Pd and Pd-CuO nanoparticles in hollow silicalite-1 single crystals for enhancing selectivity and activity for the Suzuki-Miyaura reaction

Article abstract of DOI:10.1039/c5ra05952f

Pd and Pd-CuO nanoparticles were successfully encapsulated in hollow silicalite-1 single crystals by tetrapropylammonium hydroxide (TPAOH) hydrothermal treatment with an "impregnation-dissolution-recrystallization" process. The size and number of particles in the hollow zeolite depended mainly on the nature of the metal. For palladium, the palladium nanoparticles easily aggregated into larger particles in the hydrothermal process, which displays excellent substrate selectivity for the meta- and para-substituted aryl bromides in the Suzuki-Miyaura reaction. For Pd-CuO binary metals (oxide), introducing copper oxide prevents aggregation of palladium, which shows about 3 times higher activity than encapsulated single Pd catalyst for the above reaction. The strategy using a hollow zeolite crystal as a support is a more reliable method for preparing multi-metallic (oxide) catalysts with well-dispersed nanoparticles.

Full text of DOI:10.1039/c5ra05952f

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Pd and Pd–CuO nanoparticles in hollow silicalite-1
single crystals for enhancing selectivity and activity
for the Suzuki–Miyaura reaction†
a
Cite this: RSC Adv., 2015, 5, 40297
Chengyi Dai,ab Xinmin Li,a Anfeng Zhang,ab Chun Liu, Chunshan Songabc
*
ab
*
and Xinwen Guo
Pd and Pd–CuO nanoparticles were successfully encapsulated in hollow silicalite-1 single crystals by
tetrapropylammonium hydroxide (TPAOH) hydrothermal treatment with an “impregnation-dissolution-
recrystallization” process. The size and number of particles in the hollow zeolite depended mainly on the
nature of the metal. For palladium, the palladium nanoparticles easily aggregated into larger particles in
the hydrothermal process, which displays excellent substrate selectivity for the meta- and para-
substituted aryl bromides in the Suzuki–Miyaura reaction. For Pd–CuO binary metals (oxide), introducing
copper oxide prevents aggregation of palladium, which shows about 3 times higher activity than
encapsulated single Pd catalyst for the above reaction. The strategy using a hollow zeolite crystal as a
support is a more reliable method for preparing multi-metallic (oxide) catalysts with well-dispersed
nanoparticles.
Received 3rd April 2015
Accepted 27th April 2015
DOI: 10.1039/c5ra05952f
www.rsc.org/advances
metal–organic
frameworks
(MOFs),7
organic–inorganic
1 Introduction
composites,8 silica9 and zeolites,10 are used to support Pd
nanoparticles. However, obtaining the target product with high
selectivity and activity remains difficult.
The palladium-catalyzed Suzuki–Miyaura reaction of aryl
halides with arylboronic acids is one of the most important and
powerful tools to form biaryls and has been widely used in the
synthesis of natural products, pharmaceuticals and advanced
functional materials.1 The popularity of this reaction has been
further enhanced by the increasing availability of organoboron
reagents, the low toxicity of the inorganic by-products, and the
tolerance of a broad range of functional groups.2 Because of the
great signicance of this reaction, it was recognized with the
2010 Nobel Prize in Chemistry. The generic palladium-catalyzed
Suzuki–Miyaura reaction employs phosphine ligands, which are
oen water- and/or air-sensitive.3 Although some modied
ligands could work in water, and even in air, the tedious multi-
step synthesis and high cost of these ligands restrict their
applications.4 In recent years, there has been an increasing
interest in developing efficient and green processes for the
nano-palladium catalyzed Suzuki–Miyaura reaction in the
absence of a ligand.5 A variety of supports, such as polymers,6
In the past years, hollow zeolites have attracted considerable
interests due to their well-dened hollow structure, thermal/
mechanical stability, and shape selectivity.11 However, there is
none report on hollow silicalite-1 single crystals as containers of
catalysts for the Suzuki–Miyaura reaction. Herein, using
“impregnation-dissolution-recrystallization” method, Pd and
Pd–CuO nanoparticles located on the surface of solid silicalite-1
were successfully encapsulated in the regular void of the hollow
crystal. To the best of our knowledge, this is the rst report on
the encapsulation of palladium nanoparticles in hollow
silicalite-1 single crystals. This material exhibits excellent cata-
lytic selectivity for the meta- and para-substituted aryl bromides
in the Suzuki–Miyaura reaction under ligand-free and environ-
mentally friendly conditions. In addition, we report, also for the
rst time, Pd–CuO binary metal (oxide) particles displaying
outstanding metal nanoparticle dispersion in the hollow
silicalite-1 single crystals. The binary metal (oxide) particles
show approximately 3 times higher activity than encapsulated
single Pd catalyst for the above reaction.
