Immobilization of palladium catalyst on magnetically separable polyurea nanosupport

Article abstract of DOI:10.1039/c4ra08864f

This work describes a method for preparing magnetic polyurea nanoparticles (PU NPs) and their utilization as a catalyst support. The method is based on entrapment of hydrophilic magnetic nanoparticles within the polyurea matrix. The synthetic process of these magnetic polyurea nanoparticles is based on oil-in-oil nanoemulsification of an organic polar phase comprised of N,N-dimethylacetamide (DMAc), 2,6-diaminopyridine and ionic liquid modified magnetite nanoparticles (MNPs-IL), in heptane containing a suitable surfactant. This was followed by interfacial polycondensation reaction between an isocyanate monomer, polymethylenepolyphenyl isocyanate (PAPI 27), and the amine monomer producing magnetically separable polyurea nanoparticles. Subsequently, these particles were employed as a catalyst nanosupport. Two catalytic systems based on the encapsulation of Pd(OAc)₂ within magnetic PU NPs or their adsorption on the surface of these particles were produced and subjected to hydrogenation reactions and selective hydrogenations of α,β-unsaturated compounds. Pd(OAc)₂ adsorbed on the surface of the magnetic PU NPs demonstrated high catalytic activity and selectivity, which was superior to the conventional catalyst Pd/C or palladium nanoparticles supported directly on the surface of magnetite nanoparticles. The catalyst was easily recovered from the reaction mixture by applying an external magnetic field and recycled over five times without observing any significant loss in its catalytic efficiency. This journal is

Full text of DOI:10.1039/c4ra08864f

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Immobilization of palladium catalyst on
magnetically separable polyurea nanosupport†
Cite this: RSC Adv., 2014, 4, 48299
*
Suzana Natour and Raed Abu-Reziq
This work describes a method for preparing magnetic polyurea nanoparticles (PU NPs) and their utilization
as a catalyst support. The method is based on entrapment of hydrophilic magnetic nanoparticles within the
polyurea matrix. The synthetic process of these magnetic polyurea nanoparticles is based on oil-in-oil
nanoemulsification of an organic polar phase comprised of N,N-dimethylacetamide (DMAc), 2,6-
diaminopyridine and ionic liquid modified magnetite nanoparticles (MNPs-IL), in heptane containing a
suitable surfactant. This was followed by interfacial polycondensation reaction between an isocyanate
monomer, polymethylenepolyphenyl isocyanate (PAPI 27), and the amine monomer producing
magnetically separable polyurea nanoparticles. Subsequently, these particles were employed as a catalyst
nanosupport. Two catalytic systems based on the encapsulation of Pd(OAc)2 within magnetic PU NPs or
their adsorption on the surface of these particles were produced and subjected to hydrogenation
reactions and selective hydrogenations of a,b-unsaturated compounds. Pd(OAc)2 adsorbed on the
surface of the magnetic PU NPs demonstrated high catalytic activity and selectivity, which was superior
to the conventional catalyst Pd/C or palladium nanoparticles supported directly on the surface of
magnetite nanoparticles. The catalyst was easily recovered from the reaction mixture by applying an
external magnetic field and recycled over five times without observing any significant loss in its catalytic
efficiency.
Received 18th August 2014
Accepted 24th September 2014
DOI: 10.1039/c4ra08864f
www.rsc.org/advances
Magnetic nanoparticles (MNPs) with their large surface area
to volume ratio are subjected to extensive investigation and
application in various disciplines including, catalysis,7 drug
Introduction
Heterogeneous catalysts are designed to enable their isolation
and recovery from the reaction media, while maintaining cata-
lytic efficiency. General approaches for heterogenization of
catalysts include microencapsulation within polymeric
capsules,1 and immobilization on solid surfaces such as silica
particles,2 organic polymer supports such as polystyrene3 and
organic–inorganic hybrid mesoporous supports.4 However,
conventional heterogeneous catalysts are signicantly less
active compared to their homogeneous counterparts. This led to
the development of new systems, combining the merits of both
catalytic approaches, called nanocatalysts or nanoreactors.5
Nevertheless, these systems are difficult to isolate and recover
from the reaction media, sometimes requiring expensive and
time consuming procedures. Therefore, a new approach based
on magnetic isolation has been developed.6
delivery,8 magnetic resonance imaging,9 magneto-thermal
therapy10 and biomolecular sensors.11 Recently, super-para-
magnetic Fe3O4 nanoparticles were used as highly efficient
catalyst supports because they provide easy accessibility to the
catalytic sites and facile isolation from the reaction media by
applying an external magnetic eld.12 Nonetheless, the MNPs
tend to agglomerate within a short period of time. To overcome
such difficulties, the surface of the Fe3O4 nanoparticles was
modied using various methods such as a surface capping
method using small capping agents like oleic acid13 and silane
reagents.14 Likewise, a surface coating method with organic
polymer shells such as polyvinyl pyrrolidone (PVP),15 polyani-
line (PANI)16 and polyglycidyl methacrylate (PGMA)17 or an
inorganic shell such as silica produced through the sol–gel
method.12d,18 In the development of this eld, a great emphasis
has been paid towards the immobilization of numerous cata-
lysts on the surface of Fe3O4 and their applications in catalysis.
