Palladium Nanoparticles Supported on Nitrogen-Doped Carbon Nanotubes as a Release-and-Catch Catalytic System in Aerobic Liquid-Phase Ethanol Oxidation

Article abstract of DOI:10.1002/cctc.201501379

Pd nanoparticles supported on carbon nanotubes were applied in the selective oxidation of ethanol in the liquid phase. The characterization of the surface and bulk properties combined with the catalytic tests indicated the dissolution and redeposition of Pd under the reaction conditions. A dynamic interplay within the Pd life cycle was identified to be responsible for the overall reactivity. Nitrogen-doped carbon nanotubes were found to act as an excellent support for the Pd catalyst system by efficiently stabilizing and recapturing the Pd species, which resulted in high activity and selectivity to acetic acid.

Full text of DOI:10.1002/cctc.201501379

DOI: 10.1002/cctc.201501379
Communications
Palladium Nanoparticles Supported on Nitrogen-Doped
Carbon Nanotubes as a Release-and-Catch Catalytic
System in Aerobic Liquid-Phase Ethanol Oxidation
[
a]
[a]
[a]
[a]
[a]
Weiwen Dong, Peirong Chen, Wei Xia, Philipp Weide, Holger Ruland,
[b]
[c]
[a]
Aleksander Kostka, Klaus Kçhler, and Martin Muhler*
[
4]
Pd nanoparticles supported on carbon nanotubes were applied
in the selective oxidation of ethanol in the liquid phase. The
characterization of the surface and bulk properties combined
with the catalytic tests indicated the dissolution and redeposi-
tion of Pd under the reaction conditions. A dynamic interplay
within the Pd life cycle was identified to be responsible for the
overall reactivity. Nitrogen-doped carbon nanotubes were
found to act as an excellent support for the Pd catalyst system
by efficiently stabilizing and recapturing the Pd species, which
resulted in high activity and selectivity to acetic acid.
of their comparable activity and easy recyclability. Combining
homogeneous and heterogeneous catalysis, most commonly
through irreversible immobilization of Pd complexes on solid
[
5]
supports, leads to highly active catalysts. Recently, “release
and catch” catalysts, in which the catalytic moiety is released
in solution during the reaction and recaptured at the end of
[
6]
the reaction, have been proposed as promising alternatives.
In such a catalyst system, tailor-made support materials play
an essential role in the reversible immobilization and recaptur-
ing of the active species.
Supports with porous structures can spatially confine metal
NPs to prevent sintering, but they suffer from mass-transfer
[
7]
The selective oxidation of alcohols in the liquid phase is an im-
portant reaction in organic synthesis. In comparison with the
traditional oxidation routes in which stoichiometric amounts of
the oxidants such as dichromates and permanganates are
limitations. Therefore, carbon nanotubes (CNTs) with a 1D
structure are emerging as a superior support for NPs and espe-
cially for “release-and-catch” systems that are applied in liquid-
phase reactions because of their high surface area and stability
at extreme pH values and because of the possibility to tune
used, aerobic oxidation utilizing O or air as the oxidant has
2
[
8]
significant advantages from both economic and environmental
their surface properties by chemical functionalization. By in-
troducing heteroatoms or defects on the CNT surface, the in-
teraction between the deposited metal species and the CNTs
can be optimized, and this can lead to favorable catalytic per-
[
1]
points of view. Palladium has been widely employed as a cat-
II
alyst either as a Pd complex in homogeneous catalysis or as
[
2]
supported Pd nanoparticles (NPs) in heterogeneous catalysis.
[
9–11]
Although taking advantage of all catalytic sites in homogene-
formance.
As shown in our previous studies, nitrogen-func-
II
ous Pd complexes, an excess amount of ligands is usually re-
tionalized CNTs (NCNTs) as a support were able to improve the
dispersion and, more importantly, to modify the electronic
II
0
quired to stabilize both the Pd complexes and the Pd inter-
mediate generated in situ to prevent aggregation, which im-
[
11]
structure of the supported Pd NPs. These NCNTs were found
[3]
[10,12]
pairs the separation of the catalysts and products. Therefore,
supported Pd NPs have been regarded as alternatives because
to be highly stable under severe conditions
and thus offer
great potential as a support for Pd NPs in liquid-phase reac-
tions.
