EP2132377A1

EP2132377A1 - Method for the breakdown of lignin

Info

Publication number: EP2132377A1
Application number: EP08706378A
Authority: EP
Inventors: Tobias Voitl; Philipp Rudolf Von Rohr
Original assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Current assignee: Eidgenoessische Technische Hochschule Zurich ETHZ
Priority date: 2007-03-02
Filing date: 2008-02-25
Publication date: 2009-12-16
Also published as: BRPI0808430A2; CA2679854A1; JP2010520157A; CN101680167A; CH702124B1; US20100121110A1; US7906687B2; WO2008106811A1

Abstract

The invention describes a method for the direct production of molecules with a minimum molecular weight of 78 g/mol by means of the breakdown of lignin, lignin derivatives, lignin fragments, and/or lignin-containing substances or mixtures in the presence of at least one polyoxometallate and preferably in the presence of a radical scavenger in a liquid medium.

Description

DESCRIPTION

TITLE Process for the degradation of lignin

TECHNICAL FIELD The present invention relates to the targeted conversion of lignin to chemicals.

STATE OF THE ART

Alternative processes for producing chemicals from biomass are becoming increasingly important as fossil fuels become scarcer. For example, Develops methods that win from the cellulose, or hemicellulose content of plants fuels. Renewable, plant-based raw materials include cellulose and hemicellulose as well as lignin, with lignin being the second most abundant organic substance on earth after cellulose.

Lignin is a major component of plants that provide rigidity to the cell scaffold. The lignin content can vary from plant to plant.

The chemical structure of lignin also has plant-specific differences. Thus, the macromolecule lignin depending on the plant type of different

Coniferyl alcohol, sinapyl alcohol and Cumarylalkohol together, in many cases dominated by the proportion of coniferyl alcohol (especially softwoods). In addition, there are numerous methods for separating the lignin from the other cell wall components, which in part considerably modify the chemical structure of the natural lignin. While various processes for the production of chemicals from cellulose have reached the big-industrial scale, there is currently little economic opportunity to use the lignin content of biomass as a raw material for the production of chemicals. In papermaking, large quantities of lignin are produced as by-products or by-products. Of the approximately 50 million tonnes of lignin produced each year in papermaking, only 1-2% are processed commercially.

The use of lignin is limited mainly to relatively inexpensive dispersants and binders. Vanillin is the only lignin-based phenolic product that is commercially produced. Part of the global production of vanillin (12,000 tonnes / year) is achieved by oxidative degradation of lignosulfonic acids.

Numerous methods of using lignin as a raw material are described in the literature. A frequently used approach is the gasification of biomass at high temperatures (800-1000 ⁰ C) with air and / or water vapor to synthesis gas (CO, H _2, CO ₂ and CH _4). From this synthesis gas various basic chemicals, such as methanol, ether, formic acid and higher molecular weight hydrocarbons (by Fischer-Tropsch) can be represented in further stages.

Here, the originally existing complex chemical structure is completely lost. This decomposition and recombination strategy requires a lot of energy and produces unwanted by-products. In addition, only relatively cheap, cheap molecules can be produced with this method. For economic profitability therefore very large sales are necessary. Alternatively, various methods (e.g., oxidative and non-oxidative hydrolysis, hydrogenolysis, pyrolysis) for directly producing monomeric or low molecular weight chemicals are known. While these methods are capable of breaking down lignin into smaller fractions, no method is selective enough to produce a single product in high yield.

PRESENTATION OF THE INVENTION

The inventive method proposed here differs conceptually significantly from previous approaches.

The experiments described in the literature to convert lignin to monomeric chemicals are limited to purely physical methods (high temperature and pressure), simple acid- or base-catalyzed hydrolysis or biotechnology. On closer examination of the chemistry, it becomes clear that these relatively simple methods are not suitable for achieving a high yield of monomeric chemicals.

The reason for this lies in the fact that the desired products in the conversion of lignin are degradation products. By simple methods, these degradation products can not be obtained in large quantities due to subsequent reactions.

The invention takes into account in two respects the chemically complex task of maximizing the yield of degradation products. First, the reaction of lignin is carried out in the presence of a catalytic system. This catalytic system includes so-called polyoxometalates, which allows the selective cleavage of bonds even at relatively low temperatures.

Secondly, so-called radical scavengers are added to the reaction mixture. These prevent repolymerization reactions, or combination reactions of lignin fragments, and stabilize the desired target products. As a benchmark for the invention presented herein, the alkaline oxidation of lignin can be used. The reaction of 10 g of a hardwood lignin (steam digestion) under 1.4 MPa oxygen and 170 ° C using a Cu (II) / Fe (III) catalyst results in 4.7 wt% vanillin and 9.5 wt% syringaldehyde. In a very similar study on hardwood lignin, the yield of aldehydes (vanillin and syringaldehyde) was 15% by weight.

