Synthesis and catalytic application of palladium nanoparticles supported on kaolinite-based nanohybrid materials

Article abstract of DOI:10.1039/c6dt00982d

Palladium nanoparticles (PdNPs) were deposited on the surface of the modified clay mineral, kaolinite. To improve compatibility, abundance and control of the size of the nanoparticles, kaolinite was modified by the grafting of an amino alcohol (triethanol

Full text of DOI:10.1039/c6dt00982d

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DOI: 10.1039/C6DT00982D
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Synthesis and catalytic application of palladium nanoparticles
supported on kaolinite-based nanohybrid materials†
Gaelle Ngnie,*a Gustave. K. Dedzo,*a,b and Christian Detelliera
Received 00th January 20xx,
Accepted 00th January 20xx
Palladium nanoparticles (PdNPs) were deposited on the surfaces of the modified clay mineral, kaolinite. To improve
compatibility, abundance and control of the size of the nanoparticles, kaolinite was modified by the grafting of an amino
alcohol (triethanolamine (TEA)) and an ionic liquid (1-(2-hydroxyethyl)-3-methylimidazolium (ImIL)). Characterization
techniques (XRD, TGA, solid state 13C NMR and FTIR spectroscopy) confirmed the effective grafting of these compounds on
the internal surfaces of kaolinite. After the syntesis of PdNPs onto clay particles, TEM allowed the visualization of
abundant PdNPs with sizes ranging from 4 to 6 nm, uniformly distributed onto the platelets of modified kaolinite.
Unmodified clay showed low abundance and random distribution of the nanoparticles. The catalysts obtained were
effective for catalytic reduction of 4-nitrophenol (4-NP), the material with TEA being the most effective. These materials
have achieved excellent performance during the Heck and particularly the Suzuki-Miyaura coupling reactions, with
reaction yields up to 100%. These catalysts showed a very slight loss in activity for three consecutive catalytic cycles (less
than 10% decrease of the activity compared to the first cycle). This was an evidence that the prior grafting modification of
kaolinite helps in significantly improving the quality of the synthesized NPs and also promotes their strong attachment
onto the clay mineral surface.
DOI: 10.1039/x0xx00000x
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constraints such as uneasy filtration. It is therefore common to
incorporate these NPs in solid matrices such as activated
carbons, silicates or aluminosilicate materials such as clay
Introduction
Many chemical reactions require extreme operating conditions
such as high temperatures and pressures, particular
atmosphere (inert atmosphere) and the use of toxic and
difficult to handle solvents in order to be carried out in good
yields. The use of catalysts greatly improve the yield obtained
upon these harsh reaction conditions and render economically
feasible many chemical reactions, while limiting the harmful
effects of pollution by chemicals. Metal nanoparticles (NPs)
are an important class of effective catalysts for a broad range
of chemical reactions. Their performances are mainly related
to the nanometric sizes of particles that display new properties
and very high reactivity compared to bulkier metals. The
production of nanoscale catalysts of uniform sizes has become
an important research area in material chemistry.1-5
NPs synthesis requires the use of polymers or other
chemical compounds to control their size and limit their
aggregation. This polymer layer reduces the reactivity of NPs
by limiting direct contact between the reagents and the
surface of the metallic nanoparticles. In addition, the recycling
of NPs after a reaction cycle can be difficult because of
minerals.6 Clay minerals are increasingly popular as catalyst
supports because of their availability, stability and good
compatibility with metal nanoparticles.7-10 Despite its
abundance, kaolinite is poorly used in this important research
area because of its lack of swelling ability, which limits the
amount of incorporated NPs. Kaolinite is a dioctahedral
lamellar aluminosilicate with molecular formula Al2Si2O5(OH)4.
This 1:1 clay mineral is very stable, with a small surface area
and almost no ionic exchange capacity.11 Recent works have
shown that it is possible to modify permanently this clay
mineral12-17 with simple procedures. Ionic liquid, amine and
thiol functionalized kaolinites were found very efficient for the
sequestration of anionic compounds and heavy metals.15,18-22
These properties could be used advantageously for the
deposition of nanoparticles on the surface of this clay mineral.
