Electrocatalytic hydrogenation of lignin models at Raney nickel and palladium-based electrodes

Article abstract of DOI:10.1139/v00-009

The electrocatalytic hydrogenation (ECH) of β-O-4 lignin model compounds has been carried out at constant current at Raney nickel and palladium-based cathodes in aqueous sodium hydroxide solution at temperatures ranging from 25 to 75°C. It was found that the hydrogenolysis of phenolic β-arylethyl-aryl ethers with one β-O-4 linkage takes place at both types of electrodes to give a mixture of phenolic compounds. The ECH of 1-(4-hydroxy- 3-methoxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol (model 1) at Raney nickel gives guaiacol (9), acetovanillone (11b), α-methylvanillyl alcohol (12b), and products derived from their further hydrogenation. The β-O-4 bond is more difficult to cleave upon replacing one hydrogen of the methylene group of 1 by a hydroxymethyl group (see 2). However, model 4 with two β-O-4 linkages obtained by the introduction of an α-methylvanillyl group at the phenolic position of 1 is hydrogenolysed at the same rate as 1 at Raney nickel. Only the C-O aryl bond attached to the hydroxyethylphenol moiety is cleaved. At palladium, the hydrogenolysis of 1 requires only half of the theoretical amount of electricity due to the generation of chemisorbed hydrogen by dehydrogenation of 1 followed by hydrogenolysis of the resulting ketone. A general reaction scheme is proposed for the ECH of model 1.

Full text of DOI:10.1139/v00-009

3
07
Electrocatalytic hydrogenation of lignin models at
Raney nickel and palladium-based electrodes
André Cyr, Florence Chiltz, Paul Jeanson, Anna Martel, Louis Brossard,
Jean Lessard, and Hugues Ménard
Abstract: The electrocatalytic hydrogenation (ECH) of β-O-4 lignin model compounds has been carried out at constant
current at Raney nickel and palladium-based cathodes in aqueous sodium hydroxide solution at temperatures ranging
from 25 to 75°C. It was found that the hydrogenolysis of phenolic β-arylethyl-aryl ethers with one β-O-4 linkage takes
place at both types of electrodes to give a mixture of phenolic compounds. The ECH of 1-(4-hydroxy-3-
methoxyphenyl)-2-(2-methoxyphenoxy)-1-ethanol (model 1) at Raney nickel gives guaiacol (9), acetovanillone (11b),
α-methylvanillyl alcohol (12b), and products derived from their further hydrogenation. The β-O-4 bond is more diffi-
cult to cleave upon replacing one hydrogen of the methylene group of 1 by a hydroxymethyl group (see 2). However,
model 4 with two β-O-4 linkages obtained by the introduction of an α -methylvanillyl group at the phenolic position of
1
is hydrogenolysed at the same rate as 1 at Raney nickel. Only the C—O aryl bond attached to the
hydroxyethylphenol moiety is cleaved. At palladium, the hydrogenolysis of 1 requires only half of the theoretical
amount of electricity due to the generation of chemisorbed hydrogen by dehydrogenation of 1 followed by
hydrogenolysis of the resulting ketone. A general reaction scheme is proposed for the ECH of model 1.
Key words: Lignins, lignin models, electrocatalytic hydrogenation, Raney nickel electrodes, palladium electrodes.
Résumé : L’hydrogénation électrocatalytique (HEC) de composés modèles contenant une(des) liaison(s) β-O-4 des
lignines a été effectuée sur cathodes de nickel de Raney et de palladium dans des solutions aqueuses d’hydroxyde de
sodium et à des températures variant de 25 à 75°C. L’hydrogénolyse, sur ces électrodes, d’éthers phénoliques
β-aryléthyl-aryle ayant une liaison β-O-4 a donné un mélange de composés phénoliques. L’HEC du 1-(4-hydroxy-3-
méthoxyphényl)-2-(2-méthoxyphénoxy)-1-éthanol (modèle 1) sur nickel de Raney a donné le guaiacol (9),
l’acétovanillone (11b), l’alcool α -méthylvanillique (12b) et des produits de leur hydrogénation subséquente. La liaison
β-O-4 du modèle 2 dans lequel un des hydrogènes du groupe méthylène de 1 a été remplacé par un groupement
hydroxyméthyle s’est avérée plus difficile à hydrogénolyser. Cependant, la vitesse d’hydrogénolyse, sur nickel de
Raney, du modèle 4 ayant deux liaisons β-O-4 et obtenu par l’introduction d’un groupement α -méthylvanillyle sur la
fonction phénol du composé 1 était la même que celle du modèle 1. Seule la liaison C—O aryle fixée au reste
hydroxyéthylphénol a été hydrogénolysée. Sur cathode de palladium, l’hydrogénolyse du modèle 1 n’a consommé que
la moitié de la quantité théorique d’électricité dû à la formation d’hydrogène chimisorbé par déshydrogénation de 1
suivie de l’hydrogénolyse de la cétone ainsi formée. Un schéma général de réaction est proposé pour l’HEC du modèle 1.
