Magnetically recoverable catalysts based on polyphenylenepyridyl dendrons and dendrimers

Article abstract of DOI:10.1039/c4ra00878b

Here, a systematic study of magnetite nanoparticle (NP) formation in the presence of functional polyphenylenepyridyl dendrons and dendrimers of different generations and structures (such as focal groups, periphery and a combination of phenylene and pyridyl moieties) has been reported. For certain dendron/dendrimer concentrations and structures, well-dispersible, multi-core, flower-like crystals are formed which display ferrimagnetic-like behavior. It is noteworthy that the least complex second generation polyphenylenepyridyl dendrons with a carboxyl focal group already allow formation of flower-like crystals. Magnetically recoverable catalysts were obtained via Pd NP formation in the dendron/dendrimer shells of magnetite NP and tested in selective hydrogenation of dimethylethynylcarbinol to dimethylvinylcarbinol. Dependences of catalytic activity and selectivity on the dendron/dendrimer generation and structure, type of Pd species, and Pd NP size have been demonstrated. High selectivity and activity of these catalysts along with easy catalyst recovery and successful repeated use make them promising in catalytic hydrogenation.

Full text of DOI:10.1039/c4ra00878b

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PAPER
Magnetically Recoverable Catalysts Based on
Polyphenylenepyridyl Dendrons and Dendrimers†
Cite this: RSC Adv., 2014, 4, 23271
E. Yu. Yuzik-Klimova,a N. V. Kuchkina,a S. A. Sorokina,a D. G. Morgan,b B. Boris,b
L. Zh. Nikoshvili,c N. A. Lyubimova,c V. G. Matveeva,c E. M. Sulman,c B. D. Stein,d
e
e
f
f
a
*
W. E. Mahmoud, A. A. Al-Ghamdi, A. Kostopoulou, A. Lappas, Z. B. Shifrina
be
*
and L. M. Bronstein
Here, a systematic study of magnetite nanoparticle (NP) formation in the presence of functional
polyphenylenepyridyl dendrons and dendrimers of different generations and structures (such as focal
groups, periphery and a combination of phenylene and pyridyl moieties) has been reported. For certain
dendron/dendrimer concentrations and structures, well-dispersible, multi-core, flower-like crystals are
formed which display ferrimagnetic-like behavior. It is noteworthy that the least complex second
generation polyphenylenepyridyl dendrons with a carboxyl focal group already allow formation of
flower-like crystals. Magnetically recoverable catalysts were obtained via Pd NP formation in the
dendron/dendrimer shells of magnetite NP and tested in selective hydrogenation of
dimethylethynylcarbinol to dimethylvinylcarbinol. Dependences of catalytic activity and selectivity on the
dendron/dendrimer generation and structure, type of Pd species, and Pd NP size have been
demonstrated. High selectivity and activity of these catalysts along with easy catalyst recovery and
successful repeated use make them promising in catalytic hydrogenation.
Received 30th January 2014
Accepted 13th May 2014
DOI: 10.1039/c4ra00878b
www.rsc.org/advances
syntheses functionalization, i.e., iron oxide NP are rst
synthesized and then functionalized either by ligand exchange
reaction32,33 or by placement of polymer chains on the nano-
Introduction
Magnetically recoverable catalysts have attracted considerable
attention due to the combination of catalytic properties with
easy catalyst recovery from reaction mixtures and their repeated
use.1–5 Magnetically recoverable catalysts allow conservation of
energy, more environmentally friendly processes and cheaper
target products.6–12 Normally, these catalysts consist of func-
tionalized magnetic nanoparticles (NP). Among various
magnetic materials, magnetite (Fe3O4) is oen preferred due to
its strong magnetic response and stability in ambient condi-
tions.13–20 Functionalization of these NP can be carried out in
numerous ways involving surfactants, polymers, or other func-
tional capping molecules.21–31 In the majority of cases, it is post-
particle surface,21,22 or by covalent attachment of functional
molecules to the NP surface.34–36 In all functionalization proto-
cols, however, it is difficult to control the distribution of func-
tional ligands on the NP surface. From this point of view the
most straightforward method allowing uniform distribution of
functional capping molecules on the NP surface is direct
formation of magnetic NP in the presence of functional
ligands37 followed by incorporation of catalytic species in the
ligand shells.
