Study on the selective oxidation of methane over highly dispersed molybdenum-incorporated KIT-6 catalysts

Article abstract of DOI:10.1039/d1cy00311a

A series of molybdenum-incorporated mesoporous silica (Mo-KIT-6) catalysts were successfully synthesized by a one-pot hydrothermal synthesis method, and were applied in the selective oxidation of methane to formaldehyde using oxygen as an oxidizing agent under atmospheric pressure. Comparatively, the corresponding supported catalysts (Mo/KIT-6) were prepared by incipient-wetness-impregnation method. The results of the small angle XRD, nitrogen adsorption/desorption isotherms, UV-vis, H₂-TPR and UV-Raman spectroscopy characterization combined with the catalytic activity tests demonstrated that molybdenum atoms were inserted into the framework of the mesoporous materials for the Mo-KIT-6 catalysts and the highly dispersed MoO bonds dominantly existed, which were responsible for the efficient selective formation of formaldehyde. However, for Mo/KIT-6 catalysts, the molybdenum oxide species were mainly loaded on the surface or inside the outer pore channels of the support and abundant emergence of the Mo-O-Mo bond played a major role in the activation of methane to CO_x. Furthermore, with equivalent molybdenum content, the methane selective oxidation performance of 8Mo-KIT-6 was obviously better than that of 4.6Mo/KIT-6, and the formaldehyde yield (2.1%) of 8Mo-KIT-6 was 2.3 times as much as that (0.9%) of 4.6Mo/KIT-6.In situandoperandoUV-Raman results demonstrated that the structures of the MoO_xactive sites have a strong effect on the formation and elimination of carbon deposition during the separated redox reaction with methane and O₂, respectively. The polymerized MoO_xactive sites are favorable for the formation of graphitic carbon (G), which is called ordered carbon, while the isolated MoO_xactive sites are favorable for the formation of disordered carbon (D). The reduced highly dispersed MoO_xactive sites incorporated in the framework of silica are more easily reoxidized than those on the supported catalysts.

Full text of DOI:10.1039/d1cy00311a

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Study on the selective oxidation of methane over
highly dispersed molybdenum-incorporated KIT-6
catalysts†
Cite this: DOI: 10.1039/d1cy00311a
Pei Chen, Zean Xie, Zhen Zhao, *ab Jianmei Li, Bonan Liu, Baijun Liu, *a
a
b
a
a
b
b
b
Xiaoqiang Fan, Lian Kong and Xia Xiao
A
series of molybdenum-incorporated mesoporous silica (Mo–KIT-6) catalysts were successfully
synthesized by a one-pot hydrothermal synthesis method, and were applied in the selective oxidation of
methane to formaldehyde using oxygen as an oxidizing agent under atmospheric pressure. Comparatively,
the corresponding supported catalysts (Mo/KIT-6) were prepared by incipient-wetness-impregnation
2
method. The results of the small angle XRD, nitrogen adsorption/desorption isotherms, UV-vis, H -TPR and
UV-Raman spectroscopy characterization combined with the catalytic activity tests demonstrated that
molybdenum atoms were inserted into the framework of the mesoporous materials for the Mo–KIT-6
catalysts and the highly dispersed MoO bonds dominantly existed, which were responsible for the
efficient selective formation of formaldehyde. However, for Mo/KIT-6 catalysts, the molybdenum oxide
species were mainly loaded on the surface or inside the outer pore channels of the support and abundant
x
emergence of the Mo–O–Mo bond played a major role in the activation of methane to CO . Furthermore,
with equivalent molybdenum content, the methane selective oxidation performance of 8Mo–KIT-6 was
obviously better than that of 4.6Mo/KIT-6, and the formaldehyde yield (2.1%) of 8Mo–KIT-6 was 2.3 times
as much as that (0.9%) of 4.6Mo/KIT-6. In situ and operando UV-Raman results demonstrated that the
x
structures of the MoO active sites have a strong effect on the formation and elimination of carbon
Received 19th February 2021,
Accepted 27th April 2021
deposition during the separated redox reaction with methane and O , respectively. The polymerized MoOx
2
active sites are favorable for the formation of graphitic carbon (G), which is called ordered carbon, while
the isolated MoO
dispersed MoO active sites incorporated in the framework of silica are more easily reoxidized than those
on the supported catalysts.
x
active sites are favorable for the formation of disordered carbon (D). The reduced highly
DOI: 10.1039/d1cy00311a
x
rsc.li/catalysis
1
2
13
dry reforming and partial oxidation of methane. These
routes are used to convert methane into the intermediate
1
. Introduction
Methane is the main component of shale gas, combustible
ice, natural gas and bio-gas with its wide source and rich
product of syngas (including H and CO), and then the syngas
2
was converted into the required chemical raw materials,
mostly methanol. These processes usually take place at high
temperature and high pressure, which requires a large energy
supply, as well as facility investment. Therefore, it is
economical to convert methane into chemicals with high
value added in one step. The selective oxidation of methane
to oxygenates is one approach of the methane direct
1
–5
reserves,
the efficient utilization of methane.
a tetrahedral geometry molecule with four identical C–H
and more and more attention has been paid to
6–10
However, methane as
−
1
bonds has the largest C–H bond energy (439 kJ mol ) of all
3
sp hybrid hydrocarbons, and is known as the least active
alkane. The utilization of methane can be divided into
indirect and direct conversion. The indirect conversion routes
conversion. The target products of the reaction are usually
1
1
14–21
include methane steam reforming, methane carbon dioxide
methanol or formaldehyde,
which are easy to be over-
oxidized under the harsh reaction conditions for the
activation of methane. Therefore, this reaction has always
been called the ‘Holy Grail’ in the catalysis realm.
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum,
5
Chang Ping, Beijing 102249, China. E-mail: zhenzhao@cup.edu.cn,
zhaozhen1586@.163.com, bjliu@cup.edu.cn
The inert SiO -supported MoOx catalysts with relatively
b
2
Institute of Catalysis for Energy and Environment, Shenyang Normal University,
high selectivity of oxygenates have been investigated
Shenyang 110034, China
2
2–27
extensively in the selective oxidation of methane,
unlike
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00311a
other metallic oxide supports, such as TiO , MgO and
2
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Al O , which can easily oxidize oxygenate products to CO
x
2. Experimental section
2
3
2
8,29
24
and H O.
