Na-doped OMS-2-catalzyed highly selective aerobic oxidation of ethyl lactate to ethyl pyruvate under mild conditions

Article abstract of DOI:10.1016/j.apcata.2020.117813

The direct oxidation of lactate to pyruvate is an important process for catalytic conversion of biomass, although low selectivity and harsh reaction conditions limit its potential for industrial application. In this study, catalytic oxidation of ethyl lactate (EL) to ethyl pyruvate (EP) over OMS-2-based materials was conducted using O₂ as the green oxidant. A 78 % EL conversion and 95 % EP selectivity were obtained in CH₃NO₂ at 50 °C over a Na-doped OMS-2 catalyst. The conversion and selectivity of the reaction were affected by the dopant loading of catalyst and reaction temperature. Kinetic study found that original OMS-2 showed slightly higher initial reaction rate than modified OMS-2, and the latter gave higher EP yield due to its more labile lattice oxygen. Characterization of the catalyst indicates that the enhancement of surface oxygen vacancies and reducibility from the Na-doping might contribute to the superior catalytic activity.

Full text of DOI:10.1016/j.apcata.2020.117813

AppliedCatalysisA,General605(2020)117813
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Na-doped OMS-2-catalzyed highly selective aerobic oxidation of ethyl
lactate to ethyl pyruvate under mild conditions
Luyao Taoa,b, Xiuru Bia,b, Liping Zhanga,b, Gexin Chena, Peiqing Zhaoa, Jun-Li Yangc,*,
Xu Menga,*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou 730000, China
b University of Chinese Academy of Sciences, Beijing 100049, China
c CAS Key Laboratory of Chemistry of Northwestern Plant Resources, Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences (CAS), Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The direct oxidation of lactate to pyruvate is an important process for catalytic conversion of biomass, although
low selectivity and harsh reaction conditions limit its potential for industrial application. In this study, catalytic
oxidation of ethyl lactate (EL) to ethyl pyruvate (EP) over OMS-2-based materials was conducted using O2 as the
green oxidant. A 78 % EL conversion and 95 % EP selectivity were obtained in CH3NO2 at 50 °C over a Na-doped
OMS-2 catalyst. The conversion and selectivity of the reaction were affected by the dopant loading of catalyst
and reaction temperature. Kinetic study found that original OMS-2 showed slightly higher initial reaction rate
than modified OMS-2, and the latter gave higher EP yield due to its more labile lattice oxygen. Characterization
of the catalyst indicates that the enhancement of surface oxygen vacancies and reducibility from the Na-doping
might contribute to the superior catalytic activity.
Biomass conversion
Catalytic oxidation
OMS-2
Ethyl lactate
Ethyl pyruvate
1. Introduction
Direct catalytic oxidation of lactates in gas- or liquid-phase is an
atom-economic option for the production of pyruvates. Mo-based cat-
As a sustainable carbon source, lignocellulosic biomass recently has
attracted increasing attention due to the potential replacement of fossil
raw [1,2]. Various platform molecules derived from biomass can be
converted to valuable chemicals via catalytic transformation [3,4].
Among them, pyruvic acid and pyruvate can be converted to important
chemicals and intermediates that are already used in pharmaceutical,
food additives, perfume, plastic and electronic material industrials
[5–7]. They are also employed as raw for the synthesis of bioactive
compounds, like antivirus and anticancer drugs [8,9]. Moreover, pyr-
uvates are used as a dietary supplement in food industry [10]. The
traditional and commercial pathway for producing pyruvates employs
tartaric acid as raw material via the dehydrative decarboxylation [11].
