Environmentally benign selective hydrogenation of α,β-unsaturated aldehydes and reduction of aromatic nitro compounds using Cu based bimetallic nanoparticles supported on multiwalled carbon nanotubes and mesoporous carbon

Article abstract of DOI:10.1039/c6ra04241d

Bimetallic catalysts of Cu with Fe, Co, Ni, Zn or Ru supported on multiwalled carbon nanotubes (MWCNT) and mesoporous carbon (MC) were prepared by in situ reduction of CuCl₂ and the metal chlorides using NaBH₄. The catalytic activity of these materials was investigated in the hydrogenation of α,β-unsaturated aldehydes and reduction of aromatic nitro compounds using different hydrogen sources. Cu-Ni and Cu-Ru supported on MWCNT catalysts were found to be better than MC supported catalysts in terms of yield of the expected product and recyclability. Among the different hydrogen sources used, molecular H₂ was found to be the best for the hydrogenation of α,β-unsaturated aldehydes to corresponding alcohols (conversion 83-86%, selectivity 72-78%), whereas NaBH₄ was the best for the reduction of aromatic nitro compounds to corresponding amines (conversion 100%, selectivity 98%). The reaction conditions were optimised for good yield and selectivity of the product. Generality of the catalysts was also investigated in the reduction of different α,β-unsaturated aldehydes and aromatic nitro compounds. The relationship between catalytic activity and physico-chemical properties was investigated by characterising the catalytic materials using powder X-ray diffraction (powder XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM).

Full text of DOI:10.1039/c6ra04241d

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Environmentally benign selective hydrogenation of
a,b-unsaturated aldehydes and reduction of
aromatic nitro compounds using Cu based
bimetallic nanoparticles supported on multiwalled
carbon nanotubes and mesoporous carbon†
Cite this: RSC Adv., 2016, 6, 82359
ab
a
a
c
a
H. N. Hareesh, K. U. Minchitha, K. Venkatesh, N. Nagaraju and N. Kathyayini*
Bimetallic catalysts of Cu with Fe, Co, Ni, Zn or Ru supported on multiwalled carbon nanotubes (MWCNT)
and mesoporous carbon (MC) were prepared by in situ reduction of CuCl and the metal chlorides using
NaBH . The catalytic activity of these materials was investigated in the hydrogenation of a,b-unsaturated
2
4
aldehydes and reduction of aromatic nitro compounds using different hydrogen sources. Cu–Ni and
Cu–Ru supported on MWCNT catalysts were found to be better than MC supported catalysts in terms of
yield of the expected product and recyclability. Among the different hydrogen sources used, molecular
H
2
was found to be the best for the hydrogenation of a,b-unsaturated aldehydes to corresponding
alcohols (conversion 83–86%, selectivity 72–78%), whereas NaBH was the best for the reduction of
4
aromatic nitro compounds to corresponding amines (conversion 100%, selectivity 98%). The reaction
conditions were optimised for good yield and selectivity of the product. Generality of the catalysts was
also investigated in the reduction of different a,b-unsaturated aldehydes and aromatic nitro compounds.
The relationship between catalytic activity and physico-chemical properties was investigated by
characterising the catalytic materials using powder X-ray diffraction (powder XRD), X-ray photoelectron
spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM) –
energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM).
Received 16th February 2016
Accepted 28th July 2016
DOI: 10.1039/c6ra04241d
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carbon nanotubes (MWCNT) and mesoporous carbon (MC) are
used as catalyst supports due to their (i) well-dened and
1
. Introduction
Heterogeneous catalysis plays a pivotal role in the reduction of tunable structures (ii) high catalytic activities arising from their
environmental pollution. In this regard the main objectives of mesoporosity (iii) ability to avoid mass transfer limitations and
heterogeneous catalysis are (a) application of mild reaction (iv) high surface area available for the uniform dispersion of
7
–9
conditions (b) usage of eco-friendly solvents (c) prevention of active catalyst.
hazardous and toxic wastes (d) development of recyclable
Selective hydrogenation of C]C bond of a,b-unsaturated
catalysts and (e) formation of products with high percentage aldehydes result corresponding unsaturated alcohols, which
1–3
yield and selectivity. In heterogeneous catalysis porous solids nd applications as intermediates in avors, fragrance and
1
0–12
are used as supports for the uniform dispersion of the active pharmaceuticals.
metal or metal oxide catalysts and further to reduce the cost of susceptible to hydrogenation than the C]O bond.
the active catalyst. In several instances the supported catalysts Cu-based materials are employed as catalysts in industry for
It is a known fact that C]C bond is more
13,14
Low-cost
4–6
exhibited improved catalytic activity. Recently, multiwalled selective hydrogenation of C]O bond in carbonyl compounds.