Palladium nanoparticles are well known for their
effectiveness in catalyzing reduction reactions and
organometallic cross-coupling reactions such as the Heck and
Suzuki-Miyaura reactions.23-26 The Heck reaction is the
arylation of olefins by aryl halides in the presence of a catalytic
amount of palladium (0) catalyst and a base.27,28 This reaction
is largely employed for the synthesis of substituted olefins,
dienes and a wide range of unsaturated compounds.29-31 The
Suzuki-Miyaura coupling reaction involves the cross-coupling
of organohalides with organoboron reagents in the presence
of a palladium (0) catalyst and a base. This coupling is mostly
a.Center for Catalysis Research and Innovation and Department of Chemistry and
Biomolecular Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
b.Laboratory of Analytical Chemistry, Faculty of Science, University of Yaounde I,
B.P. 812, Yaoundé, Cameroon.
† Electronic Supplementary Information (ESI) available: TGA/DTGA of K-ImIL/Pd
and K-ImIL/Pd/Cl catalytic reduction of 4-NP on K-ImIL/Pd and K-ImIL/Pd/Cl. See
DOI: 10.1039/x0xx00000x
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used to form aryl-aryl bonds which are abundant in was added 15 mL of aqueous solution of NaBH4 (1V0iemw AMrti)cleuOnndliener
DOI: 10.1039/C6DT00982D
pharmaceuticals, polymers, agrochemicals and natural vigorous stirring for 15 min. The dark gray solid obtained was
products.32-37
recovered by centrifugation and washed three times with
Recently we have shown that it is possible to modify halloysite water (3 x 40 mL). Materials obtained are denoted K-
nanotubes by the grafting of an ionic liquid which ImIL/PdNPs, K-TEA/PdNPs and K/PdNPs for K-ImIL, K-TEA and
subsequently promotes the synthesis of PdNPs localized Kaolinite precursors respectively.
exclusively inside the lumen of the clay mineral.38 In this work,
Catalytic application
we used the same synthetic approach to decorate kaolinite
platelets with PdNPs. In addition to an ionic liquid, we used a 4-NP reduction. A suspension of either the natural clay mineral
neutral compound, triethanolamine, for the prior modification or the clay supported PdNPs was prepared by dispersing 4 mg
of kaolinite. The grafted and PdNPs decorated material were of the solid in 1 mL of deionized water. The mixture was stirred
characterized by XRD, 13C NMR, TGA, ICP-AES and FTIR. The in an ultrasonic bath for 2 min to obtain well dispersed
activity of the catalysts obtained was initially checked for the particles in water. For the catalytic experiments, 2.5 mL of an
reduction of 4-NP in the presence of NaBH4 as a standard test aqueous solution of 4-NP 0.1 mM was introduced in a
for catalytic efficiencies. The materials were then tested for spectrophotometer cuvette. 20 µL of an aqueous solution of
the Heck and Suzuki-Miyaura coupling reactions. The NaBH4 1 M was added in the solution. The mixture took an
recyclability of the catalyts was also investigated.
intense yellow color characteristic of 4-nitrophenolates ions
generated by acid-base reaction of 4-NP with the alkaline
solution. 50 µL of the suspension of the catalyst was then
added in the solution and the cuvette was immediately
introduced in the spectrophotometer to monitor the reaction.
Suzuki-Miyaura coupling reaction. Iodobenzene (0.05 mL, 0.45
mmol) and phenylboronic acid (0.08g, 0.67 mmol) were added
in a round bottom flask containing a precise amount of the
catalyst and K2CO3 (0.22 g, 1.57 mmol) in 4 mL of water. The
mixture was stirred at 110 oC under reflux for 6 h. The reaction
mixture was then allowed to cool down and diluted with
diethylether (5 mL). The catalyst was recovered by
centrifugation and washed several times with methanol. The
organic fraction was washed with water (10 mL) and dried over
anhydrous MgSO4. Evaporation of residual solvent gave pure
Experimental
Chemicals
2-chloroethanol (99%), triethanolamine (TEA) (98%), K2PdCl4
(98%), NaBH4 (≥ 98.5 %), iodobenzene (98%), phenylboronic
acid (95%), potassium carbonate (≥ 99 %), styrene (≥ 99%),
tetrabutylammonium iodide (TEAI) (98 %), 4-nitrophenol
(99.8%) were purchased from Sigma-Aldrich. All other
chemicals
(dimethyl
sulfoxide
(DMSO),
anhydrous
diethylether, methanol and methylene dichloride) were of
analytical grade. The ionic liquid 1-(2-Hydroxyethyl)-3-
methylimidazolium (ImIL) was synthetized according to a
procedure previously reported.14, 15
1
biphenyl as a white solid, H NMR (300 MHz, CDCl3) δ 7.65 (d,
4H, J = 9.69 Hz), 7.49 (t, 4H, J = 7.65 Hz), 7.41-7.36 (m, 2H); 13
NMR (75 MHz, CDCl3) δ 141.2, 128.7, 127.2, 127.1 ppm.
