Quinone-mediated synthesis of hydrogen peroxide from carbon monoxide, water and oxygen

Article abstract of DOI:10.1016/S1381-1169(01)00261-8

An improved method for the synthesis of hydrogen peroxide from carbon monoxide, water and oxygen catalyzed by palladium complexes in presence of a quinone co-catalyst is described. Even if the reaction balance is formally the same, the mechanism of formation of hydrogen peroxide in the new process is totally different to that proposed for the reaction carried out in absence of quinones. Using 1,10-phenanthroline as a palladium ligand in a water/organic biphasic reaction medium, a dramatic enhancement of the catalyst stability was obtained operating in presence of 1,4-naphthoquinone, which prevented the formation of inactive binuclear palladium species. Additional improvements, with respect to the reaction carried out in absence of quinone, are the use of cheaper palladium ligands and the safer process conditions, since air can be used as a reagent instead of pure oxygen.

Full text of DOI:10.1016/S1381-1169(01)00261-8

Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
Quinone-mediated synthesis of hydrogen peroxide
from carbon monoxide, water and oxygen
Cecilia Querci, Rino D’Aloisio, Rossella Bortolo, Marco Ricci, Daniele Bianchi∗
EniChem S.p.A., Istituto G. Donegani, Centro Ricerche Novara, Via Fauser 4, I-28100 Novara, Italy
Received 24 January 2001; accepted 24 April 2001
Abstract
An improved method for the synthesis of hydrogen peroxide from carbon monoxide, water and oxygen catalyzed by
palladium complexes in presence of a quinone co-catalyst is described.
Even if the reaction balance is formally the same, the mechanism of formation of hydrogen peroxide in the new process is
totally different to that proposed for the reaction carried out in absence of quinones. Using 1,10-phenanthroline as a palladium
ligand in a water/organic biphasic reaction medium, a dramatic enhancement of the catalyst stability was obtained operating in
presence of 1,4-naphthoquinone, which prevented the formation of inactive binuclear palladium species. Additional improve-
ments, with respect to the reaction carried out in absence of quinone, are the use of cheaper palladium ligands and the safer
process conditions, since air can be used as a reagent instead of pure oxygen. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; N-ligands; Quinones; Hydrogen peroxide
1. Introduction
monoxide, water and oxygen, catalyzed by palladium
complexes with bidentate nitrogen ligands in a bipha-
sic system [3]
Hydrogen peroxide is a chemical used in large vol-
umes for chlorine free bleaching and in a wide variety
of chemical processes as environmentally friendly ox-
idant, mainly due to its unique feature of producing
water as the only by-product [1].
Industrially, hydrogen peroxide is almost univer-
sally produced by the alternate hydrogenation and oxi-
dation with air of an alkylanthraquinone [2]. Although
the process is efficient from a yield standpoint, it is
quite complex and is carried out in two separate steps,
using a stoichiometric amount of expensive high mole-
cular weight quinones (e.g. 2-ethylanthraquinone).
Recently, we have reported a new process for the
direct synthesis of hydrogen peroxide from carbon
CO + H2O + O2 → CO2 + H2O2
The selected catalyst showed high efficiency and sta-
bility, making the process promising for a possible in-
dustrial application [4].
In this paper, we describe an improvement of the
above mentioned process, obtained by operating in the
presence of a suitable quinone, acting as a co-catalyst.
This new process differs from the classical quinones
alternate reduction and oxidation since the quinone is
used in catalytic amount and the reduction and oxi-
dation steps are carried out simultaneously in a single
reactor.
∗ Corresponding author.
E-mail address: daniele.bianchi@enichem.it (D. Bianchi).
The main advantages obtained by the new method
are: (i) the use of a cheaper catalyst (the most effective
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00261-8

