Palladium and silk fibroin-containing magnetic nano-biocomposite: a highly efficient heterogeneous nanocatalyst in Heck coupling reactions

Article abstract of DOI:10.1007/s11164-021-04462-2

Supported metal catalysts, for instance, palladium, are one of the foundations of chemical reactions, especially in C–C bond formation. The present study reports preparation of a magnetically separable palladium-supported nano-biocomposite with a low cost and easy immobilization technique. Fibroin, a natural biodegradable polymer, was used through an in situ method to cover the Fe₃O₄ nanoparticles to make a nano-biocomposite followed by anchoring palladium on the fibroin surface. The morphology and the structure of palladium-supported nano-biocomposite Fe₃O₄@fibroin-Pd were characterized by FT-IR, XRD, TGA, SEM, EDX, and TEM techniques. Consequently, the nanocatalyst activity was evaluated in the Heck coupling reactions. Only a very small amount of the nanocatalyst was employed in the reaction, and it showed excellent catalytic activity; in most cases more than 90% efficiency. The significant advantages of employing this nanocatalyst include high catalytic activity, short reaction times, easy separation of the nanocatalyst with an external magnet and great reusability. The results demonstrated that the used nanocatalysts were very active for four consecutive reaction rounds.

Full text of DOI:10.1007/s11164-021-04462-2

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04462-2
Palladium and silk fbroin‑containing magnetic
nano‑biocomposite: a highly efcient heterogeneous
nanocatalyst in Heck coupling reactions
Ahmad Nouri Parouch1 · Nadiya Koukabi1 · Elham Abdous1 ·
Seyed Amin Shobeiri1
Received: 8 January 2021 / Accepted: 5 April 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
Supported metal catalysts, for instance, palladium, are one of the foundations of
chemical reactions, especially in C–C bond formation. The present study reports
preparation of a magnetically separable palladium-supported nano-biocomposite
with a low cost and easy immobilization technique. Fibroin, a natural biodegrad-
able polymer, was used through an in situ method to cover the Fe3O4 nanoparticles
to make a nano-biocomposite followed by anchoring palladium on the fbroin sur-
face. The morphology and the structure of palladium-supported nano-biocomposite
Fe3O4@fbroin-Pd were characterized by FT-IR, XRD, TGA, SEM, EDX, and TEM
techniques. Consequently, the nanocatalyst activity was evaluated in the Heck cou-
pling reactions. Only a very small amount of the nanocatalyst was employed in the
reaction, and it showed excellent catalytic activity; in most cases more than 90%
efciency. The signifcant advantages of employing this nanocatalyst include high
catalytic activity, short reaction times, easy separation of the nanocatalyst with an
external magnet and great reusability. The results demonstrated that the used nano-
catalysts were very active for four consecutive reaction rounds.
Keywords Biopolymer-supported Pd · Silk fbroin · Nano-biocomposite ·
Nanocatalyst · Heck coupling reaction
Introduction
In recent years, nanotechnology is gaining more attention to the utilization of natural
sources like agro wastes [1], biological sources [2], and biopolymers for the prep-
aration of nanoparticles [3]. There is a growing tendency for using biodegradable
*
Nadiya Koukabi
n.koukabi@semnan.ac.ir
1
Department of Chemistry, Semnan University, 35131-19111 Semnan, Iran
Vol.:(0123456789)
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A. Nouri Parouch et al.
polymers in numerous felds such as drug delivery systems [4], medical load-bear-
ing applications [5], commodity plastics [6], electronics [7], anticancer activities [8].
Using biodegradable materials results in construction of environmentally friendly
products which is an important principle of green chemistry. The frst steps toward
synthesizing a green catalyst are designing and preparing a combination of biode-
gradable materials, namely biopolymers, with suitable materials to reach a biocom-
posite. Diferent biocomposites were synthesized using various biopolymers includ-
ing proteins [9] and polysaccharides [10]. Besides, Fe3O4 nanoparticles (NPs) were
used in preparation of magnetically separable heterogeneous catalysts, and diferent
organic and inorganic shells were employed to produce core–shell nanostructures
[11, 12]. The necessity to develop new strategies for coating the Fe3O4 NPs is bar-
