Preparation of siloxene nanosheet-supported palladium as sustainable catalyst for Mizoroki-Heck reaction

Article abstract of DOI:10.1002/aoc.3221

Siloxene nanosheets were successfully modified with palladium nanoparticles by reducing palladium chloride with hydrazine hydrate. The palladium nanoparticles-siloxene nanosheets as a catalyst for the Mizoroki-Heck reaction exhibited high activity, recoverability and stability. The structural morphology of the catalyst was investigated using transmission electron microscopy. High efficiency of the catalyst was proved in the Mizoroki-Heck reaction after five catalytic recycles.

Full text of DOI:10.1002/aoc.3221

Full Paper
Received: 14 June 2014
Revised: 31 July 2014
Accepted: 24 August 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3221
Preparation of siloxene nanosheet-supported
palladium as sustainable catalyst for
Mizoroki–Heck reaction
Fei-Bao Zhang*, Su-Fang Lv, Jian-Xiong Jiang and Yong Ni*
Siloxene nanosheets were successfully modified with palladium nanoparticles by reducing palladium chloride with
hydrazine hydrate. The palladium nanoparticles–siloxene nanosheets as a catalyst for the Mizoroki–Heck reaction
exhibited high activity, recoverability and stability. The structural morphology of the catalyst was investigated using
transmission electron microscopy. High efficiency of the catalyst was proved in the Mizoroki–Heck reaction after five
catalytic recycles. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: siloxene nanosheets; Mizoroki–Heck; catalyst; silane coupling agent
HCl (100 ml, 37%) was stirred at 273 K for 2 days under an
argon atmosphere. The products were filtered and washed
Introduction
Siloxene nanosheets (SNS) have attracted increasing attention in
fundamental chemistry and physics mainly for their electronic
properties and photophysics properties arising from the
with methanol, followed by the residue being dried under
vacuum for 2 days at 373 K to afford SNS (0.48 g, 80%) as a
gray solid.
[
1–4]
graphene-like honeycomb lattice structure or buckled sheets.
SNS can be applied in photovoltaic cells, nonvolatile memory,
lighting, ultrasonic generators and biosensors with solution
Synthesis of functionalized SNS (AS-SNS)
[5–13]
Approximately 1 g of (3-aminopropyl)triethoxysilane (APTS)
was first dispersed in 500 ml of a mixture of water and
ethanol (2:8 v/v). Around 200 mg of SNS was introduced to
this mixture with ultrasonication for 1 h, and then the mixture
was heated to 343 K in a water bath and refluxed for 4 h. The
solid product was collected by centrifugation and washed
three times using the same mixture of water–ethanol to
remove the residual coupling agent. After vacuum drying
overnight, the functionalized SNS (AS-SNS) were obtained.
processibility
because of their good chemical versatility and
tunability. They might be promising candidates for a catalyst or a
catalyst support due to the high specific surface area and surface
tunability. Therefore, considering their low cost, large surface area,
stability in air and ease of modification to load metal clusters, SNS
[
14–16]
may be a promising material for catalytic applications.
Recently, various organic reactions catalyzed by nanosheet-based
[
17–20]
nanomaterials have been reported.
However, there are few
reports of metal–SNS hybrids, catalytic properties and catalyst
supports of SNS.
In the work reported in this paper, we developed a simple proce-
dure to generate SNS-supported Pd nanoparticles to be used as a sus-
tainable heterogeneous Pd catalyst for Mizoroki–Heck reactions. The
results were still promising after the catalyst was recycled for ten runs.
Preparation of Pd/AS-SNS nanocomposites
First, ethanol solutions of AS-SNS were sonicated for 1 h in
deionized water, and then a certain amount of PdCl dissolved
2
in acidified water (0.05 M HCl) was added at 323 K with magnetic
stirring. Subsequently, hydrazinium hydrate was added and
stirred for another 3 h. The composites were filtered using a
Millipore membrane (nylon, 0.45 mm, 47mm), washed with
ethanol, dried under vacuum. They are denoted as Pd-ASSN-2.5
Experimental
Materials
(400mg, 2.5 wt% Pd, thermo-AAS analysis).
2
CaSi was purchased from Alfa Aesar China Chemical Co. Palladium
For comparison, Pd-ASSN-10 (10 wt%), Pd-ASSN-5 (5 wt%)
and Pd-ASSN-1 (1 wt%) were also prepared according to the
same procedure.
chloride and hydrazinium hydrate were purchased from Aladdin
China Chemical Co. All reagents were used without further
purification.
*
Correspondence to: Fei-Bao Zhang and Yong Ni, Key Laboratory of Organosilicon
Chemistry and Material Technology of Ministry of Education, Hangzhou Normal
University, Hangzhou 310012, China. E-mail: feibaozhang@126.com
Catalyst Preparation
6 3 3
Preparation of starting SNS (Si H (OH) )
Si H (OH) was prepared according to a method reported in
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry
of Education, Hangzhou Normal University, Hangzhou 310012, China
6
3
3
[
21,22]
the literature.
A mixture of CaSi2 (1 g, 10.4 mmol) and
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

