Copper nanoparticles grafted on carbon microspheres as novel heterogeneous catalysts and their application for the reduction of nitrophenol and one-pot multicomponent synthesis of hexahydroquinolines

Article abstract of DOI:10.1039/c7nj03562d

Novel carbon microsphere-supported metallic copper nanoparticles (Cu-NP/C) were synthesized using a low-cost and facile method based on carbonising a styrene-based, chelate-forming cation exchange resin loaded with Cu²⁺ ions. The metal-organic

Full text of DOI:10.1039/c7nj03562d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Copper nanoparticles grafted on carbon microspheres as novel
heterogeneous catalysts and their application for the reduction of
nitrophenol and one-pot multicomponent synthesis of
hexahydroquinolines
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Tibor Pasinszki,*a Melinda Krebsz,a Győző György Lajgut,a Tünde Kocsis,b László Kótai,b Sushama
Kauthale,c Sunil Tekalec and Rajendra P. Pawarc
www.rsc.org/
Novel carbon microspheres-supported metallic copper nanoparticles (Cu-NP/C) were synthesized using a low cost and
facile method based on carbonising a styrene-based chelate-forming cation exchange resin loaded with Cu2+ ions. The
metal-organic framework served as both copper and carbon source. Cu-NP/C microspheres were characterized by XRD,
Raman, SEM, TGA, EDAX, and BET surface area analyser, and employed as heterogeneous catalysts for the reduction of 4-
nitrophenol to 4-aminophenol by NaBH4 and for the synthesis of medicinally significant hexahydroquinoline derivatives
based on the one pot multi-component reaction of aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate. The
sustainable and cheap starting material, the relatively easy synthetic procedure, the catalytic efficiency, and the easy
separation and reusability of Cu-NP/C microspheres opened the door for their wide scale applications.
minimally-poisonous, highly active, selective, stable, robust,
and easily separable from a reaction mixture is hardly
desirable. Cu NPs provide a very promising alternative of heavy
Introduction
Catalysis is in the heart of chemical transformations, therefore
there are continuous efforts to develop new catalysts and to
increase catalytic performances for various applications.
Metallic nanoparticles (NPs) have become essential tools for
many chemical processes employed in the industry and
academia during the last decades.1–5 NPs possess high specific
surface area, various shapes and sizes, high reactivity, and
often exhibit activity different from that of the corresponding
bulk materials due to distinctive quantum properties. Handling
of NPs, however, is not easy, because NPs are often prone to
self-aggregation and it is difficult to separate them from
reaction media, because classical convenient filtration
methods are not applicable. One economical way of
overcoming or minimising these limitations is to anchor NPs on
supports, which latter also serve as modifiers of catalytic
properties. Nowadays, economic considerations and health
issues are gaining more and more importance, and the
development of catalytic NPs that are inexpensive, non- or
metal NP catalysis,1 and the aim of the present work is to
provide a novel route to carbon microsphere supported Cu
NPs, and to test their catalytic activity in two selected
reactions, namely the 4-nitrophenol (4NP) reduction and one-
pot multicomponent synthesis of hexahydroquinolines. The
combination of reactive NPs with microsized support is
important for keeping the high activity of NPs while providing
a possibility for easy catalyst handling and separation. We
show in this paper that both Cu NPs and carbon support can
be simultaneously and economically prepared by carbonising
copper ion loaded styrene based cation exchange resins
without using any additional reducing agent. Several carbon-
supported Cu-based NPs were prepared previously using wet-
chemical, photochemical, electrochemical, and microwave
methods, as well as thermal treatments, and their synthesis is
reviewed last year.1
The reduction of 4NP by NaBH4 to 4-aminophenol (4AP) is an
industrially important reaction6 and a widely used model
reaction to test catalytic performance of metal nanocatalysts
due to the following reasons: 1) 4AP is the sole product of this
reaction, 2) the reaction does not proceed at ambient
temperature without a catalyst, 3) the reaction can be easily
observed by a colour change from yellow (4NP) to colourless
(4AP) and monitored by UV/Vis spectroscopy following the
decrease of the absorption band of 4-nitrophenolate anion at
400 nm.6,7 The reaction was shown to follow a pseudo-first-
order kinetics at the presence of a large excess of NaBH4, and
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the catalytic rate could be evaluated using first-order kinetics acid. The Cu content on these microspheres’ surfaces was
with respect to the 4-nitrophenolate concentration.6–9
measured to be 0.2% after dissolving surface Cu in 34%
Hexahydroquinolines (HHQs) are biologically active aqueous nitric acid.