However, the use of excess dehydrating reagent KHSO4 results into
serious environmental pollution and energy consumption, which is in-
compatible with green chemistry. Microbial fermentation using yeasts
has been developed, although strict control of process parameters and
difficulties of product isolation and purification decrease its practicality
[12]. Thus, there is an incentive to explore catalytic and clean trans-
formation to overcome the aforementioned drawbacks.
alysts and V2O5/TiO2 were prepared and studied in oxidation of lactates
in vapor-phase with O2 as the green oxidant [13–16]. Nevertheless,
requirement of high reaction temperature (generally above 200 °C) not
only decreased the selectivity of the reaction, but also increased the
production cost of the process. Liquid-phase oxidation of lactates to
pyruvates over catalysts with various oxidants has emerged. For ex-
ample, Hayashi with co-workers studied catalytic performance of var-
ious transition metals on the oxidation of ethyl lactate (EL) to ethyl
pyruvate (EP) at 130 °C, which shows high EP selectivity could only be
obtained at low EL conversion and the yield of EP was no more than 50
% [17]. Later on, Shiju`s group prepared mesoporous vanadia-titania,
V-N-doped carbon nanosheet and activated carbon-supported vanadia,
and found that they could be used as efficient heterogeneous catalysts
in the oxidation of EL with O2 as the green oxidant at 130 °C [18–20].
Zhou`s group reported a TS-1/H2O2 catalytic system for the oxidation
of EL with high conversion and selectivity under very mild conditions
[21]. Very recently, V2O5 and VO(acac)2 were successfully applied in
EL oxidation at 80 °C or even room temperature when TBHP was em-
ployed as an efficient oxidant [22]. Although selective conversion of EL
⁎ Corresponding author.
E-mail addresses: yangjl@licp.cas.cn (J.-L. Yang), xumeng@licp.cas.cn (X. Meng).
https://doi.org/10.1016/j.apcata.2020.117813
Received 20 July 2020; Received in revised form 19 August 2020; Accepted 29 August 2020
Availableonline01September2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.

L. Tao, et al.
AppliedCatalysisA,General605(2020)117813
to EP by catalytic oxidative dehydrogenation has been realized, per-
oxides, additives and complex catalysts are still needed. O2 that is most
abundant and cheapest oxidant has relatively high activation barrier,
and is generally activated by noble metal, like Pd and Pt, under mild
conditions. In this respect, the development of a highly selective process
for oxidative conversion of EL to EP over a non-noble catalyst with
molecular oxygen as efficient green oxidant under mild conditions
(≤50 °C) is challenging and promising.
2.3. Catalyst characterization
The crystal phase of as-prepared catalysts was analyzed by power X-
ray diffraction (XRD) employing a X-Pert PRO X-ray diffractometer with
Cu Kα radiation in the 2θ range of 10−90°. The X-ray photoelectron
spectroscopy (XPS) measurements were conducted on a Kratos AXIS
Ultra DLD high performance electron spectrometer with non-
monochromatized Al Kα excitation source (hv =1486.6 eV). Binding
energies were calibrated by using the contaminant carbon
(C1 s = 284.8 eV). Nitrogen adsorption-desorption measurements were
carried out at 76 K using an ASAP 2020 M analyzer utilizing the BET
model for the calculation of specific surface areas. Tecnai G2 F20 S-
Twin 200 KV high-resolution transmission electron microscopy and
EVO 18 scanning electron microscope were used to characterize the
morphology of catalysts. The elemental composition analysis towards
the catalysts was examined by inductively coupled plasma emission
spectroscopy (ICP-AES) on Varian Vista MPX. The reducibility of the
catalysts was analyzed by H2-TPR. 50 mg of OMS-2 or Na-OMS-2-
2.5 mol% was replaced in a quartz reactor which was connected to a
TPR instrument and the reactor was heated to 550 °C with a heating
rate of 10 °C/min. The reducing atmosphere was 5% H2/N2 with a flow
rate of 30 mL/min. The outlet gas was analyzed by TCD. The basicity of
the catalysts was analyzed by the use of CO2-TPD. 50 mg of OMS-2 or
Na-OMS-2−2.5 mol% was replaced in a quartz reactor and heated in Ar
at 400 °C for 2 h. Then, the sample was cooled down to 100 °C and
saturated with CO2 gas in a flow rate of 30 mL/min of 10 % CO2/Ar for
1 h. After, the sample was flushed with Ar at 100 °C for 1 h and heated
in Ar to 400 °C at ramp rate of 10 °C/min. The evolution of CO2 was
analyzed by TCD. TGA was performed on a METTLER TGA/DSC 1. The
sample was heated under N2 from R.T. to 1000 °C at a rate of 5 °C/min.
