Palladium nanoparticles immobilized on halloysite nanotubes covered by a multilayer network for catalytic applications

Article abstract of DOI:10.1039/c8nj02932f

The synthesis of pure fine chemicals for industrial purposes is one of the most attractive challenges of chemical research. The use of catalytic pathways mediated by palladium nanoparticles (PdNPs) for C-C bond formation is a useful way to obtain these kinds of compounds. To achieve this objective, the PdNPs can be efficiently loaded on a functionalized natural nanostructured support such as halloysite nanotubes (HNTs). Hybrid materials based on thiol functionalized halloysite nanotubes and highly cross-linked imidazolium salts were successfully developed and used for the stabilization of PdNPs. The HNT/Pd hybrids were thoroughly characterized from a physico-chemical point of view and tested as a catalyst in the Suzuki and Heck C-C coupling reactions under microwave irradiation to obtain innovative materials for fine chemicals synthesis. Catalytic tests highlighted the fact that the HNT/Pd hybrids show high performance and full recyclability (up to ten cycles) in both reactions. Regarding the Suzuki reaction, under the best experimental conditions, the remarkable values of a turnover number of 194?000 and a turnover frequency of 3?880?000 h^-1 were achieved without metal contamination in the final products.

Full text of DOI:10.1039/c8nj02932f

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Pettignano, New J. Chem., 2018, DOI: 10.1039/C8NJ02932F.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Palladium nanoparticles immobilized on halloysite nanotubes
covered by multilayer network for catalytic applications
Marina Massaro,a,* Carmelo G. Colletti,a Gabriella Buscemi,a Salvatore Cataldo,b Susanna
Guernelli,c Giuseppe Lazzara,b Leonarda F. Liotta,d Filippo Parisi,b Alberto Pettignano,b and Serena
Rielaa,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The synthesis of pure fine chemicals for industrial purposes, is one of the most attractive challenges of the chemical
research. The use of catalytic pathways mediated by palladium nanoparticles (PdNPs) for the C-C bond formation, is a
useful way to obtain this kind of compounds. To achieve these objective, PdNPs can be efficiently loaded on functionalized
natural nanostructured support such as halloysite nanotubes (HNTs). Hybrid materials based on thiol functionalized
halloysite nanotubes and highly cross-linked imidazolium salts were successful developed and used for the stabilization of
PdNPs. The HNT/Pd hybrids were thoroughly characterized from a physico-chemical point of view and tested as catalyst in
the Suzuki and Heck C-C coupling reactions under microwave irradiation, to obtain innovative materials for fine chemicals
synthesis. Catalytic tests highlighted that the HNT/Pd hybrids show high performance and full recyclability (up to ten
cycles) in both reactions. As far as is regarding the Suzuki reaction, in the best experimental conditions, the remarkable
values of turnover number of 194,000 and turnover frequency of 3,880,000 h-1 were achieved without metal
contamination in the final products.
immobilization of PdNPs such as functionalized silica,3 carbon
nanoforms,4 renewable cellulose5 and sugars.6
Introduction
In this context, clay minerals have gathered particular interest
In the last years, great interest has been paid to the fine
chemical synthesis since these compounds are essential feed
stocks for pharmaceuticals, agrochemicals and “life style”
products.1
owing to their intrinsic properties as bioavailability, no-toxicity
and low price.
Halloysite nanotubes (HNTs), are a particular aluminosilicate
clay with a predominantly hollow tubular structure, which
have gathered particular interest owing to their unique
features, such as, high specific surface area, low toxicity,
natural availability (tons) and low price (5$ per Kg). HNTs
consist in 10-15 aluminosilicate layers with an external surface
constituted by siloxane groups and an inner lumen composed
Generally, the synthesis of fine chemicals leads to the
formation of large amounts of by-products which need to be
reduced to avoid environmental contamination. One way is to
use novel catalytic reactions instead of more laborious
methods of classic organic synthesis. Among the several
catalytic routes usually adopted for the synthesis of fine
chemicals, the C-C coupling reactions, catalysed by palladium
nanoparticles (PdNPs), have been proven to be powerful and
attractive synthetic methods due to their high selectivity and
tolerance of a wide range of functional groups.2
8
of aluminol groups.7, Owing to this peculiar composition,
HNTs are positively charged in the inner surface whereas they
are negatively charged on the external one in a wide pH range.