Among recent achievements are immobilization of metal
nanoparticles, palladium based catalysts and rhodium catalyst
on the surface of MNPs.15a,19 These catalysts proved to be highly
efficient in a wide range of catalytic transformations including
hydroformylation, hydrogenation, and other C–C coupling
reactions.6b,7 This concept not only simplies the work-up
Institute of Chemistry, Casali Center of Applied Chemistry and Center for Nanoscience
and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
E-mail: Raed.Abu-Reziq@mail.huji.ac.il; Fax: +972-2-6585469
† Electronic supplementary information (ESI) available: SEM and TEM images,
Size distribution, EDX graph, IR spectrum and XRD pattern of MNPs,
MNP-IL-C4, PU NPs and magnetic PU NPs. XPS spectra of Pdad and Pden
systems before and aer hydrogenation reactions. Catalytic results of
hydrogenation reactions catalysed by Pdad and Pden systems. See DOI:
10.1039/c4ra08864f
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procedure but also minimizes the use of additional organic
solvents and chemicals during the separation process.
Results and discussion
Preparation of magnetite nanoparticles modied by ionic
Polyurea nanoparticles and nanocapsules have been mainly
utilized for pharmaceutical and biomedical applications due to
their stability and biocompatibility.20 Conversely, magnetite
nano-particles were encapsulated or entrapped mostly in silica
SiO2 nanospheres,21 polystyrene,22 polymethacrylate/poly-
hydroxymethacrylate,23 poly (urea-formaldehyde)24 and poly-
acrylamide.25 The current demands toward the production of
robust, expeditious and efficient catalyst supports, being rapidly
and easily separated, have motivated us into developing new
magnetically separable polyurea nanoparticles, in which the
features of both polyurea and magnetic nanoparticles are fused.
Polyurea systems have added an advantage over other catalyst
supports due to its facile and relatively prompt synthetic
process. The immobilization of catalytic species such as metal
nanoparticles could be achieved either by encapsulation or
adsorption on the surface of the magnetic polyurea
nanocomposites.
Recently, we have reported a new nanoreactor based on
encapsulation of palladium nanoparticles stabilized by hyper-
branched polyamidoamine modied with palmitoyl groups
along with hydrophobic magnetite nanoparticles within poly-
urea core-shell nanocapsules. The nanocapsules were synthe-
sized by nanoemulsication of oil in water followed by
interfacial polymerization between amine and isocyanate
monomers. This catalytic nanoreactor exhibited catalytic
activity in hydrogenation reactions in aqueous medium.26 Our
continuing interest in developing efficient catalyst supports led
us to investigate the efficiency of magnetic polyurea nano-
particles as a catalyst support. To the best of our knowledge,
entrapment of MNPs within polyurea nanoparticles with solid
core, synthesized from oil in oil nanoemulsions, has not been
reported yet.
Herein, we developed a new magnetically separable polyurea
NPs containing coordinative groups in the shell that can
conjugate and support the catalytic species. Synthesis, charac-
terization and catalytic applications of supported palladium
acetate, Pd(OAc)2 on biocompatible and nitrogen containing
magnetic polyurea nanoparticles is described. The structure of
this nano-system is composed of a solid core containing
hydrophilic magnetite nanoparticles and a polyurea matrix.
This system has been prepared by nano-emulsication of an oil
phase containing the MNPs and 2,6-diaminopyridine with
another oil phase including a proper surfactant, followed by
addition of polymethylene polyphenyl isocyanate (PAPI 27) to
initiate an interfacial poly-condensation reaction and to create
the polyurea matrix. Subsequently, Pd(OAc)2 entrapped within
or adsorbed on the surface of these magnetic polyurea nano-
particles, is utilized in hydrogenation reactions and selective
hydrogenations of a,b-unsaturated compounds. The catalytic
system based on adsorption of Pd(OAc)2 exhibited in both
transformations high catalytic activity and recyclability over ve
times without any signicant loss in its catalytic efficiency.
liquid
MNPs were synthesized by employing various techniques such as,
thermal decomposition,27 high pressure hydrothermal methods,28
a microemulsion route29 or co-precipitation from aqueous sol-
utions.12c In this study, MNPs were synthesised via co-precipitation
of iron salts, FeCl2 and FeCl3 in a basic aqueous solution under
inert conditions at 85–90 C according to Massart's method.30 In
ꢀ
agreement with previous reports,13c,30,31 MNPs formed via the
aforementioned route were spherical in shape and small in
diameter (5–20 nm) as conrmed by transmission electron
microscopy (TEM) analysis (ESI, Fig. S1a†). However, particles in
this size range are intrinsically unstable and tend to agglomerate
in order to reduce the energy associated with the high surface area
to volume ratio. In addition, non-modied MNPs are scarcely
soluble in any medium. In order to surpass these restrictions,
various surface modication and protection strategies of the bare
MNPs have been introduced.12c,18a We have covalently graed the
ionic liquid 1-butyl-3-(3-(trimethoxysilyl) propyl)-1H-imidazol-3-
cholride (IL-C4) on the surface of magnetite NPs in order to
stabilize and to enhance their solubility in a wide range of organic
and aqueous solvents. The ionic liquid IL-C4 was synthesized by
reacting 3-chloropropyltrimethoxysilane with 1-butyl imidazole at
120 ꢀC in solvent free and inert atmosphere conditions.