In our preceding paper,
a
family of new poly-
phenylenepyridyl dendrons of different structures has been
developed.38 As a proof of concept for the second generation
dendrons with dicarboxylate focal points, it was proved that
they can serve as capping molecules for magnetically recover-
able catalysts. In the current paper, a systematic study of
magnetite NP formation in the presence of dendrons and den-
drimers of different generations and structures is presented.
Magnetically recoverable catalysts were obtained via Pd NP
formation in the dendron/dendrimer shells of iron oxide NP
and tested in selective hydrogenation of dimethylethynylcarbi-
nol (DMEC) to dimethylvinylcarbinol (DMVC). The depen-
dences of catalytic activity and selectivity on the dendron/
dendrimer generation and structure, type of Pd species, and the
Pd NP size are demonstrated. High selectivity and activity of
aA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of
Sciences, 28 Vavilov St., Moscow, 119991, Russia
bIndiana University, Department of Chemistry, Bloomington, IN 47405, USA. E-mail:
lybronst@indiana.edu
cTver State Technical University, Department of Biotechnology and Chemistry, 22 A.
Nikitina St, Tver, 170026, Russia
dIndiana University, Department of Biology, Bloomington, IN 47405, USA
eKing Abdulaziz University, Faculty of Science, Department of Physics, Jeddah, Saudi
Arabia
fFoundation for Research and Technology-Hellas, Institute of Electronic Structure and
Laser, Heraklion, 71110, Crete, Greece
† Electronic supplementary information (ESI) available: XRD, TEM images, STEM
and EDS maps, FTIR and TGA data. See DOI: 10.1039/c4ra00878b
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these catalysts along with easy catalyst recovery and successful temperature was increased with a heating rate 10ꢀ minꢁ1 and
ꢀ
repeated use make these nanomaterials promising for hydro- the ask was allowed to reux for 1 h at 283–285 C. The ask
genation and likely for a number of other catalytic reactions.
was then removed from the heating mantle and allowed to cool
to room temperature. To isolate NP, a part of the reaction
solution was precipitated by ethanol and washed several times
with ethanol and acetone until colorless and transparent
supernatant was obtained. The latter was removed, while the
Experimental part
Materials
Iron(III) acetylacetonate (99+%) was purchased from Acros precipitate was dispersed in chloroform upon sonication for 20
Organics and was used as received. Palladium(II) acetate was min. Aer that the solution was centrifuged for 7–10 min to
purchased from Strem Chemicals and also used as received. remove aggregates if any. The rest of the reaction solution was
DMEC ($98%), toluene (99.8%), benzyl ether ($98%), and 2- stored in a refrigerator and was stable for many months. This
propanol ($99.5%) were purchased from Sigma-Aldrich and sample is notated NP1-4. “NP” stands for iron oxide NP; the rst
used without purication. Acetone (99.5%) and chloroform number indicates a dendron/dendrimer according to Schemes 1
(99.8%) were purchased from MACRON Chemicals and used as & 2 and the second number indicates the synthesis number
received.
according to Table 1. This table lists the reaction conditions and
the sizes of the iron oxide NP formed.
Synthetic procedures
Interaction of iron oxide NP coated by dendrons/dendrimers
with palladium acetate
dendrons and dendrimers used as capping molecules in this The iron oxide NP solution (0.5 mg mLꢁ1) was placed in a ask
The syntheses of functional polyphenylenepyridyl dendrons
and dendrimers are described in our preceding papers.38,39 The
work are presented in Schemes 1 and 2.
and purged with argon under stirring. The 1 mg mLꢁ1 solution
of Pd acetate in chloroform was also purged with argon and
added to the iron oxide NP solution dropwise. This addition was
notated I. The weight ratio of Pd acetate to iron oxide NP was 2/
3. Aer addition the ask was closed and kept under stirring
overnight at room temperature. The NP were collected with
magnetic separation using a rare earth magnet and washed with
chloroform 2–3 times. Then the sample was dried for 30 min in
a vacuum oven and dispersed in 5 mL of ethanol under soni-
cation. Alternatively, the Pd acetate solution was added to the
NP solution at once, sonicated for 10 min and stirred overnight
at room temperature. This procedure was notated II.
