Hydrophobic periphery tails of polyphenylenepyridyl dendrons control nanoparticle formation and catalytic properties

Article abstract of DOI:10.1021/cm502336n

(Figure Presented) Here we report control of iron oxide and palladium nanoparticle (NP) formation via stabilization with polyphenylenepyridyl dendrons of the second and third generations with dodecyl periphery. These nanomaterials are developed as magnetically recoverable catalysts. To accurately assess the influence of the dodecyl exterior for the same dendron generation, we also designed a second generation dendron with partial dodecyl periphery. For all dendrons studied, the multicore iron oxide mesocrystals were formed, the sizes and morphology of which were controlled by the dendron generation. Analysis of the static and dynamic magnetic properties, in combination with transmission electron microscopy observations, demonstrate that magnetism is sensitive on the structure-directing capabilities of the type of the dendron which was employed for the mesocrystal stabilization. Close proximity of single cores in such multicore mesocrystals promotes the coupling of the neighboring magnetic moments, thus boosting their magnetization and allowing easy crossover between superparamagnetic and ferrimagnetic behaviors at room temperature. The particularly dramatic role of the dendron structure was also witnessed via the Pd NP formation, which was found to depend on both the dendron generation and its dodecyl periphery. In the case of the catalyst based on the second generation dendron with full dodecyl periphery, no Pd NPs were observed by TEM indicating that these species are of a subnanometer size and are not visible on or near the iron oxide NPs. For the catalyst based on the second generation dendron with partial dodecyl periphery, hydrogen reduction leads to much larger Pd NPs (2.7 nm) due to an unimpeded exchange of Pd species between dendrons and nondense dendron coating with asymmetrical dendrons. The third generation dendron with full dodecyl periphery allows nearly monodisperse 1.2 nm Pd NPs in the shells of iron oxide mesocrystals and the best catalytic properties in selective hydrogenation of dimethylethynylcarbinol. This study suggests a robust approach to control NP formation in magnetically recoverable catalysts for a wide variety of catalytic reactions using dendrons combining rigidity and flexibility in one molecule.

Full text of DOI:10.1021/cm502336n

Article
pubs.acs.org/cm
Hydrophobic Periphery Tails of Polyphenylenepyridyl Dendrons
Control Nanoparticle Formation and Catalytic Properties
†
‡
∥
∥
Nina V. Kuchkina, David Gene Morgan, Athanasia Kostopoulou, Alexandros Lappas,
∥
,⊥
‡
†
§
Konstantinos Brintakis, Bethany S. Boris, Ekaterina Yu. Yuzik-Klimova, Barry D. Stein,
#
#
▽
▽
Dmitri I. Svergun, Alessandro Spilotros, Mikhaill G. Sulman, Linda Zh. Nikoshvili,
▽
,†
,‡,○
Esther M. Sulman, Zinaida B. Shifrina,* and Lyudmila M. Bronstein*
†
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov St., Moscow, 119991 Russia
‡
§
Department of Chemistry and Department of Biology, Indiana University, Bloomington, Indiana 47405, United States
∥
Institute of Electronic Structure and Laser, Foundation for Research and Technology − Hellas, 71110 Heraklion, Crete, Greece
Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
⊥
#
EMBL, Hamburg Outstation, Notkestraße 85, D-22603 Hamburg, Germany
▽
Department of Biotechnology and Chemistry, Tver State Technical University, 22 A. Nikitina St., 170026, Tver, Russia
○Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
*
S Supporting Information
ABSTRACT: Here we report control of iron oxide and palladium nanoparticle (NP) formation via stabilization with polyphenyl-
enepyridyl dendrons of the second and third generations with dodecyl periphery. These nanomaterials are developed as magnetically
recoverable catalysts. To accurately assess the influence of the dodecyl exterior for the same dendron generation, we also designed
a second generation dendron with partial dodecyl periphery. For all dendrons studied, the multicore iron oxide mesocrystals were
formed, the sizes and morphology of which were controlled by the dendron generation. Analysis of the static and dynamic magnetic
properties, in combination with transmission electron microscopy observations, demonstrate that magnetism is sensitive on the
structure-directing capabilities of the type of the dendron which was employed for the mesocrystal stabilization. Close proximity of
single cores in such multicore mesocrystals promotes the coupling of the neighboring magnetic moments, thus boosting their
magnetization and allowing easy crossover between superparamagnetic and ferrimagnetic behaviors at room temperature. The
particularly dramatic role of the dendron structure was also witnessed via the Pd NP formation, which was found to depend on both
the dendron generation and its dodecyl periphery. In the case of the catalyst based on the second generation dendron with full
dodecyl periphery, no Pd NPs were observed by TEM indicating that these species are of a subnanometer size and are not visible on
or near the iron oxide NPs. For the catalyst based on the second generation dendron with partial dodecyl periphery, hydrogen
reduction leads to much larger Pd NPs (2.7 nm) due to an unimpeded exchange of Pd species between dendrons and nondense
dendron coating with asymmetrical dendrons. The third generation dendron with full dodecyl periphery allows nearly monodisperse
1
.2 nm Pd NPs in the shells of iron oxide mesocrystals and the best catalytic properties in selective hydrogenation of dimethyl-
ethynylcarbinol. This study suggests a robust approach to control NP formation in magnetically recoverable catalysts for a wide variety
of catalytic reactions using dendrons combining rigidity and flexibility in one molecule.
Received: June 27, 2014
Revised: September 5, 2014
©
XXXX American Chemical Society
A
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a
Scheme 1. Formulas of Dendrons 1 (a), 2 (b), and 3 (c)
a
Pyridine rings are shown in red, phenylene rings are in black, and a carboxyl focal group is shown in blue. Dodecyl chains are in green.
