Synergetic effect of two halogen promoters present in acidic reaction medium or catalyst on the H2O2 formation (in H2-to-H2O2 oxidation) and destruction over Pd/C (or Al2O3) catalyst

Article abstract of DOI:10.1016/j.jcat.2006.12.012

A strong synergetic effect of two different halide anions (F^- and I^-, Cl^- and I^-, Cl^- and Br^-, and F^- and Br^-) at optimum concentration in the catalyst or in an acidic reaction medium was observed in the H₂-to-H₂O₂ oxidation over Pd/C, Pd/Al₂O₃, and halogenated Pd/Al₂O₃ catalysts. The synergetic effect promotes the net H₂O₂ formation by inhibiting the H₂O₂ decomposition and hydrogenation reactions. The effect is most pronounced for the combination of fluoride (or chloride) and iodide anions. The Br (1 wt%)-F (1 wt%)-Pd (5 wt%)/Al₂O₃ catalyst showed very high H₂O₂ yield (59%)/selectivity (60%) in the H₂-to-H₂O₂ oxidation.

Full text of DOI:10.1016/j.jcat.2006.12.012

Journal of Catalysis 246 (2007) 434–439
www.elsevier.com/locate/jcat
Research Note
Synergetic effect of two halogen promoters present in acidic reaction
medium or catalyst on the H O formation (in H -to-H O oxidation) and
2
2
2
2 2
destruction over Pd/C (or Al O ) catalyst
2
3
∗
Vasant R. Choudhary , Prabhas Jana
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411 008, India
Received 1 September 2006; revised 27 November 2006; accepted 16 December 2006
Available online 1 February 2007
Abstract
A strong synergetic effect of two different halide anions (F and I , Cl and I , Cl and Br , and F and Br ) at optimum concentration in
the catalyst or in an acidic reaction medium was observed in the H -to-H O oxidation over Pd/C, Pd/Al O , and halogenated Pd/Al O catalysts.
−
−
−
−
−
−
−
−
2
2
2
2
3
2 3
The synergetic effect promotes the net H O formation by inhibiting the H O decomposition and hydrogenation reactions. The effect is most
2
2
2 2
pronounced for the combination of fluoride (or chloride) and iodide anions. The Br (1 wt%)–F (1 wt%)–Pd (5 wt%)/Al O catalyst showed very
2
3
high H O yield (59%)/selectivity (60%) in the H -to-H O oxidation.
2
2
2
2 2
©
2006 Elsevier Inc. All rights reserved.
Keywords: Synergetic effect; Hydrogen peroxide; H -to-H O oxidation; H O decomposition; H O hydrogenation; Pd/Al O ; Pd/C; Halogen promoter
2
2
2
2
2
2
2
2 3
1
. Introduction
Hydrogen peroxide is a versatile, environmentally clean ox-
and
H2O2 + H2 → 2H2O (consecutive H2O2 hydrogenation).
(
4)
idizing agent; thus its use, particularly in the production of bulk
and fine/specialty chemicals by a number of organic oxidation
Reactions (2)–(4) are responsible for the reduction in the net
formation of H2O2. Although this process has been extensively
patented, it has not yet been commercialized, and it is often
considered a “dream process.” Apart from its hazardous nature,
reactions, has been increasing steadily [1,2]. Oxidation of H by
2
O2 to H2O2 is an environmentally clean process and involves
the direct formation of H2O2 from H2. It has a high potential
to replace the currently practiced hydroquinone auto-oxidation
process [3], which involves indirect oxidation of hydrogen to
H2O2 and has several limitations. Therefore, direct H2-to-H2O2
oxidation,
low H O selectivity is the main problem associated with this
2
2
process.
The use of halogen promoter anions, particularly bromide
5–8] or chloride [5,8–11] anions in acidic reaction medium
5,6,8,9,11] or in a catalyst [7,10], has been reported to improve
H O selectivity in the H -to-H O oxidation over supported
Pd catalysts. Among the halides (F , Cl , Br , and I ), bro-
mide and chloride anions were found to be highly effective pro-
[
[
H2 + O2 → H2O2,
(1)
2
2
2
2
2
−
−
−
−
is of great practical importance. Unfortunately, this process
also involves the thermodynamically highly favored undesir-
able parallel and consecutive water-forming reactions [4,5]:
moters for drastically increasing H O formation, while largely
2
2
decreasing the H2O2 decomposition and hydrogenation activ-
ity of supported Pd catalysts [5,7]. In contrast, fluoride anions
showed no promoting effect on H2O2 formation, but did cause
an increase in the undesirable H2O2 decomposition activity of
the Pd catalysts with little or no effect on their H2O2 hydrogena-
tion activity. Iodide anions, on the other hand, strongly inhibited
all reactions, causing catalyst deactivation due to poisoning. In
H2 + 0.5O2 → H2O (parallel H2 combustion),
H2O2 → H2O + 0.5O2 (consecutive H2O2 decomposition),
3)
(2)
(
*
Corresponding author. Fax: +91 20 25902612.
E-mail address: vr.choudhary@ncl.res.in (V.R. Choudhary).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.12.012