C
Synthesis of organo-functionalized clay materials
Synthesis of grafted kaolinite. Kaolinite (KGa-1b, Georgia) was
obtained from the Source Clays Repository of the Clay
Minerals Society (Purdue University, West Lafayette, Indiana,
USA). The DMSO pre-intercalate (K-DMSO) was prepared
according to a procedure previously reported.12,14 The grafting
of TEA and ImIL was adapted from the procedures previously
published.15,39 In practice, 0.5 g of K-DMSO was dispersed in 2
g of ImIL (for K-ImIL) or TEA (for K-TEA) and stirred vigorously
Heck coupling reaction. Iodobenzene (0.05 mL, 0.45 mmol)
and styrene (0.07 mL, 0.67 mmol) were added in a round
bottom flask containing a precise amount of the catalyst and
the phase transfer catalyst tetrabutylammonium iodide: TEAI
(0.11 g, 0.45 mmol) in 5 mL of aqueous 2.5 M KOH. The
mixture was stirred at 100 oC under reflux for 29 h. The
reaction mixture was then allowed to cool down and diluted
with diethylether (5 mL). The catalyst was recovered by
centrifugation. The organic fraction was washed with water
(10 mL), dried over anhydrous MgSO4 and the residual solvent
o
under nitrogen atmosphere at 180 C for 4 h. The solid was
then washed 4 times with isopropanol (4 x 15 mL) and dried in
o
the oven at 80 C for 2 h. The solids were redispersed in 100
1
evaporated. Pure stilbene was recovered as a white crystal, H
mL of distilled water and stirred for three days. The modified
clays were recovered by centrifugation, washed with distilled
water (4 x 15 mL) and dried in the oven at 80 oC overnight.
Synthesis of kaolinite supported palladium nanoparticles. The
synthesis of kaolinite supported PdNPs was adapted from the
procedure previously published.38 0.2 g of clay (modified or
unmodified) was dispersed in a round bottom flask containing
100 mL of 0.5 mM K2PdCl4 aqueous solution. The mixture was
homogenized by magnetic stirring at room temperature for 30
min, refluxed for 1 h and finally stirred at room temperature
for 2 h. The solid was recovered by centrifugation at 4500 rpm
and redispersed in 50 mL of deionized water. To this solution
NMR (300 MHz, CDCl3) δ 7.53 (d, 4H, J = 7.06 Hz), 7.35-7.40 (m,
4H), 7.25-7.29 (m, 2H), 7.13 (s, 2H); 13C NMR (75 MHz, CDCl3) δ
137.3, 128.7, 127.60, 126.5 ppm.
Recyclability of the catalysts. Suzuki-Miyaura coupling: for
Run 1, 10 mg of the catalyst; 0.9 mmol of iodobenzene; 1.34
mmol of phenylboronic acid; 3.14 mmol of K2CO3; 8 mL of H2O;
110 °C; 6 h. The recovered catalyst was washed with water and
o
ether and dried in an oven at 80 C overnight. The recovered
material was then used for the next run without any further
treatment. The procedure was repeated for Run 2 and Run 3.
Heck coupling: for Run 1, 20 mg of the catalyst; 0.9 mmol of
iodobenzene; 1.34 mmol of styrene; 0.9 mmol of TEAI; 10 mL
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of aqueous 2.5 M KOH; 100 °C; 29 h. The catalyst was then In order to improve the compatibility and the uniform
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recovered and the next runs achieved by following the distribution of the nanoparticles on kaoDlinOiIt:e1,0.t1h03e9c/Cla6yDTm00in9e8r2aDl
procedure described for the Suzuki-Miyaura coupling.
was first modified by grafting of TEA or ImIL. The grafting in
the interlayer space would also substantially increase the
number of active sites on the kaolinite clay mineral. The XRD
Characterization
Powder XRD patterns were recorded using a Rigaku Ultima IV patterns of the materials obtained upon grafting and after
diffractometer operating with Cu-Kα radiation (λ = 1.54056 Å) several washings in water are shown in Fig. 1.
