Sustainable production of pyruvic acid: Oxidative dehydrogenation of lactic acid over the FeMoO/P catalyst

Article abstract of DOI:10.1039/d0nj00118j

Fe-Mo bimetallic oxide catalysts doped with elemental P (FeMoO/P) were synthesized using a hydrothermal method, followed by calcination in air. The resultant catalysts were used to catalyze the oxidative dehydrogenation of lactic acid to pyruvic acid using air as an oxidant. UV-Vis characterization revealed that Fe₂O₃ was highly dispersed on the surface of MoO₃ for the FeMoO/P sample due to the enhanced band gap energy. The FeMoO/P sample showed distinct diffraction lines of MoO₃, while the diffraction of the typical (011) line slightly shifted to a higher 2 theta value. This indicated the incorporation of Fe³⁺ into the MoO₃ lattice, forming a surface substituted Fe-O_x-Mo solid solution. Furthermore, the other characterization techniques such as XPS and H₂-TPR disclosed a strong interaction between Fe₂O₃ and MoO₃ by binding energy shift and reducible peak change, respectively. The enhanced oxidizability of Fe species is responsible for the activation of the α-OH group in the lactic acid molecule, transferring hydrogen to the adjacent FeO_x site to achieve a crucial step for the oxidative dehydrogenation of lactic acid. For comparison, the Fe-Mo bimetallic oxide catalyst offered far better activity than its corresponding monometallic oxide/physically mixed oxide due to the increasing oxidizability of FeMoO/P by electron transfer between Fe and Mo species. On-line analysis of the tail gas revealed that no hydrogen was detected, precluding direct dehydrogenation of lactic acid to form pyruvic acid during the catalytic reaction. Furthermore, the experimental results in the absence of O₂, replaced by N₂, showed that a small amount of pyruvic acid formed, and it gradually decreased with time on stream, demonstrating lattice oxygen as an active species for the oxidative dehydrogenation of lactic acid. Encouragingly, the oxidative dehydrogenation reaction of lactic acid over the FeMoO/P catalyst proceeded efficiently at around 60 h on stream and with an excellent selectivity (>70%).

Full text of DOI:10.1039/d0nj00118j

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Sustainable production of pyruvic acid: oxidative
dehydrogenation of lactic acid over the FeMoO/P
catalyst†
Cite this:DOI: 10.1039/d0nj00118j
Chunyu Yin,a Xinli Li,*a Zhi Chen,a Weixin Zou,b Yanli Peng,a Song Wei,a
b
Congming Tang *a and Lin Dong
Fe–Mo bimetallic oxide catalysts doped with elemental P (FeMoO/P) were synthesized using a hydro-
thermal method, followed by calcination in air. The resultant catalysts were used to catalyze the oxidative
dehydrogenation of lactic acid to pyruvic acid using air as an oxidant. UV-Vis characterization revealed
that Fe2O3 was highly dispersed on the surface of MoO3 for the FeMoO/P sample due to the enhanced
band gap energy. The FeMoO/P sample showed distinct diffraction lines of MoO3, while the diffraction of
the typical (011) line slightly shifted to a higher 2 theta value. This indicated the incorporation of Fe3+
into the MoO3 lattice, forming a surface substituted Fe–Ox–Mo solid solution. Furthermore, the other
characterization techniques such as XPS and H2-TPR disclosed a strong interaction between Fe2O3 and
MoO3 by binding energy shift and reducible peak change, respectively. The enhanced oxidizability of Fe
species is responsible for the activation of the a-OH group in the lactic acid molecule, transferring hydro-
gen to the adjacent FeOx site to achieve a crucial step for the oxidative dehydrogenation of lactic acid.
For comparison, the Fe–Mo bimetallic oxide catalyst offered far better activity than its corresponding
monometallic oxide/physically mixed oxide due to the increasing oxidizability of FeMoO/P by electron
transfer between Fe and Mo species. On-line analysis of the tail gas revealed that no hydrogen was
detected, precluding direct dehydrogenation of lactic acid to form pyruvic acid during the catalytic
reaction. Furthermore, the experimental results in the absence of O2, replaced by N2, showed that a small
amount of pyruvic acid formed, and it gradually decreased with time on stream, demonstrating lattice
oxygen as an active species for the oxidative dehydrogenation of lactic acid. Encouragingly, the oxidative
dehydrogenation reaction of lactic acid over the FeMoO/P catalyst proceeded efficiently at around 60 h
on stream and with an excellent selectivity (470%).
Received 8th January 2020,
Accepted 9th March 2020
DOI: 10.1039/d0nj00118j
rsc.li/njc
other chemicals, such as acrylic acid,13,14 propionic acid,15,16 pyruvic
acid (PA),17,18 poly-lactic acid, 2,3-pentanedione,19 acetaldehyde,20
Introduction
Obtaining chemicals and fuels from biomass and its derivatives and lactates. Among these lactic acid derived chemicals, PA is widely
has attracted a great interest in recent years since the concept used in the production of food spices, food additives, amino acids,
of sustainable development is widely accepted all over the antioxidants and chemical intermediates.17,21
world.1–4 Lactic acid (LA) has been produced on a large scale
A traditional industrial approach for the production of PA
for several decades by using biological maize fermentation includes the dehydration and decarboxylation of tartaric acid
technology. Currently, glycerol,5–10 sugars,11 and cellulose12 with a stoichiometric amount of KHSO4, consuming high energy.17
are also used to synthesize LA by chemical catalysis consisting In contrast, oxidative dehydrogenation of LA to PA via hetero-
of oxidation/hydrolysis reactions. Undoubtedly, these technologies geneous catalysis is viewed as a potential route, which has been
provide an excellent basis for the catalytic conversion of LA into reported by several research groups.17,18 However, a serious
challenge exists for this route since both LA and PA have high
reactivity during the catalytic reaction, resulting in parallel side
reactions and tandem side reactions. Thus, developing highly
selective catalysts and optimizing reaction parameters are still
desirable.
