Selective Aerobic Oxidation of Lactate to Pyruvate Catalyzed by Vanadium-Nitrogen-Doped Carbon Nanosheets

Article abstract of DOI:10.1002/cctc.201900819

The catalytic oxidative dehydrogenation of lactates with molecular oxygen is a promising yet challenging route for producing high-value pyruvates from biomass. Here we report a simple synthetic strategy for preparing nitrogen-doped carbon nanosheets (NCNs) starting from two abundant precursors, cheap melamine and glucose, and using a simple thermal-annealing process. The resulting NCNs feature numerous edges and holes for anchoring vanadium oxides (V-NCNs). This creates cooperative catalytic sites that boost the catalytic oxidation of ethyl lactate to ethyl pyruvate. Additionally, we systematically studied the surface nitrogen species of NCNs by varying the pyrolysis temperature, and found that the active pyridinic N-oxide species formed in a high thermal-annealing treatment, acting synergistically with vanadium active sites in converting ethyl lactate with oxygen into ethyl pyruvate under mild conditions.

Full text of DOI:10.1002/cctc.201900819

Accepted Article
Title: Selective aerobic oxidation of lactate to pyruvate catalysed by
vanadium-nitrogen-doped carbon nanosheets
Authors: Wei Zhang, Paula Oulego, Thierry Slot, Gadi Rothenberg, and
N. R. Shiju
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Selective aerobic oxidation of lactate to pyruvate catalysed by
vanadium-nitrogen-doped carbon nanosheets
Wei Zhang, [a] Paula Oulego, [b] Thierry K. Slot, [a] Gadi Rothenberg [a] and N. Raveendran Shiju *[a]
Abstract: The catalytic oxidative dehydrogenation of lactates with
molecular oxygen is a promising yet challenging route for producing
high-value pyruvates from biomass. Here we report a simple synthetic
strategy for preparing nitrogen-doped carbon nanosheets (NCNs)
starting from two abundant precursors, cheap melamine and glucose,
and using a simple thermal-annealing process. The resulting NCNs
feature numerous edges and holes for anchoring vanadium oxides (V-
NCNs). This creates cooperative catalytic sites that boost the catalytic
oxidation of ethyl lactate to ethyl pyruvate. Additionally, we
systematically studied the surface nitrogen species of NCNs by
varying the pyrolysis temperature, and found that the active pyridinic
N-oxide species formed in a high thermal-annealing treatment, acting
synergistically with vanadium active sites in converting ethyl lactate
with oxygen into ethyl pyruvate under mild conditions.
typically requires expensive noble-metal catalysts such as Pd or
Pt.[14-16] For practical purposes, as well as from a sustainability
viewpoint, abundant-element catalysts are preferred. Several
catalysts were reported, including iron phosphates [
17-19]
, single
oxides (ZrO
SnO -MoO
MoVNbO based complex oxides
2
and SnO
2
) [17] and binary oxides (e.g., Ni-Nb-O,
, and MoO -TiO
)[18-21] as well as
2
3
and Fe -MoO
2
O
3
3
3
2
[
22-23]
Vanadium is a suitable option, as its availability is high and its
toxicity is low. Moreover, with a molecular weight of just 50.9, a
low-loading vanadium catalyst has only a small ecological
footprint. Mae et al. found that vanadium oxytrichloride (VOCl
3
)
can catalyze the aerobic oxidation of ethyl lactate to ethyl
3
pyruvate at room temperature, but VOCl is unstable and easily
hydrolyzed by the water byproduct.[24] Vanadium-based catalysts
are also easily recovered, simplifying product purification.[25]
OH
O
Lignocellulosic biomass platform molecules are valid sustainable
feedstocks for many value-added chemicals. They can replace
fossil raw materials, because the volumes of biomass produced
O
O
+
1/2 O2
+
H O
2
(1)
O
O
[
1-
can match the demand for chemicals (this is not true for fuels).
