Excellent performance of lithium doped sulfated zirconia in oxidative dehydrogenation of ethane

Article abstract of DOI:10.1039/a808708c

Sulfated zirconia doped with lithium catalysts are found to be effective for the oxidative dehydrogenation of ethane to ethylene, giving 98% ethane conversion and 70% selectivity towards ethylene as well as 68% ethylene yield at 650°C.

Full text of DOI:10.1039/a808708c

Excellent performance of lithium doped sulfated zirconia in oxidative
dehydrogenation of ethane
Shaobin Wang, K. Murata, T. Hayakawa, S. Hamakawa and K. Suzuki
National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305-8565, Japan.
E-mail: swang@nimc.go.jp
Received (in Cambridge, UK) 6th November 1998, Accepted 23rd November 1998
Sulfated zirconia doped with lithium catalysts are found to
be effective for the oxidative dehydrogenation of ethane to
ethylene, giving 98% ethane conversion and 70% selectivity
towards ethylene as well as 68% ethylene yield at 650 °C.
wetness impregnation as described above. The catalytic tests
were carried out in a fixed-bed vertical-flow reactor constructed
from a high-purity alumina tube (id = 6 mm) packed with 1.0
g catalyst samples and 2 g quartz sand and mounted inside a
tube furnace. The feed, consisting of 10% ethane, 10% oxygen
and 80% nitrogen, was introduced into the reactor at a flow rate
of 60 ml min21. The reactants and products were analysed using
a gas chromatography (GC-8A) equipped with a Porapak Q
column using flame ionisation detector (FID) for hydrocarbons
and a 5Å molecular sieve column for CO, CO2, CH4, O2, N2,
and H2 using thermal conductivity detector (TCD). Before
catalyst testing, an empty tube loaded with only quartz sand was
evaluated for the oxidative dehydrogenation of ethane under
various conditions. It is found that the homogeneous contribu-
tion was negligible since ethane conversion was < 2% at
650 °C.
Table 1 presents the performance of ethane ODH over
supports (ZrO2 and SZ) and LiCl-based catalysts with 3.5 wt%
Li as well as Li2ZrO3 catalyst. It is found that the main products
of ethane oxidative dehydrogenation are C2H4, CO and CO2
with lower amounts of other products such as CH4, EtOH, and
C3H6. Ethane conversion increases with the increasing tem-
perature over all catalysts, however, ethylene selectivity
exhibits varying patterns with temperature depending on
catalyst. For zirconia, sulfated zirconia and Li/ZrO2, ethylene
selectivity increases with temperature.
The selective oxidation of alkanes into their corresponding
olefins has been of great interest in recent years. Ethane is an
abundant component in natural gas, however, it receives the
least attention due to the existing thermal cracking process.
Conversion of ethane into ethylene by oxidative dehydrogena-
tion (ODH) has several advantages and been studied over
several oxide-based catalysts. Catalysts containing vanadium,
molybdate and lithium are found to be effective for this reaction.
However, most of these systems are based on alumina, silica,
titania and magnesia.1–4 Little work has concentrated on
zirconia, in particular, sulfated zirconia based catalysts. Sul-
fated zirconia as a super acid solid is known as a very active
catalyst for the conversion of small hydrocarbons.5,6 One of the
authors has discovered the excellent performance of Li-doped
sulfated zirconia in the oxidative coupling of methane (OCM).7
The oxidative coupling of methane to C2-hydrocarbons is
inevitably accompanied by the concurrent conversion of ethane
to ethylene. Here we report the results of sulfated zirconia based
catalysts for the oxidative dehydrogenation of ethane into
ethylene.
The zirconia sample was prepared by two steps.7 The
resultant ZrO2 was then used to prepare sulfated zirconia (SZ)
by wetness impregnation using ammonium sulfate, and then
calcined at 700 °C for 3 h with a corresponding anion content of
6 wt%. After this, zirconia samples were impregnated with
lithium chloride, and calcined at 700 °C for 3 h. The loading of
lithium was varied between 2 and 8 wt%. A series of Li-doped
sulfacted zirconia (SZ) catalysts were also prepared using
different Li precursor compounds at 5 wt% Li content by
However, a decreasing trend of ethylene selectivity is
observed on Li/SZ at 600–650 °C. Li2ZrO3 shows similar
ethylene selectivity at the same temperature range. For zirconia
and sulfated zirconia, ethane conversions are > 60% with
ethylene selectivity of ca. 30% at 650 °C. However, sulfated
zirconia exhibits slightly higher catalytic activity and selectivity
towards ethylene. Doped lithium chloride on supports re-
markably improves ethane conversion and selectivity to
Table 1 Catalytic behavior of ethane ODH over zirconia-based catalysts at 650 °C
Conversion (%)
Selectivity (%)
Yield (%)
Catalyst
SBET/m2 g21 T/°C
C2H6
O2
0.01
0.4
2.4
100
100
100
69.7
86.3
100
69.9
83.0
95.3
50.2
69.5
96.3
6.0
C2H4
CO
CO2
CH4
C2H4
Quartz sand
600
620
650
0.2
0.4
2.0
97.8
98.0
97.7
10.8
15.0
29.2
16.6
18.0
30.8
54.6
58.3
60.1
82.5
79.5
69.8
85.7
86.4
83.3
2.2
2.0
2.3
0.3
0.8
1.7
2.2
2.5
3.1
0.6
0.8
1.4
0.5
0.6
1.7
0.5
0.7
0.7
0.20
0.39
1.95
5.6
8.3
19.2
7.0
ZrO2
21.2
72.5
4.6
600
620
650
600
620
650
600
620
650
600
620
650
600
620
650
51.6
55.4
65.9
42.3
53.8
70.4
48.8
66.5
87.2
62.4
83.7
97.6
5.9
24.5
24.0
19.2
38.6
35.6
18.8
7.3
6.0
9.3
5.4
6.4
64.4
60.2
49.2
42.6
43.9
47.3
33.7
28.4
21.8
8.5
10.1
14.3
0
0
8.0
SZ
9.6
21.7
26.6
38.8
52.4
51.5
66.5
68.1
5.0
LiCl/ZrO2
LiCl/SZ
Li2ZrO3
2.0
11.9
13.8
12.9
8.0
< 1
8.3
20.3
8.2
15.1
7.2
16.9
Chem. Commun., 1999, 103–104
103