The different surface chemistry allows the selective
functionalization at the inner or outer side making possible the
synthesis of several nanomaterials with hierarchical
Up to now, several materials have been reported for the
10
nanostructure.9, HNTs are no-toxic for a number of living
organisms,11-13 they are widely used in many fields14-16
including such as filler for polymer17-19 or hydrogel matrices,20,
21
drug carrier22-24 and delivery,13 adsorbent25-28 as well as
catalyst support.29-31 In this context, several metal catalysts
loaded on the HNTs with excellent catalytic performances have
been reported and reviewed.9, 30, 32-34
Previously,30,
31, 35
we have synthesized palladium-based
catalytic systems using halloysite nanotubes with different
ionic liquids to obtain the so-called supported ionic liquid
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phase (SILP). These nanomaterials have been used as catalyst vinylimidazolium salt
in the C-C cross coupling reactions.
In this context highly cross-linked imidazolium salts allow to conditions, under microwave irradiation at 50 °C for 1 h, gave
reach higher loading of organic moieties onto the support and material . Since the bis-vinylimidazolium salt was added in
2
(ca. 6.4 equiv, per SH group), in the
presence of azobisisobutyronitrile (AIBN) in solvent-free
3
therefore they have been found capable to better stabilize excess relative to the moles of –SH groups, the double bonds
catalytically active Pd species compared to not cross-linked that do not react in the thiol-ene reaction can undergo a self-
imidazolium salts functionalized materials.36 To the best of our addition reaction, leading to the formation of a imidazolium
knowledge, few papers report about the functionalization of cross-linked network.26
halloysite with a polymeric shell for the immobilization of Compound
PdNPs and none of them envisages the use of cross-linked nanoparticles
imidazolium salts. tetrachloropalladate ions and the subsequent reduction with
3
was used for the immobilization of palladium
through anion metathesis with
Therefore, on the ground of this finding, herein we report the NaBH4 afforded the supported catalyst HNT/Pd. Palladium
design, synthesis, characterizations and catalytic activity of loading of the catalytic system was determined by means of
multilayered covalently SILP nanomaterials based on halloysite inductively coupled plasma-optical emission spectrometry
functionalized with bis-vinylimidazolium salts. The above (ICP-OES) and was found to be 1.5 wt%, according to
hybrid materials were synthetized by the in situ radical stoichiometry.
polymerization of the bis-vinylimidazolium salt with thiol Compound 3 was analyzed by means of FT-IR spectroscopy and
functionalized halloysite. The obtained hybrids were used for thermogravimetric analysis (TGA).
the immobilization of palladium nanoparticles and the catalytic Compared to the thiol functionalized HNTs, compound
3
performances were evaluated in the C-C cross coupling exhibited new bands around 1560, 1472 and 1390 cm−1 due to
reactions, namely Suzuki and Heck reactions. The combination
of halloysite and the highly cross-linked imidazolium salts helps moieties (Figure 2a). The FT-IR spectrum of compound
to efficiently stabilize PdNPs, obtaining an extremely active also characterized by the OH stretching vibrational bands of
C
=
C, C=
N and, C
−
N stretching vibrations of imidazolium
3
was
−
nanocatalyst for fine chemical synthesis.
halloysite. The frequencies of these bands were not altered in
the functionalized nanotubes. Specifically, there was observed
the broad peak of the water OH stretch as well as the OH
stretching vibration bands of the Al-OH and Si-OH groups.
From these data, one can state that diimidazolium salts were
grafted only on HNT external surface.
Results and Discussion
The synthesis of the catalytic material was accomplished in
two steps (Figure 1). The reaction between the
mercaptopropyl-functionalized halloysite HNT-SH 1, (degree of
functionalization of 0.4 mmol g-1) and excess of bis-
Figure 1. Schematic representation of the synthesis of HNT/Pd 4 catalyst.
2 | J. Name., 2012, 00, 1-3
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by BET method, whereas the pore volume and pore size
distributions were estimated by applying the BJH method to
the adsorption branch.37,
38
In Figure 3 the isotherms
registered for pristine HNTs and the hybrid material HNT/Pd
are compared. The HNTs displayed a type IV isotherm (IUPAC
classification) with a H2 hysteresis loop typical of mesoporous
material (Figure 3) and was characterized by SSA equal to 71
m2/g, mean pore size of 12.6 nm and pore volume of 0.17
cm3/g. A clear decrease in surface area and cumulative pore
volume was observed for the HNT/Pd that showed SSA of 9.7
m2/g and pore volume equal to 0.05 cm3/g, suggesting that
after the functionalization with bis-vinylimidazolium salt and
successive Pd loading, the surface of the halloysite was
uniformly covered and the porous structure almost filled.