Graing the IL-C4 on the surface of the MNPs was carried out by
reacting the trimethoxysilane groups of the IL-C4 with the surface
hydroxyl groups of the MNPs in ethanol for 36 h at room
temperature (Scheme 1). The resulting MNPs-IL-C4 shows no
signicant increase in the particle size, as depicted in TEM (ESI,
Fig. S1b†). The loading of IL-C4, as determined by thermogravi-
metric analysis (TGA), was 0.32 mmol gꢁ1. Moreover, those
modied particles were highly soluble in polar organic solvents
like dimethyl sulfoxide (DMSO), dimethylformamide (DMF),
methanol, N,N-dimethyl acetamide (DMAc) and water.
Synthesis and characterization of PU NPs and magnetic
polyurea nanoparticles (MNPs@PU NPs)
An optimal composition for preparing PU NPs has been estab-
lished. This system has been accomplished from an oil-in-oil (O/O)
Scheme 1 Modification of magnetic NPs with IL-C4.
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Scheme 2 Illustration of preparation of polyurea nanoparticles.
nanoemulsion via an interfacial polycondensation process of the The complete polymerization of the amine with the isocyanate
isocyanate monomer, PAPI 27, and diamine monomer, 2,6-dia- monomers was conrmed via infrared (IR) analysis (ESI,
minopyridine, with a molar ratio of 1 : 1. The synthetic route of PU Fig. S3†). The presence of absorbance peaks at 3360 cmꢁ1 and
NPs is depicted in Scheme 2. Concisely, fabrication of the nano- 1690 cmꢁ1 which are assigned to the N–H and C]O stretching
particles from the nanoemulsion system was initiated by emulsi- vibration respectively, indicate the formation of polyurea.
fying the dispersed oil phase, the surfactant solution composed of Additionally, the absence of the absorbance peak of the isocy-
heptane and the lipophilic polymeric surfactant poly(1-ethenyl anate group (–C]N]O) at 2260 cmꢁ1 indicates the complete
pyrrolidin-2-one/hexadec-1-ene) (Agrimer AL 22), followed by consumption of the isocyanate monomer.
ultrasonication for 10 minutes. Subsequently, PAPI 27 dissolved in
Thermogravimetric analysis (TGA) of the pure MNPs, MNPs-
xylene was slowly added while sonicating the nanoemulsion IL-C4, pure PU NPs and MNPs-IL-C4@PU NPs are shown in
system for 3 minutes. The interfacial polycondensation reaction Fig. 3. The thermal decomposition analyses were carried out in
occurred through the nucleophilic attack of amine groups on the the temperature range of 25–950 ꢀC under air. It has been
carbonyl groups of the isocyanate monomer. Aer establishing the noticed that there is an increase in the organic content in the
ideal PU NPs composition, it was utilized for the incorporation of modied MNPs-IL-C4 (Fig. 3b), with 7.3% weight loss compared
the MNPs-IL-C4 into the cores of PU NPs by dissolving 0.4 wt% to the bare MNPs (Fig. 3a) with weight loss of 3.5% which could
MNPs-IL-C4 in the dispersed oil phase following the same proce- be due to absorbed moisture and solvent residues. Further-
dure. The surface morphology and size distribution of the NPs more, the pure PU NPs showed 100% weight loss, which is
were examined by scanning electron microscope (SEM) analysis, concord to the fact that only organic content is present (Fig. 3d),
which revealed the formation of spherical particles with a smooth whereas the magnetic PU NPs contained 87% organic content
surface in both pure PU NPs (Fig. 1a) and MNPs-IL-C4@PU NPs (Fig. 3c). Both PU NPs systems experienced several steps of
systems (Fig. 1b). Dynamic light scattering (DLS) studies revealed degradation. In the PU NPs system (Fig. 3d), the initial weight
polydispersed systems with an average particles size of 230 nm and loss is observed from 25–125 ꢀC probably due to the presence of
450 nm in both PU NPs and magnetic PU NPs systems respectively solvents such as heptane. The weight loss at temperatures
(ESI, Fig. S2†). These results are in good agreement with the SEM higher than 300 ꢀC is contributed to the decompositions of the
results.
polyurea shell and the surfactant Agrimer AL 22. Thermal
Furthermore, the characterization of the PU NPs and the decomposition of the MNPs-IL-C4@PU NPs system (Fig. 3c)
magnetic PU NPs were carried out by TEM as illustrated in exhibits analogous behaviour to the pure PU NPs. However, the
Fig. 2. The TEM micrographs conrmed the formation of pol- presence of the magnetic nanoparticles can accelerate the
ydispersed PU NPs (Fig. 2a) in addition to the successful decomposition of the organic materials especially when the
incorporation of the MNPs within the core of PU NPs (Fig. 2b). analysis is performed under air.
Fig. 1 SEM micrographs of (a) pure PU NPs and (b) MNPs-IL-C4@PU Fig. 2 TEM micrographs of (a) pure PU NPs and (b) MNPs-IL-C4@PU
NPs.
NPs.
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Fig. 4 Micrographs of Pdad on the surface of MNPs-IL-C4@PU NPs
determined by (a) SEM and (b) TEM.
Adsorption of Pd(OAc)2 on MNPs-IL-C4@PU NPs
Fig. 3 TGA analysis for (a) bare MNPs; (b) MNPs-IL-C4; (c) MNPs-IL-
C4@PU NPs and (d) pure PU NPs.
Aer establishing a method for the production of MNPs-IL-
C4@PU NPs, their efficiency as catalyst support was examined.