+
0
It is noticed that a synergetic effect of Cu /Cu in Cu-based
catalysts remarkably control the catalytic performance in the
hydrogenation of carbonyl compounds.
a
Centre for Incubation, Innovation Research and Consultancy, A Joint Initiative of Sri
10,15–17
Sharada Peetham, Sringeri & Jyothy Institute of Technology Campus, Tataguni off
Kanakapura Road, Bengaluru – 560082, Karnataka, India. E-mail: nkathyayini45@
gmail.com; Fax: +91 80 27577211; Tel: +91 9886506387
Ni, Pd, Pt, Ru and Pt–Ru catalysts supported on CNTs and Cu
supported on mesoporous carbon are investigated for their
catalytic activity in the hydrogenation of a,b-unsaturated alde-
hydes, but the selectivity and % yield of the product were found
b
Visvesvaraya Technological University – Research Resource Centre, Jnana Sangama,
Belagavi – 590018, Karnataka, India
c
Department of Chemistry, St. Joseph's College P.G. Centre, 46, Langford Road,
18–21
to be low and the catalysts are expensive.
Hence develop-
Shanthinagar, Bengaluru – 560027, Karnataka, India
ment of low cost and efficient catalysts for the selective hydro-
genation of conjugated aldehydes without effecting to C]C
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra04241d
1
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bond to the corresponding unsaturated alcohol is a challenging nanotubes) and F/MC (functionalized mesoporous carbon).
and signicant task, from both academic and industrial point Further, these functionalized materials used as supports for the
of view.
preparation of monometal and bimetal containing catalysts by
34
Similarly, the selective reduction of a nitro group in the reduction method, using NaBH4 as a reducing agent. The
presence of other labile groups in an aromatic compound is an experimental details of catalysts preparation are given in ESI.†
industrially important organic transformation in the prepara- Preparation of acid activated MWCNT/MC and the metal ion
tion of amines, which in turn are used in the synthesis of many supported carbon material is pictorially represented in Scheme
agrochemicals, pharmaceuticals, dyes, rubbers, polymers and 1. All the prepared materials were characterized by powder XRD,
22,23
pigments.
Supported noble metals and their compounds are X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–
investigated for their catalytic activity in the selective reduction Teller (BET) surface area, SEM–EDS and TEM techniques.
24–26
of nitro group.
However the amount of the noble metals
used in the case of supported catalysts is quite high and hence
the process is expensive. Further these methods also have some
draw-backs such as long reaction time and water sensitivity of
3
. Results and discussion
3.1. Catalytic activity studies
the reagents. Further, selective reduction of various aromatic Catalytic activity of the prepared materials was investigated in
nitro compounds using Cu and Ni nanoparticles based cata- the selective hydrogenation of C]O bond of a,b-unsaturated
lysts, as well as the promotional effect of Cu in bimetallic aldehydes (cinnamaldehyde and crotonaldehyde) and reduction
catalyst with respect to the stability and selectivity has been of nitro group in an aromatic compounds under reuxing
27–33
discussed.
conditions. The experimental details of the reactions are
It is also evident from the reported literature that Cu based included in ESI.†
catalysts along with the noble metals are suitable catalytic
3.1.1. Hydrogenation of a,b-unsaturated aldehydes. The
materials for hydrogenation/reduction reactions. In this present expected hydrogenation products of cinnamaldehyde and cro-
investigation we have explored the possibility of using very low tonaldehyde are presented in the reaction Scheme 2.
percentage of noble metal along with inexpensive Cu to develop
Initially it was noticed that acid activated MWCNTs/MC,
bimetallic catalysts with good selectivity and yield for the title single metal and bimetal supported carbon materials, except
reactions. We have also made an attempt to correlate the Cu–Ru and Cu–Ni catalysts, did not show any signicant cata-
structure-catalytic activity of the material by suitable charac- lytic activity in the hydrogenation of I and V into any of the
terization techniques.
expected products. Further, MWCNT/MC supports containing
% of Fe, Co and Zn were also catalytically inactive did not show
1
any catalytic activity, whereas those containing 1% of Ni or Ru
metals showed activity to an extent of only 10% conversion of
the I and V aldehydes into the corresponding alcohols (II and
VI). The catalyst with 10% Cu exhibited a slightly better activity
2
. Experimental
2.1. Preparation and characterization of MWCNT and MC
supported single and bimetal catalysts
Multiwalled carbon nanotubes (MWCNT) were purchased from (10–14% conversion) than Ni and Ru incorporated materials.