Ozkan et al. investigated the surface MoOx
2
2.1 Catalyst synthesis
species on the MoO /SiO catalysts. The highly dispersive
3
2
The molybdenum-incorporated KIT-6 catalysts were prepared
by one-pot hydrothermal synthesis method using tetraethyl
orthosilicate (TEOS) and a triblock copolymer P123 (EO PO -
EO20, MW = 5800, Aldrich) as the silicon source and template,
respectively. The specific procedure was as follows: First, 19 g 2
M hydrochloric acid was mixed with 2 g P123 and stirred slowly
for at least 4 h until the template dissolved thoroughly in the
acid medium. Second, a certain amount of ammonium
heptamolybdate (AHM, the quantity of AHM was determined
by the quantity of the silicon source, the molar ratio Mo : Si = x :
silicomolybdic species with terminal MoO bond gradually
transformed to the polymolybdate species with the Mo–O–
Mo bond, which resulted in the non-selective oxidation of
2
0
70
3
0
methane. Valverde et al. prepared the Mo/HZSM-5 catalyst
through the impregnation and washing process for
removing the physically adsorbed polymolybdate species,
the prevailing existence of the well-dispersed tetrahedral
monomeric molybdenum species led to higher HCHO
2
6
selectivity. Aoki et al. found that a silicomolybdic acid-like
structure formed on the surface of the sol/gel catalyst was
prone to suppress the successive oxidation of methanol. Bao
100, x = 0.1, 1, 3, 5, 8, 10, respectively) was dissolved in 57 g
31
deionized water, and then the dissolved salt solution was
poured into the template solution. After stirring slowly for 2 h,
x
et al. investigated the MoO species on the Mo/HMCM-22
catalysts before and after the selective oxidation of the
methane reaction and found that the tetrahedral
molybdenum species chemically bonding to HMCM-22 was
kept unchanged, while the polymerized octahedral
molybdenum species were easy to leach out during the
reaction at high temperature. Hence, the highly dispersed
MoO species chemically bonding to SiO took advantage of
2.5 g n-butyl alcohol was dripped into the mixture and kept
stirring vigorously for 1 h. Finally, 6.4 g TEOS was added
dropwise into the above solution. After stirring for 24 h, the
precursor solution was transferred to a Teflon-lined autoclave
and aged for another 24 h at 100 °C. After filtration with
deionized water, drying at 100 °C for 6 h and calcination in air
at 550 °C for 6 h, a series of molybdenum-incorporated SiO2
catalysts were synthesized and denoted as xMo–KIT-6.
For comparison, KIT-6-supported molybdenum catalysts
were prepared by ultrasound-assisted incipient-wetness-
impregnation method. The parent KIT-6 was synthesized
following the above-mentioned steps, except for the addition
of molybdenum salt. Typically, a certain amount of AHM was
dissolved in 2.6 g of deionized water. The completely
dissolved AHM solution was added to 2.0 g KIT-6 support
drop by drop, and the solution was adequately mixed with
the parent using a mortar for 20 min. Then, the impregnated
samples were treated under ultrasound for 30 min and
vacuum dried under conditions of −0.09 MPa and 50 °C for 4
h. At last, the sample was dried thoroughly at 100 °C for 6 h,
and calcined at 550 °C in air for 4 h. The catalysts
synthesized with this method were denoted as xMo/KIT-6
x
2
the high HCHO selectivity and stability.
However, inspired by the initial synthesis of the ordered
3
2,33
mesopore materials M41S,
some researchers began to
replace the amorphous silica with the ordered mesopore
3
4–36
37
silica as support.
Wang et al.
compared the
performance of the SBA-15, MCM-41 and Cab-O-Sil-supported
molybdenum oxide catalysts for methane selective oxidation,
and the research results revealed that the nano-sized
molybdenum oxide clusters inside the mesoporous channels
of SBA-15, as well as its larger pore diameter took effect in
the enhancement of the HCHO selectivity. Therefore, the
silica with ordered mesopore is recognized as the most ideal
support in the literature for the thermal catalytic
2
9
heterogeneous reaction of methane-selective oxidation. The
inert surface of silica is beneficial to the desorption of the
target product, and its high specific surface area contributes
to the high dispersion of the active sites, which is critical to
avoiding the over-oxidation of methane.
(
the molar ratio of Mo : Si = x : 100 in the recipe).
2.2 General catalyst characterization
In this work, the mesoporous silica material KIT-6 with
high hydrothermal stability, high specific surface area and
symmetrical structure of three-dimensional interconnected
Nitrogen adsorption/desorption isotherm measurements were
carried out using a Micromeritics TriStar II 3020 porosimetry
analyzer. The samples were first dried at 120 °C in air for 30 min
and then outgassed in vacuum at 350 °C for 4 h before the
experiments. Transmission electron microscopy (TEM) images of
the catalysts were taken on a JEM 2100 transmission electron
microscope equipped with a field emission source at an
accelerating voltage of 200 kV. The samples were evenly dispersed
in ethanol after ultrasound treatment, and several drops of the
suspension were enough to apply on the carbon films. The SEM-
EDS mapping measurements were obtained on SU8100. X-ray
diffraction (XRD) experiments were performed on a Rigaku UItima
IV diffraction system with Cu Kα radiation (40 kV, 40 mA). Small-
angle X-ray diffraction (SAXRD) patterns were collected in the 2θ
range from 0.5° to 5°, and wide-angle X-ray diffraction (WAXRD)
3
8
channels was adopted as the support. Meanwhile, a series
of the incorporated molybdenum atom bonded to the
framework of the silica catalysts were prepared through one-
pot hydrothermal synthesis method, and they exhibited
significantly better selective oxidation performance than
those of the corresponding impregnated catalysts with the
3
8–40
same amount of molybdenum.
The purposes of this work
are to investigate the catalytic performance of the
molybdenum-incorporated KIT-6 catalysts in the methane-
selective oxidation with oxygen under atmospheric pressure,
and focus on the relationship between the structure of the
active species and the methane activation ability, as well as
the selectivity of formaldehyde.
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patterns were collected in the 2θ range from 10° to 80°. The exact
molybdenum content in each sample was measured by
inductively coupled plasma mass spectrometry (ICP-MS) on
Agilent 7900 ICP-MS after the sample underwent microwave-
assisted digestion in the mixture of aqua regia and hydrofluoric
acid. The ultraviolet-visible (UV-vis) diffuse reflectance
spectroscopy (DRS) measurements were conducted on a Hitachi
UH4150 spectrophotometer equipped with an accessory of the
integration sphere diffuse reflectance. Barium sulfate was selected
as a reference in the tests. Fourier transform infrared spectra (FT-
IR) were measured on a TENSOR II spectrometer. The spectra
to 550 °C again under a gas flow of 16.7% O /N for 30 min
2
2
pre-treatment to recover its active sites. The oxidation process
of the sample was carried out after the above reduction
process at 600 °C. The sample was cooled to 200 °C under
pure N , and was then exposed to 20% O /N flow for the
2
2
2
amount of time required.
