Biochar as heterogeneous support for immobilization of Pd as efficient and reusable biocatalyst in C–C coupling reactions

Article abstract of DOI:10.1002/aoc.5205

Biochar is a stable and carbon-rich solid which has a high density of carbonyl, hydroxyl and carboxylic acid functional groups on its surface. In this work, the surface of biochar nanoparticles (BNPs) was modified with 3-choloropropyltrimtoxysilane and further 2-(thiophen-2-yl)-1H-benzo[d]imidazole was anchored on its surface. Then, palladium nanoparticles were fabricated on the surface of the modified BNPs and further the catalytic application was studied as recyclable biocatalyst in carbon–carbon coupling reactions such as Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions. The structure of the catalyst was characterized using scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, X-ray diffraction and atomic absorption spectroscopy. The catalyst can be reused several times without a decrease in its catalytic efficiency. In addition to the several advantages reported, application of biochar as catalyst support for the first time is a major novelty of the present work.

Full text of DOI:10.1002/aoc.5205

Received: 21 April 2019
DOI: 10.1002/aoc.5205
Revised: 2 August 2019
Accepted: 4 August 2019
F U L L P A P E R
Biochar as heterogeneous support for immobilization of Pd
as efficient and reusable biocatalyst in C–C coupling
reactions
Parisa Moradi1 | Maryam Hajjami1
| Fatemeh Valizadeh‐Kakhki2
1 Department of Chemistry, Faculty of
Biochar is a stable and carbon‐rich solid which has a high density of carbonyl,
hydroxyl and carboxylic acid functional groups on its surface. In this
work, the surface of biochar nanoparticles (BNPs) was modified with
3‐choloropropyltrimtoxysilane and further 2‐(thiophen‐2‐yl)‐1H‐benzo[d]imid-
azole was anchored on its surface. Then, palladium nanoparticles were fabri-
cated on the surface of the modified BNPs and further the catalytic application
was studied as recyclable biocatalyst in carbon–carbon coupling reactions such
as Suzuki–Miyaura and Heck–Mizoroki cross‐coupling reactions. The structure
of the catalyst was characterized using scanning electron microscopy, transmis-
sion electron microscopy, energy‐dispersive X‐ray spectroscopy, thermogravi-
metric analysis, X‐ray diffraction and atomic absorption spectroscopy. The
catalyst can be reused several times without a decrease in its catalytic efficiency.
In addition to the several advantages reported, application of biochar as catalyst
support for the first time is a major novelty of the present work.
Science, Ilam University, PO Box
69315516, Ilam, Iran
2 Department of Soil and Water
Engineering, Faculty of Agriculture, Ilam
University, PO Box 69315516, Ilam, Iran
Correspondence
Maryam Hajjami, Department of
Chemistry, Faculty of Science, Ilam
University, PO Box 69315516, Ilam, Iran
Email: mhajjami@yahoo.com; m.
hajjami@ilam.ac.ir
KEYWORDS
biochar nanoparticles, C–C coupling reactions, Heck reaction, palladium, Suzuki reaction
1 | INTRODUCTION
their preparation which are expensive and not environ-
mentally friendly. Biochar is a novel type of nanoparti-
Recently, nanotechnology has emerged in science, medi-
cine, industry, pharmacy and chemistry.[1–4] In this
regard, various nanoparticles have also been applied as
catalyst supports due to their special properties such as
high surface area and insolubility.[5–7] When the size of
particles is decreased, their surface area is increased,
which will be lead to high capacity of catalyst loading.[8]
Therefore, nanoparticles are widely employed as solid
supports for heterogenization of homogeneous cata-
lysts.[1–8] For example, iron oxide,[9] mesoporous silica
materials,[10] carbon nanotubes,[11] ionic liquids,[12] poly-
mers,[13] graphene oxide,[14,15] heteropolyacids,[16]
boehmite nanoparticles,[17] etc., have been used as cata-
lyst supports; however, all of these materials require
chemical procedures and chemical starting materials for
cles, which is a stable carbon solid and used in various
fields in past few years for science and engineering.[18–
20] In fact, biochar (which is called black carbon) is char-
coal which is used for soil amendment, reduction in
greenhouse gas emissions from soil, adsorption, water
retention, agricultural waste recycling, climate change
mitigation and energy production.[21–24] Biochar is made
via pyrolysis of biological sources such as woody mate-
rials, agricultural wastes, green waste, animal manures
and other waste products.[21,22] Therefore, it is inexpen-
sive and environmentally friendly. A number of studies
have highlighted the benefits of biochar as soil amend-
ment.[25] Surfaces of biochar nanoparticles are covered
with carbonyl, hydroxyl and carboxylic acid functional
groups.[25,26] The presence of a high density of carbonyl,
Appl Organometal Chem. 2019;e5205.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5205

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MORADI ET AL.
hydroxyl and carboxylic acid groups on the biochar sur-
face allows the modification of its surface via reaction
with dopamine, alkoxysilane and other reagents which
can be used as a support for the immobilization of cata-
lysts or other substances. Despite various studies of the
morphology, application, properties and preparation of
biochar,[18–27] currently no reports are available of the
application of biochar as a catalyst support. Biochar is a
stable solid in air atmosphere, at high temperature and
in aqueous solution, and therefore biochar is an ideal cat-
alyst support for organic processes occurring under harsh
conditions. Also, biochar has several advantages such as
non‐toxicity, ready availability, environmentally friendly,
high stability and high surface area.[21–26]
2.2 | Preparation of catalyst
Initially, 2‐(thiophen‐2‐yl)‐1H‐benzo[d]imidazole (TBA)
as ligand for immobilization of palladium was synthe-
sized according to the procedure reported by Nagawade
and Shinde[41] (Scheme 1).