Reinste Nano ventures Pvt. Ltd and mesoporous carbon (MC) These results prompted to check the effect of combining Cu
from Sigma Aldrich. The Powder XRD patterns, SEM – EDS and with other transition metals on their catalytic activity in the
TEM images of these materials are given as ESI (S1–S3†).
hydrogenation of I and V. Bimetallic compositions with 10% Cu
MWCNT and MC materials were activated by conc. HNO
3
and 1% of Fe, Co, Zn showed slightly increasing in catalytic
and conc. H SO (1 : 3). The materials thus obtained were activity (20–28% conversion) than the catalyst with only Cu but
2
4
abbreviated as F/MWCNT (functionalized multiwalled carbon a combination of 10% Cu and 1% Ni or Ru exhibited very
Scheme 1 Schematic representation of synthesis of metal/s nanoparticles supported on F/MWCNT and F/MC.
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Scheme 2 Hydrogenation of a,b-unsaturated aldehydes in the presence of Cu/X/MWCNT or Cu/X/MC as catalysts (X ¼ Fe/Co/Ni/Zn or Ru).
+
0
signicant rise in the catalytic activity in the hydrogenation of C]O bond. A higher ratio of Cu /Cu leads to higher selectivity
a,b unsaturated aldehydes. Further, the % conversions of I and for II or VI, unsaturated alcohols. This is consistent with the
V as well as the selectivity towards alcohol products (II and VI) powder XRD and XPS results on these materials discussed later
0
were much higher with Cu–Ru than Cu–Ni catalyst. The results in this paper. Gong J. et al., have reported that, Cu species are
are presented in Table 1 (entries 3, 4, 5 and 6). It is also inter- associated with both decomposition of molecular hydrogen as
+
esting to note that the selectivity of the product II and VI well as reduction of C]C double bond, where as Cu species are
1
0
depends further on the type of the carbon support used.
selective in the hydrogenation of C]O double bond. Thus an
+
In the hydrogenation of cinnamaldehyde (I), Cu–Ru/MWCNT increase in the Cu species in the catalyst would increase the
showed higher conversion (83%) and selectivity (78%) towards II, selectivity for unsaturated alcohol. It is understood from the
cinnamyl alcohol, than Cu–Ru/MC for which these values were literature that, the presence of Au or Ru along with Cu ,
0
respectively 80% and 60%. Similarly, in the hydrogenation of V, increases the later activity for molecular hydrogen decomposi-
86% conversion and 72% selectivity for VI, was noticed in the tion. This increases the overall activity and selectivity for the
presence of Cu–Ru/MWCNT catalyst. However, Cu–Ru/MC cata- unsaturated alcohol in the presence of Cu–Ru catalysts. In the
lyst showed 89% conversion and selectivity only 63% for the case of Cu–Ni catalyst the lower selectivity for unsaturated
alcohol. The difference in the selectivity's between the MWCNT/ alcohol can thus be attributed not only to higher concentration
0
MC supported catalysts is attributed to the difference in BET of Cu species but also to the lower catalytic activity of Ni species
10,12,35
surface area and surface morphology associated with their mes- for molecular hydrogen decomposition.
oporous structure and hence the metal support interactions. Effect of the nature of hydrogen sources. As Cu–Ru/MWCNT
These characteristic properties of the materials are presented in catalyst showed better activity in terms of selectivity for unsat-
the later part of discussion in this paper. It was noticed that, in urated alcohols, further studies on the effect of the nature of the
presence of Cu–Ni/MWCNT and Cu–Ni/MC catalyst, the % hydrogen sources are investigated. The results concerning these
conversion of the aldehydes was in the range 40–53, whereas the studies in the hydrogenation of cinnamaldehyde and croto-
selectivity for the corresponding alcohols was in the range 45–52. naldehyde by Cu–Ru/MWCNT catalyst are presented in the
These values are thus much lower than those for Cu–Ru catalysts Table 2.
supported on the same carbon materials.
The selected reducing agents are known to generate molec-
The correlation between the selectivity for unsaturated ular hydrogen through catalytic decomposition over metal/
+
0
alcohol (II and VI) and the ratio of Cu /Cu in Cu–Ru and Cu–Ni metal oxide surfaces, and due to their intrinsic difference in
catalyst supported on carbon materials is notable. It is reported their chemical reactivity the extent of their reaction alters with
+
36–39
that Cu sites are responsible for the selective hydrogenation of the nature of the metal.