The total flow of the processing gas remained constant at 30
−1
ml min , and the Ramen spectra were recorded at every step.
2.4 Catalytic activity measurements
−1
were collected in a wavenumber range of 400–1300 cm with a
The catalytic activity tests were undertaken in a fixed-bed
quartz reactor (inner diameter: 7 mm) at atmospheric
pressure in the temperature range from 600 to 750 °C. First,
the catalyst needed to be crushed (10 MPa) and sieved (40–60
mesh). Then, 0.1 g calcined catalyst was pretreated at 600 °C
−1
resolution of 4 cm . H temperature-programmed reduction (H -
2
2
TPR) measurements of the samples were carried out on a
Micromeritics Auto Chem II 2920. Typically, the catalyst of 0.1 g
was first pretreated with O flow at 550 °C for 1 h in order to
2
−
1
remove the adsorbed impurities. The gas was then switched to Ar
for 30 min with N and O flow (20 ml min , respectively).
2 2
flow and the temperature dropped to 80 °C. After purging Ar for 1
The reaction mixture gas included CH
4 2
(33.3 vol%), O (16.7
−1
h, the gas was switched to 10% H /Ar flow (40 ml min ) and the
vol%) and the balance gas was N . The total flow rate was 60
2
2
−
1
temperature increased from 80 to 850 °C at the heating rate of 10
ml min . The reactants and products were online analyzed
by an Agilent 7890B gas chromatograph equipped with one
flame ionization detector (FID) and two thermal conductivity
−1
°
C min . To prevent water vapor from entering the TCD detector,
a cold trap was equipped during the record processes.
All the samples were characterized after calcination.
detectors (TCD). CH
4
, C
2
H
6
, C
2
H
4
, CO, CO
2
, O
2
and N
2
were
and
separated by Hayesep Q and Molsieve 5A columns, C
2
H
6
C H were detected by the front FID, and CH , CO, CO , O ,
2
4
4
2
2
2
.3 In situ UV Raman
Raman spectra were measured on a LabRAM HR Evolution
Raman spectrometer (HORIBA) with 325 nm laser
N
2
were detected by the back TCD. HCHO, H
2
O and CH
3
OH
were separated by HP PLOT U column and detected by side
TCD. All pipelines and valves between the bottom of the
furnace and the inlet of GC were heated to 120 °C to keep
the products from condensation. The schematic of the
methane-selective oxidation reaction is shown in Fig. S1.†
The methane conversion, product selectivity and yield
were calculated based on the mole flow rates of the methane
feed, products (considering the number of carbon atoms in
each product) and methane in the effluent. The calculation
formulas were expressed as follows.
a
excitation. For ex situ UV Raman test, the catalyst powder was
dehydrated at 200 °C and cooled to room temperature under
sealed conditions. For in situ UV Raman, the catalyst (ca. 20
mg) was placed inside an in situ cell reactor, which was
equipped with
adjustment of the reaction conditions.
The specific steps for in situ UV Raman of the elimination
process of carbon deposition under reaction flow were as
follows. The sample of the catalyst powder, which was loaded
in the in situ cell reactor, was first heated to 675 °C at a rate of
a quartz window and allowed for the
nðCH4Þin − nðCH Þ
4
out
−
1
xCH4 ð%Þ ¼
× 100
2
0 °C min and held for 30 min under the gas flow of 16.7%
nðCH Þ
4
in
O /N , then the gas flow was switched to pure N flow. After
2
2
2
M × nðproductÞ
nðHCHOÞ þ nðCOÞ þ nðCO2Þ þ nðCH3OHÞ þ 2 × nðC2H4Þ þ 2 × nðC2H6Þ
Sproduct ð%Þ ¼
× 100
3
0 min treatment, the sample was exposed to 33.3% CH /N
4 2
xCH × SHCHO
4
YHCHO ð%Þ ¼
flow for 10 min. Finally, the gas was switched to 33.3% CH
6.7% O /N flow for the amount of time required.
The specific steps for in situ UV Raman of the reduction
4
/
100
1
2
2
À
À
Á
FðCH Þ × 0:001 × M × xCH4
−
1
4
Mo
TOFCH4
s
¼
¼
and oxidation processes of the active sites on the 8Mo–KIT-6
catalysts were as follows. The catalyst powder of 8Mo–KIT-6
loaded in the cell reactor was heated to 550 °C at a rate of 20
6
0 × 22:4 × mcat × wMo × 100
Á
FðCH Þ × 0:001 × M × YHCHO
−
1
4
Mo
−
1
TOFHCHO
s
2 2
°C min under gas flow of 16.7% O /N for 30 min pre-
6
0 × 22:4 × mcat × wMo × 100
treatment, and then the temperature of the sample was
adjusted to that required under pure N2 flow. Finally, the
when the product is C H or C H , M = 2, otherwise M = 1.
2
4
2
6
sample was exposed to 5% CH
4
/N
2
flow for 30 min for the
Where xCH4 is the conversion of methane (%), n(CH ) and
4 in
reduction treatment. Before the exposure to the reduction
atmosphere at the next temperature, the catalyst was heated
n(CH4)out is the mole flow rate of methane in the feed and
effluent (mol/s), respectively. Sproduct is the selectivity of the
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product (%). n(product) is the mole flow rate of the product
(mol/s). YHCHO is the yield of formaldehyde (%). TOFCH4
represents the methane conversion rate per Mo site per second
−
1
(
s ), TOF
represents the formaldehyde formation rate per
HCHO
−1
Mo site per second (s ), and the reaction systems were under
chemical kinetic control (Fig. S2†). F(CH ) is the total volume
flow rate of methane at the inlet of the reactor (20 ml min ),
4
−1
MMo is the relative atomic mass of molybdenum (95.94 g
−1
mol ), mcat is the weight of the catalyst (0.1 g), wMo is the mass
fraction of molybdenum in the catalysts calculated by ICP-MS.
