Robust and economic reduction protocol employing immensely stable and leach-proof magnetically separable nanocomposites

Article abstract of DOI:10.1039/c6ra17387j

Magnetically recoverable nanocomposites i.e. metal loaded over modified ferrite nanoparticles have been synthesized via a facile three step pathway. Modification of ferrite nanoparticles which serve as the core has been achieved using dopamine hydrogen chloride. Owing to this, introduction of terminal amine groups on the surface of ferrite nanoparticles takes place which provides binding sites for the stabilization of metal nanoparticles providing leach-proof nanocomposites. The synthesized nanocomposites have been characterized using various characterization techniques. Substantiation of the modification of the pristine magnetic ferrite nano particles has been done from the emergence of strong stretching vibration bands of N-H and O-H in the range of 3200-3400 cm^-1, N-H stretching band in the range of 1630-1650 cm^-1, C-C vibrations of the benzene ring in the range of 1480-1500 cm^-1 and C-O stretching vibrations in the range of 1070-1100 cm^-1. In the XRD patterns additional peaks corresponding to loaded metals (Cu and Ag) along with peaks corresponding to spinel ferrites have been observed confirming the successful formation of the composite. EDS patterns and FE-SEM elemental mapping confirmed the purity of the samples by displaying the absence of any impurity. Elemental mapping also confirmed the uniform binding of the loaded metals over the surface of modified ferrite nanoparticles. Catalytic efficiency of the synthesized nanocomposites has been explored for the reduction of nitroarenes. Both the Cu and Ag loaded samples exhibited excellent activity and efficient recyclability for the reduction of nitroarenes in the presence of NaBH₄ as a reducing agent.

Full text of DOI:10.1039/c6ra17387j

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Robust and economic reduction protocol
employing immensely stable and leach-proof
Cite this: RSC Adv., 2016, 6, 91275
magnetically separable nanocomposites†
*
Ankita Goyal and Sonal Singhal
Magnetically recoverable nanocomposites i.e. metal loaded over modified ferrite nanoparticles have
been synthesized via a facile three step pathway. Modification of ferrite nanoparticles which serve as
the core has been achieved using dopamine hydrogen chloride. Owing to this, introduction of
terminal amine groups on the surface of ferrite nanoparticles takes place which provides binding
sites for the stabilization of metal nanoparticles providing leach-proof nanocomposites. The
synthesized nanocomposites have been characterized using various characterization techniques.
Substantiation of the modification of the pristine magnetic ferrite nano particles has been done from
the emergence of strong stretching vibration bands of N–H and O–H in the range of 3200–3400
cmꢀ1, N–H stretching band in the range of 1630–1650 cmꢀ1, C–C vibrations of the benzene ring in
the range of 1480–1500 cmꢀ1 and C–O stretching vibrations in the range of 1070–1100 cmꢀ1. In the
XRD patterns additional peaks corresponding to loaded metals (Cu and Ag) along with peaks
corresponding to spinel ferrites have been observed confirming the successful formation of the
composite. EDS patterns and FE-SEM elemental mapping confirmed the purity of the samples by
displaying the absence of any impurity. Elemental mapping also confirmed the uniform binding of the
loaded metals over the surface of modified ferrite nanoparticles. Catalytic efficiency of the
synthesized nanocomposites has been explored for the reduction of nitroarenes. Both the Cu and Ag
loaded samples exhibited excellent activity and efficient recyclability for the reduction of nitroarenes
in the presence of NaBH4 as a reducing agent.
Received 7th July 2016
Accepted 19th September 2016
DOI: 10.1039/c6ra17387j
www.rsc.org/advances
reactions. Dispersion and stabilization of active species are two
crucial factors affecting the catalytic efficiency of metal-based
nanocatalysts.3 Metal nanoparticles have a strong tendency to
agglomerate easily, leading to decrease in surface area and
hence degradation of catalytic performance and reduction of
lifetime of the catalyst. It is very hard to achieve good dispersion
and stabilization in case of bare metal nanoparticles. So it is
highly required and quite interesting to efficaciously protect
and stabilize these nanoparticles to be effectively used as
regenerable catalyst.4
To arrest the agglomeration of metal nanoparticles variety
of surfactants and ligands have been used by researchers.5–9
The use of surfactants and ligands have undoubtedly solved
the problem of aggregation and dispersion but, also,
degraded the catalytic ability by passivating the catalytic sites.
The emergence of core–shell nanostructures have provided
a unique and useful way-out to protect the catalytic properties
of these metal nanoparticles by proposing the idea of affixing
them over some core materials.10 In this context, the use of
some rigid inorganic materials having high thermal stability
and chemical inertness, as a core can be of great use. Also the
separation of catalyst from the product stream has always
remained an issue for any catalytic process. Many research
1. Introduction
Now-a-days catalysis has encouraged almost all kinds of organic
and inorganic transformations. Out of the two basic catalytic
systems, heterogeneous catalysis is preferred over homoge-
neous, owing to effortless management of the reaction, facile
separation and regeneration of the catalyst.1 Despite these
advantages heterogenized catalytic reactions are comparatively
slow and usually less effective in contrast to homogenized
catalytic reactions. The origin of nanocatalysis has solved the
problems associated with both the catalytic systems by bridging
the gap between homogeneous and heterogeneous catalysis.
