Cooperative Catalysis for Selective Alcohol Oxidation with Molecular Oxygen

Article abstract of DOI:10.1002/chem.201602964

The activation of dioxygen for selective oxidation of organic molecules is a major catalytic challenge. Inspired by the activity of nitrogen-doped carbons in electrocatalytic oxygen reduction, we combined such a carbon with metal-oxide catalysts to yield

Full text of DOI:10.1002/chem.201602964

DOI: 10.1002/chem.201602964
Communication
&
Heterogeneous Catalysis
Cooperative Catalysis for Selective Alcohol Oxidation with
Molecular Oxygen
Thierry K. Slot, David Eisenberg,* Dylan van Noordenne, Peter Jungbacker, and
Gadi Rothenberg*[a]
for oxygen activation is reduced. Still, chemical solution carries
Abstract: The activation of dioxygen for selective oxida-
tion of organic molecules is a major catalytic challenge. In-
spired by the activity of nitrogen-doped carbons in elec-
trocatalytic oxygen reduction, we combined such
a carbon with metal-oxide catalysts to yield cooperative
catalysts. These simple materials boost the catalytic oxida-
tion of several alcohols, using molecular oxygen at atmos-
pheric pressure and low temperature (808C). Cobalt and
copper oxide demonstrate the highest activities. The high
activity and selectivity of these catalysts arises from the
cooperative action of their components, as proven by vari-
ous control experiments and spectroscopic techniques.
We propose that the reaction should not be viewed as oc-
curring at an ‘active site’, but rather at an ‘active dough-
nut’–the volume surrounding the base of a carbon-sup-
ported metal-oxide particle.
a hefty price tag: we have produced an extra reagent that is
only necessary because of our inability to control the reactivity
of the oxygen in the air. The third way is using a noble metal
such as Pt or Ir that can activate molecular oxygen, but these
are often too expensive and scarce for large-scale applica-
tions.[7]
However, there are alternatives to platinum-group metals:
Nitrogen-doped carbons are known to activate dioxygen elec-
trochemically, catalysing the oxygen reduction reaction (ORR)
in fuel cells.[8–10] Recently, we reported the synthesis and appli-
cation of such a carbon that has a similar electrochemical ac-
tivity to platinum.[11] Indeed, nitrogen-doped carbons can en-
hance the conversion of oxidations with air, either on their
own or as supports for metal-oxide catalysts, reported by the
groups of Beller, Arai, He, Ma and Wang.[6,12–19] But how does
this happen? Inspired by this work, we reasoned that the OÀO
bond in the dioxygen molecule is activated (i.e., lengthened)
in the interaction with the nitrogen-carbon surface, becoming
more “peroxide-like”.[20] Thus, in theory, placing an adjacent
active site for catalysing this “peroxide”-mediated oxidation
would result in cooperative catalysis, giving both high conver-
sion and high selectivity.[21]
Selective oxidation of organic molecules is the catch-22 of sus-
tainable chemistry.[1] On one hand, the best and most sustaina-
ble oxidant, dioxygen, is abundant and freely available. On the
other hand, the high kinetic barrier for activating oxygen pre-
vents any spontaneous oxidation. Whereas most people would
say this is a good thing (fewer fires on a daily basis) the down-
side of this high barrier is that once dioxygen is activated, reac-
tivity is high and selectivity is low. Due to its free-radical path-
ways, air oxidation all too often leads to large quantities of un-
wanted CO and CO2.[2]
We now report that combining nitrogen-doped carbon with
various transition-metal oxides creates such a cooperative cata-
lyst. These simple materials catalyse the oxidation of cinnamyl
alcohol to cinnamaldehyde using molecular oxygen, with good
conversion and high selectivity. The composite catalyst is pre-
pared by impregnating mesoporous, nitrogen-doped carbon
with metal salts. Importantly, we decouple the metal-doping
step from the carbon growth step. Due to this decoupling, and
by using undoped carbon controls, we can study the metal-ni-
trogen interactions (or lack thereof) and reveal the true nature
of the active sites.