Oxidative dehydrogenation of LA to PA usually includes
a School of Chemistry and Chemical Engineering, Chongqing University of
Technology, Chongqing 400054, P. R. China. E-mail: lixinli@cqut.edu.cn,
tcmtang2001@163.com
b Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis,
Nanjing University, Nanjing 210093, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj00118j liquid and vapor approaches. For the former, the major superiority
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Table 1 Textural properties of samples
is a low reaction temperature (ca. o100 1C), which can suppress
a great number of side reactions, achieving a high selectivity
toward PA.17,22,23 The weakness is that the catalytic reaction
occurs in alkaline media, such as NaOH or LiOH, leading to the
formation of pyruvates. To obtain PA, it is necessary to treat
pyruvates with inorganic acids through acidification reactions,
leading to generation of a large number of waste salts. Compared
Samples
SBET (m2 gÀ1
)
VP (cm3 gÀ1
)
Pore diametera (nm)
FeO
MoO
FeMoO/P
FeMoO/P*
15.2
9.7
23.7
21.8
0.11
0.05
0.12
0.19
12.4
8.8
13.7
18.1
a
Calculated from the desorption branch data using the Barrett–Joyner–
with the former, the latter only requires one step for achieving PA Halenda (BJH) model.
through the direct transformation of LA, avoiding stoichiometric
consumption of base and acid.18,24–26 However, the major
weakness is the high reaction temperature (ca. 4190 1C) and
relatively low selectivity to PA.
However, their bimetallic oxide (FeMoO/P, 23.7 m2 gÀ1) and the
mixture (FeMoO/P*, 21.8 m2 À1) show obviously increased
specific surface areas. Similarly, the pore volume and pore size
of FeMoO/P and FeMoO/P* also increased when compared with
their corresponding single metal oxides.
XRD and FT-IR analysis. The structures of the samples were
characterized with a wide angle X-ray diffraction (XRD) instrument
and a Fourier transform infrared (FT-IR) spectrometer, and the
results are shown in Fig. 1 and Table 2. From the XRD patterns
depicted in Fig. 1a, FeO presents three characteristic diffraction
peaks at 2y = 33.31, 35.81 and 54.31, respectively, which agreed well
with Fe2O3, with the corresponding faces (104), (110) and (116).
MoO presents three characteristic diffraction peaks at 2y = 23.11,
g
In contrast, vapor phase oxidative dehydrogenation of LA to
PA does not consume stoichiometric base and acid, making it a
benign environmental process. Thus, for nearly two decades,
much work has focused on this topic. The early work reported
that different structural iron phosphates offered a different
activity for the oxidative dehydrogenation of LA to PA, and the
M phase consisting of both Fe2+ and Fe3+ ions displayed the
best performance with LA conversion of 60% and PA selectivity
of 62%.25,27 Subsequently, in the same research group, modified
iron phosphates having a small amount of Pd/Mo were developed,
and the activity was further improved.28,29 Recently, bimetallic
oxides, such as Nb–Ni–O26 and Mo–Ti–O,18 have also been further
developed to improve activity for the oxidative dehydrogenation of
LA using their tunable oxidative-reducibility. Besides, trimetallic
oxides, such as Mo–V–Nb, have also been investigated for this
reaction by Mao and co-workers.24 However, both relatively low PA
selectivity and deficiency in stability of the catalyst are observed
due to the high reactivity of both LA and PA molecules. A possible
way for improving the catalytic performance in this reaction
required disclosure of the co-effect between redox and acidic–basic
properties during the catalytic reactions. However, knowledge on
this topic is still quite limited so far. Therefore, a persistent
effort for improving the catalytic performance in the oxidative
dehydrogenation of LA to PA has to go on.
In the present work, we constructed P doped Fe–Mo bimetallic
oxide catalysts for the oxidative dehydrogenation of LA to PA
using the hydrothermal synthesis method. Furthermore, the
cooperative effect between acidic–basic properties and the redox
properties of the catalyst on the oxidative dehydrogenation of LA
has been deeply discussed to develop highly efficient catalysts.
Results and discussion
Characterization
BET analysis. Measuring the textural property of a heterogeneous
catalyst is an important step to understand the area-specific catalytic
rate and mass transfer of reactants and products.30,31 In this section,
the specific surface area, pore volume and pore distribution of the
catalysts were investigated using nitrogen adsorption–desorption at
77 K using a JW-BK100C instrument, and the results are depicted in
Table 1 and Fig. S1 (ESI†). From the BET data given in Table 1,
the single metal oxides, such as FeO and MoO, have lower
Fig. 1 XRD patterns (a) of fresh samples before reaction and their corres-
ponding FT-IR spectra (b).
Table 2 Structural properties of samples
MoO(011) plane
Sample
MoO
2y (1)
d (nm)
Crystalline size (nm)
27.53
27.56
0.3237
0.3234
22.08
21.34
specific surface areas, 15.2 m2 gÀ1 and 9.7 m2 gÀ1, respectively. FeMoO/P
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25.81 and 27.41, respectively, which agreed well with MoO3, with
the corresponding faces, (100), (002) and (011). FeMoO/P was
prepared using the hydrothermal method, followed by calcination
in air, and it almost remained consistent with the sample of MoO
in terms of the diffraction peaks, whereas the characteristic
diffraction peaks ascribed to Fe2O3 were hardly observed.
