EP3765527A1

EP3765527A1 - Method for preparing cello-saccharides from cellulose

Info

Publication number: EP3765527A1
Application number: EP19709063.2A
Authority: EP
Inventors: Gérard MORTHA; Claire Boisset; Luis Serrano; Nathalie MARLIN
Original assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Current assignee: Centre National de la Recherche Scientifique CNRS; Institut Polytechnique de Grenoble
Priority date: 2018-03-13
Filing date: 2019-03-12
Publication date: 2021-01-20
Also published as: FR3078971A1; FR3078971B1; WO2019175159A1

Abstract

The invention concerns a method for preparing cello-polysaccharides comprising the steps consisting of: a) providing a cellulose with an average viscometric degree of polymerisation DPvi, b) bringing said cellulose into contact with an aqueous solution of chlorine dioxide with a pH greater than or equal to 8, by means of which a suspension of cello-polysaccharides with an average viscometric degree of polymerisation DPvf in a liquid phase is obtained, where DPvf ≤ 0.60 x DPvi c) recovering said cello-polysaccharides in suspension and, if applicable, the cello-oligosaccharides present in the liquid phase.

Description

Process for preparing cellosaccharides from cellulose

The present invention relates to a process for preparing cellosaccharides by depolymerization of cellulose.

Cellosaccharides are particularly useful as raw materials for the chemical industry. They have a higher added value than the cellulose itself. The cellosaccharides make it possible in particular to synthesize modified cellulose, for example cellulose esters or ethers, such as carboxymethylcellulose (CMC).

Various processes for the preparation of cellooligosaccharides by hydrolysis and depolymerization of cellulose are known, and in particular:

- The enzymatic hydrolysis of cellulose, based on the use of several cellulases to convert all the cellulose into glucose. This process is, however, limited by the high cost of the enzymes, the recovery of which is difficult, and by a long reaction time. The production of oligosaccharides is low because enzymatic hydrolysis leads to a great deal of glucose.

In particular, the acetolysis of cellulose allows simultaneous hydrolysis and peracetylation of the cellulose chain. This method makes it possible to obtain yields of the same order as acid hydrolyses, but it requires an additional step of deacetylation. It has the same disadvantages of corrosivity, harmfulness and control as acid hydrolysis. Nevertheless, the oligosaccharide mixtures obtained are easier to purify.

the acid hydrolysis of the cellulose (by acid catalysis in the heterogeneous phase, or in the presence of a mineral acid). The reagents used are toxic and dangerous. This process leads to corrosion of the installations because the medium is very acidic. In addition, the effluents are very acidic and difficult to treat and recycle. Moreover, there is no selectivity for the formed cellooligosaccharides and secondary reactions take place: a mixture of many cellooligosaccharides and secondary products resulting from the decomposition of glucose in an acid medium (5- (hydroxymethyl) furfural (HMF ) and levulinic acid) is obtained, which makes the purification of the desired cellooligosaccharides very delicate. Finally, like the enzymatic pathway, the production of oligosaccharides is low, and glucose is the main reaction product.

the depolymerization carried out in the presence of catalysts but the cost is very high.

In addition, these processes generally comprise a step of mechanical grinding of the cellulose making it possible to reduce its crystallinity and to make it more accessible to the various depolymerization reagents used. This prior grinding step has the disadvantage of being expensive.

In addition, in the paper industry, millions of tons of paper pulp are produced worldwide with a process using chlorine dioxide in an acid medium (pH 2 to 4). Chlorine dioxide is used as a delignifying and whitening agent for lignocellulosic fibers. The purpose of this bleaching is to eliminate lignin (at the origin of the brown color) and the hemicelluloses, without modifying the cellulose chemically. In an acid medium, chlorine dioxide does not react (or very weakly) on polysaccharides, and its selectivity towards lignin is unique in acidic medium. Chlorine dioxide is therefore used exclusively in acidic medium, and there is a prejudice to use it in other conditions that could lead to the degradation of cellulose, which is not sought in the paper industry.

The development of alternative processes for the preparation of cellosaccharides that do not have the disadvantages of the acidic, enzymatic or catalyzed depolymerization methods described above is required.

For this purpose, the invention relates to a method for preparing cellopolysaccharides comprising the steps of:

a) supplying a cellulose with a viscosimetric average degree of polymerization DPvi, b) contacting said cellulose with an aqueous solution of chlorine dioxide at a pH of greater than or equal to 8, whereby a suspension of cellopolysaccharides with a mean viscometric degree of polymerization DPvf in a liquid phase is obtained,

where DPvf <0.60 x DPvi

c) recovering said cellopolysaccharides,

the DPvi and DPvf being measured by viscometry at 25 ° C (according to Tappi T230 om-99).

In this application:

with "cellosaccharide" (also called cellodextrins) is meant cellopolysaccharide and / or cellooligosaccharide.

by "cellooligosaccharide" is meant that the degree of polymerization is greater than or equal to 2 (glucose is excluded) and less than 20 as measured by MALDITOF mass spectrometry. In the process according to the invention, the cellooligosaccharides are solubilized in the aqueous solution at 20 ° C. obtained at the end of step b).

with "cellopolysaccharide" is meant that the degree of viscometric polymerization is greater than or equal to 20 and lower than that of the cellulose provided in step a). In the process according to the invention, the cellopolysaccharides are not solubilized in the aqueous solution at 20 ° C obtained at the end of step b). They are in suspension.

The cellulose provided in step a) is generally chosen from cellulose pulp, cotton, for example in the form of cotton linters, and microcrystalline cellulose, preferably cellulose pulp or cotton pulp, cellulosic pulp. papermaking being particularly preferred.