There are three ways around this problem. The first is simply
working at low conversions, keeping the selectivity high. This
engineering solution is practical, yet incurs large recycling
streams and low per-pass yields. The second is using an inter-
mediate oxidant, such as hydrogen peroxide, peracetic acid or
tert-butyl hydroperoxide.[3–6] In this way, some of the activity of
the oxygen is transferred to a chemical carrier, and the barrier
First, we prepared both the nitrogen-doped (N:C) and the
control carbon (Cundoped) with similar microstructures.[11] Briefly,
magnesium nitriloacetate (MgNTA) was precipitated from a so-
lution of magnesium carbonate and nitrilotriacetic acid. The
MgNTA salt was pyrolysed under argon (9008C/1 hr), washed
with dilute citric acid to remove the MgO particles, which
serve as meso-/macropore templates, and annealed under
argon (10008C/1 hr) to yield the final carbon. The resulting ni-
trogen content was determined to be 7.6 at% (XPS, see below;
detailed experimental procedures are included in the Support-
ing Information). We then used vacuum pore impregnation to
distribute metal oxide sites on the N:C support, screening 10
[a] T. K. Slot, Dr. D. Eisenberg, D. van Noordenne, P. Jungbacker,
Prof. Dr. G. Rothenberg
Van ’t Hoff Institute for Molecular Sciences
University of Amsterdam, Science Park 904
1098 XH Amsterdam (The Netherlands)
E-mail: d.eisenberg@uva.nl
g.rothenberg@uva.nl
Homepage: http://hims.uva.nl/hcsc
Supporting information and the ORCID number(s) for the author(s) of this
article are available under http://dx.doi.org/10.1002/chem.201602964.
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transition metals, which are known alcohol oxidation catalysts
in the presence of peroxides (Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, W,
Pb), as well as two alkaline-earth metals (Mg and Sr) for com-
only 2% conversion. Table 1 shows the conversion, selectivity
and yield for the different catalysts. Control experiments con-
firmed that <3% conversion was obtained without any cata-
lyst, and <5% conversion was obtained in the presence of
equivalent amounts of either Cundoped or a commercially avail-
able activated carbon (Ccomm) with a similar surface area
(1500 m2gÀ1). Even when impregnated with Cu or Co, either of
which could be expected to activate oxygen,[25] these carbons
gave much lower conversions compared with N:C.
parison. In each case, the loadings were about 1.3 mmolgÀ1
.
This loading was chosen based on the known nitrogen content
of the support (5.0 mmolgÀ1), giving a nitrogen:metal equiva-
lent ratio of 4:1. Thus, the nitrogens (which are responsible for
oxygen activation[8,9]) would outnumber the metal ions, and
some would remain unoccupied.
For our test reaction, we chose the oxidative dehydrogena-
tion of cinnamyl alcohol (1) to cinnamaldehyde (2, Table 1).
This alcohol substrate can undergo several different oxidations
(epoxidation, allylic alcohol oxidation, ring hydroxylation), as
well as combustion to CO, CO2 and water.[17,22] Moreover, the
aldehyde can react further on nitrogen-doped carbons, giving
esters or acids.[13,23,24] The variety of possible products makes
this a good reaction for testing selective oxidation.
The initial catalyst screening showed that Cu/N:C and Co/
N:C were especially active. Therefore, we focused on these cat-
alysts. First, we measured the intrinsic catalytic activity of
equivalent amounts of copper oxide and cobalt oxide on
Ccomm, as well as on Cundoped (Table 1, entries 18–19 and 21–22).
Cobalt oxide on carbon shows little or no activity, and al-
though copper oxide shows some activity, it is nowhere near
that of the Cu/N:C catalyst (see entries 21 and 5). Kinetic stud-
ies using the initial rates method showed that the reaction in
the presence of the Co/N:C catalyst was three times faster
than that in the presence of the plain N:C support (Figure S1).
To rule out the possibility that carbon is simply not the ideal
support for cobalt oxide, we synthesized an equivalent catalyst
on g-Al2O3.[26] Here, one might expect a much better activity as
the fit between the oxide support and the metal oxide crystals
would enable the formation of smaller (and thus more active)
particles.[26–29] The fact that this material gave <5% conversion
supports our hypothesis that the nitrogen-doped carbon is re-
sponsible for oxygen activation. We also studied the effect of
metal-oxide loading on the catalytic activity. All metal oxides
were prepared at two different loadings, 0.43 mmolgÀ1 and
1.3 mmolgÀ1. The higher loadings (shown in Table 1) gave con-
sistently higher yields by 20–30%. This suggests that the
number of available metal-oxide sites is limiting the reaction in
this loading range.
