Palladium nanoparticles-decorated triethanolammonium chloride ionic liquid-modified TiO2 nanoparticles (TiO2/IL-Pd): A highly active and recoverable catalyst for Suzuki–Miyaura cross-coupling reaction in aqueous medium

Article abstract of DOI:10.1002/aoc.4909

In this work, an easily obtained procedure was successfully implemented to prepare novel palladium nanoparticles decorated on triethanolammonium chloride ionic liquid-functionalized TiO₂ nanoparticles [TiO₂/IL-Pd]. Different methods were carried out for characterizations of the synthesized nanocatalyst (HR-TEM, XPS, XRD, FE-SEM, EDX, FT-IR and ICP). TiO₂/IL-Pd indicated good catalytic activity for the Suzuki–Miyaura cross-coupling reaction of arylboronic acid with different aryl halides in aqueous media at ambient temperature. The recycled catalyst was investigated with ICP to amount of Pd leaching after 6 times that had diminished slightly, Thus, was confirmed that the nanocatalyst has a good sustainability for C–C Suzuki–Miyaura coupling reaction. The catalyst can be conveniently separated by filtration of the reaction mixture and reused for 6 times without significant loss of its activity. It supplies an environmentally benign alternative path to the existing protocols for the Suzuki–Miyaura reaction.

Full text of DOI:10.1002/aoc.4909

Received: 27 January 2019
DOI: 10.1002/aoc.4909
Revised: 19 February 2019
Accepted: 21 February 2019
F U L L P A P E R
Palladium nanoparticles‐decorated triethanolammonium
chloride ionic liquid‐modified TiO2 nanoparticles (TiO2/
IL‐Pd): A highly active and recoverable catalyst for Suzuki–
Miyaura cross‐coupling reaction in aqueous medium
Hojat Veisi
| Sheida Ahany Kamangar | Pourya Mohammadi
| Saba Hemmati
Department of Chemistry, Payame Noor
University, Tehran, Iran
In this work, an easily obtained procedure was successfully implemented to
prepare novel palladium nanoparticles decorated on triethanolammonium
chloride ionic liquid‐functionalized TiO2 nanoparticles [TiO2/IL‐Pd]. Different
methods were carried out for characterizations of the synthesized nanocatalyst
(HR‐TEM, XPS, XRD, FE‐SEM, EDX, FT‐IR and ICP). TiO2/IL‐Pd indicated
good catalytic activity for the Suzuki–Miyaura cross‐coupling reaction of
arylboronic acid with different aryl halides in aqueous media at ambient tem-
perature. The recycled catalyst was investigated with ICP to amount of Pd
leaching after 6 times that had diminished slightly, Thus, was confirmed that
the nanocatalyst has a good sustainability for C–C Suzuki–Miyaura coupling
reaction. The catalyst can be conveniently separated by filtration of the reac-
tion mixture and reused for 6 times without significant loss of its activity. It
supplies an environmentally benign alternative path to the existing protocols
for the Suzuki–Miyaura reaction.
Correspondence
Hojat Veisi, Department of Chemistry,
Payame Noor University, Tehran, Iran.
Email: hojatveisi@yahoo.com
KEYWORDS
ionic liquid, palladium, Suzukie, TiO2 nanoparticles
1 | INTRODUCTION
dangerous, expensive, poisonous, inflammable, and
ligands includes hindered phosphines and carbenes in
In organic synthesis, transition metal catalyzed organic
conversions are the most significant methods.[1,2] Among
these metals, Palladium as an important catalyst for cata-
lyzing of Suzuki‐Miyaura cross‐coupling reaction was
applied. Nowadays, this reaction is an effective approach
to the formation of the C‐C bonds in organic synthesis.[3–
hazardous organic solvents such as DMF, dioxane, and
toluene. In these conditions, it happened usually with
long times, at high temperatures, and low efficiency.
These C‐C bond‐coupling reactions were often performed
using the homogeneous palladium complexes, which gen-
erate problems in the product purification and the cata-
12]
Pd has found extensive applications in fine chemical
lyst
recovering.
These
difficulties
are
the
industries, scholarly laboratories, produce of the biologi-
cally active pharmaceuticals and various natural material
syntheses.[13,14] This cross‐coupling reaction was done
commonly with valuable palladium in the presence of
environmentally and economically concern in the synthe-
sis of the industrial scale.[15–17] Since to the active sites of
the homogeneous catalysts were distributed in the reac-
tion media, these catalysts are usually more active than
Appl Organometal Chem. 2019;e4909.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4909

