WO2014096357A2

WO2014096357A2 - Lignin depolymerization method

Info

Publication number: WO2014096357A2
Application number: PCT/EP2013/077677
Authority: WO
Inventors: Stéphane GRELIER; Georges KOUMBA YOYA; Véronique COMA; Frédérique HAM-PICHAVANT
Original assignee: Universite De Bordeaux I; Centre National De La Recherche Scientifique (C.N.R.S); Institut Polytechnique De Bordeaux
Priority date: 2012-12-20
Filing date: 2013-12-20
Publication date: 2014-06-26
Also published as: WO2014096357A3

Abstract

The present invention relates to a lignin depolymerization method including a step for placing at least one manganese salt, tungstophosphoric acid or one of the salts thereof, and an oxidizing agent in the presence of lignin, in at least one solvent.

Description

Process for the depolymerization of lignin

The present invention relates to a process for the depolymerization of lignin.

Lignin is the second most abundant renewable biopolymer after cellulose, and together they make up more than 70% of the total biomass.

The valuation of lignin is therefore an important issue.

In the paper industry, large quantities of lignin are obtained as by-products or wastes, of which only a small part is recovered. Lignin is mainly used as a binder or dispersant, or for the preparation of biogas by a high temperature treatment (800 'Ό - Ι ΟΟΟ' Ό). There are also lignin depolymerization methods involving chlorinated derivatives which are unsatisfactory from an environmental point of view.

At present, there are few economically viable processes using lignin as a raw material for producing chemical compounds.

There is therefore a need for a new lignin beneficiation process that is economically viable and environmentally acceptable.

The object of the present invention is to propose a new process for the depolymerization of lignin which is efficient and adapted from an environmental point of view, making it possible to access depolymerization products which are valuable in various industrial fields, such as biorefineries. agrifood or cosmetics.

Another object of the present invention is to provide a method for accessing, in a controlled manner, depolymerization products of various sizes such as vanillin, vanillic acid, guaiacol, or diformyl guaiacol.

The process of the present invention provides the use of tungstophosphoric acid, or a salt thereof, in combination with a manganese salt and an oxidizing agent to depolymerize lignin.

More particularly, the subject of the present invention is a process for the depolymerization of lignin, comprising a step of placing lignin, at least one salt of manganese, of tungstophosphoric acid or of one of its salts, together with an oxidizing agent in at least one solvent. The inventors have observed, surprisingly, that the in situ combination of tungstophosphoric acid, or a salt thereof, a manganese salt and an oxidizing agent has the effect of effectively depolymerizing lignin.

The modes of bringing into contact with the depolymerization process of the invention will be described later.

In the context of the present application, the term "reaction medium" generally refers to the medium in which the depolymerization reaction takes place and which comprises lignin, at least one species comprising manganese, a species resulting from the solubilization of tungstophosphoric acid, a species resulting from the solubilization of an oxidizing agent, and at least one solvent.

By "bringing into presence" means the addition of a reagent in the reaction medium.

Said reaction medium may also comprise other reagents as described in the present application.

In addition, depending on the progress of the depolymerization, the reaction medium also comprises the depolymerization products.

lignin

The structure of the lignin is in the form of a complex three-dimensional network resulting from the polymerization of base units which have a phenylpropane motif as backbone. Lignin therefore more generally refers to "lignins", depending on the basic units that constitute it. Among the basic units of lignin, also called monolignols, there may be mentioned mainly paracoumaryl alcohol, coniferyl alcohol and sinapyl alcohol.

In the context of the present invention, the term "depolymerization" of lignin is understood to mean a reaction during which covalent bonds of the polymeric structure of lignin are broken, leading to depolymerization products smaller than the starting lignin.

The covalent bonds of the lignin which are broken during the process of the invention are typically C-C and / or C-O bonds.

By "smaller size" is meant that the depolymerization products of lignin obtained according to the process of the invention have a lower molecular weight than that of the starting lignin.

However, in the context of the present invention, the meaning of the term "depolymerization" should not be limited to the transformation of lignin into base units as described above. Indeed, the depolymerization of lignin according to the In particular, the process of the invention makes it possible to obtain depolymerization products of intermediate size between the base units and the starting lignin.

Typically, the depolymerization products of lignin obtained according to the process of the invention are formed of a few basic units, or even a few tens of base units.

Generally, because of the complex structure of lignin, the process of the invention provides a mixture of depolymerization products of various sizes and structures, which can carry different functional groups, such as ketone, aldehyde, phenol or alcohol groups. example.

