Silica-dendrimer core-shell microspheres with encapsulated ultrasmall palladium nanoparticles: Efficient and easily recyclable heterogeneous nanocatalysts

Article abstract of DOI:10.1021/la203066d

We report the synthesis, characterization, and catalytic properties of novel monodisperse SiO₂@Pd-PAMAM core-shell microspheres containing SiO₂ microsphere cores and PAMAM dendrimer-encapsulated Pd nanoparticle (Pd-PAMAM) shells. First, SiO₂ microspheres, which were prepared by the Stoeber method, were functionalized with vinyl groups by grafting their surfaces with vinyltriethoxysilane (VTS). The vinyl groups were then converted into epoxides by using m-chloroperoxybenzoic acid. Upon treatment with amine-terminated G4 poly(amidoamine) (PAMAM) dendrimers, the SiO ₂-supported epoxides underwent ring-opening and gave SiO ₂@PAMAM core-shell microspheres. Pd nanoparticles within the cores of the SiO₂-supported PAMAM dendrimers were synthesized by letting Pd(II) ions complex with the amine groups in the cores of the dendrimers and then reducing them into Pd(0) with NaBH₄. This produced the SiO ₂@Pd-PAMAM core-shell microspheres. The presence of the different functional groups on the materials was monitored by following the changes in FTIR spectra, elemental analyses, and weight losses on thermogravimetric traces. Transmission electron microscopy (TEM) images showed the presence of Pd nanoparticles with average size of 1.56 ± 0.67 nm on the surface of the monodisperse SiO₂@Pd-PAMAM core-shell microspheres. The SiO ₂@Pd-PAMAM core-shell microspheres were successfully used as an easily recyclable catalyst for hydrogenation of various olefins, alkynes, keto, and nitro groups, giving ～100% conversion and high turnover numbers (TONs) under 10 bar H₂ pressure, at room temperature and in times ranging from 10 min to 3 h. In addition, the SiO₂@Pd-PAMAM core-shell microspheres were proven to be recyclable catalysts up to five times with barely any leaching of palladium into the reaction mixture.

Full text of DOI:10.1021/la203066d

ARTICLE
pubs.acs.org/Langmuir
SilicaÀDendrimer CoreÀShell Microspheres with Encapsulated
Ultrasmall Palladium Nanoparticles: Efficient and Easily Recyclable
Heterogeneous Nanocatalysts
Ankush V. Biradar,†,‡ Archana A. Biradar,†,‡ and Tewodros Asefa*,†,‡
†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway,
New Jersey 08854, United States
‡Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway,
New Jersey 08854, United States
S Supporting Information
b
ABSTRACT: We report the synthesis, characterization, and catalytic properties of
novel monodisperse SiO2@Pd-PAMAM coreÀshell microspheres containing
SiO2 microsphere cores and PAMAM dendrimer-encapsulated Pd nanoparticle
(Pd-PAMAM) shells. First, SiO2 microspheres, which were prepared by the
St€ober method, were functionalized with vinyl groups by grafting their surfaces
with vinyltriethoxysilane (VTS). The vinyl groups were then converted into
epoxides by using m-chloroperoxybenzoic acid. Upon treatment with amine-
terminated G4 poly(amidoamine) (PAMAM) dendrimers, the SiO2-supported
epoxides underwent ring-opening and gave SiO2@PAMAM coreÀshell micro-
spheres. Pd nanoparticles within the cores of the SiO2-supported PAMAM
dendrimers were synthesized by letting Pd(II) ions complex with the amine groups
inthe cores ofthedendrimersand then reducing them into Pd(0) withNaBH4. This
produced the SiO2@Pd-PAMAM coreÀshell microspheres. The presence of the
different functional groups on the materials was monitored by following the changes in FTIR spectra, elemental analyses, and weight
losses on thermogravimetric traces. Transmission electron microscopy (TEM) images showed the presence of Pd nanoparticles with
average size of 1.56 ( 0.67 nm on the surface of the monodisperse SiO2@Pd-PAMAM coreÀshell microspheres. The SiO2@Pd-
PAMAM coreÀshell microspheres were successfully used as an easily recyclable catalyst for hydrogenation of various olefins, alkynes,
keto, and nitro groups, giving ∼100% conversion and high turnover numbers (TONs) under 10 bar H2 pressure, at room temperature
and in times ranging from 10 min to 3 h. In addition, the SiO2@Pd-PAMAM coreÀshell microspheres were proven to be recyclable
catalysts up to five times with barely any leaching of palladium into the reaction mixture.
the very sites on the metals where catalysis takes place will be
covered by these surface passivating organic groups. The degree
of passivation of the surface of metallic nanoparticles can become
more optimized for catalysis with dendrimers as stabilizing
agents. This is because the branching and porous structures of
dendrimers will inhibit them from forming a well-packed self-
assembled monolayer (SAM)-like structure that alkanethiol and
alkylamine groups typically form around the metallic nanoparti-
cles. Thus, dendrimer-encapsulated metallic nanoparticles can
exhibit better catalytic activities compared to other types of
ligand-stabilized nanoparticles.
1. INTRODUCTION
The use of metal nanoparticles (MNPs) in catalysis has rapidly
increased in recent years because of their efficient and intrinsic
size-dependent catalytic properties as well as their ability to
catalyze a range of chemical reactions.1À8 For many MNPs to
catalyze reactions or result in efficient catalysis, the reacting
substrates must directly interact with the metal surfaces. This
metalÀsubstrate interaction would be greater if the MNPs were
synthesized “naked”. Unfortunately, however, atoms of “naked”
MNPs have a greater tendency to aggregate into a bulk material
due to their high surface energies, which results in loss of, or
decrease in, their intrinsic catalytic activity and selectivity over
time.9 In particular, Pd nanoparticles (PdNPs), which are well-
known for their catalytic activities, can easily aggregate to form
Pd-black because of the very high surface energy of palladium.10
Although the degree of aggregation of PdNP or other MNP
catalysts can be overcome or minimized by passivating the
metals’ surfaces with organic ligands,11 this too will, unfortu-
nately, be accompanied by the loss of catalytic activity because
Since the first reports by the groups of Crooks,12 Tomalia,13
and Esumi,14 dendrimers have indeed been successfully used to
stabilize a number of MNPs including PdNPs.15 Furthermore,
the resulting dendrimer-encapsulated nanoparticles (DENs)2,16
Received: April 6, 2011
Revised:
September 16, 2011
Published: September 27, 2011
r
2011 American Chemical Society
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were shown to have various applications in sensing, molecular
electronics, photonics, and nanomedicine.17 In addition, with the
pioneering works of Crooks and co-workers, DENs consisting of
transition metal nanoparticles within their cores12b were shown
to serve as stable nanocatalysts. DEN catalysts, also known as
dendrimer-encapsulated catalysts (DECs), have branching struc-
tures that allow reactants to diffuse in and reach the entrapped
nanoparticle catalytic sites while at the same time stabilize the
nanoparticles from aggregation. For instance, Mizugaki et al.
recently demonstrated the synthesis and catalytic properties of
PAMAM dendron-stabilized PdNPs for hydrogenation of dienes
and acetylenes to monoenes.18 In another example, Ooe et al.
synthesized triethoxybenzamide-terminated poly(propyleneimine)
(PPI) dendrimer-stabilized PdNPs and showed their use as sub-
strate specific catalysts for hydrogenation reactions.19
dendrons onto the surfaces of the channels of mesoporous silicas
were achieved using a different synthetic strategy as reported by
Jiang and Gao,30 Wang et al.,31 and Reynhardt et al.32 In these cases,
the dendrimers were synthesized in situ within amine-functionalized
SBA-15 by multistep procedures involving the Michael-type addi-
tion reaction between the amino groups of the material and methyl
acrylate, followed by amidation of the resulting ester groups with
ethylenediamine. Furthermore, Jiang and Gao30 used the covalently
supported dendrimers to synthesize PdNPs in situ for catalysis.