compounds and find applications in medicinal chemistry as
anticancer, antimicrobial, antihypertensive, and antimalarial
agents.10–14 HHQs, in general, are synthesized by the four
component Hantzsch’s condensation reaction, involving
various aldehydes, dimedone, ethyl acetoacetate, and
ammonium acetate. Several homogeneous and heterogeneous
catalysts such as inorganic salts, Bronsted and Lewis acids,
ionic liquids, metal oxides, and zeolites were reported to be
active in the condensation reaction.15–21 Many of these
methods, however, involve synthetic problems associated with
difficult separation processes, expensive, hardly available, or
non-recoverable catalysts, expensive reagents, or long reaction
time. Considering the widespread applications of HHQ
derivatives, there is a need to develop new and efficient
catalysts for the green synthesis of these heterocyclic
compounds. Therefore, our aim was to develop a facile and
cheap catalyst system for Hantzsch’s HHQ synthesis by
Fig. 1. Powder X-ray diffractogram of selected Cu-NP/C microspheres
selecting nanosized copper as the catalytically active metal and
carbon microspheres as the catalyst support.
(carbonisation time and temperature is indicated).
In this paper, we present a novel method for synthesizing
copper NPs decorated carbon microspheres, their characteri-
sation for the determination of their phase composition and
morphology, and their application as heterogeneous catalysts
in 4NP reduction and HHQ synthesis.
The phase composition of synthesized Cu-NP/C microspheres
was determined by powder X-ray diffraction (XRD).
Representative XRD diffractograms are shown in Fig. 1. The
XRD study unambiguously indicated that only metallic copper
was formed as the sole crystalline phase during carbonisation
between 600 and 1000 °C, and besides Cu a small amounts of
Cu2O appeared at 500 °C carbonisation (8, 6, and 3 at% of total
Cu content applying a heating time of 2, 4, and 8 hours,
respectively; see Table S1, ESI). The average crystallite size of
copper particles was calculated to be 15 nm at 500 °C, using
the Scherrer equation, and it increased gradually up to 35 nm
by increasing the carbonisation temperature (see Table S1,
ESI). EDAX investigations indicated the presence of copper on
the surface or close to the surface of microspheres (see Fig. S2,
ESI). No graphitisation was observed by XRD.
Results and discussion
Synthesis and characterization of Cu-NP/C microspheres
In order to synthesize Cu NP loaded carbon microspheres (Cu-
NP/C),
a commercial styrene-based chelate forming ion
exchanger resin containing iminodiacetate (-CH2N(CH2COOH)2)
functional groups with a binding capacity of 1 mol divalent
metal cation per 1 dm3 resin was selected as both the carbon
source and Cu ion binder (see Experimental part). We
expected that the carbonisation of copper ion loaded resin
produces not only the carbon support but a reductive
environment for the formation of metallic copper. Since
copper ions were distributed in the resin according to
iminodiacetate functional groups, the formation of only small
Cu particles was expected due to diffusion at the carbonisation
temperature. We note that iminodiacetate chelating
functional groups selectively bind one equivalent of Cu(II) ions
(see Scheme S1, ESI). To find information about the stability
and decomposition of loaded resin, thermogravimetric (TG)
analysis was carried out. This latter indicated resin
decomposition between 180 and 450 °C and a complete
weight loss of 68.2% up to 900 °C (see Fig. S1, ESI). Therefore,
the carbonisation of the resin was investigated in the 500–
1000 °C temperature range in 100 °C increments, and the
effect of heating time was studied by performing experiments
for 2, 4, and 8 hours. The Cu content of microspheres prepared
by carbonisation at 600 °C for 4 hours was determined to be
8.1% using ICP-OES after digesting micropheres in hot chromic
Fig. 2. Raman spectra of selected Cu-NP/C microspheres (carbonisation time
and temperature is indicated, see also Figure S3, ESI).
