Easy conversion of nitrogen-rich silk cocoon biomass to magnetic nitrogen-doped carbon nanomaterial for supporting of Palladium and its application

Article abstract of DOI:10.1002/aoc.6039

In this study, magnetic nitrogen-doped carbon (MNC) was fabricated through facile carbonization and activation of natural silk cocoons containing nitrogen and then combined with Fe₃O₄ nanoparticles to create a good support material for palladium. Palladium immobilization on the support resulted in the formation of magnetic nitrogen-doped carbon-Pd (MNC-Pd). The prepared heterogeneous catalyst was well characterized using FT-IR, TGA, EDX, FE-SEM, XRD, VSM, and ICP-OES techniques. Thereafter, the synthesis of biaryl compounds was conducted to investigate the catalyst performance via the reaction of aryl halides and phenylboronic acid. Further, the catalyst could be used and recycled for six consecutive runs without any significant loss in its activity.

Full text of DOI:10.1002/aoc.6039

Received: 26 March 2020
DOI: 10.1002/aoc.6039
Revised: 26 August 2020
Accepted: 31 August 2020
F U L L P A P E R
Easy conversion of nitrogen-rich silk cocoon biomass to
magnetic nitrogen-doped carbon nanomaterial for
supporting of Palladium and its application
Parisa Akbarzadeh
| Nadiya Koukabi
Department of Chemistry, Semnan
In this study, magnetic nitrogen-doped carbon (MNC) was fabricated through
University, Semnan, 35131-19111, Iran
facile carbonization and activation of natural silk cocoons containing nitrogen
and then combined with Fe3O4 nanoparticles to create a good support material
for palladium. Palladium immobilization on the support resulted in the forma-
tion of magnetic nitrogen-doped carbon-Pd (MNC-Pd). The prepared heteroge-
neous catalyst was well characterized using FT-IR, TGA, EDX, FE-SEM, XRD,
VSM, and ICP-OES techniques. Thereafter, the synthesis of biaryl compounds
was conducted to investigate the catalyst performance via the reaction of aryl
halides and phenylboronic acid. Further, the catalyst could be used and
recycled for six consecutive runs without any significant loss in its activity.
Correspondence
Nadiya Koukabi, Department of
Chemistry, Semnan University, Semnan
35131-19111, Iran.
Email: n.koukabi@semnan.ac.ir
Funding information
Research Council of the Semnan
University
K E Y W O R D S
magnetic nanoparticles, nitrogen-doped carbon, palladium nanoparticle, silk cocoon, Suzuki–
Miyaura cross-coupling reaction
1 | INTRODUCTION
catalytic purposes.[9,10] The chemical environment of the
nitrogen atom is a crucial factor to determine important
Nowadays, there is a growing tendency to utilize the
heteroatom-doped carbon-based materials as a catalyst
support,[1–3] because of easy synthesis procedure, low
costs, high surface/volume ratio, perfect chemical and
thermal resistance, perfect control over morphological
structures, and suitability for noble metals immobiliza-
tion.[4] In addition to catalytic applications, they can be
used in different areas including sensors, fuel cells and
lithium ion batteries electrodes, adsorbents, and
properties, and it is categorized into two major groups:
structural nitrogen (i.e., pyridinic, pyrollic, or
quarternary-graphitic nitrogen), chemical nitrogen
(i.e., nitrosyl or amine group).[11] Considering the princi-
ples of green and sustainable chemistry, utilizing abun-
dant, inexpensive, and renewable resources to prepare
nitrogen-doped carbon compounds would be a desirable
goal.[12] The pyrolysis of nitrogen-rich biopolymers is one
of the most suitable procedures for the preparation of the
aforementioned substances.[13,14] Silk, a natural com-
pound made by Bombyx mori, is an environmentally
friendly protein-based polymer, and it has been used in
textiles industries for thousands of years.[15] Silk is
mainly composed of hydrophobic fibroin as the core.
Also, it contains hydrophilic sericin surrounding the
fibrous fibroin to cement them together.[16] As silk fiber
is a natural filamentous protein-based polymer, it is made
from repetitive amino acid sequences.[17] Due to high
supercapacitors.[5–7]
In
catalytic
applications,
heteroatom-doped carbon-based materials have opened a
new window for developing new catalysts which can be
used in organic synthesis. The nitrogen-containing natu-
ral compounds have attracted researcher's attention in
order to prepare a nitrogen-doped carbon catalyst.[8]
Recently, the nitrogen-doped carbon with enriched het-
erogeneous sites and improved porosity has drawn con-
siderable attention as a support for noble metals for
Appl Organomet Chem. 2020;e6039.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6039

