Gold nanoparticles deposited on MnO2 nanorods modified graphene oxide composite: A potential ternary nanocatalyst for efficient synthesis of betti bases and bisamides

Article abstract of DOI:10.1016/j.mcat.2019.110415

The decoration of novel nanostructures such as nano particle and nanorod on the surface of graphene oxide (GO) generate potential heterogeneous nanocatalyst. Highlighting this, in the present work, we have designed a ternary GO-MnO₂-Au nanocomposite by decorating MnO₂nanorods on the surface of graphene oxide via hydrothermal method, followed by deposition of Au nanoparticles on GO-MnO₂ surface. The prepared nanocomposite was thoroughly characterised by different instrumental techniques such as X-Ray diffraction (XRD),Fourier transform infrared spectroscopy (FTIR),Raman spectroscopy, Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), High resolution Transmission electron microscopy (HRTEM), X-Ray photo electron spectroscopy (XPS), N₂ adsorption desorption Brunauer–Emmett–Teller (BET) isotherm and Inductively coupled plasma - optical emission spectrometry (ICP-OES). FESEM and TEM images demonstrated that the MnO₂ forms rod like structure having diameter of 60–100 nm and are uniformly distributed over the GO surface. HRTEM image clearly signifies gold (Au) nanoparticles having diameter of 7 ± 1.9 nm homogeneously distributed throughout the GO-MnO₂ surface. Elementary state of Au and tetravalent nature of Mn as well as reduction of functional group after the decoration was confirmed from XPS studies. The catalyst GO-MnO₂-Au was found to be the superior catalyst for synthesis of biologically active molecules such as Betti bases and Bisamides. The high catalytic activity of the materials can be attributed to the small and homogeneous distribution of gold nanoparticles, high redox potential of rod shaped MnO₂ and the synergistic effect between GO, MnO₂ and Au. All the reaction conditions were optimised by varying catalyst dosage, effect of solvent and temperature. The GO-MnO₂-Au was easily recycled with minimal leaching and the product yield was found to be 85–90% after 4th cycle demonstrating the stability and durability of our nanocomposite.

Full text of DOI:10.1016/j.mcat.2019.110415

MolecularCatalysis474(2019)110415
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Gold nanoparticles deposited on MnO2 nanorods modified graphene oxide
composite: A potential ternary nanocatalyst for efficient synthesis of betti
bases and bisamides
⁎
Pratap S. Nayaka, Bapun Barika, L. Satish K. Acharya, Aniket Kumarb, Priyabrat Dasha,
b School of Material Science and Engineering, Chonnam NationalUniversity, Gwang-Ju, Republic of Korea
a Department of Chemistry, National Institute of Technology Rourkela, Odisha, 769008, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The decoration of novel nanostructures such as nano particle and nanorod on the surface of graphene oxide (GO)
generate potential heterogeneous nanocatalyst. Highlighting this, in the present work, we have designed a ternary GO-
MnO2-Au nanocomposite by decorating MnO2nanorods on the surface of graphene oxide via hydrothermal method,
followed by deposition of Au nanoparticles on GO-MnO2 surface. The prepared nanocomposite was thoroughly char-
acterised by different instrumental techniques such as X-Ray diffraction (XRD),Fourier transform infrared spectroscopy
(FTIR),Raman spectroscopy, Field emission scanning electron microscopy (FESEM), Transmission electron microscopy
(TEM), High resolution Transmission electron microscopy (HRTEM), X-Ray photo electron spectroscopy (XPS), N2
adsorption desorption Brunauer–Emmett–Teller (BET) isotherm and Inductively coupled plasma - optical emission
spectrometry (ICP-OES). FESEM and TEM images demonstrated that the MnO2 forms rod like structure having diameter
of 60–100 nm and are uniformly distributed over the GO surface. HRTEM image clearly signifies gold (Au) nano-
GO-MnO2-Au
Hydrothermal
Betti bases
Bisamides
particles having diameter of 7
1.9 nm homogeneously distributed throughout the GO-MnO2 surface. Elementary
state of Au and tetravalent nature of Mn as well as reduction of functional group after the decoration was confirmed
from XPS studies. The catalyst GO-MnO2-Au was found to be the superior catalyst for synthesis of biologically active
molecules such as Betti bases and Bisamides. The high catalytic activity of the materials can be attributed to the small
and homogeneous distribution of gold nanoparticles, high redox potential of rod shaped MnO2 and the synergistic effect
between GO, MnO2 and Au. All the reaction conditions were optimised by varying catalyst dosage, effect of solvent and
temperature. The GO-MnO2-Au was easily recycled with minimal leaching and the product yield was found to be
85–90% after 4th cycle demonstrating the stability and durability of our nanocomposite.
1. Introduction
iodine, N-methyl-2-pyrrolidone (C5H9NO), N-methyl-2-pyrrolidone (NMP)-
HSO4, ionic liquid (IL), ferric hydrogen sulphate (Fe(HSO4)3), cholineper-
Syntheses of organic moieties via multicomponent reactions (MCRs)
have drawn significant attention due to the process simplicity and time
saving operations [1]. In addition, the general features of MCRs match well
with green chemistry criteria such as atom economy, waste prevention,
avoidance of hazardous compounds, and energy efficiency [2]. Among
various multicomponent reactions, the synthesis of Betti base and Bisamide
are found to be much crucial due to its unique and wide applications. These
compounds are important because of their essential medicinal properties
such as anti-inflammatory [3], anti-fungal [4], anti-microbial [5] and an-
ticancer activity [6]. The potential catalysts developed for the synthesis of
these compounds are trifluoromethanesulfonic acid (CF3SO3H), boric acid
(H3BO3), phosphotungstic acid (H3PW12O40) [7,8] zinc sulphide (ZnS),
nano ferrouso ferric oxide (Fe3O4), triethylamine sulfonic acid (Et3N-SO3H),
oxydisulfatemonohydrate (CHPS), and task specific ionic liquids (TSILs)
[9–11]. These catalysts have shown many draw backs like lack of en-
vironmental safety, use of toxic solvents, recyclability, stability and cost
[12]. In addition, to the best of our information, it is still a challenge to use
aromatic primary amines as reaction substrates in water owing to their
electron-withdrawing properties and steric hindrance [13]. This is because
the activity of amine is strongly facilitated with water due to hydrogen
bonding with amine. To overcome these problems, lanthanide triflate [14],
acid functionalised silica coated magnetic nanoparticles [15] and surfactant
[16] were successfully used under water condition or solvent-free condition
to synthesize these compounds with high yield. However, these catalysts are
not user friendly or not recoverable.
As a consequence, it is beneficial to develop environmentally benign
⁎ Corresponding author.
E-mail address: dashp@nitrkal.ac.in (P. Dash).
https://doi.org/10.1016/j.mcat.2019.110415
Received 18 February 2019; Received in revised form 17 May 2019; Accepted 18 May 2019
2468-8231/©2019PublishedbyElsevierB.V.

