Highly water-dispersible magnetite-supported Pd nanoparticles and single atoms as excellent catalysts for Suzuki and hydrogenation reactions

Article abstract of DOI:10.1039/c6ra14257e

The molecule 4-(diphenylphosphino)benzoic acid (dpa) anchored on the surface of magnetite nanoparticles permits the easy capture of palladium ions that are deposited on the surface of the magnetite nanoparticles after reduction with NaBH₄. Unexpectedly, a significant fraction of dpa is removed in this process. Samples of Fe₃O₄dpa@Pd_x containing different Pd loadings (x = 0.1, 0.3, 0.5 and 1.0 wt%) were prepared, and their catalytic efficiency for the Suzuki C-C coupling reaction was studied. The best catalyst was Fe₃O₄dpa@Pd_0.5, which gave the highest TOF published to date for the reaction of bromobenzene with phenylboronic acid in a mixture of ethanol/water (1/1). Interestingly, the same reaction carried out in water also produced excellent yields of the resulting C-C coupling product. The behaviour of other bromide aryl molecules was also investigated. The best catalytic results for the aqueous phase reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) were obtained using Fe₃O₄dpa@Pd_0.1. The presence of Pd SACs (single atom catalysts) seems to be responsible for this performance. In contrast, the same Fe₃O₄dpa@Pd_0.1 catalyst is absolutely inactive for the hydrogenation of styrene in ethanol.

Full text of DOI:10.1039/c6ra14257e

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DOI: 10.1039/C6RA14257E
Journal Name
ARTICLE
Highly water-dispersible Pd nanoparticles and single atoms
magnetite-supported as excellent catalysts for Suzuki and
hydrogenation reactions
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
a
a
c
DOI: 10.1039/x0xx00000x
A. Guarnizo, I. Angurell,* G. Muller, J. Llorca, M. Seco, O. Rossell, and M. D. Rossell*
www.rsc.org/
The molecule 4-(diphenylphosphino)benzoic acid (dpa) anchored on the surface of magnetite nanoparticles permits the
easy capture of palladium ions that are deposited on the surface of the magnetite nanoparticle after reduction with
NaBH
different Pd loading (x= 0.1, 0.3, 0.5 and 1.0 wt%) were prepared and their catalytic efficiency for the Suzuki C-C coupling
reaction studied. The best catalyst was Fe dpa@Pd0.5 that gave the highest TOF published up to now for the reaction of
4 3 4 x
. Unexpectedly, a significant fraction of dpa is removed in this process. Samples of Fe O dpa@Pd containing
3 4
O
bromobenzene with phenylboronic acid in a mixture of ethanol/water (1/1). Interestingly, the same reaction carried out in
water produced also excellent yields of the resulting C-C coupling product. The behaviour of other bromide aryl molecules
was also investigated. The best catalytic results for the aqueous phase reduction of 4-nitrophenol (4-NP) to 4-aminophenol
3 4
(4-AP) were obtained using Fe O dpa@Pd0.1. The presence of Pd SACs (single atom catalyst) seems to be responsible for
3 4
this performance. In contrast, the same Fe O dpa@Pd0.1 catalyst is absolutely inactive in the hydrogenation of styrene in
ethanol.
9,10
11
with amino
or phosphine units. However, the benefits
Introduction
For the last few decades Pd nanoparticles (NPs) have attracted
enormous attention as transition metal catalyst in the array of
transformation processes in organic chemistry, mainly for C-C cross-
coupling reactions. Given that easy aggregation of nanoparticles
can affect their catalytic behaviour it is crucial to protect them with
produced by the linkers on the deposition of the metal catalysts
should be contrasted with the potential catalytic inhibition
produced by their steric hindrance that difficults the approach of
the reagents to the catalyst. In this context, it is evident that the
possibility of partial removal of the linker after metal
immobilization becomes a desirable condition.
1
2,3
4
Very recently, we have reported the synthesis and use of 4-
mercaptophenyldiphenylphosphine (Sdp) as linker to immobilize Pd
appropriate linkers, namely, thiol-protected linkers, dendrimers,
5
6
dendrons or by the use of ionic liquids. Other strategy involves the
12
nanoparticles onto the surface of magnetite nanoparticles.
immobilization of the Pd NPs on solid supports, usually SiO , TiO2
2
Subsequent treatment of the nanoparticles with an aqueous
hydrogen peroxide solution permitted the removal of 70wt% of
Sdp. The partial loss of the linker made the resulting nanoparticles
catalytically more efficient for Suzuki-Miyaura C-C cross coupling
reaction and for hydrogenation of 4-nitrophenol and styrene.
In this paper we expand this study to the formation of
Pd/magnetite nanoparticles and single-atoms using the partially
and C prepared by different procedures and having different areas
and morphologies. This interest has increased in the last years with
7
the easy deposition of Pd onto the surface of magnetite NPs
allowing to obtain catalytic systems magnetically separable that
avoid the requirement of catalyst filtration after completion of the
reaction. In addition, the nanocatalyst can be efficiently recycled
8
and reused in the vast majority of examples. For anchoring small
water
soluble
and
commercially
available
4-
and homogeneous nanoparticles it is worth functionalizing
previously the magnetite surface with linkers terminated generally
(
diphenylphosphino)benzoic acid (Fig. 1) as a linker in order to make
the above processes “greener” by minimizing the use of (toxic)
organic solvents. In addition to this, the increased demand for scare
13
a.
metals as palladium and future availability is a matter for concern
that would be dispelled if a control of the palladium particle size is
achieved for proper atomic economy.