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AKBARZADEH AND KOUKABI
biocompatibility and biodegradability, most researches
on silk are concentrated at biomedical and clinical
fields.[18] A silkworm cocoon, complex polymeric fibrous
network, is employed as a precursor for carbonization to
prepare nitrogen-doped carbon compounds.[19] In fact, it
contains fibroin and sericin providing carbon and nitro-
gen resources for preparing nitrogen-doped carbon com-
pounds. At the temperature of 350ꢀC, the protein
secondary structure comprising β-sheet fibroin and
α-helix sericin can be converted into sp2-hybridized struc-
ture or a hexagonal lattice structure of carbon atoms.[20]
This can improve the thermal stability of the nitrogen-
doped carbon framework.[21] Herein, we report a novel
and inexpensive procedure to prepare nitrogen-rich car-
bon materials via a thermal carbonization route under a
controlled temperature using B. mori silk cocoons as a
carbon and nitrogen resources and potassium hydroxide
(KOH) as an activating agent. After KOH pellets are dis-
solved, the silk surface is saturated with KOH, and the
reaction of K+ ions with C and O of silk during the high
thermal process produces K2CO3 pyrolyzed into K2O and
CO2. The escaping CO2 gas bubbles causes the formation
of porous structures. Carbonizing silkworm cocoons
under a controlled temperature offers several advantages.
First, the carbon that comes from silk cocoons is consid-
ered a sustainable resource. Second, carbonizing silk-
derived materials creates a multilayer architecture with
hierarchical porous structure saturated with uniformly
doped nitrogen. Third, with this synthesis route, there is
no need for templates, and activating reagents can be eas-
ily removed. So far, silk-derived carbon materials have
been used in areas such as supercapacitor and lithium
batteries.[22,23] Nevertheless, its application in catalytic
systems, especially organic chemistry, is rarely investi-
gated.[24,25] This is owing to the fact that there is difficulty
in the separation of these nitrogen-doped carbon mate-
rials from the reaction medium. So the need to design
new modified nitrogen-doped carbon materials is
unavoidable in order to overcome the aforementioned
drawback. Magnetite Fe3O4 nanoparticles have drawn
remarkable consideration as a powerful catalyst support
due to their unique characteristics including chemical
and thermal resistance, low toxicity, simple and inexpen-
sive preparation, large surface area, and simple isolation
from the reaction medium by an outer magnet. Impor-
tantly, magnetic separation is considered a green proce-
dure making it possible to save energy, time, solvents,
and so forth. In fact, the problems of high-speed centrifu-
gation, filtration, or other tedious workup processes have
been solved.[26] By combining nitrogen-doped carbon
compounds with magnetic Fe3O4 nanoparticles, we can
gain the benefits of both systems. According to our
knowledge, no report has been published about magnetic
catalysts derived from carbonized raw silk which
includes both fibroin and sericin.
The Suzuki–Miyaura cross-coupling reaction of aryl
boronic acid with aryl halides has received considerable
attention as a very useful, preeminent, and powerful
method for selective formation of C–C bond in organic
synthesis, especially for the preparation of biaryls.[27–29]
A significant number of reviews have focused on these
reactions, due to their high impact and experimental
importance in the generation of fine chemicals and
pharmaceuticals.[30–32] In this regard, many effective cat-
alytic systems have been designed and developed for this
reaction.[33–37] However, considering both environmental
and economic issues, the design and development of
phosphine-free palladium catalysts continue to be a seri-
ous challenge in organic synthesis and industry.[38]
This paper is a continuation of previous researches on
the synthesis and application of new green separable het-
erogeneous catalysts.[39–44] This report shows our efforts
to make a novel palladium catalyst using both nitrogen-
doped carbon materials prepared by hydrothermal car-
bonization process and as-prepared magnetic Fe3O4 as a
support. This combination offers benefits of simple sepa-
ration, great reusability, and perfect ability to load a con-
siderable amount of palladium. Afterwards, the catalyst
performance was assessed in the synthesizing of biaryl
compounds via reaction of aryl halides and
phenylboronic acid.
2 | EXPERIMENTAL
2.1 | Materials and instruments
All chemicals employed in this study were of analytical
grade and were bought from Sigma-Aldrich and Merck
companies. Silk cocoons were purchased from an online
shop “Albatross.ir.” The products purity was assessed uti-
lizing TLC with commercial aluminum backed plates of
Merck silica gel 60 F254. Fourier transform infrared (FT-
IR) spectra were recorded using Shimadzu 8400s spectro-
photometer in KBr pellets within
a
range of
400–4,000 cm−1. X-ray diffraction (XRD) patterns were
recorded by Philips using Cu/Kα radiation. Field emis-
sion scanning electron microscopy (FE-SEM) images
were obtained by TESCAN MIRA II equipped with an
energy dispersive X-ray (EDX) spectroscopy. Transmis-
sion electron microscopy (TEM) image was obtained with
a CM120 microscope (Philips). Magnetic properties were
assessed by vibrating sample magnetometry (VSM; Lake-
shore 7,407) at room temperature from −10,000 to
+10,000 Oe. Thermogravimetric analysis (TGA) was con-
ducted using a DuPont 2000 thermal analyzer operated