Furthermore, the diffraction of the typical (011) line indexed
to MoO3 slightly shifted to a higher 2 theta value (see Table 2).
This indicated the incorporation of Fe3+ into the MoO3 lattice,
forming a surface substituted Fe–Ox–Mo solid solution.32 The
result can be supported by the characterization data of XPS
since the Fe/Mo atomic ratio obtained from XPS is 11 : 9, higher
than the 1 : 2 ratio of the raw materials. In addition, FeMoO/P
had almost the same size as MoO, suggesting a highly dispersed
nature of FeO species on the surface of MoO3. As for the sample
of FeMoO/P*, it showed respective characteristic diffraction peaks
ascribed to both MoO and FeO samples, meaning a mechanical
mixing. FT-IR spectra in the fingerprint region (400–1300 cmÀ1
)
can be used to differentiate the structure of samples with slight
variations. A difference in characteristic absorption bands at 469,
882, and 985 cmÀ1 was observed by comparing the four samples,
which suggested a strong interaction between MoO and FeO in
FeMoO/P (shown in Fig. 1b).
UV-Vis, XPS and H2-TPR analysis. In order to further under-
stand the interaction between MoO and FeO in FeMoO/P, UV-Vis
absorption spectra were obtained. According to a recent report,18
the enhanced band gap energy can be utilized to characterize the
dispersion of some metallic oxides. We recorded the UV-Vis
spectra and derived Tauc plots, depicted in Fig. S2a and b (ESI†),
respectively. Pure FeO and FeMoO/P* have almost the same band
gap of 2.0 eV, agreeing well with that reported by Liu et al.33 For the
FeMoO/P sample, the band gap is drastically shifted to a higher
energy of 2.8 eV, further confirming the higher dispersion of FeO
species. The interaction between MoO and FeO in the FeMoO/P
sample can also be investigated by XPS measurement, and the
results are shown in Fig. 2. Indeed, the binding energy of Fe 2p3/2
in FeMoO/P was shifted from 710.98 eV (FeO) to 711.69 eV, an
increase of 0.71 eV (Fig. 2a). At the same time, the binding energy
of Mo 3d5/2 in FeMoO/P was shifted from 233.58 eV (sample MoO)
to 232.85 eV, a decrease of 0.73 eV (Fig. 2b). The evident electron
transfer between FeO and MoO demonstrated the existence
of strong interaction, and also indicated a variation of redox
properties, which is possibly favorable for the oxidative dehydro-
genation of LA to PA during the catalytic conversion of LA. The
XPS survey spectra also demonstrated the existence of the P
element with a binding energy of 133.5 eV in the three samples
of MoO, FeMoO/P and FeMoO/P* (Fig. 2c). Next, H2-TPR experi-
ments were performed to further study the redox properties, and
the results are depicted in Fig. 3. In the four samples, pure FeO
displayed the lowest reduction peak, centered at 385 1C, and
pure MoO displayed reducible peaks at high temperatures,
Fig. 2 XPS spectra of fresh samples before reaction (a, Fe 2p; b, Mo 3d; c,
survey of all elements).
Fig. 3 H2-TPR profiles of fresh samples before reaction.
ca. 4550 1C. However, the low temperature reducible peak variation of redox properties. From these results, it is clear that
ascribed to the FeO species in the prepared sample FeMoO/P the enhanced oxidizability of Fe species is favorable for the
shifted toward high temperatures such as 529 1C, higher than activation of the a-OH group in the lactic acid molecule,
that in the FeMoO/P* sample (439 1C), also indicating a strong transferring hydrogen to the adjacent FeOx site (see the succedent
interaction in the Fe–Ox–Mo solid solution, which resulted in a reaction mechanism).
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Table 3 Conversion of isopropanol
Sel. (%)
Cat.
Isopropanol conv. (%)
Propylene
Acetone
FeO
MoO
FeMoO/P
FeMoO/P*
40.4
45.3
50.8
48.9
97.4
96.1
92.2
93.1
2.6
3.9
7.8
6.9
Acidic–basic properties. It is known that surface acidic–basic
properties of catalysts play an important role during the
catalytic conversion of LA. For example, weak–medium acidity
is favorable for the dehydration of LA to acrylic acid,34–36 and
medium acidity is favorable for the decarbonylation of LA to
acetaldehyde.20,37 Stronger acidity tended to accelerate LA poly-
merization, resulting in the formation of various cokes. Basicity
is favorable for LA condensation to 2,3-pentanedione.19,38–40
Thus, determining the acidity–basicity of the catalyst is necessary
for the conversion of LA. As NH3/CO2-TPD is the usual means for
measuring acidity–basicity, it was naturally first used in this
work. The results are shown in Fig. S3 (ESI†). NH3/CO2-TPD
signals are very weak for all samples, suggesting that the technique
struggled to characterize the very weak acidity–basicity of samples.
