A PLA-TiO2 particle brush as a novel support for CuNPs: A catalyst for the fast sequential reduction and: N -arylation of nitroarenes

Article abstract of DOI:10.1039/c6nj04007a

The present study describes the synthesis and characterization of dispersible copper nanoparticles on a PLA-TiO₂ particle brush and their catalytic efficiency in conducting sequential reactions. The prepared CuNP@PLA-TiO₂ was characterized using various techniques like SEM, TEM, EDAX, FT-IR, TGA, XRD and UV-vis spectroscopy. The support controlled the size of the nanoparticles between 2-6 nm and prevented their agglomeration during catalysis. A zeta potential of -70.3 mV was measured for the dispersed catalyst, which indicated its high stability in solution. The nanoparticles were effectively used for the synthesis of biphenyl amines from nitro arenes by sequentially carrying out reduction and N-arylation in a single pot. The short reaction time of N-arylation and recyclability are the hallmark of the developed catalytic system.

Full text of DOI:10.1039/c6nj04007a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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PLA-TiO2 particle brush as novel support for CuNPs: A catalyst for fast
sequential reduction and N-arylation of nitroarenes
Nirav Barota, Tauhid Shaikha, Harjinder Kaur*
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad, India E-mail:
a
hk_ss_in@yahoo.com, Fax: +91 79 26308545; Tel: +91 79 26300969 * corresponding author
both researchers have contributed equally
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Abstract
The present study describes the synthesis and characterization of dispersible copper
nanoparticles on PLA-TiO2 particle brush and its catalytic efficiency in conducting sequential
reactions. The prepared CuNPs@ PLA-TiO2 was characterized by various techniques like SEM,
TEM, EDAX, FT-IR, TGA, XRD and UV-Vis spectroscopy. The support controlled the size of
nanoparticles between 2-6 nm and prevented their agglomeration during catalysis. Zeta potential
of -70.3 mV was measured for the dispersed catalyst which indicated its high stability in
solution. The nanoparticles were effectively used for the synthesis of biphenyl amines from nitro
arenes by sequentially carrying out reduction and N-arylation in a single pot. Short reaction time
of N-arylation and recyclability are the hallmark of developed catalytic system.
Keywords: Copper nanoparticles, PLA-TiO2 particle brush, Catalytic activity, Secondary
aromatic amines, Sequential reduction and N-arylation of nitroarenes, microwave heating
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1. Introduction
Catalysis of various organic transformations in industry is of fundamental importance in order to
minimize waste and to avoid toxic chemicals. Usage of nanoparticles in organic synthesis in the
past few decades have made noteworthy contributions in this direction. Among transition metals
copper was first to be used in coupling reactions as it is essential to all life forms and is generally
harmless. But, due to its poor recyclability compared to palladium its application as
heterogeneous catalyst decreased. With the advent of nanotechnology and greater understanding
of means of particle stabilization, interest in copper as catalyst has increased once again. Copper
nanoparticles are being used for various organic reactions. Copper at nano-scale is highly
reactive and difficult to stabilize as large number of active sites compared to bulk material make
them agglomerate and oxidizes quickly. Thus, stabilizing copper nanoparticles is a challenging
area of research.1-3 Due to high catalytic activity of nanoparticles, heterogenization of
nanoparticles by suitable support can enhance sustainability of the resources by facilitating their
recovery and reuse.4-7 The choice of the support material is one of the most important factors
affecting physicochemical behavior of the metal nanoparticles.8 Both, thermal and chemical
stability the nanoparticles has been enhanced by stabilizing them onto various inorganic and
organic supports. Of late synthesis of inorganic-polymer hybrid material as support for
nanoparticles have emerged as an immense research area to investigate structure-property
relationship at nanoscale and to obtain material with potential applications in photonics,
catalysis, energy, drug delivery etc.9-13 One such hybrid support is polymer-metal oxide particle
brush which offers large surface area for the stabilization of catalysts and encapsulation of
drugs.14-17 This hybrid material can be synthesized by grafting polymers on the surface of
inorganic nanoparticles. Literature studies showed that metal oxides such as silica18, ferrite19,20
,
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TiO221 etc. have been grafted by various polymers for stabilizing them or to achieve some special
applications.