Hence, Pd(OAc)2 was physically adsorbed on the surface of the
magnetic PU NPs (Pdad) by mechanically stirring the MNPs-IL-
C4@PU NPs with palladium acetate in 80 mL THF for 24 h. The
resulted suspension was magnetically separated, washed twice
with heptane and redispersed in heptane to reach a 20 g
suspension total. This process is illustrated in Scheme 3. The
loading of the adsorbed palladium according to inductively
coupled plasma mass spectrometry measurements (ICP-MS)
was 0.09 mmol gꢁ1. To ascertain the presence of the palladium
on the surface of the magnetic PU NPs, morphology and size
distribution, SEM, TEM and energy-dispersive X-ray (EDS)
analysis were carried out. SEM and TEM micrographs depicted
in Fig. 4a and b respectively, revealed that the surface of the
MNPs-IL-C4@PU NPs became coarse. Additionally, no changes
in the size distribution and no leaching of MNPs-IL-C4 from the
core occurred. Scanning transmission electron microscope/
energy dispersive X-ray spectroscopy (STEM/EDS) analysis
indicated the presence of palladium on the surface (ESI,
Fig. S4†).
Immobilization of palladium acetate Pd(OAc)2
Palladium is arguably the most widely used transition metal in
catalysis.32 Since noble metals are extremely precious, several
strategies for the heterogenization of homogeneous catalysts
and metal NPs have been developed. For example, Bradley and
coworkers reported an active and recyclable cross-linked resin
captured palladium catalyst which was utilized in Suzuki cross-
coupling reactions.33 Microencapsulation of palladium species
within polyurea microcapsules were reported by DeAlmeida
et al.34 and Zhou et al.35 and were proven, respectively, to be
effective and recyclable for Suzuki coupling reactions and
reductive ring-opening hydrogenolysis of epoxides. In these
studies, the catalysts were recovered from the reaction mixture
by either ltration or centrifugation techniques. In our study,
magnetic polyurea NPs were designed and utilized as a catalyst
support in which the catalysts could be either entrapped in the
core or adsorbed on the surface.
Pure magnetite NPs and Pdad/MNPs-IL-C4@PU NPs were also
analyzed by X-ray powder diffraction (XRD). The XRD pattern of
the pure magnetite NPs displayed the characteristic peaks at 2q
¼ 18.3, 30.1, 35.5, 43.3, 53.5, 57.1, 62.6, 71 and 74 with cubic
unit cell (ESI, Fig. S5a†). Likewise, the XRD pattern of the Pdad
on MNPs-IL-C4@PU NPs revealed identical characteristic peaks
with ve additional peaks at 2q ¼ 40, 46.6, 67.5, 81.3 and 85.4
which match the characteristic peaks of fcc palladium crystal
structure well. These peaks are attributed to (100), (200), (220),
(311) and (222) reections respectively (ESI, Fig. S5b†).
Entrapment of Pd(OAc)2 within the MNPs-IL-C4@PU NPs
Second approach for immobilization of catalysts is based on
incorporation of catalytic species within the core of the solid
support. In this context, palladium acetate entrapped within the
magnetic PU NPs (Pden) has been established. This was ach-
ieved by following the same procedure applied for the incor-
poration of MNPs. The palladium acetate was dissolved in the
DMAc along with the MNPs-IL-C4 and nanoemulsied in
heptane. This was followed with interfacial polycondensation to
Scheme 3 Preparation of Pdad on MNPs-IL-C4@PU NPs.
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surface of the MNPs-IL-C4. Hence, EDS analysis was carried out
and it conrmed the presence of the palladium on the MNPs-IL-
C4 (ESI, Fig. S7b†).
Catalysis
Hydrogenation reaction was utilized as a model reaction in
order to probe the catalytic efficiency of both Pdad and Pden
catalytic systems.
Scheme 4 Preparation of palladium incorporated within magnetic PU
NPs.
The catalytic performance of Pdad was initially tested with
various substrates under reaction conditions of stirring at 50 ꢀC
for 2 h with a total pressure of 40 psi of hydrogen. The results
are summarized in ESI, Table S1.† The catalyst exhibited high
catalytic activity for the hydrogenation of styrene and its deriv-
atives (ESI, Table S1,† entries 1–6) by fully converting them into
the desired products. X-ray photoelectron spectroscopy (XPS)
was utilized to characterize and conrm the oxidation state of
the palladium before and aer the hydrogenation reactions.
The binding energy of Pdad system before a hydrogenation
reaction of styrene exhibits two strong characteristic peaks
centered at 338.2 eV and 343.4 eV, which are attributed to Pd
3d5/2 and Pd 3d3/2 respectively (ESI, Fig. S8a†). In addition, the
binding energy peaks at 335.6 eV and 340.86 eV corresponds to
the reduction of Pd(II) to Pd(0) during the reaction (ESI,
Fig. S8b†).36
Fig. 5 Micrographs of entrapped Pd(OAc)2 within MNPs-IL-C4@PU
NPs determined by (a) SEM and (b) TEM.
