Activated carbon as an efficient support for gold nanoparticles that catalyze the hydrogenation of nitro compounds with molecular hydrogen

Article abstract of DOI:10.1016/j.mencom.2015.11.015

A catalyst based on activated carbon with deposited gold nanoparticles and impregnated with ethylenediamine for selective hydrogenation of nitroarenes has been suggested.

Full text of DOI:10.1016/j.mencom.2015.11.015

Mendeleev
Communications
Mendeleev Commun., 2015, 25, 443–445
Activated carbon as an efficient support for gold nanoparticles that
catalyze the hydrogenation of nitro compounds with molecular hydrogen
Grigory N. Bondarenko and Irina P. Beletskaya*
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
Fax: +7 495 939 1139; e-mail: beletska@org.chem.msu.ru
DOI: 10.1016/j.mencom.2015.11.015
A catalyst based on activated carbon with deposited gold nanoparticles and impregnated with ethylenediamine for selective hydro-
genation of nitroarenes has been suggested.
Nowadays gold nanoparticles (AuNPs) are widely used in organic
reactions due to high catalytic activity comparable to that of
homogeneous gold complexes, relative stability and selectivity to
the substrate.1 Naturally, the observed selectivity depends con-
siderably on the size and morphology of AuNPs.2,3 An advantage
of these catalysts is the absence of ligands (especially toxic phos-
phine ones), easy separation from the product and the possibility
of their reuse. As a rule, these advantages are achieved upon
immobilization of AuNPs on inorganic or organic supports that
allows one to avoid aggregation of NPs and to use simple
separation methods, such as centrifugation or filtration. However,
the immobilization process is often laborious.4 Furthermore, the
support is sometimes not inert, both to AuNPs and to the substrate.5
Reduction of nitrobenzene, a widely used industrial reaction,
is often employed as a model process for studies on catalyst
efficiency and recyclability. Previously, a number of systems
based on AuNPs fixed on solid supports of SiO2,6 ZrO2,7 TiO2,8–13
Fe2O3,9 Al2O3,14 CeO2 and other metal oxides15,16 were suggested
for hydrogenation of nitro compounds. In most cases, the target
anilines were obtained in high yields and with good selectivity.
In some cases, the smallest particles (up to 2 nm) had the highest
activity,12,13 in other cases 7–9 nm clusters were the most active,6,16
and in some systems the reaction selectivity was affected by the
solvent.7 The catalyst activity depended on the methods of its
preparation and deposition onto the support,13 whereas the
presence of oxide and hydroxy groups affected the formation of
side azo and azoxy compounds.16
8000
6000
4000
2000
0
44.9
52.3
10
20
30
40
50
60
2q/degree
Figure 1 X-ray diagram of Au/CBAU-A catalyst.
Gold(iii) was completely reduced with hydrogen under these
conditions, as it is illustrated by the absence of characteristic
peaks of gold cations in the XRA diagram exhibiting only signals
at q 44.9 and 52.3, which correspond to Au(0) (Figure 1). The
particle size calculated by the Debye–Scherrer method based on
XRA data was rather large, namely, 15 2 nm.
Furthermore, ordered gold clusters up to 50 nm in size were
detected by means of SEM (Figure 2). Though the particles
obtained were rather large, we decided to test them for hydro-
genation of nitrobenzene. Such gold clusters should be more
(a)
(b)
1 µm
100 nm
Currently, ecological standards and demands of chemical
industry require a catalyst to be readily manufactured, inex-
pensive, easily utilized, and certainly well recyclable. Therefore,
we paid attention to activated carbon (AC), a cheap and widely
available inert support. Unique features of this support include a
huge surface and polymodal size distribution of pores, ability to
adsorb both large and small organic molecules, e.g., gases and
clusters of heavy metals with various sizes.17 Meanwhile, gold
nanoparticles fixed on AC were reported to be inefficient in
hydrogenation of nitro compounds.16,18
In this study, we turned to the use of Au/C as the cheapest
and most available catalyst for hydrogenation of nitroarenes.
We developed a method of catalyst preparation preventing the
adsorption of impurities by activated carbon that involves its
impregnation with an AuCl3·3H2O solution in acetone followed
by evaporation of the excess solvent and heating to 300°C for
3 h in a stream of hydrogen. The material thus obtained was
studied by scanning electron microscopy (SEM) and X-ray
powder analysis (XRA).†
Figure 2 High resolution SEM picture of freshly prepared Au/CBAU-A
catalyst. (a) General view of catalyst surface at 50000× magnification.
