MgO catalysts by lithium chloride as a

Reac Kinet Mech Cat (2013) 110:373–385 DOI 10.1007/s11144-013-0600-3

Activity enhancement of Li/MgO catalysts by lithium chloride as a lithium precursor for the oxidative coupling of methane Fereshteh Raouf • Majid Taghizadeh Mohammad Yousefi

•

Received: 14 May 2013 / Accepted: 27 June 2013 / Published online: 9 July 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013

Abstract A series of Li/MgO catalysts were prepared through the wet impregnation method for the oxidative coupling of methane with varying content of Li but with slight differences from others. The prepared catalysts were characterized by a variety of techniques, including X-ray diffraction, CO2-TPD, and BET methods. The catalytic performance and physical structure properties of the synthesized Li/ MgO catalysts were compared with the available knowledge on Li/MgO. The reaction was carried out using a fixed bed reactor at 675–800 °C, while lithium loading and feed gas ratio were varied. The methane conversion was similar with only small fluctuation to the amount of lithium but selectivity toward C2 hydrocarbons increased with Li percentage. The methane conversion increased with the oxygen concentration, while ethane and ethylene selectivity decreased with increasing oxygen concentration. Keywords

OCM Li/MgO Oxygen concentration Lithium loading Yield

Introduction The major component of natural gas is methane that is widely distributed at sites around the world. Conversion of methane into higher hydrocarbons via indirect or direct methods is an important route for the production of synthetic fuels and chemicals. Oxidative coupling of methane (OCM) is the most important approach F. Raouf M. Taghizadeh (&) Chemical Engineering Department, Babol University of Technology, P.O. Box 484, 4714871167 Babol, Iran e-mail: [email protected] M. Yousefi Department of Gas Conversion, Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran

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for the direct conversion of methane to ethane, ethylene, and higher hydrocarbons in the presence of oxygen. While the world’s natural gas resources are limited, this method has attracted great attention because of its numerous applications. Since the early studies by Keller and Bhasin, numerous researchers have focused on this area. Various catalytic materials comprising simple, mixed and complex oxides have been tested for this reaction. A large number of catalyst systems have been so far found to be effective in this reaction. Basic oxides like alkaline oxides, alkaline earth oxides, rare earth oxides and binary transition metal oxides promoted by alkali metal ions and supported on SiO2 or TiO2 are among the most active catalysts for OCM [1–7]. Among all the investigated OCM catalysts with simple structure, lithium-doped magnesium oxide has been considered an effective catalyst with good conversion of methane and high selectivity to the desirable higher hydrocarbons; so, it is one of the more promising catalyst for OCM [8–13]. This catalyst contains irreducible metal oxide and it is believed that MgO is involved in hydrogen atom abstraction from alkanes and alkenes, resulting in the formation of CH3 radicals that sustain higher hydrocarbon production by coupling [3, 14–16]. CH3 radicals in the gas phase may either couple to or enter chain branching reactions that ultimately result in the formation of COX [17, 18]. Li insertion into MgO lattices has been thought to be one of the governing factors responsible for methane activation. The required minimum temperature for CH4 conversion has been concordantly reported to be at approximately 600 °C but, for reaching the maximum selectivity and conversion, increased temperature up to 750 °C is required [6, 19]. Therefore, lowering the operating temperature without lowering the yield of C2 hydrocarbons would reduce running costs and possibly reduce capital costs of the process as well. Li-doped MgO still is the object of intensive studies for research groups worldwide. The performance of the catalysts for the oxidative methylation of acetonitrile to acrylonitrile is significantly affected by the Li precursor, where the catalysts prepared with LiCl and LiOH on MgO have the best performance for this reaction. Most of the Li-doped MgO catalysts were prepared by wet impregnation of lithium precursors over magnesium oxide, in which one of the Li2CO3, LiOH, LiNO3 and C2H3O2Li forms was employed as lithium sources [20]. Halogen-containing catalyst materials show higher C2 selectivity at high methane conversions and they could improve total productivity of C2 [21, 22]. Addition of chloride ions to an oxidative coupling catalyst can have a marked effect on its properties, particularly with respect to the ethylene to ethane product ratio. It is suspected that homogenous reactions might facilitate ethane dehydrogenation to ethylene and create large C2H4/C2H6 ratios observed in these systems [17, 23, 24]. Otsuka et al. [25] reported a significantly enhanced catalytic performance over manganese oxide, which was promoted by lithium chloride. The use of Cl2 or other chlorine oxidizing agents like HCl in the methane coupling process is not industrially attractive due to inherent corrosion problems. To obtain catalysts with different amounts of doped lithium and acceptable catalytic performance in OCM, a series of Li/MgO catalysts were prepared by the wet impregnation method. LiCl was used as lithium precursor with ethanol as a

