Drastic increase of selectivity for H2O2 formation in direct oxidation of H2 to H2O2 over supported Pd catalysts due to their bromination

Article abstract of DOI:10.1039/b405415f

Incorporation of bromide anions (1.0 wt%) in supported Pd catalysts (viz. Pd supported on Al₂O₃, ZrO₂, SiO₂, H-β or Ga₂O₃) leads to a drastic increase in their selectivity for H₂O₂ formation in the direct oxidation of H₂ to H₂O₂ by O₂ (at room temperature) in an aqueous acidic (0.03 M H₃PO₄) reaction medium; the selectivity increase is accompanied by a large decrease in the H₂O₂ decomposition activity of the catalysts.

Full text of DOI:10.1039/b405415f

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Drastic increase of selectivity for H O formation in direct oxidation of
2
2
H to H O over supported Pd catalysts due to their bromination
2
2
2
Vasant R. Choudhary,* Chanchal Samanta and A. G. Gaikwad
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune – 41008,
India. E-mail: vrc@ems.ncl.res.in; vrc@che.ncl.res.in; Fax: 1 91-20-25893041
Received (in Cambridge, UK) 15th April 2004, Accepted 6th July 2004
First published as an Advance Article on the web 13th August 2004
Incorporation of bromide anions (1.0 wt%) in supported Pd
catalysts (viz. Pd supported on Al , ZrO , SiO , H-b or
Ga ) leads to a drastic increase in their selectivity for H
formation in the direct oxidation of H to H by O
room temperature) in an aqueous acidic (0.03 M H PO )
improvement in their H O selectivity or causes adverse effects on
2 2
2
O
3
2
2
their performance in the H
-to-H O oxidation.
2 2 2
2
Results showing the influence of different halide anions (F ,
2
O
3
2 2
O
2
(at
2
2
2
2
2
O
2
Cl , Br , and I ) incorporated in supported Pd catalysts by their
halogenation on their performance in the H -to-H oxidation
and H decomposition reactions in an aqueous acidic (0.03 M
PO ) reaction medium at room temperature (27 uC) are
presented in Table 1. Results in Fig. 1 show the influence of the
2 2
O
3
4
2
reaction medium; the selectivity increase is accompanied by a
large decrease in the H decomposition activity of the
catalysts.
O
2 2
O
2
H
3
4
2
2
concentration of Br added to the Pd/Al
performance in both the H -to-H O oxidation and H O
2 2
2 3
O catalyst on its
Demand for hydrogen peroxide, as an environmentally-friendly
oxidizing agent, in the paper/pulp industries, for water purification
or disinfection and waste-water treatments and also for the
preparation of fine and bulk chemicals has been increasing day-by-
day. Efforts are therefore being made to replace the presently
practised anthraquinone process for H O production, which
2
2
2
decomposition reactions. The Pd (5%)/Al O was obtained from
2
3
Lancaster (UK). The ZrO , Ga O , H-b or SiO supported Pd
2
2
2
3
(2.5%) catalysts were prepared by impregnating the respective
support with palladium acetate from its acetonitrile solution,
drying, calcining at 500 uC and then reducing by ammoniacal
hydrazine. The halogenation of Pd catalyst was done by impre-
gnating the catalyst with ammonium halide from its aqueous solu-
2
2
1
2
involves indirect oxidation of H in multiple steps and has several
limitations/disadvantages, by a direct environmentally-friendly Pd-
2–5
oxidation route.
catalyzed H
2
-to-H
2
O
2
However, the main
problem associated with the direct H -to-H O oxidation is the
tion, drying and calcining in a flow of N at 400 uC. The direct
2
2
2
2
H -to-H O oxidation was carried out in a magnetically stirred
2
2 2
3
glass reactor (capacity: 250 cm ) at the following reaction con-
poor selectivity for H O formation because of the high activity of
2
2
2
,3,6
3
ditions: reaction medium ~ 150 cm aqueous 0.03 M H PO ,
palladium for H
is, therefore, of both practical and scientific interest to kill or
drastically reduce the H decomposition activity of Pd-catalysts
by their modification so that their H O selectivity in the H -to-
2
O
2
decomposition (H
2
O
2
A H
2
O 1 0.5 O
2
).