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VEISI ET AL.
heterogeneous catalysts, but there are some difficulties
like separating and reusing.[18–21] Hence, it is important
that the stable heterogeneous catalysts should be
designed with simple isolation from the reaction mixtures
and high specific surface area of the active phase. To
eliminate these difficulties, metal nanoparticles are dis-
persed on appropriate solid supports with high surface
area. Owing to several advantages of heterogeneous cata-
lysts including simple isolation and recovering, Pd NPs
were immobilized on numerous supports like Fe3O4
NPs,[22] hydroxyapatite,[23] carbon,[24] metal–organic
frameworks,[25] silica,[26] zeolites,[27] organic polymers,[28]
and other synthetic adsorbents such as ion‐exchange
resins.[29] However, there is still the requirement to ame-
liorate the catalytic activities and reaction conditions like
a low reaction temperature and the application of water
as a solvent.[3,16,30–38] The most of the new heterogeneous
catalysts, among the inorganic oxides, are silica‐based
supports. This is principle because of several beneficial
features of the silica, including well mechanical, thermal
and chemical stability, suitable availability and porosity,
and the reality that organic compounds can be covalently
connected to the surface to create catalytic centers. The
selection of support is crucial to form an efficient immo-
bilization approach because the support affects the cata-
lytic performance of immobilized catalysts.[39] SiO2–
TiO2 is one of the inorganic supports formed of double
oxides, exhibited well mechanical and thermal stabil-
ity.[40] These synergic properties appear due to the combi-
nation of properties of the silica and titania phase.[41]
Lately, noteworthy studies have been performed on utiliz-
ing binary silica‐titania as solid support for the heteroge-
neous catalytic reactions.[42] It has been utilized in
various catalytic reactions like selective oxidation and
hydrogenation.[43]
SCHEME 2 TiO2/IL‐Pd nanocatalyst for Suzuki‐Miyaura cross‐
coupling reaction
great activity and stability of the nanocatalyst for the inves-
tigated reaction. By filtration, the nanocatalyst in the reac-
tion mixture easily isolated from the reaction medium and
used six times without significant loss of its activity.
2 | EXPERIMENTAL
2.1 | Synthesis of the TiO2/IL
At first, 1.0 g of TiO2 nanoparticles were dispersed in
100 ml of toluene for 30 min. In the next step, 1 ml of
3‐chloropropyltrimethoxysilane (CPTS, 5 mmol) was
added to the above mixture and it was stirred strongly
at 40 °C for 18 hr. The precipitate (TiO2/Si‐Pr‐Cl) was
filtrated, washed with ethanol/toluene and dried in vac-
uum at 60 °C. The obtained TiO2/Si‐Pr‐Cl was dispersed
in 50 ml ethanol by an ultrasonic bath for 20 min. Then,
1 mL triethanolamine was added to the reaction solution,
stirred, and heated at 80 ° C for 24 hr.[44] Following the
end of the reaction, it was cooled down to ambient
temperature. The obtained precipitate (TiO2/IL) was
filtrated, rinsed with EtOH five times and dried under
vacuum at 60 °C.
Here,
a new nanocatalyst, i.e. Pd nanoparticle
immobilized on the TiO2/IL surface, was synthesized as a
heterogeneous nanocatalyst (Scheme 1). As shown in
Scheme 2, this nanocatalyst was investigated in the
Suzuki‐Miyaura coupling reaction. The nanocatalyst was
synthesized via supporting Pd nanoparticles on TiO2NPs
modified triethanolamine and functionalized with
chloropropyl groups. The triethanolamine groups on the
modified TiO2 NPs surface treat as powerful and effective
support for the palladium nanoparticles, which supply
2.2 | Preparation of TiO2/IL‐Pd(0)
For a typical run, 300 mg of TiO2/IL was dispersed in
30 ml CH3CN for 20 min. Then, 30 mg PdCl2 in 40 ml
CH3CN was added dropwise to the above mixture and
the reaction solution was stirred strongly at ambient tem-
perature for 12 hr. After that, TiO2/IL‐Pd (II) was filtrated
and washed with ethanol/water to eliminate the unbound
substrates. TiO2/IL‐Pd (II) was reduced to TiO2/IL‐Pd(0)
using NaBH4. Finally, the product shown in Scheme 1,
was rinsed with deionized water three times and dried
at ambient temperature. For determination of the amount
of Pd loaded on the catalyst was used ICP‐AAS analysis,
and was estimated to be 0.9 mmol/g.
SCHEME 1 Preparation of the TiO2/IL‐Pd nanocatalyst

