

14.91 19.27

Table 10. Adduct content determined thanks to the simulated spectrum.

The quantity of the adduct DMPO-COO-, higher than DMPO-OH°, allows to confirm the presence of hydroxyl radicals in the solution. The small content of DMPO-OH° could also come from the nucleophilic addition of water. Thus this combination is necessary while determining the presence of radicals.

1.2.2.4. NaIO4 + DMPO + Model compound with UV light exposure Under UV light exposure (Figure 21), in presence of a cellulose model compound, radicals are also created. This could mean that during the cellulose oxidation, some radicals interfere and could depolymerise it.

Figure 21. X-band EPR spectra of NaIO4 + DMPO + Model compound under UV light exposure

39

Thanks to this analysis, some hypothesis could be drawn. Periodate is sensitive to light exposure, especially UV radiations. The cellulose is processed in a glassware covered by an aluminium barrier to avoid light interferes with the solution. But the pulp washing is done under light exposure, so during a moment, the solution containing pulp and periodate is exposed to the room light radiations. Therefore, an oxidation in a dark room is tested, as well as its washing with distilled water. The room is completely dark and the lab work is done thanks to a red-light lamp. Results on DPv and COOH content are similar to the ones obtained previously. Cellulose depolymerisation during periodate oxidation could not be explained by the presence of hydroxyl radicals generating by UV exposure. Further investigations are thus required to explain why cellulose is depolymerised during the oxidation.

2. Dissolution of the oxidized cellulose into an aqueous sodium hydroxide solution First trials of dissolution have been made on the oxidized cellulose, using 2% (in weight) of cellulose in the solvent. The solvent is an aqueous sodium hydroxide solution containing 10% (in weight) NaOH. The never dried pulp was introduced in the solution rapidly and let to dissolve during 2 hours at room temperature. Different oxidized celluloses (different periodate/AGU ratios) have been used. A photo of a resulting solution after 2 hours dissolution is given in figure 22. .

Figure 22. On the right, small sparklings are obtained after regeneration in sulfuric acid. On the right, a picture of the gel that is recovered after filtration of the dissolved pulp in soda (r=1)

As seen in figure 22, dissolution is not fully achieved, and small sparklings are obtained after an attempt of regeneration of the liquid fraction, in a diluted sulfuric acid solution. Moreover, dissolution of 2% cellulose in weight is not enough for cellulose regeneration, the solution should be more concentrated.

The dissolution procedure has been thus modified: the oxidized cellulose, never dried before dissolution, is recovered after the two-step oxidation, after filtration. The dry matter of the oxidized cellulose is around 30% at this step. Then this cellulose substrate (wet) is introduced slowly during around 1.30 hour in the sodium hydroxide solution. 5 g oxidized cellulose (counted in oven dried material) are put to dissolve in 100g of sodium hydroxide aqueous solution, so that the percentage of cellulose is 5%. Dissolution is made a room temperature and it lasts around 2 hours. Figure 23 presents photos of the resulting solutions after 2 hours dissolution.

40

Figure 23. Dissolved pulp (5% consistency) in sodium hydroxide (10% w). From left to right, ratio equal to 0.3 – 0.621 – 1

Using this novel dissolution procedure, dissolution seems to be improved. The obtained solutions are viscose and homogenous. The dissolution yield is gravimetrically determined after separation of the dissolved and undissolved fractions through centrifugation, and after weighing the washed undissolved fraction.

The dissolution yields are given in table 11.

r 0.3 0.621 1 1.5 2

Yield (%) 14 29 38 44 56 Table 11. Dissolution yields for different oxidation ratio conditions.

Cellulose dissolution yield is linked to the periodate/AGU molar ratio applied during the cellulose oxidation and thus to the COOH content. The figure 24 presents the evolution of the dissolution yield in function of the number of COOH units for 100 AGU.

Figure 24. Dissolution yield as a function of COOH units for 100 AGU.

The dissolution yield rises with the COOH content in the cellulose till to 5 COOH /100 AGU. Then the COOH content is constant but the yield still increases. It can be due to the decrease of the DPv that reduces the molecular weight of cellulose and improves its dissolution (Figure 25).