Iron oxide NP syntheses
Syntheses of iron oxide nanoparticles in the presence of den-
drons/dendrimers were carried out according to the following
protocol. In a typical experiment, the three-neck round-bottom
ask (with elongated necks) equipped with a magnetic stir bar,
a reux condenser, and two septa, one of which contained an
inserted temperature probe protected with a glass shield and
the other had a long inserted needle, was loaded with 0.353 g (1
mmol) of Fe(acac)3, 0.140 g (0.11 mmol) of dendron 1, and 7 mL
of benzyl ether. The ask was placed in a Glas-Col heating
mantle attached to a digital temperature controller which in
turn was placed on a magnetic stirrer. The ask was degassed by
Reduction of Pd complexes with hydrogen
argon bubbling for 30 min under stirring. Then the temperature Ethanol dispersion (5 mL) of Pd-containing sample was placed
was raised to 60 ꢀC at 10ꢀ minꢁ1 and kept under stirring at this in a three-neck round-bottom ask equipped with a stir bar, a
temperature for 30 min to allow solubilization. Then the septum with a long needle, and a reux condenser. The ethanol
Scheme 1 Structures of polyphenylenepyridyl dendrons of the second (1) and third (2 and 3) generations. Pyridine rings are shown in red,
phenylene rings are in black, and carboxyl focal groups are shown in blue.
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Scheme 2 Structures of polyphenylenepyridyl dendrimers of the third generation with tetra- (4) and tri-substituted (5, 6) cores. Color coding is
similar to that in Scheme 1.
solution was diluted with 5 mL of distilled water. Then the an accelerating voltage of 80 kV on a JEOL JEM1010 trans-
reaction solution was purged with argon under stirring for 1 h mission electron microscope. Images were analyzed with the
and then bubbled with hydrogen under stirring for 2 h. Aer National Institute of Health developed image-processing
that the solution was transferred to a vial, the catalyst was package ImageJ to estimate NP diameters. Between 150 and 300
separated with a magnet and washed several times with NP were used for this analysis. High resolution TEM (HRTEM)
ethanol. Then the sample was dried in a vacuum oven overnight images and energy dispersive X-ray spectra (EDS) were acquired
and dispersed in chloroform.
at accelerating voltage 300 kV on a JEOL 3200FS transmission
electron microscope equipped with an Oxford Instruments
INCA EDS system. The same TEM grids were used for both
analyses.
Characterization
Electron-transparent NP specimens for transmission electron
Fourier Transform Infrared (FTIR) spectra were recorded on
microscopy (TEM) were prepared by placing a drop of dilute a Nicolet 510P FT-IR spectrometer. The samples were prepared
solution onto a carbon-coated Cu grid. Images were acquired at by evaporating their chloroform solutions on a KBr disk.
Table 1 Reaction conditions of iron oxide NP syntheses with dendrons/dendrimers and the NP characteristicsa
Dendron/
Dendron/
dendrimer
loading (g)
Dendron/
dendrimer
loading (mmol)
dendrimer
Size of
ower-like
NP (nm)
Standard
deviation of
NP sizes (%)
Yield of
ower-like
NP (%)
concentration
Yield of
NP (%)
Yield of
aggregates (%)
Notation
(mg mLꢁ1
)
NP1-1
NP1-2
NP1-3
NP1-4
NP1-5
NP2-1
NP2-2
NP2-3
NP3-1
NP4-1
NP5-1
NP5-2
NP6-1
NP6-2
NP6-3
0.279
0.837
1.395
0.140
0.07
0.616
0.308
0.154
0.615
0.607
0.592
0.296
0.596
0.397
0.297
0.22
0.66
1.1
0.11
0.055
0.22
0.11
0.055
0.22
0.055
0.073
0.0365
0.073
0.049
0.037
40
120
199
20
10
88
44
22
88
87
85
42
85
57
42
72
5
2
10
83
87
8
22.7
77.3b
52.6b
21.8
12.8
18.0
22.6
15.1
17.1
16.1
20.4
20.5
11.0
24.8
19.5
20.6
22.8
20.7
20.4
63.0
—
—
61.1
59.3
100.0
74.0
42.7
75
70
0
65
68
65
61
59
63
0
13
93
17
15
17
18
16
17
92
90
93
20.5
55.8b
21.1
16.1
23.1
22.6
34.4
85.5
76.0
89.6
—
—
—
32.0
101.2b
105.4b
57.5b
0
0
a
b
0.353 g (1 mmol) Fe(acac)3 and 7 mL of benzyl ether were used in all procedures. The iron oxide NP are fully aggregated.