As cellulose used as starting material, any kind of paper pulp can be used. It can be obtained mechanically or chemically from virgin fibers (kraft process, sulfite, bisulphite, soda, by an organosolve process, ...), or from the recycling of recovered paper and paperboard. The pulp is preferably kraft paper pulp, especially bleached kraft pulp.

The pulp is a suspension of lignocellulosic fibers in water, generally at a concentration of 20 to 400 grams, especially 50 to 300 grams, typically 50 to 150 grams of lignocellulosic fibers per liter of water.

The consistency of the pulp (which corresponds to the percentage by weight of dry pulp in the aqueous suspension, that is to say the mass of dry cellulosic fibers in grams that contains 100 g of the suspension of cellulosic fibers) is generally between 0.5 and 40%, especially from 1 to 20%, for example between 5 and 15%.

The paper stock may be unbleached or bleached (partially bleached or whitened).

The process may comprise, prior to step a), a pulp bleaching step aO). Any type of bleaching process known to those skilled in the art can be implemented. In particular, the pulp can be delignified by an oxygen stage or partially bleached by a TCF type sequence: OOQP, OZ, OZEop, OZEp, OZE ... or of the EOF type: Odeop, ODEp, ODE, ODEpDEp, or other types of partial bleaching sequence, for example those involving chelating, acidic or reducing stages. These notations of bleaching stages are known from the literature, for example, the two complementary works, published by TAPPI Press, GA, USA: "Dence CW, Reeve D., Pulp Bleaching, Principles and Practices, 4th Edition, 1996." , and "Hart PW, AW Rudie, The Bleaching of Pulp, 5th Edition, 2012". Preferably, the bleaching is carried out by contacting the cellulose paper pulp with an aqueous solution of chlorine dioxide at a pH of less than 7 (that is to say the usual conditions for the bleaching of cellulosic pulp in industry paper).

The pulp (unbleached, partially bleached or wholly bleached) provided in step a) preferably has a kappa number (oxidation potassium permanganate, standardized method according to ISO 302: 2015) between 40 and 0.1, more preferably between 5 and 0.1. This index makes it possible to evaluate the rate of oxidizable functions of the pulp, including mainly the residual lignin, as well as the demand for oxidizing bleaching reagent. The lower the kappa number, the lower the level of lignin and the lower the amount of bleaching reagent to be used.

Preferably, the cellulose provided in step a) contains less than 0.5% by weight, or even less than 1% by weight of lignin.

The process comprises a cellulose depolymerization step, namely the step b) of bringing the cellulose into contact with an aqueous solution of chlorine dioxide at a pH greater than or equal to 8. Under these conditions, the cellulose is depolymerized into cellopolysaccharides with a mean degree of viscometric polymerization DPvf less than or equal to DPvi, and cellooligosaccharides with a degree of polymerization DPo of between 2 and 20. More precisely, under the conditions of step b), cellopolysaccharides with a mean viscometric degree of polymerization DPvf less than 0.60 x DPvi are obtained.

Cellopolysaccharides are not soluble in the liquid phase and are therefore suspended therein. At the end of step b), the degree of viscosimetric average polymerization DPvf of the suspended cellopolysaccharides is typically greater than or equal to 20, in particular it is greater than or equal to 20 and less than or equal to 0.60 × DPvi, for example it is greater than or equal to 20 and less than or equal to 0.50 x DPvi. These cellopolysaccharides are recovered in step c) of the process.

In contrast, the cellooligosaccharides with a degree of polymerization as measured by MALDITOF mass spectrometry (DPo below) of less than 20 are solubilized in the liquid phase. These cellooligosaccharides can be recovered in step e) of the process described below.

Viscosimetry, a standard method according to Tappi T230 om-99, is the usual method for evaluating the degree of polymerization of cellulose and of cellopolysaccharides. Typically, cellulose or cellopolysaccharides are dissolved in cupriethylenediamine (CuED), whereby a stable complex is formed. The DPvi and DPvf are measured by viscometry at 25 ° C. As the viscosimetric method is only applicable to solid cellulose samples, the measurement of the degree of DPo polymerization of the cellooligosaccharides, soluble in the liquid phase, was carried out by MALDITOF mass spectrometry.

Preferably, step b) is carried out by contacting the cellulose with an aqueous solution of pH greater than or equal to 8, and then adding chlorine dioxide, preferably an aqueous solution of chlorine dioxide. The aqueous solution basic makes it possible to obtain a pH greater than or equal to 8 after addition of chlorine dioxide. Under these conditions, the contacting of the cellulose with the chlorine dioxide takes place at pH greater than or equal to 8, whereas according to the conventional use of chlorine dioxide in pulp bleaching processes, this setting contact is carried out in acidic medium. Furthermore, the base is first brought into contact with the cellulose, and before introduction of chlorine dioxide. Thus the contacting of the basic aqueous solution and the chlorine dioxide is carried out in the presence of cellulose, which disadvantages or even avoids the decomposition of chlorine dioxide by the base. The prior introduction of the base generally allows a complete reaction of the chlorine dioxide with cellulose at high pH throughout the time of its action.

Step b) is carried out by contacting the cellulose with an aqueous solution of chlorine dioxide at a pH greater than or equal to 8, generally from 8.0 to 13.5, in particular from 9.0 to 13.0, of preferably from 10.0 to 12.5. The pH is adjusted by addition of a base, preferably chosen from alkali metal hydroxides; alkaline earth metal hydroxides; alkali metal oxides; alkaline earth metal oxides; alone or in mixture. It may especially be NaOH, MgO, Mg (OH) ₂ , Ca (OH) ₂ , KOH, etc., preferably NaOH.