F.-B. Zhang et al.
Catalytic Tests and Recycling
and two other Si―OH groups either undergo a condensation reac-
tion with further Si―OH or can be in a free state and form hydro-
gen bonds. When surface OH groups on nanosheets react with
the APTS coupling agent, γ-aminopropyl functional groups are in-
troduced onto the nanosheets and they produce a thin organic
shell on the surface of the nanosheets which can interact with pal-
ladium and reduce the agglomeration of nanoparticles (Scheme 1).
Aryl halide (0.5 mmol), ethyl acrylate (0.6 mmol), triethylamine
(1.0 mmol) and Pd-ASSN-10 (0.005 mmol Pd) were placed in a
Schlenk tube equipped with a magnetic stirring bar. DMF (2 ml)
was added to the tube and the reaction mixture was stirred at
383 K under nitrogen atmosphere. The reaction was monitored using
TLC. After complete consumption of the starting material, the
reaction was stopped, cooled to room temperature, separated and
purified using column chromatography with silica gel (SiO
2
, ethyl
TEM Analysis
acetate–hexane). The product was dried under vacuum, weighed
and characterized using NMR spectroscopy. Product yields from
the Heck reaction were determined using column chromatography.
A similar procedure was employed for other reactions (Table 3).
To investigate catalyst recyclability, the reaction was repeated
with 4-iodotoluene as the substrate on a 2.0mmol scale maintain-
ing the same conditions as described above, except using the
recovered catalyst. After the reaction was completed, the solid
particles were collected by centrifugation, washed with DMF,
ethanol and water, and dried under vacuum. The dried catalyst
was reused for further catalytic runs.
The morphologies of SNS and Pd/SNS composite were investigated
using TEM (as shown in Fig. 1). Clearly, it can be seen that the sheets
are almost transparent with flake-like shapes which indicates suc-
cessful exfoliation, leading to an ultrathin morphology (Fig. 1(a)).
AS-SNS shows no obvious difference in shape and structure from
SNS (Fig. 1(b)). After surface functionalization via silylation of SNS
with organosilane, the immobilized functional groups bonded onto
the surface of SNS act as anchors for metal adsorption. Hydrophilic
2
amino groups (―NH ) show great affinity for the aqueous palla-
dium chloride, and therefore facilitate the deposition of metal
precursor and the formation of small and highly dispersed Pd nano-
particles on the nanosheets. From Fig. 1(c, d) it can be seen that Pd
nanoparticles are well dispersed on the surface of SNS with a diam-
eter of 2–5 nm. Size-similar and highly dispersed Pd nanoparticles
can be synthesized easily with this method. For comparison, Pd
nanoparticles supported on unmodified SNS were also prepared
using a similar method, as shown in Fig. 1(e, f).
Characterization
1
13
H NMR and C NMR spectra were obtained with a Bruker
Avance 400 instrument and chemical shifts were recorded
downfield of tetramethylsilane. Transmission electron micros-
copy (TEM; Hitachi 600, Japan) was used to observe the morphol-
ogy and degree of agglomeration. GC-MS analyses were
performed using an Agilent 26890 N/59731 equipped with a
DB-5 column (30 m × 2.5 mm × 0.25 μm).
Catalytic Activity of Pd/SNS in Heck Coupling Reaction
The Heck coupling reaction was selected to evaluate the catalytic
performance of the Pd-ASSN catalyst (Table 1). In our study,
4
-iodotoluene with n-butyl acrylate was chosen as benchmark
Results and Discussion
substrate in the coupling model reaction. Initially, the influence of
various Pd metal concentrations on the coupling was investigated.
In the model reaction, catalysts of Pd supported on modified SNS
(Table 1, entries 1–4), Pd supported on unmodified SNS (entry 5)
and Pd(OAc) (entry 6) were all tested under same reaction condi-
tion. Performing the reaction in the presence of Pd-ASSN-2.5 leads
to a satisfactory yield of 95% (Table 1, entry 2).
In the Heck reaction, iodobenzene and n-butyl acrylate
were initially selected as test substrate to examine the
catalytic activity of the supported Pd materials. The reaction
conditions were optimized, and the results are presented in Table 2.
It is found that the best system for this reaction is DMF as solvent in
combination with triethylamine as base, which gives a 99%
conversion of iodobenzene at 90°C within 12 h (entry 5).
[
21]
According to the reported method, the starting Weiss SNS, Si6H3
OH) , were prepared with Zintl phase CaSi , a layered material with
(
3
2
Si corrugated (111) layers linked by Ca ions. The structure of the
siloxene has been reported to consist of Si (111) layers terminated
above and below by OH groups and H atoms.
face OH groups present on the nanosheets, it is easy for SNS to
readily agglomerate in organic solvent. Coupling agent molecules
can be adsorbed on the surface of the nanosheets by their
hydrophilic end and can react with the surface OH groups on the
nanosheets (as shown in Scheme 1) The APTS coupling agent
2
[
1,21]
Due to the sur-
2 5
undergoes hydrolysis and the Si―OC H group transforms into
Si―OH. One Si―OH group of the coupling agent reacts with the
hydroxyl groups of the siloxene surface and forms a covalent bond,
Scheme 1. Process of formation of Pd-ASSN composite materials.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2014)