AKBARZADEH AND KOUKABI
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over the range of 25ꢀC to 800ꢀC using a heating rate of
10ꢀC/min under air flow. The quantity of palladium
placed on the catalyst surface was measured by an induc-
tively coupled plasma optical emission spectrometry
(ICP-OES) on a Varian VISTA-PRO.
and reflux conditions, NH4OH solution was added to
the above mixture, and the reaction proceeded under
N2 atmosphere for 2 h. The reaction resulted in the
formation of black precipitates which could be
isolated by an external magnet followed by washing
and vacuum drying.
2.2 | Preparation of NC
2.4 | Synthesis of MNC-Pd
First, silk cocoons were boiled in water for 30 min, and
then they were washed with water and dried at 60ꢀC for
12 h. In the second step, the silk was exposed to thermal
processing in an alumina crucible at 500ꢀC with a
heating rate of 5ꢀC min−1 for 30 min. The first thermal
process resulted in a solid product that was combined
with KOH as an activating reagent by grinding at a ratio
of 1:1 (w/w). Ultimately, for final carbonization, the mix-
ture was heated in an alumina crucible at 800ꢀC with the
previous heating rate under N2 atmosphere for 2 h. The
carbonized product was treated with an aqueous solution
of HCl 5% for 12 h, followed by washing with distilled
water and drying at 80ꢀC.[45]
MNC (0.1 g), Pd (OAc)2 (0.005 g), and 50 ml of deionized
water were mixed together in a round-bottom flask, and
then the mixture was sonicated and stirred for 12 h at
room temperature. Next, 5 ml of aqueous NaBH4 (0.1%w/
w) was added to this solution dropwise to reduce the Pd2
+ to Pd0. The final palladium complex MNC-Pd was mag-
netically separated and washed by deionized water and
ethanol and dried in a vacuum oven at 80ꢀC.
2.5 | Typical procedure for Suzuki–
Miyaura reaction using MNC-Pd
A mixture of aryl halide (1.0 mmol), phenylboronic
acid (1.2 mmol), K2CO3 (3 mmol), and MNC-Pd
(0.032 mol%) in 5 ml ethanol:water (1:1) was heated
up and stirred at 80ꢀC for different time length (see
Table 2). The progress of reaction was monitored by
TLC. After finishing the reaction and cooling to room
2.3 | Synthesis of MNC
For the synthesis of MNC, 1 g of NC was added to
250 ml of an aqueous solution containing FeCl2.4H2O
(0.858 g) and FeCl3.6H2O (2.33 g). Then, under stirring
TABLE 1 Optimization of reaction condition for Suzuki–Miyaura coupling reactionsa
Entry
Amount of catalyst (mol %)
Base
Temperature (ꢀC)
Solvent
Time (min)
Yield (%)b
1
-
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
KOH
80
80
80
80
80
80
80
80
60
100
r.t
H2O
120
40
30
30
30
30
30
30
30
30
120
30
30
30
30
-
60
70
72
70
60
80
97
70
97
-
2
0.020
0.032
0.064
0.032
0.032
0.032
0.032
0.032
0.032
0.032
0.032
0.032
0.032
0.032
H2O
3
H2O
4
H2O
5
DMF
6
CH3COOEt
EtOH
7
8
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
EtOH:H2O (1:1)
9
10
11
12
13
14
15
80
80
80
80
85
50
54
80
NaOH
Et3N
Bold values indicate the optimal conditions.
aReaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), base (3 mmol), and MNC-Pd.
bIsolated yield.