1
equipped with a generator operating at 45 kV and 40 mA. H
K-ImIL and K-TEA displayed a d001 of 13.1 Å and 11.2 Å
and 13C NMR high resolution spectra in solution were recorded respectively. The d001 of K-ImIL is consistent with that obtained
using a Bruker AVANCE 300 MHz spectrometer in deuterated in previous work.15 In the case of K-TEA the value is greater
chloroform. Solid-state 13C NMR CP/MAS spectra were than what obtained by other authors using ordered kaolinite
collected on a Bruker AVANCE 200 MHz spectrometer, (10.7 Å or 10.8 Å).22,39,40 The d060 at 62.4o (2θ) confirmed that
operating at a spinning rate of 4.5 kHz. Scanning electron the a,b plane was not affected by the grafting reaction.
microscope (SEM) images were taken on a JEOL JSM-7500F Structurally, K-ImIL can be considered as a nanohybrid
FESEM in low secondary electron imaging (LEI) mode with a 1.5 kaolinite with cationic pillars grafted onto aluminol group
kV acceleration voltage. Energy Dispersive X-ray Spectroscopy through Al-O-C bonds. Exchangeable chloride anions are
(EDS) device coupled to SEM was used to record the EDS associated to the immobilized cations in order to provide
spectra of defined area of the samples. Transmission electron electrical neutrality to the entire structure. In the case of K-
microscopy (TEM) microscopic images were obtained on a TEA, Al-O-C bonds are also formed and the pillars are neutral.
JEOL JEM-2100F electron microscope. The powders were Because of the basicity of the nitrogen atom, the charge of the
dispersed in ethanol in an ultrasound bath for about 2 min and grafted molecule can vary depending on the pH of the
1 drop of the suspension deposited on copper grid and air medium.
dried before imaging. Thermal gravimetric analyses (TGA) were
Subsequently to the deposition of PdNPs (confirmed by the
recorded using a TA instrument Q5000 under nitrogen flow (25 dark gray color of the materials after the reaction with NaBH4),
mL min-1) at a heating rate of 10 ºC/min. TGA curves used for the d001 and other characteristic reflection planes remained
the determination of chemical formula of modified clays were unchanged. Two conclusions can be drawn from this
recorded under house air flow (25 mL min-1) at a heating rate observation: (i) PdNPs are not formed in the interlayer despite
of 10 °C/min with additional 5 min isotherm at 950 °C. KBr the presence of organic compounds having good affinity with
pellets were prepared for the IR analysis and the spectrum the anionic palladium precursor. Nanoparticles present in the
recorded on a Thermoscientific Nicolet 6700 FT-IR apparatus. interlayer space should substantially increase the d-value.
Mass percentage of palladium in clay was determined by Contrarily to what one might have expected, the characteristic
inductively coupled plasma atomic emission spectrometry peaks of PdNPs located on the external surfaces of clay
(ICP-AES) using a Varian Vista Pro ICP-Emission Spectrometer. platelets (39.4o, 45.8o and 66.7o (2θ)) are not visible on the
Samples were mineralized in aqua regia (Mineralization in XRD patterns (35o - 70o (2θ) region highlighted in Fig. 1 (B)).
concentrated HF yield similar results) before analysis. Samples This is an indication of either the small size of the particles
were analysed in duplicate.
obtained and/or low abundance of nanoparticles. A similar
feature was obtained recently during the synthesis of PdNPs
onto halloysite38 (ii) PdNPs formation do not modify the
structure of the nano-hybrid material. This suggests that the
presence of nanoparticles brings new properties that will be
added to the existing ones (permanent and high anionic
exchange capacity in the case of K-ImIL and acid base
properties for K-TEA).
Results and discussion
Synthesis and characterization of clays supported PdNPs
(A)
(B)
13.2
PdNPs
The thermal gravimetric analyses (TGA) of the materials are
13.2
K-ImIL
presented in Fig. 2. The well-marked mass loss between 100 oC
*
*
K-ImIL/Pd
1
o
11.2
and 400 C confirms the presence of organic matter in the
nanohybrid materials before and after PdNPs deposition. ImIL
11.2
K-TEA
o
o
K-TEA/Pd
decomposition started at 300 C with a maximum at 360 C.