Various polyoxometalates have been used for delignification and patented, as described, for example, in US 5,302,248, US 5,552,019 or WO95 / 26438. The unbleached pulp is treated here in a first process step with an aqueous polyoxometalate solution (eg: Na ₇ AlVW ₁₁ O ₄ Q) under an inert atmosphere, wherein the lignin is finally converted to carbon dioxide and water.

These processes exclusively pursue the goal of liberating pulp from unwanted lignin. The lignin should be completely converted to CO ₂ and H ₂ O without decomposing the cellulose. The possibility of directly obtaining degradation products or chemicals by treatment with polyoxometalates was not considered.

The invention thus aims to convert lignin directly into chemicals by means of polyoxometalates. In this case, so-called free-radical scavengers can be used in addition to polyoxometalates.

The terms lignin and polyoxometalates each describe whole classes of substances and are therefore briefly presented below.

It is further defined how the terms "scavengers" and "chemicals" are understood in the context of this document.

Lignin:

Independently of origin and pretreatment, chemicals can be obtained from all lignin types using the method described in this invention. It is also possible to carry out a targeted pretreatment of the lignin used, for example to modify the solubility in organic or inorganic solvent. It is also possible to use an already partially decomposed lignin.

It is also possible to use lignin-containing biomass, such as wood, without prior separation of the lignin portion of the biomass.

Pol yoxometalates:

Polyoxometallates belong to the class of metal-oxygen cluster anions. Polyoxometalates are characterized, among others, by their mostly simple synthesis, their structural modification and their specific redox behavior. For many reaction systems important key properties, such as solubility in organic / inorganic media, redox potential and pH, can be adapted by targeted synthesis. The synthesis of the polyoxometalates is usually carried out in one stage by heating an acidic, aqueous solution containing all the components in the desired, stoichiometric ratio. The polyoxometalate, or the Polyoxometalatmischung, thus resulting from the amounts of the components used. It also makes it clear that there is a wide variety of polyoxometalates. The following two names for structurally different polyoxometalates have become established in the literature: "isopolyanions" and "heteropolyanions". These anions are represented by the following general empirical formula:

[M _m O _y ] ^{p "} isopolyanions

[XχM _m O _y ] ^{q "} (x ≠ m) heteropolyanions

The "M" part is called the addenda atom and the part "X" is called the heteroatom. In this document, the term polyoxometalates includes both anion types: isopoly and heteropolyanions.

An important subgroup within the polyoxometalates are the heteropolyacids. Heteropolyacids are very strong acids, which are composed of heteropolyanions with protons as countercations. In the case that all countercations consist of other elements, one also speaks of heteropoly salts. A number of these polyoxometalates have proven to be particularly interesting for catalysis.

The most common addenda atoms ("M") are molybdenum or tungsten, less common are vanadium or niobium, or mixtures of these elements in the highest oxidation states (d °, d ¹ ). Heteroatoms ("X") are essentially all elements of the periodic table in question. The elements P ⁵⁺ , Si ⁴⁺ , As ⁵⁺ , Ge ⁴⁺ , B ³⁺ are most frequently mentioned in connection with catalytically used polyoxometalates. In comparison to the very large number of polyoxometalates, only a few polyoxometalates are currently used as catalysts, these being mainly limited to the class of keggin anions and their derived structures. It is therefore usually sufficient to use a very simplified nomenclature. This treats polyoxometalates as quasi-coordinated complexes. If a heteroatom is present, this is considered to be the central atom of the complex, which is surrounded by the addenda parts as ligands. In the molecular formula of the heteropolyanions, the heteroatoms are placed in front of the addenda atoms and the countercations in front of the heteroatoms. The heteropolyanion is placed in square brackets and thus separated from the countercations.

This will be clarified in the following examples:

[SiW ₁₂ O ₄₀ ] ^{4 "} 12-tungsten silicate

H ₃ [PMo ₁₂ O ₄₀ ] 12-phosphomolybdic acid

Sometimes, in the simplified representation, the counter cations, the charge of the polyanion, and even the oxygen atoms are not explicitly stated. Na ₆ [P ₂ Mo _1S O ₆₂ ] can thus be represented as [P ₂ Mo ₁₈ O ₆₂ ] or even P ₂ Mo ₁₈ .

Any polyoxometalate (isopoly and heteropolyanions), its acids, salts and partial salts can be used regardless of the method of preparation of this invention. It is also possible to use mixtures of different polyoxometalates. In fact, depending on the pH in solution, many species may be in equilibrium side by side, with a crystalline isolated compound not necessarily being the major component.

Preferably, the polyoxometalates used can be reversibly oxidized and reduced, which is the case for all polyoxometallates of the Keggin and Wells / Dawson type.

Structure is given. A preferred class of polyoxometalates are components of the form [Y _3-1S] ⁰⁺ [X _1-4 M _1-36 O O ₁ _-60] ^{11 ",} each" X "released from the elements of the

Periodic system is selectable and / or also a molecular part with four or less

Can be atoms. "M" is freely selectable from the group of metals, "Y" represents countercations and "n" is integer, another preferred class is

Polyoxometalates of the form [Y ₃ -I ₈ ] ^ [M _1-36 O 1 O _-60 ] ^{11 '} , wherein "Y", "M" and "n" are as defined above.

