Facile synthesis of palladium nanoparticles immobilized on magnetic biodegradable microcapsules used as effective and recyclable catalyst in Suzuki-Miyaura reaction and p-nitrophenol reduction

Article abstract of DOI:10.1016/j.carbpol.2019.115029

This study reports (i) preparation and characterization of palladium nanoparticles (Pd NPs) stabilized on magnetically separable chitosan/agar microcapsules (Pd NPs@Fe₃O₄/CS-AG microcapsules) and (ii) investigation of catalytic behaviors of the prepared nanoparticles in Suzuki-Miyaura C-C cross-coupling reactions and reduction of p-nitrophenol (p-NP). Fourier transforms infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), elemental mapping, inductively coupled plasma-optical emission spectrometry (ICP-OES) and thermogravimetry/derivative thermogravimetry (TG/DTG) techniques were used for analyzing the Pd NPs@Fe₃O₄/CS-AG microcapsules. Characterization studies showed that Pd NPs@Fe₃O₄/CS-AG microcapsules were successfully synthesized and the size of the particles was in the range of 28–39 nm. Pd NPs@Fe₃O₄/CS-AG microcapsules displayed highly effective catalytic performance in the conversion of various aryl halides into the corresponding biaryl derivatives by Suzuki-Miyaura reaction under solvent-free conditions in 5 min. Pd NPs@Fe₃O₄/CS-AG microcapsules were also successfully employed in p-NP reduction to p-aminophenol (p-AP) in water at room temperature in 180 s. Moreover, reproducibility studies showed that Pd NPs@Fe₃O₄/CS-AG microcapsules were highly efficient and could be used many times in both catalytic reactions due to their magnetically separable nature. According to the results of this work, Pd NPs@Fe₃O₄/CS-AG microcapsules are highly efficient, economical, practical and environmentally superb catalysts for Suzuki-Miyaura reaction and p-NP reduction.

Full text of DOI:10.1016/j.carbpol.2019.115029

Accepted Manuscript
Title: Facile synthesis of palladium nanoparticles immobilized
on magnetic biodegradable microcapsules used as effective
and recyclable catalyst in Suzuki-Miyaura reaction and
p-nitrophenol reduction
Authors: Talat Baran, Mahmoud Nasrollahzadeh
PII:
DOI:
Article Number:
S0144-8617(19)30696-4
https://doi.org/10.1016/j.carbpol.2019.115029
115029
Reference:
CARP 115029
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Accepted date:
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13 June 2019
25 June 2019
Please cite this article as: Baran T, Nasrollahzadeh M, Facile synthesis of palladium
nanoparticles immobilized on magnetic biodegradable microcapsules used as effective
and recyclable catalyst in Suzuki-Miyaura reaction and p-nitrophenol reduction,
Carbohydrate Polymers (2019), https://doi.org/10.1016/j.carbpol.2019.115029
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Facile synthesis of palladium nanoparticles immobilized on magnetic
biodegradable microcapsules used as effective and recyclable catalyst in
Suzuki-Miyaura reaction and p-nitrophenol reduction
Talat Barana and Mahmoud Nasrollahzadeh*b
aDepartment of Chemistry, Faculty of Science and Letters, Aksaray University, 68100 Aksaray, Turkey
bDepartment of Chemistry, Faculty of Science, University of Qom, 3716146611, Qom, Iran
*Corresponding author. E-mail address: mahmoudnasr81@gmail.com (M. Nasrollahzadeh).
Graphical Abstract
1

Highlights:




Preparation of palladium nanoparticles stabilized on magnetically separable chitosan/agar micro beads.
Catalytic reduction p-NP in water at room temperature in a very short time.
The catalyst shows excellent activity and generality in Suzuki-Miyaura reaction.
Suzuki-Miyaura reaction can be performed under solvent-free conditions in 5 min.