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AKBARZADEH AND KOUKABI
TABLE 2 Suzuki–Miyaura
coupling reactions of aryl halides with
PhB (OH)2 catalyzed by MNC-Pda
Time
Entry Aryl halide
(min)
Product
Yieldb (%) TON
TOF
1
2
3
4
5
6
7
30
35
35
40
25
35
60
97
90
94
92
97
92
89
3,031 6,062
2,812 4,848
2,937 5,064
2,875 4,356
2,968 7,068
2,875 4,956
2,781 2,781
8
40
92
2,875 4,356
9
45
30
90
94
70
2,812 3,749
2,937 5,875
2,187 1,093
10
11
2 h
Abbreviations: TON, turnover number (yield of product/per mol of Pd); TOF, turn over frequency
(TON/time of reaction [h]).
aReaction condition: aryl halide (1.0 mmol), phenyl boronic acid (1.2 mmol), K2CO3 (3 mmol), catalyst
(0.032 mol%), and EtOH/H2O (1/1; 5 ml) at 80ꢀC.
bIsolated yield.
temperature, the prepared catalyst was magnetically
isolated with an outer magnet, and the product was
extracted with ethyl acetate. Next, the organic layer
was washed by water and dried with anhydrous
MgSO4. Copies of GC-FID chromatograms, 1H NMR
and 13CNMR of some selected products is available in
supporting information.
3 | RESULTS AND DISCUSSION
The silk fibers can be transformed into N-doped car-
bon fibers with highly ordered graphitic structures
during the heat treatment. Scheme 1 illustrates the
structure of the pristine B. mori silk fibers and the
transformation of silk fibroin molecular under heat

AKBARZADEH AND KOUKABI
5 of 14
SCHEME 1 Procedure for preparation of MNC-Pd
treatment and the synthetic route used for the fabrica-
tion of the catalyst. The single silk fiber includes two
parallel fibroin fibers, which are conglutinated by
hydrophilic sericins. At molecular scale, silk fibroin
comprises primarily hydrogen-bonded β-sheet crystal-
lites embedded in a less-ordered α-helix domain, lead-
ing to the exceptional strength and toughness of silk
fibers. Under high temperature heat treatment (800ꢀC)
in the presence of KOH, as an active agent, the adja-
cent amide bonded protein chains can transform to
aligned porous polyaromatic carbon structures
(NC) due to intermolecular dehydration. After that,
of the magnetic catalyst were validated using FT-IR,
TGA, EDX, SEM, TEM, XRD, and VSM techniques.
3.1 | Characterization of the catalyst
3.1.1 | FT-IR analysis
To characterize different functional groups presented in
the structures of the NC and MNC-Pd nanoparticles, FT-
IR spectroscopy was utilized. As shown in Figure 1a,
there are three distinctive peaks at 3,444, 1,593, and
1,117 cm−1 that could be attributed to the stretching
vibrations of O–H and N–H, C O and C N, and C–OH,
respectively.[46] The existence of the O–H, C O, and C–
OH bonds can be attributed to the reaction of carbon
with KOH during the activation process.[47] FT-IR results
suggest that the amide group nitrogen in silk undergo
carbonization and form functional groups like N–H
and C N.
MNC was prepared according to
a chemical co-
precipitation procedure of FeCl2.4H2O and FeCl3 6H2O
in ammonia solution. Next, palladium nanoparticles
were simply immobilized on the surface of MNC sup-
port in deionized water, the reduction of Pd (II) to
Pd(0) was performed in the presence of NaBH4.
Finally, the catalyst was separated using a permanent
magnet and dried under vacuum. The quantity of pal-
ladium on the surface of the MNC was measured
using ICP-OES technique, and it was calculated to be
0.32 mmol g−1. The predicted physicochemical features
In addition, the band at 715 cm−1 is related to the
substituted aromatic ring. There is a band located at
580 cm−1 which is more noticeable in the curve 1b rather