There is also a significant decrease in the dehydroxylation
temperature of kaolinite attributed to the presence of grafted
compounds in the interlayer space. The partial pyrolysis of the
grafted compounds in the interlayer space ensures good heat
diffusion through the layered structure and thus facilitates the
dehydroxylation. Similar behavior is observed with K-TEA. This
behavior was reported to be a simple method for an indication
of the successful grafting.41
1
7.1
7.1
10
K/Pd
50
K
0
5
10
15
2
20
25
30
0
20
30
40
60
70
2
Fig. 1 XRD Patterns of kaolinite and grafted kaolinite (A), and PdNPs decorated kaolinite
and grafted kaolinite (B). The d001 of unreacted kaolinite is marked by (*) in (A) and
region between 35o and 70o are highlighted for K-ImIL/Pd, K-TEA/Pd and PdNPs in (B).
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DOI: 10.1039/C6DT00982D
100
(a)
(b)
(A)
90
80
70
60
50
3620
3470
3695
47
445
353
K-TEA/PdNPs
K-TEA
(b')
(a')
360
426
0
200
400
T (oC)
600
800
100
90
80
70
60
50
(a)
K-ImIL/PdNPs
(B)
(b)
K-ImIL
K
58
(b')
(a')
370
3750
3500
3250
(cm-1)
3000
2750
432
Fig. 3 FTIR of kaolinite and nanohybrid kaolinite before and after PdNPs deposition.
0
200
400
T (oC)
600
800
The band at 3695 cm-1 is frequently assigned to the stretching
vibration of interlayer AlO-H oriented perpendicular to the c-
direction and that interact mostly with the neighboring O-H of
the octahedral layer. The band at 3620 cm-1 is assigned to the
internal OH pointing in the siloxane macroring.42 Only the two
OH pointing in the interlayer space (bands at 3670 cm-1 and
3653 cm-1) are expected to be strongly affected by the
presence of grafted compounds. The presence of the
stretching vibration bands of the aliphatic C-H of TEA and ImIL
(between 3000 cm-1 and 2700 cm-1) and aromatic C-H of ImIL
(between 3200 cm-1 and 3000 cm-1) are also noticeable. The
intensities of the bands associated to water molecules (broad
band attributed to the stretching vibration at 3400 cm-1 and
the bending vibration band at 1630 cm-1 (Results not shown)
become more intense after incorporation of PdNPs especially
in the case of K-ImIL. These results are in perfect agreement
with the results of TGA that show the high water content of K-
ImIL/PdNPs.
Fig. 2 TGA of (A) ImIL and (B) TEA modified kaolinite. TGA curves of nanohybrid
materials before (a) and after PdNPs deposition (b). DTGA curves of nanohybrid
materials before (a’) and after PdNPs deposition (b’).
After decorating the clay particles with PdNPs, one can
mention two major changes in the case of K-ImIL: (i) a small
o
decrease in the mass loss temperature of ImIL (from 360 C to
o
353 C). This decrease can be attributed to the presence of
PdNPs that increases the thermal conductivity of the material;
(ii) the high amount of water present in K-ImIL/PdNPs (8.2%),
at least two times higher than the mass percentage
determined in K-ImIL (3.75%). The presence of PdNPs greatly
improves the hydrophilic character of K-ImIL. In the case of K-
TEA, the amount of water in the material increases slightly (3%
for K-TEA and 3.5% for K-TEA/PdNPS). This hydrophilic
difference between TEA and ImIL modified materials could
result from the steric hindrance induced by the presence of
TEA in the interlayer space compared to ImIL.
The FTIR spectra of the materials before and after PdNPs
deposition (Fig. 3) are almost identical. There are significant
modifications of the O-H stretching vibrational frequencies of
kaolinite (between 3700 cm-1 and 3600 cm-1)42, confirming the
insertion of organic compounds into the interlayer space. The
band at 3695 cm-1 and 3620 cm-1 remained unchanged as
expected since they do not interact with species present in the
interlayer space.
Even though 13C NMR is not a characterization technique
sufficient enough to confirm the grafting since the chemical
shifts of grafted or intercalated compounds are difficult to
differentiate,40 it is helpful to confirm the structure of the
grafted compound in the material. In the 13C NMR spectra of
Fig. 4, it appears that the characteristic peaks of ImIL were
clearly displayed on the spectrum of K-ImIL (aliphatic carbons
at 34.5 ppm, 47.6 ppm and 60 ppm and imidazolium carbons
at 121.1 ppm and 135.5 ppm).
