Promotion by strontium fluoride of neodymium oxide catalysis of the oxidative coupling of methane

Article abstract of DOI:10.1039/a800367j

Catalytic performance measurements showed that CH₄ conversion and C₂ selectivity were higher over SrF₂/Nd₂O₃ catalyst than over pure Nd₂O₃ for the oxidative coupling of methane (OCM). XRD indicated that there was some exchange of F^- and O^2-, leading to the formation of tetragonal and rhombohedral NdOF phases during the catalyst preparation. After a certain amount of SrF₂ was added to Nd₂O₃, the increase in surface basicity and conductivity, as well as the role of F^- on the dispersion of surface active sites, would be favourable to a decrease in the secondary deep oxidation of methyl radicals and C₂ hydrocarbons, giving improved C₂ selectivity. Since there is a certain correlation between methane conversion and C₂ selectivity for an oxygen-limited OCM reaction, methane conversion is also expected to increase with the increase in C₂ selectivity.

Full text of DOI:10.1039/a800367j

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Promotion by strontium fluoride of neodymium oxide catalysis of the
oxidative coupling of methane
Ruiqiang Long*¤ and Huilin Wan
Department of Chemistry and State Key L aboratory for Physical Chemistry of the Solid Surface,
Xiamen University, Xiamen, 361005, PeopleÏs Republic of China
Catalytic performance measurements showed that CH conversion and C selectivity were higher over SrF /Nd O catalyst than
4
2
2
2 3
over pure Nd O for the oxidative coupling of methane (OCM). XRD indicated that there was some exchange of F~ and O2~,
2
3
leading to the formation of tetragonal and rhombohedral NdOF phases during the catalyst preparation. After a certain amount
of SrF was added to Nd O , the increase in surface basicity and conductivity, as well as the role of F~ on the dispersion of
2
2 3
surface active sites, would be favourable to a decrease in the secondary deep oxidation of methyl radicals and C hydrocarbons,
2
giving improved C selectivity. Since there is a certain correlation between methane conversion and C selectivity for an oxygen-
limited OCM reaction, methane conversion is also expected to increase with the increase in C selectivity.
2
2
2
OCM to C hydrocarbons is an attractive means of using the
abundant natural-gas resource. Since Otsuka et al.1 reported
that rare earth oxides (REO), with the exception of Ce, Pr,
e†ect of alkaline-earth-metal Ñuoride, we also investigated the
2
catalytic performance for the OCM reaction over di†erent
alkaline-earth-metal Ñuoride promoted neodymium oxide
catalysts. The results showed that CH
and Tb oxides, exhibit very high C selectivity for OCM
conversion and C2
4
2
under the condition of extremely high CH /O ratio, REO,
selectivity over these alkaline-earth-metal Ñuoride promoted
Nd O catalysts were apparently better than over pure
4
2
either alone or in combination with alkali- or alkaline-earth-
metal oxides or carbonates, have been extensively studied as
OCM catalysts.2h8 From previous work, it has been generally
accepted that the major reaction path of the OCM reaction is
as follows: a methane molecule reacts with a surface oxygen
species to produce a gaseous methyl radical, which may then
couple with a second methyl radical in the gas phase to give
ethane; further dehydrogenation of ethane to form ethene
takes place in the gas phase or on the surface of cata-
lyst.2,3,9h11 Together with this major reaction path, methyl
2
3
Nd O and that SrF was the best promoter among these
2
3
2
alkaline-earth-metal Ñuorides, hence we have focused on the
SrF ÈNd O catalyst in this study. The surface properties (e.g.
2
2 3
speciÐc surface area, surface element composition, surface
basicity) and bulk properties (e.g. bulk composition and struc-
ture, and conductivity) of the SrF -promoted Nd O catalyst
2
2 3
were also studied and compared with pure Nd O . The
2
3
reason that surface basicity and conductivity were selected for
study is that these two properties have been considered to be
two fundamental characteristics of a good OCM catalyst.8
The relationship between the physicochemical properties and
catalytic performance of the catalysts is discussed. The results
radicals and C hydrocarbons also have a chance of being
deeply oxidized to carbon dioxide or monoxide (CO ) on the
catalyst surface, so it is necessary to minimize the secondary
2
x
deep oxidation reaction in order to get a good C
and yield.
