Direct generation of hydrogen peroxide from formic acid and O2 using heterogeneous Pd/γ-Al2O3 catalysts

Article abstract of DOI:10.1039/b803149e

Hydrogen peroxide formation is achieved with remarkable productivity at ambient conditions (25°C and atmospheric pressure) in aqueous medium using a heterogeneous catalytic system; formic acid is decomposed in the presence of a continuous flow of O₂ over Pd/γ-Al₂O₃ catalyst leading to the generation of hydrogen peroxide; the addition of a negligible amount of bromide ion improves the selectivity of the reaction. The Royal Society of Chemistry.

Full text of DOI:10.1039/b803149e

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COMMUNICATION
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Direct generation of hydrogen peroxide from formic acid and O using
2
heterogeneous Pd/c-Al O catalystsw
2
3
Mohammad S. Yalfani, Sandra Contreras, Francesc Medina* and Jesus Sueiras
Received (in Cambridge, UK) 25th February 2008, Accepted 13th May 2008
First published as an Advance Article on the web 27th June 2008
DOI: 10.1039/b803149e
Hydrogen peroxide formation is achieved with remarkable
productivity at ambient conditions (25 1C and atmospheric
pressure) in aqueous medium using a heterogeneous catalytic
system; formic acid is decomposed in the presence of a contin-
can be used as a hydrogen source for production of hydrogen
peroxide leading a remarkable selectivity toward hydrogen
peroxide. However, this system utilizes hydrazine which is
classified as toxic and a possibly carcinogenic compound. It
also requires addition of halide ions as prerequisite to proceed.
Besides, presence of a mineral acid creating an extremely low
pH is of importance in order to stabilize hydrogen peroxide.
2 2 3
uous flow of O over Pd/c-Al O catalyst leading to the gen-
eration of hydrogen peroxide; the addition of a negligible
amount of bromide ion improves the selectivity of the reaction.
9
Recently, the same authors, proposed hydroxylamine as
Hydrogen peroxide has been known as a benign and efficient
oxidant for various purposes such as synthesis of organic
compounds, pulp and paper industry, bleaching and also
oxidation of organic pollutants in industrial wastewater via
the Fenton process. Hydrogen peroxide is industrially synthe-
sized through alkyl anthraquinone as an intermediate from
hydrogen source using Pd/Al O catalyst. Although this sys-
2 3
tem is sufficiently simple, hydroxylamine is known to be
explosive.
Formic acid is a compound which has not been classified as
either carcinogenic or explosive. It can be decomposed accord-
ing to dehydration (HCOOH - CO + H
2
O) or dehydrogena-
2 2
tion (HCOOH - CO + H ). The process over supported-
1
10
hydrogen and oxygen. Recently, due to negative economic
1
1
and environmental aspects of this process, there has been
strong interest in replacing that with one in which hydrogen
metal catalysts has also been investigated. The studies
indicated that catalytic decomposition of formic acid is pre-
dominantly inclined toward production of hydrogen and
carbon dioxide when the process occurs at low temperature
peroxide can be produced by direct reaction between H
2
2
2
and
2 2
H and O are among
–4
O
or as in situ in the reaction media.
1
0,12
the first choices as substrate for direct generation of hydrogen
3
peroxide, what can be considered as an environmentally-
(o380 K) and in aqueous medium.
utilized as a source of hydrogen in hydrogenation processes.
Therefore, it could be
1
3
friendly process. In this approach, heterogeneous catalysis is
recognized as a promising technique, where Pd has shown a
Regarding to this fact, Hyde et al. carried out hydrogenation
of organic compounds using hydrogen released from decom-
position of formic acid. It has also been shown that formic
3
significant performance in formation of hydrogen peroxide.
However, there are some drawbacks in this process, the main
ones being the risk of explosivity of the O /H mixture and the
2
acid can be considered as a promising substitute for H in
1
4
hydrodechlorination of chlorinated organic compounds.
2
2
low solubility of the gases in solution, in particular hydrogen.