INTRODUCTION
■
the dendrons/dendrimers when iron oxide or Pd NPs were formed
did not allow us to single out key parameters which would be
crucial for the control of the catalyst formation properties.
Although numerous active and selective homogeneous catalysts
are described in literature, less than 20% of such processes are
used in industry while heterogeneous catalysis is clearly pre-
For conventional dendrons and dendrimers, it was reported
that inert tails in the periphery prevent interdendrimer interac-
tions via complexation with metal ions but also add steric pro-
tection to single dendrimers. This results in prevention of metal
ion exchange between dendrimers and formation of dendrimer
1
vailing. This should be ascribed to the difficulty of the separation
of homogeneous catalysts from reaction solutions. The separa-
tion can be performed, as is discussed in a recent review, but it is
2
time and energy consuming. In recent years a combination of
catalytic species and magnetic NPs has been suggested which
allows robust and inexpensive magnetic separation of cata-
16−18
encapsulated small monodisperse NPs.
It is demonstrated
that NPs designed for catalytic applications should be sufficiently
small to have a large surface area with many active centers, but at
the same time, not too small because of unfavorable curvatures
3
−10
lysts.
Earlier we reported magnetically controllable catalysts
where magnetic iron oxide nanoparticles (NPs) were synthesized
using polyphenylenepyridyl dendrons and dendrimers with
mixed pyridyl−phenylene or purely pyridyl periphery as capping
19−22
for adsorption of reacting molecules
or a different character
of NPs (semiconductor vs metallic) which changes catalytic center
1
1,12
23
molecules.
Dendrimers are complex monodisperse macro-
properties. Thus, an optimal catalytic NP size is required. However,
molecules with a regular and highly branched three-dimensional
in complex composite materials control over catalytic NP formation
is rarely achieved.
2,13,14
architecture and well-defined chemical structure.
Dendrons
are essentially wedges or slices of a dendrimer and due to
coordination groups in a focal point they are especially suited as
Here we report formation of multicore iron oxide mesocrystals
stabilized by polyphenylenepyridyl dendrons of the second and
third generationswithdodecylchainsinthe periphery (Scheme 1).
The iron oxide mesocrystals display ferrimagnetic behavior at
room temperature and possess shell functionality due to pyridine
groups. We demonstrate that these dendrons control both iron
oxide NP morphology and the sizes of Pd NPs. Moreover, this
control is carried out via a rational design of the dendron structure,
i.e., both periphery and generation. To probe the influence of the
number of hydrophobic chains in the periphery, we designed, for
the first time, a second generation dendron with partial dodecyl
15
ligands during NP formation. The benefit of synthesizing iron
oxide NPs directly in the presence of functional dendrons vs
ligand exchange is that the dendron adsorbs on the NP sur-
face upon formation and makes NPs evenly functionalized. In
other words, the NP is already formed well dispersed and no
further functionalization is required. Although in our preced-
1
1,12
ing work
we were able to influence the Pd NP size and the
catalytic properties varying the structure of these dendrons/
dendrimers and the reaction conditions, the ion exchange between
B
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periphery (Scheme 1). It is established here that the Pd NP size
dramatically influences the catalytic properties tested in selective
hydrogenation of dimethylethynylcarbinol (DMEC) to dimethyl-
vinylcarbinol (DMVC).
(EDS) were acquired 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 all analyses.
X-ray fluorescence measurements to determine the Pd content were
performed with a Zeiss Jena VRA-30 spectrometer (Mo anode, LiF
crystal analyzer and SZ detector). Analyses were based on the Co Kα
line and a series of standards prepared by mixing 1 g of polystyrene with
EXPERIMENTAL SECTION
■
1
0−20 mg of standard compounds. The time of data acquisition was
Materials. Iron(III) acetylacetonate (99+%) was purchased from
Acros Organics and was used as received. Palladium(II) acetate was
purchased from Strem Chemicals and also used as received. Super-
hydride solution (1.0 M lithium triethylborohydride in THF), benzyl
constant at 10 s.
X-ray diffraction (XRD) patterns were collected on an Empyrean
from PANalytical. X-rays were generated from a copper target with a
scattering wavelength of 1.54 Å. The step-size of the experiment was 0.02.
The magnetic properties of the samples were studied by a
Superconducting Quantum Interference Device (SQUID) magneto-
meter (Quantum Design MPMS XL5). The measurements have been
performed in dried nanoparticles on cotton in a gelatin capsule.
The isothermal hysteresis loops, M(H), were measured at fields −1 ≤
H ≤ +1 T. The dc magnetic susceptibility as a function of temperature,
χ(T), was attained down to 5 K under zero-field cooled (ZFC) and field-
cooled (FC) protocols, at H = 50 Oe. The frequency dispersion of the
ac susceptibility (f = 0.099, 0.999, 9.999, 100.024, and 997.34 Hz; ac
field 1 Oe) was also recorded as a function of temperature. Suitable
phenomenological models (eqs S1−S3, Supporting Information) were
ether (98%), and the Lindlar catalyst (Pd/CaCO poisoned by lead)
3
were purchased from Sigma-Aldrich and used without purification.
Acetone (99.5%) and chloroform (99.8%) were purchased from
MACRON Chemicals and used as received.
Synthetic Procedures. Synthesis of Dendrons. Syntheses of
11
dendrons 1 and 2 were described in our preceding paper. Dendron 3
was synthesized according to the procedure described in the Supporting
Information (Scheme S1).