Said depolymerization products can be defined as

"Oligomers" of lignin or "fragments" of lignin.

Thus, by "depolymerization process" is also meant a "fragmentation process" of lignin.

The depolymerization products obtained according to the process of the invention have a size of between 500 g / mol and 20,000 g / mol, preferably between 750 g / mol and 15,000 g / mol, advantageously between 1,000 g / mol and 10 000 g / mol, preferably between 1000 g / mol and 5000 g / mol.

The lignin depolymerization product mixture is typically characterized by its weight average molecular weight M _w , which is the weighted average weight average of each product, and has the formula:

M =

^W Σn _iMi

i

in which n denotes the number of products i and M, denotes the molecular mass of the product i.

For a mixture of depolymerization products obtained according to the method of the invention, the mass-average molecular weight can be measured by Size Exclusion Chromatography (SEC), using a device that typically comprises a reservoir of solvent, a pumping system, an injector, a set of chromatography columns and detectors (UV for example) arranged at the column outlet.

The mixture of depolymerization products typically has a weight average molecular weight M _w of between 500 g / mol and 20,000 g / mol, preferably between 750 g / mol and 15,000 g / mol, advantageously between 1,000 g / mol. and 10,000 g / mol, preferably between 1,000 g / mol and 5,000 g / mol. According to some embodiments, the mixture of depolymerization products may have a weight average molecular weight M _w of between 400 g / mol and 20,000 g / mol, preferably between 400 g / mol and 15,000 g / mol, advantageously between 400 g / mol and 10,000 g / mol, preferably between 400 g / mol and 5,000 g / mol.

The mixture of depolymerization products obtained according to the process of the invention may also be characterized by the molar ratio between the weight average molecular weight M _{w of} said mixture and the weight average molecular weight of the starting lignin M _Ng .

Typically, using the process of the invention, a molar ratio M _w / M | _ig between 1/2 and 1/10, and which can be up to 1/20, or even up to 1/35, according to some embodiments described below.

In the context of the process of the invention, it is possible to use as raw material any type of available lignin, whether raw or pretreated, typically to modify its solubility in an aqueous or organic medium.

Advantageously, the lignin has been pretreated so as to make it soluble in an aqueous medium.

Preferably, lignin is a sulfonated lignin or a Kraft lignin. Sulfonated lignins (also known as lignosulphonates) are water-soluble compounds from the industrial production of paper pulp using bisulphite ions.

Kraft lignins (also known as thiolignins) are water-soluble compounds derived from the industrial production of pulp using sulphate ions.

The lignins that can be used in the context of the present invention are described in particular in Lignins and Lignans: Advances in Chemistry, Cyril Heitne and John Schmidt, CRC Press, Taylor & Francis Group, 2010.

It is also possible to use a source of industrial lignins or mixtures of compounds comprising lignins, typically mixtures of biomass comprising lignin and other constituents, such as cellulose and / or hemicellulose, or else a mixture of sulfonated lignins and Kraft lignins.

The lignin used in the process of the invention typically has a mass average molecular weight (M _ng ) of between 1,000 and 30,000 g / mol, preferably between 5,000 and 25,000 g / mol, advantageously between 10,000 and 20,000 g / mol.

The sulfonated lignin used in the process of the invention typically has a weight average molecular weight ( _Mw ) of between 10,000 and 20,000 g / mol. The Kraft lignin used in the process of the invention typically has a weight average molecular weight ( _Mw ) of between 1000 and 2000 g / mol.

The mass average molecular weight of the starting lignin (M | _ig ) represents the weighted mass average of the weight of each lignin size, and has the formula:

in which n denotes the number of lignin molecules of size j and M, denotes the molecular mass of a lignin of size j.

This value is usually indicated by the lignin supplier or can be determined by size exclusion chromatography as described above.

Tungstophosphoric acid

The tungstophosphoric acid has the formula H ₃ PW ₁₂ 04o.

The structure of tungstophosphoric acid was determined by Keggin in 1993 by X-ray diffraction (Keggin et al., Nature 1993, 131, 908).

Tungstophosphoric acid is soluble in aqueous solution and is present in different ionic forms depending on the pH.

Thus, at pH = 1, the dominant species is the [PW ₁₂ O ₄₀ ] ^{3 ~} anion, at pH = 7.3, the dominant species is [PW ₉ 0 ₃₄ ] ^{9 "} , and at pH> 9 , the dominant species is (P0 ₄ ^{3 "} , W0 ₄ ^2" ).

