PROCESS FOR THE ELECTROCHEMICAL CLEAVAGE OF LIGNIN AT A DIAMOND ELECTRODE

Abstract

The invention relates to a process for the electrochemical cleavage of lignin by means of a diamond electrode and also to a process for producing vanillin and derivatives thereof by electrochemical cleavage of lignin in a solution having a pH≦11.

Description

The invention relates to a process for the electrochemical cleavage of lignin by means of a diamond electrode and also to a process for producing vanillin and derivatives thereof by electrochemical cleavage of lignin in solutions having a pH≦11.

Lignin is a high-molecular-weight aromatic substance which in woody plants fills up the spaces between the cell membranes and may be converted to wood. The lignin content of the dried plant material is about 27 to 33% by weight in conifer wood and 22% by weight in deciduous wood.

Lignin must be seen as a higher-molecular-weight derivative of phenylpropane. Depending on the wood species, the phenyl ring is substituted by one to two methoxy groups and the propane units with hydroxyl groups. In coniferous woods, there is predominantly the guaiacyl type, and in deciduous wood, in addition, the syringyl and coumaryl type. By means of various possibilities of linking, inter alia, lignin and coumarin structures, cyclic ethers and lactones are formed.

Alkali lignin is used in North America as binder for wood- and cellulose-based hardboards, as dispersion medium, for clarification of sugar solutions, stabilizing asphalt emulsions and also foam stabilization. By far the greatest amount of the alkali lignin, however, is used as energy source for the wood pulp process by combustion of the black liquors.

Vanillin is widely used in place of the expensive natural vanilla as aroma substance for chocolate, confectionery, liqueurs, bakery products and other sweet foods and also for the production of vanilla sugar. The vanillin content of wood which has been processed into wine barrels contributes to the aromatizing of wine. Smaller amounts are used in deodorants, perfumes and for flavor enhancement of pharmaceuticals and vitamin preparations. Vanillin is also an intermediate in the synthesis of various medicaments such as, for example, L-dopa, methyldopa and papaverine.

In EP-B 0 245 418, the electrochemical cleavage of lignin for production of vanillin and its derivatives such as guaiacol and acetovanillone (3-methoxy-4-hydroxyacetophenone) is described. In this case, an aqueous-alkaline solution is employed, wherein heavy metal electrodes are used. The workup proceeds with the use of toxic organohalogen solvents such as chloroform. From (eco)toxicological aspects, this is very disadvantageous, as is also the use of heavy metal electrodes. The use of high sodium hydroxide concentrations, as described in EP 0245418 B1, leads to vanillin and its derivatives being present as phenolate or hydroxybenzaldehyde derivatives. The phenolates and hydroxybenzaldehyde derivatives of vanillin and of the vanillin derivatives are, however, very sensitive to oxidative processes which should proceed in the alkaline environment. For the workup, therefore, a neutralization must proceed in advance, so that to obtain vanillin an increased expenditure on workup is necessary.

There is therefore a great requirement for being able to degrade lignin by oxidation in such a manner that workup of the resultant preliminary products requires lower expenditure and therefore is cheaper than the processes known hitherto.

This object is achieved by a process for the degradation of lignin having a yield of hydroxybenzaldehyde derivatives and/or phenol derivatives higher than 5% by weight, wherein an aqueous solution or suspension of lignin is electrolyzed at a diamond electrode.

Advantageously in the process according to the invention, an aqueous solution is employed which has a pH≦11.

Advantageously in the process according to the invention, an aqueous, acidic solution is employed.

Advantageously in the process according to the invention, the diamond electrode used is a boron-doped diamond electrode.

Advantageously in the process according to the invention, the lignin degradation products are continuously removed from the electrochemical cell.

Advantageously in the process according to the invention, the lignin degradation products are removed by steam distillation.

Advantageously in the process according to the invention, the lignin degradation products are removed by continuous extraction using an organic solvent.

Advantageously in the process according to the invention, the lignin degradation products are selected from the group of guaiacol, vanillin and acetovanillone.

The lignin used for the degradation is any lignin known to those skilled in the art. Preference is given to lignin which is present in products which are selected from the group of straw, bagasse, black liquor, kraft lignin, lignin sulfonate, organosolv lignin and corresponding residues from the paper industry or fiber production. Particular preference is given to the lignin present in kraft lignin and in lignin sulfonate.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which are formed in the degradation of lignin can be obtained at more than 5% by weight using the process according to the invention.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which are formed in the degradation of lignin are selected from the group of guaiacol, vanillin and acetovanillone. Particular preference is given to vanillin or guaiacol.

The hydroxybenzaldehyde derivatives and/or phenol derivatives which are obtained by the process according to the invention can be continuously removed from the reaction products. Preferably, these hydroxybenzaldehyde derivatives and/or phenol derivatives are continuously removed from the reaction mixture by distillation or extraction. Particular preference is given to steam distillation.

For the electrolysis, the lignin is present in aqueous solution, wherein the aqueous solution has a pH of ≦11, preferably 9, particularly preferably is acidic. Very particularly preferably, the pH is 3. Preferably, the pH is adjusted to a pH 3 using readily water-soluble inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, or organic acid such as para-toluenesulfonic acid or mixtures of various acids. Particular preference is given to sulfuric acid.