Mots clés : Lignines, modèles de lignines, hydrogénation électrocatalytique, électrodes de nickel de Raney, électrodes
de palladium.
Cyr et al. 315
Introduction
low molecular weight chemicals. Since most of the lignins
generated by the pulp and paper industry are still exploited
only as fuel, there is an economic interest to develop meth-
ods of cleaving lignins into valuable monomers (2). Gen-
erally, oxidative methods have the objective of producing
vanillin and syringaldehyde. Typical oxidation treatments in-
Historically, chemical treatments of lignins, such as oxi-
dation in alkaline nitrobenzene or permanganate solutions,
hydrogenolysis, and ethanolysis (1) have given an insight
into the lignin structure, but have also revealed the potential
use of this biopolymer as a feedstock for the preparation of
2
, 3
volve nitrobenzene (3), metal oxides (3), oxygen , and
Received July 16, 1999.
1
1
A. Cyr, F. Chiltz, P. Jeanson, A. Martel, J. Lessard, and H. Ménard. Centre de recherche en électrochimie et électrocatalyse,
Département de chimie, Université de Sherbrooke, Sherbrooke, QC J1K 2R1, Canada.
L. Brossard. Institut de recherche d’Hydro-Québec (IREQ), 1800 boul. Lionel-Boulet, Varennes, QC J3X 1S1, Canada.
1Authors to whom correspondence may be addressed. (JL) Telephone: (819) 821-7091. Fax: (819) 821-8017.
e-mail: jlessard@courrier.usherb.ca. (HM) Telephone: (819) 821-7084. Fax: 819) 821-8017. e-mail: hmenard@courrier.usherb.ca
2
H.B. Marshall and C.A. Sankey. U.S. Patent 2,516,827 (1945).
J.R. Salvesen, D.L. Brink, D.G. Diddams, and P. Owzarski. U.S. Patent 2,434,626 (1948).
3
Can. J. Chem. 78: 307–315 (2000)
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Can. J. Chem. Vol. 78, 2000
4
electrolysis (4, 5). Catalytic hydrogenation has also been
studied as a method for production of phenolic monomers
(
(which are soluble in aqueous alkali) on Raney nickel and
palladium-based electrodes. It has been recognized that the
β-O-4 linkage (see 4 in Fig. 1) is the most important type of
structural element in native lignins. We therefore studied the
following models (see Fig. 1 for structures): (i) three com-
pounds having one β-O-4 linkage and referred to as phenolic
monoethers: 1-(4-hydroxy-3-methoxyphenyl)-2-(methoxyphenoxy)-
1-ethanol (1), its hydroxymethyl derivative 2, and its keto
derivative 3; (ii) phenolic diether 4 with two β-O-4 linkages;
(iii) 1-(3,4-dimethoxyphenyl)-2-(methoxyphenoxy)-1-ethanol (5),
a non-phenolic monoether (β-O-4 linkage); and (iv) 4-
phenoxyphenol (6), a model of 4-O-5 linkages in which two
aryl groups are directly bound to an oxygen atom. Raney
nickel has proven to be a versatile hydrogenation electro-
catalyst (13), and supported palladium was chosen in view
of its well known activity in catalytic hydrogenolysis (17).
6). A variety of catalysts have been used, including Raney
nickel (7), copper chromite (8), supported rhodium (9, 10),
ruthenium or palladium (11), and nickel boride . Interesting
5
compounds such as 4-n-propylguaiacol and dihydroconiferyl
alcohol have been generated from aspen wood using Raney
nickel (7) or supported rhodium (9, 10). With a palladium
catalyst, a high selectivity has been reported for the forma-
tion of dihydroconiferyl alcohol (11). The use of nickel
boride has led to the production of 4-ethylguaiacol and
4
-ethylsyringol from wood.5
In a recent article (12), we prepared basic nonphenolic
models of α-O-4 and β-O-4 bonds in lignins to demonstrate
the potential of electrocatalytic hydrogenation (ECH) as an
alternative method for lignin depolymerization. ECH is an
advantageous combination of electrochemical and catalytic
methods (for a review on ECH, see ref. 13). In ECH, chemi-
sorbed hydrogen (H)M is generated in situ on the
electrocatalyst surface by electroreduction of water at a low
overpotential (rxn. [1]) and reacts with the adsorbed organic
substrate (rxns. [2]–[4]). However, hydrogenation (rxns.
2]–[4]) competes with hydrogen desorption (rxns. [5] and
or) [6]) and this may decrease the current efficiency (13).
This method of generating chemisorbed hydrogen allows
one to carry out reactions similar to catalytic hydrogenation
but at lower temperatures and pressures (13). Furthermore,
the concentration of chemisorbed hydrogen can be con-
trolled by adjustment of the current density.