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X-ray diffraction (XRD) patterns were collected on an molecules, thus, the compounds are less expensive. In addition,
Empyrean from PANalytical. X-rays were generated from a the higher the Fe(acac)3/dendron(dendrimer) molar ratio, the
˚
copper target with a scattering wavelength of 1.54 A. The step- lower the fraction of valuable dendrons/dendrimers used in
size of the experiment was 0.02.
syntheses, making this approach appealing for practical appli-
single
Thermal gravimetric analysis (TGA) was performed on cations. Therefore, the possibility of obtaining
a
TGAQ5000 IR manufactured by TA Instruments under nitrogen. morphology iron oxide NP with the second generation dendron
About 2–3 mg of NP were placed into a 100 mL platinum pan by 1 with a carboxyl focal group and the mixed pyridine–phenylene
consecutive evaporations of small volumes of concentrated NP periphery (Scheme 1) has been explored.
solutions. The experiments wereꢁc1arried out upon heating to
1000 C with a rate of 10.0ꢀ min with the purpose of deter-
mining the amount of dendron/dendrimer on the NP surface.
The magnetic properties of the samples were studied by a
ꢀ
Morphology and composition of iron oxide NP formed in the
presence of dendron 1
Superconducting Quantum Interference Device (SQUID) At the molar ratio precursor/dendron 1 : 0.22 which was typi-
magnetometer (Quantum Design MPMS XL5). The measure- cally used in similar protocols,38 a mixture of multi-core and
ments have been performed in dried nanoparticles on cotton in single core NP was obtained (Fig. 1a). The Fast Fourier Trans-
a gelatin capsule. The isothermal hysteresis loops, M(H), were form (FFT) pattern of the HRTEM image (inset in (b)) reveals the
measured at elds ꢁ1 # H # +1 Tesla. The dc magnetic single crystalline arrangement inside the particles. Apparently,
susceptibility as a function of temperature, c(T), was attained the multicore, ower-like NP were formed due to oriented
down to 5 K under zero-eld cooled (ZFC) and eld-cooled (FC) attachment upon single core aggregation.41 The XRD pattern
protocols, at H ¼ 50 Oe.
(Fig. S1, the ESI†) conrmed the magnetite phase which is
consistent with our previous results.38 Thus, it was assumed
that the dendron 1 functionality might be not sufficient to
efficiently stabilize ower-like iron oxide NP at this
concentration.
In order to increase the fraction of ower-like NP, the
precursor/dendron molar ratio was increased to 1 : 0.66 and
1 : 1.1, i.e., ratios used when traditional surfactants (oleylamine
and oleic acid) are employed.42 However, only aggregated NP
were obtained and the size increases to 52.6 nm at molar ratio
1 : 1.1 (Fig. 1d) and to 77.3 nm at 1 : 0.66 (Fig. 1c). Alternatively,
when the fraction of the dendrons decreases to molar ratios
equal to 1 : 0.11 and 1 : 0.055 (Fig. 1e and f), the aggregation is
minimal, while NP sizes are 21.8 nm and 20.5 nm, respectively,
indicating that the dendron loading inuences strongly the
nal morphology and size of the ower-like NP.
Catalytic studies
Catalytic testing was carried out in a 60 mL isothermal glass
batch reactor installed in a shaker and connected to a gas-
ometric burette (for hydrogen consumption control). The total
volume of liquid phase was 30 mL. Reaction conditions were:
ambient hydrogen pressure, stirring rate 850 shakings/minute,
ꢀ
temperature 90 C. Toluene was used as a solvent.
Samples were periodically taken and analyzed via GC-MS
(Shimadzu GCMS-QP2010S) equipped with a capillary column
HP-1MS (30 m ꢂ 0.25 mm i.d., 0.25 mm lm thickness). Helium
was used as a carrier gas at ow rate 1 mL minꢁ1. Analysis
ꢀ
conditions: oven temperature 60 C (isothermal), injector and
ꢀ
ꢀ
interface temperature 280 C, ion source temperature 260 C,
range from 10 up to 200 m/z. For catalyst recycling, the catalyst
was separated using a rare earth magnet, washed with toluene
and ethanol and dried in vacuum oven for 3 hours.