Synthesis of Iron Oxide Nanoparticles. In a typical procedure, the
three-neck round-bottom flask (with elongated necks) equipped with a
magnetic stir bar, a reflux 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
tested against the temperature evolution of the relaxation time, τ =
−1
(
2πf) . The position of the maximum in the dissipative part of the
(
1 mmol) of Fe(acac) , 0.427 g (0.22 mmol) of dendron 1, and 7 mL of
3
ac susceptibility, χ″(T), was determined as the temperature at which the
benzyl ether. The flask was placed in a Glas-Col heating mantle attached
to a digital temperature controller which in turn was placed on a
magnetic stirrer. The flask was degassed by argon bubbling for 30 min
under stirring. Then the temperature was raised to 60 °C at 10°/min and
kept under stirring at this temperature for 30 min to allow solubilization.
Then the temperature was increased with a heating rate 10°/min, and
the flask was allowed to reflux for 1 h at 283−285 °C. The flask was then
removed from the heating mantle and allowed to cool to room tem-
perature. To isolate NPs, 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 precipitate was dispersed in chloroform upon
sonication for 20 min. No aggregates were formed with dendrons 1−3
first derivative of χ″ becomes zero.
The X-ray scattering data have been collected at the P12 beamline
of the European Molecular Biology Laboratory (EMBL) on the storage
ring Petra III of the Deutsches Elektronen Synchrotron (DESY,
Hamburg). Using a Pilatus2M detector (Dectris, Switzerland), the
scattering was recorded in the range of the momentum transfer 0.07 <
−1
s < 4.3 nm , where s = 4π sin θ/λ, 2θ is the scattering angle, and λ =
0
.1 nm is the X-ray wavelength.
Dendrons 1 and 2 have been dissolved in tetrahydrofuran (THF) at
three different concentrations (10, 5, and 2.5 mg/mL). The solutions
were then measured with exposure time of 1 s in a vacuum capillary to
diminish the parasitic scattering. The scattering profiles were corrected
for the background scattering from the THF solvent and processed
1
2
unlike other dendrons or dendrimers described elsewhere. The yield
of iron oxide NPs was in the range 87−90%. The rest of the reaction
solution was stored in a refrigerator and was stable for many months.
This sample is notated NP1-1. “NP” stands for iron oxide NPs; the first
number indicates the specific dendron according to Scheme 1, and the
second number indicates the synthesis number according to Table 1.
24
using an automated pipeline. The concentration series has been
used to check for interparticle interactions and to extract the structural
24
parameters corresponding to infinite dilution. The distance
distribution functions P(r) were calculated using an indirect Fourier
25
transform program GNOM.
The low-resolution shapes of the dendron ensembles were recon-
26
structed ab initio from the scattering patterns by DAMMIF. This
program represents the object as an assembly of beads inside a spherical
search volume. Starting from a random assembly, DAMMIF employs
simulated annealing to build compact scattering equivalent models
fitting the experimental data Iexp(s) to minimize discrepancy:
Table 1. Characteristics of Iron Oxide NP Stabilized by
Dendrons with Dodecyl Periphery
a
NP
notation
dendron amount
(g/mmol)
NP size
(nm)
standard deviation
(%)
2
NP1-1
NP1-2
NP1-3
NP2-1
NP3-1
0.427/0.22
0.214/0.11
0.107/0.055
0.912/0.22
0.274/0.22
24.5
22.6
18.8
31.6
21.6
20.8
16.8
20.7
13.3
21.3
⎡
⎢
⎤
⎥
Iexp(s ) − cI (sj)
1
j
calc
2
χ
=
∑
N − 1
⎢
⎣
σ(sj)
⎥
⎦
j
(1)
where N is the number of experimental points, c a scaling factor, and
Icalc(s ) and σ(s ) are the calculated intensity from the model and the
experimental error at the momentum transfer s , respectively.
a
j
j
Loading of other reagents: 0.353 g (1 mmol) of Fe(acac) , 7 mL of
benzyl ether.
3
j
The content of aggregates in the two samples was estimated by the
analysis of the forward scattering intensity I and the radii of gyration of
0
This table lists the reaction conditions and the sizes of the iron oxide
NPs formed with all three dendrons.
the monomers (self-assembled compact particles) Rgmono and aggregates
RgAgg. For the monomer scattering, I0mono ∼ Nmono(Vmono) where N is
2
Methods. Electron-transparent NP specimens for transmission
electron microscopy (TEM) were prepared by placing a drop of dilute
solution onto a carbon-coated Cu grid. Images were acquired at an
accelerating voltage of 80 kV on a JEOL JEM1010 transmission electron
microscope. Images were analyzed with the National Institute of Health
developed image-processing package ImageJ to estimate NP diameters.
Between 150 and 300 NPs were used for this analysis. High resolu-
tion TEM (HRTEM), tomography, and energy dispersive X-ray spectra
the number of monomers and V their volume. The forward scattering
from the aggregates can be estimated as I0agg = I0whole − I0mono, where
I0whole is the forward scattering of the entire system. Taking into account
that the number of monomers in an aggregate k is approximately k ∼
(V /V ), the volume fraction of compact particles is calculated as
νmono = k(I0mono/I0agg)/(1 + k(I0mono/I0agg)).
Catalytic Studies. Catalytic testing was carried out in a 60 mL
Agg
mono
isothermal glass batch reactor installed in a shaker and connected to a
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Figure 1. HRTEM images of iron oxide NPs formed in the presence of dendrons 1 (a), 2 (b), and 3 (c) as capping molecules. Insets show FFT patterns
of the HRTEM images.