By "tungstophosphoric acid or a salt thereof" is meant tungstophosphoric acid H ₃ PWi ₂ 0 ₄ o and any salt form thereof, obtained by replacing the H ⁺ ion against another against ion, typically Na ⁺ , Li ⁺ , or K ⁺ .

Preferably, according to the process of the invention, the depolymerization is carried out at a pH adjusted from 1 to 7, preferably from 2 to 6, advantageously from 3 to 5.

According to another embodiment, the depolymerization is carried out at a pH adjusted from 9 to 14, preferably from 10 to 13, advantageously from 12 to 13.

In particular, tungstophosphoric acid is commercially available in the form of H ₃ [P (W ₃ O ₁₀ ) 4] xH ₂ O (CAS No. 12501-23-4) in Sigma-Aldrich.

Preferably, tungstophosphoric acid is brought into its acid form H ₃ PW ₁₂ O ₄₀ .

According to an advantageous embodiment, tungstophosphoric acid is used as a catalyst, that is to say that for a tungstophosphoric acid molecule put in the presence in the reaction medium, there is observed on average the rupture of more than one covalent bond within the lignin.

Typically, in catalysis, the molar ratio between the catalyst and the starting reagent is usually of the order of 1%.

However, in the context of the present invention, the amount of lignin material

(starting reagent) should be weighted by the number of covalent bonds that must be broken to depolymerize said lignin. This number is generally of the order of the number of base units constituting said lignin.

This is why we can talk about "catalyst" about tungstophosphoric acid, although, according to some embodiments, it is brought into contact in an amount (in mol) equivalent or greater than the amount (in mol) of lignin.

Preferably, the molar ratio between tungstophosphoric acid and lignin is between 0.1 and 10, preferably between 0.5 and 5.

This embodiment is particularly advantageous because it allows the depolymerization of a large amount of lignin economically. In addition, the use of a catalytic amount of tungstophosphoric acid facilitates the recovery of the products and their purification.

Inasmuch as it is sometimes difficult to determine the number of moles of lignin brought into contact at the beginning of the process, it is also possible to define a mass ratio between tungstophosphoric acid and lignin, which is typically between 0.01 and 0. , 5, preferably between 0.1 and 0.2.

Manganese salt

The inventors have observed that the in situ presence of tungstophosphoric acid (or a salt thereof), a manganese salt and an oxidizing agent has the effect of depolymerizing lignin.

Without wishing to be bound to a particular theory, the inventors have observed that the bringing together in situ of the species resulting from the solubilization of tungstophosphoric acid and of the manganese salt results in the formation of a reactive species whose redox potential is able to overcome the energetic barriers associated with the splitting of covalent bonds of lignin, resulting in the depolymerization of lignin.

The species resulting from the bringing into contact with H ₃ PW ₁₂ 0 ₄ o and manganese salts has in particular been described in Patel et al. Eur. J. Inorg. Chem. 201 1, 1871-1875.

In one embodiment, the manganese salt is an Mn (II) salt. According to one variant, the manganese salt is an organic manganese salt.

By "organic salt" is meant a salt in which the counterion of the manganese species is an organic ion.

According to this variant, the manganese salt is, for example, manganese acetate (II) of formula Mn (CH ₃ COO) ₂ .

According to another variant, the manganese salt is a mineral salt of manganese.

By "mineral salt" is meant a salt in which the counterion of the manganese species is a mineral ion.

According to this variant, the manganese salt is, for example, manganese chloride (II) of formula MnCl ₂ .

According to one embodiment, the molar ratio between the manganese salt and the tungstophosphoric acid is between 1 and 5, preferably between 2 and 4.

The manganese salt is typically placed in excess of the tungstophosphoric acid, which allows the optimal formation of the reactive species capable of depolymerizing the lignin.

Alternatively, instead of a manganese salt, it is possible to use any source of manganese, more particularly Mn (II), adapted to the implementation of the invention.

Oxidizing agent

The process of the invention also comprises bringing into the presence of an oxidizing agent.

By "oxidizing agent" is meant a substance capable of oxidizing the species present in the reaction medium.

Examples of oxidizing agents that may be mentioned include peroxides, such as hydrogen peroxide (H ₂ O ₂ ) and benzoyl peroxide, or compounds such as O ₂ , O ₃ , KMnO ₄ and NalO _4. or any oxidant conventionally used in organic chemistry.

Preferably, the oxidizing agent is hydrogen peroxide, oxygen or air. According to a preferred embodiment, the oxidizing agent is hydrogen peroxide (H ₂ O ₂ ).