In order to obtain insight as to the catalytic efficiency of the
entrapped palladium within magnetically retrievable PU NPs
(Pden) the same hydrogenation reactions were carried out under
equal reaction conditions. The outcome displayed in ESI, Table
S2† clearly indicates that the palladium inside the PU NPs is less
active compared to that adsorbed. Even though reaction time
was prolonged to 6 h, only styrene (ESI, Table S2,† entry 1) fully
reacted. However, this cannot be said for the other substrates in
which no signicant change in the yield was detected (ESI,
Table S2,† entries 2–4). This decrease in the reactivity of Pden
system might be attributed to inaccessibility of the substrates to
the catalytic sites. Furthermore, XPS analysis depicts the pres-
ence of mainly Pd(II) cations before and aer catalytic hydro-
genation (ESI, Fig. S9†). It is known that Pd(II) is less reactive
than Pd(0) in hydrogenation of olens.37
Since Pdad system exhibited excellent activity compared to
the Pden system, it was further investigated in the hydrogena-
tion of various unsaturated organic compounds. Table 1
displays high activity of Pdad in the hydrogenation of vinyl
arenes (Table 1, entry 1) and simple aliphatic alkenes (Table 1,
entries 2–7), in addition to aromatic alkynes (Table 1, entries 8–
11), in which full conversions were achieved. Simultaneously,
and under identical reaction conditions, hydrogenation of 3-
methylstyrene was carried out using commercial Pd/C (10%)
yielding 64% of the desired product (Table 1, entry 12).
produce polyurea NPs. Preparation route is illustrated in
Scheme 4. The resulted suspension was magnetically separated,
washed with heptane and redispersed in heptane to reach a 20 g
suspension total. SEM and TEM micrographs of Pden system are
depicted in Fig. 5. The STEM/EDS analysis (ESI, Fig. S6†) indi-
cated the presence of palladium in the PU NPs matrix.
Supporting palladium on the surface of MNPs-IL-C4
Heterogenization of metal NPs can be further achieved by
means of a deposition and immobilization directly onto the
surface of solid supports and functionalized MNPs. Herein, in
order to conrm that polyurea matrix is indeed essential for
supporting metal nanoparticles and for obtaining high catalytic
activity and selectivity, we further investigated the capability of
the MNPs-IL-C4 to operate as a catalyst support. In this regard,
the Pd NPs were directly immobilized on the surface of the
modied ionic liquid MNPs simply by exposing the MNPs-IL-C4
dispersed in ethanol to a solution of Na2PdCl4 followed by
reduction with either NaBH4 or hydrazine hydrate affording
supported Pd NPs (Scheme 5).
The supported palladium NPs system was analyzed by TEM
(ESI, Fig. S7a†). The palladium NPs were not easily recognized
in the TEM analysis when they were immobilized directly on the
Selective hydrogenation reaction of a,b-unsaturated
compounds
Selective, and particularly chemoselective, catalytic hydrogena-
tion of a,b-unsaturated organic compounds is an essential and
desirable process in synthetic chemistry and chemical indus-
tries.38 The hydrogenation of C]C and C]O bonds is
Scheme 5 Palladium NPs supported on MNPs-IL-C4.
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Table 1 Hydrogenation of alkenes and alkynes by Pdad/MNPs-IL-
reactions and with the optimized reaction conditions, the scope
of the Pdad catalytic system was further examined in the selec-
tive hydrogenation of a,b-unsaturated organic compounds as
displayed in Table 2. First, we attempted to examine the cata-
lytic performance of Pdad system in the selective hydrogenation
of cinnamaldehyde at room temperature (Table 2, entry 1) 17%
conversion with 15% yield in favour of the saturated carbonyl
compounds was obtained. When the same reaction was carried
out at 50 ꢀC for 2 h, the reaction proceeded to completion with
92% yield (Table 2, entry 2). It is noteworthy that the selective
hydrogenation of unsaturated aldehydes led to the production
of minor amounts of unsaturated alcohols (Table 2, entries 2–
4). Whereas, the Pdad system proved to be highly efficient and
extremely selective for the hydrogenation of C]C bonds while
keeping the carbonyl and the nitro group intact, when unsatu-
rated ketones (Table 2, entries 5–9), unsaturated esters (Table 2,
entries 11, 12) and unsaturated nitro compound (Table 2, entry
10) were employed. In addition, the selective hydrogenation of
cinnamaldehyde with the commercially available Pd/C 10%
(Table 2, entry 13) was examined. 85% conversion with 67%
yield of the saturated carbonyl product was obtained. In addi-
tion, when the same reaction was performed under homoge-
neous conditions using Pd(OAc)2, 20% conversion was obtained
with 14% yield (Table 3, entry 1).
C4@PU NPsa
Entry Substrate
1
Productb
Yieldc (%)
94
2
3
100
100
4
100
5
6
100
100
7
8
96
100
For further conrmation that our catalyst support is highly
efficient and that the presence of PU matrix is indispensable for
obtaining high conversions and selectivities, the Pd NPs sup-
ported on MNPs was tested in the hydrogenation of cinna-
maldehyde as a control reaction. When hydrazine hydrate was
used as the reducing agent only 14% conversion was obtained
(Table 3, entry 2). On the other hand, when NaBH4 was applied,
40% conversion with 29% yield was obtained (Table 3, entry 3).
In order to substantiate that the catalysis is due to the sup-
ported palladium and not by leached palladium species, hot
ltration test was carried out. Selective hydrogenation of cin-
namaldehyde was conducted with Pdad system and H2 pressure
9
100
100
100
10
11
ꢀ
of 40 psi at 50 C. Aer 30 min the catalyst was magnetically
12d
64
separated, and the reaction was allowed to react for further 1.5
h. We have observed that no further reaction occurred aer
performing the hot separation process. This outcome highly
indicates that there is no palladium leaching.
a
Reaction conditions: 1.1
g of the Pdad/MNPs-IL-C4@PU NPs
suspension in heptane containing 0.008 mmol of palladium catalyst,
1.6 mmol substrate in 5 mL heptane, 40 psi hydrogen, 2 h at 50 ꢀC.
b
c
Characterized by 1H-NMR spectroscopy. Determined by 1H-NMR
d
Recyclability of Pdad catalytic system
spectroscopy and GC. Reaction conditions: 0.5 mol% Pd/C (10%), 40
psi hydrogen, 2 h at 50 ꢀC and 6 mL heptane.