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Raman spectroscopy confirmed the carbonisation of the resin
(Fig. S3, ESI), and selected spectra are shown in Fig. 2. The
appearance of characteristic D, D′, G, and 2D Raman bands at
1329–1347, 1162–1240 (shoulder), 1588–1599, and 2660–
2700 cm–1, respectively, and the D/G band intensity ratio are
characteristic of amorphous carbon.22 The graphitisation of the
carbonaceous material, according to Raman spectra, is very
low, in agreement with XRD. Raman bands characteristic of
amorphous carbon appear on top of a broad luminescence
background at 500 °C carbonisation, what indicate imperfect
carbonization at this temperature. This background, however,
gradually decreases and disappears by increasing
carbonisation time and temperature, respectively (Figs. 2 and
S3, ESI). The measured Raman intensity, in general, decreases
with increasing carbonisation temperature, which indicates
gradual replacement of sp3 carbon atoms by sp2 carbon atoms.
Raman spectra of microspheres obtained by carbonisation
between 600 and 1000 °C for 4 and 8 hours is very similar at
the given temperature, indicating similar structural
organisation of carbon microsphere surfaces.
Fig. 3. SEM image of the Cu-NP/C microsphere surface obtained by
carbonisation of the copper loaded ion exchange resin at 600, 800, and
1000 °C for 8h (see also Figure S5-7, ESI).
The shape and surface morphology of synthesized
microspheres were studied by Scanning Electron Microscopy
(SEM). Cu-NP/C microspheres inherited the spherical
morphology of ion exchange resins (see Fig. S4, ESI), but their
diameter decreased down to the half, to 300–500 µm, due to
shrinkage during the heat treatment. According to SEM Cu NPs
were distributed all over the surface of carbon microspheres.
After breaking some microspheres in a mortar, SEM confirmed
the distribution of Cu NPs inside the microspheres, too. Tiny
Cu NPs are clearly visible on the surface of microspheres
obtained by carbonisation at 500 and 600 °C, but they tend to
be embedded into the carbon matrix at higher temperatures.
A carbon collar forms around Cu NPs at about 800 °C, and NPs
disappear in a seemingly fluffy carbon matrix at higher
temperatures (Figs. 3, S5, S6, and S7, ESI). SEM investigations
thus suggest that Cu-NP/C microspheres prepared at around
600 °C are the most promising candidates for catalytic
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applications. Cu NPs are known to be slowly oxidized by air activity of Cu NPs in 4NP reduction. Cu-NP/C microspheres
oxygen,1 and it was fortuitous for this study that microspheres prepared by carbonisation at 1000 °C for 4 hours were also
synthesized two years ago and stored in a closed container but investigated, but negligible catalytic activity was observed (k=
under air could be reinvestigated by XRD. The old samples 4 min–1g–1; Fig. S12, ESI), what is in line with SEM investigations
prepared at 600 °C indicated the partial oxidation of copper indicating embedding of Cu NPs in the carbon matrix at high
(20 wt% of Cu was oxidized to Cu2O, possibly the surface temperature.
content), but microspheres prepared at 1000 °C were
practically unchanged (see Figs. S8 and S9, ESI). This confirmed
that Cu NPs were embedded in the carbon matrix and
protected from air in the case of high temperature synthesis.
The specific surface areas of microspheres were determined
using nitrogen adsorption measurements and found to be
between 3 and 9 m2g–1 (Table S1, ESI). The total pore volume
for the sample prepared by carbonisation at 600 °C for 4 hours
was measured to be only 0.0017 cm3g–1, and the average pore
width was estimated to be 2.2 nm. The measured nitrogen
sorption isotherms suggested poorly developed micro- and
mesopores (see Fig. S10, ESI).