Departament de Química Inorgànica i Orgànica, Secció de Química Inorgànica,
Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona (Spain). e-mail:
inmaculada.angurell@qi.ub.es.
b.
Institut de Tècniques Energètiques i Centre de Recerca en Nanoenginyeria,
Universitat Politècnica de Catalunya, Diagonal 647, 08028 Barcelona (Spain).
Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials
c.
Science and Technology, Überlandstrasse 129, 8600 Dübendorf (Switzerland). e-
mail: marta.rossell@empa.ch.
†
Electronic Supplementary Information (ESI) available: XPS of catalysts (Fig. S1),
tables of conversion in Suzuki-Miyaura based on temperature (in ethanol/water)
(
(
Table S1), base (Table S2), solvent (Table S3) and temperature in water solution
Table S4). See DOI: 10.1039/x0xx00000x
Fig. 1 4-(diphenylphosphino)benzoic acid (dpa)
J. Name., 2013, 00, 1-3 | 1
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Journal Name
Results and discussion
Synthesis of the catalysts
Table 1 Composition of magnetite-dpa-Pd hybrids
View Article Online
DOI: 10.1039/C6RA14257E
Catalyst
% dpa (nmol/mg NPs)
%Pd
0
0.68
0.097
0.32
0.51
1.01
Fe O dpa
3.2 (105)
0 (0)
3
4
The starting bare magnetite nanoparticles were synthesized
Fe O @Pd
3
4
14
following
a
literature procedure.
Functionalized magnetite
Fe O dpa@Pd
2.4 (78.7)
1.9 (63.0)
1.7 (55.2)
1.1 (35.0)
3
4
0.1
nanoparticles Fe O dpa were obtained by sonication of Fe O in
Fe O dpa@Pd
3
4
3
4
3
3
4
4
0.3
methanol in presence of dpa during 2 hours (Scheme 1a). Elemental
analyses (C, H) of the resulting nanoparticles showed that the load
of the organic component was about 3.2 wt%. The next step
consisted of immobilizing Pd onto the magnetite surface (Scheme
Fe
O
dpa@Pd0.5
Fe O dpa@Pd
3
4
1.0
Images of the hybrid nanoparticles Fe O dpa@Pd have been
3
4
x
obtained by TEM (for x = 1.0) and HAADF-STEM (High-Angle
Annular Dark-Field Scanning Transmission Electron
Microscopy) (for x = 0.1, 0.3, and 0.5). The samples display
1
b). Thus, Fe O dpa was dispersed in water and sonicated for 10
3 4
min. Then, a water solution of K [PdCl ] followed by another of
2
4
sodium borohydride were added to obtain the Fe O dpa@Pd spherical Fe O4 nanoparticles with an average particle size
3
4
x
3
nanoparticles where x is the weight Pd percentage in the NPs. With around 12 nm. In addition, smaller Pd species appear
immobilized on the magnetite surface. As expected, the total
Pd load determines the Pd nanoparticle size. Thus, for x= 1.0
the images clearly reveal the presence of nanoparticles with an
average size of 4.5 nm (Fig. 2). In contrast, when the Pd
loading is 0.1 wt%, all Pd species exist exclusively as isolated
single atoms; neither subnanometre clusters nor nanoparticles
this method, we prepared several samples with different Pd
contents (0.1%, 0.3%, 0.5% and 1.0%). The first one was included in
order to obtain magnetite NPs decorated exclusively with isolated
palladium single atoms. These species are denoted as SACs (single-
atom catalyst) and exhibit generally exceptional catalytic activity.
A crucial and surprising observation in the course of the were detected at all (Fig. 3). Samples containing intermediate
15
Fe O dpa@Pd NPs fabrication was the significant fraction of dpa palladium contents, x = 0.3 and 0.5, evidenced numerous
3
4
x
single metal atoms accompanied with other subnanometer
clusters and larger nanoparticles. This fact is somewhat
surprising because the presence of large nanoparticles of ~5
nm along with a significant number of Pd SACs has not
precedents (Fig. 4). The presence of palladium was confirmed
by Energy-dispersive X-ray Spectroscopy (EDS) measurements;
lost during the reduction process with NaBH . For example, the
4
Fe O dpa@Pd only kept 1.7% dpa from 3.2% of the starting
3
4
0.5
Fe O dpa NPs. That is, dpa favours the dispersion of Pd NPs on the
3
4
magnetite surface and then the ligand undergoes partial
elimination without the need of using other agents. Consequently,
the poor linker content in Fe O dpa@Pd NPs anticipated excellent sharp Pd Lα1 and Lβ1 peaks are clearly visible at 2.84 and 2.99
3
4
x
keV, respectively (Fig. 4c and Fig. 4f).
catalytic activity that we have explored in the Suzuki-Miyaura
reaction, reduction of 4-nitrophenol and styrene hydrogenation
processes.