Biochar nanoparticles modified with 3‐cholo-
ropropyltrimtoxysilane (CPTMS@biochar) were obtained
according to a new reported procedure;[42] subsequently
CPTMS@biochar (1.0 g) was dispersed in toluene and
1.5 mmol of TBA was added to this mixture. The mixture
was stirred at 90°C for 48 h. The solid product
(TBA@biochar) was isolated after washing with ethanol
and drying at 50°C. TBA@biochar (1.0 g) was dispersed
in ethanol and mixed with 0.5 g of Pd(OAc)2. The mixture
was stirred at 80°C for 20 h. Finally, NaBH4 (0.5 mmol)
was added to the reaction mixture and was allowed to
run for another 2 h. The solid product (Pd(0)‐TBA@bio-
char) was obtained after washing with water and ethanol
and drying at 50°C.
In this regard, we report a new complex of palladium,
immobilized on biochar nanoparticles, as an efficient and
reusable biocatalyst for C–C cross‐coupling reactions.
The C–C coupling reaction is one of the most powerful
tools for the preparation of natural products, advanced
materials, pharmaceuticals, biologically active com-
pounds, polymers, hydrocarbons and liquid crystal mate-
rials.[27–32]
Suzuki–Miyaura
and
Heck–Mizoroki
2.3 | General procedure Suzuki reaction
catalysed by Pd(0)‐TBA@biochar
reactions are usually reported with homogeneous or
heterogeneous palladium catalysts.[33–40] The use of
palladium–phosphine catalysts involves expensive, toxic
and air‐ and moisture‐sensitive procedures. Therefore, to
embrace green chemistry principles, we report a procedure
for the immobilization of palladium on biochar nanoparti-
cles as a phosphine‐free, stable and recyclable biocatalyst
for the Suzuki–Miyaura and Heck–Mizoroki reactions.
Amounts of 1 mmol of aryl halide, 1 mmol of
phenylboronic acid or 1 mmol of 3,4‐difluorophe-
nylboronic acid, 3 mmol of sodium carbonate and 5 mg
of Pd(0)‐TBA@biochar (containing 0.715 mol% of Pd)
were stirred in PEG‐400 at 80°C and the progress of the
reaction was monitored by TLC. After completion of the
reaction, the mixture was cooled to room temperature
and the catalyst was separated by simple filtration and
washed with ethyl acetate. The reaction mixture was
extracted with water and ethyl acetate. The organic layer
was dried over Na2SO4 (1.5 g). Then the solvent was evap-
orated and pure biphenyl derivatives were obtained in
good to excellent yields.
2 | EXPERIMENTAL
2.1 | Preparation of biochar
An amount of 500 g of dried chicken manure was placed
in a porcelain crucible. The pyrolysis temperature was
selected from 400 to 800°C, which is the common temper-
ature range of fast pyrolysis. The set temperature was
reached after about 30 min of heating with the carrier
gas (N2) sweeping at 0.3 l min−1, and the porcelain cruci-
ble was fed into the heating zone with an N2 flow rate of
0.03 l min−1. After 1 and 2 h, the pyrolysis process was
ended, and the porcelain crucible was removed from the
heating zone and cooled with N2 sweeping at
0.3 l min−1 for 30 min. The solid product was biochar,
which was ground through a 40 mesh sieve (0.45 mm).
Prior to additional experiments, no pre‐treatment was
performed. The obtained biochar samples were abbrevi-
ated as CMB400–1, CMB400–2, CMB600–1, CMB600–2,
CMB800–1 and CMB800–2 according to the pyrolysis res-
idence time (1 and 2 h) and temperature.
2.4 | General procedure for Heck reaction
catalysed by Pd(0)‐TBA@biochar
A mixture of aryl halide (1 mmol), butyl acrylate or
methyl acrylate or acrylonitrile (1.2 mmol), Na2CO3
(3 mmol) and Pd(0)‐TBA@biochar (10 mg, containing
1.43 mol% of Pd) was stirred in PEG‐400 at 120°C and
the progress of the reaction was monitored by TLC. After
SCHEME
1
Synthesis
of
2‐(thiophen‐2‐yl)‐1H‐benzo[d]
imidazole (TBA)

MORADI ET AL.