Catalytic activity of Cu-NP/C microspheres for the reduction of 4-
nitrophenol
The Cu-NP/C sample prepared by carbonisation at 600 °C for 4
hours was selected for catalytic efficiency evaluation. 4NP was
Fig. 4. Concentration of 4NP versus time for the 4NP reduction by NaBH4 at
treated with large excess of NaBH4 in water at the presence of
the presence of Cu-NP/C microspheres (A) (amount of catalyst used is
various amounts of catalysts (initial concentrations of 4NP and
indicated; initial 4NP and NaBH4 consentrations are 0.1 and 10 mM; catalyst
NaBH4 were 0.1 mM and 10 mM, respectively) at 20 °C, and
was added to mixed solutions of 4NP and NaBH4 (a and b); 4NP solution
the reaction was monitored by UV/Vis spectroscopy. The
stirred with the catalyst for 10 min before adding NaBH4 solution (c, d and
characteristic absorption band of nitrophenolate anion
e), temperature= 20 °C). Time dependent evolution of UV/Vis spectra of 4NP
appeared at 400 nm. Upon addition of Cu-NP/C microspheres,
anion at the absence (B) and at the presence (C) of Cu-NP/C catalyst
the absorption band decreased with time and that of 4AP at
showing the reduction of 4NP by NaBH4.
300 nm increased. The 4NP concentration decrease versus
reaction time is shown in Fig. 4. No reaction between 4NP and
NaBH4 was observed at the absence of Cu-NP/C microspheres
(Fig. 4). When 4NP and NaBH4 solutions were mixed and the
catalyst was added, an induction time of about 9 min was
observed for the reaction. However, when the catalyst was
added to the 4NP solution, stirred for 10 minutes, and NaBH4
solution was consecutively added, the reaction proceeded
without any induction time. The induction time of 4NP and
NaBH4 reaction was noted previously6,9 and was explained by a
slow nanoparticle surface reconstruction at the presence of
4NP.9 Our observations are in line with this. At the presence of
a large excess of NaBH4, the 4NP reduction reaction is ruled by
a first order kinetics, indicated by the linear correlation of –
Catalytic activity of Cu-NP/C microspheres in the one-pot multi-
component synthesis of hexahydroquinolines
The catalytic activity of Cu-NP/C microspheres prepared by
carbonisation at 600 °C for 4 hours were investigated for the
synthesis of HHQs via the four component Hantzsch’s
condensation reaction (Scheme 1). The reaction was
performed on a preparative scale, and thus was not monitored
by spectroscopy. The reaction mixture was worked up using
standard preparative methods and isolated yields were
determined in all cases. Instead of varying, the reaction time
was fixed to two hours and conclusion on catalytic activity was
made on the basis of isolated product yields. For reactions
with apparently smaller rate, reaction time was increased in
selected cases. In order to study the effect of solvent and
catalyst concentration on product yield, the reaction between
benzaldehyde (1 mmol), dimedone (1 mmol), ethyl
acetoacetate (1.2 mmol) and ammonium acetate (1.5 mmol)
was selected. The effect of solvent on the rate of reaction was
studied in the presence of 5 mg Cu-NP/C catalyst (ca. 5 wt% of
benzaldehyde used). Among various screened solvents, namely
methanol, ethanol, isopropanol, 1,2-dichloroethane (DCE),
THF, dioxane, and MeCN, ethanol was found to be the best in
the studied model reaction concerning the yield of 2-amino-4-
phenyl-3-cyano-7,7-dimethyl-5-oxo-1,4,5,6,7,8-hexahydroqui-
noline (Table 1). Without catalyst the yield of the reaction was
ln(ct/c0) versus time (min) plots, where
c is the 4NP
concentration at the given time (see Fig. S11, ESI). The
apparent rate constant, kapp, can be determined from the slope
of the fitted line. Increasing the amount of the catalyst used,
the rate constant increased due to the increasing number of
active sites for the reaction. The ratio of the rate constant over
the total weight of the Cu-NP/C catalyst, k = kapp/mcat, where m
is the mass of catalyst used, was determined to be between 68
and 100 min–1g–1. In separate experiments Cu NPs were
removed from the microsphere surface using 1:1 diluted nitric
acid. These ‘bare’ carbon microspheres did not catalyse the
4NP reduction, what confirmed the importance and catalytic
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very low, below 5% for each solvent. Using ethanol as the most enaminoester (II), 3. the reaction of arylidene intermediate (I)
appropriate solvent, the amount of catalyst was tested, and with β-enaminoester (II), and 4. proton transfer and
was found that 10 mg catalyst was sufficient to complete the cyclization.