Characterization of Fe O dpa@Pd NPs. The FT-IR spectrum of the
(
a)
(b)
3
4
x
-
1
catalysts are practically identical showing a sharp peak at 594 cm ,
which is the IR signature for ferrite particles and weak signals at
-1
-1
1
100 cm and 2921 cm (fig. S1) due to aryl C-H vibrations. The Pd
content in Fe O dpa@Pd was determined by ICPoes (Inductive
3
4
x
Coupled Plasma optical emission spectroscopy) measurements and
the dpa content was estimated by OEA (Organic Elemental
Analysis). The analytical composition of the hybrids, as well as of
the NPs loaded with Pd in the absence of linker, is shown in Table 1.
Fig. 2 (a) TEM images of Fe O dpa@Pd . (b) Histrogram of the
3
4
1.0
Pd particle size distribution.
(
a)
(b)
Fig. 3 (a) HAADF-STEM images of Fe O dpa@Pd . Isolated Pd
3
4
0.1
Scheme 1 Synthesis of catalysts: (a) MNPs functionalization
with dpa and Pd(II) capture and (b) deposition of palladium
nanoparticles and partial dpa lost.
atoms (white circles) are uniformly dispersed on the magnetite
support. (b) EDS spectrum. The Cu and C signals originate from
the TEM grid.
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6RA14257E
(
a)
(b)
(c)
(
d)
(e)
(f)
Fig. 4 (a,b) HAADF-STEM images of Fe O dpa@Pd showing the presence of different Pd species. (c) EDS spectrum of Fe O dpa@Pd0.3.
3
4
0.3
3 4
(
d,e) HAADF-STEM images of Fe O dpa@Pd showing the presence of different Pd species. (f) EDS spectrum of Fe O dpa@Pd The
3 4 0.5 3 4 0.5.
circles and squares in images indicate Pd single-atoms and clusters/nanoparticles, respectively.
X-ray Photoelectron Spectroscopy (XPS) was used to determine the 4-bromobenzene and phenylboronic acid (1:1) was analysed
and optimized in terms of time, temperature (Table S1), base
Table S2) and solvent (Table S3). The best results were
obtained with Fe O dpa@Pd NPs after 20 min at 65 °C in
surface elemental composition for Fe O dpa@Pd nanocomposites
3
4
x
(
(
Fig. S1). The surface Pd/Fe atomic ratios are compiled in Table 2.
3
4
0.5
For the Fe O dpa@Pd catalysts with x=0.1, 0.3 and 0.5, the Pd/Fe
3
4
x
EtOH/water (1:1) and K PO . Table 3 lists the catalytic results.
Note that Fe O dpa NPs in absence of Pd (entry 1) showed
catalytic inactivity, while entry 2 revealed the poor catalytic
behaviour of the NPs loaded with Pd in the absence of linker.
Entries 3-6 display the results obtained with the rest of the Pd-
containing NPs. Clearly, the best catalyst is the sample
3
4
ratio progressively increases from Pd/Fe=0.003 to 0.010 as
expected since all the Pd single atoms and subnanometer clusters
observed by HAADF-STEM are sampled. However, for the
3
4
Fe O dpa@Pd sample, the Pd/Fe ratio is lower because not all the
3
4
1.0
Pd in the Pd nanoparticles is sampled given the larger dimensions of
the Pd nanoparticles, in total accordance to high-resolution TEM containing 0.5% of Pd (entry 5). The catalyst constituted only
results discussed above. The oxidation state of Pd in each catalyst is by single atoms (Fe dpa@Pd0.1) (entry 3) and the Fe dpa
O
4
O
3 4
3
also included in Table 2, where the relative contributions from @ Pd0.3 (entry 4) are also excellent although the sample loaded
with 1.0 Pd% is clearly less efficient (entry 6) due the large
nanoparticles size.
Pd(0), Pd(II) and Pd(IV) species and their binding energies (Pd 3d5/2
)
are indicated. There is a clear trend between the oxidation state of
Pd and its content in the catalysts; the higher the Pd loading the
more oxidized is Pd. The sample with the highest Pd content,
Fe O dpa@Pd1.0, exhibits photoemission peaks ascribed to Pd(IV),
We next proceeded to examine the scope and limitation of the
catalysts. Working with Fe O dpa@Pd NPs several bromo-
3
4
0.5
aryl substrates were reacted with phenylboronic acid (1:1) and
3
4
the coupling products were obtained in excellent yields (Table
which confirms the tendency of Pd oxidation in this series. On the
other hand, the sample Fe O dpa@Pd0.1, which only contains Pd
-5
4
). Note that the Pd used is as low as 9.4x10 eq. To our
3
4
knowledge the TOFs exhibited with Fe O dpa@Pd NPs for
3 4 0.5
single atoms, exhibits a Pd(0) component at a significantly higher
the Suzuki-Miyaura C-C coupling are by far the highest
binding energy value (335.8 vs. 335.2-335.3 eV), which suggests reported up to now.
that there is an electronic transfer from the Pd single atoms to the
1
6,17
Table 2 Pd/Fe surface atomic ratios, % of Pd species and binding
energies obtained by XPS.
