A novel palladium catalyst for the synthesis of hydrogen peroxide from carbon monoxide, water and oxygen

Article abstract of DOI:10.1016/S1381-1169(99)00218-6

Hydrogen peroxide was synthesized from CO, water, and oxygen in a biphasic system using Pd complexes with bidentate nitrogen ligands as catalysts. The effect of the Pd ligand, acid cocatalyst, nature of solvent, temperature, and gas phase composition on the hydrogen peroxide yield and catalyst efficiency was studied. Among the phenanthroline derivatives, 2,9-dimethyl-4,7-diphenylphenanthroline was the most efficient ligand. The selected reaction medium, for catalysts comparison, was a mixture of water, 1-butanol, and 1,2,4-trichlorobenzene (a hydrophobic solvent added to improve the phase separation). The reaction was strongly accelerated by the presence of a water-immiscible alcohol that possibly played an active role in the reaction mechanism, acting as a ligand itself. The Pd complex with ligand 2,9-dimethyl-4,7-diphenylphenanthroline exhibited high stability and catalytic activity, with turnover number of ≤ 600 mole hydrogen peroxide/mole Pd-hr. The ligand structure, nature of the weakly-coordinating anion, and solvent played a key role in determining the catalytic activity and the operational stability of the complexes.

Full text of DOI:10.1016/S1381-1169(99)00218-6

Ž
.
Journal of Molecular Catalysis A: Chemical 150 1999 87–94
www.elsevier.comrlocatermolcata
A novel palladium catalyst for the synthesis of hydrogen
peroxide from carbon monoxide, water and oxygen
)
Daniele Bianchi , Rossella Bortolo, Rino D’Aloisio, Marco Ricci
EniChem, Centro Ricerche NoÕara, Istituto G. Donegani, Via Fauser 4, I-28100 NoÕara, Italy
Received 22 December 1998; accepted 12 April 1999
Abstract
The synthesis of hydrogen peroxide from carbon monoxide, water and oxygen in a biphasic system using palladium
complexes with bidentate nitrogen ligands as catalysts was investigated. After testing a series of phenanthroline derivatives,
Ž .
2,9-dimethyl-4,7-diphenylphenanthroline 8 was selected as the most efficient ligand. The palladium complex with ligand 8
Ž
showed high stability and catalytic activity turnover number up to 600 moles of hydrogen peroxide per mole of palladium
.
per hour and, on the basis of a preliminary study, carried out in continuous operation mode, it appears a promising catalyst
for the development of an industrial process. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; N-ligands; Hydrogen peroxide
1. Introduction
reaction, disclosed in 1979 by Yermakov et al.
Ž
Ž .. w x
Eq. 1 3 , is less critical in terms of safety
Hydrogen peroxide is a chemical that is be-
coming more and more popular as an environ-
mentally friendly oxidant, due mainly to its
unique feature of producing water as the only
since it allows to operate in a wider range of
non-ignitable gas phase composition.
O2 qCOqH2O H2O2 qCO2
1
Ž .
w x
by-product 1 . Industrially, hydrogen peroxide
Despite the favourable thermodynamics of
is produced primarily by the alternate oxidation
and reduction of alkylantraquinone derivatives
y
1
Ž
.
the reaction DG8 sy134 kJ mol , the
Ž 298
w x
Ž
original catalyst palladium triphenylphosphine
complex was quite inefficient and a turnover
number TON, moles of hydrogen peroxide per
mole of palladium of only 5 could be obtained.
2 . Since this method is quite complex a stoi-
chiometric amount of quinone must be used and
.
Ž
.
recycled extensive studies have been carried
.
out in order to find a direct route, such as the
synthesis from hydrogen and oxygen or from
carbon monoxide, oxygen and water. The latter
The fast catalyst inactivation was mainly due to
the oxidation of the phosphine ligand with con-
sequent precipitation of reduced palladium. A
Ž
moderate improvement of the catalyst life TON
.
)
up to 87 was obtained by using the more stable
triphenylarsine as the palladium ligand 4,5 ,
Corresponding author. E-mail:
w
x
daniele bianchi@hq.enichem.geis.com
–
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 99 00218-6

(
)
88
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
being the process still not suitable for a practical
application.
