Effect of Mo dispersion on the catalytic properties and stability of Mo-Fe catalysts for the partial oxidation of methanol

Article abstract of DOI:10.3390/molecules25102410

Mo-Fe catalysts with different Mo dispersions were synthesized with fast (Cat-FS, 600 r·min^?1) or slow stirring speed (Cat-SS, 30 r·min^?1) by the coprecipitation method. Improving the stirring speed strengthened the mixing of the solution and increased the dispersion of particles in the catalyst, which exhibited favorable activity and selectivity. The byproduct (dimethyl ether (DME)) selectivity increased from 2.3% to 2.8% with Cat-SS, while it remained unchanged with Cat-FS in a stability test. The aggregation of particles and thin Mo-enriched surface layer decreased the catalyst surface area and slowed down the reoxidation of reduced active sites with Cat-SS, leaving more oxygen vacancies which promoted the formation of DME by the nonoxidative channel.

Full text of DOI:10.3390/molecules25102410

molecules
Article
Effect of Mo Dispersion on the Catalytic Properties
and Stability of Mo–Fe Catalysts for the Partial
Oxidation of Methanol
Shuai Zhang and Minghan Han *
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China; vstar_shuai@163.com
*
Correspondence: hanmh@tsinghua.edu.cn; Tel.: +86-10-62781469
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 13 February 2020; Accepted: 19 May 2020; Published: 21 May 2020
Abstract: Mo–Fe catalysts with different Mo dispersions were synthesized with fast (Cat-FS,
00 r·min− ) or slow stirring speed (Cat-SS, 30 r
1
·
min ) by the coprecipitation method. Improving the
−1
6
stirring speed strengthened the mixing of the solution and increased the dispersion of particles in the
catalyst, which exhibited favorable activity and selectivity. The byproduct (dimethyl ether (DME))
selectivity increased from 2.3% to 2.8% with Cat-SS, while it remained unchanged with Cat-FS in a
stability test. The aggregation of particles and thin Mo-enriched surface layer decreased the catalyst
surface area and slowed down the reoxidation of reduced active sites with Cat-SS, leaving more
oxygen vacancies which promoted the formation of DME by the nonoxidative channel.
Keywords: Mo–Fe catalyst; Mo dispersion; stability; oxygen vacancies
1
. Introduction
Formaldehyde is an important industrial chemical [
1,
2]. The production of formaldehyde from
methanol selective oxidation is based on a Mo–Fe catalyst [3,4], which offers a high formaldehyde
yield and long catalyst lifetime. These advantages result in low methanol unit consumption and a
high-quality product, i.e., formaldehyde aqueous solution. The lifetime of an industrial Mo–Fe catalyst
is about 8 to 18 months; it is recognized that the deactivation of Mo–Fe catalysts is due to Mo loss and
phase separation into MoO and Fe O , which decreases activity and selectivity [
3
,
5
,
6
]. In industrial
use, the Mo/Fe mole ratio of the catalyst is usually above 2.0, rather than the stoichiometric ratio of
iron molybdate to compensate for Mo loss during the reaction [ ]. Therefore, the catalyst consists not
only of Fe (MoO ) , but also of MoO . However, MoO should be prevented from forming plate-like
3
2
3
7,8
2
4 3
3
3
structures in the preparation process due to its low activity and selectivity [9–12].
The reaction for methanol selective oxidation to formaldehyde (Equation (1)) and the main
byproduct (DME) are
CH OH + 0.5 O = CH O + H O
(1)
3
2
2
2
2
CH OH = CH OCH + H O
(2)
3 3 3 2
Factors like Mo and Fe precursors, concentration of initial solutions, temperature, pH during
precipitation, stirring speed, aging of the precipitate and calcination conditions are important [13 16].
–
Many researchers have focused on the effect of the structure on catalytic activity rather than its stability.
In this work, catalysts with different Mo dispersions, that is, different contents of amorphous Mo-rich
surface layers or plate-like MoO with identical Mo/Fe mole ratios, were synthesized by controlling
3
the stirring speed during catalyst preparation. The stirring speed affected the dispersion of iron
