Synthesis and characterization of bifunctional lipophilic and basic mesoporous organosilica supported palladium nanoparticles as an efficient and ecofriendly nanocomposite in aqueous Heck reaction

Article abstract of DOI:10.1039/c6ra17452c

In the present study, a highly ordered mesoporous organosilica nanocomposite having modified pore channels with both lipophilic and basic units, SBA-R/Im-NH₂, was synthesized through surfactant-templated sol-gel methodology and post modification process. The nanocomposite with ionic liquid properties was used as a potential host for supporting palladium nanoparticles. The structure and composition of the target nanocomposite, SBA-R/Im-NH₂·Pd, were supported by infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), elemental analysis (CHNS), energy dispersive X-ray (EDX) and Brunauer-Emmett-Teller (BET) measurements. The efficiency of this porous inorganic-organic hybrid nanocomposite as a powerful catalyst in one of the most important carbon-carbon bond-forming process, the Heck coupling reaction of aryl halides with olefins, in aqueous media was also examined. This method has the advantages of high yields, cleaner reactions, simple methodology, short reaction times, easy workup, and greener conditions. In addition, the nanocatalyst can be easily separated from the reaction mixture and reused several times without significant decrease in activity and promises economic as well as environmental benefits.

Full text of DOI:10.1039/c6ra17452c

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PAPER
Synthesis and characterization of bifunctional
lipophilic and basic mesoporous organosilica
supported palladium nanoparticles as an efficient
and ecofriendly nanocomposite in aqueous Heck
reaction
Cite this: RSC Adv., 2016, 6, 81614
Mina Jafari Nasab and Ali Reza Kiasat*
In the present study, a highly ordered mesoporous organosilica nanocomposite having modified pore
2
channels with both lipophilic and basic units, SBA-R/Im-NH , was synthesized through surfactant-
templated sol–gel methodology and post modification process. The nanocomposite with ionic liquid
properties was used as a potential host for supporting palladium nanoparticles. The structure and
composition of the target nanocomposite, SBA-R/Im-NH
2
$Pd, were supported by infrared spectroscopy
FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy
TEM), elemental analysis (CHNS), energy dispersive X-ray (EDX) and Brunauer–Emmett–Teller (BET)
(
(
measurements. The efficiency of this porous inorganic–organic hybrid nanocomposite as a powerful
catalyst in one of the most important carbon–carbon bond-forming process, the Heck coupling reaction
of aryl halides with olefins, in aqueous media was also examined. This method has the advantages of
high yields, cleaner reactions, simple methodology, short reaction times, easy workup, and greener
conditions. In addition, the nanocatalyst can be easily separated from the reaction mixture and reused
several times without significant decrease in activity and promises economic as well as environmental
benefits.
Received 7th July 2016
Accepted 17th August 2016
DOI: 10.1039/c6ra17452c
www.rsc.org/advances
10,11
paramount importance.
The recent progress regarding the
Introduction
catalytic synthesis of ne chemicals via tandem multistep
In recent times, mesoporous materials with controllable pore reactions catalyzed by bifunctional mesoporous silica is
structure and surface chemistry have attracted much attention impressive. There is specic interplay between both functional
from both academia and industry. In particular, mesoporous supported on mesoporous silica which is responsible for the
silicas are of great importance for the fabrication of inorganic– high selectivity and activity in many molecular catalysts.
organic hybrid materials due to their ordered arrays of design-
On the other hand, the study of Heck reaction is receiving
able pores. Furthermore, these materials display high thermal considerable attention due to its wide range of applications in
and mechanical stability, large and readily accessible pore natural product synthesis, pharmaceuticals, material sciences,
volumes, and high specic surface areas, which can be easily molecular wires, liquid crystals, herbicides, and bioactive
1
–7
functionalized.
synthesized by surfactant-directed self-assembly of various vinyl halides with olens is arguably one of the most powerful
Therefore, this class of material that is compounds. The palladium-catalyzed Heck reaction of aryl/
12,13
organosilane precursors, shows great potential for high tools for C–C bond formation in synthetic organic chemistry.
performance catalysts with desired catalytic activity, and pro- Along this line, palladium-complexes systems with several
longed lifetime.