The weight percentage of chlorine dioxide relative to the dry cellulose mass (cellulose mass at the beginning of step b) is from 0.1 to 20%, especially from 1 to 15%, preferably from 2 to 12%. . By "cellulose mass" is meant the dry mass of cellulose contacted at the beginning of step b).

The amount of chlorine dioxide introduced is also generally expressed in the amount of active chlorine, according to the following formula:

Quantity of active chlorine (kg) = 2.63 x Amount of chlorine dioxide (kg)

The quantity of active chlorine introduced is determined according to the nature of the cellulose used as starting material and the degree of polymerization of the desired cellosaccharides, that is to say the desired degree of viscosimetric polymerization DPvf for the cellopolysaccharides and the degree of DPo polymerization desired for the cellooligosaccharides. Generally, the amount of active chlorine introduced is between 0.26% and 52.6% by weight relative to the weight of the cellulose (dry, brought into contact at the beginning of step b).

The contact time between the cellulose and the chlorine dioxide at pH greater than 8.0 is at least a few seconds, advantageously at least 10 seconds, especially at least one minute, preferably at least 10 minutes, and generally less than at 24 hours, especially less than 12 hours, preferably less than 2 hours, typically step b) lasts about one hour. It is adapted according to the degree of polymerization of the desired cellosaccharides.

Step b) is generally carried out at a temperature greater than 5 ° C., or even greater than 15 ° C., in particular from 20 ° C. to 90 ° C., preferably from 40 ° C. to 80 ° C., for example 60 ° C. C at 80 ° C.

Step b) is advantageously carried out with stirring in order to promote the depolymerization.

The degree of viscosimetric polymerization DPvf of the cellopolysaccharides (and the degree of polymerization DPo of the cellooligosaccharides) can (be) adapted (s) according to the wishes of the user depending on the conditions of step b) and the degree of polymerization viscosimetric means of the cellulose provided in step a). The degree of viscosimetric polymerization DPvf (and DPo) is (are) all the lower (s) as:

the degree of viscosimetric average polymerization of the cellulose provided in step a) is low, and / or

the duration of the contacting of step b) is important, and / or

the quantity of chlorine dioxide brought into contact is high, and / or

the temperature during step b) is high, and / or

the stirring of the medium during step b) is intensive.

In a preferred mode of the process, cotton linters are used as cellulose and step b) is carried out at a temperature of 60 to 80 ° C for one hour with an aqueous solution of chlorine dioxide at pH from 9.0 to 13.0 at a mass concentration of chlorine dioxide relative to the cellulose mass of 4 to 8%.

The method comprises a step c) of recovering the cellopolysaccharides suspended in the liquid phase, generally by filtration. This filtration may for example be carried out using a conventional fabric, such as existing industrial press filters already present in the pulp mill. Alternatively, step c) can be performed by centrifugation, but the filtration on canvas, as commonly practiced in pulp mills, is generally simpler to implement than centrifugation.

The process may comprise one or more steps of washing the cellopolysaccharides recovered in step c), generally with water, optionally followed by a drying step. The cellopolysaccharides recovered in step c) therefore have a mean viscometric degree of polymerization DPvf of less than 0.60 x DPvi and generally greater than or equal to 20.

The cellopolysaccharides recovered in step c) carry carboxylic acid and / or carboxylate functions, but in a small amount. These functions are created during oxidative depolymerization by chlorine dioxide. Typically, the amount of carboxylic acid and / or carboxylate functions of the cellopolysaccharides recovered in step c) is less than 0.20 millimoles per gram of cellopolysaccharides (as dry extract), ie less than 3.24 carboxyl functions per 100 units. anhydroglucose (monomer unit of the cellulosic chain), in other words there are fewer than 1.1 carboxylic acid and / or carboxylate functions per 100 R-OH alcohol groups present on the cellulosic chain. The content of carboxylic acid and / or carboxylate functions of the cellopolysaccharides can be determined by adsorption with methylene blue and then UV spectrometry.

The mass distribution of the cellopolysaccharides recovered in step c) is advantageously narrow, that is to say that it has a dispersity D _{M of} less than 3.0, typically of the order of 2 to 2.5 where the dispersity D _M (also referred to as the dispersity index) is the ratio of the weight average molar mass to the number average molar mass of the cellopolysaccharides (the dispersity is 1 for a monomer, or for a polymer consisting of chains single length). Thus, it is known to obtain, by the process according to the invention, a mass distribution of recovered cellopolysaccharides generally narrower than the mass distribution of cellopolysaccharides obtained by hydrolysis in acidic aqueous medium. Without wishing to be bound to a particular theory, the inventors assume that one of the explanations would be that in a basic aqueous medium, the cellulose swells better than in acidic medium and that more areas of the cellulosic matrix are thus accessible to be hydrolyzed. by chlorine dioxide. On the contrary, during a hydrolysis of the cellulose in an acidic aqueous medium, it is known that certain areas of the cellulose do not swell and remain crystalline, preventing access of the acid allowing the hydrolysis of these zones.

The process for preparing cellopolysaccharides according to the invention has the following advantages:

- The raw materials (cellulose, NaOH, chlorine dioxide, water) have a low cost.

In particular, millions of tons of paper pulp are produced worldwide with a process that uses chlorine dioxide (in acidic conditions), among other chemical products, which makes this reagent very cheap (approx. 1, 2 euro / kg). Advantageously, the process can be implemented in an existing bleaching line, only by modifying the pH of the chlorine dioxide solution. No change in reaction temperature or facilities (pH adapted towers and infrastructures, and use of existing tanks and vats for filtrate recovery) is not required. The raw material remains identical, namely the existing cellulosic raw material. The process implemented with pulp as cellulose is therefore particularly inexpensive.