3
. Results
3
.1 The effect of Mo content on the mesoporous structure
The N2 adsorption/desorption isotherms and pore-size
distributions of the xMo–KIT-6 and xMo/KIT-6 catalysts are
displayed in Fig. 1. The isotherms of the xMo–KIT-6 (x < 5)
4
1
catalysts are all typical type IV since a rapid capillary
condensation occurred in the high-pressure zone, and their
4
2
hysteresis loops belong to type H1, indicating a large
number of long channels with narrow pore diameter in these
samples. For the xMo–KIT-6 (x > 3) catalysts, the hysteresis
loop converted to type H2 and became ambiguous with the
increase of Mo. The same phenomenon can be found for the
xMo/KIT-6 catalysts. It demonstrated that too much Mo leads
to the change of the mesoporous structure, which was further
illustrated in their TEM images (Fig. 2). The mesoporous
structure gradually collapsed with the increase of the
molybdenum content.
The SAXRD patterns of the Mo–KIT-6 and Mo/KIT-6
catalysts are displayed in Fig. 3. The well-resolved intense
peak and a shoulder peak occurring in the region of 0.5–1.3
degree are indicative of the (211) and (220) plane diffraction
peaks, which is attributed to the highly ordered bicontinuous
three-dimensional cubic symmetric structure (Ia3d) of KIT-
4
3
6
.
As shown in Fig. 3(a), with the increase of the
molybdenum content, the diffraction peaks of Mo–KIT-6
shifted to a small angle. At the same time, the unit-cell
parameter a (listed in Table S1†) increased with the rise of
0
the molybdenum content. They were all attributed to the
incorporation of the Mo atom with larger atomic radius than
that of Si in the framework of KIT-6, resulting in expanding
the size of the pore. It was consistent with the result of the
pore-size distributions (Fig. 1(c)). Furthermore, the peak
intensity gradually decreased, indicating that the ordered
three-dimensional cubic structure became destroyed with the
4
4,45
increase of the molybdenum dopant.
In contrast, for the
KIT-6-supported Mo catalysts, when the Mo content was 0.1,
the diffraction peaks of the 0.1Mo/KIT-6 catalyst remained
almost unchanged. When the Mo content was greater than
0
.1, the diffraction peaks shift to the high angle direction,
which demonstrated that the pore sizes become smaller (as
shown in Fig. 1(d)). This is because the molybdenum oxide
species is mainly loaded on the surface or inside the pore
channels of KIT-6 without destroying the basal mesoporous
Fig.
1
The
N
2
adsorption/desorption isotherms and pore-size
distributions of the Mo–KIT-6 (a and c) and Mo/KIT-6 (b and d) catalysts.
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Fig. 3 The SAXRD patterns of the Mo–KIT-6 (a) and Mo/KIT-6 (b)
catalysts with different molybdenum contents.
were highly dispersed on both systems of catalysts. At the same
time, the SEM-EDS mapping (Fig. S3†) of the 8Mo–KIT-6 and
Fig. 2 The TEM images of 1Mo–KIT-6 (a), 0.1Mo/KIT-6 (b), 8Mo–KIT-6
(
c), 4.6Mo/KIT-6 (d), 10Mo–KIT-6 (e) and 9.3Mo/KIT-6 (f).
x
4.6Mo/KIT-6 catalysts also illustrate that the MoO species were
highly dispersed on the nanoscale.
The UV-vis DRS is an effective characterization method for
analyzing the local coordination structure of the transition
metal-incorporated zeolites. Fig. 5 shows the UV-vis DRS of
the Mo–KIT-6 and Mo/KIT-6 catalysts with different
molybdenum contents. The adsorption band from 200 to 400
structure, but plugs the partial pores and results in the
decrease in the SBET (Table S1†).
Table S1† displays the specific physical parameters of the
catalysts. For the Mo–KIT-6 catalysts, when the molybdenum
content was low, the specific surface area, pore volume and
pore diameter of the catalysts were generally higher than
those of the pure KIT-6 support. For the Mo/KIT-6 catalysts,
the specific surface area, pore volume and pore diameter
were all lower than those of KIT-6. In addition, the unit-cell
parameter was almost unchanged as the molybdenum
loading increased.
2
−
nm is due to the charge transfer from ligand (O ) to metal
3
.2 The effect of the Mo content on the structure of the
MoO species
x
Fig. 4 shows the WAXRD patterns of the doped and supported
catalysts with higher molybdenum content. There was only one
broad diffraction peak appearing at about 23°, which can be
assigned to the amorphous silica. No diffraction peak of the
MoO crystal phase emerged on the Mo–KIT-6 and Mo/KIT-6
3
catalysts, which indicated that the molybdenum oxide species
Fig. 4 The WAXRD patterns of the part-doped and supported catalysts.
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6
+ 46
(
Mo ). Generally, absorption bands between 250 and 280
nm have been assigned to Mo(Td), and bands between 300
4
7
and 330 nm are assigned to Mo(Oh). In Fig. 5(a), the band
at around 225 and 276 nm belong to the isolated tetrahedral
6
+
Mo , while the band at 320 belongs to the polymeric
6
+
octahedral Mo . With the increase of the molybdenum
content, the species of the octahedral molybdenum oxides in
the catalysts increased gradually, but the isolated tetrahedral
molybdenum oxides still dominated in the Mo–KIT-6
catalysts. In Fig. 5(b), the Mo/KIT-6 samples give three bands
at around 258, 327 and 341 nm. With the increase of the
molybdenum content, the latter two bands turned
predominant, indicating the abundant accumulation of the
polymeric molybdenum oxide species.
−
1
As shown in Fig. 6, the FT-IR absorption peaks at 446 cm
−1
and 800 cm are attributed to the ring structure of the Si–O
bonds, and a peak at 963 cm is assigned to the Si–OH groups
−1
4
8–50
or Si–O–M vibration.
For the incorporated catalysts
(
Fig. 6(a)), the support KIT-6 showed the strongest peak at 963
−1
cm . The introduction of molybdenum largely weakened the
peak intensity of the silanol group, and with the increase of the
molybdenum content, a slight blue shift (6 cm ) occurred over
these catalysts, which indicated that the molybdenum atoms
interacted with the hydroxyl groups on the surface of the
support or were incorporated into the framework of KIT-6 to
−1
Fig. 6 The FT-IR profiles of the Mo–KIT-6 (a) and Mo/KIT-6 (b)
catalysts with different molybdenum contents.
form the di-oxo molybdate ((O) Mo(O–Si) ) and mono-oxo
2
2
49
molybdate (OMo(O–Si)
4
)
species.
For the supported
catalysts (Fig. 6(b)), the hydroxyl peak of KIT-6 was also
significantly higher than those of other catalysts. At the same
time, a slighter blue shift (compared with the doped catalysts)
−1
occurred at 963 cm , which also demonstrated the formation
of Si–O–Mo bonds at the cost of protonic sites corresponding
to the Si–OH groups of KIT-6.