Herein, we designed a Pd-based catalytic system for the
aqueous-phase oxidation of ethanol on the basis of CNT-sup-
ported Pd NPs (Pd/CNTs) and found strong evidence for the
dissolution and redeposition of Pd. The selective oxidation of
EtOH produces acetic acid (AcOH) as a major product if the re-
action is performed in the aqueous phase. The generated acid
dissolves the Pd NPs under the oxidizing reaction conditions,
[
a] W. Dong, Dr. P. Chen, Dr. W. Xia, P. Weide, Dr. H. Ruland, Prof. Dr. M. Muhler
Laboratory of Industrial Chemistry
Ruhr-University Bochum
4
4780 Bochum (Germany)
and
MPI for Chemical Energy Conversion
Stiftstrasse 34-36
4
5470 Mülheim an der Ruhr (Germany)
II
E-mail: muhler@techem.rub.de
and this leads to the formation of Pd cations. Pd/NCNTs with
[
b] Dr. A. Kostka
an appropriate N content showed the highest activity and
yield of acid owing to moderate stabilization of the Pd species
by the N-containing functional groups.
Lehrstuhl Werkstoffwissenschaft
Ruhr-Universität Bochum
4
4780 Bochum (Germany)
In this study, oxygen-functionalized CNTs (OCNTs) and two
[
c] Prof. Dr. K. Kçhler
Department Chemie, Anorganische Chemie
Technische Universität München
Lichtenbergstrasse 4
types of N-doped CNTs (i.e., NCNT-NH and NCNT-G) were em-
3
ployed. The major difference between the CNTs was their sur-
face chemistry, because higher amounts of nitrogen atoms and
8
5747 Garching (Germany)
defects were found on NCNT-G than on either NCNT-NH or
3
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cctc.201501379.
[
11,13]
OCNTs.
Pd NPs were deposited onto the CNTs by a colloidal
[
11,14]
method and were further reduced in H /He flow.
The sup-
2
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Communications
ported Pd NPs had a mean diameter of approximately 3 nm re-
gardless of the support (Table S1, Supporting Information). The
prepared Pd/CNTs were tested in the aerobic oxidation of
EtOH, as shown in Figure 1. In addition to AcOH, small
amounts of acetaldehyde and ethyl acetate were formed in
the liquid phase. An unaccounted gap in the selectivity is pos-
metallic Pd are only detected in Pd/OCNT and are hardly ap-
parent in Pd/NCNT-NH3 and Pd/NCNT-G, which suggests
[
11]
a higher degree of dispersion on the NCNTs. After the reac-
tion (denoted with the suffix -AR), downshifts in the Pd reflec-
tions are clearly observed for Pd/OCNT-AR and Pd/NCNT-NH3-
AR, but not for Pd/NCNT-G-AR owing to very low intensity. The
lattice expansion of Pd is presumably due to the formation of
sibly due to the formation of CO . Among the three catalysts,
2
PdC , as reported for spent Pd/C resulting from reprecipitation
x
[
16]
of dissolved Pd during the reaction. On the basis of trans-
mission electron microscopy (TEM) studies, the average Pd par-
ticle size in Pd/OCNT-AR significantly increased to approxi-
mately 16 nm (Table S1), which indicates sintering of the Pd
NPs after the reaction (Figure S3a,b). In contrast, the Pd spe-
cies in Pd/NCNT-NH -AR and Pd/NCNT-G-AR are too dispersed
3
to be observed, and only a few large particles can be found
(
Figure S3c–f). Elemental analysis was performed to determine
the Pd loadings before and after the reaction and the Pd con-
tents in the filtrates of the reaction mixtures. None of the anal-
yses indicated a significant loss of Pd, which confirmed that
some Pd species in Pd/NCNT-NH -AR and Pd/NCNT-G-AR exist
3
in a highly dispersed state hardly detectable by TEM.
X-ray photoelectron spectroscopy (XPS) was employed to
study the nature of the Pd species on Pd/CNTs before and
after the reaction. The recorded Pd3d spectra are shown in
Figure 2. Three contributions at binding energies (BEs) of
3
35.5, 336.5, and 338 eV can be assigned to metallic Pd, PdO,
[
11]
and Pd(OAc) , respectively. For the freshly reduced Pd/CNTs,
2
Pd is largely in the metallic state on OCNT and NCNT-NH3,
whereas a considerably larger fraction remains as Pd(OAc) on
2
NCNT-G; this indicates that the reduction of Pd(OAc) is signifi-
2
cantly hindered on N-rich NCNT-G. After the reaction, an in-
crease in the intensity of the peak at a binding energy of
3
38 eV attributed to Pd(OAc)2 was observed for all the Pd/
Figure 1. a) Conversion and b) yield of AcOH in the selective oxidation of
EtOH as a function of temperature (EtOH/metal=1500 mol/mol, 10 mL of
CNTs. Pd(OAc) species were clearly generated under the oxi-
2
5
wt% aqueous EtOH, 3.0 MPa, 5 h).