- The process does not require a preliminary step of grinding the cellulose. He is usually exempt from such a step.

DPvf cellopolysaccharide yields are good. They can reach more than 91% (weight of suspended cellopolysaccharides recovered relative to the initial weight of cellulose).

the process allows the industrial production of large quantities of high purity cellopolysaccharides, whereas the methods of the prior art are generally limited to the preparation of small quantities.

the cellopolysaccharides (and the cellooligosaccharides) have a good purity. In fact, the combined action of the alkaline medium and chlorine dioxide generally eliminates the last traces of residual lignin and chromophores, and the other secondary residual compounds that can be found in trace amounts in cellulosic pulps ( saponifiable fats, fatty acids, pectins, proteins, soluble mineral matter, ...). Typically, the cellulose and hemicellulose purity of the cellopolysaccharides is greater than 95%, especially greater than 98%, for example greater than 99%, generally close to 100% and the whiteness of the cellopolysaccharides is very high (greater than 90% ISO - measured according to ISO 2470-1: 2016).

the molar mass distribution of the cellopolysaccharides and the DPO of the cellooligosaccharides are controlled.

Advantageously, the method comprises a step d) of recovering the liquid phase.

Generally, the process induces a relatively large formation of carboxylic acid and / or carboxylate functions in the liquid phase: typically greater than

60% of all the acid functions measured. The acids of the liquid phase can be assayed by acid-base titration. The liquid phase recovered during step d) comprises in particular cellooligosaccharides, at least one organic acid and chlorite, which are valuable compounds. More precisely, it contains:

cellooligosaccharides with degree of polymerization DPo as measured by MALDITOF mass spectrometry of less than 20, generally from 2 to 1 1,

- glucose,

organic acids (in particular formic acid, lactic acid and acetic acid),

- chlorite,

a base (the one that was used to adjust the pH during step b)).

The reaction routes of depolymerization of cellulose are explained in the diagram below. In a first step, the cellulose is hydrolysed into insoluble cellopolysaccharides in the medium and then into soluble molecules, cellooligosaccharides and small saccharides such as cellobiose. The saccharides are then degraded into glucose. Glucose is degraded to acids, such as lactic or acetic acids by dehydration of 1,6-anhydroglucose, and levulinic or formic acids by dehydration of 5-hydroxymethylfurfural.

Acetic acid Lactic acid Formic acid

Scheme: Reaction pathways of depolymerization of cellulose The processes for producing cellosaccharides by acidic or catalytic depolymerization of cellulose lead mainly to glucose and acids. In the process according to the invention, on the contrary, it is sought to minimize the degradation of the cellosaccharides to glucose, and the degradation of glucose to acid.

Generally, the liquid phase recovered in step d) comprises, in dry extract:

from 10 to 50% by weight, typically from 20 to 40% by weight of cellooligosaccharides with a DPo degree of polymerization of less than 20,

from 20 to 65% by weight, typically from 30 to 55% by weight of formic acid,

less than 5% by weight, typically less than 3%, or even less than 1, 5% by weight of glucose.

Preferably, the process comprises, after step d), a step e) of recovering cellooligosaccharides from the liquid phase. These cellooligosaccharides have a DPo degree of polymerization of less than 20, generally from 2 to 1 1.

This recovery of the cellooligosaccharides of degree of polymerization DPo less than 20 from the liquid phase can be carried out by acidification of the liquid phase with a pH lower than or equal to 8 (by addition of an acid, for example of H ₂ SO ₄ ) then separation by steric exclusion chromatography or by ultrafiltration.

These cellooligosaccharides of low degree of polymerization are recoverable in the chemical industry. Cellooligosaccharides of degree of polymerization DPo of 3 or 4 may for example serve as substrates for assaying the cellulase activity and perform biochemical characterizations, those with a degree of polymerization of 7 to 9 are useful for their elicitrices activities on the vine, and those of degree of polymerization of 2 to 20 are useful as precursors of fermentable substrates and as prebiotics.

The method thus makes it easy to obtain:

cellopolysaccharides with a degree of viscometric polymerization DPvf of greater than or equal to 20 (those insoluble in the liquid phase formed during step b, recovered in step c)) and

- Cellooligosaccharides of degree of polymerization DPo less than 20 (those solubilized in the liquid phase formed in step b), recovered in step e)).

The process may also comprise, after step d), a step f) of recovering the formic acid from the liquid phase. Formic acid is a commodity of the chemical industry. It is particularly used in the textile and leather industries, in electroplating and for the formulation of insecticides, lacquers, solvents and household products as a substitute for mineral acids ("anti-limescale"). In the diluted state, it is used in human food (food additive E236). Formic acid is also a preservative and antibacterial agent in livestock feed. In the poultry industry, it is sometimes added to the diet to destroy Escherichia coli. Beekeepers use formic acid as an acaricide. In addition, formic acid is a potential dihydrogen carrier for the fuel cell feed.

The process may also comprise, after step d), a step g) of recovering chlorite from the liquid phase. Chlorite can for example be used as an oxidizing agent active in step aO) pulp bleaching, and therefore in the chain of conventional bleaching of the pulp mill (main line). If it is necessary to neutralize it to regenerate chlorine dioxide, use sulfuric acid already present on site (chlorine dioxide generator effluent), or a conventional stage filtration effluent with chlorine dioxide. chlorine (acid medium) from the main bleaching line. Thus, in the paper industry, the process according to the invention can be implemented in quasi-closed circuit, for water as for minerals, and the need for additional chlorine dioxide is negligible compared to the flow consumed by the main line of bleaching in the factory.