Siloxene nanosheet-supported Pd catalyst for Mizoroki–Heck reaction
Figure 1. TEM images of as-prepared materials: (a) SNS; (b) AS-SNS; (c, d) Pd-ASSN-2.5 (inset in (c) shows a statistical size distribution of Pd nanoparticles); (e, f)
Pd/SNS.
To generalize the application of the catalyst, the optimized condi-
tions were employed in Heck reactions of n-butyl acrylate or styrene
with various aryl halides (Table 3). The reactions proceed well with a
wide range of aryl iodides and bromides with electron-donating
and electron-withdrawing groups or without. For most cases, the re-
action is completed in 12–32h with good yields.
Catalyst Recovery and Heterogeneity Tests
Since the recyclability of a catalyst is very important for practical
applications, the recovery and reusability of the catalyst under
investigation were tested in the Heck reaction of iodobenzene
with n-butyl acrylate under the optimized reaction conditions
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

F.-B. Zhang et al.
Table 1. Various catalyst formulations in the model reaction of Heck
a
coupling
100
b
Entry
Catalyst
Pd (wt%)
Yield (%)
1
2
3
4
5
6
Pd-ASSN-1
Pd-ASSN-2.5
Pd-ASSN-5
Pd-ASSN-10
Pd-SN-5
1
2.5
5
90
95
91
89
83
72
8
0
10
5
60
40
Pd(OAc)
2
a
Reaction scale: 1 mmol of substrates.
b
Determined using GC.
0
2
4
6
8
10
Runs
Figure 2. Long-term stability of Pd-ASSN-2.5 catalyst.
Table 2. Optimization of Heck coupling reaction in the presence of
Pd-ASSN-5
a
(Scheme 2). When the reaction was completed, the mixture was
centrifuged and the liquid was poured out. The catalyst was col-
lected and washed with acetone and ethanol, and dried under
vacuum. Then, the recovered catalyst was reused in the same
Heck reaction for ten runs. The catalyst maintained a high activ-
ity even in the fifth cycle under optimized conditions (95%). The
EntrySolvent
Base
Temperature Time
Yield
(%)
b
(°C)
(h)
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
NaAc
Na CO
90
90
90
90
90
90
90
90
12
12
12
12
12
12
12
12
90
95
2
3
n-Butylamine
Ethylenediamine
Triethylamine
Triethylamine
Triethylamine
Triethylamine
78
results are shown in Fig. 2. Compared with the activity of Pd
80
[23–25]
nanoparticles supported on other nanosheets,
the activity
≥99
—
of Pd/SNS is not the highest. However, we find that the activity
of Pd/SNS still remains high after ten catalytic cycles. Moreover,
Pd/SNS is a cheap and easily obtained carrier. Therefore, we
can foresee Pd/SNS being a promising catalyst support material.
c
2
H O
Ethanol
Diethyl
ether
65
c
—
c
9
1
THF
Triethylamine
Triethylamine
90
90
12
12
—
c
0
Acetone
—
Conclusions
a
Reaction scale: 1 mmol of substrates.
b
Determined using GC.
No reaction.
We have introduced a new catalyst support of SNS obtained from the
layered silicon compound CaSi . When modified using a silane cou-
2
c
pling agent, palladium nanoparticles could be highly dispersed on
the surface of the nanosheets, and the catalytic properties of the
resulting material were tested in the Mizoroki–Heck reaction. High
efficiency and stability of this catalytic system was shown for Heck
coupling reactions with a number of aryl halides. This study suggests
that SNS could be a promising support in heterogeneous catalysis.
Table 3. Catalytic performance of Pd-ASSN-5
Entry
X
R
1
R
2
Reaction
time
Reaction
temperature Yield
(h)
(°C)
(%)
Acknowledgements
1
2
3
4
5
6
7
8
9
I
I
I
I
H
COOC
COOC
COOC
COOC
COOC
COOC
COOC
Ph
H
4 9
H
4 9
H
4 9
H
4 9
H
4 9
H
4 9
H
4 9
24
12
12
24
24
24
32
32
24
36
36
24
24
80
92
99
98
88
96
75
—
92
97
95
72
96
56
CH
NO
OCH
3
90
90
This research was supported by Zhejiang Provincial Natural Science
Foundation of China under Grant No. Y4100131 and the Scientific
Research Fund of Dunhuang Academy under Grant No. 200808.
2
3
90
Br
Br
Cl
I
NO
CN
H
2
110
110
120
90
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H
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