Hantzsch’s transformation within 2 hours (Table 1).
The reusability of the catalyst is important for practical
applications, and it was studied by selecting the reaction
between benzaldehyde, dimedone, ethyl acetoacetate and
ammonium acetate, as well as the most favorable reaction
conditions (ethanol, reflux, 10 mg catalyst/1 mmol aldehyde)
and 2 hours reaction time. Experiments revealed that the Cu-
NP/C catalyst could be consecutively reused four times
without significant loss of catalytic activity (Fig. 5). The slight
decrease in the yield of the corresponding product 2-amino-4-
phenyl-3-cyano-7,7-dimethyl-5-oxo-1,4,5,6,7,8-
O
Ar
O
O
O
O
Cu-NP/C
OEt
NH4OAc
ArCHO
OEt
solvent, reflux
N
H
O
Scheme 1. Synthesis of hexahydroquinolines using Cu-NP/C catalyst.
Table 1. Solvent screening and optimisation of catalyst concentration
Entrya
Conditions
Catalyst (mg)
Isolated yield (%)
hexahydroquinoline can be attributed to a small catalyst loss
during its recovery and deactivation. The efficiency of the Cu-
NP/C catalyst is compared to those of a number of previously
reported heterogeneous catalysts in Table S3, ESI.
1
2
3
4
5
6
7
8
THF, reflux, 5h
Dioxane, reflux, 5h
DCE, reflux, 3h
MeCN, reflux, 5h
iPrOH, reflux, 3h
MeOH, reflux, 2h
EtOH, reflux, 2h
EtOH, reflux, 2h
EtOH, reflux, 2h
EtOH, reflux, 2h
5
5
5
5
5
5
5
10
15
20
38
58
63
74
76
82
87
92
94
94
9
10
a See Scheme 1; Ar= phenyl; Reagents: benzaldehyde (1 mmol), dimedone (1
mmol), ethyl acetoacetate (1.2 mmol), ammonium acetate (1.5 mmol), and
solvent (2 mL). See Fig. S13–15, ESI, for MS and NMR spectra.
Table 2. Yields of various hexahydroquinolines using the Cu-NP/C catalyst
Entrya
11
12
13
14
15
16
17
18
19
20
21
Ar group
Isolated yield (%)
Phenyl
92
88
86
94
90
89
87
89
91
88
87
82
2,4-dichlorophenyl
2-chloro-4-fluorophenyl
4-chlorophenyl
2-fluorene
4-methoxyphenyl
2-hydroxyphenyl
3-hydroxyphenyl
3-nitrophenyl
4-nitrophenyl
4-bromophenyl
2-thiophenyl
Fig. 5. Graphical representation of the reusability of the Cu-NP/C catalyst.
Experimental
General information
22
Chemicals used were reagent grade from commercial sources
and used without further purification. Identification of
synthesized hexahydroquinolines was based on melting point
and spectroscopic measurements. Melting points were
measured in open capillaries using an Optics technology
melting point apparatus and were uncorrected. 1H NMR, 13C
NMR, and DEPT spectra were obtained on Bruker Avance 250
and 400 spectrometers and referenced to tetramethylsilane
(TMS). Infrared (IR) spectra were recorded on a Shimadzu FTIR
spectrometer (Prestige IR21) using neat samples. Samples
were analyzed for exact mass on a mass analyzer (Waters-
TQD).
a
See Scheme 1 and Table S2, ESI; Conditions: reflux, 2h, ethanol, 20 mg
catalyst, aldehyde (2 mmol), dimedone (2 mmol), ethyl acetoacetate (2.4
mmol) and ammonium acetate (3 mmol), solvent 5 mL. See Fig. S13-34, ESI,
for MS and NMR spectra.
