Biomimetic oxygen activation by MoS2/Ta3N5 nanocomposites for selective aerobic oxidation

Article abstract of DOI:10.1002/anie.201206542

Learning from nature: The development of an MoS₂/Ta ₃N₅ nanocomposite as a catalyst for selective aerobic oxidation by O₂ activation was inspired by the nitrogenase enzymes in nature. The superior performance of this biomimetic catalyst, which shows potential for the selective oxidation of multifunctional substrates (see picture), results from the integration of Ta₃N₅ and MoS₂ at the nanoscale and the synergistic enhancement of their activity. Copyright

Full text of DOI:10.1002/anie.201206542

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Angewandte
Communications
DOI: 10.1002/anie.201206542
Functional Nanocomposites
Biomimetic Oxygen Activation by MoS /Ta N Nanocomposites for
2
3
5
Selective Aerobic Oxidation**
Qingsheng Gao,* Cristina Giordano, and Markus Antonietti
The selective oxidation of petroleum-based feedstocks to
useful functionalized chemicals is an important family of
Remarkably, we discovered tunable oxidation ability associ-
[
10]
ated with tailored nitridation, namely, improved activity
and tunable selectivity for alkene epoxidation on TaON and
Ta N nanoparticles (NPs) with H O . This discovery opens up
[
1]
chemical transformations. Of these transformations, the
selective oxidation of alcohols, alkenes, amines, and sulfides
are among the most challenging reactions in green chemis-
3
5
2
2
opportunities to develop superior tantalum-based catalysts
with well-defined properties, especially for reactions involv-
[2]
try. There is significant interest in the design of new, cost-
effective, and environmentally friendly heterogeneous cata-
ing cheap O as the oxidant. Access to such catalysts is needed
2
lysts that use molecular oxygen (O ) under mild conditions, to
to enable the important factors for catalytic turnover and
selectivity to be uncovered. However, the absence of O2
activation in such (oxy)nitrides synthesized so far seriously
limits further exploration.
2
avoid the use of a large excess of toxic and expensive
[
1,3]
stoichiometric metal oxidants.
catalysts based on novel metals and transition-metal oxides
Although a number of
[4]
have been introduced, the precise design of catalysts with
well-defined behaviors that depend on surface properties and
electron features is still desired. Such catalysts are significant
not only for use with multifunctional substrates, but also for
insightful studies of catalytic mechanisms. These challenges
are expected to be met through facet engineering and
component control at the catalyst surface and in the active
Biomimetic studies point to a new way to develop
catalysts by learning from nature. In nature, the active
center of nitrogenase enzymes contains metal atoms usually
[
11]
bound to sulfur, such as active MoÀS and FeÀS clusters. In
nitrogen fixation, MoÀS and FeÀS sites activate inert N to
2
+
[11,12]
react with H , with the generation of NH and H .
This
3
2
process inspired the use of MoS for electro- and photo-
x
[
5]
sites on the level of nanochemistry.
electrocatalytic H evolution based on electron transfer from
2
+
[12,13]
0
+
Crystal-facet engineering has been successfully intro-
duced to exploit novel metal nanocatalysts with high-sur-
face-energy planes. This approach has led to high activity and
MoS to H .
The close energy potentials of E (H /H ) =
2
2
0
0 V and E (O /CO ) = À0.16 V versus the normal hydrogen
2
2
[
14]
electrode suggest that MoS could be used as a biomimetic
x
[
5b,e,6]
selectivity in oxidation catalysis.
However, it is difficult
O -activation reagent to exploit bifunctional tantalum-based
2
to control facet growth in metal-oxide catalysts with low-
symmetry crystal structures owing to the complexity of their
nanocatalysts for aerobic oxidation reactions.
