Oxidative Condensation of Methane on Sr2 –xLaxTiO4 Catalysts: Effect of the Degree of Substitution of Sr and La

Article abstract of DOI:10.1134/S0023158419030108

Abstract: The Sr_{2 –x}La_xTiO₄ (x = 0–2.0) catalysts were synthesized based on strontium titanate with a layered perovskite structure. The effect of the degree of substitution of La for Sr on the physicochemical (phase composition and textural characteristics) and catalytic properties of oxides in the oxidative condensation of methane at temperatures of 700–800°C were studied. It was found that multiphase Sr_2?–?xLa_xTiO₄ samples with the degree of substitution x = 0.8–1.8 were most active and selective in the test reaction; this was likely related to the presence of lanthanum oxide and strontium oxide impurities in them, their optimum distribution over the surface, and the specific surface area.

Full text of DOI:10.1134/S0023158419030108

ISSN 0023-1584, Kinetics and Catalysis, 2019, Vol. 60, No. 6, pp. 862–867. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Kinetika i Kataliz, 2019, Vol. 60, No. 3, pp. 403–408.
Oxidative Condensation of Methane on Sr2 – xLaxTiO4 Catalysts:
Effect of the Degree of Substitution of Sr and La
R. V. Petrova, *, Yu. A. Ivanovaa, S. I. Reshetnikova, and L. A. Isupovaa
a Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia
*e-mail: petrov@catalysis.ru
Received November 30, 2018; revised January 14, 2019; accepted January 15, 2019
Abstract—The Sr2 – xLaxTiO4 (x = 0–2.0) catalysts were synthesized based on strontium titanate with a lay-
ered perovskite structure. The effect of the degree of substitution of La for Sr on the physicochemical (phase
composition and textural characteristics) and catalytic properties of oxides in the oxidative condensation of
methane at temperatures of 700–800°C were studied. It was found that multiphase Sr2 – xLaxTiO4 samples
with the degree of substitution x = 0.8–1.8 were most active and selective in the test reaction; this was likely
related to the presence of lanthanum oxide and strontium oxide impurities in them, their optimum distribu-
tion over the surface, and the specific surface area.
Keywords: oxidative condensation of methane, synthesis of catalysts, layered perovskites, strontium titanate
DOI: 10.1134/S0023158419030108
INTRODUCTION
promising. Thus, catalysts based on the mixed com-
pounds of alkaline earth and rare earth elements are
effective in this region. They mainly contain La2O3
promoted with alkaline earth metal ions of Sr, Mg, Ba,
etc. [10–13].
Traditional methods of methane conversion into
liquid hydrocarbons are carried out through complex
multistage processes, the implementation of which is
associated with large capital investments. One-step
methods (without a stage of synthesis gas production)
for methane conversion with the formation of C–C
bonds as a result of the oxidative condensation of
methane (OCM) seem more reasonable. In the course
of an OCM process, methane is oxidized at high tem-
peratures (800–900°C) on catalysts with the forma-
tion of target products (ethane and ethylene) and by-
products (carbon oxides) [1, 2]. The reaction scheme
can be represented as follows:
Materials based on Sr–La2O3 are of greatest inter-
est; they exhibit high activity in the OCM reaction [12,
14–17]. Along with supported oxide systems, special
attention should be paid to mixed oxide systems of SrO
and La2O3 with a perovskite-like structure (perovskite
is CaTiO3). Due to their crystal structure (the closest
cubic packing of oxygen ions and metal cations), they
have a number of unique properties (ability to ion
exchange and intercalation of oxygen ions, high ther-
mal stability, etc.), which are important in catalysis.
Layered perovskite-like systems consisting of per-
ovskite layers (ABO3) alternating with oxide layers
(AO) are another variety of mixed oxides. The intro-
duction of metal cations of different valences and sizes
into the structure of perovskite-like oxides affects the
metal–oxygen bond strength and opens up an addi-
tional opportunity to regulate their properties, which
influence the catalytic activity. Ivanov et al. [16] found
that the Sr2TiO4 oxide system prepared by a mechano-
chemical method can serve as a basis for the develop-
ment of promising catalysts for the OCM reaction.