3 of 13
completion of the reaction, the mixture was cooled to
room temperature and the catalyst was separated by sim-
ple filtration and washed with diethyl ether. The reaction
mixture was extracted with water and diethyl ether. The
organic layer was dried over Na2SO4 (1.5 g). Then the sol-
vent was evaporated and pure products were obtained in
moderate to good yields.
2.5.7 | Butyl 3‐(4‐methoxyphenyl)acrylate
1H NMR (400 MHz, CDCl3): δH = 7.67–7.63 (d, J = 16 Hz,
1H), 7.5–7.48 (d, J = 8 Hz, 2H), 6.92–6.90 (d, J = 8 Hz,
2H), 6.34–6.30 (d, J = 16 Hz, 1H), 4.23–4.19 (t,
J = 8 Hz, 2H), 3.84 (s, 3H), 1.74–1.66 (quint, J = 4 Hz,
2H), 1.50–1.41 (sextet, J = 8 Hz, 2H), 1.00–0.96 (t,
J = 8 Hz, 3H) ppm.
2.5 | Selected spectral data
2.5.1 | 1,4‐Diphenylbenzene
2.5.8 | Methyl 3‐(4‐methoxyphenyl)
acrylate
1H NMR (400 MHz, CDCl3): δH = 7.69–7.65 (d, J = 16 Hz,
1H), 7.50–7.48 (d, J = 8 Hz, 2H), 6.93–6.91 (d, J = 8 Hz,
2H), 6.35–6.31 (d, J = 16 Hz, 1H), 3.85 (s, 3H), 3.81 (s,
3H) ppm.
1H NMR (400 MHz, CDCl3): δH = 7.72–7.70 (d, J = 8 Hz,
4H), 7.68–7.66 (d, J = 8 Hz, 4H), 7.52–7.46 (q, J = 8 Hz,
4H), 7.42–7.36 (q, J = 8 Hz, 2H) ppm.
2.5.2 | 2‐Phenylnaphthalene
2.5.9 | Methyl 3‐(p‐tolyl)acrylate
1H NMR (400 MHz, CDCl3): δH = 8.08 (s, 1H), 7.97–7.89
(m, 3H), 7.80–7.75 (m, 3H), 7.57–7.50 (m, 4H), 7.44–7.40
(tt, J = 7.2 Hz, 1.6 Hz, 1H) ppm.
1H NMR (400 MHz, CDCl3): δH = 7.71–7.67 (d, J = 16 Hz,
1H), 7.45–7.43 (d, J = 8 Hz, 2H), 7.22–7.20 (d, J = 8 Hz,
2H), 6.44–6.40 (d, J = 16 Hz, 1H), 3.82 (s, 3H), 2.39 (s,
3H) ppm.
2.5.3 | [1,1′‐Biphenyl]‐4‐carbaldehyde
1H NMR (400 MHz, CDCl3): δH = 10.09 (s, 1H), 7.99–7.97
(d, J = 8 Hz, 2H), 7.79–7.77 (d, J = 8 Hz, 2H), 7.68–7.66
(d, J = 8 Hz, 2H), 7.53–7.49 (t, J = 8 Hz, 2H), 7.47–7.43
(t, J = 8 Hz, 1H) ppm.
2.5.10 | 3‐(p‐Tolyl)acrylonitrile
1H NMR (400 MHz, CDCl3): δH = 7.41–7.37 (d, J = 16 Hz,
1H), 7.37–7.34 (m, 2H), 7.25–7.20 (m, 2H), 5.86–5.82 (d,
J = 16 Hz, 1H), 2.41 (s, 3H) ppm. Supporting Information.
2.5.4 | [1,1′‐Biphenyl]‐3‐carbaldehyde
3 | RESULTS AND DISCUSSION
1H NMR (400 MHz, CDCl3): δH = 10.12 (s, 1H), 8.40 (s,
1H), 8.15–8.14 (d, J = 2 Hz, 1H), 7.91–7.87 (td,
J = 4 Hz, J = 2 Hz, 1H), 7.68–7.65 (m, 2H), 7.62–7.57
(dd, J = 12 Hz, J = 4 Hz, 1H), 7.53–7.49 (t, J = 8 Hz,
2H), 7.45–7.43 (m, 1H) ppm.
Pd(0)‐TBA@biochar was prepared based on the concise
route outlined in Scheme 2. Firstly, biochar was prepared
from pyrolysis of chicken manure. Secondly, the surface
of the biochar was modified with CPTMS. Subsequently,
the terminal chloro groups attached on surface of biochar
were functionalized with TBA (TBA@biochar). Finally,
the catalyst (Pd(0)‐TBA@biochar) was prepared by com-
plexation of TBA@biochar with palladium acetate. This
work has several novelties such as: (1) first time applying
biochar as a support for a catalyst, (2) first report of using
a reusable biocatalyst based on biochar in organic reac-
tions and (3) first time of supporting a metal complex
on biochar nanoparticles.