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
catalytic processes for Betti reaction. At the same time, synthesis of
Bisamides under solvent-free condition is a noteworthy protocol as sol-
vent–free conditions play a crucial role in green chemistry, since there is no
need for solvents which are detrimental to the environment [4,17]. Such
demand for an environmentally benign procedure with the use of a stable
and reusable catalyst encouraged us to develop a safe alternative method for
the synthesis of Betti bases and Bisamides. In catalysis, homogeneous cat-
alysts are more efficient than heterogeneous catalysts. However, they have
major drawbacks such as high cost, no recovery, and tedious workup. This
has triggered the interest more towards heterogeneous catalysts which are
cost effective, easily recoverable, and reusable and involve simple procedure
to form the product [18,19]. Besides, heterogeneous catalysis has emerged
as a useful methodology to reduce waste production owing to the simplicity
of the process, lower contamination of the products with the active catalytic
species, and less use of toxic solvents [20–22].Owing to their unique
properties like high surface-to-volume ratio, high surface area and facile
separation, nanostructures of metals and metal oxides have shown great
promise as catalysts in many organic reactions. Towards this direction, nano
crystalline metal oxides are found to be suitable catalyst for organic reaction
[16]. For example, karmakar et al reported an effective approach for
synthesis of Betti bases using nano MgO [7]. In another work, nano ZnO was
used to synthesize Betti base. One major problem associated with these
metal oxides are agglomeration due to their high surface area dispersed in
solution which resulted in lower yield of product [23]. However, recent
advancements in the synthesis of uniformly sized nanostructures offer nu-
merous opportunities to improve the catalytic performance of these mate-
rials [24–26]. This motivated us to develop novel approaches for preparing
nano crystalline metal oxides based highly active, recyclable and stable
heterogeneous nanocatalyst for the synthesis of Betti base and Bisamides.
Among other metal oxides, manganese dioxide (MnO2) has been widely
used as catalyst owing to its eco-friendly nature, existence of multiple
crystalline phases and oxidation states, low cost, high activity, non-toxicity
and high redox ability [27,28]. MnO2 can exist in many polymorphs such as
α, β and γ form via the linking of MnO6 octahedral units [29]. Out of all, β-
form exhibit rod like morphology and found to be the most stable form of
MnO2. The presence of large number of surface Mn4+ ions, homogeneous
distribution, shape anisotropy, high electronic effects and surface un-
saturation in MnO2 nanorods are some of the important factors responsible
for its good catalytic activity [30]. However, the increase in surface energy
due to large surface area causes various stability issues such as tendency to
aggregate, changes in shape, change in surface states leading to an in-
evitable loss of their original catalytic activities [31,32]. One novel ap-
proach to address these issues is found to be the design of composite na-
nocomposite with other metal nanoparticles such as Pt, Au, Ag and Pd. This
type of integration improves the catalytic activity because of synergistic
effect between the two components [33–35]. However, it has been shown
that such nanocatalyst generally do not retain high catalytic reactivity
without a solid support. Generally, they become unstable and structural
deformation can occur during the reaction [36]. To address these problems,
design of a ternary composite would be an ideal methodology in which the
binary composite can be anchored onto a solid support, particularly carbon
support. The carbon support will provide permeable pores, chemical inert-
ness, and good mechanical stability where as metal nanoparticle will pro-
vide more catalytic centres and further enhance the composite’s stability
[37–39]. Surprisingly, such ternary heterogeneous composite has drawn
little attention in the synthesis of biologically active molecules via MCRs.
Among various carbon materials, graphene oxide has shown great
promise as a suitable catalytic support material [40–42].The presence of
various oxygen containing functional groups such as hydroxyl, epoxy and
carboxyl with large surface area makes GO as a possible support for dec-
oration of catalytically active nanorod and nanoparticle onto it [25,43,44].
At the same time, its high surface area, ultrahigh electrical conductivity,
excellent mechanical properties, high thermal conductivity and the lattice
defects play an important role in catalysis. The surface defects and oxygen
containing functional groups in graphene oxide support can act as strong
binding traps for metal nanocatalyst which leads to long term stability of the
material [45,46]. The electronic interaction arising due to the transfer of
charge from metal to GO substrate in the composite materials results in the
enhancement of its catalytic activity [47,48]. These unique properties of GO
fulfil the basic requirements of an ideal catalyst support material. Similarly,
among various metal nanoparticles, incorporation of Au nanoparticles can
simultaneously enhance charge transfer ability due to local field enhance-
ment and also enhance the stability of the composite due to its chemical
stability [49]. Keeping all these factors into mind, it was envisaged that the
design of GO-MnO2-Au will provide a low-cost, stable and efficient het-
erogeneous nanocatalyst for MCRs. Good electronic interaction between the
three components, increased redox potential and possible synergistic effects
between them will help to enhance the catalytic activity. Moreover, this
methodology will minimize the possibility of nanocatalyst aggregation
during recovery and improves the durability of the nanocatalyst [50–52].
Herein, we report the successful demonstration of a nanorod deco-
rated graphene oxide-based GO-MnO2-Au ternary nanocomposite as a
promising heterogeneous catalyst for synthesis of Betti bases and
Bisamides. The reaction involves synthesis of amino alkyl napthol and
Bisamides by two separate routes from one substrate (aldehydes). The
addition of napthol aromatic amines with aromatic aldehydes followed
by elimination of water molecules led to the synthesis of amino alkyl
napthol (Betti base) whereas condensation of aromatic aldehydes with
two molecules of amide led to Bisamides synthesis. All the reactions
proceeded with high yields in shorter period of time as compared to
other reported catalysts. During the synthesis of Betti bases, higher
yield of product was obtained in water solvent using primary aromatic
amine as substrate. Similarly, the reaction went well under solvent-free
condition during the synthesis of Bisamides. The catalyst can be reused
with minimal loss in activity. Our current methodology demonstrates a
novel heterogeneous catalyst along with a versatile, stable, green and
convenient synthetic protocol that easily afforded the synthesis of Betti
bases and Bisamide. Several noticeable benefits are: (1) green reaction
condition reaction like water solvent or solvent-free condition; (2)
shorter reaction time with high yield, (3) easy catalyst recovery; and (4)
easy to handle, safe and stable catalyst with little leaching problem
[53,54]. These types of advantages were not obtained in some of the
reported literatures. Moreover, our current study can be successfully
extended to the synthesis of other important multicomponent reactions.
2. Experimental Section
2.1. Materials
HAuCl4, MnSO4 and KMnO4, were purchased from Hi-Media.
Graphite powder, H3PO4 and H2O2were purchased from Sigma-Aldrich.
NaBH4, ethanol, NaNO3, H2SO4 (98%), HCl and silica gel were pur-
chased from Avra Chemicals. All chemicals were used as received
without further purification.
2.2. Preparation of GO
Graphene oxide was prepared from the graphite flakes by modified
Hummer’s method. Firstly 1.4 g graphite flakes was taken in a 1000 ml
round bottom flask followed by addition of 2 g NaNO3 and 40 ml sulphuric
acid. After adding all, the mixture was continuously stirred in ice-cold
(0–5 °C) bath. The solution was stirred for 2 h followed by adding KMnO4
carefully and maintaining the temperature below 14 °C. Then, the solution
was brought to room temperature. After some time, the solution became
pasty brownish and then again stirred for 2 h. During constant stirring,
temperature increased in each half an hour and then 100 ml water was
added resulting in the change of colour to brown. Later on, 200 ml water
was added and then stirred continuously. The solution was finally treated
with 20 ml H2O2 to terminate the reaction with formation of yellow colour.
The solution was purified by centrifugation and rinsed with HCl and with DI
water for various times. After filtration it was dried overnight in oven to get
the synthesized GO.
2