In the preferred embodiments, each "X" is chosen freely from the group of elements P, Si, As, Ge, B, Al, Co, S ₅ Fe. Each "M" part is preferably free Mo, W, V, Ti ₃ Co, Cu, Zn, Fe, Ni, Mn, Cr, lanthanides, Ce, Al, Ga, In, Tl. The counter cations "Y" are preferably free from the group H. , Zn, Co, Cu, Bi ₅ Na, Li, K, Rb, Cs, Ba, Mg, Sr, ammonium, C _1-12 -alkylammonium, C _1-12 -alkylamine In a particularly preferred embodiment, polyoxometalates of Form [Yf ⁺ [XM ₁₂ O ₄₀ ] ^{11 '} , where "Y", "X", "M" and "n" are the same definitions as above, the most preferred polyoxometalates are of the form [Yf ⁺ [ XM ₁₂ O ₄₀ ] "^" , wherein "Y" and "n" are as defined above, "X" is Si, P, Ge or As and each "M" is freely selected from Mo, V or W. In another Polyoxometalates of the abovementioned definitions, which have been applied to a suitable support material, are used in a preferred embodiment The application of a polyxoxmetallate to a support material such as SiO ₂ , Al ₂ O ₃ and zeolites and its use as a heterogeneous K Atalysator for eg oxidations with oxygen has already been described in the literature. For possible polyoxometalates, refer to the systems as described in Hill (1998) Chemical Reviews 98: 1-389; Pope (1983) Heteropoly and Isopoly Oxometalates, Springer, Berlin and Okuhara et al. (2001) Applied Catalysis A: General 222: 63-77, the disclosure of which is expressly incorporated herein.

For the sake of simplicity, the abbreviation POM for polyoxometalates according to the abovementioned definitions is used below.

Radical scavengers:

In a preferred embodiment of the invention, one or more components are used as Radikallanger.

Radical scavengers serve in the context of this invention to scavenge the radicals formed during the degradation of lignin and thus to reduce repolymerization reactions. In particular, this should increase the yield of desired chemicals.

Radical scavengers are generally all components which, under the given conditions, form radicals in one or more steps or already exist as free radicals.

The operation of the radical scavenger is illustrated in the following exemplary and incomplete reaction scheme. The mechanisms proposed here reflect the current state of knowledge and are not to be construed as limiting the invention.

For example, consider two radicals R »and R '% which are formed during the cleavage of lignin and thus represent lignin fragments. It is then possible that the coupling of the lignin fragments with one another leads to the formation of stable bonds and thus counterproductively affects the desired cleavage of lignin fragments (equation 1). When radical scavengers are added which form radicals S or are already present as free radicals, it is possible that the lignin fragments R * and R '* are converted to the coupling products RS and R'-S (Equation 2 and 3). It is thus possible to reduce the repolymerization of lignin fragments by targeted coupling with radicals S »: R '+ R'- → RR ¹ (1)

R »+ S- → R-S (2)

R ' ^« + S- → R'-S (3)

It is e.g. it is possible to use radical scavengers which are already present as free radicals in the form S *. For example, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), which is e.g. used in radical free radical polymerization as radical scavenger.

It is also possible to use radical scavengers which can form free radicals by homolytic cleavage of a covalent bond (Equation 4). From a thermodynamic point of view, a relatively low binding energy of the S-S 'bond in comparison to the binding energies of the possible coupling products R-S, R'-S, R-S' and R'-S 'is then to be preferred when selecting the radical scavenger. For example, Dimethyl ether, diethyl ether, peroxides.

S-S '→ S * + S'- (4)

It is also possible to use free radical scavengers, which are free radicals in several steps form. In particular, several steps can be taken in conjunction with catalysts to form a radical S *. For example, acid-catalyzed condensation of methanol to dimethyl ether in the presence of POM as an acid catalyst with subsequent homolytic cleavage of the ether bond to the radicals CH ₃ O * and CH ₃ 'drove.

On the one hand, the radical scavenger can serve as a hydrogen donor and, on the other hand, prevent the repolymerization of the lignin fragments. Thus, suitable components of the general form S-H are those in which H represents a hydrogen available for reaction and S denotes the radical of the radical scavenger, i. minus the considered hydrogen atom. The radical scavenger can have several accessible hydrogen atoms.

The operation of the radical scavenger is illustrated in the following exemplary second reaction scheme. By way of example, radicals which have been formed by separation of an ether bond of the lignin (equation 5), where R in turn represents lignin fragments, are now considered.

The following reaction scheme again serves only to illustrate a possible radical scavenging mechanism. Activation of the radical scavenger does not have to be done in one step and not by pure hydrogen abstraction. In fact, radicals, especially in the presence of POMs and / or oxygen, can participate in a wealth of reactions. For example, radicals of the form R * (or S *) in the presence of oxygen form radicals of the form ROO * (or SOO *). Due to the sometimes very strong tendency of POMs to pick up electrons (and protons), POM can also interact with radical scavengers. It should be noted at this point that phenolic degradation products of lignin are per se radical scavengers.