a
Table 1 Hydrogenation of cinnamaldehyde and crotonaldehyde using Cu/X/MWCNT or Cu/X/MC catalysts (X¼ Ni or Ru)
Products selectivity (in%)
Entry
Catalysts
Conversion% I (V)
II (VI)
III (VII)
IV (VIII)
Other
1
2
3
4
5
6
10% Cu/MWCNT
10% Cu/MC
Cu–Ru/MWCNT
Cu–Ni/MWCNT
Cu–Ru/MC
10 (12)
13 (14)
83 (86)
48 (53)
80 (89)
40 (51)
90 (100)
83 (100)
78 (72)
45 (49)
60 (63)
50 (52)
5 (0)
7 (0)
12 (16)
36 (42)
09 (13)
39 (42)
0 (0)
6 (0)
6 (9)
17 (8)
21 (18)
6 (5)
5 (0)
4 (0)
4 (5)
2 (1)
10 (6)
5 (1)
Cu–Ni/MC
a
Reaction conditions: 1 mmol of cinnamaldehyde & crotonaldehyde, 0.1 g catalyst, H
2
, 20 mL ethanol : water (9 : 1) as solvent for 1 h. I ¼
cinnamaldehyde, II ¼ cinnamyl alcohol, III ¼ hydrocinnamaldehyde, IV ¼ hydro cinnamyl alcohol, V ¼ crotonaldehyde, VI ¼ crotyl alcohol, VII
¼
hydrocrotonaldehyde, VIII ¼ butyl alcohol.
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a
Table 2 Hydrogenation of cinnamaldehyde and crotonaldehyde by Cu–Ru/MWCNT catalyst using different hydrogen sources
Product selectivity (%)
ꢀ
Hydrogen source
Time (min) (temp C)
Conversion (%) I (V)
II (VI)
III (VII)
IV (VIII)
Other
H
2
60 (100)
30 (RT)
30 (RT)
60 (100)
60 (100)
83 (86)
78 (68)
69 (79)
66 (70)
63 (71)
73 (79)
12 (16)
8 (6)
18 (15)
3 (5)
6 (10)
4 (6)
0 (0)
5 (2)
7 (5)
2 (0)
a
4
NaBH
NH NH
COOH
100 (100)
95 (100)
15 (26)
20 (15)
11 (13)
27 (19)
15 (13)
b
2
2
c
CH
3
d
HCOOOH
44 (54)
10 (8)
a
Reaction conditions: 1 mmol of cinnamaldehyde & crotonaldehyde, 0.1 g Cu–Ru/MWCNT catalyst, 20 mL ethanol : water (9 : 1) as solvent. a – 2
mmol, b – 5 mmol, c – 10 mmol, d – 10 mmol. Conversion and selectivity obtained from GC analysis.
Table 3 Reduction of 4-bromo nitrobenzene to 4-bromoaniline catalytic activity (<6% conversion), whereas, 1% of Ni and Ru
a
using Cu/X/MWCNT or Cu/X/MC catalysts (X ¼ Ni or Ru)
metals showed slight increase in activity (10–15% conversion) of
-bromo nitrobenzene to 4-bromoaniline (X). Further, the
materials with 10% Cu exhibited a slightly better activity (18–
4
Selectivity
of X (XI)
Isolated
yield%
Entry
Catalysts
Conversion%
20% conversion).
1
2
3
4
5
6
7
8
9
F/MWCNT
F/MC
NC
NC
20
18
15
12
10
13
90
58
80
54
—
—
—
—
8
From these results, the effect of combining Cu with other
transition metals on their catalytic activity in the reduction of 4-
bromo nitrobenzene was evaluated. Bimetallic compositions
with 10% Cu and 1% of Fe, Co and Zn showed slight increase in
catalytic activity (conversion 40–45%) than 10% Cu catalyst.
Noteworthy that, combination of 10% Cu and 1% Ni or Ru,
exhibited a signicant rise in the catalytic activity in the
reduction of 4-bromo nitrobenzene. It was further noticed that
conversions of 4-bromo nitrobenzene (IX) as well as the %
selectivity and yield towards 4-bromoaniline (X) were much
higher with Cu–Ru than Cu–Ni catalysts supported on carbon
10% Cu/MWCNT
10% Cu/MC
1% Ru/MWCNT
1% Ru/MC
1% Ni/MWCNT
1% Ni/MC
Cu–Ru/MWCNT
Cu–Ni/MWCNT
Cu–Ru/MC
100 (0)
95 (5)
100 (0)
100 (0)
97 (3)
95 (5)
93 (7)
88 (12)
80 (20)
82 (18)
6
—
—
—
—
75
49
66
42
1
1
1
0
1
2
%
Cu–Ni/MC
a
Reaction conditions: 1 mmol of 4-bromo nitrobenzene, 0.1 g of
catalyst Cu–X (10 : 1), 0.5 mmol CoCl
2
, H
2
, 20 mL ethanol : water materials. The results are presented in Table 3 (entries 9, 10, 11
and 12). It is also to be noted that the selectivity of X depends
further on the type of the carbon support.
(9 : 1) as solvent, 2 h. NC – no conversion.