Nanocatalysis offers advantages associated with both the cata-
lytic systems as nanocatalysts are endowed with a large surface
area to volume ratio.2
Metal based nanocatalysts have long been fascinating the
scientic community to be used as catalysts in a variety of
Department of Chemistry, Panjab University, Chandigarh, India-160014. E-mail:
sonal1174@gmail.com; sonal@pu.ac.in; Fax: +91-172-2545074; Tel: +91-172-
2534421; +91-9872118810
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra17387j
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cohorts have used ferrite nanoparticles as core materials and were bought from Fisher Scientic. Dopamine hydrochlo-
which added the advantage of facile and effective separation ride was purchased from Sigma-Aldrich. Methanol (99.5%),
of the catalyst due to their inherent magnetic character.11–14
ethyl acetate (99.5%), ethylene glycol (99%), hydrochloric acid
Conversely, the use of ferrite nanoparticles as a core material (37%) and silver nitrate (pure) were provided by Merck. Sodium
without modication or functionalization is not an effective borohydride (97%), hydrazine hydrate (75%) all the nitro-
idea due to lack of binding sites on the surface of ferrite phenols and other nitro derivatives were contributed by Avra
nanoparticles to affix metal nanoparticles.15 So prior to metal Synthesis. All the chemicals were used as such.
loading over the surface of ferrite nanoparticles it is required to
modify it which can be achieved by integrating the inorganic–
organic moieties. Many reports have been available wherein
ferrite cores have been pre-modied with various organic
functionalities to incorporate effective binding sites followed by
metal loading over the surface.16–19 Chang and Chen proposed
the synthesis of novel magnetically recoverable Au nanocatalyst
via loading of Au(III) ions on chitosan-coated iron oxide
magnetic nanocarrier. The synthesized magnetically retrievable
Au nanocatalyst was found to be quite stable with excellent
catalytic ability even aer 11 recycles.16 Chen et al. prepared Au
nanoparticles loaded over polyaniline modied ferrite (Fe3-
O4@PANI@Au) composites exhibiting excellent magnetic and
catalytic properties via a three-step procedure. The Fe3O4@-
PANI@Au composite catalyst showed excellent catalytic perfor-
mance for various concentrations of dye wastewater for the
reduction of Congo red with NaBH4.17 Well-dispersed Ag@Pd
supported on magnetite nanoparticles were obtained by Jiang
et al. through a simple colloidal impregnation method. The as-
synthesised nanocomposite exhibited greatly enhanced cata-
lytic reactivity and reusability towards 4-nitrophenol hydroge-
nation.18 Guin et al. stablized Pd nanoparticles on to the surface
of amine terminated ferrite–dopamine to be used as catalyst for
hydrogenation reactions.19 Out of the various organic func-
tionalizations, the modication ferrites with dopamine have
proved to be highly benecial as minimal decrease in magnetic
character has been observed to be there in contrast to others
due to its lower weight. Also the presence of terminal amine
groups provides efficient binding to metal nanoparticles.20
So in the present investigation Cu and Ag nanoparticles
have been stabilized on the surface of dopamine modied
cobalt ferrite nano particles. The catalytic activity of the
synthesized magnetically retrievable nanocomposites has
been evaluated for the reduction of nitroarenes. Cobalt ferrite
nanoparticles have been found to be inactive catalysts for the
reduction of nitroarenes as reported previously.21 Cobalt
ferrite nanoparticles have been used as core material for
having very good magnetic character with chemical inertness
and thermal stability. So the view point of using them as core
material to utilize their excellent magnetic character without
any interference in the catalytic performance has been
applied. Also the optimum amount of metal catalyst loaded
over ferrite core to be used as catalyst has been interestingly
scrutinized.
2.2. Physical characterization
The presence of dopamine shell over the ferrite nanoparticles
was conrmed from the observation of the presence of various
vibrational bands in the FT-IR spectrum for which iS50-FTIR
(Model no. AUP1200343) instrument with the resolution of 1
cmꢀ1 and scan range of 4000 cmꢀ1 to 400 cmꢀ1 was employed.
The knowledge of the crystallographic data and various lattice
planes present in the sample was obtained by recording the
powder X-ray diffraction patterns using the Panalytical's
X'Pert Pro diffractometer equipped with vertical theta–theta
goniometer and x'Celerator solid-state detector. Scanning rate
of 4ꢁ minꢀ1 in the 2q range of 10–80ꢁ was used for the
ꢀ
measurement. Cu K-alpha-1 radiation (l ¼ 1.54 A) was used
whereas nickel metal acted as beta lter. The visualization of
the obtained nanocomposites was done using high resolution
transmission electron microscopy (HR-TEM). For the HR-TEM
analysis FEI Tecnai (G2 F20) operating at 200 keV was used.
Surface morphology was determined using eld emission
scanning electron microscopy (FE-SEM) employing Hitachi
(SU-8010) operated at 15 kV. Elemental mapping associated
with FE-SEM was used to establish the presence of metal ions
on to the surface of ferrite–dopamine nanoparticles. To
evaluate the time dependent conversion of the nitrophenols
for the surface area analysis of the samples, BET surface area
analyzer ((11-2370) Gemini, Micromeritics, USA) was
employed. Vibrating sample magnetometer (VSM) (155, PAR)
was used for the evaluation of the magnetic characteristics of
the synthesized samples. UV-visible spectroscopy was
employed for which Jasco double beam UV-visible spectro-
photometer (Model-V-750-PC) was used. The spectrophotom-
eter was equipped with single monochromator, deuterium
and halogen lamps as light sources, photomultiplier tube
detector with a wavelength range of 190–900 nm. The reduced
products aer the reduction of nitroarenes were analyzed
using GC-MS analysis for which THERMO GC (TRACE 1300)
with a Thermo TG 5MS 5% phenyl methylpolysiloxane capil-
lary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm) and a triple
quadrupole Thermo MS (TSQ 8000) mass spectrometer was
employed.
2.3. Fabrication of metal decorated modied magnetic
nanocomposites (M@Dop@CoFe2O4)
Synthesis of M@Dop@CoFe2O4 nanocomposites was achieved
in three steps. Schematic representation of the synthetic
procedure is shown in Fig. 1.