Hydrogen peroxide is typically available as a 35 weight percent aqueous solution, which equates to a molar concentration of about 10 M.

According to another preferred embodiment, the oxidizing agent is gaseous, and typically comprises oxygen (O ₂ ). The oxidizing agent can be oxygen (O ₂ ) or air (which comprises 20% oxygen vol). Preferably, typically when the oxidizing agent is hydrogen peroxide, the molar ratio between the oxidizing agent and the lignin is between 1 and 1000, preferably between 100 and 500.

Since it is sometimes difficult to determine the number of moles of lignin brought into contact at the beginning of the process, it is also possible to define a ratio (in mmol / g) between the quantity of oxidizing agent material added and the mass of lignin put in the presence.

Said ratio is typically greater than 1 mmol / g, preferably between 5 and 100 mmol / g, advantageously between 5 and 30 mmol / g.

Preferably, the molar ratio between the oxidizing agent and the tungstophosphoric acid is between 1 and 1000, preferably between 100 and 500.

Preferably, typically when the oxidizing agent is gaseous, for example when the oxidizing agent is oxygen or air, said oxidizing agent is brought into contact with lignin under a pressure of said gaseous oxidizing agent.

By "bringing into contact with an oxidizing agent" is meant the introduction of said oxidizing agent into the reaction medium of the process of the invention. By "bringing into contact with an oxidizing agent" is also meant bringing said oxidizing agent into contact with the reaction medium. Said introduction can be carried out at the beginning of the depolymerization as during depolymerization. It can be carried out by adding the oxidizing agent at once, in portions, dropwise in the form of an aqueous solution (which does not generally modify the dilution of the reaction medium), by bubbling in the reaction medium. or by pressurizing in an autoclave, for example.

Generally, the oxidizing agent is added to the reaction medium after bringing into contact with other reagents, such as lignin, tungstophosphoric acid (or a salt thereof) and the manganese salt.

Preferably, the oxidizing agent is added dropwise in the form of an aqueous solution. This mode of addition makes it possible to more effectively control the depolymerization of lignin.

According to one embodiment, when the oxidizing agent is in gaseous form, such as air, O ₂ or O ₃ , it is brought into the reaction medium by continuous bubbling in said medium. Alternatively, typically when the oxidizing agent is gaseous oxygen or air, it is brought into the presence of the reaction medium by pressurizing in an autoclave.

The presence of an oxidizing agent in the reaction medium has the effect of accelerating the depolymerization of the lignin. Without wishing to be bound to a particular theory, the inventors have observed that the placing in contact with an oxidizing agent in situ allows the transformation (or regeneration) of inactive species (in reduced form) into reactive species (in oxidized form). As such, said oxidizing agent promotes the cleavage reactions of covalent bonds within the lignin.

Free radical trap

According to a particular variant of the process of the invention, the process further comprises the addition of a compound capable of trapping free radicals.

This variant is particularly suitable in the case where the oxidizing agent is hydrogen peroxide.

By "compound capable of trapping free radicals", also called "free radical trap" is meant a substance capable of capturing the free radicals radical species possibly present in the reaction medium.

By "addition of a compound capable of trapping free radicals" is meant the introduction of said compound into the reaction medium of the process of the invention. Said addition may be carried out at the beginning of the depolymerization as during depolymerization. It can be carried out by adding said compound at once, in portions, or else dropwise in the form of an aqueous solution (which does not generally modify the dilution of the reaction medium).

Preferably, the compound capable of trapping free radicals is added at the beginning of the depolymerization, with the other reagents such as lignin, manganese salt and tungstophosphoric acid.

There are many types of compounds that can trap free radicals.

Compounds capable of trapping free radicals generally carry at least one function selected from the group consisting of thiol (-SH), nitroxide (-N-O) and phenol (Ph-OH) functions.

According to a preferred embodiment, the compound capable of trapping free radicals is N-acetylcysteine.

Preferably, the molar ratio between the compound capable of trapping free radicals and lignin is between 1 and 100, preferably between 10 and 50.

Since it is sometimes difficult to determine the number of moles of lignin brought into contact at the beginning of the process, it is also possible to define a ratio (in mmol / g) between the amount of free radical scavenging compound material added and the mass of lignin brought together. Said ratio is typically greater than 0.1 mmol / g, preferably between 0.5 and 5 mmol / g, advantageously between 0.5 and 3 mmol / g.

The addition of a compound capable of trapping free radicals in the reaction medium has the effect of limiting the radical repolymerization of the depolymerization products present in the reaction medium in the form of radicals and thus makes it possible to access depolymerization products lower than those accessible in the absence of any compound capable of trapping free radicals.