In comparison with all the catalysts mentioned in Table 3,
Cu–Ru/MWCNT showed improved catalytic activity and selec-
3
.1.2. Reduction of aromatic nitro compounds. Initially
tivity towards the formation of 4-bromoaniline in presence of
molecular H and CoCl
Cu/X/MWCNT or Cu/X/MC catalysts (where X ¼ Fe/Co/Ni/Zn or
Ru) were investigated for their catalytic activity in the reduction
of 4-bromo nitrobenzene (Table 3). The expected products of
reduction of 4-bromo nitrobenzene are presented in the reac-
tion Scheme 3.
Initially a reaction was conducted using H
ence of plain acid activated carbon materials and 0.5 mmol of
anhydrous CoCl . These materials did not show any catalytic
activity in the reduction of IX into any of the expected products
X–XII) (Table 3, entries 1 & 2). However, single metal supported
2
2
.
Role of Lewis acid. Under the same reaction conditions, the
catalytic reduction of 4-bromo nitrobenzene was conducted in
the presence of Cu–Ru/MWCNT catalyst with different Lewis
acids like FeCl , FeCl , AlCl , ZnCl and SnCl results are pre-
3
2
3
2
2
2
gas in the pres-
sented in ESI Table (ST-1†). High yield and good selectivity of 4-
bromoaniline (X) was obtained with weak Lewis acids like CoCl
and FeCl whereas, strong Lewis acids like FeCl , ZnCl , SnCl
and AlCl
reactivity with nitro compound. However, in presence of CoCl
and FeCl no debromination product (XI) was observed, but in
presence of FeCl , AlCl , ZnCl and SnCl , up to 26% of debro-
2
2
2
3
2
2
3
gave less yield and selectivity of X, due to their high
(
2
carbon materials (1% of Fe, Co and Zn) exhibited very low
2
3
3
2
2
mination product was observed. This indicates the existence of
strong synergetic effect between Lewis acid and Cu–Ru/MWCNT
catalyst.
A plausible reaction mechanism for the reduction of 4-
bromo nitrobenzene over Cu–Ru/MWCNT with
2
H was
4
1
proposed based on the reported literature by Z. Zhao et al., is
shown in ESI Fig. S8.† It is reported that, zero valent metal
0
0
0
nanoparticles (Cu , Ni and Ru ) present on the support acti-
Scheme 3 Reduction of 4-bromo nitrobenzene in the presence of
Cu/X/MWCNT or Cu/X/MC catalysts (X ¼ Fe/Co/Ni/Zn or Ru).
vate H to produce active H for the reaction and Lewis acid
2
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a
Table 4 Reduction of 4-bromo nitrobenzene to 4-bromo aniline by Cu–Ru/MWCNT catalyst using different hydrogen sources
ꢀ
Entry
Hydrogen sources
Time (temperature C)
Conversion
Selectivity% X (XI)
% yield
1
2
3
4
5
H
NaBH
Hydrazine hydrate
Formic acid
Acetic acid
2
2 h, 100
30 min, RT
30 min, RT
3 h, 80
90
100
100
10
93 (7)
100 (0)
92 (8)
70 (30)
60 (40)
75
92
78
—
—
4
3 h, 80
8
a
Reaction conditions: 1 mmol of 4-bromo nitrobenzene, 0.1 g Cu–Ru/MWCNT catalyst, 20 mL ethanol : water (9 : 1) solvent.
activate aromatic NO group for hydrogenation by weakening conversion of IX, selectivity and yield of X were respectively 90,
2
NO
2
, N]O and N–O bonds through its electron accepting 93 and 75 (Table 4 entry 1) in a reaction carried out for 2 h at
ꢀ
properties. Possible steps involved in the hydrogenation is as 100 C using H
2
as a reducing agent where as these numbers not
follows: (i) formation of active hydrogen over catalyst surface, only increased signicantly but also obtained under much
ii) N–O of NO is activated by the coordination of catalyst with milder reaction conditions (RT, 30 min) with NaBH and
N atom and the Lewis acid with O, and then activated H NH NH as reducing agents Table 4 (entries 2 & 3). However,
(
2
4
2
2
interacts with the activated N–O bond, and thereby formation formic acid and acetic acid showed very low conversions Table 4
of intermediate 1-bromo 4-nitrosobenzene (A), with the elim- (entries 4 & 5).
ination of H O molecule (iii) further, the 1-bromo 4-nitro-
A plausible reaction mechanism for the reduction of
to N-(4-bromophenyl) aromatic nitro compounds to corresponding amine using Cu–
hydroxyl amine intermediate (B). Similarly, intermediate ‘B’ is Ru/MWCNT catalyst with NaBH as hydrogen source showed in
further activated and reduced to produce the 4-bromoaniline Fig. 1. The mechanism was proposed based on the reported
2
sobenzene (A) is reduced with H
2
4
4
2
(
X).
literature. The reduction of nitro compound involves electron
ꢁ
Effect of the nature of hydrogen sources. The catalytic activity of transfer from [BH
4
]
ion to the nitro compound through the
Cu–Ru/MWCNT catalyst was also investigated using different catalyst. Thereby larger amount of hydride can be transferred or
hydrogen sources such as NaBH , hydrazine hydrate, formic a larger amount of H gas can be produced in the reduction of
4
2
acid and acetic acid. It is interesting to note that the % nitro compound.