ABSTRACT
This study reports (i) preparation and characterization of palladium nanoparticles (Pd NPs) stabilized on
magnetically separable chitosan/agar micro beads (Pd NPs@Fe3O4/CS-AG microcapsules) and (ii) investigation
of catalytic behaviors of the prepared nanoparticles in Suzuki-Miyaura C-C cross-coupling reactions and
reduction of p-nitrophenol (p-NP). Fourier transforms infrared spectroscopy (FT-IR), scanning electron
microscopy (SEM), X-ray diffraction (XRD), elemental mapping, inductively coupled plasma-optical emission
spectrometry (ICP-OES) and thermo gravimetric-differential thermal analysis (TG-DTG) techniques were used
for analyzing the Pd NPs@Fe3O4/CS-AG microcapsules. Characterization studies showed that Pd
NPs@Fe3O4/CS-AG microcapsules were successfully synthesized and the size of the particles was in the range
of 28-39ꢀnm. Pd NPs@Fe3O4/CS-AG microcapsules displayed highly effective catalytic performance in the
conversion of various aryl halides into the corresponding biaryl derivatives by Suzuki-Miyaura reaction under
solvent free conditions in 5 min. Pd NPs@Fe3O4/CS-AG microcapsules were also successfully employed in p-
NP reduction to p-aminophenol (p-AP) in water at room temperature in 180 seconds. Moreover, reproducibility
studies showed that Pd NPs@Fe3O4/CS-AG microcapsules were highly efficient and could be used many times
in both catalytic reactions due to their magnetically separable nature. According to the results of this work, Pd
NPs@Fe3O4/CS-AG microcapsules are highly efficient, economical, practical and environmentally superb
catalysts for Suzuki-Miyaura reaction and p-NP reduction.
Keywords: Chitosan; Agar; Nanocatalyst; Reduction; Palladium; Suzuki-Miyaura reaction
1. Introduction
Pd-catalyzed Suzuki-Miyaura cross-coupling reaction is one of the most important modern organic synthesis
methods for the preparation of biaryls compounds, which are used in the synthesis of pharmaceuticals, polymers,
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advanced materials and natural products [1-3]. There are many reports in the literature concerning the
preparation of different homogeneous or heterogeneous metal catalysts and their catalytic performance in the
Suzuki coupling reaction [4-6]. Although homogeneous catalysts have good catalytic performance, they are not
preferred due to the requirement for their purification and recyclability problems [7, 8]. Therefore,
heterogeneous catalysts have been more widely used in Suzuki-Miyaura reactions compared to homogeneous
catalysts because the separation of the latter catalysts from the reaction medium could be performed by
conventional separation methods such as filtration or centrifugation [9,10]. However, these separation methods
have some challenges such as energy and time. Additionally, if the particle size of catalyst is very small, catalyst
loss is inevitable by conventional separation techniques and this directly affects their recoverability and
reusability. To overcome these challenges, the design of magnetically separable catalysts has gained importance
recently due to the advantages of these catalysts such as low cost, ease of recovery and simple work-up [11].
More importantly, the magnetite nature of these catalysts (i) prevents their loss, (ii) increases the reproducibility
and (iii) facilitates the commercialization of the catalyst.
Organic pollutants and nitroarenes are toxic compounds in wastewater because of their high stability and low
solubility in water. Degradation reactions of organic pollutants to non-toxic products and nitroarenes to useful
compounds are very important reactions [12-16]. Among nitroarenes, p-nitrophenol (p-NP) is a hazardous and
toxic pollutant for ecosystem and human health due to its chemical stability and water solubility [17,18]. The
inhalation or ingestion of p-NP can cause some illnesses such as drowsiness, cyanosis, headaches and nausea
[19]. Therefore, the removal of p-NP from aqueous media is important for human health. Recently, different
methods such as adsorption [20], membrane separation [21], electrocoagulation [22] and biological treatment
[23] to remove p-NP have been reported. Among these techniques, catalytic reduction has been preferred due to
its being safer and more economical compared to other method. Metallic nanoparticles play an important role in
the catalytic reduction compared to other metal counterparts due to their large surface areas and small sizes [24].
