Catalytic activity of V-substituted Cs2te0.2H 0.6 + xPMo12-xVxOn heteropoly compounds in selective oxidation of isobutane

Article abstract of DOI:10.1134/S0023158412030068

A series of Cs₂Te_0.2H_0.6 + xPMo ₁₂ - xVxOn (x = 0-3) heteropoly compounds has been.prepared and tested in the partial oxidation of isobutane. Catalytic tests show that at 350°C very high selectivity to methacrylic acid (60.1%) can be achieved at isobutane conversion of 12.2% over a Cs_2.0Te_0.2H _1.6PMo₁₁VOn catalyst with only one molybdenum atom per.unit cell substituted by vanadium. The presence of Te⁴⁺ in the heteropoly compounds appears to interfere with the dehydrqgenation step and favor the formation of methacrolein and methacrylic acid. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0023158412030068

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2012, Vol. 53, No. 3, pp. 404–408. © Pleiades Publishing, Ltd., 2012.
Catalytic Activity of VꢀSubstituted Cs Te H
PMo V O
2
0.2 0.6 +
x
12 – x x
n
1
Heteropoly Compounds in Selective Oxidation of Isobutane
Wenli Ding, Qiubin Kan, Heng Liu, Lulu Liu, and Jingqi Guan
College of Chemistry, Jilin University, Changchun, PR. China
eꢀmail: guanjg@jlu.edu.cn
Received July 15, 2011
Abstract—A series of Cs Te H
xPMo12 – xV O (
x
= 0–3) heteropoly compounds has been.prepared and
tested in the partial oxidation of isobutane. Catalytic tests show that at 350 very high selectivity to methꢀ
acrylic acid (60.1%) can be achieved at isobutane conversion of 12.2% over a Cs Te H PMo VO cataꢀ
2
0.2 0.6 +
x n
°
C
2.0 0.2 1.6
11
n
4
+
lyst with only one molybdenum atom per.unit cell substituted by vanadium. The presence of Te in the hetꢀ
eropoly compounds appears to interfere with the dehydrqgenation step and favor the formation of methacꢀ
rolein and methacrylic acid.
DOI: 10.1134/S0023158412030068
1
Harold and Samuel first described the selective oxiꢀ the effect of Te in the heteropoly compounds on the
dation of isobutane with heteropoly catalysts in 1979 activity and selectivity of catalysts in the partial oxidaꢀ
1]. Later, several research groups carried out the oxiꢀ tion of isobutanc. Especially the role of Te in Vꢀsubstiꢀ
[
dation of isobutane catalyzed by heteropoly comꢀ tuted heteropoly compound system is far from being
pounds [2–11]. Many investigations have been done understood.
on the modification of molybdophosphoric salts with
The aim of the work reported here was to explore
the Keggin structure. The salts can be modified by the ways leading to improvement of selectivity to a
substituting protons with potassium or cesium, substiꢀ desired product in the oxidation of isobutane over hetꢀ
2+
3+
2+
tuting transition metal ions such as Cu , Fe , Ni
for counterꢀcations, or by altervalent replacement
typically Vꢀsubstitution for Mo). Improvement of
eropolyacid catalysts.
EXPERIMENTAL
(
catalytic behavior can be observed due to an effective
modification process. In addition, other catalytic sysꢀ
tems, such as V/MCMꢀ41 and Te–Moꢀbased mixed
oxides have also been used as catalysts for this reaction
Catalyst Preparation
A series of H3 + xPMo12 – xV O (x = 0–3) hetꢀ
x
40
eropoly compound catalysts was prepared using
already described methods [23], In a typical synthesis,
[
12–20]. It has been found that high selectivity to
methacrolein (MAL) can be achieved over Te–Moꢀ
based mixed oxide catalysts, while high selectivity to
methacrylic acid (MAA) was observed with heteropoly
compounds.
7
.1 g of Na HPO4 was dissolved in 100 mL water and
2
mixed with NaVO3 that had been dissolved in 100 mL
of hot water. The mixture was cooled and acidified to
a red color with 5 mL of concentrated sulfuric acid. To
this mixture a solution of Na MoO4 was added.