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2.3 | Typical procedure for Suzuki
reaction
The catalytic activity of the novel TiO2/IL‐Pd
nanocatalyst was studied for Suzuki‐Miyaura cross‐
coupling reaction. The 0.2 mol% TiO2/IL‐Pd nanocatalyst,
the 1.0 mmol aryl halides, 1.1 mmol phenylboronic acid,
2.0 mmol K2CO3, and 3 ml EtOH/H2O (1:1) were Trans-
ferred to a reaction flask and stirred at ambient tempera-
ture (Scheme 1). The progress of the reaction completion
was followed with TLC (acetone/n‐hexane; 1:4). After the
end of the reaction, 5 ml ethanol was added and then
heated at 70 °C as solvation of product. Finally, the
nanocatalyst was filtered, washed with ethanol. Then as
purification of the product was used column
chromatography.
FIGURE 2 EDX data for TiO2/IL‐Pd.
Additionally, SEM along with elemental mapping was
used for obtaining several qualitative information on dis-
tribution of numerous elements of the catalyst matrix.
Figure 3 shows the representative SEM images and ele-
mental maps corresponding to the prepared nanocatalyst.
The maps indicate that Pd NPs (according to XPS out-
come) are completely dispersed in the composite matrix.
Also, selected‐area elemental analysis approves homoge-
nous presence of Ti, Si, C, N and Pd throughout the
nanocomposites, and therefore, the samples are prepared
regular and uniform.
The high‐resolution TEM (HR‐TEM) image of TiO2/
IL‐Pd nanocatalyst is shown in Figure 4a. As observed
in the image, the morphology of Pd nanoparticles was
spherical and completely dispersed and deposited on to
the surface of TiO2 nanoparticles, the particle size of Pd
nanoparticle was found about 3 nm. Also the TEM
images of the fresh and recovered TiO2/IL‐Pd
Nanocatalyst after 3 runs were given in Figure 4b and
4c, respectively. No clear agglomeration in the particles
was observed, and the morphology and particle size have
not changed much in the recovered catalyst after 3 runs.
These results showed that almost no change in the
2.4 | Characterization
The size and morphology of the TiO2 and TiO2/IL‐Pd
synthesized samples were investigated using scanning
electron microscopy (SEM) that were shown in Figure 1.
The shape of particles is almost spherical and their aver-
age size is of about 30 nm (Figure 1a). When the surface
of the TiO2 particles was coated with IL and Pd, their
average size was of about 33 nm (Figure 1b), results were
indicated which modification of the surface of particles
has not led to their aggregating.
To additional verify, the Pd‐coated TiO2 nanoparticles
were synthesized favorably, the nanocomposite was
investigated by EDX analysis and mapping images. The
signals in Figure 2 showed elements of Ti, Si, C, N, O
and Pd into the nanocatalyst perpetrated. These analyzes
displayed that the IL and Pd NPs were suitably dispersed
on the TiO2 nanoparticle surface.
FIGURE 1 FE‐SEM image of TiO2 (a), and TiO2/IL‐Pd (b)