Accordingly, the adsorption/desorption isotherms of HNT/Pd
were nearly coincident without any evident hysteresis loop
(Figure 3).
(a)
The morphology of HNT/Pd hybrid nanomaterial was
investigated by transmission electron microscopy (TEM) and
scanning electron microscopy (SEM). TEM analysis performed
on the catalyst shows that, Pd nanoparticles were obtained
with a narrow particle size distribution (9.4 ± 3.7 nm) and
uniform dispersion on the polymeric matrix (Figure 4).
Furthermore, the HNT/Pd material exhibits the characteristic
hollow tubular structure of halloysite with a shell of polymeric
matrix.
(b)
SEM investigation showed a compact structure where HNTs
are fully covered by the organic moiety that further confirms
the formation of a cross-linked network in agreement with
TGA results. In addition, the energy dispersive X-ray
spectroscopy (EDS) analysis of the HNT/Pd catalyst showed the
presence of bromo and absence of chloro atoms. These
findings indicate that the labile tetrachloropalladate ions have
successful reacted with bromide anions during the anion
exchange process.
Figure 2. (a) FT-IR spectra of HNT-SH and compound 3; (b) Mass change for
compound 3 with respect to pristine HNT and its temperature derivative.
Thermogravimetric analysis was used to determine the
amount of organic matter in the hybrid compound
its thermal stability. As Figure 2b shows, compound
3
besides
3
contains
ca. 25 wt% of organic moieties indicating the successful
functionalization with the formation of a cross-linked network.
The onset for the thermal degradation is at 290 °C with a
maximum degradation rate at 340 °C and a two steps
degradation mechanism. The observed degradation
temperature range is in agreement with recent TGA findings
for bis-imidazolium salts demonstrating that the grafting did
not alter significantly the thermal stability.35
Dynamic light scattering experiments have been performed in
aqueous dispersion to characterize the diffusion dynamic of
the HNT/Pd catalyst. A single translational diffusion mode was
observed with an apparent hydrodynamic diameter of 612 nm.
This value clearly shows that HNTs in the catalyst does not
move as single nanotubes but it shows entanglement due to
the organic network. ζ−potential measurements for HNT/Pd
catalyst in water showed a value of -17.6 mV that indicates a
reduction of surface charge upon functionalization (for p-HNT
ζ−potential value of -20 mV).
Figure 3. N2 adsorption/desorption isotherms registered for (ꢀ) pristine HNT
and for (ꢁ) HNT/Pd hybrid nanomaterial.
Surface textural properties of the materials were investigated
by means of N2 adsorption/desorption measurements at -196
°C. In particular, the specific surface area (SSA) was calculated
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(a)
(b)
(c)
Figure 4. (a) TEM images of the HNT/Pd material; the inset shows the palladium nanoparticles size distribution (n=200); (b) SEM image of the HNT/Pd
catalyst; (c) EDS pattern.
3
4
5
10
10
10
>99
>99
>99
Catalytic activity of HNT/Pd
The catalytic activity of the hybrid nanomaterial HNT/Pd was
tested in C−C cross-coupling reactions; in particular, the Heck
and Suzuki reactions were studied.
The applicability of the Suzuki reaction has been checked on a
set of substituted aryl bromides having either electron-
donating or electron-withdrawing groups and phenyl boronic
acid (Table 1) using HNT/Pd at 0.6 mol% loading. In most of the
cases, the conversions were almost quantitative with excellent
isolated yields (entries 1−10). The use of the less reacꢀve aryl
chlorides gave the 20% of conversion under the same reaction
conditions; however, increasing the irradiation time from 10 to
30 minutes allowed to obtain the correspondent 1,1'-biphenyl-
4-carbaldehyde in 40% of yield from 4-chloro-benzaldehyde
(entry 13). This finding represents a very promising result given
that this reaction with conventional heating does not occur
often and generally higher catalyst loading is necessary.39
6
10
>99
7
8
10
10
>99
>99
9
10
>99
Table 1. Suzuki–Miyaura reactions catalyzed by HNT/Pd catalyst.a
10
11
12
10
20
30
28
25
40
Entry Substrate
Product
Time
Conversion
(%)b
(min)
1
10
>99
aReaction conditions: aryl halide (0.55 mmol), phenylboronic acid (0.54 mmol), K2CO3
(0.61 mmol), EtOH/H2O (1:1, 1.2 mL), and catalyst (0.6 mol%).
bConversion determined by 1H-NMR in CDCl3.