2. Experimental
2.1. Materials
2.3.1. Synthesis of CoFe2O4 nanoparticles. Magnetic cobalt
Ferric nitrate nonahydrate (98%), cobaltous nitrate hexahydrate ferrite nanoparticles were synthesized using the well-known and
(99.5%), cupric nitrate trihydrate (99.5%) citric acid (99.57%) well explored sol–gel methodology. Sol–gel methodology has long
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Fig. 1 Schematic representation of the synthesis of metal decorated modified magnetic ferrite nanoparticles (M@Dop@CoFe2O4).
been known to produce nanoparticles with uniform and narrow surface metal atoms. To achieve such linkage, cobalt ferrite
size distribution without the adoption of complex synthetic nanoparticles (1 g) were dispersed in 20 mL of distilled
procedure and not too high temperature requirement.21 In water by sonication for 30 min. 1 g of dopamine hydrochlo-
a typical synthesis desired metal salts (Fe : Co ¼ 2 : 1) i.e. cobalt ride dissolved in minimum amount of distilled water was
nitrate, ferric nitrate and citric acid (in the ratio 1 : 1 to precursor added to the ferrite nanoparticles dispersion. The obtained
metal salts) were separately dissolved in minimal distilled water mixture was continuously sonicated for 2 hours while the
to obtain clear solutions. The obtained clear metal salt solutions temperature of the sonication bath was maintained at 25–
were then mixed together at 60 ꢁC and stirred for 5 minutes, 30 ꢁC. Aerwards the obtained modied Dop@CoFe2O4
followed by the addition of citric acid solution. Citric acid was nanoparticles were repeatedly washed with distilled water
used as complexing agent. The stirring and heating at 60–70 ꢁC and acetone and were dried under vacuum for 2 hours
was continued. Aer 10–15 minutes xed amount of ethylene at 60 C.22
ꢁ
glycol was added which promoted gelation and temperature was
2.3.3. Decoration of metal nanoparticles over modied
raised to 80–90 ꢁC. Thickening of solution took place and the cobalt ferrite (M@Dop@CoFe2O4). Decoration of metal nano-
obtained gels were further dried to get loose powder. The ob- particles over the surface of modied cobalt ferrite nano-
tained powders were then annealed in a muffle furnace for two particles was achieved by reduction. 1 g of the modied cobalt
hours.
ferrite nanoparticles were dispersed in 30 mL of distilled water
2.3.2. Modication of cobalt ferrite nanoparticles using sonication. To the obtained dispersion required amount
(Dop@CoFe2O4). The linkage of dopamine on the surface of of metal salt dissolved in 15 mL of distilled water was added and
magnetic nanoparticles is based on the chelation of the dispersion was subjected to stirring. Metal loading was varied
hydroxyl groups of dopamine with the under-coordinated from 2.5 wt% to 20 wt%. Hydrazine hydrate was then added
Fig. 2 FT-IR spectra for synthesized nanocomposites (M@Dop@CoFe2O4) M ¼ (a) Cu and (b) Ag with different metal loadings from 2.5 wt% to 20
wt%.
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Fig. 3 XRD patterns for synthesized nanocomposites (M@Dop@CoFe2O4) M ¼ (a) Cu and (b) Ag with different metal loadings from 2.5 wt% to 20
wt%.
dropwise to adjust the pH of the solution to 9. Aer the pH nanocomposites were then thoroughly washed with distilled
adjustment 0.3 g of NaBH4 was added and stirring was water and acetone and dried under vacuum at 60 ꢁC for 2
continued for 24 hours at room temperature. The obtained hours.22
Fig. 4 Typical transmission electron micrographs for 10 wt% Cu@Dop@CoFe2O4 (a) and (b) at low resolution (c) with visible interplanar spacing
at high resolution (d) SAED pattern with corresponding electron diffraction rings and (e) EDS pattern for elemental composition.
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2.4. Catalytic procedure for the reduction of nitrophenols
3. Results and discussion
Reduction of nitrophenols was performed in aqueous medium.
In a typical catalytic procedure nitrophenol solution was taken
in a beaker and kept for magnetic stirring. Freshly prepared
sodium borohydride solution was then added which changed
the colour from pale yellow to dark yellow indicating the
formation of nitro-phenolate ion. The catalytic reaction started
at once with the addition of catalyst and continuous fading of
dark yellow colour started to occur indicating the conversion of
nitrophenol to subsequent aminophenol. The time dependent
conversion was checked using UV-visible spectroscopy by taking
out xed amount of sample aliquots at different time intervals
and diluting the same for absorbance measurements.
3.1. FT-IR spectroscopy
To investigate the successful synthesis of metal loaded modied
magnetic nanocomposites (M@Dop@CoFe2O4) the synthesized
samples were inspected using FT-IR spectroscopy. FT-IR spectra
for all the synthesized nanocomposite samples with variable
metal loading are shown in Fig. 2(a) and (b). Ferrite nano-
particles which serve the core of synthesized nanocomposites
conrmed their presence by exhibiting the characteristic bands
in the range of 500–600 cmꢀ1 corresponding to the M–O cluster
present in the tetrahedral site of the ferrite sub-lattice.23 The
corroboration of the modication of the pristine magnetic nano
particles was done from the emergence of strong stretching
vibration bands of N–H and O–H in the range of 3200–3400
cmꢀ1. N–H stretching band in the range of 1630–1650 cmꢀ1
,
2.5. Catalytic procedure for the reduction of nitroarenes
C–C vibrations of the benzene ring in the range of 1480–1500
cmꢀ1 and C–O stretching vibrations in the range of 1070–1100
cmꢀ1 further conrmed the formation of modied magnetic
nano particles.24,25 As the metal loading has been varied from
2.5 wt% to 20 wt%, the increase in metal loading can also be
visualized from the lowering in the intensity of the character-
istic bands of dopamine and ferrite with increase in metal
loading for both Cu and Ag.