However, this material also has the DENs within the “small”
cylindrical channel pores of SBA-15, possibly limiting the reactants
access to all of the DENs. Covalent attachment of dendrimers on a
more accessible outer surface of relatively polydisperse and small
size (average size ∼40 nm) amorphous silica nanoparticles was
demonstrated recently by Hagiwara et al.33 The resulting supported
PAMAMs were used to anchor a metal complex (palladium(II)
acetate), and ionic liquids noncovalently for the SuzukiÀMiyaura
reaction. There have been other examples on silica-supported
dendrimers containing metal complexes for heterogeneous catalysis
of various reactions, including some of the pioneering works in this
area by the groups of Alper and Sayari.32b,34 A nice review article
in this area, particularly on palladium-containing dendrimers and
dendrons, has also recently been published by Andrꢀes et al.29
Other notable examples on supported DECs include the
works of Chandler35 and Crooks36 groups. In these cases, metal
oxides such as silica, alumina, and titania were used either to
support or to encapsulate DENs. The dendrimer shells were then
removed by high-temperature treatment during catalyst activa-
tion, leaving metal oxide-supported MNPs or catalysts. In a
related but slightly different approach, Knecht and Wright used
the solÀgel process to encapsulate DENs within silica shell using
silicic acid.37 This resulted in ∼80 nm polydisperse silica nano-
particles containing several randomly distributed DENs composed
of PAMAM-entrapped Au nanoparticles or quantum dots within
the silica nanoparticles. Many of the DENs in the resulting materials
appeared to be visibly buried within a thick layer of silica as seen
from their reported transmission electron microscopy (TEM)
images.37 The accessibility of the DENs in these materials to external
probe molecules or reactants for catalysis is not clear.
Herein we report on the synthesis and highly efficient recycl-
able catalytic properties of novel SiO2@Pd-PAMAM coreÀshell
microspheres consisting of etched silica microsphere cores and
highly accessible DEN shells. The efficient catalytic activities of
the SiO2@Pd-PAMAM coreÀshell microspheres are confirmed
by their ability to catalyze the hydrogenation of various olefins,
alkynes, keto, and nitro groups, yielding ∼100% conversion with
high turnover numbers (TONs) under 10 bar H2 pressure, at
room temperature and in reaction times ranging from 10 min to
3 h. Furthermore, the microspheres are shown to be recyclable
catalysts in up to five cycles with barely losing their catalytic
activities. The SiO2@Pd-PAMAM coreÀshell microspheres’ high
catalytic activity is possibly the result of the accessibility of the DECs
or PdNPs on the outer surface in the coreÀshell nanostructure as
well as the stability of the PdNPs. The stability and ease of
recyclability of the SiO2@Pd-PAMAM coreÀshell microspheres
as catalysts are most likely due to the attachment of the DECs on
relatively bigger etched SiO2 microspheres via covalent bonds.
Although the synthesis and catalytic activities of various DECs
have been successfully demonstrated, dendrimers, even those
with many generations, are relatively too small to easily separate
from solutions and reuse after catalytic reactions—a feature
that is often sought for “true” heterogeneous catalysts.20 Thus,
research in the past several years has also focused on developing
easier ways for recycling DECs. Solvent precipitation21 and
membrane filtration22 were among the common methods con-
sidered to separate and recycle DECs; however, they were shown
to result in some losses of the materials as well as their catalytic
activities. Surface modification of the surfaces of DECs by
multi-step chemical reactions in order to tune the DECs’ solubi-
lity and help their separation from solutions have also been
reported.23 Similarly, dialysis was used to separate dendrimers
from solutions.24 However, both surface modification and dia-
lysis involve time-intensive processes and relatively costly re-
agents and membranes. Although separation of dendrimers by
electrophoresis and chromatography is also possible, it is com-
plicated by interaction of dendrimers with the stationary support
materials, leading to the retention and incomplete separation of
the dendrimers.25 In another work, Crooks group26 used selec-
tive solution phase extraction to separate a mixture of DENs
containing Ag and Au nanoparticles from each other from an
aqueous solution. Similarly, the group led by Baker reported the
separation of different generations (G1ÀG9) of amino-, acet-
amide-, hydroxyl-, and carboxylate-terminated dendrimers using
reversed-phase HPLC.27 Although these two research groups
showed elegant methods for separating a mixture of two or more
types of dendrimers (or DENs), analyzing them as well as
extracting the entrapped MNPs from within the DENs, the
methods have limited applications for recycling DECs in catalysis.
Thus, in order to overcome these inherent difficulties in
separation and recycling of DECs from solutions, heterogenizing
DECs on solid support materials has been receiving increased
attention. Recently, Somorjai and Toste groups have introduced
a synthetic strategy that involves the immobilization of Gn-
PAMAM dendrimer-based DECs containing Pt nanoparticles
(PtNPs) within the pores of mesoporous material, SBA-15.28
The resulting material, labeled as Gn-PAMAM-SBA-15, was
shown to combine the advantages of both “homogeneous” and
“heterogeneous” catalysts and exhibited the “true” features of
heterogeneous catalysts.20 However, the long cylindrical channel
pores of SBA-15 mesoporous silica support in which the DECs
are encapsulated still pose some limitations for reactants to reach
all the PtNPs (or the DECs). Moreover, the DECs in this case are
noncovalently immobilized in the support material; thus, they are
more likely to leach out.29 Covalent attachment of dendrimers or
2. EXPERIMENTAL SECTION
Chemicals and Reagents. Anhydrous ethanol, aqueous ammo-
nium hydroxide solution (28%), tetraethyl orthosilicate (TEOS, 98%),
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Pd/C (1%), sodium borohydride (NaBH4), m-chloroperoxybenzoic
acid, styrene, phenylacetylene, potassium hydroxide, cyclohexene, n-hexene,
acetone, acetyleacetonate, styrene oxide, 4-chloroaniline, trans-stilbene,
4-nitrostyrene, acetonitrile, methanol, and sodium tetrachloropalladate(II)
(Na2PdCl4) were obtained from Sigma-Aldrich. Palladium(II) acetylaceto-
nate (99%) was purchased from Strem Chemicals, Inc. Methylene chloride
and hydrochloric acid were obtained from Fisher Scientific. Poly-
(amidoamine) (PAMAM) dendrimers (G4) as 10À25% in methanol
was obtained from Dendritech NanoTechnologies, Inc., Midland, MI.