Addressing these problems, we studied a series of supported
vanadium oxides in the conversion of lactates to pyruvates, and
found that titania-supported vanadia outperformed other
2
]
A good example is lactic acid and lactates, which are already
produced on a large scale from biomass fermentation.[ Lactates
can be converted into various commodity chemicals, including
acrylic acid, pyruvic acid and pyruvates, lactide, acetaldehyde
and propanediol.[4-5] In particular, pyruvate esters are important
chemicals used in many sectors, including agrochemicals,
foodstuffs, cosmetics, and pharmaceuticals.[6-7] As pyruvate is a
key metabolite, it can be biologically produced from sugars in
microorganisms.[8-9] However, the main drawback of metabolic
process is its low productivity and costly purification of pyruvate.
3]
supports.[
26]
We also found that adding a small amount of
activated carbon can inhabit the ethyl pyruvate polymerization in
the titania-catalyzed oxidation of ethyl lactate.[27] Recently, we
demonstrated that vanadia supported on activated carbon can act
synergistically with homogeneous pyridine additives in the
aerobic oxidation of ethyl lactate to ethyl pyruvate, giving high
conversion and selectivity.[28] Based on these findings, we
hypothesized that integrating the vanadia sites into nitrogen-
doped carbon (NCs) structures could be efficient heterogeneous
catalysts for the selective oxidation of lactate to pyruvate under
mild conditions.
[
10]
Alternatively, pyruvates can be produced by direct oxidative
dehydrogenation of lactates with molecular oxygen (eq 1). This
route is more sustainable than the currently used pyrolysis of
Nitrogen-doped carbons are readily prepared by pyrolysing
various nitrogen-rich precursors together with carbon. The
nitrogen dopant plays a vital role in the surface activation of
tartaric acid with stoichiometric KHSO
4
as a dehydrating agent.[11-
1
2]
But the direct aerobic oxidation of lactate to pyruvate is a
challenging process. Oxygen is the most abundant and the
cheapest oxidant, but its high activation barrier and its tendency
to form free-radicals means that the pyruvate product is easily
over-oxidized.[13] This over-oxidation can be suppressed, but that
[
29-
dioxygen, boosting the efficiency of the carbon-based catalyst.
3
0]
Moreover, the electron-rich character of nitrogen-doped
carbons facilitates the anchoring of metal/metal oxide active sites,
thereby enhancing catalytic performance.[31] Inspired by this, we
prepared nitrogen-doped carbon nanosheets (NCNs) using
melamine and glucose as precursors via a simple two-step
process. These NCNs are excellent supports for anchoring
vanadium oxides. The resulting V-NCNs catalysts combining
abundant nitrogen dopant and well-dispersed vanadium sites
showed a synergistic effect, improving their catalytic activity in the
selective aerobic oxidation of lactate to pyruvate. Moreover,
owing to the strong interaction between vanadium and nitrogen
dopant, the V-NCNs are stable and can be reused at least five
times without losing any activity.
[a]
W. Zhang, T.K. Slot, Prof. Dr. G. Rothenberg, Dr. N.R. Shiju,
Van ’t Hoff Institute for Molecular Sciences
University of Amsterdam
Science Park 904, 1098 XH Amsterdam (The Netherlands)
E-mail: n.r.shiju@uva.nl; Web: http://hims.uva.nl/hcsc
Dr. P Oulego
[b]
Department of Chemical and Environmental Engineering
University of Oviedo
C/ Julián Clavería, s/n. E-33071, Oviedo (Spain)
Supporting information for this article is given via a link at the end of
the document.
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glucose-derived carbon matrix during the hydrothermal process.
These fragments provide nitrogen atoms doped in the carbon
skeleton, and decompose at 600–700°C, yielding N-doped
nanosheets.[ Vanadium oxides were then supported onto the
NCN supports by wet impregnation of an ethanolic solution of
32]
2
vanadyl acetylacetonate, VO(acac) , followed by drying and
calcining (see the experimental section in the Supporting
Information for details).
Transmission electron microscopy (TEM) of the NCNs shows
ultrathin sheet-like morphology with many wrinkles in large
domains (Figure 1a). High-resolution TEM shows numerous
nanoholes in the sheet structure (Figure 1b). Adding Pluronic ® F-
127 as the soft-template leads to the formation of mesopores. The
polymerization of melamine precursor occurred during the
hydrothermal synthesis, which was confirmed by Fourier-
transform infrared spectroscopy (FTIR). As shown in Figure S1,
the characteristic absorption band of s-heptazine units (~810 cm-
Scheme 1 The synthesis route to nitrogen-doped carbon nanosheet supports
(NCNs) and their combination with vanadium oxide to give V-NCNs.
