Hydrogen processing by FeIII-exchanged montmorillonite: A unique geochemical protocol

Article abstract of DOI:10.1002/anie.200461070

The production of hydrogen by the relay of electrons from 1^- to H⁺ in an acidic, aqueous medium and the consumption of hydrogen by reductive N acylation open up enormous opportunities in hydrogen chemistry (see scheme).

Full text of DOI:10.1002/anie.200461070

Communications
Geochemistry
III
Hydrogen Processing by Fe -Exchanged
Montmorillonite: A Unique Geochemical
Protocol**
Boyapati M. Choudary,* Mannepalli L. Kantam,
Kalluri V. S. Ranganath, and Kottapalli K. Rao
Hydrogen processing, such as the production of hydrogen by
the reduction of protons and consumption of hydrogen by
oxidation, is of renewed interest since hydrogen is regarded as
a fuel for the future. Hydrogen has long been known as a
biofuel—it can be produced by many microbes under
anaerobic conditions and then consumed later on as a
[
1]
fuel —and [Fe]-hydrogenase is known to produce hydrogen,
[
1–4]
whereas [NiFe]-hydrogenase consumes it.
During hydro-
gen production by [Fe]-hydrogenase, the electrons are relayed
by hydride clusters. Hydrogenase model compounds, devel-
oped from complex and tedious synthetic protocols, are
known to facilitate heterolytic H2 activation and H/D
exchange reactions or produce hydrogen catalytically in
[
5]
cyclic voltammetry.
approaches to the production of hydrogen by photoreduction
Initial results from divergent
[
6]
[7–9]
of protons or by water cleavage
are quite encouraging.
[*] Dr. B. M. Choudary, Dr. M. L. Kantam, K. V. S. Ranganath,
Dr. K. K. Rao
IICT Commun NO:040923
Indian Institute of Chemical Technology
Hyderabad 500 007 (India)
Fax: (+91)40-2716-0921
E-mail: choudary@iict.res.in
[**] K.V.S.R thanks the Council of Scientific and Industrial Research,
India, for a research fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3
22
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200461070
Angew. Chem. Int. Ed. 2005, 44, 322 –325

