Note: Descriptions are shown in the official language in which they were submitted.
CA 02722235 2010-10-21
PF 60845
Process for the electrochemical cleavage of lignin at a diamond electrode
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 s 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
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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
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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 s 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 RuOxTiOx 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
PF 60845 CA 02722235 2010-10-21
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process (chemical vapor deposition). Such electrodes are commercially
available such
as, for example, from the manufacturers: Condias, Itzehoe; 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. Wi thrich 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 Ti02. Ti02 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.
CA 02722235 2010-10-21
PF 60845
Examples
Example 1
5 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 x 10 cm) and boron-
doped diamond anodes (expanded metal, 5 x 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 pm. 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.