1)
polymeric carbon nitride.[33] Subsequent pyrolysis removed the
fragments, leading to the formation of nanoholes. This was
was detected for NCN precursor, confirming the in-situ formed
3 4
C N
verified by the disappearance of the heptazine vibration at a high
pyrolysis temperature (up to 800 °C).
The average pore sizes of V-NCNs materials decrease as
follows (see Table S1): V-NCN-900 (48 nm) ≈ NCN-800 (45 nm)
>
NCN-700 (14nm) ≈ NCN-600 (13 nm). This agrees with the
FTIR results and confirms that the enlarged pores are caused
mainly by the decomposition of carbon nitride when the
temperature was increased to 800 °C.
The wrinkles of sheet-like structures consist of 8−9 nitrogen-
doped carbon layers (Figure 1c). The NCNs sample was further
loaded with vanadium and annealed in argon at 400 °C for 4 h,
yielding vanadium-nitrogen co-doped carbon nanosheets (V-
NCNs). The analysis of the morphology of V-NCNs-800 catalyst
by the scanning transmission electron microscopy (STEM)
indicates that the ultrathin nanosheet remains stable when
vanadia was introduced in NCNs support (Figure 1d). The
corresponding energy-dispersive X-ray spectroscopy (EDX)
elemental mapping (Figure 1, e-h) showed that the elements V,
N, C and O were homogeneously distributed over the V-NCNs-
800 nanosheets. The selected area electron diffraction (SAED)
image of V-NCNs-800 shows a set of diffraction rings of graphic
carbon (Figure 1i), implying that amorphous vanadia species are
well dispersed on the NCNs surface.
We then tested the V-NCNs catalysts in the oxidative
dehydrogenation of ethyl lactate to ethyl pyruvate. Previously, we
Figure 1 (a) TEM image of nitrogen-doped carbon nanosheets (NCNs-800) (b)
representative high-resolution TEM images of NCNs-800, with yellow arrows
marking some mesopores (c) magnified HRTEM image showing the lamellar
stacking of the nanosheets (d) STEM image of NCNs-800 supported vanadia
catalyst (V-NCNs-800) (e-h) the corresponding element mapping of V, C, N and
O in V-NCNs-800 catalyst (i) SAED image of V-NCNs-800 (note: V-NCNs-800,
pyrolysis at 800 °C).
showed that adding molecular sieve-3
Å (MS-3A) as a
dehydrating agent can suppress the competing hydrolysis, giving
a much higher selectivity to pyruvate.[ In a typical reaction, ethyl
lactate (5 mmol) and oxygen (1 atm) were reacted in the presence
of 25mg of V-NCNs catalyst (together with 1 ml diethyl succinate
as the solvent and 100 mg MS-3A ) at 130 °C for 9 h. Control
experiments confirmed that the pristine NCNs (without vanadium)
were inactive, giving only 5% conversion of the background
thermal reaction. Loading the NCNs with ~1.0 wt.% vanadium
25]
Scheme 1 outlines the synthesis of vanadia-nitrogen co-doped
carbon nanosheets (V-NCNs). First, we prepared nitrogen-doped
carbon nanosheets (NCNs) by a simply pyrolysing various
nitrogen-rich precursors together with carbon. For this, we
hydrothermally treated a mixture of melamine, glucose and
Pluronic® F-127 at 180 °C for 12 h, followed by a pyrolysis
process at 800 °C. Melamine is an abundant and non-toxic
(
(
determined by ICP analysis) increased the conversion to 99%!
Figure 2). This shows that these novel integrated V-NCNs
catalysts combing abundant nitrogen dopant and well-dispersed
3 4
nitrogen precursor, which can form C N fragments in the
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vanadium sites showed high efficiency as synergistic catalysts for
selective aerobic oxidation of lactate to pyruvate. More
importantly, owing to the strong interaction between vanadium
and nitrogen dopant, the V-NCNs catalysts are stable and can be
reused at least five times without losing any activity, showing
superior durability and recyclability.