The heat flow data were dynamically normalized using the in-
stantaneous weight of the sample at the respective temperature.
Manganese oxide octahedral molecular sieve (OMS-2) is a micro-
porous nanostructural material which is composed of edge- and corner-
shared MnO6 octahedra [23]. It has unique mixed-valence of Mn
(Mn4+, Mn3+ and Mn2+) and a 2 × 2 open tunnel structure with K+
inside that maintains the framework and balances the valence. Due to
its structural properties, redox ability, adsorption and semi-
conductivity, OMS-2 has been applied as catalysts, ion-exchange agents,
battery materials and adsorption materials [24–32]. In terms of clean
synthesis and environmental catalysis, OMS-2-based materials have
been widely used as heterogeneous oxidation catalysts because of
readily activation of green oxidant (like O2) via electron transfer be-
tween mixed valent Mnn+ and O2 [33–40]. Rich amounts of surface
oxygen vacancies, mobile lattice oxygen and superior redox ability are
commonly believed to be beneficial for its catalytic activity in aerobic
oxidative conversion. As a consequence, alternative preparation and
modification methods of OMS-2 have been developed for enhancement
of its catalytic activity [41–44]. In our previous research, the use of
different oxidants during OMS-2 preparation and doping with transition
metals both were able to better the catalytic performance of OMS-2 in
clean synthesis of organic compounds [45–48]. Herein, we conducted
oxidation of EL to EP using O2 as the terminal oxidant catalyzed by
OMS-2 and a series of modified ones. It was found that the direct
aerobic oxidation over Na-doped OMS-2 (Na-OMS-2) showed the
highest conversion of EL and excellent selectivity of EP when the re-
action was performed at 50 °C. Based on characterization of as-syn-
thesized catalysts, the abundant surface oxygen vacancies, enhanced
reducibility and labile lattice oxygen on Na-OMS-2 might be re-
sponsible for the good catalytic performance.
2.4. Typical reaction procedure
In a typical reaction, Na-OMS-2 (80 mg, 20 mol%) was added into a
Schlenk tube with an O2 balloon. The air in the reaction tube was re-
moved and replaced by O2. After that, ethyl lactate (0.5 mmol) and
CH3NO2 (2 mL) were added into the tube by syringe. The reaction
mixture was allowed to stir at 50 °C for 12 h under oxygen. The reaction
was monitored and analyzed by gas chromatography (GC) and 1,4-di-
oxane was used as internal standard.
2. Experimental section
2.1. Materials
KMnO4, MnSO4·H2O, NaNO3, ethyl lactate, ethyl pyruvate, CH3NO2,
dimethyl carbonate (DMF), EtOH (anhydrous), N,N-dimethylforma-
mide (DMF), PhCl, toluene and acetonitrile were purchased from China
and used without further purification. OMS-2, δ-MnO2, AMO and CuOx/
OMS-2 were all synthesized by literature methods [40,49,50]. V-OMS-2
and Mo-OMS-2 were synthesized by one-pot reflux method (For details,
see SI).
3. Results and discussion
3.1. Catalytic performance
Initially, we used traditional reflux method to synthesize original
OMS-2 catalyst and conducted the oxidation of EL over it under 1 atm of
O2 for optimization of reaction solvent (Fig. 1). The reaction did not
give any desired product EP in the absence of solvent, although OMS-2
showed catalytic activity. When green solvents, such as DMC and EtOH,
were used, the low yields of EP were observed. Next, it was found that
polar solvents, like DMF, PhCl, toluene and acetonitrile, did not better
the reaction, though moderate conversion of EL was offered and the
highest selectivity of EP (61 %) was obtained in MeCN. To our delight,
the highest 80 % conversion of EL and a 58 % selectivity of EP were
provided when MeNO2 was employed as the solvent.