Herein, we describe the development of a new MoS2/
Ta N5 catalyst in which Ta N5 NPs are integrated with
[
7]
structures. On the other hand, the ability to effectively vary
the surface properties and electronic features of metal oxides
by doping with other elements of different electronegativity,
such as N, P, and S, enables new strategies for catalyst
3
3
ultrathin MoS layers on the nanoscale by a hydrothermal
2
method. The MoS2 nanolayers act as a biomimetic O2-
activation reagent in the MoS /Ta N NPs, which showed
2
3
5
[
8]
design. For example, the introduction of N into metal oxides
can increase the energy of the HOMO orbital and narrow the
high activity and selectivity in the aerobic oxidation of
alcohols as a result of the synergistic effect between MoS and
2
[9]
band gap to thus enhance the catalytic activity, although
controlled nitridation is difficult by current synthetic strat-
Ta N . The MoS /Ta N NPs were also active in the aerobic
3
5
2
3
5
oxidation of alkenes, amines, and sulfides. The different
activities observed for these different substrates imply the
potential use of this catalyst with multifunctional substrates.
For example, high selectivity for hydroxy-group oxidation
2
+
egies. Recently, we proposed Ca - and SiO -assisted urea
2
methods for the controlled nitridation of transition metals.
(
> 90%) was observed in the oxidation of unsaturated
[
*] Dr. Q. S. Gao, Dr. C. Giordano, Prof. Dr. M. Antonietti
Department of Colloid Chemistry, Max Planck Institute of Colloids
and Interfaces, Research Campus Golm
alcohols.
Well-defined Ta N5 NPs of approximately 20 nm in
3
1
4424 Potsdam (Germany)
diameter were prepared by our previously reported SiO2-
E-mail: qingsheng.gao@mpikg.mpg.de
assisted urea method (see Figure S1 in the Supporting
[
10a]
Dr. Q. S. Gao
Department of Chemistry, Jinan University
Information).
Hydrothermal treatment of the Ta N NPs
3 5
with varying amounts of ammonium heptamolybdate (AHM)
and thiourea at 1808C for 20 h (see the Supporting Informa-
tion) gave a series of MoS /Ta N nanocomposites that varied
5
10632 Guangzhou (P.R. China)
E-mail: tqsgao@jnu.edu.cn
**] We acknowledge financial support from the BMBF (Project No.
35F0353A-E), the Max Planck Society, and the NSFC (21203075).
2
3
5
[
in their MoS content. The color of the composites changed
2
0
from red to black as the MoS content increased (Figure 1a;
Q.S.G. thanks Dr. X. Liu and K. Otte of MPIKG for SEM, and Prof.
C. Y. Liu and M. Meng of Jinan University for fruitful discussions.
2
see also Figure S2 in the Supporting Information). Inductively
coupled plasma analysis and CHNS elemental analysis were
used to determine the Mo and S content, respectively. The
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206542.
1
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Chemie
The preservation of the Ta N phase in the as-obtained
3
5
MoS /Ta N nanocomposites is well confirmed by their X-ray
2
3
5
diffraction patterns (see Figure S3 in the Supporting Infor-
mation). Scanning electron microscopy (SEM) also confirmed
the retention of the shape and size of the NPs in the products.
Thus, both SEM and TEM images of MoS /Ta N -5.0
2
3
5
displayed well-defined NPs with a size of about 20 nm
Figure 1c,d). Furthermore, the Ta N NPs coated with ultra-
(
3
5
thin MoS layers were revealed well by TEM (Figure 1e), in
2
which the characteristic lattice fringes of MoS and Ta N
5
2
3
were detected. The combination of these materials on the
nanoscale was expected to have a great effect on their
catalytic behavior.
The aerobic oxidation of benzyl alcohol (BA) was carried
out as the test reaction for our MoS /Ta N nanocatalysts
2
3
5
(
Table 1). Bare Ta N NPs did not promote the conversion of
3 5
[
a]
Table 1: Study of the reaction conditions.
Entry
1
Catalyst
Ta N
Atm.
