Ultrasonic-assisted green synthesis of β-amino carbonyl compounds by copper oxide nanoparticles decorated phosphate functionalized graphene oxide via Mannich reaction

Article abstract of DOI:10.1016/j.cattod.2019.07.050

A facile chemical synthetic route has been demonstrated for the synthesis of copper oxide nanoparticles decorated phosphate functionalized graphene oxide (CuO/PGO). The synthesized nanocatalyst was used as an efficient and active candidate for the synthesis of β-amino carbonyl compounds viaa green synthetic ultrasonic route. The structural properties of the samples were investigated by means of a number of sophisticated techniques like X-ray diffraction (XRD), Fourier-transform Infrared (FTIR) spectroscopy, High Resolution Transmission Electron Microscope (HRTEM), N₂ adsorption-desorption measurements, X-ray photoelectron spectroscopy (XPS) analysis, Ammonia temperature programmed desorption analysis (NH₃-TPD) and Raman spectroscopy. HRTEM analysis confirmed the presence of spherical CuO nanoparticles distributed uniformly throughout the PGO surface. XPS analysis demonstrated the presence of Cu²⁺ species and minor reduction of oxygen functional groups on GO. A higher surface area of 162 m²/g for CuO/PGO was found from N₂ adsorption-desorption isotherms. Later on, the presence of acidic groups on CuO/PGO that play an essential role in the catalytic activity was examined by NH₃-TPD and pyridine adsorbed IR analysis. The total acidity on the surface of synthesized nanocatalyst was found to be of 0.59 mmol g^?1 which includes both Lewis as well as Br?nsted acidic sites. A higher product yield of 95% in a shorter period of time of 15 min was achieved which is superior to many reported catalytic systems. A combined strategy involving greener and easier ultrasonic route and use of an efficient acidic graphene oxide-based catalyst resulted in higher catalytic activity and stability with good recyclability.

Full text of DOI:10.1016/j.cattod.2019.07.050

Accepted Manuscript
Title: Ultrasonic-Assisted Green Synthesis of ␤-Amino
Carbonyl Compounds by Copper Oxide Nanoparticles
Decorated Phosphate Functionalized Graphene Oxide via
Mannich Reaction
Authors: L. Satish K. Achary, Pratap S. Nayak, Bapun Barik,
Aniket Kumar, Priyabrat Dash
PII:
S0920-5861(19)30406-7
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2019.07.050
CATTOD 12379
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
19 May 2019
27 July 2019
29 July 2019
Pleasecitethisarticleas:AcharyLSK, NayakPS, BarikB, KumarA, DashP, Ultrasonic-
Assisted Green Synthesis of ␤-Amino Carbonyl Compounds by Copper Oxide
Nanoparticles Decorated Phosphate Functionalized Graphene Oxide via Mannich
Reaction, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.07.050
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Ultrasonic-Assisted Green Synthesis of β-Amino Carbonyl Compounds
by Copper Oxide Nanoparticles Decorated Phosphate Functionalized
Graphene Oxide via Mannich Reaction
a
a
a
b
a
L. Satish K. Achary , Pratap S. Nayak , Bapun Barik , Aniket Kumar and Priyabrat Dash*
aDepartment of Chemistry, National Institute of Technology, Rourkela, Orissa, 769008,
bSchool of Materials Science and Engineering, Chonnam National University, Gwang-Ju, 61186, Republic of
Korea.
Email id (corresponding author): dashp@nitrkl.ac.in
Graphical Abstract
Highly efficient and active CuO nanoparticle decorated Phosphate functionalised graphene oxide (PGO-
CuO) nanocomposite has been synthesized in a water-isopropanol system and used as an active catalyst
for the synthesis of β-amino carbonyl compounds via Mannich reaction by using a greener ultrasonic
method.
1

Highlights





CuO nanoparticles were decorated on PGO via a simple wet chemical synthetic route.
PGO-CuO catalyst was thoroughly characterized by various analytical techniques.
A greener ultrasonic route was used for synthesis of β-amino carbonyl compounds.
Higher catalytic efficiency was obtained under solventless condition.
The catalyst could be recycled for six times without any effective loss in activity.
Abstract
A facile chemical synthetic route has been demonstrated for the synthesis of copper oxide nanoparticles
decorated phosphate functionalized graphene oxide (CuO/PGO). The synthesized nanocatalyst was used
as an efficient and active candidate for the synthesis of β-amino carbonyl compounds via a green
synthetic ultrasonic route. The structural properties of the samples were investigated by means of a
number of sophisticated techniques like X-ray diffraction (XRD), Fourier-transform Infrared (FTIR)
spectroscopy, High Resolution Transmission Electron Microscope (HRTEM), N
measurements, X-ray photoelectron spectroscopy (XPS) analysis, Ammonia temperature programmed
desorption analysis (NH -TPD) and Raman spectroscopy. HRTEM analysis confirmed the presence of
spherical CuO nanoparticles distributed uniformly throughout the PGO surface. XPS analysis
2
adsorption-desorption
3
2
+
demonstrated the presence of Cu species and minor reduction of oxygen functional groups on GO. A
2
higher surface area of 162 m /g for CuO/PGO was found from N
2
adsorption-desorption isotherms.
Later on, the presence of acidic groups on CuO/PGO that play an essential role in the catalytic activity
was examined by NH
3
-TPD and pyridine adsorbed IR analysis. The total acidity on the surface of
2