(b) View of a part of catalyst surface with a hexagonal gold nanoparticle at
300000× magnification.
†
Preparation of 5 wt% Au/C catalyst. Salt AuCl3·3H2O (0.052 g) was
dissolved in acetone (5 ml). Pre-dried BAU-A activated carbon (0.544 g)
was added to the solution. The mixture was stirred for 2.5 h until the
solution discoloured, then the solvent was removed in vacuo. The dried
catalyst was placed in a tube furnace and heated at 300°C for 4 h in a
stream of hydrogen (15–20 ml min–1). The catalyst thus obtained was
impregnated with ethylenediamine (En) using the technique known for
Pd/C.20 X-ray powder diffraction analysis was carried out on a DRON-3M
diffractometer (CuKa irradiation, l = 1.78896 Å). The microstructure of
samples was studied by field emission scanning electron microscopy
(FE-SEM) on a Hitachi SU8000 electron microscope. The catalyst was
prepared from activated carbon, GOST (USSR State Standard) 6217-74.
It is mostly manufactured from birch wood and has good mechanical
strength characteristics, an average specific surface of 740–840 m2 g–1
and a maximum adsorption pore volume of 0.33 cm3 g–1 for benzene.25
© 2015 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2015, 25, 443–445
(a)
(b)
Table 1 Optimization of conditions for hydrogenation of nitrobenzene with
molecular hydrogen.a
Yieldb for cycle (%)
AuNPs/support
Entry
P/atm T/°C t/h
(mol%)
1
2
3
4
5
1
2
3
4
5
6
7
Au/C (1)
Au/C (1)
Au/C (1)
Au/C (1)
Au/C (1)
10
20
40
40
40
50
50
50
20
80
80
80
20
8
10
43
54
52
2 µm
500 nm
4
Figure 4 High resolution SEM image of an Au/CBAU-A catalyst sample.
(a) General view of catalyst surface at 20000× magnification. (b) View of a
part of catalyst surface with a hexagonal gold nanoparticle at 100000×
magnification.
20
20
20
20
100 86 54
100 100
Au/C (En) (1) 40
Au/C (En) (1) 10
100 100 100 100 98
R1
R2
NO2
R1
R2
NH2
a Reaction conditions: 0.01 mmol of nitrobenzene, 1 mol% of the Au catalyst,
H2 (10 atm), AuNPs/C(En), 1 mol%
EtOH, 80 °C, 16 h
3 ml of EtOH, H2. bThe yield of aniline was determined by NMR spectroscopy.
R3
R3
stable during the reaction and in subsequent recycles than 1–3 nm
nanoparticles that undergo leaching or coalescence with time.
Moreover, 50 nm nanoparticles are safe for living organisms
since they possess no genotoxic activity.19
As shown in Table 1, the first attempts to use the catalyst
(1 mol%) in nitrobenzene hydrogenation showed poor efficiency:
the yield of aniline after 20 h was only 10% (entry 1). We
succeeded in raising the yield somewhat (43–54%) by varying
hydrogen pressure, reaction time and temperature (entries 2–4).
Finally, optimum conditions were found that allowed nitrobenzene
to be quantitatively consumed in 20 h at 80°C and a hydrogen
pressure of 40 atm, however, the catalyst recyclization was
insufficient (entry 5).
1a–i
2a–i
1f, 2f R1 = R3 = H, R2 = OH
1g, 2g R1 = R3 = H, R2 = Ac
1h R1 = NO2, R2 = R3 = H
2h R1 = NH2, R2 = R3 = H
1i, 2i R1 = R3 = H,
1a, 2a R1 = R2 = R3 = H
1b, 2b R1 = R3 = H, R2 = Me
1c, 2c R1 = R3 = H, R2 = Br
1d, 2d R1 = R2 = H, R3 = Cl
1e R1 = NO2, R2 = H, R3 = Br
2e R1 = NH2, R2 = H, R3 = Br
R2 = CH=CHCO2H
Scheme 1
Table 2 Hydrogenation of nitroarenes using Au/C catalyst.
Entry
Substrate
Product
mol% Au
Yielda (%)
1
2
1a
1b
1c
1d
1e
1e
1f
2a
2b
2c
2d
2e
2e
2f
1
1
1
1
1
2
1
2
1
2
2
2
100
98
We solved the recyclization problem by modifying the catalyst
by its treatment with ethylenediamine. Previously, this modifica-
tion proved to be efficient in prolongation of activity and re-
cyclization of palladium catalysts on carbon supports.20,21 On the
other hand, this treatment appears no less promising in the case
of gold that forms stable complexes with various polyamines.22
Furthermore, the amino group of ethylenediamine can interact
with the nitro group of the substrate to give an adduct on the
catalyst surface (Figure 3) that is analogous to the adduct identified
previously in hydrogenation of nitrobenzene in the presence of
Ru/CNT.23 In fact, this treatment of the catalyst with ethylene-
diamine allowed us not only to perform successful recyclization
and thus to increase the TON of the reaction at least fivefold but
also to decrease hydrogen pressure to 10 atm (Table 1, entries 6, 7).