2
Fig. 7 shows the H -TPR curves of the xMo–KIT-6 and xMo/
KIT-6 catalysts. For the incorporated catalysts (Fig. 7(a)), when
the molybdenum content was low (x < 5), almost no reduction
peak was detected in the test temperature range, which
demonstrated that the MoO species were highly dispersed on
x
51
these catalysts. With the increase of the molybdenum content
3 < x < 8), a reduction peak appeared above 800 °C, which
(
4
+
may be generated by the reduction of the Mo species to
Mo .
0
52,53
With the continued increase of the molybdenum
content (x > 5), in addition to the further broadening of the
reduction peak at higher temperatures, a new peak centered at
ca. 550 °C, which is probably attributed to the reduction of
52,53
MoO
3
to MoO
2
,
demonstrating the formation of the
oligomeric molybdenum oxide species or small MoOx
29,51,54–56
clusters.
Fig. 7(b)), with the increase of the molybdenum content (x >
.9), the reduction peak at higher temperature gradually
Similarly, for the supported catalysts
(
0
Fig. 5 The UV-vis DRS of Mo–KIT-6 (a) and Mo/KIT-6 (b) of different
molybdenum contents.
widened, and the same reduction peak appeared at ca. 550 °C.
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Fig. 7 The
b) catalysts.
2
H -TPR profiles of the xMo–KIT-6 (a) and xMo/KIT-6
Fig. 8 Ex situ UV Raman spectra of the dehydrated Mo–KIT-6 (a) and
Mo/KIT-6 (b) catalysts of different molybdenum contents.
(
Raman spectroscopy is very sensitive to the structure of
the sample, and has been frequently applied in investigating
the morphology of the surface anchoring MoO with the
molybdenum loading. However, only a weak vibration band
of Mo–O–Mo occurred on 10Mo–KIT-6 for the incorporated
catalysts. What is more, two Raman bands on 9.3Mo/KIT-6
x
5
7
MoO bond. Fig. 8 elucidates the ex situ UV Raman
spectra of the dehydrated Mo–KIT-6 and Mo/KIT-6 catalysts
with different molybdenum contents. For the parent KIT-6
assigned to crystalline MoO were centered at 832 and 1004
3
−
1
cm , which were absent on the incorporated catalysts.
−
1
sample, the band at 976 cm is attributed to the vibration of
5
7–59
3.3 Catalytic performance
Si–OH,
and 1004 cm , which correspond to the crystalline MoO
For the dehydrated Mo–KIT-6 catalysts, the band at 973–989
3
the pure MoO exhibits distinct bands at 673, 832
−
1
56
3
.
The catalytic activity of the catalysts for the selective oxidation
of the methane reaction is influenced by numerous factors,
−
1
−1
cm and the shoulder band at 966–970 cm belong to the
MoO symmetric vibration and asymmetric stretch of the di-
oxo molybdate species ((O) Mo(O–Si) ), respectively.
The weak and wide vibration band at ∼1027 cm belongs to
the MoO vibration in the mono-oxo molybdate species
such as the structure of the MoO species, the interaction with
x
the support, reaction conditions, and others. In this work, we
investigated the catalytic performance of the Mo–KIT-6
catalysts in terms of the reaction temperature, molybdenum
content, reaction time and the structure of the active site.
3.3.1 The effect of temperature on methane conversion
and product distribution. Fig. 9 represents the methane
conversion as a function of temperature for the xMo–KIT-6 and
xMo/KIT-6 catalysts. Generally, the methane conversion
increased with the reaction temperature. However, for the
9.3Mo/KIT-6 catalyst, when the temperature was higher than
5
9–61
2
2
−
1
(
OMo(O–Si) ). The intensity of the vibration band increased
4
with molybdenum content, reached the strongest on the
Mo–KIT-6 catalyst, and then decreased. For the Mo/KIT-6
8
samples, the vibration bands of the two highly dispersed di-
oxo and mono-oxo molybdate species on the surface of the
supported catalysts were much weaker compared with those
of the incorporated catalysts. It should be noted that a band
675 °C, a large number of MoO species on the surface of KIT-6
x
−
1
of the 4.6Mo/KIT-6 catalyst was centered at 863 cm , which
belongs to the stretching vibration of Mo–O–Mo, indicating
the existence of the highly polymeric MoO species or MoO
were prone to aggregate and form highly polymerized MoO or
x
3
nanoparticle MoO species due to the weak interaction between
the MoO species and KIT-6 (Fig. S4†), resulting in the decrease
x
3
x
6
2–66
nanoparticles,
and the band intensity increased with
of catalytic activity. With the further increase of temperature,
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Fig. 10 Selectivity of formaldehyde as a function of temperature for
the Mo–KIT-6 (a) and Mo/KIT-6 (b) catalysts. Reaction conditions:
Fig. 9 Conversion of methane as a function of temperature for the
Mo–KIT-6 (a) and Mo/KIT-6 (b) catalysts. Reaction conditions: catalyst
−
1
−
1
4 2 2
catalyst weight = 0.1 g, CH /O /N = 2/1/3, flow rate = 60.0 ml min .
weight = 0.1 g, CH
4
/O
2
/N
2
= 2/1/3, flow rate = 60.0 ml min
.
the positive effect of temperature on the activation of methane
was greater than the negative effect of the decrease in the
performance of the catalyst. Thus, the methane conversion
would continue to rise. Finally, the maximum methane
conversion was 28.9% and 28.1% at 750 °C over the 5Mo–KIT-6
and 2.8Mo/KIT-6 catalysts, respectively.
Fig. 10 reveals the selectivity of formaldehyde as a
function of reaction temperature for the xMo–KIT-6 and xMo/
KIT-6 catalysts. The selectivity of formaldehyde decreased
rapidly with temperature for both the incorporated catalysts
and the high loading supported catalysts (x > 2.8), and the
selectivity of the supported catalysts with lower loading (x <
dioxide selectivity values of 23.6% and 56.7% were obtained
on 8Mo–KIT-6 and 2.8Mo/KIT-6, respectively.
Fig. S7† depicted the yield of formaldehyde as a function
of temperature for the xMo–KIT-6 and xMo/KIT-6 catalysts.
The highest yield of formaldehyde (2.1% and 1.4%) was
obtained on the 8Mo–KIT-6 and 9.3Mo/KIT-6 catalysts at 675
°C, respectively. Obviously, the yields of the incorporated
catalysts were superior to those of the supported catalysts,
indicating that the incorporated catalysts were beneficial to
the production of formaldehyde.