Synthesis of Silica Microspheres. The synthesis of the spherical
silica microspheres was carried out by following previously reported
methods.38,39 In a 250 mL plastic bottle, aqueous ammonia solution
(10 mL), ethanol (100 mL), and distilled water (3.6 mL) were added
and stirred vigorously for 2 min. Then, TEOS (5.84 g) was added into
the solution under moderate stirring to hydrolyze TEOS. After 3 h of
stirring, the solution was centrifuged and the supernatant was
decanted carefully. The precipitate was washed three times with
3 Â 40 mL of 1:1 ethanol/water solution by sonication, centrifuga-
tion, and decantation. Finally, the precipitate was dispersed by
sonication in 40 mL 1:1 ethanol/water solution. This resulted in
450 nm silica microspheres.
Synthesis of Etched Silica Microsphere Cores (SiO2). The
450 nm silica microspheres synthesized above were etched with 0.01 M
aqueous potassium hydroxide (KOH) solution. This is performed to
create nanocorrugated surface and increase the microspheres’ surface
area and consequently help the microspheres anchor the PAMAM
dendrimers more effectively. Typically, 250 mg of the silica micro-
spheres was suspended in 0.01 M KOH solution (100 mL) and stirred
for 2 h. The resulting etched silica microspheres were centrifuged,
and the supernatant was decanted. The particles were washed three
times with 3 Â 20 mL Millipore water via sonication, centrifugation,
and decantation. They were then dried in a desiccator under vacuum
at room temperature (RT). The etched silica microspheres were
labeled as SiO2.
Grafting Vinyltriethoxysilane (VTS) on the Silica Micro-
spheres. The etched SiO2 microspheres were dried at 100 °C for 12 h
prior to grafting by vinyltriethoxysilane (VTS). In a 500 mL round-
bottom flask, 500 mg of the etched SiO2 microspheres was dispersed in
dry toluene (250 mL). VTS (3 mL) was then added into the flask, and
the solution was refluxed at 80 °C for 6 h. The resulting vinyl-
functionalized silica microspheres were collected by centrifugation and
washed via sonication, centrifugation, and decantation using 3 Â 10 mL
of toluene, 3 Â 10 mL of methylene chloride, and finally 3 Â 10 mL of
ethanol. The precipitate was then dried at RT to produce the vinylsilica
microspheres.
Epoxidation of Vinylsilica Microspheres into Vinyl Oxide
(Epoxysilica) Microspheres. In a 200 mL round-bottom flask, 500 mg
of the above synthesized vinylsilica microspheres was dispersed in
acetonitrile (50 mL). After adding excess m-chloroperoxybenzoic acid
(500 mg) into the solution, it was stirred at RT for 24 h. The resulting
epoxysilica microspheres were collected by centrifugation. After decant-
ing the supernatant, the precipitate was washed three times with aceto-
nitrile (3 Â 10 mL) and then three times with ethanol (3 Â 10 mL) via
sonication, centrifugation, and decantation. The resulting vinyl oxide-
functionalized silica microspheres were then dried and labeled as
epoxysilica microspheres.
PAMAM dendrimer-functionalized silica microspheres were collected by
centrifugation and washed via sonication, centrifugation, and decantation
with anhydrous acetonitrile (50 mL), followed by anhydrous ethanol
(50 mL), and let to dry. This produced SiO2@PAMAM dendrimer coreÀ
shell microspheres, which were found to have 5.9 μmol PAMAM/g sample
by thermogravimetric analysis (TGA) (see Supporting Information).
Entrapping Palladium Nanoparticles within the PAMAM
Dendrimers of SiO2@PAMAM CoreÀShell Microspheres
(Synthesis of SiO2@Pd-PAMAM CoreÀShell Microspheres).
SiO2@PAMAM dendrimer coreÀshell microspheres (250 mg) were
dispersed by sonication in distilled water (10 mL). The solution was
then treated with 2 M HCl (3 mL) for 30 min at RT, until its pH became
2.00. This was performed in order to quaternize the remaining terminal
primary amine groups in the PAMAM shell of the SiO2@PAMAM
microspheres. This will, in turn, help the amine groups on the external
surface of the coreÀshell microspheres not to anchor Pd(II) ions and
not to lead to the uncontrolled growth of Pd particles on the outer
surface of the core-shell microspheres. After this step, 0.05 M K2PdCl4-
(aq) solution (250 μL) was slowly added, making the G4 PAMAM
dendrimer to palladium(II) ratio 3:5 . The solution was stirred at RT for
30 min to let Pd(II) ions anchor within the cores of the PAMAM
dendrimers. Then, 1 M NaBH4(aq) solution (500 μL) was added. This
caused the solution to change its color from yellowish to golden brown,
indicating the transformation of Pd(II) to Pd(0) (or Pd nanoparticles).
The solid material was separated by centrifugation; washed twice with
2 Â 20 mL distilled water via sonication, centrifugation, and decantation;
and then dried at room temperature in a desiccator. The resulted
material was labeled as SiO2@Pd-PAMAM coreÀshell microspheres.
The sample was kept in a desiccator until its use in catalytic reactions.
Synthesis of SiO2@Pd Microspheres. A reference coreÀshell
microsphere, labeled as SiO2/Pd, containing no PAMAM shell was also
synthesized by following a method reported by Wan et al.41 In a typical
synthesis, etched silica microspheres (SiO2) (100 mg) were dispersed in
distilledwater (10 mL) by sonication for 1 min and then treated with
20 wt % aqueous HNO3 for 24 h. After filtration, the particles were washed
with distilled water. After drying overnight at 110 °C, the resulting SiO2
microspheres (100 mg) were stirred with Pd(II) acetylacetonate/
acetylacetone solution (2 mL) at room temperature for 24 h. The
concentration of Pd(II) ions with respect to SiO2 microspheres was set
to 0.05 wt % Pd in order to produce small size Pd(0) nanoparticles, as
reported in ref 41. The solvent was then slowly evaporated at 110 °C
under stirring. The obtained solid was dried overnight at 110 °C and then
calcined in air at 800 °C for 4 h. Before the sample was used in catalysis, it
was treated with NaBH4 in order to reduce any PdO, possibly formed on
the sample's surface during and after synthesis, into Pd(0). The solid
sample was then let to dry at RT resulting in SiO2@Pd microspheres,
which were kept in a desiccator until use in catalytic reactions.
