Polyoxometalate catalysts with co-substituted VO2+ and transition metals and their catalytic performance for the oxidation of isobutane

Article abstract of DOI:10.1039/c8cy01101j

Cs-Salts of the Keggin-type phosphomolybdic acid with simultaneous doping of VO²⁺ and different transition metals are prepared, characterized and tested as catalysts for the oxidation of isobutane to methacrylic acid under isobutane-rich condit

Full text of DOI:10.1039/c8cy01101j

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Polyoxometalate catalysts with co-substituted
VO2+ and transition metals and their catalytic
performance for the oxidation of isobutane†
Cite this: Catal. Sci. Technol., 2018,
8, 5774
a
a
Yanchun Liu,a Jingfang He,ab Wenling Chu
and Weishen Yang
*
*
Cs-Salts of the Keggin-type phosphomolybdic acid with simultaneous doping of VO2+ and different transi-
tion metals are prepared, characterized and tested as catalysts for the oxidation of isobutane to methacrylic
acid under isobutane-rich conditions. The results show that Cs2.0V0.3Cu0.2PMo12O40 catalyst with co-
substituted VO2+ and Cu2+ in its secondary structure exhibits higher catalytic performance than those with
co-doped VO2+ and other transition metals such as Fe, Ni and Ce. This difference is ascribed to the syner-
gistic effect between Cu2+ and VO2+ only in the secondary structure. This synergistic effect is related to
the charge transfer between Mo6+ in the Keggin anions and Cu2+ counter cation, which promotes the re-
oxidation of the catalyst and consequently its catalytic activity.
Received 30th May 2018,
Accepted 3rd October 2018
DOI: 10.1039/c8cy01101j
rsc.li/catalysis
performance due to considerable changes in redox properties
including re-oxidation ability. For example, Fe and Cu ions
1. Introduction
The selective oxidation of isobutane to methacrylic acid
(MAA) is of great interest because of its low-cost raw material,
simplicity of the one-step process and negligible environmen-
tal impact. To date, heteropoly compounds (HPCs) have
exhibited superior catalytic properties for this reaction due to
their adjustable acidic and redox properties by incorporating
appropriate transition metals into the catalyst within the pri-
mary structure1–3 and/or the secondary structure.4,5
Over the past twenty years, significant progress has been
made toward a better understanding of the structure of HPCs
and their complex catalytic mechanism. A particular feature
of isobutane oxidations described in patents6–8 is that almost
all reactions require isobutane-rich conditions. However, a
major obstacle of HPC catalysts for isobutane oxidation un-
der isobutane-rich conditions is that the upper limit of isobu-
tane conversion is difficult to reach >10% at higher selectiv-
ity to MAA (∼50%) due to insufficient re-oxidation ability of
the catalyst, which is the rate-determining step under
isobutane-rich conditions. The results highlight that the
main problem in the oxidation of isobutane to methacrylic
acid is improving the re-oxidation ability of catalysts.
are usually doped into HPCs as counter ions to obtain supe-
rior catalytic activity for oxidative reactions because they are
involved in charge transfer with atoms of heteropoly anions
and may act as high capacity agents for oxygen transfer.9,10
M. Langpape et al. reported11 that in the oxidation of isobu-
tane to MAA under oxygen-rich conditions, incorporation of
Cu in phosphomolybdic HPCs appeared to have a positive ef-
fect on the activity and a negative effect on the selectivity to
MAA, whereas iron-substituted HPC exhibited almost no ef-
fect on the activity but a positive effect on the selectivity. An-
other successful example has been reported by Mizuno's
group;12 they found that a Cu-doped Cs2.5H0.5PMo12O40 cata-
lyst has a positive effect on both the activity and selectivity
for isobutane oxidation under isobutane-rich conditions and
suggested that copper promotes the re-oxidation of the cata-
lyst. Superior catalytic properties of HPCs with the incorpora-
tion of
V were also observed for the oxidation of
isobutane.13–15 Huynh and co-workers found16 that the sub-
stitution of vanadyl cations in the secondary structure in-
creased the catalytic activity of isobutane oxidation under
isobutane-rich conditions without losing too much selectivity
to MAA. However, our previous study17 showed that the intro-
duction of VO2+ in the secondary structure resulted in a con-
siderable positive effect on selectivity to MAA under
isobutane-rich conditions, which is in good agreement with
the results reported by Casarini.18 In addition, Mizuno et al.
also showed that Cs2.5HPMo12O40 catalysts with incorporated
Mn, Fe, Co, Ni, and Cu are effective catalysts with optimal ac-
tivity obtained with Ni2+ and the highest selectivity to MAA
with Ni2+ or Fe3+ for the oxidation of isobutane to MAA under
It has been reported that the incorporation of transition
metals into the HPC framework can influence the catalytic
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China.