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Scheme 1. Synthesis scheme of catalyst GO-MnO2-Au.
Fig. 2. FTIR spectra of GO, GO-MnO2 and GO-MnO2-Au.
Fig. 1. XRD spectra of GO, GO-MnO2 and GO-MnO2-Au.
and stirred for 30 min. After that 20 ml sodium borohydride (NaBH4) so-
lution (0.01 M) was added slowly under constant stirring for 20 min for
reduction of gold salt. The resulting solution was stirred for 6 h under ni-
trogen atmosphere at 80 °C and then cooled to room temperature. After that
it was centrifuged and washed several times with water to remove im-
purities. Then the resulting material was dried in the oven to get desired
GO-MnO2-Au nanocomposite (Scheme 1).
2.3. Preparation of GO-MnO2 nanorod composite
In a typical system, 0.016 g of MnSO4·H2O were mixed with 0.23 g
of GO in distilled water (50 ml) and then ultra-sonicated for 2 h forming
a brown homogeneous solution. In this procedure, Mn2+ of MnSO4.H2O
bond to the negative O atom present in functional groups of GO through
electrostatic interaction. Then, KMnO4 solution was prepared by dis-
solving 0.32 g of KMnO4 in 50 ml distilled water. The obtained KMnO4
solution was added to the MnSO4.H2O solution under constant vigorous
stirring. After that whole solution was transferred to the autoclave,
sealed and heated at 120 °C for 12 h. After completion of reaction, the
solution was brought to room temperature. The formed brown-black
precipitates were collected and washed several times with ethanol to
remove the excess of impurities.
2.4.1. General procedure for synthesis of Betti base
In a typical procedure, benzaldehyde, 2-napthol and aniline having
same ratio (1 mmol) were taken in a round bottom flask and mixed and
stirred at room temperature, followed by the addition of GO-MnO2-Au
(15 mg) catalyst in water. The progress of the reaction was monitored
by thin layer chromatography (TLC). After completion of reaction, the
catalyst was separated by centrifugation and then washed and dried for
further use. The unreacted materials present in the product were re-
moved by washing with water and dissolved in ethyl acetate. The final
pure product was separated by column chromatography.
2.4. Preparation of GO-MnO2-Au composite
In brief, 96 mg of GO-MnO2 was taken in 50 ml of distilled water fol-
lowed by addition of 0.1 ml of 0.01 M prepared poly vinyl pyrrolidone
(PVP) solution. The solution was stirred and heated to boiling. Then, 12 ml
of 1 wt% HAuCl4 solution was added quickly under nitrogen atmosphere
2.4.2. General procedure for synthesis of Bisamides
Briefly, 1 milimole of benzaldehyde and 2 milimole of acetamide
were taken in a round bottom flask followed by addition of a catalytic
3

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Fig. 3. FESEM images of (a) GO (b) GO-MnO2 and (c–f) GO-MnO2-Au.
Fig. 4. (a–d) TEM images of GO-MnO2-Au and (e, f) HRTEM images of GO-MnO2-Au.
ͦ
amount of GO-MnO2-Au (15 mg) in solvent-free condition at 80 C. The
progress of the reaction was checked by TLC. After the reaction was
completed, product was separated out and washed and then dissolved
in ethyl acetate. The final product was separated by column chroma-
tography.
diffraction system with Ni filtered Cu-Kα source. The composition in-
formation was measured using EDX (JEOL JSM-6480LV). Raman
spectra were recorded using
a BRUKER RFS 27 spectrometer.
Transmission electron micrographs (TEM) of the sample were recorded
using PHILIPS CM 200 equipment using carbon coated copper grids.
Field emission scanning electron microscopy (FESEM) of the sample
was recorded by Nova Nano SEM/FEI-450. Nitrogen adsorption/deso-
rption isotherm was obtained at 77 K on a Quantachrome Autosorb-IQ
3-B apparatus. The specific surface area and pore size distribution were
3. Characterisations
FTIR spectra of the product were recorded using a Perkin-Elmer-
1000 FTIR spectrophotometer using NaCl support. XRD patterns were
analysed by using Rigaku, ultimate-iv, japan multipurpose X-ray
acquired by emulating BET equation and BJH method, respectively. 1
H
NMR and 13C NMR spectra were recorded on a Bruker spectrometer at
4

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Fig. 5. (a-f) Elemental mapping of GO-MnO2-Au.
Fig. 6. Raman spectra of GO, GO-MnO2 and GO-MnO2-Au.
4. Results and discussion
Table 1
Surface area and pore volume of GO,GO-MnO2 and for GO-MnO2-Au.
The MnO2 nanorod was decorated on GO surface via hydrothermal
method. During the synthesis of GO-MnO2 composite, initially, large
number of nuclei aggregated into a mixture of α and γ form of MnO2
having dendrites structure. With time this structure broke down into
micro rods and phase transformation occur leading to the formation of
β-form of MnO2 [55].The suggests that β-MnO2 is the most thermo-
dynamically stable form of MnO2. During the formation, following re-
action takes place:
Catalyst
Surface area (g2. m−1
)
Pore volume (nm)
MnO2
58
3.2
5.8
5.3
GO-MnO2
GO-MnO2-Au
132
118
400 MHz using TMS as an internal standard. A commercial electron
energy analyzer(PHOIBOS 150 from Specs GmbH, Germany) and a non-
monochromatic Mg Kα X-ray source have been used to perform XPS
measurements with the base pressure of 10 mbar supplied by Thermo-
scientific, UK, equipped with an Al Ka (1486.6 eV). Catalytic reactions
were monitored by thin layer chromatography on 0.2 mm silica gel F-
254 plates. ICP-OES were performed using Perkin Elmer (Optima2100
DV) in the range of 0.005–100.0 mg/L. All the reaction products are
known compounds and were identified by comparing their physical and
spectral characteristics with the literature reported values.
3Mn2+ + 2MnO4−+ 2H2O → MnO2+4H+
The ternary nanocomposite (GO-MnO2-Au) was then synthesized by
simple reduction of gold salt onto the GO-MnO2 composite. As the
morphology of nanoparticles and their distribution on the surfaces of
nanorods are critical to the catalytic properties, care was taken to
uniformly modify the surfaces of nanorods with monodispersed nano-
particles. In the ternary composite, GO provides the stability to MnO2
5