V.R. Choudhary, P. Jana / Journal of Catalysis 246 (2007) 434–439
435
the presence of fluoride or iodide anions in the catalyst [7] or
in an acidic reaction medium [5], very little or no formation of
H2O2 occurred in the H2-to-H2O2 oxidation over the supported
Pd catalysts [5,7]. In this communication, we report for the first
time a strong synergetic effect of two different halide anions
on the H2O2 decomposition and hydrogenation reactions over
the Pd/C, Pd/Al O , and halogenated Pd/Al O catalysts in an
2
3
2
3
acidic (0.1 M H PO ) aqueous medium. Because of the pres-
3
4
ence of the second halide, the net rate of H O formation in-
2
2
creased, whereas the H O decomposition and hydrogenation
2
2
−
−
−
−
−
−
−
−
(F and I , Cl and I , Cl and Br , and F and Br ) present
rates decreased, depending on the nature of the two different
halide anions and their concentration in the reaction medium or
in the catalyst, as follows.
in the catalyst or in an acidic reaction medium at their optimum
concentration on the H2-to-H2O2 oxidation, largely promoting
the H2O2 formation by inhibiting the H2O2 decomposition and
hydrogenation reactions over Pd/C, Pd/Al2O3, and halogenated
Pd/Al2O3 catalysts.
−
−
3
.1. Synergetic effect of I and F anions
In the presence of fluoride anions in either the reaction
2
. Experimental
The Pd/C and Pd/Al O catalysts (with 5 wt% Pd loading) in
−
3
medium (4.0 mmol F /dm ) or the catalyst (5.0 wt% F), the H2
conversion in the H2 oxidation over all the catalysts was com-
plete (100%), but without any formation of H2O2. This is most
probably due to the very high rates of the consecutive H2O2
decomposition and hydrogenation reactions (Table 1). In addi-
tion, when iodide alone was present in the medium (0.1 mmol
2
3
their reduced form were obtained from Lancaster, UK, in finely
powdered form. The presence of a metallic Pd phase in the Pd
catalysts was confirmed by XRD. The halogenated Pd/Al2O3
catalysts containing single or two different halogens were pre-
pared by impregnating using an incipient wetness technique one
ammonium halide (NH4F, NH4Br, or NH4I) or a mixture of two
different ammonium halides from aqueous solution, drying, and
calcining in a flow of N2 at 400 C for 2 h.
The catalytic direct H2-to-H2O2 oxidation over the Pd cat-
alysts was carried out in a magnetically stirred jacketed glass
−
3
I /dm ) or in the catalyst (1.0 wt% I), both the H2O2 yield and
selectivity in the H2-to-H2O2 oxidation were very poor. How-
ever, for all catalysts, the rate of net H2O2 formation in the
H2-to-H2O2 oxidation was increased very markedly, whereas
that of the H2O2 decomposition and hydrogenation was de-
creased appreciably, when both the fluoride and iodide anions
were present in the medium or in the catalyst (Table 1). The
H2-to-H2O2 oxidation was carried out for 1 h. For the higher
reaction periods, the rate of H2O2 formation is expected to de-
crease due to the increased H2O2 concentration, which causes
an increase in the rate of H2O2 destruction.
◦
3
3
reactor (capacity, 100 cm ) in a reaction medium of 50 cm