E-mail: cwl@dicp.ac.cn, yangws@dicp.ac.cn; Fax: +86 411 84379301,
Fax: +86 411 84379073; Tel: +86 411 84379301, +86 411 84379073
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cy01101j
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oxygen-rich conditions.19 Clearly, the effects of the incorpora-
tion of transition metals on the catalytic performance of HPC
catalysts during the oxidation of isobutane are still ambigu-
ous due to the complex structure of transition metal-
substituted HPCs and/or reconstruction under various reac-
tion conditions.20–22
2.2 Characterization of catalysts
The phase structure of the as-synthesized samples was identi-
fied by X-ray diffraction (XRD, Rigaku D/MAX 2500/PC diffrac-
tometer) with CuKα radiation. Patterns were recorded in the
2θ range of 5–60° at a scan speed of 2° min−1.
Fourier transform infrared (FT-IR) spectra (Nicolet 6700
spectrometer) were recorded at a resolution of 4 cm−1 with 32
scans.
A Lambda950 spectrophotometer (PerkinElmer) was used
for obtaining ultraviolet-visible (UV-vis) diffuse reflectance
spectra of different catalyst samples.
We recently reported the catalytic performance of HPCs
with VO2+ as the counter ion for isobutane oxidation under
isobutane-rich conditions, in which the catalyst exhibited
higher selectivity to MAA but poor isobutane conversion.17
Thus, to couple the advantages in selectivity gained by
V-containing Cs-salt of HPCs from our previous study with ex-
cellent re-oxidation ability of transition metals reported in
the literature, we prepare a Cs-salt of 12-molybdophosphate
with the co-incorporation of VO2+ and different transition
metals. Encouraged by the reported good synergy between
VO2+ and Cu2+ counter ions in a mixed vanadyl–copper salt
of H3PMo12O40 for the oxidative dehydrogenation of iso-
butyric acid,23 herein, special attention is focused on the rela-
tionship between the catalytic performance and structural
properties of HPCs from a new point of view, considering the
synergistic effect of co-doped VO2+ and transition metals as
counter ions of heteropoly anions. The results may prompt
us to explore this field further to provide more information
for designing effective polyoxometalate-based catalysts for
the oxidation of isobutane.
H2-TPR measurement was carried out on an AutoChem II
2920 instrument. The sample (50 mg) was loaded in a quartz
tube reactor and pre-treated with helium at 200 °C for 1 h
and then cooled to 50 °C. The H2 consumption was continu-
ously monitored using a thermal conductivity detector (TCD)
from 50 °C to 750 °C in a 5% H2/N2 mixture gas at a flow rate
of 30 mL min−1
.
X-ray photoelectron spectra (XPS) of the as-synthesized
samples were obtained on a ThermoFisher ESCALAB 250 Xi
spectrometer equipped with a monochromated Al Kα radia-
tion source (1486.6 eV) using power of 15 kV and 10.8 mA.
The binding energy of each element was calibrated using car-
bon C1s as the reference line (284.6 eV).
X-ray fluorescence (XRF) chemical analysis of the prepared
solids was performed at room temperature on a Zetium
apparatus.
The acid characteristics of the catalysts were studied by
NH3-TPD tests, in which about 0.1 g of catalyst was pre-
treated under a He flow (30 mL min−1) at 200 °C for 1 h. After
cooling to 100 °C, NH3 was chemisorbed for another 1 h at
an NH3/He flow of 30 mL min−1. After purging the reactor
with He to a flat baseline, the TPD profiles were recorded in
He flow from 100 °C to 600 °C at a ramp of 10 °C min−1
using a TCD detector.
The IR spectra of adsorbed pyridine (Py-IR) were obtained
on a Tensor27 (Bruker) FT-IR spectrophotometer. Prior to the
test, a self-supporting pellet of 13 mm diameter was degassed
at 250 °C for 2 h to remove the physisorbed water. After
cooling to room temperature, the spectrum of the sample
was recorded as a background spectrum. Afterwards, the
sample was exposed to pyridine vapour for 4 min and then,
the IR spectrum of the adsorbed pyridine at room tempera-
ture was recorded. Subsequently, IR spectra were obtained
following pyridine desorption by evacuation for 1 h at 25 °C,
150 °C and 300 °C. The IR spectra of all samples were
obtained by subtracting the background spectrum from the
absorbance spectra recorded after pyridine adsorption.