For the reduction of other nitroarenes methanol was used as
solvent due to the solubility issue of nitroarene derivatives in
aqueous medium. To the solution of nitroarene xed excess of
NaBH4 was added followed by the addition of catalyst. Reaction
progress was monitored by GC-MS analysis by analyzing the
samples aliquots drawn at different time intervals. Prior to GC-
MS analysis the organic content in the sample aliquot was
extracted using ethyl acetate.
Fig. 5 Typical transmission electron micrographs for 10 wt% Ag@Dop@CoFe2O4 (a) and (b) at low resolution (c) with visible interplanar spacing at
high resolution (d) SAED pattern with corresponding electron diffraction rings and (e) EDS pattern for elemental composition.
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no. 00-001-1167). Peaks corresponding to silver metal appears
at 2q (ꢁ) values of 38.10ꢁ, 44.41ꢁ, 64.17ꢁ and 77.55ꢁ corre-
sponding to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) lattice planes
respectively.28 The growth and sharpening of the peaks corre-
sponding to Ag, was also observed with increase in loading
from 2.5 wt% to 20 wt%.
3.2. Powder X-ray diffraction studies
For the illustration of the structural characteristics of the
synthesized nanocomposite samples powder X-ray diffraction
technique was employed. Fig. 3(a) and (b) shows the XRD patterns
of all the synthesized nanocomposites (M@Dop@CoFe2O4 (M ¼
Cu and Ag)) for different metal loadings.
For copper decorated modied ferrite nanocomposites as
can be seen from Fig. 3(a), all the peaks appearing in the XRD
patterns matched well with the standard patterns correspond-
ing to cobalt ferrite (JCPDS card no. 00-001-1121)26 and copper
metal (JCPDS card no. 00-001-1242). Peaks corresponding to
copper appears at 2q (ꢁ) values of 43.47ꢁ, 50.67ꢁ and 74.67ꢁ
corresponding to (1 1 1), (2 0 0) and (2 2 0) lattice planes
respectively.27 As the loading of Cu was increased from 2.5 wt%
to 20 wt% the peaks corresponding to copper metal were
observed to grow sharper and clearer as highlighted in the
gure.
3.3. Structural and morphological characteristics:
crystallinity and elemental analysis
Structural and morphological characteristics of the synthe-
sized nanocomposites were evaluated using HR-TEM and FE-
SEM electron microscopic techniques. Fig. 4 and 5 shows
the typical HR-TEM images for Cu@Dop@CoFe2O4 and
Ag@Dop@CoFe2O4 nanocomposites loaded with 10 wt% of
metal respectively. Fig. 4(a) and (b) and 5(a) and (b) are the
low resolution TEM images of the nanocomposite samples
wherein metal loaded agglomerated modied ferrite nano-
particles can be seen. It can be clearly noticed from the
images that 20–25 nm sized nanoparticles of cobalt ferrites
were trapped within the clouds of dopamine over which 4–7
nm sized metal nanoparticles were present. Fig. 4(c) and 5(c)
Similarly, for silver decorated modied ferrite nano-
composites as is shown in Fig. 3(b), all the peaks appearing in
the XRD patterns matched well with the standard patterns
corresponding to cobalt ferrite and silver metal (JCPDS card
Fig. 6 Typical FE-SEM elemental mapping for 10 wt% Cu@Dop@CoFe2O4 nanocomposite sample displaying elemental composition.
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Fig. 7 Typical FE-SEM elemental mapping for 10 wt% Ag@Dop@CoFe2O4 nanocomposite sample displaying elemental composition.
represents the high resolution images wherein lattice inter- crystallinity of the samples owing to the presence of concen-
planar distance is clearly noticeable. Fig. 4(d) and 5(d) tric rings corresponding to different lattice planes. EDS
represents the SAED patterns which conrmed the patterns for both the nanocomposite samples are shown in
Fig. 8 Room temperature hysteresis loops for synthesized nanocomposites (M@Dop@CoFe2O4) M ¼ (a) Cu and (b) Ag without any metal and
with different metal loadings from 2.5 wt% to 20 wt%.
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Table 1 The values of saturation magnetization (Ms), remanence (Mr), coercivity (Hc) and squareness ratio (S ¼ Mr/Ms) for various synthesized
samples
Saturation magnetization
Remanence (Mr)
Squareness ratio
Sample
(Ms) (emu gꢀ1
)
(emu gꢀ1
)
Coercivity (Hc) (Oe)
(S ¼ Mr/Ms) ꢂ 10ꢀ2
Pristine CoFe2O4
Dop@CoFe
2.5 wt% Cu
5 wt% Cu
7.5 wt% Cu
10 wt% Cu
20 wt% Cu
2.5 wt% Ag
5 wt% Ag
50.2
35.7
14.7
17.6
18.5
23.1
27.1
15.9
16.2
18.2
19.1
21.3
20.0
14.0
5.4
6.3
6.9
8.2
12.8
5.9
6.1
6.5
8.3
8.8
1116.0
1367.5
1156.0
1218.5
1165.5
1165.0
1723.5
1567.0
1656.0
1511.0
1874.0
1697.0
39.1
39.2
36.7
35.8
37.3
35.5
47.2
37.1
37.7
35.7
43.5
41.3
7.5 wt% Ag
10 wt% Ag
20 wt% Ag
Fig. 9 The fitting curves for the BET surface area of M@Dop@CoFe2O4 (M ¼ Cu and Ag) for different metal loadings.