Catalytic Hydrogenation Reaction. The hydrogenation reac-
tions were carried out in a 50 mL high-pressure reactor (Parr Instrument
Co.). The reactor was equipped with a heating arrangement, overhead
stirrer, thermowell, and pressure gauge as well as a transducer, a gas inlet,
a gas outlet, and a sampling valve. The reactor has controllers to set
temperatures, pressure and agitation speeds to maximum values of
350 °C, 4000 psig pressure and 1500 rpm, respectively. The reactor was
charged with reactant (0.01 mol), methanol (20 mL), and SiO2@Pd-
PAMAM coreÀshell microsphere catalyst (or control sample) (5 mg)
and tightly closed, leak-proof. The reactants included styrene, phenyla-
cetylene, cyclohexene, n-hexene, acetone, acetyleacetonate, styrene
oxide, 4-chloroaniline, trans-stilbene, and 4-nitrostyrene. The reactor
was flushed three times with nitrogen and then three times more with
hydrogen before each reaction. Then hydrogen gas was introduced into
the reactor to the desired pressure, and the heater was turned on to
achieve the desire reaction temperature under a slow stirring rate at
∼100 rpm for uniform heat distribution. After attaining the desired
Ring-Opening of Epoxy Groups of Epoxysilica Micro-
spheres by Amine Groups of G4 PAMAM Dendrimer. In a
100 mL round-bottom flask 400 mg of above prepared epoxysilica
microspheres was mixed with G4 PAMAM dendrimer (0.5 mL) and
anhydrous acetonitrile (50 mL), and the solution was stirred for 24 h at
RT. The 0.5 mL of 10À25% stock solution of G-4 PAMAM dendrimer
in methanol, which was obtained from Dendritech NanoTechnologies,
Inc., was calculated to have 3.5À8.8 μmol of PAMAM. The resulting
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Scheme 1. Synthesis of SiO2@Pd-PAMAM Dendrimer CoreÀShell Microsphere Catalysts
temperature, the reaction was initiated by increasing the agitation speed
to 500 rpm. The absorption of hydrogen gas due to reaction was
monitored from the pressure drop in the reactor. The reaction was
stopped when the gas absorption ceased. The reactor was cooled, and
the reaction mixture was analyzed by gas chromatography (GC) and gas
chromatographyÀmass spectrometry (GC-MS).
Instrumentation. FTIR spectra were obtained from samples
loaded onto a KBr disk (25 Â 4 mm) using a Thermo-Nicolet IR200
spectrometer. Thermogravimetric analysis (TGA) was performed using
a TA Q50 thermogravimetric analyzer with a temperature ramp of
10 °C/min from room temperature to 780 °C under nitrogen gas flow.
The UVÀvis diffuse reflectance spectra of the samples were obtained
with a Lambda 850 spectrophotometer (PerkinElmer) by sandwiching
the powder samples between two 3 Â 3 cm quartz slides. Powder X-ray
diffraction (XRD) patterns were recorded on a Siemens, Daco-Mp
instrument having Cu Kα radiation with wavelength of 1.54 Å. The
diffractometer was set to 40 kV accelerating voltage and 30 mA, and the
data were obtained by setting a wide scan range of 2θ from 20° to 80°
with step size of 0.015° and dwell time of 5 s. Transmission electron
microscope (TEM) images were obtained with a TOPCON microscope
operating at 200 kV on samples prepared on carbon/Formvar-coated Cu
grids. The catalytic reactions were analyzed by using an Agilent 6850 GC
instrument equipped with an HP-1 column (1% dimethylpolysiloxane,
30 m length, 0.25 mm i.d., and 0.25 μm film thickness) and a flame
ionization detector. The products were further confirmed by a HP 5890
(MSD-5972) GC-MS instrument containing a HP-5 MS 50 m  0.200
mm  0.33 μm capillary column. Elemental analyses of the materials
were carried out by Robertson Microlit Laboratories (Ledgewood, NJ).
Figure 1. FTIR spectra of (a) SiO2, (b) vinylsilica, and (c) epoxysilica
microspheres and (d) SiO2@PAMAM and (e) SiO2@Pd-PAMAM
coreÀshell microspheres. See Supporting Figures S1ÀS4 for enlarged
spectra.
room temperature. After centrifugation and washing with anhy-
drous acetonitrile, epoxysilica microspheres were obtained. On
these epoxysilica microspheres, amine-terminated G4 PAMAM
dendrimers were covalently anchored through nucleophilic ring-
opening of the epoxides by the primary amine groups of the G4
PAMAM dendrimers, resulting in SiO2@PAMAM coreÀshell
microspheres. The reaction does not need any catalyst as the
silica surface itself or its residual silanol groups served as a mild
catalyst for the epoxide ring-opening reaction.42 Furthermore,
since the terminal primary amine groups of the PAMAM den-
drimers are more accessible, they are capable of opening up the
epoxides under the reaction conditions we used.42a,43 Once one of
the few primary amine groups of PAMAM dendrimers anchor
ontotheepoxysilicamicrospheres, the other primaryaminegroups
that are not close enough to the epoxy groups remain as they are
on the SiO2@PAMAM coreÀshell microspheres. These residual
primary amine groups were converted to quaternary ammonium
ions by stirring the microspheres in dilute HCl solution in order to
prevent the amine groups from anchoring Pd(II) ions. This, in turn,
prevented the uncontrolled growth of bigger Pd particles on the
external surface of PAMAM shell. After this step, Pd(II) ions were
anchored by using the residual amido groups within the dendrimer
cores. The Pd(II) ions were then reduced into Pd(0) by following
Crooks' method.24 This generated Pd nanoparticles (PdNPs) within
the cores of the PAMAM shells, resulting in the SiO2@Pd-PAMAM
coreÀshell microspheres (or SiO2 supported DEC nanocatalysts).
The materials after each step of syntheses were characterized
by FTIR spectroscopy (Figure 1 and Figures S1ÀS4). The FTIR
3. RESULTS AND DISCUSSION
Synthesis and Structural Analysis. For the synthesis of
SiO2@Pd-PAMAM and SiO2@Pd coreÀshell microspheres,
first 450 nm silica microspheres, which were synthesized by the
St€ober method,38 were etched with KOH (Scheme 1). This
produced ∼305 nm size nanocurrugated/nanoporous silica micro-
spheres, labeled here as SiO2. We have chosen the etched silica
microspheres here as support materials for dendrimers or
dendrimer-encapsulated catalysts (DECs) because from our
previous studies39,40 we found that such etched silica micro-
spheres were more conducive to support metallic nanoparticles
as well as biomolecules by the virtue of their nanocurrugated
surface, nanoporous structure, high surface area, and relatively
larger density of surface silanol groups.
To attach G4 PAMAM dendrimers or the DECs onto SiO2,
first, vinyltriethoxysilane (VTS) was grafted on the surface of the
etched silica microspheres. This produced vinyl-functionalized
silica (vinylsilica) microspheres. The surface vinyl groups were
then converted to epoxides using m-chloroperoxybenzoic acid at
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Table 1. BET Surface Area Results Obtained from N2 Gas
Adsorption Experiments of the Different SiO2-Based Micro-
spheres Including SiO2@Pd-PAMAM Microspheres
organic groups, including the dendrimers, on the SiO2 micro-
spheres in subsequent steps of the syntheses. The wt % epoxy and
PAMAM groups on the samples were calculated from the
difference in the values of these weight losses as shown in Table 2.