2.1. General procedure for the synthesis of hy-
(
)
drogen peroxide batch mode
On the other hand, during the last 15 years
nitrogen ligands have turned out to be valuable
substitutes of more traditional phosphorus lig-
ands in a number of homogeneously catalysed
Ž
. Ž
.
Ž
Pd OAc 0.07 mmol , an acid C7F15COOH
2
.
Ž
.
.
or toluene-4-sulfonic 2.8 mmol and one
among the ligands 1–8 0.28 mmol were dis-
solved in a mixture of the organic solvent
Ž
w x
reactions 6 . Recently, as a part of an industrial
research aimed at developing a new process for
the production of hydrogen peroxide, we have
found that palladium complexes containing
bidentate nitrogen ligands efficiently catalyse
the reaction 1 with productivity quite compara-
ble to that of the current commercial process
Ž
. Ž
.
1,2,4-trichlorobenzene or toluene 40 ml and
Ž
.
the cosolvent 1-butanol or 2-methyl-2-butanol
Ž
ing complex solution was added to water 80
ml to form a biphasic system and poured into
.
60 ml , and stirred for 6 h at 258C. The result-
Ž
.
an autoclave containing a glass liner. The reac-
tor was then pressured to 6500 kPa partial pres-
sure of O2 and 600 kPa partial pressure of CO
in order to form a non-flammable gas mixture.
The reactions were carried out for 1 h at
708C, with the mixture being stirred by a mag-
netically-driven impeller. After depressuring the
reactor, the hydrogen peroxide concentration in
the aqueous solution was measured by titration
with potassium permanganate.
w x
7 . Hydrogen peroxide formation is likely to
Ž .
occur through known reactions: i the reduc-
Ž .
tion of Pd II complex, by carbon monoxide and
water, affording carbon dioxide and Pd 0
species; ii the oxidation of this Pd 0 species
by oxygen with formation of a Pd II peroxo-
Ž .
Ž .
Ž .
Ž .
Ž
.
complex; iii the reaction of the latter species
with an acid, producing hydrogen peroxide and
Ž .
restoring the initial Pd II complex.
We now report the influence of the reaction
Ž
conditions palladium ligand, acid cocatalyst,
2.2. Kinetic measurements
nature of the solvent, temperature and gas phase
.
composition on the hydrogen peroxide yield
Ž
.
Ž . Ž
2.5 mmol , C7F15COOH 60
and the catalyst efficiency. We also report a
study of the catalyst stability, carried out in
continuous operation mode.
Pd OAc
2
.
Ž . Ž
.
mmol and 2,9-dm-4,7-dpphen 8 20 mmol
were dissolved in a mixture of 1,2,4-trichlorobe-
Ž
.
Ž
.
nzene 85 ml and 2-methyl-2-butanol 15 ml ,
and stirred for 6 h at 258C. Alternatively, a
second catalytic system was prepared in the
same way by using toluene-4-sulfonic acid and
toluene instead of C7F15COOH and 1,2,4-tri-
chlorobenzene, respectively. In both cases, the
2. Experimental
The ligands used in this study were: 1,10-
X
Ž
. Ž .
Ž
.
phenanthroline phen 1 , 2-2 -bipyridine bipy
Ž .
phen
Ž
Ž
.
2 , 4,7-diphenyl-1,10-phenanthroline 4,7-dp-
resulting solution was added to water 80 ml to
form a biphasic system. The comparison be-
tween the two catalysts was carried out in the
same reactor described above, equipped with an
automatic system to maintain constant the inlet
pressure and composition of the gas mixture.
The reactions were carried out at 708C and 8100
.
Ž .
3 , 2,9-dimethyl-1,10-phenanthroline
Ž .
4 , 2,9-n-dibutyl-1,10-phenan-
throline 2,9-dbphen 5 , 2,9-diphenyl-1,10-
Ž
.
2,9-dmphen
Ž
. Ž .
Ž
. Ž .
phenanthroline 2,9-dpphen 6 , 2,9-dichloro-
Ž
. Ž .
1,10-phenantroline 2,9-dClphen 7 , 2,9-di-
methyl-4,7-diphenyl-1,10-phenanthroline 2,9-
dm-4,7-dpphen 8 . Ligands 1–4 and 8 were
Ž
. Ž .