Fig. 1 A plausible reaction mechanism for the reduction of 4-bromo nitrobenzene to 4-bromoaniline using Cu–Ru/MWCNT catalyst with
NaBH as hydrogen source.
4
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Table 5 Reduction of different aromatic nitro compounds using Cu–Ru/MWCNT catalyst
%
yield
ꢀ
Entry
1
Substrate
Product
Time/temp ( C)
30 min/80
30 min/RT
60
55
2
3
30 min/80
65
4
5
6
7
8
10 min/RT
10 min/RT
10 min/RT
10 min/RT
88
92
82
85
70
ꢀ
30 min/80 C
9
1 h/RT
72
1
1
0
1
1 h/RT
56
78
10 min/RT
1
2
30 min/RT
55
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Optimization of the reaction conditions and recyclability studies indicated that there was no signicant leaching of Cu and Ru
using Cu–Ru/MWCNT and Cu–MC catalysts. The effect of solvent, nanoparticles from the surface of MWCNT. Whereas, MC based
temperature and duration of the reaction as well as amount of catalysts showed increased leaching of Cu and Ru particles with
the catalyst was investigated in the hydrogenation of cinna- increase in number of recycles (Fig. S7 and S12(b)†). The recy-
maldehyde and reduction of 4-bromo nitrobenzene using Cu– clability of the catalysts demonstrates Cu and Ru species are the
Ru/MWCNT as the catalyst to get the best yield and selectivity of catalytic active sites dispersed over carbon supports that play
the cinnamyl alcohol and 4-bromo aniline respectively. The a synergetic role in this reaction.
results are given as ESI in Fig. S4–S12.† A maximum yield of the
Generality of Cu–Ru/MWCNT catalyst in the reduction of
expected products was obtained in ethanol and water (9 : 1)
aromatic nitro compounds. Several substituted aromatic nitro
compounds were subjected in the reduction reactions using
Cu–Ru/MWCNT catalyst in presence of NaBH as hydrogen
4
ꢀ
mixture for both the reactions but at 100 C for hydrogenation
and room temperature for reduction reaction. These observed
results could be due to the hydrogen bonding between carbonyl
oxygen and hydroxyl group of the alcohols and also it may be
source. The results are presented in Tables 5 and 6. 4-Chloro
and 4-bromo nitrobenzene (entries 5 and 6, Table 5) were
completely reduced to the corresponding anilines without any
dehalogenation, which was oen encountered with several
16,40
due to higher solubility of H in polar solvents.
2
Cu–Ru/MWCNT catalyst was found to be recyclable up to 5
times without any alteration in its, selectivity for the expected
products II and X (Fig. S7 and S12(a)†). Further, separation of
the catalyst from the reaction mixture was more difficult in the
case of Cu–Ru/MC catalyst than Cu–Ru/MWCNT catalyst. Here
it is worth mentioning that, analysis of the recycled catalysts
made by ICP-OES method to estimate Cu and Ru contents
22,43
hydrogenation reactions.
4-Cyano nitrobenzene (entry 10,
Table 5) was reduced to 4-cyano aniline keeping the –CN group
intact. The ketocarbonyl and carboxylic functionalities present
in the aromatic ring also remained unaffected during reduction
of the corresponding nitrobenzenes by employing Cu–Ru/
Table 6 Reduction of some heterocyclic nitro compounds with Cu–Ru/MWCNT catalyst
Sl. no Substrate Product
Time/temp
30 min/RT
Yield
78
1
2
3
30 min/RT
30 min/RT
60
70
4
5
30 min/RT
55
45
45 min/RT
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49
MWCNT catalyst (entries 9 & 12, Table 5). To extend the scope of structure of carbon, fcc metallic copper and cubic phases of
the catalyst various heterocyclic aromatic nitro compounds are Cu O were observed on MC. The diffraction peaks at metallic Ru
2
ꢀ
ꢀ
ꢀ
ꢀ
also reduced to corresponding amines using Cu–Ru/MWCNT (42.17 and 78.33 ) and metallic Ni (44.7 and 76.27 , JCPDS no.
42,50
catalyst (Table 6). Spectroscopic data of the some heterocyclic 87-0712)
are evident from the corresponding powder XRD
compounds reported in Table 6 are presented in ESI (SD-1†).
patterns (Fig. 2e–g).