It
3
4
amount of catalyst ~ 0.5 g, gaseous feed ~ 4.6 mol% H in O ,
2
2
3
21
2
O
2
gas flow rate ~ 15.5 cm min , temperature ~ 27 uC, pressure ~
0.95 atm, and reaction period ~ 3 h; by the procedure given
elsewhere. The H O in the reaction mixture was determined quan-
2
2
2
3
H O
2 2
oxidation can be drastically improved. We report here that
2
2
the above difficult-to-achieve goal can very well be accomplished by
incorporating bromide ions in Pd-catalysts (viz. Pd supported on
alumina, silica, zirconia, gallia and H-b zeolite). The incorporation
of other halides in the catalysts, however, either produces a little
titatively by an iodometric titration method. The H O decom-
2 2
position activity of the catalysts was measured in terms of the
pseudo first order H O decomposition rate constant (k ) at 27 uC,
2
2
a
3,6
using the same reaction medium, by the procedure given earlier.
Table 1 Performance of supported Pd catalysts, with or without addition of different halides, in the H
decomposition reactions at 27 uC (concentration of halide in halogenated catalysts ~ 1.0 wt%)
2
-to-H
2
O
2
oxidation and H
O
2 2
H
2 2 2
-to-H O oxidation
Halide added
to the catalyst
Colour of the
catalyst
H
2
O
2
decomposition
a
) (%)
b
) (%)
c
) (%)
3 21 d
Catalyst
X (H
2
Y (H
2
O
2
S (H O
2 2
a
activity k 6 10 (min )
0
Pd (5 wt%)/Al
2
O
3
None
Br
F
Cl
Black
Gray
Brownish black
Gray
Brown
54.7
48.9
62.3
63.4
9.3
0.5
29.7
0.0
0.0
3.4
3.9
25.6
0.0
3.6
5.4
24.8
2.2
0.0
26.4
0.0
0.9
60.8
0.0
0.0
33.3
8.9
68.8
0.0
7.1
10.8
64.5
3.9
0.0
46.9
0.0
0.0
0.0
60.5
0.0
4.36
0.58
398.8
368.2
0.19
3.8
0.25
5.7
2.7
I
0
Pd (2.5 wt%)/ZrO
2
None
Br
F
Cl
None
Br
F
None
Br
F
Cl
None
Br
F
Cl
Black
43.7
37.3
51.8
50.5
50.0
38.5
55.5
60.5
56.3
54.1
51.3
50.3
38.5
54.7
58.4
d
Faint gray
Black
Gray
Grayish black
Pale cream
Gray
Black
Faint gray
Black
Blackish gray
Black
0
Pd (2.5 wt%)/Ga
2
O
3
4.22
0.38
5.7
0
Pd (2.5 wt%)/H-b
13.4
0.04
60.8
30.5
54.2
0.73
102.3
88.7
0.0
0.0
23.3
0.0
0
Pd (2.5 wt%)/SiO
2
Faint black
Black
Black
0.0
0.0
a
b
c
Total conversion of H
2
.
Yield of H
2
O
2
.
2 2 a 2 2
Selectivity for H O . k is pseudo first order H O decomposition rate constant.
2
0 5 4
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 4 – 2 0 5 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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maximum and the H
the H decomposition activity of the catalyst is decreased
sharply.
2
conversion is decreased to a small extent but
2 2
O
The XRD analysis of the catalysts (Table 1) indicated that the
0
palladium present in all the catalysts was in metallic (Pd ) form.
The highly improved performance of the supported Pd catalysts
after their bromination is attributed to the inhibition caused by the
bromide ions (present in the catalysts) to the H
reaction, which is a consecutive reaction, responsible for the
indirect oxidation of H to water. The inhibition is expected
2 2
O decomposition
2
because of the interaction of bromide anions with palladium,
changing its electronic properties, depending upon the concentra-
tion of bromide anions. The direct oxidation of H to water (H 1
2 2
0
.5 O A H O) over the supported Pd catalysts may also be
2
2
inhibited to some extent due to the presence of bromide anions in
the catalyst. It may be noted that the use of halide promoters added
2
into the acidic reaction medium for the H -to-H
7
oxidation is
2
O
2
undesirable because of their highly corrosive nature. This serious
problem is avoided in the present case by incorporating the halide
directly in the solid catalyst. Further detailed studies are necessary
to understand the role played by the different halide anions
incorporated in the Pd-catalysts for the H -to-H O oxidation and
2
2 2
H
2
O
2
decomposition reactions.