Fig. 4(e) and 5(e) respectively, which conrmed the presence nanoparticles due to comparable size, so, for the verication
of desired elements in the nanocomposite sample. It is diffi- of the uniform presence of metal nanoparticles over the
cult to distinguish between the ferrite and the loaded metal surface of modied ferrite nanoparticles elemental mapping
Table 2 The values of slope (A), intercept (I), monolayer adsorbed gas quantity (Qm), BET constant (C), total surface area (Stotal), mass of
adsorbent sample (M) and specific surface area (SBET) for various M@Dop@CoFe2O4 (M ¼ Cu and Ag) for different metal loadings
Sample
A (g cmꢀ3
)
I (g cmꢀ3
)
Qm (cm3 gꢀ1
)
C
Stotal (m2)
M (g)
SBET (m2 gꢀ1
)
5 wt% Cu
10 wt% Cu
20 wt% Cu
5 wt% Ag
10 wt% Ag
20 wt% Ag
0.2962
0.0574
0.0649
0.3704
0.2893
0.2503
0.0036
0.0006
0.0003
0.0046
0.0027
0.0036
3.3354
17.2229
15.3274
2.6663
3.4241
3.9382
83.8331
14.52
75.02
66.76
11.61
14.91
17.15
0.0520
0.1838
0.1340
0.1373
0.0902
0.1095
279.40
408.17
496.02
84.590
165.36
156.67
89.7634
196.0938
80.7780
106.8769
71.0098
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Fig. 10 Typical UV-visible spectra for the reduction of ortho-, ((a) and (d)) meta- ((b) and (e)) and para-nitrophenol ((c) and (f)) in the presence of
10 mg of 10 wt% M@Dop@CoFe2O4 (M ¼ Cu and Ag) respectively.
Fig. 11 The absorbance vs. time curves for the reduction of ortho-, meta- and para-nitrophenols in the presence of varying amounts (5 mg (a–c),
10 mg (d–f) and 15 mg (g–i)) of Cu@Dop@CoFe2O4 where metal loading was varied from 2.5 wt% to 20 wt%.
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associated with FE-SEM was done. Fig. 6 and 7 display the FE- respectively. The values of saturation magnetization (Ms),
SEM elemental mapping images which conrmed the remanence (Mr), coercivity (Hc) and squareness ratio (S ¼ Mr/Ms)
uniform presence of the loaded metal nanoparticles i.e. Cu procured from the obtained hysteresis loops for all the samples
and Ag respectively. Different elements present in the samples and for pristine CoFe2O4 nanoparticles are given in Table 1.
have been represented using different colors. Also the pres- With the modication of cobalt ferrite nanoparticles with
ence of Co, Fe, O and N were observed to be there due to the dopamine hydrochloride decrease in the saturation magneti-
dopamine modied CoFe2O4 nanoparticles over which metal zation and remanence value was observed due to coating of
loading has been done. No extra element corresponding to non-magnetic dopamine over ferrite30 while coercivity was
any impurity was found to be there which conrmed the found to increase. With the decoration of metal nanoparticles
purity of the nanocomposite samples.
(M ¼ Cu and Ag) over the surface of modied ferrite, further
decrease in the values of saturation magnetization and rema-
nence was observed. With increase in metal loading in case of
both the metals, increase in the saturation magnetization and
remanace was observed to be there. This increase in the
magnetization values could be due to the interactions between
CoFe2O4 and increasing amount of metal loaded over the
surface of ferrite nanoparticles. Not any particular or signicant
increasing or decreasing trend in the values coercivity and
squareness ratio was found to be there. Although, a decrease in
saturation magnetization was there for nanocomposites in
comparison to pristine ferrite nanoparticles but still the nano-
composites were sufficiently magnetic so that they could easily
be recovered out of the reaction mixture using an external
magnet.
3.4. Magnetic characterization
Magnetic characteristics of the synthesized nanocomposites
samples were established by recording room temperature
hysteresis loops employing VSM. Cobalt ferrite nanoparticles
which serve the core of the nanocomposite samples have
previously been characterized and saturation magnetization
has been found to be 50.2 emu gꢀ1 21 Good magnetic character
.
and high thermal and chemical stability were the major reasons
behind choosing it to be used as core material.29 The room
temperature hysteresis loops for all the synthesized samples
(Dop@CoFe2O4 and M@Dop@CoFe2O4 (M ¼ Cu and Ag)) for
different metal loadings are shown in Fig. 8(a) and (b)
Fig. 12 The absorbance vs. time curves for the reduction of ortho-, meta- and para-nitrophenols in the presence of varying amounts (5 mg (a–
c), 10 mg (d–f) and 15 mg (g–i)) of Ag@Dop@CoFe2O4 where metal loading was varied from 2.5 wt% to 20 wt%.
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3.5. Surface area analysis
QmNs
V
Stotal
¼
(3)
(4)
Surface area of any heterogeneous catalyst is an important factor
which contributes towards the comparative evaluation of the
catalytic performance. Brunauer–Emmett–Teller (BET) theory was
used as the basis for the measurement of the specic surface area
of the synthesized nanocomposite samples. Using the equations
given in our previous report graphs were plotted between 1/[Q
{(P0/P) ꢀ 1}] vs. P/P0. P and P0 are the equilibrium and saturation
pressure and Q is the quantity of the gas adsorbed on the adsor-
bate.31 Typically the corresponding plots for 5 wt%, 10 wt% and 20
wt% of Cu and Ag loaded over modied cobalt ferrite nano-
particles are shown in Fig. 9. From the obtained plots the values of
slope (A) and intercept (I) were deduced which were further used
to calculate Qm (monolayer adsorbed gas quantity) and C (BET
constant) according to the following relations:
Stotal
M
SBET
¼
where, N is Avogadro's number, s (0.1620 nm2) is molecular
cross-sectional area, V is the molar volume of the adsorbate gas
(22.4 L) and M is the mass of the adsorbent sample. The value of
A, I, Qm, C, M, Stotal and SBET are given in Table 2.