Fe O support, in accordance to previous reports.
3
4
Catalytic studies
%
(BE Pd 3d5/2, eV)
Pd(II)
Suzuki-Miyaura reaction. Palladium-catalyzed Suzuki-Miyaura
cross-coupling is a well-known process for the synthesis of
biaryls using aryl halides with arylboronic (Scheme 2) acids
that have many applications in agrochemicals, natural
Catalyst
Pd/Fe
Pd(0)
Pd(IV)
Fe O @Pd
0.006 42 (335.2) 58 (337.5)
0.003 48 (335.8) 52 (337.3)
0.005 30 (335.3) 70 (337.4)
0.010 18 (335.2) 82 (337.4)
-
-
-
-
3
4
Fe O dpa@Pd
3
4
0.1
0.3
0.5
1.0
Fe O dpa@Pd
3
4
18
products and pharmaceutical intermediates. For testing the
catalytic activity of Fe O dpa@Pd NPs the reaction between
Fe O dpa@Pd
3
4
3
4
x
Fe O dpa@Pd
0.007 18 (335.3) 58 (337.5) 24 (338.7)
3
4
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Journal Name
4 Substrate effect on Suzuki-Miyaura cVoieuwpAlirnticgle Ownliitnhe
Table
Fe O dpa@Pd as catalyst.
DOI: 10.1039/C6RA14257E
3
4
0.5
-
1
Bromoaryl
TOF (h )
110 000
Scheme 2 Suzuki-Miyaura coupling.
96 300
Table 3 Activity of different catalyst in Suzuki-Miyaura coupling.
3
5 000
-
1
Entry
Catalyst
TOF (h )
0
870
1
2
3
4
5
6
Fe O dpa
3
4
Fe O @Pd
38 400
28 300
3
4
Fe O dpa@Pd
51200
64600
110000
1630
3
4
0.1
0.3
0.5
1.0
Fe O dpa@Pd
3
4
Fe O dpa@Pd
3
4
Fe O dpa@Pd
3
4
TOF (mol product/mol catalyst·time). Bromobencene (3 mmol),
phenylboronic acid (3.6 mmol), K PO (9 mmol), catalyst
Bromoaryl (3 mmol), phenylboronic acid (3.6 mmol), K PO (9
3 4
-5
3
4
mmol), catalyst (9.4×10 mmol Pd), 65 °C, EtOH:water (60 mL).
-5
(
9.4×10 mmol Pd), 65 °C, EtOH:water.
Table 5 Reuse of Fe O dpa@Pd as catalyst in the Suzuki-Miyaura
3
4
0.5
coupling.
The lifetime of catalytic systems and their reusability is
important for practical applications. The reusability of the
catalyst was examined upon the reaction of bromobenzene
and phenylboronic acid. In detail, the NPs were collected after
completion of the first reaction by using an external magnet
and washed with ethanol and dried. Then, the recovered
catalyst nanoparticles were used for the next round by mixing
them with a new substrate, base, and solvent. The third round
was carried out following the same procedure and the results
are displayed in Table 5. As can be seen, the loss of the
catalytic efficiency is remarkably high and greater than that
-
1
Cycle
TOF (h )
110000
19200
1
2
3
4800
Bromobencene (3 mmol), phenylboronic acid (3.6 mmol), K PO
3
4
-5
(
9 mmol), catalyst (9.4×10 mmol Pd), 65 °C, EtOH:water 1:1.
(
a)
12
detected, for example, by employing Sdp linker. The strong
decrease of the catalytic activity is concomitant with the
reduction of Pd concentration (33 wt%) found in the residual
NPs. The measured leaching of Pd suggests that the catalytic
process is essentially homogeneous, which was subsequently
confirmed by a hot filtration test. Thus, bromobenzene with
o
phenylboronic acid were reacted at 65 C. After 10 min. the
solid catalyst was separated magnetically and the filtrate was
transferred to another Schlenk flask. Without the presence of
catalyst the reaction progressed to reach 95% of conversion
after 30 min. This result is in good accord with recently
reported data arguing that palladium species leached out from
nanoparticles into the reaction mixture are indeed the actual
(
b)
19
catalytic species instead of the original Pd NPs. In addition,
the HAADF-STEM images of the catalyst after 3 cycles showed
the presence of only palladium single atoms (Fig. 5)
strengthening the idea that strong bonding between Pd single-
1
6
atoms and naked Fe O takes place. This behavior explains
3
4
the remaining activity of the catalyst after several cycles.
The Suzuki reaction with chloroaryl substrates is difficult to removal. (b) HAADF-STEM of Fe
Fig. 5 (a) Conversion of 4-bromobencene after Fe
3
O
4
dpa@Pd0.5
O dpa@Pd0.5 after the third
3 4
achieve. In this paper only traces of the final product have recycling process. Only Pd single-atoms were present (white
been observed by reaction of chlorobenzene and circles).
chlorotoluene with phenylboronic acid in the presence of
Fe O dpa@Pd .