2 selectivity
showed that SrF modiÐed the surface and bulk properties of
2
Nd O , leading to an increase in surface basicity and conduc-
2
3
Many investigators have found that the catalytic per-
formance, especially the selectivity to ethene, can be signiÐ-
cantly improved by chlorine, present either in the form of a
chloride component built into the catalyst or as a volatile
chlorinated compound (organic or inorganic) in the reactant
feed.3,7,8 Considering that metal Ñuorides (especially alkaline-
earth-metal Ñuorides) are usually more stable than the corre-
sponding chlorides, and that F~ (r \ 0.133 nm) and O2~
tivity. This would favour a decrease in the secondary deep
oxidation of methyl radicals and C hydrocarbons, and result
2
in an improvement in C selectivity. Moreover, the increase in
2
methane conversion is attributed to the improvement in C
selectivity, because there is a certain relationship between
2
methane conversion and C selectivity for an oxygen-limited
2
OCM reaction.
(
r \ 0.132 nm) have similar ionic radii so that anionic
exchange between Ñuoride and oxide may easily take place
leading to the formation of lattice defects, such as anionic
Experimental
Preparation and evaluation of catalyst
(
vacancies that are requisites for the adsorption and activation
Each catalyst sample was prepared by the method of grinding
and calcination, as described elsewhere.13 Di†erent mole
ratios of SrF ÈNd O , CaF ÈNd O , BaF ÈNd O and
of O over the compound catalysts with stable cationic
2
valences), we have developed a new series of metal Ñuoride
(
mainly alkaline-earth-metal Ñuoride) promoted REO cata-
2
2 3
2
2 3
2
2 3
Sr(NO ) ÈNd O were mixed and carefully ground into Ðne
lysts in recent years. These catalysts (e.g. SrF ÈLa O ,
3 2
2 3
2
2 3
powder. The solid mixture was mixed with a certain amount
of deionized water to form a paste, then dried at 393 K for 4 h
and calcined at 1173 K for 6 h. The resulting solid was
SrOÈLaF , BaF ÈCeO , Cs OÈCeF ÈCeO ) showed good
3
2
2
2
3
2
catalytic performance in OCM and in the oxidative dehydro-
genation of light alkanes (C H and C H ).12,13 In previous
2
6
3 8
crushed and sieved to 40È60 mesh particles. The pure Nd O
work, we found that, when metal Ñuoride was added to
oxides, partial substitution of cations and/or anions occurred.
In order to understand clearly the nature of the promoting
2 3
and SrF used for the catalytic performance evaluation were
2
also treated similarly. The materials, such as Nd O (99.95%),
2 3
CaF (98.5%), SrF ([98%), BaF (98.5%) and Sr(NO )
2
2
2
3 2
(
99.5%), were obtained from Shanghai Chemicals Company.
¤
Current address: Department of Chemical Engineering, Uni-
The catalytic reaction was carried out in a Ðxed-bed quartz
versity of Michigan, Ann Arbor, MI 48109-2136, USA.
reactor, 5.0 mm id, with CH /O \ 2È8 (mole ratios, without
4
2
J. Chem. Soc., Faraday T rans., 1998, 94(8), 1129È1135
1129

View Article Online
dilution gas) with a Ñow rate of 66.7 ml min~1; methane
99.99%) and oxygen (99.5%) were used without further puriÐ-
cation. In each experimental run, 0.20 ml (unless otherwise
indicated) of catalyst was used, and the effluent gas was
analysed at room temperature with an on-line Shang Fen 102
GD gas chromatograph equipped with a thermal conductivity
quartz reactor was treated in a Ñow of helium at 1173 K for
30 min. The amount of carbon dioxide desorbed was deter-
mined with a gas chromatograph equipped with a thermal
conductivity detector.
The bulk composition and structure of the catalyst were
measured by XRD. The experiment was carried out at room
temperature on a Rigaku RotaÑex D/Max-C system with
Cu-Ka (j \ 0.154 06 nm) radiation. The samples were loaded
to a depth of 1 mm on a sample holder. XRD powder patterns
were recorded in the range 2h \ 20È70¡.
The conductivity was determined by measuring the speciÐc
resistance of the catalyst disk between two parallel platinum
sheet electrodes in the frequency region 0.1È100 kHz on an
M378 system. Before the measurement, the catalyst was
pressed to a disk with a diameter of 13 mm and a thickness of
2.5È3 mm under a pressure of 4.5 ] 103 kg cm~2 and calcined
at 1173 K for 12 h. Platinum was then sprayed onto the
surface of the sample under vacuum (10~2 Torr) in an IB-3
10N Coater instrument, in order to minimize the contact
resistance between the platinum electrode and the sample. The
sample was laid in a quartz cell and heated from room tem-
(
detector, with 5A molecular sieve column for O and CO, and
2
a GDX 502 column for CH , C H , C H and CO . Water
4
2 4 2 6
2
and hydrogen gas were also present but were not measured.