Figure 2. TEM tomography projections for NP2-1 at −31° (a), 0.6° (b), and −52.6° (c). Scale bar is 20 nm.
gasometrical buret (for a hydrogen consumption control). The total
volume of the liquid phase was 30 mL. Reaction conditions were as
follows: ambient hydrogen pressure, stirring rate of 850 shakings per
minute, temperature 90 °C. Toluene was used as a solvent.
second generation dendron 3 with partial dodecyl periphery to
evaluate the influence of the amount of hydrophobic tails for the
same dendron generation.
Influence of Dodecyl Periphery on the Iron Oxide
Nanoparticle Formation. HRTEM and TEM images of iron
oxide NPs prepared in the presence of dendrons 1, 2, and 3 as
capping molecules are presented in Figure 1 and Figure S1
(Supporting Information). These iron oxide NPs display mainly
Probes were periodically taken and analyzed via GC-MS (Shimadzu
GCMS-QP2010S) equipped with a capillary column HP-1MS (30 m ×
0
.25 mm i.d., 0.25 μm film thickness). Helium was used as a carrier gas
at flow rate 1 mL/min. 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.
Selectivity to DMVC was defined as SDMVC = YDMVC × X− × 100%,
where X is the DMEC conversion and Y is the DMVC yield.
(
27
multicore, flower-like morphologies. Only a few single core
spherical NPs were observed in NP1-1 and NP3-1, while in
NP2-1 they are nearly absent. This is in contrast with the majority
of iron oxide NP samples formed in the presence of phenyl-
enepyridyl dendrons and dendrimers with mixed phenylene−
1
DMVC
−1
−1
Activity was calculated as TOF = CDMEC × CPd × τ , where C
CPd are the molar amounts of DMEC and Pd and τ is the reaction time.
and
DMEC
It is noteworthy that the molar ratio of DMEC to Pd was about
12
31000 mol/mol for all the catalytic systems.
pyridyl periphery, where the mixture of single and multicore
NPs was observed. Fast Fourier Transform (FFT) patterns of
multicore NPs shown in the insets of Figure 1 demonstrate single
crystalline orientation. The XRD patterns reveal that the iron
oxide NPs are magnetite (Figure S2, Supporting Information).
The tomography images for NP2-1 presented in Figure 2 and
video S1 (Supporting Information) show that multicore NPs are
3D structures and there are spaces between single cores, thus
RESULTS AND DISCUSSION
■
We studied the influence of hydrophobic periphery of the second
and third generation dendrons on the possibility to control the
iron oxide and Pd NP formation in the dendron environment.
The former NPs are the support for magnetically recoverable
catalysts, while the latter ones provide catalytic centers. In addi-
tion to exploring full dodecyl periphery in 1 and 2 and assessing
the influence of the dendron generation, we also designed a new
28−30
revealing that these NPs are mesocrystals.
The close look
at Figure 2a shows two characteristic distances for the NP2-1
D
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Figure 3. (a) Experimental data (one point out of every eight is displayed) and the scattering patterns computed from the ab initio model (lines) for
and 2. The appropriate interval was selected for the fitting in order to exclude any interparticle interactions. (b) Distance distribution functions
computed by GNOM for 1 and 2. (c) Three dimensional models obtained by DAMMIN corresponding to dendrons 1 and 2.
1
sample: 2 nm in the center of the multicore particle and
.8 nm along the connecting single cores. For NP1-1, only a
single distance of 0.8 nm is observed (Figure S3, Supporting
(Figure S6c, Supporting Information), so additional specimens
0
were taken in the other procedure at 200 and 225 °C. The TEM
images of all these specimens presented in Figure S6 (Supporting
Information) show a similar mechanism, but the Fe(acac)3
decomposition with the formation of seeds was actively
happening already by 200 °C: not only seeds but also single
cores can be observed at this temperature. According to ref 29,
2
7
Information) as was described in our preceding paper.
Evidently, larger NP2-1 consists of larger single cores whose
fitting in a multicore NP requires a larger distance in the center.
Thus, the presence of flexible hydrophobic tails in the rigid
dendron periphery does not prevent oriented attachment of
in the solid state Fe(acac) decomposes at 182 °C. Thus,
3
28
single cores (necessary condition of mesocrystal formation)
dendron 1 facilitates the Fe(acac) decomposition compared to
3
27
but rather facilitates it.
the dendrimer studied earlier.
The data presented in Table 1 show that, for dendrons with
dodecyl periphery, the iron oxide NP sizes can be somewhat
influenced by varying the dendron loading (samples NP1-1,
NP1-2, and NP1-3, see Figure S4, Supporting Information) and
the dendron periphery (compare NP1-1 and NP3-1). On the
other hand, the NP sizes are strongly affected when the dendron
generation is varied (compare NP1-1 and NP2-1). The TGA
traces of NP1-1 and NP1-2 (Figure S5, Supporting Information)
demonstrate that iron oxide NPs prepared at lower dendron
loading contain less stabilizing molecules on their surface, but no
other changes in thermal behavior were observed.