This behavior, when iron oxide NP aggregation occurs at the
higher capping molecule loadings instead of better NP stabili-
zation, can be attributed to the dendron aggregation at higher
concentrations leading, in turn, to the formation of larger iron
oxide NP and their subsequent aggregation.
Results and discussion
The dendron 1 adsorption on the NP surface was further
To assess the inuence of dendron/dendrimer structures on the supported by the FTIR study. The FTIR spectra of dendron 1 and
morphology of iron oxide NP, the dendrons of the second and NP1-1 (see Fig. S2 and the text underneath, ESI†) show that the
third generations with the carboxyl focal group and different dendrons are present in the NP1-1 sample, although the
interior structure and the dendrimers of the third generation majority of the dendron FTIR bands become weaker or nearly
with different central cores and exterior (Schemes 1 and 2) were disappear as in the case of the bands responsible for vibrations
explored. Fig. 1 shows that iron oxide NP prepared in the of substituted arenes and pyridine rings. This might occur due
presence of dendron 1 have a mixed morphology. The sample to dendron adsorption on the iron oxide NP surface. A TGA trace
ꢀ
consists of multicore, ower-like NP and single core NP. The of NP1-1 (Fig. S3, ESI†) shows a weight loss of 56% at 1000 C
goal was to identify the proper conditions and the type of due to dendron loss. It is noteworthy, however, that 1 itself
capping molecules to obtain iron oxide samples with a ower- decomposes with the formation of 32 wt% of coke (Fig. S3, ESI†)
like morphology only, as more interesting magnetic properties but the coke formation can be different in NP1-1due to iron
were expected.40
oxide catalytic inuence during thermal decomposition. Thus,
The molar ratio of the iron precursor to capping molecules the exact amount of dendrons on the NP surface cannot be
has been varied to assess its inuence on the iron oxide NP determined, but it is estimated that it is not less than 56 wt%.
formation. It is worth noting that the lower the dendron/den-
Magnetic measurements were carried out for NP1-1 and NP1-
drimer generation, the shorter the syntheses of these capping 4, prepared with different amounts of dendron 1. The blocking
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Fig. 1 TEM images of the Fe3O4 NP formed in the presence of 1 at the molar ratios Fe precursor/dendron 1 : 0.22 (a), 1 : 0.66 (c), 1 : 1.1 (d), 1 : 0.11
(e), and 1 : 0.055 (f). Insets in (a, e and f) show higher magnification images. HRTEM image and the corresponding FFT pattern (inset) of the NP of
panel (a) are shown in (b).
temperature, TB, as indicated from the zero eld and eld formed with no single-core particles le out of the aggregate
cooled susceptibility curves (Fig. 2a) and the coercive eld, HC, (Fig. S4. ESI†). Much smaller ower-like particles were obtained
(Fig. 2b and c) are equal for the two samples due to the similar for decreased molar ratios (1 : 0.11 and 1 : 0.055) (Table 1). The
NP size. The TB for both samples is above 300 K, revealing that fraction of single core iron oxide NP increases with the decrease
these NP are ferrimagnetic at room temperature. This is also in of the dendron loading. On the other hand, for the third
agreement with the non-zero coercive eld, HC, derived from the generation dendron 3, which has fully phenylene middle layer
isothermal magnetization curves at the same temperature pre- (with no pyridine moieties) and thus lower functionality, at the
sented in Fig. 2b. This holds promise for easy magnetic molar ratio of Fe to the dendron 1 : 0.22 (at 88 mg mLꢁ1
switching from ferrimagnetic to superparamagnetic behaviour concentration) mainly well-dispersible iron oxide NP (a mixture
at a minor temperature change and therefore controllable of multi-core, ower-like NP and single cores) were obtained.
magnetic separation.
This indicates that both the generation and functionality of the
Thus, the least expensive, second generation poly- dendrons play a crucial role in controlling the size and the
phenylenepyridyl dendrons with a carboxyl focal group allow morphology of the iron oxide NP formed.
successful formation of ower-like crystals, but it was desirable
to increase the multicore NP fraction.