By comparison, the effluents from the depolymerization processes of the prior art require treatments. In fact, the effluents resulting from an acidic depolymerization are very acidic and must be neutralized, and those obtained by enzymatic depolymerization are biochemically active, which requires subsequent treatment.

The appended figures and the examples which follow illustrate the invention.

FIG. 1: Distributions of molar masses M (g / mol) of cotton linters (starting material - curve 1) and of the cellopolysaccharides obtained by implementing step b) in FIG.

30 ° C, 70 ° C or 90 ° C, with either 2% or 6% CI0 ₂ (Curve 2: 2% CI0 ₂ , 90 ° C - Curve 3: 2% CI0 ₂ , 70 ° C - Curve 4: 2% CI0 ₂ , 30 ° C - Curve 5: 6% CI0 ₂ , 90 ° C - Curve 6: 6% CI0 ₂ , 70 ° C - Curve 7: 6% CI0 ₂ , 30 ° C ) (example 1).

2: abundance of the cellooligosaccharides as a function of their degree of polymerization DPo measured by MALDI-TOF mass spectrometry when step b) was carried out at 90 ° C., and at an IC ₂ concentration of 2% (triangles), 6% (round) or 12% (squares) for a duration of one hour (example 2).

FIG. 3 Abundance of cellooligosaccharides as a function of their degree of polymerization DPo measured by MALDI-TOF mass spectrometry when step b) was carried out at 30 ° C. (triangles), 70 ° C. (round) or 90 ° C. ( squares), and at a concentration of ClO ₂ of 6% for a period of one hour (Example 2). FIG. 4 Abundance of cellooligosaccharides as a function of their degree of polymerization DPo measured by MALDI-TOF mass spectrometry when step b) was carried out at 30 ° C. (triangles), 70 ° C. (round) or 90 ° C. ( squared), and at an IC ₂ concentration of 2% for a period of one hour (Example 2).

Examples

The following products were used in the examples.

Celsur bleached cotton linters (in the form of fibers) having a solids content of 93.65% (Example 1) or 93.40% (Example 2) were used as starting cellulose.

The chlorine dioxide solution used was produced by reaction between sodium chlorite, NaClO ₂ (Roth supplier, purity greater than 80%) and sulfuric acid, H ₂ SO ₄ , followed by absorption of the gas in deionized water. The IC0 ₂ concentration of the solution, determined by titration of the chlorinated compounds by an iodometric method, was 6 to 8 g / L.

An aqueous solution of sodium hydroxide 1M NaOH (prepared from Roth NaOH, purity greater than 99%).

A phosphate buffer solution containing sodium hydrogen phosphate, Na ₂ HPO ₄ (Roth supplier, purity greater than 98%).

A phosphate buffer solution containing potassium hydrogen phosphate, K ₂ HPO ₄ (Sigma Aldrich supplier, purity greater than 99%).

A solution containing 10% by weight of potassium iodide, KI (prepared from Roth potassium iodide of greater than 98% purity).

A 0.1 M solution of sodium thiosulfate, Na ₂ S ₂ 0 ₃ (prepared from Roth sodium thiosulfate with a purity greater than 99.5%).

A 2M solution of sulfuric acid, H ₂ SO ₄ (prepared from Roth H ₂ SO ₄ with a purity greater than 96%).

0.002M, 0.1M and 2M solutions of hydrochloric acid, HCl (prepared from a solution of Roth HCl, purity greater than 37%).

A 1M solution of Cupriethylene diamine, noted CuED (Supplier Applichem Panréac).

A dimethylacetamide solution, denoted DMAc (Roth supplier, with purity higher than 99%).

Lithium chloride, LiCl, (Roth supplier, purity greater than 99%). A solution of phenylisocyanate, C ₆ H ₅ NCO (Fluka supplier, purity higher than 99%).

0.04 mM methylene blue solution (Roth Supplier).

A barbital buffer solution prepared from Fluka's 5,5-diethylbarbituric acid solution with purity greater than 99%.

A solution using tetrahydrofuran, THF.

A solution of ammonium carbonate, CO ₃ (NH ₄ ) ₂

The water was deionized water.

Example

Cotton linters were immersed in distilled water for 12 hours to swell the fibers and then defibrated in a disintegrator at 1200 rpm.

Step b) was carried out in polyethylene bags immersed in a thermostated bath and using:

2 or 6% by weight of CI0 ₂ relative to the starting cellulose mass

(expressed as dry matter), corresponding respectively to 5.25 or 15.75% by mass of active chlorine, with respectively 2.37 or 7.1% by weight of NaOH.

10 g of cellulose (expressed as dry matter) in each experiment, with a consistency of 4% (10 g of fiber and 240 g of liquid containing the reagents).

The temperature of the thermostated bath was 30, 70 or 90 ° C and the duration of the contacting of one hour. The pH, controlled before contacting and after the time of contacting, was of the order of 11 to 12.

After step b), the medium was filtered on a filter funnel provided with a sintered glass filter of porosity index No. 2 (step c)). The solid residue remaining on the filter funnel was washed with distilled water to neutral pH and total elimination of chlorinated species, dried in the open air and weighed.

A. Influence of the process conditions on the cellopolysaccharides obtained in suspension in the liquid phase during step b) and recovered in step c)

The mass recovery yield of the solid residue (cellopolysaccharides) was determined after determination of its dry matter content.

The yields are provided in Table 1:

Table 1: Mass yields of cellopolysaccharides recovered in step c)

The average degree of polymerization of the cellopolysaccharides was determined by viscometry at 25 ° C., based on the intrinsic viscosity of a solution of the cellopolysaccharides in a 1: 1 (v / v) CuED / water mixture, that is, that is, at a final concentration of 0.5 mol / L in Cu and 1 mol / L in ED, as provided by Tappi T230 om-99. DPv was measured according to the Tappi T230 om-99 method.