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VEISI ET AL.
FIGURE 3 Elemental mapping of TiO2/IL‐Pd shows the presence of Ti, Si, C, N and Pd atoms in the nanocomposite
structure of the catalyst occurs during the reaction pro-
cess was stable.
as 60.9 m2 g−1 and 53.6 m2 g−1 for TiO2 and TiO2/IL‐Pd,
respectively (Figure 6). This decrease could be due to the
surface modification of TiO2 with the IL‐Pd. The N2
adsorption–desorption isotherms diagrams for TiO2 and
TiO2‐IL‐Pd nanoparticles were shown in Figure 6. Based
on the IUPAC category, TiO2 shows a type II isotherm,
which is for non‐porous materials and the TiO2/IL‐Pd
nanoparticles displays a type V isotherm, which is related
to mesoporous materials. This reducing in porosity was
possibly attributed to the agglomeration of TiO2/IL‐Pd
nanoparticles and increase of the particle size, which
could be due to modification of the surface.
The catalyst was characterized by Fourier transform
infrared (FT‐IR) spectroscopy (Figure 7). The absorption
peaks appeared at about 650 and 1100 cm−1 are related to
Ti‐O and Si‐O stretching vibrations of the TiO2 and coated
SiO2, respectively (Figure 7a, b). The peaks appeared at
2950 and 2930 cm−1 are related to C─H stretching vibra-
tions which proves the presence of alkylsilane groups on
TiO2‐SiO2 and formation of TiO2‐SiO2‐Cl nanostructure
(Figure 7c, d). Additionally, the peak found at 1439 cm‐1,
in curve b, can be related to the appearance of the quater-
nary amine,[44] that confirmed ILs have been favorably
supported on the TiO2 NPs surface. The IR spectra of
TiO2/IL‐Pd NPs showed a small shift in vibration band
located at 3410 cm−1 which is relevant to O‐H groups of the
catalyst. This small shift of O‐H group band to lower
The XPS technique can determine the chemical com-
position and oxidation chemical valence states of compo-
nents existence on the surface of the nanocatalyst
(Figures 5). The XPS survey spectrum of the TiO2/IL‐Pd
composite informed of Palladium (Pd), Titanium (Ti),
Oxygen (O), Nitrogen (N), carbon (C) and Silicon (Si) ele-
ments. The observed peaks in binding energy of 335.7 and
340.9 eV are attributed to metal Pd(0) which related to
3d5/2 and 3d3/2 of Pd. This result demonstrated that Pd
is only existent in its reduced form, which proved the
favored reduction of Pd (II) to Pd(0) in the method. In
addition, the binding energies of XPS spectra were
observed at about 464.7 and 459.0 eV which matched well
with the Ti4+ 2p1/2 and 2p3/2 states, respectively and bind-
ing energy assigned at 530.01 eV is related to O 1 s. This
result indicated that Ti metal was informed to TiO2. This
result indicated that Pd was just present in its reduced
form, which confirmed the successful reduction of Pd
(II) to Pd(0) in the process. Figure 5 indicated 2p Ti of
three different nanocatalyst in the spectra, two peaks
placed at 464.7 and 459.0 eV were identified, which
agreed quite with Ti4+ 2p1/2 and 2p3/2 states, respectively.
The BET surface area was measured from the N2
adsorption data using the Brunauer–Emmett–Teller
equation. BET surface area measurements were obtained

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FIGURE 4 HR‐TEM of TiO2/IL‐Pd nanocatalyst (a), TEM of TiO2/IL‐Pd nanocatalyst fresh prepared (b), and recovered catalyst after six
runs (c)
wavelengths is because of the interaction between Pd and
O‐H groups of the catalyst (Figure 7e).
Different solvents were tested and the reactions were car-
ried out using H2O/ethanol (1:1) as the best solvent. Also,
the type of the base was assessed and found that K2CO3
base was the most effective one. The reaction in the
absence of catalyst was tested and as expected, no product
was obtained. (Table 1, entry 10) and it was found that
the existence of catalyst was necessary for the reaction
to occur. The coupling reactions were done at different
amounts of catalyst from 0.1 to 0.2 mol %. Table 1 shows
that the decreasing amount of catalyst leads to a decrease
in the yield of the reaction. Also, it was observed that
increasing the amount of catalyst (more than 0.2% mol)
did not improve the yield. Therefore, the amount of
0.2% mol of the Pd catalyst is sufficient to do the reaction
in the desired conditions. (Table 1, entry 7). The Suzuki
cross‐coupling reactions of differently substituent aryl
halides (electron donating and electron withdrawing)
with phenylboronic acids were examined using
0.2 mol% of the nanocatalyst under optimized reaction
2.5 | Evaluation of the catalytic activity of
TiO2/IL‐Pd nanocatalyst via
Suzuki‐Miyaura cross‐coupling reaction
As shown in Scheme 2, the evaluation of catalytic activity
of the TiO2/IL‐Pd nanocatalyst was done through Suzuki
coupling reaction. In order to create a more efficient cat-
alytic process, parameters such as the effect of the solvent
(protic and aprotic), type of base (organic and inorganic)
and the quantity of nanocatalyst on Suzuki coupling were
examined. This reaction was done at ambient tempera-
ture applying the model reaction i.e. the reaction of
phenylboronic acid and bromobenzene (Table 1). The
type of solvent has been proven to play an essential role
in the rate and yield of Suzuki cross‐coupling reactions.