ricade aggregation and air oxidation to increase stabilization and decreased vulner-
ability during catalytic applications [13]. Coating Fe3O4 NPs with polymers which
have multiple functional groups allows us for further surface modifcations with var-
ious catalytic agents for extensive catalytic applications [14]. Between the materials
which can be used as a shell, fbroin is a great candidate because of high biocom-
patibility, biodegradability, stability, low costs, simple surface functionalization, and
low toxicity [15, 16]. Therefore, fbroin ofers a good opportunity in manufactur-
ing and developing green and durable catalysts. Bombyx mori silk is an amphiphi-
lic fbrous protein, mainly composed of fbroin and sericin as the core and coating
layer, respectively [17]. Sericin is a protein with a molecular weight that varies from
10–20 to 310–400 kDa [18]. Also, fbroin is a linear polypeptide with 5263 amino
acids residues composed of glycine, alanine, serine, tyrosine, valine, and only 4.7%
of the other 15 amino acid types [19]. Although there are various reports about uti-
lizing silk fbroin in diferent felds including, drug delivery [20], biomaterial [21],
enzyme immobilization [22], tissue regenerations [23], and biomedical [24] appli-
cations, a few reports are related to its catalytic applications. Importantly, fbroin
has functional groups such as carbonyl, amine, and hydroxyl groups on its surface.
These functional groups can be used for surface modifcation to expand catalytic
applications. A few papers investigated fbrous silk fbroin catalytic application as
a support for nanoparticles of palladium [25], iron [26], and gold [27]. Unfortu-
nately, two unavoidable problems are related to using bulky fbroin as catalyst sup-
port. Firstly, difculty in separating the bulky fbroin from the reaction mixture and
products. Secondly, bulky silk fbroin provides much less surface area and active
sites in comparison with nanoparticles. The present study is a continuation of our
previous studies toward developing new eco-friendly heterogeneous catalysts [28,
29] and ofered a facile method to make Fe3O4@fbroin-Pd then assessed in the
Heck reaction. For this purpose, the frst step was to cover the Fe3O4 NPs using puri-
fed fbroin powder for making a magnetic nano-biocomposite. In the second step,
the fbroin-palladium complex formed through a green procedure. With the novel
Fe3O4@fbroin-Pd nanocatalyst in hand, Heck reaction was carried out to investi-
gate its catalytic capability comprising activity and reusability (Scheme 1). Besides,
parameters like amount of the nanocatalyst, solvent, reaction time, temperature, and
base system were also investigated. Considering the importance of palladium in
organic syntheses, this novel and unique nanocatalyst would gain wide attention.
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Palladium and silk fbroin‑containing magnetic…
Scheme 1 Procedure for preparation and application of Fe3O4@fbroin-Pd
Experimental
Materials and methods
All chemicals were bought from Merck and Sigma-Aldrich and were employed
without any purifcation. Bombyx mori cocoons were bought from Rasht, Iran.
Iron (II) chloride tetrahydrate (FeCl2.4H2O), iron (III) chloride hexahydrate
(FeCl3.6H2O), ammonium hydroxide (NH4OH), sodium carbonate (Na2CO3),
calcium chloride (CaCl2), iodobenzene, methyl acrylate, butyl acrylate, triethyl-
amine (Et3N), dimethylformamide (DMF), aryl halides, ethyl acetate, magnesium
sulfate (MgSO4), and palladium acetate (Pd(OAc)2). Dialysis tubing cellulose
membrane (cut-of = 14,000) was bought from Sigma-Aldrich.
SEM–EDX analysis was accomplished using MIRA3 TESCAN-XMU digital
scanning microscope. TEM images were collected using a transmission elec-
tron microscope (TEM; Philips EM 208S‐100 kV). X‐ray difraction (XRD)
analysis was done on a Siemens D5000 (Siemens AG, Munich, Germany) using
Cu‐Ka radiation of wavelength 1.54°A. The thermogravimetric analysis (TGA)
was performed using a Du Pont 2000 thermal analysis apparatus heated from 25
to 1000 °C at ramp 10 °C/min under air atmosphere. The FT‐IR spectra were
recorded by a Shimadzu 8400 s spectrometer using KBr pressed powder disks.
The amount of palladium in the catalyst was measured with an Agilent model 240
AA Shimadzu (USA) fame atomic adsorption spectrometer. The NMR spectra
obtained using a Brucker Avance 400 MHz instruments (1H‐NMR 400 MHz and
13C‐NMR 125 MHz) in pure dimethyl sulfoxide.
1 3