2.5.5 | 3,4‐Difluoro‐1,1′‐biphenyl
1H NMR (400 MHz, CDCl3): δH = 7.56–7.54 (d, J = 8 Hz,
2H), 7.49–7.45 (t, J = 8 Hz, 2H), 7.44–7.38 (m, 2H), 7.34–
7.31 (m,1H), 7.28–7.21 (m, 1H) ppm.
2.5.6 | Butyl 3‐(4‐methylphenyl)acrylate
1H NMR (400 MHz, CDCl3): δH = 7.70–7.66 (d, J = 16 Hz,
1H), 7.46–7.44 (d, J = 8 Hz, 2H), 7.22–7.20 (d, J = 8 Hz,
2H), 6.44–6.40 (d, J = 16 Hz, 1H), 4.25–4.21 (t,
J = 8 Hz, 2H), 2.39 (s, 3H), 1.75–1.68 (quint, J = 8 Hz,
2H), 1.51–1.42 (sextet, J = 8 Hz, 2H), 1.01–0.97 (t,
J = 8 Hz, 3H) ppm.
3.1 | Catalyst characterization
After the preparation of the catalyst, it was characterized
using scanning electron microscopy (SEM), energy‐
dispersive X‐ray spectroscopy (EDS), thermogravimetric
analysis (TGA), X‐ray diffraction (XRD) and atomic

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MORADI ET AL.
SCHEME 2 Synthesis of Pd(0)‐
TBA@biochar
FIGURE 1 SEM images of (a) biochar and (b) Pd(0)‐TBA@biochar
absorption spectroscopy (AAS). SEM images of biochar
and Pd(0)‐TBA@biochar are shown in Figure 1. As
shown in Figure 1, biochar and catalyst were prepared
with particles of 30–70 nm of diameter with quasi‐
spherical morphology. The SEM images of biochar
(Figure 1a) and of catalyst (Figure 1b) show good agree-
ment in size and shape of particles, which confirms that
the biochar is not changed during the modification. Also,
TEM images of Pd(0)‐TBA@biochar are shown in
Figure 2. It can be seen the catalyst was obtained with
average size of less than 100 nm. As shown in Figure 2,
palladium particles are present in the structure of the cat-
alyst, and the average size of these particles is between 6
and 8 nm.

MORADI ET AL.
5 of 13
The TGA diagram of biochar is shown in Figure 4a and
that of Pd(0)‐TBA@biochar is shown in Figure 4b. The
small weight loss of about 5% below 100°C is related to
evaporation of solvents and hydroxyl groups on the surface
of biochar.[43] The organic contents were decomposed at
200–600°C. As shown in Figure 4b, the weight loss of
Pd(0)‐TBA@biochar is higher than that of biochar nano-
particles which is associated with the immobilized organic
layers on the surface of biochar. This result is strong evi-
dence that organic layers and palladium complex were
supported on the surface of biochar nanoparticles.
XRD patterns of biochar and Pd(0)‐TBA@biochar are
shown in Figure 5 with the diffracted beam as the inten-
sity as a function of the Bragg angle (2θ). The XRD pat-
tern of biochar (Figure 5a) indicates crystallinity with a
sharp peak at 2θ = 30°.[18] The XRD pattern of biochar
also shows several weak peaks at 2θ = 40.7°, 43.7°,
48.7° and 66.7°.[18] As shown in Figure 5b, the XRD pat-
tern of Pd(0)‐TBA@biochar shows a good agreement with
XRD pattern of biochar nanoparticles, from which result
it is revealed that the surface modification of the biochar
did not destroy its crystallinity. Also the XRD pattern of
Pd(0)‐TBA@biochar (Figure 5b) shows three peaks
(39.5°, 46.1° and 67.5°) which are indexed to Pd(0) on
the surface of biochar nanoparticles.[6,33]
3.2 | Catalytic activity of Pd(0)‐
TBA@biochar in C–C coupling reactions
FIGURE 2 TEM images of Pd(0)‐TBA@biochar
In order to determine the catalytic activity of Pd(0)‐
TBA@biochar, we investigated the catalytic C–C coupling
reaction of aryl halides with phenylboronic acid (PhB(OH)2)
or 3,4‐difluorophenylboronic acid (3,4‐diF‐C6H3B(OH)2)
(Scheme 3) and also butyl acrylate, methyl acrylate or acrylo-
nitrile (Scheme 5 in the presence of this catalyst.
To optimize the Suzuki coupling reaction, the reac-
tion of 4‐iodotoluene with PhB(OH)2 was selected as a
model reaction. This model reaction was investigated in
the presence of various amounts of Pd(0)‐TBA@biochar
In order to determine the content of elements in the
catalyst, EDS analysis of Pd(0)‐TBA@biochar was per-
formed (Figure 33). As depicted, the EDS spectrum of
the catalyst shows the presence of C, Si, O, S, N and as
well as Pd species in the catalyst. Also, the exact amount
of palladium (which is immobilized on the modified bio-
char nanoparticles) was obtained using the AAS tech-
nique and was found to be 1.43 × 10−3 mol g−1
.