In nature most of the reactions occur sequentially, in the same reaction medium. In
industry also, one pot reactions that can be carried out by successively adding reactants are very
important as they enhance efficiency. In context of sustainability and green synthesis, these
protocols are very important as lengthy separation and purification of intermediates can be
avoided. These are especially useful when intermediates are sensitive and may decompose. A
number of requirements need to be fulfilled to development such protocols. The reactions should
occur in a common solvent which require optimization if reactants of different steps have
different polarity. In case of heterogeneous catalysis supporting material should be such that
reactants with different polarities have accessibility to catalytic sites. With this aim in our mind,
we decided to synthesize a dispersible yet filterable nano-catalyst and used it for synthesis of
diphenyl amines from nitro amines in a sequential reaction to avoid isolation of intermediate
primary amines. A TiO2-polylactic acid (TiO2-PLA) particle brush was synthesized and used as
support for copper nanoparticles. PLA was chosen as it is derived from bioresources and TiO2 is
a semiconductor which can strongly attach to PLA through co-ordination of surface Ti atoms
with free carboxylate group of latter22. PLA fibers were grown on TiO2 surface by a method
developed in our lab which involved polycondensation of lactic acid assisted by azeotropic
separation of water23. The assembly thus synthesized was used to stabilize copper nanoparticles
(CuNPs). The prepared CuNPs@PLA-TiO2 catalyst was characterized by various instrumental
techniques viz. UV-visible spectroscopy, TEM-EDAX, SEM, XRD, FT-IR, TGA and AAS etc.
and employed to study reduction of nitro arenes and N-arylation of aromatic amines. All the
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reaction parameters for the two reactions were optimized individually, prior to setting up a one
pot sequential reaction for the direct synthesis of secondary amines from nitro amines.
2. Experimental
2.1. Materials and instruments
Chemicals used were of the analytical grade or of the highest purity available. Copper
nitrate, aryl halides, aryl boronic acid and aryl amines were purchased from Aldrich or Merck.
Ethyl acetate, methanol, NaBH4 and K2CO3 were purchased from Finar Chemicals and hydrazine
hydrate was purchased from Ranbaxy Laboratories Ltd. Water used in all experiments was
purified by Millipore-Q system. Freshly prepared 3:1 HCl/HNO3 (aqua regia) was used to clean
all glassware thoroughly cleaned before use.
UV–Visible absorption spectra were acquired on a Jasco V-570 UV–visible spectrophotometer.
JEOL JEM 2100 instrument and were used for particle size measurement through High
resolution transmission electron microscopy (HR-TEM) as well as Energy dispersive atomic X
ray analysis (EDAX). A drop of methanol with dispersed catalyst was placed onto a 200 mesh
carbon coated copper grid. The water was evaporated and particle size measurement was done.
The powder XRD of the catalyst was recorded on Brucker D2 Phaser using Cu Kα radiation (λ =
1.54184 Å) and a filter of nickel at 30 kV and 10 mA with step size 0.03 and count time 0.1s.
Scanning Electron microscopy analysis (SEM) was carried out through JSM 6100 (JEOL) with
acceleration voltage range 0.3 to 30 kV and working distance of 6 to 48 mm. Atomic absorption
spectroscopy (AAS) measurements were carried out on an Atomic absorption spectrophotometer
SL-243 Elico instrument: Voltage 619 V, nebulizer flow 7 ml / min, wavelength 324.8 nm, slit
width 0.5 nm, current 5.0 mA, air-acetylene flame. IR spectra were recorded on Bruker FT-IR
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spectrophotometer using KBr pellets. The disc containing 1 mg of sample was scanned within a
range of 600 to 3600 cm-1. Zeta potential and particle size measurement were carried out on
1
Malvern Zetasizer Nano Z. H NMR spectra were measured in Bruker AV 400 MHz using
CDCl3 as solvent and TMS as internal standard. Mass spectra were recorded by injecting the
samples in ESI ionizer of Shimadzu LCMS 2010 with single quadruple analyzer.
Synthesis of TiO2 nanoparticles and TiO2/Poly lactic acid (PLA) nanocomposites are discussed
in Electronic supplementary information (ESI).
2.2. Synthesis of CuNPs@PLA- TiO2 nanocomposites
Into a 100 ml RBF TiO2-PLA nanocomposites (1.0 g) was stirred in 20 mL deionized water. A
solution of CuNO3.3H2O (0.241 g, 1.0 mmol) in 20 ml water was added. After 10 min, reducing
agent hydrazine hydrate (0.1 g, 2.0 mmol) was added in one portion. The stirring was continued
for 4 h. The resulting colloidal solution was centrifuged and washed with deionized water. The
catalyst was re-dispersed in 10 mL water and stored under inert conditions.