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FIGURE 5 XPS spectrum related to the elemental survey scan of TiO2/IL‐Pd and in the Ti 2p, Pd 3d regions (inset)
with phenylboronic acid. (Table 2, entries 1–17). The aryl
halides that have an electron‐donating substitute in the
para‐position showed less activity than those have an
electron‐withdrawing substitute. Also for evaluation of
catalytic activity of the prepared nanocatalyst, a range of
deactivated and activated aryl chlorides was considered.
Based on the results, moderate yields were obtained at
the mentioned reaction condition (Table 2, entries 3, 6
and 9), because the bond energy of C–Cl in aryl chlorides
are higher than C‐Br in aryl bromide, yield of coupling of
aryl chloride was less than that of aryl bromide. (Table 2,
entries 3, 6 and 9). As expected according to of the results
obtained, aryl boronic acids with electron‐donating sub-
stitute produced good yields. (Table 2, entries 15–17).
Also, the Suzuki Miyaura cross coupling reactions of
heteroaryl halides derivatives such as 2‐bromothiophene
and 2‐iodothiophene with phenylboronic acid give the
corresponding coupled products in 86 and 90% yields,
respectively (Table 2, entries 18 and 19).
FIGURE 6 The N2 adsorption–desorption isotherms of TiO2 (a),
and TiO2/IL‐Pd (b)
conditions (Table 2). The H2O/EtOH was used as a reac-
tion solvent for optimization of coupling reaction condi-
tion. This solvent has advantages such as eco‐friendly
and economic. Under the optimized conditions, types var-
ious aryl halides such as phenyl iodides, phenyl bro-
mides, and phenyl chlorides were used for coupling
A comparison between the efficacy of our reported cat-
alyst and some other palladium‐based catalysts for aryl

