Green synthesis of metal oxide nanoparticles and their catalytic activity for the reduction of aldehydes

Article abstract of DOI:10.1016/j.procbio.2018.12.001

In the present work, a green synthesis of Metal Oxide nanoparticles was demonstrated using the freshly prepared aqueous extract of the immature fruit of Cocos nucifera and the MO nanoparticles were characterized by the analytical techniques such as UV–vis, FT-IR, XRD, SEM, TEM and EDAX. Characterization techniques confirmed that the biomolecules involved in the formation of nanoparticles and also they stabilized the nanoparticles. The synthesized MO nanoparticles were used as catalysts for the reduction of aromatic aldehydes. The reduction was done at mild reaction conditions using ammonium formate as a green hydrogen donor and the corresponding alcohols were obtained in 2–24 h with excellent yields. The reduction reaction was optimized using various solvents, loading of catalyst and at different temperatures.

Full text of DOI:10.1016/j.procbio.2018.12.001

Accepted Manuscript
Title: Green synthesis of Metal Oxide nanoparticles and their
catalytic activity for the reduction of aldehydes
Authors: A. Muthuvinothini, S. Stella
PII:
S1359-5113(18)31098-5
DOI:
Reference:
https://doi.org/10.1016/j.procbio.2018.12.001
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Process Biochemistry
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16 November 2018
1 December 2018
Please cite this article as: Muthuvinothini A, Stella S, Green synthesis of Metal
Oxide nanoparticles and their catalytic activity for the reduction of aldehydes, Process
Biochemistry (2018), https://doi.org/10.1016/j.procbio.2018.12.001
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Green synthesis of Metal Oxide nanoparticles and their catalytic activity
for the reduction of aldehydes
A. Muthuvinothini and S. Stella*
Department of Chemistry & Research Centre, Sarah Tucker College (Autonomous) affiliated
to Manonmaniam Sundaranar University, Tirunelveli – 627012, Tamil Nadu, India.
Corresponding author’s Email: stella.selvaraj@gmail.com.
Graphical Abstract
Highlights
 Metal Oxide (MO) nanoparticles were synthesized using a green synthetic method and
characterized.
 The catalytic property of the synthesized MO nanoparticles were examined.
 A green protocol for the reduction of aromatic aldehydes is demonstrated.
ABSTRACT
In the present work, a green synthesis of Metal Oxide nanoparticles was demonstrated
using the freshly prepared aqueous extract of the immature fruit of Cocos nucifera and the MO
nanoparticles were characterized by the analytical techniques such as UV-Vis, FT-IR, XRD,
SEM, TEM and EDAX. Characterization techniques confirmed that the biomolecules involved
in the formation of nanoparticles and also they stabilized the nanoparticles. The synthesized
MO nanoparticles were used as catalysts for the reduction of aromatic aldehydes. The reduction
was done at mild reaction conditions using ammonium formate as a green hydrogen donor and
1