R-O-R → RO * + R * (5)

RO * + S-H → R0H + S * (6)

R * + S-H → RH + S * (7)

S * + S * → SS (8) S- + R ^« → SR (9)

S- + RO ^« → ROS (10)

By abstraction of a hydrogen atom from the scavenger, the desired degradation products of the lignin ROH and RH can be obtained (equations 6 and 7). In addition, there is a possibility that the remainder of the radical scavenger (i.e., minus one hydrogen atom) will be integrated into the product, thus reducing radical repolymerization reactions (equations 9 and 10).

In principle, for example, molecules which can give off hydrogen atoms under the given conditions are suitable as radical scavengers. Therefore, considering the chemical structure, H _{2 is} the simplest radical scavenger. Also possible are alcohols and acids.

It is also possible, as explained above, to use as free-radical scavengers substances which under the given conditions form radicals without hydrogen abstraction, for example peroxides, as well as hydrogen peroxide and the like.

The structure of lignin includes a variety of interesting for the industry, mostly phenolic compounds. This invention aims to obtain these phenolic compounds in an economically interesting amount directly from lignin. This invention clearly distinguishes itself from other methods in that the chemicals produced here are not recombined from synthesis gas. Further, in this invention, molecules having a minimum molecular weight of MW are _{produced at} f = 78 g / mol.

In a preferred embodiment, molecules are prepared which contain at least one benzene ring in their structure. In a further preferred embodiment, molecules are produced which have at least one and at most three benzene rings in their structure. Most preferred is an embodiment in which molecules are made containing only one benzene ring in their structure. In the above preferred embodiments, the benzene ring is used as a backbone used. The functional groups and possible bonds between Benzqlringen can be chosen arbitrarily. In particular, the radical scavengers which may be used can be integrated into the structure of the chemicals produced. It is possible to obtain a mixture of several chemicals by the method explained in this invention.

One embodiment provides for converting lignin into a suitable reactor by means of POM. For this purpose, lignin and POM are dissolved or suspended in a suitable liquid medium. The mixture is placed on conditions which promote the degradation of lignin for a sufficient time. Here, the temperature may be between 0 ° C and 500 ⁰ C, but preferably between 25 ° C and 300 ° C. The degradation of lignin can take place here under inert gas, oxygen-containing, ozone-containing or hydrogen-containing gas at pressures up to 300bar. The pH value can be in the range 0-10 or be set. In the degradation of lignin, products from the group of chemicals defined above are to be formed. Here, therefore, the products are obtained in the range between complete decomposition of lignin and quasi-unchanged lignin. This approach represents a new optimization task in terms of suitable POM systems, temperatures, reaction times, etc. The difficult task of intercepting degradation products can be solved, inter alia, by the use of radical scavengers.

One embodiment therefore envisages converting lignin by means of POM in the presence of radical scavengers. This procedure is analogous to the first embodiment. The main difference is that at a suitable time a radical scavenger is added to the reaction mixture. The radical scavenger can be present as a solid, liquid or gas. Radical scavengers are generally all components which under the given conditions form radicals in one or more steps or are already present as free radicals. As scavengers, for example, substances can be used which can give off one or more hydrogen atoms under the conditions used. It is one Special feature that POM can promote the activation of radical scavengers (or the formation of radicals) due to their mostly strong electron (and proton) affinity. It is also possible to use stable radicals, such as nitroxyl radicals, as radical scavengers. It is also possible to use radical formers, such as, for example, dibenzoyl peroxide from the group of peroxides, as radical scavengers.

Another embodiment provides to convert lignin by means of POM in the presence of two liquid phases. In this case, the procedure is analogous to the first embodiment. The main difference is that two only partially or immiscible liquid phases are in contact during the degradation of lignin. Due to different solubilities of POM, lignin and lignin-based degradation products in the selected liquid phases, a partial or complete separation of the components lignin, POM and lignin-based degradation products can take place. It is possible, for example, to choose a system with two liquid phases, wherein lignin and POM are dissolved or suspended mainly in the first liquid phase (eg water) and the second liquid phase offers a higher solubility for degradation products of lignin (eg chloroform) , Thus, the desired degradation products of lignin can be separated from the reaction medium before they react further in subsequent reactions. It is thus possible, by suitable choice of two liquid phases, to increase the yield of products compared to a system with only one liquid phase.

A further embodiment provides to convert lignin by means of POM in the presence of radical scavengers and two liquid phases.

A further embodiment provides for using a metallic catalyst in conjunction with one of the abovementioned embodiments. Combinations of metallic catalysts, mostly palladium, with polyoxometalates have been studied in a number of oxidations and reductions. An additional metallic

Catalyst allows the degradation of lignin at lower temperatures and has a positive effect on the selectivity of the target products. It is also the case that the yield of desired products using an additional increase metallic catalyst. It is possible to use a metallic catalyst in combination with polyoxometalates for the most complete oxidation of non-desired degradation products of lignin during the workup.