(I)
CH + O → C H , C H + H2O,
(
)
4
2
2
4
2
6
(II)
CH + O → CO, CO + H2O.
(
)
4
2
2
Despite numerous studies, this process has not yet
received practical implementation due to insuffi-
ciently high yields of C2 hydrocarbons mainly because
of the low efficiency (selectivity and stability) of the
catalysts developed. Therefore, studies on the synthe-
sis of promising catalysts are in continuous progress.
As a result of extensive research, it was found that a
wide range of oxide systems has catalytic activity: alka-
line earth metal oxides (modified with alkali metal
ions [3]), rare earth metal oxides, mixed alkaline earth
and rare earth metal oxides [4–6], and complex oxides
containing manganese oxide [7–9]. From an eco- perovskite structure and to study the effect of the
nomic point of view, catalysts that are active in a degree of substitution of La for Sr on the physico-
region of lower temperatures (<800°C) are most chemical (phase composition and textural characteris-
The aim of this work was to synthesize complex
oxide systems Sr2 – xLaxTiO4 (x = 0–2) with a layered
862

OXIDATIVE CONDENSATION OF METHANE
863
tics) and catalytic properties of catalysts in the oxida- 0.05° over a range of 2θ from 10° to 70°. The compo-
tive condensation reaction of methane.
sition of phases was determined by comparing the
experimental X-ray diffraction patterns with the
JCPDS database. The structural parameters were cal-
culated using the FullProff program.
EXPERIMENTAL
Sample Preparation
The samples were prepared by precipitation from a
solution of strontium nitrate Sr(NO3)2 in the presence
of a suspension of titanium dioxide TiO2, which was
preliminarily crushed in a centrifugal planetary ball
mill at an acceleration of 40 g for 90 s. A 0.4 M solution
of K2CO3 was used as a precipitating agent. The pre-
cipitation was carried out at room temperature to pH 9.5;
then, the precipitate was filtered off, washed to pH 7, and
dried for 10 h at 120°C. All of the samples obtained were
calcined at a temperature of 1100°C for 4 h [17].
RESULTS AND DISCUSSION
Synthesis and Study of the Physicochemical
Properties of Samples
To study the principal possibility of preparing
oxide systems of Sr and La titanates with a layered per-
ovskite structure, we synthesized the samples of with
different ratios between Sr and La (x = 0–2.0 with a
step of 0.2) using a precipitation method.
Figure 1 shows the XRD analysis data for the
Sr2 xLaxTiO4 samples. The Sr2TiO4 (I4/MMM)
–
sample has a well-crystallized structure similar to that
of layered perovskite without foreign phase impurities.
In the structure of the samples with the degrees of sub-
stitution x = 0.2–0.6, TiO2 (P42/MNM) and La2O3
(P3-M1) impurity phases appeared in addition to the
main Sr2TiO4 phase. In the sample with x = 0.8, the
formation of a SrTiO3 (PM3-M) perovskite-like phase
was observed, and the Sr2TiO4 phase ceased to be the
main one (Fig. 1a). The Sr2TiO4 phase was absent
from the samples with x ≥ 1.0. The main phases in the
samples with x = 1.0–1.2 were SrTiO3 and La2O3. A
further increase in the degree of substitution to x = 1.4
led to the formation of a new phase of La2TiO5
(PNAM), which became the main phase in the sam-
ples with x = 1.6–2.0, and, on the contrary, the SrTiO3
phase gradually disappeared. In the region of high
degrees of substitution (x > 1.4), a phase of the lantha-
num oxycarbonate La2O2(CO3) also occurred due to
the ability of La2O3 to readily adsorb CO2 and to inter-
act with it [1] (Fig. 1b). Intense reflections of the SrO
phase (2θ = 29.77°, 34.69°, 49.88°, and 59.33°) [18]
are difficult to distinguish in the diffraction patterns of
the samples with high degrees of substitution x ≥ 0.8
due to overlapping with signals from other phases;
however, the probability of the presence of SrO in the
samples cannot be excluded.