molybdate particles in the catalysts. These factors jointly influenced the activity, selectivity and stability
of the catalysts.
Molecules 2020, 25, 2410; doi:10.3390/molecules25102410
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Most authors agree that the mechanism over Mo–Fe catalysts is a redox mechanism involving the
reduction of the active site of the catalyst during the reaction and reoxidation of the reduced catalyst,
that is, a Mars-van Krevelen mechanism [9,17]. In this work, the reoxidation of the reduced catalyst is
discussed based on a characterization of the behavior of the oxygen species, such as oxygen vacancy
and lattice oxygen, of fresh and used catalysts, to understand the connection between the structure
and catalytic performance.
2
. Experimental
2
.1. Materials
Ammonium heptamolybdate tetrahydrate (AHM), iron nitrate nonahydrate and methanol were
purchased from Sinopharm Chemical Reagent Ltd. Corporation, Shanghai, China. Nitric acid and
ammonia, with mass fractions of 65–68% and 25–28%, respectively, were obtained from Beijing
Chemical Works.
2
.2. Preparation of Catalysts
The catalysts were prepared by the coprecipitation method with different stirring speeds. The speed
−
1
−1
for Cat-SS was 30 r
·
min , and for Cat-FS was 600 r
·
min . AHM and iron nitrate were dissolved in
−
1
−1
deionized water at concentrations of 0.29 mol
ratio of 2.60. The ferric nitrate solution was added dropwise to the AHM solution over 30 min at
0 C. The pH value was tested by phs-3C pH meter, and maintained at pH = 2.0 by adding ammonia.
The precipitate was collected by suction filtration, washed, dried in an oven at 60 C for 12 h, and then
·
L
and 0.94 mol
·
L , respectively, and a Mo/Fe mole
◦
6
◦
◦
calcined in a muffle furnace at 500 C for 24 h.
2
.3. Catalytic Test and Characterization
The reaction was carried out in a microreactor (6 mm i.d.
into 100–300 mesh powder and 0.50 g was placed in the middle of the microreactor. The flow rate of air
×
70 cm). The calcined catalyst was ground
−
1
−1
min with methanol mole
was 106.6 mL
·
min , and methanol was pumped in at a flow rate of 0.011 mL
·
◦
◦
ratios of N : O = 1: 13.2: 12.9. An activity test was carried out between 250 C and 300 C. A stability
2
2
◦
◦
test was carried out at 300 C for 150 h. The product was sampled online at 120 C and analyzed using a
gas chromatograph (GC, 9790IIT-2, FULI, ZheJiang, China) equipped with a TCD detector and a packed
column (Poropak N, 3 mm × 5 m, Hichina Zhicheng Technology Ltd., Gansu, China).
In addition to the main product, i.e., formaldehyde, DME was also detected as a byproduct.
The relative contents of both products were determined by the normalization method. The conversion
of methanol and the selectivity of the products were calculated as:
Methanol conversion (%) = moles of converted methanol/moles of methanol feedstock × 100% (3)
Products selectivity (%) = moles of products/moles of converted methanol × 100%
The crystal structure was identified by Powder X-ray diffraction (XRD) using an X-ray powder
diffractometer (Bruker-AXS D8 Advance, Karlsruhe, Germany) with a Cu K radiation source with an
min . Ascanningelectronmicroscope(SEM, JEM 7401F, JEOL, Tokyo, Japan)
(4)
α
−1
operatingspeedof10degree
·
was used to characterize the morphology of the catalysts. A transmission electron microscope (TEM,
JEM-2010, JEOL, Tokyo, Japan) was used to examine the difference between the catalyst bulk and
interface. High-angle annular dark-field scanning TEM (HAADF-STEM) was performed using a JEOL
ARM200F microscope (JEOL, Tokyo, Japan) with a STEM aberration corrector operating at 100 kV.
Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the component in selected areas of
the catalysts. An inductively coupled plasma optical emission spectrometer (ICP-OES, Spectro Arcos
FHX22, Kleve, Germany) was used to determine the Mo/Fe mole ratio of the whole catalyst. The specific
surface area and pore size distribution of the catalysts were determined by N adsorption using a
2