Ž
kPa of total pressure PCO s600 kPa; PO s
2
.
purchased from Aldrich. Ligands 5, 6 and 7
7500 kPa . The hydrogen peroxide concentra-
were synthesised according to the procedures
tion in the aqueous phase was determined peri-
w
x
described in literature 8,9 .
odically during the course of each run by tem-

(
)
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
89
porarily stopping the gas flow for 1 min, which
allowed the liquid phases to separate, withdraw-
ing a measured volume of aqueous phase and
pumping back into the reactor an equal volume
of deionised water.
that the reaction was strongly accelerated by the
presence of a water-immiscible alcohol which
possibly plays an active role in the reaction
mechanism, acting as a ligand itself.
The selected reaction medium, for the cata-
lysts comparison, was a mixture of 1-butanol,
(
Ž
2.3. Synthesis of hydrogen peroxide continuous
1,2,4-tricholorobenzene a hydrophobic solvent
)
.
operation mode
added in order to improve the phase separation
and water. The catalysts were generated in situ
Ž
. Ž
.
Ž
.
Pd OAc 2.5 mmol , toluene-4-sulfonic acid
from Pd acetate , 4 equiv of one among the
2
2
Ž
.
Ž . Ž
60 mmol and 2,9-dm-4,7-dpphen 8
20
ligands 1–8, and 40 equiv of sulfuric acid,
added as a cocatalyst. All the reactions were
performed at 708C and 7100 kPa of total pres-
.
mmol were dissolved in a mixture of toluene
Ž
.
Ž
.
85 ml and 2-methyl-2-butanol 15 ml , and
stirred for 6 h at 258C. The resulting solution
was added to water 80 ml to form a biphasic
system.
Ž
sure with an excess of oxygen PCO s600 kPa;
Ž
.
.
PO s6500 kPa in order to prevent the precipi-
2
tation of reduced palladium, which becomes
The reaction was carried out in a stainless
predominant under reducing conditions.
Ž
.
steel Hastelloy C autoclave equipped with a
bottom outlet and an automatic system to main-
tain constant the inlet pressure and composition
of the gas mixture. The reactor was fed with an
As reported in Table 1, we observed three
different reactivities, depending on the nature of
the substitution in the 2,9-positions of the lig-
Ž .
and. All the unsubstituted ligands phen 1 , bipy
Ž
Ž .
Ž .
aqueous solution of toluene-4-sulfonic acid 87
mM and hydrochloric acid 1 mM , saturated
with 2-methyl-2-butanol. The aqueous phase
containing the produced hydrogen peroxide
2 and 4,7-dpphen 3 , in presence of CO, lead
to the formation of stable red complexes, being
as a result ineffective in the synthesis of hydro-
gen peroxide.
.
Ž
.
Ž
.
was selectively withdrawn from a non-stirred
area created on the bottom of the reactor which
allowed the phase separation. The feeding and
withdrawal flow rate were 80 mlrh correspond-
ing to a residence time of 1 h. The reaction was
carried out at 708C and 8100 kPa of total pres-
The ligands bearing bulky substituents, such
Ž .
Ž .
as 2,9-dbphen 5 and 2,9-dpphen 6 , due to
their steric hindrance adjacent to the N-donor
function, do not form stable species but gave
Table 1
Effect of the liganda
Ž
.
sure PCO s600 kPa; PO s7500 kPa for 10 h
in a continuous operation2 mode.
y1
b
Ž
.
Ligand
Acid
TOF h
phen 1
Ž .
H2 SO4
H2 SO4
H2 SO4
H2 SO4
H2 SO4
H2 SO4
H2 SO4
H2 SO4
0c
Ž .
bipy 2
4,7-dpphen 3
Ž .
0c
0c
Ž .
2,9-dmphen 4
4d
3. Results and discussion
Ž .
Ž .
2,9-dbphen 5
0c
2,9-dpphen 6
Ž .
0e
The reaction was carried out in a biphasic
system, being the catalyst soluble in the organic
phase and the produced hydrogen peroxide solu-
ble in the aqueous phase. The biphasic opera-
tion minimises the ligand oxidation by reducing
the contact with hydrogen peroxide and makes
possible the separation and recycling of the
catalyst by simple decantation. After screening
a wide variety of organic solvents, we found
2,9-dClphen 7
32
431
Ž .
2,9-dm-4,7-dpphen 8
a LigandracidrPd molar ratio: 4r40r1; solvent: trichloroben-
Ž
.
zener1-butanolrwater 35r25r40 vrvrv ; temperature: 708C;
PCO : 600 kPa; PO : 6500 kPa.