Finally, this solution was vigorously stirred while
Earlier we were able to show that tellurium is a very
important element that serves to achieve high selectivꢀ
ity to MAL, whereas a phase containing V–Te4 is
active and selective component in the Mo–V–Teꢀ
based oxide catalysts for selective oxidation of isobuꢀ
tane to MAA [12–15], The mixed oxide catalysts
appear to have a multifunctional surface structure;
2
+
8
5 mL of concentrated sulfuric acid was added slowly.
Once the water solution was cooled the heteropolyacid
was extracted with 400 mL of ethyl ether. After separaꢀ
tion, a stream of air was passed through the hetꢀ
eropolyetherate layer to free ether. The solid was disꢀ
solved in 50 mL of water, and then allowed to crystalꢀ
lize. The formed crystals were collected by nitration,
washed with water, and dried in air.
6+
5+
Mo or
V
ions are responsible for the activation of
isobutane and abstraction of methylene hydrogen,
while the abstraction of the methyl hydrogen and
α
ꢀhydrogen (from isobutene) is associated with Te4
+
A series of Cs Te0.2H PMo12 – V O heteropoly
2 0.6 + x x x n
cation and the chemisorption of olefin and insertion of compounds were prepared as follows. An aqueous
oxygen occur on Mo6 ion [12, 21, 22]. However, solution of telluric acid was added dropwise to an
+
available information is not sufficient to understand aqueous solution of
H PMo12 – V O40, followed by
3 + x x x
the addition of aqueous solution of cesium carbonate at
1
o
The article is published in the original.
80 C. The resulting suspension or solution was evapoꢀ
404

CATALYTIC ACTIVITY OF VꢀSUBSTITUTED
405
4
Cs Te H PMo V O
2
0.2 3.6
9
3
n
3
2
Cs Te H PMo V O
2
0.2 2.6
10
2
n
1
Cs Te H PMo VO
11
2
0.2 1.6
n
Cs Te H PMo O
2
0.2 0.6
12
n
10
20
30
40
50
60
1200
1000 800
Wavenumber, cm
600
400
2
θ
, degrees
–1
Fig. 2. The FTꢀIR spectra of Cs Te0.2H0.6 + xPMo12 – xVxOn
2
heteropoly compounds:
Cs2.0Te0.2H1.6Mo VO ,
1
—Cs2.0Te0.2H0.6PMo O
2—
12
n,
Fig. 1. XRD patterns of Cs Te0.2H0.6 + xPMo12 – xVxOn
3—Cs2.0Te0.2H2.6PMo V O ,
2
11
n
10 2 n
heteropoly compounds.
4—Cs2.0Te0.2H3.6PMo V On.
9 3
rated to dryness at 80
lected, dried at 110
20
may be Cs Te H PMo xVxOz, but in this paper the products. Mass balance of carbon
°
C
. The powder samples were colꢀ then fed via heated lines to an onꢀline gas chromatogꢀ
°C for 12 h, and then calcined at raphy for analysis. Methacrolein, methacrylic acid,
3
°C
for 2 h. The actual composition of the samples COx (CO, CO2), and acetic acid (HAC) were the main
97% was typically
catalysts are designated as Cs Te H xPMo12 – xVxOn
2
0.2
y
12 –
≥
.
observed.
2
0.2 0.6 +
Catalyst Characterization
RESULTS AND DISCUSSION
XRD and FTꢀIR Studies
Powder Xꢀray diffraction patterns (XRD) were colꢀ
lected using a Shimadzu XRDꢀ6000 scanning
The Xꢀray diffraction patterns of the
Cs Te H xPMo12 – xV O heteropoly compounds
are shown in Fig. 1. Only the diffraction lines typical
to molybdophosphoric salts with, a cubic structure
(4 /min) with CuK radiation (40 kV, 30 mA). The
°
α
2
0.2 0.6 +
x
n
Fourier transform infrared spectra (FTꢀIR) were
recorded at room temperature using a NICQLET
Impact 410 spectrometer. Specific surface areas of the
catalysts were computed using the BET adsorption
[
2–6] were present on. the difiractograms. No peaks
due to MoO3 were observed on the XRD patterns.
isotherms of N2 measured at –196
ASAP2010). Scanning electron micrographs (SEM
°
C (Micronieritics
The FTꢀIR spectra of catalysts are compared in
)
–1
Fig. 2. The bands at 1.060, 965, 866 and 797 cm can
were taken through a FESEM XLꢀ30 field emission
scanning electron microscope. Xꢀray, photoelectron
spectra (XPS) were recorded on aVG ESCA LAB
be readily seen which, are characteristic of a Keggin
structure [2–6].