To implement the invention in a process plant, it is possible, for example, to carry out the operations described below. The following diagram serves only to illustrate possible operations and does not claim to be exhaustive.

The reaction of lignin according to the above-mentioned embodiments (reaction stage). Separation of the products from the reaction medium (for example by extraction).

• Processing the products (separating the product mixture).

Separation of radical scavenger (e.g., by distillation).

• Return of the radical catcher.

• Oxidation of lignin fragments, which were not converted to target products, by means of oxygen to carbon dioxide and water.

Use of non-target lignin fragments or lignin for energy production (e.g., thermal energy from oxidation).

• Reoxidation or regeneration of POM (i.d.R coincident with oxidation of lignin fragments). • Return of the POM to the reaction stage.

• solids discharge at a convenient location (for example by filtration).

• Addition of solvents in a meaningful place.

• Discharge of solvents in a meaningful place.

It is possible to carry out the separation of the products at the same time as the conversion of lignin, for example by extraction in one operation (reactive extraction).

The essence of the invention is thus inter alia, in summary, a process for the direct production of molecules having a minimum molecular weight of 78 g / mol by the degradation of lignin, lignin derivatives, lignin fragments and / or lignin-containing substances or mixtures in the presence of at least one polyoxometalate in a liquid medium.

In this case, a radical scavenger is preferably used at least in sections, during the degradation, wherein this radical scavenger is preferably a system or a mixture of systems which contains radicals, such as e.g. atomic hydrogen provides and / or which the repolymerization resp.

Combination reactions of lignin fragments prevented or at least hindered, wherein the radical scavenger can be used gaseous, liquid and / or as a solid, and wherein the radical scavenger is particularly preferably selected from: molecular hydrogen, a peroxide such. Hydrogen peroxide or

Dibenzoyl peroxide, an alcohol such as methanol and / or ethanol, a stabilized free radical such as a nitroxyl radical, an organic acid such as ascorbic acid, a phenol such as butylhydroxytoluene, an ether such as dimethyl ether, an ester such as ethyl acetate, an anhydride such as

Acetic anhydride or mixtures of such systems.

A second, liquid phase can be added to the mixture, wherein this second liquid phase preferably has a substantially different polarity from the first medium.

It is also possible additionally to use a metallic catalyst, in particular from the group of transition-element-containing catalysts, preferably from the group VIIIB group of metal-containing and / or group IB metal-containing catalysts, it being possible for the catalyst to be bound to and / or in a preferably porous substrate ,

The reaction can be carried out under an inert gas, under an oxygen-containing gas phase, under a hydrogen-containing gas phase or under an ozone-containing gas phase in the pressure range from 0 to 300 bar, preferably at more than 5 bar.

Preferably, the polyoxometalate is H ₃ PMo ₁₂ O ₄₀ , in liquid medium to water, wherein an alcoholic radical scavenger is used, preferably selected from methanol and / or ethanol, wherein particularly preferably the volume ratio of water to alcoholic radical scavenger in the range of 1:10 to 10: 1 and wherein preferably a pressure of more than 2 bar prevails. In general, it is advantageous if the liquid medium is water, optionally in combination with an alcohol. It can be used per 100 ml of liquid medium between 2-200 g Polyoxometallate, more preferably between 8-12 g. It is therefore preferable to use concentrations in the range from 0.01 mol / 1 to 1 mol / l. Typically, subsequent to the degradation of the lignin to the molecules, a separation of the molecules from the reaction medium, particularly preferably by extraction and / or distillation.

Further preferred embodiments are described in the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

The invention will be explained in more detail with reference to embodiments in conjunction with the figure. Fig. 1 shows chemicals from the degradation of lignin by means of POM and radical scavengers.

WAYS FOR CARRYING OUT THE INVENTION

The method according to the invention for the production of chemicals from lignin by means of POM and radical scavengers will be explained below on the basis of representative experiments.

In particular, the effectiveness of POM and radical scavengers is demonstrated. In comparison with a method according to the prior art, a higher product yield can be achieved with the method according to the invention.

Versuchsdurcliführung: Experiments 01-11 and blank (see Table 1):

In a typical experiment, 9.13g of the POM H ₃ PMo ₁₂ O _{40 was dissolved} in 100ml of water or a water / alcohol mixture, which is formally equivalent to a 0.05 molar solution. This was then transferred to a 500 ml autoclave (Premex Reactor AG, CH-Lengnau). Before closing the reactor, Ig powder of lignin was added. The reaction mixture was then charged and drained with 5 bar of oxygen or nitrogen three times to displace the initial air. Finally, the reactor was charged with 5 bar of oxygen or 10 bar of nitrogen. The reaction mixture was heated to 170 ° C at a bake speed of 1200rpm at a rate of 8K / min. The mixture was held for 20 minutes at 17O ⁰ C. The liquid phase was removed after 20 minutes. The sampling was carried out by a water-cooled cooling coil. The sample was filtered once and then extracted three times with 10 ml of chloroform. To the organic phase was added 30μl of n-decane as internal standard for GC / MS analysis (Fisons instruments GC8000 / MD800; column of Restek Rtx-5MS 30m x 0.25mm x 0.25μm).