3
4
0.5
4
| J. Name., 2012, 00, 1-3
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Catalysis in neat water. There are only very few articles on the Table 6 Suzuki-Miyaura coupling with different substrates using
View Article Online
application of magnetically separable Pd-catalyzed Suzuki-Miyaura Fe
reaction in neat water, and in some of the examples the process
required the use of large amounts of ionic liquids or phase transfer
O
dpa@Pd0.5 as catalyst.
Bromoaryl
DOI: 10.1039/C6RA14257E
3 4
-
1
TOF (h )
2
0
reagents. That is, there is still much room for developing new
magnetically recoverable catalyst in pure water. Here, we report
the catalytic behaviour of Fe O dpa@Pd NPs in water for the
3
150
030
0
3
4
0.5
Suzuki coupling under atmosphere. Although the boronic acids are
soluble in water, the efficiency of Suzuki reaction in neat water can
be affected by side reactions, mainly, homocoupling reactions of
the boronic acids. The reaction of 4-bromobenzene and phenyl
boronic acid was used as model reaction and optimized in terms of
time and temperature. We next examined the scope of this process
using several substrates (Table 6). It is well-known that the coupling
reactions of aryl bromides containing electron–donating groups
proceeded more efficiently than those having electron-withdrawing
groups. This trend is not observed in our results because the
solubility of the substrates is the predominant factor in water.
Formation of homocoupling products was negligible. The overall
results shown in Table 7 are exceptional in terms of TOFs and
selectivity overcoming all those reported earlier, but the reusability
is very limited. For example, using bromobenzaldehyde as a reagent
it was found that after the third cycle the TOF decreased from 5030
5
1479
475
Bromoaryl (0.5 mmol), phenylboronic acid (0.6 mmol), K PO
3
4
-4
(
1.5 mmol), 2.0×10 mmol Pd, 65 °C, water, air atmosphere.
Hydrogenation of 4-nitrophenol. Nitrophenol is a by-product
produced from pesticides and synthetic dyes that causes damage to
central nervous human system so that its removal from the
-1
to 106 h . Palladium leaching was clearly evidenced by a hot
filtration test in which the important loss (28 wt%) of Pd content (by
ICPoes analysis) in residual NPs confirmed the homogeneous nature
of the catalytic process.
33
environment is a crucial task. Recently, FeO -supported platinum
x
34
35
SAC and carbon nitride- palladium SAC have been reported to
show excellent activity for the hydrogenation of nitroarenes but, to
our knowledge, the Pd SACs supported on magnetite NPs have not
Table 7 Comparison of Suzuki-Miyaura coupling catalyzed by Pd NPs from the literature.
o
-1
Support
Substrate
BB
Solvent
Base
K3PO4
K3PO4
KOH
Temperature ( C)
TOF(h )
Ref.
This work
This work
12
Fe O dpa@Pd
EtOH:H O
65
65
110000
1470
60
3
4
0.5
0.5
0.5
2
Fe O dpa@Pd
BB
H O
2
3
4
Fe O Sdp@Pd
4-BA
4-BN
4-BN
4-BA
4-BN
4-BA
4-BA
4-BA
4-BA
4-BA
4-BD
4-BA
BB
EtOH:H O
65
3
4
2
γ-Fe O /dendrimer
THF/Triton
NaOH
K2CO3
K3PO4
KOH
65
41.7
13.1
132
232
50
21
2
3
Fe O -SiO /phosphine
H O /DMF
2
25
22
3
4
2
Fe O -SiO /phosphine-Pd(II)
MeOH
60
23
3
4
2
Graphitic carbon nitride/none
None support/ionic phosphine
Fe O -SiO /iminophosphine
EtOH:H O
25
24
2
Glycerol
Toluene
Toluene
DMF
t-BuOK
KOH
100
100
100
65
25
100
33
11b
11a
26
3
4
2
Fe O /dopamine-phosphine
KOH
3
4
NiFe O /dopamine
K3PO4
NaOH
K3PO4
Na CO
0.4
2
4
Carbon
EtOH:H O
65
171
82.5
0.01
16500
437
24
2
Phosphine dendrimer
Dioxane
Toluene
100
130
28
27
Fe O4
3
28
2
3
Click dendrimer
EtOH: H O
K3PO4
K2CO3
29
2
Fe O -SiO / TEG-imidazolium
BB
H O
2
60
30
3
4
2
Multi-walled carbon
nanotubes/Pd/PdO
BB
BB
H O
i-Pr NH
100
70
250
31
32
2
2
Fe O /Triazole-acid
EtOH: H O
K2CO3
2920
3
4
2
4-BN: 4-bromonitrobenzene. 4-BA: 4-bromoanisol. BB, bromobencene.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Page 6 of 11
ARTICLE
Journal Name
explored for the reduction of 4-nitrophenol in water.
Table 8 Activity of catalysts in 4-NP reduction.