8,9
ligands have been extensively employed as an effective catalyst
Indeed, organic functionalization of mesoporous silica is in controlling the reactivity and selectivity in the Heck reaction.
a route to introduce or accentuate important surface properties, The main role of ligands is to stabilize the palladium(0), which
such as hydrophilicity or hydrophobicity, host–guest interac- can enter the catalytic cycle and consequently prevent the
14,15
tions, chemical resistance, and other functional accomplish- formation of inactive palladium black.
ments possess without any doubt an actual and future
Therefore, herein in continuation of our recent studies in the

eld of mesoporous organosilica nanocomposite and according
to importance of the palladium-catalyzed coupling reactions as
a powerful toolbox in the synthetic organic chemistry, we wish
to disclose, for the rst time the preparation and
Chemistry Department, College of Science, Shahid Chamran University of Ahvaz,
6
3
1357-4-3169, Iran. E-mail: akiasat@scu.ac.ir; Fax: +98 61 333728044; Tel: +98 61
33728044
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2
Scheme 1 The synthesis route of SBA-R/Im-NH $Pd.
characterization of a novel bifunctional lipophilic and basic Table 1 Elemental compositions of sample analyzed using the CHN
technique
mesoporous organosilica supported palladium nanoparticle
2
with ionic liquid property, SBA-R/Im-NH $Pd, as well as study
its catalytic application in the Heck coupling reaction of aryl
halides with olens, in aqueous media.
Samples
C (wt%)
12.5
N (wt%)
2.6
H (wt%)
2.1
SBA-R/Im-NH2
Statement of the advantages of described method, as well as
the potential activities or signicance of the compounds
prepared.
template (EO20PO70EO20, MW 5800). Then, the basic units
with ionic liquid property was introduce in the nano-
composite via a simple post-synthesis method by nucleo-
philic substitution of Cl with imidazole followed by the
reaction of N atoms of imidazole units with 2-bromo ethyl
amine. By having this fact in mind, that these lipophilic and
basic units with ionic liquid property graed to the SBA can
be produced cavities that stabilize Pd nanoparticle and avoid
its aggregation, we disclosed the generation of Pd nano-
Results and discussions
In this context, we wish to report the preparation of SBA-R/
Im-NH $Pd as a new hybrid mesoporous silica with basic,
2
ionic liquid nature and lipophilic feature which has the
ability to support Pd nanoparticles. For this purpose, rst,
bifunctional conjugated lipophilic and chlorinated units on
mesoporous silica, SBA-R/Cl, was successfully synthesized by
co-condensation of organosilane precursors, 3-chlor-
opropyltrimethoxysilane and cetyl trimethoxy silane, and
tetraalkoxysilane in the presence of triblock copolymer
particle in the presence of SBA-R/Im-NH by chemical
2
2
+
reduction of Pd by NaBH
4
. The general pathway for the
$Pd nanocomposite is rep-
preparation of the SBA-R/Im-NH
2
resented schematically in Scheme 1.
Fig. 1 The FT-IR spectra of SBA-R/Cl and SBA-R/Im-NH
2
.
2
Fig. 2 The EDX spectrum of SBA-R/Im-NH $Pd.
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2
Fig. 5 TGA-DTG analysis of SBA-R/Im-NH .
ꢀ
1
Fig. 3 XRD pattern of SBA-R/Im-NH
2
$Pd.
vibration is normally observed around 1000–1200 cm that
cannot be resolved due to its overlap with the absorbance of Si–
O–Si stretch. The presence of amino groups in the SBA-R/Im-
2
In order to characterize the catalyst, and to conrm the NH were further corroborated by a broad band at 2800–3400
ꢀ
1
immobilization of the active components on the pore surface, cm attributed to the O–H vibration of physical absorption
FT-IR spectroscopy was utilized. The FT-IR spectra of the SBA-R/ water. These features from FT-IR revealed the existence of the
ꢀ
1
Cl and SBA-R/Im-NH (Fig. 1) in the range of 400–4000 cm
lipophilic and basic species in the structure of the SBA-R/Im-
2
ꢀ
1
were shown peaks at 1220, 1078, 804, and 470 cm that related NH
hybrid nanocomposite.
2
to stretching, bending and vibration modes of Si–O–Si and
CHN elemental analysis was performed to quantitatively
peaks appearing at 2922 and 2856 cm are characteristic of determine the number of organic groups that were incorporated
ꢀ1
C–H stretching vibrations. In addition, the FT-IR spectra of the onto the surface of SBA-15. The percentages of C, H and N were
SBA-R/Im-NH
2
exhibited two new peaks display at 1560, 1645 assessed by elemental analysis and are presented in Table 1.
ꢀ
1
cm which were assigned to C]C, C]N bands of the imid- CHN elemental analysis indicates that amino and cetyl groups
azole rings, respectively. The absorbance of the C–N stretching were successfully functionalized on the surface of SBA-15.