aqueous 0.1 M H3PO4 with 0.1 g of catalyst at a gaseous feed
3
◦
of 50 mol% H2 in O2 at a gas flow rate of 6 cm /min at 27 C
and 0.95 atm, for a reaction period of 1 h. For the Pd/C and
Pd/Al2O3 catalysts, a single or a mixture of two halides was
added to the reaction medium before the reaction. However, for
the halogenated Pd/Al2O3 catalysts, no halide was added to the
reaction medium and, after the reaction, no presence of halide
anions in the reaction medium was detected. The H2O2 in the
reaction mixture was determined quantitatively by iodometric
titration. The H2 conversion, H2O2 yield, and H2O2 selectiv-
ity are defined as follows: H2 conversion (%) = {[(moles of H2
in the feed) − (moles of H2 in the effluent gases)] ÷ (moles
of H2 in the feed)} × 100. H2O2 selectivity (%) = [(moles
of H2O2 formed) ÷ (moles of H2 consumed)] × 100. H2O2
yield (%) = [(moles of H2O2 formed) ÷ (moles of H2 in the
feed)] × 100.
For the Pd/C catalyst, with increasing concentration of fluo-
3
ride in the presence of iodide (0.1 mmol/dm ) or of increasing
3
concentration of iodide in the presence of 1.0 mmol/dm flu-
oride, both the H2O2 yield and selectivity increased while the
H2 conversion decreased (Figs. 1a1 and 1b1). In the case of
the Pd/Al2O3 catalyst, the H2O2 yield and selectivity both
first increased and then passed through a maximum with
increasing fluoride concentration in the presence of iodide
3
(
0.1 mmol/dm ) (Fig. 1a2) or of increasing iodide concentra-
3
tion in the presence of 4.0 mmol/dm fluoride (Fig. 1c1) in
the medium. This indicates that an optimum concentration of
halides is needed to achieve the best results (i.e., highest H2O2
yield and/or selectivity).
The catalytic H2O2 conversion (in absence or presence
of H2) over the catalyst was carried out in the same reactor
containing 0.1 g of catalyst in 50 cm aqueous acidic (0.1 M
−
−
3
3.2. Synergetic effect of I and Cl anions
H3PO4) reaction medium with or without halide anions, by
injecting through the rubber septum 1.0 cm of a 30% aque-
3
3
In the presence of 1.0 mmol/dm chloride in the reaction
ous H2O2 solution in the reactor under vigorous stirring. The
gas evolved or consumed was measured as a function of time
at 27 C and atmospheric pressure. These experimental proce-
medium, the H2 conversion in the H2-to-H2O2 oxidation was
high (ꢀ98%) but both the H2O2 yield and selectivity were poor
for the Pd/C and Pd/Al2O3 catalysts (Table 1). However, when
◦
3
dures were described previously earlier [5,7,10].
0.1 mmol/dm iodide was added to the medium, the H2O2 yield
and selectivity both increased markedly and the rates of the
H2O2 decomposition and hydrogenation reactions decreased
appreciably.
3
. Results and discussion
The results given in Table 1 and Figs. 1 and 2 reveal
a strong synergetic effect of two different halide anions on
the formation of H2O2 in the H2-to-H2O2 oxidation and also
With increasing concentration of iodide (in the presence of
chloride), H2O2 yield and selectivity in the H2-to-H2O2 oxida-
tion over the Pd/C and Pd/Al2O3 catalysts first increased and

