NiO-polyoxometalate nanocomposites as efficient catalysts for the oxidative dehydrogenation of propane and isobutane

Article abstract of DOI:10.1039/b823369a

Novel nanocomposites of NiO and polyoxometalate (Cs_2.5H _0.5PMo₁₂O₄₀) with particle sizes in the range of 5-10 nm showed exceptional oxygen and ammonia adsorption capabilities, and the nanocomposites catalyzed the oxidative dehydrogenation of propane and isobutane efficiently under mild conditions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b823369a

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NiO–polyoxometalate nanocomposites as efficient catalysts
for the oxidative dehydrogenation of propane and isobutanew
Qinghong Zhang, Chuanjing Cao, Ting Xu, Miao Sun, Jizhe Zhang, Ye Wang*
and Huilin Wan*
Received (in Cambridge, UK) 5th January 2009, Accepted 20th February 2009
First published as an Advance Article on the web 12th March 2009
DOI: 10.1039/b823369a
Novel nanocomposites of NiO and polyoxometalate
(Cs2.5H0.5PMo12O40) with particle sizes in the range of 5–10 nm
showed exceptional oxygen and ammonia adsorption capabilities,
and the nanocomposites catalyzed the oxidative dehydrogenation
of propane and isobutane efficiently under mild conditions.
advantages of both NiO and polyoxometalates. Recently, we
have succeeded in synthesizing nanocomposites of NiO and a
polyoxometalate (Cs2.5H0.5PMo12O40, denoted as POM) with
excellent catalytic performances in the ODH of propane and
isobutane. Herein, we report the structure, adsorption properties
and catalytic behaviour of the NiO–POM nanocomposites.
NiO–POM composites with different compositions were
synthesized by a citric acid complexation method (see ESIw
for details). We fixed the composition of POM at
Cs2.5H0.5PMo12O40 here because the composites containing
POM with this composition showed outstanding catalytic
performances and good stability. XRD patterns of the
NiO–POM composites are shown in Fig. 1. For the composites
with NiO content of 85–75 wt% and POM content of
15–25 wt% (denoted as 85–75% NiO–POM), only diffraction
peaks of NiO could be observed. Moreover, these diffraction
peaks became much broader compared with those of single
NiO, indicating that the crystalline size of NiO in these
composites became smaller. With further decrease of NiO
content to r70 wt% in the composites, XRD peaks of NiO
became weaker and those of POM appeared.
Selective oxidation of light alkanes into olefins and organic
oxygenates is an attractive route for the utilization of abundant
light alkane resources. Although intensive efforts have been
made in this field, selective oxidation of C1–C4 alkanes still
remains an unsolved challenge, except for the conversion of
n-butane to maleic anhydride.1 The main reason is that the
alkane activation generally requires severe conditions, under
which the consecutive oxidation of reactive target products to
CO and CO2 can easily occur, leading to low selectivities to
target products at reasonably high conversions.2 Therefore,
the development of efficient catalysts which are capable of
working under mild conditions would be a promising route.
NiO is a typical p-type semiconductor, and various types of
oxygen species can be adsorbed on its surface under mild
conditions.3 Some studies have shown that NiO can work for
the oxidative dehydrogenation (ODH) of light alkanes at mild
temperatures (o500 1C).4 However, because NiO can be easily
reduced to Ni0, single NiO is hard to employ as a stable
catalyst with high catalytic performances. Some composite
oxides such as Ni–Ce–O, Ni–Nb–O and Ni–Ti–O with
relatively higher stability toward reduction have been investigated
for the ODH of ethane or propane, but olefin yields are still
not satisfactory.5 For the ODH of propane, the highest
propene yield over these composites was B12%.5c,d On the
other hand, polyoxometalates, which have received considerable
attention in materials science, catalysis and biological fields,6
are well known to have the ability to activate molecular
oxygen at moderate temperatures, and some substituted
polyoxometalates have been exploited for the selective oxidation
of light alkanes.7 It would be of interest to combine the
SEM and TEM observations suggest that the size and
morphology of the NiO–POM composites are different from
those of single compounds. Fig. 2 and 3 show that the 80%
and 70% NiO–POM samples are composed of uniform
nanoparticles with sizes of 5–10 nm, which are much smaller
than those of single NiO (B26 nm) or Cs2.5H0.5PMo12O40
State Key Laboratory of Physical Chemistry of Solid Surfaces,
National Engineering Laboratory for Green Chemical Productions of
Alcohols, Ethers and Esters, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen, 361005, China.