Other details were as reported previously.13
To determine the Ñuorine loss after calcination and reac-
tion, 20 mg of the fresh and used 50 mol.% SrF ÈNd O cata-
2
2 3
lysts were weighed and dissolved in ca. 2 ml of 6 M nitric acid.
The pH of the solution was then adjusted to 6È7 by adding a
certain amount of 1.1 M CH CO Na solution. The solutions
3
2
were then diluted to 50.00 ml in a volumetric Ñask. In each
case, the concentration of F~ in the solution was determined
by means of a calibrated Ñuoride-ion-selective electrode.14
Based on the weight of the catalyst and the concentration of
F~ in the solution, the content of Ñuorine in the catalyst
(
expressed as mg of F~ per gram of catalyst) was easily deter-
mined.
perature to 1173 K in a Ñow of N (20 ml min~1). The resist-
2
ance spectra were recorded at speciÐed temperatures during
Catalyst characterization
the heating period. At each speciÐed temperature, the tem-
perature of the quartz cell was maintained constant for ca. 10
min, to ensure that the resistance measurement was conducted
under thermostatic conditions.
The surface composition was analysed by means of X-ray
photoelectron spectroscopy (XPS). Spectra were recorded on a
VG ESCALAB 210 XPS/AES instrument at room tem-
perature, with Mg-Ka as the excitation radiation. The catalyst
samples were pressed into wafers (diameter, 10 mm) under a
pressure of 100 kg cm~2 and were loaded onto a sample
holder for XPS analysis. The base pressure of the XPS
analysis chamber was ca. 7 ] 10~11 Torr. All measured
binding energies were calibrated with respect to the C 1s
energy at 284.6 eV due to adventitious carbon. Eclipse 1.7T
software was used to resolve the experimental spectra. The
surface composition of the catalysts was estimated from peak
areas using appropriate instrumental sensitivity factors.
Results and Discussion
Catalytic performance
The catalytic performances of the catalysts for OCM are sum-
marized in Table 1. Over pure Nd O catalyst, only 26.6È
2
3
27.7% methane conversion and 36.3È42.7% C selectivity
2
were obtained in the temperature range 973È1073 K with a
CH /O \ 3/1 and GHSV \ 20 000 h~1. Whereas, after the
4
2
The speciÐc surface area of the catalyst was measured by
addition of a certain amount of SrF (which alone showed
2
the BET method with N adsorption at 77 K on a Sorptmatic
almost no activity for OCM) to Nd O , and ignition at high
2
2 3
temperature, methane conversion and C selectivity, as well as
2
1
900 Carlo Erba instrument. A large quantity of catalyst (5.0
g) was used, in order to increase the precision of the measure-
ment. Samples were outgassed at 523 K under vacuum
the ethene/ethane ratio, increased under identical conditions.
At the same time, the catalytic performance of 50 mol.%
(
5 ] 10~2 Torr) for 3 h before nitrogen adsorption. The spe-
SrF ÈNd O catalyst was also found to be superior to that of
2
2 3
ciÐc surface areas of Nd O and 50 mol.% SrF ÈNd O were
4
50 mol.% SrO-promoted Nd O catalyst, indicating that
2
3
2
2 3
2 3
.4 and 3.8 m2 g~1, respectively.
strontium Ñuoride played a signiÐcant promoting role for
The basicity of the catalyst was measured by temperature-
OCM with Nd O as host catalyst. A maximum C yield of
2
3
2
programmed desorption (TPD) of carbon dioxide (adsorbed
at 300 K) in a quartz reactor from 300 to 1173 K at a linear
heating rate of 20 K min~1 in a Ñow of helium (30 ml min~1).
Before the TPD experiment, the catalyst (0.2 g) packed in a
19.6% could be obtained over 50 mol.% SrF ÈNd O catalyst
2
2 3
at 1023 K, which was ca. 9% and 5% higher than over Nd O
and 50 mol.% SrOÈNd O catalysts under the same condi-
2
3
2
3
tions.
Table 1 Catalytic performance of catalysts for OCM
conv. (%)
selectivity (%)
C H
C2
yield
(%)
catalyst
T /K
CH4
O2
CO
CO2
C H
C2
2
4
2
6
SrF2
1073
2.5
0.8
no activity
3.8
1.4
0
0
24.8
46.0
16.4
0
58.8
54.0
75.2
54
1.9
0.4
1
9
023
73
1073
023
73
1073
023
73
1073
023
Nd O3
26.6
27.2
27.7
28.8
29.4
30.0
33.6
34.3
33.7
33.1
99.6
99.7
99.7
99.3
99.1
99.2
99.0
98.9
98.7
98.2
8.2
7.2
5.8
4.1
4.1
4.3
3.7
4.0
5.1
7.1
55.5
53.5
51.5
47.7
45.8
44.3
39.7
38.9
39.9
41.0
19.2
20.1
21.7
27.4
27.8
28.0
33.5
33.1
30.4
27.2
17.1
19.2
21.0
20.8
22.3
23.4
23.1
24.0
24.5
24.7
36.3
39.3
42.7
48.2
50.1
51.4
56.6
57.1
54.9
51.9
9.6
2
1
9
10.7
11.8
13.9
14.7
15.4
19.0
19.6
18.5
17.2
5
0 mol.% SrOÈNd O
2
3
1
9
5
0 mol.% SrF ÈNd O
2
2 3
1
9
9
73
23
Conditions: CH /O \ 3/1 and GHSV \ 20 000 h~1.