436
V.R. Choudhary, P. Jana / Journal of Catalysis 246 (2007) 434–439
Table 1
Results of the H -to-H O oxidation and H O conversion in the presence and absence of H over the Pd/C and Pd/Al O catalyst in the acidic medium containing
2
2
2
2
2
2
2 3
single halide or two different halides
Halide in medium/catalyst
Conversion
Yield
Selectivity
Net rate of
H O formation
Time for half H O
2
conversion (min)
2
3
a
b
(mmol/dm ) [wt%]
of H
of H O
for H O
2
2
2
2
2
2 2
(%)
(%)
(%)
(mmol/(gcat h))
I
II
In air
In H
2
Catalyst: Pd/C (with or without halide(s) in the reaction medium)
KF (1.0/4.0)
KI (0.1)
KF (1.0)
Nil
Nil
100
91
86
82
98
79
92
87
0.0
3.0
4.9
5.5
6.5
22
0.0
3.3
5.7
6.7
6.7
28
0.0
2.1
3.4
3.8
4.5
15
11
23
25
26
30
<1.0
<1.0
≈1.0
≈1.0
3.0
18
<1.0
12
KI (0.1)
KI (0.1)
Nil
KI (0.1)
Nil
KF (4.0)
KCl (1.0)
KCl (1.0)
KBr (0.1)
KBr (0.1)
c
Large {38}
56
Large {32}
22
37
30
42
15
25
c
KCl (1.0)
Catalyst: Pd/Al O (with or without halide(s) in the reaction medium)
2
3
c
KF (4.0)
KI (0.1)
Nil
Nil
100
94
0.0
4.2
0.0
4.5
25
9.5
30
0.0
2.9
12
6.5
17
Large {38}
Large {17}
Large {18}
Large {22}
<1.0
25
37
11
26
c
c
KF (4.0)
KCl (1.0)
KCl (1.0)
KI (0.1)
Nil
KI (0.1)
69
100
81
17
9.5
25
c
c
Large {12}
Catalyst: Halogenated Pd/Al O (containing one or two different halogens)
2
3
F [5.0 wt%]
I [1.0 wt%]
F [5.0 wt%]
Br [1.0 wt%]
Br [1.0 wt%]
Br [1.0 wt%]
0.0
0.0
I [1.0 wt%]
Nil
F [1.0 wt%]
F [5.0 wt%]
100
28
41
93
98
99
0.0
5.7
13
51
59
57
0.0
20
32
55
60
0.0
3.9
9.0
35
41
39
1.0
V. large {1}
V. large {1}
V. large {3}
V. large {3}
V. large {2}
<1.0
c
d
e
V. large
V. large
12
13
14
b
c
c
c
c
57
a
The values in parentheses are the concentrations of respective halide in the medium.
b
c
d
e
Values in the brackets are the concentrations of the respective halogen in the halogenated Pd/Al O catalyst.
2
3
Values in the curly brackets are the H O conversions in 1 h.
2
2
1
0% H O conversion, respectively, in 1 h.
2 2
2
0% H O conversion, respectively, in 1 h.
2
2
then passed through a maximum. However, H2 conversion de-
creased continuously and very markedly (Figs. 1b2 and 1c2).
A similar trend was observed when the chloride concentration
in the medium was increased in the presence of 0.1 mmol/dm3
iodide (Fig. 2); however, the decrease in the H2 conversion
was much smaller. These observations also point to an opti-
mum concentration of the halides for achieving the best results
in the H2-to-H2O2 oxidation. The decreases in H2 conversion
indicate that the iodide and chloride anions poison the catalyt-
ically active sites and that iodide is the most effective catalyst
poison.
the H2O2 yield (from 51 to 59%) and selectivity (from 55 to
60%) in H2-to-H2O2 oxidation, while decreasing the rates of
the H2O2 decomposition and hydrogenation reactions (Table 1).
These increases and decreases were relatively small, however.
The higher fluoride concentration (5 wt%) also caused an in-
crease in H2O2 selectivity and yield of the brominated catalyst,
with a relatively smaller increase. This also points to the need
for an optimum concentration of F in the brominated Pd/Al2O3
catalyst to achieve the best catalyst performance. These re-
sults clearly show a synergetic effect of the two halogens in
the catalyst on the H2O2 formation and destruction reactions,
thereby improving H2O2 yield and selectivity in the process.
Further work on the optimization of Br and F concentration
in the catalyst may lead to a better catalyst for the oxidation
process.
−
−
−
3
.3. Synergetic effect of Cl or F and Br anions
The present results (Table 1) and earlier studies [5,7] showed
that bromide is the best halide promoter for the H2-to-H2O2
oxidation over the Pd catalysts. The presence of bromide also
drastically reduces the rates of H2O2 decomposition and hy-
drogenation reactions over the Pd catalysts [5,7]. However, the
The synergetic effect is expected to arise from the modifica-
tion of the electronic properties of the Pd catalyst through the
interactions of the two halogen anions (which differ appreciably
in electronegativity and electron affinity) with the Pd clusters in
the catalyst; the electronegativity and electron affinity of halo-
gens occur in the order F > Cl > Br > I and Cl > F > Br > I,
respectively. The combined effect of the two halides on the elec-
tronic properties of the Pd clusters seems to retard the rate of
O–O bond cleavage of O (O + 2Pd → 2Pd·O) and H O
3
addition of chloride (1.0 mmol/dm ) to the reaction medium
3
containing 0.1 mmol/dm bromide caused an appreciable in-
crease in both the H2O2 yield and selectivity, while largely
decreasing the rates of the H2O2 decomposition and hydrogena-
tion reactions over the Pd/C catalyst (Table 1). Similarly, the
incorporation of 1 wt% fluorine in the brominated Pd/Al2O3
2
2
2
2
(2Pd + H O → 2Pd·OH), thereby reducing the rates of the
2
2
(
1.0 wt% Br) also resulted in an appreciable increase in both
H2 combustion [5,6,11,12] and H2O2 decomposition and hy-