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C. Querci et al. / Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
palladium ligand is now 1,10-phenanthroline, instead
of the expensive 2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline), (ii) an even improved catalyst stability,
and (iii) safer process conditions (air can be used in-
stead of pure oxygen in the gas mixture with carbon
monoxide).
50 ml of water, the resulting biphasic system was
stirred under carbon monoxide atmosphere for 2 h
at 25◦C. The solution was filtered off and vacuum
1
dried, affording 480 mg of a red powder. H NMR: δ
13
=
−15.72 (s, 1H, hydride); C NMR: δ 215.1 (C O);
IR: ν(C O) 1830 cm−1; ESI-MS: m/z 603 (M+). The
=
m/z value represents the most intense peak in a cluster,
the pattern of which is consistent with the presence
of two palladium atoms.
2. Experimental
The following ligands were used: 1,10-phenanthro-
line (phen) (1); 2-2ꢀ-bipyridine (bipy) (2); 4,7-diphe-
nyl-1,10-phenanthroline (4,7-dpphen) (3); 5-nitro-1,
10-phenanthroline (5-NO2phen) (4); 2,9-dimethyl-1,
10-phenanthroline (2,9-dmphen) (5); 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline (2,9-dm-4,7-dpp-
hen) (6).
The following quinones were used: 1,4-benzoqui-
none (7); phenyl-1,4-benzoquinone (8); 1,4-naphtho-
quinone (9); 2-methyl-1,4-naphthoquinone (10);
tetramethyl-1,4-benzoquinone (11); 2-ethylanthr-
aquinone (12).
2.3. General procedure for the synthesis of
hydrogen peroxide
Pd(OAc)2 (5.6 mg, 0.025 mmol), 1,10-phenanthro-
line (67.5 mg, 0.375 mmol), and C7F15COOH
(414 mg, 1 mmol) were dissolved in a mixture of
toluene (3 ml) and 1-butanol (7 ml), and stirred for 6 h
at 25◦C. The resulting complex solution was added
to water (10 ml) to form a biphasic system.
1,4-Naphthoquinone (9) (198 mg, 1.25 mmol) was
added and the mixture was poured into an autoclave
containing a glass liner. The reactor was then pres-
sured to 64 atm partial pressure of O2 and 6 atm partial
pressure of CO in order to form a non-flammable gas
mixture and reaction was carried out for 1 h at 70◦C.
At the end of the reaction, the hydrogen peroxide con-
centration was evaluated by titration with potassium
permanganate.
All these chemicals were purchased from Aldrich.
2.1. Palladium catalyzed reduction
1,4-naphthoquinone (9)
Pd(OAc)2 (5.6 mg, 0.025 mmol), 1,10-phenanthro-
line (22.5 mg, 0.125 mmol), and C7F15COOH
(207 mg, 0.5 mmol) were dissolved in a mixture of
chlorobenzene (3 ml) and 1-butanol (7 ml), and stirred
for 6 h at 25◦C. The resulting complex solution was
added to water (10 ml) to form a biphasic system.
The 1,4-naphthoquinone (9) (395 mg, 2.50 mmol)
was then added and the reaction was carried out by
stirring under CO atmosphere for 2 h at 50◦C. At the
end of the reaction, the products composition was
quinone 9, 0.23 mmol, and 1,4-dihydroxynaphthalene
(9a), 2.27 mmol, corresponding to 91% conversion
(determined by HPLC analysis).
3. Results and discussion
In the synthesis of hydrogen peroxide from
CO/H2O/O2 catalyzed by homogeneous palladium
complexes with phenanthroline ligands, the palladium
complex with 2,9-dimethyl-4,7-diphenyl-1,10-phena-
nthroline (6) turned out to be the most efficient catalyst
[3]. We suggested [4] that the formation of hydrogen
peroxide occurred through: (i) reduction of a Pd(II)
complex by carbon monoxide and water, via hydride
intermediate, affording carbon dioxide and Pd(0) spe-
cies; (ii) oxidation of this Pd(0) species by oxygen
with formation of a Pd(II) peroxo-complex; (iii) reac-
tion of the latter species with an acid, producing hydro-
gen peroxide and restoring the initial Pd(II) complex.
However, ligand 6 is quite expensive, and the use
of a simpler and cheaper ligand, such as 1,10-phen-
anthroline (1), was highly desirable for the
2.2. Isolation of the red inactive
palladium/1,10-phenanthroline complex
Pre-formed Pd(1,10-phenanthroline)(OAc)2 com-
plex [5] (510 mg, 1.25 mmol) and (1.42 g, 12.5 mmol)
of CF3COOH, were dissolved in a mixture of toluene
(15 ml) and 1-butanol (35 ml). After the addition of