aState Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian
116024, P. R. China. E-mail: cliu@dlut.edu.cn; Tel: +86-411-84986182
bPSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian
University of Technology, Dalian 116024, P. R. China. E-mail: guoxw@dlut.edu.cn;
Fax: +86-411-84986134; Tel: +86-411-84986133
2 Experimental
2.1 Preparation of silicalite-1 zeolite
cEMS Energy Institute, PSU-DUT Joint Center for Energy Research and Department of
Energy
& Mineral Engineering, Pennsylvania State University, University Park,
Silicalite-1 was synthesized from the clear solution method.
Typically, 15.4 mL of tetraethyl orthosilicate (TEOS) was mixed
with a certain amount of TPAOH solution. The molar composition
Pennsylvania 16802, USA
† Electronic supplementary information (ESI) available: 1H NMR data of products.
See DOI: 10.1039/c5ra05952f
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of the synthesis mixture was 1 TEOS : 0.27 TPAOH : 37 H2O. Aer
X-ray photoelectron spectroscopy (XPS) was conducted on an
being stirred for 3 h at 35 ꢀC, the resultant solution was heated at ESCALAB 250 (Thermo VG Corporation) using Mg Ka radiation
80 ꢀC in order to remove the ethanol generated during the (1253.6 eV, 15 kV, 10 mA, 150 W). The recorded spectra were
hydrolysis of TEOS and then water was added to maintain tted by a least square procedure to a product of Gaussian–
constant volume. Aer crystallization at 170 ꢀC for 3 days, the Lorentzian functions. The concentration of each element was
product was recovered by centrifugation and dried overnight at calculated from the area of the corresponding peak.
100 ꢀC. Finally, the template was removed by calcination in static
air at 540 ꢀC for 6 h.
The elemental analysis of catalysts was carried out on a
Perkin Elmer OPTIMA 2000DV ICP Optical Emission
Spectrometer.
2.2 Preparation of Pd@Hol S-1 and Pd–Cu@Hol S-1
2.4 Catalytic tests
Pd/S-1 was synthesized by the incipient-wetness impregnation
method. In brief, the calcined silicalite-1 was impregnated with
an aqueous solution of H2PdCl4, aer drying overnight at
All commercially available reagents (from Acros, Aldrich, Fluka)
were used without further purication. All reactions were
carried out in air. NMR spectra were recorded on a Brucker
AdvanceII 400 spectrometer using TMS as an internal standard
(400 MHz for 1HNMR). All products were isolated by short
chromatography on a silica gel (200–300 mesh) column using
petroleum ether (60–90 ꢀC). Compounds described in the
literature were characterized by 1H NMR spectra and compared
to reported data.
A mixture of aryl bromides (0.25 mmol), phenylboronic acid
(0.375 mmol), K2CO3 (0.5 mmol), catalysts (1.6 mol%), and
EtOH/H2O (1 mL/1 mL) was stirred at 80 C in air for the indi-
cated time. The reaction mixture was added to brine (15 mL)
and extracted with ethyl acetate (4 ꢁ 10 mL). The solvent was
concentrated under vacuum, and the product was isolated by
short column chromatography on silica gel.
ꢀ
100 ꢀC; the product was calcined in static air at 500 C for 4 h.