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
and Au by making different types of electrostatic interaction with
oxygen containing functional groups. Because of such interaction, the
catalytic properties of the ternary composite could substantially im-
proved as discussed in the catalytic studies section. Later on, to check
the successful synthesis of MnO2 nanorod, decoration of these rods on
GO structure, the presence of functional groups on GO and the suc-
cessful synthesis of the desired ternary nanocatalyst, various char-
acterization techniques were utilized and the results are outlined below.
Initially, the phase purity and presence of functional groups on the
nanocomposite were investigated by XRD and FTIR techniques. Fig. 1
shows the XRD patterns of individual GO, GO-MnO2 and GO-MnO2-Au
composite. The diffraction peaks at 110,110, 101, 200, 210, 211, 002
and 310 matches with the β-form of MnO2 (JCPDS 24-0735). However,
the relative intensities of peaks were found to be higher than other
reported MnO2 particle, which reveals the formation of rod like mor-
phology. XRD peaks of GO shows
a broad diffraction peak at
2θ = 10.89° corresponding to (002) plane which is very large compared
to the graphite. No distinct peaks of GO was observed in both GO-MnO2
and GO-MnO2-Au which may be due to incorporation of MnO2and Au
nanoparticles in between GO layers. After the deposition of gold na-
noparticles on GO-MnO2, the obtained XRD patterns remains almost
unchanged with weak metallic gold diffraction lines at 38.33° and
44.56° which are assigned to face-cantered cubic (fcc) bulk gold (111)
and (200) planes, respectively.
FTIR spectra were carried out to study the presence of various
functional groups in the GO-MnO2-Au nanocomposite. Fig. 2 shows the
FTIR spectra of GO, GO-MnO2 and GO-MnO2-Au. The broad absorption
peak at 3419 cm−1 in the spectra of GO can be attributed to the
stretching vibration of −OH group. The peaks corresponding to
Fig. 7. XPS Survey scan study of GO, GO-MnO2 and GO-MnO2-Au.
Fig. 8. XPS spectra (a,b) O 1s XPS spectra of GO-MnO2 and GO-MnO2-Au, (c and e) Mn 2p spectra of GO-MnO2 and GO-MnO2-Au, (d) 4f spectra of Au, (f, g, h) C 1s
XPS spectra of GO,GO-MnO2 and GO-MnO2-Au.
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P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Scheme 2. Reaction scheme for synthesis of Betti base.
Table 2
Table 4
Effect of catalyst for synthesis of Betti base synthesis.
Effect of metal oxide loading (MnO2 with respect to GO) for Betti base synth-
esis.
Entry Solvent Catalyst
Catalyst
loading
Time
Temp.(oC) Yield (%)
(min.)
1
2
3
4
5
6
Water
Water
Water
Water
Water
Water
GO-MnO2 5 wt(%)
GO-MnO2 10 wt(%)
GO-MnO2 15 wt(%)
GO-MnO2 20 wt(%)
GO-MnO2 25 wt(%)
GO-MnO2 30 wt(%)
48
40
31
25
25
27
rt
rt
rt
rt
rt
rt
41
52
71
80
79
77
Entry Solvent Catalyst
Catalyst
loading
Time(min.) Temp.(oC) Yield (%)
1
2
3
4
5
Water
Water
Water
Water
Water
–
–
180
50
rt
rt
rt
rt
rt
Trace
50
GO
2O wt(%)
2O wt(%)
Reaction condition :- Benzaldehyde (1 mmol),2-napthol (1 mmol), and aniline
(1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
GO-MnO2
25
80
GO-MnO2-Au 2O wt(%)
10
96
MnO2
2O wt(%)
39
66
over both rods and also on GO sheet. After deposition of MnO2 na-
norods and gold nanoparticles, the layer structure of GO was still
maintained. The rod shape and structure of gold nanoparticles in the
nanocomposite were further confirmed from the TEM images.
Reaction condition :- Benzaldehyde (1 mmol),2-napthol (1 mmol), and aniline
(1 mmol), catalyst (15 mg),water solvent, rt.
1732 cm-1 attributed to carbonyl group of carboxyl (C]O), 1372 cm-1
associated to phenol and alcohol groups, 1263 cm−1 attributed to
epoxy and ether groups, 1595 cm−1 to C]C and 1065 cm−1 to CeOeC
groups. In the IR spectrum of GO-MnO2, stretching vibrations of
CeOeC and OeH peaks reduced due to reduction of some oxygen
containing functional groups during deposition of MnO2. Further, one
additional peak was observed at 597 cm−1 that is assigned to Mn-O
stretching vibration [56] confirming the successful decoration of MnO2
nanorods on GO. After deposition of gold nanoparticles the intensities
of the peaks remains almost same and the external and internal struc-
ture of GO-MnO2-Au was further studied by FESEM and TEM.
TEM analysis is one of the most important techniques to understand
the structural morphology of a nanomaterial. TEM images shown in
Fig. 4a further reveal that the MnO2 have well-defined rod like mor-
phology. The lattice fringes of the β-MnO2 nanorods shown in the high-
resolution TEM (HRTEM, Fig. 4d) image indicate the high crystalline
nature of these nanorods, and the measured lattice spacing is confirmed
to be 0.30 nm, which is consistent with the d-spacing of the (200) plane
of the tetragonal β-MnO2 crystal [57]. Gold nanoparticles are found to
be spherical in shape and are highly dispersed on the GO-MnO2 surface
which prevent the restacking of GO layer, leading to the enahanced
stability of GO. Gold particle sizes are estimated to be 7
1.9 nm. To
find out the distribution of MnO2 on GO and Au on GO-MnO2 nano-
composite, elemental mapping of the nanocomposite were carried out.
Fig. 5b and c depicts the elemental mapping of gold, manganese, carbon
and oxygen. From the elemental mapping of manganese it is clearly
seen that it forms a rod like structure. From the elemental mapping of
gold it is clear seen that gold particles are homogeneously distributed
over GO and MnO2 in the ternary composite.
FESEM is a powerful technique to investigate the surface mor-
phology of a sample. Fig. 3 shows the FESEM images of GO, GO-MnO2
and GO-MnO2-Au. It is clear from FESEM image that the layer structure
with sub micrometre pore is visible in the case of pure GO (Fig. 3a).
From Fig. 3, it can be seen that MnO2 forms rods like morphology and
are homogeneously distributed throughout GO surface. The diameter of
the rods is found to be in the range of 60–100 nm and length around
400–600 nm. In Fig. 3c, gold nanoparticles are found to be dispersed
To understand the formation any surface defects, degree of
Table 3
Effect of solvent for synthesis of Betti base synthesis.
Entry
Solvent
Catalyst
Catalyst loading
Time(min.)
Temp.(oC)
Yield (%)
1
2
3
4
5
6
7
Hexane
THF
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
20wt(%)
20wt(%)
20wt(%)
20wt(%)
20wt(%)
20wt(%)
20wt(%)
15
21
19
18
15
15
10
rt
rt
rt
rt
rt
rt
rt
56
72
75
63
43
81
96
Acetonitrile
DCM
Toluene
Methanol
Water
Reaction condition :- Benzaldehyde (1 mmol),2-napthol (1 mmol), and aniline (1 mmol), GO-MnO2-Au (15 mg), rt.
7