Further careful analysis of the TGA results (see Supporting
Information) indicated that there were ∼2.25 mmol of epoxides
per gram of epoxysilica microspheres. Similar calculation gave
∼5.90 μmol of PAMAM per gram of SiO2@PAMAM coreÀshell
microspheres. Geometrical calculation along with the density of
silica of ∼2 g/cm3 46 reveals the existence of ∼3.38 Â 1013 silica
microspheres in a gram of ∼305 nm monodisperse SiO2 micro-
spheres (Supporting Information). This calculation is reasonable
considering the fact that our silica microspheres are monodis-
perse. When this is combined with the mmol of epoxides and
PAMAM per gram of sample, it gives ∼4.01 Â 107 epoxides and
∼1.05 Â 105 PAMAMs per SiO2@PAMAM coreÀshell micro-
sphere. Geometrical calculation (see Supporting Information) is
also used to obtain the theoretical surface area of 9.87 m2 for a
gram of ∼305 nm smooth spherical microspheres. This area is
half as much as the experimental BET surface area of ∼20 m2/g ,
which was obtained by N2 sorption measurement for the etched
∼305 nm silica microspheres. The difference between the two
surface areas is mainly because the etched microspheres have
nanocorrugated and nanoporous surface, while the geometrical
calculation is applied to smooth spherical silica microspheres.
The TEM image of the SiO2@Pd-PAMAM dendrimer coreÀ
shell microspheres is shown in Figure 3 and Figures S6ÀS8. The
images clearly show that large numbers of PdNPs are supported
on the surface of the SiO2@Pd-PAMAM coreÀshell microspheres.
The average size of the PdNPs is 1.56 ( 0.67 nm (Figure 3F).
The control experiment involving simple impregnation of Pd(II)
on SiO2 microspheres with no PAMAM shells around them also
produced PdNPs, or the sample labeled here as SiO2@Pd. The
formation of PdNPs directly on the SiO2 microspheres is driven
by the possible electrostatic interaction between Pd(II) ions and
the silanol (SiÀOH) groups of the SiO2 microspheres and the
subsequent reduction of the Pd(II) ions into supported PdNPs
(Figure S8). However, the number of PdNPs formed in this case
was visibly much fewer compared to those on the SiO2@Pd-
PAMAM dendrimer coreÀshell microspheres, indicating the
importance of the PAMAM dendrimers. Furthermore, and most
importantly, the PdNPs in SiO2@Pd appeared to be slightly bigger
in size, with sizes up to ∼3.37 ( 1.40 nm. In addition, their particle
size distribution appeared more polydispersed (see Figure S8)
compared to those in SiO2@Pd-PAMAM dendrimer coreÀshell
microspheres.
sample name
BET surface area, m2/g
SiO2 microspheres
20
29
25
17
15
13
etched SiO2 microspheres
vinylsilica microspheres
epoxysilica microspheres
SiO2@PAMAM
SiO2@Pd-PAMAM
Figure 2. TGA of (a) etched silica, SiO2, (b) vinylsilica, and (c) epoxysilica
microspheres and (d) SiO2@PAMAM coreÀshell microspheres.
spectrum of SiO2 microspheres showed a broad OÀH stretching
peak between 2500 and 3500 cmÀ1 and a strong SiÀOÀSi stretch-
ing band centered at ∼980 cmÀ1. The spectrum for vinylsilica
microspheres showed CÀH stretching and vibrational modes
corresponding to the vinyl groups at ∼3065 and ∼1410 cmÀ1
,
respectively, and an additional CdC stretching band as a shoulder at
∼1610 cmÀ1.44 Upon epoxidation, the stretching peak correspond-
ing to CÀH of C(O)ÀCÀH remained at ∼3065 cmÀ1, and a peak
corresponding to epoxide appeared at ∼909 cmÀ1 45
.
After the
epoxysilica microspheres were reacted with G4 PAMAM dendri-
mers, a new NÀH deformation peak corresponding to amide
groups at ∼1555 cmÀ1 appeared.
The N2 gas adsorption/desorption isotherms for the all the
samples are shown in Figure S5. The BrunauerÀEmmettÀTeller
(BET) surface areas of unetched SiO2, etched SiO2, vinylsilica
and epoxysilica microspheres, and SiO2@PAMAM and
SiO2@Pd-PAMAM coreÀshell microspheres were 20, 29, 25,
17, 15, and 13 m2 gÀ1, respectively (Table 1). Their TGA results
(Figure 2 and Table 2) showed a slight weight loss below
∼120 °C in all the cases, which was attributed to the loss of
physisorbed water from the samples. The weight loss from the
samples in the range 150À780 °C on the thermogravimetric
traces was attributed mainly to the loss of organic groups. In this
temperature range, the weight losses from vinylsilica and epox-
ysilica microspheres and SiO2@PAMAM coreÀshell micro-
spheres were 5.9, 9.5, and 17.9%, respectively. These weight
losses correspond to the loss of vinyl, epoxy, and PAMAMÀ
epoxy species, respectively. The trend in increase in wt % loss
from vinylsilica to epoxysilica microspheres and then to SiO2@-
PAMAM clearly confirmed the successful anchoring of more
The formation of PdNPs in SiO2@Pd-PAMAM dendrimer
coreÀshell microspheres was confirmed by UVÀvis spectro-
scopy, powder X-ray diffraction (XRD), and elemental analyses.
The UVÀvis spectrum of SiO2@Pd-PAMAM coreÀshell micro-
spheres showed an absorption band at ∼229 nm (Figure 4A)
corresponding to PdNPs.47 Pd(0) nanoparticles, especially very
small ones, have no strong plasmon band in the visible region,
and they rather have only broad UVÀvis absorption bands near
UV region with no discernible peak.47a,48 The strong and sharp bands
in the range of 210À300 nm, particularly at ∼210 and ∼230 nm, that
we have observed could be due to various MLCT bands of Pd2+
complexes.48 Our UVÀvis spectra for SiO2@Pd-PAMAM micro-
spheres, in fact, look very similar to the UV-vis spectrum reported for
tannin-stabilized Pd(0) nanoparticles by Huang et al.47a
Furthermore, the wide-angle XRD pattern of the SiO2@Pd-
PAMAM microspheres (Figure 4B) exhibited a broad peak at
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Table 2. Weight Losses of Organic Groups Calculated from TGA Traces for the Different Samples
% wt loss in the
% wt change compared
to the proceeding sample
sample
SiO2
composition
range 200À750 °C
mol of functional groups
SiO2
4.7
5.9
vinylsilica
SiO2À(CHdCH2)n
1.2
3.6
8.4
NAa
epoxysilica
SiO2À[CH(O)CH2]n
9.5
2.3 mmol epoxide/g
5.9 μmol PAMAM/g
SiO2@PAMAM
SiO2À[CH(O)CH2]nÀm(PAMAM)m
17.9
a The exact amount of functional group in this case (i.e., vinyl groups) is hard to determine as the weight change from SiO2 to vinylsilica is not necessarily
only due to the addition of vinyl groups but also due to the loss of some of the SiÀOH (silanol) groups during grafting of the vinyltriethoxysilane
on them.