Angewandte
Chemie
The Earthꢀs crust has abundant reserves of hydrogen
buried in it. Besides dehydrogenation of minerals, such as
production of hydrogen increases as the percentage of iron in
the montmorillonite increases (Figure 1). This result unam-
biguously demonstrates that iron is the active ingredient in
the clay support.
Mg(OH) and silanols of silicates, the interaction of water
2
II
with the reduced form of Fe , which is available in the
subsurface of the Earth, is thought to contribute to the
production and existence of such huge reserves of hydro-
[
10,11]
gen.
However, the presence of only low quantities of this
II
reduced form of Fe , which produces hydrogen stoichio-
metrically, brings into question the extent to which this
process contributes to hydrogen production in the Earthꢀs
[
11]
crust.
Another geochemically important reaction is the
formation of dihydrogen and pyrite from FeS and H S under
2
[
12]
anaerobic conditions.
Despite several attempts, the classical, stoichiometric
production of hydrogen in situ by the reduction of protons
0
with Fe powder and acids, which is currently used in industry
[
13]
for the reduction of nitroarenes to anilines and acetanilides,
Figure 1. A plot of volume of hydrogen liberated (mL) versus the
has not yet been transformed into a catalytic reaction. We
therefore conceived a nonbiological and nonphotochemical
catalytic system for the production of hydrogen by a simple
amount of iron (%) in the clay.
ꢀ
electron-transfer protocol with I as the electron source and
Hydrogen production is enhanced when higher homo-
logues of organic acids or higher temperatures are used
(Table 1). The maximum production of hydrogen was found
to be 9.37 mmol per g of catalyst using hexanoic acid (Table 1,
entry 5). When the production of hydrogen was attempted
under identical conditions, but with water as the proton
source, no reaction was observed. However, the reaction
metal-ion-exchanged montmorillonite clay as the catalyst.
This clay is known to promote electron transfer and is
[
14]
abundant in the Earthꢀs crust. The standard Gibbs energy
+
change of reaction for the formation of H and I from H and
I
2
2
ꢀ
ꢀ1
at room temperature, DG8, is + 25.79 kJmol . Even
though the free-energy change (DG8) is positive, the above
“
uphill” reaction has been realized previously by choosing a
proceeded well after the addition of a small amount of I as
2
ꢀ
[
15,16]
suitable catalyst and reaction conditions.
herein a unique geochemical protocol for the effective,
catalytic electron-transfer from I to H to generate I and
H , two products of commercial importance. The reaction
We report
initiator. This result indicates that I reacts initially with I to
2
ꢀ
[18]
form I3 in situ, as determined spectrometrically;
known that I is a better electron source to induce proton
reduction. The formation of both H and I3 at identical
it is
ꢀ
+
ꢀ
2
3
[
19]
ꢀ
2
2
occurs at room temperature, employs carboxylic acids as
proton source, iodide ions as electron source, and is catalyzed
by recyclable Fe -exchanged montmorillonite (Fe -mont)
rates in an aqueous medium, plotted in Figure 2 as a function
of time, further reinforces the proposal that I3 is indeed the
ꢀ
III
III
electron source here. These results are impressive when
ꢀ
[
Eq. (1)].
compared with other examples of electron transfer from I to
+
H , which require temperatures of 300–4008C and employ
III
þ
ꢀ
Fe
-
mont
2
H
þ 2 I ƒƒƒƒƒ! H þ I2
ð1Þ
either expensive, noble-metal catalysts or an iodine–sulfur
2
[
15,16]
thermochemical process.
There was no oxygen evolution
III
The superiority of Fe -mont over the other catalysts in
from the reaction, which further confirms that the electron
transfer occurs from the iodide only. Moreover, deactivation
of the catalyst was not noticed even after the fifth cycle in the
production of hydrogen; the XRD pattern of the catalyst
recovered after the fourth recycle remains unchanged
(Table 1, entries 1, 5, 7).
terms of turnover number in hydrogen production is clearly
evidenced by the results summarized in Table 1 (see also
Supporting Information). The activity is correlated to the
higher number of acidic sites, which increase with the
[
17]
increased valence state of the cation.
Furthermore, the
In an effort to extend the scope of this reaction and to
demonstrate the production and consumption of hydrogen in
the reaction catalyzed by Fe -exchanged montmorillonite,
III
Table 1: Production of hydrogen gas catalyzed by Fe -mont at different
III
temperatures using water/carboxylic acids as the proton source.
the reductive N-acylation of azo-, cyano-, and nitroarenes was
accomplished, in a one-pot reaction, under reflux conditions
Entry
Proton
Source
T [8C]
H evolution
(mmol)
Turnover
number
2
[
a]
(Table 2). The selective reduction of azo-, cyano-, and nitro-
[
b]
1
2
3
4
5
6
7
acetic acid
acetic acid
acetic acid
propionic acid
hexanoic acid
water
25
60
150
150
150
25
1562.5
2700.0
3370.0
5400.0
9370.0
446.0
8.0, 8.0
13.8
arenes in the presence of other susceptible functional groups,
such as carbonyl and chloro groups, is a significant achieve-
ment. The catalyst, used for four cycles, shows a slight
decrease in activity after each run (Table 2, entry 7). These
catalytic reactions effected by the cheap and readily available
[
b]
17.3, 17.2
27.7
48.0
2.3
3.7, 3.7
[
c]
III
water
60
713.8
Fe -mont are superior to the nonregenerable, stoichiometric
hydride reagents or the expensive catalysts needed to reduce
[
a] The calculation is given in the Supporting Information. [b] HI was
[
20,21]
[22–24]
used in place of NaI. [c] Fourth recycle of the catalyst.
azo-
or cyanoarenes
under hydrogen pressure. Sim-
Angew. Chem. Int. Ed. 2005, 44, 322 –325
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
323