Therefore, researchers have developed different noble metallic NPs such as Pt, Au and Ag for the effective
reduction of p-NP to p-AP [25-27].
Among many metal catalysts deployed, Pd is the most commonly used catalyst in a variety of reactions.
Despite the davantages of homogenous catalysts such as high reactivity and selectivity, they have, at times,
failed for commercialization because of difficulties encountered while attempting to separate and reuse the
expensive noble metals. These disadvantages and some industrial problems such as corrosion and deposition on
the reactor walls strongly restrict their potential application in large scale syntheses; particularly because of
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environmental and economic concerns. The heterogenization of homogeneous catalysts on solid supports is
emerging as an important area of research in heterogeneous catalysis. However, the agglomeration of Pd NPs is
inevitable. An ideal support is needed to decrease the agglomeration of such nanoparticles [28]. To prevent the
agglomeration of Pd NPs and overcome the problems concerning stability, leaching, separation, and recovery of
Pd NPs, several inorganic materials such as sodium borosilicate [29], activated carbons [30], and graphene oxide
[31] have been used as supports for Pd NPs. Among these materials, bio-polymers such as chitosan (CS) and
agar (AG) are vitally important in terms of their low cost, high surface area, excellent stability and easy
accessibility [32]. They are excellent candidates for support material in the catalytic systems due to these
properties.
In this study, highly effective magnetically separable Pd NPs were immobilized on novel chitosan/agar micro
beads and characterized by FT-IR, SEM-EDAX, XRD, ICP-OES and TG/DTG analyses. The catalytic
performance of the designed Pd NPs@Fe3O4/CS-AG microcapsules was tested in p-NP reduction and Suzuki-
Miyaura reactions. The catalyst showed superb catalytic activity in both p-NP reduction and Suzuki-Miyaura
reaction with only 180 seconds and 5 min of reaction time, respectively. The reusability tests revealed that Pd
NPs@Fe3O4/CS-AG microcapsules have superior reusability in both catalytic reactions due to their magnetically
separable nature. These findings demonstrated that the palladium nanoparticles prepared on the designed
chitosan/agar micro beads can be used in various catalytic reactions due to their significant, excellent catalytic
performance.
2. Materials and methods
2.1 General remarks
Chitosan, agar, FeSO4⋅7H2O, FeCl3⋅6H2O, NaOH, K2CO3, NaBH4, PdCl2 and other materials were obtained from
Sigma Aldrich Chemical Co. FT-IR spectra of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules were
recorded on a Perkin Elmer Spectrum 100 FT-IR spectrophotometer. X-ray diagrams of Fe3O4/CS-AG and Pd
NPs@Fe3O4/CS-AG microcapsules were recorded on a Rigaku smart lab system (at 40 kV, 30 mA, and 2θ with
a scan angle: 10-75°). Thermal stabilities of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules were
examined with EXSTAR S11 7300 (nitrogen atmosphere; heating range: 30-650 °C). The surface images of
Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules were obtained using a Zeiss Supra 55VP instrument.
Palladium ion content of Pd NPs@Fe3O4/CS-AG microcapsules was determined by inductively coupled plasma
(ICP) optical emission spectrometry (OES) using a Perkin Elmer Optima 2100 DV instrument. The
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characterization of biaryl compounds was carried out using a GC-MS Agilent GC-7890 A-MS 5975 instrument.
The catalytic activity tests of Pd NPs@Fe3O4/CS-AG microcapsules were performed using a domestic
microwave oven. The reduction of p-NP in the presence of the catalyst was monitored by UV-Vis spectroscopy
(Genesys 10S UV-VIS Spectrophotometer).
2.2 Preparation of Fe3O4 particles
Magnetite particles (Fe3O4) were prepared by stirring FeSO4⋅7H2O (4.2g) and FeCl3⋅6H2O (6.1 g) in water (100
mL) at 90°C for 30 min. Then, 10 mL of anhydrous ammonia was added into the reaction medium and the
mixture was stirred at 80 °C for 30 min. Afterwards, Fe3O4 was separated, washed with water to neutrality and
dried at 80°C.