In order to test the general applicability of Cu-NP/C catalyst for
HHQ synthesis, the reaction of various ortho, meta, and para
substituted benzaldehydes, two of them synthesized for the
first time, and thiophene-2-aldehyde with dimedone, ethyl
acetoacetate, and ammonium acetate was studied under the
optimized conditions (ethanol, reflux, 10 mg catalyst/1 mmol
aldehyde). The catalyst was found to be efficient and provided
excellent yields (82-94 %) for the investigated derivatives
(Table 2). A plausible and stepwise mechanism for the four
component reaction is suggested in Scheme S2, ESI,
analoguously to that in ref. [15]. It involves four possible steps:
1. the catalyst assists to enhance electrophilicity of carbonyl
carbon of aldehyde which reacts with the enolic form of
dimedone to form arylidene intermediate (I), 2. ethyl
acetoacetate reacts with ammonium acetate to form β-
Synthesis of Cu-NP/C microspheres
A styrene-based VARION BIM-7 commercial chelate forming
ion exchanger resin, containing 7% divinylbenzene crosslinker,
2%
acrylonitrile
modifier
and
iminodiacetate
(-
CH2N(CH2COOH)2) functional groups with a binding capacity of
1 mol divalent metal cation per 1 dm3 resin was used as the
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starting material. The resin was saturated with Cu2+ ions using corresponding pure hexahydroquinolines. The recovered
aqueous copper sulphate (CuSO4) solution according to the catalyst was washed several times with hot ethanol, dried in
procedure described in our previous studies.22,23 The an electric oven at 120 ºC for about 30 minutes, and reused
exchanged resin was first dried in air and then in a drying box several times when it was required. Spectral data of newly
at 120 °C for 1 day. The dried resin was carbonized at 500, 600, synthesized compounds:
700, 800, 900, and 1000 °C for 2, 4, and 8 h in a high purity dry Ethyl-4-(2-chloro-4-fluorophenyl)-1,4,5,6,7,8-hexahydro-
nitrogen stream using an electric furnace. The furnace was 2,7,7-trimethyl-5-oxoquinoline-3-carboxylate (Table 2, Entry
1
heated up to the desired temperature in 30 min, and cooled 13). M.p.: 180-181 °C; H NMR (DMSO-d6): δ= 0.8 (s, 3H), 1.19
down to room temperature naturally after heat treatment.
(t, 3H), 1.9 (dd, 1H), 1.99 (s, 3H), 2.1 (dd, 1H), 2.2 (s, 3H), 2.3
(dd, 1H), 2.4 (dd, 1H), 3.9 (q, 2H), 5.12 (s, 1H), 7.05 (dd, 1H),
7.13 (dd, 1H), 7.25 (dd, 1H), 9.1 (s, 1H) ppm; 13C NMR (DMSO-
d6): δ= 14.1, 18.2, 26.4, 29.0, 31.9, 34.5, 50.2, 58.9, 103.0,
109.4, 113.8, 115.6, 132.5, 141.6, 145.2, 149.7, 158.6, 161.0,
166.6, 193.9 ppm; IR (neat): 3270, 2902, 1780, 1670, 1604,
1060, 826, 890 cm–1; Mass (m/z): 392 (M+1)+.
Catalyst characterization
X-ray powder diffraction measurements were done on a
Model PW 3710 / PW 1050 Bragg-Brentano diffractometer
using Cu-Kα radiation (λ = 1.541862 Å). Raman spectra were
recorded on a HORIBA JobinYvonLabRAM HR confocal Raman
microscope using He-Ne excitation (632 nm) and a laser power
of 0.1 mW. BET specific surface area was determined using the
volumetric method and nitrogen gas at liquid nitrogen
temperature, using an ASDI RXM-100 catalyst characterization
instrument. Samples were pre-treated in vacuum at 300 °C for
2 hours. TG measurements were performed on a modified
Perkin-Elmer TGS-2 thermo balance. Sample in a platinum
sample pan was heated at 10 °C min-1 up to 900 °C in a
nitrogen atmosphere. Scanning electron microscopy (SEM) and
EDAX analysis were performed using an FEI Quanta 3D high-
resolution microscope. Resolution of the instrument was ≤1.2
nm at 30 keV accelerating voltage and in high vacuum.