2.2. Catalyst preparation
In a round bottom flask, MnSO4·H2O (17.6 g, 0.104 mol) and NaNO3
(0.336 g, 0.004 mol) was dissolved in 60 mL of deionized water fol-
lowed by addition of 6 mL of HNO3 at room temperature. Then, a
200 mL of KMnO4 (11.78 g, 0.074 mol) solution was added dropwise
into the above solution with stirring at room temperature. The mixture
was stirred at 100 °C for 24 h. After that, the brown solid was collected
by filtration and washed by water. The solid material was dried in an
oven under air at 120 °C for 8 h. Finally, the theoretical Na/Mn mole
ratio was 2.5 % and the as-synthesized material was named as Na-OMS-
2−2.5 mol%. For comparison, different amounts of NaNO3 (0.0016,
0.008 and 0.016 mol) were used following the same preparation
method to produce Na-OMS-2−1 mol%, Na-OMS-2−5 mol% and Na-
OMS-2−10 mol%, respectively.
In order to better the reaction, other green oxidants, like air and
hydrogen peroxide, were tested (Fig. 2). Air led to a slightly higher
selectivity, but decreased the conversion of EL significantly. H2O2 was
deleterious to the reaction showing a 56 % conversion. Finally, we ran
the reaction under anaerobic conditions. The reaction under N2 gave
low conversion and moderate selectivity, which indicates that the
oxygen species of OMS-2 could give a 27 % conversion. The experi-
mental results proved that the reaction under O2 was aerobic and
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L. Tao, et al.
AppliedCatalysisA,General605(2020)117813
Fig. 1. Effect of reaction solvent on conversion of EL to EP. Reaction conditions:
2 mL of solvent, 0.5 mmol of EL, 40 mg (10 mol%) of OMS-2, 90 °C, 12 h, O2
balloon.
Fig. 3. Catalytic performance of OMS-2-based catalysts on conversion of EL to
EP. Reaction conditions: 2 mL of MeNO2, 0.5 mmol of EL, 40 mg of catalyst,
90 °C, 12 h, O2 balloon.
conversion (93 %) was given by doubling the amount of catalyst from
10 mol% to 20 mol%. These observations indicate that Na doping could
enhance the catalytic activity and higher loading of catalyst (20 mol%)
led to higher conversion of EL (93 %). However, higher catalyst loading
resulted into the significant decrease of selectivity of EP (from 73 % to
53 %).
According the previous research on optimization of EL aerobic
oxidation, high reaction temperature usually led to high conversion of
EL and low selectivity of EP [17–20]. In order to enhance the se-
lectivity, efforts had been paid to proceed the reaction under mild
conditions [21,22]. After we got the highest conversion of EL by the use
of Na-OMS-2-10 mol% (0.2 eq. to substrate), lower temperatures were
investigated (Fig.4). Na-OMS-2-10 mol% was very active, and a mod-
erate conversion and a remarkable selectivity in 86 % were offered even
at room temperature. As we expected, the conversion of EL gradually
decreased with the decrease of reaction temperature and the selectivity
of EP gradually increased. When the reaction was run at 50 °C in
MnNO2 under O2, the good selectivity in 77 % and 84 % conversion
were obtained (the yield of EP is 65 %). From the respect of EP yield,
this catalytic result was same as the result from the use of 10 mol% of
Na-OMS-2-10 mol% catalyst at 90 °C. Thus, the optimized reaction
temperature was determined as 50 °C.