4
2
1130
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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Table 2 Catalytic performance of 50 mol.% MF ÈNd O (M \ Ca, Sr, and Ba) catalysts
2
2
3
conv. (%)
selectivity (%)
C2 yield
(%)
catalyst
CH4
O2
CO
CO2
C H
C H
C2
2
4
2
6
CaF ÈNd O
29.6
34.3
30.8
98.9
98.9
99.3
6.8
4.0
3.5
45.2
38.9
42.6
26.1
33.1
30.6
21.9
24.0
23.3
48.0
57.1
53.9
14.2
19.6
16.6
2
2
3
SrF ÈNd O
2
2
3
BaF ÈNd O
2
2
3
Conditions: T \ 1023 K, CH /O \ 3/1, and GHSV \ 20 000 h~1.
4
2
Table 2 shows the catalytic performance over di†erent
alkaline-earth-metal Ñuoride promoted Nd O catalysts at
for 45 h, the content Ñuorine in the catalyst decreased to 56.7
mg g~1.
2
3
1
023 K. The methane conversion and C selectivity were
Over the catalysts studied (except for those with SrF2
content exceeding 75 mol.%), the conversions of oxygen were
more than 98%, indicating that OCM was carried out under
an oxygen-limited condition. This makes it difficult to
compare the real activity of these catalysts and the reason for
the increase in methane conversion becomes complex as a
2
found to increase according to the sequence Ca \ Ba \ Sr, i.e.
SrF was the most e†ective promoter among the alkaline-
earth-metal Ñuorides for the Nd O
reaction.
2
3 catalyst in the OCM
2
As shown in Fig. 1, at 1023 K, methane conversion and C
2
selectivity increased with SrF loading, reached a maximum at
result of doping the Nd O host catalyst with strontium Ñuo-
2
2 3
3
3È50 mol.%, and then decreased as the loading increased
ride. It is interesting to note that, for almost all of the catalysts
further. Over 50 mol.% SrF ÈNd O catalyst, when the
2 3
CH /O ratio was increased from 2/1 to 8/1, the C selectivity
increased from 44.4% to 73.4%, with an accompanying
under the same methane-to-oxygen mole ratio, the higher the
2
C selectivity, the higher the methane conversion, under di†er-
4
2
2
2
ent conditions. For example, over 50 mol.% SrF ÈNd O
2
2 3
decrease in CH conversion from 41.5% to 17.7% at 1023 K
catalyst, C selectivity was found to increase at Ðrst, pass
through a maximum, then decrease as the temperature
4
2
(
Fig. 2).
In the absence of catalyst, the conversion of methane was
increased from 923 to 1073 K, and the methane conversion
varied with temperature in the same manner (Table 1). This
phenomenon was also observed over other catalysts at di†er-
ent temperatures (Table 1). Hence, it is suggested that C2
selectivity may have something to do with methane conver-
sion for an oxygen-limited OCM reaction. According to the
reaction equations:
less than 0.7% at 923È1073 K with CH /O \ 3/1 and
4
2
GHSV \ 20 000 h~1. This showed that the conversion of
methane by its homogeneous oxidation with O was negligi-
2
bly small compared with OCM over the catalysts studied.
The catalytic performance of the 50 mol.% SrF ÈNd O
2
2 3
catalyst with respect to time on stream indicated that CH
4
conversion and C selectivity gradually decreased over a
2at
1023
K
with CH /O \ 3/1 and
CH ] 1/4O \ 1/2C H ] 1/2H O
(1)
(2)
(3)
(4)
period of 30
h
4
2
2 6
2
4
2
2
GHSV \ 20 000 h~1, leading to a decrease in C yield from
CH ] 1/2O \ 1/2C H ] H O
4
2
2 4
2
1
9.6% to 15.2%. The C yield then remained almost
2
CH ] 3/2O \ CO ] 2H O
unchanged for the following 15 h. The gradual change in the
catalytic behaviour of the catalyst was accompanied by the
loss of some F~ from the sample as HF. The analytical result
showed that the content of Ñuorine in the fresh 50 mol.%
SrF ÈNd O was 78.6 mg g~1, which was smaller than that
4
2
2
CH ] 2O \ CO ] 2H O
4
2
2
2
it is obvious that the amount of oxygen consumed in convert-
ing a deÐnite amount of methane to ethene and ethane [eqn.