The Pd loading on the Pd/S-1 was 0.97 wt%, as measured by
inductively coupled plasma (ICP) mass spectrometry.
The as-prepared Pd/S-1 was treated with 0.3 M TPAOH
(20 mL of solution per gram of zeolite) at 170 ꢀC for 72 h, aer
dried overnight at 100 ꢀC and calcined in static air at 500 ꢀC
for 4 h, the Pd@Hol S-1 was obtained. The Pd loading on
the Pd@Hol S-1 was 1.3 wt%, as measured by ICP mass
spectrometry.
ꢀ
Pd–CuO binary metals (oxide) encapsulated in the hollow S-1
were prepared by the similar method with single metal, which
used H2PdCl4 and CuCl2 as Pd and Cu source, respectively, and
co-impregnation method to synthesize Pd–CuO/S-1. The as-
prepared Pd–CuO/S-1 was treated with 0.3 M TPAOH (20 mL
ꢀ
of solution per gram of zeolite) at 170 C for 72 h, aer drying
overnight at 100 ꢀC and calcining in static air at 500 ꢀC for 4 h,
the Pd–CuO@Hol S-1 was obtained. The Pd and Cu loading on
the Pd–CuO@Hol S-1 was 1.2 and 0.82 wt%, respectively, as
measured by ICP mass spectrometry.
3 Results and discussion
Fig. 1a shows a typical TEM image of the prepared Pd/S-1, which
clearly exhibits that palladium nanoparticles are located on the
crystal surface. Aer TPAOH treatment, a large regular hollow
void was created in the interior of the silicalite-1 and the
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a thickness of the shell was about 20 nm. The zeolite nanocubes
Rigaku Smartlab diffractometer using a nickel-ltered CuKa X- were still single-crystals and the palladium particles with 11.1
ray source at a scanning rate of 0.02ꢀ over the range between 5ꢀ nm in average particle size were encapsulated in the large cavity
and 80ꢀ.
(Fig. 1b). Fig. 1c and d are images of this sample with high
Transmission electron microscopy (TEM) images were taken resolution and the zeolite lattice fringes align clearly parallel to
on a Tecnai G2 20 S-twin instrument (FEI Company) with an each other, conrming the nanocubes are still single-crystals.
acceleration voltage of 200 kV. The samples for TEM analysis From Fig. 1d, it can be seen that the lattice fringes are not
were prepared by dipping the carbon-coated copper grids into aligned parallel to each other, conrming the Pd particle is
ethanol solutions of the samples and drying at ambient agglomerated from smaller clusters. The lattice spaces are
condition.
measured to be 0.194 and 0.225 nm, which matches the values
Ar isotherms were measured in a Quantachrome autosorb- of the d-spacing of Pd (200) and Pd (111), respectively (JCPDS:
iQ2 gas adsorption analyzer at 87 K. Prior to the measure- 46-1043). The energy-dispersive X-ray spectrometry (EDX)
ment, the samples were degassed in vacuum at 300 ꢀC for 10 h. elemental analysis along the line shown in Fig. 1e shows the
The Brunauer–Emmett–Teller (BET) method was applied to core rich in Pd and the shell rich in Si and O, thereby con-
calculate the total surface area (SBET), while the t-plot method rming that palladium is mainly located in the core of hollow
was used to discriminate between micro- and meso-porosity. In silicalite-1 (Fig. 1f).
the t-plot, the reported mesopore surface area (Smeso) consists of
contributions from the outer surface of the particles as well as During TPAOH treatment, the silicate oligomers are leached
mesopores and macropores. from interior of the crystal and recrystallize at the surface of
The Pd@Hol S-1 formation process is shown in Fig. 2.
Scanning electron microscopy (SEM) images were obtained silicalite-1 crystal. In this process, the palladium particles move
on a Hitachi S-5500 instrument with an acceleration voltage of from surface of the crystal to the interior of cavity, just like the
3 kV. Some samples were sputtered with a thin lm of gold.
process of cell phagocytosis (Fig. 2 step I and step II). During the
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sizes of nanocubes are larger than that of the parent samples,
and their shells were not damaged (Fig. 3).