C. Querci et al. / Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
97
development of an industrial process. But, in the
absence of the methyl substitution in the positions
2 and 9 of the phenanthroline, red palladium com-
plexes formed, which were stable under the reaction
conditions and were responsible of the fast irre-
versible deactivation of the catalyst. Thus, using
1,10-phenanthroline (1) as the ligand, a red solid was
Fig. 1. Palladium catalyzed reduction of 1,4-naphthoquinone.
1
isolated which, on the basis of MS and H NMR re-
sults (see Section 2), turned out to be a multinuclear
hydride species (possibly, a binuclear one), the forma-
tion of which was prevented by the steric hindrance of
2,9-substituted ligands. Since it is likely that reduced
palladium species are involved in the formation of
such inactive complexes, a possible way to prevent
the catalyst deactivation appeared to be the addition to
the reaction media of suitable ligands, other than car-
bon monoxide, effective in the stabilization of Pd(0).
At this purpose, we explored the use of quinones [6].
In order to evaluate the effect of quinones on our
system, we tested the series of compounds 7–12 using
the complex Pd(phen)X2 as a catalyst, operating under
reducing reaction conditions (CO/H2O, in absence of
O2) which favors the formation of red, inactive com-
plexes.
peroxide (1-butanol/chlorobenzene/water). Since the
hydrophilic ligand phenanthroline 1 afforded a com-
plex soluble in the aqueous phase, the catalyst was
moved into the organic phase by using a highly hy-
drophobic acid co-catalyst (perfluorooctanoic acid)
acting as a phase transfer anion (the partition of the
complex in the organic layer was <1 and >99% with
trifluoroacetate and perfluorooctanoate anion, respec-
tively).
Operating under these conditions, we found that
quinones 7–10, added in excess with respect to the pal-
ladium (quinone/Pd molar ratio = 100) not only prev-
ented the formation of red, inactive species, but were
also reduced in a catalytic way to the corresponding
hydroquinones [7]. Fig. 1 shows the reaction of quin-
one 9 that is reduced to 1,4-dihydroxynaphthalene (9a)
in the presence of Pd(phen)(C7H15COO)2 catalyst.
The reaction was carried out in the biphasic sys-
tem previously selected for the synthesis of hydrogen
Fig. 2. Quinone-mediated palladium catalyzed synthesis of hydrogen peroxide. N–N: bidentate nitrogen ligand; X: CF3COO–, C7F15COO–.