View Article Online
DOI: 10.1039/C6RA14257E
The aqueous phase reduction reaction of 4-nitrophenol (4-NP)
to 4-aminophenol (4-AP) (Scheme 3) using metal-supported
NPs is a model reaction that permits to compare the efficiency
of the catalysts employed. Esumi et al have proposed that the
catalytic reduction of 4-NP proceeds in two steps: i) diffusion
and adsorption of 4-NP to the catalyst surface and ii) electron
-1 -1
Catalyst
Fe O @Pd
k’(s g )
24500
90000
58000
50600
16300
3
4
Fe O dpa@Pd
3
4
0.1
Fe O dpa@Pd
3
3
3
4
4
4
0.3
Fe
Fe
O
O
dpa@Pd0.5
dpa@Pd1.0
-
36
transfer mediated by the catalyst surface from BH to 4-NP.
4
The rapid reduction generally is carried out using NaBH and
4
monitored by time-resolved UV/Visible spectra. We have
tested this reaction with the catalysts Fe O dpa@Pd NPs and
3
4
x
the results are listed in Table 8. The TOFs measured are also
very high, including that obtained with a sample having Pd
deposited on their magnetite surface without the assistance of
dpa (entry 1). In all cases the reaction takes place with rapid
release of H bubbles accompanied with a decrease of the
2
absorption at 400 nm and the appearance of other at 317 nm,
confirming the transformation of 4-nitrophenol to 4-
aminophenol (Fig. 6a). The UV-Vis spectra showed an
isosbestic point (313 nm) indicating that no by-product is
formed during the reduction of 4-NP. Since the concentration
Fig. 7 Activity of Fe O dpa@Pd per cycle in 4-NP reduction.
3
4
0.1
of
NaBH4
exceeds
that
of
4-nitrophenol
Table 9 Comparison of reduction of 4-NP catalyzed by Pd NPs from
the literature.
(c(NaBH )/c(nitrophenol) = 100:1) the reduction can be
4
considered as pseudo-first-order reaction with regard to 4-
nitrophenol only. The plot matches the first-order reaction
kinetics and the rate constant is calculated from the equation
ln(c /c )=kt. Figure 6b shows the linear relationship between
ln(c /c ) and the reaction time for Fe O dpa@Pd . For a
quantitative comparison, we have introduced the activity
-
1
-1 -1
Catalyst
Fe O dpa@Pd
k (s ) k' (s g )
Ref.
-3
3.6×10
3.2×10
1.2×10
4.4×10
2.0×10
107333 This work
152153
7827
6100
2349
3
4
0.1
-3
-2
-2
-2
t
o
rGO/Pd–Fe O /PPy
37
38
12
3
4
t
0
3
4
0.3
MOF
Fe O Sdp@Pd ]
OX
[
3
4
-1
parameter k’ = k(s )/m where mPd is the total mass of the
palladium added as catalyst. From the catalytic results (Table
Pd
PdNPs/Fe O /dopamine-
11a
3
4
phosphine
8
) it can be seen that the best sample is Fe O dpa@Pd that
-2
-3
-3
-2
3
4
0.1
PdNPs /Fe O -Ag core-shell
3.3×10
8.9×10
8.0×10
1.2×10
6.5×10
1736
1415
1026
750
39
40
41
42
43
3
4
contains only 0.10% of palladium. As commented above this
catalyst is constituted by Pd SACs, which enormous catalytic
activity has been recently recognized in different reactions.
PdNPs/Polypirrol capsule
PdNPs/Fibrous nano-silica
PdNPs/Mesoporous silica
Pd-AuNPs/graphene
-3
130
Table 9 permits to compare the efficiency of a good number of
catalytic species and it can be seen that the efficiency of the
Fe O dpa@Pd NPs overcomes largely the majority of them.
3
4
0.1
Scheme 3 Reduction of 4-NP.
The reusability of the catalysts has been examined. The
catalytic efficiency is maintained during the first six cycles for
Fe O dpa@Pd (Fig. 7). Then, it began to drop. ICPoes
3
4
0.1
analysis indicated that the Pd content of the resulting NPs
after six cycles decreased only 6%. This fact along with the
constant activity shown for the catalyst through the cycles
seems to indicate that reduction of nitrophenol occurs
heterogeneously.
(
a)
(b)
Hydrogenation of styrene. Very recently, we have described the
catalytic activity of Fe O dopPPh @Pd NPs supported on magnetite
3
4
2
x
nanoparticles using styrene hydrogenation as a model catalytic
reaction in isopropanol as solvent at room temperature in a
1
7
hydrogen atmosphere. Surprisingly we observed that a sample
containing a very low Pd weight content (0.18wt%) and constituted
uniquely by Pd SACs was absolutely inactive for the hydrogenation
of styrene and other alkenes. This is the first time that Pd single
Fig. 6 Catalytic reduction of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP) with Fe O dpa@Pd . (a) UV-Vis spectra
3
4
0.3
of time dependent reaction. (b) ln C / C vs. time of reaction.
t
0
6
| J. Name., 2012, 00, 1-3
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Page 7 of 11
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Journal Name
atom catalyst (SACs) were shown to be catalytically inactive. In Table 11 Styrene hydrogenation per cycle with Fe
ARTICLE
O
3
dpa@Pd 0.5 as
4
View Article Online
contrast, samples with higher Pd content, exhibiting small clusters catalyst.
and nanoparticles, gave exceptional catalytic results. These findings
prompted us to analyse the catalytic activity of Fe O dpa@Pd NPs
for the hydrogenation of styrene in order to check whether our
catalysts are able to mimic the catalytic behaviour shown by the
Fe O dopPPh @Pd NPs. The results are compiled in Table 10. The
DOI: 10.1039/C6RA14257E
-
1
Cycle
TOF (h )
4880
3
4
x
1
2
3
4793
4760
3 bar H , 5 mmol styrene, 25 °C, EtOH.