Under the reaction conditions, formaldehyde, carbon
monoxide and carbon dioxide were the main reaction
products. Only trace amounts of methanol, ethane and
ethylene were detected at higher temperature, as shown in
Table S2.† It should be noted that, when the highest
formaldehyde selectivity (73.5% and 65.9%) of the two series
of catalysts was obtained on 5Mo–KIT-6 and 9.3Mo/KIT-6,
respectively, both the methane conversion and carbon
dioxide selectivity were close. For the 9.3Mo/KIT-6 catalyst,
the carbon monoxide selectivity was much higher than that
of 5Mo–KIT-6. Therefore, due to the decomposition of
formaldehyde to carbon monoxide, the highest formaldehyde
selectivity of the supported catalysts was lower than that of
the doped catalysts.
4
.6) increased first and then declined with temperature. The
highest selectivity (73.5% and 65.9%) of the two series
catalysts was achieved on 5Mo–KIT-6 and 9.3Mo/KIT-6 at 600
°C, respectively. However, contrary to the formaldehyde
selectivity, the carbon monoxide selectivity increased rapidly
with temperature as depicted in Fig. S5,† and the highest
selectivity of carbon monoxide was obtained at 750 °C on
1
0Mo–KIT-6 (77.9%) and 9.3Mo/KIT-6 (81.5%) for the
incorporated and supported catalysts, respectively. Compared
with the doped catalysts, the carbon dioxide selectivity of the
supported catalysts was significantly higher as shown in Fig.
S6.† When the temperature was 600 °C, the highest carbon
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.3.2 Effect of the Mo content on methane conversion and
product distribution. Fig. 11 shows the relationship between
the catalytic performance and actual molybdenum content of
the Mo–KIT-6 and Mo/KIT-6 catalysts at 675 °C. In Fig. 11(a),
with the increase of the molybdenum content, the HCHO
Paper
3
active sites on the incorporated catalysts remained unchanged,
while great alterations occurred for the supported catalysts with
the increase of the molybdenum content at the high reaction
temperature, which is consistent with the results of the UV
Raman spectra and UV-vis DRS characterization.
selectivity dropped gradually. The CH
selectivity increased when the molybdenum content was below
.5, and then all of them almost remain unchanged. It could
4
conversion and the CO
Fig. 12 shows the variation of the methane consumption
rate and formaldehyde formation rate on per Mo atom
(apparent TOF, s ) with nominal molybdenum content of
−
1
1
be inferred that the type of active sites on the incorporated
catalysts did not change much with the increase of the
molybdenum content. In Fig. 11(b), with the increasing of the
molybdenum content, the formaldehyde selectivity of the
supported catalysts dropped rapidly, and the selectivity of
carbon monoxide and carbon dioxide rose gradually. It may be
attributed to the formation of a large number of highly
the Mo–KIT-6 and Mo/KIT-6 catalysts at 675 °C. Since the
MoO on the catalysts were in different forms, their catalytic
x
activities were different, and the apparent TOF reflected an
average activity value of each catalyst. As shown in Fig. 12,
the highest methane conversion and formaldehyde formation
rate were obtained at the lowest molybdenum content, which
x
was attributed to the highly dispersed isolated MoO active
polymerized MoO species for the over-oxidation of methane to
sites for the efficient selective oxidation of methane. The TOF
of the methane conversion and formaldehyde formation of
the doped and supported catalysts decreased dramatically
with the increase of the molybdenum content. When the
molybdenum content was beyond 2.0, the TOF dropped
slowly, indicating that the formation of the polymerized,
particulate, or crystalline MoOx species would weaken the
x
CO . When the molybdenum content was further increased to
x
3.4, the methane conversion started to fall, but the selectivity of
formaldehyde suddenly turned to slowly go up with the further
increase of molybdenum loading, indicating that the crystalline
3 x
MoO aggregated from the highly polymeric MoO species at
high temperature was inactive for methane conversion, but
beneficial
formaldehyde.
to
the
formation
It could be concluded that most of the
or
stabilization
of
3
0,40,67,68
Fig. 12 TOF of the methane consumption (filled symbols) and
formaldehyde formation (empty symbols) as a function of the nominal
molybdenum content over the Mo–KIT-6 (a) and Mo/KIT-6 (b)
catalysts. Inset displays the dependance of Mo surface density on
Fig. 11 Methane conversion and main product selectivity as a function
of the molybdenum content for the Mo–KIT-6 (a) and Mo/KIT-6 (b)
catalysts. Reaction conditions: T = 675 °C, catalyst weight = 0.1 g,
nominal Mo content. Reaction conditions: T = 675 °C, catalyst weight
−1
−1
CH
4
/O
2
/N
2
= 2/1/3, flow rate = 60.0 ml min
.
= 0.1 g, CH
4
/O
2
/N
2
= 2/1/3, flow rate = 60.0 ml min
.
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methane activation ability, and thus diminish formaldehyde
production. In addition, the highest TOF value of both
methane conversion and formaldehyde formation of the
incorporated catalysts were greater than those of the
supported catalysts, which demonstrated the superiority of
the framework-incorporated catalysts with highly dispersed
and isolated active sites prepared with hydrothermal
synthesis self-assembly method.
The 8Mo–KIT-6 catalyst with the highest catalytic
performance for highly selective production of formaldehyde
was chosen for the stability test under 675 °C, and the results
are displayed in Fig. 13. Within 13 h, the selectivity of carbon
monoxide, carbon dioxide and formaldehyde remained stable,
while the conversion of methane decreased slightly with time.
The molybdenum oxide species on the catalyst surface were
prone to aggregate under high operating temperature over time,
which resulted in the decrease of the number and exposure area
of active sites, thus reducing the methane conversion.
3
.3.3 Selective oxidation of methane over the 8Mo–KIT-6
and 4.6Mo/KIT-6 catalysts. The selectivity of formaldehyde
and carbon monoxide for the two catalysts with the same Mo
actual content (8Mo–KIT-6 and 4.6Mo/KIT-6) were compared
versus the variation of the methane conversion (Fig. 14).
There was no obvious difference between the two types of
catalysts in the carbon dioxide selectivity (Fig. S8†). However,
the 8Mo–KIT-6 catalyst showed higher formaldehyde
selectivity and lower carbon monoxide selectivity than those
of the 4.6Mo/KIT-6 catalyst at the same conversion, which
indicated that the doped catalyst is more beneficial to the
selective production of formaldehyde from methane, and
formaldehyde is more likely to be decomposed to carbon
monoxide on the supported catalyst.