2. Experimental
2.1 Preparation of catalysts
H3PMo12O40 heteropoly acid was commercially obtained from
Sinopharm Chemical Reagent Co., Ltd, and it was calcined at
200 °C for 3 h to remove adsorbed and crystalline water be-
fore use. The H4PMo11VO40 sample was prepared using
MoO3, V2O5 and 85% H3PO4 according to the literature proce-
dure.24 Other reagents were of analytical grade and used
without further purification.
The transition metal-doped Cs2.0VxPMo12O40 (x = 0–0.4)
catalysts were prepared as follows: stoichiometric amounts of
VOSO4·5H2O and corresponding transition-metal nitrate were
added dropwise to an aqueous solution of H3PMo12O40 parent
acid, followed by the addition of an aqueous solution of ce-
sium carbonate at 80 °C. The resulting suspension was evap-
orated to dryness at 60 °C using a rotavapor under reduced
pressure, and the dried catalyst was calcined at 300 °C in air
for
3
h. Here, the catalysts are designated as
Cs2.0VxMyPMo12O40 (M = Cu, Fe, Ni and Ce), in which x and y
are the amounts of VO2+ and transition metal ions, respec-
tively. In our study, the values of x and y were adjusted to
keep the catalyst neutral.
2.3 Selective oxidation of isobutane
Cs2.5H1.5PMo11VO40 and Cs2.0Cu0.2H1.6PMo11VO40 catalysts
with doped Cu2+ ions were prepared according to the process
mentioned above except that H4PMo11VO40 was used as the
parent acid. If no special statement was given, the H+ ions in
all the catalysts were abbreviated.
Selective oxidation of isobutane was carried out at atmo-
spheric pressure in a quartz flow reactor (ID: 12 mm). The
particle catalysts (2 g) with 20–40 mesh were loaded in the
middle of the reactor. The catalyst was initially heated under
a N2 flow to 330 °C for a certain time. Then, the standard
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feed (isobutane/O2/H2O/N2 = 27/13.5/10/49.5) for the oxidation
was fed into the reactor at a total flow rate of 20 mL min−1.
The reaction temperature was controlled at 350 °C for isobu-
tane oxidation. The feed and products were analyzed on-line
by a TCD gas chromatograph equipped with two sequential
columns: Porapak Q for hydrocarbons and CO2 and molecu-
lar 13X for O2, N2 and CO. The liquid products such as ace-
tone, aldehydes and acids were captured by an ice cooling
trap and analyzed by an off-line gas chromatograph with an
FFAP capillary column using a flame ionization detector. All
data were collected after
conditions.
2 h under stable catalytic
Fig. 1 FT-IR spectra of V-containing Cs-salt of HPCs with the incor-
poration of different transition metals. a: Cs2.0V0.3Cu0.2PMo12O40; b:
3. Results and discussion
3.1 XRD
Cs2.0V0.2Ce0.2PMo12O40; c: Cs2.0V0.2Fe0.2PMo12O40; d: Cs2.0V0.3Ni0.2
PMo12O40; and e: Cs2.0Cu0.2PMo11VO40
-
.
The powder XRD patterns of V-containing Cs-salt of HPC with
the incorporation of different transition metals are shown in
Fig. S1.† It is clear that all the synthesized materials exhibited
similar peak patterns to those of their parent salt
Cs2.0HPMo12O40 with a stable cubic secondary structure,25 in-
dicating the presence of the Keggin unit in the catalysts. Fur-
thermore, the cell parameter of the V-containing Cs-salt of
HPCs with the incorporation of different transition metals
was calculated based on the strong (110), (211), (222), (400)
and (332) reflection lines, which are the characteristic peaks
of the Keggin unit, and the average crystal size was obtained
from all the characteristic reflection planes using Scherrer's
equation. The results are listed in Table 1. No clear changes
in the 2θ values and cell parameters were observed for each
catalyst with doping of transition metals, implying that the
incorporation of transition metals did not influence the lat-
tice of heteropoly anions.
structure induces alterations in the IR spectrum due to de-
crease of the oxoanion symmetry.28 Consequently, a split in
the asymmetric P–O stretching band appears, and shoulder
peaks are observed at 1080 and 998 cm−1, as shown for the
Cs2.0Cu0.2PMo11VO40 sample with the incorporation of V into
the Keggin unit. The FT-IR spectra of the transition metal
(Cu, Fe, Ni and Ce)-incorporated samples show no difference
in the structure with respect to that of pure
Cs2.0HPMo12O40,17 which strongly confirms that the transi-
tion metals are not incorporated in the primary Keggin anion
while occupying the secondary structure as counter cations.