Without wishing to be bound to a particular theory, the inventors have observed that the addition of a compound capable of trapping free radicals in situ makes it possible to capture the free radicals of the depolymerization products in the form of radicals, which tend, according to a to some extent, to combine to provide larger products, which is desirable to avoid when reaching a high degree of depolymerization.

Typically, the use of a compound capable of trapping free radicals provides access to a molar ratio M _w / M | _ig (as defined above) of the order of 1/20, while in the absence of a compound capable of trapping free radicals, a molar ratio of the order of 1/10 is typically reached.

Implementation of the process

The lignin depolymerization process according to the invention is carried out in the presence of at least one solvent.

According to one embodiment, the solvent is a mixture of water and a polar organic solvent.

The use of such a mixture as a solvent has the advantage of solubilizing the lignin and all of the reagents brought together. A homogeneous reaction mixture is thus obtained in which the chemical interactions are facilitated and the depolymerization is favored.

According to this embodiment, the solvent preferably comprises at least 50% by weight of water, preferably between 60% and 95%, advantageously between 70% and 90%, relative to the total mass of said solvent.

The additional solvent is typically composed of a polar organic solvent, preferably sufficiently volatile to be separated from the depolymerization products by evaporation, optionally under reduced pressure.

The solvent advantageously comprises at least 5% by weight of a C ₁ -C ₄ alcohol, preferably between 10% and 50%, advantageously between 10% and 30%, relative to the total weight of said solvent. As alcohol CrC ₄ , there may be mentioned methanol, ethanol, propan-1-ol, propan-2-ol, n-butanol, sec-butanol, isobutanol and tert- butanol.

Preferably the solvent comprises 70% to 90% water and 10% to 30% ethanol.

According to another embodiment, the solvent is water.

According to the process of the invention, the depolymerization is carried out according to a dilution such that for 1 g of lignin, a volume of solvent of 10 ml to 100 ml, preferably 30 ml to 70 ml, is used.

Those skilled in the art will be able to adapt the dilution of the reaction medium depending on the solubility of the starting lignin, the reaction temperature and / or the viscosity of the reaction mixture.

The order of introduction of the reagents into the reaction medium is indifferent.

Typically, tungstophosphoric acid (or a salt thereof) and the manganese salt in the solvent are first contacted. The lignin and the optional compound capable of trapping the free radicals are then added. The oxidizing agent is then added, preferably throughout the depolymerization over a period of time. Alternatively, the oxidizing agent is placed in the presence of the reaction medium, preferably by pressurization.

The depolymerization process of the invention is typically carried out in the open air. Alternatively, the depolymerization process of the invention is carried out under a controlled atmosphere, typically under pressure.

The depolymerization process of the invention is generally carried out at a temperature of between 20 ° C and 200 ° C.

According to one embodiment, typically when the oxidizing agent is hydrogen peroxide, the depolymerization process of the invention is carried out at a temperature between 20 ° C and Ι ΟΟ 'Ό, preferably between 50' Ό and θδ'Ό, preferably between 80 ° C and 95 ° C.

In another embodiment, typically when the oxidizing agent is oxygen or air, the invention depolymerisation process is performed at a temperature between 100 ^<€ and 200 ° C, preferably between 150 ° C and 190 ^< €, preferably between 160 ° C and 180 ^< €.

Said temperature ranges make it possible to solubilize the reagents efficiently, to obtain advantageous depolymerization kinetics, while minimizing the energy cost of the process.

The depolymerization process of the invention is typically carried out at atmospheric pressure. Alternatively, typically when the oxidizing agent is gaseous, the depolymerization process of the invention is generally carried out by placing the reaction medium in the presence of a pressure of said gaseous oxidizing agent, which is typically from 1 to 10 bar.

The depolymerization process of the invention can be carried out batchwise or in a continuous manner. It can operate in a closed reactor (autoclave for example) or semi-open reactor. Typical implementations of the process of the invention will now be described.

According to a particular embodiment, in a reactor equipped with a heating system, a stirring means, and optionally a refrigerant system, tungstophosphoric acid (or one of its salts), salt, is introduced. of manganese and all or part of the solvent.

The mixture obtained is, for example, heated, typically at ΘΟ 'Ό, for a time sufficient to solubilize the reagents and ensure the formation of the reactive species (typically for one hour). The pH of the mixture obtained can be adjusted, preferably between 1 and 7, to obtain the appropriate reactive species.

To this mixture is then added the lignin, the possible complement of solvent, and optionally the compound capable of trapping free radicals.