Fig. 2. Effect of green oxidants and anaerobic conditions on conversion of EL to
EP catalyzed by OMS-2. Reaction conditions: 2 mL of MeNO2, 0.5 mmol of EL,
40 mg of OMS-2, 90 °C, 12 h (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article).
probably followed the Mars-van Krevelen mechanism [35]. So, mole-
cular oxygen was the best green oxidant.
After the optimization of the reaction solvent and oxidant, a series
of manganese oxides and modified OMS-2 materials were screened as
the catalyst in the oxidation of EL to EP in MeNO2 at 90 °C under O2
(Fig. 3). Compared with OMS-2 having a-MnO2 phase, δ-MnO2 and
amorphous MnOx (AMO) only resulted into moderate conversion and
selectivity, although AMO gave a better selectivity (64 %) than OMS-2.
Acid-modified one (H-OMS-2) only showed similar catalytic perfor-
mance like AMO, which means acid sites are not beneficial for the
conversion of EL to EP. Mo and V, that were generally used as catalyst
species in previous reports [15,18], doped OMS-2 both did not further
enhance the conversion and selectivity of the reaction. CuOx/OMS-2,
that reportedly has enhanced redox ability due to the synergistic effect
between Cu and Mn, just offered a 37 % conversion and 53 % se-
lectivity. According the previous research towards selective oxidative
dehydrogenation of alcohols, solid base catalysts were able to improve
the transformation [51]. Based on the report from Liu`s group, Na ion
could make OMS-2 possess basicity and higher redox ability, which
enhanced the activity for gas-phase ethanol oxidation [52]. Thus, we
tried to prepare Na-doped OMS-2 via ion-exchange in one-pot proce-
dure and use it as the catalyst in aerobic oxidative dehydrogenation of
EL. Interestingly, Na doping gave the highest 93 % conversion of EL and
highest 73 % selectivity of EP. Furthermore, a better result on EL
Lastly, we investigated the effect of Na dopant loading on catalytic
Fig. 4. Effect of reaction temperature on conversion of EL to EP catalyzed by
Na-OMS-2-10 mol%. Reaction conditions: 2 mL of MeNO2, 0.5 mmol of EL,
80 mg (0.2 eq.) of Na-OMS-2-10 mol%, 12 h, O2 balloon.
3

L. Tao, et al.
AppliedCatalysisA,General605(2020)117813
Fig. 7. Reaction profile and initial rate of the aerobic oxidative conversion of EL
to EP over OMS-2 and Na-OMS-2-2.5 mol%. Reaction conditions: 4 mL of
MeNO2, 0.5 mmol of EL, 80 mg (20 mol%) of OMS-2 or Na-OMS-2-2.5 mol%,
12 h, 50 °C, O2.
Fig. 5. Effect of dopant loading of Na-OMS-2 on conversion of EL to EP.
Reaction conditions: 2 mL of MeNO2, 0.5 mmol of EL, 80 mg (20 mol%) of Na-
OMS-2-xmol%, 12 h, 50 °C, O2 balloon.
performance (Fig. 5). Through tuning the amount of NaNO3 used in the
synthesis of Na-OMS-2, Na-OMS-2-1 mol%, Na-OMS-2-2.5 mol% and
Na-OMS-2-5 mol% were also prepared. It was found that the catalytic
performance of Na-OMS-2-xmol% was influenced by dopant loading
severely (Fig. 5). The dopant loadings of 5 mol% and 10 mol% had very
similar catalytic activity and selectivity. With dopant loading decreased
from 10 mol% to 1 mol%, the conversion of EL under the optimized
reaction conditions decreased from 84 % to 62 % and the selectivity of
EP increased from 77 % to 98 %. For Na-OMS-2 catalysts doped by
1 mol% and 2.5 mol% of NaNO3, they both showed nearly 100 % se-
lectivity, but Na-OMS-2−2.5 mol% gave the higher conversion than the
former and afforded the highest 74 % yield of EP. Considering the
yields around 60 % from other Na-doped catalysts, Na-OMS-2-2.5 mol%
demonstrated the best catalytic performance.