Recyclability of the heterogeneous catalysts, while preserving
their catalytic efficiency, is one of the major and notable inter-
ests due to the environmental concerns. Hence, by incorpo-
rating the MNPs-IL-C4 into the core of the PU NPs, the
separation of the product and recovery of the catalyst was easily
accomplished simply by applying an external magnetic eld.
The reusability of the catalyst was assessed in the hydrogena-
tion of 3-methyl styrene and selective hydrogenation of 3-
methylcyclohexenone, under the optimized reaction condi-
tions. Aer each run the catalyst was separated and recycled by
applying an external magnetic eld, washed with heptane and
used directly in the next cycle without further purications. The
recovered catalyst exhibited remarkable efficiency and
thermodynamically viable, producing valuable saturated
carbonyl and unsaturated alcohol compounds which can be
utilized as intermediates for synthesizing ne chemicals,
pharmaceuticals, avoring materials and biological active
compounds.39 Various metal nanoparticles, amongst them
palladium, have been utilized to serve this aim.40 Traditionally,
Pd based catalysts are employed for the selective hydrogenation
of C]C bonds yielding saturated carbonyl compounds.
Encouraged by the results obtained in the hydrogenation
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Table 2 Selective hydrogenations of a,b-unsaturated organic compounds catalyzed by Pdad/MNPs-IL-C4@PU NPsa
Entry
Substrate
Productb
Conversionc %
Yieldd
15
%
1e
17
2
100
92
3
4
5
100
100
96
98
98
96
6
7
100
100
100
100
8
100
100
9
100
100
100
100
10
11
12
100
100
100
100
13f
85
67
a
Reaction conditions: 1.1 g of the Pdad/MNPs-IL-C4@PU NPs suspension in heptane containing 0.008 mmol of palladium catalyst, 1.6 mmol
substrate in 5 mL heptane, 40 psi hydrogen, 2 h at 50 ꢀC. Products were characterized by 1H-NMR spectroscopy. Determined by 1H-NMR
spectroscopy and GC. Determined by GC. Reaction carried out at room temperature. Reaction conditions: 0.5 mol% Pd/C (10%), 40 psi
hydrogen, 2 h at 50 ꢀC and 6 mL heptane.
b
c
d
e
f
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Table 3 Selective hydrogenation of cinnamaldehyde
NPs produced from reduction of Pd(OAc)2 during the hydroge-
nation reaction. It is worth noting that no leaching of palladium
was observed aer the third and h run according to ICP-MS
studies.
Conclusion
Entry
Catalyst
Conversionb (%)
Yieldc (%)
This work demonstrated the development of a new magnetically
retrievable catalytic PU NPs based on adsorption of palladium
acetate on the surface of the magnetic and nitrogen containing
PU NPs. This new catalytic Pdad system exhibited remarkable
activity and higher efficiency in hydrogenation of aromatic and
aliphatic alkenes compared to the encapsulated palladium
system. The scope of the catalytic Pdad system was further
probed in the selective hydrogenation of aromatic and aliphatic
a,b-unsaturated organic compounds demonstrating high and
consistent activity and selectivity. The catalytic Pdad system,
both in hydrogenation and selective hydrogenation reactions
was easily isolated and reused up to 5 times without observing
any signicant loss in catalytic activity. In future studies, the
present catalytic system will be subjected to various other
catalysts and examined in other essential transformations.
1a
2d
3e
Pd(OAc)2
Pd(0)/MNPs-IL-C4
Pd(0)/MNPs-IL-C4
20
14
40
14
12
29
a
Reaction conditions: 0.009 mmol of catalyst, 1.8 mmol
cinnamaldehyde, 40 psi hydrogen, 2 h at 50 ꢀC, 6 mL heptane and 0.3
mL CH2Cl2. Products were characterized by 1H-NMR spectroscopy.
b
c
d
Determined by 1H-NMR spectroscopy and GC. Reducing agent:
hydrazine hydrate, Reaction conditions: 2 g of the dispersed catalyst
in heptane contaning 0.008 mmol palladium catalyst, 1.6 mmol
e
cinnamaldehyde in 3 mL heptane, 50 ꢀC, 40 psi H2, 2 h. Reducing
agent: sodium borohydride. Reaction conditions: same as d but 1.6 g
of dispersed catalyst in heptane contaning 0.008 mmol palladium
catalyst were used.
preserved its catalytic activity and selectivity while maintaining
the yield at 100% even aer 5 subsequent cycles.
SEM of the recycled catalytic system revealed and conrmed
that the morphology and particle size distribution were not
Experimental section
Materials and methods
dramatically changed aer three (Fig. 6a) and ve cycles Poly(1-ethenyl pyrrolidin-2-one/hexadec-1-ene) (Agrimer AL22)
(Fig. 6b). In addition, TEM (Fig. 7a) and EDS (Fig. 7b) micro- and polymethylenepolyphenyl isocyanate (PAPI 27) were
graphs unambiguously revealed the formation and presence of contributed by FMC Corporation. FeCl3$6H2O, FeCl2$4H2O,
Pd nanoparticles of 2–3 nm on the surface of the magnetic PU ammonium hydroxide 25% and 2,6-diaminopyridine were
purchased from Acros Fischer Scientic. (3-chloropropyl) tri-
methoxysilane, 1-butyl imidazole, tris(2-aminoethyl)amine 96%
was purchased from Sigma Aldrich. All the substrates for the
hydrogenation reactions were purchased from either Sigma
Aldrich or Acros and were used without further purications.