Figure 3. (A, B) Low magnification and (C) high magnification TEM images of etched SiO2 microspheres. (D, E) Enlarged TEM images of SiO2@Pd-
PAMAM coreÀshell microspheres. (F) Size distribution of Pd nanoparticles in SiO2@Pd-PAMAM coreÀshell microspheres.
Figure 4. (A) UVÀvis spectra of SiO2@Pd-PAMAM coreÀshell microspheres. (B) Powder X-ray diffraction pattern of SiO2@Pd-PAMAM coreÀshell
microspheres.
39.25° that was attributed to the (111) reflection of Pd.49 The
broad XRD peak is to be expected for such small Pd nanoparticles
encapsulated within the dendrimers of the SiO2@Pd-PAMAM
microspheres. The XRD pattern of the Pd nanoparticles in
SiO2@Pd-PAMAM microspheres appears similar to the XRD
pattern recently reported for small Pd nanoparticles within
mesoporous silica by Jiang and Gao.30
which showed 534 ppm (0.053 wt %) Pd. This was calculated to
be 2.5 mmol Pd per gram of SiO2@Pd-PAMAM coreÀshell
microspheres.
Catalytic Studies. The catalytic properties of the SiO2@Pd-
PAMAM microspheres were investigated by using the micro-
spheres as catalyst in hydrogenation reaction of various sub-
strates under 10 bar hydrogen pressure and at room temperature
(RT) (Table 3). Control experiments involving attempted hy-
drogenation reaction without any catalyst or with silica
The presence and wt % of Pd in SiO2@Pd-PAMAM coreÀ
shell microspheres was further determined by elemental analysis
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Table 3. Hydrogenation of Various Substrates by SiO2@Pd-PAMAM Dendrimer CoreÀShell Microsphere Catalysta
a Reaction conditions: substrate: 0.01 mol; methanol: 20 mL; catalyst: 5 mg (0.0053 wt % Pd in the SiO2@Pd-PAMAM dendrimer coreÀshell
c
microspheres; 10 bar H2 pressure; temperature: 20 °C. b Catalyst used was Pd@SiO2. Catalyst used was 1% Pd/C. d Third recycle of 1% Pd/C.
e Temperature: 50 °C. f 4-Hydroxypentan-2-one. g 1-Phenylethanol. h Phenylacetaldehyde. i In 15 min, all the double bonds were hydrogenated. j In 2.5 h,
35% of the nitro groups were additionally hydrogenated. k In 8 h, 88% of the nitro groups were additionally hydrogenated. l In 15 min, 100% selectively to
4-nitroethylbenzene product was obtained. m In 2.5 h, 35% of 4-ethylaniline and 65% 4-nitroethylbenzene were obtained. n In 8 h, 88% 4-ethylaniline and
12% 4-nitroethylbenzene were obtained. See Figure S10 also for GS-MS spectra.
microspheres containing no Pd (SiO2 and SiO2@PAMAM) gave
almost no reaction or no hydrogenation product. However, the
SiO2@Pd-PAMAM microspheres catalyzed the hydrogenation
reactions of various substrates well, giving the corresponding
hydrogenated products, as confirmed by GC and GC-MS
(Tables 3 and 4 and Figure S9). For instance, the hydrogenation
of styrene in the presence of SiO2@Pd-PAMAM coreÀshell
microspheres resulted in ethylbenzene with ∼100% conversion
in 10 min at RT and with a high net rate of 23 880 hÀ1. This
showed that the catalytic activity of the SiO2@Pd-PAMAM
coreÀshell microspheres was even higher than that of the very
active shaped Pd nanoparticle catalysts in nanoporous silica that
we have recently reported.50 The catalytic efficiency of the
SiO2@Pd-PAMAM coreÀshell microspheres was also compared
with commercially available 1 wt % Pd/C catalyst. When the
latter was used as catalysts for hydrogenation of styrene under the
same conditions, ∼100% conversion in 10 min was obtained;
however, the TOF of the Pd/C catalyzed reaction was found to
be about half as much as that of SiO2@Pd-PAMAM coreÀshell
microspheres. Whereas the 1 wt % Pd/C for this reaction yielded
a net rate of 10 260 hÀ1 (Table 3, entry 3), the SiO2@Pd-
PAMAM coreÀshell microspheres gave a net rate of 24 000 hÀ1
.
Furthermore, the SiO2@Pd-PAMAM coreÀshell microspheres
were found to be highly recyclable at least five times with almost
no change in their catalytic property giving similar net rate, where
as the 1 wt % Pd/C lost its activity, significantly giving a net rate
of 4800 hÀ1 after the third recycle (see Figure 5).
Another control experiment involving a catalytic reaction with
a catalyst prepared by physisorption of premade Pd-PAMAM
nanoparticles (or DECs) on the surface of SiO2 microspheres was
performed. This catalyst gave significantly lower catalytic activity
in catalytic hydrogenation of styrene, and most importantly, it
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Table 4. Results for the Recycle Catalytic Reactions for
SiO2@Pd-PAMAM CoreÀShell Microsphere Catalysts for
the Hydrogenation of Styrenea
demonstrated using the SiO2@Pd-PAMAM coreÀshell micro-
spheres as catalyst. The reaction gave ∼100% conversion in 3 h,
yielding 65% of 1-phenylethanol and 35% of phenylacetaldehyde
products (Table 3, entry 9). The catalytic activity of the
SiO2@Pd-PAMAM microspheres was also tested with 4-chloro-
nitrobenzene—a less reactive compound to undergo hydrogenation.53
The SiO2@Pd-PAMAM coreÀshell microspheres catalyzed the
reaction with 100% conversion and 96% selectivity to 4-chlor-
oaniline product in 2.5 h and at a slightly elevated temperature of
50 °C (Table 3, entry 10). The SiO2@Pd-PAMAM coreÀshell
microspheres were also found to catalyze the hydrogenation of
trans-stilbene to generate 1,2-diphenylethane successfully with
∼100% conversion in 25 min (Table 3, entry 11).
entry recycle no. t, min % conv % selectivityb net rate, hÀ1 c
1
2
3
4
5
6
0
1
2
3
4
5
7
7
7
7
7
7
76.5
76.5
76.2
76.2
76.2
76.0
∼100
∼100
∼100
∼100
∼100
∼100
23 880
23 880
23 810
23 810
23 810
23 780
a Reaction conditions: styrene: 0.01 mol; methanol: 20 mL; catalyst:
5 mg (0.0053 wt % Pd in the SiO2@Pd-PAMAM coreÀshell micro-
spheres); 10 bar H2 pressure; temperature: 20 °C; reaction time: 10 min.