FIGURE 3 EDX spectrum of Pd(0)‐
TBA@biochar

6 of 13
MORADI ET AL.
the aromatic ring of aryl halide (involving of Cl, Br and I) was
examined (Table 2). Both electron‐donating groups such as
OH, NH2, OCH3 or CH3 and electron‐withdrawing groups
such as NO2, CN, CHO, Cl, COOH or CF3 reacted conve-
niently, and the corresponding products were obtained in
good to excellent yields and with high turnover frequency
(TOF) values. The major advantages are high TOFs and
excellent yields of products in short reaction times. In order
to extend this work, the coupling of aryl halides with 3,4‐
diF‐C6H3B(OH)2 was investigated (Table 2, entries 22–29).
In this stage corresponding products were obtained in good
to excellent yields. These results revealed that this catalyst
can be used for the synthesis of a wide range of biphenyls
by coupling of various aryl halides with phenylboronic acid
derivatives.
FIGURE 4 TGA diagrams of (a) biochar and (b) Pd(0)‐
TBA@biochar
The catalytic cycle for the Suzuki reaction in the pres-
ence of Pd(0)‐TBA@biochar is shown in Scheme 4. Ini-
tially, an oxidative addition of aryl halide to palladium(0)
catalyst forms the intermediate
I which includes
palladium(II). Next, transmetallation of I gives intermedi-
ate II. Finally, the reductive elimination of II leads to
biphenyls as products and regeneration of the catalyst.
Additionally, we studied the catalytic activity of Pd(0)‐
TBA@biochar in the Heck C–C reaction involving the
coupling of aryl halides with various alkenes such as butyl
acrylate, methyl acrylate or acrylonitrile (Scheme 5).
In order to optimize the Heck C–C coupling reaction
conditions, the coupling of iodobenzene with butyl acrylate
was selected as a model reaction. The results summarized
in Table 3 indicate that the best outcome was obtained with
the use of Na2CO3 (3 mmol, 0.318 mg) as base and PEG‐400
as solvent in the presence of Pd(0)‐TBA@biochar (10 mg,
1.43 mol%) at 120°C (Table 3, entry 3). When the model
reaction was examined at 100°C (Table 3, entry 14), the
reaction was negatively affected (the yield of product was
decreased from 98 to 64%). The yield of obtained product
was decreased from 98 to 84% when the amount of Pd(0)‐
TBA@biochar was decreased to 7 mg (Table 3, entry 2).
Also, among the various solvents (water, toluene, 1,4‐
dioxane, PEG, DMF and DMSO), the best results were
obtained in PEG‐400 as solvent (Table 3, entries 4–9).
Under the optimized reaction conditions, we next
investigated this reaction with other aryl halides bearing
FIGURE
TBA@biochar
5
XRD patterns of (a) biochar and (b) Pd(0)‐
and the optimal amount of catalyst was found to be
5 mg (0.715 mol% of Pd) of Pd(0)‐TBA@biochar
(Table 1, entry 3). As evident from Table 1, lower amounts
of Pd(0)‐TBA@biochar led to a lower yield of product
(Table 1, entry 2) and also a higher amount of this catalyst
gave no significant improvement in the time or yield of
reaction (Table 1, entry 4). In the next step, the model reac-
tion was carried out in various solvents and with various
bases (Table 1, entries 5–11). Based on the obtained results,
PEG‐400 as solvent and sodium carbonate (Na2CO3) as
base gave the best conversion of starting materials to prod-
ucts. In the final step, the effect of temperature on the
model reaction was studied, and the best results were
obtained at 80°C (Table 1, entries 11–13).
With the optimal reaction conditions in hand, the scope of
other meta‐, ortho‐ and para‐substituted functional groups on
SCHEME 3 Suzuki C–C coupling
reaction in the presence of Pd(0)‐
TBA@biochar

MORADI ET AL.
7 of 13
TABLE 1 Optimizing reaction conditions for Suzuki reaction in the presence of Pd(0)‐TBA@biochar
Entry
Solvent
Catalyst (mg)
Base
Temperature (°C)
Time (min)
Yield (%)a
1
PEG
—
3
5
7
5
5
5
5
5
5
5
5
5
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
KOH
80
80
80
80
80
80
80
80
80
80
80
60
40
100
60
No reaction
2
PEG
88
3
PEG
20
97
4
PEG
15
95
5
H2O
100
70
88
6
DMSO
DMF
1,4‐Dioxane
PEG
95
7
90
91
8
120
60
71
9
72
10
11
12
13
PEG
NaOEt
Et3N
60
41
PEG
60
69
PEG
Na2CO3
Na2CO3
60
46
PEG
60
Trace
aIsolated yield.