The resulting mixture is stirred and can be heated, typically at 90 ° C. The oxidizing agent is then added, typically dropwise.

Stirring (and optional heating) of the reaction medium is continued until the desired degree of depolymerization is obtained.

Once the depolymerization is complete, the depolymerization products of the lignin can be recovered, using standard purification techniques in the field, namely filtration, extraction, distillation and / or chromatographic separation. In particular, the solvent may be removed by evaporation, possibly under reduced pressure.

This embodiment is particularly suitable for the depolymerization of the sulfonated lignin.

According to another embodiment, a mixture comprising a solvent and tungstophosphoric acid (or a salt thereof) is prepared. The manganese salt is then brought into contact, preferably having previously adjusted the pH, typically between 6 and 8. The resulting mixture is, for example, heated, typically at 90 ° C, for a time sufficient to solubilize the reagents and provide formation of the reactive species (typically for one hour).

To this mixture is added the lignin and the pH of the mixture obtained can be adjusted, preferably between 1 1 and 13.

The mixture obtained is then introduced into an autoclave, or any other device for heating the mixture under pressure.

The oxidizing agent is then placed in the presence of the reaction medium.

According to this mode, the oxidizing agent is generally gaseous, and is typically oxygen or air. The mixture is then heated under a pressure of said gaseous oxidizing agent, which is generally from 2 to 10 bar, for example 5 bar.

According to this mode, the heating temperature is generally between 100 ° C. and 200 ° C., preferably greater than 150 ° C., which makes it possible to reduce the reaction time, in particular with respect to that of the embodiment described above. .

Once the depolymerization is complete, the heating is stopped, the autoclave is depressurized and the lignin depolymerization products can be recovered, using conventional purification techniques in the field, namely filtration, extraction, distillation. and / or separation by chromatography. In particular, the solvent may be removed by evaporation, possibly under reduced pressure.

This embodiment is particularly suitable for the depolymerization of sulfonated lignin and Kraft lignin.

The present invention also relates to the use of tungstophosphoric acid, or a salt thereof, in combination with a manganese salt and an oxidizing agent for depolymerizing lignin.

In general, unless otherwise stated, by "Z is between the value x and the value y" it is meant that Z can be any value in the range [x, y] formed by the x and y values, inclusive. EXAMPLES

Reagents

H ₃ PW ₁₂ O ₄ O: Sigma-Aldrich (M = 2880.2 g / mol)

Mn (CH ₃ COO) ₂ : Sigma-Aldrich (M = 245.1 g / mol (tetrahydrate))

Lignosulfonate: Sigma-Aldrich

Kraft lignin: prepared from black liquor from Smurfit Kappa plant H ₂ 0 ₂ (35% aqueous solution): Sigma-Aldrich

EXAMPLE 1 Depolymerization in the Presence of H ₃ PW ₁₂ 0 ₄ 0, a Manganese Salt and an Oxidizing Agent (Hydrogen Peroxide)

144 mg of H ₃ PW ₁₂ 0 ₄ 0 and 30 mg of Mn (CH ₃ COO) ₂ are mixed in 40 ml of water and the mixture obtained is heated at 90 ° C. for 1 hour.

1 g of lignosulfonate is then added to the mixture.

Finally, 1 mL of hydrogen peroxide (35% aqueous solution) is slowly added. The reaction mixture is stirred at 90 ° C.

At a reaction time, a sample of 1 ml of the reaction medium is taken, brought to room temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis.

The SEC analysis is carried out using three columns of type TSK-gel (3000 PW, 4000 PW, 3000 PW) coupled in series, with as eluent a solution of NaN ₃ (3%) in osmosis water and the pH is adjusted to 12 with a 1M sodium hydroxide solution. The flow rate is 1 mL / min and the detection is by UV at a wavelength of 280 nm. Is thus measured at each time t the average molecular weight M _w of the sample. The results as a function of time are collated in Table 1.

Table 1

The average molar mass drop of the mixture indicates the depolymerization of the lignin observed under the conditions of Example 1. EXAMPLE 2 Depolymerization in the Presence of H3PW12O40, a Manganese Salt, an Oxidizing Agent (Hydrogen Peroxide) and a Free Radical Trap

1 mmol (157 mg) of N-acetyl-L-cysteine (NAC) in 10 ml of EtOH and 1 g of lignosulfonate are then added to the mixture.

Finally, 1 ml of hydrogen peroxide (35% aqueous solution by weight) is slowly added. The reaction mixture is stirred at 90 ° C.