The EDX spectrum (Fig. 2) of SBA-R/Im-NH $Pd shows the
2
elements, including C, N, O, Si and Pd that are present in the
structure of this material. The palladium peak clearly conrms
the immobilized Pd nanoparticles on the surface of SBA-R/Im-
2
NH .
The Pd content of the nanocomposite was determined by
atomic absorption spectroscopy and conrmed by energy
ꢀ1
dispersive X-ray (EDX) (loading ca. 0.362 ꢁ 0.001 mmol g ).
Fig. 3 shows the low and high-angle XRD patterns of the as-
prepared SBA-R/Im-NH $Pd composite. The low-angle XRD
2
pattern of sample exhibit three well-resolved peaks, one major
ꢂ
peak at about 0.98 together with two additional peaks can be
observed, which conrm the long range order and excellent
textural uniformity of the mesoporous material and also the
high angle of sample show the peaks at 2q values of 40.1, 47.0,
68.4 and 82.1 originated from Pd (111), (200), (220) and (311)
diffraction peaks (no. 46-1043) and the broad peak at 2q of 15–
ꢂ
5 due to amorphous SiO .
2
16
2
The TEM images provide the direct observation of the
morphology and the distribution of the Pd nanoparticles in
SBA-R/Im-NH . The TEM micrograph in Fig. 4a clearly shows
that SBA-R/Im-NH has a highly ordered mesostructure with
2
2
ranging from 12 to 30 nm. The dark spots of the Fig. 4b is the Pd
nanoparticles that highly dispersed Pd nanoparticles are
distributed on the surface of modied silica without
aggregation.
2 2
Fig. 4 The TEM images of (a) SBA-R/Im-NH , (b) SBA-R/Im-NH $Pd.
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Table 2 Comparison of the textural parameters of SBA-R/Im-NH and
SBA-Cl
BET surface area
Diameter
(nm)
Pore volume
2
ꢀ1
3
ꢀ1
Samples
(m g
)
(cm g
)
SBA-Cl
SBA-R/Im-NH
753
488
9.7
4.9
1.25
0.59
2
The thermal stability of the functionalized SBA was exam-
ined by thermo-gravimetric analysis. The TGA thermograms
and the respective DTG curves (Fig. 5) for the SBA-R/Im-NH
2
ꢂ
presents the initial weight loss below 200 C and this can be
attributed to the desorption of physically adsorbed water as well
as dehydration of the surface –OH groups. The nearly 22%
ꢂ
weight loss from 200 to 790 C is probably due to the decom-
position of organic groups, while relatively slow weight loss at
elevated temperatures can be related to the decomposition of
the silica shell. Thus, the TGA curves conrm the successful
graing of organic groups onto the surface of silica and the
content of the organic units was also dened by CHN, EDX data
and is in agreement with thermogravimetric analysis data.
The nitrogen adsorption–desorption isotherms and the BJH
pore size distribution (based on adsorption branch of the
isotherms) for SBA-R/Im-NH is shown in Fig. 6. The isotherms
2
are characterized by the type IV with a H1-type hysteresis loop
dened by IUPAC indicating that the ordered mesoporous
17
structure of SBA-15 is well remained. However, it has
a dramatically decreased surface area and porosity relative to
SBA-Cl (Table 2 and Fig. 6).
The basic capacity of the nanocomposite was determined by
potentiometric and conrmed by acid–base titration. According
to the potentiometric method, SBA-R/Im-NH (0.1 g) was situ-
2
ated in an aqueous solution of NaCl (1 M, 25 mL, initial pH ¼ 6)
and the resulting mixture was stirred for 24 h, the pH of solu-
tion increase to 6.70. The basic capacity of the nanocomposite is
ꢀ
10
ꢀ
0
.5 ꢃ 10
mmol OH per gram of composite. This result was
also veried by back-titration method.
In order to investigate the possible catalytic properties of
SBA-R/Im-NH $Pd in the C–C bond formation, the Heck reac-
2
tion was carried using iodobenzene and styrene as a starting
materials using H O as solvent and potassium carbonate as
2
ꢂ
base at 90 C. The best result was obtained when the reaction
2
done in the presence of 70 mg of SBA-R/Im-NH $Pd. It is worth
to mention that the reaction in the absence of catalyst aer
prolonged reaction time is not produced the product (Table 3).