Both COOH content and low DPv are parameters influencing cellulose dissolution.

41

The maximum dissolution yield achieved is around 55%.

Figure 25. Dissolution yield qs a function of DPv.

3. Regeneration of dissolved cellulose in sulfuric acid into yarn

3.1. Yarn formation The regeneration is proceeded thanks to a needle plunged into the dissolved cellulose, then plunged into diluted sulfuric acid solution. The regeneration, when it occurs, is immediate and a yarn can be taken out of the bath. Then it can be dried and even rolled (figure 26).

Figure 26. Regenerated cellulose obtained for a ratio equal to 1 in sulfuric acid 4N.

First regenerations are tested on oxidized celluloses made using periodate/AGU molar ratios of 0.3, 0.621 and 1, in sulfuric acid solutions of concentrations 2N, 4N or 8N. The table 12 summarizes whenever regeneration is possible or not. The content of COOH in the different oxidized celluloses are also given.

r 1 0.621 0.3

COOH / 100 AGU 5.4 4.7 4.3

Acid 2N Little yarn No No

Acid 4N Yes Little yarn No

Acid 8N Yes Yes Little yarn

Table 12. Summary of possible regeneration for different oxidized celluloses

Regeneration is also possibly linked to the COOH content.

42

For higher ratio (1.5 – 2), the carboxyl content is similar as for the ratio 1 and the regeneration is possible. Hand resistant yarns are obtained (Figure 27) and will be tested et compared with viscose yarns at Gemtex (Textile Research Laboratory, Roubaix) in some weeks.

Figure 27. Yarn drying

3.2. First analyses of the regenerated cellulosic yarns The yarns obtained present a certain resistance when they are hand-manipulated. Some observations of the intern structure of the yarn can be done by optical microscopy. The diameter for each yarn can be measured (Figure 28). However, the same needle is used for regeneration, the diameter can be influenced by the hand-pression of the user.

Figure 28. Optical microscopy observation of yarns (x20), respectively containing 5.4 - 5.3 - 5.3 COOH/ 100 AGU. Diameter (µm) = 261.3 - 283.75 - 265.4

43

Figure 29 and Figure 30 present the optical structure of yarns with a COOH content equal to 5.3 with and without polarized light. It appears homogenous with no weak points.

Figure 29. Optical microscopy observation of a yarn with Figure 30. Polarized optical microscopy observation COOH content = 5.3 / 100 AGU (x10) to see the porosity of a yarn with COOH content = 5.3 /100 AGU (x20)

The structure of the yarn obtained from cellulose with 5.3 COOH/100AGU (but with another periodate content for the oxidation) also present a heterogeneous organization (Figures in appendix 9).

Some yarns have been analysed via FT-IR. For example, the FT-IR spectrum, in figure 31, shows the presence of the absorption band corresponding to ionized COOH groups (COO-) at 1720 cm-1. This band is not present on the spectrum of the non-oxidized cellulose, analysed in the Dissolucell project. Besides, FT-IR spectra of all oxidized cellulose are similar, all of them exhibit the COOH characteristic band (see appendix 10).

Figure 31. FT-IT analysis made on the yarn regenerated after oxidation at ratio equal to 1.5

4. Recycling of the filtrates originating from cellulose periodate oxidation As already discussed, periodate is expensive and harmful. The high doses of periodate used during the cellulose oxidation will be a key issue for the industrialization of the process. However, it has been shown that yarns could be generated via this process.

44

As periodate consumption is very low, less than 10% in all oxidation trials, the possibility of periodate recycling has been investigated. For that the filtrate recovered after a first oxidation, and containing large amount of residual periodate, is reused to oxidize the dissolving pulp. Then the filtrate is again reused, and in total, 4 oxidative cycles are done reusing each time the filtrate of the previous oxidation. The first oxidation is done using a periodate/AGU molar ratio of 1. The initial volume of periodate solution was 2L, after each filtration, the volume of the filtrate is reduced so the amount of cellulose is adjusted in consequence. Moreover, no new periodate is added in the recycled filtrates. As a consequence, the oxidations using recycled filtrates are made with slightly lower periodate/AGU molar ratios. Figure 32 presents the consumption of periodate for each oxidation cycle, in mmoles.