E-mail: wangye@xmu.edu.cn, hlwan@xmu.edu.cn;
Fax: +86-59-2183047; Tel: +86-592-2186156
w Electronic supplementary information (ESI) available: Experimental
details, EDS results, BET surface areas, adsorption amounts of O2 and
NH3, FT-IR spectra of adsorbed NH3, plots of selectivity versus
conversion, catalytic stability and FT-IR spectra before and after
reactions. See DOI: 10.1039/b823369a
Fig. 1 XRD patterns. (a) NiO, (b) 85% NiO–POM, (c) 80%
NiO–POM, (d) 75% NiO–POM, (e) 70% NiO–POM, (f) 50%
NiO–POM, (g) 30% NiO–POM, (h) POM (Cs2.5H0.5PMo12O40).
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Fig. 2 SEM images. (A) 80% NiO–POM, (B) 70% NiO–POM,
(C) NiO, (D) POM (Cs2.5H0.5PMo12O40). The scale bar in (A)–(C)
denotes 100 nm, while that in (D) denotes 200 nm.
Fig.
4
O2–TPD and NH3–TPD profiles. (a) NiO, (b) 85%
NiO–POM, (c) 80% NiO–POM, (d) 75% NiO–POM, (e) 70%
NiO–POM, (f) 50% NiO–POM, (g) POM (Cs2.5H0.5PMo12O40).
the stability of adsorbed oxygen on NiO by combining other
oxides always leads to marked decrease in the amount of oxygen
adsorption even based on the same amount (gram) of sample.5
We found an unexpected NH3 adsorption ability of the
NiO–POM nanocomposites. NH3–TPD results in Fig. 4B
show that there is no or only a small amount of NH3
adsorption over single NiO or POM. However, a large amount
of NH3 desorption was observed at 150–500 1C over the
NiO–POM nanocomposites with NiO contents of 85–70%.
As compared to single POM, the nanocomposites exhibited
significantly higher amount of NH3 adsorption per surface
area (see Table S2, ESIw). FT-IR studies of adsorbed NH3
suggest that the acidic sites over the nanocomposites are
mainly the Lewis type in nature, whereas the POM possesses
mainly the Brønsted acid sites (see Fig. S2, ESIw).
We have investigated the chemical states of Ni and Mo in
the NiO–POM nanocomposites by XPS studies. In the 80%
NiO–POM, the binding energy (EB) of Ni2p was at 854.2 eV,
which was higher than that in single NiO (EB = 853.8 eV). The
EB of Mo 3d5/2 in the composite was at 232.4 eV, lower than
that in Cs2.5H0.5PMo12O40 (EB = 232.9 eV). This suggests the
partial oxidation of Ni2+ to Ni3+ and the partial reduction of
Mo6+ to Mo5+ in the nanocomposite.8 Thus, an electron
transfer from Ni2+ to Mo6+ sites may occur in the composite.
We speculate that there may also be a migration of oxygen
anion from Mo to Ni sites, leaving oxygen vacancies around
the coordinatively unsaturated Mo5+ sites. This may create a
number of oxygen species on NiO phase near Ni3+ sites in the
nanocomposite. Because of the strong interaction with the
POM component, the oxygen species on NiO may become
more stable than those on single NiO. The coordinatively
unsaturated Mo5+ sites probably function as the Lewis acid
sites responsible for the unique NH3 adsorption over the
NiO–POM composite. These Mo5+ sites may also work for
the adsorption and activation of molecular oxygen.
Fig. 3 TEM images. (A) 80% NiO–POM, (B) 70% NiO–POM,
(C) NiO, (D) POM (Cs2.5H0.5PMo12O40).
(1–2 mm). EDS analyses revealed that all the elements including
Ni, Cs, P, Mo and O distributed homogeneously over the
NiO–POM composite (see Fig. S1, ESIw). The surface areas of
the 85–70% NiO–POM composites were larger than those of
single NiO and POM (see Table S1, ESIw).
We found that the NiO–POM nanocomposites showed inter-
esting oxygen adsorption behaviour. For single NiO, we ob-
served three O2 desorption peaks at 220, 325 and 490 1C in the
O2–TPD profile (Fig. 4A). These peaks were reported to arise
from the oxygen species chemisorbed on NiO surface and were
À
assigned to O2 (the first peak) and OÀ species (the second and
the third peaks).3 On the other hand, there was almost no
desorption of O2 from the POM. For the 85% and 80%
NiO–POM nanocomposites, the O2 desorption pattern was
the same as that of NiO, but the desorption temperatures
increased by B200 1C. This indicates that the oxygen species
become more stable than those on single NiO. The amount of
O2 adsorbed per gram of sample for these two nanocomposites
was larger than that for single NiO although the adsorption
amount per surface area became lower for the nanocomposites
(see Table S2, ESIw). To our knowledge, the effort to improve
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Table 1 shows the catalytic performances of the NiO–POM
nanocomposites for the ODH of propane at 450 1C. Single
NiO only catalyzed the formation of CO and CO2 under the
conditions in Table 1, whereas single POM showed a very low
propane conversion. The NiO–POM nanocomposites could
catalyze the selective formation of propene at good propane
conversions. Moreover, the nanocomposite prepared by the
citric acid complexation method exhibited much higher
selectivity than the corresponding physical mixture of NiO
and POM. We further compared propene selectivities over the
80% NiO–POM and NiO at different propane conversions,
and the result confirmed that the 80% NiO–POM was a
significantly more selective catalyst for the ODH of propane
(see Fig. S3, ESIw). The 80% NiO–POM catalyst was found to
be stable during the reaction, and the propene yield did not
undergo significant changes with time on stream (see Fig. S4,
ESIw). To our knowledge, the propene yield (20%) obtained
over the present nanocomposite is the highest one reported to
date under such a mild temperature. Furthermore, our FT-IR
studies for the NiO–POM composites before and after
the catalytic reaction under conditions of Table 1 indicated
that there was no significant change in the structure of the
nanocomposites (see Fig. S5, ESIw).