3.3 UV-vis diffuse spectra
UV-vis diffuse spectroscopy provided additional information
about the substitution of transition ions in HPCs. For the
spectrum of the pure Cs2.0HPMo12O40 sample shown in
Fig. 2, two typical reflectance bands at 210–230 nm and 500–
600 nm are ascribed to the O2− → Mo6+ charge transfer in the
Keggin anion.29,30 Similar peaks are observed over the cata-
lysts with the co-incorporation of VO2+ and transition metals,
indicating the presence of Keggin ions. It has been reported
that the position and intensity of UV-vis reflection bands
strongly depend on the overall symmetry of the Keggin
unit.31,32 Cu2+- and/or VO2+-doped samples show decrease in
intensity for the peaks at 500–600 nm due to decrease in the
symmetry of the Keggin unit induced by the interaction be-
tween the doping metals (such as Cu2+ or VO2+) and Keggin
anions. However, in the UV-vis spectra of the VO2+-doped
3.2 FT-IR spectra
The FT-IR spectra of V-containing Cs salt of HPC with doping
of different transition metals are compared in Fig. 1. The
main characteristic peaks of the Keggin structure in the 600–
1100 cm−1 range are clear in the spectrum of each transition
metal-doped sample, indicating the preservation of the basic
Keggin structure26,27 after the incorporation of the transition
metal and calcination at 300 °C. Notably, a minor band at
∼1035 cm−1 is observed over the series of samples with the
co-substitution of VO2+ and transition metals, which can be
ascribed to the V–O stretching bands of the vanadyl species
in the secondary structure.20 Moreover, it has been revealed
that the substitution of any transition metal in the Keggin
Table 1 Cell parameter, crystallite size and elemental chemical state of the samples from XRD data and XPS analysis
Sample
Cell parameter (Å) Crystallite size (nm) Mo 3d5/2 (Mo6+/Mo5+
)
V 2p3/2 (V5+/V4+
)
Mo5+/(Mo6+ + Mo5+
)
V5+/(V4+ + V5+
)
Cs2V0.3Cu0.2PMo12O40 11.756
Cs2V0.2Fe0.2PMo12O40 11.754
Cs2V0.3Ni0.2PMo12O40 11.756
Cs2V0.2Ce0.2PMo12O40 11.756
Cs2.0Cu0.2PMo11VO40 11.778
39.5
37.3
42.2
40.1
39.2
232.7/231.8
232.7/231.6
232.7/231.6
232.6/231.7
232.6/231.7
517.3/516.3
517.1/516.1
517.0/516.1
517.1/516.2
517.2/516.2
0.08
0.08
0.07
0.07
0.08
0.40
0.30
0.27
0.32
0.83
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to a significant red shift and decrease in the reflectance
intensity as a result of the destruction of the center-symmetry
of the Keggin unit, which is similar to that shown in our pre-
vious report17 and literature.35 Unlike the results for
Cs2.0Cu0.2PMo11VO40 catalyst, the characteristic bands of the
Keggin unit for the samples with the incorporation of differ-
ent transition metals do not shift to lower frequencies, fur-
ther confirming that the transition metals are confined to the
secondary structure and not incorporated in the primary
structure of the Keggin unit.
Fig. 2 UV-vis diffuse spectra of as-synthesized Cs-salts of HPCs.