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After the grafting of TEA or ImIL (result not shown) the
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DOI: 10.1039/C6DT00982D
platelets retain similar morphology, confirming that the
121.1
34.5
grafting has almost no effect on the a,b plane of kaolinite as
shown in XRD results. In addition to visual confirmation related
to the gray color of the material upon the synthesis of PdNPs,
one can clearly identify palladium nanoparticles of spherical
shapes contrasting with the surface of the clay mineral. The
EDS spectra clearly confirmed the presence of palladium
associated to these materials (results not shown). In the case
of unmodified kaolinite, PdNPs with shapes between 6 nm to
10 nm (Inset of figure 5) are rare and randomly distributed on
the clay surfaces. This scarcity is due to the small amount of
palladium precursor adsorbed on the surface of kaolinite
during the synthesis of NPs since kaolinite is characterized by a
poor anion exchange capacity. Identical behavior has been
observed with halloysite using a similar synthetic procedure.38
Interestingly, modified materials (K-ImIL/PdNPs and K-
TEA/PdNPs) show abundant and uniform nanoparticle sizes
60
47.6
135.5
K-ImIL
58.5
60.5
K-TEA
150
100
50
0
 (ppm)
Fig. 4 13C NMR spectra of nanohybrid kaolinites.
This confirms that the structure of the ionic liquid is (Insets of figure 5) ranging between 4 nm and 6 nm. The
maintained after grafting. For K-TEA, there is a very broad peak abundance of NPs can be due to the ability of materials to
made of two fused peaks assigned to the carbons of accumulate the precursors (tetrachloropalladate ions) by an
triethanolamine at 58.5 ppm and 60.5 ppm. Contrarily to the anionic exchange mechanism, thanks to functionalities
results obtained from the literature, the spectrum of K-TEA introduced by the grafting of organic moieties. In the particular
does not show residual DMSO, an indication that the water case of TEA, this property is related to the partial protonation
washing of the material has been particularly effective in this of triethanolamine in the experimental conditions of
case.39
accumulation of PdCl42-. The presence of organic compounds
TEM pictures of kaolinite and modified kaolinite before and also helps to control the nanoparticle sizes and prevent
after PdNPs synthesis were recorded (Fig. 5). As expected for a agglomeration.
crystallized sample as KGa-1b, there are almost hexagonal
platelets with shapes clearly defined.
ICP-AES analysis of samples were performed to determine
the amount of Pd loaded onto clay particles. The results
showed that K/Pd, K-ImIL/Pd and K-TEA/Pd contain by weight
1.47 ± 0.16%, 2.45 ± 0.07% and 2.42 ± 0.01% of palladium
respectively. The values obtained for the grafted clays were
very close to the total amount of palladium precursor used for
the synthesis (2.59%). Almost all the palladium precursor was
retained in the functionalized clay during the synthesis. As
expected, for the unmodified kaolinite, the palladium
percentage is lower, due to its poor anionic exchange capacity.
100
75
50
25
0
2
4
6
8
10
12
12
12
Particle diameter (nm)
K/Pd
K
50 nm
50 nm
20 nm
20 nm
100
80
Catalytic Application of kaolinite Supported PdNPs
60
.
40
20
0
The catalytic efficiencies of the catalysts were first evaluated
for 4-NP reduction and then applied for cross-coupling
reactions.
2
4
6
8
10
Particle diameter (nm)
Catalytic Reduction of 4-NP. Catalytic reduction of 4-NP is
frequently used as a fast and efficient model reaction to
evaluate various metal catalysts.43,44 It is well known that in
presence of PdNPs, borohydride anions can reduce 4-NP to
yield 4-aminophenol (4-AP). The results obtained on kaolinite
and modified kaolinites are presented in Fig. 6. One can easily
notice that without PdNPs, no effect on 4-NP reduction is
observed as the absorbance of the nitrophenolate anions at
400 nm remained unchanged.
K-TEA/Pd
K-TEA/Pd
20 nm
100
80
60
40
20
0
.
2
4
6
8
10
Particle diameter (nm)
K-ImIL/Pd
K-ImIL/Pd
10 nm
Fig. 5 TEM pictures of kaolinite (K) and pdNPs supported kaolinite. Inset: PdNPs
diameter distribution.