Experimental
Organic substrates and products
Phenolic monoethers 1 (18), 2 (19), 3 (18), and 5 (5) are
known compounds and were synthesized according to
published procedures. Phenolic diether 4 is also a known
compound and was prepared from O-isopropyl-α -bromo-
acetovanillone (prepared according to ref. 20) by described
procedures (18, 21). The following compounds (Fig. 2) were
obtained from commercial sources: 4-phenoxyphenol (6),
guaiacol (9), ethylphenol (10a), acetovanillone (11b), and
4′-hydroxypropiophenone (11c) from Aldrich; cyclohexanol
[
(
(
7) and phenol (8) from BDH; 4-ethylguaiacol (10b), 4-n-
–
–
[
1]
2 H O + 2 e + M → 2 (H)M + 2 OH
2
propylphenol (10c), and 4-n-propylguaiacol (10d) from
Lancaster; 4′-hydroxyacetophenone (11a) from Eastman.
The alcohols 12a, b (Fig. 2) were prepared by catalytic hy-
drogenation of the corresponding ketone using nickel-
aluminum alloy in 10% sodium hydroxide at 15°C (in situ
generation of Raney nickel and chemisorbed hydrogen) (22).
[
[
2]
3]
R + M
º
(R)M
(R)M + 2 (H)M º (RH )M
2
[
[
4]
5]
(RH )M º RH + M
1
13
2
2
All compounds had H NMR, C NMR, IR, and mass spec-
tral data in accordance with their structure.
–
–
(H)M + H O + e → H + M + OH
2
2
Electrodes and electrolyses
Raney nickel electrodes were made by mixing and grind-
ing 45% of Raney alloy (Fluka), 45% of fractal nickel pow-
[
6]
(H)M + (H) M º H + 2 M
2
ECH at electrodes made of Raney-type materials has been
applied successfully to the selective hydrogenation of many
types of molecules (13). In principle, all catalytic powders
used in catalytic hydrogenation can also be used in ECH,
provided that the catalyst can be immobilized on (or sup-
ported by) a conductive material or cast into a solid conduc-
tive piece. The simplest electrode set up is to spread the
catalytic powder onto a conductive plate, but such a design
is not fully satisfactory from a practical point of view. Sev-
eral approaches have been developed to prepare electrodes
from catalytic powders, including electromagnetic deposi-
tion (14), electrocodeposition (15), lanthanum phosphate
6
der (Inco Type 250), and 10% of lanthanum phosphate .
About 40 g of this powder was pressed into a plate 3mm
2
2
thick at about 1000 kg/cm into a 5 × 71/2 cm mould from
2
which 2 × 21/2 cm plates were cut. These plates were
heated at 800°C for 4 h under an argon atmosphere. The
electrical contact was made with a nickel stripe. The plates
were leached in NaOH 30% at 75°C for 24 h to remove alu-
minum. In order to eliminate the chemisorbed hydrogen
formed in the leaching process (reduction of water by alumi-
num), the Raney nickel plate was treated with a solution of
trans-cinnamic acid (5 g in 200 mL of 1 M NaOH) at room
temperature for at least 18 h (hydrogenation to phenyl-
propionic acid) prior to electrolysis.
6
polymerization, and embodiment in a reticulated vitreous
carbon support (16). The latter two methods have been used
in this work.
The palladium-based electrode consisted of a 2 × 2.5 ×
3
As an extension of our previous work (12), we report in
0.6 cm piece of 100 ppi reticulated vitreous carbon
this paper the results of the ECH of phenolic lignin models
(Electrosynthesis Co.) into which 0.20 g of catalyst
4
J.H.P. Utley and C.Z. Smith. U.S. Patent 4,786,382 (1988).
B. Loubinoux. French Patent 80 15272 (1980).
J.-M. Lalancette, E. Potvin, and H. Ménard. U.S. Patent 4,886,591 (1989).
5
6
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2000 NRC Canada

Cyr et al.
309
Fig. 1. Lignin models studies.
(
(
palladium 5% on alumina from Aldrich) was embedded
16). This physical embodiment of the particles into the vit-
Hitachi 100-40 spectrophotometer at λ = 272 nm, and
HP3390A integrator) with the help of calibration curves for
compounds 1–6, 8, 9, 10a–c, 11a–c and 12a, b. The best
overall separation was achieved with the following eluent
program at a flow rate of 1.0 mL/min: 90% of 4% acetic
reous carbon reticulated structure occurred spontaneously
when the powder was stirred in the catholyte solution for
about one hour in the presence of the vitreous carbon matrix.
The particles remained in contact with the carbon matrix de-
spite vigorous stirring, provided that there was no sudden
perturbation (16).
The electrolyses were carried out with a 410 Potentio-
static Controller coupled to a 640 digital coulometer
(
H-cell with a Nafion-324 membrane (E.I. duPont de
Nemours & Co.) and condensers was connected to a temper-
ature-regulated circulating bath. Both compartments were
filled with 30 mL of 1 M NaOH (except for the electrolysis
of 5:1 M NaOH in 1:1 ethanol–water) and 0.75 mmol of or-
ganic substrate (0.38 mmol for compound 4 as it contains
two β-O-4 links) was added to the catholyte stirred magneti-
cally. A graphite rod was used as anode. A constant current
was applied (5 or 20 mA) and samples of the catholyte were
taken at regular intervals. The electrolysis of 1 at 75°C and 5
mA was conducted under a nitrogen atmosphere in order to
avoid chemical degradation (23).
acid in water – 10% CH CN for 5 min, then adding 0.5%
3
CH CN per min up to 45% CH CN.