Molecules 2020, 25, 2410
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Quadrasorb-S1 instrument (Quantachrome, Florida, USA) and the Brunauer-Emmett-Teller (BET) and the
Barrett-Joyner-Halenda (BJH) method, respectively. X-ray photoelectron spectroscopy (XPS) analysis was
performed on a Thermal Scientific ESCALAB 250Xi instrument using an Al Kα X-ray source.
The O -TPD of the catalysts was measured by O temperature programmed desorption (TPD,
2
2
◦
Quantachrome Instruments, Chembet PULSAR). The sample was flushed with He at 300 C for 30 min,
and then cooled to 30 C and kept under flowing 5% O /He for 30 min. Physically adsorbed O was
removed by flushing with He at 100 C for 30 min. Then, chemically adsorbed O on the catalyst was
measured by heating from 100 to 800 C at a rate of 10 C
were determined by temperature programmed reduction (TPR, Quantachrome Instruments, Chembet
PULSAR). The samples were flushed with He at 300 C for 30 min and then cooled to 30 C. 10% H /He
◦
2
2
◦
2
◦
◦
−1
min . The H -TPR profiles of the catalysts
2
·
◦
◦
2
was switched on and the flow rate through the reactor was controlled at 100 mL/min. The temperature
◦
−1
◦
was increased at a rate of 10 C
recorded by a PC.
·
min from 30 to 1000 C. The H consumption (TCD signal) was
2
3
. Results and Discussion
3
.1. Characterization of the Catalysts
◦ ◦ ◦ ◦ ◦ ◦
Figure 1 shows the XRD patterns of two catalysts, where 2θ = 20.4 , 21.7 , 22.9 , 24.9 , 26.6 , 30.2 ,
◦
◦
◦
◦
◦
◦
◦
4
0.9 and 49.5 belonged to the diffraction peak of Fe (MoO ) , and 2
θ
= 23.1 , 25.7 , 27.3 , 34.1 , 38.9
2
4 3
◦
and 55.1 were assigned to MoO . It was shown that the catalyst consisted of Fe (MoO ) and MoO .
3
2
4 3
3
Figure 1. X-ray diffraction patterns of Cat-SS and Cat-FS.
Figure 2a,b show SEM images of the two catalysts. It can be clearly seen that the catalysts
comprised particle-and plate-like structures. Cat-SS has more plate-like structures and showed an
obvious agglomeration of particles. However, Cat-FS has few plate-like structures and the particles
were dispersed uniformly.
Figure 2c,d display the EDS spectra of particles and plates marked in the SEM images of Figure 2a,b,
respectively. The Mo/Fe mole ratio of particles and plates are shown in Table 1. The mole ratio of
particles was between 1.6 and 1.8, while the mole ratio of the plates ranged from 4.8 to 5.5. The mole
ratio of the particles was lower than the plate-like structure. It was concluded that the particles and
plates could be assigned to Fe (MoO ) and MoO , respectively. Due to that the particles stuck to the
2
4 3
3
plates, the mole ratio of MoO was not theoretical infinite.
3