Fig. 14 Selectivity of formaldehyde (a) and carbon monoxide (b) as a
function of methane conversion for the 8Mo–KIT-6 and 4.6Mo/KIT-6
catalysts. Reaction conditions: T = 600–750 °C, catalyst weight = 0.1 g,
−
1
4 2 2
CH /O /N = 2/1/3, flow rate = 60.0 ml min .
6
9
Moreover, the catalytic performance of the two catalysts at
performances of the gas-phase free radical reaction and
gas–solid catalytic reaction without highly dispersed MoOx
active sites were quite low, which indirectly evidenced the
indispensable role of the KIT-6 support for the dispersion of
the MoOx active sites. Therefore, we considered that the
difference in the catalytic activity mainly resulted from the
different morphology of the active sites on the two types of
catalysts. Under the identical reaction conditions, the
methane conversion of 8Mo–KIT-6 and 4.6Mo/KIT-6 were
equal, but the formaldehyde selectivity of 8Mo–KIT-6 was 2.3
times as much as that of 4.6Mo/KIT-6, indicating that the
highly dispersed and isolated framework-incorporated
molybdate oxide species active sites are in favor of inhibiting
the over-oxidation or decomposition of formaldehyde.
6
75 °C was further compared in the following Table 1, which
also provides the activity data of the methane-selective
oxidation reaction in an empty tube and on a pure support,
as well as on pure MoO . It could be observed that the
3
4. Discussion
In this work, a well-ordered mesoporous silica material KIT-6
with three-dimensional cubic interconnected pore channels
was used as a promising parent to develop highly dispersed
and isolated molybdenum-incorporated catalysts with high
catalytic performance in methane-selective oxidation. The
molybdenum-incorporated KIT-6 catalysts prepared with a
simple one-pot hydrothermal synthesis method show
Fig. 13 The stability of the catalytic performance and production
selectivity over the 8Mo–KIT-6 catalysts. Reaction conditions: T =
6
6
4 2 2
75 °C, catalyst weight = 0.1 g, CH /O /N = 2/1/3, flow rate =
−
1
0.0 ml min
.
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Table 1 The catalytic performance of the catalysts prepared by different methods for the selective oxidation of methane
Selectivity (%)
Yield (%)
HCHO
a
Sample
Blank
Conv. (%)
HCHO
CH
3
OH
C
2
H
4
C
2
H
6
CO
0.0
0.0
41.3
72.9
60.1
CO
2
Total
0.1
0.2
0.9
7.3
7.3
39.6
38.5
16.2
12.6
28.8
0.0
0.0
0.0
0.2
0.8
0.0
0.0
0.6
0.3
0.1
23.6
13.9
7.8
2.8
0.3
36.8
47.6
34.1
11.2
9.9
63.2
52.4
65.9
88.8
90.1
0.05
0.08
0.1
0.9
2.1
MoO
KIT-6
3
4
8
.6Mo/KIT-6
Mo–KIT-6
−
1 a
Notes: reaction conditions: T = 675 °C, catalyst weight = 0.10 g, CH
4
/O
/N
2 2
= 2/1/3, flow rate = 60.0 ml min
.
Formaldehyde, methanol,
ethylene, ethane, carbon monoxide.
superiority in methane conversion and the selective oxidation
of methane to formaldehyde in the temperature range from
high temperature (Fig. S4†), which would lead to the decrease
in the number of isolated MoO active sites and were prone
to excessive oxidation of methane, resulting in the decline in
the selectivity of formaldehyde. Although the supported
catalysts could obtain the highest TOF (0.115) of methane
conversion at the lowest Mo loading as shown in Fig. 12(b),
this value was much lower than the highest value (0.205) of
the doped catalysts at the lowest Mo content. It meant that
the support of the doped catalysts with Mo atoms in the
framework (proved by the results of SAXRD, UV-vis DRS and
UV Raman spectroscopy) exhibited more excellent methane
activation ability than the corresponding supported catalysts
(in view of the fact that the active sites were dominantly
6
00 to 750 °C, exhibiting obviously higher formaldehyde
yields than those of the supported catalysts synthesized with
the traditional impregnation method.
The results of UV-vis DRS and UV Raman spectroscopy
elaborated the existence of different forms of MoOx (the
isolated monomers, the polymolybdates, and nanoparticles).
In particular, the isolated MoO
x
include the di-oxo molybdate
species ((O) Mo(O–Si) ) and mono-oxo molybdate species
2
2
4
(OMo(O–Si) ), which possess terminal MoO active sites.
However, with the increase of Mo content, a large number of
Mo–O–Mo bridges began to form at the cost of MoO
3
1
bonds. As illustrated in the UV-Raman spectra of Fig. 8, for
the xMo–KIT-6 catalysts, when x < 10, the active centers were
mainly MoO bonds. However, for the xMo/KIT-6 catalysts,
only when x < 4.6, the MoO bonds would dominate.
Otherwise, there was a large amount of Mo–O–Mo bond
derived from highly polymerized MoOx or even crystalline
x
isolated MoO species on both kinds of catalysts when the
molybdenum content was the lowest, the effects of support
and structure of the active sites on the catalytic activity could
not be neglected). Moreover, their inert support KIT-6 showed
weak catalytic selective oxidation performance (Table 1), and
the MoOx species mainly accumulated in the outer pore
channels or on the surface of KIT-6 for the supported
catalysts. Therefore, with the same Mo content, it was easier
to form a large number of Mo–O–Mo active sites, which
aggravated the excessive oxidation of methane, as shown in
MoO
3
. Meanwhile, the UV-vis DRS results showed that the
species of the doped catalysts
quantity of the isolated MoO
x
prepared with hydrothermal synthesis method was much
greater than that of the supported catalysts prepared by
impregnation method. As listed in Table S1,† the band area
ratios of the isolated MoO to the polymerized MoO (I/P) of
x
Fig. 14. However, when the highly polymerized MoO further
aggregated to form the crystalline MoO3 at high reaction
temperature, they could be favorable for the formation or
stabilization of formaldehyde (Fig. 11(b)), despite their weak
x
x
the doped catalysts were all greater than those of the
supported catalysts, especially for the 8Mo–KIT-6 and 4.6Mo/
KIT-6 catalysts with the same Mo content. There was a two-
fold difference value between their r(I/P). The above results
demonstrated that the hydrothermal synthesis method is
more conducive to the formation of catalysts with rich
isolated MoO active sites.
3
0,40,67,68
C–H activation performance.