performance, we run the reaction over original OMS-2 at 50 °C and
compared the initial reaction rate of it and Na-OMS-2-2.5 mol% under
the optimized reaction conditions. As shown in Fig. 7, original OMS-2
gave a higher initial reaction rate (0.065) than Na-OMS-2-2.5 mol%
(0.035). However, the yield of EP over OMS-2 did not increase and
stopped at about 50 %, although the reaction time was extended to
600 min. Na-OMS-2-2.5 mol% with lower initial reaction rate provided
the higher yield of EP in 70 % during the same reaction period.
Lastly, we compared our catalytic result with the previous reports
that used heterogeneous catalysts in liquid-phase and the comparison
was listed in Table 1. The results from the present work feature the
merits including green oxidant, mild conditions and high selectivity.
Under the optimized conditions, the reaction progress was mon-
itored by the time-profile experiment at 50 °C (Fig. 6). Ethyl lactate
converted very fast in initial period and reached the highest conversion
after around 200 min. After that, the conversion of EL slowly increased
to 78 %. The yield of EP smoothly raised with the increase of conversion
of EL and the selectivity of EP nearly reached to 100 % after 500 min.
From the analysis of GC, there was no by-products or over-oxidized
products in the reaction. In order to further investigate the catalytic
3.2. Catalyst characterization
After finding out the best catalyst, we characterized the catalysts in
order to understand the catalytic performance. Firstly, XRD was used to
analyze the composition and crystal phase of Na-OMS-2-xmol% mate-
rials (Fig. 8). Na-OMS-2 with dopant loading ranging from 1 to 10 mol%
all showed typical cryptomelane phase (JCPDS file #29-1020) and
there were no other diffraction peaks and peak shift observed. With the
increase of dopant loading, the diffraction peaks became increasingly
sharp and narrow, which means that the crystal size and crystalline of
Na-doped materials became larger and stronger than original OMS-2.
These results indicate that Na ion possibly substituted partial K ion in
tunnel of OMS-2 framework because eight-coordinated cations with
large crystal radius size are easily introduced into the tunnel [54].
Compared with original OMS-2, Na-doping enhanced the surface area
Table 1
Catalytic performance comparison of oxidative EL conversion to EP over het-
erogeneous catalysts in liquid-phase with previous reports.
Catalyst
Temp. (oC)
Oxidant
Con. (%)
Sel. (%)
Ref.
Na-OMS-2
ZrO2
50
O2
78
52
62
60
95
80
98
98
99
70
98
95
83
80
85
85
90
95
100
100
80
98
This work
[17]
130
130
130
130
130
50
O2
SnO2-MoO3
V-TiO2
O2
[17]
O2
[18]
V-NCNs
O2
[19]
VOx/C + pyridine
TS-1
O2
[20]
H2O2
TBHP
TBHP
O2
[21]
V2O5
80
[22]
VO(acac)2
TiO2
25
[22]
Fig. 6. Reaction profile of the aerobic oxidative conversion of EL to EP.
Reaction conditions: 4 mL of MeNO2, 0.5 mmol of EL, 80 mg (20 mol%) of Na-
OMS-2-2.5 mol%, 12 h, 50 °C, O2 balloon.
130
80
[53]
Zn(NO3)2+VOC2O4
O2
[55]
4

L. Tao, et al.
AppliedCatalysisA,General605(2020)117813
patterns of the catalyst, Na doping decreased the amounts of Mn4+ of
Na-OMS-2-xmol% compared with original OMS-2, which means more
oxygen vacancies were generated on Na-OMS-2-xmol%. Na doping also
lowered the Mn valence from 3.83 to 3.40 (Table S2, in SI). Among
them, Na-OMS-2-2.5 mol% had the most abundant of low valent man-
ganese species. And, the dopant loading did not obviously affect the
concentration of different valent Mn (Table 2). Surface oxygen vacancy
is generally accepted to be critical for aerobic oxidation, because O2 is
captured and activated by it to form active oxygen species to proceed
the reaction. From O1 s XPS patterns of the catalyst, Na doping en-
hanced the amount of lattice oxygen (Table 3). It is believed that the
lattice oxygen became much more labile by Na doping. These results of
XPS were in accordance with the observation of H2-TPR. The enhanced
redox ability is related to absorbed active oxygen species which is
generated from oxygen absorbed on oxygen vacancies and activated by
Mnn+/Mn(n−1)+ species.