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DOI: 10.1039/C6DT00982D
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h
K/Pd
K-TEA/Pd
K-ImIL/Pd
K
K-ImIL
K-TEA
I
(B)
C
T
l a
y
s
u
p
p
,
o r t e d P d N P s
1.5
1.2
0.9
0.6
0.3
0.0
K
O
,
H
0
2
. 5
M
E
A
I
1
0
o C
, 2 9 h
Scheme 1 (A) Suzuki-Miyaura coupling reaction of iodobenzene with phenylboronic
acid and (B) Heck coupling reaction of iodobenzene with styrene catalyzed by kaolinite
supported PdNPs.
K/Pd
K-ImIL/Pd K-TEA/Pd
Catalysts
The reaction yields for the Suzuki-Miyaura and the Heck
coupling reactions are summarized in Table 1. The catalysts
were effective for the Suzuki coupling reaction. K-TEA/Pd has
the higher yield (93%), followed by K/Pd (64%). K-ImIL/Pd with
a yield of only 40% was the less efficient catalyst. This result is
very surprising since this material does have a large amount of
PdNPs as confirmed by the TEM pictures, ICP-AES analysis and
the performances during the catalytic reduction of 4-NP. This
could be due to the presence in the interlayer spaces of
residual borohydride ions acting as the counter anions of the
grafted imidazolium cations. To remove this anion a simple
procedure was used: 0.1 g of K-ImIL/Pd was dispersed in 40 mL
of 0.1 M NaCl aqueous solution and stirred for 1 h to ensure
the complete replacement of borohydrides by chlorides. The
solid was washed with deionized water until the solution was
free of chloride ions (silver nitrate test) and the solid was dried
in the oven. The material thus obtained was denoted K-
ImIL/Pd/Cl. During the washing step, no PdNPs leaching was
observed as confirmed by the ICP-AES analysis that shows
similar amount of Pd (2.56 ± 0.01%) compared to K-ImIL/Pd
(2.45 ± 0.07%). The characterization of this material (TGA and
reduction test of 4-NP) shows that it is almost identical to K-
ImIL/Pd (Fig. S1 and Fig. S2). By following similar experimental
conditions, a yield of 96% was obtained with K-ImIL/Pd/Cl
(Table 1). This clearly confirmed that the presence of
borohydride in the catalyst reduces its activity for Suzuki
coupling. One can also mention that the yields of the reaction
obtained with the four different classes of catalysts under the
same reaction conditions increased with the amount of
catalyst used. These catalysts were significantly less effective
for the Heck reaction. K-TEA/Pd was the most efficient catalyst
with a yield of 51%, followed by K/Pd (9%). As with the Suzuki
coupling reaction, the presence of borohydrides inhibits the
activity of the catalyst (5% yield for K-ImIL/Pd).
0
100
200
300
400
500
t (s)
Fig. 6 Variation of 4-NP maximum absorbance at 400 nm for various catalysts during
the catalytic reduction of 4-NP in presence of NaBH4.
The presence of the modifier (ImIL or TEA) has no effect on the
reduction of the nitro function. For the materials supported
PdNPs, a fast decrease of the amount of 4-NP confirms the
presence and the catalytic efficiency of PdNPs onto clay
platelets. While this reduction is less efficient with K/Pd, it
appears to be more efficient with K-TEA/Pd. Apparent pseudo-
first order kinetic constants were determined (Inset of Fig. 6)
in order to accurately compare the efficiencies of the catalysts.
As expected with the small amount of PdNPs incorporated,
K/Pd was the less efficient catalyst (kapp of 0.65 x 10-2 s-1). K-
TEA/Pd presents the most important apparent rate constant
(1.23 x 10-2 s-1), followed by K-ImIL/Pd (1.00 x 10-2 s-1). The high
efficiency of K-TEA/Pd can be explained by the better
accessibility of PdNPs because of the reduction of the stacking
of kaolinite platelets. The XRD patterns (Fig. 1) of the modified
clays clearly show that the structural disorder of kaolinite was
more pronounced for K-TEA/Pd (kaolinite 001 reflection plane
is less intense and broad compared to the pristine material)
compared to the well crystalline K-ImIL/Pd. The XRD pattern of
K-ImIL/Pd (Fig. 1 (B)) displayed four orders of reflection of the
basal plane (d001, d002, d003 and d004).