Scanning electron microscope (SEM) was utilized to determine
the morphology of the PU-NPs. The experiments were per-
formed on high resolution scanning electron microscope (HR
SEM) Sirion (FEI Company) using Shottky type eld emission
source and secondary electron (SE) detector. The images were
scanned at voltage of 5 kV. Transmission electron microscope
(TEM) and electron diffraction spectroscopy (EDS) were per-
formed with (S) TEM Tecnai F20 G2 (FEI Company, USA) oper-
ated at 200 kV. The infrared spectra were recorded at room
temperature in transmission mode using a Perkin Elmer spec-
trometer 65 FTIR instrument. Thermogravimetric analysis
(TGA) was performed on Mettler Toledo TG 50 analyzer.
Measurements were carried out at temperature range that
extended from 25–950 ꢀC at a heating rate of 10 ꢀC minꢁ1 under
air. Dynamic light scattering (DLS) was utilized to determine the
size distribution of the PU-NPs. These measurements were
performed on Nano Series instrument of model Nano-Zeta Sizer
(Malvern Instruments) model ZEN3600. Gas chromatography
(GC) (Aglient Technologies, 7890A) with a capillary column (HP-
5, 30 meters) was used to determine the conversion of the
hydrogenation reactions. 1H-NMR and 13C-NMR spectra were
recorded with Bruker DRX-400 instrument in CDCl3. Powder X-
Fig. 6 SEM images of recycled catalyst after (a) 3 cycles and (b) 5
cycles.
Fig. 7 (a) TEM micrograph and (b) EDS analysis of Pd NPs adsorbed on
the surface of MNPs-IL-C4@PU NPs.
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ray diffraction (XRD) measurements were performed on the D8
Advance diffractometer (Bruker AXS, Karlsruhe, Germany) with
a goniometer radius 217.5 mm secondary graphite mono-
chromator, 2ꢀ Sollers slits and 0.2 mm receiving slit. Low-
background quartz sample holders were carefully lled with the
powder samples. XRD patterns within the range 2q ¼ 1ꢀ to 90ꢀ
were recorded at room temperature using CuKa radiation (l ¼
Preparation of polyurea nanoparticles
The polyurea nanoparticles were prepared in a typical proce-
dure of interfacial polymerization of an oil-in-oil emulsion
technique. A continuous phase composed of a surfactant solu-
tion of 5 g (5 wt %) Agrimer AL22 dissolved in 90 g of heptane
was homogenized at 10 000 rpm for 30 seconds. A dispersed
phase composed of a mixture of 4.65 g of N,N-dimethylaceta-
mide and 2,6-diaminopyridine (0.35 g, 3.2 mmol) was then
rapidly added during homogenization. The emulsication
process was carried out for further 1 min and 30 seconds at
10 000 rpm followed by sonication for 10 min using ultrasonic
cell disrupter with an output of 130 Watt and 20 KHz. Eventu-
ally, 1.1 g PAPI 27 dissolved in 8.9 g xylene was added slowly to
the emulsion system during sonication. The mixture was then
stirred for 4 h at room temperature. The resulting PU NPs were
collected by centrifugation at 11 000 rpm for 15 min, washed
couple of times with heptane and nally redispersed in heptane
to reach a 20 g suspension total.
˚
1.5418 A) with following measurement conditions: tube voltage
of 40 kV, tube current of 40 mA, step-scan mode with a step size
of 2q ¼ 0.02ꢀ and counting time of 1 s per step. The X-ray
Photoelectron Spectroscopy (XPS) measurements were per-
formed using a Kratos Axis Ultra X-ray photoelectron spec-
trometer (Karatos Analytical, Manchester, UK) using an Al Ka
monochromatic radiation source (1486.7 eV), pass energy 20 eV,
step size 0.1 eV. The binding energies were calibrated using the
C 1s energy as 285.0 eV.
Synthesis of 1-butyl-3-(3-(trimethoxysilyl)propyl)-1H-imidazol-
3-cholride (IL-C4)31
Preparation of magnetic PU NPs (MNPs-IL-C4@PU NPs)
3-Chloropropyltrimethoxysilane (14.2 g, 114.3 mmol) and 1-
butylimidazole (22.73 g, 114.3 mmol) were stirred under inert
atmosphere at 120 ꢀC for 24 h. The mixture was cooled down to
room temperature and yellow-orange viscous liquid (35.32 g)
was obtained (96% yield). 1H NMR (400 MHz, CDCl3) d 0.524 (t, J
¼ 8 Hz, 3H), 0.847 (t, J ¼ 7.2 Hz, 3H), 1.25 (m, 2H), 1.81 (m, 2H),
1.91 (m, 2H), 3.45 (s, 9H), 4.27 (q, J ¼ 7.2 Hz, 4H), 7.39 (s, 1H),
7.52 (s, 1H), 10.57 (s, 1H) ppm. 13C NMR (100 MHz, CDCl3) d
5.94, 13.11, 19.27, 23.89, 32.01, 49.31, 50.12, 51.29, 121.97,
122.37, 136.48 ppm.