Figure 2 Selectivity to ethyl pyruvate plotted against ethyl lactate conversion
over three supported vanadium oxide catalysts: V-CNs (cyan), V-mpg-C N
3 4
(
yellow) and V-NCNs (red). The inset shows the corresponding ethyl pyruvate
2
yield. Reaction conditions: ethyl lactate 5 mmol, catalyst 25 mg, 1 atm O , diethyl
succinate (solvent, 1 ml), molecular sieve-3 Å (dehydrating agent, 100 mg),
130 °C, 9h.
To validate the nitrogen surface effect, we prepared pure
carbon nanosheets via the same method yet without the nitrogen
precursor (denoted as CNs, see Figure S2). The nitrogen-free
CNs supported vanadia (V-CNs) gave only 60% conversion under
otherwise identical conditions. This shows that the surface
nitrogen species play a vital role in the catalytic oxidation of ethyl
lactate. To prove this, we prepared separate samples of vanadium
oxide supported on mesoporous graphic carbon nitride (V-mpg-
Figure 3 (a) Comparisons of the aerobic oxidation of ethyl lactate to ethyl
pyruvate over the various V-NCNs-x catalyst. Reaction conditions: ethyl lactate
5
2
mmol, catalyst 25 mg, 1 atm O , diethyl succinate (solvent, 1 ml), molecular
sieve-3 Å (dehydrating agent, 100 mg),130 °C and 9 h. (b) The time-resolved
conversion profile of ethyl lactate over the V-NCNs-x catalysts.
3 4
C N ). The surface vanadium content of these samples was
identical to that of the V-NCNs (detailed experimental procedures
are included in the Supporting Information). As shown in Figure 2,
The pyrolysis temperature can impact the nitrogen species in
NCNs supports, affecting the catalytic activity.[35] We studied this
influence by varying the pyrolysis temperature of the NCNs. The
resulting supports are denoted as NCNs-x, where x represents
the pyrolysis temperature (600–900 °C, respectively). Using these
supports, we prepared the corresponding vanadium oxide
catalysts (V-NCNs-x) under identical conditions. All four V-NCNs-
x samples were highly active, giving ethyl pyruvate with high
selectivity (Figure 3a). The activity order was V-NCNs-800 > V-
NCNs-900 > V-NCN-700 > V-NCN-600, showing the importance
of the pyrolysis temperatures. Plotting the reaction profiles for
these catalysts confirmed the trend (Figure 3b), showing that a
pyrolysis temperature of 800 °C was optimal for converting lactate
to pyruvate.
3 4
V-mpg-C N gave a quantitative conversion of ethyl lactate but
with more side-products, affording only 75% of ethyl pyruvate
compared to the 85% obtained with V-NCNs. Note that vanadia
supported on pure carbon nanosheets showed the highest
selectivity to ethyl pyruvate of 91 %, confirming our earlier
observations that the presence of carbon favors the production of
ethyl pyruvate.[27-28] We therefore conclude that combining
vanadia active sites with nitrogen-rich carbon support creates
cooperative catalysts,[ boosting the catalytic oxidation of ethyl
lactate to ethyl pyruvate.
34]
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assigned to the disordered/defected carbon (D band) and
2
[41-42]
graphitic sp carbon (G band), respectively.
the peak intensity ratio of I /I was determined to be 0.81
suggesting a graphitic structure, in agreement with the XRD
results (Figure S3).[32] By comparison, the I
/I was remarkably
For V-NCNs-600,
D
G
D
G
increased to 0.95, 0.95 and 0.97 for V-NCNs-700, V-NCN-800
and V-NCNs-900, respectively, indicating an increase in the
defect and disorder degree with pyrolysis temperature.[ Finally,
we examined the bulk structure using powder X-ray diffraction
43]
(
PXRD, Figure S3). In all four cases, no crystalline vanadia peaks
are detected, which is in good agreement with the SAED pattern
(
Figure 1i).