SAXS Study of the Solution Behavior of Dendrons 1, 2,
and 3. To assess the relationship between the solution behavior
of dendrons and multicore NP formation, dendrons 1 and 2 were
examined by SAXS. The experimental scattering profiles from 1
and 2 are displayed in Figure 3, and the structural parameters
obtained from the data analysis are summarized in Table S1
(
Supporting Information). The SAXS profiles in Figure 3 display
−1
sharp upturns at very small angles (s < 0.5 nm ), which indicate
that large clusters or aggregates coexist in solution with smaller
particles. The low resolution structure of the individual particles
can be reconstructed from the higher-angle portions of the
scattering data beyond the influence of the aggregates. The
appropriate intervals of the experimental curves were processed
by GNOM to compute the distance distribution functions
In all cases, the iron oxide NPs were colloidally soluble in
chloroform for weeks and easily dispersible again if chloroform
had evaporated. Moreover, even the much larger iron oxide NPs
shown in Figure 1b are well dispersed in solution without aggre-
gation. This is a striking difference with iron oxide NPs stabilized
with dendrons having mixed phenylene−pyridyl periphery
(
Figure 3b) and used in the modeling procedure. The latter
functions were backtransformed to extrapolate the scattering
from the individual particles to zero angle, and low resolution
12
where larger multicore NPs are usually aggregated. Therefore,
the hydrophobic periphery here allows much better stabilization
of iron oxide NPs, preventing aggregation.
31
shapes were further reconstructed ab initio by DAMMIN
Figure 3c). While dendron 1 exhibits a globular shape, dendron
has an elongated shape, and the typical models yield good fits to
(
2
The mechanism of the multicore (flower-like) iron oxide
mesocrystal formation in the case of the dendrimer with mixed
phenylene−pyridyl periphery as a capping molecule was
discussed in ref 27, based on the TEM analysis of the specimens
withdrawn from the reaction solution when the temperatures
reached 250 and 270 °C and in 15 min at 285 °C (reaction
temperature).
the experimental data with discrepancy χ = 1.0 and 1.3,
respectively (Figure 3). Dendron 1 shows a small and compact
structure in solution with a maximum linear dimension of 1.4 nm.
This dimension is close to an individual dendron size. Alter-
natively, dendron 2 forms column-like structures with a diameter
of about 1.3 nm and a height of 4 nm, suggesting the dendron
self-assembly. It is noteworthy that dodecyl chains do not
contribute into the SAXS scattering because of their low density.
The fraction of large aggregates in the case of dendrons 1
does not exceed 2 vol %, while for dendrons 2, the fraction
of aggregates is much larger (22 vol %), although the cluster size
According to this mechanism, seeds (small NPs) form at
50 °C, and then at 270 °C they become oriented due to the
2
dendrimer self-assembly and start growing into larger single
cores due to fusion. Further, the single cores are oriented in a
single crystallographic pattern assisted by the dendrimer but
without fusion leading to multicore mesocrystals. For dendron 1,
we also withdrew specimens at different temperatures; however,
at 250 °C the multicore NP formation had already taken place
is close. The number of individual dendron particles in a cluster is
5
given by the ratio Vcluster/V
for dendron 1 to 0.3 × 10 for dendron 2.
and varies from about 2.3 × 10
mono
5
E
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These data demonstrate that dendron 2 has much higher
propensity to self-assemble and to aggregate. According to our
2
7
preceding paper, this tendency is consistent with the forma-
tion of a higher fraction of multicore NPs and larger NPs in the
case of 2.
Spin Dynamics and Ferrimagnetism in Iron Oxide
Multicore Mesocrystals. In multicore mesocrystals, the con-
trolled oriented attachment of single core NPs increases their
proximity and promotes the coupling of the neighboring mag-
netic moments. This coupling boosts magnetization and ferri-
magnetic behavior at room temperature and may allow an easy
crossover between superparamagnetic and ferrimagnetic behavior.
To assess the magnetic behavior of these multicore meso-
crystals, initially, dc magnetic measurements were carried out for
NP1-1, NP2-1, and NP3-1. The temperature dependence of the
dc susceptibility, χ(T), is shown in Figure 4a. It is worth noting
the presence of two broad humps in ZFC curves, one at Tmax,h
(
∼300 K) and a weaker one at T
(∼40 K). T
can be
max,l
max,h
attributed to the blocking temperature of the superspins of the
mesocrystals, and this should be affected by intramesocrystal
characteristics (e.g., dipolar interactions between the single
32
cores and intraparticle exchange interactions). For NP1-1 and
NP3-1, the blocking temperatures are nearly the same due to the
comparable NP sizes (24.5 and 21.6 nm, respectively, Table 1),
while it is higher for NP2-1 whose mean size is 31.6 nm. At the
same time, the blocking temperatures for all the samples are
above 300 K, revealing that they are ferrimagnetic at room
temperature. Similar ferrimagnetic behavior has been observed
27
for magnetite NPs in our preceding paper and for maghemite
3
2
nanoparticle assemblies capped with poly(acrylic) acid. In the
latter case, it was assigned to intracluster characteristics. This is
also in agreement with the nonzero coercive field, H , derived
C
from the isothermal magnetization curves at the same temper-
ature presented in Figure 4b. The smaller coercive field at 5 K,
where the thermal agitation is reduced, for the largest diameter
sample NP2-1 (Figure 4c), is evidence for stronger dipolar
interactions compared to the other samples that are stabilized
with dendrons of the second generation. This is in agreement
with the extended plateau in the FC χ(T) curve (Figure 4a curves
with filled symbols), as it has been suggested before on the basis
of experimental studies supported by Monte Carlo simulations in
32
relevant systems of assembled NPs.
Furthermore, it is worth noting the presence of the weak
broad hump at Tmax,l for the samples NP2-1 and NP3-1 while this
feature is absent in the case of NP1-1, the origin of which
required further investigation. Therefore, the frequency depend-
ence of the ac susceptibility has been measured down to 5 K for
Figure 4. ZFC−FC dc susceptibility, χ(T), curves (open and filled
symbols, respectively) as a function of temperature under a magnetic
field of 50 Oe (a) and isothermal magnetization curves at 300 K (b) and
the sample (NP2-1) with the more prominent cusp around Tmax,l
.