(1) and (2)] is less than that used to convert the same amount
2
2 3
(
82.2 mg g~1) in the sample before calcination. After reacting
Fig. 1 E†ect of SrF content on (a) CH conversion and (b) C selec-
Fig. 2 E†ect of CH /O ratio on (a) CH conversion and (b) C
2
4
2
4 2 4 2
selectivity over 50 mol.% SrF ÈNd O catalysts at 1023 K with
tivity over SrF ÈNd O catalysts at 1023 K with GHSV \ 20 000 h~1
2
2
3
2
2 3
and CH /O \ 3
GHSV \ 20 000 h~1
4
2
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1131

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Table 3 E†ect of the amount of catalyst on the catalytic performance
conv. (%)
selectivity (%)
C H
volume
/ml
C2 yield
(%)
catalyst
CH4
O2
CO
CO2
C H
C2
2
4
2
6
Nd O3
0.2
27.2
27.5
27.4
28.0
34.3
33.9
24.9
14.0
99.7
99.8
99.7
99.6
98.9
99.7
80.5
38.0
7.2
10.4
11.7
14.9
4.0
4.9
8.6
53.5
49.9
48.9
44.8
38.9
38.6
35.3
37.0
20.1
20.9
20.7
20.3
33.1
32.1
29.3
15.8
19.2
18.8
18.7
20.0
24.0
24.4
26.8
34.9
39.3
39.7
39.4
40.3
57.1
56.5
56.1
50.7
10.7
10.9
10.8
11.2
19.6
19.2
14.0
7.1
2
0
0
0
.1
.05
.02
5
0 mol.% SrF ÈNd O
0.2
2
2 3
0
0
0
.1
.05
.02
12.3
Conditions: T \ 1023 K, and CH /O \ 3/1 with a Ñow rate of 66.7 ml min~1.
4
2
Table 4 Electron binding energies of the elements on the fresh catalysts
E /eV
b
catalyst
Nd O
Nd 3d5@2
982.1
982.1
Sr 3d5@2
È
F 1s
O 1s
C 1s
È
684.5
531.8, 530.7, 529.0
531.9, 530.9, 528.7
289.2, 284.6
289.2, 284.6
2
3
50 mol.%
133.7
SrF ÈNd O
2
2 3
of methane to CO [eqn. (3) and (4)]. For an oxygen-limited
OCM reaction, there will be little oxygen left in the gas phase
after the reactant Ñows over a short distance of catalyst bed,
hence a large amount of methane can no longer be activated.
increase in the length of catalyst bed being exposed to gas-
phase oxygen, the chance that ethane will react with surface
active oxygen species to produce ethene will increase, leading
to an improvement in ethene-to-ethane ratio over 50 mol.%
SrF ÈNd O catalyst, as compared with pure Nd O catalyst.
x
Owing to the increase in C selectivity as a result of adding
2
2
2 3
2 3
strontium Ñuoride to Nd O , a smaller amount of oxygen
A similar phenomenon was also found on the BaF -promoted
2
3
2
would be used to convert the same amount of methane
REO with variable valence.17
(
mainly to C products) over this catalyst than that over
2
undoped Nd O catalyst (mainly to CO ), leading to the
Surface composition
2
3
x
results that the reaction zone of the catalyst bed for the doped
catalyst appeared to be longer, and that methane conversion
was higher, than for the undoped catalyst.
The electron binding energies (E ) and surface composition of
b
the elements on the fresh Nd O and 50 mol.% SrF ÈNd O
2
3
2
2 3
In order to conÐrm this hypothesis, we have compared the
catalytic performance for di†erent lengths of catalyst bed over
Nd O and 50 mol.% SrF ÈNd O under the same Ñow rate
samples are summarized in Tables 4 and 5. In the C 1s
spectra, besides the carbon peak due to adventitious carbon at
284.6 eV, another peak was observed at higher energy (289.2
2
3
2
2 3
of CH ÈO mixture. For the Nd O catalyst, when the cata-
eV) on the pure Nd O sample, this corresponds typically to a
4
2
2 3
2 3
lyst volume was decreased from 0.2 to 0.02 ml, while the
GHSV was increased from 20 000 to 200 000 h~1, the catalytic
performance remained almost unchanged (Table 3), suggesting
that Nd O was a very active catalyst which could convert
methane efficiently in a short catalyst bed. In contrast, a
decrease in methane and oxygen conversions as well as in
ethene-to-ethane ratio was found over 50 mol.% SrF ÈNd O
carbon atom belonging to a carbonate group.18 The O 1s
spectra in the catalyst could be resolved into three peaks
located at 531.8, 530.7 and 529.0 eV, respectively (Table 4).