The structure of the active site will alter with various
reaction conditions. Therefore, in situ UV Raman spectroscopy
was utilized to deeply study the changes of the active sites on
the 8Mo–KIT-6 and 4.6Mo/KIT-6 catalysts with the same
molybdenum content under different conditions. As the coke
formed under the actual reaction conditions (high
temperature, as well as the existence of oxygen) is readily
removed in situ by oxygen in the reactant feed (Fig. S9†), the
reduction and oxidation processes involved in the title reaction
were investigated individually to uncover the distinguishing
attributes between the coke formation and elimination
processes on the active sites with different coordination
structures. As shown in Fig. 15(a) A → B, when the 8Mo–KIT-6
catalyst was exposed to the reduction flow at 675 °C, two
It could be inferred from the catalytic activity test results
of the methane-selective oxidation that the MoO sites in
the incorporated catalysts play a crucial role in efficient
activation of methane conversion and selective production of
3
0,70,71
formaldehyde,
and the 8Mo–KIT-6 with the best
strong MoO
selective oxidation performance had
a
vibration peak as revealed in Fig. 8, which also confirmed the
hypothesis. However, with further increase of the
molybdenum content, the excess molybdenum oxide species
outside the framework accumulated on the surface or inside
the pore channels of 10Mo–KIT-6 tended to agglomerate and
−1
Raman bands at 978 and 1018 cm attributed to the highly
form larger polymerized MoO
x
with Mo–O–Mo bonds under
dispersed MoO active sites disappeared rapidly and two
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−
1
−1
intense bands emerged at 1393 cm and 1569 cm , which
belong to the disordered carbon (D) and graphitic carbon (G)
of the formed coke,
To further focus on the separate reduction and oxidation of
the MoO species on the 8Mo–KIT-6 catalyst under different
x
72,73
respectively. The band area ratio of G
temperatures, in situ Raman spectroscopy (Fig. 16) under CH
4
to D (G/D) was about 1.1. When the reduced catalyst was
exposed to the reaction flow (Fig. 15(a) C → F), the active sites
of MoO emerged rapidly, and the coke could be eliminated
after 30 min. For the supported catalyst of 5Mo/KIT-6 in
Fig. 15(b) A → B, the bands assigned to the isolated MoO
vibration also disappeared after the reduction process and two
bands of coke occurred at the same time. However, the band
area ratio of G to D (G/D) was 1.7, which was higher than that
of the 8Mo–KIT-6 catalyst, indicating the graphitization degree
of coke on the supported 4.6Mo/KIT-6 catalyst was higher.
Upon exposure to the reaction atmosphere (Fig. 15(b) C → G),
and O flow, respectively, was recorded. As shown in Fig. 16(a),
2
the Raman spectrum in N flow at 400 °C was similar to that
2
4 2
under 5% CH /N flow. When the temperature rose to 450 °C,
the band intensity of the di-oxo and mono-oxo molybdate
species was obviously weakened, which was consistent with the
2 4
H -TPR results in Fig. 7, signifying that CH was reacted with
MoO above 400 °C. With the further increase of temperature,
−1
the intensity of the bands at 982 and 1025 cm decreased
more obviously and almost completely disappeared at about
−1
600 °C (the band at ∼972 cm was assigned the vibration of
Si–OH). For the oxidation process, in Fig. 16(b), after 10 min
both the oxidation of the reduced MoO
x
active sites and the
2 2
treatment in 20% O /N flow at 200 °C, the reduced MoO
elimination of coke of the 4.6Mo/KIT-6 catalyst were slower
than those of the 8Mo–KIT-6 catalyst. That is to say, the MoO
sites of the incorporated catalyst are much easier to be
activated and less likely to generate carbon deposits than those
of the supported catalyst.
active sites were rapidly oxidized, and the coke was also
significantly eliminated. After 110 min exposure in oxidizing
flow, the MoO active sites were completely recovered and at
the same time, the carbon depositions were eliminated
completely. The above processes demonstrated that the
activation of methane is the rate-determining step of the title
Fig. 15 In situ UV Raman spectra of the dehydrated 8Mo–KIT-6 (a)
and 4.6Mo/KIT-6 (b) catalysts at 675 °C under (A) oxidizing flow of
Fig. 16 In situ UV Raman spectra of the dehydrated 8Mo–KIT-6
2
catalyst (a) under N gas flow at (A) 400 °C, and under reducing flow
1
6.7% O
exposed to real reaction condition of 33.3% CH
min, (D) 10 min, (E) 20 min, (F) 30 min, (G) 60 min.
2
/N
2
, and under (B) reducing flow of 33.3% CH
4
/N
2
, and
of 5% CH /N at (B) 400 °C, (C) 450 °C, (D) 500 °C, (E) 600 °C, and (b)
4
2
4
/16.7% O /N
2
2
after (C)
exposed to oxidizing flow of 20% O
30 min, (H) 60 min, (I) 110 min.
2 2
/N at 200 °C after (F) 10 min, (G)
1
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reaction, and the regeneration of the reduced MoO bonds is
active sites are favorable for the formation of disordered
carbon (D). The elimination of disordered carbon (D) is
much easier than graphitic carbon (G). Thus, the catalysts
easy in the presence of O , respectively.
2
with isolated MoO active sites should be much stronger for
the tolerance of carbon deposition than the catalysts with
x
5
. Conclusions
(
1) In this work, a series of three-dimensional interconnected
pore structure, highly dispersed and framework-incorporated
catalysts (Mo–KIT-6) were prepared by one-pot
polymerized MoO
high dispersed MoO
framework of silica are more easily reoxidized than those
on the supported catalysts.
x
active sites. In addition, the reduced
active sites incorporated in the
x
a
hydrothermal self-assembly method. For comparison, the
KIT-6-supported MoOx catalysts (Mo/KIT-6) were also
obtained by the traditional incipient-wetness-impregnation Conflicts of interest
method. The catalytic performances of these two systems of
catalysts for the selective oxidation of methane were
There are no conflicts to declare.
comparatively explored. The characterization results
demonstrated that the hydrothermal synthesis method is
Acknowledgements
more conducive to the formation of catalysts with rich
The authors are grateful for financial support from the
National Natural Science Foundation of China (91845201;
1808241; 91545117).
isolated MoO active sites than impregnation method.
2) For the incorporated catalysts (Mo–KIT-6), the highly
dispersed and isolated di-oxo molybdate species
(
2
(
(
(O) Mo(O–Si) ) and mono-oxo molybdate species
OMo(O–Si) ) are mainly present in the framework of KIT-
4
2
2
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(
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