3
3
Product identification was performed by GC–MS
(Hewlett–Packard 5890, 30 m DB-5 column and HP5971
mass-selective detector) and complementary quantitative
analyses were done by GC–FID (Perkin–Elmer 8500, 30m
DB-1 capillary column and HP3390A integrator), when
needed, using internal standards. Response factors were ap-
proximated for compounds for which authentic samples
were not available (10e, 11d, 12c–e). The thermally unstable
compounds 1, 2, and 12a–e were characterized as their ace-
tate derivatives. Acetylation of the hydroxyl groups was ac-
complished overnight with a 1:1 pyridine – acetic anhydride
mixture (24). For the GC analyses of the acetylated deriva-
tives, part of the catholyte was neutralized with 1 M HCl
then extracted with ethyl acetate, the organic layer was dried
with magnesium sulfate and the solvent removed under vac-
uum.
Electrosynthesis Co.). A two-compartment jacketed glass
Work-up procedures and analyses
Results and discussion
Part of the catholyte was neutralized with 10% acetic acid
in water, then diluted to a fixed volume with 20%
acetonitrile in water and analyzed by HPLC (C18 column
packing, Beckman 100A pumps, Beckman 420 controller,
ECH at Raney nickel
The ECH of model 1 in alkaline solution was studied as a
function of temperature and current density (Table 1). At
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Can. J. Chem. Vol. 78, 2000
Table 1. Electrocatalytic hydrogenation (ECH) of model 1 at a Raney nickel electrode and in aqueous 1 M NaOH: yield (%) of
products.
T = 25°C
T = 50°C
T = 75°C
a
–1
Q = 18 F mol–1
Q = 18 F mol–1
Products
Q = 18 F mol
I =
I =
I =
5
1
mA
20 mA
2
5 mA
3
20 mA
4
5 mA
5
20 mA
6
Entry
Unreacted (1)
Cyclohexanol (7)
Phenol (8)
6.0
60
0
Traces
14
ndb
2.5
7.0
37
10
ndb
4.0
13
1.0
2.5
7.0
37
0.3
2.0
43
0.2
2.0
40
Guaiacol (9)
24
4
4
4
-Ethylphenol (10a)
-Ethylguaiacol (10b)
′-Hydroxyacetophenone (11a)
0.7
Traces
0
Traces
1.6
Traces
21
8.0
Traces
1.2
7.0
6.0
21
2.0
Traces
0.6
7.0
3.0
31
24
7.0
1.5
2.0
12
3.0
18
Traces
Traces
1.3
1.0
41
2.0
3.0
5.0
2.5
4.0
Acetovanillone (11b)
-(1-Hydroxyethyl)phenol (12a)
α-Methylvanillyl alcohol (12b)
4
a
–1
Q in F mol is the charge corresponding to the number of moles of electrons per mole of substrate.
b
Not detected.
Fig. 2. Compounds obtained from ECH of models 1 to 5.
of guiacol (9) and α -methylvanillyl alcohol (12b) (see
Figs. 1, 2, and 4 for structures) as expected and the electro-
hydrogenolysis is much less efficient at a current of 20 mA
–
1
(
entry 2: 40% conversion after a charge of 18 F mol ) than
at a current of 5 mA (entry 1: 94% conversion). The effect
of increasing the current becomes less and less important as
the temperature is increased to 50°C (compare entries 3 and
4
) then to 75°C (compare entries 5 and 6). At a given cur-
rent, an increase of temperature leads to higher conversion
rates and to further hydrogenation of guaiacol (9) to phenol
(
(
(
8) and cyclohexanol (7) and of α -methylvanillyl alcohol
12b) to 4-(1-hydroxyethyl)phenol (12a), 4-ethylguaiacol
10b) and 4-ethylphenol (10a) (compare entries 1 and 3,
compare entries 2, 4, and 6). Thus, an increase of tempera-
ture and a lower current density favor the hydrogenation
process (rxns. [2]–[4]) over hydrogen desorption (hydrogen
evolution, rxns. [5] and (or) [6]) as observed with other sub-
strates (13). As shown in Table 2, independent studies con-
firmed the hydrogenation, under the ECH conditions, of
guaiacol (9) to phenol (8) and cyclohexanol (7) (entry 4), of
phenol (8) to cyclohexanol (7) (entry 5), and of α -methyl-
vanillyl alcohol (12b) to 4-(1-hydroxyethyl)phenol (12a),
4
-ethylguaiacol (10b) and 4-ethylphenol (10a) (entry 1).
Figure 3 (variation of the product distribution during the
ECH of model 1 at 50°C and 5 mA) also shows that alcohol
1
2b is hydrogenolysed further to 4-ethylphenol (10a).