2
b Turnover frequency moles of hydrogen peroxide per mole of
Ž
.
palladium per hour .
c The formation of stable red complexes was observed.
d The complex was soluble in the aqueous phase.
e The precipitation of reduced palladium was observed.

(
)
90
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
Table 2
other metals gave inactive species confirming
Effect of the acid promotera
the unique features of palladium as a catalyst.
As shown in Table 2 for the palladium com-
plex with ligand 8, an improvement of the
catalyst efficiency was obtained by using an
acid promoter, such as trifluoroacetic or perflu-
orooctanoic acid, the anion of which coordi-
nates only weakly to the metal. In the case of
the perfluorooctanoic acid, even the water solu-
ble palladium complex formed with the hy-
drophilic ligand 4 was moved into the organic
phase by the highly hydrophobic anion acting as
y1
b
Ž
.
Ligand
Acid
TOF h
Ž .
2,9-dm-4,7-dpphen 8
2,9-dm-4,7-dpphen 8
Ž .
2,9-dm-4,7-dpphen 8
2,9-dm-4,7-dpphen 8
Ž .
2,9-dm-4,7-dpphen 8
2,9-dm-4,7-dpphen 8
Ž .
2,9-dm-4,7-dpphen 8
2,9-dmphen 4
H2 SO4
Tol–SO3H
HCl
431
442
0
Ž .
172c
528
634
170c
4
Ž .
HPF6
CF3COOH
C7 F15COOH
C8 F17SO3H
H2 SO4
Ž .
Ž .
Ž .
2,9-dmphen 4
C7 F15COOH
681
a LigandracidrPd molar ratio: 4r40r1; solvent: trichloroben-
zener1-butanolrwater 35r25r40 vrvrv ; temperature: 708C;
Ž
.
PCO : 600 kPa; PO : 6500 kPa.
Ž
a phase transfer agent the partition of the com-
2
b Turnover frequency moles of hydrogen peroxide per mole of
Ž
plex in the organic phase was -1% and )99%
with trifluoroacetate and perfluorooctanoate an-
.
palladium per hour .
c The precipitation of reduced palladium was observed.
.
ion, respectively , showing a dramatic raising of
TOF from 4 to 681 hy1. Other fluorinated acids,
such as perfluorooctanesulfonic and hexafluo-
rophosporic acids, accelerated the dissociation
of the complex with precipitation of reduced
palladium. Conversely, the strongly coordinat-
ing chloride anion inhibited the reaction by
forming stable unreactive complexes.
In Fig. 1, the effect of the temperature on the
catalytic activity of the complex with ligand 8 is
reported. A sharp increase of TOF was mea-
sured in the temperature range between 0 and
708C. The process was not carried out at higher
Ž .
unstable Pd 0 complexes which quickly disso-
ciated with formation of reduced palladium. The
best results were obtained with 2,9-dm-4,7-dp-
phen 8 , which showed the optimum steric and
donating capacity for the palladium catalysed
synthesis of hydrogen peroxide. The lower
turnover frequency TOF found for the 2,9-
Ž .
Ž
.
Ž .
dClphen
7
indicates that such electron-
withdrawing substituents have a severe negative
effect on the catalytic activity, while 2,9-dmphen
4 afforded a water soluble complex, almost
Ž .
inactive under these reaction conditions.
In addition to the above described phenan-
throline derivatives, we tested a series of mono-
Ž
dentate triphenylamine, 4-phenylpyridine, 2,
.
6-dicholoropyridine, phenanthridine and poly-
dentate nitrogen ligands 2,6-di 2-pyridil pyri-
dine, 2,3-di 2-pyridil pyrazine, 3,6-di 2-pyri-
dil -1,2,4,5-tetrazine, 6,7-dimethyl-2,3-di 2-
pyridyl quinoxaline, 2,4,6-tri 2-pyridil triazine,
picolinic acid without finding any appreciable
activity in the synthesis of hydrogen peroxide.
The same set of ligands was used for an
w
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
x
Ž
extensive screening of different metals Fe, Ru,
.
Co, Rh, Ir, Ni, Pt, Cu, Ag , in order to find a
possible alternative to the palladium. Operating
in the same reaction conditions, only a slight
Fig. 1. Effect of the temperature. Ligand 8rC7 F15COOHrPd
molar ratio: 4r40r1; solvent: trichlorobenzener1-butanolrwater
Ž
.
activity TOFs5 was detected using the plat-
Ž
.