3.4 H2-TPR
To gain insights into the change in redox properties with the
incorporation of transition metals into the secondary struc-
ture of V-containing Cs-salt of HPCs, we performed H2-TPR
measurements, and the results are illustrated in Fig. 4. Im-
portantly, all the Cu-incorporated heteropoly compounds
exhibited H2 consumption peaks at about 200–350 °C due to
the reduction of Cu2+ to Cu, and it was clear that the reduc-
tion degree and temperature strongly depend on the incorpo-
ration of V species and their location. It was observed that
the V-free Cs2.0Cu0.2PMo12O40 sample exhibited the lowest re-
duction temperature and highest H2 consumption. This fact
suggested that the incorporation of Cu2+ promoted the
charge transfer and further provided a high capacity agent
for oxygen transfer, which was in good agreement with the re-
sults reported in literatures.9,10 Further doping of VO2+ into
the secondary structure led to a shift in the reduction peak
for the Cs2.0V0.3Cu0.2PMo12O40 sample towards a higher tem-
perature, whereas the Cs2.0Cu0.2PMo11VO40 catalyst with V
species in the primary structure showed significantly lower
H2 consumption. The H2-TPR data indicated that the pres-
ence of V species leads to stronger interaction of Cu2+ with
the entire solid, including VO2+ in the secondary structure
and V-substituted polyanions. However, the samples with the
incorporation of Fe3+, Ni2+ and Ce4+ were characterized by
samples such as Cs2.0V0.3PMo12O40 and Cs2.0V0.3Cu0.2PMo12-
O40, besides a decrease in peak intensity, a clear red-shift is
also observed. This change in the reflectance peak induced
by the O2− → Mo6+ charge transfer indicates possible weaken-
ing of this bond due to the incorporation of VO2+ ions into
the secondary structure. However, the reason for the red-shift
depending more on the addition of VO2+ than that of Cu2+ is
not clear. Interestingly, the co-doping of Cu2+ and VO2+ in
the secondary structure leads to an increase in the intensity
of the reflectance peak at 500–600 nm. This is probably due
to the synergistic effect between Cu2+ and VO2+ ions, which
promotes the mobility of oxygen in the HPC lattice, and the
holes in the octahedral lacuna are filled by the available oxy-
gen; furthermore, the symmetry of the Keggin ions
increases.33,34
The UV-vis diffuse spectra of V-containing HPCs with the
incorporation of different transition metals are shown in
Fig. 3. Compared to the Cs2.0V0.3PMo12O40 sample, the cata-
lysts with further incorporation of transition metals into their
secondary structure do not show significant differences ex-
cept that an increase in the intensity of the reflectance band
is observed only for the Cs2.0V0.3Cu0.2PMo12O40 catalyst, which
is probably due to the synergistic effect between VO2+ and
Cu2+ ions, as explained above. For the Cs2.0Cu0.2PMo11VO40
catalyst, the presence of one V atom in the Keggin unit leads
Fig. 3 UV-vis diffuse spectra of V-containing Cs-salts of HPCs with
the incorporation of different transition metals (the dashed line is for
the Cs2.0V0.3PMo12O40 sample).
Fig. 4 H2-TPR profiles of V-containing Cs-salt of HPCs with the incor-
poration of different transition metals (inset: expansion of the 100–450
°C region).
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negligible H2 consumption at the lower temperature range of
50–450 °C, suggesting rather weaker redox ability compared
to that of the Cs salt of V-containing HPC with the incorpo-
ration of Cu2+ ions. The above results clearly demonstrate
that there are more oxygen vacancies, more reducible spe-
cies and better low-temperature reducibility over the catalyst
with the co-substitution of VO2+ and Cu2+ in the secondary
structure, which may be favourable for enhancing the oxy-
gen mobility and its re-oxidation. Since the Cs salts of
molybdophosphoric acid decompose above 540 °C,28,36 the
reduction peaks above 550 °C are ascribed to the reduction
of free metal oxides originating from the destruction of
polyanion; also, the Cu-doped catalysts showed lower stabil-
ity than those with the incorporation of Fe, Ni and Ce into
the secondary structure.
and V5+ species, respectively. The presence of V5+ can be
explained by the oxidation of V4+ to V5+ during calcination in
air. As shown in Table 1, the coexistence of Cu2+ and VO2+ in
the secondary structure favoured this oxidation, resulting in
the highest V5+/(V4+ + V5+) ratio. This further indicated that
higher redox ability is induced by the synergistic effect be-
tween the Cu2+ and VO2+ ions.
3.6 NH3-TPD and pyridine-adsorbed FT-IR spectra
The acidity of the samples with the incorporation of different
transition metals was evaluated by NH3-TPD. As shown in
Fig. S3,† all six analyzed samples showed similar desorption
behaviours in the investigated temperature range, which was
ascribed to the desorption of NH3 from weak (100–350 °C),
medium (350–450 °C) and strong (450–600 °C) acidic sites,
and the corresponding acidic content is listed in Table S1.†
Clearly, the distribution of the acid content and strength
slightly differed among the Cs salts of V-containing HPC
doped by different transition metals. The Cs2.0V0.3Cu0.2PMo12-
O40 sample with co-doped Cu2+ and VO2+ in its secondary
structure exhibited the highest total acidic content. Moreover,
the desorption of NH3 for the catalysts with incorporated
Cu2+ shifted to a lower temperature, indicating weaker acid
strength compared to those of the samples with incorporated
Ni, Fe and Ce.
3.5 X-ray photoelectron spectroscopy (XPS)
The XPS technique was used to determine the chemical state
and atomic concentration of the surface elements of the as-
synthesized samples, and the corresponding results are
shown in Fig. S2† and Table 1. Based on XRF characteriza-
tion, the bulk chemical compositions of the prepared cata-
lysts are listed in Table S1,† and the results of chemical analy-
sis are in good agreement with the theoretical
stoichiometries.