The SEM images of Cs Te H PMo O and
2.0 0.2 0.6
12
n
MKꢀII. Xꢀray electron spectrometer using Al
K radiꢀ
α
Cs Te H PMo V O are shown in Fig. 3. Sphere
2.0 0.2 3.6
9
3
n
ation (1486.6 eV, 10.1 kV). The binding energies for
the samples in spectra were referenced to the C1 line
at 284.7 eV. The measurement error of the spectra was
0.2 eV.
morphology of the samples can be easily recognized.
s
The average particle size of the Cs Te H PMo O
2.0 0.2 0.6
12
n
spheres near to 1.1
average particle size of the Cs Te H PMo V O
n
μ
m can be estimated, while the
±
2.0 0.2 3.6
9
3
spheres appears to be about 2.6 m.
μ
Table 1 shows the specific surface area (SBET) of the
catalysts. It can be seen that for all samples the surface
area of the Cs Te H xPMo12 – xVxOn catalysts gradꢀ
Catalytic Tests
The reaction was performed in a stainless steel
tubular fixed bed reactor (16 mm i.d., 400 mm long) at
atmospheric pressure. A mass flow controller reguꢀ
lated reaction feed, and a mini–pump fed water. The
catalytic reaction conditions was as follows: molar
2
0.2 0.6 +
ually decreases with increasing level of substitution of
Mo by V. Moreover, the SBET values slightly decrease as
the content of Cs in the Cs Te H PMo O catalyst
2.0 0.2 0.6
12
n
increases.
To gain further insight into the surface structure
gas hourly space velocity 1000 mL h g . The experꢀ and properties of these heteropoly compounds. XPS
ratio of the feed gas
i
ꢀC H : O : N : H O =1 : 1 : 2 : 1,
4
10
2
2
2
−
1
−1
cat
iments were carried out at 350 C. The products were spectra of Cs 3d5/2, Mo 3d5/2, V 2p3/2, Te 3d5/2 and P 2p
°
KINETICS AND CATALYSIS Vol. 53
No. 3
2012

406
DING et al.
(а)
(b)
100 nm
100 nm
Fig. 3. SEM images of Cs2.0Te0.2H0.6PMo O (a) and Cs2.0Te0.2H3.6PMo V O (b).
12
n
9 3 n
region with binding energies were investigated and the
results are given in Table 1.
The Cs 3d5 binding energies of surface cesium are
/2
centered on 724.1 eV for the Cs Te H xPMo12 – xVxOn
2
0.2 0.6 +
+
catalysts, which indicate that Cs cations are the most
The Mo 3d5/2 peak of the catalysts could be
important surface Cs species [11].
resolved into two components at 231.7 and 232.8 eV,
5+
6+
which can be assigned to Mo and Mo species,
respectively [11, 12]. The positions of the peaks indicate
Catalytic Properties
6+
that Mo cations are the main surface species present
Table 2 shows the catalytic data for isobutane oxiꢀ
on the surface of the Cs Te H xPMo12 – xV O cataꢀ
2
0.2 0.6 +
x
n
5
+
dation over Cs2.0Te0.2H1.6PMo11VO obtained at differꢀ
n
lysts along with small amounts of Mo
ions
5+
5+
6+
ent reaction temperatures. The conversion of isobuꢀ
tane gradually increases, while selectivity to MAL and
[
Mo /(Mo + Mo ) 0.1].
≤
The V 2p3/2 peak of the Cs Te H xPMo12 – xVxOn MAA progressively decreases with the increasing reacꢀ
2
0.2 0.6 +
catalysts could be resolved into two components at tion temperature. At relatively low reaction temperaꢀ
4
+
5
V
16.2 and 517.3. eV, which can be related to
V
and tures, small amounts of isobutene can be observed,
species, respectively [11, 12]. It can be thus while at higher temperatures isobutene is readily oxiꢀ
inferred that vanadium exists on the surface of the dized, it suffers further transformations and cannot be
Cs Te H
xPMo12 – xVxOn catalysts essentially as found among the reaction products.
5+
2
0.2 0.6 +
5
+
V
ions.