Subsequently, we chose isopropanol as a probe molecule to
continuously explore the surface acidity–basicity of samples at
240 1C, which determines acidity according to the obtained
product of propylene, and basicity according to the obtained
product of acetone.41 We chose isopropanol as a probe molecule
since it has three carbon atoms and one alcoholic hydroxyl
group similar to the lactic acid molecule. Especially, the reactive
site is focused on the alcoholic hydroxyl group. Table 3 shows
the conversion of isopropanol at 240 1C. From the results shown
in Table 3, the conversion of isopropanol on all samples is less
than 50% at a low feed flow rate of isopropanol (0.5 g hÀ1),
suggesting weak acidic–basic properties on the surface of the
catalysts, agreeing well with the results measured by NH3/CO2-
TPD. It was further found from the distribution of products
such as propylene and acetone that acidic sites dominated when
compared with the basic sites. Besides, basic sites on FeMoO/P
are more numerous than on the others. The basicity of the
catalyst is favorable for the first step of esterification between
–COOH of the LA molecule and the P–OH group of the catalyst,
protecting the –COOH group during the catalytic reaction, to
efficiently prohibit the LA decarbonylation reaction to form acetal-
dehyde; while acidity of the catalyst is favorable for the step of
hydrolysis of the pyruvate intermediate to form pyruvic acid.
Fig. 4 Effect of catalyst particle size on the oxidative dehydrogenation of
LA. Catalyst: FeMoO/P, 0.38 mL, 0.38–0.50 g; reaction temperature,
230 1C; carrier gas air, 3.0 mL minÀ1; feed flow rate, 2.0 g hÀ1, LA
feedstock, LA 10 wt% in water; TOS, 1–7 h; LA, lactic acid.
diffusion limitation. As the catalyst reduced to 20–40 mesh in
particle size, LA conversion is comparable to that in 40–60 mesh,
indicating that the internal diffusion limitation can be easily
removed. Fig. 4b shows that LA conversion was stable with time
on stream, and no induction period occurred for this reaction.
Next, 20–40 mesh sizes of the catalyst were used to further
investigate the external diffusion limitation. At a fixed LA/O2
molar ratio and contact time, LA conversion was obviously
influenced by LA flow rate (Fig. 5a), suggesting the occurrence
of an external diffusion limitation. As the LA flow rate increased
to 1.0 mL hÀ1 loading in a 1.5 cm height of the catalyst, the
external diffusion limitation was efficiently removed. Similarly,
LA conversion was also stable with time on stream (Fig. 5b).
In the subsequent experiments for activity evaluation, catalyst
particle size was chosen as 20–40 mesh, and LA flow rate was
chosen as 2.0 mL hÀ1 loading in a 3.0 cm height of the catalyst in
a fixed bed reactor to eliminate the mass transfer effect.
Catalyst evaluation
Comparison of catalysts. According to the discussion on
Effect of mass transfer during the catalytic reaction. In order mass transfer during the catalytic oxidative dehydrogenation
to understand the authentic performance of the prepared reaction of LA, catalyst particle sizes of 20–40 mesh as well as the
catalysts,42,43 preliminary experiments on the internal diffusion LA flow rate of 2 mL hÀ1 were used to evaluate the catalytic
limitation and the external diffusion limitation were carried performances of samples, which fully excluded the effect of mass
out, and the results are shown in Fig. 4 and 5. From Fig. 4a, the transfer. The performances of the four samples in the oxidative
effect of catalyst particle size on the oxidative dehydrogenation of LA dehydrogenation of LA are shown in Table 4 and Fig. 6. For LA
to PA was clearly observed, i.e., LA conversion increased drastically conversion, the increased order for these samples is: FeO o
with the decrease of the catalyst particle size, suggesting an internal MoO B FeMoO/P* o FeMoO/P. The difference between the lowest
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Fig. 6 Time course for LA conversion (a) and PA selectivity (b). Reaction
conditions are the same as those in Table 4.
Fig. 5 Effect of LA flow rate on the oxidative dehydrogenation of LA.
Catalyst: FeMoO/P, 0.05–1.0 g; reaction temperature, 230 1C; particle size,
20–40 meshes; carrier gas air, 3.0 mL minÀ1; feed flow rate, 2.0 g hÀ1, LA
feedstock, LA 10 wt% in water; TOS, 1–7 h; LA, lactic acid.
Further understanding of the relationship between the structure,
the constituents and the surface property of the catalysts and their
activity is an important way to improve catalytic performances
during the catalytic oxidative dehydrogenation of LA.17 Firstly,
MoO was evaluated for its activity in the catalytic oxidative
dehydrogenation of LA at 200 1C by the Pidko and Hensen
group, and it achieved a PA yield of o10%.18 We enhanced the
reaction temperature from 200 1C to 230 1C, and achieved a
slightly higher PA yield of 15.6%. In general, a low PA yield
necessitated us to add other contents into MoO, and we con-
structed composite catalysts to improve catalytic performance.
In the conversion of LA, Fe based catalysts have been investigated in
our previous work,44 and we found that acidic–basic properties and
redox properties were incorporated. For example, in the absence of
oxygen, Fe based catalysts displayed acidity for LA decarbonylation
to acetaldehyde at low temperature (B300 1C),44 and redox
properties for LA deoxygenation to propionic acid at high tempera-
Table 4 Comparison of catalysts in the oxidative dehydrogenation of LAa
LA
conv.
Sel.b (%)
LA consumption PA formation
rate  10À4
rate  10À4
Catalyst (%) PA AD AA Others mmol (m2 h)À1 mmol (m2 h)À1
FeO
MoO
58.3 44.3 7.0 15.7 33.0
73.7 21.2 8.3 0.5 70.0
1.58
4.49
2.91
1.27
0.70
0.95
2.16
0.84
FeMoO/P 88.4 74.2 6.9 7.1 11.8
FeMoO/P* 71.9 66.1 8.5 1.7 23.7
a
Catalyst, 0.38 mL; reaction temperature, 230 1C; particle size,
20–40 meshes; air, 3.0 mL minÀ1; feed flow rate, 2.0 mL hÀ1; LA
feedstock, LA 10 wt% in water; TOS, 1–7 h. LA, lactic acid; PA, pyruvic
b
acid; AD, acetaldehyde; AA, acrylic acid.