A. Nouri Parouch et al.
Fibroin preparation
Fibroin was purifed and degummed according to previous reports [30]. To remove
the sericin proteins, small slices of Bombyx mori cocoons boiled for 1 h in an aque-
ous solution of Na2CO3 5% (w/v) and then it was fltered through a flter paper fol-
lowed by washing with hot water and drying overnight. The molecular weight of
pure fbroin is around 95 kDa for the Na2CO3-degumming Bombyx mori cocoons
[31]. The extracted fbrous fbroin was added to a mixture of deionized water, cal-
cium chloride, and ethanol with the mole ratio of 8:1:2 and kept at 90 °C to com-
pletely dissolved. The fbroin solution was dialyzed with a dialysis tube against
deionized water for 3 days, and fbroin powder could be prepared by the freeze-dry-
ing technique.
Synthesis of magnetic nano‑biocomposite
The magnetic nano-biocomposite (MNB) was synthesized through an in situ reac-
tion according to the literature with some modifcation [32]. FeCl2·4H2O (1.789 g)
and FeCl3·6H2O (4.865 g) were dissolved in 90 mL of distilled water followed by
sonication until complete disappearance of solid particles. To this mixture under
continuous stirring, 10 mL of the 10 wt% fbroin and then NH4OH (10 mL) solution
were added. Upon the formation of a black suspension, it was refuxed at 100 °C for
6 h under a nitrogen atmosphere and continuous stirring. Finally, MNB was isolated
from the reaction mixture by an external magnet followed by washing with distilled
water several times and then drying in the air.
Preparation of Fe3O4@fbroin‑Pd
The Fe3O4@fbroin-Pd was prepared according to a green pathway without using
organic solvents. At frst, MNB (0.3 g) was added in distilled water (6 mL) and
sonicated for 30 min. In the following, 6 mL palladium acetate solution (0.05 g
Pd(OAc)2 in 6 mL water) was added to the above mixture. Finally, the last mixture
was refuxed at 100 °C for 24 h under an N2 atmosphere with constant stirring. After
magnetically separating and washing with distilled water, Fe3O4@fbroin-Pd nano-
catalyst was obtained.
Results and discussion
Catalyst characterization
SEM/EDX
The surface morphology of both MNB and Fe3O4@fbroin-Pd was studied
and imaged utilizing a scanning electron microscope (SEM) equipped with an
1 3

Palladium and silk fbroin‑containing magnetic…
energy-dispersive X-ray (EDX) facility to investigate the chemical composition.
A very thin layer of gold has covered the samples for both SEM and EDX analy-
ses before imaging to prevent charging efects and to have enhanced electrical
conductivity. The SEM images Fig. 1a, b transparently show that the MNB had
a spherical formation with diameters of 16–20 nm. Figure 1c, d proves that no
aggregation happened after treating with palladium acetate during the fnishing
step. EDX data also ofer valuable information about the elements in the nano-
biocomposite and the nanocatalyst. Figure 1e, f shows the EDX pattern of MNB
and Fe3O4@fbroin-Pd, respectively. The peaks confrmed that the fnal nanocata-
lyst was composed of carbon (C) and nitrogen (N) indication of fbroin structure,
iron (Fe) and oxygen (O) indication of Fe3O4, and palladium (Pd) element.
TEM
Transmission electron microscopy (TEM) can provide more details about the
internal structure and morphology of the prepared nanocomposite. As was previ-
ously observed in SEM images, in the TEM images the particles had also spheri-
cal morphology with diameters in the range of 16–20 nm for the MNB (Fig. 2).
The images of the MNB are presented on diferent scales, and at the highest mag-
nifcation, it can be better seen that the surface of nano-biocomposite is rather
blurry due to the fbroin coating.
Fig. 1 SEM images of fbroin-based magnetic nano-biocomposite (MNB) (a and b), Fe3O4@fbroin-Pd
(c and d). EDX patterns of MNB (e) and Fe3O4@fbroin-Pd (f)
1 3