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suspension medium to evaluate the effects of operating parameters on total OCM productivity.

Experimental Catalyst preparation The catalyst samples were prepared by the conventional wet impregnation method using lithium chloride as lithium precursor and ethanol as liquid media. The magnesium oxide powder (99 % Merck) was impregnated with the solution of lithium chloride (99 % Merck) by the immersion of MgO powder in pure ethanol (99 % Merck) followed by gradual addition of lithium chloride solution in deionized water at 60 °C. The suspension was stirred and followed by its evaporation to dryness at 100 °C. The resulting mixture was dried at 120 °C for 16 h before being calcined in air at 850 °C for 4 h. Catalytic activity test Catalytic tests were carried out in a laboratory-scale continuous flow system with a fixed quartz reactor (ID = 20 mm) which contained 1.5 g catalyst (catalyst bed length of 12 mm) operating at atmospheric pressure. Fig. 1 shows the schematic detail of the reactor and thermocouple assembly used in this study. The temperature was measured by K-type thermocouple placed in an appropriate thermowell. The tubular reactor was filled with catalyst (25–30 mesh) and it was activated in situ by being heated at 550 °C for 1 h under nitrogen flow. The reactant feed gases used in this study were high purity methane (99.9 %), oxygen (99.99 %) and nitrogen (99 %). The inlet volumetric flow rate of each gas was controlled using an individual volumetric flow controller and total flow rate of the reactant gas mixture was 150 mL/min with a feed composition of CH4:O2:N2 = 4:2:4. The reactor’s free space was filled with quartz chips (25–30 mesh) for reducing free volume and minimizing subsequent reactions of desired products at the post-catalytic volume. A Hewlett Packard (HP) 6890 series online gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) analyzed the reactor effluent gases. Methane conversion (XCH4 ), selectivity to desired products (SC2 ) and total process yield were calculated based on total carbon content as follows: XCH4 ¼ S C2 ¼

Input CH4 Output CH4 100 Input CH4

ð1Þ

Total concentration of ethane and ethylene in products 100 Total products concentration

ð2Þ

XCH4 SC2 100

ð3Þ

YC 2 ¼

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Fig. 1 Schematic of the fixedbed reactor for OCM reaction

Catalyst characterization A Siemens D5000 instrument with Fe anode (FK 60-04) was used for X-ray ˚ at 35 kV and diffraction analysis of the fresh catalyst. The wavelength of 1.93604 A 25 mA radiations in step mode between 5° and 80° with step size of 0.020° was used. The crystalline phases in the catalyst were identified by comparing diffraction lines of the samples with those of instruments in the literature. The surface basicity and base strength distribution of the Li/MgO were determined by the CO2-TPD using BELCAT-A. To remove adsorbed carbon dioxide from the surface, the catalysts were pretreated by heating from 100 to 850 °C with the 10 °C/min flow rate under helium flow of 30 mL/min as a carrier gas, and then cooled to the room temperature. The chemisorption of CO2 was carried out by a CO2 flow of 30 mL/

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min for 100 min. Specific surface area of the catalysts was determined by BET method using a CHEMBET-3000 with N2 adsorbent at liquid nitrogen temperature.