Magnetic PU NPs were prepared by incorporation of magnetite
NPs-IL-C4 into the PU NPs. 0.02 g of MNPs-IL-C4 were dispersed
in the mixture of 4.65 g N,N-dimethylacetamide and 2,6-dia-
mino pyridine (0.35 g, 3.2 mmol) and sonicated in a bath son-
icator for 60 min. The incorporation process has been achieved
by following the same procedure and using the same amount of
surfactant solution as it described in preparation of PU NPs.
Lastly, the reaction mixture was mechanically stirred at room
temperature for 4 h. The resulting magnetic PU NPs were
separated by external magnetic eld and redispersed in
heptane.
Preparation of magnetite nanoparticles supported IL-C4
(MNPs-IL-C4)
Encapsulation of palladium acetate within MNPs-IL-C4@PU
NPs
11.73 g FeCl3$6H2O and 4.4 g FeCl2$4H2O were dissolved and
mechanically stirred in 400 mL deionized watꢀer under inert
atmosphere. The mixture was heated at 85–90 C followed by
quick addition of 18 mL concentrated ammonia 25% gener-
ating black suspension of magnetite NPs. The mixture was
heated for further 15 min, and then it was allowed to cool to
room temperature. The black magnetite nanoparticles were
separated via external magnetic force and washed 4 times with
250 mL deionized water and once with 200 mL ethanol. The
magnetite nanoparticles were suspended in 400 mL ethanol and
sonicated for 90 min. The suspension was mechanically stirred
and a solution of 100 mL of ethanol containing 30 mmol of the
functionalized IL-C4 and 15 mL of concentrated ammonia
(25%) were added and the reaction mixture was allowed to stir
under nitrogen for 36 h at room temperature. The modied
magnetite NPs were magnetically separated, washed three times
Pd(OAc)2 (36 mg, 0.16 mmol) was dissolved in the dispersed
phase containing 0.02 g MNPs-IL-C4, 4.65 g N,N-dimethyl acet-
amide and 2,6-diaminopyridine (0.35 g, 3.2 mmol) and soni-
cated for 60 min. Encapsulation was achieved by following the
same procedure as for preparation of MNPs-IL-C4@PU NPs. The
catalytic magnetic PU NPs were separated by external magnetic
eld, washed three times with heptane and eventually redis-
persed in heptane to reach a 20 g suspension total. The loading
of Pd in this system was found to be 0.1 mmol gꢁ1 according to
inductively coupled plasma mass spectrometry measurements
(ICP-MS).
Adsorption of Pd(OAc)2 on the surface of the MNPs-IL-C4@PU
NPs
with ethanol and once with methanol. Then MNPs were sus- The prepared MNPs-IL-C4@PU NPs were magnetically sepa-
pended in 400 mL methanol and mechanically stirred for rated from heptane, washed with THF, redispersed in 80 mL
another 3 h at room temperature. Eventually, 100 mL diethyl THF and sonicated for 20 min. The suspension was mechan-
ether was added to the suspension and the modied magnetite ically stirred at room temperature and Pd(OAc)2 (36 mg, 0.16
NPs were magnetically separated, washed with diethyl ether and mmol) in 2 mL THF was added to the suspension and all was
dried under vacuum for 12 h. 5 g of black magnetite NPs powder mixed for 24 h. The resulting catalytic system was separated via
was obtained.
external magnetic eld, washed twice with heptane and
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redispersed heptane to reach a 20 g suspension total. The
palladium loading in this system is 0.09 mmol gꢁ1 according to
ICP-MS analysis.
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magnetite nanoparticles (Pd(0)/MNPs-IL-C4)
0.5 g of the ionic liquid-modied magnetite nanoparticles were
dissolved in 100 mL of ethanol and sonicated for 60 min. The
suspension was mechanically stirred at room temperature and
60 mg (0.172 mmol) of Na2PdCl4 dissolved in 2 mL of distilled
water was added. The resulted mixture was stirred for 24 h at
room temperature, and then 2 mL hydrazine hydrate or 0.26
mmol sodium borohydride were added. Aer stirring for 6 h,
the supported palladium nanoparticles were magnetically
separated and washed twice with 30 mL water, twice with 20 mL
ethanol and twice with 20 mL heptane. The washed nano-
particles were dissolved in 15 mL heptane. The loading of
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hydrazine and 0.15 mmol gꢁ1 when it was sodium borohydride.
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General procedure for the hydrogenation and selective
hydrogenation reaction catalyzed by Pdad and Pden systems
A 25 mL glass lined autoclave was charged with 1.6 mmol
substrate dissolved in 5 mL heptane and the proper amount of
the catalytic system dispersed in heptane containing 0.008
mmol palladium. The autoclave was sealed, purged three times
with hydrogen, and pressurized toꢀ40 psi with hydrogen. The
reaction mixture was stirred at 50 C for 2–6 h. The autoclave
was cooled to room temperature and the gas was released. The
catalytic system was magnetically separated and the solution
was decanted, ltered through Celite and evaporated under
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General procedure for the selective hydrogenation catalyzed
by immobilized palladium on MNPs-IL-C4
A 25 mL glass lined autoclave was charged with 1.6 mmol
substrate dissolved in 3 mL heptane and the desired amount of
Pd(0)/MNPs-IL-C4 dispersed in heptane (containing 0.008 mmol
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was decanted, ltered through Celite and evaporated under
reduced pressure.
´
˜
(d) J. Puig, C. E. Hoppe, L. A. Fasce, C. J. Perez, Y. Pineiro-
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