Methods of Analysis
Methane conversion and selectivity for products
(ethane and ethylene) were determined in a fixed-bed
catalytic flow system [17]. A prepared mixture of reac-
tants from a cylinder entered a catalytic reactor
through a gas-flow regulator at a nearly atmospheric
pressure. The quartz reactor (inner diameter, 6 mm) was
heated with a high-temperature electric furnace with a
fluidized bed of sand. At the reactor outlet, the reaction
mixture was sampled for online analysis, which was per-
formed on series-connected LKhM-80 (Russia) and
Tsvet-500 (Russia) chromatographs equipped with ther-
mal conductivity detectors. The CO2 and C2-hydrocar-
bon gases were separated on a column packed with a
HayeSep polymer sorbent, and H2, O2, N2, CH4, and
CO were separated on an analogous column with zeolite
5A at column temperatures of 60 and 90°C, respectively.
Helium was used as a carrier gas.
The activity of catalysts in the OCM reaction was
determined at contact times of 0.02–0.06 s. The initial
reaction mixture contained 46 mol % CH4, 11.5 mol %
O2, and 42.5 mol % N2. The experimental conditions
were as follows: temperature, T = 700–800°C; pres-
sure, P = 1 atm; catalyst volume, 0.5 mL; fraction,
0.25–0.5 mm (diluted with 0.25–0.5 mm quartz in a
volume ratio of 1 : 4); and volumetric feed rate of the
reaction mixture, 30 L/h.
The porous structure of the samples was analyzed
by mercury porosimetry under pressure on an
AutoPore IV 9500 V1.09 porosimeter (Micromeritics,
the United States).
The specific surface area (Ssp, m2/g) was deter-
mined by the BET method from the thermal desorp-
tion of argon.
The phase composition was investigated by X-ray
diffraction (XRD) analysis. The X-ray diffraction patterns
were obtained on a Bruker D8 diffractometer (Bruker,
Germany) using CuKα radiation (λ = 1.5418 nm). Each
According to published data, the SrLaTiO4 com-
pound with a layered perovskite structure is difficult to
obtain [19] due to significant atomic coordination dif-
ferences in the layers of SrO0 and LaO+, which lead to
a strong internal electric field voltage in the oxide lat-
tice [20]. Nevertheless, La enters the structure of
strontium titanate. An analysis of the positions of the
main reflections of the Sr2TiO4 crystal structure (θ =
31.35° and 32.57°) showed that the intensity maxi-
mums of reflections in the samples with x = 0.1–0.8
are shifted toward smaller angles. This may indicate
that lanthanum atoms enter the crystal structure of
Sr2TiO4 and replace Sr atoms and the lattice parame-
sample was scanned by points at regular intervals of ters increase because the radius of La3+ (0.819 Å) is
KINETICS AND CATALYSIS Vol. 60 No. 6 2019

864
PETROV et al.
(а)
L—La2O3
T—TiO2
Intensity
P
P—SrTiO3
O—Sr2TiO4
L
L
L
P
P
L
P
L
L
L
P
P
L
L
6
5
4
T
L
L
T
3
2
1
L
L
O
O
O
O
O
O
O
O
O
O
O
O
O
20
30
40
50
60
2θ, deg
(b)
L—La2O3
Intensity
T—TiO2
S—La2TiO5
P—SrTiO3
C—La2O2(CO3)
S
S
L
S
L
S
S
S
L
T
S
T
T
L
T
T
L
C
S
C
L
S
C
S
S
C
C
11
10
9
8
7
T
L
P
L
P
L
P
L
L
L
P
P
P
L
L
6
20
30
40
50
60
2θ, deg
Fig. 1. XRD analysis data for the Sr
La TiO catalysts with different values of x: (a) x = (1) 0, (2) 0.2, (3) 0.4, (4) 0.6, (5) 0.8,
x 4
2 – x
and (6) 1.0; (b) x = (7) 1.2, (8) 1.4, (9) 1.6, (10) 1.8, and (11) 2.0.