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AKBARZADEH AND KOUKABI
FIGURE 1 FT-IR spectrum
of (a) NC and (b) MNC-Pd
than 1a, and it corresponds to Fe-O proving the existence
of magnetite nanoparticles on the NC.
nitrogen, iron, oxygen, and palladium in the catalyst
structure (Figure 4). The content of palladium obtained
from EDX results is 3.4 wt% that is in good harmony with
the ICP-OES analysis.
3.1.2 | SEM analysis
The surface morphology and structural changes resulting
from the chemical modifications of NC, MNC, and MNC-
Pd were recorded using SEM and TEM techniques pres-
ented in Figures 2 and 3, respectively. Based on previous
report,[48] the silk microfibers are essentially nonporous,
judging from their smooth framework surface. After the
carbonization via KOH activation at 800ꢀC under N2
atmosphere, the organic silk fibers convert into carbon-
ized nitrogen-doped carbon fibers (NC), as shown in Fig-
ures 2a,b and 3a,b. There are many small pores randomly
distributed across the surfaces. This indicated that the
fibrous morphology could be well retained after the high
temperature pyrolysis treatment. These pores derived
from the thermal treatment process in the presence of
KOH in which small molecule gases released and left
spaces in the NC support. From the SEM and TEM
images presented Figures 2c,d,e and 3c, it is evident that
Pd and Fe3O4 nanostructures are uniformly dispersed on
the surface of the synthesized NC support. In Figure 3c,
Pd NPs can easily be recognized as it appears brighter
than Fe3O4 NPs.
3.1.4 | VSM analysis
Magnetic property of MNC-Pd was measured using VSM
technique at room temperature. According to Figure 5,
the curve shows that the values of the saturation magne-
tization of MNC-Pd (37.98 emu/g) is below that of pure
Fe3O4 nanoparticles (61.60 emu/g). It proves the exis-
tence of nonmagnetic compounds such as NC and Pd
NPs in the catalyst. However, the catalyst could be swiftly
and effectively removed from the reaction medium using
an outer magnet.
3.1.5 | XRD analysis
Crystalline structure and the purity of the samples were
investigated utilizing XRD analysis. The XRD patterns of
NC, MNC, and MNC-Pd were displayed in Figure 6.
Based on previous report,[41] the silk fiber cocoon dis-
plays a main sharp peak at 19.6ꢀ that corresponds to the
(020) diffraction peak of silk, exhibiting a typical β-sheet
crystalline structure. After carbonization, the XRD pat-
tern of NC shows a typical diffraction peaks located at
26.3ꢀ and 42.6ꢀ corresponding to (002) and (100) planes
of hexagonal conjugated carbon structure (Figure 6a).[22]
The XRD pattern of MNC demonstrated in Figure 6b
shows the bands corresponding to Fe3O4 structures. Six
distinctive peaks are located at 2θ = 30.86ꢀ, 36.26ꢀ,
43.15ꢀ, 53.81ꢀ, 57.19ꢀ, and 63.1ꢀ, which are related to
3.1.3 | EDX analysis
The EDX analysis was employed as a powerful technique
to identify catalyst purity and the chemical composition
of the synthesized MNC-Pd. The EDX spectrum proves
the existence of expected elements comprising carbon,

AKBARZADEH AND KOUKABI
7 of 14
FIGURE 2 SEM of (a,b)
NC, (c,d) MNC, and (e) MNC-Pd