Based on the above results, we reasoned that higher pyrolysis
temperatures (800–900 °C) lead to an increase in the
graphitization degree, forming nitrogen-doped carbon nanosheets
with abundant edges and holes. However, various nitrogen types
were formed in NCs including pyridinic N, pyrrolic N, quaternary
N and pyridine-N-oxide, and the role of each nitrogen remains
unclear.[44] Previously, Wang[45] and Cao[46] reported that the
pyridinic nitrogen was responsible for the enhanced efficiency of
Pd/NCs catalysts. Beller et al. identified the graphitic nitrogen as
Figure 4 (a) XPS spectra of N 1s in the V-NCNs-x catalysts (x= 600, 700, 800
and 900) (b) types of nitrogen functionalities occurring in V-NCNs-x samples
and (c) nitrogen contents of NCNs-x with different pyrolysis temperatures (N1:
2
sp hybridized-N, red; N2: pyrrolic-N, green; N3: amino-N, yellow; N4: graphitic-
N, pink; N5: pyridine oxide-N, blue). The nitrogen species were quantified based
on XPS data of the materials treated at different temperatures. (d) high-
resolution C 1s spectra of V-NCNs-x. (e) Raman spectra of V-NCNs-x samples.
the active promoter in the hydrogenation of arenes over Ru/CNs
catalysts. [47] In our case, there is no clear correlation between the
catalytic activity and the amount of pyridinic or graphitic nitrogen.
Notably, we found that more nitrogen species in NCNs were
oxidized to pyridinic-N-O starting from 800 °C (see Figure 4). We
hypothesize that the new oxidized N active sites could stabilize
To understand this effect further, we ran ex-situ X-ray
photoelectron spectroscopy (XPS) measurements to probe the
surface chemical composition and valence states in NCNs-x
samples varying at different pyrolysis temperatures ranging from
the vanadium sites and tune their electronic properties, thereby
favoring the catalytic efficiency.[48]
600–900 °C. As shown in Figure 4a, the N1s signals of NCNs-x
samples can be fitted with several peaks, reflecting the existence
of six different nitrogen forms in the materials (Figure 4, b-c):
2
pyridinic-N or sp -hybridized-N in triazine rings (N1, ~398.7 eV,
red), pyrrolic-N (N2, ~399.8 eV, green), amino-N (N3, ~400.6 eV,
yellow), quaternary-N or graphitic-N (N4, ~401.3 eV, pink) and
pyridine oxide-N (N5, ~403.5 eV, blue), respectively.[36-38]
We found that the pyrolysis temperatures affect the nitrogen
species in NCNs surface. For the NCNs-600, two deconvoluted
2
peaks belong to sp hybridized N and bridging amino N in tri-s-
triazine periodic networks (N1 and N3), respectively.[39-40] As the
pyrolysis temperature increased, the triazine structure started to
decompose and converted into other nitrogen species. The
characteristic peaks of amino groups (N3) disappeared at 700 °C,
accompanying two new N signals of pyrrolic-N (N2) and graphitic-
N (N4). When the temperature increased to 800 °C, nitrogen
dopant will be gradually converted to pyridine-N oxide (N5). The
thermal transformation was also reflected by C1s spectra (Figure
4d). The spectrum of C1s in NCNs-600 can be deconvoluted into
two components including adventitious C (~284.6 eV), C─N
2
(
~286.6 eV) and sp -bonded C (N═C─N, ~287.8). Further
increased the pyrolysis temperature, new peaks located at 289.1
eV and 291.16 eV can be attributed to C─O and C═O species,
respectively. Again, note that the XPS data is not in-situ, therefore
the correlation can serve only orientative purposes.
Figure 5 (a) Comparisons of TOF of ethyl lactate oxidation and area-specific
conversion rate of ethyl lactate over the V-NCNs-x catalysts. (b) High-resolution
O 1s spectra of V-NCNs-x samples. (c) Schematic diagram of pyridine N-oxide
in the V-NCNs-900 catalyst.
We ran Raman spectroscopy to detect the degree of
graphitization in V-NCNs-x samples. As shown in Figure 4e, all
-1
the samples showed two bands centered at 1338 and 1592 cm ,
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To check this hypothesis, we studied the catalytic
performance of V-NCNs-x catalysts by comparing the turnover
frequency (TOF) at low conversions (<15%). As shown in Figure
vanadium loading (from 1.2 wt. % to 1.0 wt.%, measured by ICP-
AES analysis, see Table S1).