The AC surface stayed nearly unchanged after the treatment
withethylenediamine:goldnanoparticlesofregularmicrocrystal-
line shape were observed in the catalyst (Figure 4).
3
100
4
93
5
79b
100b
52
6
7
8
1f
2f
100
41
9
1g
1g
1h
1i
2g
2g
2h
2i
10
11
12
100
100b,c
98
a The yield was determined by NMR spectroscopy. The yield matches the
conversion of the starting nitro compound in all cases. b The yield of the
corresponding phenylenediamine is specified. c 60 atm H2.
As shown in Table 2, hydrogenation with 1 mol% of the
catalyst occurs rather smoothly both in the case of non-substituted
nitrobenzene (entry 1) and in the case of compounds containing
an electron-donating substituent, 4-nitrotoluene (entry 2) and
4-bromonitrobenzene (entry 3). In the case of 3-chloronitro-
benzene, we managed to selectively hydrogenate only the nitro
group (entry 4), avoiding dechlorination that is rather common in
similar reactions.24 Some substrates, namely, 5-bromo-1,3-di-
nitrobenzene (entries 5, 6), 4-nitrophenol (entries 7, 8) and 4-nitro-
acetophenone (entries 9, 10), require the catalyst amount to be
increased to 2 mol%. In the case of 1,3-dinitrobenzene, hydrogen
pressure had to be additionally increased to 60 atm (entry 11).
If a readily reducible double bond is introduced into the molecule,
only the nitro group is hydrogenated, as shown for 4-nitrocinnamic
acid (entry 12). The selectivity was 100% in all cases. It should
also be noted that when a starting molecule contains two nitro
groups, only the corresponding phenylenediamine is formed
even in the case of incomplete conversion (entries 5, 6, 11), i.e.,
both nitro groups are reduced at once.
Reduction of various nitroaromatic compounds was carried
out using this catalytic system and the conditions identified
previously (Scheme 1, Table 2).‡
O
O
H
H
Cact
N
N
N
Figure 3 Reaction of ethylenediamine with nitrobenzene.
‡
General procedure for hydrogenation of nitroaromatic compounds. Nitro-
arene 1 (0.1 mmol), anhydrous ethanol (3 ml) and the catalyst (3.9 mg or
7.9 mg, see Table 2) were placed in an RHP30ML high pressure reactor
(www.openscience.ru) equipped with a magnetic stirrer. The reactor was
purged with argon, purged and filled with hydrogen, then heated to
80°C and its content was stirred for 20 h. After the reaction completion,
the mixture was centrifuged for 10 min and the solution was decanted.
The catalyst was washed with ethanol (2×5 ml) in the same manner. The
combined centrifugate was concentrated in a rotary evaporator, the residue
was dissolved in CDCl3 and analyzed by 1H NMR (400 MHz).
Thus, we suggested a simple modification of inexpensive and
recyclable Au/C catalyst based on AC-supported gold nanoparticles
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Mendeleev Commun., 2015, 25, 443–445
9 M. Boronat, P. Concepción, A. Corma, S. González, F. Illas and P. Serna,
and impregnated with ethylenediamine for selective hydrogena-
tion of nitroarenes. Note that the rather considerable size of gold
nanoparticles (15–50 nm) does not prevent catalyst recycling.
Hydrogenation of dinitrobenzenes with molecular hydrogen on
this catalyst results in total reduction of all nitro groups to
exclusively give the target phenylenediamines. Functional groups
such as halogen, ketone and olefin easily reduciable on Ni or
Pd catalysts remain intact in our procedure, which makes it
essentially prospective for fine organic synthesis.
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This study was supported by the Russian Science Foundation
(grant no. 14-23-00186). The authors are grateful to Department
of Structural Measurements of the N. D. Zelinsky Institute of
Organic Chemistry of the Russian Academy of Sciences for
electron microscopy studies of samples and to Assistant Professor
F. M. Spiridonov (Department of Chemistry, M. V. Lomonosov
Moscow State University) for X-ray powder diffraction analysis
of samples.
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Received: 10th August 2015; Com. 15/4704
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