In a typical reaction, one equivalent of 1 was dissolved in
ethanol and charged to an autoclave under 1 atm of oxygen
(ca. 5 equiv oxygen per substrate) with 20 mg of catalyst
(2.6 mol% of transition metal, based on 1). The mixture was
stirred at 808C for 16 h, and the products were analysed by
gas chromatography (see the Supporting Information for full
experimental details). Control experiments run at 258C gave
Table 1. Catalytic oxidation of cinnamyl alcohol with molecular oxygen.[a]
Entry
Catalyst[b]
Conversion
of 1[c] [%]
Selectivity
of 2[c] [%]
Yield
of 2[d] [%]
1
2
3
none
Ccomm
Cundoped
N:C
2
5
5
49
84
87
94
90
96
93
79
94
97
91
94
95
91
92
93
90
90
91
81
89
92
84
1
4
5
One key question for the performance of every heterogene-
ous catalyst is that of possible leaching. To test for leaching,
we ran a simple filtration experiment (Figure 1) using the “cat-
in-a-cup” concept.[30] We ran the reaction for 5 h, then removed
the catalyst and let the reaction run for another 20 h, and then
re-introduced the catalyst. The stepped reaction profile in
Figure 1 shows that the reaction stops when the catalyst is
taken out. Moreover, the catalyst retains its activity after re-in-
troduction, as shown by the similar reaction rates (0.47 and
0.51 mmhÀ1, respectively).
To understand how this Co/N:C catalyst works, we character-
ized its composition and structure. X-ray photoelectron spec-
troscopy (XPS) showed that Co/N:C contains 7.3 at% nitrogen
and 1.4 at% cobalt (Table 1), both of which give an upper limit
for surface concentrations of 4.06 and 0.88 mmol/m2, respec-
tively. Furthermore, XPS revealed that the binding energies of
the nitrogen atoms (N 1s orbital) are unaffected by the cobalt
impregnation (Figure 2, top). This suggests that there is no sig-
nificant NÀCo coordination, albeit without precluding the
N···Co proximity required for cooperative catalysis. Moreover,
the Co (3/2p) XPS signals are the same for the N:C and the
Cundoped samples (Figure 2, bottom). Thus, the active sites are
not simple metal–nitrogen sites. Earlier studies hypothesized
4
17
56
48
55
80
31
27
22
23
20
19
14
13
16
11
8
15
50
46
51
63
29
26
20
22
19
17
13
12
14
10
6
5[e]
6[e]
7[f]
8[g]
9
Cu/N:C
Co/N:C
Co/N:C
Co/N:C
Ni/N:C
Fe/N:C
Pb/N:C
Mn/N:C
Zn/N:C
Cr/N:C
Mo/N:C
W/N:C
Mg/N:C
Sr/N:C
Co/Cundoped
Co/Ccomm
Co/g-Al2O3
Cu/Cundoped
Cu/Ccomm
10
11
12
13
14
15
16
17
18
19
20
21
22
22
4
4
10
25
3
4
9
21
[a] Reaction conditions: 1 atm O2, 1.0 mmol 1, 20 mg catalyst, 5 mL etha-
nol; stirred in an autoclave, 808C, 16 h. [b] Catalyst notation: [Metal ion]/
[dopant]:[support]. Example: Co/N:C designates a cobalt oxide on nitro-
gen-doped carbon; all metal loadings are 1.3Æ0.1 mmolgÀ1. [c] Based on
GC analysis (n-octane internal standard). [d] Conversionꢁselectivity.
[e] Results are the average of triplicate experiments. [f] 48 h reaction time.
[g] 120 h reaction time.
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Figure 1. Filtration test reaction profile. Reaction conditions: 1 atm O2,
2.0 mmol cinnamyl alcohol and 40 mg catalyst in 10 mL ethanol; stirred in
a 20 mL tube, 808C.