The real χ′(T) and the imaginary χ″(T) parts of the susceptibility
are shown in Figure 5. Two features in the imaginary part of the
susceptibility, χ″(T), are particularly interesting; a sharper peak
5
K (c) for NP1-1 (triangles), NP2-1 (circles), and NP3-1 (squares).
36
at T ∼ 40 K and a much broader one at T
∼ 280 K. Similar
after successful fitting attempts with the Arrhenius, Vogel−
37
max,l
max,h
dynamic behavior has been observed in systems such as nickel
Fulcher, and power-law phenomenological descriptions
3
3
34
35
38
ferrite, NiO, and Co Ni
NPs, where the high-temper-
(Figure S7, Supporting Information and Table 2).
5
0
50
ature feature was attributed to the blocking temperature of the
superspins, while the low-temperature one was attributed to the
freezing of the surface spins.
In order to further understand the origin of the dynamic
susceptibility anomalies, one needs to consider the temperature
dependence of the characteristic relaxation times, τ, as they are
evaluated from the frequency dependence of the χ″(T) maxi-
mum. This was proven to be very informative for the low-
temperature magnetic regime when we assessed the dynamics
It appears that the frequency dependence of the χ″(T)
maximum at T
does not follow the Arrhenius law (eq S1 and
max,l
Figure S7a, Supporting Information), which normally describes
the temperature dependence of the characteristic relaxation time,
τ, in particulate systems with very weak interparticle interactions.
The fit value obtained for the activation energy, E /k (= 420 K)
a
B
(Table 2), was found to be large with respect to the energy scale
(k T) in this low-temperature regime. It implies that in the
B
multicore mesocrystal the dipolar interactions among the single
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Figure 6. Temperature dependence of the imaginary part, χ″(T), of the ac
susceptibility under dc magnetic fields from 50 to 750 Oe (a) and the field
dependence of the freezing temperature, Tmax,l, for the NP2-1 sample. (b)
Figure 5. Frequency dependent real part, χ′ (T), (a) and imaginary part,
χ″(T), (b) of the ac susceptibility, for the NP2-1 sample.
2
/3
The line is a fit with the Almeida−Thouless law (ΔT ∼ H ) and the
max
/3
T
(0) extrapolated temperature for H = 0 Oe. H2 versus reduced
max,l
cores may play an important role in their magnetic behavior.
According to the latter, the phenomenological Vogel−
Fulcher law has been utilized to fit the frequency dependence
temperature, Tred (Tred = 1 − Tmax,l(H)/Tmax,l(0)) curve (c).
of the χ″(T) maximum at T
(eq S2 and Figure S7b,
magnetic field, H, on the system dynamical behavior was
investigated (Figure 6a). The temperature Tmax,l was found to
decrease with raising the magnetic field strength according to the
max,l
−6
Supporting Information). The resultant values for τ (∼10 s)
0
are comparable to those in the literature for particles with
intermediate-strength dipolar interactions.
39,40
46
Almeida−Thouless law (Figure 6b). This behavior has been
At the same time, a power-law analysis (Supporting
Information eq S3 and Figure S7c) could give the relaxation
time in spin-glass systems, where strong dipolar interactions
observed either in superparamagnetic or weakly interact-
39
ing ferrimagnetic particles, as well as for the spin-glass state
4
0,47
of surface disordered spins.
To resolve this, we utilized a
41,42
33
2/3
(superspin-glass response)
or disordered surface spins
criterion which rests on the relative offset of the H vs Tred
extrapolated line with respect to the origin. Tred = 1 − Tmax,l(H)/
Tmax,l(0) is the reduced temperature and Tmax,l(0) = 30.8 K is the
extrapolated temperature for H = 0 Oe. In the general case, the
existence of such an offset corroborates to superparamagnetic
are observed. This latter approach results in reasonable values for
−
4
the τ (∼10 s) and the critical exponential zv = 6.9. It is worth
0
noting that such long τ ’s have been observed before for
0
33,43
the surface spin-glass freezing processes in Ni-ferrite
and
44
48,49
γ-Fe O nanoparticles.
behavior (Figure 6c) .
2
3
In order to distinguish which one among the Vogel−Fulcher or
power laws (as both produce reasonable fit values) provide a more
suitable phenomenological description of the spin dynamics, one
can calculate the relative variation of the χ″(T) peak-temperature
position per frequency decade, ψ (= ΔT/(T Δlog f)), known
as the Mydosh parameter. The calculated value of the ψ for the
sample NP2-1 is 0.126. It falls in the range that predicts a blocking
temperature with intermediate strength dipolar interactions
among the magnetic moments (0.05 < ψ < 0.13) and not due to
Overall, the above experimental evidence for NP2-1 together
with the literature data are in favor of describing the frequency
dependence of the χ″(T) maximum at T
on the basis of the
max,l
Vogel−Fulcher law. The absence of the individual single core
NPs in the NP2-1 sample suggests that the sharp maximum of
the imaginary part of the susceptibility at ∼40 K can originate
from mesocrystals involving a larger distance between single
cores at the center of the NP assembly, as is discussed in the
previous section (Figure 2a) on the effect of the dodecyl
periphery. Due to their relative longer interparticle separa-
tion this may point to weaker dipolar interactions than those
giving rise to the high-temperature broader maximum and the
45
a spin-glass state, the range of which is 0.005 < ψ < 0.05.