At a reaction time, a sample of 1 ml of the reaction medium is taken, brought to ambient temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis. The results as a function of time are collated in Table 2.

Table 2

Example 2 shows that the addition of a free radical scavenger has the advantageous effect of allowing even more efficient lignin depolymerization than under the conditions of Example 1.

Example 3 Comparative, in the absence of manganese salt

144 mg of H ₃ PW ₁₂ 0 ₄ 0 and 1 g of sulphonated lignin are mixed in 50 ml of an acetate buffer solution (pH = 4 to 50 mM).

1 ml of hydrogen peroxide (35% aqueous solution) is slowly added. The reaction mixture is stirred at room temperature.

At a reaction time, a sample of 1 ml of the reaction medium is taken, brought to room temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis. The results are collated in Table 3.

Table 3

Comparative Example 3 shows that in the absence of manganese salt, the depolymerization of lignin is extremely slow and stagnant. This shows the synergy of tungstophosphoric acid and manganese salt in the depolymerization of lignin. Example 4 Comparative, in the Absence of Oxidizing Agent

1 g of lignosulfonate is then added to the mixture.

At a reaction time, a sample of 1 ml of the reaction medium is taken, brought to room temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis. The results as a function of time are collated in Table 4.

Table 4

Comparative Example 4 shows that in the absence of an oxidizing agent, no depolymerization of the lignin is observed.

Example 5 Comparative, in the Absence of Tungstophosphoric Acid

30 mg of Mn (CH ₃ COO) ₂ are mixed in 40 ml of water and the mixture obtained is heated at 90 ° C. for 1 hour.

1 g of lignosulfonate is then added to the mixture.

Finally, 1 mL of hydrogen peroxide (35% aqueous solution by weight) is slowly added (drops). The reaction mixture is stirred at 90 ° C.

At a reaction time, a sample of 1 ml of the reaction medium is taken, brought to room temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis. The results as a function of time are collated in Table 5.

Table 5

Comparative Example 5 shows that in the absence of tungstophosphoric acid, no depolymerization of lignin is observed. Example 6 Depolymerization in the Presence of H3PW12O40, a Manganese Salt and an Oxidizing Agent (Oxygen)

150 mg of H3PW12O40 are dissolved in 50 ml of distilled water. The pH of the mixture obtained is 2.5. Using a 3M sodium hydroxide solution, the pH is adjusted to 7.3, then the medium is heated at 90 ° C. for 10 min, and 30 mg of Mn (CH ₃ COO) 2.4H ₂ O are added.

The heating at 90 ° C is continued for 1 h, then 1 g of lignosulphonate is then added to the mixture and the pH is adjusted to 12.6 with a 3M sodium hydroxide solution.

The mixture obtained is then introduced into an autoclave and heated at 170 ° C for 1 h under a pressure of 0 ₂ set at 5 bar initially.

After 1 h at 170 ° C, the autoclave is depressurized and the measured pH is 12.45.

A sample of 1 mL of the reaction medium is taken, brought to ambient temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis.

The results are collated in Table 6.

Table 6

To the remainder of the reaction medium, 50 ml of ethanol are added and the pH is adjusted to 2 by addition of a solution of 6M HCl. The medium is then extracted with chloroform (3 times 50 ml), the organic phases are combined, dried over Na ₂ SO ₄ and the solvents are evaporated. Purification of the crude reaction product by flash chromatography on silica gel (eluent = ethyl acetate / cyclohexane 3/7) made it possible to isolate vanillin (M = 152 g / mol) and diformyl guaiacol (M = 180 g / mol) as depolymerization products.

The particular reaction conditions of Example 6 (pH, temperature and oxygen under pressure) make it possible to obtain efficient and rapid depolymerization of the lignosulfonate.

EXAMPLE 7 Depolymerization in the Presence of H3PW12O40, a Manganese Salt and an Oxidizing Agent (Air)

150 mg of H3PW12O40 are dissolved in 50 ml of distilled water. The pH of the mixture obtained is 2.5. Using a 3M sodium hydroxide solution, the pH is adjusted to 7.3, then the medium is heated at 90 ° C. for 10 min, and 30 mg of Mn (CH ₃ COO) 2.4H ₂ O are added. The heating at 90 ° C. is continued for 1 h, then 1 g of lignosulphonate is then added to the mixture and the pH is adjusted to 12.5 with a 3M sodium hydroxide solution.

The mixture obtained is then introduced into an autoclave and heated at 170 ° C. for 1 h under an air pressure set at 5 bar initially.