To evaluate the importance of the dendron focal groups and
the increase of the amount of functional groups in one mole-
cule, iron oxide NP formation in the presence of third genera-
tion dendrimers (with structures 4, 5, 6 presented in Scheme 2)
was studied.39 These macromolecules contain no carboxyl focal
groups. Considering that a third generation dendron is a
quarter of the dendrimer 4 with a tetra-substituted core, the
molar ratio of the precursor to dendrimer was chosen 1 : 0.055
equal to the 1 : 0.22 ratio used for dendron capped iron oxide
NP (Table 1). The iron oxide NP in NP4-1 (Fig. S5a, ESI†) were
compared with those in NP2-1 (Table 1) because of similar
Inuence of the dendron/dendrimer structure and generation
on the iron oxide NP morphology
In order to assess the inuence of the dendron generation on
iron oxide NP formation, the third generation dendron 2 whose
structure is similar to that of 1 has been explored. At the molar
ratio precursor/dendron 1 : 0.22 (dendron concentration ¼ 88
mg mLꢁ1) similar to those of NP1, large ower-like NP were
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different Fe/dendrimer molar ratios: 1 : 0.073 and 1 : 0.365
(Fig. S5, ESI†) was used. For 5, the ratio 1 : 0.073 is analogous to
the 1 : 0.22 molar ratio of the dendron loading. TEM images
presented in Fig. S5b and c (ESI)† demonstrate that for the
dendrimers with the tri-coordinate core similar, although
slightly larger particles were formed. Probably a larger size can
be attributed to a more open the dendrimer 5 structure allowing
weaker NP stabilization and therefore larger NP (Table 1). When
the periphery of the dendrimer with a tri-coordinate core is fully
pyridine (6), it strongly increases dendrimer intermolecular
interactions leading to NP aggregation at all molar ratios used
as shown in Table 1 (TEM images are not shown).
Formation of catalysts based on iron oxide NP stabilized by
dendrons/dendrimers
To synthesize catalysts based on magnetic iron oxide NP, the
latter were reacted with Pd acetate and further reduced using
hydrogen as is shown in Scheme 3. The complexation of iron
oxide NP with Pd species leads to the iron oxide NP aggregation
due to interparticle complexation. This aggregation is crucial
for easy magnetic separation.33,38 The reduction of PdAc-con-
taining species in these aggregates with hydrogen resulting in
Pd NP, does not change the aggregate size and morphology
(Fig. 3).
A dark-eld STEM image and EDS mapping of the NP1-1–
PdNP sample presented in Fig. S6 (ESI)† show superposition of
Pd on Fe indicating that Pd NP are grown in the iron oxide NP
dendron shells.
Pd NP are also incorporated in the shell of the iron oxide NP
capped with dendrimer 4 (Fig. 4 and 5) as indicated from the
elemental mapping (Fig. 5). In this case, Pd NP formed are even
smaller (<1 nm). Thus, all dispersible ower-like iron oxide NP
successfully interact with Pd acetate and allow formation of Pd
NP, the sizes of which depend on the structure of the dendron/
dendrimer.
The method of Pd acetate addition determines the nal
morphology of the aggregates. For the iron oxide NP capped
with dendron 1, when the chloroform solution of Pd acetate was
added slowly (dropwise) to the NP solution, the iron oxide NP
self-assemble due to complexation into long rod-like aggregates
(Fig. S7 and S8a, ESI†). When the iron oxide NP and Pd acetate
solutions were mixed at once, the aggregates formed are irreg-
ular and fuzzier (Fig. S8b, ESI†). Alternatively, with the den-
drons 3, rod-like Pd-containing aggregates are formed even
Fig. 2 ZFC-FC susceptibility curves as function of temperature under
a magnetic field of 50 Oe (a) and isothermal magnetization curves at
300 K (b) and 5 K (c) for NP1-1 (red) and NP1-4 (black). In (a) ZFC curves
are shown with filled symbols, while FC curves are shown with empty
symbols.
interior and exterior structure (i.e., mixed phenylene and pyridyl
periphery) in 2 and 4. The comparison of the iron oxide NP
morphology shows that dendrimers allow much smaller ower-
like NP without substantial aggregation. Thus, the higher total
amount of functional groups and/or the presence of focal
carboxyl groups facilitate interaction between dendrons and
leads to the formation of larger particles and aggregates.