2.3 Synthesis of magnetite-loaded chitosan/agar composite microcapsules (Fe3O4/CS-AG microcapsules)
Firstly, 1.0 g of chitosan was dissolved in 100 mL of 2% v:v acetic acid, followed by the addition of 1.0 g of
agar to the reaction medium and stirring at room temperature overnight to obtain chitosan/agar composite (CS-
AG). The prepared F3O4 particles were introduced into chitosan/agar suspension and stirred for overnight to
provide a homogenous mixture. Subsequently, the resulting mixture (chitosan/agar/Fe3O4) was poured into a
solution of ethanol (60 mL), H2O (40 mL) and NaOH (12 g) to obtain gelatinous micro beads. The obtained
gelatinous, magnetic chitosan/agar micro beads were collected and rinsed with water to neutrality. Subsequently,
they were transferred into a crosslinking solution (50 mL of MeOH and 2 mL of 25% glutaraldehyde solution)
and refluxed for 6 h, at the end of which time, the cross-linked magnetically chitosan/agar micro beads were
filtered, rinsed with methanol to remove the unreacted glutaraldehyde solution and then dried at room
temperature.
2.4 Synthesis of magnetically recoverable palladium nanoparticles (Pd NPs@Fe3O4/CS-AG microcapsules)
1.0 g of Fe3O4/CS-AG beads and 0.3 g of PdCl2 were added to 50 mL of water and the mixture obtained was
stirred for 4 h at 80ºC to convert Pd(II) to Pd NPs. It was observed that after the reaction was completed, the
solution color turned into dark gray. Then, the obtained catalyst was recovered from the medium using a magnet,
rinsed with distilled water and used for the investigation of catalytic activity (Scheme 1).
5

Scheme 1. Preparation of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules and their application in the
p-NP and Suzuki-Miyaura coupling reaction
2.5 General synthesis procedure for Suzuki-Miyaura reactions
Aryl halide (1.0 mmol), phenylboronic acid (1.8 mmol), K2CO3 (3.5 mmol) and Fe3O4/CS-AG beads@Pd NPs
(1.5 × 10-2 mmol) were placed into a Schlenk tube and the mixture was subjected to microwave irradiation at
400W for 5ꢀmin. The mixture was then diluted with water and Pd NPs@Fe3O4/CS-AG microcapsules were
collected by an external magnet and the filtrate was extracted with toluene three times. Coupled products were
obtained by evaporation of the organic phase and then characterized by GC/MS.
2.6 Catalytic reduction of p-nitrophenol in the presence of Pd NPs@Fe3O4/CS-AG microcapsules
An aqueous solution of p-NP (1.0 mL, 1.25 × 10−4 M) was added to freshly prepared NaBH4 (1.0 mL, 0.05 M)
and the mixture obtained was stirred at room temperature for 1 min. 1.5 × 10-2 mmol of Pd NPs@Fe3O4/CS-AG
microcapsules were then added to the reaction medium and the progress of the conversion of p-NP to p-AP was
monitored by UV-Vis spectroscopy. After completion of the reaction, Pd NPs@Fe3O4/CS-AG microcapsules
were collected by an external magnet and washed with water to reactivate them.
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3. Results and discussion
3.1 Characterization studies
Figure 1 displays the FT-IR spectra of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules. After the
formation of Fe3O4/CS-AG microcapsules, a new sharper peak appeared at 1648 cm-1, which corresponds to the
imine groups. In addition, the stretching vibration of glutaraldehyde carbonyl groups (1730 cm-1) [33] and NH2
bending of chitosan (1590 cm-1) [34] did not appear in the spectrum of Fe3O4/CS-AG. These observations reveal
that the cross-linking of Fe3O4/CS-AG with glutaraldehyde has been successfully achieved. On the other hand,
considerable changes were not observed in the Pd NPs@Fe3O4/CS-AG microcapsule spectrum. However, the
characteristic peaks of Fe3O4/CS-AG microcapsules shifted to lower wavenumber due to the strong bonding of
Pd ions.