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AKBARZADEH AND KOUKABI
FIGURE 3 TEM of (a,b) NC and (c) MNC-Pd
(220), (311), (400), (422), (511), and (440) planes of the
cubic magnetite with spinel structures, respectively
(JCPDS 65–3107). Also, according to Figure 5c, existence
of Pd(0) is proved (2ϴ = 40.2ꢀ, 46.7ꢀ, 68.2ꢀ and 82ꢀ that
related to (111), (200), (220), and (311)).[49]
3.2 | Catalytic activity of MNC-Pd in
Suzuki–Miyaura cross-coupling
C C bond formation through Suzuki–Miyaura cross-
coupling reaction was chosen to investigate the catalytic
performance of the MNC-Pd (Scheme 2).
The reaction of iodobenzene and phenylboronic acid
was chosen as a sample reaction to determine the optimal
reaction conditions including the amount of catalyst,
base and solvent type, and temperature. At first, the
model reaction was conducted in different levels of
MNC-Pd in H2O at 80ꢀC. It was found that the efficient
amount of the catalyst is 0.032 mol% which provided a
high yield within 30 min. Further increase in the catalyst
amount did not raise the product efficiency or lessen the
reaction time. Also, less catalyst amount lowered the
reaction yield with longer reaction time. When the model
reaction was carried out without catalyst, at 80ꢀC, the
desired product was not formed (Table 1, entries 1–6). To
3.1.6 | TGA analysis
Thermal analysis of the synthesized NC and MNC-Pd
was evaluated with thermogravimetric analysis within a
range of 30ꢀC–800ꢀC under air flow at a heating rate of
10ꢀC/min. The thermograms of NC and MNC-Pd are dis-
played in Figure 7. Based on previous report,[24] the silk
cocoon decomposes at about 300ꢀC. The result of TGA of
NC shows that it has very high thermal stability in com-
parison with the primary silk cocoon. Also, the prepared
MNC-Pd shows high thermal stability as can be seen
from Figure 6.

AKBARZADEH AND KOUKABI
9 of 14
FIGURE 4 EDX analysis of MNC-Pd
FIGURE 5 VSM
magnetization curve of MNC-Pd
determine the optimal temperature, different tempera-
tures were investigated for the model reaction, and it was
found that the 80ꢀC could afford the maximum efficiency
(Table 1, entries 3, 7–9). Next, the solvent effect was
investigated on the model reaction, and different solvents
such as H2O, DMF, CH3COOEt, and EtOH were utilized
to maximize the efficiency. A mixture of H2O:EtOH in
1:1 ratio could afford the best result (Table 1, entries
3, 12–16). The efficacy of the base on the reaction was
also checked. For this purpose, the sample reaction was
accomplished in the presence of various bases such as
K2CO3, Na2CO3, KOH, NaOH, and Et3N, which K2CO3
was selected as the best base (Table 1, entries 12–15).
Consequently, the most effective reaction medium was

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AKBARZADEH AND KOUKABI
FIGURE 6 The XRD patterns of (a) NC, (b) MNC, and (c) MNC-Pd
FIGURE 7 TGA curves of (a) NC
and (b) MNC-Pd
provided at 80ꢀC using 0.032 mol% MNC-Pd and 3 mmol
of K2CO3 in EtOH/H2O (1/1) after 30 min (Table 1,
entry 11).
founded from Table 2 are in consistent with predictions,
and the reaction in the presence of aryl bromide and aryl
chloride is slower than aryl iodides. MNC-Pd was insuffi-
ciently effective in aryl chloride reactions and longer
reaction time was inevitable for increased performance.
In addition, turnover frequency (TOF) and turnover
numbers (TON) for all products included in Table 2 are
calculated.
Based on the obtained optimization results, the MNC-
Pd was employed in the Suzuki–Miyaura reaction with
various aryl halides including iodides, bromides, and
chlorides. As shown in Table 1, all the precursors with
electron-rich or electron-withdrawing substituents were
successfully transformed to the desired products in high
yields. Of the halogens, the order for activity of leaving
group is as follows: Cl < Br < I; therefore, the results
3.3 | Reaction mechanism
The proposed mechanism for the Suzuki–Miyaura cross-
coupling reaction in the presence of MNC-Pd is shown in
Scheme 3. Based on substrates chemistry together with
some literature data, the possible mechanism has three
steps. First step includes the oxidative addition of aryl
halide to MNC-Pd which produces aryl palladium
SCHEME 2 Suzuki–Miyaura reaction catalyzed by MNC-Pd