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Figure 2. SEM images of Cat-SS (
a) and Cat-FS (b), (c) EDS data of the particles and plates marked in
(
a), (d) EDS data of the particles and plates marked in (b).
Table 1. The Mo/Fe mole ratio and surface areas of catalysts.
Mo/Fe Mole Ratio-EDS
Mo/Fe Mole Ratio
2
−1
Surface Area/m ·g
Plate
Particles
ICP
XPS
Cat-SS
Cat-FS
5.5
4.8
1.6
1.8
2.50/2.50
2.50/2.51
2.49/2.51
2.54/2.53
15.6
18.8
Note: the ICP and XPS column have two Mo/Fe mole ratios. The left data refer to the fresh catalyst and the right
data to the used catalyst after the stability test in Figure 4.
The Mo/Fe mole ratio of the whole Cat-FS was identical with Cat-SS, which was accurately
analyzed by ICP-OES, as shown in Table 1. However, the EDS data, as shown in Table 1, illustrates
that the Mo/Fe mole ratio of the Fe (MoO ) particles of Cat-FS was higher than those of Cat-SS,
2
4 3
which implied that there was more Mo on these particles. The XPS results, as shown in Table 1, show
that the Mo/Fe mole ratio of Cat-FS was slightly higher than that of Cat-SS, which also implies that
more Mo enriched on the surface layer of particles in Cat-FS than in Cat-SS, because the XPS method
focused more on the surface layer.
Figure 3a shows the STEM images of particles with two catalysts. A 3–4-nm-thick amorphous
surface layer was observed on the bulk Fe (MoO ) crystallites on Cat-SS. Cat-FS has a thicker
2
4 3
amorphous layer than Cat-SS. Figure 3b,c show a STEM-EDS line scan, giving the amount of Mo,
Fe and O in the area across the particle marked in the HAADF-STEM image of Cat-SS and Cat-FS,
respectively. It is shown that there was no gradient in the Fe or Mo content on the bulk Fe (MoO ) ,
2
4 3
but that the amount of Fe decreased more than that of Mo on the surface layer. This illustrates that the
amorphous layer was enriched in Mo and depleted in Fe compared with the bulk composition [14],
that is, there was a thicker Mo-enriched surface layer on the particles of Cat-FS. The strengthened
mixing of the solution at a fast stirring speed caused the excess Mo species to be better dispersed on
Cat-FS than on Cat-SS, and increased the thickness of the amorphous layer on Cat-FS. More uniformly
dispersed particles and more polyporous amorphous layers in the Cat-FS may have increased the
surface area of the catalyst, as shown in Table 1.

Molecules 2020, 25, 2410
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Figure 3.
line scan of particles in Cat-FS (
(
a
) STEM images of particles with Cat-SS and Cat-FS, HAADF-STEM image and STEM-EDS
) and Cat-SS ( ) showing the Mo, Fe and O distribution along the
b
c
arrow marked in the HAADF-STEM image.
3
.2. Activity and Stability of the Catalyst
Figure 4a shows the activity of catalysts synthesized at different stirring speeds. When the
methanol conversion reached 90%, the reaction temperature increased along with the stirring speed,
which implies that the activity of the catalyst increases as the stirring speed increases. Figure 4b shows
the selectivity of Cat-FS and Cat-SS. The Cat-FS had higher formaldehyde selectivity than both Cat-SS
and the catalyst used in industry. Improving the stirring speed strengthened the mixing of the solution
and increased the dispersion of particles in the catalyst. The fast stirring speed used in the catalyst
preparation resulted in a thicker Mo-enriched amorphous surface layer and higher specific surface
area, which may exhibit favorable activity and selectivity.
◦
The stability of the catalysts was also tested at 300 C for 150 h, as shown in Figure 4c,d. With Cat-FS,
the methanol conversion and formaldehyde selectivity remained unchanged for 150 h. However,
although methanol conversion of Cat-SS was unchanged, the selectivity of the formaldehyde decreased
from 96.7% to 96.2%, and that of DME increased from 2.3% to 2.8%. These results illustrated that
Cat-SS had not only poor activity and selectivity, but also poor stability.

Molecules 2020, 25, 2410
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Figure 4.
(a) Activity and (b) selectivity of catalysts. Stability of catalysts: (c) conversion and (d) selectivity
of the catalyst.
3
.3. Analysis of Fresh and Used Catalysts
Figure 5a shows the XRD patterns of catalysts after stability testing; the XRD patterns are almost
identical with those of fresh catalysts. All the diffraction peaks were assigned to Fe (MoO ) and MoO ,
2
4 3
3
implying that the structure of two catalysts had not changed. Figure 5a illustrates the Raman spectra
−
1
of fresh and used catalysts, where the lines at 968, 936, 783, 352 and 337 cm belong to Fe (MoO ) ,
2
4 3
−1
and those a of 996, 820, 669, 337 and 289 cm to MoO . The used catalyst was similar to the fresh
3
catalyst, implying that no new phase composition was generated after stability testing. Moreover,
the Mo/Fe mole ratio of the used catalysts was not significantly lower than that of the fresh catalysts
analyzed by ICP and XPS, as shown in Table 1. These results exclude the effect of changes in the Mo/Fe
mole ratio and the structure of catalysts on the DEM selectivity of Cat-SS in the stability testing.
Figure 5. (a) X-ray diffraction patterns of Cat-SS and Cat-FS; (b) Raman spectra of fresh and used catalysts.