From the values given in Table 2, both for Cu and Ag loaded
nanocomposites it can be clearly noticed that specic surface
area increases drastically from 5 wt% to 10 wt% and 10 wt% and
20 wt% metal loaded samples exhibited almost comparable
values. Also specic surface area values for Cu loaded nano-
composites were comparatively higher in comparison to Ag
loaded samples.
1
Qm
¼
(1)
A þ I
3.6. Catalytic studies
A
C ¼ 1 þ
(2) For the evaluation of the catalytic property of the synthesized
nanocomposites, reduction of nitroarenes was carried out.
Nitroarenes have been the precedence pollutants as these are
common byproducts from the production of pesticides, herbi-
cides, and synthetic dyes.32 Nitroarenes can be converted to
I
Further, the values of total surface area (Stotal) and specic
surface area (SBET) were calculated according to the eqn (3) and (4).
Fig. 13 The ln(At/A0) vs. time curves for the reduction of ortho-, meta- and para-nitrophenols in the presence of varying amounts (5 mg (a–c), 10
mg (d–f) and 15 mg (g–i)) of Cu@Dop@CoFe2O4 where metal loading was varied from 2.5 wt% to 20 wt%.
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corresponding amines via catalytic reduction in the presence of the UV-visible spectra it can be clearly seen that the absorption
NaBH4. Aromatic amines are very useful as these can serve as maxima corresponding to nitrophenols gradually decreased
reactants and intermediates in variety of organic trans- with the passage of time. The absorbance values were noted
formations and are an important raw material for pharmaceu- for all the successive readings and plots were plotted between
tical industry.33 Reduction of all the nitroarenes was performed absorption maxima and time for all the catalytic experiments.
in the presence of excess of NaBH4 (50 equivalents as optimum The absorbance vs. time curves for the reduction of ortho-,
amount) as reducing agent.
meta- and para-nitrophenols in the presence of varying
3.6.1. Reduction of nitrophenols. Reduction of nitro- amounts (5 mg, 10 mg and 15 mg) of M@Dop@CoFe2O4 (M ¼
phenols was performed in aqueous medium and before per- Cu and Ag) where metal loading was varied from 2.5 wt% to 20
forming the catalytic experiment with the nanocomposites, wt% are shown in Fig. 11 and 12 respectively. It can be closely
catalytic performance of bare CoFe2O4 and modied i.e. noticed from Fig. 11 and 12, that irrespective of the nature of
dopamine functionalized CoFe2O4 nanoparticles was checked the metal loaded over the surface of modied cobalt ferrite
and both were found inactive for the reduction of nitro- nanoparticles, catalytic performance effectively got improved
phenols. Further reduction of all the three nitrophenols was with increase in the metal loading from 2.5 wt% to 10 wt%
performed in the presence of synthesized nanocomposites i.e. while not much considerable augmentation in the catalytic
M@Dop@CoFe2O4 (M ¼ Cu and Ag) where metal loading was performance was observed to be there when metal loading was
varied from 2.5 wt% to 20 wt%. Further catalyst loading i.e. the raised above 10 wt%. With increase in the catalytic amount
amount of the catalyst used in the catalytic run was also varied from 5 mg to 10 mg for all the metal loadings for both the
from 5 mg to 15 mg.
nanocomposites noteworthy improvement was there in the
The typical UV-visible spectra for the reduction of ortho-, catalytic performance. While, with further increase in the
meta- and para-nitrophenol in the presence of 10 mg of 10 wt% catalytic amount from 10 mg to 15 mg not much signicant
M@Dop@CoFe2O4 (M ¼ Cu and Ag) are shown in Fig. 10. In improvement in the catalytic performance was observed to be
Fig. 14 The ln(At/A0) vs. time curves for the reduction of ortho-, meta- and para-nitrophenols in the presence of varying amounts (5 mg (a–c), 10
mg (d–f) and 15 mg (g–i)) of Cu@Dop@CoFe2O4 where metal loading was varied from 2.5 wt% to 20 wt%.
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Table 3 The values of completion times and apparent rate constants for the reduction of ortho-nitrophenola
Completion time (min) (T) and rate constant (K ꢂ 10ꢀ1
)
Metal loading (wt%)
Nanocatalyst
M@Dop@CoFe
2.5 wt%
5 wt%
7.5 wt%
10 wt%
20 wt%
Catalytic
M
amount
T
K
T
K
T
K
T
K
T
K
Cu
5 mg
8.00
3.50
2.50
16.00
5.00
4.50
3.15
7.77
9.03
2.15
6.38
5.02
4.00
2.50
2.00
6.00
4.00
3.00
9.16
2.50
2.00
1.75
4.00
3.00
2.25
10.20
14.39
17.37
7.55
10.09
12.63
1.50
1.00
1.00
2.00
1.50
1.50
17.40
25.96
27.74
15.66
18.64
18.57
1.50
1.00
1.00
2.00
1.50
1.50
17.69
26.17
27.76
16.47
19.96
19.36
10 mg
15 mg
5 mg
10 mg
15 mg
9.43
14.26
4.77
6.78
10.05
Ag
a
Reaction conditions: 2-nitrophenol: 0.72 mmol, NaBH4: 50 eq., water, 25 ꢁC, catalyst: M@Dop@CoFe2O4 (M ¼ Cu and Ag; metal loading 2.5–20
wt%; catalyst amount: 5, 10 and 15 mg).