With the optimized conditions at hand, we investigated the
use of a wide range of starting materials (Scheme 2) in Heck
Table
2
3 Optimization of the amount of the SBA-R/Im-NH $Pd
nanocomposite in the proposed reaction
Entry
Catalyst (mg)
Time (min)
Yield (%)
Fig. 6 Pore size distributions of SBA-Cl (a), SBA-R/Im-NH
2
(b) and
1
2
3
4
—
50
70
100
360
120
40
—
70
98
98
nitrogen adsorption–desorption isotherms of SBA-Cl (c), SBA-R/Im-
NH (d).
2
38
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2
Scheme 2 Heck reaction of aromatic aryl halides with alkenes catalyzed by SBA-R/Im-NH $Pd.
Table 4 Heck coupling reaction of various aryl halides and alkenes under optimized conditions
Entry
Aryl halide
Olen
Product
Time (min)
40
Yield (%)
98
1
2
3
45
90
97
80
4
5
6
7
8
75
96
90
98
120
180
40
Trace
70
92
9
30
95
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To investigate whether the catalytically active species is really
heterogeneous or not, the hot ltration test was also explored.
During the reaction of iodobenzene and styrene, the catalyti-
cally active nanocomposite was removed from the reaction by

ltration aer 15 min using a hot frit, and the ltrate was
monitored for continued activity. The GC analysis shows that
aer removal of the catalyst, the reaction did not proceed,
indicating that no catalytically active Pd remained in the ltrate.
The heterogeneous character of the nanocomposite in this
reaction was also conrmed by the analysis of the crude reac-
tion media by ICP. The analysis indicated negligible Pd
leaching.
Fig. 7 Recyclability of catalyst.
In order to show the merit of our procedure for the synthesis
of Heck of aromatic aryl halides with alkenes, we have shown
the advantages of present procedure by comparing our results
with those previously reported in the literature (Table 6).
2
Table 5 Recyclability of the SBA-R/Im-NH $Pd
a
b
S. No.
Yield (%)
TON
TOF
1
2
3
4
5
98
96
93
88
88
1400
35
1371.4
1328.6
1257.1
1257.1
34.3
33.2
31.4
31.4
Experimental
General
a
b
Tetraethylorthosilicate (TEOS), 3-chloropropyltrimethoxysilane
Turnover number. Turnover frequency.
(
CPTMS), cetyl trimethoxy silane (CTMS), 2-bromo ethyl amine
hydrobromide, imidazole and Pluronic P123 triblock copolymer
EO20PO70EO20, MW 5800) were supplied by Aldrich. Other
(
ꢂ
$Pd at 90 C. Whereby,
the reaction proceeded smoothly to afford the desired product
in high yield (Scheme 2, Table 4).
reaction using 70 mg of SBA-R/Im-NH
2
chemical materials were acquired from Fluka and Merck
companies were used as received without further purication.
The purity determination of the products and reaction moni-
toring were performed by TLC on silica gel Poly Gram SIL G/UV
For the catalytic synthesis of active pharmaceutical inter-
mediates, contamination of the product due to the leaching of
the metal is a serious issue. Therefore, the catalyst stability was
analyzed by means of recyclability studies and hot ltration test.
The recycling performance of the catalyst was investigated using
the reaction of iodobenzene and styrene. As shown in Fig. 7, the
catalyst could be reused up to ve times with a slight loss in
yield from 98% to 88% and also TON (turnover number) and
TOF (turnover frequency) of the catalyst for each stage recycling
is calculated (Table 5). The results conrm that the catalyst has
good stability and recyclable applicability under experimental
conditions. The catalyst was easily recovered through simple
vacuum ltration/Whatman lter paper. The catalyst aer
washing with dichloromethane and drying in oven was sub-
jected for the use in next reaction.
254 plates. Melting points were determined in open capillaries
with a BUCHI 510 melting point apparent. FT-IR spectra of the
powders were recorded using BOMEM MB-Series 1998 FT-IR
spectrometer. X-ray diffraction (XRD) patterns of samples were
taken on Philips X-ray diffraction model PW 1840. The particle
morphology was examined by SEM (LEO 1455VP scanning
electron microscope, operating at 1–30 kV) and TEM (Zeiss-
EM10C transmission electron microscope, 80 kV). A Metrohm
digital pH-meter model 632 with a combined glass electrode
was used for pH adjustments. The TGA curve of the SBA-R/Im-
NH
2
ꢂ
$Pd was recorded on a BAHR, SPA 503 at heating rates of
ꢀ1
10 C min . The thermal behavior was studied by heating 1–5
mg of samples in aluminum-crimped pans under nitrogen gas
ꢂ

ow, over the temperature range of 25–1000 C.
Table 6 A comparisons of the results of the present system with the some recently reported procedures
S. No.
1
Substrate
Catalytic system
Reaction conditions
Yield
97%
Ref.