LA conversion of FeO (58.3%) and the highest LA conversion of ture (B390 1C).45,46 Furthermore, we checked the activity of pure FeO
FeMoO/P (88.4%) is up to 30.1%. However, the tendency of PA in the presence of oxygen, and achieved a better PA yield of 25.8%
selectivity of these samples does not follow LA conversion, and the than MoO. Therefore, we introduced a versatile Fe element into
new increased order of PA selectivity is: MoO o FeO o FeMoO/P* o MoO, and constructed composite catalysts. As expected, the compo-
FeMoO/P. Similar to LA conversion, the bigger difference value of site catalyst of FeMoO/P exhibited excellent performance compared
53% between the lowest PA selectivity of MoO (21.2%) and the to MoO and FeO alone. From the textural properties of the catalysts
highest PA selectivity of FeMoO/P (74.2%) is observed. These results shown in Table 1, FeMoO/P has a greater pore size and pore volume
clearly demonstrate that the oxidative dehydrogenation of LA to PA is than the single metal oxides, lowering the mass transfer resistance
a catalytic reaction under the conditions of 230 1C and air as an of reactants and products, which favors the catalytic reaction of
oxidant, and is sensitive to the catalysts. In addition, the area-specific oxidative dehydrogenation of LA.
catalytic rate for the formation of PA (2.16 Â 10À4 mmol (m2 h)À1) on
The constituent elements of catalysts were detected by XPS
spectra (Fig. 3c), and the detected elements in FeMoO/P were
FeMoO/P is two times higher than the others.
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Fe, Mo, O and P, respectively. XRD characterization confirmed PA concentration reached 2.1 wt% in the aqueous solution.
the incorporation of Fe3+ into the MoO3 lattice to form a surface With further lengthening of reaction time, LA conversion as well
substituted Fe–Ox–Mo solid solution for FeMoO/P. Electron as PA formation rate drastically decreased, suggesting that
transfer between Fe and Mo verified by XPS characterization the catalyst deactivated rapidly. Meanwhile, tail gas analysis
(Fig. 2a and b) efficiently regulates the redox properties of the confirmed no occurrence of hydrogen, demonstrating that PA
catalysts, which has been demonstrated by H2-TPR experiments was not formed by direct dehydrogenation of LA (Fig. 7c). The
(Fig. 3). The reduction peak of Fe species shifts from the low used catalysts with different times on stream were characterized
temperature of 385 1C to the higher temperature of 529 1C, by XRD measurements, as shown in Fig. 7d. Characteristic
suggesting that the reducibility decreases, and the oxidizability diffraction peaks indexed to oxides decreased rapidly, and then
increases indirectly. Besides, the oxygen content of FeMoO/P is disappeared, whereas the used catalysts, which ran for 7 h on
evidently higher than that of the single metal oxides (shown in stream under the air atmosphere, remained similar to the fresh
Table S1, ESI†). Thus, FeMoO/P offered excellent activity, sample in characteristic diffraction peaks. Moreover, we also
ascribed to an increase of oxidizability. In addition, FeMoO/P found that the sample running under a N2 atmosphere restored
displays relatively higher isopropanol conversion, suggesting a its initial structure when it was placed under an air atmosphere
higher density of acidic–basic sites compared with other samples. (Fig. 7e). These results suggested that lattice oxygen can be used
Surface basicity of catalysts favors adsorption and esterification for the oxidative dehydrogenation of LA, accompanying the
of LA molecules during the oxidative dehydrogenation of LA to PA departure of the H2O molecule. Also, gas phase O2 in air was
in the presence of oxygen, whereas surface acidity favors the adsorbed on the surface of the catalyst, and transformed into
hydrolysis of the absorbed pyruvate intermediate from the lattice oxygen.
surface of the catalyst to form pyruvic acid. Therefore, appro-
priate redox and acidic–basic properties are the main factor for dehydrogenation of LA from the discussion on the preliminary
improving the oxidative dehydrogenation of LA to PA. experiments. Further study on the effect of the ratio of LA/O2 during
Oxygen has been found to play an important role in the oxidative
Ratio of LA/O2. In the preliminary experiment, we replaced the catalytic oxidative dehydrogenation of LA is a way to improve
oxygen with nitrogen (purity: 99.999 wt%) as the reaction atmo- catalytic performance. Fig. 8 plots LA conversion and PA selectivity
sphere, and tested the catalytic performance for the oxidative as a function of the ratio of LA/O2 at 5 h on stream. LA conversion as
dehydrogenation of LA to PA, and the results are shown in well as PA selectivity with time on stream at different molar ratios of
Fig. 7a and b. At the initial reaction time, LA conversion reached LA/O2 is shown in Fig. S4 (ESI†). It was clearly observed that LA
nearly 90%, and quite a bit of PA was formed. For example, conversion continuously increased with a decrease of LA/O2 molar
Fig. 7 Performance of the FeMoO/P catalyst: (a) PA concentration with time on stream, (b) LA conversion with time on stream, (c) GC profiles of the tail
gas over the FeMoO/P catalyst, compared with reference air and pure hydrogen and (d) XRD patterns. Conditions: reaction temperature, 230 1C; particle
size, 20–40 meshes; carrier gas N2 or air, 3.0 mL minÀ1; feed flow rate, 2.0 mL hÀ1; LA feedstock, LA 10 wt% in water; LA, lactic acid; PA, pyruvic acid.