It is noteworthy that, during the ECH of model 1, aceto-
vanillone (11b) was present along with methylvanillyl alco-
hol (12b) at the beginning of the electrolysis, whatever the
temperature and the current. This is clearly seen in Fig. 3
which shows that an equimolar amount of ketone 11b and
–
1
alcohol 12b was present after 2 F mol in an electrolysis
carried out at 50°C and 5 mA. Furthermore, the amount of
ketone 11 formed is more important the higher the temperature
(
compare entries 2, 4, and 6 of Table 1). This demonstrates
that a secondary benzyl alcohol can be dehydrogenated to
the corresponding ketone on a polarized Raney nickel catalyst
(reverse of rxns. [4], [3], and [2]) and that dehydrogenation
room temperature (entries 1 and 2), the hydrogenolysis of
the β-O-4 bond occurs readily to give about equal amounts
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Cyr et al.
311
Table 2. Electrocatalytic hydrogenation, at a Raney nickel electrode and in aqueous 1 M NaOH, of compounds isolated from the ECH
of model 1: yield (%) of products.a
Products
Compound
Qb
12b
18 F mol–1
11b
18 F mol–1
3
9
8
8 F mol–1
3c
18 F mol–1
18 F mol–1
Entry
1
2
4
5
Cyclohexanol (7)
Phenol (8)
Guaiacol (9)
—
—
—
13
6.0
3.5
21
9.0
38
—
—
—
11
1.0
2.0
1.8
9.0
53
0
0
48
0
0
1
27
1.5
15
3.0
19
70
—
—
—
—
—
—
33
64
—
—
—
—
—
—
—
4
4
4
-Ethylphenol (10a)
-Ethylguaiacol (10b)
′-Hydroxyacetophenone (11a)
Acetovanillone (11b)
-(1-Hydroxyethyl)phenol (12a)
4
α-Methylvanillyl alcohol (12b)
a
Experimental conditions: T = 50°C and I = 5.0 mA.
b
–1
Q in F mol is the charge corresponding to the number of moles of electrons per mole of substrate.
c
No unreacted 3 was detected.
Fig. 3. Evolution of the product distribution with the charge
(
11a)), 4-ethylguaiacol (10b), and 4-ethylphenol (10a). Af-
–
1
(
F mol ) for the ECH of model 1 at a Raney nickel electrode
–1
ter 18 F mol , there was still 38% of alcohol 12b and 21%
of ketone 11b present together with 9% of alcohol 12a,
at 50°C and 5mA: ᭹ unreacted 1, ᭡ guaiacol (9), ᭺ α -
methylvanillin (12b), ᭿ 4-ethylphenol (10a), Q acetovanillone
3
.5% of ketone 11a, 6% of 4-ethylguaiacol (10b), and 13%
(
11b).
of 4-ethylphenol (10a). The ECH of ketone 11b (entry 2)
did give alcohol 12b which is hydrogenolysed further to 12a
(
also dehydrogenated to 11a), 10b, and 10a as expected.
Thus, there is a benzyl alcohol – ketone interconversion
dehydrogenation–hydrogenation) during the ECH of sec-
(
ondary alcohols at Raney nickel in aqueous alkali and this
interconversion is faster the higher the temperature. Such
dehydrogenation of alcohol 1 to ketone 3 during the ECH of
1
certainly occurs even if ketone 3 is not detected during the
ECH of 1. In fact, the electrohydrogenolysis of ketone 3 at
0°C and 5 mA occurs readily (Table 2, entry 3): after 8 F
5
–
1
mol , the conversion is complete and the products formed
are: guaiacol (9) (48%); α -methylvanillyl alcohol (12b)
(
15%) together with acetovanillone (11b) (27%), its dehy-
drogenation product; 4-(1-hydroxyethyl)phenol (12a) (1.5%)
and its dehydrogenation product, 4′-hydroxyacetophenone
(
11a) (1%). Ketone 3 also undergoes rapid degradation un-
der the conditions used for ECH.
A scheme for the formation of most of the products iso-
lated and detected in the ECH of 1 (Table 1) and in the ECH
of some of the products isolated (Table 2), at a Raney nickel
electrode, is shown in Fig. 4. Two possible pathways for the
formation of α-methylvanillyl alcohol (12b) are illustrated:
(
1) direct hydrogenolysis of the β-O-4 bond of 1 (1 → 12b +
guaiacol (9)), and (2) dehydrogenation of 1 to ketone 3 fol-
lowed by its hydrogenolysis to acetovanillone (11b) and
guaiacol (9), 11b being hydrogenated to 12b (1 → 3 → 11b
→ 12b). The hydrogenolysis of ketone 3 should be faster
than that of alcohol 1 because, in the former, the β-O-4 bond
is conjugated to the carbonyl group of an acetophenone (to a
π-electron system). So pathway (2) is probably the main
route to alcohol 12b. The two most probable routes to
4-ethylguaiacol (10b) are: (i) hydrogenolysis of α -methyl-
vanillyl alcohol (12b) (12b → 10b) and (or) (ii) dehydration
of 1 to quinonemethide 13 followed by easy hydrogenolysis
of the β-O-4 bond (13 → 14 + 9), then rapid tautomerization
of 14 to the more stable styrene 15 which would be rapidly
is faster at higher temperatures. The fact that
dehydrogenation of a benzyl secondary alcohol could
effectively take place under the electrolysis conditions was
confirmed by carrying out the ECH of alcohol 12b (Ta-
ble 2, entry 1) under the same conditions (alcohol 12b is sta-
ble under the electrolysis conditions in the absence of an
electrode): acetovanillone (11b) is formed rapidly in the cell,
then is slowly converted back to alcohol 12b which, as
already observed, is hydrogenolysed to 4-(1-hydroxyethyl)phe-
nol (12a) (also dehydrogenated to 4′-hydroxyacetophenone
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Can. J. Chem. Vol. 78, 2000
Fig. 4. Reaction pathways for the formation of the various products in the ECH of model 1.