In summary, the incorporation of bromide ions, particularly at
optimum concentration, in the supported Pd catalysts by their
bromination causes a drastic improvement in their performance
Fig. 1 Influence of the amount of Br added to Pd (5 wt%)/Al
catalytic activity in the H -to-H oxidation and H decomposition
reactions (at 27 uC).
2 3
O on its
(
both the H
2
O
2
selectivity and yield) in the direct oxidation of H
2
to
2
2
O
2
2 2
O
2 2
H O .
Chanchal Samanta and V. R. Choudhary are grateful to the
CSIR (New Delhi) for the award of a Senior Research Fellowship
and the Emeritus Scientist Scheme, respectively.
The results in Table 1 can be summarized as follows:
) All the supported Pd catalysts, before their bromination, show
little or no selectivity for H O in the H -to-H O oxidation but
1
2
2
2
2 2
high activity for the H
) After bromination of the Pd catalysts, their H
selectivity is drastically increased, whereas their activity for the
decomposition is drastically reduced. The total H conver-
sion is also decreased but to a smaller extent. The H con-
centration achieved for the brominated Pd catalysts was 8.6 ¡
1.0 mmol dm . The catalysts could be reused without a significant
loss in their activity in the reaction. Also no leaching of Pd from the
catalysts during the reaction is observed, indicating long catalyst
life.
2 2
O decomposition.
Notes and references
2
2 2
O yield and/or
1
G. Goor, W. Kunkel, O. Weiberg, in Ullmann’s Encyclopedia of
Industrial Chemistry, Vol. A13 (Eds.: B. Elvers, S. Hawkins,
M. Ravenscroft, G. Schulz), VCH, Weinheim, 1989, pp. 443–466.
H
2
O
2
2
2 2
O
2 L. Fu, K. T. Chuang and R. Fiedorow, Stud. Surf. Sci. Catal., 1992, 72,
3.
23
3
3
(a) V. R. Choudhary, A. G. Gaikwad and S. D. Sansare, Angew. Chem.,
Int. Ed., 2001, 40, 1776; (b) A. G. Gaikwad, S. D. Sansare and
V. R. Choudhary, J. Mol. Catal., 2002, 181, 143; (c) V. R Choudhary,
A. G. Gaikwad and S. D. Sansare, Catal. Lett., 2002, 83, 81;
(d) V. R. Choudhary, S. D. Sansare and A. G. Gaikwad, Catal. Lett.,
3) The incorporation of iodide in the Pd/Al O catalyst results in
2 3
a marked decrease both in its H
oxidation) and H decomposition activities.
) The incorporation of fluoride or chloride anions in the Pd
catalysts caused an increase in the H conversion (except for the Pd/
H-b) but no improvement in the H yield and/or selectivity.
Because of fluorination or chlorination, the H decomposition
activity of the Pd catalysts is increased markedly.
The results in Fig. 1 reveal that there is an optimum con-
2 2 2 2
conversion (in the H -to-H O
2002, 84, 235.
2
O
2
4 (a) J. H. Lunsford, J. Catal., 2003, 216, 455; (b) D. P. Dissanayake and
J. H. Lunsford, J. Catal., 2002, 206, 173; D. P. Dissanayake and
J. H. Lunsford, J. Catal., 2003, 214, 113.
4
2
5
(a) P. Landon, P. J. Collier, A. J. Papworth, C. J. Kiely and
G. J. Hutchings, Chem. Commun., 2002, 2058; (b) P. Landon, P. J. Collier,
A. F. Carley, D. Chadwick, A. Burrows, A. J. Papworth, C. J. Kiely and
G. J. Hutchings, Phys. Chem. Chem. Phys., 2003, 5, 1917.
2 2
O
2 2
O
6
7
V. R. Choudhary and A. G. Gaikwad, React. Kinet. Catal. Lett., 2003,
80, 27.
(a) L. W. Gosser, US Pat., 4681751, 1987; (b) L. W. Gosser and
J. T. Schwartz, US Pat., 4772485, 1988; (c) U. Luckoff, H. Paucksch and
G. Luft, US Pat., 5505921, 1996.
2
1
centration of Br in the brominated Pd/Al
achieving highest H yield (i.e. H -to-H
H -to-H O oxidation. With the increase in the amount of Br
2
O
3
($0.2 mmol g ) for
2
O
2
2
2 2
O conversion) in the
2
2 2
2 3 2 2
added to the Pd/Al O catalyst, the H O yield is passed through a
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 4 – 2 0 5 5
2 0 5 5