Figure 3. Scanning electron micrographs of (a) Co/N:C, (b) Cu/N:C, (c) a
single CoOx particle on N:C and (d–f) the corresponding EDS maps for ele-
ments N, O and Co, respectively.
the existence of CoÀN bonds.[14] However, the proposed CoÀN
binding peaks overlap with the abundant pyridinic nitrogen
peaks, making such assignments difficult to confirm.[14,31]
a subject of open debate.[8,34] Our carbon contains high pro-
portions of the leading functionalities (38% pyridinic, 46%
graphitic; Figure S2), which should support catalytic activity no
matter the identity of the best functionality.
We then studied the morphology and distribution of the cat-
alyst particles using scanning electron microscopy (SEM). As
shown in Figure 3a–b, the Co/N:C catalyst has a hierarchical
porous surface, on which spherical cobalt-oxide particles are
dispersed. The size of these particles range from relatively
large (100 nm) to 30 nm. Whereas the nitrogen atoms are
evenly distributed (Figure 3d), cobalt oxide is available only at
the surface of the particles. This emphasizes the importance of
the immediate regions around the metal-oxide particles for
combined oxygen reduction and oxidative dehydrogenation
catalysis.[35]
Based on these results, we can formulate a simplified picture
of the catalytic reactions on the surface (Figure 4). First,
oxygen is activated by the nitrogen-doped sites, lengthening
the OÀO bond and creating a “peroxide-like” species. This can
react with the labile allylic hydrogen of the cinnamyl alcohol.
The fact that no other oxidation occurs, nor is there any dehy-
drogenation of the ethanol solvent, shows that this reaction
requires an activated substrate.[36] Oxygen activation may
occur through an electron transfer from the support. This step
is not spatially limited, because the support is conductive and
nitrogen sites are omnipresent (see EDS mapping in Fig-
ure 3d). However, the oxidative dehydrogenation step requires
the physical transfer of two hydrogens from the cinnamyl alco-
hol to form the water molecule. Such an atom transfer process
can only happen in close proximity to the cobalt-oxide site.
Considering the relatively large size of the CoOx particles, we
suggest that the active regions are doughnut-shaped volumes
surrounding the base of the particles (Figure 4). Oxygen mole-
cules that are activated outside these areas are unlikely to par-
ticipate in the subsequent dehydrogenation step. Whereas in
theory the metal oxide can also be the site of O2 activation
with concurrent hydrogen transfer from the carbon, this is less
likely. N-doped carbons are known for their ORR activity,
Figure 2. X-ray photoelectron spectra of N 1s orbital in Co/N:C and pristine
N:C (top), and Co-3/2p orbital on Co/N:C and Co/Cundoped (bottom). The
metal loadings for both Co/N:C and Co/Cundoped are 1.3 mmolgÀ1
.
In contrast, the cobalt impregnation does shift the O 1s XPS
signals (Figure S2), reflecting the formation of cobalt oxide.
Energy-dispersive X-ray spectroscopy (EDS) mapping confirms
that the particles are composed of cobalt and oxygen (Fig-
ure 3c–f, see discussion below). Similarly, temperature-pro-
grammed reduction (Figure S3) showed a major reduction
peak at 3508C and a minor one at 4508C, corresponding to
Co3O4 with some CoO.[32] Finally, X-ray diffraction showed only
broad peaks near 248 and 448, which are typical for carbon
(Figure S4). No additional peaks were observed, indicating that
the CoOx particles are partially amorphous.