−
1
synthesized nanocatalyst was found to be of 0.59 mmol g which includes both Lewis as well as
Brönsted acidic sites. A higher product yield of 95% in a shorter period of time of 15 min was achieved
which is superior to many reported catalytic systems. A combined strategy involving greener and easier
ultrasonic route and use of an efficient acidic graphene oxide-based catalyst resulted in higher catalytic
activity and stability with with good recyclability.
Keywords: Ultrasonic assisted, β-amino carbonyl compounds, phosphate functionalized graphene oxide
and CuO nanoparticles.
1. Introduction
Acid catalysis has become an important pathway for the synthesis of many important industrial
chemicals [1]. A wide range inorganic/organic acids and stoichiometric amounts of acidic sites are
normally used for various industrial reactions. Friedel–Crafts alkylations, nitrations, aromatic
halogenations, acylations, condensation and oligomerisations are some of the well known acid catalyzed
reactions [2-5]. Most commonly used catalysts for these reactions are Brönsted acids like H
2
SO , HCl,
4
HClO and HF and Lewis acids like BF , AlCl and ZnO-Al [6-8]. However, the use of these acids
4
3
3
2
O
3
have a number of drawbacks such as hazardous in handling, corrosiveness, lack of reusability and
production of large volume of toxic and corrosive wastes. These drawbacks facilitated researchers to
design suitable alternative catalysts. In this regard, solid acid materials are found to be promising
material in which various support materials are normally used for the design of such catalysts. Owning
to the unique properties of carbon-based materials like graphene and carbon nanotubes (CNT), various
acid groups that can act as active sites have been incorporated on their surface inorder to design active
and efficient nanocatalyst. For example, Xiao and co-workers reported sulfonated graphene as an
enhanced solid nanocatalyst for acid catalyzed esterification reaction [9]. In another work, Yue et al.
showcased the enhanced catalytic properties of aminoacid functionalised multiwalled carbon nanotubes
3

towards asymmetric reduction of aromatic ketones [10]. In all the cases, carbon-based materials have
shown promising support material for acid catalyzed reactions.
Among other carbon-based materials, graphene oxide (GO) has become an important support material as
it has unique structural and physicochemical properties [11-13]. Especially in the field of catalysis, GO
has been used as a best support material due to its unique structural properties like high specific surface
area and easy surface modification [14]. The modification of its surface becomes facile because of the
presence of hydrophilic oxygen functional groups including carbonyl, carboxylic group, epoxide and
hydroxyl groups. This helps for the immobilization of the active species which are responsible for
catalysis [15]. In this regard, various acidic groups such as phosphate and sulfonate functional group
have been used for the modification of graphene oxide surface [16-18]. In particular, phosphate groups
would be an suitable candidate to design an efficient solid nanocatalyst as it provides several advantages
such as high charge carrier concentration, thermal stability, oxidation resistance and possible proton
conduction in the dry state by forming dynamic hydrogen bond networks [19]. Therefore, more studies
are required to study the sutability of phosphate functionalized GO (PGO) catalyst for various catalytic
reactions.
Over the past few years, there is a considerable interest in the development of nitrogen-based
heterocycles due to their diverse activities. Among these heterocycles, β-amino carbonyl compounds are
found to be an important and essential type of nitrogen-based heterocyclic compound which attracts
most of the pharmaceutical industry because of their interesting biological activities such as antitumor,
antibacterial, anticancer, and anti-inflammatory activity [20, 21]. They also act as fundamental
building blocks for the preparation of important molecules like amino alcohols, peptides, lactams and as
precursors to synthesize amino acids. [22]. Also, these compounds are regared as impotant synthetic
intermediate for various natural products like alkaloids and polyketides [23]. The most preferable path
4

for the synthesis of these β-amino carbonyl compounds is Mannich reaction which involves a three
component condensation of aldehyde, ketone and amine. Many acidic groups (sulfonic acid and
heteropolyacids) as well as metal/metal oxide nanoparticles (sulfated alumina and zirconia) have been
reported as efficient catalyst for this type of reaction [24-30]. However, these catalysts have various
drawbacks like more time consumption for reaction, inadequate yields, non-greener method and use of
expensive catalyst. Moreover the methodologies used for the synthesis have drawbacks such as longer
reaction time and high power consumption. As both acids as well as metal oxide nanoparticles were
successfully used for synthesis of β-amino carbonyl compounds we envisioned that by modifying metal
oxide nanoparticles on acidic GO (PGO) surface would potentially make a novel nanocatalyst for this
type of Mannich reaction. In this regards, CuO nanoparticle modified PGO would be a potential
nanocatalyst for such reaction as CuO nanoparticle have high surface area, good chemical and thermal
stability and reactive morphologies [31]. It also provides several other advantages like low cost, ease of
handling and good reusability. To the best of our knowledge, such system have earlier not been reported.
With an aim to design a novel and highly efficient heterogeneous nanocatalyst for synthesis of β-amino
carbonyl compounds via greener route, herein, we report an ultrasonic assisted methodology using CuO
decorated PGO (CuO/PGO) as potential catalyst. Ultrasonic assisted methodology was chosen as it
generates high temperatures and pressures in a very short period of time (about 10-15 sec) making this
as a more efficient and greener route for organic syntheses. The β-amino carbonyl compounds were
synthesized via condensation of benzaldehyde, acetophenone and aniline in the presence of our designed
nanocatalyst (CuO/PGO) as shown in scheme1 and the same reaction pathway was applied for the
synthesis of other derivatives. The designed nanocatalyst was highly active towards all the reactions
within a shorter period of time as compared to other traditionally available catalysts with less
5