18
PhI/styrene
Palladium/SBA-15 nanocomposites
Trimethylamine (TEA), Pd (0.04 mol%),
ꢂ
1
40 C, 3 h
ꢂ
2
3
4
5
6
PhI/styrene
PhI/styrene
PhI/styrene
PhI/styrene
PhI/styrene
Pd–SBA
TEA, Pd (0.02 mol%), 140 C, 2 h
88%
97%
88%
95%
75%
19
20
21
22
23
ꢂ
Palladium immobilized on silica gel
Pd–SBA-15
SBA-15–TAT–Pd(II)
K
2
3
CO , Pd (0.01 mmol%), 115 C, 8 h
ꢂ
TEA, Pd (0.04 mol%), 120 C, 2 h
TEA, Pd (0.62 mmol%), 120 C, 1 h
ꢂ
PdCl
2
TBAB (n-Bu
4
N + OAc), Pd (0.02 mmol%),
ꢂ
ꢂ
120 C, under sonicated, 25 C, 5 h
ꢂ
7
PhI/styrene
SBA-R/Im-NH
2
$Pd
K
2
3
CO , Pd (0.025 mmol%), 90 C, 40 min
98%
This work
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Direct synthesis of SBA-15 functionalized by chloro and
lipophilic units, SBA-R/Cl
and being concentrated under reduced pressure, the product
was puried by silica gel chromatography using n-hexane/AcOEt
(40 : 1) as an eluent.
For the direct synthesis of SBA-15 functionalized by chloro and
lipophilic units, TEOS was used as a silica source, CPTMS and
CTMS, were used as chloropropyl and lipohilic cetyl groups Conclusions
sources respectively. Typically, 3 g of Pluronic P123 as a struc-
In conclusion, a novel bifunctional lipophilic and basic meso-
porous organosilica supported palladium nanoparticle with
ionic liquid property, SBA-R/Im-NH $Pd was prepared, charac-
terized and applied as effective heterogeneous nanocatalyst in
Heck coupling reaction of aryl halides with olens in aqueous
media. The lipophilic and basic units with ionic liquid property
graed to the SBA produced cavities that stabilize Pd nano-
particle and avoid its aggregation.
The efficiency of this porous inorganic–organic hybrid
nanocomposite as a powerful catalyst has been applied in the
Heck coupling reaction of aryl halides with olens, in aqueous
media. This method has the advantages of high yields, cleaner
reactions, simple methodology, short reaction times, easy
workup, and greener conditions. In addition to the savings in
time, cost, and facility of this methodology, the nanocatalyst
ture directing agent was dissolved in a solution containing 95 g
of HCl (2 M) at room temperature. Then, 6.3 g of TEOS was
ꢂ
2
added, and the mixture was authorized to stir at 40 C for 60
24
min for prehydrolysis. Aerward, (0.487 g, 2.437 mmol) of
CPTMS and (0.478 g, 2.437 mmol) of CTMS were slowly added
into the mixture. The resulting mixture was vigorously stirred at
ꢂ
4
0 C for 20 h. Then, the resulting mixture was aged without
ꢂ
stirring at 90 C for 24 h. The template was extracted from the
SBA-R/Cl channels by a Soxhlet apparatus by using ethanol for
72 h. Thereaer, the mesoporous organosilica, SBA-R/Cl, was
dried at room temperature overnight.
Preparation of bifunctional lipophilic and basic
nanocomposite, SBA-R/Im-NH
2
To a solution of imidazole (0.166 g, 2.437 mmol) in 25 mL of dry can be easily separated from the reaction mixture and reused
toluene, sodium hydride (0.058 g, 2.437 mmol) was added and several times without signicant decrease in activity and
stirred under a nitrogen atmosphere at room temperature for 2 promises economic as well as environmental benets.
25
h to supply imidazole sodium salt. To the solution, SBA-R/Cl
(
3.00 g) was added and the mixture reuxed under a nitrogen
Acknowledgements
atmosphere for 24 h. The resulting nanocomposite was ltered
and washed with ethanol (3 ꢃ 20 mL) and dried under vacuum We are grateful to the Research Council of Shahid Chamran
ꢂ
at 100 C for 8 h.
University for nancial support.
3
To the suspension of the nanocomposite (3 g) in CH CN (25
mL), 2-bromo ethyl amine hydrobromide (0.487 g, 2.437 mmol)
was slowly added and the mixture reuxed at 80 C for 12 h. The
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8
9
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2
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3614.
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2
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SO
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