TABLE 2 C–C coupling reaction for synthesis of biphenyl derivatives catalysed by Pd(0)‐TBA@biochar
Reported
Melting point melting
Phenylating
reagent
Time
Yield
TOF
TON (h−1
Entry Aryl halide
(min) (%)a
)
(°C)
point (°C)
1
Iodobenzene
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
C6H5B(OH)2
125
140
25 h
20
96
94
89
97
91
93
95
89
90
95
87
96
90
89
92
90
64
91
85
134
131
124
135
127
130
133
124
126
133
121
134
126
124
129
126
89
64
56
5
66–68
65–67
65–67
44–46
Oil
66–68[33]
66–68[33]
66–68[33]
45–47[39]
Oil[8]
2
Bromobenzene
3
Chlorobenzene
4
4‐Iodotoluene
405
40
92
76
65
63
76
43
45
5
5
2‐Iodotoluene
190
85
6
4‐Bromochlorobenzene
4‐Bromonitrobenzene
4‐Bromophenol
4‐Bromobenzonitrile
4‐Iodoanisole
70–72
111–113
159–162
82–83
80–82
Oil
71–73[6]
113–115[9]
162–164[47]
80–84[42]
82–84[42]
Oil[6]
7
105
115
120
105
170
180
25 h
24 h
125
45
8
9
10
11
12
13
14
15
16
17
18
19
2‐Iodoanisole
4‐Bromobenzaldehyde
4‐Bromoaniline
4‐Bromoanisole
3‐Bromoanisole
4‐Bromotoluene
1,4‐Dibromobenzene
3‐Bromobenzaldehyde
53–55
52–54
80–83
Oil
55–57[8]
53–54[47]
81–83[48]
Oil[9]
5
62
168
22
39
62
44–46
210–212
Oil
43–45[49]
212–214[50]
Oil[9]
240
195
115
127
119
1‐Bromo‐3‐
(trifluoromethyl)
benzene
Oil
Oil[6]
20
2‐Bromonaphthalene
C6H5B(OH)2
380
93
130
20
101–103
104–106[51]
(Continues)

8 of 13
MORADI ET AL.
TABLE 2 (Continued)
Reported
Melting point melting
Phenylating
reagent
Time
Yield
TOF
TON (h−1
Entry Aryl halide
(min) (%)a
)
(°C)
point (°C)
219–224[52]
39–41[33]
21
22
4‐Bromobenzoic acid
C6H5B(OH)2
340
85
91
96
127
134
22
95
219–223
38–41
Iodobenzene
3,4‐diF‐
C6H3B(OH)2
23
24
25
26
27
28
29
Bromobenzene
3,4‐diF‐
C6H3B(OH)2
235
90
90
92
95
91
85
96
83
126
127
133
127
119
134
116
32
38–41
42–43
118–120
105–106
Oil
39–41[33]
4‐Iodotoluene
3,4‐diF‐
C6H3B(OH)2
86
—
4‐Bromonitrobenzene
4‐Bromobenzonitrile
3‐Bromoanisole
4‐Bromochlorobenzene
4‐Iodoanisole
3,4‐diF‐
C6H3B(OH)2
145
165
75
55
119–120[33]
3,4‐diF‐
C6H3B(OH)2
46
—
—
—
—
3,4‐diF‐
C6H3B(OH)2
95
3,4‐diF‐
C6H3B(OH)2
70
115
116
59
3,4‐diF‐
60
Oil
C6H3B(OH)2
aIsolated yield.
electron‐donating and electron‐withdrawing groups at
the meta, ortho and para positions of aromatic ring of aryl
halides (Table 4). In order to extend this work, coupling
of aryl halides with methyl acrylate, acrylonitrile and
butyl acrylate was investigated, and all products were
obtained in good to excellent yields and high TOF values.
Therefore this procedure can be applied for coupling of
various aryl halides with a variety of alkenes. The cata-
lytic activity of Pd(0)‐TBA@biochar showed good selec-
tivity in the coupling of alkenes with aryl iodides in
comparison with aryl bromides or aryl chlorides.
The suggested mechanism for the Heck coupling reac-
tion in the presence of Pd(0)‐TBA@biochar is outlined in
Scheme 6. In the first step, an oxidative addition of aryl
halide to palladium(0) gives intermediate I. Then, coordina-
tion of alkene with intermediate I leads to intermediate II,
which is converted to intermediate III with an insertion.
Next, β‐elimination of intermediate III forms intermediate
IV and products are formed. Finally, the reduction of inter-
mediate IV leads to regeneration of the catalyst.
In order to investigate the chemoselectivity of Pd(0)‐
TBA@biochar, 1‐chloro‐4‐bromobenzene was coupled
with phenylboronic acid and 3,4‐difluorophenylboronic
acid in the presence of Pd(0)‐TBA@biochar (Scheme 7).
Interestingly, the chloro group was not coupled, while
the bromo group was coupled successfully and 4‐chloro‐
1,1′‐biphenyl was obtained as pure product in excellent
yield (Table 2, entries 6 and 28).
SCHEME 4 Suggested mechanism for Suzuki reaction in the
presence of Pd(0)‐TBA@biochar
SCHEME 5 Pd(0)‐TBA@biochar‐catalysed Heck C–C coupling
reaction

MORADI ET AL.