Table 2: DPvf Obtained by Viscosimetry of the Cellopolysaccharides Recovered in Step c)

The average molar masses by weight, Mw, and by number, Mn, were determined by steric exclusion chromatography from cellopolysaccharides derivatives in the form of cellulose tricarbanilate (according to the method described in MORTHA G., MARCON J., DALLERAC D., MARLIN N., VALLEY C., CHARON N., THE MASLE A., Depolymerization of cellulose during cold acidic chlorite treatment, Holzforschung, 2015, 69 (6), 731-736). The results are provided in Table 3.

Table 3: Degrees of polymerization by weight (DPw) and in number (DPn) of the cellopolysaccharides recovered in step c) obtained by steric exclusion chromatography

The results in Tables 2 and 3 and Figure 1 show that

the depolymerization (DPvf / DPvi) of the cellulose of cotton linters is important, even at low temperature (30 ° C.) and / or at low levels of IC0 ₂ (2%),

the proportion of IC0 ₂ has a strong influence on the depolymerization of cellulose,

the reduction of the dispersity index D _M observed at 2% of CI0 ₂ is noticeable, even at 30.degree. The same is observed at 6% CI0 ₂ except at 30 ° C. At this lower temperature, the depolymerization was not complete and a double population of low mass / high mass cellopolysaccharides was obtained, as shown in FIG. This is probably a kinetic limit.

- Figure 1 shows that the cellulose appears more modified after oxidation at 6% CI0 ₂ , and this even at 30 ° C.

The content of carboxylic acid groups of the cellulose used before the process and of the cellopolysaccharides obtained at the end of the process (filtered obtained in step c)) was determined by adsorption with methylene blue using a UV spectrophotometer ( UV-1800 Shimadzu UV) at 664 nm (according to the method described in Davidson GF, "The acidic properties of cotton cellulose and derived oxy celluloses." Part II, The absorption of methylene blue, Journal of Textile Institute 39, pp. T65-86 1948). The concentration of carboxylic acid groups was expressed in mmol / g of cellulose starting point (mass expressed in dry matter). The carboxyl content of the starting cellulose was measured at 0.046 mmol / g of cellulose (expressed as dry matter).

The total carboxylic acid content in the filtrate recovered in step e) was analyzed by conductimetric assay. The measurement was carried out on 20 ml of filtrate diluted to 50 ml with distilled water with 0.02 M HCl. The total acid content was determined from the conductivity curve and compared to the carboxylic acid content. in the cellopolysaccharides recovered at the end of step c).

The combination of the two methods thus made it possible to determine the amount of carboxylic acid groups in both the solid and liquid phases. The experiments were carried out on 10 g of initial cellulose (mass expressed in dry matter).

Table 4: Proportion of carboxylic acid functions in the solid and liquid phases The results in Table 4 show that:

the process enriches the solid phase with carboxylic acids by a factor between 2.8 and 4.4

the process enriches the liquid phase with carboxylic acids

a large majority of the carboxylic acids are present in the liquid phase (between 66 and 88.3%).

The crystallinity of the cellulose and of the cellopolysaccharides (filtered solid recovered in step c)) was determined by X-ray diffraction (XRD). The samples were placed in a cell of 200 μm depth and measurements were made with a PANanalytical, X'Pert PRO MPD diffractometer equipped with a detector X'Celerator 1 D. The operating conditions of the refractometer were: copper Ka radiation (1 , 5418A), (Bragg angle) between 5 and 56 °, not 0.067 °. To calculate the crystallinity index (Cl) of cellulose and cellopolysaccharides from the XRD spectra, an adjustment program was used, assuming Gaussian functions for each peak and a broad peak of about 22.5 ^%. for the amorphous contribution. The results are provided in Table 5.

Table 5: Crystallities measured by XRD

The results show that the process did not modify the crystallinity: the crystallinity of the cellopolysaccharides is identical to that of the cellulose.

B. Influence of the Process Conditions on the Composition of the Liquid Phase Recovered in Step d)

2 or 6% by weight of CI0 ₂ relative to the starting cellulose mass

The temperature of the thermostated bath was 30, 70 or 90 ° C and the duration of the contacting of one hour. The pH, controlled before contacting and after the time of contacting, was of the order of 1 1 to 13.

Another experiment was carried out with 12% CI0 ₂ (31, 55% active chlorine) and 12% NaOH at 90 ° C for 1 h.

After step b), the medium was filtered on a filter funnel equipped with a sintered glass filter of porosity No. 2 (step c)). This first filtrate, containing the cellooligosaccharides in solution, is recovered for analysis. The solid residue remaining on the filter funnel was washed with distilled water to neutral pH and total elimination of the chlorinated species, then dried in the open air and weighed.

Methods of filtrate purification and analysis:

Purification Steric Exclusion Chromatography (SEC)

Size exclusion chromatography was used to remove the inorganic fraction, in salt form, present before analysis of the organic compounds. The chromatography system was equipped with a Tosoh HW40 column (100 x 6 cm) for the purification of cellooligosaccharides and a refractometric detector for the detection of eluted products. A solution of 0.1 M ammonium carbonate was used as an eluent at a flow rate of 1.8 ml / min and the analyzes were carried out at room temperature by injecting 7 ml of filtrate adjusted to pH 8 with H ₂ SO _4. at 72%.