These catalysts are less efficient compared to the one
previously obtained with halloysite using similar synthesis
procedure (kapp of 1.80x10-2 s-1).38 These results can be
rationalized considering that the restricted area of location of
PdNPs in the case of modified halloysite (PdNPs were located
specifically inside the lumens) favored the catalytic reaction
because of the vicinity of the reactants with the reactive sites.
In the case of the kaolinite based catalysts, the stacking
remained pronounced and some reactive PdNPs were not
exposed to the reactants.
Table 1 Suzuki-Miyaura and Heck Coupling Reactions Yields
Coupling
reaction
(amount of
catalyst)
Suzuki
coupling
(3mg)
Suzuki
coupling
(5mg)
Heck
coupling
(5 mg)
Heck
coupling
(10 mg)
Suzuki-Miyaura and Heck Cross-coupling Reactions. The
coupling of iodobenzene with phenyl boronic acid in water in
the presence of K2CO3 was chosen as a model reaction to form
biphenyl (Scheme 1 (A)). Iodobenzene was reacted with
styrene to afford stilbene (Scheme 1 (B)) for the Heck coupling
reaction.
K/Pd
64
40
96
93
81
34
100
100
9
5
22
51
45
25
82
95
K-ImIL/Pd
K-ImIL/Pd/Cl
K-TEA/Pd
6 | J. Name., 2012, 00, 1-3
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Table 2 Recyclability Tests of the Catalysts on the Suzuki-Miyaura and Heck Coupling
Reactions.
to be robust enough to show only a slight loss in aVciteiwviAtryticaleftOenrlin3e
cycles of reactions. K-TEA based catalysts were the most
DOI: 10.1039/C6DT00982D
efficient both for the reduction of 4-NP and the coupling
reactions. It was also the most robust catalyst, probably
because of the strongest interactions between PdNPs.
Suzuki coupling
Heck coupling
Run 1 Run 2 Run 3
Run 1
100
Run 2
71
Run 3
67
K-ImIL/Pd/Cl
K-TEA/Pd
82
95
80
89
75
86
100
100
100
Acknowledgements
After the treatment with sodium chloride, a yield of 22% was
obtained. The poor yield observed can be explained by the
weak compatibility of the reactants and the catalysts. These
reactants are hydrophobic (iodobenbenzene and styrene) and
the use of a phase-transfer catalyst fails to ensure effective
contact with the highly hydrophilic catalysts (as confirmed by
the results of TGA and FTIR). Doubling the amount of catalysts,
significantly improved the yields, 95% for K-TEA/Pd, 82% for K-
ImIL/Pd/Cl and 45% for K/Pd. The better effectiveness of K-
TEA/Pd is probably linked to the presence of grafted TEA which
promotes the catalytic activity of PdNPs. Indeed, many reports
stated that tertiary amines are very efficient bases in the Heck
coupling reaction. 46
This work was financially supported by a Discovery Grant of
the Natural Sciences and Engineering Research Council of
Canada (NSERC). The Canada Foundation for Innovation and
the Ontario Research Fund are gratefully acknowledged for
infrastructure grants to the Centre for Catalysis Research and
Innovation of the University of Ottawa. The authors
acknowledge Dr. Nimal De Silva of the Geochemistry
Laboratory of the University of Ottawa for the support on ICP-
AES analysis.
Notes and references
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DOI: 10.1039/C6DT00982D
Graphical abstract
Synthesis and catalytic application of palladium nanoparticles
supported on kaolinite-based nanohybrid materials†
Gaelle Ngnie,*a Gustave. K. Dedzo,*a,b and Christian Detelliera
aCenter for Catalysis Research and Innovation and Department of Chemistry and Biomolecular Sciences,
University of Ottawa, Ottawa, Ontario K1N 6N5, Canada.
bLaboratory of Analytical Chemistry, Faculty of Science, University of Yaoundé I, B.P. 812, Yaoundé, Cameroon.
E-mails: gngnietu@uottawa.ca, gkennede@uottawa.ca
Synthesis and catalytic application of palladium
nanoparticles supported on kaolinite-based
nanohybrid materials†
Gaelle Ngnie,*a Gustave. K. Dedzo,*a,b and Christian
Detelliera
Modified kaolinite-supported palladium nanoparticles
displayed excellent catalytic activity for the Heck and
the Suzuki-Miyaura coupling reactions.