(e) XRD patterns of the FeMoO/P catalyst, compared for that used in N2 for 4.0 h and that used in N2 for 4.0 h and that used in air.
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the catalyst surface resulted in the decrease of the adsorbed LA
molecules, which is not favorable for the conversion of LA to PA via
oxidative dehydrogenation. Therefore, determining an appropriate
O2/LA molar ratio is very important for improving PA selectivity,
and the optimal molar ratio of LA/O2 is around 2.25–2.75.
Productivity of the FeMoO/P catalyst. The contact time was
changed between 0.15 s (corresponding LA flow rate of 4 mL hÀ1
)
and 1.2 s (corresponding LA flow rate of 0.5 mL hÀ1) at the fixed
LA/O2 molar ratio at 230 1C so that the maximum PA productivity
was determined for the FeMoO/P sample. The results are
depicted in Fig. 10. Clearly, LA conversion increased with length-
ening contact time. In contrast, PA productivity decreased. The
maximum PA productivity of B3.5 mmol gÀ1 hÀ1 was achieved
for contact times lower than 0.2 s and LA conversion was in the
range of 48–60%. Combining the high LA conversion and PA
productivity, optimal conditions can be set as a contact time of
0.3 s (the corresponding LA flow rate of 2.0 mL hÀ1 and air flow
rate of 3.0 mL minÀ1), where LA conversion reached 88.4%, and
Fig. 8 Effect of LA/O2 molar ratios. Conditions: catalyst, FeMoO/P,
0.38 mL, 0.37 g; reaction temperature, 230 1C; particle size, 20–40 meshes;
carrier gas air, 3.0 mL minÀ1; feed flow rate, 2.0 mL hÀ1; LA feedstock, LA
10 wt% in water; TOS, 5 h; LA, lactic acid; PA, pyruvic acid.
PA productivity reached 3.0 mmol gÀ1 hÀ1
.
ratio. As previously mentioned, oxygen is necessary for the oxidative
dehydrogenation of LA since it is required to compensate for losing
lattice oxygen. Indeed, at the initial stage of increasing O2/LA molar
ratio, PA selectivity drastically increased. However, PA selectivity
decreased with a further increase of O2/LA molar ratio. In other
words, a high concentration of oxygen, i.e., a high O2/LA molar ratio,
caused a worse selectivity toward PA. The reasons can be ascribed to
three aspects. First, an appropriate amount of O vacancies, which
has been confirmed according to the recent work32 using O1s XPS
spectra, as shown in Fig. 9, favors Fe3+ for activating a-OH of the
lactic acid molecule. But, high O2/LA molar ratios decreased O
vacancies, resulting in a low formation rate of PA. Further, the
parallel side reactions have a high reaction rate at high O2/LA molar
ratios, and it is known that the decarbonylation/decarboxylation of
LA to acetaldehyde is a major parallel side reaction during the
catalytic conversion of LA.14,47,48 For example, a larger amount of
acetaldehyde was generated at high O2/LA molar ratios. Secondly,
serial reactions increased their reaction rate due to the occurrence
of full oxygen and unstable pyruvic acid molecules. In the end,
competitive adsorption between LA and the oxygen molecule on
Stability. Stability testing is very important for heterogeneous
catalyst research from a perspective of industrial application.49,50
The stability experiment of FeMoO/P was performed in the case
of controlling the initial LA conversion at less than 70% in order
to achieve fuller saturation of active sites of the catalyst by
reactant molecules, and observe a genuine stability (Fig. 11).
Controlling low LA conversion has two paths: (1) adopting high
LA LHSV (liquid hourly space velocity) and (2) decreasing catalyst
usage. Here, we chose the latter. The catalyst was reduced to 30%,
and diluted with inert quartz sand. From the data shown in
Fig. 11, we can clearly observe that LA conversion remains at
60% and PA selectivity almost remains constant (470%) at the
reaction time of 60 h on stream although LA conversion abruptly
decreased near 10% from B70% to B60% at the initial reaction
time of 10 h. So far, it is a pity that much of the work on the
oxidative dehydrogenation of LA has lacked a disclosure of
catalyst stability. Fortunately, bare phosphate iron and when it
Fig. 10 LA conversion and catalyst productivity for PA as a function of
contact time. Conditions: catalyst, FeMoO/P, 0.38 mL, 0.37 g; reaction
temperature, 230 1C; particle size, 20–40 meshes; LA feedstock, LA 10 wt%
in water. LA, lactic acid; PA, pyruvic acid.
Fig. 9 XPS spectra of O1s in fresh samples before reaction.
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Fig. 11 Stability of the FeMoO/P catalyst. Conditions: catalyst, 0.12 g
FeMoO/P and 0.30
g quartz sand, 0.38 mL; reaction temperature,
230 1C; particle size, 20–40 meshes; air, 3.0 mL minÀ1; feed flow rate,
2.0 mL hÀ1; LA feedstock, LA 10 wt% in water; LA, lactic acid; PA, pyruvic acid.
is doped with 0.8 wt% Pd catalysts have shown stability for only
10 h on stream due to the presence of Ai, where LA conversion
was controlled at B50% in the initial reaction time, and PA
selectivity was observed at B70%.28 For bare phosphate iron, LA
conversion also decreased by around 10% within the reaction
time of 10 h, and longer reaction times were not studied.
Comparatively speaking, the stability of the FeMoO/P catalyst
for 60 h on stream is a desirable result.