Fig. 1 Effect of Li content in the LiCl/SZ system on catalytic activity.
ethylene. For LiCl/ZrO2, ethane conversion is enhanced to 87%
and ethylene selectivity is increased to > 60%. Similarly, ethane
conversion and ethylene selectivity over LiCl/SZ are also
significantly improved with 70% selectivity at 98% conversion,
giving 68% ethylene yield. LiCl/SZ also exhibits higher
ethylene selectivity than Li/ZrO2, suggesting that sulfation of
zirconia modifies the surface properties and could promote
ethylene selectivity. It has been found that Li/SZ shows better
catalytic performance than Li/ZrO2 in OCM reaction.7 Another
catalyst, Li2ZrO3, shows the lowest catalytic activity, but the
highest ethylene selectivity and the ethylene yield is lower than
that over the two supports (ZrO2 and SZ). Several researchers
have reported that Li-doped catalysts are much more selective
to ethylene in ethane ODH because of the increase of in number
of active sites for dehydrogenation.9,10
Fig. 2 Catalytic activity of ethane ODH as a function of time over LiCl/SZ
at 650 °C.
shown in Fig. 1. It can be seen that ethylene selectivity is
enhanced over all Li-doped SZ catalysts. However, the ethane
conversion and ethylene yield depend on lithium content. A
maximum value can be attained over 3.5 wt% Li/SZ.
Fig. 2 shows the catalytic performance of 3.5 wt% LiCl/SZ
and 5 wt% LiCl/SZ catalysts at 650 °C in terms of reaction time.
One can see that the two catalysts show different stability
behaviour. For 3.5 wt% LiCl/SZ ethane conversion and ethene
yield decrease gradually over 15 h and then remains nearly
unchanged thereafter. Ethane conversion and ethene yields are
reduced from 98 and 68% to 70 and 46%, respectively. Ethene
and COx selectivities stay at the same level during 24 h of
testing. The ethane conversion and ethene yield over 5 wt%
LiCl/SZ increases in the first 10 h and then decreases, however,
the deactivation rates are much slower than those of 3.5 wt%
LiCl/SZ. After 25 h testing, ethane conversion and ethene yield
are 50 and 42%, respectively. Like the behavior of 3.5 wt%
LiCl/SZ, ethene and COx selectivities show no alteration.
Investigations on 2 wt% LiCl/SZ and 8 wt% LiCl/SZ reveal that
2 wt% LiCl/SZ shows similar catalytic behaviour as 3.5 wt%
LiCl/SZ while 8 wt% LiCl/SZ presents the same characteristics
as that of 5 wt% LiCl/SZ. All these results seem to suggest that
LiCl loading on catalysts is a crucial factor influencing the
catalyst performance. Too high a LiCl content with result in a
decrease in activity.
Conway and Lunsford8 found that 5 g Li+–MgO–Cl2 could
produce a C2H6 conversion of 75–79% at a C2H4 selectivity of
70% with 58% ethylene yield at 650 °C under a flow rate of 60
ml min21. Ji et al.9 also obtained 60% ethane conversion with
75% ethylene selectivity, giving 45% ethylene yield over Li/La/
CaO catalysts at 650 °C. Therefore, the values obtained in this