Figure 32. Consumption of periodate (mmol) at each oxidation cycle

Figure 32 shows that the periodate consumption is not varying in a large extend, whatever the cycle. The pulp consumed always almost the same quantity of periodate, whatever the initial dose of periodate.

Figure 33 presents the percentage of periodate consumed compared to the initial amount of periodate introduced in the first oxidation.

Figure 33. Total percentage of periodate consumed after each cycle.

45

After the first oxidation, almost 10% of periodate has been consumed, then 15% when the filtrate is reused for the first time. The periodate consumption linearly increased with the filtrate reuse cycles.

The amount of pulp used in the oxidation is adjusted with the volume of the filtrate, but the periodate concentration decreases in the filtrates (as it is consumed in the previous oxidation). So the ratio, moles of periodate/AGU decreases. The evolution of this ratio as a function of the reuse cycle is presented in the Figure 34.

Figure 34. Evolution of the periodate/AGU molar ratio with the number of cycles

Logically the periodate/AGU molar ratio decreases when the filtrate is recycled. It can be interesting to adjust the quantity of cellulose, knowing the residual periodate in the recycled filtrate to be reused or to add fresh periodate to maintain the ratio constant.

Based on these 4 experiments reusing the same solution of periodate and the oxidant content measured after each oxidation step (each cycle), it is possible to have the periodate content in the solution before starting another oxidation cycle, and thus, to adjust the mass of cellulose to introduce. The figure 35 presents the periodate content in the solution (in mol) for each cycle.

Figure 35. Moles of periodate present in solution for each oxidative cycle

46

Thanks to the figure 35, the number of cycles can be determined till all periodate will be consumed and is equal to 8 (lower rounding). The periodate content can be estimated thanks to that curve and thus the mass of cellulose to introduce. For 8 cycles, it should be possible to oxidize 82 g of cellulose (in dry equivalent).

If the filtrate recycling seems to be possible, the COOH content of the oxidized cellulose should be checked, since a minimum of COOH is required to dissolved cellulose into the aqueous alkaline solution before regeneration. Cellulose DPv should be also analysed.

47

VIII – Conclusion and perspectives

The project has proven that cellulose could be dissolved in an aqueous alkaline solution, in a sufficient concentration to be regenerated into yarn. This was possible if cellulose is enriched with COOH groups.

For that, cellulose has been oxidized by the periodate/chlorite system. In our study, only the periodate oxidation conditions have been investigated and an excess of chlorite has been used for the overoxidation. To get a minimum of 50% dissolution yield, the dissolving pulp should contain at least 5 COOH/100 AGU. This oxidation rate is obtained using a periodate / AGU molar ratio of 1 and more, and a further oxidation using chlorite in excess. The resulting oxidized pulps are partially depolymerized but cellulose DPv higher that 300 are obtained which is sufficient for the production of resistant textile yarn in the viscose process.

Despite the periodate oxidation is a well-known reaction, the oxidation mechanism is still under investigation. Dialdehyde cellulose is produced but the conversion of hydroxyl into carbonyl is still low, even with high periodate doses. In parallel cellulose is depolymerised which supposes that side-reactions occur, possibly reactions involving radicals. To study this hypothesis, analyses of electron paramagnetic resonance (EPR)/spin-trapping have been made to investigate the possible presence of hydroxyl radicals, suspected to take part in sodium periodate reaction on cellulose. As the lifetime of hydroxyl radicals is short, they can’t be analysed by simple EPR. The spin-trapping method is used to detect radicals thanks to a “spin-trap” that traps radicals and form an adduct more stable. The spectrum of the adduct is characteristic of the trapped radical. Results showed that periodate alone is not a radical and that hydroxyl radicals are detected only if UV radiations are present. Under light exposure, no radical could be detected. Cellulose depolymerisation during periodate oxidation is still not explained because even in the absence of UV or visible light, cellulose still suffers from depolymerisation after the oxidation.