consumption of energy. Moreover, our catalyst was highly stable after six reaction cycles with minimal
loss in activity.
2
. Experiment
2
.1. Materials and instrumentation
Graphite powder, triethylphosphite, CDCl
Aldrich. H (30%), NaNO , ethanol, HCl, KMnO
3
, Cu(OAc)
2
·H
2
O and LiBr were purchased from Sigma-
SO were
2
O
2
3
4
, silica gel (100-200 mesh) and H
2
4
purchased from Hi-Media. 18 MΩ cm Milli-Q water was used throughout the syntheses. All the
chemicals were used without any further purification. X-ray diffraction study of the catalyst was done by
using Philips PW 1830 X-ray diffractometer with Cu Kα source (1.54 Aº). Perkin-Elmer FTIR
spectrophotometer was used to record the FTIR spectra of the catalyst using NaCl support. Philips CM
2
00 equipment was used for the transmission electron microscopy (TEM) analysis of the synthesized
sample using carbon coated nickel grids. The Raman analysis was recorded using Bruker RFS 27
spectrometer with 1064 nm wavelength incident laser light. Surface area analysis was done by nitrogen
1
13
adsorption/desorption analysis using a Quantachrome Autosorb 3-B apparatus. H NMR and C NMR
spectra were recorded using Bruker spectrometer at 400 MHz using TMS as an internal standard. A
Multi Lab 2000 (Thermo VG Scientific, United Kingdom) was used to record the XPS spectra.
Micromeritics Chemisorb 2750 TPD unit was used for the NH -TPD analysis of the catalyst. Ultrasonic
3
syntheses were carried out in an ultrasonic cleaning bath GT Sonic-L3, (multi-frequency at 45 and 65
kHz). The progress of catalytic reaction was monitored by using thin layer chromatography (TLC) on
0
.2 mm silica gel F-254 plates. All the pure products obtained were identified by comparing their
spectral characteristics with the literature reported values.
2
.2. Catalyst preparation
6

2
.2.1. Synthesis of CuO/PGO nanocatalyst
GO was synthesized via modified Hummers’ method using natural graphite flake [32]. Then PGO was
synthesized via Arbuzov reaction [33]. Later on, PGO was decorated with CuO nanoparticles to prepare
CuO/PGO nanocatalyst in water-isopropyl system (scheme 1) [34]. Briefly, 200 mg of PGO was taken
along with 50 ml of isopropyl alcohol and ultrasonicated for 30 min to afford a black dispersion. Under
continuous ultrasonic irradiation, 0.14 g of Cu(OAc)
2
·H O was added to the homogeneous dispersion
2
resulting in the formation of brown colored solution. Then it was heated at 85 ºC under continuous
stirring for 30 min. After that 20 mL of deionized (DI) water was added rapidly into the above
dispersion and allowed it for further heating at 85 ºC for 30 min. The colour of the dispersion gradually
changed to black. In the next step, the reaction mixture was cooled and centrifuged. Then the solid
product was washed with ethanol several times and dried at 70 ºC in air overnight without any
calcination process. Our primary aim was to design a highly active solid nanocatalyst for synthesis of β-
amino carbonyl compounds. Calcination of the catalyst under high temperature may reduce the active
acidic sites present on the catalyst. Hydrolysis in water-isopronol system with multiple washing in
ethanol resulted in the removal of unreated Cu(OAc)
2
·H O.
2
2
2
.3. Synthesis of 3-Anilino-1,3-diphenyl-1-propanone
.3.1. Traditional method
In brief, aniline (1 mmol), benzaldehyde (1 mmol) and acetophenone (1 mmol) were taken in a round
bottom flask and mixed thoroughly. CuO/PGO (0.03 g) catalyst was added to the resulting mixture and
then stirred at 40 ºC for 20 min without any solvent. During this process, the reaction progress was
monitored by thin layer chromatography (TLC). Finally, the solid crude was recovered and washed with
distilled water and 20ml of ethyl acetate. In this process, the unreacted organic part was removed and the
solid crude was dissolved. The mixture was sonicated for 2-3 min and the solid catalyst was separated
7

by centrifugation and dried at 90 ºC in oven. The desired product was separated and purified by column
chromatography.
2
.3.2. ultrasonic assisted method
In brief, aniline (1 mmol), benzaldehyde (1 mmol) and acetophenone (1 mmol) were taken along with
.03 g of catalyst (CuO/PGO).Then, the glass vial was placed in the bath sonicator to interact with
0
ultrasonic wave. By TLC, the reaction progress was monitored. The time period of the ultrasonic wave
irradiation was for 15-20 min. After the completion of reaction, the resulting crude was dissolved in
ethanol and the solid catalyst was recovered by centrifugation. The solid catalyst was thoroughly washed
with ethanol and then separated from the reaction mixture. Then it was dried under vacuum and reused.
The final product was purified by column and all the products were characterized by comparison of their
spectral data with those for authentic samples.
3
.
Results and discussion
3
.1.
Characterization of CuO/PGO catalyst
During Hummers’ method various hydrophilic oxygen functional groups are generated on GO surface
which make it an eligible candidate for support material in catalysis. Moreover, the introduction of
acidic groups on GO surface generate more active centers which further enhance its catalytic activity.
Envisioning the above possibility, we have employed Arbuzov reaction for the synthesis of phosphate
functionalized GO and used a simple water-isopropanol system to incorporate the CuO nanoparticles.
We expected that the introduction of metal oxide nanoparticles in the functionalised GO would further
increase the active sites. The resulting solid acid CuO/PGO catalyst was then thoroughly characterised
by different sophisticated analytical techniques such as PXRD, FTIR, TEM analysis, N adsorption
2
desorption, XPS, NH
3
-TPD and Pyridine adsorbed IR.
8