For the calcined samples shown in Fig. S2A,† the Mo 3d5/2
peak could be decomposed into two peaks corresponding to
Mo6+ and Mo5+ species at ca. 232.6 0.1 and 231.6 0.1 eV.
This suggested the formation of partially reduced catalysts
due to electron exchange occurring between Mo6+ and doped
transition metals in the secondary structure.37,38 From
Table 1, it can be seen that the reduction extent of the
samples (Mo5+/(Mo5+ + Mo6+ ratio)) has no significant depen-
dence on the doped transition metals.
The surface acidity of the samples with incorporation of
different transition metals was further examined by FT-IR
spectroscopy using pyridine as the probe. Fig. 5(A) shows the
pyridine-adsorbed FT-IR spectra at room temperature for the
as-synthesized catalysts. The typical FT-IR spectrum (not
shown) for pyridine adsorbed on pure Cs2.0HPMo12O40
showed the presence of a large number of Brønsted acid sites
(1540 cm−1) and the absence of Lewis acid sites (1450 cm−1).
This behaviour is consistent with previously reported data39
and our previous measurements.17 However, the substitution
of transition metals into the secondary structure resulted in
the appearance of bands at 1450 cm−1 originating from the
As shown in Fig. S2B,† the broad peak assigned to V 2p3/2
at about 517.5 eV could be fitted into two components at ca.
516.2 0.1 and 517.1 0.1 eV, which are related to the V4+
Fig. 5 FT-IR spectra of V-containing Cs-salt of HPCs with the incorporation of different transition metals after pyridine desorption at room tem-
perature (A) and 150 °C (B). a: Cs2.0Cu0.2PMo12O40 b: Cs2.0V0.2Ce0.2PMo12O40 c: Cs2.0V0.3Ni0.2PMo12O40 d: Cs2.0V0.2Fe0.2PMo12O40 e:
;
;
;
;
Cs2.0V0.3Cu0.2PMo12O40; and f: Cs2.0Cu0.2PMo11VO40
.
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Table 2 Catalytic performance of Cs2.0VxCu0.2PMo12O40 (x = 0–0.4) with various VO2+ contents
Selectivitya (%)
Conversion of
Catalyst
isobutane (%)
MAA
MA
Acetone
AcA
AA
CO
CO2
Cs2HPMo12O40
Cs2V0.3PMo12O40
Cs2Cu0.2PMo12O40
Cs2V0.1Cu0.2PMo12O40
Cs2V0.2Cu0.2PMo12O40
Cs2V0.3Cu0.2PMo12O40
Cs2V0.4Cu0.2PMo12O40
Cs2Cu0.2PMo12O40 + V2O5 (5%)
Cs2.0Cu0.2PMo11VO40
Cs2.5H1.5PMo11VO40
8.50
8.80
12.0
13.6
13.1
12.6
12.8
10.4
10.4
11.6
43.6
50.8
15.3
30.2
37.5
47.5
41.3
16.4
38.9
20.9
7.30
8.30
5.82
7.57
5.48
5.85
5.48
6.60
7.68
2.50
2.00
1.40
0.80
1.50
1.60
1.30
1.60
1.18
1.79
1.30
12.8
11.2
21.0
17.1
15.0
12.6
14.3
19.0
11.3
14.8
1.40
1.40
0.80
1.30
1.80
1.80
1.90
1.20
1.93
2.00
17.1
13.0
34.2
24.6
19.7
16.1
17.9
32.6
14.6
22.9
15.7
13.9
22.0
17.6
18.8
14.8
17.9
22.9
23.6
35.4
a
MAA: methacrylic acid; MA: methacrolein; AcA: acetic acid; and AA: acrylic acid.
pyridine coordinately bound to Lewis acid centers. The quan-
tification of the Lewis and Brønsted acid sites was deter-
mined on the basis of the bands at 1450 cm−1 and 1540 cm−1
according to the procedure reported by Emeis,40 and the cor-
responding semi-quantitative concentrations of acid sites
(mmol g−1-cat) are shown in the parenthesis in Fig. 5. It is
worth noting that the Cs2.0V0.3Cu0.2PMo12O40 catalyst with co-
doped VO2+ and Cu2+ in its secondary structure possessed
the greatest number of Lewis acid sites, which were evidently
far greater than those on the samples with the substitution
of other transition metals (such as Ni, Fe or Ce) and
Cs2.0Cu0.2PMo11VO40 catalyst with V species in its primary
structure. This indicated that the synergistic effect between
VO2+ and Cu2+ in the secondary structure could accommo-
date more acid sites on the surface of the
Cs2.0V0.3Cu0.2PMo12O40 catalyst, which was in agreement with
the NH3-TPD results. In addition, it can be observed that the
incorporation of Ni or Ce ions into the secondary structure
can diminish the intensity of the adsorption peak at 1540
cm−1, corresponding to reduction in the concentration of sur-
face Brønsted acidic sites. This may be due to basic Ce or Ni
anchoring at the Brønsted acidic sites.