To be implemented at industrial scale, periodate doses should be optimised. For that, the residual periodate recycling has been studied. It has been shown that the filtrate of a previous oxidation could be reused as a fresh periodate solution. However, the quality of the oxidized cellulose (COOH content and cellulose DPv) has not be verified at this moment, this will be tested during the last weeks of the internship. Moreover, a complete recovery of periodate could be envisaged using sodium hypochlorite or ozone for example to re-oxidize iodate into periodate. This should be also investigated later.

If this project shows that the formation of a hand resistant yarn is possible, there is still lot of work to produce textile grade yarn. Stretching when the yarn is wrapped around the winder is required to get yarn with high strength. A laboratory spinneret has been recently developed at LGP2 and I started some spinning with the dissolved oxidized celluloses. After getting yarns, their properties will be compared with Viscose or Lyocell fibers. These physical and chemical properties will be essential to go back to parameters that need to be optimize.

Finally, other alternatives exist to modify the dissolving ability of cellulose. For example, the use of hydroxylamine. The industrial synthesis of nylon uses the reaction of hydroxylamine with cyclohexanone (1) to give an oxime (2) which rearranges in an acidic medium to give caprolactam (3) (Figure 36). Then a ring-opening polymerization on caprolactam give nylon 6. The use of hydroxylamine could be interesting to dissolve cellulose in sodium hydroxide after a pre-oxidation with sodium periodate. the resulting aldehyde compound after periodate oxidation (“dialdehyde” cellulose, DAC) can be further converted to carboxylic groups (with chlorite oxidation) or imines (Schiff bases) [35]. The influence of hydroxylamine should be interesting if it presents a certain stability with carbonyls that need to be checked in the experimental part.

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Figure 36. The industrial synthesis of nylon uses the reaction of hydroxylamine

The reaction between hydroxylamine and aldehydes or ketones gives oxime, illustrate by the Figure 37.

Figure 37. Oxime formation during the reaction between hydroxylamine and aldehydes or ketones [36]

Oximes are easily reduced to amines and then can be used to manufacture synthetic fibers. Hydroxylamine hydrochloride is a powerful reducing agent that can be used for this purpose. It can be prepared by the hydrolysis of nitroalkanes (RCH2NO2) and the catalytic hydrogenation of nitric oxide (NO) [37].

When the reductive elimination is performed with hydroxylamine, the functionalized beads that can be produced show a very smooth texture and a perfectly spherical shape [20]. It produced beads with a high degree of oxidation and can ensure several ways to modify cellulose. It also doesn’t need ionic organic solvent to achieve cellulose dissolution.

In a study conducted by Ung-Jin Kim et al [23], they proceed to the conversion of DAC samples to oximes by Schiff base reaction with hydroxylamine. The reagent (0.0125 mol) was dissolved in 100 mL of pH 4.5 acetate buffer (0.1 M) and added to 10 mL of DAC suspension containing 0.1 g or 0.05 g of cellulose. The mixture was stirred at 20 °C for 24 h and the product was recovered by centrifugation. The elemental composition was determined for C, H, and N by atomic absorption spectroscopy. This could be also examined in another project.

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IX – References

[1] Société chimique de France, [visited 03/03/2020]. Available: https://www.societechimiquedefrance.fr/Cellulose.html.

[2] Université virtuelle de Tunis, Les particularité de la cellulose végétale [consulté le 14/12/2020], https://www.uvt.rnu.tn/resources-uvt/cours/biologie-physiologie-vegetale/chap1b/Chapitre-3/Sous-section-3-2-1.html

[3] Monica Spinu, Evaluation des paramètres physiques et physico-chimiques qui influencent l'accessibilité de la cellulose, Matériaux, École Nationale Supérieure des Mines de Paris, 2010

[4] Mazza Mathieu. Modification chimique de la cellulose en milieu liquide ionique et CO2 supercritique. PhD, Institut National Polytechnique de Toulouse, 2009