The peaks at ca. 529.0 eV can be attributed to lattice oxygen
(O2~),19,20 while the other peaks at 531.8 and 530.7 eV cannot
2
3
be identiÐed because the E values of some oxygen species (e.g.
b
O~, OH~, O 2~, CO 2~) fall in the range 530È532 eV. Since
2
2 3
2
3
catalyst with the same decrease in catalyst volume. This indi-
the ratio of atomic oxygen concentration to atomic carbon
cated that the reaction zone of the 50 mol.% SrF ÈNd O
concentration with higher C 1s E was much greater than 3
2
2 3
b
catalyst bed was longer that that of the Nd O catalyst bed,
(Table 5), it can be inferred that other oxygen species, e.g.
2
3
and, with the decrease in reaction zone, the conversions of
methane and oxygen would be decreased. It has been accepted
that a good OCM catalyst has two functions: (1) the pro-
duction of methyl radicals and (2) inhibition of the total oxi-
OH~, O~ and O 2~, may also exist on the catalyst surface
2
besides CO 2~.18h22 The Nd 3d
peak at 982.1 eV can be
3
5@2
assigned to Nd3`.18
dation of both the methyl radicals and C hydrocarbons.15
2
REO, especially La O and Nd O , are very e†ective at gen-
Table 5 Surface composition of various elements on the fresh cata-
2
3
2 3
erating methyl radicals; however, these catalysts are also very
active for the complete oxidation of these methyl radicals to
lysts
atomic concentration, C a (%)
CO .2,16 The change in surface and bulk properties (e.g.
x
x
surface composition, surface basicity and conductivity) as a
other
oxygenb
result of adding SrF to Nd O catalyst resulted in a decrease
catalyst
Nd O
Nd3`
Sr2`
F~
O2~
Cc
2
2 3
in the extent of deep oxidation of methyl radicals and C
2
hydrocarbons, leading to an increase in C selectivity. In the
next sections, more discussion about this will be presented.
15.1
11.4
È
7.9
È
5.1
12.5
14.0
62.0
48.0
10.4
13.6
2
3
2
5
0 mol.%
SrF ÈNd O
Since there is a certain relationship between methane conver-
sion and C2 selectivity for an oxygen-limited OCM reaction,
methane conversion is also found to increase with the
improvement in C2 selectivity. At the same time, with the
2
2 3
a C \ (A /S )/(& A /S ), where A \ peak area; S \ atomic sensitivity
x
x
x
i i i
factor. b Other oxygen refers to the oxygens in CO 2~, OH~, O~,
3
O 2~ etc. c C does not include adventitious carbon.
2
1132
J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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After SrF was added to Nd O catalyst, the binding ener-
might arise from the SrO formed as a result of F~ and O2~
exchange at high temperature (XRD result, see next section).
However, the desorption temperature was much lower than
2
2 3
gies of the above elements were almost unchanged, but the
content of surface carbonate was found to increase, suggesting
an increase in the catalyst surface basicity. Sr2` and F~ ions
were also detected on the catalyst surface, but the F~ ion
content on the surface was apparently lower than that in the
bulk. This indicates that F~ ion can easily move into the cata-
lyst bulk. When methane molecules react with surface oxygen
species of the catalyst to produce methyl radicals, the methyl
radicals may have a chance to react with the adjacent oxygen
the temperature of SrCO decomposition (1613 K),23 indicat-
3
ing that SrO did not exist as an isolated phase but, rather, was
dissolved and highly dispersed in the catalyst. The SrO might
exist as a new form in which strontium ion adjoins both Ñuo-
rine and oxygen ions, e.g. (FwSrwOwSrwF). Because the
electronegativity of Ñuorine is greater than that of oxygen, its
long-distance induced e†ect will lead to the result that the
oxygen ion linking with strontium ion in the catalyst contains
fewer negative charges than that in normal strontium oxide
and thus, to a decrease in oxide basicity and carbonate
decomposition temperature. It is known that most OCM cata-
lysts are basic oxides. Surface basicity was considered by
many researchers to be one of the fundamental characteristics
species and be further deeply oxidized to CO species before
x
they desorb from the catalyst surface.2,3,9h11 Since the Ñuo-
rine ions cannot react with methyl radicals under the reaction
conditions (as proved by the fact that no Ñuorinated hydro-
carbon was detected in the product), the dispersion of inert
Ñuorine ions on the catalyst surface will be beneÐcial to the
isolation of the surface active centres. When one molecule of
methane reacts with a surface oxygen species to produce a
methyl radical, the chance that this methyl radical reacts con-
tinuously with another adjacent oxygen species to produce
CO
of an OCM catalyst for better C selectivity.8,24h26 However,
2
a very strong basic site will be easily poisoned by the CO
2
generated during the reaction, leading to a higher “light-o†Ï
temperature, or a higher reaction temperature necessary to
maintain an adequate level of activity.24,27,28 Thus, a strong
basicity with intermediate basic sites for an OCM catalyst,
such as these alkaline-earth-metal Ñuoride promoted REO
catalysts, might be beneÐcial to obtaining a high C
medium temperature.