AKBARZADEH AND KOUKABI
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SCHEME 3 Possible mechanism for Suzuki–Miyaura reaction catalyzed by MNC-Pd
TABLE 3 Comparison of MNC-Pd nanocomposite with other catalysts for the Suzuki–Miyaura cross-coupling reaction
Entry
Catalyst
Condition
Time (min)
Yield (%)
Reference
[50]
[51]
[52]
[53]
[54]
[55]
[56]
1
2
3
4
5
6
7
8
Pd-imino-Py-γ-Fe2O3
GO/Fe3O4/PAMPS/Pd
Pd@CC-SO3H-NH
Pd-ATBA-MNPs
Pd/Fe3O4@SiO2
[κ3-N,N0,O-Pd(1 ꢁ 2)H2O]OAc
Pd/CNFs
DMF, 100ꢀC/
EtOH:H2O, 80ꢀC
H2O, 100ꢀC
30
120
120
40
95
100
96
100ꢀC/PEG
97
EtOH, Reflux/MW
H2O, 25ꢀC
30
99.7
84
35
EtOH:H2O, 80ꢀC
EtOH:H2O, 80ꢀC
240
30
99
MNC-Pd
97
This work
intermediate (I). Next phenylboronic acid which is acti-
vated by K2CO3, reacts with intermediate (I), and in a
transmetallation reaction, aryl palladium intermediate
(II) is formed. Finally, the coupled product is generated
by a reductive elimination of intermediate (II).
3.5 | Recycling of the catalyst
The catalyst reusability is an important factor for com-
mercial and industrial applications. For this purpose,
after the first run, the catalyst was isolated by an outer
magnet and then washed with ethanol followed by drying
3.4 | Comparison of the catalytic activity
of MNC-Pd with other reported catalysts
A catalytic performance comparison between the MNC-
Pd nanocomposite and recently reported palladium cata-
lysts is presented in Table 3. The new catalyst features
easy synthesis process, magnetic separability, high stabil-
ity, and great retrievability. Although the aforementioned
catalysts in the previous papers have their own benefits,
the newly prepared catalyst has significant potential in
terms of reaction conditions containing products yield,
temperature, reaction time, amount of catalyst, and sepa-
ration. Considering the results, it can be found that
MNC-Pd is an efficient catalyst, and it is beneficial in the
synthesizing of biaryl compounds via Suzuki–Miyaura
coupling reaction.
FIGURE 8 Recycling experiment of MNC-Pd

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AKBARZADEH AND KOUKABI
FIGURE 9 EDX spectra of the recovered MNC-Pd after being reused six times
Figure 9, the palladium content which was leached after
six cycles was insignificant. To study the leaching of Pal-
ladium, ICP-OES analysis of the recovered catalyst was
also carried out. According to the achieved results, no
appreciable decrease in the Pd content was observed. The
Pd amount in the primary catalyst and the reused catalyst
was 0.32 and 0.29 mmol/g, respectively, that demon-
strated the Pd-leaching of the catalyst is negligible.
3.6 | Leaching
In another investigation to determine the leaching of pal-
ladium from the MNC-Pd, once 15 min of the reaction
time passed, the catalyst was separated from the reaction
medium. The remaining mixture was kept to continue
the reaction under optimized conditions. According to
Figure 10, a very slight increase was observed even after
40 min. The results proved the real heterogeneous nature
of the catalyst.
FIGURE 10 Leaching experiment of MNC-Pd
at 80ꢀC under vacuum conditions. Next, the separated,
used catalyst was again added to a reaction vessel con-
taining starting materials. The reaction was carried out
for six cycles, and the results have been presented in
Figure 8. The catalyst could be used for six cycles without
significant loss in its performance. Furthermore, EDX
technique was used to evaluate the changes in the ele-
mental composition of the used MNC-Pd. As shown in
4 | CONCLUSION
In summary, natural silk cocoons were used to make
new a nitrogen-doped carbon material by thermal treat-
ment for catalytic purposes. In fact, the combination of
Fe3O4 nanoparticles with the new nitrogen-doped carbon
material created a good support for palladium to form

AKBARZADEH AND KOUKABI
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Parisa Akbarzadeh: Conceptualization; data curation;
formal analysis; funding acquisition; investigation; meth-
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The data that support the findings of this study are avail-
able from the corresponding author upon reasonable
request.
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Parisa Akbarzadeh https://orcid.org/0000-0003-1921-
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Nadiya Koukabi https://orcid.org/0000-0002-7358-783X
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