A. Nouri Parouch et al.
Fig. 2 TEM micrograph of MNB at diferent scales
Fig. 3 XRD patterns of MNB
XRD
X-ray difraction (XRD) patterns of MNB were measured by X-ray difraction
spectroscopy, and along with Fe3O4, it is presented in Fig. 3. Six difraction
peaks appeared at 2Ɵ = 30.6°, 35.6°, 43.5°, 53.4°, 57.6°, and 63.4° correspond-
ing to the (220), (311), (400), (422), (511), and (440) planes of pure Fe3O4 [33,
34].
TGA
Thermogravimetric analysis (TGA) of MNB is presented in Fig. 4. The TGA
curve of MNB shows frst weight loss below 120 ℃ due to the removal of
adsorbed water. The second weight loss happened in the range 220–411 ℃,
indicating the decomposition of organic content (fbroin) presented in the sam-
ple. The thermal stability of the nano-biocomposite is up to 220 ℃, confrming
that utilizing MNB is suitable as a stable support for catalyst systems at high
temperatures.
1 3

Palladium and silk fbroin‑containing magnetic…
Fig. 4 Thermogravimetric analysis of fbroin magnetic nano-biocomposite (MNB)
Fig. 5 FT-IR spectra of silk fbroin and MNB
FT‑IR
FT-IR spectra of fbroin and MNB are shown in Fig. 5. In the fbroin spectrum
(Fig. 5a), three distinct bands can be observed at 1650, 1519, 1396 cm−1 corresponding
to primary, secondary, tertiary amide functional groups, respectively [35]. Figure 5b
1 3

A. Nouri Parouch et al.
also shows MNB spectrum containing a new band at 561 cm−1 correspondings to Fe–O
stretching vibration because of the presence of Fe3O4 nanoparticles in MNB [36].
ICP‐OES analysis
Finally, the content of Pd immobilized on the surface of MNB was measured by induc-
tively coupled plasma optical emission spectroscopy (ICP-OES) technique. The Pd
loading content is 7.77 wt% revealed that the high loading of palladium was immobi-
lized. The palladium loading shows the high performance of fbroin-based magnetic
nano-biocomposite to keep metals.
Catalytic activity
The performance of the prepared nanocatalyst was examined in the C–C bond forma-
tion through Heck cross-coupling reaction (Scheme 2). The construction of the C–C
bond is a fundamental reaction in organic synthesis and widely used in the synthesis
of natural products, drugs, and polymeric materials [37]. The Heck cross-coupling
Scheme 2 Heck cross-coupling reaction of iodobenzene and methyl acrylate
reaction is one of the most important C–C forming reactions and has been studied to
develop efcient catalysis protocols [38, 39]. Due to wide applications, C–C bond for-
mation via Heck cross-coupling reaction was selected to investigate the catalytic per-
formance of Fe3O4@fbroin-Pd.
To determine the optimized reaction conditions, a series of experiments were car-
ried out, and the reaction of iodobenzene and methyl acrylate was chosen as a model
reaction (Table 1). Diferent reaction terms were changed, and it was found that 0.001 g
of catalyst, 100 ℃ temperature, and Et3N as a base in DMF aforded the most desirable
medium for this reaction (Table 1, entry 10).
General method of the Heck coupling reaction
A round-bottomed fask equipped with a magnetic stirrer was flled with 0.5 mmol
of aryl halide, 0.75 mmol of olefn, 1 mmol of Et3N, 0.001 g of Fe3O4@fbroin-Pd,
and 1.5 mL of DMF. The reaction continued for an appropriate time under stirring at
100 °C. The progress of the reaction was assessed using thin-layer chromatography
(TLC). Once the reaction was fnished, the nanocatalyst was separated by an external
magnet. The products were extracted with ethyl acetate, and then, the organic layer was
washed with water and dried by MgSO4. The reaction was carried out with various aryl
halides, and results are shown in Table 2. The structural features of products were char-
acterized using spectral data IR, 1H NMR, and 13C NMR.
1 3