Conv. [%]
Sel. [%]
O2
Air
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
Air
N2
O2
–
44
17
–
–
99
90
–
3
5
5
[
b]
2
3
4
5
6
7
8
9
0
1
2
3
4
5
Ta N
3
MoS2
[
c]
MoS +Ta N
5
2
3
MoS /Ta N -1.3
9
99
99
99
99
98
99
99
99
99
–
2
3
5
MoS /Ta N -1.6
32
90
98
93
64
62
56
10
–
2
3
5
MoS /Ta N -4.2
2
3
5
MoS /Ta N -5.0
2
3
5
MoS /Ta N -7.4
2
3
5
1
1
1
1
1
1
MoS /Ta N -9.7
2 3 5
MoS /Ta N -13.5
2
3
5
MoS /Ta N -20.7
2
3
5
[
[
d]
e]
MoS /Ta N -5.0
2
3
5
MoS /Ta N -5.0
2
3
5
Ta N
–
–
3
5
[a] Reaction conditions: benzyl alcohol (2 mmol), catalyst (40 mg), N,N-
dimethylacetamide (4 mL), O balloon (1 atm), 1208C, 3 h. [b] H O was
2
2
2
used as the oxidant. [c] A physical mixture of MoS and Ta N was used
2
3
5
(
MoS : 5.0 wt%). [d] Reaction time: 9 h. [e] Ta N was treated with
2
3
5
thiourea in a hydrothermal process, 1808C, 20 h.
BA, although they were active when H O was used as the
2
2
Figure 1. a) Photographs of as-obtained MoS /Ta N NPs dispersed in
2
3
5
oxidant (Table 1, entries 1 and 2); thus, O activation does not
EtOH. b) MoS loading content of the MoS /Ta N NPs with respect to
2
2
2
3
5
the AHM/Ta N feeding ratio. c) SEM, d) TEM, and e) high-resolution
appear to occur on Ta N . Bare MoS only exhibited low
3
5
2
3
5
TEM images of MoS /Ta N -5.0 NPs with 5.0 wt% MoS .
activity for the aerobic oxidation of BA, and no BA
conversion was observed with a physical mixture of MoS2
and Ta N (Table 1, entries 3 and 4). Surprisingly, the MoS /
2
3
5
2
obtained Mo/S molar ratio is close to the theoretical value
0.5), which confirms the MoS stoichiometry. The correlation
3
5
2
(
Ta N nanocomposites displayed significantly improved activ-
3 5
2
between the MoS loading and the AHM/Ta N feeding ratio
ity with 99% selectivity for the formation of benzaldehyde
under the same reaction conditions (Table 1, entries 5–12).
The catalytic activity can be optimized by adjusting the MoS2
content, whereby complete BA conversion was observed for
2
3
5
(
Figure 1b) suggests the potential of our method for the
design of various MoS /Ta N composites of defined compo-
2
3
5
sition. The small slope difference in the ranges of low and high
[
10a]
feeding ratios is due to the surface basicity of Ta N ,
since
nanocomposites with an MoS content ranging from 5.0 to
3
5
2
the basic surface will partly dissolve MoS . Herein, the series
6.0% (see Figure S4 in the Supporting Information).
We used the MoS /Ta N -5.0 NPs to further explore the
2
of MoS /Ta N nanoparticles with different compositions are
2
3
5
2
3
5
denoted as MoS /Ta N -n, in which n refers to the MoS
content (n%).
catalytic behavior of our catalysts. The dramatic decrease in
catalytic activity when the reaction was carried out in air and
2
3
5
2
the complete loss of activity in a N atmosphere (Table 1,
2
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entries 13 and 14) indicated that the oxidation of BA relies on
catalytic processes involving O and the MoS /Ta N NPs. A
2
2
3
5
possible effect on the catalyst of hydrothermal treatment with
thiourea could be disregarded following the observation that
no BA conversion occurred on Ta N NPs that had been
3
5
treated with thiourea (Table 1, entry 15). Thus, we could
confirm the biomimetic activation of O by the MoS nano-
2
2
layers. Although a slight deactivation was observed after
three cycles, probably as a result of the partial dissolution of
MoS by peroxide species formed in situ, the activity of MoS /
2
2
Ta N5 was recovered well by repeating the hydrothermal
3
process (see Figure S5 in the Supporting Information).
In a further set of experiments, we examined the efficacy
of the transformation of alcohols into aldehydes under the
optimized reaction conditions (Table 2). MoS /Ta N -5.0
2
3
5
[
a]
Table 2: Selective oxidation of alcohols.