3
Fig. 1. Effect of (a) concentration of KF added to the acidic medium containing 0.1 mmol/dm KI in case of the Pd/C and Pd/Al O catalysts, (b) concentration of KI added to the acidic medium containing
2
3
3
3
3
1
.0 mmol/dm KF or KCl in case of the Pd/C catalyst, and (c) concentration of KI added to the acidic medium containing 4.0 mmol/dm KF or 1.0 mmol/dm KCl in case of the Pd/Al O catalyst, on the H
2 3 2
conversion in the H -to-H O oxidation.
2
2 2

438
V.R. Choudhary, P. Jana / Journal of Catalysis 246 (2007) 434–439
3
Fig. 2. Effect of the concentration of KCl added to the acidic medium containing 0.1 mmol/dm KI in the H -to-H O oxidation over the Pd/C catalyst.
2
2 2
drogenation [5,11] reactions. However, the decrease in H2O2
selectivity at the higher halide concentration (Figs. 1 and 2)
indicates that the favorable modification of the electronic prop-
erties of catalyst occurs only in a particular range of halide
concentrations, probably depending on the relative concentra-
tion and nature of the halides anions. Further thorough studies
are needed to gain more insight into the very complex issues of
the synergetic effects and the decreased H2O2 selectivity at the
higher halide concentrations.
Fluorine and chlorine are highly electronegative halogens,
and thus fluorination or chlorination of the Pd catalysts also
may cause modification of the support properties, particularly
the acidity of the support (alumina or carbon) of the Pd cata-
lysts. For example, alumina contains only Lewis acid sites, but
protonic acid sites are created after fluorination [13]. It should
be noted that, apart from the halide promoter, the presence of
protons is also required for the selective formation of H2O2 [5].
The increase in the H2O2 yield and selectivity after the addi-
tion of fluorine in the brominated Pd/Al2O3 catalyst may be at
least partly attributed to the creation of protonic acidity due to
fluorination of the alumina support.
4. Conclusion
H2O2 formation with a much higher selectivity/yield in the
direct oxidation of H2 by O2 to H2O2 over supported Pd cat-
alysts in an acidic medium can be accomplished using two
−
−
−
−
−
−
−
halogens (F and I , Cl and I , Cl and Br , and F
−
and Br ) as catalyst promoters at their optimum concentrations
in the medium or in the catalyst, instead of using either of the
two halogens alone. The two halogen promoters have a syner-
getic effect. They promote H2O2 formation by inhibiting the
H2O2 decomposition and hydrogenation reactions. The syner-
getic effect is most pronounced for the combination of fluoride
(
or chloride) and iodide anions. Fluorinated and brominated
Pd/Al2O3 is a highly promising catalyst for the selective H2-
to-H2O2 oxidation with high yields.
Acknowledgments
P.J. and V.R.C. are grateful to the UGC (New Delhi) for the
award of a Senior Research Fellowship and to the CSIR (New
Delhi) for the Emeritus Scientist Scheme, respectively.
The synergetic effect depends on the concentration of the
two halides (Figs. 1 and 2); it is more pronounced when the
two halogen promoters differ greatly in electronegativity and/or
electron affinity. The presence of more than two halides is also
expected to produce a synergetic effect on the H2O2 formation
in the H2-to-H2O2 oxidation. Thus, there is a large scope for
greatly improving the H2O2 formation in H2-to-H2O2 oxidation
by optimizing the concentrations of two or more halides in the
medium or in the Pd catalyst.
References
[
[
[
1] B. Notari, Adv. Catal. 41 (1996) 253.
2] G. Centi, M. Misino, Catal. Today 41 (1998) 287.
3] W.T. Hess, in: J.K. Kroschwitz (Ed.), Kirk–Othmer Encyclopedia of
Chemical Technology, fourth ed., vol. 13, Wiley, New York, 1995, p. 961.
4] T.A. Pospelova, N.I. Kobozev, E.N. Eremin, Russ. J. Phys. Chem. 35
[
(1961) 143.
[5] V.R. Choudhary, C. Samanta, J. Catal. 238 (2006) 28.
[6] P. London, P.J. Collier, A.F. Carley, D. Chadwick, A.J. Papworth, A. Bur-
rows, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys. 5 (2003) 1917.

V.R. Choudhary, P. Jana / Journal of Catalysis 246 (2007) 434–439
439
[
7] V.R. Choudhary, C. Samanta, A.G. Gaikwad, Chem. Commun. (2004)
[10] V.R. Choudhary, S.D. Sansare, A.G. Gaikwad, Catal. Lett. 84 (2002)
81.
2054.
[
8] S. Chinta, J.H. Lunsford, J. Catal. 225 (2004) 249.
[11] Q. Liu, J.H. Lunsford, J. Catal. 239 (2006) 237.
[12] R. Burch, P.R. Ellis, Appl. Catal. B Environ. 42 (2003) 203.
[13] V.R. Choudhary, Ind. Eng. Chem. Prod. Res. Dev. 16 (1977) 12.
[9] V.V. Krishnan, A.G. Dokoutchaev, M.E. Thompson, J. Catal. 196 (2000)
366.