work are much better than the above results.
From Table 1, it can be seen that the catalytic activity is not
dependent on the surface area. XRD measurements indicate that
two phases (monoclinic and tetragonal ZrO2) coexist in sulfated
zirconia while monoclinic ZrO2 with a trace of tetragonal ZrO2
is present in ZrO2. Li/ZrO2 and Li/SZ catalysts consist of
monoclinic ZrO2 and a fraction of Li2ZrO3 crystallites. The
peak intensities for Li2ZrO3 are higher for Li/SZ, suggesting
that a larger amount of Li2ZrO3 is present in Li/SZ. Therefore,
it is deduced that the synergistic effect of multiphases in the
catalyst are responsible for the catalytic activity and selectivity.
For Li doped catalyst, chlorine present in catalyst can also play
an important role for the higher selectivity over these catalysts.
Some researches have revealed that chlorine can promote the
decomposition of ethyl radicals to ethylene.10,11 Further work
on catalyst surface characterisation is in progress to elucidate
the active sites for the reaction.
In summary, sulfation of zirconia promotes selectivity
towards ethylene in the oxidative dehydrogenation of ethane.
LiCl-doped SZ exhibits not only a high ethane conversion but
also high selectivity towards ethylene and also high stability.
Notes and references
1 H. H. Kung and M. C. Kun, Appl. Catal., 1997, 157, 105.
2 T. Blasco and J. M. Lopez-Nieto, Appl. Catal., 1997, 157, 117.
3 E. A.Mamedov and V. Cortes-Corberan, Appl. Catal., 1995, 127, 1.
4 F. Cavani and F. Trifiro, Catal. Today, 1995, 24, 307.
5 X. Song and A. Sayari, Catal. Rev., 1996, 38, 329.
6 T.-K. Cheung and B. C. Gates, J. Catal., 1997, 168, 522.
7 K. Murata, T. Hayakawa and K. Fujita, Chem. Commun., 1997, 221.
8 S. J. Conway and J. H. Lunsford, J. Catal., 1991, 131, 513.
9 L. Ji, J. Liu, X. Chen and M. Li, Catal. Lett., 1996, 39, 247.
10 R. Burch, E. M. Crabb, G. D. Squire and S. C. Tsang, Catal. Lett., 1989,
2, 249.
The effect of lithium precursor (LiNO3, LiCl, LiF and
Li2CO3) in sulfated zirconia systems on the catalytic conver-
sion, ethylene selectivity and yield was also studied. It is found
that LiNO3 and LiCl doped SZ catalysts exhibit higher ethane
conversions, but that LiNO3/SZ shows the lowest ethylene
selectivity. The other three lithium doped SZ catalysts show
similar ethylene selectivity. In terms of ethylene yields, LiCl
doped sulfated zirconia catalyst generally gives the highest
values; the overall order is: LiCl/SZ > LiNO3/SZ > LiF/SZ >
Li2CO3/SZ.
11 D. Wang, M. P. Rosynek and J. H. Lunsford, J. Catal., 1995, 151,
155.
It is also found that lithium content affects the catalyst
performance. The dependence of activity, selectivity and yield
of LiCl/SZ catalysts with different lithium content at 650 °C is
Communication 8/08708C
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Chem. Commun., 1999, 103–104