Initially, the structural investigation was carried out with XRD and FTIR to confirm the crystalinity and
phase purity as well as the presence of essential functional groups on the surface of CuO/PGO (Fig.1).
From the XRD spectra of GO, a sharp diffraction peak was found at 2θ = 10.4° which is normally
appeared for (002) planes of GO due to the presence of the uniform graphitic layer. After
functionalization with the phosphate group, the sharp (002) peak vanished which suggested exfoliation
of individual graphene layers by the incorporation of the phosphate groups. The presence of a poor
crystalline nature of functionalized GO was indicated due to a broad peak at around 2θ =22°. This
indicated that the sheets are randomly oriented and fully exfoliated by the incorporation of the phosphate
groups. After the decoration of CuO nanoparticles, some sharp peaks have been seen in the XRD spectra
of CuO/PGO nanocatalyst confirming the crystalline nature of the nanocatalyst. All the diffraction peaks
appeared in XRD pattern confirmed the presence of CuO lattice which are with good agreement with
JCPDS: 80-1268 file. Overall, the purity and crystalline nature of the CuO/PGO nanocatalyst was
inferred by the above results. Similar XRD pattern for CuO nanoparticles can also be found in other
literature [35].
To further check the purity of the material and the functional groups present on its surface, FTIR
analysis was carried out. FTIR spectrum of GO, PGO as well as the CuO/PGO nanocatalyst are shown
in Fig. 1 (b). The FTIR spectra of GO shows the absorption peaks at 1052, 3410 and1724 cm−1
suggesting the presence of C-O-C, -OH and C=O in carboxylic acid groups respectively. After
−
1
functionalization of phosphate groups, new absorption peaks at 600, 960 and 1020 cm appeared which
can be can be attributed to different vibrational mode of phosphate group [36]. These peaks can also be
seen in the FTIR spectra of desired nanocatalyst (CuO/PGO). In addition, some new peaks can be seen
-1
-1
at 510 cm and 450 cm corresponding to Cu-O stretching vibrations [37]. Besides, the absence of two
-1
sharp peaks at nearly 1401 and 1630 cm corresponding to acetate ion indicated the absence of any
9

unreacted acetate ion on our catalyst surface. Hydrolysis of acetate salt under high temperature in
presence of isopropanol and water solvent resulted in the formation of CuO nanoparticle. Therefore, the
possible presence of Cu(OAc) metal precursor in the prepared nanocatalyst (CuO/PGO) can be
2
discarded in this synthesis process. Similar procedures are available in literatures in which CuO
nanoparticles have been successfully synthesized in alcohol without calcination [38].
Both the above techniques (XRD & FTIR) confirmed the coexistence of phosphate group and CuO
nanoparticle in the CuO/PGO nanocatalyst which are essential for acid catalyzed reaction. The
dimensions (shape) and surface morphology of the designed active solid nanocatalyst (CuO/PGO) were
then thoroughly studied via TEM analysis (Fig. 2). The 2-dimensional layered structure of GO was
clearly observed in the TEM image on which phosphate groups are successfully incorporated (Fig.
2
(a)). The spherical CuO nanoparticles are seen to be uniformly distributed throughout the layer having
size range of several nanometers (5–8 nm) as shown in Fig. 2(b). The lattice spacing of the CuO
nanoparticle taken from the HRTEM image of an individual nanoparticle is found to be 0.26 nm and can
̅
be ascribed to (111) plane of CuO (Fig. 2(c)) [39]. The crystalline nature of the material can be validated
from the SAED pattern which shows clear ring structure as seen in Fig. 2(d). The elemental composition
of the nanocatalyst was also investigated by EDX spectral analysis. It was confirmed that our
nanocatalyst contains elements like C, O, P and Cu. All the above results from the TEM analysis
confirmed the successful deposition of CuO nanoparticles on PGO surface and the pattern is in good
agreement with the previously reported results [40] [41].
Development of any surface defects during the synthesis of PGO and after the decoration of CuO
nanoparticles was then confirmed by Raman analysis. Raman spectra of GO, PGO and CuO/PGO are
-1
shown in Fig. S1. Two distinct peaks can be seen at 1593 and 1362cm for GO, which are mainly due to
2
the vibration of sp carbon atoms in a graphitic 2D hexagonal lattice (G band) and the vibrations of sp
1
0