the corresponding alcohols were obtained in 2-24 h with excellent yields. The reduction
reaction was optimized using various solvents, loading of catalyst and at different temperatures.
Keywords: Cocos nucifera, metal oxide nanoparticles, reduction, aldehyde, nanocatalysis.
1 Introduction
The field of nanoscience and nanotechnology is growing and glowing in a high way
because of its applications in various fields [1]. In the ancient methods, the nanoparticles are
synthesized and stabilized by the stabilizing agents to prevent aggregation [2-4]. Nowadays,
green synthetic route is used to synthesize nanoparticles in which the biomolecules can act both
as reducing and stabilizing agents [5-7]. Commonly, biomolecules like amino acids, plant
extracts, microorganisms and enzymes are used to synthesize nanoparticles in green chemistry
[8-9]. Synthesis of nanoparticles using plant materials is gaining more attraction from the
researchers, due to its simple experimental procedures. Various plant materials are used for the
synthesis of nanoparticles which may be in the form of zero valent [10,11], metal [12-15] or
metal oxide [16,17].
Cocos nucifera is a well-known plant which is a great resource of vitamins, minerals,
proteins, carbohydrates and tannins etc. It has industrial and medicinal applications and its parts
can be used as antibacterial, anti-inflammatory, antiviral agent in many fields [18-21]. Elango
et al. have prepared Lead nanoparticles via the agricultural waste of Cocos nucifera plant [22].
Roopan et al. have reported the green synthesis of Pd NPs [23], Ni-Pd nanoparticles [24], Silver
nanoparticles [25] and Nickel nanoparticles [26] from the extract of Cocos nucifera coir and
studied their biological activities.
In the present work, immature fruit extract of Cocos nucifera was used for the
preparation of nanoparticles. The immature coconut fruit is a biowaste which falls from the
coconut tree due to pest damage, temperature changes, plant diseases, summer drop, drought
stress/pruning or other unfavourable conditions.
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The bio synthesized nanoparticles from plant extracts can be used as catalysts for
various organic transformation reactions. Very few reports are available in the literature, for
the biotransformation using biosynthesized nanoparticles. Edison et al. have reported the
reduction of ortho and para nitroaniline by sodium borohydride with silver nanoparticles
synthesized by the aqueous extract of Tamarindus indica (T. indica) seed coat [27].
Raghasudha have synthesized silver nanoparticles from the leaf extract of Syzigium cumini and
used the synthesized nanoparticles as catalyst for the reduction of p-Nitrophenol to p-
aminophenol [28]. Aswathy Aromal et al. have reported the green synthesis of gold
nanoparticles using the aqueous extract of fenugreek (Trigonella foenum-graecum). They used
the synthesized Au nanoparticles as catalyst for the reduction of nitrophenol to amino phenol
and studied their size dependent catalytic activity [29].
In the present work, we have synthesized three metal oxide nanoparticles (CuO, NiO
and Fe2O3) from the aqueous extract of immature fruit of Cocos nucifera and the metal oxides
were characterized using UV-Vis, FT-IR, XRD, SEM, EDAX and TEM analytical procedures.
The synthesized MO nanoparticles were used as catalysts for the reduction of aromatic
aldehydes.
2 Materials and Methods
The fresh immature coconut fruit which was fallen from the coconut tree was collected
from Palayamkottai area, Tirunelveli, Tamil Nadu, India. Nickel (II) Chloride hexahydrate,
Ferric Chloride, p-Chlorobenzaldehyde and Sodium hydroxide were purchased from SRL
chemicals. Copper Sulphate penta hydrate (CuSO4.5H2O) was purchased from Nice chemicals,
India. Ammonium formate was purchased from Loba chemicals. All the reagents used were of
AR Grade. All aqueous solutions were made up of double distilled water.
3