Figure 1. FT-IR spectra of (a) Fe3O4/CS-AG microcapsules and (b) Pd NPs@Fe3O4/CS-AG microcapsules
7

The XRD diagrams of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules are illustrated in Figure 2. In
the diagram of Fe3O4/CS-AG microcapsules, strong peaks appear at 30.20°, 35.52°, 43.20°, 53.82°, 57.36° and
62.98° (Figure 2a), which are attributed to (220), (311), (400), (422), (511), and (440) crystal planes of face-
centered cubic spinel structures of Fe3O4, respectively, confirming the formation of magnetite loaded CS-AG
beads [24,35]. In the case of Pd NPs@Fe3O4/CS-AG microcapsules, apart from the magnetite peaks, new peaks
are observed at 40.28°, 46.74° and 67.66°, which correspond to (111), (200) and (220) crystalline planes of face-
centered Pd [29,36] (Figure 2b). These important peaks show that Pd NPs have been successfully prepared on
the Fe3O4/CS-AG microcapsules.
Figure 2. XRD patterns of (a) Fe3O4/CS-AG microcapsules and (b) Pd NPs@Fe3O4/CS-AG microcapsules
The SEM analysis of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules was performed to determine
their surface morphologies and particle sizes (Figure 3). SEM micrographs clearly display that both Fe3O4/CS-
AG and Pd NPs@Fe3O4/CS-AG microcapsules are spherical (Figures 3a and b). Following the preparation of
palladium nanoparticles, the surfaces of Pd NPs@Fe3O4/CS-AG microcapsules were covered with nanoparticles
and it was found that Pd NPs are nearly spherical (Figure 3c). In addition, the average size of the particles was
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determined to be in the range of 28-29 nm. To confirm the presence of Fe and Pd ions on the micro beads, EDX
elemental mapping analysis of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules was also performed and
the images are given in Figures 4 and 5. The presence of C, O and Fe was confirmed by the elemental mapping
images of Fe3O4/CS-AG beads (Figure 4). Figure 5 displays the existence of Pd along with C, O and, Fe,
confirming that Pd NPs have been successfully immobilized on Fe3O4/CS-AG microcapsules. Additionally, the
amount of palladium nanoparticles loaded on Fe3O4/CS-AG beads was determined as 9.45 % (w/w) by ICP-OES
analysis.
Figure 3. Surface morphologies of (a) Fe3O4/CS-AG microcapsules and (b, c) Pd NPs@Fe3O4/CS-AG
microcapsules
Figure 4. Elemental mapping of Fe3O4/CS-AG microcapsules
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Figure 5. Elemental mapping of Pd NPs@Fe3O4/CS-AG microcapsules
The thermal behavior of Fe3O4/CS-AG and Pd NPs@Fe3O4/CS-AG microcapsules were investigated by
TG/DTG analysis and the curves are displayed in Figure 6. The maximum thermal degradation of Fe3O4/CS-AG
microcapsules was recorded 243.8°C (Figure 6a). On the other hand, the degradation temperature for Pd
NPs@Fe3O4/CS-AG microcapsules was determined as 261.2°C (Figure 6b). Preparation of palladium
nanoparticles contributed to an increase of the thermal stability of the catalyst. These results reveal that Pd
NPs@Fe3O4/CS-AG microcapsules could be safely used in different catalyst systems, which require high
temperatures. In addition, the total mass loss of 57.1 and 48.1 wt. % were determined for Fe3O4/CS-AG and Pd
NPs@Fe3O4/CS-AG microcapsules, respectively. The increase of the residual content in the spectrum of Pd
NPs@Fe3O4/CS-AG microcapsules can be explained by metal-oxide (Pd-O), confirming that the palladium
nanoparticles have been successfully prepared.
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Figure 6. Thermal behaviors of (a) Fe3O4/CS-AG microcapsules and (b) Pd NPs@Fe3O4/CS-AG microcapsules
3.2 Catalytic performance studies
3.2.1 Suzuki-Miyaura coupling reactions
The catalytic behavior of Pd NPs@Fe3O4/CS-AG microcapsules was investigated in Suzuki-Miyaura reactions.
Initially, optimum reaction conditions including catalyst loading, base, and reaction time were studied utilizing a
model coupling reaction, which is reaction of phenylboronic acid with 4-iodoanisole. Additionally, considering
economical, practical purposes and green chemistry aspects, the coupling reactions were carried out under
solvent free and microwave irradiation. As a result of the optimization experiments, the best reaction yield (99%)
was obtained using 1.5 × 10-2 mmol Pd NPs@Fe3O4/CS-AG microcapsules loading, K2CO3 base and 5 min.
reaction time under microwave irradiation at 400W. All Suzuki-Miyaura coupling reactions were then carried
out under same catalytic conditions in the presence of Pd NPs@Fe3O4/CS-AG microcapsules and the results
obtained are illustrated in Table 1. As observed, excellent reaction yields were obtained when aryl iodides were
used as the substrate (Table 1, entries 1-4). For example, p-substituted -OCH3 and m-substituted -NO2 aryl
iodides afforded excellent reaction yields of 98% and 91%, respectively. The catalytic activity of Pd
NPs@Fe3O4/CS-AG microcapsules was also evaluated in the presence of aryl bromides to extend their catalytic
scope. Ten different aryl bromides, which are electron donating or withdrawing groups, were tested in Suzuki-
Miyaura reactions. As observed in Table 1, aryl bromides produced the desired biaryl products with good
reaction yields in the presence of Pd NPs@Fe3O4/CS-AG microcapsules (Entries 5-12). For example, p-
substituted CN and NO2 aryl bromides produced high biaryl yields of 94% and 92%, respectively. Based on the
promising results obtained, the catalytic performance of Pd NPs@Fe3O4/CS-AG microcapsules was tested by
using aryl chlorides, which are known to have poor activity in Suzuki-Miyaura reactions (Table 1, entries 13-16).
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As illustrated in Table 1, the C-C coupling reactions were efficiently performed using aryl chlorides and 61-82%
reaction yields were obtained. Consequently, these findings show that biaryl compounds can be readily and
efficiently synthesized by using the designed Pd NPs@Fe3O4/CS-AG microcapsules.
Table 2 shows the comparison of the results of application of Pd NPs@Fe3O4/CS-AG microcapsules with
previously reported catalysts in the synthesis of 4-methoxybiphenyl via Suzuki-Miyaura coupling reaction
between phenylboronic acid and 4-bromoanisole. As shown in Table 2, Pd NPs@Fe3O4/CS-AG microcapsules
have excellent catalytic activity affording the highest yield of 4-methoxybiphenyl during shorter reaction times
and lowest catalyst loading. In contrast with other previously reported catalysts, 4-methoxybiphenyl can be
synthesized under solvent free conditions without any hazardous, toxic and expensive solvents. In addition, in
contrast to some of the reported catalysts, the Pd NPs@Fe3O4/CS-AG microcapsules can be easily recovered and
recycled for several times.
3.2.2 Catalytic reduction of p-NP in the presence of Pd NPs@Fe3O4/CS-AG microcapsules
The catalytic performance of Pd NPs@Fe3O4/CS-AG microcapsules was also tested in the catalytic reduction of
p-NP to p-AP in water at room temperature. As observed in Figure 7a, p-NP displayed a strong absorption peak
at 317ꢀnm. It was observed that when freshly prepared NaBH4 solution was added to the p-NP aqueous solution,
the p-NP band shifted to a higher wavelength (400 nm) and the yellow color of the solution turned to deep
yellow due to the formation of 4-nitrophenolate ions (p-NPT) [24]. Upon the addition of Pd NPs@Fe3O4/CS-AG
microcapsules to the reaction medium, the p-NPT absorption peak at 400 nm gradually decreased and
completely disappeared after 180 seconds and a new absorption band was also observed about at 300 nm. These
important findings reveal that the catalytic reduction of p-NP to p-AP was performed in only 180 seconds in the
presence of Pd NPs@Fe3O4/CS-AG microcapsules [49,50]. In addition, the complete reduction of p-NP to p-AP
was shown by the color change of the solution. At the end of the catalytic reduction, the deep yellow color of p-
NP/NaBH4 changed to colorless. On the other hand, a blank test was conducted to show the catalytic
performance of Pd NPs@Fe3O4/CS-AG microcapsules against catalytic reduction of p-NP. This test was carried
out using all reactants (p-NP and NaBH4), except Fe3O4 Pd NPs@Fe3O4/CS-AG microcapsules, and no
formation of p-AP was observed in UV-Vis analysis. This indicates that Pd NPs@Fe3O4/CS-AG microcapsules
are critically important in the p-NP reduction. Figure 7b shows the linear relations of ln(c/c0) and reaction time
(t) in the p-NP reduction. Pseudo-first-order rate was calculated by the following equation:
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ln(c/c0) = -kt
where c0 and c correspond to the p-NP initial concentration and that at selected reaction time (t), respectively,
and k(s-1) is the reaction rate. The rate constant was determined as 5.1 × 10-3 s-1. Therefore, Pd NPs@Fe3O4/CS-
AG microcapsules have good catalytic performance in the p-NP reduction due to the strong interaction between
Fe3O4/CS-AG microcapsules and palladium ions.
Figure 7. a) UV-Vis absorption spectrum for reduction of p-NP in the presence of Pd NPs@Fe3O4/CS-AG
microcapsules and b) linear dependence graph between ln(c/c0) and time.
Based on our experimental observations and with the previous reports (Table 3), Pd NPs@Fe3O4/CS-AG
shows a better catalytic activity in the reduction of p-NP in terms of catalyst load, reaction time, temperature and
sustainability.
3.3 Investigation of reusability of Pd NPs@Fe3O4/CS-AG microcapsules
Recycling and reusing of catalysts are the most important parameters from sustainability aspect and should thus
be investigated. The reusability of Pd NPs@Fe3O4/CS-AG microcapsules was examined under the optimized
conditions determined for p-NP reduction and Suzuki-Miyaura reaction. After each catalytic reaction was
complete, Pd NPs@Fe3O4/CS-AG microcapsules were collected by an external magnet (Figure 8) from the
reaction medium and then washed with water to reactivate them. The recovered Pd NPs@Fe3O4/CS-AG
microcapsules were reused in the next catalytic reactions. The results of the reusability tests indicated that
magnetically recoverable Pd NPs@Fe3O4/CS-AG microcapsules retained their catalytic performance and were
highly efficient even after ten successive runs in Suzuki-Miyaura reaction (90%). Pd NPs@Fe3O4/CS-AG
microcapsules also displayed good reusability in the reduction of p-NP by showing an efficiency of 80% after
the 8th run. The ICP-OES analysis following the reusability tests showed that very low Pd was leached out from
13

Pd NPs@Fe3O4/CS-AG microcapsules (≤ 1%). These findings reveal that Pd NPs@Fe3O4/CS-AG microcapsules
are efficient and reusable catalysts for reduction of p-NP and Suzuki-Miyaura reaction.