2
10
>99
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Next, catalyst loading has been decreased down to 0.1 mol% The recyclability of the catalyst was assessed for the reaction
successfully affording 4-biphenylcarboxaldehyde and 3- between 3-bromoanisole and phenylboronic acid (1 mol %, 10
methoxy-1,1’ biphenyl in a quantitative yield with a turnover min). After easy recovery of HNT/Pd catalyst by centrifugation,
number (TON) of 1000 (Table 2, entries 1 and 3) and a it was used for ten consecutive runs. It was found that in these
turnover frequency (TOF) of 6,250 h−1, in both cases. Same conditions the catalyst gave rise to almost complete
results were obtained by decreasing the reaction time to 5 conversion and yield (>99%) for at least ten cycles (Figure S.1).
minutes (entries 2 and 4). Further improvement has been ICP-OES measurements revealed that less than 0.075 wt% of
obtained by decreasing once again the catalyst loading (entries Pd nanoparticles were released into solution during the
5-10). Just 0.0005 mol% of the HNT/Pd catalyst are enough to reaction; and therefore, it was possible to assume that no
yield 4-biphenylcarboxaldehyde in 10 min when irradiated at leaching occurs.
120 °C (entry 11). In such a way, a TON of 188,000 and a TOF To gain some insight on the actual mechanism involved during
of 1,175,000 h-1 values, were achieved. Finally, a further the catalysis, additional experiments were performed. In three
increase in the TON and TOF values were reached by the parallel reactions (3-bromoanisole and phenylboronic acid),
coupling reaction between phenylboronic acid and 4- the catalyst (0.01 mol%) was filtered-off after 2 min. In the first
1
bromoacetophenone (entry 11). In this latter case only 3 min one, the reaction was stopped and the H-NMR analysis of the
of irradiation gave the remarkable values of TON of 194,000 crude mixture showed 80% of the corresponding biaryl
and TOF of 3,880,000 h-1. To the best of our knowledge, this compound. The second reaction was quickly filtered still hot
latter value is among the highest ever reported for Pd (hot filtration) and left to stir (without HNT/Pd nanomaterial)
supported nanoparticles.4, 40
for additional 8 min leading to a 94% conversion, slightly lower
The obtained results highlighted the importance of halloysite with respect to the conversion obtained without catalyst
in the heterogeneous catalyst. The nanoclay indeed, with its removal (Table 2, entry 7). On the contrary, the last reaction,
specific surface area and tunable surface properties, allows a was cooled down to room temperature prior to catalyst
good dispersion of the dicationic salt and consequently better removal and, after, left to stir for additional 8 min (cold
stabilization of palladium nanoparticles with respect to other filtration), in this case no extra conversion was observed. The
systems.9
above data and the absence of leaching allow to conclude that
the catalyst works with a “release and catch” mechanism. This
mechanism envisages the existence of Pd soluble species
released from the catalyst during the Suzuki reaction, that
were redeposited onto the HNT/Pd after the end of the
reaction. Owing to this mechanism, the nanoparticle leaching
is avoided.
Table 2. Suzuki–Miyaura reaction catalyzed by HNT/Pd catalyst at 120 °C.a
To confirm the above mentioned mechanism some additional
TEM analyses were performed on the reused catalyst. Figure 5
shows TEM images of the catalyst recorded after ten cycles.
Conversely to that of the fresh catalyst; in this case it is
possible to observe Pd nanoparticles slight bigger in size and
with a not uniform distribution on the support. By statistical
analysis it was found that the mean Pd diameter was ca. 14.7 ±
4.8 nm confirming the hypothesized “release and catch”
mechanism.29
Entry
Catalyst
(mol%)
0.1
R
Time
(min)
10
5
Conv.