Entry
Substrate
Selectivity [%]
Conv.
%]
T
[h]
[
1
2
3
4
5
6
99
99
99
99
96
99
85
88
99
99
74
36
1.5
1.5
1.5
1.5
1.5
1.5
7
8
99
99
86
80
3.0
4.0
9
99
80
2.0
[
a] Reaction conditions: alcohol (1 mmol), MoS /Ta N -5.0 (40 mg),
2 3 5
N,N-dimethylacetamide (4 mL), O balloon (1 atm), 1208C.
2
Figure 2. a) Effect of the Mo species and b) effect of the nitridation
level on the catalytic performance (conversion, selectivity) of tantalum-
based NPs in the aerobic oxidation of BA (the amount of the Mo
additive in the nanocomposites was fixed at 5.0 wt%). Reaction
conditions: BA (2 mmol), catalyst (40 mg), N,N-dimethylacetamide
showed high versatility. The catalyst was highly active and
extremely selective for the aerobic oxidation of all substrates
tested. Substituted benzyl alcohols containing electron-donat-
ing groups (such as -CH or -OCH ; Table 2, entries 3 and 4)
3
3
(4 mL), O balloon (1 atm), 1208C, 3 h. c) Schematic illustration of the
2
were more readily oxidized than those containing electron-
catalytic mechanism of the MoS /Ta N nanocatalyst.
2
3
5
withdrawing groups (such as -Cl and -NO ; Table 2, entries 5–
2
7
). In the latter case, the reaction is even slower than that of
unsubstituted benzyl alcohol. Our MoS /Ta N -5.0 nanocata-
of 5.0%, MoS /Ta N displayed the highest activity of the
2
3
5
2
3
5
lyst also exhibited high activity and selectivity for the aerobic
oxidation of aliphatic and alicyclic alcohols (Table 2, entries 8
and 9).
three Ta N5 catalysts formed by modification with MoS2,
3
MoO , and MoO (Figure 2a), which implies that MoS has
2
3
2
the best O -activation ability. On the other hand, the
2
Under the same conditions, MoS /Ta N -5.0 did not
nitridation level of the tantalum-based NPs also affected
2
3
5
catalyze BA oxidation in the presence of butylated hydrox-
ytoluene, a terminating reagent that suppresses autoxidation
their catalytic activity according to the order Ta N > TaON >
3
5
Ta O (Figure 2b). We suggest that the enhanced oxidation
2
5
(
a free-radical chain process). This result strongly suggests
ability of tantalum-based nanocatalysts results from the
tailored introduction of N atoms, the electronegativity of
which is lower than that of O atoms.
that the reaction in our catalytic system involves the
formation of the superoxide radical anion (CO ). Further-
À
[15]
[10a]
2
more, we found that the activity of our catalyst depends on the
The combination of MoS and Ta N on the nanoscale is
2
3
5
Mo additive used. For example, with the same loading content
the key to the superior catalytic behavior of MoS /Ta N ;
2
3
5
1
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Angewandte
Chemie
therefore, the interaction of these components at the nano-
interface should be taken in account in the proposed catalytic
Table 3: Selective oxidation of alkenes, amines, sulfides, and unsatu-
rated alcohols.
[
a]
mechanism (Figure 2c). During O activation, the donation of
Entry Substrates
Selectivity [%]
Conv.
[%]
T
[h]
2
an electron from MoS2 to molecular O2 is promoted by
electron transfer from tantalum-based NPs (see Figure S6 in
the Supporting Information). This process is similar to that
1
2
94 49
6.0
6.0
[
16]
observed at Schottky junctions.
The increased electron
density around Ta and increased HOMO energy upon the
introduction of nitrogen atoms facilitates electron trans-
48 75
93 50
[
9,10]
fer,
and the activity of the catalyst depends on the
3
6.0
nitridation level. Thus, the HOMO orbitals that lose electrons
are considered the oxidation sites on Ta N for alcohol
3
5
À
[14]
oxidization with activated CO2
.