hydrogenated to 10b. The hydrogenolysis of the C—O aryl
bond of quinonemethide 13 should be easy because the
C—O bond is conjugated to the quinonemethide π-bond
system. Quinonemethides 13 and 14 as well as styrene 15
were not detected during the ECH of model 1, which means
that, if pathway (ii) was occurring, their conversion is faster
than their generation. Dehydrations similar to the dehydra-
tion of 1 to quinonemethide 13 were shown to occur under
the drastic conditions of alkali pulping (25) but should be
slower at temperatures of 75°C or lower (5). Finally, 4-
ethylphenol (10a) might have been formed by demethoxyl-
ation of 4-ethylguaiacol (10b) and(or) by hydrogenolysis of
by elimination of methanol. The fact that 2-methoxy-
cyclohexanol was not detected during the ECH of guaiacol
(9) to phenol (8) and cyclohexanol (7) (Table 2, entry 9) in-
dicates that the hydrogenation of the aromatic ring takes
place only after the methoxy group has been removed. As a
matter of fact, cyclohexanol (7) was the only saturated com-
pound detected in this study of ECH of model 1 and the
other models.
The hydrogenolysis of the β-O-4 bond of model 2 with a
hydroxymethyl group attached to the β carbon is more diffi-
cult than that of model 1 as shown by the lower conversion
rates of entries 1 to 3 of Table 3. The mixture of compounds
formed is more complex than that obtained from the ECH of
1 because compounds with a phenylpropane structure (10c,
10e, 11c, 11d, 12c, 12d, 12e) are formed in addition to those
with a phenylethane structure (10a, 10b, 11a, 11b, 12a, 12b)
identified from the ECH of 1. In the ECH of 2, none of the
compounds, besides guaiacol (9), is obtained with a high de-
gree of selectivity. Compounds 12 (12a–e) were the most
abundant with a total yield of 12% only (see Table 3, entry 2).
Again, exhaustive electrolysis should favor compounds 10
(10a–e). Despite the relatively low yields, ECH appears
more interesting than electrooxidation which causes much
polymerization of 2 (and of 1 also) (5).
4
-(1-hydroxyethyl)phenol (12a) and there are two possible
routes to 12a: 12b → 12a and(or) 11b → 11a → 12a.
For the demethoxylation of 2-methoxyphenols (guaiacol
9) to phenol (8), 4-ethylguaiacol (10b) to 4-ethylphenol
10a), acetovanillone (11b) to 4′-hydroxyacetophenone (11a),
(
(
α-methylvanillyl alcohol (12b) to 4-(1-hydroxyethyl)phenol
12a)), direct hydrogenolysis is not very probable due to the
(
fact that the C—OCH bond should be quite strong since the
carbon is a sp carbon and the C—O bond is not conjugated
3
2
to a π-electron system. So demethoxylation most probably
takes place on the less stable (nonaromatic) 6-methoxycyclo-
hexa-1,4-dien-1-one tautomer (keto–enol tautomerism). Two
routes are possible: (i) hydrogenolysis of the C—OCH3
bond which should occur readily since the methoxy group is
Expanding model 1 by adding a methylvanillyl alcohol
unit to the phenolic end (see model 4 which has two β-O-4
bonds) appears to have little influence on the ease of
hydrogenolysis. Indeed, (i) the extent of hydrogenolysis after
a given charge (the current efficiency), for an ECH carried
3
attached to a sp carbon and the C—O bond is conjugated to
a cyclohexadienone π-electron system; (ii) hydrogenation of
the dienone to a 6-methoxycyclohexa-1,4-dien-1-ol followed
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Cyr et al.
313
Table 3. Electrocatalytic hydrogenation of model compounds 2, 4, and 6 at a Raney nickel electrode in aqueous 1 M NaOH: yield (%)
a
of products.