(%)b
>99
>99
>99
>99
90
TONc
TOFd(h-1)
1
2
3-OCH3
3-OCH3
4-CHO
4-CHO
3-OCH3
3-OCH3
4-CHO
4-CHO
4-CHO
4-CHO
1000
1000
6250
12,000
6250
3
10
5
1000
4
1000
12,000
21,600
51,250
62,500
625,000
1,840,000
1,175,000
5
0.05
0.01
5
1800
6e
7e
8f
9f
10f
10
10
10
3
80
8,200
10,000
100,000
92000
188,000
>99
>99
92
0.001
0.0005
10
94
11f
4-COCH3
3
97
194,000
3,880,000
aReaction conditions: phenylboronic acid (0.54 mmol), 4-bromobenzaldehyde or 3-
bromoanisole (0.55 mmol), K2CO3 (0.61 mmol), EtOH (0.6 mL), H2O (0.6 mL), and
catalyst were stirred under microwave irradiation for the indicated time. bDetermined
(a)
by 1H-NMR. cTON defined as mol of product/mol of catalyst. dTOF
= TON/h.
ePhenylboronic acid (0.99 mmol), 4- bromobenzaldehyde (1.00 mmol), K2CO3 (1.12
mmol), EtOH (0.6 mL), H2O (0.6 mL), and catalyst. fPhenylboronic acid (1.98 mmol), 4-
bromobenzaldehyde or 4-bromoacetophenone (2.00 mmol), K2CO3 (3.68 mmol), EtOH
(1 mL), H2O (1 mL), and catalyst were stirred under microwave irradiation for the
indicated time.
(b)
Figure 5. (a) TEM images of the catalyst after ten consecutive runs; (b)
palladium nanoparticles size distribution (n=65).
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Since the Suzuki products are often used as building blocks for Table 3. Optimization of conditions for Heck reaction.a
a lot of pharmaceuticals, it is crucial to determine if could
there be palladium contamination in the final products. In
pharmaceutically active ingredients, indeed, Pd content is
strictly regulated, current government regulations allow a
maximum of 5 ppm and 0.5 ppm of Pd for oral and parenteral
administrations, respectively.41
Therefore, the Suzuki products were mineralized and analyzed
by ICP-OES to quantify the amount of Pd leached that is about
0.2 ppm, lower than that allowed.
Entry
Solvent
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Reaction
conditions
ꢀ, 120 °C,
6 h
Conversion
(%)b
1
>99 (>99)
The above-mentioned results prompted us to continue with a
study of HNT/Pd catalytic activity in the Heck reaction. With
the final goal to find the best experimental conditions for the
Heck coupling, a series of reactions were performed by using
varying conditions (Table 3). As model was chosen the Heck
reaction between 4-iodoanisole with methyl acrylate. First of
all, the reaction was carried out following a procedure
reported in literature, using DMF as solvent with triethylamine
(TEA) as the base at 120 °C. (Table 3, entries 1 and 2). In this
case, we obtained the methyl (E)-3-(4-methoxyphenyl)acrylate
in quantitative yield. Therefore, in order to reduce the reaction
time, we performed the same reaction under microwave
irradiations (entries 3-7). Under these conditions the product
was obtained in quantitative yield after only 2 min of
irradiation.
Subsequently, safer media42 were used as alternatives to DMF.
The introduction of water in the reaction medium allows to
obtain once again quantitative yields (entry 8). On the
contrary, the use of only water (entry 9) as medium gave no
results, confirming the need of combined organic/aqueous
media to reach the highest efficiency. The use of an aqueous
mixture DMF/water (1:1) did not lead a full conversion (entry
9) showing that for the Heck reaction, only a small amount of
water is beneficial. Therefore, we focused our attention to a
greener media such as ethylene glicole and ethanol, which
2
ꢀ, 120 °C,
3 h
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
70
3
MW, 120
°C, 1 h
4
MW, 120
°C, 30 min
MW, 120
°C, 10 min
MW, 120
°C, 5 min
MW, 120
°C, 2 min
MW, 120
°C, 2 min
MW, 120
°C, 2 min
MW, 120
°C, 5 min
MW, 120
°C, 2 min
MW, 120
°C, 2 min
MW, 120
°C, 5 min
MW, 120
°C, 2 min
MW, 120
°C, 2 min
5
6
7
8
DMF/H2O
4:1
9
DMF/H2O
1:1
10
11
12
13
14c
15c
H2O
10
Ethylene
glicole
EtOH
28
>99 (95)
>99 (40)
28
/
possess different ability to interact with microwaves (tan
δ of
EtOH
1.350 and 0.941 for ethylene glicole and ethanol, respectively).