In Experiment 03, 04, 08, 09, 10 and 11 radical scavengers (methanol, ethanol) were used.

In Experiment 02, a second liquid phase in the form of 30 ml of chloroform was added before the reactor was closed. In experiment 05, 9.45 g of the POMs Na ₃ PMo ₁₂ O _{40 were} used. For the blank, no POM was used.

Experiment 12 and Wu et al. (G.X. Wu, M. Heitz, E. Chornet, Ind. Eng. Chem. Res. 33, 718 (Mar, 1994)) (see Table 1):

The experimental procedure in Experiment 12 is analogous to Wu et al. held. 10 g of lignin were dissolved in 100 ml of a 3-molar sodium hydroxide solution. The solution was placed in a 500 ml autoclave and the catalyst system consisting of 500 mg of copper sulfate and 50 mg of ferric chloride was added. The reactor was closed and filled three times with oxygen at 10 bar and emptied to displace the originally present air. Subsequently, the reaction vessel pressurized with 13.2bar oxygen. The reaction mixture was heated to 170 ° C at a bake speed of 1200rpm at a rate of 8K / min. The mixture was held at 170 ° C for 20 minutes. The liquid phase was removed after 20 minutes. The sampling was carried out by a water-cooled cooling coil. In Experiment 01, lignin was treated in a 0.05 molar aqueous H3PMol2O40 solution under inert gas. The color of the solution changed from yellowish to dark blue. This clearly shows that the polymolybdate used was reduced. By GC / MS analysis, 0.56 mg of lignin-based products (mainly vanillin) were quantified.

The yield of lignin-based products was increased by about 30% by in situ extraction (see Experiment 01 with Experiment 02). For this purpose, a second liquid phase (chloroform) was added to the reaction mixture. This has a higher solubility for the desired degradation products and can therefore any products formed the actual reaction medium (aqueous POM solution) withdraw before possible subsequent reactions destroy the products formed again.

The degradation of lignin with 0.05 molar H ₃ PMo ₁₂ O ₄ Q-LoSiUIg under oxygen (Experiment 06) also increased the yield compared to degradation under inert gas (Experiment 01). In contrast to the experiment under inert gas, only slight discoloration of the solution to yellow-greenish was found after the experiment under oxygen, ie the polyoxomolybdate was reduced only slightly or not at all. This suggests that the POM is reoxidizable under the conditions used and in this case oxygen is consumed during the degradation of lignin via the POM. Experiment 07 shows that different types of lignin can be used and that the yield of chemicals is of the same order of magnitude. The lignin from Aldrich (Batch NrO9724CE) is according to the manufacturer a softwood lignin (mainly from spruce wood), which was obtained by Kraft process. The lignin from the Granit® process, however, was obtained from agricultural plants and therefore has a different chemical structure. In experiment 05, the sodium salt Na ₃ PMo ₁₂ O ₄ O corresponding to H ₃ PMo ₁₂ O _4O was used as POM. The yield of chemicals is noticeably higher in this experiment. It therefore appears that, from a chemical point of view, minor changes in the POM system used can affect the yield of desired products. The use of an optimal POM system is therefore crucial for achieving high yields.

The positive effect of the radical scavenger on the chemical yield was demonstrated in experiments 03 and 04 compared to experiment Ol and the blank. In experiments 03 and 04, ethanol and methanol were used as radical scavengers. The yield could be increased from 0.56 mg to 1.17 mg in the case of ethanol and to 2.38 mg in the case of methanol using the free-radical scavengers.

In the blank test without POM no lignin-based products could be detected. Figure 1 shows the associated GC / MS chromatograms and illustrates that not only are the amounts of vanillin and 4-hydroxyacetyl-2-methoxy-phenol produced greater when using the radical scavengers, but also new components are prepared in comparable amount to vanillin.

The formation of vanillic acid methyl ester in Experiment 04 shows that methanol is incorporated into the product (see Equation 11).

By contrast, using ethanol as radical scavenger, the ethyl vanülate is formed (see Equation 12).

Experiments were further performed to verify the mechanism indicated in Equations 11 and 12. With deuterated methanol CD ₃ OD it could be clearly demonstrated that a methanol residue is incorporated into the product vanillic acid methyl ester. The molecular weight of vanillic acid methyl ester produced in reactions of lignin in the presence of non-deuterated methanol was determined by GC / MS analysis to be 182 g / mol. Using deuterated methanol, the molecular weight increased 185 g / mol determined. Furthermore, it was examined whether the formation of the vanillic acid methyl ester is also possible from vanillin, for example via the reaction route vanillin → vanillic acid → vanillic acid methyl ester. The treatment of vanillin in 0.05 molar H ₃ PMo ₁₂ O ₄ O LOSUTIg under nitrogen in the presence of methanol did not produce any products. It can thus be assumed that the acyl radical shown in Equations 11 and 12 results from the cleavage of the Ca-C β bond in the lignin.