To evaluate the inuence of the core structure, dendrimer 5
Scheme 3 Schematic representation of the complexation of iron
oxide NP (blue circles) coated with dendrons/dendrimers (dark red
shell) with Pd acetate (PdAc, an orange shell) and the growth of Pd NP
with similar periphery and a tri-coordinate core at the two (yellow circles) in the iron oxide NP shells.
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Fig. 5 Dark-field STEM image (a) and Fe (b) and Pd (c) maps of the
NP4-1–PdNP sample. Scale bar is 25 nm.
behavior upon complexation can be attributed to the presence
of carboxyl groups in the exterior of the iron oxide NP stabilized
with 3 and their absence for 4. Indeed, the FTIR spectrum of
NP3-1 (Fig. S12, ESI†) shows a band at 1715 cmꢁ1, indicating
that not all carboxyl groups are adsorbed on the iron oxide NP
surface which promotes self-assembly into rod-like aggregates.
Catalytic behavior of magnetically recoverable catalysts
Iron oxide NP containing Pd species were tested for their catalytic
performance in a model reaction of selective hydrogenation of
dimethylethynylcarbinol (DMEC) to dimethylvinylcarbinol
(DMVC) (Scheme 4), which is an intermediate product in
syntheses of fragrant substances and vitamins E and K.43
Fig. 3 HRTEM image of NP1-1 after interaction with Pd acetate and Pd
NP formation (NP1-1–PdNP). Yellow arrows indicate iron oxide NP,
while red arrows point to the Pd NP.
The data of catalytic testing are presented in Table 2. The
turnover frequency (TOF) values in hydrogenation with the
catalysts based on dendron 1 and prepared upon fast and slow
addition of Pd acetate show that the latter leads to a slightly
higher value, presumably, due to more accessible catalytic
centers. At the same time, the catalyst based on 3 allows much
higher activity (at the same selectivity) than the catalyst based
on 1, indicating that aggregate morphology plays a minor role in
overall catalytic properties. The major factors inuencing the
catalytic properties are likely the Pd NP size and the Pd NP
surface functionalization due to the dendron/dendrimer
adsorption. For example, in the case of NP3-1–PdNP, the Pd NP
size is 1.5 nm, while for NP1-1–PdNP, the Pd NP diameter is 1.9
nm and the former catalyst is more active (Table 2).
It is noteworthy, that the TOF is more than doubled when Pd
NP are formed in the catalyst before catalytic reaction (NP3-1–
PdNP) compared to the catalyst containing PdAc residues (NP3-
1–PdAc). In the latter case, the Pd NP are also formed, but
during a catalyst pretreatment step with hydrogen which is
happening before catalytic reaction in a toluene solution (in a
catalytic reactor). Toluene is a good solvent for dendrons/den-
drimers and allows swelling of the iron oxide NP shells. As a
result, Pd NP formed are much larger, 2.8 nm (Fig. S10, ESI†),
Fig. 4 HRTEM image of NP4-1–PdNP. Smaller particles represent Pd
NP (red arrows), while larger particles are the iron oxide NP (yellow
arrows).
upon fast Pd acetate addition, revealing that aligning of NP into
rods is aided by a higher functionality of these dendrons
(Fig. S9, ESI†).
For the particles based on 4, much smaller and uffier
aggregates are formed even upon slow addition of the Pd acetate
Scheme 4 Hydrogenation of DMEC to DMVC. 2-Methylbutane-2-ol is
solution to the NP solution (Fig. S11, ESI†). Such a different a side product. “Cat” stands for the catalyst of hydrogenation.