Molecules 2020, 25, 2410
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Figure 6a shows the XPS survey of Cat-FS and Cat-SS, which implied that both catalysts were
composed of three elements: Mo, Fe and O. Figure 6b,c show the Fe2p and Mo3d XPS results of fresh
and used catalysts, respectively. The XPS spectra exhibited similar peaks, showing that Mo and Fe
remained unchanged after the stability test lasting 150 h. Figure 6e shows the O1s XPS results of
the fresh and used Cat-SS. The peaks at 530.2 and 530.9 eV were attributed to oxygen ions in the
crystal lattice (O2 ). The second peak at 532.0 eV was assigned to adsorbed oxygen species (O /O2 ).
−
−
2−
The third peak with the highest binding energy at 533.2 eV was assigned to hydroxyl and carbonate
−
2−
species (OH /CO3 ) [18]. The appearance of the second and third peaks suggested that the reaction
was consuming the oxygen in the Cat-SS during the long-lasting testing, which generated oxygen
vacancies [19] due to slow replenishment by gas phase O in air. However, the XPS profile of the used
2
Cat-FS was quite the same as that of fresh Cat-FS, as shown in Figure 6d, suggesting that consumed
lattice oxygen on Cat-FS was replenished by gas phase O over time. Determining the behavior of
2
O species in a long-duration test is important to understanding the mechanism of the reaction that
resulted in poorer formaldehyde selectivity.
Figure 6. XPS profiles of fresh and used catalysts: (
e) O1s of Cat-SS.
a) Survey, (b) Fe2p, (c) Mo3d, (d) O1s of Cat-FS and
(
Figure 7 shows the O -TPD results of fresh and used catalysts; the three peaks were attributed to
2
weak chemisorbed oxygen species (O1), strong chemisorbed oxygen species (O2) and lattice oxygen
species (O3) [20–22]. With the used catalysts, the intensity of the O1 peak of Cat-FS was higher than that
of Cat-SS, indicating that it had more adsorbed oxygen species. The intensity of the O1 and O3 peaks
of Cat-FS were little changed after the long-duration test, but, in contrast, these nearly disappeared
for Cat-SS after the long-duration test; this was accompanied by strong intensity of the O2 peak.
The strongly chemisorbed oxygen species of the O2 peak was associated with oxygen vacancies [23
which are harmful to the selectivity to formaldehyde.
,24],
Figure 8 shows the H -TPR results of the fresh and used catalysts. MoO and Fe (MoO ) were
2
3
2
4 3
◦
reduced to MoO and FeMoO , respectively, in the temperature range from 550 to 750 C, and the
2
4
◦
reduced species were further reduced when the temperature exceeded 750 C [25]. The reduction
profile of fresh Cat-FS was similar to that of fresh Cat-SS. However, the reduction temperature of the
used Cat-FS was slightly lower than that of the fresh catalyst, indicating that a small amount of active
oxygen species was produced during the long-duration reaction. It was obvious that the reduction

Molecules 2020, 25, 2410
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temperature of the used Cat-SS was much lower than the fresh catalyst, and a small peak appeared at
◦
6
30 C, which demonstrated that many more active oxygen species were generated during the reaction.
Figure 7. O -TPD results of fresh and used Cat-FS and Cat-SS.
2
Cat-FS-Fresh
Cat-FS-Used
Cat-SS-Fresh
Cat-SS-Used
5
00
600
700
800
900
1000
Temperature (℃)
Figure 8. H -TPR results of fresh and used Cat-FS and Cat-SS.
2
Figure 9 shows the suggested mechanism of the selective oxidation of methanol to formaldehyde
and DME over a Mo–Fe catalyst, denoted as species (I). This reaction mechanism is a Mars-van
Krevelen mechanism, which can be broken down into three steps [26]: (1) the dissociative adsorption
of methanol, (2) the oxidation and desorption of products, leaving an oxygen vacancy on the surface of
the catalyst, and (3) the replenishment of the oxygen vacancy.
The initial step is the dissociative chemical adsorption of methanol to form surface methoxy (II) [27].
The methoxy intermediate can react via either an oxidative or nonoxidative channel. The nonoxidative
channel involves the reaction with physically adsorbed methanol molecules to produce DME and
a
the (III) intermediate [28]. The original structure (I) is restored by the removal of water from the
(III)a intermediate.