In addition, the formation of the radical CH ₃ O * from methanol was considered in more detail. The homolytic cleavage of the OH bond in methanol is thermodynamically unlikely due to the high binding energy of 104 kcal / mol. However, it has been found that methanol in aqueous H ₃ PMo ₁₂ O ₄₀ solution is partially converted to dimethyl ether under nitrogen. The homolytic cleavage of the CO bond in dimethyl ether is possible due to the relatively low binding energy of 84 kcal / mol. It is therefore possible that methanol can form radicals via acid-catalyzed ether formation and is thus considered as radical scavenger in the context of this invention. Direct addition of dimethyl ether as a radical scavenger to react lignin in aqueous H ₃ PMo ₁₂ O ₄₀ solution under nitrogen and in the absence of methanol resulted in the production of a small amount of vanillic acid methyl ester. It has thus been shown that dimethyl ether is effective directly as radical scavenger and the homolytic cleavage of dimethyl ether and the associated formation of vanillic acid methyl ester is possible.

(11)

To compare the results of the degradation of lignin by POM and radical scavengers with the prior art, the Wu et al. (Benchmark) method applied to the lignin used by Aldrich in our examples.

A comparison of the results revealed that the yield of lignin-based chemicals using softwood lignin compared to Wu et al. much lower (~ one power of ten). This may be due to the use of a variety of lignins. According to the manufacturer, the lignin from Sigma-Aldrich used in our work is a typical softwood lignin that has been isolated from spruce wood by means of Kraft technology. Wu et al. on the other hand used a lignin variety made of hardwood by steam digestion. It is known that hardwood lignin has a much higher proportion of syringyl units (~ 50%). The comparatively high proportion of syringyl units is also reflected in the results of Wu et al. in considerable amount of syringaldehyde produced (7.8Gew%). In contrast, softwood lignin (from spruce) has a very high proportion of guaiacyl units (-90%). This coincides with our results in which only traces of syringaldehyde were formed. It is also known that the degradation of hardwood lignin is much easier and faster than the degradation of softwood lignin. Furthermore, that of Wu et al. produced hardwood lignin is not fully characterized. The composition of the acid-insoluble lignin is described by Wu et al. with: Klason lignin = 84.7%; acid soluble lignin = 3.0% and Other (unknown) = 12.3%.

An economically viable process requires a reliable and cost-effective source of lignin. It therefore seems sensible to use the softwood lignin available in large quantities as a raw material for the production of chemicals. As already indicated, softwood lignin is a waste product in papermaking (e.g., kraft or sulfite processes). In the following, therefore, the method of Wu et al. applied to the lignin accessible to us, considered as a benchmark.

A comparison of Experiment 10, in which lignin was reacted under oxygen in the presence of POM and radical scavenger, with the prior art method (Experiment 12) shows that the inventive method, a higher product yield is achieved. The chemicals produced are mainly vanillin (16.5mg) and vanillic acid methyl ester (13.5mg).

In experiment 11, poplar lignin was used. The product yield is also in the range of 3.5 wt .-%. However, the main products here are vanillin (5.0mg), vanillic acid methyl ester (6.2mg), syringaldehyde (8.2mg) and syringic acid methyl ester (12.2mg), corresponding to the chemical structure of poplar lignin with a relatively high proportion of syringyl units.

It should be noted that the examples given in this document represent the first series of experiments. Achieving the comparatively higher yields is therefore particularly promising since the method according to the invention has not yet been optimized. Given the many possible combinations of POM, radical scavengers and, ultimately, lignin, there is still much room for improvement in yields. In addition, there are still some reaction-technical solutions available, such as in-situ extraction. Among other things, the potential becomes clear when comparing experiments 06 and 10. Here, the yield of chemicals could be increased by a factor of 30 by the use of an aqueous methanol solution alone. An advantage of the invention presented here is also the possibility of producing various chemicals depending on the radical scavenger. Thus, vanillic acid methyl ester is formed when using methanol and ethyl vanillate when using Ethanol. It therefore appears possible, by means of suitable combinations of POM and free-radical scavengers, to be able to decisively influence not only the yield but also the selectivity with respect to a target product.

Table 1: Results for the preparation of chemicals from lignin using POM and radical scavenger compared to conventional methods. based on dry lignin

Claims

1. A method for the direct production of molecules with a minimal

Molecular weight of 78 g / mol by the degradation of lignin, lignin derivatives, lignin fragments and / or ligninhaltigen substances or mixtures in

Presence of at least one polyoxometalate in a liquid

Medium.

2. The method according to claim 1, characterized in that at least in sections, during the degradation a radical scavenger is used, wherein this radical scavenger is preferably a system or a mixture of systems, which provides radicals such as atomic hydrogen and provides / or which the repolymerization resp. Combination reactions of lignin fragments prevented or at least hindered, wherein the radical scavenger can be used gaseous, liquid and / or as a solid, and wherein the radical scavenger is particularly preferably selected from: molecular hydrogen, a peroxide such as hydrogen peroxide, an alcohol such as methanol and / or Ethanol, an organic acid such as ascorbic acid, a phenol such as butylhydroxytoluene, a stabilized free one

Radical such as a nitroxyl radical, or mixtures of such systems.