will be decreased. This will favour an increase of C2
x
selectivity.
2 yield at
Surface basicity
As shown in Fig. 3, one could observe two CO desorption
2
bands, centred at 378 and 675 K, over the Nd O catalyst,
Bulk composition and structure
2
3
which were assigned to basic sites with weak and intermediate
strengths, respectively. By contrast, two, relatively stronger,
CO desorption peaks (the latter appeared at higher
XRD indicated that only the hexagonal Nd O phase was
present in the fresh 25 mol.% SrF ÈNd O catalyst (Fig. 4),
2
3
2
2
2 3
temperature) were also observed over 50 mol.% SrF ÈNd O
the SrF phase could not be found since it dissolves in the
2
2 3
2
catalyst, suggesting that the surface basicity and basic strength
of the catalyst containing strontium Ñuoride were greater than
those of pure Nd O . This is consistent with the increase in
Nd O phase. However, new phases, such as tetragonal
2
3
and/or rhombohedral NdOF, were formed in the fresh 50
2
3
surface carbonate content of the catalyst (XPS result).
Since no carbon dioxide desorption peak was detected on
the SrF sample (Fig. 3), the appearance of the new peak at
2
8
99 K over 50 mol.% SrF ÈNd O catalyst is obviously
2
2 3
related to the interaction between Nd O and SrF . The peak
2
3
2
Fig. 4 X-Ray powder patterns of fresh (a) Nd O (b)È(d) 25, 50, 80
2
3
Fig. 3 TPD of CO adsorbed on (a) SrF , (b) Nd O , (c) 50 mol.%
mol.% SrF ÈNd O , (e) SrF . (]) Rhombohedral NdOF, (L)
2
2
2
3
2
2
3
2
SrF ÈNd O catalysts
tetragonal NdOF.
2
2 3
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1133

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Table 6 Bulk composition and structurea of the fresh 50 mol.%
MF ÈNd O (M \ Ca, Sr, and Ba) catalysts
2
2 3
catalyst
composition and structurea
CaF ÈNd O
hexagonal Nd O (s), tetragonal NdOF(w),
2
2
3
3
2
cubic CaF (w)
2
hexagonal Nd O (vs), tetragonal NdOF(m),
3
SrF ÈNd O
2
2
3
2 3
rhombohedral NdOF(w), cubic SrF (s)
2
BaF ÈNd O
hexagonal Nd O (s), tetragonal NdOF(m),
2
2
2 3
rhombohedral NdOF(m), cubic BaF (vs)
2
a vs \ very strong; s \ strong; m \ medium; w \ weak.
mol.% and 80 mol.% SrF ÈNd O catalysts, besides cubic
2
2 3
SrF and hexagonal Nd O phases. This suggests that partial
2
2 3
exchange of F~ with O2~ took place during the catalyst prep-
aration. The SrO phase formed could not be detected, most
probably due to its solution in the catalyst lattice. The content
of NdOF reached a maximum in the 50 mol.% SrF ÈNd O
2
2 3
catalyst.
The XRD results on di†erent alkaline-earth-metal Ñuoride
promoted neodymium oxide catalysts are summarized in
Table 6. Exchange between F~ and O2~ also took place and
the NdOF phase appeared in the calcined catalysts. The
di†raction-line intensity of alkaline-earth-metal Ñuorides
decreased in the order Ba [ Sr [ Ca in fresh 50 mol.%
MF ÈNd O (M \ Ca, Sr, and Ba) catalysts. This can be
interpreted as follows: as the cationic radii in these alkaline-
earth-metal Ñuorides became increasingly larger than that of
Nd3` (0.0995 nm), the solubility of the corresponding Ñuo-
rides (or oxides) in neodymium oxide or oxyÑuoride became
smaller and smaller.