2.1 Synthesis of metal oxide nanoparticles
The flesh of the immature coconut fruit was cut into small pieces with an ethanol
sterilized knife and ground with a mortar and pestle. Then it was dispensed in 100 ml of distilled
water for 5 minutes. The extract was filtered using Whatman no. 1 filter paper (Fig. 1 a).
About 100 ml of the immature coconut fruit extract was mixed with 100 ml of 0.1 M
of aqueous Nickel (II) chloride hexahydrate/ Copper sulphate penta hydrate/ Ferric Chloride
salt solution separately under stirring condition (Fig. 1 b). Fe2O3 nanoparticles was settled down
immediately as brown colour precipitate. However, NiO and Cuprous Oxide nanoparticles
were formed after the addition of 5 ml of 0.1 M sodium hydroxide solution into the reaction
mixture separately (Fig. 1 c). The precipitate was then filtered and dried at 100ᵒC for 2 h in a
hot air oven.
2.2 Characterization of Metal Oxide Nanoparticles
Optical absorption spectra of the synthesized NPs were taken with the UV-Vis Double
Beam spectrophotometer 2201, Systronics using a quartz cuvette of 1 cm path length. FT-IR
spectra of the nanoparticles prepared were recorded at room temperature on IR affinity (FT-IR
spectrophotometer, Shimadzu) ranging from 4000 to 400 cm-1. The morphology of the
synthesized nanoparticle was studied by Scanning Electron Microscopy (Jeol, Karunya
University, India). The synthesized MO nanoparticles were analysed by powder X-Ray
diffraction technique using X-Ray Diffractometer (XPERT-PRO, Alagappa university, India)
with the copper Kα1 radiation (k = 1.54 A) in the 2h range of 10° – 80°. The elemental
composition and stoichiometry of the synthesized nanoparticles was studied by EDAX
instrument (Oxford Instruments, Karunya University, India). TEM analysis of Nickel Oxide
nanoparticles were done at SAIF, Cochin, India.
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2.3 Catalytic activity of nanoparticles on the reduction of aromatic aldehydes
Aromatic aldehyde (30 mg, 0.2 mmol) dissolved in 2 ml of ethanol was mixed with the
MO nanoparticles as catalyst (1 mg, 1:30 ratio). Ammonium formate (0.3 mmol, 1.5
equivalent) and sodium hydroxide (0.15 mmol) dissolved in water were added to the above
solution and heated to 60 ºC. The reaction was monitored by TLC. After completion of the
reaction, the catalyst was filtered and the organic layer was evaporated. The crude product was
dissolved in ethyl acetate and washed with sodium bisulphite solution to remove the unreacted
aldehyde. The crude products were purified by column chromatography.
3 RESULTS AND DISCUSSION
The biowaste, immature coconut fruit, was chosen to prepare the metal oxide
nanoparticles. Phytochemical analysis (Table.1) was done to identify the presence of various
phytoconstituents in the aqueous extract of immature fruit of Cocos nucifera. Phytoconstituents
such as alkaloids, steroids, reducing sugars, carbohydrates, phenolic compounds, flavonoids,
saponins and tannins were present in the immature coconut fruit extract.
NiO, Cu2O and Fe2O3 nanoparticles were synthesized using the aqueous immature fruit
extract of Cocos nucifera. Fe2O3 nanoparticles were formed at neutral pH whereas NiO and
Cu2O nanoparticles were formed at basic pH. Further, the synthesized nanoparticles were
characterized by spectral techniques.
3.1 Characterization of the synthesized nanoparticles
3.1.1 UV-Vis spectra
UV-Vis spectrum of NiO nanoparticles (Fig. 2 (a)) showed a broad absorption peak at
333 nm corresponds to the formation of nanoparticles [30-32]. UV-Vis spectrum of Fe2O3
nanoparticles in Fig. 2 (b) showed absorption band at 201 nm confirming the formation of
Fe2O3 nanoparticles [33]. Cuprous Oxide (Cu2O) nanoparticles (Fig 2 (c)) showed absorption
band in the range of 398 nm - 569 nm. Absorption at ~410 nm indicates the formation of Cu2O
5

nanoparticles [34] and the two small bands at 484 nm and 560 nm are due to the
phytoconstituents present in the plant extract.
3.1.2 FT-IR spectra of Metal Oxide nanoparticles
Fig. 3 showed the FT-IR spectra of the synthesized metal oxide nanoparticles. In the
FT-IR spectrum of Metal Oxide (MO) nanoparticles, absorption peaks around 3300 cm-1,
1600 cm-1 and 1000 cm-1 correspond to H-O-H stretching, H- bonded alcohols and phenols, C-
H alkyl stretching respectively. It confirmed the presence of phytoconstituents like
polyphenols, alkaloids, steroids which were responsible for the reduction of metal precursors
into Metal Oxide nanoparticles. The IR band at 455 cm-1 in the FT-IR spectrum of NiO
nanoparticles confirmed the Ni-O stretching vibration. The strong absorption band at
516 cm-1 in the IR spectrum of iron oxide was due to the Fe-O stretching vibration. In the FT-
IR spectrum of Cuprous Oxide nanoparticles, the peaks at 470.63 cm-1 (Cu-O symmetric
stretching),) and 601.79 cm-1 (Cu-O wagging) implied the presence of metal-oxide group in
the sample [35].
3.1.3 X-Ray Diffraction (XRD) analysis
Fig. 4 denotes the XRD spectra of the synthesized MO nanoparticles. Peaks at 2θ values
of 38.2˚, 44.4˚, 77.7˚ in the XRD spectrum of Nickel Oxide nanoparticles (JCPDS: 47-1049)
confirmed that the structure was face centred cubic [30]. XRD spectrum of Fe2O3 nanoparticles
was broad indicating that the particles fell in nanometre range. Cuprous oxide nanoparticles
showed diffraction patterns at 35.2˚, 38.4˚, 48.4˚ which were related to the single phase
monoclinic (JCPDS: 80-1916) structure. The particles size was calculated using Debye-
Scherrer formula using the FWHM of the most intense peak. The size of the NiO, Fe2O3 and
Cu2O nanoparticles were of 24 nm, 53 nm and 51 nm respectively.
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The formation of NiO nanoparticles was further confirmed by comparing the 2θ values
in the XRD spectrum of biosynthesized NiO nanoparticles with that of chemically synthesized
NiO nanoparticles (Fig. 4 (b)).
3.1.4 SEM analysis
Surface morphology of the synthesized metal oxide nanoparticles was studied by SEM
analysis. SEM image of NiO nanoparticles (Fig. 5 (a)) confirmed that the particles were highly
mono dispersed and have deformed cubic size. Fig. 5 (b) implied that Cuprous Oxide
nanoparticles were spherical in shape. SEM image of Fe2O3 nanoparticles in Fig. 5 (c) showed
that the particles were in distorted spherical shape surrounded by the stabilizing agents from
the plant extract. Since, Fe2O3 nanoparticles were formed in neutral medium in contrast to the
NiO and Cu2O NPs, the stabilizing agents are tangible in the SEM image of iron oxide
nanoparticles.
3.1.5 EDAX analysis
The elemental composition of the synthesized nanoparticles was analysed using EDAX
spectrum. Table. 2 presents the composition of metal oxide nanoparticles. EDAX spectrum
(Fig. 6 (a)) confirms the formation of NiO nanoparticles. The presence of Si [36] and K were
originated from the biomolecules of the plant extract. EDAX spectrum (Fig. 6 (c)) confirms
the formation of Cu2O (Cuprous Oxide) nanoparticles. EDAX spectrum of iron oxide
nanoparticles strongly confirms the formation of Fe2O3 nanoparticles (Fig. 6 (b)).
3.1.6 TEM analysis
TEM and SAED pattern of Nickel Oxide nanoparticles are shown in Fig. 7. The SAED
pattern of NiO nanoparticles confirms the nanoparticle formation and the exact size of the NiO
nanoparticles was 51 nm.
7