greater than the radius of Sr2+ (0.683 Å); this causes a
shift of reflections toward smaller angles.
depending on the composition. The single-phase Sr2TiO4
sample had the lowest specific surface area of
0.6 m2/g. The specific surface area of the samples
Table 1 summarizes data on the specific surface area
of the samples, which varied in a range of 0.6–3.3 m2/g increased as the degree of substitution was increased to
KINETICS AND CATALYSIS
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2019

OXIDATIVE CONDENSATION OF METHANE
865
Table 1. Phase composition of Sr2 – xLaxTiO4 oxide systems
(Fig. 2). Thus, the particle size of a single-phase sam-
ple of Sr2TiO4 (x = 0) was 3–5 μm, whereas the parti-
cle sizes of multiphase samples with x = 1 and x =1.6
were 1–2 and 0.5–1.0 μm, respectively.
depending on the La content
Ssp,
Vpore,
Main CSR,
phases
x
Impurities
–
m2/g cm3/g
Å
Thus, in the Sr2 – xLaxTiO4 system, the formation of
limited solid solutions with a layered perovskite struc-
ture is possible under the specified synthesis condi-
tions. An increase in the La content of the initial pre-
cursor to x ≤ 0.8 resulted in the formation of SrTiO3
and La2TiO5 phases in addition to a layered perovskite
phase of Sr2 – xLaxTiO4, whereas the phase of layered
perovskite was not formed at x ≥ 1.0. In this case, a
decrease in the size of particles was observed, as evi-
denced by the CSR, BET (Table 1), and SEM data
(Fig. 2).
0
0.6
0.8
0.8
1.0
–
0.08 Sr2TiO4 650
0.2
0.4
0.6
0.8
–
–
–
–
Sr2TiO4
Sr2TiO4
510 TiO2, La2O3
470 TiO2, La2O3
Sr2TiO4 300 SrTiO3, TiO2
SrTiO3
La2O3
230
580
–
1.0
1.2
1.4
1.3
1.8
2
0.122 SrTiO3
La2O3
300 TiO2
630
–
SrTiO3
La2O3
350 TiO2
470
Study of the Catalytic Activity of the Samples
0.200 La2TiO5 440 TiO2,
Figure 3 illustrates data on changes in the conversion
of methane and selectivity for C2 hydrocarbons as func-
tions of the La content (x) of the synthesized samples.
310
310
SrTiO3
La2O3
La2O2(CO3)
1.6
1.8
2.0
3.3
2.5
1.5
0.210 La2TiO5 580 SrTiO3, TiO2,
La2O2(CO3)
The conversion of methane on all of the samples
increased with the test temperature, and it changed
nonmonotonically with increasing x (Fig. 3). Thus,
methane conversion on the samples with x = 0–0.6 at
800°C was 15–20%, and the yield of C2 products was
9–12% (Fig. 3); these results are consistent with pub-
lished data [16] for the Sr2TiO4 catalyst (at a test tem-
perature of 850°C, the selectivity was 60%, and the
yield of C2 products was ~12%). A maximum methane
conversion of ~30% at 800°C was achieved on the
sample with x = 0.8; at 64% selectivity, it provided a
19.2% yield of C2 products. Upon the complete
replacement of Sr by La (x = 2.0), the yield decreased
to 13%; that is, a synergistic effect was observed in the
Sr2 – xLaxTiO4 system. Our experimental data indicate
a higher methane conversion at 800°C for multiphase
samples in the range x = 0.8–1.8; this may be due to
the presence of strontium oxide and lanthanum oxide
impurities in these samples and a higher specific sur-
0.121
La2TiO5 540 SrTiO3, TiO2,
620
La2TiO5 740 TiO2,
580
La2O3
La2O2(CO3)
–
La2O3
La2O2(CO3)
x = 1.6 and then decreased for x > 1.6. It is likely that
the change in the specific surface area of the Sr2 – xLaxTiO4
samples as a function of x reflects a change in the dis-
persity of particles, which may be due to the appear-
ance of new phases. Indeed, according to X-ray anal-
ysis data, the sizes of the coherent scattering regions
(CSRs) of the main phases passed through a minimum
(Table 1).