Fig. 5 E†ect of temperature on the conductivities of Nd O
3 and 50
2
mol.% SrF ÈNd O
2
2 3
2
2 3
log p \ log A [ 2.303E /RT , the active energy (E ) of conduc-
a
a
tion can be calculated as follows:
Nd O
2
3
log p \ [0.61È3.88 ] 103/T ; E \ 74.3 kJ mol~1
a
The structure of hexagonal Nd O can be described by
2
3
50 mol.% SrF ÈNd O
slabs of OLn tetrahedrons linked by three of their edges and
2
2 3
4
forming a complex cationic group (LnO) n` separated by
log p \ 1.52È5.75 ] 103/T ; E \ 110 kJ mol~1
n
a
ionic oxygens O2~,29 with 17% of “geneticÏ oxygen vacancies
It can be seen that the conduction activation energy of 50
in the lattice.30 The tetragonal NdOF can be expressed by the
mol.% SrF ÈNd O was also higher than that of pure
formula LnO F
(0.7 \ x O 1),31,32 the structure of which
2
2 3
x 3~2x
Nd O .
is closely similar to that of Ñuorite, the excess F~ ions can be
accommodated in a Ñuorite-like structure and occupy some of
the interstitial position as Frenkel defects.32 Whereas the
rhombohedral NdOF is found to be a single phase, whose
chemical composition is sharply deÐned and corresponds
exactly to NdOF,31 this structure is also a slightly distorted
2 3
Note that the total conductivity of a solid is made up of
electron conductivity and ion conductivity, and that ion con-
duction plays a more important role than electron conduction
at high temperature.34 When a certain amount of SrF was
2
added to Nd O catalyst, the increase in conductivity suggests
2 3
that the mobility of anionic ions in the catalyst increased. This
CaF -type structure. Taking into consideration the fact that
many compounds with Ñuorite-type structure, such as
2
will be favourable to the movement of these species (e.g. F~
ion and O~ ion, which might form during the catalyst prep-
aration or under the reaction conditions), which have shown
smaller radii and/or fewer negative charges, from the catalyst
surface into the bulk. The inference that F~ ion mainly exists
in the SrF ÈNd O catalyst bulk has been demonstrated by
alkaline-earth-metal Ñuorides, have anion Frenkel defects and
anionic vacancies,33 it is reasonable to postulate that anionic
vacancies might also exist in the NdOF lattice with a Ñuorite-
like structure. On the other hand, ionic exchange between the
oxide and Ñuoride phases in the MF ÈNd O (M \ Ca, Sr,
2
2 3
2
2 3
the above XPS result. Considering the fact that O~ has a
and Ba) catalysts, as well as the loss of some Ñuorine (as HF)
at high temperature, may also lead to the formation of anionic
vacancies and other lattice defects, such as F-centre or O~
species, in order to maintain electroneutrality. The presence of
anionic vacancies in the above catalysts will be favourable to
the movement of anionic ions as well as to adsorption and
higher activity for oxidizing light hydrocarbons than O 2~
2
and O ~,35 the movement of O~ species from the catalyst
2
surface into the bulk will decrease the chance of deep oxida-
tion of methyl radicals and C hydrocarbons and thus lead to
2
an increase in C selectivity.
2
activation of O under the reaction conditions.
2
Conclusions
The catalytic performance of SrF ÈNd O catalyst was better
Conductivity measurement
2
2 3
than that of pure Nd O for OCM. The maximum C yield
2
3
2
The conductivity measurement results for Nd O and 50
reached 19.6% over 50 mol.% SrF ÈNd O catalyst at 1023 K
2 3
for CH /O \ 3/1 and GHSV \ 20 000 h~1. After the addi-
2
3
2
mol.% SrF ÈNd O catalysts are shown in Fig. 5. It is found
2
2 3
4 2
that, in the temperature region 773È1173 K, the conductivity
tion of a certain amount of SrF to the Nd O , the increase in
2
2 3
apparently increases after adding a certain amount of SrF to
surface basicity and conductivity, as well as the role of F~ on
the dispersion of surface active sites, would be favourable to
an improvement in C2 selectivity. Since there is a certain
2
Nd O . With increase in temperature, the conductivity of the
2
3
catalysts increased. According to the Arrhenius equation:
1134 J. Chem. Soc., Faraday T rans., 1998, V ol. 94

View Article Online
relationship between methane conversion and C selectivity
for an oxygen-limited OCM reaction, methane conversion is
15 S. J. Korf, J. A. Roos, J. W. H. C. Derksen, J. A. Vreeman, J. G.
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2
XRD results indicated that tetragonal and rhombohedral
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NdOF phases are formed during the catalyst preparation.
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This work has been supported by the National Natural
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Paper 8/00367J; Received 13th January, 1998
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
1135