On the other hand, in order to distinguish experimentally the
likelihood of a possible spin-glass state at Tmax,l, the impact of a dc
Table 2. Analysis of the Relaxation Times, for the Sample NP2-1, on the Basis of the Arrhenius, Vogel−Fulcher, and Power-Law
Phenomenological Descriptions (equations S1−S3, Supporting Information) of the Spin Dynamics
Arrhenius law
E /k (K)
Vogel−Fulcher law
E /k (K)
power law
zv
τ0 (s)
τ0 (s)
T0 (K)
τ0 (s)
T* (K)
a
B
a
B
.3 × 10−
10
419.8 ± 27.0
2.1 × 10−6
99.3 ± 9.1
11.8 ± 1.4
9.2 × 10−4
6.9 ± 0.4
14.2 ± 3.3
3
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Table 3. Results of DMEC Hydrogenation Using Pd-Containing Magnetically Separable Catalysts
selectivity (%)
Pd content
a
Pd NP size
(nm)
Pd NP size
TOF50
at 50% of the DMEC
conversion
at 95% of the DMEC
conversion
−1
catalyst notation
NP1-1-PdNP
(wt %)
distribution (%)
(s
)
3.2
2.9
4.3
“
-
-
0.7
94.7
97.5
98.4
98.1
98.0
98.0
96.1
92.8
95.7
97.1
96.9
96.7
97.3
95.3
NP1-2-PdNP
NP2-1-PdNP
-
-
0.6
14.7
14.5
14.6
5.0
1.2
“
11
“
2
1
nd use
0th use
“
“
“
NP3-1-PdNP
3.5
5
2.7
-
30
-
Lindlar catalyst (Pd/CaCO3
poisoned by lead)
2.1
a
The Pd content was determined by XRF analysis.
associated ferrimagnetic behavior. For the NP1-1 mesocrys-
tals, this larger distance between single cores is not realized
(Supporting Information Figure S3), thus resulting in the
absence of additional superparamagnetic-like features in the low-
T regime. The aforementioned discussion suggests that the
observed magnetic properties depend on the type of the dendron that
has been employed for the mesocrystal stabilization in each case.
Influence of Dodecyl Periphery on Pd Nanoparticle
Formation. In this work iron oxide NPs were designed as a
support for magnetically recoverable catalysts. Because iron
oxide NPs are coated with polyphenylenepyridyl dendrons,
catalytic Pd species coordinate with pyridyl groups and then
Pd NP formation occurs upon reduction as was established in our
1
1,12
preceding papers.
Although we demonstrated different Pd
12
NP sizes for different dendrons and dendrimers, a sophisticated
design of the catalysts was not carried out. Pd NP sizes were
rather unpredictable because they depended on numerous
factors such as the Pd species loading, the rate of reduction,
and the Pd species exchange between dendrons in the iron oxide
NP shell. The last factor is most uncontrollable as it depends
on many aspects of the dendron structure. Thus, a logical step
was designing a protective exterior, i.e., dodecyl chains on the
dendron periphery.
Figure 7. HRTEM image of NP2-1-PdNP. Pd NPs are indicated by red
arrows. Larger iron oxide NPs are shown by yellow arrows.
It is worth noting that interaction of dendron-stabilized iron
oxide NPs with Pd acetate was demonstrated to lead to the
11,12
formation of aggregates which allow fast magnetic separation.
In the case of dendrons with dodecyl periphery as capping
molecules, aggregates also form, but their sizes do not exceed
Figure 8. Dark-field STEM image (a) and Fe (b) and Pd (c) EDS maps
of the NP2-1-PdNP sample.
1−2 μm (Figure S8, Supporting Information). This allows us to
consider these catalysts for further use in microreactors where the
aggregate size needs to be limited to a few micrometers and well
controlled.
To prove that indeed such morphology exists, we carried out
EDS. A dark-field STEM image and EDS mapping of the NP2-1-
PdNP sample presented in Figure 8 clearly show superposition of
Pd on Fe indicating that Pd NPs are grown in the iron oxide NP
dendron shells.
Reduction of Pd-containing magnetic nanocomposite with
1
1,12
hydrogen normally leads to Pd NP formation.
To our
surprise, in the TEM image of the NP1-1PdNP nanocomposite
no Pd NPs were observed after reduction (Figure S9, Supporting
Information). At the same time, Pd was detected in this nano-
material by elemental analysis (Table 3). Similar results were
obtained for NP1-2-PdNP (images are not shown). On the other
hand, for the nanocomposite stabilized by dendron 3 (second
generation dendron with partial dodecyl periphery), the
reduction resulted in comparatively large (2.7 nm) Pd NPs
with broad NP size distribution, while in the case of dendron 2,
nearly monodisperse 1.2 nm NPs (11% standard deviation) have
been formed. HRTEM image of the NP2-1-PdNP sample
presented in Figure 7 shows these Pd NPs (red arrows) around
and on the top of iron oxide NPs (yellow arrows).
We propose the following explanation of differences in the Pd
NP formation depending on the dendron structure. We assume
that, for dendron 1, exchange of Pd species between dendrons
is nearly prohibited. Each dendron contains six pyridines and
can coordinate not more than six Pd atoms. When reduction is
carried out with hydrogen, it penetrates well the nanocomposite
aggregates allowing uniform and fast nucleation with the for-
mation of a large amount of nuclei. In the absence of the
exchange between dendrons carrying Pd species, the Pd NP sizes
are significantly below 1 nm (subnanometer particles). These Pd
NPs cannot be seen in the nanocomposite because of the dark
background provided by iron oxide NPs and comparatively thin
dendron shells.