Powder XRD studies indicated the presence of metallic Cu as
well as Cu
indicated that the MWCNT did not lose their graphitic struc-
.2.1. Powder XRD. Cu–Ru/MWCNT and Cu–Ni/MWCNT ture. The intensities of the diffraction peaks due to metallic Cu
2
O on the supported carbon materials. Further,
3
.2. Characterization of the catalysts
3
ꢀ
ꢀ
catalysts graphitic structure of MWCNT (25.96 and 43.49 ), in Cu–Ni catalysts were more than in Cu–Ru. On the other hand
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
fcc metallic Cu (43.3 , 50.42 and 74.1 , JCPDS no. 04-0836) and the intensities of the peaks due Cu
cubic phases of Cu
O is reverse. These results
O (36.3 and 61.4 , JCPDS no. 78-2076) on clearly indicate why selectivity of cinnamyl alcohol is more in
2
2
MWCNT are evident from the powder XRD patterns (Fig. 2a– case of Cu–Ru supported catalyst than Cu–Ni catalyst.
4
4–47
The diffraction peaks of metallic Ru (2q ¼ 42.2ꢀ and
c).
6
3.2.2. XPS analysis. XPS characterization of materials
ꢀ
48
9.4 ) and metallic Ni are very weak because of low percentage provides information regarding oxidation state of the copper
loading (1%) of Ru or Ni in the corresponding catalysts. Simi- species associated with the catalysts under investigation. These
0
larly, Cu–Ru/MC and Cu–Ni/MC catalysts hexagonal graphitic studies are useful in evaluating the role copper species (Cu /
Fig. 2 Powder XRD patterns of (a) F/MWCNT, (b) Cu–Ru/MWCNT, (c) Cu–Ni/MWCNT, (e) F/MC, (f) Cu–Ru/MC and (g) Cu–Ni/MC catalysts.
Fig. 3 XPS spectra of Cu, Cu–Ru and Cu–Ni supported on MWCNT and MC catalysts for copper 2p3/2 and 2p1/2 electrons.
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+
Cu ) which contribute towards catalytic activity. The XPS spectra
of Cu 2p3/2 and 2p1/2 electrons for Cu, Cu–Ru and Cu–Ni sup-
ported on MWCNT/MC catalyst are depicted in Fig. 3. The
binding energies (BE) of Cu 2p3/2 and Cu 2p1/2 electrons in Cu/
MWCNT & Cu/MC are 932.4 eV and 952.3 eV respectively,
0
+
indicating the presence of both metallic Cu and Cu species
Table 7). However, in bimetallic catalysts the BE are shied to
higher values indicating that there is a strong interaction of Cu
(
51–53
with Ru/Ni.
Further an analysis of XPS spectra of C (1s) electron in Cu
and bimetallic catalysts deposited on MWCNT and MC (Table 7)
revealed the difference in the interaction of the metallic species
with the support. For instance the BE of C 1s electron in Cu/
MWCNT was found to be 284.8 eV, and this value is shied in
Cu–Ru and Cu–Ni catalysts supported on MWCNT indicating
5
2,53
a noticeable surface interactions.
of
On contrary to this, the BE Fig. 5 TEM image of (a and b) Cu–Ru/MWCNT and (c and d) Cu–Ni/
C
1s electron in MC supported catalysts remained MWCNT catalysts.
unchanged. These results corroborate with the recyclability of
these catalysts in hydrogenation and reduction reactions. Due
to higher interaction between the support and the metallic signicantly upon incorporation of bimetallic species (Cu–Ru
particles in MWCNT based catalysts than in MC supported and Cu–Ni) into functionalized carbon materials (ST-2,† entry 2,
catalysts the former catalysts are efficiently recyclable.
.2.3. BET surface area. In both MWCNT and MC catalysts a mesoporous structure of the materials.
the BET surface area, pore volume and pore diameter decreased eters 3.2 and 10.6 nm (Fig. S13 and S14†).
.2.4. SEM-EDS. Fig. 4a and c represents SEM images of
3 and 5, 6). The N adsorption–desorption isotherms indicated
2
18,54,55
3
with pore diam-
3
Cu–Ru/MWCNT and Cu–Ni/MWCNT, indicates the presence of
small metal particles deposited over the mesoporous tubular
structured MWCNT. Similarly, in the case of MC based catalysts
which are obtained as carbon spheres, small metal nano-
particles were also observed on the surface of their pores. The
EDS proles of the catalytic materials are included in ESI
Table 7 Binding energies of Cu 2p1/2 and C 1s electrons in MWCNT
and MC supported Cu, Cu–Ru and Cu–Ni catalyst
Binding energy
(eV) (Cu 2p3/2)
Binding energy
(eV) (C 1s)
Catalyst
(
Fig. S15†) which gives the atomic percentages of carbon,
Cu/MWCNT
Cu–Ru/MWCNT
Cu–Ni/MWCNT
Cu/MC
Cu–Ru/MC
Cu–Ni/MC
932.4
932.6
932.5
932.4
932.5
932.5
284.8
284.4
285.2
284.4
284.4
284.4
oxygen, copper, ruthenium and nickel. The atomic percentages
of the metals deposited on MWCNT and MC as mentioned in
the corresponding EDS proles are also matching with the
amount of metals deposited during synthesis procedure.