2.3. General procedure for reduction of nitroarenes
Into a 25 ml RBF, nitrobenzene (1mmol) was dissolved in a 5 ml (4:1) methanol/water mixture,
and 100 mg catalyst (CuNPs@PLA-TiO2) was added followed by NaBH4 (5 mmol). The
reaction mixture was vigorously stirred at room temperature.
2.4. General procedure for N-arylation of nitroarenes
Into a 25 ml RBF, Aryl halide (1mmol), K2CO3 (3 mmol) and aryl amine (1.2 mmol) were
dissolved in a 5 ml (4:1) methanol/water mixture, catalyst (100 mg CuNPs@PLA-TiO2) was
added. The reaction mixture was heated at reflux temperature for 6 hours.
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2.5. General procedure for Sequential reduction and N-arylation of nitroarenes
2.5.1. Microwave method
Into a 10 ml vial, nitrobenzene (1mmol), 100 mg CuNPs@PLA-TiO2 were taken in 5 ml (4:1)
methanol:water mixture and NaBH4 (5 mmol) was added. The reaction mixture was vigorously
stirred at room temperature. After 30 min reaction was monitored by TLC to check completion
of reduction and aryl halide (1 mmol) or aryl boronic acid (1 mmol), K2CO3 (3 mmol) were
added and the reaction mixture was heated in a CEM microwave (100 °C, 50 W) for 30 min.
2.5.2. Conventional method
In a 25 ml RBF, nitrobenzene (1mmol), 100 mg catalyst (CuNPs@PLA-TiO2) were taken in a 5
ml (4:1) methanol/water mixture followed by addition of NaBH4 (5 mmol). The reaction mixture
was vigorously stirred at room temperature. After 30 min, aryl halide (1 mmol) or aryl boronic
acid (1 mmol) along with K2CO3 (3 mmol) were added. The reaction mixture was heated at
reflux temperature for 6-8 hours.
The progress of all the reactions was monitored by TLC. After completion of reaction, the
reaction mixture was centrifuged. The catalyst was separated and washed with methanol and re-
dispersed in water. Washings along with the filtrate were combined and methanol was distilled
off using rotary evaporator. After a work up with dichloromethane and water, the organic layer
was separated, dried over anhydrous Na2SO4 and the solvent removed under reduced pressure to
get the crude product. The crude products were purified by column chromatography (ethyl
acetate: hexane) to afford the pure products which gave satisfactory mass and NMR spectral data
[see ESI].
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3. Result and discussion
3.1. Characterization of CuNPs@PLA-TiO2 nanocomposites
Support plays a vital role in the stabilization and catalytic efficiency of nanoparticles. Copper
nanoparticles have been supported on many conventional inorganic/organic supports such as
24
Al2O3, alumina/gallium oxide,25 Ferrite,26-28 silica,29,30 PEG,31 PVP,32 CMC,33 PVC,34
polystyrene composites35 etc. To the best of our knowledge PLA-TiO2 assembly has never been
used for stabilizing copper nanoparticles. Though, in a report by Kaur et al., copper nanoparticles
decorated onto polyaniline-zeolite hybrid material have been used to prepare hydrazine sensor36.
In another report, palladium nanoparticles have been synthesized on PLA grafted carbon
nanotubes and used to study C- C coupling reactions37. In the present studies CuNPs synthesized
on PLA-TiO2 particle brush were synthesized and analyzed extensively by various instrumental
techniques and the results are discussed below.
Size of nanoparticles is the most crucial requirement for its catalytic activity and
efficiency. Surface plasmon resonance (SPR) absorption is a phenomenon which is characteristic
of metal particles at nanoscale only. This can be easily observed by UV-visible
spectrophotometer and used to estimate size of nanoparticles38 as the absorption maximum is
size specific according to Mie theory, although; absorption maxima may vary depending upon
the nature of support.39 In the present case characteristic SPR was observed at 582 nm and hence,
stepwise synthesis of CuNPs was conveniently monitored by UV-visible spectroscopy (Fig. 1). A
series of CuNPs were prepared by varying the precursor to support ratio and the results are
shown in figure 2. Interestingly, the value of SPR band was not drastically shifted with the
change in the precursor to nanocomposite ratio, though, the intensity of the peak was found to
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increase with increasing metal percentage. This indicated that average particle size remained
constant even when the quantity of support increased. This may be due to the brush like
architecture of nanocomposite that controls the size rather than its quantity. Standard catalyst
prepared by the method given in section 2.4 was used for all other analysis and for the catalysis.