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 5
Effect of metal nanoparticle loading (Auwith respect to GO-MnO2) for Betti base synthesis.
Entry
Solvent
Catalyst
Catalyst loading
Time(min.)
Temp.(oC)
Yield (%)
1
2
3
4
Water
Water
Water
Water
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
0.5 wt(%)
1.0 wt(%)
1.5 wt(%)
2.0 wt(%)
18
10
11
13
rt
rt
rt
rt
88
96
96
95
Reaction condition :- Benzaldehyde (1 mmol),2-napthol (1 mmol), and aniline (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
Table 6
Effect of catalytic loading of GO-MnO2-Au for Betti base synthesis.
Entry
Solvent
Catalyst
Catalyst amount (mg)
Time(min.)
Temp.(oC)
Yield (%)
1
2
3
4
5
Water
Water
Water
Water
Water
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
5
16
13
10
11
12
rt
rt
rt
rt
rt
85
89
96
95
95
10
15
20
25
Reaction condition :- Benzaldehyde (1 mmol), 2-napthol (1 mmol), and aniline (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
spectrum of GO-MnO2 and GO-MnO2-Au shown in Fig. 8 (c, e) depicts
hybridisation, crystal order and extent of chemical modification in the
nanocomposite, Raman measurements were then carried out. Raman
spectroscopy could possibly give useful information in the GO-MnO2-Au
nanocomposite in comparison to GO and GO-MnO2 due to the enhanced
disorder created in it. In Raman spectra of graphene oxide (Fig. 6), two
major peaks were observed near 1340 cm−1 and 1592 cm−1, corre-
sponding to the well-documented D and G bands, respectively. The D
band at 1340 cm−1 arises due to the defect induced in the sp2 ring of
graphitic materials due to oxidation and G band is due to first order
scattering of E2g phonons of sp2 carbon [58]. The larger intensity and
line width of D band than G band signifies more disorder in the struc-
tures due to defects. When MnO2 was decorated on GO sheet its ID/IG
value increased (0.98) with respect to GO (0.96) indicating the for-
mation of slight disorderness in the structure. This suggests that elec-
tronic interaction occurred on deposition of MnO2 nanorod on GO
sheet. Further, while decorating gold nanoparticles on composites the
intensity ratio of D and G band remains almost same while little
broadening of peak happened. This possibly represents the damage in
the structure due to the electron-phonon coupling by Au ions [59].
Surface area of nanocomposite plays an important role in catalysis
by providing more surface active sites on which more number of re-
actants can be adsorbed. In order to find out the surface area, the N2
adsorption- desorption studies were carried out for GO, GO-MnO2 and
GO-MnO2-Au and the results are shown in Fig. S1. It was found that all
the samples show type-IV isotherm. The average pore diameters in
MnO2, GO–MnO2 and GO-MnO2-Au composite were found to be 3.2 nm,
5.8 nm and 5.3 nm, respectively whereas BET surface areas were 58
m2 g−1, 132 m2 g−1, and 118 m2 g−1, respectively (Table 1). The in-
crease in surface area after decorating MnO2 on GO can be attributed to
the reduction of oxygen functional groups during the synthesis while
further deposition of gold nanoparticles blocked the pore size resulting
in the decrease of the surface area [60].
two peaks at 641.6 and 654.4 eV assigned to Mn 2p3/2 and Mn 2p1/2
,
respectively. The two peaks observed for Mn is due to spin orbit cou-
pling with an energy separation of 12.8 eV which confirmed that the
Mn in GO-MnO2 and GO-MnO2-Au remained tetravalent (Mn4+
)
[63,64]. It can also be seen that the 2p peaks of Mn in GO-MnO2-Au
decreased to small extent possibly due to deposition of Au nanoparticles
on MnO2 surface. Two peaks observed for O1 s at 529.7 eV and 532.6 eV
is due to Mn-O and oxygen containing functional groups associated
with GO. All the above data clearly reveals that successful decoration of
MnO2 nanorod on GO surface. The decrease in peaks of oxygen con-
taining functional groups suggests the presence of a strong chemical
interaction between Mn with GO surface which was previously con-
firmed from FTIR analysis. Fig. 8d shows the high resolution XPS
spectra of Au 4f in which two distinct peaks separated by 3.6 eV can be
seen which suggests the spin orbit coupling of 4f orbital. The two peaks
at 87.81 eV and 84.24 eV corresponds to Au 4f7/2 and 4f5/2 in GO-
MnO2-Au samples which indicates that gold is found to be in metallic
form (Au0) [63].The peaks of 4f7/2 was shifted to 0.2 eV from the me-
tallic gold reported in literature which further suggest the possible
electron interaction between the species in the nanocomposite. After
successful characterization, the potential and efficiency of the designed
nanocatalyst (GO-MnO2-Au) as active and stable catalyst was in-
vestigated in the synthesis of Betti bases and Bisamides.
5. Catalytic study
5.1. Synthesis of Amino alkylnapthol (Betti base)
The catalytic efficiency of GO-MnO2-Au was then investigated in the
multicomponent reactions of aldehydes, aniline and β-napthol to syn-
thesize amino alkylnapthol (Scheme 2). To get the optimisation con-
dition, the reaction of benzaldehyde, aniline and β-napthol was taken as
the model reaction. Initially, different catalysts were investigated in
water media in order to find out the suitable catalyst for synthesis of
amino alkyl napthol. When no catalyst was used trace amount of pro-
duct was obtained (entry 1, Table 2). Then, we investigated the reaction
under homogeneous catalytic conditions using MnSO4.H2O, KMnO4,
and HAuCl4.3H2O. However, they did not show any good activity under
these conditions. To study the heterogeneous systems, we examined the
catalytic reactivity of unsupported MnO2 catalyst (Table 2 and 8 (entry
5). The reaction took longer time in both the cases. When we tested the
catalytic performance of GO supported GO-MnO2 and GO-MnO2-Au
under identical reaction conditions, better yield of the product was
obtained. From this experiment it can be inferred that the nanorods
Later on, the chemical composition and oxidation state of metals in
GO-MnO2-Au were analysed by X-ray photo electronic spectroscopy
(XPS).The presence of all the elements C, O, Mn and Au in GO-MnO2-Au
were confirmed from the survey scan XPS spectra (Fig. 7). The C1 s core
level XPS spectrum of GO (Fig. 8e) consist of four main peaks out of
which two peaks at284.5 and 286.7 eV, corresponds to C]C/C-C (sp2)
of aromatic rings and CeOH of hydroxyl groups. Other two peaks at
287.8 and 289.1 eV corresponds to keto group (C]O) of carbonyl and
carboxyl groups. After the decoration of MnO2 on GO (Fig. 8f) and Au
on GO-MnO2 (Fig. 8g) surface, decrease in intensities of oxygen con-
taining functional groups were found due to the binding of metal (Mn
and Au) with oxygen containing functional groups [61,62] and due to
reduction of oxygen functional groups during the synthesis. Mn 2p
8

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 7
Effect of different substrate for synthesis of Betti base synthesis.
Entry
1
Aldehyde
P-NO2
Product
Time
7
Yield
95
2
P-Br
9
95
3
4
P-OMe
P-C2H5
13
13
88
88
Entry
5
Aldehyde
P-OMe
Product
Time
Yield
93
11
6
7
M,P-OMe
12
93
95
O.M,P-OMe
13
8
9
P-C2H5
P-Cl
9
8
93
94
10
11
P-Br
H
9
7
13
96
(continued on next page)
9