2
3
4
2
x
most remarkable result was the null activity shown by sample
Fe O dpa@Pd NPs (entry 2). This result confirms the no-activity of
3
4
0.1
Table 12 Comparison of styrene hydrogenation catalyzed by Pd NPs
from the literature.
Pd SACs for the hydrogenation of styrene and it is opposed to the
44
excellent catalytic efficiency reported in a number of processes.
o
Significantly, the rest of samples followed the expected catalytic Catalyst
trend, that is, excellent performances that deteriorate with the
Solvent Pressure T ( C ) TOF Ref.
-
1
(bar)
(h )
increase of Pd content and size of the Pd nanoparticles.
Reusability studies have been carried out on the best catalytic
Fe O dpa@Pd
0.5
EtOH
3
25
4880 This
work
3
4
sample Fe O dpa@Pd . The loss of activity is very small (Table 11) [Fe O Sdp@Pd]
which is confirmed by the final Pd content after 3 rounds, less than Polivinilpiridina/PdCl2
EtOH
EtOH
EtOH
DMF
3
1
3
1
25
25
25
25
3100 12
6944 45
3
4
0.5
3
4
ox
3
% in relation to the starting sample. These results strongly support
PdNPs/Polietilenglicol
Polyestyrene/Pd
660
766
46
47
the catalytic heterogeneous behaviour for this process. In Table 12
are listed selected catalysts for hydrogenation of styrene including complex
reaction conditions and TOFs where the Pd catalysts reported here
are among the best ones.
PdNPs/Stearate/Oleyl Cyclohexane
amine
21
50
7704 48
MOF/Pd
Pd/C
None
Ethyl acetate
1
3
35
25
703
163
49
50
Conclusions
In conclusion, in this work we demonstrate that the use of the
dpa molecule as a linker for the deposition of palladium Acknowledgements
species on the magnetite surface has permitted to solubilize
This work was financially supported by the MICINN (projects
CTQ2012-31335 and CTQ2015-65040-P). J. L. is a Serra Húnter
Fellow and is grateful to the ICREA Academia program. Access
to the TEM facilities at IBM Research-Zurich, Switzerland,
under the IBM/Empa Master Joint Development Agreement is
gratefully acknowledged. A. G. thanks University of Tolima
partially the resulting hybrid in water. This fact along with the
partial removal of dpa during the reduction process facilitates
the approach of the substrates to the catalyst. In addition to
this, the small size of the Pd NPs supported on the magnetite
in Fe O dba@Pd
and Fe O dba@Pd
along with the
3
4
0.3
3
4
0.5
presence of Pd SACs, explains the exceptional results for the
Suzuki C-C coupling reaction and hydrogenation of 4-
nitrophenol both in neat water and in a mixture of
ethanol/water.
(Colombia) for financial support.
Experimental
All manipulations were performed under purified nitrogen using
standard Schlenk Techniques. Organic Elemental Analysis (OEA) was
performed in an Elemental Analyzer Thermo Scientific Flash 2000
A7 model. The Infrared Spectrum (IR) was plot with a NICOLET
Impact 400 FT-IR. And the abbreviations used for signals were: w
(
weak), m (medium), s (strong), only significant signals are
Scheme 4 Styrene hydrogenation.
mentioned. Thermogravimetric Analysis (TGA) was performed on
IGA 851 Mettler-Toledo instrument, nitrogen flow (50 mL/min),
from 30 up to 1000 °C, and 10 °C/min as gradient. ICPoes
(Inductively Coupled Plasma optical emission spectrometry) analysis
was achieved in a Perkin Elmer 3200RL model. Samples were
Table 10 Activity of the catalyst in styrene hydrogenation.
-
1
Entry
Catalyst
TOF (h )
404
1
2
3
4
5
Fe O @Pd
3 4
Fe O dpa@Pd
0
dissolved in HCl/HNO 3:1 (v/v) mixture. High-angle annular dark-
3
4
0.1
0.3
0.5
1.0
3
Fe O dba@Pd
4690
4880
3630
field scanning transmission electron microscopy (HAADF-STEM) and
energy-dispersive X-ray spectroscopy (EDS) were carried out using a
double spherical aberration-corrected JEOL JEM-ARM200F
microscope operated at 200 kV, and equipped with a JEOL Dry
3
4
Fe O dba@Pd
3
4
Fe O dba@Pd
3
4
3
bar H , 5 mmol styrene, 25 °C, EtOH, 1h.