Results and discussion X-Ray diffraction XRD patterns of the Li/MgO catalysts with different levels of Li loading are shown in Fig. 2. All the samples were calcined at 850 °C in an air atmosphere. XRD analysis of catalysts indicated that the catalysts were predominantly mixtures of magnesium oxide and lithium precursors or decomposition products of these precursors (e.g., LiCl, Li2O2, Li2CO3, and LiOH). By comparing positions and intensities of the diffraction peaks against a library of known crystalline materials, the target materials were identified. It could be observed from these patterns that catalysts possessed sharp diffraction lines at 2h values of 46° and 54° that corresponded to MgO. These patterns also showed that the crystallinity of MgO decreased with increasing Li loading, which provided evidence that there was some kind of interaction between Li species and the MgO, leading to the destruction of the MgO skeleton in some extents; also, modification of the properties of Li species was carried on the MgO. The weak diffraction line at 2h value of 29.1° represented the presence of Li2CO3. The observed Li2CO3 compound formation might be presumably be due to peak results from decomposition of LiCl precursor in the catalysts. The peaks of Li2CO3 were observed in both 5 % Li/MgO and 3 % Li/MgO in which 5 % Li/MgO had more visible peak. It could be proposed that the LiOH was present on the surface of the catalysts with diffraction line at 2h values of 50° and 53° in all Li/

Intensity (a.u.)

MgO Li2CO3 LiOH

5%Li/MgO 3%Li/MgO 1%Li/MgO 10

30

50

70

90

2θ θ (degree) Fig. 2 XRD pattern of Li/MgO with different levels of Li content

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MgO samples. No other phases of Li, including LiCl were detected indicating that the LiCl was completely decomposed. Temperature programmed desorption of carbon dioxide (CO2-TPD) By CO2-TPD peak temperatures and amount of desorbed CO2, the strength and density of surface basicity of the catalysts were measured. The temperature programmed desorption of CO2 profiles of the catalysts is shown in Fig. 3, which revealed the base strength distribution in these catalysts. Carbon dioxide was used as a quest molecule to measure the surface basicity and numbers of weak, medium and strong sites. To facilitate the discussion, the basic sites were divided to weak, moderate and strong sites; total basicity refers to the total amount of desorbed CO2. According to this division for base strength distribution, their distinction can be roughly classified based on the temperature at which desorption peaks were recorded in the three temperature range, weak (\200 °C), medium (200–400 °C) and strong ([400 °C) basic sites [26–28]. For better distinction between strong and very strong basic sites in unusual classification, the desorption peak revealed in the temperature range greater than of 600 °C was assumed to belong to the very strong basic sites (so strong sites refer to the 400–600 °C) [28]. The medium basic sites were associated with desorption peak in 200–400 °C and there was no CO2 desorption peaks in the region of medium temperature. The desorption peaks in the temperature range from 150 to 200 °C appeared in all Li/MgO samples, which indicated that a site with lower basic strength was present in all these catalysts. At the higher temperature ramp for 1 % Li/MgO, the peak was demonstrated in 800 °C. In the case of 3 % Li/MgO, two peaks at 600 and 750 °C were clearly visible. All these peaks referred to very strong basic sites, but for the 5 % Li/MgO, the peak appeared at 510 °C pointed to strong basic sites.

1%Li/MgO

Intensity (a.u.)