3. The method according to any one of the preceding claims, characterized in that a second, liquid phase is added to the mixture, wherein preferably this second liquid phase has a substantially different from the first medium polarity.

4. The method according to any one of the preceding claims, characterized in that additionally a metallic catalyst, in particular from the group of transition element-containing catalysts, preferably from the group of group VIIIB metal-containing and / or group IB metal-containing, is used, wherein the catalyst may be bound to and / or in a preferably porous substrate.

5. The method according to any one of the preceding claims, characterized in that the reaction is carried out under an inert gas, under an oxygen-containing gas phase, under a hydrogen-containing gas phase or under an ozone-containing gas phase in the pressure range of 0-3 OObar.

6. The method according to any one of the preceding claims, characterized in that the reaction in a temperature range of 0 ° C-500 ° C, preferably between 25 ° C and 300 ⁰ C, is performed.

7. The method according to any one of the preceding claims, characterized in that at least one of the polyoxometalates used has a Keggin structure.

8. The method according to any one of the preceding claims, characterized in that at least one of the polyoxometalates used of the form (I) or (II) is:

(I) [Y ₃ -i8] ^{n +} [X _1-4 M _1-36 O _10-6 Or

(II) [Y _3- i ₈ ] ^{n +} [M _1-36 O _10-60 ] ¹¹ - where each "X" is selected from the group of atoms or molecules less than 4

Atoms, preferably from the group consisting of P, Si, As, Ge, B, Co, S and Fe, is selected, each "M" from the group of metals, preferably from Mo, W, V, Ti, Co, Cu , Zn, Fe, Ni, Mn, Cr, lanthanides, Ce, Al, Ga, In, Tl is "Y" countercations, preferably selected from: H, Zn, Co, Cu, Bi, Na, Li, K, Rb, Cs, Ba, Mg, Sr, ammonium, Cl-12-alkylammonium and Cl-12-alkylamine , represents and "n" is integer.

9. The method according to any one of the preceding claims, characterized in that the polyoxometalate of the form (III) is:

(III) [Y _y ] ^{n +} [XM ₁₂ O ₄₀ ] ¹¹ - wherein "X" is selected from the group consisting of Si, P and Ge wherein "M" is selected from the group consisting of Mo, V and W, where the countercations "Y" are selected from the group consisting of H, Zn, Co, Cu, Bi, Na, Li, K, Rb, Cs, Ba ₅ Mg,

Sr, ammonium, C _1-12 -alkylammonium, Ci-π-alkylamine, and combinations thereof are selected and "n" and "y" are integers.

10. The method according to any one of the preceding claims, characterized in that the polyoxometalate on a substrate, particularly preferably on

Silica, alumina, activated carbon, or zeolite, wherein the substrate is preferably porous and a portion of the polyoxometalate is in the

Pores of the substrate is located.

11. The method according to any one of the preceding claims, characterized in that the molecules have at least one and at most three benzene rings.

12. The method according to any one of the preceding claims, characterized in that it is at least one of the molecules to vanillin or a vanillin derivative such as an acid methyl or acid ethyl ester.

13. The method according to any one of the preceding claims, characterized in that it is the polyoxometalate is HsPMo ₁₂ O ₄₀ , the liquid medium is water and an alcoholic radical scavenger is used, preferably selected from methanol and / or ethanol, wherein particularly preferably the volume ratio of water to alcoholic radical scavenger in the range of 1:10 to 10: 1 and wherein preferably a pressure of more than 2 bar prevails.

14. The method according to any one of the preceding claims, characterized in that it is the liquid medium is water, optionally in combination with a liquid radical scavenger.

15. The method according to any one of the preceding claims, characterized in that a polyoxometalate or a Polyoxometallatmischung is dissolved in the liquid medium, wherein preferably the concentration in the range of 0.01 mol / 1 to 1 mol / 1.

16. The method according to any one of the preceding claims, characterized in that during or subsequent to the degradation of the lignin to the molecules, a separation of the molecules from the reaction medium, particularly preferably by extraction and / or distillation.

17. The method according to any one of the preceding claims, characterized in that at least time-sectionwise during the degradation of a radical scavenger is used, wherein this radical scavenger is a system or a mixture of systems which directly or indirectly provides radicals and which the repolymerization resp. Combination reactions of lignin fragments prevented or at least impeded, wherein the radical scavenger gaseous, liquid and / or can be used as a solid, and wherein the radical scavenger is particularly preferably selected from: molecular hydrogen, a peroxide such as dibenzoyl peroxide or hydrogen peroxide, an alcohol such as Methanol and / or ethanol, a stabilized free radical such as a nitroxyl radical, an organic acid such as ascorbic acid, a phenol such as butylhydroxytoluene, an ether such as dimethyl ether, an ester such as ethyl acetate, an anhydride such as acetic anhydride or mixtures of such systems.