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 7 (continued)
Entry
12
Aldehyde
P-NO2
Product
Time
7
Yield
95
13
14
O-NO2
P-CN
8
9
95
94
15
H
C4H9NH2
14
89
Reaction condition: Benzaldehyde (1 mmol),2-napthol (1 mmol), and aniline (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
Reaction condition: Benzaldehyde (1 mmol),2-napthol (1 mmol), and pyrrolidine (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
Reaction condition: Benzaldehyde (1 mmol),2-napthol (1 mmol), and piperidine (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
Table 8
Effect of catalyst for synthesis of Bisamide synthesis.
Entry
Catalyst
Catalyst loading
Solvent
Temp.(oC)
Time (min.)
Yield (%)
1
2
3
4
5
–
–
Solvent less
Solvent less
Solvent less
Solvent less
Solvent less
80
80
80
80
80
90
44
21
4
Trace
23
GO
15 wt(%)
15 wt(%)
15 wt(%)
15 wt(%)
GO-MnO2
GO-MnO2-Au
MnO2
75
97
32
58
ͦ
Reaction condition: - Benzaldehyde (1 mmol), and acetamide (2 mmol), catalyst (15 mg), solventless, 80 C.
Scheme 3. Reaction scheme for synthesis of Bisamide.
Table 9
Effect of solvent for synthesis of Bisamide synthesis.
Entry
Solvent
Catalyst
Catalyst loading
Time(min.)
Temp.(oC)
Yield (%)
1
2
3
4
5
6
7
Hexane
THF
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
15 wt(%)
15 wt(%)
15 wt(%)
15 wt(%)
15 wt(%)
15 wt(%)
15 wt(%)
15
21
19
18
15
15
4
80
80
80
80
80
80
80
48
61
69
53
82
75
97
Acetonitrile
DCM
Water
Methanol
Solvent less
°
Reaction condition: - Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-MnO2-Au (15 mg), 80 C.
10

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
from 41 to 80% and then remained almost constant with further in-
crease in loading. The constant or slight decrease in catalytic properties
is due to the agglomeration of nanorods which cover the active site in
the ternary composite [68]. This result motivated us to modify the Au
loading to improve the reaction rate as well as product yield. To get
high yield of product in a fixed amount we varied the weight percen-
tage (wt %) of Au from 0.5 to 2.0 with respect to GO-MnO2and found
that at 1 wt% catalyst show high yield of 96% (Table 5, entries
2).Further, to check the effect of amount of GO-MnO2-Au catalyst, the
amount was varied and found that 15 mg (Table 6, entry 3) as the
optimised catalyst for synthesis of amino alkyl napthol. The reaction
was also carried out in solvent-free condition at room temperature but
the yield of the product was not satisfactory at room temperature. Thus,
the optimised catalyst for the synthesis of Betti base was; 15 mg of 20 wt
% of MnO2 with respect to GO and 1 wt% of gold with respect to GO-
MnO2 in water medium at room temperature. The Mn and Au loading
on the above optimised nanocatalyst, measured by ICP-OES, was found
to be ˜12.73 wt % and ˜ 0.96 wt%, respectively. The mol% of the metal
catalyst was found to be 3.48 mol % Mn and 0.08 mol% Au from ICP-
OES analysis in the ternary composite [69–71]. After the optimisation
of catalyst, a series of products were then synthesized by taking alde-
hyde as a substrate. Encouraged by this success, to further evaluate the
scope of this reaction, different aromatic aldehydes, amines, 2-napthol
and amides were used under similar conditions (Tables 7 and 13).Al-
dehydes with functionality gave much satisfactory results, however
groups like NO2, Br and Cl were found to be more reactive and com-
pleted the reaction in shorter period of time (Table 7, entry 1,2 and 9).
Benzamides were found to be highly reactive than acetamide that might
be due to more basicity of Benzamides. It was observed that the sub-
stituent at para position ((Table 13, entry 3) showed better yields as
compared to that at ortho positions (Table 13,entry 7).
Fig. 9. Effect of temperature with yield for synthesis of bisamides.
Reaction condition : Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-
°
MnO2-Au (15 mg), solventless, 80 C.
Table 10
Effect of metal oxide loading (MnO2 with respect to GO) for Bisamide synthesis.
Entry Catalyst
Catalyst
Solvent
Temp.(oC) Time
Yield (%)
loading
(min.)
1
2
3
4
5
GO-MnO2 5 wt (%)
GO-MnO2 10 wt (%)
GO-MnO2 15 wt (%)
GO-MnO2 20 wt (%)
GO-MnO2 25 wt (%)
Solvent less 80
Solvent less 80
Solvent less 80
Solvent less 80
Solvent less 80
35
29
21
22
25
51
69
75
74
74
Reaction condition: - Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-
MnO2-Au (15 mg), solventless, 80 °C.
5.2. Synthesis of Bisamides
supported on stable support provide a higher product yield in less time.
However, highest yield (96%) of the desired product was obtained in
presence of GO-MnO2-Au as catalyst (entry 4, Table 2) which tells that
ternary composite is superior to that of a binary compound. Catalysts
are strongly influenced by solvents because of polarity, protictity (hy-
drogen bond forming ability) and basicity. On account of this, we had
investigated the role of solvent by varying the solvent media for the
model reaction and the result is summarised in Table 3. Both polar and
non-polar solvents are used for optimisation. It was found that less
product yield (56%) obtained in nonpolar solvent (Table 3 entry1)
whereas the yield of the product increased moderately in solvents like
tetra hydro furan (THF), dichloro methane (DCM), toluene and acet-
onitrile (Table 3 entry 2–6). The yield of Betti reaction increased with
increase in polarity of the solvent and water shows high affinity to-
wards this reaction because it can form hydrogen bonding, charge
stabilisation and dipolar effects [65–67]. Additionally, solubility of
substrate in water is high and formation of transition state would be
sooner.
Similarly, the reaction between benzaldehyde and acetamide was
taken as a model reaction to get the optimisation conditions for the
synthesis of Bisamide (Scheme 3). Here, at first the effect of different
type of catalysts was investigated. The reaction was allowed to proceed
both in absence and presence of catalyst in solvent-free conditions at
80 °C and it was found that the presence of catalysts is essential for the
reaction. The yield was found to be less in presence of GO (23%) as
compared to GO-MnO2 (74%) and GO-MnO2-Au (97%) (Table 8 entries
1–4).
Later on, the reaction in different polar, non-polar and also in sol-
vent-free condition was carried out. The highest product yield was
found in case of reaction occurring in solvent-free condition as com-
pared to solvent media (Table9, entry7). Temperature is an essential
parameter which has a greater impact on reaction progress. To in-
vestigate the effect of temperature on catalytic activity of GO-MnO2-Au,
the temperature was varied. It was found that with rise in temperature
the product yield was increased up to 80 °C and then for further in-
crease in temperature yield remained almost same (Fig. 9).
After optimising the solvent media, all other optimisations were
done by taking water as media. The effect of MnO2 on product yield was
investigated by changing the loading of MnO2 from 5 to 30 % (Table 4,
entries 1–5). It was observed that the yield of the product increased
To get the optimised condition for the amount of active centres like
MnO2 and Au on GO surface, the wt% of both the elements were then
varied. The yield of product was found to increase from 51% to 75%
Table 11
Effect of metal nanoparticle loading (Au with respect to GOMnO2)) for Bisamide synthesis.
Entry
Catalyst
Catalyst loading
Solvent
Temp.(oC)
Time (min.)
Yield (%)
1
2
3
4
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
0.5 wt (%)
1.0 wt (%)
1.5 wt (%)
1.5 wt (%)
Solvent less
Solvent less
Solvent less
Solvent less
80
80
80
80
8
4
6
9
70
97
95
94
Reaction condition: - Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-MnO2-Au (15 mg), solventless, 80 °C.
11