Palladium and silk fbroin‑containing magnetic…
Table 1 Optimization of the reaction condition
Entry
Amount of
Base
Solvent
Temperature Time (h:min)b
( °C)
Yield (%)c
catalysta (g)
1
2
3
4
5
6
7
8
0.0007
0.001
0.0015
0.003
–
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
Et3N
Et3N
Et3N
Et3N
Et3N
KOH
NaOH
K3PO4
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
DMF
DMSO
DMF
DMF
DMF
100
100
100
100
100
100
100
100
100
100
100
120
80
00:25
00:12
00:15
00:20
24:00
00:35
0030
00:25
00:25
00:09
00:35
00:10
00:15
24:00
55
75
75
78
-
65
60
70
70
96
60
94
85
-
9
10
11
12
13
14
R.T
aReaction was performed in the presence Fe3O4@fbroin-Pd
bDetected by TLC
cIsolated yield
Table 2 Heck coupling reactions of aryl halides with olefnsa
Yield
(%)c
96
Entry
Aryl halide
Olefin
Product
Time (h:min)b
1
00:09
00:12
00:05
00:20
00:35
00:15
00:20
2
3
4
5
6
7
92
92
87
45
94
85
a Reaction condition: aryl halide (0.5 mmol), olefin (0.75 mmol), Et3N (1 mmol), catalyst (0.001 g), DMF (1.5 mL), at 100 °C.
b Detected by TLC.
c Isolated yield.
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A. Nouri Parouch et al.
Table 3 Comparison of Fe3O4@fbroin-Pd results in the Heck coupling reaction with similar researches
Entry Catalyst
Solvent
Tempera- Time (h:min) Yield (%) Ref
ture (°C)
1
2
3
4
5
6
CMC-PdII
Pd/Fe3O4@PIL-NH2
Pd@OMP-DVB
Pd-NPs/P.a.kurdica gum H2O
Fe3O4/DAG/Pd
DMF
Solvent-free 125
DMF
110
03:00
00:25
00:30
00:25
01:00
00:09
85
94
99
98
90
96
[40]
[41]
[42]
[43]
[44]
This work
80
80
110
100
DMF
DMF
Fe3O4@fbroin-Pd
Comparison of Fe3O4@fbroin‑Pd results with some similar works
A direct comparison of Fe3O4@fbroin-Pd and recently reported papers in the Heck
coupling reaction proves the strong performance of the nanocatalyst (Table 3).
Catalyst recyclability
In the ensuing lines, the recyclability of Fe3O4@fbroin-Pd is assessing. The yield of
the model reaction catalyzed by untouched Fe3O4@fbroin-Pd was compared with
the yields obtained by recovered Fe3O4@fbroin-Pd. After completion of the frst
round Heck coupling reaction, the nanocatalyst was separated by an external magnet
and washed with water and ethanol followed by drying at 100 ℃ overnight. This
recycling process was performed up to four reactions round, and the yields are pre-
sented in Fig. 6. The results proved that Fe3O4@fbroin-Pd could be employed for
four consecutive reaction runs without a considerable decrease in activity.
Fig. 6 Reusability of the Fe3O4@fbroin-Pd
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Palladium and silk fbroin‑containing magnetic…
Scheme 3 Proposed mechanism for the Heck cross-coupling reaction in the presence of Fe3O4@fbroin-Pd
Leaching
In supported metal catalysts, including palladium, leaching is a major problem
that leads to loss of active sites in the nanocatalyst. To investigate whether or not
the attachment of palladium on the fbroin surface is strong enough, the reaction
between iodobenzene and methyl acrylate was again performed under optimal con-
ditions. After half of the reaction time, the nanocatalyst was separated using an
external magnetic feld. The reaction was continued for 2 h under the same condi-
tions without adding any fresh nanocatalyst. The reaction progress was monitored by
TLC, and it showed that the coupling reaction no longer performed. Above results
revealed that the palladium leaching phenomenon was insignifcant in the Heck
reaction, demonstrating the palladium deposited on magnetic nano-biocomposite
Fe3O4@fbroin acts as a heterogeneous catalyst in the reaction. A plausible mecha-
nism for the Heck reaction, based on previous reports [45], is proposed in Scheme 3.
Conclusion
Herein, we demonstrated a successful preparation of Fe3O4@fbroin-Pd by employ-
ing a simple, economical, and eco-friendly strategy for the Heck coupling reaction.
The prepared nanocatalyst possessed high surface areas, excellent thermal stabil-
ity, easy separation ability, uniform morphology, and high catalytic activity. The
core–shell structure of Fe3O4@fbroin was the main reason for the high functionali-
zation capacity of fbroin as a support for metal. Thus, the successful example opens
up a new avenue in constructing new core–shell type magnetic nanocatalysts with
good catalytic performance and leads to preparation of other metal functional cata-
lysts with special requirements for organic reactions.
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A. Nouri Parouch et al.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04462-2.
Acknowledgements The authors acknowledge the fnancial support from the Semnan University
Research Council.
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