Fig. 12 Performance of the FeMoO/P catalyst at 230 1C: (a) LA conversion
with time on stream, and PA concentration with time on stream (b).
Reaction mechanism. Further investigation of the reaction
mechanism for the oxidative dehydrogenation of LA to PA can help
to optimize catalyst design and reaction parameters, achieving
better catalytic performances. (1) Differentiating the oxygen source:
is gas phase oxygen or lattice oxygen utilized during the catalytic
oxidative dehydrogenation of LA? On the FeMoO/P catalyst, when
nitrogen (N2) replaces air as a reaction atmosphere, PA is formed,
with a decrease in PA forming rate with time on stream, indicating
that lattice oxygen is consumed upon the oxidative dehydro-
genation of LA to PA (shown in Fig. 12). With further consumption
of lattice oxygen in the catalyst, PA formation rate drastically
reduces, and LA conversion also quickly decreases. When TOS
arrives at 4 h, PA is hardly detected in the liquid samples. Then,
when nitrogen is substituted by air as a reaction atmosphere, LA
conversion drastically increases, and PA concentration in the
liquid sample also increases drastically, indicating that a large
amount of PA is formed in the presence of oxygen. At this
moment, lattice oxygen can be confirmed as having a role in the
oxidative dehydrogenation of LA. Whether gas phase oxygen is
directly used in the oxidative dehydrogenation of LA or it is first
transformed to lattice oxygen the oxidative dehydrogenation of
LA is not confirmed yet. Another experiment in which inert
quartz sand was substituted for the catalyst in the presence of
air was carried out. As a result, only a trace of PA was detected
(see in Table S2, ESI†), demonstrating that gas phase oxygen was
Scheme 1 Reaction mechanism for the oxidative dehydrogenation of LA
to PA over the FeMoO/P catalyst.
not directly used, but it was transformed into lattice oxygen to carboxyl group (–COOH) in the LA molecule reacted with P–OH
be used subsequently in the oxidative dehydrogenation of LA. (basic site) in the catalyst to form lactate (I), accompanied with
A catalytic cycle of LA converted to PA over the FeMoO/P the removal of the H2O molecule. In this step, the carboxyl
catalyst is depicted in Scheme 1. First, LA is combined with the group was protected. Then, the secondary alcohol was deprotonated
surface of the catalyst via an acid–base reaction, in which the in adsorbed lactate by Fe species to form the Fe–O intermediate (II),
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and the hydride transformed to O in the adjacent Fe–O, forming Preparation of catalysts
Fe–OH. According to the XPS characterization of samples, the
Fe–Mo bimetallic oxide catalysts decorated by elemental P (denoted
as FeMoO/P) as well as the corresponding single metallic oxides
(FeO/MoO) were synthesized by a hydrothermal method under
alkaline conditions, followed by calcination in air. In a typical
experiment to prepare FeMoO/P, 8 mmol of Fe(NO3)3Á9H2O,
electron transfer of Fe species toward Mo leads to an increase of
oxidation state, which favors the formation of the Fe–O inter-
mediate (II). In the next step, the negative hydrogen ion in the
methyl site was left and further transferred its two electrons
to Fe species, which reduced the Fe center to produce a H2O
molecule, resulting in the formation of pyruvate (III). Meanwhile,
pyruvate (III) adsorbed on the surface of the catalyst was hydrolysed
to produce PA by acid catalysis and regenerated the basic OH group
on the phosphide. The appropriate acid sites existing on the surface
of the catalysts was verified by isopropanol conversion experiments,
which also favors the hydrolysis of the pyruvate intermediate (III).
After desorption of water, the Fe (+3) center was reduced to the
+2 state. The molecular oxygen in the air oxidized the surface
Fe2+ species back to the +3 state, and it was transformed into
lattice oxygen. As a result, a catalytic cycle was completed over
the FeMoO/P catalyst in the presence of oxygen.
The effect of acidity–basicity of catalysts together with their
redox properties on reaction performance for LA conversion
was further discussed. In the reported work,14,20,51 the catalysts
have only acidic–basicity, and not redox properties. These catalysts
are used to catalyze LA conversion, and the obtained major
products are acrylic acid, acetaldehyde, and 2,3-pentanedione in
LA conversion via LA dehydration, LA decarboxylation, and LA
condensation.36,52–55 More importantly, PA is not reported as a
minor side product, suggesting that the redox of the catalyst is
necessary for formation of PA from LA. In addition, the catalysts
combining acidic–basicity and redox properties have catalyzed LA
conversion to form PA at lower reaction temperature (ca. 230 1C)
than acidity–basicity catalyzed LA conversion alone (ca. 4300 1C).
These results indicate that controlling the lower reaction tempera-
ture is favorable for enhancing PA selectivity, and decreasing the side
reactions to form acrylic acid and acetaldehyde.
1.33 mmol of ammonium phosphomolybdate ((NH4)3PMo12
-
O40ÁH2O) and 36 mmol of CO(NH2)2 were dissolved in 60 mL of
distilled water and stirred at room temperature for 20 min.
Next, the resultant solution was transferred to a 100 mL of
PTFE-lined stainless steel hydrothermal reactor, placed in an oven,
and kept at 120 1C for 8 h. Then, the precipitate was formed, and
washed several times with distilled water and absolute ethanol,
and dried at 50 1C for 3 h. Besides, the corresponding single
metallic oxides, FeO and MoO, were prepared in a similar manner.