5
The use of membranes to separate the reaction mixture or the
In this work, we perform generation of hydrogen peroxide
in aqueous medium from formic acid and oxygen using Pd/g-
use of other sources of hydrogen could overcome these
Al
2
O
3
catalysts at ambient conditions. This new route could be
difficulties. Furthermore, the use of in situ generated H
2
O
2
in
2 2
applied to produce in situ H O
for the subsequent oxidation
5
,6
oxidation processes would open new possibilities, for which
the in situ generation should be performed in operating con-
ditions compatible with those of subsequent oxidation reac-
purposes. Selection of supported-Pd catalyst has been based
on its appreciable results in development of hydrogen peroxide
formation process and decomposition of formic acid as
3,12
7
tions. Bortolo et al. found that alcohols can act as reducing
well.
The production rate and the selectivity of the reaction
agents of palladium complexes with production of hydrogen
peroxide. The main drawback of this process is use of a
homogeneous catalyst which has problems concerning to
separation and recycling of catalyst and products from the
2 2
for H O at both short and long time are of high importance in
this route. The effect of halide ion on the selectivity of the
reaction is also studied. According to our knowledge no result
concerning to this route for generation of hydrogen peroxide
has been published.
8
reaction medium. Choudhary et al. indicated that hydrazine
The results illustrated in Table 1 show formation of hydro-
gen peroxide during the reaction of formic acid with O
15
2
over
Departament d’Enginyeria Quı´mica, Universitat Rovira i Virgili, Av.
Paı ¨s os Catalans 26, 43007 Tarragona, Spain. E-mail:
francesc.medina@urv.cat; Fax: +34 977 559621; Tel: +34 977
Pd/g-Al O catalysts. The reactions were carried out at
2
3
ambient conditions (25 1C and atmospheric pressure) in aqu-
1
6
559787
eous medium for 1 hour using various Pd content catalysts.
Without using the catalyst no H O was detected through the
w Electronic supplementary information (ESI) available: MS profiles
of the outlet gases during decomposition of formic acid in presence
2
2
reaction. The highest short-time production is achieved by
the highest Pd content catalyst with 5% Pd. After 1 h reaction,
2 3
and in absence of oxygen over Pd5%/Al O . See DOI: 10.1039/
b803149e
This journal is ꢀc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3885–3887 | 3885

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Table 1
2 2 2 2 3
H O formation results during the reaction of formic acid with O over Pd/g-Al O catalysts
2 2
H O
Formic acid
conv./mmol
H
mmol
2
O
2
formation/
selectivity
(%)
a
D (%)
b
TOF /h
À1
Catalyst
t/h
Pd5/g-Al
2
O
3
5.8
0.083
0.31
3.60
0.22
3.10
0.27
2.52
0.18
2.0
0.0310
0.0420
0.0135
0.0445
0.0135
0.0890
0.0075
0.140
10.0
1.2
6.2
1.5
5.0
3.5
4.2
7.0
2.5
1.7
1
.0
0.083
.0
0.083
.0
0.083
.0
0.083
.0
15.4
Pd2.5/g-Al
Pd1/g-Al
2
O
3
12.3
19.9
25.2
26.7
1
15.4
2
O
3
1
47.6
Pd0.5/g-Al
Pd0.1/g-Al
2
O
3
1
118.2
105.6
2
O
3
0.15
1.60
0.0037
0.0265
1
a
b
Metal dispersion. mmol H
À1 À1
mmol(Pd dispersed) h .
2
O
2
the highest amount of H O produced is obtained by the
2
case, water formation occurs by direct reaction of HCOOH
and oxygen as well as decomposition of hydrogen peroxide.
Most importantly, the decomposition rate of formic acid was
remarkably higher in comparison with the reaction carried out
in the absence of oxygen. Thus, in the presence of oxygen,
decomposition of formic acid is accelerated.
2
Pd0.5%/g-Al O catalyst. This catalyst also presents the high-
2
3
À1
est efficiency, measured as TOF value (118 h ).
Conversion of formic acid is decreased by reducing the Pd
content in the catalysts (Table 1). Selectivity of the reaction for
H
2
O
2
is calculated as mmol H
HCOOH converted. According to this, selectivity of H
in Table 1) was higher at short time for the catalysts than at long
time. However, for Pd0.5%/g-Al catalyst the H produc-
tion showed highest selectivity (7%) at long time (1 h).
2
O
2
formed divided by mmol
2
O
2
(shown
2 2
In the previous works of generation of H O , the require-
ment of halide ion and mineral acid has been stressed in order
3
,8
2
O
3
2
O
2
to achieve the process with high selectivity. As shown in
Table 2, addition of different dosages of bromide ion to the
reaction medium displays interesting changes in the H O
The phenomenon can be simply presumed to be first decom-
position of formic acid over the Pd catalyst (Scheme 1A). This
process is expected to lead to production of H and CO at these
2
2
production process. First, decomposition of formic acid espe-
cially at long time is appreciably diminished. Considering that
decomposition of formic acid occurs prior to hydrogen per-
oxide formation, it could be described that halide ions block
the active sites of the catalyst to adsorb formic acid and
hamper this trend. In other words, halide ion behaves as a
poison for formic acid decomposition reaction. In order to
confirm the above hypothesis, we fulfilled the catalytic decom-
position of formic acid in Ar atmosphere at 50 1C (to observe
clearly the phenomenon) using Pd5%/g-Al O with and with-
2
2
1
2
conditions as has been proved in the literature. Analysis of the
overhead gases of the reaction indicates only O and CO . No CO
and H peaks were observed in the gas chromatogram. In
addition, mass analysis profile of the outlet gases displays only
masses related to CO (44), O (32) and 28 as a fragment of CO
see Fig. 1S, ESIw). Only trace amount of MS (2) related to H is
2
2
2
2
2
2
(
2
observed indicating almost total consumption of released hydro-
gen. In order to further evidence the above results, we performed
the decomposition of formic acid over the catalyst (Pd5%/g-
Al O ) in Ar atmosphere and observed only CO and H2
2
3
À
À4
À
out Br ion (10 mol). In the presence of Br the decomposi-
tion rate of formic acid was clearly lower than in the absence of
À
2
3
2
corresponding masses in mass analysis profile of the outlet gases
Br . A similar trend was observed for the decomposition of
À
(see Fig. 2S, ESIw). These results confirm that the formic acid
2 2
H O , mainly at low Br content. Consequently, bromide can
decomposition on the catalyst at our reaction conditions rather
follows dehydrogenation than dehydration. As second step, the
2 2
control decomposition of H O and improve its selectivity. As
shown in Table 2, by decreasing the concentration of bromide
À5
introduced O
A side-reaction of water formation is also expected to occur with
a significant rate which drastically influences H selectivity.
2
reacts with released H
2
to form hydrogen peroxide.
to 10 mol in the reaction medium, H
O
2 2
selectivity using
Pd5%/g-Al
2
O
3
reaches a respectable value of 21% at long
À6
2
O
2
time. Further decreasing of bromide to 10 mol results again
in increasing of formic acid decomposition and thereupon
decreasing of H O selectivity. H O selectivity for the catalysts
Another assumption that can be considered is the direct
reaction of formic acid with O (Scheme 1B) (with different
2
2
2
2
2
stoichiometries) resulting in H O or H O (HCOOH + O -
with lower Pd content (as shown in Table 2) was influenced by
2
2
2
2
H O + CO or HCOOH + 1/2O - H O + CO ). In this
bromide but not to the same extent as for Pd5%/g-Al O .
2
2
2
2
2
2
2
3
Productivity of the catalysts with lower Pd amount can also
be described regarding to their ability to decompose hydrogen
peroxide. As shown in Fig. 1, the decomposition rate of hydro-
gen peroxide is abated by decreasing the Pd amount in catalysts.
In other words, hydrogen peroxide molecules demonstrate more
resistance when they are exposed to the low Pd catalysts.
Our catalytic system shows a marked stability property
because no Pd leaching was detected in aqueous medium after
the end of each run. Hence, the catalyst can be easily recovered
at the end of the reaction.
Scheme 1 Suggested approaches toward generation of H
2 2
O from
formic acid and O over Pd/g-Al ; A: two steps, B: one step.
2
2 3
O
3
886 | Chem. Commun., 2008, 3885–3887
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Table 2
H
O
2 2
formation results during the reaction of formic acid with O
2
over Pd/g-Al
2
O
3
catalysts in presence of KBr
formed/
mmol
Formic acid
conv./mmol
H
2
O
2
2 2
H O
selectivity (%)
Catalyst
KBr/mol
t/h
À4
Pd5/g-Al
2
O
O
O
O
3
3
3
3
10
0.083
0.22
0.36
0.19
0.70
0.38
3.55
0.28
0.43
0.44
0.54
0.23
0.68
0.21
0.36
0.0074
0.0099
0.0197
0.1430
0.0445
0.0670
0.0074
0.0345
0.0025
0.0148
0.0123
0.0271
0.0099
0.0123
3.4
2.8
10.4
20.5
11.7
1.9
2.7
8.0
o1
2.8
5.4
4.0
4.7
3.4
1
.0
0.083
.0
0.083
.0
0.083
.0
0.083
.0
0.083
.0
0.083
.0
À5
Pd5/g-Al
Pd5/g-Al
Pd1/g-Al
2
2
2
10
1
À6
10
1
À5
10
1
À4
Pd0.5/g-Al
Pd0.5/g-Al
Pd0.1/g-Al
2
O
2
O
2
O
3
3
3
10
1
À5
10
1
À5
10
1
2
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1
5 The Pd/g-Al
pient wetness impregnation. An aqueous solution of PdCl
ified by hydrochloric acid was added to an aqueous slurry of g-
Al (prepared by sol–gel method) stirring vigorously. The
mixture was aged stirring for 1 h, heated under vacuum at 60 1C
and eventually dried at 110 1C for 12 h. The precipitate was
2
O
3
catalysts were synthesized by conventional inci-
Fig. 1
H
2
O
2
2 3
decomposition profile over Pd/g-Al O catalysts.
2
acid-
As conclusion, we attained a new route to produce hydrogen
peroxide with several advantageous properties including simpli-
city, cleanness and most importantly is environment-friendly. It
performs at ambient conditions and in aqueous medium. The
productivity and selectivity of the system are noteworthy. In the
presence of a negligible amount of bromide ion the selectivity of
2
O
3
calcined in static air at 400 1C for 3 h and reduced under a flow
À1
of pure hydrogen (20 ml min ) at 200 1C for 2 h. H
2
chemisorp-
tion was performed using a Micromeritics ASAP 2010 apparatus.
6 The hydrogen peroxide formation reactions were implemented at
ambient conditions (T = 25 1C and atmospheric pressure) in a
magnetically stirred three-necked glass reactor with a capacity of
1
H
2
O
2
increases remarkably. Nevertheless, both productivity and
selectivity of the system have still a significant potential for
improvement. The decomposition of formic acid to H and
introduces formic acid as eligible
1
mmol formic acid in H O. The amount of catalyst was 0.1 g in all
00 ml. The volume of the reaction was always 50 ml containing 25
2
2
10,12
the reactions. Oxygen was passed bubbling into the reaction
medium with a flow rate of 20 ml min . The temperature of the
CO at ambient conditions,
2
À1
replacement for pure hydrogen in the hydrogen peroxide forma-
tion process. This hydrogen peroxide generation approach could
be proposed to be used for oxidation of organic compounds.
The authors would like to acknowledge URV for the PhD
fellowship given to M. S. Y. and funding AIRE 2006/02, and
also AECI, ref. A/5188/06. Spanish Ministry of Science and
Education is appreciated for the funding given to Dr Sandra
Contreras in Ramon y Cajal program.
2 2
reaction was controlled using a water-bath. H O formation and
formic acid decomposition were monitored by sampling at regular
time intervals and analysing by iodometric titration and high
performance liquid chromatography HPLC (Shimadzu LC-2010
equipped with a Acclaim OA column and SPD-M10A detector and
using 100 mM Na SO at pH 2.65 adjusted by methanesulfonic
2 4
acid as mobile phase), respectively. The overhead exhausted gases
of the reaction were analyzed online using a gas chromatography
(
Shimadzu GC-14B with a TEKNOKROMA Propapack R col-
umn, TCD detector and He as carrier gas) and a mass analyser
PFEIFFER VACUUM, GSD 301 O , Omnistart). The reaction
solution at the end of each run after filtration was analysed by
atomic absorption spectroscopy to detect leached Pd. H de-
composition reactions were performed at ambient conditions using
a 50 ml aqueous solution containing 100 ppm H and 0.1 g
(
2
Notes and references
2 2
O
1
W. T. Hess, Kirk–Othmer Encyclopedia of Chemical Technology,
ed. J. K. Kroschwitz and M. Howe-Grant, Wiley, New York, 4th
edn, 1995, vol. 13, pp. 961–995.
2 2
O
catalyst. H
2
O
2
degradation was monitored by iodometric titration.
This journal is ꢀc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3885–3887 | 3887