3%Li/MgO

100

5%Li/MgO

200

300

400

500

600

Temperature (°C) Fig. 3 CO2 TPD spectra of fresh catalysts

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700

800

900


379

As revealed in Fig. 3, the basicity and base strength distributions were influenced by the lithium content in the catalysts. Also, increasing the amount of Li to the Li/ MgO catalyst shifted the CO2 desorption peaks to lower temperature and decreased the amount of desorbed CO2. The weak strength mostly corresponded to OHgroups on the catalyst surface, while those with medium and strong strength were related to the oxygen of Mg–O (or Li–O) and surface O2- ions, which were in good agreement with XRD results. BET surface area The catalysts were characterized with respect to BET surface area by nitrogen adsorption at 77 K using an accelerated surface area and porosity apparatus. Prior to the analysis, 0.65 g of the catalyst was degassed at 120 °C for 3 h in order to desorb moisture and other weakly adsorbed residues. The adsorption isotherm of nitrogen was collected at 77 K. Table 1 shows different BET surface areas of Li/MgO catalysts. The surface area of these catalysts was around 4.2 m2g-1. Catalytic performance of Li/MgO Among different Li/MgO catalysts investigated for OCM, some are relatively progressive in activity and selectivity which are listed in Table 2. According to Table 1, the performance of synthesized catalysts compared to some other Li/MgO (Table 2) showed better results, especially in methane conversion. All of these samples were prepared by wet impregnation method but there were two important differences. The most important difference between synthesized samples’ preparation method and conventional wet impregnation was lithium precursor. Normally lithium is used in nitrate or carbonate forms but, because of chloride effective impacts on the total productivity, lithium chloride is chosen as a lithium precursor. However, the XRD results showed no chlorinated compounds on the catalyst surface, but it affected total OCM productivity. Seemingly, these catalysts were more successful in hydrogen abstraction from methane. The second difference was precursor-mixing medium. In this work, ethanol was employed as a liquid medium and this choice had an impact on the catalyst surface area (BET). The specific surface area of the synthesized Li/MgO was about 4.2 m2g-1, while the surface area of Li/MgO with proper performance studied by other researches was equal to 2 m2g-1 (Table 2). Table 1 Activity and selectivity of the catalysts for oxidative coupling of methane Catalyst

BET surface area (m2g-1)

Best temperature (°C)

Products distribution at the best temperature XCH4 ð%Þ

SC2 ð%Þ

Y C2 ð%Þ

C2 H6

C2 H4

1% Li/MgO

4.19

750

44.2

39.9

17.62

2.49

3% Li/MgO

4.21

675

40.01

43.3

17.33

4.27

5% Li/MgO

4.15

725

35.12

60.8

21.35

5.12

Reaction conditions: atmospheric pressure; flow rate 150 mL/min; CH4/O2 = 2; 1.5 g catalyst

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Li content

0

1

3

7

2.18

7

6.5

3.2

24.8

5.3

No

1

2

3

4

5

6

7

8

9

10

803

717

775

860

800

800

720

720

650

650

Temperature (°C)

2

0.75

1

–

–

–

2

1

8

60

Surface area (m2g-1)

25.2

64

100

60

99.6

25.2

49.8

49.8

49.8

49.8

Flow rate (ml/min)

50:10:40

10:3:87

CH4:O2 = 10:1

11:8.6:80.4

CH4:O2 = 5:1

50:10:40

7.7:4.7:87.6

7.3:3.5:89.2

42:2:56

42:2:56

CH4:O2:diluent (%)

Table 2 Reports of Li/MgO catalysts for OCM prepared by the wet impregnation method

23

2.5

20

15

15

17

42.8

37

4.4

1

XCH4 (%)

54

–

76

81

72

80

45.6

49.8

54.7

0

SC2 (%)

12.4

–

15.1

12.1

12.2

13.7

19.4

18.4

2.4

0

YC2 (%)

MgO–Li2CO3

MgO–Li2CO3

MgO–Li2CO3

MgO–Li2CO3

MgO–Li2CO3

MgO–LiOH

[30]

[8]

[1]

[29]

[18]

[25]

[9]

[9] MgO–Li2CO3

[9] MgO–Li2CO3

[9]

Ref

MgO–Li2CO3

MgO–Li2CO3

Precursor

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(a) 80 Selectivity (%)

60 50 40 30 20 10 700

750

CH4 conversion (%)

(b) 60 1% Li/MgO 3% Li/MgO %5 Li/MgO

70

0 650

381

50 40 30 20 1% Li/MgO 3% Li/MgO %5 Li/MgO

10 0 650

800

700

750

800

Temperature (°C)

Temperature (°C )

Fig. 4 Selectivity toward C2 hydrocarbons (a) and methane conversion (b) versus operating temperature over Li/MgO with different Li loadings at atmospheric pressure and CH4:O2 = 2 (20 % in total feed)

6

(b) 30 1% Li/MgO 3% Li/MgO %5 Li/MgO

1% Li/MgO 3% Li/MgO %5 Li/MgO

25

Yield (%)

C2 H 4 /C 2 H 6

(a) 8

4

20 15 10

2 5 0 650

700

750

Temperature (°C)

800

0 650

700

750

800

Temperature (°C)

Fig. 5 Ethylene to ethane ratio (a) and C2 hydrocarbons yield (b) versus operating temperature over Li/ MgO with different Li loading at atmospheric pressure and CH4:O2 = 2 (20 % in total feed)