A comparison of the pyridine adsorption at 150 °C for all
the investigated samples is presented in Fig. 5(B). The in-
crease in evacuation temperature causes drastic reduction
and even disappearance of the Lewis acid sites on the surface
of the samples, indicating that the strength of the Lewis acid
sites originating from transition metals is fairly weak com-
pared to the strength of the Brønsted acid sites. Specifically,
at higher temperatures, the Brønsted acid sites are dominant
over all the samples.
3.7 Selective oxidation of isobutane
To assess the catalytic activity of the synthesized samples, se-
lective oxidation of isobutane under isobutane-rich condi-
tions was carried out at 350 °C with a space velocity of 600
mL g−1-cat h. The variations in activity and selectivity are
listed in Table 2. Compared to the pure Cs2HPMo12O40 cata-
lyst, the sample with incorporated Cu in its secondary struc-
ture was significantly more active, in which an increase in
the isobutane conversion from 8% to 12% was observed, but
the selectivity to MAA was rather poor. In contrast, the single
VO2+-doped catalyst exhibited strong positive effect on the se-
lectivity to MAA, but the activity remained constant. Interest-
ingly, simultaneous doping of Cu2+ and VO2+ for the
Cs2.0HPMo12O40 sample had clear advantages for isobutane
conversion and selectivity compared to single Cu2+ or VO2+-
doped samples. Also, with the increase in VO2+ content, the
selectivity to MAA showed a significant increase, exhibiting a
maximum value for the Cs2.0V0.3Cu0.2PMo12O40 compound
with 47.5% selectivity to MAA. It is proposed that the increase
in catalytic performance for the oxidation of isobutane in-
duced by the association of VO2+ and Cu2+ cations is due to
the synergistic effect of VO2+ and Cu2+ in the secondary struc-
ture, which is involved in re-oxidation of Mo5+ formed during
the reaction process and this promotes the charge transfer
and circulation of Mo6+. To further investigate this synergistic
effect, we mechanically mixed V2O5 and Cs2.0Cu0.2PMo12O40,
but no improvement in catalytic performance was observed
compared to that of the Cs2.0Cu0.2PMo12O40 catalyst. In addi-
tion, for the Cs2.0Cu0.2PMo11VO40 catalyst with the same Cu
content but with V species initially inside the Keggin anion,
Table 3 Catalytic performance of V-containing Cs-salt of HPCs with the incorporation of different transition metals
Selectivitya (%)
Conversion
Catalyst
of isobutane (%)
MAA
MA
Acetone
AcA
AA
CO
CO2
Cs2V0.3Cu0.2PMo12O40
Cs2V0.2Fe0.2PMo12O40
Cs2V0.3Ni0.2PMo12O40
Cs2V0.2Ce0.2PMo12O40
12.6
8.70
7.70
6.30
47.5
37.6
40.5
40.6
5.85
8.62
9.08
1.30
2.10
2.30
2.40
12.6
11.2
13.1
11.2
1.80
1.30
1.30
1.60
16.1
18.1
15.5
15.1
14.8
21.0
18.1
17.5
12.0
a
MAA: methacrylic acid; MA: methacrolein; AcA: acetic acid; and AA: acrylic acid.
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idation of light alkanes,44 the oxidation of isobutane begins
by electron transfer from the isobutane substrate to HPCs to
yield reduced species; this is followed by oxygen transfer
from polyoxometalate to the activated substrate to produce
an oxygenated MAA product. The coexistence of VO2+ and
Cu2+ in the secondary structure results in a synergistic effect
that is beneficial for enhanced activity for the oxidation of
isobutane. According to the H2-TPR results, the reduction of
Cu2+ ions located in the secondary structure at a lower tem-
perature (∼350 °C) facilitates the redox process of catalyst
under reducing conditions, promoting rapid electron transfer
between the Cu and Mo cations and speeding up the supple-
mentation of lattice oxygen for the re-oxidation process (Cu2+
+ Mo5+ → Mo6+ + Cu+), which was believed to be the rate-
determining step under isobutane-rich conditions. Finally,
Cu+ is oxidized to Cu2+ by oxygen to complete the redox cycle.