Spectrophotometric measurements were done using a Perkin-
Elmer UV/Vis Lambda 35 spectrometer applying a split width
of 1 nm. Cu content was measured by inductively coupled
Ethyl-4-(9H-fluoren-2-yl)-1,4,5,6,7,8-hexahydro-2,7,7-
trimethyl-5-oxoquinoline-3-carboxylate (Table 2, Entry 15)
.
M.p.: 224-225 °C; 1H NMR (DMSO-d6) δ= 0.9 (s, 3H), 1.0 (s, 3H),
1.19 (s, 3H), 1.98 (dd, 1H), 2.2 (dd, 1H), 2.3 (s, 3H), 2.5 (dd, 1H),
2.6 (dd, 1H), 3.8 (q, 2H), 4.0 (s, 2H), 4.9 (s, 1H), 7.19-7.35 (m,
4H), 7.5 (dd, 1H), 7.6-7.8 (dd, 2H), 9.1 (s, 1H) ppm; 13C NMR
(DMSO-d6): δ= 14.1, 18.3, 26.4, 29.1, 32.1, 36.1, 36.3, 50.3,
59.0, 103.8, 110.1, 119.3, 119.6, 124.3, 125.0, 126.2, 126.4,
126.6, 138.8, 141.0, 142.4, 142.8, 144.8, 146.8, 149.4, 166.9,
194.3 ppm; IR (neat): 3279, 2957, 1703, 1649, 1210, 1072 cm–
1; Mass (m/z): 428 (M+1)+.
Conclusions
In conclusion, we have developed a pyrolytic method for the
synthesis of novel copper nanoparticle/carbon microsphere
(Cu-NP/C) composites from Cu-loaded iminodiacetate
plasma optical emission spectrometry (ICP-OES) using
Spectro Genesis instrument.
a
functionalized
styrene-divinylbenzene
copolymer
(ion
exchanger). The method is simple, cost-effective, and easy to
scale-up for large scale production. Microspheres have been
characterized for their phase composition and surface
morphology using various methods. Based on these
observations, microspheres obtained by carbonising the ion
exchanger at 600 °C for 4 hours have been selected for
catalytic evaluation. The selected Cu-NP/C microspheres have
demonstrated excellent catalytic activities for the reduction of
4-nitrophenol and for the one pot four component synthesis of
medicinally important hexahydroquinolines. Due to the facile
and economical synthesis, reusability, activity, separability,
and eco-friendliness, Cu-NP/C microspheres are expected to
replace expensive noble metals in certain catalytic
applications.
Reduction of 4-nitrophenol (4NP)
Two stock solutions were prepared by dissolving 4NP and
NaBH4 separately in distilled water. A quartz cuvette with an
optical path length of 1 cm was consecutively charged with the
catalyst (1, 2, or 4 mg), 1.5 cm3 0.2 mM 4NP solution (stirred
for 0 or 10 min before adding NaBH4), and 1.5 cm3 20 mM
NaBH4 solution. Concentrations of 4NP and NaBH4 in the initial
solution were 0.1 mM and 10 mM, respectively. The reaction
mixture was stirred and monitored by UV/Vis spectroscopy
following the absorption band of 4NP anion at 400 nm. The
stirring was stopped while recording the UV/Vis spectrum.
General procedure for the synthesis of Hantzsch’s
hexahydroquinoline derivatives
A mixture of aldehyde (2 mmol), dimedone (2 mmol), ethyl
acetoacetate (2.4 mmol), ammonium acetate (3 mmol) and
Cu-NP/C catalyst (20 mg) in ethanol (5 mL) was refluxed for 2
h. The progress of the reaction was monitored by TLC (30%
ethyl acetate: n-hexane). After completion of the reaction, the
reaction mass was diluted with hot ethanol (10 mL) and
filtered off to separate catalyst. The filtrate was concentrated
on rotary evaporator to obtain the crude product which was
purified by recrystallization from ethanol to afford the
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank T. Váczi, I. Kovács, G. Varga, Sz. Klébert and Z. May
for their assistance in instrumental measurements.
6 | J. Name., 2012, 00, 1-3
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New Journal of Chemistry
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Table of content
Carbon microspheres-supported copper nanoparticles were synthesized, characterized, and applied
for the reduction of 4-nitrophenol and for the synthesis of hexahydroquinolines.