Prior to their use for catalyzing the oxidative dehydrogenation of
LA and catalyst characterization, all catalysts were calcined at
500 1C for 3 hours in air. In addition, the mixed catalyst (denoted
as FeMoO/P*) was prepared by direct mixing using two corres-
ponding single metallic oxides (FeO and MoO).
Catalyst characterization
The specific surface areas of catalysts were measured through
nitrogen adsorption at 77 K using a JW-BK100C instrument.
The pore size of catalysts is calculated from desorption branch
data using the Barrett–Joyner–Halenda (BJH) model. Powder
X-ray diffraction measurement was conducted on a Dmax/
Ultima IV diffractometer operating at 40 kV and 20 mA with
Cu-Ka radiation. The FTIR spectra of the catalysts were recorded
in the range of 500–4000 cmÀ1 on a FTIR-8400S spectrometer.
Diffuse reflectance UV-Vis spectra were recorded on a Shimadzu
UV-2401 PC spectrometer in diffuse-reflectance mode using an
integrating sphere (internal diameter 60 mm) and BaSO4 was
used as the reference. Redox properties of the samples were
estimated by H2-TPR on a Finesorb-3010 Instrument. X-ray
photoelectron spectroscopy (XPS) measurements were performed
using a Thermo Scientific K-Alpha spectrometer, equipped with a
monochromatic small-spot X-ray source and a 1801 double focusing
hemispherical analyzer with a 128-channel detector. The binding
energy was corrected for surface charging by taking the C 1s peak of
contaminant carbon as a reference at 284.6 eV. Surface acidic and
basic properties of the samples were estimated by NH3-TPD and
CO2-TPD, respectively, on a Finesorb-3010 Instrument. Besides,
isopropanol dehydration/dehydrogenation on the catalyst to form
the corresponding propylene/acetone at a reaction temperature of
240 1C was also used to characterize the acidic–basic properties
of the catalyst.
Nitrogen is used as the reaction atmosphere (0–4 h on
stream), and air is used as the reaction atmosphere (4.5–10 h
on stream). LA, lactic acid; PA, pyruvic acid.
Experimental
Materials
L-(+)-Lactic acid (80 wt% and 98.5 wt%, fermentation type with
maize) was obtained from Henan Jindan Lactic Acid Technology
Co., Ltd; the former (80 wt%) was directly used for the oxidative
dehydrogenation of LA to pyruvic acid without further purification,
and the latter (98.5 wt%) was used as a standard reference sample.
Pyruvic acid, acetaldehyde, acetone, acrylic acid, and isopropanol
were purchased from Sinopharm Chemical Reagent Co., Ltd.
Acetaldehyde, acetone, and acrylic acid were used for gas
Catalyst evaluation
chromatograph reference samples, and isobutyl alcohol was The vapor phase oxidative dehydrogenation of LA to pyruvic acid
used as an internal standard sample. Pyruvic acid was used for was performed with a continuous up-down fixed-bed quartz
the standard reference sample in analysis with a liquid chromato- tubular reactor (4 mm inner diameter) under atmospheric
graph. Ferric nitrate, ammonium phosphomolybdate and urea pressure. The catalyst (ca. 0.35 g, 20–40 meshes) was placed
were purchased from Energy Chemical Company, which were used between two layers of quartz wool inside the reactor. First, the
for the preparation of catalysts.
catalyst was heated from room temperature to 230 1C at a rate of
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3 1C minÀ1, and kept at the temperature for 1.0 h by flowing air
(3.0 mL minÀ1). Then, the feedstock (10 wt% aqueous solution
of LA, 2.0 mL hÀ1) was pumped into the reactor and driven
through the catalyst bed by air (0.1 MPa, 3.0 mL minÀ1). The
contact time of the reactant over the catalyst was estimated to be
around 0.30 s according to previous ref. 44 and 56. The liquid
products were analyzed off-line using an SP-6890 gas chromato-
graph with a FFAP capillary column connected to a FID for acrylic
acid and acetaldehyde, and a LC-20AD liquid chromatograph with a
reversed-phase C18 column connected to a UV detector for lactic
acid and pyruvic acid.
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Conclusions
The catalytic oxidative dehydrogenation of lactic acid to pyruvic
acid over the FeMoO/P catalyst and its single components was
investigated under various reaction conditions. It was found that
pyruvic acid was formed from lactic acid via oxidative dehydro-
genation, and not from direct dehydrogenation. FeMoO/P offered
a more excellent activity than its corresponding single compo-
nents and FeMoO/P* prepared by a mechanical mixing method,
which was ascribed to forming the surface substituted Fe–Ox–Mo
solid solution and electron transfer between Fe and Mo, leading
to novel redox and acidic–basic properties that are favorable for
the oxidative dehydrogenation of lactic acid. The experiments
substituting nitrogen for air as a reaction atmosphere demon-
strated that lattice oxygen participated in the oxidative dehydro-
genation of lactic acid. Promoting adsorption of gas phase oxygen
and its transformation into lattice oxygen can efficiently enhance
catalytic performance, which is dependent on optimizing LA/O2
molar ratios.
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the Youth/Key Project of Science
and Technology Research Program of Chongqing Education
Commission of China (KJZD-K201901104 and KJQN201801109),
the Opening Foundation of Jiangsu Key Laboratory of Vehicle
Emissions Control (OVEC 049), the Natural Science Foundation
of Chongqing (cstc2019jcyj-msxmX0198), the Special Project
of Technology Innovation and Application Development of
Chongqing (cstc2019jscx-msxmX0002), and the Initial Scientific
Research Foundation of Chongqing University of Technology
(2017ZD47 and 2017ZD48).
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