Separative Steric Exclusion (SEC) Chromatography

Size exclusion chromatography was used to fractionate the organic compounds after removing the inorganics from the filtrates as described above. Purified filtrates were injected onto three GE Heathlcare Superdex-30 Hiload 26/600 columns (60 x 26 cm) followed by a refractometric detector. The 0.1 M ammonium carbonate was used as an eluent at a flow rate of 1.2 mL / min. The analyzes were carried out at room temperature using 1 mL of purified filtrate.

MALDI-TOF mass spectrometry

MALDI-TOF assays were performed to identify the cellooligosaccharides contained in the purified filtrates using an Applied Biosystems Voyager-DETM STR Biospectrometry ™ workstation. A solution of 2,5-dihydroxybenzoic acid (DHB) in 30% trifluoroacetic acid was used as a template. All analyzes were performed in positive mode.

Chromatographic analyzes of sugars by high performance anion exchange (HPAEC)

Identification of the compounds (glucose and cellobiose) was performed using HPAEC with pulsed amperometric detection (PAD) on a Dionex CarboPac PA1 column (4 mm x 250 mm). The neutral monosaccharides were eluted in isocratic mode using 18 mM aqueous sodium hydroxide solution at a flow rate of 1 mL / min. The concentration of the compounds was determined by comparison with a calibration curve made with the individual compounds.

For quantification of the cellooligosaccharides, the filtrates were subjected to a post-hydrolysis process. Each sample was treated with a solution of H ₂ SO ₄ to 72% using a sample ratio / H ₂ SO ₄ : 5/1 (v / v) at 120 ° C for 60 min. After the post-hydrolysis, the samples were injected into the HPAEC system for quantification.

Analysis of organic acids: Initial phase of liquid-liquid extraction

Before analyzing the acidic compounds present in the filtrates, a liquid-liquid extraction method was used to remove the other organic compounds and to facilitate the detection of organic acids, according to the protocol of S. De Baere, V. Eeckhaut, M. Steppe, C. De Maesschalck, P. De Backer, F. Van Immerseel, S. Croubels, "Development of a HPLC-UV method for the quantitative determination of the short-chain fatty acids and lactic acid produced by intestinal bacteria during in vitro fermentation ". 15. After the last neutralization (addition of 10 μL of concentrated HCl), the samples were directly injected into the HPLC system.

High Performance Liquid Chromatography (HPLC)

The contents of low molecular weight compounds, with the exception of glucose and cellobiose, were determined by HPLC (Shimadzu apparatus equipped with a UV detector). 20 μl of sample were injected onto an 812H column of Macherey-Nagel Nucleogel Chloride (7.8 × 300 mm) using a 1 mM H ₂ SO ₄ solution in a mobile phase at 0.7 mL / min. ⁵ C. compounds of high purity (available from Sigma-Aldrich) were used for the calibration curves.

Analysis of chlorinated mineral compounds:

At the end of step b), the content of chlorinated mineral compounds in the filtrate was analyzed by iodometric assay and is provided in Table 6.

Table 6: contents of chlorinated mineral compounds present in the filtrate for initial concentrations of IC0 ₂ of 1, 23.10 ² , 3.70 × 10 ² and 7.41 .10 ² M corresponding to 2, 6 and 12% of IC0 ₂ applied. The final concentration of CI0 ₂ is low (10 ³ - 10 ⁴ M). Almost all chlorine dioxide is consumed during the reaction. On the other hand, the consumption of residual chlorite reaches 50% of the amount of initial chlorine dioxide in some cases.

Separation of the cellooligosaccharides recovered in step e) (in the filtrate), quantification and analysis of their DP.

The filtrate was purified by purifying steric exclusion chromatography in order to remove the large quantity of salts present (CI0 ₃ , ClO ₂ , NaOH, and then the mineral acids added to adjust the pH for the purposes of the chromatography). The samples were concentrated and injected into the chromatograph using an ammonium carbonate solution as eluent. 3 fractions were collected. The one with the lowest retention time (F1) comprising cellooligosaccharides up to glucose, and the other two corresponding to higher retention times were composed of salts and acids (F2 and F3).

F1 sample was injected in separative steric exclusion chromatography.

Whatever the implementation conditions of step b), it was observed that fraction F1 contained more cellooligosaccharides than glucose.

Figures 2 to 4 illustrate the influence of the conditions of step b) on the degree of polymerization of the cellooligosaccharides obtained.

Figure 2 shows the results when step b) was performed at high temperature (90 ° C). At 12% CI0 ₂ , DPO cellooligosaccharides of 3 to 10 were produced, but in smaller amounts compared to the 6% CI0 _{2 test} , probably due to the fact that at high temperature and high concentration of C0 2 ₂ occurs a degradation cellooligosaccharides into glucose and acids. When step b) was carried out at high temperature (90 ° C) and with low proportions of IC0 ₂ (2%), low DP cellooligosaccharides (2-7) were obtained, probably because the cellulose was only partially hydrolysed.

Figures 2 and 3 show the variation of the DPs of the cellooligosaccharides at a fixed proportion of IC0 ₂ , by varying the temperature. Even the mildest conditions (2% CI0 ₂ , 30 ° C) led to many DP cellooligosaccharides of 2 to 1 1. An increase in temperature to the same proportion of IC0 ₂ (70 ° C, 2% CI0 ₂ ) caused an increase in the abundance of cellooligosaccharides, but at the highest temperature (90 ° C, 2% CI0 ₂ ), cellooligosaccharide degradation occurred. Using 6% CI0 ₂ instead of 2%, the lowest temperature (30 ° C) had no effect on cellooligosaccharide abundance and the highest temperature high (90 ° C) further led to degradation. Nevertheless, an average temperature (70 ° C) favors an increase in the abundance of cellooligosaccharides without causing too much glucose degradation compared to the 2% CI0 ₂ experiment.