The type of nitrogen functionality (graphitic, pyridinic or
other) may well affect the catalytic activity. However, the exact
nature of oxygen-activating active sites in N-doped carbons is
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making them the more likely candidates for the O2 activation
half-reaction in this cooperative catalyst. Thus, we suggest that
the ‘active doughnut’ hypothesis is useful in describing coop-
erative catalysis on supported catalysts.[37–39]
Table 2. Oxidation of different benzylic and allylic alcohols.[a]
Conversion (Selectivity)[c]
Entry Catalyst[b] Substrate
16 h [%]
48 h [%]
a
b
Cu/N:C
Co/N:C
19 (92)
27 (91)
27 (96)
34 (94)
1
2
3
4
a
b
Cu/N:C
Co/N:C
>99 (>99)
>99 (>99)
>99 (94)
>99 (99)
a
b
Cu/N:C
Co/N:C
18 (94)
25 (93)
23 (95)
33 (91)
a
b
Cu/N:C
Co/N:C
8 (83)
8 (38)
15 (74)
38 (65)
a
b
Cu/N:C
Co/N:C
2 (25)
2 (18)
2 (19)
2 (22)
5
6
7
a
b
Cu/N:C
Co/N:C
5 (62)
5 (79)
5 (58)
5 (75)
a
b
Cu/N:C
Co/N:C
1 (83)
<1 (52)
2 (74)
1 (51)
[a] Reaction conditions: 1 atm O2, 1.0 mmol substrate, 20 mg catalyst,
5 mL ethanol; stirred in an autoclave at 808C, 16 h. [b] Catalyst notation:
Figure 4. Cartoon showing the ‘active doughnut’ volume concept in cooper-
ative catalysis: dioxygen activation at a nitrogen-doped carbon surface site
combined with oxidative dehydrogenation at the metal-oxide surface.
[Metal ion]/[dopant]:[support]. Example: Co/N:C designates
a cobalt
oxide on nitrogen-doped carbon; all metal loadings are 1.3Æ
0.1 mmolgÀ1. [c] Selectivity based on GC analysis (n-octane internal stan-
dard); 16 or 48 h reaction time.
We further examined the catalytic scope of the MOx/N-
doped materials with various benzylic and allylic alcohols, run-
ning reactions for both 16 and 48 h (Table 2). For each sub-
strate, control experiments were run without a catalyst to ex-
clude any possibility of autocatalysis. Oxidation of 1-phenyl-1-
propanol gave propiophenone in near-quantitative yields
(entry 2). Primary benzylic alcohols (entries 1,3) gave moderate
yields, yet still much higher when compared with those of pyri-
dinic or furfuryl alcohols (entries 4 and 5). This may reflect the
added stability of the substrate through hydrogen bonding to
the ring heteroatom. Reactions with geraniol and the much
less active cyclohexanol gave little conversion, confirming the
need for an extended p-system.
Acknowledgements
We thank P. Mettraux and Prof. N. Setter (EPFL, Switzerland) for
XPS measurements, and Dr. J. Pandey for help with SEM meas-
urements. This work is part of the Research Priority Area Sus-
tainable Chemistry of the University of Amsterdam (http://sus-
chem.uva.nl).
Keywords: active sites · cooperative effects · heterogeneous
catalysts · oxygen · supported catalysts
Overall, we have demonstrated here that the low selectivity
problem associated with air oxidation of activated alcohols can
be solved by combining an oxygen-activating species and
a simple metal-oxide catalyst. The proximity requirement of
the two is most likely responsible for suppressing free-radical
chain byproducts. By applying a rational screening, we can
study the influence of metal oxides on the nitrogen-doped
carbon without changing the support morphology. We use no
noble metals, and indeed, only minute amounts of first-row
transition metals that are present as stable oxides. The good
conversions and excellent selectivities obtained in the cases of
cobalt and copper show the potential of this approach in in al-
cohol oxidations with oxygen. Considering that this reaction
proceeds well under mild conditions (808C and approximately
1 atm), we foresee opportunities in fine-chemical synthesis, es-
pecially for sensitive multi-functional substrates.
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Received: June 21, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
&
Heterogeneous Catalysis
T. K. Slot, D. Eisenberg,*
D. van Noordenne, P. Jungbacker,
G. Rothenberg*
&& – &&
Cooperative Catalysis for Selective
Alcohol Oxidation with Molecular
Oxygen
Mmm…catalytic doughnuts: The com-
bination of the oxygen reduction prop-
erties of nitrogen-doped carbons with
copper- or cobalt-oxide catalysts creates
cooperative catalytic regions or ’active
doughnut’ sites. These sites enable se-
lective oxidation of alcohols (e.g., cin-
namyl alcohol to cinnamaldehyde)
using dioxygen, with good conversion
and excellent selectivity.
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!