Ž
35r25r40 vrvrv ; total pressure: 7100 kPa PCO : 600 kPa;
inum complex with ligand 8, whereas all the
.
PO : 6500 kPa .
2

(
)
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
91
Table 3
temperature in order to prevent side reactions
such as the hydrogen peroxide decomposition
and the ligand oxidation.
The study of the effect of the gas phase
composition on the catalyst activity and stability
was carried out by operating with non-ignitable
mixtures of oxygen and carbon monoxide below
Effect of the alcohol cosolventa
y1
c
Cosolvent % cosolventb TOF h
Ž
.
Ligand
Ž .
Ž .
Ž .
Ž .
2,9-dm-4,7-dpphen 8 none
2,9-dm-4,7-dpphen 8 1-butanol
2,9-dm-4,7-dpphen 8 1-butanol
2,9-dm-4,7-dpphen 8 2-methyl-2- 40
0
40
100
0
634
15d
578
butanol
none
Ž .
2,9-dmphen 4
2,9-dmphen 4
Ž .
2,9-dmphen 4
0
40
0
681
124
Ž
the Lower Explosion Limit LELs12.5% CO
Ž .
1-butanol
.
molar fraction and above the Upper Explosion
Limit UELs94% CO molar fraction .
2-methyl-2- 40
butanol
Ž
.
As shown in Fig. 2, under oxidising condi-
tions the reaction was accelerated by increasing
the carbon monoxide molar fraction up to 9%;
beyond this value the complex became unstable
and partial precipitation of reduced palladium
started to occur. This phenomenon was empha-
a LigandrC7 F15COOHrPd molar ratio: 4r40r1; solvent:
trichlorobenzeneralcoholrwater; temperature: 708C; PCO
:
600 kPa; PO : 6500 kPa.
2
bCosolventrtrichlorobenzene vrv ratio.
c Turnover frequency moles of hydrogen peroxide per mole of
Ž
.
palladium per hour .
d The precipitation of reduced palladium was observed.
Ž
sised operating under reducing conditions 98%
.
CO molar fraction where the immediate pre-
cipitation of reduced palladium was observed
with production of traces of hydrogen peroxide.
As reported above, the selection of the most
efficient catalysts was carried out in a biphasic
system using a mixture of 1-butanol and 1,2,4-
whereas it was rapidly reduced to Pd-black
operating in pure 1-butanol.
The analysis of the solvent at the end of the
reaction revealed that 1-butanol was partially
Ž
oxidised to butanal 2% conversion was found
Ž
.
trichlorobenzene 2:3 v:v as organic phase. As
shown in Table 3, the presence of the alcohol
cosolvent was necessary to activate the reduc-
tion of the palladium complexes, which was
completely unreactive in pure trichlorobenzene,
.
after 10 h . A comparative test, carried out in
absence of carbon monoxide, showed that pri-
mary and secondary alcohols can act themselves
w
x
as reducing agents of the palladium 10 , with
formation of hydrogen peroxide and the corre-
sponding carbonyl derivative, according to Eq.
Ž .
2 .
O2 qR2CHOH H2O2 qR2CO
2
Ž .
The reaction is catalysed by the palladium com-
plex since 1-butanol does not undergo sponta-
neous oxidation under the given reaction condi-
tions.
This side reaction could be avoided by using
Ž
.
a tertiary alcohol cosolvent 2-methyl-2-butanol
which was equally effective at least in the acti-
vation of the complex with the ligand 8 and
resulted in stability toward the palladium catal-
ysed oxidation.
Fig. 2. Effect of the gas-phase composition. Ligand
An equally crucial role in determining the
catalytic activity and the stability of the catalyst
was played by the organic solvent present as the
second component of the organic medium.
8rC7 F15COOHrPd molar ratio: 4r40r1; solvent: trichloroben-
Ž
.
zener1-butanolrwater 35r25r40 vrvrv ; temperature: 708C;
ŽU .
total pressure: 7100 kPa.
LEL: Lower Explosion Limit for
carbon monoxideroxygen mixtures.