Products
Model
2
4
6
T
Qb
25°C
50°C
18 F mol–1
75°C
75°C
50°C
6 F mol–1
5 mA
5
18 F mol–1
20 mA
4c
I
5 mA
1
5 mA
2
20 mA
3
Entry
Unreacted substrate
Cychohexanol (7)
Phenol (8)
69
0
0
11
0
0
0
0.1
0
0.3
0
0.2
0.2
0.8
0.7
1.3
1.5
27
22
5.0
0
9.0
15
9.0
0
—
—
6.0
26
—
—
13
0.3
—
—
—
9.0
0
0.5
3.5
26
0.8
0.5
1.0
0.6
0
1.3
0.3
2.0
0.8
3.5
2.5
3.0
2.0
1.0
5.0
29
84
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Guaiacol (9)
4
4
4
-Ethylphenol (10a)
-Ethylguaiacol (10b)
-n-Propylphenol (10c)
2.5
1.0
2.0
1.0
1.0
3.5
1.0
3.0
1.5
4.0
2.0
2.5
1.0
Dihydroconiferyl alcohol (10e)
′-Hydroxyacetophenone (11a)
Acetovanillone (11b)
4
4
4
4
-Hydroxypropiophenone (11c)
′-Hydroxy-3′-methoxy-propiophenone (11d)
-(1-Hydroxyethyl)phenol (12a)
α-Methylvannillyl alcohol (12b)
4
1
1
-(1-Hydroxypropyl)phenol (12c)
-(4-Hydroxy-3-methoxyphenyl)-1-propanol (12d)
-(4-Hydroxy-3-methoxyphenyl)-1,3-propanediol (12e)
a
Some compounds giving rise to GC–MS peaks could not be identified and they were not taken into account in the calculation of the yields.
b
c
–1
Q in F mol is the charge corresponding to the number of moles of electrons per mole of β -O-4 bonds.
Other products identified were ketone 4a (2.0%), ketone 3 (2.0%), and model 1 (0.2%). There was as much as 7% of compound 1 accumulated after
–
1
3
F mol .
Fig. 5. Influence of temperature and current on the rate of
hydrogenolysis of model 4 : ᭡ 25°C, 20 mA, ᭹ 50°C, 20 mA,
out at 50°C and at a current of 20 mA, is about the same for
model 4 as for model 1 as shown in Fig. 5 and (ii) the
᭿
75°C, 20 mA, ᭺ 50°C, 5mA, ٗ model 1: 50°C, 20 mA.
–
1
product distribution after 18 F mol for an ECH carried out
at 75°C and a current of 20 mA is similar for model 4 (entry
4
, Table 3) and model 1 (entry 6, Table 1). This is because
the same type β-O-4 bond is hydrogenolysed in both model
and model 1, the C—O aryl bond attached to a hydroxy-
4
ethylphenol moiety. Figure 5 shows that the efficiency of
hydrogenolysis increases with increasing temperature and
decreases with increasing current as observed for the ECH
of model 1. Again dehydrogenation of secondary benzylic
alcohols did occur as already noted since both the ketone re-
sulting from dehydrogenation of model 4 and ketone 3 re-
sulting from dehydrogenation of model 1 are formed in the
ECH of 4.
The fact that model 1 and its corresponding dehydro-
genation product, ketone 3, are formed in the ECH of 4 indi-
cates that hydrogenolysis of one of the β-O-4 bonds, the
C—O aryl bond attached to the hydroxyethylphenol moiety,
is faster than the hydrogenolysis of the other β-O-4 bond, the
C—O aryl bond attached to the O-alkylated hydroxyphenol
moiety. In fact, model 5 in which the phenolic group had
been methylated (structure akin to the O-alkylated hydroxy-
phenol moiety of model 4) was recovered quantitatively after
electrolysis under the same conditions as those used for the
ECH of models 1, 2, and 4. There are two possible explana-
tions for the faster hydrogenolysis of the C—O aryl bond
attached to the hydroxyethylphenolate moiety: (i) the hydro-
genolysis takes place on the ketone resulting from
©
2000 NRC Canada

3
14
Can. J. Chem. Vol. 78, 2000
a
b
Table 4. Electrocatalytic hydrogenation of model 1 on palladium-based electrodes in aqueous 1 M NaOH: yield (%) of products.
Products
Electrode
Qc
Pd–C
1 F mol–1
1
Pd–C
18 F mol–1
Pd–Al O
2 3
1 F mol–1
Entry
2
3
Unreacted substrate
Guaiacol (9)
Acetovanillone (11b)
ndd
33
42
Traces
Traces
ndd
41
24
0.7
14
ndd
51
51
0
4
′-Hydroxyacetophenone
α-Methylvanillyl alcohol (12b)
0
a
Pd 5% on charcoal or alumina in contact with a reticulated carbon matrix (see Experimental).
b
Experimental conditions : 50°C and 5 mA. The lower mass balance for ECH on Pd–C compared to ECH on Pd–Al O is due to the stronger
2
3
adsorption of organics on charcoal than on alumina (16).
c
–1
Q in F mol is the charge corresponding to the number of moles of electrons per mole of substrate.
d
Not detected.
dehydrogenation of 4 and the dehydrogenation of the
hydroxy group at the benzylic position of the electron-rich
(open circuit), 1 was first dehydrogenated to ketone 3 (see
Fig. 4). After a period of time corresponding to the duration
of an electrolysis, some guaiacol (9) and acetovanillone
(11b) were formed in small amounts thus showing that a
small quantity of the hydrogen atoms generated at the sur-
face of the palladium catalyst by dehydrogenation of 1 is
used in the hydrogenolysis of ketone 3. On the polarized cat-
alyst, about 50% of the hydrogen atoms generated at the sur-
face of the palladium electrode by dehydrogenation of 1
must have been used for the hydrogenolysis of ketone 3 in
(
negatively charged) phenolate ring is faster than that of the
other benzylic alcohol; and (or) (ii) hydrogenolysis takes
place from the quinonemethide resulting from elimination of
the hydroxy group at the benzylic position of the phenolate
ring which would indeed be faster than the elimination of
the other benzylic alcohol because a negatively charged oxy-
gen atom (phenolate anion) is a better electron donor than an
alkoxy group.