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C. Querci et al. / Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
In a possible reaction mechanism, the intermediate
observed the fast formation of the red inactive species.
The analysis of the final reaction mixtures showed that
the quinones 7 and 8 were quantitatively reduced to
the corresponding hydroquinones, not effective in the
stabilization of reduced palladium species. Quinones
11 and 12 were not transformed, confirming the re-
sults obtained operating under reducing conditions in
absence of oxygen. Also the quinone 9 was recovered
unaltered, but in this case we observed the vigorous
formation of hydrogen peroxide and the maintenance
of the palladium catalyst in the active form.
These findings can be explained by the ability of
1,4-dihydroxynaphthalene (9a), resulting from the
first reduction step (Fig. 2, cycle A), to react with
oxygen to give hydrogen peroxide, thus, regenerating
the starting quinone 9 which can re-enter into the
catalytic cycle (Fig. 2, cycle B). Conversely, hydro-
quinone and phenylhydroquinone arising from the
reduction of quinones 7 and 8, respectively, have a
too high reduction potential (E0) and cannot undergo
the oxidation process.
complex L2Pd(0)-quinone 13 is formed instead of the
red inactive species [8,9]. Complex 13 is not stable un-
der the reaction conditions and, in presence of the acid
co-catalyst, forms the hydroquinone 9a, with regener-
ation of the starting Pd(II) complex (Fig. 2, cycle A).
On the contrary, quinones 11 and 12 did not bind to
the reduced palladium (probably because of their steric
hindrance) and failed in preventing the formation of
the red inactive species.
The same quinones series was tested in the presence
of oxygen (CO/H2O/O2) under the reaction conditions
suitable for the synthesis of hydrogen peroxide. As
reported in Table 1, the only quinone really effective
in promoting the formation of hydrogen peroxide was
the 1,4-naphthoquinone 9. In all the other cases, we
Table 1
Effect of the quinonea
c
Quinone
E0 (V)b
TOF (h−1
)
2-Methyl-1,4-dihydroxynaphthalene, coming from
the reduction of 2-methylnaphthoquinone (10), was
easily oxidized, but due to the steric hindrance of
its methyl group, quinone 10 showed a low binding
capability towards the Pd(0) species and failed in
preventing the formation of the inactive complex.
So, despite the same overall stoichiometry, pro-
cesses run in the absence [3] and in the presence
of a suitable quinone are likely to have completely
different mechanisms. In the former case, Pd(0)
species are likely to be oxidized by oxygen, and hy-
drogen peroxide is formed upon hydrolysis of the
resulting peroxo-complex. On the contrary, in the
presence of the quinone, Pd(0) species are oxidized by
the quinone itself, and hydrogen peroxide is formed
upon auto-oxidation of the resulting hydroquinone.
The different mechanism was confirmed by study-
ing the ligand effect on the reaction carried out in
absence and in presence of quinones. Table 2 reports
the catalytic activities of a series of palladium com-
plexes with the bidentate nitrogen ligands 1–6 in the
synthesis of hydrogen peroxide carried out in pres-
ence or in absence of the selected quinone 9 (used
in catalytic amount). Trifluoroacetic acid and perflu-
orooctanoic acid were used as acid co-catalysts for
hydrophobic and hydrophilic ligands, respectively, in
order to ensure the segregation of the catalysts into the
7
0.712
5d
8
9
0.698
0.484
0.422
0.466
0.150
13d
263
20d
0d
10
11
12
0d
a Phenanthroline/C7F15COOH/quinone/Pd molar ratio: 5:20:
50:1; solvent: 1-butanol/chloro benzene/water 35:15:50 (v/v/v);
temperature: 70◦C; PCO: 6 atm; PO2 : 64 atm.
b Reduction potential.
c Turnover frequency (calculated as moles of hydrogen peroxide
per mole of palladium per hour).
d Formation of red inactive complexes was observed.