(
)
92
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
In Table 4 are reported the turnover numbers
and the residual catalytic activities of the com-
plex with the ligand 8, measured after 3 h of
reaction in different organic media. The residual
activity was determined by recycling the or-
ganic phase in a successive catalytic cycle, after
removal of the aqueous phase containing the
produced hydrogen peroxide and replacing with
an equal amount of pure water. In spite of their
strong accelerating effect on the synthesis of
Ž
.
hydrogen peroxide Table 4, entries 1, 4 , chlo-
rinated solvents caused a rapid inactivation of
the catalyst. Only a negligible improvement of
the catalyst stability was obtained, replacing the
perfluorinated acid with the non-halogenated
Fig. 3. Synthesis of hydrogen peroxide by palladium catalysis:
Ž
.
kinetic profile.
I
Ligand 8; acid: C7 F15COOH; solvent:
Ž .
B Ligand 8;
2,4-trichlorobenzener2-methyl-2-butanolrwater;
acid: toluene-4-sulfonic acid; solvent: toluener2-methyl-2-
butanolrwater; PdrCly molar ratio: 1r1. Temperature: 708C;
Ž
.
toluene-4-sulfonic acid entry 2 , indicating the
prevailing effect of the solvent.
Ž
.
total pressure: 8100 kPa PCO s600 kPa; PO s7500 kPa .
2
This phenomenon was explained by the pres-
ence of an undesired side-reaction: the reduction
of the organic chloride catalysed by the palla-
dium with carbon monoxide and water, accord-
Ž
.
low concentration molar ratio ClrPdG4r1 ,
can act as a strongly coordinating ligand of
palladium, leading to the formation of stable
complexes with consequent irreversible inacti-
Ž .
ing to Eq. 3 .
RClqCOqH2O RHqHClqCO2
3
Ž .
w
x
vation of the catalyst 11 . The higher the ob-
served chloride release rate 1,2-dichloroethane
)trichlorobenzene)chlorobenzene , the faster
the inactivation rate of the catalyst entries 2–4 .
The mass spectroscopy analysis of the ex-
hausted catalyst, recovered from a chlorinated
Ž
Using 1,2,4-tricholobenzene as a solvent, a
progressive accumulation of mono- and
dichlorobenzenes was indeed detected in the
reaction mixture by GLC-MS analysis. The hy-
drochloric acid generated in this way, even at a
.
Ž
.
Table 4
Effect of chlorinated solventsa
Entry
Solvent
Acid
TONb
Residual catalytic
c
Ž
.
activity %
1
2
3
4
5
6
7
8
1,2,4-trichlorobenzene
1,2,4-trichlorobenzene
chlorobenzene
1,2-dichloroethane
toluene
C7 F15COOH
Tol–SO3H
Tol–SO3H
Tol–SO3H
C7 F15COOH
Tol–SO3H
C7 F15COOH
Tol–SO3H
939
658
738
251
293
42
31
35
57
0
69d
0d
toluene
toluenee
420
876
100
100
toluenec
a
Ž
.
Ligand 8racidrPd molar ratio: 4r40r1; solvent: 2-methyl-2-butanolrwater 35r25r40 vrvrv ; temperature: 708C; PCO : 600 kPa; PO
:
2
6500 kPa.
b
Ž
.
Turnover number moles of hydrogen peroxide per mole of palladium after 3 h of reaction .
c Measured after 3 h of reaction.
d The precipitation of reduced palladium was observed.
eA weighed amount of hydrochloric acid was added to the reaction mixture with a molar ratio ClrPds1r1.

(
)
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
93
Ž
reaction medium tricholorobenzener2-methyl-
.
2-butanol , revealed indeed the presence of
chlorine containing polynuclear species with
molecular weight higher than 1000.
The attempt to replace halogenated solvents
Ž
with toluene were initially unsuccessfully entry
.
5, 6 , since in the chloride-free medium the
complex became unstable and the rapid precipi-
tation of reduced palladium was detected. A
similar behaviour was observed with a series of
different non-chlorinated solvents such as xyl-
ene, anisole, acetophenone and benzonitrile.
Much better results, in terms of catalyst sta-
bility, were obtained operating in the halogen-
free solvent, adding only a weighed amount of
Fig. 4. Synthesis of hydrogen peroxide by palladium catalysis:
continuous operation. Ligand 8; acid: toluene-4-sulfonic acid;
solvent: toluener2-methyl-2-butanolrwater; PdrCly molar ratio:
Ž
1r1. Temperature: 708C; total pressure: 8100 kPa PCO s600
. Ž
kPa; PO s7500 kPa .