The conversion of CH4 and selectivity of C2 hydrocarbons over Li/MgO catalysts with different Li loadings as a function of operation temperature at atmospheric pressure are shown in Fig. 4 while the other operation parameters were similar. However, these catalysts have a similar specific surface area; visible difference in catalytic performance could be seen. So it can be concluded that the increase of Li loading changed the nature of catalysts (strength and crystalline phase of basicity sites). Fig. 4b shows the variation of methane conversion versus temperature. At lower temperatures, no significant decrease in the selectivity over 1 % Li/MgO and 3 % Li/MgO was observed, but the selectivity decreased against further temperature increases. An upward trend was observed for the selectivity toward C2 hydrocarbons in 5 % Li/MgO. This behavior was also observed in the yield curves (Fig. 5b). According to Figs. 4 and 5, the variation of parameters like selectivity, conversion and yield on 1 % Li/MgO catalyst was very slight relative to operating temperature of the reaction and no severe fluctuations could be observed. The addition of 5 wt % of lithium resulted in an increase of the steady-state C2 hydrocarbons selectivity. The selectivity, conversion as well as the yield increased by increasing the temperature from 675 to 725 °C simultaneously. This increase has led to improvements in yield by 100 % between 675 and 725 °C (Fig. 5b). After

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Reac Kinet Mech Cat (2013) 110:373–385 80 70 60 50 40 30 20 10 0 650

25 20

Yield (%)

Selctivity (%)

382

15 10 5

10% O2

12% O2

15% O2

20% O2

10 % O2

25% O2

700

12 % O2

15 % O2

0 650

750

20 % O2

25 % O2

700

Temperature (°C )

750

Temperature (°C )

Fig. 6 Selectivity toward C2 hydrocarbons and yield versus operating temperature over 5 % Li/MgO at different O2 concentrations

(b)

12

CO selectivity (mol %)

CO2 selectivity (mol %)

(a) 10 8 6 4 2 0 670

10 % O2

12 % O2

680

690

15 % O2

20 % O2

700

710

25 % O2

720

730

6 10 % O2

12 % O2

15 % O2

20 % O2

25 % O2

5 4 3 2 1 0 670

680

Temperature (°C )

690

700

710

720

730

Temperature (°C )

Fig. 7 Selectivity toward CO2 (a) and CO (b) versus operating temperature over 5 % Li/MgO at different O2 concentrations

this temperature, although the conversion was improved, a strong decrease in the C2 selectivity can be observed caused by side reactions and the production of undesired products. According to Figs. 4 and 5, 5 % Li/MgO catalyst was more selective toward desired products with an impressive difference in the product selectivity. This catalyst was selective at near low temperature; the optimum operation temperature in the mentioned conditions was 725 °C. Further increases in temperature caused more COx formation that had a visible impact on the total selectivity. Effect of oxygen concentration on OCM productivity In general, the selectivity of ethane, ethylene, carbon monoxide, and carbon dioxide varies with oxygen concentration in the feed. The principal reactions in the oxidative coupling of methane are [9, 30–32]: 1 2CH4 þ O2 ! C2 H6 þ H2 O 2 1 C2 H6 þ O2 ! C2 H4 þ H2 O 2