carbon atoms of defects and disorder (D band), respectively. The intensity ratio of these two bands
/I ) has been considered an indicator of the degree of carbon defects. When the Raman spectra of
PGO and CuO/PGO were compared to the Raman spectra of GO it was found that I /I value of GO
(
I
D G
D
G
was 0.96 where as for PGO and CuO/PGO the value increased to 1.05 and 1.09 respectively. Moreover,
in comparison to GO and PGO, the G and D band of CuO/PGO nanocatalyst was slightly shifted to
−
1
−1
1
581 cm and 1349 cm respectively. This suggested the increased disorderness on the GO surface.
These type of observations have been seen from various previously reported studies [42]. It can be
concluded that functionalization of phosphate group followed by decoration of CuO nanoparticles on the
parent material (GO) generated defect sites which results in the destruction of conjugated carbon lattice.
After surface modification of GO with phosphate groups as well as with CuO nanoparticles, the surface
became rough which was seen from the HRTEM image. This change in surface morphology would
facilitate an increase in surface area of the final CuO/PGO material. Hence, N adsorption-desorption
2
measurements were carried out and the results are shown in Fig. 3. All the isotherms were formed to be
type IV isotherm according to the IUPAC classification [43]. Surface area for GO was found to be 154
2
-1
2
-1
m g which increased to 179 m g for PGO due to the reduction of GO surface during
2
-1
functionalization. However, the surface area of CuO/PGO was decreased to 163 m g . This decrease in
surface area is mainly due to the surface covering with CuO nanoparticles. However, the resulting
surface area of the designed nanocatalyst is higher than the parent material (GO) which indicated that
more reactant molecules would be facilitated for interaction during the catalytic reaction.
X-ray photoelectron spectroscopy (XPS) is a vital and powerful technique to investigate the presence of
different chemical environment in an unknown sample [44]. The presence of phosphate groups as well
as CuO nanoparticles in CuO/PGO nanocatalyst was previously confirmed from FT-IR and TEM
analysis. To further confirm the successful synthesis of the desired nanocatalyst (CuO/PGO), presence
1
1

of different functional groups and oxidation state of Cu, XPS analysis was carried out (Fig. 4). In Fig.
a, the presence of phosphorus and copper in the nanocatalyst can be confirmed from the new peaks
4
appeared at 132.4 and 935.1eV which can be attributed to P2p and Cu2p core level spectra, respectively.
Minor reduction of oxygen functional groups can be observed from core level XPS spectrum of C1s of
CuO/PGO ((fig. 4 (b,c)). This reduction was mainly occurred during the fuctionalisation of phosphate
group on the GO surface, which is the main reason for its higher surface area as compared to GO. The
core level XPS spectrum of Cu2p shown in Fig. 4 (e) has two distinct peaks (doublet) at 934.2 eV and
9
54.1 eV with the spin-orbital splitting of 19.9 eV and are attributed to Cu 2p3/2 and Cu 2p1/2
,
respectively . The binding energy difference between these two peaks was found to be 19.9 eV which
2
+
suggested the presence of the Cu in the nanocatalyst [45]. Besides these, two satellite peaks were
found at 943.3 and 963.2 eV due to Cu 2p3/2 and Cu 2p1/2, further confirming the presence of CuO
nanoparticles in the designed nanocatalyst. Most importantly, the peak appeared for C-P at 285.3eV
confirmed the presence of phosphate group on the GO surface.
Besides higher surface area, presence of different acidic groups (Lewis and Brönsted) on the
nanocatalyst surface and their amount play an important role in the enhacement of its catalytic activity.
In CuO/PGO, the presence of surface functional groups like alcohols and carboxylate groups on GO
surface (as evidenced from XPS) provide Brönsted acidic sites [46]. In addition, GO surface has been
functionalised with phosphate groups which is a well known Brönsted acidic group. On the other side,
CuO also provides acidity as the copper site in CuO have great tendency to interact and co-ordinate with
different functional group such as carbonyl (-CO), nitrile (-CN) and hydroxyl (-OH) and behaves as
2
+
Lewis acid [47]. The Lewis acidity of Cu is originated due to the acquirement of a stable and
completely filled d-subshell on receiving an electron [48]. Therefore, in CuO/PGO nanocatalyst the
1
2

overall acidity originates from two types of acidic sites, i.e, Brönsted acidic sites from GO and
phosphate group, and Lewis acidic sites from CuO nanoparticles.
To experimentally confirm the presence of such acidic sites in our nanocatalyst, NH
carried out. Fig. 5 (a) shows the NH -TPD analysis of PGO and CuO/PGO nanocatalyst. In NH
technique, basic NH molecules adsorb on the catalyst surface and the acid groups are quantified by the
amount of NH molecules desorbed at different temperature. As this technique involves the adsorption
3
-TPD analysis was
3
3
-TPD
3
3
and desorption of gas molecules with respect to temperature, total surface acidic density can be
calculated by determining the amount of ammonia desorbed in milimole/gram on the surface of the
catalyst. Based on this calculation, the total acidic sites present catalyst were found to be 0.59 mmol g-1
-1
for CuO/PGO and 0.65 mmol g for PGO (table S1). Normally, strong acidic sites are assigned for
those peaks which appear above 400 °C where as medium and weak acid sites for peaks around 200-400
-1
°
C and below 200 °C, respectively [49]. Based on these observations, acidic site of 0.45 mmol g can be
-1
assigned to strong acid sites and 0.14 mmol g to medium acidic sites. However, there is no desorption
peak found in weak acidic sites in the TPD profile. It is worth to mention here that three peaks seen
around 300 °C in the NH
3
-TPD profile of CuO/PGO can be attributed to the desorption of chemisorbed
2
+
NH molecules from the catalyst surface due to the acidic nature of Cu in the nanocatalyst. The
3
possibility of these peaks due to acetate ions can be dismissed as the hydrolysis in water and
isopropanaol solvent followed by continuous washing led to the removal of these species as evidenced
from FTIR studies and other reported literatures [50]. From the above result it can be inferred that our
catalyst (CuO/PGO) had both Lewis as well as Brönsted acidic sites on its surface which play an
important role in its catalytic activity for the synthesis of 3-Anilino-1, 3-diphenyl-1-propanone as
discussed in more details in the catalytic study section. In addition, the combination of Brönsted acidic
1
3