[5] Anne Michud, Blandine Giustini (2010), Les fibres cellulosiques à usage textile, [visited 03/21], available on http://cerig.pagora.grenoble-inp.fr/memoire/2010/fibre-cellulose-textile.htm

[6] Björn Lindman, Gunnar Karlström, Lars Stigsson, On the mechanism of dissolution of cellulose, Journal of Molecular Liquids, Vol 156; Iss 1, 6 pages, 27 April 2010]

[7] Kontturi, Osterberg, Cellulose model films and the fundamental approach, Chemical Society Reviews, Vol 35, Issue 12, p 1287-1304, décembre 2006

[8] Woodings, Regenerated cellulose fibers, Woodhead publishing, 1st edition, 352 pages, 2001

[9] Y. Xu. Yunhui Xu, Chen Huang, Effect of Sodium Periodate Selective Oxidation on Crystallinity of Cotton Cellulose, College of Light-Textile Engineering and Art, Anhui Agricultural University, Hefei, China, Advanced Materials Research, Vol 197-198, 5 pages, 2011

[10] Moon R J, Martini A, Nairn J, et al. Cellulose nanomaterials review: structure, properties and nanocomposites, Chemical Society Reviews, 2011, 40(7): 3941-3994.

[11] Benoit Arnoult-Jarriault, Extraction des hémicelluloses de pâtes papetières pour la production de pâtes à dissoudre, Thèse de doctorat en Génie des procédés, Université Grenoble Alpes, 216 pages, 17 décembre 2015

[12] Jérôme Le Roux, Modification Des Fibres Cellulosiques et Amélioration Des Propriétés Hydrophiles Des Pates Bisulfites, Ecole Doctorale Des Sciences Chimiques en chimie organique, Université de Bordeaux, 195 pages, mars 2003

[13] Gordon Floe, Pöyry Management Consulting, Dissolving pulp: The great come back; It’s All About Cotton, TAPPI PEERS Dissolving Pulp Forum 19 pages, 2011

[14] Herbet Sixta, Hand book of pulp, Vol 1, Wiley-VCH, 1291 pages, 2006

[15] Linda Östberd and Ulf Germgard, Some aspects on the activation of dissolving pulps and the influence on the reactivity in a following viscose, Department of Engineering and Chemical Sciences, Karlstad University, SE-65188 Karlstad, Sweden, May 24, 2012

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[16] Viscose manufacturing process, 2019, [Visited 04/04/2021], available on https://www.deniminnovation.com/2019/02/09/viscose-manufacturing-process/

[17] Anne Michud et Blandine Giustini, Les fibres cellulosiques à usage textile, Élèves ingénieurs 2e Année, Grenoble-INP Pagora, Cerig, Mai 2009, [visited in March 2020], available on : http://cerig.pagora.grenoble-inp.fr/memoire/2010/fibre-cellulose-textile.htm

[18] Henrikki Liimatainen et al., Enhancement of the Nanofibrillation of Wood Cellulose through Sequential Periodate−Chlorite Oxidation, ACS Publication, dx.doi.org/10.1021/bm300319m, Biomacromolecules 2012, 13, 1592−1597

[19]

Saito, T., et al., Individualization of Nano-Sized Plant Cellulose Fibrils by Direct Surface Carboxylation Using TEMPO Catalyst under Neutral Conditions, Biomacromolecules, 2009, 10(7): p. 1992-1996.

[20] Lindh, Daniel O. Carlsson, Maria Strømme, and Albert Mihranyan, Convenient One-Pot Formation of 2,3-Dialdehyde Cellulose Beads via Periodate Oxidation of Cellulose in Water, Biomacromolecules 2014 15 (5), 1928-1932, DOI: 10.1021/bm5002944

[21]

Kim, U. J.; Kuga, S.; Wada, M.; Okano, T.; Kondo, Periodate oxidation of crystalline cellulose, T. Biomacromolecules 2000, 1, 488−492.