Communications
II
III
Scheme 1. Fe /Fe redox cycle in the production and consumption of
hydrogen.
ꢀ
3
Figure 2. The activity profile diagram for the formation of both H and I at
2
identical rates in aqueous medium at 608C as a function of time. A mixture of
catalyst (1.0 g), sodium iodide (40 mmol), and carboxylic acid/water (50 mL)
III
(
when water is used as proton source 0.0250 g of iodine was added initially) was
equimolar ratio of FeCl or FeSO instead of Fe -mont,
3
4
ꢀ
ꢀ1
ꢀ1
heated to 608C. x=formation rate of d/dt H or I3 [mmolmin gcat ].
2
under otherwise identical conditions, afforded the desired
products in less than 5% yield. These results reinforce our
observation that the montmorillonite is an excellent support
that facilitates the redox reaction.
Table 2: Reductive N-acylation of azo-, cyano-, and nitroarenes catalyzed
III
[a]
by Fe -mont.
In conclusion, the geochemical approach presented here,
which provides unambiguous evidence for the production of
hydrogen by reduction of protons with the iodide anion as an
electron source, even at room temperature, opens up enor-
mous opportunities in hydrogen chemistry. Production of
hydrogen by geochemicals, as described here, provides
evidence that the vast quantities of hydrogen found in the
Earthꢀs crust may be formed by a similar process. The
cogeneration of H and I by simple electron transfer has great
potential for the economical production of iodine and hydro-
gen. This methodology has also been used for the catalytic
and selective reductive N-acylation of a variety of functional
groups.
Entry
Substrate
t [h]
Yield [%]
[
b]
1
2
3
4
5
6
7
8
9
2-azidoanisole
0.50
0.50
0.50
0.50
20.0
20.0
12.0
12.0
16.0
12.0
98, 92
98
4-azidonitrobenzene
4-azidochlorobenzene
2-azidobenzoic acid
benzonitrile
benzyl cyanide
nitrobenzene
p-nitroanisole
p-chloronitrobenzene
1-nitronaphthalene
100
92
60
80
82, 70,
74
[
c] [d]
5
2
2
63
70
1
0
[a] For azoarenes: NaI/acetic acid under reflux; for cyano- and nitro-
arenes: NaI/propionic acid under reflux. [b] HI/acetic acid under reflux.
[
c] Fourth recycle of the catalyst. [d] FeCl /FeSO .
3
4
Experimental Section
[
25–28]
ilarly, the reduction of nitroarenes
is usually performed
General procedure for the production and consumption of hydrogen
in the reductive N-acylation of azo-, cyano-, and nitroarenes: A
mixture of carboxylic acid (acetic acid for azoarenes and propionic
acid for cyano- and nitroarenes; 40 mmol), sodium iodide (40 mmol),
by nonregenerable, stoichiometric carbonyl complexes or by
expensive catalysts under hydrogen or CO pressure.
Fe -mont was obtained by following a reported proce-
dure. The X-ray photoelectron spectrum (Fe
catalyst sample generated by the treatment of freshly
prepared Fe -mont with NaI highlights the predominance
of Fe species. However, the heterogeneous catalyst obtained
after reductive acylation of nitrobenzene shows larger
amounts of Fe in the montmorillonite (Figure 1 in the
Supporting Information). These results provide strong evi-
dence that, upon treatment with NaI, the Fe centers in Fe -
mont are predominantly reduced to Fe , which is, in turn,
reoxidized to Fe during the reductive acylation of nitro-
benzene. The redox cycle Fe /Fe , involving the relay of two
electrons from I3 to H to form hydrogen, is thus established
in the present system (Scheme 1). Conversely, the reductive
acylation of nitroarenes conducted in the presence of an
III
III
substrate (10 mmol), and Fe -montmorillonite (1.0 g) was placed in a
[
29]
2p
3
band) of a
/2
100-mL, two-necked, round-bottomed flask, which was sealed with a
rubber septum and fitted with a reflux condenser. The reaction
mixture was then heated to reflux. After completion of the reaction
(as monitored by GC), the reaction mixture was filtered. The filtrate
was taken up in ethyl acetate (50 mL) and quenched with a 1%
Na S O solution (20 mL) to remove the iodine. It was then washed
III
II
III
2
2
3
with a 5% sodium bicarbonate solution to remove the unreacted
carboxylic acid. The resulting mixture was concentrated under
vacuum and the crude product was purified by column chromatog-
raphy using ethyl acetate/hexane (1:5) as eluent to yield the pure
product.
III
III
II
III
III
II
ꢀ
+
Received: June 24, 2004
Keywords: acylation · clays · geochemistry · hydrogen · iodine
.
3
24
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Angewandte
Chemie
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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