sites on GO and PGO and the Lewis acidic sites of CuO nanoparticles could also show possible
synergistic effect in the nanocatalyst resulting in the higher activity [51].
To further validate the above results of NH
3
-TPD analysis, pyridine-IR study was done (Fig. 5 (b)). In
this study, the IR spectra of before and after adsorption of pyridine on CuO/PGO suface were recorded.
−
1
From the figure three distinct peaks at 1541, 1489, and 1437 cm can be observed which were also seen
in the IR spectra of PGO (Fig. 1 (b)). Additionally, two new peaks can be seen at 1469, and 1447 cm−1
which are mainy due to the development of Lewis acidic sites on the catalytic surface. Among the
−
1
observed peaks, the intense peak present at 1541 cm can be attributed to Brönsted acidic sites. This
peak is mainly due to the the formation pyridinium ion resulting from the interaction of pyridine with
−
1
Brönsted acidic sites [52]. A low intense peaks appeared nearly at 1437, 1447 and 1469 cm are due to
the presence of Lewis acidic site [53]. As discussed before, the source of these acidic sites are from GO,
phosphate groups and CuO nanoparticles.
3
.2.
Catalytic activity
After the successful synthesis and characterization of CuO/PGO, its catalytic activity was then
investigated towards the synthesis of 3-anilino-1,3-diphenyl-1-propanone via one pot three component
Mannich reaction of benzaldehyde, aniline and acetophenone (scheme 2). Design of a sustainable, cost-
effective, environment friendly and green reaction pathway is the need of the hour for any organic
transformation reaction. Ultrasonic route is found to be one such suitable pathway owing to its
advantages like reduced energy consumption, easy reaction condition and short reaction time. In this
regard, we had initially focused our investigation to establish a facile method for the synthesis of the
desired product. The initial experiments were carried out with different catalysts under different
synthetic conditions (table 2). It can be seen that trace amount of desire product was obtained in all the
reaction conditions when there was no catalyst (entry 1, table 2) indicating the necessity of catalyst to
1
4

carry out the reaction. However, when the reaction was carried out in presence of catalysts like GO,
PGO, Cu nanoparticles, CuO nanoparticle and CuO/PGO, higher yield of product was obtained (entry 3-
8
, table 2). The equations used for calculation of product yield is given below [54].
Yield = (map /mtp) × 100 ............................ (1)
Where map is the actual mass of product and mtp is the theoretical maximum mass of product.
Among these, the presence of CuO/PGO nanocatalyst resulted in a higher product yield (59% - 95%) in
less reaction time (10-20 min) using all the available synthetic methods. Further, by comparing the
efficiency of different synthetic routes such as grindstone, ultrasonic assisted, microwave and traditional
stirring methods, it was found that ultrasonic assisted synthesis of 3-anilino-1,3-diphenyl-1-propanone
led to the highest product yield (93%) in a reaction time of 15 min (entry 8, table 2) demonstarating the
effective role of ultrasonic process. Normally, the ultrasonic assisted reactions involve the interaction of
ultrasonic radiation having a frequency above 20 kHz with the organic substrates. During this
interaction process, transient cavities containing air or the vapours of liquid are formed which collapse
vigorously after a few cycles of ultrasound. This process leads to enhancement of local temperatures
and pressures inside the cavitating bubbles and at their interfaces. Therefore, these are known as the hot
spots having higher temperature and pressure [55]. Overall, in this process the mechanical energy of
mass flow is converted to kinetic energy of random molecular translation and rotation [56]. This above
process helps in the breaking of chemical bonds while using ultrasonication route resulting in the
generation of high product yield in shorter period of time. We have also carried out the reaction for a
prolonged time (45-60 min) and found that maximum yield of 95% can be achieved within 15 min. When
the reaction was continued for 60 min, the product yield was not changed significantly (91%). We have
1
5

also calculated the TOF for all catalysts and the results are presented in table 3. The equation used for
calculation of TOF is given below [57].
TOF = Np / Na × Time (h) ....................... (2)
Where N
TPD.
p
is the moles of substrate converted to product and Na is the mole of acidic sites calculated from NH -
3
Later on, to evaluate the effect of various reaction parameters on the product yield, the effect of solvents
and instrumental frequency were investigated. The instrumental frequency was varied (45kHz and
6
5kHz) to know the optimized power of the ultrasonic irradiation. The product yield was found to be
more (95%) in case of 65kHz frequency within same reaction time (15 min) as compared to instrument
frequency (45kHz). The reaction was also carried out in both presence and absence of solvent under
ultrasonic assisted method. Table 4 presents the obtained catalytic results. A higher product yield (95%)
was found in solventless condition using ultrasonic assisted method with less time for the completion of
the reaction (enrty 5, table 4). This methodology is advantageous as many problems associated with the
using solvents such as complex work-up procedure, safety, cost and environmental concern can be
avoided by using solventless condition.
To further study the effect of CuO loading on CuO/PGO, the catalytic experiments were carried out
(entries 1-4, Table S2) by varing the metal oxide loading from 5 to 20 wt%. The results showed that the
product yield increased with the increase in CuO loading up to 10 wt% on the PGO catalyst. After that
no significant change in product yield was observed with increase in metal oxide loading (entry 3-4,
Table S1). Hence, 10 wt % CuO/PGO was considered as the optimized catalyst for the catalytic process.
Further, to know the optimized and suitable amount of catalyst to be taken for high product yield, the
catalyst amount was varied from 10 mg to 50 mg (Fig. 6). The product yield was found to be increased
1
6