The SiO2@Pd-PAMAM coreÀshell microspheres’ selecti-
vity to catalytically hydrogenate different functional groups was
tested using 4-nitrostyrene, which has a nitro group and a vinyl
group that can undergo hydrogenation (Table 3, entry 12). In the
initial period of the reaction, only the vinyl groups underwent
catalytic hydrogenation in the presence of the SiO2@Pd-PA-
MAM coreÀshell microspheres almost completely in ∼15 min.
Then, the nitro groups followed and underwent hydrogenation
into amine groups almost completely in ∼2.5 h.
b Selectivity to ethylbenzene product. The net rate was calculated by
c
dividing the mmol of product formed in 7 min by mmol of catalyst used
in the reaction and then normalizing all the results for 1 h for comparison
purposes. It is worth mentioning that the hydrogenation reaction of
styrene with our catalyst is extremely fast, getting almost into completion
within 10 min of reaction time. The reaction was monitored by
measuring the drop in H2 pressure in the reactor due to the hydrogena-
tion reaction as precisely as possible using a pressure gauge that comes
with the Parr reactor. Furthermore, the reaction times were collected
with stop watches as precisely as possible. Moreover, because of the
unavoidable slight loss of catalyst during recovery, the amount of
reactant used in the next cycle was scaled down accordingly in order
to maintain the reactant/catalyst ratio as constant as possible in all of the
five cycles for comparison purposes.
The reusability of SiO2@Pd-PAMAM coreÀshell micro-
spheres as catalyst was tested using styrene hydrogenation as a
model reaction (Table 4). This was performed by collecting the
SiO2@Pd-PAMAM core-shell microspheres from the preceding
reaction via simple centrifugation and then reusing them as
catalyst in the next cycle. The SiO2@Pd-PAMAM coreÀshell
microspheres were proven to be effectively recyclable catalysts at
least in five cycles with barely losing their catalytic activities.
Their TOF remained between 23 780 and 23 880 hÀ1 after five
cycles. On the other hand, the SiO2@Pd microspheres (control
sample), which contained no PAMAM on their shells, also catalyzed
the hydrogenation of styrene to ethylbenzene with similar catalytic
efficiency, giving 98% conversion in 10 min and 23 750 hÀ1 TOF in
the first cycle. However, they quickly lost their catalytic activities
after recycling, giving only 61, 54, and 40% conversions and 14 783,
13 086, and 9693 hÀ1 TOFs, respectively, after the first, second and
third cycles (Table 5). These TOF values were significantly lower
than the corresponding values obtained for the SiO2@Pd-PAMAM
coreÀshell microspheres.
resulted in severe leaching of the DECs into the reaction solution.
These microspheres were also found to contain less Pd-PAMAM
per silica microsphere. This clearly indicates the advantage of
covalent attachment of the Pd-PAMAM DECs onto the SiO2
microspheres in order to increase the stability of the DECs.
To study the scope of the catalytic activity of the SiO2@Pd-
PAMAM microspheres, the catalytic hydrogenation reactions
of substrates other than styrene were explored (Table 3). For
instance, the SiO2@Pd-PAMAM coreÀshell microspheres
catalyzed the hydrogenation of phenylacetylene to ethylbenzene.
The hydrogenation of phenylacetylene was found to be slightly
slower than that of styrene (Table 3, entry 4). This was, however,
to be expected considering the fact that the hydrogenation of
phenylacetylene to ethylbenzene goes through two hydrogenation
steps, i.e., phenylacetylene to styrene and then styrene to ethylben-
zene. The catalyst, nevertheless, still exhibited high catalytic
activity and TOF even for such two-step hydrogenation reaction.
The hydrogenation of another reactant, cyclohexene, took place
with almost ∼100% conversion in 20 min in the presence of the
SiO2@Pd-PAMAM coreÀshell microspheres under the same
reaction condition (Table 3, entry 5). In addition, the SiO2@
Pd-PAMAM coreÀshell microspheres catalyzed n-hexene with
∼100% conversion in 60 min (Table 3, entry 6). More interest-
ingly, although ketones undergo hydrogenation rather relatively
slowly,51 the SiO2@Pd-PAMAM coreÀshell microspheres cata-
lyzed the hydrogenation of acetone and acetylacetone efficiently and
with high TOF. The catalytic hydrogenation of acetone with
SiO2@Pd-PAMAM coreÀshell microspheres gave a sole product,
isopropanol, with ∼100% conversion in 2 h at 50 °C, whereas the
catalytic hydrogenation acetylacetonate gave 4-hydroxypentan-2-
one with 95% conversion in 2 h (Table 3, entries 7 and 8).
Furthermore, leaching tests for the SiO2@Pd-PAMAM coreÀ
shell microspheres in the catalytic reactions were conducted. The
catalytic reaction of the supernatant, after separating the
SiO2@Pd-PAMAM coreÀshell microspheres from the reaction
mixture, gave only 1% styrene conversion in an additional 10
min, indicating the “true” heterogeneous catalytic nature of the
SiO2@Pd-PAMAM coreÀshell microspheres. This result was
corroborated by ICP-OES analysis of palladium in the reaction
mixture after the first, third, and fifth reaction cycles, which
showed <1 ppm palladium in the solutions.
It is worth mentioning that under the experimental conditions
we performed the hydrogenation reaction using the SiO2@Pd-
PAMAM microspheres as catalysts, the arene groups of styrene
and the substrates did not undergo hydrogenation, while the
vinyl and nitro functional groups did. It is not clear whether this is
because the arene groups are too big to penetrate through DENs
and reach to the Pd nanoparticles, or the arene groups do not
undergo catalysis with our SiO2@Pd-PAMAM microsphere
catalysts under the conditions we employed. Whatever the case
might be, this difference in reactivity between the vinyl and arene
groups can be taken advantage of and potentially utilized to
The hydrogenation of styrene oxide into 1-phenylethanol—an
important compound in the fragrance industry52—was also
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Figure 5. (A) Comparative graph for % conversion of styrene in 10 min versus recycle time of SiO2@Pd-PAMAM coreÀshell catalysts and commercial
Pd/C (1%) catalyst. (B) Catalytic efficiency of recycled Pd/C catalyst showing significant catalytic activity loss after recycling. A similar graph for
SiO2@Pd-PAMAM coreÀshell catalysts was difficult to plot because of the materials’ very high catalytic activities and the fast H2 consumption by the
reaction.