In these Heck coupling, we obtained poor results when glicole
was used (entry 11), while the use of ethanol gave the methyl
(E)-3-(4-methoxyphenyl)acrylate in quantitative manner (entry
12). In attempt to further improve the greenness of the
reaction, the Heck coupling was performed in solvent-free
conditions; in this case although we obtained quantitative
conversion of 4-iodoanisole in the correspondent phenyl
acrylate; the yield of the reaction was low (entry 13). Finally,
the role of the base was investigated. The reaction of 4-
iodoanisole and methyl acrylate in EtOH, at 120 °C, for an
irradiation time of 2 min, in the presence of K2CO3 as base led
to a mixture of products where only 28% was methyl (E)-3-(4-
methoxyphenyl)acrylate. The product deriving from the
Michael reaction followed by transesterification between
ethanol and methyl acrylate was also present in the mixture.43,
DMF/H2O
4:1
12
aReaction conditions: 4-iodoanisole (0.50 mmol), methyl acrylate (0.75 mmol), TEA (1.0
mmol), solvent (1 mL) and catalyst (0.1 mol%). bDetermined by 1H-NMR, in parenthesis
yields are reported. c K2CO3 (1 mmol).
The above data suggest that the best experimental conditions
to obtain the Heck products in high conversion and yields are
ethanol as solvent and TEA as base. However, in this solvent,
low yields were obtained in the screening of several substrates
coupled with methyl acrylate or styrene (see Table S.1). For
this reason, we opted to work in a mixture DMF/water 4:1, in
the presence of TEA as base and for an irradiation time of 2
min at 120 °C. In all cases, the HNT/Pd catalysts was used at
0.1 mol% loading (Table 4). Several substrates were taken into
account, both with electron-donating and withdrawing groups.
44
Considering the low conversion obtained in this case, we
performed an additional experiment where the Heck reaction
was carried out in a mixture DMF/water 4:1 and K2CO3 as base.
Under these conditions, we still obtained a low conversion
value (entry 15).
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Table 4. Heck reactions catalyzed by HNT/Pd catalyst.a
under traditional heating instead of the microwave ones
(entries 5-8).
The different behavior between methyl acrylate and styrene
could be due to an effect of microwave irradiation on a more
electron-rich substrate which could lead to a degradation of
both reactants and products.
Furthermore, the styrene coupling lead to a decrease in the
regioselectivity, since in the reaction mixture it was also found
the gem isomer product as usually observed for this kind of
substrates.4
Entry
Product
Conversion
(%)b
1
>99
Finally, the recyclability of the novel HNT/Pd hybrid
nanocomposites, was tested in the Heck reaction between 4-
iodoanisole and methyl acrylate chosen as model. The catalyst
was employed at 0.5 mol% and re-used for at least eight
consecutive runs giving complete conversion, while a slight
decrease in the conversion (from 99 to 80%) was observed in
the ninth cycle (Figure S.2). This decrease could be explained
taking into account the mechanism operating in the Heck
reaction. It was hypothesized39 that, in this case, although still
exists a “release and catch” mechanism, the catalyst works
mainly in homogenous conditions where, the dissolved Pd
species could be strongly stabilized in solution by the presence
of triethylamine and dimethylformamide and therefore they
could be leached during the course of the reaction. From ICP-
OES measurements it was found that about 20 wt% of Pd was
leached into solution after eight cycles, confirming the above
hypothesis.
2
3
>99
94
4
5
6
>99
On the ground of the above results it could be interesting to
compare the performances of the HNT/Pd catalyst with other
HNT supported PdNPs catalysts developed for C-C coupling
>99 [85]c
>99 [92]c
reactions.29-31, 35, 45
.
There are several studies concerning PdNPs stabilized by
halloysite nanotubes modified with a polymeric network or
covalently functionalized with imidazolium salts. However,
they have shown limited catalytic activity such as lower
product yields, limited recyclability (up to 5 cycles), higher
reaction times and low TON and TOF values. For example, Pd
supported on halloysite dispersed in a PNIPAAM matrix
showed a TOF value of 180 h-1,45 while similar system, where
the polymer is covalently grafted on the HNT external surface
7
8
>99 [90]c
29
leads to an increase of the TOF up to 39063 h-1 still lower
than that obtained in the present work.