[22] Isogai, Akira, and Yaxin Zhou, Diverse nanocelluloses prepared from TEMPO-oxidized wood cellulose fibers: Nanonetworks, nanofibers, and nanocrystals, Current Opinion in Solid State and Materials Science 23.2 (2019): 101-106

[23]

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X – Appendix

Appendix 1. Diagram of manipulations

The diagram illustrates the experimental manipulations described. On the left, from top to bottom, the general protocol, and the pulp and process analysis.

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Appendix 2. Protocol for the DPv

Calculation of DPv

- Calculate the ratio ƞ/ƞ0 using the relationship : ƞ/ƞ0= t/𝑡0

Where 𝑡0 is the CUED solution (0.5 mol/l) flow time and t the flow time of the solution containing cellulose at concentration C.

- Using the table, read the value corresponding to [ƞ]C with C expressed in g/ml.

- Calculate [ƞ] = IVL (Limiting Viscosity Number) by dividing [ƞ]C by C. The result is expressed in ml/g.

- Calculate the cellulose DPv using the formula:

𝐷𝑃𝑣 = 101

0,905log (0,75∗𝐼𝑉𝐿)

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Appendix 3. Number of carboxyl after methylene blue quantification for samples oxidized 2 hours at room temperature or at 50°C with LiCl

In complement with the result without LiCl presented in the report, but here, results are less encouraging.

Figure 38. Conditions: 2h - 50°C - LiCl

Figure 39. Conditions: 2h – Room temperature - LiCl

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Appendix 4. Conductivity monitoring to measure COOH rate with an a medium composed of 100% distilled water

These graphs are made from the COOH quantification by conductimetry. The method is explained in the report. The conductivity monitoring gives 2 equivalent volumes of NaOH that allow to determine the COOH content.

Figure 40. Conductivity monitoring to measure COOH rate for a sample oxidized at a ratio r=1

Figure 41. pH monitoring proceed during the conductivity monitoring

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Appendix 5. Conductivity monitoring to measure COOH rate with an a medium with 50% ethanol

In order to have a more precise determination of COOH content than in appendix 4, the solution that is titrated can contain half ethanol. However, it seems to present the same precision and the same drawbacks. The method using methylene blue gives the same COOH content.

Figure 42. Conductivity monitoring

Figure 43. pH monitoring

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Appendix 6. Values of DPv for samples oxidized with LiCl are presented.

The DPv should be preceded by a NaBH4 reduction as the measure is always underestimated without previous reduction.

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Appendix 7. Comparison between carbonyls and carboxyl content for the oxidation proceeded at room temperature.

When the solution is heated, the periodate consumption leads to an increase in carbonyls formed, without correlation with COOH formed after overoxidation. Therefore, it is highly possible that periodate is degraded when it is heated at this temperature and a certain content does not participate to cellulose oxidation.

Conditions r COH units/100 AGU COOH units/100 AGU

Room T°C 1 0.621

0.3

5.1 14.4 5.6

5.5 4.7 4.3

Room T°C 2h – LiCl

1 0.621

0.3

5.3 5.6 3.5

3.5 2.5 3.6

50 °C 2h – LiCl

1 0.621

0.3

65.8 40.7 25.5

- 5.8 4.1

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Appendix 8. DPv for pulp oxidized with a ratio (NaIO4/AGU) equal to 1 – 1.5 – 2

Samples were analysed with and without prior reduction with sodium borohydride.

r DPv

Oxidized pulp reduced 1

1.5 2

523 361 336

Over oxidized pulp 1

1.5 2

363 348 289

Over oxidized pulp reduced 1

1.5 2

409 392 343

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Appendix 9.Optical analysis of the generated yarns

The structure of the yarn obtained from cellulose with 5.3 COOH/100 AGU (but with another periodate content for the oxidation) also present a heterogeneous.

Figure 44. Optical microscopy observation of a yarn from r=2 oxidation (x10) to see the porosity

Figure 45. Polarized optical microscopy observation of a yarn from r=2 oxidation (x20)

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Appendix 10. FT-IR analysis of a yarn with a COOH content of 5.3 COOH / 100 AGU

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