Fig. 8. XRD patterns of OMS-2 and Na-doped OMS-2 catalysts.
3.3. Catalyst recycling
of the catalyst from 59 to 86 m2 g−1 (Table S1, in SI). Then, SEM and
TEM images showed that the optimized catalyst Na-OMS-2-2.5 mol%
had uniform nano-rod morphology which is same as original OMS-2
(Fig. 9 and Fig. S2 in SI). It is believed that the doping did not change
the morphology of the catalyst.
To confirm whether the catalysis was derived from the solid mate-
rial or leached metal species, we performed the hot-filtration experi-
ment (Fig. S5, in SI). The catalyst was removed via filtration, when the
reaction showed a conversion of EL in roughly 56 %. Then, the filtrate
was kept stirring at 50 °C for another 7 h and no further conversion of
EL detected, which proves that the observed catalysis was truly het-
erogeneous. Subsequently, the recycling experiment was carried out by
the use of 2.5 mmol of EL and 400 mg of Na-OMS-2-2.5 mol% under the
optimized conditions (Fig. 12). It was found that the selectivity of the
reaction maintained very well (above 95 %), however, the conversion
of EL decreased after the first run and was stable at around 50 % in the
third and next runs. We analyzed the reaction solution by ICP-AES after
removing the catalyst in the first run and no sodium or potassium
species was detected (below 0.001 %) in the filtrate. However, 366 ppm
of manganese was detected. The metal leaching might cause the deac-
tivation of the catalyst.
H2-TPR and CO2-TPD were used to examine the surface reducibility
and basicity of optimal catalyst (Na-OMS-2-2.5 mol%). For comparison,
original OMS-2 was also tested. As shown in Fig. 10, Na-doped OMS-2
and original OMS-2 had very similar reduction patterns and they both
showed a board peak range 200−450 °C which means the reduction of
MnO2 to MnO occurred. Obviously, the reduction peak of Na-OMS-2-
2.5 mol% shifted to lower temperatures (200 °C) compared with that of
OMS-2 (250 °C). The results indicate that Na-doped OMS-2 was easier to
reduce and had enhanced reducibility [52]. Next, CO2-TPR analysis
proved that Na-doped OMS-2 had more basic sites and higher basicity
than original OMS-2. It is believed that enhanced reducibility resulted
in higher activity and higher conversion of EP in the oxidation of EL.
And, basicity generated from Na-doping might help cleavage of hy-
droxyl OeH bond of ethyl lactate when it was adsorbed on the catalyst
surface [52]. The thermal stability was analyzed under N2 by TGA/DSC,
which showed that OMS-2 and Na-doped OMS-2 had nearly same pat-
terns of weight losses in the temperature of 25−780 °C. Specifically,
physisorbed water was lost at 50−200 °C, chemisorbed O2 and H2O
were lost at 380−550 °C and lattice oxygen species were released at
580−600 °C. When the temperature was above 620 °C, the framework
of OMS-2 was collapsed and decomposed. The TGA analysis results was
consistent with the previous report and Na-doping did not change the
thermal stability of the material [23].
3.4. Proposed mechanism
On the basis of our experimental results (Fig. 2) and previous re-
ports, we proposed a possible reaction mechanism which is consistent
with Mars-van Krevelen mechanism (Fig. 13) [13,18,33,35]. Initially,
ethyl lactate adsorbed on the catalyst surface via active site Mnn+, and
oxygen species was adsorbed and activated by oxygen vacancies. The
hydrogen atom of hydroxyl group was captured by surface activated
oxygen species and the secondary H atom was caught by the lattice
oxygen of Mn-O bond to generate the EL radical and reduced Mn(n−1)+
.