–
1
The aryl-O-aryl bond of phenoxyphenol (6) is easily
hydrogenolyzed to phenol at a Raney nickel cathode and at
order to explain the low charge corresponding to 1 F mol
(50% of the theoretical charge) consumed for complete con-
version of model 1. Electrocatalytic hydrogenolysis of
ketone 3 did indeed occur readily at these palladium cath-
odes to give guaiacol (9) and acetovanillone (11b) plus an
unidentified compound resulting form the degradation of 3
in basic medium. Because of such degradation, the yield of 9
and 11b was not determined. When an electrolysis at Pd–C
5
0°C and 5 mA (Table 3, entry 5). This type of linkage is
most frequent in the Kraft lignin (26). A study of the ECH
of this compound as function of the electrode material has
been reported elsewhere (16). It must be noted that phenol
was the sole product formed. Neither benzene nor p-cresol
were detected. Thus, only the C—Oaryl bond attached to the
phenol ring was cleaved. This can be readily explained if the
hydrogenolysis takes place via the cyclohexadienone tauto-
–
1
was pursued up to 18 F mol (entry 2), slow electrocatalytic
hydrogenation of acetovanillone (11b) to alcohol 12b did
occur as shown by the decrease of the amount of 11b and in-
crease of the amount of 12b.
3
mer in which the C—Oaryl bond is attached to a sp carbon
and conjugated to a cyclohexadienone π-electron system as
suggested above for the demethoxylation of 2-methoxy-
phenols by ECH.
Conclusions
Finally, an electrolysis has been attempted on 2,2′-biphenyl
which is representative of 5—5′ bonds in lignins. Such car-
Electrocatalytic hydrogenation has been applied success-
fully to the low-temperature hydrogenolysis of representa-
tive lignin phenolic models with β-O-4 bonds at Raney
nickel and palladium electrodes in basic aqueous medium.
The hydrogenolysis does not take place if the phenolic group
is alkylated. This fact suggests that hydrogenolysis would
occur on intermediates in which the C—O aryl bond is con-
jugated to a π-electron array such as in ketone 3 or quino-
nemethide 13, and that the formation of those intermediates
would be significantly faster with the electron-rich phenolate
anion than with the O-alkylated derivative less rich in elec-
trons. 4-Phenoxyphenol (6) is cleaved easily and exclusively
to phenol at a Raney nickel electrode, showing that hydro-
genolysis of a 4-O-5 linkage by ECH is also possible and
most probably takes place on the cyclohexadienone tautomer
of the phenol in which the C—O aryl bond is again conju-
gated to a π-electron array. The 4-O-5 type of linkage is
abundant in Kraft lignins produced by the pulp and paper in-
dustry. The results of this paper infer that lignins could be
2
bon–carbon bonds between two sp carbons are particularly
strong and, as expected, the unreacted substrate was quanti-
tatively recovered.
ECH at palladium
The results are presented in Table 4. The ECH of model 1
at a cathode made of 5% Pd–C or 5% Pd–Al O particles
2
3
embedded in a matrix of reticulated vitreous carbon (16, see
Experimental) gave an unexpected result (entries 1 and 3):
the substrate was completely converted to guaiacol (9) and
–
1
acetovanillone (11b) after only 1 F mol , that is after con-
sumption of only half of the theoretical amount of electricity
required for complete hydrogenolysis. The reaction was
quantitative with Pd–Al O (entry 3) but the mass balance
2
3
was 72% only with Pd–C due to the strong adsorption of
organics on activated charcoal as reported previously (16).
In a blank test performed under the same conditions as those
of entry 3 (Pd–Al O ) but with a nonpolarized electrode
2
3
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Cyr et al.
315
depolymerized and converted to useful basic organics by
ECH.
12. B. Mahdavi, A. Lafrance, A. Martel, J. Lessard, H. Ménard,
and L. Brossard. J. Appl. Electrochem. 27, 605 (1997).
1
3. J.M. Chapuzet, A. Lasia, and J. Lessard. In Electrocatalysis
frontiers of electrochemistry series). Edited by J. Lipkowski
(
and P.N. Ross. VCH Publishers Inc., New York. 1998.
pp. 155–196.
14. I.V. Kirilyus and M.A. Zhuk. Sov. Electrochem. 8, 967 (1972).
15. G. Belot, S. Desjardins, and J. Lessard. Tetrahedron Lett. 25,
5347 (1984).
Acknowledgments
Financial support from the Natural Sciences and Engi-
neering Research Council of Canada and Hydro-Québec is
gratefully acknowledged.
1
6. P. Dabo, A. Cyr, J. Lessard, L. Brossard, and J. Lessard. Can.
J. Chem. 77, 1225 (1999).
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