9 of 13
TABLE 3 Optimizing reaction conditions for Heck reaction in the presence of Pd(0)‐TBA@biochar
Entry
Solvent
Catalyst (mg)
Base
Temperature (°C)
Time (min)
Yield (%)a
1
PEG
—
7
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
NaOEt
Et3N
120
180
180
70
60
70
70
70
70
70
70
70
70
70
70
No reaction
2
PEG
120
84
98
93
62
74
82
79
52
35
73
68
83
64
3
PEG
10
12
10
10
10
10
10
10
10
10
10
10
120
4
PEG
120
5
Toluene
H2O
Reflux
Reflux
120
6
7
DMF
DMSO
1,4‐Dioxane
PEG
8
120
9
Reflux
120
10
11
12
13
14
PEG
120
PEG
KOH
120
PEG
K2CO3
Na2CO3
120
PEG
100
aIsolated yield
TABLE 4 Coupling of aryl halides with butyl acrylate, methyl acrylate and acrylonitrile catalysed by Pd(0)‐TBA@biochar
Time
(min)
Yield (%)a
TOF
(h−1
)
Melting point
(°C)
Reported melting point
(°C)
Entry
Aryl halide
Alkene
TON
1
Iodobenzene
4‐Iodotoluene
2‐Iodotoluene
4‐Iodoanisole
2‐Iodoanisole
Iodobenzene
4‐Iodotoluene
2‐Iodotoluene
4‐Iodoanisole
2‐Iodoanisole
Iodobenzene
4‐Iodotoluene
2‐Iodotoluene
4‐Iodoanisole
2‐Iodoanisole
Butyl acrylate
Butyl acrylate
Butyl acrylate
Butyl acrylate
Butyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Methyl acrylate
Acrylonitrile
70
98
96
91
93
90
95
88
91
93
89
97
95
92
93
90
68
67
64
65
63
66
61
64
65
62
68
66
64
65
63
59
50
19
30
24
72
39
25
49
15
63
20
21
22
15
Oil
Oil[53]
Oil[6]
2
80
Oil
3
200
130
160
55
Oil
Oil[54]
Oil[42]
Oil[8]
4
Oil
5
Oil
6
Oil
Oil [55]
55–57[56]
[57]
7
95
54–56
Oil
8
155
80
9
86–88
Oil
88–91[56]
Oil[58]
Oil[54]
Oil[54]
Oil[6]
10
11
12
13
14
15
240
65
Oil
Acrylonitrile
200
180
175
255
Oil
Acrylonitrile
Oil
Acrylonitrile
Oil
Oil[59]
‐
Acrylonitrile
Oil
aIsolated yield.
(Figure 6b). The obtained results for the reusability of
Pd(0)‐TBA@biochar in Suzuki and Heck coupling
reactions are summarized in Figure 6. It was found that
this catalyst can be successfully recovered for up to
seven successive reaction runs. As depicted, the catalyst
shows no significant change in its activity after
seven runs.
3.3 | Reusability of catalyst
The reusability of Pd(0)‐TBA@biochar was studied in
the Suzuki coupling reaction of iodobenzene with
phenylboronic acid (Figure 6a). Also, the recyclability
of this catalyst was examined in the Heck reaction using
the coupling of iodobenzene with butyl acrylate

10 of 13
MORADI ET AL.
63 min). In this step, the exact amount of palladium remain-
ing in the catalyst was calculated using the AAS technique
which was found to be 1.42 × 10−3 mol g−1, which shows
good agreement with fresh catalyst (1.43 × 10−3 mol g−1).
Then, the C–C coupling reaction of iodobenzene with
phenylboronic acid was repeated and in half the time of the
reaction, the catalyst was separated and the filtered solution
was allowed to react for 125 min. In this stage, 63% of product
was obtained. In this step, the recovered catalyst after
125 min was analysed using AAS, with which the exact
amount of palladium was found to be 1.42 × 10−3 mol g−1.
As shown, the amount of palladium in the recovered catalyst
shows a good agreement with fresh catalyst. These results
confirmed that the leaching of palladium did not happen.
Therefore, the C–C coupling reaction proceeded using het-
erogeneous palladium species as catalyst.
SCHEME 6 Suggested mechanism for Heck reaction in the
presence of Pd(0)‐TBA@biochar
Also, the hot filtration test was performed for the Heck
reaction of iodobenzene with butyl acrylate. In this exper-
iment, in half the time of the reaction (after 35 min), 58%
of butyl cinnamate was obtained. Then, the coupling of
iodobenzene with butyl acrylate was repeated under opti-
mized conditions and in half the time of the reaction, the
catalyst was removed and the filtered solution was
allowed to react for 70 min. In this stage, 61% of product
was obtained which confirmed that the leaching of palla-
dium did not happen.
SCHEME 7 Selectivity in C–C coupling reaction in the presence
of Pd(0)‐TBA@biochar
The poisoning test is a powerful method for determining
homogeneity/heterogeneity
of
catalysis
systems.