Scanning electron microscopy (SEM) data con-
firmed a reduction in the particle size of the samples face area of these samples. Thus, Yamashita et al. [13]
(а)
(b)
1 μm
1 μm
Fig. 2. SEM images of samples with La contents (x) of (a) 0 and (b) 1.4.
KINETICS AND CATALYSIS Vol. 60 No. 6 2019

866
XCH , %
PETROV et al.
tural characteristics) and catalytic properties of the
samples.
SC , %
4
2
800°C
700°C
30
20
10
We demonstrated the principal possibility of the
synthesis of Sr2 – xLaxTiO4 double oxide systems with a
layered perovskite structure in samples with x < 0.8.
An increase in the lanthanum content of the reaction
mixture hinders the formation of a layered perovskite
phase, and this phase was absent from samples with
x ≥ 1, whereas SrTiO3 and La2TiO5 phases were
formed; along with these phases, there were La2O3
and, probably, SrO impurity phases. An increase in
the La content leads to a nonmonotonic increase in
the dispersity of the samples due to their multiphase
nature.
60
50
40
30
20
10
0
800°C
700°C
0
0.5
1.0
1.5
2.0
x
The synthesized samples of catalysts based on
strontium titanate of different composition were
tested in the oxidative condensation of methane at
temperatures of 700 and 800°C. It was established
that the Sr2 – xLaxTiO4 catalyst samples with the degree
of substitution x = 0.8–1.8 were most active and selec-
tive at these temperatures. This was likely due to an
optimum concentration and mutual distribution of the
active impurity oxides La2O3 and SrO and a higher
specific surface area of these samples.
Fig. 3. Changes in methane conversion and selectivity for
the formation of C hydrocarbons at temperatures of 700
2
and 800°C and a contact time of 0.06 s.
reported an 18% yield of C2 products on the
Sr0.15La0.85O catalyst at a temperature of 750°C.
The maximum values of methane conversion and
selectivity for C2 products—26 and 62%, respec-
tively—at a test temperature of 700°C were achieved in
a narrower range of compositions (x = 1.2–1.4).
An increase in contact time (0.02–0.06 s) in the
tests led to an increase in the yield of C2 products due
to an increase in the conversion of methane because
selectivity for C2 products remained almost
unchanged.
FUNDING
This work was performed within the framework of a state
contract at the Boreskov Institute of Catalysis, Siberian
Branch, Russian Academy of Sciences (project no. AAAA-
A17-1170 41710090-3).
The activity of both individual and supported SrO
[21] and La2O3 oxides [21, 22] in the OCM reaction
has been widely discussed in the literature. It is likely
that, in the samples with a low degree of substitution to
x = 0.8, Sr atoms are bound in perovskite structures,
and they do not make an additional contribution to the
OCM reaction. The substitution of lanthanum for
strontium violates the structure of layered perovskite
and leads to the formation of separate oxide phases of
La2O3, TiO2, and, probably, SrO. The assignment of
SrO reflections in the X-ray diffraction patterns of
multiphase samples is complicated due to overlapping
with the reflections of the perovskite phases of Sr2TiO4
and SrTiO3. Therefore, it is possible that a higher
activity of samples with the degrees of substitution x =
1.2–1.4 at temperatures of 700–800°C is due to a
higher specific surface area, the presence of La2O3 and
SrO oxides active in OCM, and their optimum disper-
sity and mutual distribution relative to each other.
REFERENCES
1. Onsager, O.T., Lodeng, R., Soraker, P., Anundskaas, A.,
and Helleborg, B., Catal. Today, 1989, vol. 4, p. 355.
2. Lomonosov, V.I. and Sinev, M.Yu., Kinet. Catal., 2016,
vol. 57, no. 5, p. 647.