2
2
SD100GV silicon drift detector with 100 mm detection area for EDS
analysis. The microscope was setup in STEM mode with a probe
semiconvergence angle set to 25.3 mrad which yields a calculated
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Please Rd So Cn oA tda vd aj un sc te ms argins
Page 8 of 11
ARTICLE
Journal Name
probe size of about 80 pm. The annular semidetection range of the magnet and dried under reduced pressure. ICPoes: 1.1%, 0.50%,
View Article Online
DOI: 10.1039/C6RA14257E
annular dark-field detector was set to collect electrons scattered 0.32%, and 0.097% of palladium for Fe O dpa@Pd , where x = 1,
3
4
x
between 90 and 370 mrad. Transmission electron microscopy (TEM) 0.5, 0.3, and 0.1, respectively).
images were registered at 200 kV in a JEOL 2010F instrument having Suzuki-Miyaura coupling. In a typical Suzuki-Miyaura reaction 3.6
a point resolution of 0.21 nm. Samples were prepared by placing a mmol of phenylboronic acid, 9 mmol of base, and a quantity of Pd
-5
drop of solution on a holey-carbon-coated Cu TEM grid and allowing catalyst (e.g. 2.0 mg of Fe O Sdp@Pd0.5, 9.4×10 mmol) were
3
4
the solvent to evaporate in air. In addition, for the HAADF-STEM weighed into a Schlenk tube. The mixture was purged with nitrogen
experiments, the samples were gently plasma cleaned with an Ar and 60 mL of solvent was added. After that, the tube was brought
(
75%) / O (25%) plasma for a few seconds to remove hydrocarbon to a preheated plate at 65 °C, and 3 mmol (1 eq) of substrate was
2
contamination from the surfaces of the nanoparticles. XPS (X-ray added. When the desirable reaction time was reached, the mixture
Photoelectron Spectroscopy) analysis was carried out in a SPECS was allowed to cool down. An extraction with ethyl acetate was
instrument with Al X-ray source and a Phoibos 150 analyzer. The needed. The organic phase was removed, dried over sodium
-7
spectrum was recorded under 10 Pa. Binding energies were sulphate and analyzed by GC. The identity of products was
referred to the C 1s signal at 284.8 eV. All reactants were purchased confirmed by GC-MS analysis.
from suppliers in “for synthesis” quality or higher and used without Catalytic reduction of p-nitrophenol. 30 μL of p-nitrophenol (7.4
further purification. The solvents were HPLC quality and/or were mM) and 30 μL of NaBH (0.40 M) were added into a quartz cuvette
4
dried trough pure solve instrument from Innovate Tehcnology USA containing 2 mL of water. Then, 30 μL of an aqueous suspension
Inc. Deionized water was obtained from Milipore Helix 3 water containing the catalyst nanocomposite (3 to 5 mg approximately in
purification system.
5 mL of water) was injected into the cuvette to start the reaction.
Synthesis of ferrite nanoparticles (Fe O ). 6.95 g (25.3 mmol) of The intensity of the absorption peak at 400 nm in UV-Vis
3
4
iron(II) sulphate heptahydrate was mixed with 10.0 g of iron(III) spectroscopy was used to monitor the process of the conversion of
sulphate hydrate and dissolved in water (250 mL). Then an p-nitrophenol to p-aminophenol. After each cycle of reaction,
ammonia 25 % solution was slowly added to adjust pH up to 10. The another 30 μL of p-nitrophenol and 30 μL of NaBH were added to
4
reaction mixture was stirred for 1 h at 50 °C. The nanoparticles the reaction to study the reuse of the catalyst. The catalytic
obtained were separated magnetically and washed with water until reduction reactions were conducted at room temperature.
pH 7 was attained. The resulting nanoparticles were dried under Hydrogenation of styrene. Hydrogenation reactions were carried
reduced pressure at 60 °C for 2 h.
out in a hydrogen atmosphere at room temperature. Typically,
Synthesis of Fe O @Pd. 100 mg of magnetite nanoparticles were under nitrogen, ferrite catalyst (5.0 mg approximately) was
3
4
sonicated in methanol (10 ml) for 30 min. Then, 4.1 mg (12.5 µmol) dispersed in ethanol freshly distilled (20 mL), and then, styrene (2.5
of K [PdCl ] dissolved in water (2 mL) were added and stirred (1000 mmol) was poured. After that, the mixture was transfer into a
2
4
rpm) for 2 h. After that, the nanoparticles were washed with water Fisher-Porter reactor, filled with hydrogen (3 bar), and stirred at
3 × 10 mL), acetone (10 mL), and water (10 mL). Next, the NPs constant speed. The conversion was determined by GC analysis.
(
were dispersed in water (8 mL) and an aqueous solution (2.5 mL) of
sodium borohydride 0.05 M was added. Then, the nanoparticles
were washed with water (3 × 30 mL) and acetone, removed by an
external magnet and dried under reduced pressure. ICPoes: 0.68 %
Pd.
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RSC Advances
Page 10 of 11
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DOI: 10.1039/C6RA14257E
The linker dpa allows to deposit palladium on magnetite nanoparticles that show excellent catalytic
behavior in water or water/ethanol solvents.

Page 11 of 11
RSC Advances
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DOI: 10.1039/C6RA14257E
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68x85mm (150 x 150 DPI)