AAS analysis of the catalyst showed 0.85 mmol of copper per gram of catalyst.
Fig. 1. Stepwise study of CuNPs synthesis by UV-Visible spectroscopy a) PLA-TiO2
nanocomposites b) after addition of copper nitrate c) immediately after addition of hydrazine
hydrate d) after 4 hrs.
Fig. 2. Effect of variation in metal precursor concentration on SPR
High resolution tunneling electron microscopy (HR-TEM) is another powerful tool to study the
size and shape of nanoparticles. TEM pictures of catalyst dispersed in water were taken and the
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results are shown in figure 3. Average diameter of CuNPs was 2-6 nm and they were mostly
spherical in shape. The TEM pictures of PLA-TiO2 were also taken which showed average
diameter of TiO2 nanoparticles was 1-2 nm (Fig. S2, ESI). A typical EDX analysis (Fig. S3, ESI)
showed only Cu and Ti metal peaks in the catalyst while absence of any other peaks negated the
possibility of other metallic impurities. The nature of spectra was found to be similar to the
earlier reported studies.40,41
.
Fig. 3. HRTEM- ED studies of CuNPs @PLA-TiO2
Fig. 4. XRD spectrum of CuNPs @PLA-TiO2
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XRD analysis of the catalyst was carried out to analyze its constituents. The results shown in
figure 4 clearly indicated that copper was in zero oxidation state. The peaks at 43.2033 (111),
50.4025 (200) and 74.0049 (220) were assigned to copper nanoparticles (JCPDS No. 04-0836)
and correspond to the FCC lattice of copper (0). Average size of CuNPs calculated from the
Debye-Scherrer equation was 4.54 nm from the reflection at 2θ = 43.2033 which is close to
results obtained from HR-TEM analysis. Two other peaks were also observed. The one at
25.0382 (101) (JCPDS No. 21-1272.1) was assigned to the anatase phase of TiO242 and the other
at 16.5632 was characteristic of PLA.43 No other peaks for TiO2 could be detected due to its very
low concentration.
Fig. 5. SEM images of CuNPs @PLA-TiO2
We tried to understand the morphology of the catalyst by Scanning electron microscopy
(SEM). It showed spherical globules intertwined with each other. The reason became clear from
particle size analysis (an average of three readings, see Figure S6, ESI) carried out on Malvern
particles size analyzer. The average diameter of PLA-TiO2 support when dispersed in water was
found to be 55 to 85 nm whereas after impregnation with the CuNPs the average size of catalyst
at half width was found to be 500 to 830 nm. This increase in the particle diameter was attributed
to close association of PLA-TiO2 brushes due to electrostatic interactions between the copper
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nanoparticles and the ester linkages of PLA chain of same or different brush. This resulted in a
network (figure 5) in which copper nanoparticles were dispersed. Zeta potential is a measure of
the magnitude of the electrostatic or charge repulsion/attraction between particles, and is one of
the fundamental parameters known to estimate stability of colloidal dispersion. Zeta potential
measurement showed a potential of -30.1 mV for water dispersion of PLA-TiO2 (Fig. S4) which
indicated very high stability of support. Whereas, for the copper loaded system (CuNPs@ PLA-
TiO2) zetapotential was found to be -70.3 mV (Fig. S5). These studies indicated that though
some agglomeration of support (PLA-TiO2) which itself was in nanometric scale has taken place,
the overall stability of the catalytic system in water is good.
3.2.Sequential reductive N-arylation of nitroarenes
Preparation of secondary aryl amines is of great importance as they are widely used in
agrochemicals, pharmaceuticals, conducting polymers, dyes, intermediates etc. Ullmann type C-
N coupling of aryl halides with aryl amines is generally used to get the desired product using
copper as catalyst. A number of reports of homogeneous and heterogeneous copper catalysts
used for C-N coupling have been reported in literature.44-46 Aromatic amines required for this
step are themselves synthesized by reduction of nitroarenes. Separation and isolation of amines is
always difficult and tedious as they are susceptible to oxidation under atmospheric conditions.
As reduction of nitroarenes has been reported by copper nanoparticles by many workers, we
decided to carry out the two steps viz. reduction of nitro arene and N-arylation of resulting aryl
amine in sequential manner. But before proceeding to sequential studies, parameters for
individual reactions were optimized.
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3.2.1 Reduction of nitroarenes
Scheme 1 Reduction of various nitroarenes catalyzed by CuNPs@PLA-TiO2
A number of reducing agents have been reported for the reduction of nitroarenes usin CuNPs.