dizing conditions according to Equation (1):
Pd þ 1=2 O þ 2 AcOH ! PdðOAcÞ þ H O
ð1Þ
2
2
2
Pd/NCNT-NH showed the highest degree of EtOH conversion
3
and the highest yield of AcOH over the whole temperature
range (80–1808C). Notably, Pd/NCNT-G achieved lower yields
of AcOH and lower EtOH conversion, especially at high tem-
peratures (160 and 1808C). Although Pd/NCNT-G demonstrated
slightly higher yields of acetaldehyde at T>1008C than the
other two catalysts (Figure S1c), a poorer carbon balance (de-
termined from analysis of the liquid-phase compounds) of ap-
proximately 70% was observed for Pd/NCNT-G than for the
other catalysts with approximately 90%, which indicated pro-
in which AcOH is the product of EtOH oxidation. The observed
increase in the concentration of Pd(OAc) after the reaction is
more pronounced on NCNT-NH than on OCNT, and on NCNT-
G, metallic Pd was almost completely transformed into
Pd(OAc) . The strong interactions between Pd(OAc) and the
surface N species seem to play important roles in both the for-
mation and the stabilization of Pd(OAc) under the reaction
conditions. The XPS results are in good agreement with the
TEM images, which indicates the formation of Pd(OAc) under
2
3
2
2
2
2
nounced production of gaseous products, most likely CO as
the reaction conditions and its redeposition at the termination
of the reaction.
2
a result of total oxidation. Our previous studies showed that
dissociation of O preferentially takes place on surface defects,
Recycling tests were conducted with the three catalysts at
1608C (Figure S4). Both Pd/OCNT and Pd/NCNT-NH3 showed
stable performance, whereas Pd/NCNT-G was deactivated
strongly and displayed very poor performance in the second
run. Although a significant increase in the Pd particle size of
Pd/OCNT-AR was observed by XRD and TEM, the sample did
not lose activity in the second run. Clearly, sintering did not
result in deactivation of Pd/OCNT under the employed reaction
2
[
12,15]
which makes the oxidation of NCNT-G more favorable.
Therefore, the higher amount of dissociated oxygen species on
Pd/NCNT-G could be the reason for the less-selective oxidation
behavior.
The XRD patterns of the Pd/CNT catalysts after H reduction
2
are shown in Figure S2 before and after the catalytic reaction
(
1608C, 5 h). For the fresh samples, characteristic reflections of
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II
Table 1. Conversion and yield of acetic acid for Pd +CNT mixtures in the
[a]
first and the second run.
Catalyst
Run 1
Run 2
Conv. [%] AcOH yield [%] Conv. [%] AcOH yield [%]
II
II
Pd
63
53
77
60
40
33
49
29
46
69
68
29
29
50
47
9
Pd +OCNT
II
Pd +NCNT-NH
3
II
Pd +NCNT-G
[a] Reaction conditions: EtOH/metal=1500 mol/mol, 10 mL of 5 wt%
aqueous EtOH, 3.0 MPa, 1608C, 5 h.
II
catalysts; this showed that Pd alone as a precursor is able to
II
form the catalytically active Pd species. Among the four Pd +
II
CNT mixtures, Pd +NCNT-NH achieved the highest conversion
3
II
as well as the highest selectivity to AcOH, whereas with Pd +
OCNT the lowest activity was observed; this is indicative of
a negative effect of the support, likely as a result of the abun-
dance of acidic oxygen-containing functional groups on OCNT,
[
18]
which are detrimental for alcohol oxidation. The detrimental
effect is less prominent in Pd/OCNT, as the number of exposed
functional groups is reduced owing to the deposition of Pd.
The catalytic results in the second run resemble those of the
corresponding Pd/CNTs (Figure S4), that is, similar performance
II
II
of Pd +OCNT and Pd +NCNT-NH , but strong deactivation of
3
II
Pd +NCNT-G. Similarly, the significant influence of porous
solids on the activity and stability of homogeneous Pd(OAc) /
2
[
19]
pyridine catalysts was studied by Steinhoff et al.,
who
showed that added molecular sieves can provide Brønsted ba-
0
sicity and heterogeneous surfaces to anchor the Pd clusters to
inhibit bulk aggregation.
The XRD patterns of the solid after the reaction (Figure S5)
reveal the presence of metallic Pd in all four cases. It is con-
II
ceivable that Pd is first reduced by EtOH during the reaction
0
to form Pd clusters, which undergo further growth to give
NPs or even bulk Pd. The functional groups on the CNTs can
help to stabilize these Pd particles. Similar to the observations
for the Pd/CNTs (Figure S2), broader Pd reflections are recorded
II
II
for Pd +NCNT-NH -AR and Pd +NCNT-G-AR, which indicates
Figure 2. X-ray photoelectron Pd3d spectra of the Pd/CNT catalysts before
and after the reaction (1608C, 5 h): a) Pd/OCNT, b) Pd/OCNT-AR, c) Pd/NCNT-
3
that NCNTs can efficiently improve the dispersion of the Pd
NH
3
, d) Pd/NCNT-NH
3
-AR, e) Pd/NCNT-G, and f) Pd/NCNT-G-AR.