Table 5. Catalytic Activities of SiO2@Pd Microspheres in
Multiple Cycles of Catalytic Testa
295 K to perform ethylene hydrogenation. The authors further
reported that at such intermediate pressures of 100 Torr of H2 gas
the richest surface chemistry with infrared-visible sum frequency
generation (SFG), which was used to monitor the surface vibra-
tional spectrum in situ during ethylene hydrogenation on Pt(111)
at 295 K, was obtained. Thus, these low pressures are chosen
mainly to perform experiments of these types in ultrahigh vacuum
(UHV) systems to probe reaction mechanisms.
entry
times recycled
% conv
% selectivityb
net rate, hÀ1 c
1
2
3
4
5
6
0
1
2
3
4
5
98
61
54
40
23
19
∼100
∼100
∼100
∼100
∼100
∼100
23 750
14 783
13 086
9 693
On the other hand, experiments in our case using very low H2
pressure are not very feasible because the alkene is in liquid form
and the reaction mixture is in solution phase. In hydrogenation of
alkenes in liquid phase, the hydrogen pressures are often set to a
much higher pressures than 100 Torr to get reasonable yield/
TOF.56 For instance, in the work by Oyamada et al.,56a the
authors used 0.15 MPa or 1.48 atm (a much higher pressure than
100 Torr) for hydrogenation of various organic compounds.
They obtained (the highest) TON of 2700 in 1.5 h (or a TOF
value of 1800 hÀ1), which was much lower than the one obtained
in our case, which was 24 000 hÀ1; however, the pressure used in
our case was high, i.e., 9.8 atm. In the work by Xu et al.,56b the
authors used 1.5 MPa or 14.8 atm H2 pressure for hydrogenation
of various nitroaromatices in solution phase using Pt nanopar-
ticles. Although the H2 pressure used in their case was actually
higher than the one used in our case, the conversion of the nitro
groups in their catalytic reactions also reached ∼90À100% only
in 1À10 h.
5 573
4 604
a Reaction conditions: substrate: 0.01 mol; methanol: 20 mL; catalyst:
5 mg (0.05 wt % Pd in the SiO2@Pd microspheres; 10 bar H2 pressure;
temperature: 20 °C; and reaction time: 10 min. b Selectivity to ethyl-
benzene product. c The net rate was calculated by dividing the mmol of
product formed in the specified time by mmol of catalyst used in the reac-
tion and then normalizing all the results for 1 h for comparison purposes.
perform selective catalysis of only vinyl or nitro groups in the
presence of arene groups. A similar idea of size selective catalysis
has actually been successfully demonstrated by an elegant work
of Crooks and co-workers,54 who showed that the rational design
of the periphery of dendrimers would allow preferential size-
dependent selective hydrogenation of smaller α-allylic alcohols.
It is also worth noting that the conditions we employed for
hydrogenation reaction, 10 atm H2 pressure, is relatively, but not
surprisingly, high. There are literature reports on hydrogenation
reactions using lower H2 pressure, for instance 100 Torr of H2
(∼0.13 atm).55 However, in many of these cases, the reactions
are often done under vacuum and for gas phase hydrogenation of
substances such as ethylene. Furthermore, in most of these cases,
although the H2 pressure is set to be low, the other gases such as
ethylene or argon make up the overall pressure. For instance, in
the report by Molero et al.,55a the authors used ethylene
pressures between 50 and 300 Torr and H2 pressures from 45
to 600 Torr at 300 and 475 K to do ethylene hydrogetation in gas
phase. In another example reported by Cremer et al.,55b the
authors used near 1 atm of total pressure of ethylene and H2 at
To further evaluate the catalytic activities of the SiO2@Pd-
PAMAM coreÀshell microspheres, we also conducted the hydro-
genation of styrene under lower pressure (1 atm or 1.01 barr) of
H2 in the presence of the SiO2@Pd-PAMAM coreÀshell micro-
spheres. We obtained only 7% conversion in 4.5 min. Soon after
this, the H2 pressure dropped to 0.27 atm and the reaction barely
continued afterwards. When setting the pressure in the reaction
mixture to 1 atm again, the reaction gave 6% more reactant
conversion, followed by a significant drop in the H2 pressure as
well as in the reaction rate.
In addition, we compared the net rate of SiO2@Pd-PAMAM
coreÀshell microsphere catalyst (i.e., ∼24 000 h-1) with values
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reported for similar catalysts in the literature. For instance,
compared with the catalytic activity of nanoporous silica encap-
sulated Pd nanoparticle catalysts that we have recently reported50
and that gave net rate of 2800À5400 hÀ1, the catalytic activity of
the SiO2@Pd-PAMAM coreÀshell microspheres was higher.
Furthermore, compared with the highest TOF value of 4.37 s-1
for Pd catalysts reported by Mastalir et al.,57 the one we obtained
for SiO2@Pd-PAMAM coreÀshell microspheres (∼24 000 h-1)
was again higher. In the work by Mekasuwandumrong,58 typical
TOF values ranged from 4.3 to 66.2 sÀ1 (which were 258 to
3972 hÀ1 when converted into hÀ1 for comparison purposes) for
different silica-supported palladium nanomaterials were re-
ported. This result also showed that the catalytic activity of the
SiO2@Pd-PAMAM microspheres was higher than that of these
Pd catalysts reported by Mekasuwandumrong.58 In the work by
Jackson et al.,59 ∼100% conversion of alkene in much longer
reaction time of 40 min, but under much lower reaction pressure
than those used in our case, was reported. In the work by Nijhuis
et al.,60 a catalyst with 2% Pd on Davisil 643 was reported to
catalyze hydrogenation at 1 bar pressure of alkyne into alkene
almost completely in 10 min, with further conversion of the
resulting alkene into alkane after ∼20 min more. Comparing the
above results, we notice that our SiO2@Pd-PAMAM micro-
sphere catalysts perform well, if not better than some of them. In
fact, only in case of Pt nanoparticles,61 we find a much higher net
rate (i.e., 65 041 hÀ1 for hydrogenation of nitrobenzene) than
those obtained in our case. Nonetheless, since the catalysts and
reaction conditions in each system, including ours, are different,
it is difficult to make a full scale comparison or conclusion on the
relative efficiency of our SiO2@Pd-PAMAM microsphere cata-
lysts with respect to others.
’ ACKNOWLEDGMENT
We gratefully acknowledge the generous financial assistance
by the US National Science Foundation (NSF) for this work,
under Grant Numbers CAREER CHE-1004218, NSF DMR-
0968937, and NSF NanoEHS-1134289. T.A. also thanks NSF for
recognizing him as one of its ten NSF American Competitiveness
and Innovation Fellows (NSF-ACIF) for 2010 and the accom-
panying grant for his group's research work, and for recently
awarding him the NSF Special Creativity grant.
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In summary, we have developed a synthetic route to novel
SiO2@Pd-PAMAM coreÀshell microspheres or heterogeneous
nanocatalysts. Their synthesis involved supporting PAMAM
dendrimers covalently onto etched silica microspheres and then
encapsulating small and monodisperse palladium nanoparticles
(PdNPs) within the supported PAMAM dendrimers. The re-
sulting SiO2@Pd-PAMAM coreÀshell microspheres exhibited
highly efficient and recyclable catalytic activities in catalytic hydro-
genation of various substrates and functional groups. Further-
more, because of their silica microsphere supports, their coreÀ
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MAM coreÀshell microspheres showed better catalytic activities
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with their corresponding control samples, including SiO2@Pd coreÀ
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’ ASSOCIATED CONTENT
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Supporting Information. FTIR spectra, transmission
b
electron micrographs, N2 gas adsorption isotherms, and GC-MS
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*Tel: (732) 445-2970; Fax: (732) 445-5312; e-mail: tasefa@rci.
rutgers.edu.
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