3. Ito, T. and Lunsford, J.H., Nature, 1985, vol. 314,
p. 721.
4. Otsuka, K., Jinno, K., and Morikawa, A., Chem. Lett.,
1985, vol. 14, p. 499.
5. Lin, C.H., Campbell, K.D., Wang, J.X., and
Lunsford, J.H., J. Phys. Chem., 1986, vol. 90, p. 534.
6. Otsuka, K., Jinno, K., and Morikawa, A., J. Catal.,
1986, vol. 100, p. 353.
7. Jones, C.A., Leonard, J.J., and Sofranko, J.A., J.
Catal., 1987, vol. 103, p. 311.
8. Sofranko, J.A., Leonard, J.J., Jones, C.A., Gaffney, A.M.,
and Withers, H.P., Catal. Today, 1988, vol. 3, p. 127.
9. Minachev, Kh.M., Usachev, N.Ya., Belanova, E.P.,
and Shirokova, G.A., Catal. Today, 1992, vol. 13,
p. 565.
CONCLUSIONS
10. Zhang, Z.-L., Au, C.T., and Tsai, K.R., Appl. Catal.,
Thus, we synthesized catalysts of various stoichio-
metric composition based on strontium titanate
(A2TiO4, where A is the cation of Sr, La, or Sr/La) and
studied the effect of the degree of La substitution for Sr
on the physicochemical (phase composition and tex-
1990, vol. 62, p. 29.
11. Yingli, B., Kaiji, Z., Yutao, J., Chiwen, T., and Xiang-
guong, Y., Appl. Catal., 1988, vol. 39, p. 185.
12. Sollier, B.M., Gymez, L.E., Boix, A.V., and Miry, E.E.,
Appl. Catal., A, 2017, vol. 532, p. 65.
KINETICS AND CATALYSIS
Vol. 60
No. 6
2019

OXIDATIVE CONDENSATION OF METHANE
867
13. Yamashita, H., Machida, Y., and Tomita, A., Appl.
Catal., A, 1991, vol. 79, p. 203.
prapto, S., Bull. Chem. React. Eng. Catal., 2017, vol. 12,
p. 299.
19. Hosono, H., Tanabe, K., Takayama-Muromachi, E.,
Kageyama, H., Yamanaka, S., Kumakura, H., Nohara, M.,
Hiramatsu, H., and Fujitsu, S., Sci. Technol. Adv. Ma-
ter., 2015, vol. 16, no. 033503, p. 1.
14. Song, J., Sun, Y., Ba, R., Huang, S., Zhao, Y., Zhang, J.,
Sun, Y., and Zhu, Y., Nanoscale, 2015, vol. 7, p. 2260.
15. Karakaya, C. and Kee, R.J., Prog. Energy Combust. Sci.,
2016, vol. 55, p. 60.
20. Pham, A. and Li, S., Phys. Chem. Chem. Phys., 2017,
16. Ivanov, D.V., Isupova, L.A., Gerasimov, E.Yu., Dovli-
tova, L.S., Glazneva, T.S., and Prosvirin, I.P., Appl.
Catal., A, 2014, vol. 485, p. 10.
17. Ivanova, Yu.A., Petrov, R.V., Reshetnikov, S.I., and
Isupova, L.A., Vest. TGU, Ser. Khimiya, 2017, no. 8,
p. 38.
vol. 19, p. 11373.
21. Ghose, R., Hwang, H.T., and Varma, A., Appl. Catal., A,
2013, vol. 452, p. 147.
22. Zavyalova, U., Holena, M., Schlogl, R., and Baerns, M.,
ChemCatChem, 2011, vol. 3, p. 1935.
18. Widiarti, N., Suryana, L.A., Wijayati, N., Rahayu, E.F.,
Harjito, H., Wardhana, S.B., Prasetyoko, D., and Su-
Translated by V. Makhlyarchuk
KINETICS AND CATALYSIS Vol. 60 No. 6 2019