The catalytic results obtained for the oxidation of
isobutane over the Cs Te H xPMo12 – xVxOn cataꢀ
lysts at 350 C (Table 3) indicate that heteropoly comꢀ
2
0.2 0.6 +
The Te 3d5/2 peak of catalysis could be fitted with
two components at 576.2 and 577.3 eV, which can be
ascribed to Te and Te species, respectively [11, 12].
It can be suggested that a Te cation occurs in the catꢀ
alyst surface as the main tellurium ion. At the same
°
4
+
6+
pounds with incorporated the Teꢀions are active and
selective catalysts in the partial oxidation of isobutane.
The conversion of isobutane and selectivity to MAA is
considerably improved by substitution of vanadium for
molybdenum in the molybdophosphoric system.
However, the best catalytic performance can be
4+
0
time the presence of Te (binding energy of 573 eV)
was not observed on the catalysts.
The P 2p binding energies of surface phosphorus achieved over Cs Te H PMo VO catalyst with,
2.0 0.2 1.6 11 40
are very similar (ca. 133.7 eV) for all of the samples, one molybdenum ion per unit cell substituted by vanaꢀ
which indicate that P atoms are mainly present as P5+ dium. An increase in the extent of vanadium substituꢀ
in the Cs Te H
xPMo12 – xVxOn catalysts [11].
tion for molybdenum in the heteropoly compound
2
0.2 0.6 +
Table 1. BET surface area and XPS data for the heteropoly compounds
Binding energy, eV
Atomic ratios
S
,
BET
2
Sample
m /g
Cs 3 5
d
Mo 3d5/2 V 2p3/2 Te 3d5/2 P 2
p
Cs/P
Mo/P
V/P
Te/P
/2
Cs Te H PMo O
12 n
8.8
7.5
7.3
6.2
4.5
724.1
724.2
724.1
724.2
724.1
233.0
233.2
233.1
233.2
233.2
–
577.2
577.2
577.2
577.3
577.1
134.3
134.3
134.2
134.2
134.2
1.8
2.3
1.9
1.9
2.0
11.6
11.7
10.5
9.7
–
0.18
0.18
0.19
0.17
0.18
2.0 0.2 0.6
Cs Te H PMo O
12 n
–
–
2.5 0.2 0.1
Cs Te H PMo VO
n
516.7
516.8
516.9
0.8
1.8
2.5
2.0 0.2 1.6
11
Cs Te H PMo V O
10 2 n
2.0 0.2 2.6
Cs Te H PMo V O
9 3 n
8.8
2.0 0.2 3.6
KINETICS AND CATALYSIS Vol. 53
No. 3
2012

CATALYTIC ACTIVITY OF VꢀSUBSTITUTED
407
Table 2. Catalytic properties of Cs Te H PMo VO
n
2.0 0.2 1.6
11
Selectivity, %
CO
Reaction
temperature,
Converꢀ
C sion, %
°
=
C3
=
MAL
MAA
CO2
HAC
iꢀC4
300
310
320
330
340
350
360
5.2
6.5
2.2
1.3
9.3
9.1
8.9
8.6
8.3
8.0
6.5
69.1
68.2
66.8
64.6
62.5
60.1
55.2
10.3
11.7
12.7
14.2
15.5
16.8
19.3
8.4
8.8
traces
traces
traces
traces
traces
traces
0.8
0.6
0.8
1.0
1.2
1.5
1.7
2.5
7.8
0.8
9.7
9.3
0.5
10.8
12.1
13.3
15.6
10.8
12.2
14.6
traces
traces
traces
Note: Experimental conditions: contact time 3.6 s, catalyst weight 2 g, iꢀC H : O : N : H O = 1 : 1 : 2 : 1.