Table 4 The values of completion times and apparent rate constants for the reduction of meta-nitrophenola
Completion time (min) (T) and rate constant (K ꢂ 10ꢀ1
)
Metal loading (wt%)
Sample
M@Dop@CoFe
2.5 wt%
5 wt%
7.5 wt%
10 wt%
20 wt%
Catalytic
M
amount
T
K
T
K
T
K
T
K
T
K
Cu
5 mg
16.00
5.00
5.50
40.00
6.00
5.50
1.53
5.46
3.94
0.67
3.82
3.00
5.50
3.00
3.50
7.50
3.50
3.50
4.76
8.51
6.13
3.14
6.82
5.13
4.00
2.50
2.25
6.00
3.00
2.25
7.44
2.00
1.50
2.00
3.00
2.50
2.00
12.63
17.87
16.80
9.12
10.19
14.11
2.00
1.50
1.50
2.50
2.50
1.50
12.78
18.89
21.25
9.38
11.24
14.94
10 mg
15 mg
5 mg
10 mg
15 mg
11.06
10.71
4.77
8.43
8.07
Ag
a
Reaction conditions: 3-nitrophenol: 0.72 mmol, NaBH4: 50 eq., water, 25 ꢁC, catalyst: M@Dop@CoFe2O4 (M ¼ Cu and Ag; metal loading 2.5–20
wt%; catalyst amount: 5, 10 and 15 mg).
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there. Also for all the cases i.e. for different metal loadings and varied from 2.5 wt% to 20 wt% are shown in Fig. 13 and 14
for varying catalytic amounts, catalytic performance of respectively.
Cu@Dop@CoFe2O4 nanocomposites was better than that of
Ag@Dop@CoFe2O4 nanocomposites.
The values of completion times and apparent rate constants
for the reduction of ortho-, meta- and para-nitrophenols in the
3.6.2. Kinetics of reduction. Reduction reactions were presence of varying amounts of M@Dop@CoFe2O4 (M ¼ Cu
performed in the presence of large excess i.e. 50 equivalents of and Ag), where metal loading was varied from 2.5 wt% to 20
NaBH4 as reducing agent, so the reaction rate could considered wt%, are given in Tables 3–5 respectively. For both the catalytic
to be solely dependent upon the concentration of the reactant systems, signicant improvement in the catalytic performance
nitrophenols.34 Owing to which the reduction reactions could with increase in metal loading from 2.5 wt% to 10 wt%
be considered to follow pseudo-rst order kinetics and follow can also be conrmed from the decreasing values of comple-
the given rate law equation
tion time and increase rate constant values. Almost same
completion time values and comparable rate constant
values were observed for 10 wt% and 20 wt% metal loaded
samples. Also lower completion times and higher rate
constant values were observed for all the Cu@Dop@CoFe2O4
nanocomposites in comparison to Ag@Dop@CoFe2O4
nanocomposites.
3.6.3. Reduction of other nitroarenes. From the above
given results obtained from the reduction of nitrophenols, 10
wt% cu loaded over the surface of modied cobalt ferrite
nanoparticles was found to be the best catalyst. To further
establish the substrate scope and versatility of the present
protocol, reduction of various chloro, amino, methyl and bromo
derivatives was carried out in the presence of methanol as
solvent. The time dependent progress of the reduction of
nitroderivatives other than nitrophenols was not associated
Ct
ln
¼ ꢀkapp
t
(5)
C0
wherein, Ct and C0 are the concentration of the reactant nitro-
phenol at any time (t) and at the start of the reaction i.e. at time
(t ¼ 0). kapp is the apparent rate constant value for pseudo-rst
order reduction reactions. As absorbance is directly propor-
tional to the concentration, the kapp values were acquired from
the slope of the straight lines obtained aer plotting ln(At/A0) vs.
time (t), where, At and A0 are the absorption maxima of the
reactant nitrophenol at any time (t) and at the start of the
reaction i.e. at time (t ¼ 0). The ln(At/A0) vs. time plots for the
reduction of ortho-, meta- and para-nitrophenols in the presence
of varying amounts (5 mg, 10 mg and 15 mg) of
M@Dop@CoFe2O4 (M ¼ Cu and Ag) where metal loading was
Table 5 The values of completion times and apparent rate constants for the reduction of para-nitrophenola
Completion time (min) (T) and rate constant (K ꢂ 10ꢀ1
)
Metal loading (wt%)
Sample
M@Dop@CoFe
2.5 wt%
5 wt%
7.5 wt%
10 wt%
20 wt%
Catalytic
M
amount
T
K
T
K
T
K
T
K
T
K
Cu
5 mg
60.00
6.00
5.00
80.00
7.00
6.00
0.68
4.78
6.90
0.45
4.17
6.14
15.00
4.00
3.50
40.00
4.50
4.00
2.51
8.06
9.95
0.98
6.80
9.11
6.00
3.00
2.50
20.00
3.50
3.00
6.25
3.00
2.00
2.00
3.50
3.00
2.50
11.71
14.21
16.48
8.88
10.40
15.02
2.50
2.00
2.00
3.50
3.00
2.50
15.38
16.96
17.08
10.75
10.52
16.84
10 mg
15 mg
5 mg
10 mg
15 mg
11.21
12.31
1.93
9.32
11.89
Ag
a
Reaction conditions: 4-nitrophenol: 0.72 mmol, NaBH4: 50 eq., water, 25 ꢁC, catalyst: M@Dop@CoFe2O4 (M ¼ Cu and Ag; metal loading 2.5–20
wt%; catalyst amount: 5, 10 and 15 mg).
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Table 6 The values of time taken for complete conversion, conversion% and selectivity% for all the nitroarenesa
Serial no.
Substrate
Product
Timeb (min)
Conversion%/selectivityb%
100/100
Spectra no.
GC-MS-1
1.
6.0
2.
3.
20.0
4.5
100/100
100/100
GC-MS-2
GC-MS-3
4.