The Ar adsorption–desorption isotherms of Pd@Hol S-1
show the presence of a H2 hysteresis loop with an abrupt step
around p/p0 ¼ 0.45 in the desorption branch (Fig. 4 inset). This
shows that during the TPAOH treatment, most of the hollow
structure of the zeolite is preserved, which is consistent with the
TEM and SEM results. The micropore size distributions derived
from the argon adsorption isotherms indeed conrm that the
original micropore size is not affected during the alkaline
treatment (Fig. 4). X-ray photoelectron spectroscopy (XPS)
reveals that Pd/S-1 exhibits two energy bands at 341.0 and 335.7
eV, which are values for the Pd 3d3/2 and 3d5/2 electrons of
metallic Pd0, while the peaks around 342.4 and 337.1 eV
correspond to Pd2+ species (Fig. 5). Compared to the Pd/S-1
sample, the Pd0 content of Pd@Hol S-1 was increased from
24.9% to 41.3%.
In situ XRD and TEM were used to investigate the thermal
stability of Pd@Hol S-1 (Fig. 6 and 7). The diffraction peaks were
ꢀ
almost unchanged when the temperature rose to 800 C, sug-
gesting that the MFI framework structure of hollow S-1 was not
destroyed and the Pd nanoparticles were not sintered during
heating. Fig. 7 shows TEM images of Pd/S-1 and Pd@Hol S-1
Fig. 1 TEM images of prepared Pd/S-1 (a) and Pd@Hol S-1 (b–d); (e)
STEM image of Pd@Hol S-1; and (f) EDX analysis along the line shown
in (e).
Fig. 3 SEM images of prepared Pd/S-1 (a) and Pd@Hol S-1 (b).
Fig. 2 Pd@Hol S-1 formation process.
Fig. 4 Ar adsorption and desorption isotherms at ꢂ186 ꢀC (inset) and
pore size distributions of Pd/S-1 and Pd@Hol S-1. The isotherms of
encapsulation process, the palladium nanoparticles aggregated
into larger particles (Fig. 2 step III, Fig. 1b). The SEM images of
Pd@Hol S-1 have been shifted upward by 150 cm3 ꢂ1. The pore size
g
distributions were determined by non-local density functional theory
Pd/S-1 and Pd@Hol S-1 show that, aer alkaline treatment, the (NLDFT).
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Table 1 The results of catalyzed Suzuki–Miyaura reactiona
Entry Ar–Br
Product
Time (h) Yieldb (%)
1
2
1.5
1.0
96
96c
3
4
5
1.0
2.0
4.0
95
96
93
Fig. 5 Pd 3d XPS spectra for Pd/S-1 and Pd@Hol S-1.
6
2.0
2.0
2.0
2.0
2.0
2.0
0.5
2.0
97
0
7
8
32c
49d
93e
0
9
10
11
12
13
96d
0
Fig. 6 In site XRD patterns of Pd@Hol S-1.
14
15
16
0.5
2.0
96d
0
0.5
0.5
94d
96f
Fig. 7 TEM images of Pd/S-1 (a) and Pd@Hol S-1 (b) after calcinate in
static air at 800 ꢀC for 30 min.
17
a
Reaction conditions: aryl bromide (0.25 mmol), phenylboronic acid
(0.375 mmol), Pd@Hol S-1 (1.6 mol%), K2CO3 (0.5 mmol), EtOH/H2O
ꢀ
aer calcination in static air at 800 C for 30 min. For Pd/S-1,
b
c
d
(1 mL/1 mL), under air. Isolated yields. Pd/S-1 (2 mol%). 5% dry
palladium particles grow signicantly compared to the sample
aer calcination in static air at 500 ꢀC for 4 h (Fig. 1a). For
Pd@Hol S-1, the particle sizes of palladium were almost
Pd/C (1 mol%). e Pd(OAc)2 (1 mol%). f Pd–CuO@Hol S-1 (2 mol%).
40300 | RSC Adv., 2015, 5, 40297–40302
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