After 1 h at 170 ° C, the autoclave is depressurized and the measured pH is 12.35.

The results are collated in Table 7.

Table 7

SEC analysis detected the presence of vanillin (M = 152 g / mol) and diformyl guaiacol (M = 180 g / mol).

Example 7 shows that, under the conditions of Example 6, the use of air as an oxidizing agent gives results equivalent to the use of oxygen.

Example 8 Comparative, in the Absence of Tungstophosphoric Acid and Manganese Salt

This example uses the conditions of Example 7, with the exception of tungstophosphoric acid and manganese salt.

1 g of lignosulphonate is dissolved in 50 ml of distilled water. The resulting mixture, pH = 7.14, is heated at 90 ° C for 1 h, then the pH is adjusted to 13 with a 3M sodium hydroxide solution.

After 1 h at 170 ° C, the autoclave is depressurized and the measured pH is 10.65.

By SEC analysis of the mixture obtained, a low level of depolymerization of lignin is observed, the depolymerization products being of greater size than that of the depolymerization products of Example 7. In particular, no trace of vanillin or of diformyl guaiacol is observed.

The presence of tungstophosphoric acid and manganese salt thus makes it possible to achieve a better level of depolymerization of the sulphonated lignin. EXAMPLE 9 Depolymerization in the Presence of H3PW12O40, a Manganese Salt and an Oxidizing Agent (Air)

The heating at 90 ° C is continued for 1 h, then 1 g of kraft lignin is then added to the mixture and the pH is adjusted to 12.6 with a 3M sodium hydroxide solution.

After 1 h at 170 ° C, the autoclave is depressurized and the measured pH is 12.4.

The results are collated in Table 9.

Table 9

The particular reaction conditions of Example 9 (pH, temperature and oxygen under pressure) make it possible to obtain efficient and rapid depolymerization of kraft lignin.

Example 10 Comparative, in the Absence of Tungstophosphoric Acid and Manganese Salt

This example uses the conditions of Example 9, with the exception of tungstophosphoric acid and manganese salt.

1 g of kraft lignin is dissolved in 50 ml of distilled water. The resulting mixture, pH = 7.14, is heated at 90 ° C for 1 h, then the pH is adjusted to 12.8 using a 3M sodium hydroxide solution.

After 1 h at 170 ° C, the autoclave is depressurized and the measured pH is 9.20. A sample of 1 mL of the reaction medium is taken, brought to ambient temperature, and filtered through a 0.45 μηι diameter filter for SEC analysis.

The results are collated in Table 10.

Table 10

A low level of depolymerization of lignin is observed, the depolymerization products being of greater size than that of the depolymerization products of Example 9. In particular, no trace of vanillin or of diformyl guaiacol is observed.

The presence of tungstophosphoric acid and manganese salt thus makes it possible to achieve a better level of depolymerization of kraft lignin.

Claims

Process for the depolymerization of lignin, comprising a step of bringing lignin, at least one salt of manganese, tungstophosphoric acid or a salt thereof, and an oxidizing agent into contact in at least one solvent .

The process of claim 1 wherein the lignin is a sulfonated lignin.

The process of claim 1 wherein the lignin is a Kraft lignin.

A process according to any one of claims 1 to 3 wherein the oxidizing agent is selected from the group consisting of hydrogen peroxide, benzoyl peroxide, O ₂ , air, O ₃ , KMnO ₄ and NalO ₄ .

The process of claim 4 wherein the oxidizing agent is selected from the group consisting of hydrogen peroxide, oxygen and air.

A process as claimed in any one of claims 1 to 5 further comprising adding a compound capable of trapping free radicals.

The process of claim 6 wherein the free radical scavenging compound is N-acetylcysteine.

The process of any one of claims 1 to 7, wherein the manganese salt is an Mn (II) salt.

The process of claim 8 wherein the manganese salt is Mn (CH ₃ COO) ₂ or MnCl ₂ .

Process according to any one of Claims 1 to 9, in which the molar ratio between tungstophosphoric acid and lignin is between 0.1 and 10, preferably between 0.5 and 5.

1 1. Process according to any one of claims 1 to 10, wherein the molar ratio between the oxidizing agent and the lignin is between 1 and 1000, preferably between 100 and 500.

The process of any one of claims 1 to 11, wherein the solvent is a mixture of water and a polar organic solvent.

The process of any one of claims 1 to 11, wherein the solvent is water.

14. Use of tungstophosphoric acid, or a salt thereof, in combination with a manganese salt and an oxidizing agent to depolymerize lignin.