4 10 2 2 2
Table 3. Isobutane oxidation at 350°C
Selectivity, %
CO
Catalyst
Conversion, %
MAL
MAA
CO2
HAC
Cs Te H PMo O
12 n
8.5
6.6
9.3
7.7
8.0
7.8
7.6
38.3
15.4
60.1
52.7
48.9
26.7
21.2
16.8
22.3
23.5
18.7
23.1
13.3
15.6
16.2
6.7
26.1
1.7
2.0 0.2 0.6
Cs Te H PMo VO
2.5 0.2 1.1
11
n
Cs Te H PMo VO
12.2
10.7
9.6
2.0 0.2 1.6
11
n
Cs Te H PMo V O
10 2 n
1.5
2.0 0.2 2.6
Cs Te H PMo V O
9 3 n
3.7
2.0 0.2 3.6
catalysts deteriorates the catalytic behavior. Furtherꢀ the role of tellurium is less than completely underꢀ
more, by comparing the catalytic performance of stood. Over the cesium salts, the selectivity to MAL
Cs Te H PMo O and Cs Te H PMo VO it and MAA increases with the introduction of small
2.0 0.2 0.6
12
n
2.5 0.2 1.1
11
n
can be readily recognized that Cs2.5ꢀcontaining heleꢀ amount of tellurium. This observation implies that the
4+
ropoly compound shows poor catalytic properties, main role of Te ions lies in the transformation of the
indicating that activity and selectivity decrease when reaction intermediate into MAA, which accounts for a
more than two cesium cations per unit cell are introꢀ considerable increase in selectivity to MAA. It appears
4+
duced in exchange for protons.
that Te cations are active in the dehydrogenation
Figure 4 shows the results of the isobutane oxidaꢀ
tion catalyzed by Cs H PMo VO and Cs Te ꢀ
2
2.0
11
40
2.0 0.2
H PMo VO at 350
°
C
. It can be seen that the subꢀ
100
1.6
11
40
A
B
HAC
CO
CO
MAL
MAA
stitution of protons by tellurium in the cesium salt has
a strong positive effect on the selectivity to MAA and
MAL. At the same time, a minor increase in activity
was observed for the Teꢀsubstituted heteropoly comꢀ
pounds.
2
8
0
0
6
It has been generally accepted that the isobutane
6+
activation is associated with the presence of Mo surꢀ
4+
40
20
0
face site [21], while
V
ion can enhance catalytic
activity and favor the formation of the redox cycle
4
+
6+
5+
5+
V + Mo
↔
V + Mo to improve the redox capaꢀ
6+
bility of Mo species. In addition, a (Mo ) site is assoꢀ
2
ciated with the insertion of oxygen atom. Our previous
experiments indicated that Te–Moꢀbased mixed
oxides are very active and selective in the partial oxidaꢀ
tion of.isobutane [13–16]. However, in contrast to the
results obtained over heteropoly compounds, the main
product in the isobutane oxidation over Te–Moꢀbased
mixed oxides was MAL rather than MAA, Therefore,
1
1.6 11.8 12.0 12.2 12.4 12.6 12.8 13.0
Conversion, %
Fig. 4. Selective oxidation of isobutane over CS
Cs H PMo VO (A) and Cs2.0Te0.2H1.6PMo VOn
2
2.0
11
40
11
(B) catalysts at 350°C.
KINETICS AND CATALYSIS Vol. 53
No. 3
2012

408
DING et al.
step, because the efficiency of these ions in the
abstraction of hydrogen has already been repeatedly
reported for many oxidation reactions of alkenes and
alkanes [22].
7. Li, W., Oshihara, K., and Ueda, W., Appl. Catal., A
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. Langpape, M., Millet, J.M., Ozkan, U.S., and Bouꢀ
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The data outlined above suggest that by substituting
one vanadium atom per unit cell for molybdenum it is
possible to improve catalytic activity of the molybꢀ
dophosphoric system in the selective oxidation of
isobutane. However, an increased level of replacement
of protons by cesium deteriorates the catalytic perforꢀ
mance. It was found that the substitution of protons by
small amounts of tellurium in the cesium salt is favorꢀ
able for the formation of MAL and MAA. Under the
reaction conditions used, the Cs Te H PMo VO
1
0. Cavani, F., Etienne, E., Favaro, M., Galli, A., Trifiro, F.,
and Hecquet, G., Catal. Lett., 1995, vol. 32, p. 215.
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1 . M o u l d e r, J . F. , S t i c k l e , W. F. , S o b o l , P. E . , a n d
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2. Guan, J., Wu, S., Wang, H., Jing, S., Wang, G., Zhen, K.,
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2.0 0.2 1.6
11
n
catalyst showed the best selectivity to MAL + MAA
68.1%) with yield of 8.3% at 350
This work was supported by Jilin province (20090591
(
°C.
1
1
1
1
1
1
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KINETICS AND CATALYSIS Vol. 53
No. 3
2012