In this mechanism, the
4
5
6
99 41
99 52
99 50
3.0
3.0
3.0
combination of MoS and Ta N on the nanoscale is indis-
2
3
5
pensable for the reaction, as confirmed by the lack of BA
conversion detected in the presence of a physical mixture of
these catalyst components (Table 1, entry 4). Thus, MoS2/
Ta N nanocatalysts integrate the features of both MoS and
3
5
2
tantalum-based NPs; O2 activation on MoS2 and alcohol
oxidation by the tantalum sites are synergistically enhanced
through interaction on the nanoscale.
7
8
99 50
99 43
86 33
3.0
3.0
3.0
Our MoS /Ta N NPs could be considered in a sense as
2
3
5
a universal catalyst for the aerobic oxidation of various
functional groups, including alcohols, alkenes, amines, and
sulfides. The MoS /Ta N -5.0 nanocatalysts were found to be
2
3
5
9
active for the epoxidation of cyclooctene, styrene, and cis-
stilbene (according to structure shown) with O as the oxidant
2
[b]
1
1
0
99 10
99 11
99 78
93 99
3.0
3.0
3.0
1.5
(
Table 3, entries 1–3). For the example of cyclooctene epox-
idation, the performance of the catalyst is associated with the
[c]
1
MoS loading, the Mo species, and the nitridation level of the
2
tantalum-based NPs (see Figure S7 in the Supporting Infor-
mation). These observations are consistent with the results for
BA oxidation and further support the proposed catalytic
12
1
3
mechanism. In the case of amine and sulfide substrates, MoS2/
14
99 99
99 66
3.0
3.0
Ta N shows lower activity than for BA oxidation under the
3
5
15
same conditions (Table 3, entries 4–9). As commonly
accepted, amines and sulfides can be oxidized more readily
than alcohols; however, they unfortunately have a poisoning
16
98 99
3.0
[
17]
effect on many catalysts. In the presence of thioanisole or
phenethylamine, the conversion of BA was dramatically
decreased (Table 3, entries 10 and 11). This observation
confirms the ready poisoning of MoS /Ta N by -SH and
[a] Reaction conditions: substrate (1 mmol), MoS
N,N-dimethylacetamide (4 mL), O balloon (1 atm), 1208C. [b] The
reaction was carried out in the presence of thioanisole (1 mmol). [c] The
reaction was carried out in the presence of phenethylamine (1 mmol).
/Ta N -5.0 (40 mg),
3 5
2
2
2
3
5
-
NH groups, probably as a result of their strong combination
2
[
18]
with Mo exposed on MoS edges.
2
The better activity of MoS /Ta N NPs for alcohol
2
3
5
oxidation relative to that for alkene oxidation suggests the
potential of this catalyst for the selective oxidation of
unsaturated alcohols. Remarkably high selectivity (> 90%)
was observed for hydroxy-group oxidation on MoS /Ta N -
be considered universally active for the oxidation of alkenes,
amines, and sulfides. The different performance observed for
the oxidation of various functional groups makes MoS /Ta N
5
2
3
a highly selective catalyst for use with multifunctional
substrates, such as unsaturated alcohols. Owing to the facile
synthesis and tunable properties of the MoS /Ta N NPs, we
2
3
5
5
.0, as well as high activity (Table 3, entries 12–16). This well-
defined catalytic behavior of MoS /Ta N NPs will promote
2
3
5
2
3
5
their use for the oxidation of multifunctional substrates. They
will also be useful in catalyst design and mechanistic studies.
In summary, MoS was used as a biomimetic O -activation
believe that our efforts will pave the way for the design of
a range of efficient metal-based nanocatalysts.
2
2
reagent to construct a new catalyst with high activity and
selectivity in the aerobic oxidation of alcohols. The superior
catalytic behavior results from the combination of Ta N and
Received: August 14, 2012
Published online: October 12, 2012
3
5
MoS on the nanoscale and the synergistic enhancement of
the effects of these two components. The resulting catalyst can
Keywords: molybdenum sulfide · nanocomposites ·
O–O activation · selective oxidation · tantalum nitrides
2
.
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.
Angewandte
Communications
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