Polyvinylpyridine (PVP) poisoning and mercury poisoning
tests are often employed for extinguishing solution‐phase
catalysis.[44–46]
In
order
to
confirm
the
homogeneity/heterogeneity of Pd(0)‐TBA@biochar, cou-
pling of iodobenzene with phenylboronic acid in the pres-
ence of PVP was examined. For this, a mixture of
phenylboronic acid (1 mmol), iodobenzene (1.0 mmol),
Na2CO3 (3 mmol), 0.005 g of PVP, PEG‐400 (3 ml) and
Pd(0)‐TBA@biochar (5 mg, 0.715 mol%) was stirred at
80°C. This reaction was completed, and any changes in the
reaction yield or time were not observed, which suggests that
Pd(0)‐TBA@biochar is heterogeneous in nature.
Results of poisoning and hot filtration tests showed
good agreement for the heterogeneous nature of Pd(0)‐
TBA@biochar.
FIGURE 6 Recycling experiment of Pd(0)‐TBA@biochar in (a)
Suzuki reaction and (b) Heck reaction using coupling of
iodobenzene with (a) phenylboronic acid and (b) butyl acrylate
3.5 | Comparison of catalysts
3.4 | Leaching of catalyst
In order to compare the activity of Pd(0)‐TBA@biochar with
that of previously reported catalysts, the results for the C–C
coupling reaction of iodobenzene with phenylboronic acid
in the presence of Pd(0)‐TBA@biochar were compared with
those obtained with previous catalysts (Table 5). Pd(0)‐
TBA@biochar is a more effective catalyst than the other cat-
alysts. Additionally, previous catalysts require chemical
In order to examine the leaching of palladium from Pd(0)‐
TBA@biochar in the reaction mixture, poisoning test and
hot filtration test were performed for the carbon–carbon cou-
pling reaction of iodobenzene with phenylboronic acid under
optimized conditions. In the hot filtration test, 59% of biphe-
nyl was obtained in half the time of the reaction (after

MORADI ET AL.
11 of 13
TABLE 5 Comparison of Pd(0)‐TBA@biochar in the coupling reaction of iodobenzene with phenylboronic acid with previously reported
procedures
Reaction
conditions
Time
(min)
TOF
(h−1
)
Yield
(%)
Entry
Catalyst (mol%)
1
PANI‐Pd (2.2 mol%)
K2CO3, 1,4‐dioxane–H2O
240
10
91[60]
(1:1), 95°C
2
N,N′‐Bis(2‐pyridinecarboxamide)‐1,2‐
benzene
H2O, K2CO3, 100°C
180
32
97[61]
palladium complex (1 mol%)
3
4
5
6
7
8
9
Pd (II)–NHC complex (1 mol%)
NHC–Pd (II) complex (1.0 mol%)
Pd/Au NPs (4.0 mol %)
DMF, Cs2CO3, 100°C
THF, Cs2CO3, 80°C
EtOH/H2O, K2CO3, 80°C
PEG, K2CO3, 100°C
H2O, KOH, 100°C
24 h
12 h
24 h
70
4
99[62]
88[55]
88[63]
95[42]
95[64]
94[65]
99[66]
7
1
Pd (0)‐Schiff‐base@MCM‐41 (1.6 mol%)
Pd NP (1.0 mol%)
50.8
4
12 h
120
CA/Pd (0) (0.5–2.0 mol%)
H2O, K2CO3, 100°C
94
40
Polymer anchored Pd(II) Schiff base complex
(0.5 mol%)
K2CO3, DMF–H2O (1:1),
80°C
300
10
11
12
Pd@SBA‐15/ILDABCO (0.5 mol%)
Pd‐MPA@MCM‐41 (1.7 mol%)
Pd(0)‐TBA@biochar (0.715 mol%)
K2CO3, H2O, 80°C
90
129
27.94
64
97[67]
95[47]
96
K2CO3, PEG, 100°C
PEG‐400, Na2CO3, 80°C
120
125
(this work)
procedures and chemical starting materials for their prepara-
tion which are expensive and not environmentally friendly,
but biochar is made via pyrolysis of biological sources such
as woody materials, agricultural wastes, green waste, animal
manures and other waste products, and therefore it is inex-
pensive and environmentally friendly. The products were
obtained in higher yields and higher TOF values in shorter
reaction times when Pd(0)‐TBA@biochar was used. Also
application of Pd(0)‐TBA@biochar as a heterogeneous cata-
lyst was compared with homogenous catalyst in the C–C cou-
pling reaction. The result showed that Pd(0)‐TBA@biochar is
more effective than homogenous catalyst in terms of reaction
time and obtained yield of product (Table 5, entry 12).
Additionally, Pd(0)‐TBA@biochar is economical and
environmentally friendly as it can be recovered and
reused several times.
ACKNOWLEDGMENTS
The authors are grateful to the research facilities of Ilam
University, Ilam, Iran, for financial support of this
research project.
ORCID
Maryam Hajjami https://orcid.org/0000-0002-5225-4809
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Valizadeh‐Kakhki F. Biochar as heterogeneous
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reusable biocatalyst in C–C coupling reactions.
Appl Organometal Chem. 2019;e5205. https://doi.
org/10.1002/aoc.5205
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
article.