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 12
Effect of catalytic loading of GO-MnO2-Au for Bisamide synthesis.
Entry
Catalyst
Catalyst loading(mg)
Solvent
Temp.(oC)
Time (min.)
Yield (%)
1
2
3
4
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
GO-MnO2-Au
5
Solvent less
Solvent less
Solvent less
Solvent less
80
80
80
80
11
6
80
87
97
95
10
15
20
4
5
Reaction condition: - Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-MnO2-Au (15 mg), solventless, 80 °C.
with the increase in MnO2 loading on GO from 5% to 15% (Table10,
entries 1–3) and almost remained same on further increase in wt% of
MnO2 with respect to GO. With increasing gold loading the yield of the
reaction was increased linearly up to 1 wt% after which the yield in-
creased slightly. The increased activity was due to the presence of ac-
tive sites of gold nanoparticles [72] whereas decreased activity was due
to covering of Au nanoparticles on the active site of MnO2 nanorods and
also agglomeration of Au nanoparticles [73,74]. From the investigation
it was found that 1 wt% of gold with respect to GO-MnO2 shows high
activity (97% yield) towards the synthesis of Bisamides under solvent-
free condition at 80 °C(Table 11,entries1–4). Later on, to optimise the
amount of catalyst we had varied the amount from 5 to 20 mg (Table 12
entry 3) and found that 15 mg shows better yields with less time.
Therefore, the optimised conditions for the synthesis of Bisamides were:
15 mg of 15 wt (%) of MnO2 with respect to GO and 1 wt% of gold with
respect to GO-MnO2in solvent-free condition at 80 °C. The Mn and Au
loading on the above optimised nanocatalyst, measured by ICP-OES,
was found to be ˜9.49 wt% and ˜ 0.95 wt%, respectively. From same
ICP-OES study, the mol% of the metal catalyst was found to be 2.60 mol
% Mn and 0.079 mol% Au in the ternary composite [69–71,75]. After
optimisation of the catalyst, the scope and limitations of this procedure
were studied by carrying out a series of reactions by taking aldehydes as
variable substrates (Table 13).The reactions went well with a good
product yield with almost all the aldehydes. However, it was seen from
the investigation that electron withdrawing group at para position of
benzaldehyde accelerate the rate of reaction for Bisamide synthesis.
To demonstrate the advantages of our catalyst for the synthesis of
biologically important molecules such as Betti bases and Bisamides, we
compared the activity of our catalyst with other reported catalyst and
the data is summarised in table (Table14 and 15). Our catalyst shows
better results than other reported catalyst like high yield in less reaction
time, environmental friendly and green conditions. For example, when
we compare the activity, stability and reactivity with other reported
catalyst like TiO2, ZnS and CTAB (Table 14, entry 3–5) for Betti base
synthesis we found high yield (96%) of products in shorter period of
time. Moreover, stability and recyclability is an issue in thee reported
systems. Similarly, for the synthesis of Bisamides, some of the pre-
viously used catalysts involve acid catalysts or ionic liquid based cat-
alysts like SiO2-IL and CHPS-TSIL (Table 15, entry 7, 8). However, acid
catalysts are not environmental friendly while ionic liquids are quite
expensive. Thus it can be confirmed that our catalyst (GO-MnO2-Au) is
more superior for synthesis of both Betti bases and Bisamides with
superior properties.
from BET measurements, it adsorbs more reactant molecules on to it
through π-π stacking and electro static interaction leading to en-
hancement in catalytic property [79]. In the end, when MnO2 and Au
nanoparticles are decorated over graphene oxide sheet it shows high
activity possibly due to synergistic effects between GO,MnO2 and Au.
To study such synergistic effect, the catalytic activity of both prepared
nanocomposite and the physical mixture of its individual components
were tested. It was found that the reaction containing GO-MnO2-Au
nanocomposite shows highest yield in both the synthesis of Betti base
(96% yield) and Bisamides (97% yield) whereas the product yield was
found to be less in case of physical mixture of all the three components
such as GO, MnO2 nanorod and Au for the synthesis of both Betti bases
(73% yield) and Bisamides (78% yield). Therefore, all in all, the higher
catalytic activity of GO-MnO2-Au can be attributed to high redox po-
tential of MnO2, small and homogeneous distribution of Au nano-
particles, high surface area and possible synergistic effect between GO
and MnO2 nanorod as well as Au nanoparticles [80–84].
Heterogeneous nature of catalyst is the most important point of
research in catalysis. These heterogeneous catalysts are generally
composed of one or more catalytically active components and a func-
tional support, in which the interaction between the catalytic compo-
nents and the support materials endow composite catalysts with much
improved catalytic properties, such as significantly enhanced catalytic
activity, selectivity for target products, chemical stability and pro-
longed lifetime [18,85–89]. Such enhancement was seen in our cases
where GO supported binary and tertiary composite showed higher ac-
tivity than that of unsupported systems (Table 2 and 8, entry. 7). To
further confirm the heterogeneity of our catalyst, a leachibility study of
was done. For this a hot filtration test was carried out by taking ben-
zaldehyde (1 mmol), β-napthol (1 mmol) and aniline (1 mmol) for
amino alkyl napthol and benzaldehyde (1 mmol) and acetamide
(2 mmol) for Bisamides synthesis. The synthesis of amino alkyl napthol
reaction was performed at room temperature for 5 min in water solvent.
After that catalyst was filtered in hot condition and the yield of product
was found to be 52%. No further yield beyond 55% is observed after
10 min of the reaction. For synthesis of Bisamides, the reaction was
performed for 2 min and yield was found to be 57%. Similar to above,
the catalyst was then filtered in hot condition and reaction was con-
tinued with the filtrate. Small increase in yield upto 60% was found
after 10 min. No further increase in yield after filtration suggests the
heterogeneous nature of our catalyst.
5.3. Reproducibility
From all the above data it can be concluded that GO-MnO2-Au
catalyst showed higher activity in comparison to other systems. The
tetravalent nature of Mn in MnO2 and high redox ability (due to re-
duction of Mn7+and oxidation of Mn2+to Mn4+) play an important role
for such increased catalytic activity [76]. Significant enhancement of
catalytic property after deposition of gold nanoparticles is due to highly
dispersion and small size of gold nanoparticles on surface of GO-MnO2
[77]. Electric dipole is the driving force in case of O = Mn = O for
homogeneous distribution of Au nanoparticles. When gold nano-
particles are decorated on MnO2 surface its redox potential values in-
creases drastically leading to increase in catalytic property [78]. Be-
sides, due to the high surface area of our nanocomposite as observed
Reusability and durability of the catalysts are important factors for
practical applications [90]. To verify these issues, we investigated se-
paration, leaching and recycling of the GO-MnO2-Au catalyst
(Table 16). The catalyst after one reaction were recovered by cen-
trifugation and washed with water and ethanol, then dried and reused
for another reaction under the same conditions. The catalyst was suc-
cessfully recycled and reused for four consecutive cycles with small
decrease in activity. The product yield was found to be 86% and 88%
for Bisamides and Betti bases from 96% and 97% respectively. The ICP-
OES analysis indicated very small amount of metal i.e. 0.08% Mn and
0.186 wt% Au during Betti base reaction while 0.06 wt% Mn and
12