>99 [80]c
In addition, the HNT/Pd catalyst possesses an impressive
catalytic activity not shown by other SILP systems previously
reported by us, where more catalyst loading and higher
reaction times were required to obtain satisfying yields.30, 31, 35
Therefore, the combination of the high cross-linked
imidazolium salts and halloysite allowed us to achieve an
efficient support in which PdNPs can be efficiently stabilized in
smaller size and thus more active than that on other HNT
catalytic systems.30, 31, 35
aReaction conditions: aryliodide (0.50 mmol), alkene (0.75 mmol), TEA (1 mmol),
mixture DMF/H2O (4:1) (1 mL) and catalyst (0.1 mol%). bDetermined by 1H-NMR, in
square brackets selectivity towards trans alkene determined by 1H-NMR. cReaction
conditions: Traditional heating at 120 °C for 3 h.
High conversions and yields were obtained for the synthesis of
methyl acrylate derivatives (entries 1-4), where the only
reaction product was, as expected, the trans alkene; whereas,
in the case of styrene derivatives, poor yields were observed.
In these cases, we obtained an increase in the yield working
Experimental Section
Materials and Methods
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All reagents needed were used as purchased (Sigma Aldrich), with water to remove the unreacted materials. The solid
without further purification. obtained was re-suspended in ethanol (5 mL) and to the
The HNT-SH and 1,5-bis (3-vinylimidazolium-1-pentyl) obtained dispersion a solution of NaBH4 (29.80 mg, 0.79 mmol)
bromide were previous synthetized following a procedure in ethanol (4 mL) was added dropwise. The obtained
reported in literature.31, 46
dispersion was left to stir at room temperature for 6 h. After
Microwave-assisted syntheses were carried out with a CEM this time, the solvent was filtered off and the powder was
DISCOVER monomode system in a closed vessel. washed several times with water and then dried overnight at
1
2
Thermogravimetric analyses were performed on a Q5000 IR 60 °C.
apparatus (TA Instruments) under a nitrogen flow of 25 cm3 General procedure for the Suzuki reaction under microwave
min-1 for the sample and 10 cm3 min-1 for the balance. The irradiation
weight of each sample was ca. 10 mg. Measurements were
HNT/Pd catalyst (ranging from 0.0001 to 0.6 mol%),
carried out by heating the sample from room temperature up
phenylboronic acid (0.54 mmol, 0.99 equiv.), K2CO3 (0.61
to 900 °C at a rate of 10 °C min-1.
mmol, 1.12 equiv.), aryl bromide (0.55 mmol, 1 equiv.),
FT-IR spectra in KBr were recorded with an Agilent
ethanol (0.6 mL) and water (0.6 mL) were placed in a
Technologies Cary 630 FT-IR spectrometer. Specimens for
microwave test tube provided with a cap. The mixture was
these measurements were prepared by mixing 5 mg of the
inserted into microwave apparatus at a temperature of 120 °C
sample powder with 100 mg of KBr.
under constant stirring for the appropriate time. The reaction
ζ−potential and dynamic light scattering (DLS) measurements
were taken by using Zetasizer NANO-ZS (Malvern Instruments)
mixture was cooled at room temperature and the crude was
poured into water and extracted by centrifuge with diethyl
at 25.0 ± 0.1 °C.
ether. The organic layers were dried with MgSO4 and the
N2 adsorption–desorption measurements were carried out at -
solvent removed under vacuum. The residue was checked by
1H-NMR to calculate conversion.
Recycling procedure of HNT/Pd catalyst in the Suzuki
Reaction
196 °C by using
a
volumetric adsorption analyzer
(Micromeritics ASAP 2020 Plus). Before the analysis, the
samples were pretreated at 80 °C for 10 h under reduced
pressure (0.1 mbar). The BET method was applied in the p/p0 =
0.05–0.30 range to calculate the specific surface area, whereas
the pore size distributions were estimated from the adsorption
isotherm using the BJH method.
HNT/Pd catalyst (1 mol%), phenylboronic acid (0.54 mmol),
K2CO3 (0.61 mmol), 3-bromoanisole (0.55 mmol), ethanol (0.6
mL) and water (0.6 mL) were placed in a microwave test tube
provided with a cap. The mixture was inserted into microwave
oven at a temperature of 120 °C under constant stirring for 10
min. The crude solution was poured into water and extracted
by centrifuge with diethyl ether; the organic layers were dried
with MgSO4; the solvent was removed under vacuum and the
For TEM observation the samples were prepared using a drop
of suspension solution on formvar-coated copper grid (400
mesh) and allowing the drop to dry completely in a vacuum
desiccator. The TEM images of the samples were obtained
using a Philips TEM CM 100 transmission electron microscope
at accelerating voltage = 80 kV.