50
H
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Chemistry of Materials
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When the dendron has only half of the dodecyl chains as
shown in Scheme 1 for 3, the exchange of Pd species between
dendrons is not prevented. Moreover, because this dendron is
asymmetrical, its packing on the iron oxide NP surface is likely to
be not dense. Both factors lead to 2.7 nm Pd NPs, although in both
NP1-1-PdNP and NP3-1-PdNP, the Pd content is nearly the same
smaller catalytic NPs allow higher catalytic activity due to higher
NP surface area. Nevertheless, there is a number of studies
demonstrating that an optimum catalytic NP size is required for
19−23
the best catalytic activity.
Apparently, Pd NPs significantly
below 1 nm formed in NP1-1-PdNP and NP1-2-PdNP are below
20
the threshold for efficient catalytic NPs, while the 1.2 nm Pd
NPs represent an optimum size, the influence of which can be
attributed to the maximum amount of the effective catalytic sites
(
Table 3). The third generation dendron with fully dodecyl
periphery, 2, contains 14 nitrogen atoms which are readily avail-
able to complex with Pd acetate and to stabilize Pd NPs. Consider-
ing that each 1.2 nm Pd particle contains approximately 50 Pd
atoms, the exchange between dendrons does take place, but it is
minimal and well-controlled (narrow NP size distribution) reveal-
ing that dodecyl chains significantly diminish this process.
Thus, both generation and periphery of the dendrons allows us
to control the Pd NP size.
19
on the NP surface.
A magnetic separation was applied to NP2-1-PdNP for test-
ing in 10 consecutive catalytic reactions. No catalyst loss was
observed at every separation as the catalyst was not removed
from the reaction vessel. It was situated at the side of the reactor
using a rare earth magnet and washed in the same way directly in
the reactor. This would be impossible if using filtration (typically
52
employed for heterogeneous catalysts ). The data of Table 3
show that TOF and selectivity remain unaffected, revealing
exceptional stability of this catalyst. It is worth noting that the
NP2-1-PdNP catalyst is significantly more active and selective
than the Lindlar catalyst (Table 3) which is conventionally
Influence of the Pd NP Sizes on Catalytic Behavior. To
assess the influence of the nanomaterial morphology on catalytic
performance, NP1-1-PdNP, NP1-2-PdNP, NP2-1-PdNP, and
NP3-1-PdNP were tested in selective hydrogenation of DMEC
to DMVC (Scheme 2). DMVC is an intermediate for fragrant
11
51
used in selective hydrogenation or any other magnetically
substances and vitamins E and K.
12
recoverable catalyst studied by us earlier.
Scheme 2. DMEC Hydrogenation with DMVC Being a Target
Product and 2-Methylbutane-2-ol Being a Side Product
CONCLUSION
■
We demonstrated control over iron oxide and Pd NP forma-
tion using polyphenylenepyridyl dendrons of the second and
third generations with dodecyl periphery. For iron oxide NPs,
the dendron generation allows significant variation of the NP
size, while stabilizing well comparatively large NPs. The Pd NP
formation is finely regulated by both the generation and the
amount of dodecyl chains on the dendron periphery. At the same
time, it has also been shown that the dendron generation influences
the internal structure of iron oxide multicore mesocrystals and, as
such, it provides a sensitive nanoscale control of the associated
magnetic properties. Furthermore, the catalytic activity was found
to depend on the size of the Pd NPs that were immobilized on the
mesocrystals.
Thus, we proposed a strategy for the smart design of novel
catalysts. Exceptional catalytic properties of these nanomaterials,
as well as the easy catalyst recovery, successful repeated cycle use,
and the catalyst stability make them promising for hydrogenation
and great potential for a number of other catalytic reactions
carried out with Pd catalysts.
ASSOCIATED CONTENT
Supporting Information
■
*
S
Figure 9. Kinetic curves of DMVC accumulation with Lindlar catalyst
Synthetic procedures, TEM images, XRD, TGA, SAXS data, and
magnetic characteristics. This material is available free of charge
via the Internet at http://pubs.acs.org.
(1), NP1-2-PdNP (2), NP1-1-PdNP (3), NP3-1-PdNP (4), and NP2-1-
PdNP (5).
The data of catalytic testing are presented in Table 3 and
Figure 9. The TOF values in hydrogenation with NP1-1-PdNP
and NP1-2-PdNP (i.e., two catalysts based on dendron 1 when
Pd NP sizes are below resolution for these nanomaterials) are
exceptionally low. The conversion after ∼8 h did not exceed 65%.
In the case of NP3-1-PdNP with 2.7 nm Pd NPs, the TOF
is higher and conversion reaches 98% but selectivity does not
exceed 95.1% at 95% of the DMEC conversion. For NP2-1-
PdNP containing 1.2 nm Pd NPs, the reaction is very fast and the
selectivity is 96.9% at 99% conversion.
AUTHOR INFORMATION
Corresponding Authors
E-mail: shifrina@ineos.ac.ru (Z.B.S.).
E-mail: lybronst@indiana.edu (L.M.B.).
Author Contributions
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
■
*
*
ACKNOWLEDGMENTS
The financial support of this work was provided in part by
funding from the European Community’s Seventh Framework
Considering that the Pd NP environment is similar in all
three dendrons, we believe the major factor influencing the
catalytic properties is the Pd NP size. The traditional thinking is
■
I
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Chemistry of Materials
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Programme [FP7/2007-2013] under Grant Agreement No. CP-
IP 246095, the NSF Grant CHE-1048613, the Ministry of
Education and Science of Russia, and the Russian Foundation for
Basic research under Grant Numbers 14-03-00876 and 14-03-
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