3.2 Reduction of aromatic aldehydes
The synthesized nanoparticles were used for the reduction of aromatic aldehydes.
p-Chlorobenzaldehyde was taken as the model substrate. The reduction was carried out using
ammonium formate as the hydrogen donor in basic pH at 60 ºC. Among the nanoparticles used,
NiO showed high catalytic activity as evident from the high yield of the p-Chlorobenzyl alcohol
(90%) in 2 h (Scheme. 1). This was mainly due to the large surface area and high activity of
NiO nanoparticles as heterogeneous catalyst than the other MO nanoparticles. Cu2O and Fe2O3
nanoparticles gave the alcohol in 32% and 20% yield respectively. When the reaction was done
without the catalyst, there was no conversion of the product in 2 h. When the reaction was
carried out at room temperature, the yield was only 57%.
Since NiO nanoparticles has shown remarkable catalytic activity, it was used for the
subsequent optimization process. Further, the effect of solvent on the reduction of p-
Chlorobenzaldehyde was studied using various hydrophilic solvents like ethanol, methanol,
tetrahydrofuran, acetonitrile and a mixture of water and methanol. Among the solvents used,
ethanol was found to be the best solvent to reduce the aldehyde to a higher extent (Scheme. 2).
Further, when the reduction was carried out by varying the amount of catalyst loading, it
was observed that increase in the amount of catalyst did not alter the yield of the reaction (Fig.
8).
Then, the reaction was optimized by using various nickel catalysts synthesized by various
methods (Table. 3). Montmorillonite clay supported Nickel nanoparticles [37]gave the reduced
product in low yield (34%). Cysteine capped Nickel nanoparticles [38] gave the alcohol in
moderate yield (58%). When the reduction was done using NiCl2 salt as catalyst, the reaction
did not proceed. NiO nanoparticles prepared from immature fruit of Cocos nucifera has only
shown promising catalytic activity.
8