Separation and analysis of the content of low molecular weight compounds in the filtrate Glucose and cellobiose concentrations were determined by HPAEC. According to the MALDI-TOF analyzes, only the samples resulting from the experiments in which step b) was carried out at 70 ° C., 2% CI0 ₂ , or 30 ° C., 2% CI0 ₂ contained cellobiose, but a concentration too low to be detected by HPAEC.

Glucose was present in all samples at 5-16 mg / L

(Table 7). The combination of high temperature and high IC0 ₂ concentration would cause degradation of cellulose and cellooligosaccharides to glucose and then to acids. Intermediate conditions (70 ° C, 2% CI0 ₂ , or 70 ° C, 6% CI0 ₂ ) gave the lowest proportions of glucose and the highest contents of cellooligosaccharides.

The contents of low molecular weight compounds (lactic acid, levulinic acid, acetic and formic acid, furfural, HMF, ethanol) were determined by HPLC.

The results are provided in Table 7 (lines 3 to 8).

Table 7: Concentration (mg / L) of low molecular weight organic compounds in the filtrate (lanes 3 to 8), compared with the total organic material extracted from the fibers (lane 9).

The comparison between the total organic matter assayed in solution (obtained by summing the concentrations of sugars and organic acids assayed by HPLC, line 8 of Table 7), and the concentration of total organic matter extracted from the fibers (measured by the difference in weight of the solids before and after reaction, reduced to the volume of the filtrate extracted, line 9), shows very irregular variations. It can be seen that if the materials found in solution are systematically lower than the materials extracted from the solid, the recovery rates vary a lot and not systematically. It is possible to advance several explanations:

or an imperfect and variable recovery of the compounds in the filtrates (retention of certain compounds in the solid phase during the extraction of the first filtrate, but subsequent extraction of the latter during the hyper-washing of the fibers making it possible subsequently to recover the solid phase perfectly washed after reaction),

or the formation of volatile compounds which escape analysis in the filtrates, formed in variable quantities according to the reaction conditions.

It can thus be concluded that, while the general trend remains a greater extraction of organic compounds under more stringent treatment conditions, the yields obtained, given the heterogeneous nature of the medium, can vary greatly depending on the conditions of implementation. process, including the conditions for contacting the reagents, filtration and washing of the recovered solid phase.

In general, HPLC measurements show that the most severe operating conditions (90 ° C, 12 or 6% of IC0 ₂ ) make it possible to produce the filtrates which are the most highly concentrated in total organic compounds of low molecular weight. Moreover, glucose is present in all the filtrates at a content of 5 to 16 mg / L, but its concentration is much lower than that of the other organic compounds assayed. The predominantly low molecular weight organic compounds are formic acid and cellooligosaccharides.

The results are also expressed in relative proportions (Table 8).

Table 8: Relative Proportions (%) of Low Molecular Weight Organic Compounds in the filtrate

The proportion of cellooligosaccharides present in the filtrate is in this example between 20 and 40%. The highest proportions were obtained for median operating conditions (70 ° C, 2% CI0 ₂ , or 70 ° C, 6% CI0 ₂ ). Under more severe conditions (90 ° C, 12% CI0 ₂ or 90 ° C, 6% CI0 ₂ ), cellulose and cellooligosaccharides would be degraded mainly glucose and acids. Under the least severe conditions, the production of cellooligosaccharides is less favored compared to the production of organic acids.

Claims

A process for the preparation of cellopolysaccharides comprising the steps of:

where DPvf <0.60 x DPvi

c) recovering said cellopolysaccharides in suspension,

the DPvi and DPvf being measured by viscometry at 25 ° C.

2. The method of claim 1, wherein the cellulose is selected from a papermaking pulp, cotton and microcrystalline cellulose, preferably unbleached or bleached cotton or pulp cellulosic pulp.

The method of claim 2, wherein the cellulose is a bleached pulp cellulosic pulp, the process comprising, prior to step a), a pulp bleaching step aO).

4. Method according to any one of claims 1 to 3, wherein step b) is carried out by contacting the cellulose with an aqueous solution of pH greater than or equal to 8, and then adding chlorine dioxide preferably an aqueous solution of chlorine dioxide.

The method of any one of claims 1 to 4, wherein:

during step b), the weight percentage of chlorine dioxide relative to the cellulose mass is from 0.1 to 20%, in particular from 1 to 15%, preferably from 2 to 12%, and / or

step b) is carried out at a pH of 9.0 to 13.0, preferably from 10.0 to 12.5, and / or step b) lasts from 10 seconds to 24 hours, in particular from one minute to 12 hours, preferably 10 minutes to 2 hours, and / or

step b) is carried out at a temperature greater than 5 ° C, or even greater than 15 ° C, in particular from 20 ° C to 90 ° C, preferably from 40 ° C to 80 ° C, for example 60 ° C at 80 ° C.

6. Process according to any one of claims 1 to 5, wherein, at the end of step b), the degree of viscosimetric average polymerization DPvf suspended cellopolysaccharides is greater than or equal to 20.

7. Process according to any one of claims 1 to 6, comprising a step d) of recovery of the liquid phase comprising cellooligosaccharides, at least one organic acid and chlorite.

8. Process according to claim 7, comprising, after step d), a step e) of recovering the cellooligosaccharides.

9. Process according to claim 8, in which the recovery of the cellooligosaccharides is carried out by acidification of the liquid phase with a pH of less than or equal to 8, then separation by steric exclusion chromatography or by ultrafiltration.

The method of any one of claims 7 to 9, comprising, after step d):

a step f) of recovering formic acid, and / or

a step g) of recovering chlorite.