.
B
Hydrogen peroxide concentration in
Ž
.
hydrochloric acid Cl:Pd molar ratios1:1 ,
2
ŽU .
the aqueous phase withdrawn from the reactor;
turnover
which exerted a remarkable stabilising effect
Ž
.
number moles of hydrogen peroxide per mole of palladium .
w
x
against the formation of reduced palladium 12
Ž
.
entry 7, 8 .
In Fig. 3 are reported the kinetic profiles of
the reaction carried out using the ligand 8 oper-
In conclusion, complexation of palladium
with a suitable bidentate nitrogen ligand pro-
vides an efficient catalyst for the synthesis of
hydrogen peroxide from carbon monoxide, oxy-
gen and water. The structure of the ligand as
well as the nature of the weakly-coordinating
anion and the solvent play a key role in deter-
mining the catalytic activity and the operational
stability of the complexes, which were both
dramatically improved with respect to the palla-
dium-based catalytic systems already described
in literature. The development of a continuous
operation process, more suitable for an indus-
trial application, is presently under considera-
tion.
Ž
ating either in a halogenated environment acid:
C7F15COOH; solvent: 1,2,4-trichlorobenzener
.
2-methyl-2-butanolrwater and in a halogen-
Ž
free environment acid: toluene-4-sulfonate, sol-
vent: toluener2-methyl-2-butanolrwater . Un-
.
der the latter conditions it was possible to
achieve high TON, producing hydrogen perox-
ide aqueous solutions up to 8% wrw concentra-
tion.
The stability of the catalytic system was con-
firmed with a preliminary test in continuous
operation mode. In this case, the aqueous phase,
containing the produced hydrogen peroxide, was
continuously withdrawn from the bottom of the
reactor while an aqueous solution of toluene-4-
sulfonic acid and hydrochloric acid, saturated
with 2-methyl-2-butanol, was fed from the top,
as described in Section 2. The catalyst, quantita-
tively distributed in the organic phase, was
maintained inside the reactor. As shown in Fig.
4, the hydrogen peroxide productivity was con-
References
w x
1
Ž
.
G. Strukul, in: G. Strukul Ed. , Catalytic Oxidations with
Hydrogen Peroxide as Oxidant, Kluver, Dordrecht, 1992, p.
1.
w x
2
Ž
.
G. Goor, in: G. Strukul Ed. , Catalytic Oxidations with
Hydrogen Peroxide as Oxidant, Kluver, Dordrecht, 1992, p.
13.
Ž
stant for 10 h of operation H2O2 concentration
w x
3
V.N. Zudin, V.A. Likholobov, Y.I. Yermakov, Kinet. Katal.
in the aqueous phases1.8% wrw; TONs
Ž
.
20 1979 1559.
.
1600 , without any appreciable loss of the cat-
w x
Ž .
4
F.E. Jacobson, BOC U.S. Patent, 4711772, 1987.
w x Ž .
5 F.E. Jacobson, Chem. Abstr. 108 1988 40590x.
alytic activity.

(
)
94
w x
D. Bianchi et al.rJournal of Molecular Catalysis A: Chemical 150 1999 87–94
w x
9 M. Yamada, Y. Nakamura, S. Kuroda, I. Shimao, Bull.
6
A. Togni, L.M. Venanzi, Angew. Chem., Int. Ed. Engl. 33
Ž
.
Ž .
Chem. Soc. Jpn. 63 1990 2710.
1994 497.
w x
w
x
Ž
.
7
D. Bianchi, R. Bortolo, R. D’Aloisio, M. Ricci, Angew.
10 K.P. Peterson, R.C. Larock, J. Org. Chem. 63 1998 3185.
Ž
.
w x
11 A. Bontempi, E. Alessio, G. Chanos, G. Mestroni, J. Mol.
Chem. Int. Ed. Engl. 33 1999 706.
w x
8
Ž .
Catal. 42 1987 67.
x Ž .
C.O. Dietrich-Buchecker, P.A. Marnot, J.P. Savage, Tetrahe-
Ž
.
w
dron Lett. 50 1982 5291.
12 F. Ragaini, S. Cenini, J. Mol. Catal. A: Chem. 109 1996 1.