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TABLE 2 Suzuki–Miyaura coupling reaction of different aryl
halides with phenylboronic acida
Time
Entry RC6H4X R2C6H4B(OH)2
X
(min.) Yield (%)b
1
H
H
I
30
94
92
70
92
89
65
94
92
78
94
90
94
90
82
93
88
86
90
86
2
H
H
Br 70
3
H
H
Cl 24 (h)
4
4‐CH3
4‐CH3
4‐CH3
4‐COCH3
4‐COCH3
4‐NO2
4‐OCH3
4‐OCH3
4‐NO2
4‐NO2
4‐CH3
4‐NO2
H
H
I
30
5
H
Br 60
6
H
Cl 24 (h)
7
H
I
45
8
H
Br 60
9
H
Cl 24 (h)
10
11
12
13
14
15
16
17
18
19
H
I
30
Br 60
30
Br 90
H
H
I
H
H
I
I
30
90
FIGURE 7 FT‐IR spectroscopy of TiO2 (a), TiO2‐SiO2‐Cl (b),
TiO2/IL (c), TiO2/IL‐Pd2+(d) and TiO2/IL‐Pd(0) (e)
4‐OCH3
3‐OCH3
2‐OCH3
H
Br 90
Br 90
H
2‐Thienyl
2‐Thienyl
I
75
TABLE 1 Optimization of the reaction conditions in Suzuki‐
Miyaura coupling reaction of bromobenzene with phenylboronic
acida
H
Br 100
aReactions were performed under aerobic conditions (air) in 5 ml of H2O/
EtOH (1:1), 1.0 mmol arylhalide, 1.1 mmol phenylboronic acid and 2 mmol
K2CO3 in presence of the catalyst catalyst (0.2 mol% of Pd) at 25 °C.
Time
(min.) Yield (%)b
bIsolated yield.
Entry Pd (mol%) Solvent
Base
1
0.20
0.20
0.20
0.20
0.20
0.20
0.20
0.15
0.10
0.0
Toluene
DMF
K2CO3 70
K2CO3 80
K2CO3 70
K2CO3 100
70
85
65
50
92
70
92
80
72
0
2
3
EtOH
H2O
2.6 | Recyclability of supported catalyst
4
5
EtOH/H2O Et3N
90
Moreover, the leaching metal is a significant parameter
for stability and industrial applicability of catalysts, to this
end, the ICP technique was used. The amount of palla-
dium loaded on the catalyst after 6 runs were found as
9.3 wt%, which was a slight change (9.6 to 9.3 wt %).
The TiO2/IL‐Pd nanocatalyst showed good catalytic per-
formance after 6 times of use, so that it retained more
than 90% of their activity even at the 6th run of catalytic
reuse. And also, elemental analysis (CHNS) carried out
for detection of IL (triethanolammonium chloride) on
surface of TiO2. The obtained results showed that total
C content 5.2 (w%), N content 0.6 (w%) for TiO2/IL‐
Pd(0) at first run. This results shown that successful
functionalization of TiO2 with IL. After 6 runs were
obtained C content 4.9 (w%), N content 0.5 (w%) for
TiO2/IL‐Pd(0), which indicates that the IL is well
established on the surface of TiO2.Thus, the catalyst
6
EtOH/H2O NaOAc 70
EtOH/H2O K2CO3 70
EtOH/H2O K2CO3 70
EtOH/H2O K2CO3 70
EtOH/H2O K2CO3 60
EtOH/H2O No base 90
7
8
9
10
11
0.20
Trace
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid
(1.1 mmol), catalyst, base (2 mmol), solvent (3 ml) and 25 °C.
bIsolated yield.
halide coupling is presented in Table 3. It is apparent that
our catalyst works well in reasonably short reaction times
and under mild reaction conditions.

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VEISI ET AL.
TABLE 3 Comparison of catalytic activity of TiO2/IL‐Pd with other Pd catalysts
Yield in lit.
Entry
Catalyst
Reaction condition
(This work)a
Ref.
[45]
1
2
Pd/C
Pd/C
EtOH/H2O (1:1), 75 °C, Flow (1 ml/min) 4‐OMe‐Ph‐I, Ph‐B (OH)2
81 (94)
91 (90)
[46]
EtOH/H2O (1:1), r.t, Ar, 24 hr
4‐OMe‐Ph‐Br, Ph‐B (OH)2
[47]
[48]
3
4
Pd@GOF
EtOH, 40 °C, 24 hr, Ph‐Br, Ph‐B (OH)2
97 (92)
60 (90)
Fe3O4@SiO2‐4‐AMTTPd (II)
EtOH/H2O (1:1), r.t, 3.5 hr
4‐OMe‐Ph‐Br, Ph‐B (OH)2
[49]
[50]
5
6
Fe3O4@C–Pd@mCeO2
Pd@Cu‐BDC/Py‐SI
EtOH/H2O, 80 °C, 3 hr,
Ph‐Br, Ph‐B (OH)2
95 (92)
95 (90)
DMF/H2O (2:1), 80 °C, 1 hr
4‐OMe‐Ph‐Br, Ph‐B (OH)2
aReaction conditions: bromobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), catalyst (0.2 mol% of Pd), K2CO3 (2 mmol), EtOH:H2O (1:1) (3 ml) and 25 °C.
ORCID
Hojat Veisi https://orcid.org/0000-0001-9060-8973
Pourya Mohammadi
https://orcid.org/0000-0002-9715-
1811
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ACKNOWLEDGEMENTS
We are thankful to Payame Noor University (PNU) for
partial support of this work.
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