linearly with increase in the amount of the catalyst up to 30 mg after which no significant change in the
yield was obtained. We have also varied the ratio of starting materials and carried out the reaction in
similar condition and found that 1 milimole each of starting materials with a ratio of (1:1:1) results in
high yield of product in the reaction.
In order to investigate the efficiency and performance of our CuO/PGO catalyst using the optimized
reaction conditions obtained from the above experiments, a wide range of substituted β-amino carbonyl
compounds were synthesized (Table 5). We have varied the substituents in the aromatic aldehyde and
aniline to check its effects on product yield and it was found that aromatic aldehydes with electron
withdrawing groups led to a higher product yield in less time as compared to electron-donating
substituents. Further, we have calculated the tunr over frequency (TOF) of the nanocatalyst based of the
total acidic sites for all derivatives (table 5). It can be observed from the table that high TOF value was
obtained in all the cases which is quite useful for real time industrial applications. The resulting products
1
13
were analyzed well and confirmed by H and C NMR spectroscopy by comparing previously reported
data.
Another factor which affect the catalytic activity and product yield during a catalytic reaction is the
synergistic effect of individual active sites present on the catalyst such as -COOH, -OH, phosphate
group and CuO nanoparticle on CuO/PGO. To validate this characteristic effect a control experiment
was carried out by protecting the hydroxyl and carboxylic groups with methoxytrimethylsilane (MTS)
[
58]. Initially, the catalyst was allowed to react with MTS for a particular time and then used as catalyst
for the reaction. After the reaction, the product yield was found to be 45% which was less than that of
fresh unreacted catalyst. This product yield of 45% mainly originated due to the presence of CuO
nanoparticles which was very less as compared to unreacted CuO/PGO nanocatalyst. Besides, a physical
mixture of PGO and CuO nanoparticles resulted in a product yield of 65% (entry 6, Table 2) which is
1
7

lower than that of CuO decorated PGO. These results confirmed the presence of synergistic effects
between the individual component. Such synergistic effect would result in the significant adsorption of
reactant molecules on CuO/PGO surface resulting in the enhancement of its catalytic activity. Existence
of similar type of synergistic effects have been reported in previous literatures [59].
From all the results as discussed above, it can be concluded that the presence of acidic sites (Lewis and
Brönsted), the synergistic effect among the individual component and higher surface area of CuO/PGO
surface play an important role in its overall catalytic activity. Moreover, use of ultrasonic irradiation
during the synthesis of β-amino carbonyl compounds stimulated the initiation of the reaction. A
plausible mechanism can be elucidated considering all the above results and is presented in scheme 3.
The acidic nature (both Brönsted and Lewis) of the catalyst and efficient chemical adsorption of
reactant molecules on the catalyst surface initiate the interaction of acidic groups with aldehyde (IA)
and amine substrates under ultrasonic irradiation. Due to this interaction, the carbonyl group was
protonated and activated (mostly by the Brönsted acidic sites present on CuO/PGO). At the same time
2
+
the aniline molecule (IB) also became activated by the Lewis acidic site (Cu ) [60]. Then the
nucleophilic attack on activated aldehyde took place to form an iminium intermediate (II) via
dehydration. The intermediate was stabilised by both the acidic sites thorugh intermolecular bonding
with -OH and -NH groups which further increased its electrophilicity. The intermediate then undergo
nucleophilic attack by the activated acetophenone (generated by the interaction of active acidic sites of
the CuO/PGO catalyst) to form the desired product. At the same time CuO nanoparticles binds properly
with the organic substrates lowering the energy barrier required for all the intermediate steps [61].
2
+
Overall, the presence of both Cu and phosphate groups help in the activation, stabilisation and
formation of β-amino carbonyl compound. In addition, the efficient production of the desired product
was influenced by cavitation induced by ultrasonic irradiations. Generation of huge amount of heat
1
8

energy and local pressure during the growth and collapse of microbubbles help in the efficient synthesis
of β-amino carbonyl compounds. Mainly, during the formation of the intermediate (II), the dehydration
process was favored by both ultrasonic irradiation as well as the acid groups present on the catalyst.
Similar reaction mechanism for the synthesis of β-amino carbonyl compounds has been reported
previously [62].
Recyclability and stability are the most important and essential characteristics of an efficient and useful
catalyst. The optimized conditions were used to investigate the reusability of CuO/PGO catalyst. The
catalyst was recovered after the completion of the reaction. It was washed properly with ethyl acetate to
remove the organic parts and dried under vacuum at 80 °C for 3 h and reused six times with minimal
loss of catalytic activity (Fig. 7). With the use of fresh sample, the product yield was found to be 95%
th
which later became 82% after 6 catalytic run. These results suggest higher reusability of our
nanocatalyst. The reused catalyst was further characterized by powder XRD and FTIR analysis. The
similar pattern in XRD spectra and similar peaks in the FTIR spectra confirm the stability of the catalyst
(Fig. S2). The strong bonding of individual active sites in the nanocatalyst makes it more active and
reusable with minimal leaching of active sites like phosphate and CuO nanoparticles during the reaction.
To confirm this, hot filtration test was carried out [63]. Briefly, benzaldehyde (1 mmol), aniline (1
mmol) and acetophenone (1 mmol) were taken in a round bottom flask and 0.015 gm of catalyst
(CuO/PGO) was added and the reaction was put in sonication at a frequency of 65KHz for 7 min (Fig.
8
). After that the catalyst was separated in hot condition and the product yield was found to be 53%.
After the filtration process, the filtrate was allowed to continue under sonication for further reaction
under same conditions. After 15 min of sonication the product yield was found to be 55% indicating
minimal change in product yield. This change in product yield is not significant which confirms that
1
9