1
residue was checked by H-NMR to calculate conversion. The
residual solid was washed several times with water, dried and
reused for the next cycle.
AESEM FEI QUANTA 200F microscope apparatus, with EDS
probe, was used to study the morphology of the functionalized
HNTs. Before each experiment, the sample was coated with
gold under argon by means of an Edwards Sputter Coater
S150A in order to avoid charging under electron beam.
1H-NMR spectra were recorded at 300 MHz in CDCl3 solutions
at room temperature on a Bruker 300 MHz spectrometer.
ICP-OES analyses were carried out with an Optima 2100,
Perkin Elmer, equipped with an auto sampler model AS-90.
Analyses were conducted using a calibration curve, obtained
by dilution (range: 0–10 ppm) of palladium standard solution
for ICP-OES analyses. The operative wavelengths were 340.458
and 363.470 nm.
General procedure for the Heck Reaction via traditional
heating
In a 3 mL glass vial with screw cap, HNT/Pd catalyst (0.1 mol%),
aryliodide (0.5 mmol, 1 equiv.), alkene (0.75 mmol, 1.5 equiv.),
triethylamine or K2CO3 (1.0 mmol, 2 equiv.), and DMF (1 mL)
were placed and heated at 120 °C for the appropriate time.
The reaction mixture was allowed to cool down and was
poured in 10 mL of water and then extracted with diethyl
ether (3×10 mL). The combined organic layers were dried with
MgSO4 and the solvent removed under vacuum and the
residue was purified by short flash column chromatography
(SiO2, petroleum ether/ethyl acetate). Conversions were
calculated by 1H-NMR.
General procedure for the synthesis of HNT/Pd hybrid
material
General procedure for the Heck Reaction via microwave
irradiation
PdCl2 (20 mg, 0.11 mmol) and NaCl (132 mg, 2.26 mmol) in
water (1 mL) were heated at 80 °C until the PdCl2 was
dissolved. The obtained clear reddish solution of sodium
tetrachloropalladate(II) was cooled at room temperature and
In a microwave test tube, provided with a cap, HNT/Pd catalyst
(0.1 mol%), aryliodide (0.50 mmol, 1 equiv.), alkene (0.75
mmol, 1.5 equiv.), triethylamine or K2CO3 (1.0 mmol, 2 equiv.),
and the appropriate solvent (1 mL) were introduced. The
added dropwise to a suspension of compound
3 (500 mg) in
water (3 mL). The suspension was left to stir at room
temperature for 20 h, then filtered and washed several times
mixture was inserted into microwave apparatus at
a
temperature of 120 °C under constant stirring for the required
8 | J. Name., 2012, 00, 1-3
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time. The reaction mixture was allowed to cool down and was occurs in the final products, which value is lower than that
poured in 10 mL of water and then extracted with diethyl allowed by current government legislation.
ether (3×10 mL). The combined organic layers were dried with These findings, are promising for future application of the
MgSO4 and the solvent removed under vacuum and the material synthesized for industrial purpose, for example for
residue was purified by short flash column chromatography the synthesis of fine chemicals under flow conditions.
(SiO2, petroleum ether/ethyl acetate). Conversions were
calculated by 1H-NMR.
Conflicts of interest
Recycling procedure of HNT/Pd catalyst in Heck reaction
There are no conflicts to declare.
In a microwave test tube, provided with a cap, HNT/Pd catalyst
(0.5 mol%), 4-iodoanisole (0.50 mmol, 1 equiv.), methyl
acrylate (0.75 mmol, 1.5 equiv.), triethylamine (1.0 mmol, 2
Notes and references
equiv.), and 1 mL of a mixture DMF/H2O (4:1) were introduced.
The mixture was poured into microwave oven at
a
a
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Catalyst was washed, sonicated and centrifuged with diethyl
ether, water, methanol and ethyl acetate and then dried in an
oven at 60 °C before using it for the next cycle. The recovered
organic layers were collected and evaporated under reduced
pressure to remove volatiles then poured in 20 mL of water
and extracted with diethyl ether (3×20 mL). The combined
organic layers were dried over MgSO4 filtered and evaporated
under vacuum. The residue was checked by 1H-NMR to
calculate conversion.
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