We tried three reagents i. e. ammonium formate, hydrazine hydrate and sodium borohydride. No
conversion was observed with ammonium formate whereas; hydrazine hydrate was effective
under reflux conditions. However, complete conversion was observed with sodium borohydride
at room temperature and the results are shown in table 1. Our earlier studies have shown that
methanol/water is a good solvent for reduction of nitro compounds48 as well as C-N coupling of
aryl amines47 hence, no other solvent was tried. However, we optimized the ratio of
methanol/water for the best yield. From the results reported (Figure S10, ESI) it can be
concluded that complete conversion of all nitroarenes can be obtained in 4:1 methanol:water. It
has been observed that presence of a small amount of water promotes solubility of base as well
as efficiency of the catalyst especially in coupling reactions.
Table 1 Reduction of nitroarenes under various conditions
Temp.
(°C)
Yielda
(%)
Entry
Reducing agent
time
1
2
HCOONH4 (5 mmol) reflux
24 h trace
24 h trace
N2H4.H2O (3 mmol)
N2H4.H2O (3 mmol)
r.t.
3
reflux
24 h
100
30
min
4
NaBH4 (5 mmol)
r.t.
100
aNitroarene (1 mmol), 0.1 g catalyst, 5 ml (methanol/water 4:1).
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The scope of the reaction was explored by reducing a number of substituted nitroarenes. Most
reactants were reduced completely to corresponding amines within 30 minutes (Table 2) with
high yield. Ortho substituted nitroarenes such as 2-nirto phenol and 2-nitro aniline were reduced
at slower rate due to steric hindrance generating less effective resonance stabilization in
comparison to para substituented (Table 2, entries 9, 10). In case of 4-nitro benzaldehyde, 4-
amino benzyl alcohol was obtained showing reduction of both the functionalities.
To understand the kinetics of the reaction, reduction of 4-nitro phenol at two different
concentrations was monitored by UV-visible spectrophotometric measurements at 35 °C.
Aqueous solution of 4-nitrophenol in presence of sodium borohydride (pH 12) clearly showed a
peak at 400 nm (λmax) due to the formation of 4-nitro phenolate.48 As the catalyst was added,
reduction began immediately. Spectra measurement at regular time interval showed a gradual
decrease in the λmax (Fig. 8). Simultaneously, a new peak appeared at 300 nm which
corresponded to the formation of 4-amino phenol. The reduction reaction was found to be pseudo
first order as the sodium borohydride was taken in excess and rate of reduction would be
independent of its concentration. The rate constant was calculated by plotting a graph of ln
(At/Ao) against time t (min) (Fig. 6).49 The calculated rate constants for the CuNP catalyzed
reaction was found to be 3.44 × 10-2 sec-1 and 1.79× 10-2 sec-1 respectively for I mmol and 2
mmol solutions. The value is close to the best reported in literature for copper catalyzed
reduction of nitrobenzene.
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Table 2 Scope of the catalyst for reduction of various nitroarenes
Entry Substrate Product T (min)
aYield (%)
100
1
2
30
30
95
90
94
96
90
3
4
5
6
30
30
30
30
7
60
98
8
9
50
55
95
97
aNitroarenes (1 mmol), NaBH4 (5 mmol), 0.1 g catalyst,
5 ml MeOH:H2O (4:1), 40 °C, 30 min.
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Fig. 6. Time dependent UV-Visible spectra and ln (At/A0) versus t for reduction of 4-nitrophenol
at different conc. A) 2 mmol. B) 1 mmol
3. Detailed study of C-N cross coupling reaction
Scheme 2 CuNPs@PLA-TiO2 catalyzed C-N cross coupling reaction
N-arylation of aryl amines is widely used process in agrochemical, pharmaceutical, dye and
polymer industry. Palladium catalysed Buchwald coupling is the most popular protocol to carry
this process. However, of late development of environmentally benign methods for N-arylation
or C-N coupling using copper nanoparticles in the absence of ligands have attracted the attention
of various researchers.50 N-arylation has of aryl amines have been reported with haloarenes as
well as with arylboronic acids. In the present case, the reaction between aryl halides and aryl
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amine catalyzed by CuNPs@PLA-TiO2 yielded diphenyl amine as the sole product whereas, N-
arylation with phenylboronic acid gave substantial amount of biphenyl (20 %) due to
homocoupling. Subsequently, all other reactions were tried with various aryl halides only.
methanol-water mixture in 4:1 ratio was used as solvent and K2CO3 as base and it gave excellent
results. The effect of catalyst concentration on N-arylation was examined in detail. The best yield
for standard reaction between iodobenzene and aniline was obtained with 100 mg of catalyst
quantity in 6 h under conventional heating and 30 min in closed vessel under microwave heating
in good to high yields.