3.2.5. TEM analysis. Fig. 5 and 6 represents TEM images of
Cu–Ru/MWCNT, Cu–Ru/MC, Cu–Ni/MWCNT and Cu–Ni/MC. It
Fig. 4 SEM image of (a) Cu–Ru/MWCNT, (b) Cu–Ru/MC, (c) Cu–Ni/ Fig. 6 TEM image of (a and b) Cu–Ru/MC and (c and d) Cu–Ni/MC
MWCNT and (d) Cu–Ni/MC catalysts.
catalysts.
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is clear from Fig. 5a–d, metal nanoparticles are deposited on
inner and outer walls of MWCNT. In the case of Cu–Ru/
9 H. N. Hareesh, K. U. Minchitha, N. Nagaraju and
N. Kathyayini, Chin. J. Catal., 2015, 36, 1825–1836.
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However, the density of metal nanoparticles deposited on Cu– Ind. Eng. Chem., 2013, 19, 250–255.
Ni/MWCNT was found to be greater than in the case of Cu–Ru/ 12 A. J. Marchi, D. A. Gordo, A. F. Trasarti and C. R. Apesteguia,
MWCNT. Similarly, in Cu–Ru/MC and Cu–Ni/MC catalysts, Appl. Catal., A, 2003, 249, 53–67.
metal/s nanoparticles are observed on the surface of the mes- 13 J. Shi, R. Nie, P. Chen and Z. Hou, Catal. Commun., 2013, 41,
oporous carbon spheres as dark spots (Fig. 6a–d).
101–105.
14 H. Rojas, G. Diaz, J. J. Martinez, C. Castaneda, A. G. Cortes
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4
. Conclusion
1
22–128.
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tion method. The resultant catalysts were tested in 16 H. Liu, Q. Hu, G. Fan, L. Yang and F. Li, Catal. Sci. Technol.,
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nitroarenes. Cu–Ru/MWCNT catalyst containing only 1% noble 17 W. Li, G. Fan, L. Yang and F. Li, Catal. Sci. Technol., 2016, 6,
metal was found to be an excellent and selective catalyst for the 2337–2348.
hydrogenation of a,b unsaturated aldehydes as well as reduc- 18 Z. Liu, Y. Yang, J. Mi, X. Tan and Y. Song, Catal. Commun.,
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containing only 1% noble metal exhibited good selectivity for 19 H. Vu, F. Goncalves, R. Philippe, E. Lamouroux, M. Corrias,
the unsaturated alcohol formation in the range of 72–78%. A
synergistic effect between Cu and Ru was proposed. Ru sites
Y. Kihn, D. Plee, P. Kalck and P. Serp, J. Mol. Catal. A: Chem.,
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were responsible for the dissociative activation of H
2
molecules 20 M. Jahjah, B. Caron, S. Menuel, E. Monier, L. Djakovitch
and Cu sites contribute on adsorption-activation of C]O bond
and C. Pinel, ARKIOC, 2011, 7, 406–415.
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catalyst to reduce wide variety of nitroarenes with NaBH as the 22 A. Saha and B. Ranu, J. Org. Chem., 2008, 73, 6867–6870.
+
4
hydrogen source. This catalyst is more selective to –NO
and unaffected to the other sensible functional groups (–Br, –Cl
or –CN). Further, catalyst can be recyclable upto 5 times without 24 J. Spencer, R. P. Rathnam, H. Patel and N. Anjum,
2
group 23 H. Feng, Y. Li, S. Lin, E. V. Van der Eycken and G. Song,
Sustainable Chem. Processes, 2014, 2, 14–19.
loss of its catalytic activity.
Tetrahedron, 2008, 64, 10195–10200.
2
2
5 R. J. Rahaim and R. E. Maleczka, Org. Lett., 2005, 7, 5087–
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Acknowledgements
6 R. G. de Noronha, C. C. Romao and A. C. Fernandes, J. Org.
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The authors thank Department of Science and Technology –
Science and Engineering Research Board (DST-SERB) for the 27 C. Antonetti, M. Oubenali, A. M. R. Galletti, P. Serp and

nancial support given for this research work. Project number:
G. Vannucci, Appl. Catal., A, 2012, 421–422, 99–107.
SERB/F/3690/2013–14.
28 E. Kim, H. S. Jeong and B. Moon Kim, Catal. Commun., 2012,
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2
3
9 R. Bendi and T. Imae, RSC Adv., 2013, 3, 16279–16282.
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