Molecules 2020, 25, 2410
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Figure 9. Suggested mechanism of the selective oxidation of methanol to formaldehyde over Mo–Fe catalysts.
The formation of formaldehyde occurs due to the oxidative channel of the second step. Hydrogen
is abstracted from adsorbed methoxy by a terminal oxygen in Mo=O to produce a transient adsorbed
CH =O intermediate that desorbs from the active site, which forms an intermediate with two
2
(a)
hydroxyl (III)b [29]. After the removal of the water, there is an oxygen vacancy with two electrons on
the surface (IV).
The catalytic reaction needs the participation of lattice oxygen, which leads to the formation of
an oxygen vacancy, and the active center on the surface is left in a reduced state. Before it can be
used again, the active center must be reoxidized. The reoxidation can be carried out either by the
incorporation of oxygen from the gas phase (Pathway I) or the diffusion of lattice oxygen ions from the
bulk and the electrons from the surface to the bulk (Pathway II) [22,30]. If the active center cannot be
regenerated in time, it will remain in a reduced state. In this state, it will produce DME as a byproduct
through the nonoxidative channel rather than formaldehyde through the oxidative channel, which
needs to abstract the hydrogen in the methoxy by a terminal oxygen. This is because the methoxy on
an oxidized site has a relatively stronger C–H bond than that on a reduced site [29].
The effect of the Mo-enriched layer on the catalytic properties is unknown and will be studied in
the future, but we can explain the increase in DME selectivity during the lifetime test with Cat-SS now.
It probably adsorbed more oxygen from the gas phase when the Mo species formed a Mo-enriched
surface layer rather than forming plate-like MoO , which led to a faster rate of O transfer from air
3
2
into lattice oxygen with Cat-FS. However, it is inferred that less O transfer from air into lattice oxygen
2
caused the reduced active center of Cat-SS, generating more nonoxidative channel DME during the
long-duration reaction. It is necessary for the oxygen vacancy to be reoxidized in time for the activity
and selectivity to remain constant in the long term. Raun [31] also found that the Mo structure could
influence the stability of Mo–Fe catalysts. A lower rate of catalyst deactivation was observed for the
large h-MoO compared with α-MoO .
3
3
In summary, the Cat-SS agglomerated particles with a thinner Mo-enriched surface layer and
more plate-like MoO than Cat-FS, which also decreased the surface area of the Cat-SS. These factors
3
reduced the activity and selectivity, as well as the stability of the catalyst. A ow Cat-SS surface area
slowed down the adsorption of gas phase oxygen, which caused Cat-SS to consume lattice oxygen and
produced oxygen vacancies during the oxidation-reduction cycle. Therefore, in order to enhance the
activity and stability of the catalyst, the stirring speed during the preparation should be maximal to

Molecules 2020, 25, 2410
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avoid forming aggregated particle structures and increase the dispersion of Mo species to enable the
formation of a thick Mo-enriched surface layer, rather than MoO plates.
3
4
. Conclusions
Mo–Fe catalysts were prepared by the coprecipitation method with a slow or fast stirring speed,
and their catalytic performance were studied. The catalyst consisted of particle-like Fe (MoO ) and
2
4 3
plate-like MoO . Fast stirring during catalyst preparation enhanced the dispersion of the particles
3
and formed a thicker Mo-enriched surface layer on the particles. However, the catalyst prepared
at a slow stirring speed aggregated severely and had a thinner Mo-enriched surface layer on the
particles, which decreased the surface area and led to poor activity, selectivity and stability. This resulted
in slower reoxidation of the reduced active sites and formed oxygen vacancies during the reaction,
which promoted the formation of DME by the nonoxidative channel as a byproduct.
Author Contributions: M.H. and S.Z. conceived and designed the experiments; S.Z. performed the experiments
and wrote the original draft. M.H. and S.Z. analyzed the data and revised the manuscript together. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: The authors are grateful for the discussion of the experimental results with our group members.
Conflicts of Interest: There are no conflict to declare.
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Sample Availability: Samples of the compounds Cat-SS and Cat-FS are available from the authors.
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