Figure 8. Magnetically recoverable Pd NPs@Fe3O4/CS-AG microcapsules
4. Conclusions
In summary, nanosized Pd NPs@Fe3O4/CS-AG microcapsules, which can be easily separated from the reaction
medium via an external magnet, were successfully prepared and characterized. The catalytic activity of the
magnetic nanocatalyst was then evaluated in the Suzuki-Miyaura reactions and p-NP reduction. Pd
NPs@Fe3O4/CS-AG microcapsules showed superb catalytic performance in the Suzuki-Miyaura reaction by
giving high reaction yields in an extremely short reaction time (5 min). The designed Pd NPs@Fe3O4/CS-AG
microcapsules displayed superior catalytic performance in the p-NP reduction to p-AP at room temperature in
180 seconds. Moreover, Pd NPs@Fe3O4/CS-AG microcapsules maintained their catalytic activity for ten
consecutive runs in the Suzuki-Miyaura reaction and nine consecutive runs in p-NP reduction. Consequently,
this study revealed that Pd NPs@Fe3O4/CS-AG microcapsules have notable advantages such as magnetic
recoverability, high reusability, low cost, thermal stability and eco-friendly nature compared to the catalysts
reported in the literature. Additionally, based on these findings, the catalytic activity of Pd NPs@Fe3O4/CS-AG
microcapsules can be investigated under different catalytic reactions in further studies.
14

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20

Table 1.
Catalytic performance of Pd NPs@Fe3O4/CS-AG microcapsules in the Suzuki-Miyaura C-C coupling reaction.a
Entry
1
2
3
4
5
6
7
8
X
I
I
I
R
Yield (%)
99
80
83
91
80
88
96
92
72
79
85
94
61
69
74
82
TOF
792
640
664
728
640
704
768
736
576
632
680
752
488
552
592
656
4-OMe
4-Me
4-NH2
3-NO2
2-OMe
3-OMe
4-OMe
4-NO2
3-NH2
4-NH2
3-NO2
4-CN
I
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
9
10
11
12
13
14
15
16
3-OMe
4-OMe
3-NO2
4-CN
aReaction conditions: 1.0 mmol aryl halides, 1.8 mmol phenylboronic acid, 3.5 mmol K2CO3 and 1.5 × 10-2 Pd
NPs@Fe3O4/CS-AG microcapsules, 5 min, 400 W.
21

Table 2.
Pd NPs@Fe3O4/CS-AG microcapsules catalyzed Suzuki-Miyaura coupling reaction compared to reported
methods.
Entry Catalyst
Temp. (°C)
50
Time
12 h
Yield (%)a Ref.
Fe3O4-Pd
1
90
37
2
3
4
5
6
OCMCS-3aPd
Pd-1/FSG
PS-PEG600-NHC-Pd
pEVPBr-Pd
Polymer-supported Pd-NHC complex
100
100
50
90
50
48 h
12 h
12 h
9 h
97
86
76
68
38
39
40
41
42
12 h
68.5
7
8
9
10
11
12
Pd NPs@CMC/AG
Pd-PVP
Pd(OAc)2, polymer-supported ferrocenylphosphine
AG-Pd
60, US
90
110
30 min
24 h
6 h
94
89
67
97
94
80
43
44
45
46
47
48
50, MW
6 min
CL-Gly-Pd
MW, 400 W 7 min
80-100 5 h
MW, 400 W 5 min
Agarose hydrogel supported Pd nanoparticles
13
Pd NPs@Fe3O4/CS-AG microcapsules
96
This work
a Isolated yields.
Table 3.
Comparison of catalytic potential of NPs@Fe3O4/CS-AG microcapsules in the reduction of p-NP with other
catalysts.
Entry Catalyst
Solvent
Temp. (°C)
Time
Ref.
1
2
3
4
5
6
7
8
9
Au-GO
Cu NPs
Ag-PNA-BIS-2
AgNC-1
Ni@Pd/MIL-101
Au NPs/GNs
Pd NPs/GNs
Au/SRG
Pd NPs@Fe3O4/CS-AG microcapsules
H2O
THF/H2O
H2O
r.t.
50
r.t.
r.t.
30 min
2 h
9 h
2 h
2 h
22 min
15 min
4 min
3 min
51
52
53
54
55
56
56
57
H2O
Ethyl acetate 30
H2O
H2O
H2O
H2O
r.t.
r.t.
r.t.
r.t.
This work
22