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 13
Effect of substrate variation for Bisamide synthesis.
Entry
1
Aldehydes (R1)
H
Amides (R2)
CH3
Product
Time
7
Yield
96
2
3
H
Ph
Ph
5
4
97
97
P-NO2
4
P-Cl
Ph
6
95
5
6
P-Cl
CH3
CH3
10
12
93
91
P-OMe
7
8
O-OMe
P-F
CH3
CH3
14
9
90
94
9
P-Br
CH3
CH3
11
16
93
88
10
P-C2H5
11
12
P-OEt
CH3
Ph
13
10
86
92
P-OMe
Reaction condition :-Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-MnO2-Au (15 mg), solventless, 80 °C.
0.162 wt% Au during Bisamides were leached. These results indicate
the remarkable stability of catalyst. FESEM and TEM image of the re-
cycled catalyst showed that the catalyst was stable after the recycling
(Fig. S2). Therefore, the small decrease of catalytic activity seems to
result from either incomplete separation of the GO-MnO2-Au catalyst or
leaching of Mn and Au species during the successive recycling.
5.4. Probable reaction mechanism
The mechanism of these multicomponent reactions generally pro-
ceeds through addition as well condensation route (Scheme 4 and 5).
The Lewis acidic property of our catalyst GO-MnO2-Au catalyst acti-
vates the carbonyl oxygen of aldehydes and form an intermediate which
then reacts with aniline (in Betti reaction) and acetamide (in Bisamides)
13

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Table 14
Comparisation with other catalyst for Betti base synthesis.
Entry Catalyst
Solvent
Temp.(oC) Time
Yield (%)
1
2
3
4
5
6
7
8
9
TsOH
Solventless rt
Solventless rt
Solventless rt
Solventless rt
Solventless rt
Solventless rt
4.0 (hrs.) 35
Boric acid
TiO2
4.5 (hrs.) 15
4.0 (hrs.) 25
4.5 (hrs.) 13
ZnS
CTAB
5.2(hrs.)
59
Nano Fe3O4
Nano MgO
ZnO NPs
4.0 (hrs.) 74
1.5(hrs.) 62
Water
Water
Water
rt
rt
rt
4.0 (hrs.) 12
ZnO reverse micelle
(100 nm)
1 (hrs.)
98
96
10
ZnO reverse micelle
(200 nm)
Water
rt
30 (min)
11
12
Cerium(iv) nitrate
GO-MnO2-Au
Methanol
Water
rt
rt
30 (min)
10(min)
96
96
Scheme 4. Mechanistically synthesis of Betti base.
Table 15
Comparisation with other catalyst for Bisamide synthesis.
Entry
Catalyst
Solvent
Temp (oC)
Time
Yield (%)
1
2
3
4
5
6
7
8
CF3SO3H
MDC
Reflux
Reflux
110
125
100
85
0.2-48h
16-80h
15-50min
30min
3min
67-98
38-92
86-96
93
H3BO3
Tolune
Neat
TEA sulfonic acid
Iodine
Tolune
Neat
NMP HSO4
SiO2-IL
90
Neat
5min
92
CHPS-TSIL
GO-MnO2-Au
Neat
70
5-20min
4min
88-97
97
Solvent less
80
Table 16
Recyclibility of catalyst GO-MnO2-Au for sour successive runs.
(a)Recyclibity Test for synthesis of Betti bases.
Scheme 5. Mechanistically synthesis of Bisamide.
the electron from oxygen atom of substrate (aldehyde) and form co-
ordinate bond which activates the reaction. After that the metal gold
and metal oxide nanorod of MnO2 plays crucial role to accelerate the
catalytic activity by electronic effect. In addition, solvent water helps to
stabilise the reactants by forming hydrogen bond that play a role in
enhancement of the catalytic activity.
cycle
1
2
3
4
Yield of product (%) for Betti base(a)
97
95
91
88
(b) Recyclibity test foe synthesis of Bisamide
6. Conclusions
In summary, ternary GO-MnO2-Au nanocomposite was successfully
synthesized by hydrothermal route and used as a catalyst for the one
pot multicomponent synthesis of Betti bases and Bisamides using al-
dehydes as a substrate. Synthesis of Betti bases showed better results at
room temperature in water as a solvent while synthesis of Bisamides
showed at 80 °C in solvent free condition. The formation of nanorods
and β form of MnO2 were confirmed by FESEM, TEM and XRD. The
diameter of the rods was found to be 60–100 nm and average size of the
gold nanoparticles was found to be 7 1.9 nm. The oxidation state of
Mn (+4) and Au (0) was confirmed from XPS. The linkage between Mn-
O and also small reduction of functional group present over GO were
confirmed from XPS and FTIR. Both the biological active compounds
were synthesized by ternary composite with shorter time in green water
solvent and solvent-free conditions. The high activity of the catalyst can
be attributed to high redox potential with uniform rod shapes of MnO2,
small and uniform dispersion of gold nanoparticles and synergistic ef-
fect between GO, MnO2 and gold nanoparticles.
cycle
1
2
3
4
Yield of the product (%) For Bisamide(b)
96
93
89
86
Reaction condition : (a) Benzaldehyde (1 mmol),2-napthol (1 mmol), and ani-
line (1 mmol), GO-MnO2-Au (15 mg),water solvent, rt.
Reaction condition : (b) Benzaldehyde (1 mmol), and acetamide (2 mmol), GO-
MnO2-Au (15 mg), solventless, 80 °C.
to generate imine and acyl imminium intermediates respectively after
dehydration [13,91].The intermediate imine further adsorbed on sur-
face of catalyst and activated by metals (Mn and Au). More electrophilic
nature of intermediate fascinate the nucleophilic addition of 2-napthol
to give desired product and regenerates the catalyst. The product Bi-
samide obtained after the removal of catalyst from acyliminium inter-
mediate followed by addition of one more molecule of amide. Forma-
tion of intermediate in both cases is the rate determine step. The
syengergstic effects between GO, MnO2 and Au in GO-MnO2-Au helps to
enhance the catalytic property. Lewis acidic nature of catalyst accepts
14

P.S. Nayak, et al.
MolecularCatalysis474(2019)110415
Appendix A. Supplementary data
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