These facts provide a reasonable explanation for the enhance-
ment of catalytic activity with the Cs2.0V0.3Cu0.2PMo12O40 cata-
lyst. Moreover, the synergistic effect between VO2+ and Cu2+
in the secondary structure leads to an increase in the amount
of surface acid sites on the Cs2.0V0.3Cu0.2PMo12O40 catalyst, as
shown in Fig. 5; this can activate the C–H bond and favour
the adsorption of the acid product MAA on the catalyst sur-
face and consequently increase its activity and selectivity.
Bayer and co-workers have also shown23 that the synergistic
function of the simultaneous presence of VO2+ and Cu2+
leads to more active and selective catalysts for the oxidative
dehydrogenation of isobutyric acid. Contrary to that observed
for Cu2+ ions, the Fe3+, Ni2+ or Ce4+ ions were not reduced be-
low 600 °C, as shown by their H2-TPR profiles. Thus, it ap-
pears that the Fe, Ni and Ce ions do not participate in the re-
dox mechanism like Cu ions. As a result, no enhancement in
catalytic activity was observed compared to that for the
Cs2.0V0.3Cu0.2PMo12O40 catalyst. Also, the lower selectivity to
MAA over the CsVMPMo12O40 (M = Fe, Ni or Ce) catalyst may
be ascribed to the lower number of surface acid sites.
Fig. 6 Selectivity to MAA as a function of isobutane conversion over
V-containing Cs salt of HPCs with the incorporation of different transi-
tion metals.
clear enhancement in selectivity to MAA and increase in the
yield of MAA product were observed; this indicated that this
beneficial synergistic effect was also present, but it was much
less intense with respect to that for the Cs2V0.3Cu0.2PMo12O40
sample. It is well-known20,41 that under isobutane-rich reaction
conditions, vanadium species in the Keggin anions are not sta-
ble, and restructuring of the compound occurs with the migra-
tion of the V species from the Keggin anions to counter cat-
ionic positions, in which the interaction between the Cu2+ and
V species leads to the generation of a synergistic effect. The
foregoing observations suggest that only V species at the sec-
ondary structure of HPC could interact synergistically with
Cu2+ counter ions, inducing a significant positive effect on the
oxidation of isobutane to MAA.
The effect of the substitution of other transition metal
ions (e.g., Fe3+, Ni2+, and Ce4+) as counter cations on the cata-
lytic activity and selectivity to the products was investigated,
and the results are summarized in Table 3. Compared to the
catalyst with co-substituted VO2+ and Cu2+ in its secondary
structure, the V containing Cs-salt of HPCs with the incorpo-
ration of Fe, Ni and Ce did not exhibit enhanced activity and
selectivity to MAA, indicating the absence of a synergistic ef-
fect between VO2+ and these transition metals located in the
secondary structure of HPCs.
Thus, the present results demonstrated that the co-
incorporation of Cu2+ and VO2+ in the secondary structure in
the Cs2.0V0.3Cu0.2PMo12O40 catalyst enhanced its catalytic per-
formance for the oxidation of isobutane under isobutane-rich
conditions due to the synergistic effect, which plays an im-
portant role in the promotion of the re-oxidation ability of
HPC catalysts.
Fig. 6 shows the selectivity dependence of the catalytic ac-
tivity for the Cs-salt of V-containing HPCs with the incorpora-
tion of different transition metals, in which the results fur-
ther showed that the Cs2.0V0.3Cu0.2PMo12O40 catalyst with co-
doped Cu2+ and VO2+ in its secondary structure exhibited su-
perior catalytic activity and selectivity to MAA for the oxida-
tion of isobutane under isobutane-rich conditions.
It has been proposed that the formation of oxygen vacan-
cies due to proton elimination associated with oxygen from
the heteropoly anions42 is easier in the presence of Cu2+.43
This proposal is in good agreement with our H2-TPR results
shown in Fig. 4. According to the mechanism for selective ox-
Conclusions
The catalytic activity for the oxidation of isobutane to
methacrylic acid varied with the substitution of different
transition
metals
in
VO2+-containing
HPCs.
The
Cs2.0V0.3Cu0.2PMo12O40 catalyst with the co-incorporation of
Cu2+ and VO2+ in its secondary structure exhibited the best
catalytic performance, which was mainly due to the synergis-
tic effect of Cu2+ and VO2+ involved in the re-oxidation of the
catalyst and accommodation of surface acid sites; conse-
quently, clear increase in its catalytic activity and selectivity
was observed. These findings should be helpful for providing
5780 | Catal. Sci. Technol., 2018, 8, 5774–5781
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a better design for catalysts and deeper insights into the na-
ture of active sites for the selective oxidation of isobutane to
methacrylic acid under isobutane-rich conditions.
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