5.
6.0
100/100
100/100
GC-MS-4
GC-MS-5
22.0
6.
6.0
100/100
GC-MS-6
7.
8.
9.0
8.0
100/100
100/100
GC-MS-7
GC-MS-8
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Table 6 (Contd.)
Serial no.
Substrate
Product
Timeb (min)
Conversion%/selectivityb%
Spectra no.
GC-MS-9
9.
4.0
100/100
10.
11.
3.0
4.0
100/100
100/100
GC-MS-10
GC-MS-11
12.
5.0
100/100
GC-MS-12
13.
14.
7.0
0.5
100/100
100/100
GC-MS-13
GC-MS-14
15.
2.0
100/100
GC-MS-15
a
Reaction conditions: nitroarene (0.72 mmol), NaBH4 (50 eq.), methanol, 25 ꢁC, catalyst: Cu@Dop@CoFe2O4; metal loading 10 wt%; catalyst
amount: 10 mg. b Completion time, conversion% and selectivity% were obtained using GC-MS analysis.
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with any colour change. So GC-MS was employed for evaluating successive catalytic runs hot ltration experiment was per-
the completion times and % conversion. 100% conversion and formed. Wherein, reduction of 4-NP was performed in the
100% selectivity was achieved for all the nitro derivatives as presence of 10 mg of 10 wt% Cu@Dop@CoFe2O4 as catalyst and
conrmed from the absence of even trace amounts of any side reaction was stopped aer 1 minute aer the start of the reac-
products in the GC-MS analysis. The details of the reduced nitro tion by just ltering out the catalyst using a syringe lter. It was
derivatives along with the completion time values are given in then observed that till one minute ꢃ70% conversion was ach-
Table 6. All the GC-MS data for the reduction of all the nitro- ieved and aer the separation of the catalyst, almost no further
arenes is given in ESI le† (GC-MS-1 to GC-MS-15) as mentioned conversion of nitrophenol to aminophenol was observed in the
in Table 6.
supernatant solution (Fig. 16(a)). So this hot ltration experi-
3.6.4. Mechanistic pathway. In polar protic solvents such ment established the stability of the synthesized nano-
as methanol and water, decomposition reaction of sodium composite and also ruled out the possibility of metal leaching
borohydride starts at once, generating H2 gas.35 Also NaBH4 got which is the major issue associated with the heterogeneous
ꢀ
ionized to give BH4 ions. Reaction starts with the addition of catalysis.
catalyst in to the nitroarene solution. Reactant nitro derivative
Recyclability is the key feature of any heterogeneous
and borohydride ions get adsorbed on the surface of the metals catalyst. As the synthesized nanocomposite catalysts are
loaded over the modied cobalt ferrite nanoparticles to yield magnetic in nature so these can be very easily recovered out
metal hydride complexes. Reduction reaction takes place at of the reaction mixture aer the completion of reaction using
these metal hydride complexes where nitro moieties get con- an external magnet. So aer the recovery of the catalyst
verted to corresponding amines. Aer the completion of the the catalyst was washed with acetone and distilled water
reaction the products formed get desorbed from the surface and repeatedly to remove any reactant or product adsorbed and
catalyst was magnetically separated out of the reaction mixture was vacuum dried. So recyclability of the catalyst was checked
using an external magnet and washed thoroughly with distilled up to 5 catalytic runs. For the recyclability experiment
water and acetone and becomes free for the next reaction. The reduction of 4-NP was performed in the presence of 10 mg of
schematic representation of the reaction mechanism is shown 10 wt% Cu@Dop@CoFe2O4 nanocomposites. No signicant
in Fig. 15.
dilapidation in the catalytic performance in terms of
3.6.5. Hot ltration experiment and recyclability. To completion time and percent conversion was observed to be
discard the possibility of leaching of metal (active species) in to there. Recyclability experiment has been illustrated in
the reaction mixture and weakening of the catalyst over Fig. 16(b).
Fig. 15 Schematic representation of the reaction mechanism for the reduction of nitroarenes in the presence of M@Dop@CoFe nanocatalyst.
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Fig. 16 (a) Hot filtration experiment ruling out the possibility of metal leaching and (b) recyclability of the catalyst up to 5 catalytic runs.
established to be versatile catalysts that can be employed for
industrial applications as well.
4. Conclusions
A highly robust and economic reduction protocol has been
procured which employs metal stabilized over modied
magnetic ferrite nanoparticles (M@Dop@CoFe2O4) as catalyst.
Acknowledgements
Varying amounts of both Ag and Cu metal nanoparticles were
affixed over the surface of modied ferrite nanoparticles. Irre-
spective of the nature of the metal loaded over the surface of
modied ferrite nanoparticles, catalytic performance effectively
got improved with increase in the metal loading from 2.5 wt% to
10 wt% while not much considerable augmentation in the
catalytic performance was observed to be there when metal
loading was raised above 10 wt%. Cu@Dop@CoFe2O4 nano-
composites possessed better catalytic activity than that of
Ag@Dop@CoFe2O4 nanocomposites. The synthesized nano-
composites have been established as very stable, magnetically
recyclable and leach-proof competent catalyst for the reduction
of nitroarenes. Substrate scope was evaluated for different
nitroderivatives, so the synthesized nanocomposites have been
Financial supports from Council of Scientic and Industrial
Research (CSIR) are gratefully acknowledged. The authors wish
to thank Sophisticated Analytical Instrumentation Facility
(SAIF), Panjab University (PU) for providing the instrumenta-
tion facility for the required characterizations. The authors are
also grateful to Kunash Instruments Pvt. Ltd., Thane (W) for
performing the surface area analysis of the samples.
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91294 | RSC Adv., 2016, 6, 91275–91294
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