CuO/PGO is the active candidate for this catalytic process and no leaching of active sites during the
reaction had occured.
Later on, the advantages, efficiency and superiority of our designed nanocatalyst was demonstrated by
comparing with other traditionally reported catalysts and are shown in table 6. It was found from the
comparison data that our catalyst is superior as it shows higher activity within very short period of time
via a green ultrasonic method as compared to other available catalyst such as γ‐Al
2
O
3
/CH
3
SO
3
H [29],
H [68].
K
3
PO
4
[64], Tin(ll)chloride [65], Fe
3
O
4
@ZrO
2
/SO 2
4
[66]
,
CaCl
2
[67] and nano-Fe
3
O
4
@PEG-SO
3
‐
4
. Conclusions
In summary, a novel nanocatalyst (CuO/PGO) was successfully synthesized and characterized by
decorating CuO nanoparticles on phosphate functionalized graphene oxide. The acidic nature of the
synthesized catalyst was investigated by NH -TPD and Pyridine IR techniques by which total acidity
3
−1
was found to be 0.59 mmol g . Later on, CuO/PGO catalyst was utilized as efficient catalyst for the
synthesis of β-amino carbonyl compounds via one-pot multi-component reaction under greener
ultrasonic route. As compared to other reported systems, our catalyst showed a higher product yield
(95%) in a shorter period of time (15 min). This higher activity can be attributed to the presence of
acidic sites, higher surface area, ultrasonication method and synergistic effects between the individual
components. The catalyst showed excellent recyclability up to six consecutive runs without
significant loss of catalytic activity with minimal leaching. The current methodology shows that
by decorating suitable nanoparticles on acidic GO surface, highly efficient, stable and reusable
catalyst can be designed which will have practical importance in various organic
transformations.
Acknowledgements
2
0

The authors are thankful to Ministry of Human Resource Development (MHRD), Govt. of India, for
funding.
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3

Figure Captions
Fig. 1. (a) XRD pattern of GO, PGO and PGO-CuO and (b) FTIR spectra of GO, PGO and PGO-CuO
nanocomposite.
Fig. 2 HRTEM image of (a, b, d) PGO-CuO and (c) Histogram plot for particle size (e) SAED image of
PGO-CuO nanocomposite.
Fig. 3 N adsorption-desorption study of GO and PGO-CuO nanocomposite.
2
Fig. 4. XPS Survey spectra of (a) PGO-CuO (b) C1s XPS spectra of GO, (c) C1s XPS spectra of PGO-
CuO,(d) P 2p XPS spectra of PGO-CuO and (e) Cu2p XPS spectra of PGO-CuO nanocomposite.
Fig. 5 (a) Ammonia TPD profile of PGO and PGO-CuO and (b) Pyridine adsorbed IR spectra of PGO
and PGO-CuO nanocomposite.
Fig. 6 Influence of the PGO-CuO amount on reaction yield at a constant time.
Fig. 7 Reusability study of PGO-CuO catalyst.
Fig. 8 Leaching study of PGO-CuO catalyst.
2
4

Fig.1. (a) XRD pattern of GO, PGO and PGO-CuO and (b) FTIR spectra of GO, PGO and PGO-CuO
nanocomposite.
Fig. 2 HRTEM image of (a, b, d) PGO-CuO and (c) Histogram plot for particle size (e) SAED image of
PGO-CuO nanocomposite.
2
5

Fig.3 N adsorption-desorption study of GO and PGO-CuO nanocomposite.
2
Fig.4. XPS Survey spectra of (a) PGO-CuO (b) C1s XPS spectra of GO, (c) C1s XPS spectra of PGO-
CuO,(d) P 2p XPS spectra of PGO-CuO and (e) Cu2p XPS spectra of PGO-CuO nanocomposite.
2
6

Fig . 5 (a) Ammonia TPD profile of PGO and PGO-CuO and (b) Pyridine adsorbed IR spectra of PGO
and PGO-CuO nanocomposite.
Fig. 6 Influence of the PGO-CuO amount on reaction yield at a constant time.
2
7

Fig. 7 Reusability study of PGO-CuO catalyst.
Fig. 8 Leaching study of PGO-CuO catalyst.
2
8

Scheme Captions
Scheme 1: Schematic diagram for the synthesis of PGO-CuO catalyst.
Scheme 2. A model reaction for the synthesis of 3-Anilino-1, 3-diphenyl-1-propanone.
Scheme 3: Plausible reaction mechanism for the synthesis of 3-Anilino-1, 3-diphenyl-1-propanone.
Scheme 1. Schematic diagram for the synthesis of PGO-CuO catalyst.
Scheme 2: A model reaction for the synthesis of 3-Anilino-1,3-diphenyl-1-propanone.
2
9