II
NPs. Upon using Pd without CNTs as the catalyst, a black pre-
cipitate was obtained after the reaction, and the XRD pattern
of this substance disclosed the formation of bulk PdO and Pd,
II
conditions. More importantly, the amount of N in the catalysts
has a crucial influence on activity and stability.
as indicated by sharp Pd reflections for Pd -AR.
To further verify the importance of dissolved Pd species, the
reaction was performed by using different amounts of Pd/
The presence of Pd(OAc) on the catalysts after the reaction
2
II
indicates that Pd dissolves as a Pd complex that is stabilized
NCNT-NH . The average rates of EtOH conversion normalized
3
by AcOH. The leached Pd species can also serve as active sites,
to the Pd amount are shown in Figure 3. The significant de-
crease in the normalized rates with increasing mass of catalyst
suggests that the activity is not proportional to the total
amount of Pd, despite the same Pd particle size, which points
to the critical role of soluble Pd species, as their concentrations
are likely limited by saturation. These results imply that soluble
Pd species generated in situ play a major role in the catalytic
process. The influence of concentration can be understood in
analogy to the homeopathic effect for Pd-catalyzed coupling
as observed in the Heck reaction and other CÀC coupling reac-
[
16,17]
tions.
To elucidate the role of Pd(OAc) in the oxidation of
2
EtOH, Pd(OAc) was dissolved in water and then different CNTs
2
II
were added. The aqueous Pd +CNT mixtures were subse-
quently mixed with EtOH by using the same EtOH and metal
concentrations as those in the tests shown in Figure 1, and the
catalytic results are shown in Table 1. Pd(OAc) without CNTs
2
achieved high conversion comparable to that of the Pd/CNT
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In summary, we identified the “release-and-catch” behavior
of the carbon nanotube (CNT)-supported Pd nanoparticle (Pd/
CNTs) catalysts applied in the liquid-phase oxidation of EtOH.
II
0
The evolution and stabilization of Pd and Pd species during
the reaction strongly depends on their interactions with the
functional groups on the support. Highly active Pd/CNT cata-
lysts for EtOH oxidation can be obtained by introducing an ap-
propriate amount of N-containing groups on the surface of the
CNTs. Our study provides new insight into the understanding
and rational design of Pd-catalyzed alcohol oxidation systems
by bridging the gap between heterogeneous and homogene-
ous catalysis.
3
Figure 3. Pd-based rate of EtOH conversion over Pd/NCNT-NH as a function
of catalyst mass (10 mL of 5 wt% aqueous EtOH, 3.0 MPa, 1608C, 5 h).
Experimental Section
General methods
reactions provided that the dissolution rate of the Pd NPs
OCNTs were prepared by treating purified CNTs (Baytubes C 150 P)
[20]
strongly affects the rate of EtOH conversion.
in HNO vapor at 2008C for 48 h. Subsequent treatment of OCNT
3
UV/Vis studies of the filtrates of the reaction mixtures after
in flowing NH3 at 4008C for 6 h produced NCNT-NH . NCNT-G
3
II
the reaction proved the presence of Pd in solution (Figure S6).
(
Bayer AG) were directly grown from a nitrogen-containing organic
On the basis of all the experimental evidence, a Pd life cycle is
proposed for the oxidation of EtOH (Scheme 1). Pd NPs sup-
ported on CNTs are partially dissolved by the reaction with O2
precursor. As determined by quantitative XPS analysis, NCNT-G had
a surface N content of 4.6 at%, which is much higher than that of
NCNT-NH (0.9 at%). Detailed experimental description of the cata-
3
II
lyst preparation, characterization, and activity test can be found in
the Supporting Information.
and in situ generated AcOH to produce Pd complexes stabi-
II
lized by AcOH and/or surface N species. The reduction of Pd
0
in solution by EtOH leads to the formation of Pd clusters,
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft
(
(
DFG) in the framework of the Cluster of Excellence RESOLV
EXC1069) is gratefully acknowledged. W.D. thanks the Interna-
tional Max Planck Research School for Surface and Interface En-
gineering in Advanced Materials (IMPRS-SurMat) for a research
grant.
Scheme 1. Evolution of Pd species during aerobic EtOH oxidation in the
liquid phase.
Keywords: alcohols · deposition · nanotubes · oxidation ·
palladium
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Received: January 15, 2016
2
Published online on March 1, 2016
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim