COMMUNICATION
A novel route for in-situ H2O2 generation from selective reduction of O2
by hydrazine using heterogeneous Pd catalyst in an aqueous medium
Vasant R. Choudhary,* Chanchal Samanta and Prabhas Jana
Received (in Cambridge, UK) 22nd July 2005, Accepted 5th September 2005
First published as an Advance Article on the web 3rd October 2005
DOI: 10.1039/b510543a
reduction by hydroxylamine, using acetonitrile–water mixture as
the reaction medium.11 Because of separation problem, the use of
homogeneous catalyst and/or organic solvent in the reaction is
problemistic. It is, therefore, of both scientific and practical interest
to develop a new route for the in-situ H2O2 generation, which
requires no organic solvent, produces only environment-friendly
and easily separable by-product(s) and provides H2O2 with high
yield under mild conditions, preferably using an easily separable
heterogeneous catalyst. We have accomplished this difficult task.
We report here our new route for the in-situ H2O2 generation
from the selective reduction of O2 by hydrazine (N2H4) from its
hydrate/salt in an acidic aqueous medium, using Pd/C or Pd/Al2O3
(or Ga2O3) catalyst, in the presence of Br2 in the medium or in
the catalyst. The presence of both the mineral acid (protons) and
Br2 is a requirement for the selective oxidation of hydrazine to
H2O2. In this method, the by-products formed are only N2 and
water, which are environmentally benign and pose no problem
for their separation. Also, a complete conversion of hydrazine
with high H2O2 yield can be accomplished in a short reaction
period (¡30 min) at room temperature (25 uC) and atmospheric
pressure.
Hydrogen peroxide in high yields can be generated with high
efficiency at mild conditions (25 uC and atmospheric pressure)
with the formation of only environment-friendly by-products
(N2 and H2O) by a reduction of O2 by hydrazine from its
hydrate/salt with its complete conversion in a short reaction
period (¡0.5 h) using a easily separable supported Pd catalyst
(Pd/Al2O3, Pd/Ga2O3 or Pd/C) in an acidic aqueous medium in
the presence of bromide anions; the presence of both acid
(protons) and bromide anions is essential for the selective
reduction of O2 by hydrazine to H2O2 and in their absence, the
reaction leads only to the formation of water.
Hydrogen peroxide is an environmentally clean oxidizing agent;
the by-product of its use is only water (which is environmentally
benign). Because of the increasing environmental concerns, the use
of H2O2 in a number of organic oxidation processes (e.g. epoxida-
tion of olefins, hydroxylation of aromatics/olefins, oxidation of
benzylic –CH2OH to –CHO or –COOH, etc.) for the synthesis of
fine/speciality chemicals is widely increasing.1–3 At present, H2O2 is
produced commercially mainly by the autoxidation of hydro-
anthraquinone in a complex organic solvent, involving cyclic
oxidation–hydrogenation steps.4 This process however, is cost
effective only on a large scale (.20 000 tpa). The transport,
storage and handling of bulk H2O2 are also quite hazardous.
Hence, the use of in-situ generated H2O2 for organic oxidation
reactions is not only of scientific interest but also of great practical
importance. A few studies have been reported on the use of in-situ
generated H2O2 in liquid-phase organic oxidation reactions,3,5–7
The conventional method for in-situ H2O2 generation is based
on the autoxidation of hydroanthraquinones by O2 (hydro-
anthraquinone + O2 A H2O2 + anthraquinone).3 However, the
use of the autoxidation is limited because of the complex organic
solvent system used to keep both the hydroquinone and quinone in
their dissolved form. Moreover, the separation of the solvent,
quinone and hydroquinone from the products of organic oxidation
reaction is problematic. Direct oxidation of H2 by O2 using Pd
catalyst is highly hazardous8 and hence its use for the in-situ H2O2
generation for organic oxidation reactions is dangerous. Sheriff
and coworkers9,10 used Mn(II)/4,5-dihydroxybenzene-1,3-disulfo-
nate as a homogeneous catalyst9 and Mn(II)-exchanged montmor-
illonite clay as a heterogeneous catalyst10 for the reduction of
dioxygen by hydroxylamine to H2O2 with high turnover numbers
(.104). Recently, they have reported the use of Mn(III) and
Mn(IV) complexes as homogeneous catalysts for the O2-to-H2O2
Results (Tables 1–4) clearly show the formation of H2O2 with
high rate in the reduction of O2 by hydrazine from its sulfate or
hydrate (N2H4 + 2 O2 A 2 H2O2 + N2) over the supported Pd
catalysts12 in an aqueous acidic medium in the presence of bromide
anions in the medium or in the catalyst at 25 uC and atmospheric
pressure. However, since the H2O2 formed per mole of N2H4
converted is less than 2.0, the non-selective conversion of N2H4 to
water (N2H4 + O2 A 2 H2O + N2) also occurs simultaneously,
depending upon the reaction conditions. The procedure for
carrying out the oxidation reaction is given elsewhere.13 No
decomposition of N2H4 to N2 and H2 over the catalysts was
observed at the reaction conditions employed for the reaction.
Table 1 H2O2 formed in the reduction of O2 by hydrazine sulfate (at
25 uC) over Pd/Al2O3 in water as the reaction medium in the presence
or absence of halide (reaction period 5 0.5 h)
Halide in
medium
Halide conc./ H2O2
mmol dm23 formedc
H2O2
Rate of H2O2
yield (%) generationd
None 0.0
KF or KCl 0.94
0.0
0.0
0.0
1.17
1.22
0.0
0.0
0.0
59
61
0.0
0.0
0.0
53.8
56.1
54.3 ¡ 1.8
29.9
KI
0.94
0.94
4.7
4.7
2.7
KBr
KBr
MBra
KBrb
a
1.18 ¡ 0.04 57–61
0.65 33
b
c
Chemical Engineering and Process Development Division, National
Chemical Laboratory, Pune, 411008, India. E-mail: vrc@ems.ncl.res.in;
vrc@che.ncl.res.in; Fax: + 91-20-2589.41
M 5 H, Na or NH4. For Pd//C catalyst. mol/mol of N2H4
d
converted. mmol of H2O2 formed per gram of catalyst per hour.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 5399–5401 | 5399
Choudhary, Vasant R.