In summary, we have prepared nitrogen-doped carbon
nanosheets (NCNs) from melamine and glucose precursors via a
simple thermal-annealing process. These supports have
numerous edge and vacancy sites and are excellent for anchoring
vanadium oxides, boosting the catalytic efficiency for the aerobic
oxidation of ethyl lactate to ethyl pyruvate. The surface nitrogen
dopant plays a vital role in this reaction, stabilizing the vanadium
sites and tuning the catalyst electronic properties. Specifically, the
active pyridinic N-oxide species formed in a high thermal-
annealing treatment acts synergistically with vanadium active
sites in converting ethyl lactate with oxygen into ethyl pyruvate
under mild conditions.
5a, the TOFs of the oxidation reaction on V-NCNs-x catalysts are
proportional to the pyrolysis temperature of the NCN supports.
Moreover, despite the higher nitrogen content and specific
surface area in V-NCNs-600 and V-NCN-700, their TOFs are
lower compared to V-NCNs-800 and V-NCN-900. We also plotted
the area-specific conversion rate of ethyl lactate (rEL, the right axis
in Figure 5a) against the pyrolysis temperature. Clearly, with a
temperature of below 700 °C, the rEL almost leveled up. Increasing
-1
-2
the temperature to 800 °C gave rEL of 4.5 mmol h m , which is
-1
-2
thrice than that of V-NCNs-700 (rEL=1.72 mmol h m ). Annealing
00 °C (V-NCNs-900) increased the area-specific conversion rate
9
-1
-2
of ethyl lactate of 5.0 mmol h m . This may reflect the oxidized-
N content increase from 8% to 11% (see Figure 4c).
As shown in Figure 5b, the spectra of O 1s in V-NCNs-x can be
fitted into three peaks: the peaks at 530.5 eV and 531.7 eV
Acknowledgments
I
represent the vanadia lattice oxygen (O ) and the chemisorbed
W. Zhang thanks the China Scholarship Council for a Ph.D.
fellowship (201506140058). P. Oulego thanks the Spanish
Ministry of Economy and Competitiveness (MINECO) (Project
CTM2015-63864-R) and European Union (FEDER) for funding.
This work is part of the Research Priority Area Sustainable
Chemistry of the University of Amsterdam, http://suschem.uva.nl.
oxygen(OII), respectively;[ The peak at 533.7 eV was attributed
to the pyridinic N-oxygen or carbonyl oxygen (OIII), in which the
pyridinic N-oxide species dominate at high temperature
26]
(
>800 °C).[48-50] In our case, the atom ratio of OIII/ (O
these series increased as follows: V-NCNs-900 (0.33) > V-NCNs-
00 (0.30) > V-NCNs-700 (0.19) > V-NCNs-600 (0.11), in good
I
+OII+OIII) in
8
agreement with the catalytic efficiency in Figure 5a. This indicates
the oxidized N plays a key role. Furthermore, the surface variation
of nitrogen dopant also affects the electron charge density of
vanadium sites. The V 2p3/2 peak can be deconvoluted into three
distinct peaks at 516.2 eV, 517.3 eV and 518.2 eV, corresponding
Conflict of interest
The authors declare no conflict of interest.
to V , V and V species, respectively (Figure S4). The V5+/
3
+
4+
5+
3
+
4+
5+
(
V + V + V ) ratio gradually decrease from 0.41 to 0.23 when
Keywords: heterogeneous catalysis • biomass conversion •
platform molecules • catalytic oxidation • synergistic effect •
XPS
the temperature is increased from 600 to 900 °C, implying more
surface V species are reduced by the nitrogen dopant. The
increased redox V active sites are beneficial for the catalytic
oxidation reaction.[ We conclude that the catalytic performance
is determined by the structure of surface nitrogen species, and
particularly by the pyridinic N-oxide species located at the defects
are more active for the oxidation, acting synergistically with
vanadium active sites (Figure 5c).
25]
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[
[
[
[
[
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ChemCatChem
10.1002/cctc.201900819
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Wei Zhang, Paula Oulego, Thierry K.
Slot, Gadi Rothenberg and N.
Raveendran Shiju*
Page No. – Page No.
Selective aerobic oxidation of lactate
to pyruvate catalysed by vanadium-
nitrogen-doped carbon nanosheets
Spot on! Combining vanadium oxide and nitrogen-doped carbons results in
cooperative catalysis for lactate oxidation.
This article is protected by copyright. All rights reserved.