Table 2 Catalytic performances of the NiO–POM nanocomposites
for the oxidative dehydrogenation of isobutanea
Selectivityb/%
i-C4H8
MAc
CO2
Catalyst
NiO
Temp./1C
Conv./%
400
450
400
450
450
500
450
500
400
43
50
16
48
15
21
4.7
8.4
o1.0
11
0
0
0
0
67
69
38
80
10
18
7
80%NiO–POM
70%NiO–POM
50% NiO–POM
63
20
79
71
93
87
—
0
11
11
0
0
—
13
—
POM
a
Reaction conditions: W = 0.5 g; P(i-C4H10) = 5.6 kPa; P(O2) =
b
11.2 kPa; P(N2) = 84.2 kPa; F(total) = 90 mL minÀ1
.
Other
c
products are mainly CH4. MA denotes methacrolein.
the adsorption of oxygen and ammonia and superior catalytic
behaviours in the ODH of propane and isobutane. A stable
propene yield of 20% can be obtained in the ODH of propane
at 450 1C. For the ODH of isobutane, the selectivity to
isobutene and methacrolein reaches 90% at an isobutane
conversion of 15%.
The NiO–POM nanocomposites also showed superior
catalytic performances for the ODH of isobutane. Over the
70% NiO–POM nanocomposite, the selectivities to isobutene
were 79% and 71% at isobutane conversions of 15% and 21%
at 450 and 500 1C, respectively (Table 2). The total selectivity
to isobutene and methacrolein reached 90% and 82% at the
same time. These performances are significantly better than
those reported for other catalysts.9 The 70% NiO–POM was
also stable during the ODH of isobutane (see Fig. S6, ESIw).
We suggest that the superior performances of the NiO–POM
nanocomposites in the ODH reactions are related to the
enhanced stability of the oxygen species. Moreover, the
This work was supported by the NSFC (No. 20625310,
20773099 and 20873110), the National Basic Research
Program of China (No. 2005CB221408), and the Program
for New Century Excellent Talents in Fujian Province (Q.Z.).
Notes and references
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À
disappearance of O2 species over the nanocomposites with
NiO content r75 wt% (Fig. 4A) may also contribute to their
higher selectivity.
5 (a) P. Boizumault-Moriceau, A. Pennequin, B. Grzybowska and
Y. Barbaux, Appl. Catal., A, 2003, 245, 55; (b) E. Heracleous and
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In conclusion, we have succeeded in synthesizing
a
NiO–POM nanocomposite with particle sizes in the range of
5–10 nm. The nanocomposite exhibits unique capabilities for
6 (a) A. Proust, R. Thouvenot and P. Gouzerh, Chem. Commun.,
2008, 1837; (b) E. Cartuyvels, G. Absillis and T. Parac-Vogt, Chem.
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Table 1 Catalytic performances of the NiO–POM nanocomposites
for the oxidative dehydrogenation of propanea
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Selectivityb/%
C3H6
C3H6
CO
CO2
Catalyst
Conv./%
yield/%
NiO
100
72
44
23
0
20
45
65
75
81
95
14
13
0
59
80
50
27
16
17
3.0
86
0
8.2
20
85% NiO–POM
80% NiO–POM
75% NiO–POM
70% NiO–POM
50% NiO–POM
POM
4.6
6.4
5.9
2.2
1.9
0
15
11
8.2
2.4
1.4
7.9
3.0
1.5
55
80% NiO–POMc
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G. Selleri, F. Trifiro and A. Trovarelli, J. Catal., 1996, 160, 317;
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P. Amoros, D. Beltran-Porter and V. Corberan, Catal. Today,
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a
Reaction conditions: T = 450 1C; W = 0.5 g; P(C3H8) = 4.1 kPa;
P(O2) = 16.2 kPa; P(N2) = 81.1 kPa; F(total) = 50 mL minÀ1
.
Other products mainly include CH4, C2H6 and C2H4. Prepared by
physical mixing.
b
c
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This journal is The Royal Society of Chemistry 2009
2378 | Chem. Commun., 2009, 2376–2378