C. Querci et al. / Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
99
Table 2
Effect of the liganda
b
Entry
Ligand
Acid
Quinone
TOF (h−1
)
1
2
3
4
5
6
7
8
9
phen (1)
phen (1)
bipy (2)
bipy (2)
4,7-dpphen (3)
5-NO2phen (4)
2,9-dmphen (5)
2,9-dmphen (5)
2,9-dm-4,7-dpphen (6)
2,9-dm-4,7-dpphen (6)
C7F15COOH
C7F15COOH
C7F15COOH
C7F15COOH
CF3COOH
None
1,4-Naphthoquinone
None
1,4-Naphthoquinone
1,4-Naphthoquinone
1,4-Naphthoquinone
None
1,4-Naphthoquinone
None
0c
263
0c
225
185
180
434d
90
528d
75
CF3COOH
C7F15COOH
C7F15COOH
CF3COOH
10
CF3COOH
1,4-Naphthoquinone
a Ligand/acid/quinone/Pd molar ratio: 5:20:50:1; solvent: 1-butanol/chlorobenzene/water 35:15:50 (v/v/v); temperature: 70◦C; PCO
6 atm; PO2 : 64 atm.
:
b Turnover frequency (calculated as moles of hydrogen peroxide per mole of palladium per hour).
c The formation of red inactive complexes was observed.
d The fraction of 1-butanol ratio was reduced 1-butanol/chlorobenzene/water 25:25:50 (v/v/v) in order to prevent the precipitation of
reduced palladium.
organic layer of the biphasic system. The complexes
with 2,9-unsubstituted ligands 1–4 resulted inactive in
absence of quinones ([3] and entries 1, 3), due to the
formation of red inactive species, but became active in
presence of 9. Conversely, the complexes with ligands
5 and 6, which were strongly effective in absence of
quinones (entries 7, 9), displayed a lower activity in
presence of 9. Thus, this behavior seems to confirm
the presence of two different reaction pathways, and
consequently different reaction rates, if palladium is
oxidized by the oxygen or the quinone.
to increase the CO fraction up to 14%, As reported
in Fig. 3, this resulted in a higher catalytic activity
(TOF = 680) and in safer reaction conditions, being
the lower explosion limit (LEL) of carbon monoxide
in air (17.5%) higher than that in pure oxygen (12.5%).
In a preliminary study, the stability of the catalytic
system was evaluated by running a series of reac-
tion cycles in a discontinuous operation mode. At
the end of each cycle, the aqueous phase, containing
After selection of 1 as the most effective ligand,
the reaction conditions were optimized in order to
improve the catalyst efficiency and stability. The
ligand 1/perfluorooctanoic acid/quinone 9/Pd(OAc)2
molar ratio was tuned to 15:40:50:1, achieving an
increase of turnover frequency (TOF) higher than
500 moles of hydrogen peroxide per mole of palla-
dium per hour. The best catalyst stability was ob-
tained operating in a chlorine free reaction medium
(1-butanol/toluene/water) [10].
In the case of the process carried out in the absence
of quinone, the use of a large excess of oxygen in the
gas phase was necessary in order to prevent the precip-
itation of reduced palladium, which becomes predom-
inant under reducing conditions (operating with pure
oxygen, the maximum allowed CO fraction was 8.5%)
[4]. In the new process, the strong stabilizing effect of
the quinone allowed to use air instead of oxygen and
Fig. 3. Effect of the gas phase composition. Phenanthro-
line/C7F15COOH/1,4-naphthoquinone/Pd(OAc)2 molar ratio: 15:
40:50:1; solvent: 1-butano/toluene/water 35:15:50 (v/v/v); temper-
ature: 70◦C; total pressure: 70 atm. LEL: lower explosion limit for
carbon monoxide in mixture with oxygen or air; ( ) oxygen; (᭿)
air.

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C. Querci et al. / Journal of Molecular Catalysis A: Chemical 176 (2001) 95–100
the produced hydrogen peroxide, was removed by de-
cantation and replaced with pure water. The organic
layer, containing the catalyst, the quinone and the acid
co-catalyst, was then reused in a following cycle. After
10 consecutive cycles (carried out in 10 h), an overall
turnover number (TON) of 4150 moles of hydrogen
peroxide per mole of palladium was achieved and the
catalyst did not show any appreciable loss of activity.
However, in the aqueous phase we detected traces of
phthalic acid, produced by the partial oxidation of the
quinone 9: thus, quinones more stable under oxidizing
conditions are highly desirable.
The development of a continuous process, based on
the above reported improvement, is presently under
consideration.
Acknowledgements
Prof. Giovanni Mestroni (Department of Chemical
Sciences, University of Trieste, Italy) is gratefully ac-
knowledged for helpful suggestions and discussions.
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4. Conclusions
The use of palladium complexes with bidentate ni-
trogen ligands in the presence of a catalytic amount of
a suitable quinone, provides an efficient catalyst for the
synthesis of hydrogen peroxide from carbon monox-
ide, oxygen and water. The improvements with respect
to the reaction carried out in absence of quinone are
the use of a cheap palladium ligand such as phenan-
throline, an improved catalyst stability, and the safer
process conditions.
The features of an effective quinone co-catalyst
should be: (i) a reduction potential value (E0) rang-
ing between 0.4 and 0.5 V, to make possible both the
reduction and oxidation processes; (ii) the absence of
substituents in positions 2 and 3 on the ring to allow
an efficient Pd(0) binding and stabilization; (iii) the
presence of hydrophobic substituents to ensure a se-
lective partition of the quinone in the organic layer
of the biphasic system; (iv) its high stability toward
oxidation.
[10] D. Bianchi, R. Bortolo, R. D’Aloisio, C. Querci, M. Ricci,
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