3.2.5 Reduction of benzaldehyde substrates
Having optimized the reaction conditions for the reduction of p-Chlorobenzaldehyde,
various aromatic aldehydes were reduced using NiO nanoparticles (Scheme. 3). The electronic
nature of the substituents in the benzene ring did not affect the yield of the reaction. The
electron donating methoxy and methyl substituents and electron withdrawing nitro group did
not make a pronounced effect on the course of the reaction and the products were obtained in
good yields (Table. 4). However, the reaction time varied for each substrate (2-24 h).
p-Bromobenzaldehyde, p-Chlorobenzaldehyde and 2-Hydroxybenzaldehyde were reduced in
83%, 90% and 81% yield respectively.
Conclusion
In summary, metal oxide nanoparticles were synthesized using the aqueous immature
fruit extract of Cocos nucifera in a green synthetic route. The catalytic activity of the
synthesized nanoparticles was examined for the reduction of aromatic benzaldehydes. It was
found that Nickel Oxide nanoparticles showed excellent yields compared to the other
synthesized nanoparticles. The reduction was carried out without any harsh conditions using
ammonium formate as green hydrogen source.
Acknowledgement
This work was supported by the financial assistance from DST-SERB, India
(ECR/2016/001523).
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Figure Captions
Fig. 1. Synthesis of metal oxide nanoparticles (a) Aqueous immature coconut fruit extract
(b) Addition of metal salt into extract (i) CuSO4. 5H2O (ii) FeCl3 (iii) NiCl2. 6H2O
(c) Formation of Metal Oxide nanoparticles
Fig. 2. UV-Visible spectra of (a) NiO nanoparticles (b) Fe2O3 nanoparticles (c) Cu2O
nanoparticles
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17

18

Fig. 3. Fourier-transform infrared spectra of (a) NiO nanoparticles (b) Fe2O3 nanoparticles
(c) Cu2O nanoparticles
19

Fig. 4. (a) Powder X-Ray Diffraction spectra of (i) NiO nanoparticles (ii) Fe2O3 nanoparticles
(iii) Cu2O nanoparticles
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Fig. 4 (b) X-Ray diffraction spectra of biosynthesized and chemically prepared NiO
nanoparticles
21

Fig. 5. Scanning Electron Microscopic images of (a) NiO nanoparticles (b) Cu2O nanoparticles
(c) Fe2O3 nanoparticles
22

Fig. 6. Energy dispersive analysis x-ray spectra of (a) NiO nanoparticles (b) Fe2O3
nanoparticles (c) Cu2O nanoparticles
23

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Fig. 7. Transmission Electron Microscopic images of NiO nanoparticles at different
magnifications (a) 50 nm (b) 20 nm (c) Selected area electron diffraction image of NiO
nanoparticles
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Fig. 8. Reduction of p-Chlorobenzaldehyde by varying catalyst loading
90.5
90
89.5
89
88.5
1
1.5
2
2.5
3
3.5
Loading of catalyst (mg)
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Supporting Information
(i) Benzyl alcohol
Colourless liquid. FT-IR Data: 3606 cm-1, 3204 cm-1, 2167.84 cm-1, 1640.35 cm-1,
-1
1
1426.26 cm (Fig. 1). H NMR (400 MHz, CDCl3) (Fig. 2): 2.8 ppm (s, 1H, -OH), 4.6 ppm
(s, 2H, -CH2-) and 7.24-7.33 ppm (m, Ar-H). 13C NMR: 64.84 ppm-65.08 ppm, 115.46 ppm,
120.06 ppm, 126.70 ppm - 129.57 ppm, 140.61 ppm - 140.91 ppm and 156.39 ppm (Fig. 3).
Fig. 1 FT-IR spectrum of Benzyl alcohol
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Fig. 2 1H NMR spectrum of Benzyl alcohol
Fig. 3 13C NMR spectrum of Benzyl alcohol
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(ii) 4-Methoxybenzyl alcohol
Colourless liquid. FT-IR data: 3356.87 cm-1, 3002 67 cm-1, 2936.42 cm-1, 1462.91 cm-
1
-1
1
and 1586.34 cm (Fig. 4). H NMR (400 MHz, CDCl3) (Fig. 5): 3.73 ppm (s, 3H, -CH3),
4.47 ppm (s, 2H, -CH2-), 3.4 ppm (s, 1H, -OH), 6,81-6.83 ppm (d, Ar-H) and 7.18-7.20 ppm
(d, Ar-H). 13C NMR: 159.23 ppm, 133.13 ppm, 128.67 ppm, 113.97 ppm, 65.06 ppm and 55.32
ppm (Fig. 6).
Fig. 4 FT-IR spectrum of 4-Methoxybenzyl alcohol
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