For the electrolysis, use can be made of any electrolysis cells which are known to those skilled in the art, such as partitioned or non-partitioned flow cells, capillary gap cells or plate stack cells. Particular preference is given to the non-partitioned flow cell. To achieve optimum space-time yields, a bipolar arrangement of a plurality of electrodes is advantageous.

For the process according to the invention, the anode is a diamond electrode. These diamond electrodes comprise a diamond layer applied to a support material, wherein the support material is selected from the group of niobium, silicon, tungsten, titanium, silicon carbide, tantalum, graphite, or ceramic supports such as titanium suboxide. Particularly preferably, the support material is niobium or silicon. The diamond layer on the support can in addition be doped with further elements. Preference is given to boron- or nitrogen-doped diamond electrodes. Particular preference is given to boron-doped diamond electrodes.

As cathode material, use can be made of any conventional cathode material having a low oxygen overvoltage selected from the group of RuO_xTiO_x mixed oxide electrodes (DSA), platinated titanium, platinum, nickel, molybdenum or stainless steel. Preference is given to the combination of boron-doped diamond cathode with stainless steel as cathode. Particular preference is given to the use of boron- or nitrogen-doped diamond electrodes. Very particular preference is given to boron-doped diamond electrodes.

Use can be made of diamond electrodes which have been produced by the CVD process (chemical vapor deposition). Such electrodes are commercially available such as, for example, from the manufacturers: Condias, ltzehoe; Diaccon, Furth (Germany) and Adamant Technologies, La-Chaux-de-Fonds (Switzerland). Less expensive diamond electrodes which have been produced by the HTHP process (high temperature high pressure: industrial diamond powder is mechanically introduced into the surface of a support plate) can likewise be used. HTHP-BDD electrodes are commercially available from pro aqua, Niklasdorf (Austria), their properties are described by A. Cieciwa, R. Wüthrich and Ch. Comninellis in Electrochem. Commun. 8 (2006) 375-382.

The temperature for the process according to the invention is between 20 and 150° C., preferably in the range from 90 to 120° C.

For the process according to the invention, the current density is preferably in the range from 5 to 3000 mA/cm², particularly preferably in the range from 10 to 200 mA/cm². To avoid a coating on the electrodes, when diamond electrodes are used as anode and/or cathode material, the polarity can be changed at short intervals of time. The polarity change can proceed in an interval of 30 seconds to 10 minutes, preference is given to an interval of 30 seconds and 2 minutes.

The efficiency of the electrolysis of lignin in aqueous solution at boron-doped diamond electrodes can be increased by the addition of additives such as TiO₂. TiO₂ is preferably used in catalytic amounts.

For the electrolysis of lignin, a metal-comprising or metal-free redox mediator can be added, preference is given to transition metal-free mediators, e.g. nitrosodisulfonates such as Fremy's salt (dipotassium nitrosodisulfonate).

For the mixing of the cell contents, any mechanical stirrer known to those skilled in the art can be used, but also other mixing methods can be used such as the use of an Ultraturrax or ultrasound.

EXAMPLES

Example 1

A suspension of 30 g of kraft lignin in an electrolyte comprising 570 g of dilute sulfuric acid (0.1 M) is electrolyzed in a non-partitioned cell which is provided with an electrode stack of boron-doped diamond cathodes (expanded metal, 5×10 cm) and boron-doped diamond anodes (expanded metal, 5×10 cm) at a spacing of 0.5 cm, at a current density of 80 mA/cm² and a temperature of 30° C. for 1 hour with stirring at 9500 rpm. The cell voltage which is established is in the range of 2-6 V. The aqueous phase is extracted with methyl tert-butyl ether (MTBE), the solid is filtered off with suction and washed with MTBE. The aqueous phase is repeatedly extracted with MTBE, the organic phases are combined, dried and the solvent is removed. Gas-chromatographic analysis of the crude organic product gives the following typical composition (GC percentage areas): 27% guaiacol, 24% vanillin, 24% acetovanillone and 25% other compounds. For the GC analysis, the stationary phase used was a dB1 column from J&W Scientific having a length of 30 m and diameter of 0.25 mm and a layer thickness of 1 μm. This column is heated by means of a temperature program from 80° C. to 250° C. in the course of 5 min in 8° C. steps. The carrier gas used is helium having a flow rate of 20 to 30 mL/min.

Example 2

Procedure as in example 1 with the following variation: 6 g of kraft lignin, 594 g of dilute sulfuric acid (0.1 M), 3 g of Fremy's salt, electrolysis for 30 minutes at 25° C. Typical composition of organic extracts (GC percentage areas): 37% guaiacol, 23% vanillin, 40% acetovanillone.

Claims

1. A process for the degradation of lignin comprising electroyzing an aqueous solution comprising lignin in an electrochemical cell with a diamond electrode to produce a lignin degradation product, wherein the lignin has a yield of hydroxybenzaldehyde derivatives and/or phenol derivatives higher than 5% by weight, and the aqueous solution has a PH≦11.

2. The process according to claim 1, wherein the aqueous solution is acidic.

3. The process according to claim 1, wherein the diamond electrode is a boron-doped diamond electrode.

4. The process according to claim 1, wherein the lignin degradation product is continuously removed from the electrochemical cell.

5. The process according to claim 1, wherein the lignin degradation products arc product is removed by steam distillation.

6. The process according to claim 1, wherein the lignin degradation product is removed by continuous extraction using an organic solvent.

7. The process according to claim 1, wherein the lignin degradation product is selected from the group consisting of guaiacol, vanillin and acetovanillone.