In heterogeneous catalytic oxidation, the oxygen and metal species
on the catalysts surface are believed to be critical for the catalytic
performance [41]. So, XPS was employed to analyze the oxygen and
manganese species of original OMS-2 and Na-doped OMS-2 with dif-
ferent dopant loading (Fig. 11 and Fig. S3 in SI). From Mn2p XPS
After dissociation, the desired product ethyl pyruvate and H2O were
formed while the surface oxygen vacancies of the catalyst were gen-
erated. Finally, the reduced catalyst was reoxidized by O2 and adsorbed
oxygen replenished the oxygen vacancies, regenerating the highly ac-
tive oxygen to finish the catalytic cycle.
Fig. 9. SEM images of Na-OMS-2-2.5 mol%.
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L. Tao, et al.
AppliedCatalysisA,General605(2020)117813
Fig. 10. H2-TPR, CO2-TPD and TGA profiles of OMS-2 and Na-OMS-2-2.5 mol%.
4. Conclusion
Table 2
The Mn2p XPS data of the as-synthesized materials.
It was found that OMS-2-based catalysts were highly active for the
conversion of ethyl lactate to ethyl pyruvate in liquid-phase and Na-
doped OMS-2 could further better the selectivity of the reaction to
roughly 100 %. Moreover, O2 could be activated by the catalyst playing
a role as the terminal oxidant instead of peroxide at very mild reaction
temperature because Na-doping enhanced the amount of oxygen va-
cancies, reducibility and lattice oxygen lability. The catalytic system
offers an atom-economic, environmental friendly and practical process
for catalytic conversion of biomass platform molecules. Enhancement of
catalytic ability of manganese oxide-based catalysts via unreducible
metal-doping provides a new pathway for exploration of organic
transformation and increasing the reaction selectivity.
Catalyst
Binding energy of Mn 2p (eV)
Manganese species (%)
Mn4+
Mn3+
Mn2+
Mn4+
Mn3+
Mn2+
OMS-2
643.36
643.77
642.44
642.33
641.04
641.08
60.1
39.4
35.5
44.5
4.4
Na-OMS-2-
1 mol%
16.1
Na-OMS-2-
2.5 mol%
Na-OMS-2-
5 mol%
643.81
643.95
644.03
642.11
642.18
642.22
640.71
640.86
640.86
40.3
42.3
41.9
38.8
40.4
42.1
20.9
17.3
16.0
Na-OMS-2-
10 mol%
Table 3
The O 1s XPS data of the as-synthesized materials.
CRediT authorship contribution statement
Catalyst
Binding energy of Mn 2p (eV)
Oxygen species (%)
Luyao Tao: Investigation, Data curation, Writing - original draft.
Xiuru Bi: Investigation, Formal analysis. Liping Zhang: Formal ana-
lysis. Gexin Chen: Data curation, Formal analysis. Peiqing Zhao:
Funding acquisition. Jun-Li Yang: Supervision, Conceptualization,
Project administration. Xu Meng: Supervision, Conceptualization,
Project administration, Writing - original draft, Writing - review &
editing.
Lattice O
Surface O
Lattice O
Surface O
OMS-2
530.03
529.67
529.58
529.55
529.59
531.12
531.37
531.30
531.29
531.34
66.4
78.0
78.7
77.3
76.7
33.6
22.0
21.3
22.7
23.3
Na-OMS-2-1 mol%
Na-OMS-2-2.5 mol%
Na-OMS-2-5 mol%
Na-OMS-2-10 mol%
Fig. 11. The Mn2p XPS patterns of as-synthesized materials (left) and the O1 s XPS. patterns of as-synthesized materials (right).
6

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