Table 3 Effect of catalyst conc. on N-arylation of aniline
Entry Catalyst
Copper Base
(mol %)
Yield
(%)
1
2
3
4
100 mg PLA-TiO2
-
K2CO3 trace
25 mg catalyst
50 mg catalyst
100 mg catalyst
1.98 K2CO3
4.0 K2CO3
8.232 K2CO3
35
60
100
Iodobenzene (1 mmol), aniline (1.2 mmol), 0.1 g catalyst,
K2CO3 (3 mmol), 5 ml (4:1) MeOH: H2O, reflux temp., 6 h.
4. Sequential reductive N-arylation of nitroarenes
After optimizing the individual steps we carried out the two steps in sequential manner. First
reduction was carried out at 40 °C and after completion of this step (30 – 60 min) aryl halides
along with the base were added and the reaction continued under microwave (30 min) or
conventional heating (6 h). The general applicability of the protocol was studied by taking a
number of different aryl halides and their amination was carried out. The products were
confirmed by their NMR and Electron spray mass ionization data (see ESI). The yield and the
time required for various reactions is shown in table 5. The reaction with iodo and bromo
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benzene went smoothly (Table 5, entry 1). However, chloro benzene required longer time.
Compounds containing electron donating as well as electron withdrawing groups were tried and
corresponding coupled products were obtained in good yield (Table 4, entries 2, 3). In case of 4-
nitro aryl halides 4-nitroanisole was obtained as major product which indicates ring activation
towards methoxylation rather than amination. Replacing methanol with DMSO as solvent gave
the corresponding amine products (Table 4, entry 8).
Table 4 N-arylation of various aryl halides and various aryl amines
Entry
R1
X
R2
Micro Conventional
wave
Yield (%)
1
2
3
4
5
6
7
8
9
H
I
H
H
H
H
H
H
H
H
H
80
75
73
75
70
70
59
30c
65
98
92
85
94
86
83
70
30
70
4 -CH3
4 –OCH3
H
I
I
Br
Br
Br
Cl
Cl
4 -CH3
4 –OCH3
H
4 –NO2
4 –NO2, 2- Cl
OCH3
10
H
I
2-
63
87
aminopyr
imidine
aryl halide(1 mmol), aryl amine(1 mmol), CuNPs@PLA-
TiO2(0.1g) K2CO3 (3 mmol), 5 ml (4:1) MeOH: H2O
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3.5 Stability and reusability of the catalyst
In order to check the stability of CuNPs dispersed in water, the colloidal solution was analyzed
by UV-Vis spectroscopy at regular interval of times over a period of 28 days and the results are
shown in figure 7. It can be seen that there was no significant shift in the absorption maxima of
the SPR band of CuNPs. The catalyst was found to be stable even after the reaction. Even FT-IR
spectrum of fresh and used catalyst does not show any change. (Fig. S11, S12, ESI) indicating
that the composition of catalyst is unchanged.
Fig. 7. UV-visible spectra of catalyst dispersion taken over a period of 28 days and its reusability
The recyclability of the catalyst was investigated for the N-arylation reaction of iodobenzene and
aniline. After completion of reaction, the catalyst was removed from the reaction mixture by
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centrifugation washed with methanol and reused after adding fresh amount of reagents. The
recovered catalyst was reused several times without any significant loss of activity (Fig. 7).
4. Conclusion
In summary, grafting of PLA on TiO2 can provide a environmentally friendly support for
synthesizing CuNPs with small size variation of 2 – 6 nm. The support enhanced the stability and
dispersability of CuNPs in water. The synthesized nanoparticles showed excellent catalytic
activity and recyclability for reductive amination of nitroarenes which was carried out in
sequential manner under conventional and microwave heating in a benign mixed solvent system.
Acknowledgement
The authors are grateful to the DST nanomission for the financial support, UGC, New Delhi for
the BSR fellowship to Nirav Barot and MANF to Tauhid Shaikh, CSMCRI, Bhavnagar, for the
1
TEM-EDAX analysis, SAIF, Panjab University, Chandigarh, for SEM and H NMR, Charotar
University of Science and Technology, Changa, for XRD analysis.
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One pot synthesis of diarylamines from nitro arenes by sequential reduction and n-arylation
6x3mm (300 x 300 DPI)