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Table 2 Results of DMEC hydrogenation using Pd-containing magnetically separable catalysts
Dendron/
dendrimer type
Pd state/
content (%)
Notation of
the catalyst
Pd NP
size, nm
Standard deviation
of NP sizes, %
Selectivity at 95%
of DMEC conversion, %
TOF, sꢁ1
2.2
1
Pd NP/6.1
(PdAc added slowly)
Pd NP/6.2
NP1-1–PdNP-I
NP1-1–PdNP-II
1.8
1.9
25.1
26.2
97.5
97.9
2.5
(PdA added fast)
PdAc/4.7
Pd NP/4.9
3
4
NP3-1–PdAc
NP3-1–PdNP
NP4-1–PdAc
NP4-1–PdNP
—
1.5
—
—
18.3
—
4.3
9.3
1.6
6.7
7.2
7.1
2.4
97.5
97.5
97.0
96.8
97.1
97.0
94.6
PdAc/5.1
Pd NP/5.3
0.9
23.4
2nd use
3rd use
Lindlar catalyst
(2 wt% Pd in Pd/CaCO3)
—
—
—
compared to the 1.5 nm Pd NP formed by hydrogen reduction in NP3-1–PdNP should be close to the optimal size allowing the
ethanol–water mixture (NP3-1–PdNP). Thus, a clear dependence highest TOF demonstrated in this work. It is worth noting that
of TOF on the Pd NP size is observed. At the same time, the the NP3-1–PdNP and NP4-1–PdNP catalysts are signicantly
activity of NP4-1–PdAc was much lower than that of NP3-1– more active and selective than the Lindlar catalyst (Table 2)
PdAc, indicating once more that aggregate morphology has which is traditionally used in a selective hydrogenation.38
nearly no inuence on the catalytic performance. Alternatively,
aer hydrogen reduction of NP4-1–PdAc to NP4-1–PdNP TOF
increases dramatically due to formation of 0.9 nm Pd NP
Conclusions
Formation of ower-like multicore crystals in the presence of
second and third generation polyphenylenepyridyl dendrons
and third generation polyphenylenepyridyl dendrimers as
capping molecules has been demonstrated. It was proven that
the dendron/dendrimer structures and their concentrations
control the iron oxide NP size, morphology and the tendency to
aggregation. The higher the concentration of the capping
molecules, the higher the probability of the aggregate formation
and the larger the particle size. On the other hand, both the
generation and functionality of the dendrons play a role in the
size and morphology of iron oxide NP formed.
With NP1-1 as an example, it was demonstrated that these
NP are ferrimagnetic-like with a blocking temperature about
300 K, allowing easy magnetic switching. The interaction of
these iron oxide NP with Pd acetate leads to the formation of
magnetically recoverable aggregates. The hydrogen reduction of
the Pd species results in Pd NP without affecting the aggregate
size and morphology. It was also established that the catalytic
properties in selective hydrogenation of DMEC are strongly
inuenced by the size of Pd NP, which, in turn, is determined by
the structure of capping molecules. The highest TOF of 9.3 sꢁ1
at the 97.5% selectivity is obtained for the catalyst based on
dendron 3 and containing 1.5 nm Pd NP. Repeated uses (up to
three catalytic cycles) of the magnetically recoverable catalyst
demonstrate a stable catalytic performance making this class of
catalysts promising for applications in various catalytic
reactions.
(Fig. 6).
When NP4-1–PdNP was magnetically separated aer hydro-
genation and used again in two more catalytic cycles, the TOF
and selectivity remained unaffected revealing exceptional
stability of this catalyst (Table 2).
However, the lower activity of NP4-1–PdNP compared to that
of NP3-1–PdNP was puzzling. The conventional wisdom is the
smaller the NP, the higher the activity due to higher NP surface
area. However, there are studies indicating that for the best
catalytic performance, an optimal catalytic NP size is
needed.44–47 Apparently, the Pd NP size of 0.9 nm in NP4-1–PdNP
is below this optimal size, while the 1.5 nm Pd NP formed in
Acknowledgements
Fig. 6 Kinetic curves of the DMVC accumulation with NP4-1–PdAc (1)
and NP4-1–PdNP (2) as well as those for NP4-1–PdNP in the 2nd (3)
and 3rd (4) uses.
The nancial support of this work was provided in part by
funding from the European Community's Seventh Framework
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Programme [FP7/2007–2013] under grant agreement no. CP-IP 21 S. Wan, J. Huang, M. Guo, H. Zhang, Y. Cao, H. Yan and
246095, the NSF grant CHE-1048613, the Ministry of Education
and Science of the Russian Federation and the Russian
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31057, 14-03-31669 and 14-03-00876, and the Deanship of 6121.
Scientic Research (DSR), King Abdulaziz University, Jeddah, 23 B. D. Korth, P. Keng, I. Shim, S. E. Bowles, C. Tang,
under grant no. (GR-33-7). L.B., W.M., and A.A. therefore
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