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ð4Þ ð5Þ


383

In addition, the main side reactions are: CH4 þ 2O2 ! CO2 þ 2H2 O

ð6Þ

3 ð7Þ CH4 þ O2 ! CO þ 2H2 O 2 To examine the effect of oxygen concentration on methane conversion and C2 selectivity, the experiments were carried out with 5% Li/MgO catalyst in temperature range of 675–725 °C with changing oxygen concentrations (10, 12, 15, 20 and 25 mol%). The yield of reaction and C2 hydrocarbon selectivity are illustrated in Fig. 6. The C2 hydrocarbon selectivity decreased when the oxygen concentration increased. Methane conversion increased with the oxygen concentration; i.e. higher oxygen concentration was favorable for methane activation. The gradual decrease in C2 hydrocarbon selectivity with increasing oxygen concentration due to C2 hydrocarbon and other intermediates was prone to be overoxidized to COx under higher oxygen concentration (Eqs. 6 and 7) at elevated temperatures. As shown in Fig. 6, the yield of the reaction increased when the oxygen concentration decreased from 25 to 15 %, and more decrement to 10 % had a reverse trend because of poor methane conversion. The effects of temperature and oxygen content in the feed gas on 5 % Li/MgO activity were demonstrated; changes of both parameters affected the CH4 conversion, C2 hydrocarbons yield, and the amount of unwanted products. It seems that the desired products were reacted with the oxygen feed and produced certain by-products such as carbon monoxide and carbon dioxide. As could be seen in Fig. 7, selectivity toward carbon monoxide was greater than carbon dioxide and this factor showed that deep oxidation occurred on the intermediate and converted them to a non-reversible way.

Conclusion Among various lithium loaded MgOs, 5 % Li/MgO catalyst was found to be the most suitable one for OCM, with which methane conversion and C2 hydrocarbon selectivity were about 35.1 and 60.8 %, respectively. This catalyst had a higher BET surface area compared with the catalysts prepared by conventional wet impregnation methods. This activity was dependent on the use of ethanol as a solution medium too. There were no chlorine compounds on the catalyst surface, but catalysts were more successful in hydrogen abstraction from methane due to the use of LiCl. 5 % Li/MgO had a higher activity and selectivity compared to other Li/ MgO, over this catalyst methane and oxygen conversions increased with increasing temperature. The selectivity and yield toward desired products had the largest value at 725 °C, so this is the best operating temperature for this catalyst. Ethane and ethylene in the presence of extra oxygen were involved in subsequent reactions, so the selectivity decreased. On the other hand, the C2 formation rate increased with increasing temperature while the C2 selectivity decreased with increasing temperature at temperatures higher than 725 °C. This issue indicated that the combustion rate of the products increased faster than the methane activation reaction above

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725 °C. As was expected, not only the C2 formation rate but also the C2 selectivity increased with increasing methane concentration in the feed composition.

References 1. Hoogendam GC, Seshan K, Van Ommen JG, Ross JRH (1994) Oxidative coupling of methane over doped Li/MgO catalysts. Catal Today 21:333–340 2. Choudhary VR, Rane VH, Chaudhari ST (1997) Surface properties of rare earth promoted MgO catalysts and their catalytic activity/selectivity in oxidative coupling of methane. Appl Catal A Gen 158:121–136 3. Choudhary VR, Rane VH, Chaudhari ST (2000) Factors influencing activity/selectivity of La-promoted MgO catalyst prepared from La- and Mg- acetates for oxidative coupling of methane. Fuel 79:1487–1491 4. Dedov AG, Loktev AS, Moiseev II, Aboukais A, Lamonier JF, Filimonov IN (2003) Oxidative coupling of methane catalyzed by rare earth oxides: Unexpected synergistic effect of the oxide mixtures. Appl Catal A Gen 245:209–220 5. Gao Z, Shi Y (2010) Suppressed formation of CO2 and H2O in the oxidative coupling of methane over La2O3/MgO catalyst by surface modification. J Nat Gas Chem 19:173–178 6. Arndt S, Laugel G, Levchenko S, Horn R, Baerns M, Scheffer M, Schlogl R, Schomaacker R (2011) A Critical Assessment of Li/MgO-Based Catalysts for the oxidative coupling of methane. Catal Rev Sci Eng 53(4):424–514 7. Balint I, Miyazaki A, Gingasu D, Papa F (2012) Relevance of the basicity of MO–Sm2O3 (M=Zn, Mg, Ca, Sr) mixed oxides for the efficiency of methane conversion to C2? hydrocarbons. Reac Kinet Mech Cat 105(1):5–11 8. Coulter K, Szanyi J, Goodman DW (1995) Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO. Catal Lett 35(1–2):23–32 9. Ito T, Wang JX, Lin CH, Lunsford JH (1985) Oxidative dimerization of methane over a lithiumpromoted magnesium oxide catalyst. J Am Chem Soc 107:5062–5068 10. Aigler JM, Lunsford JH (1991) Oxidative dimerization of methane over MgO and Li?/MgO monoliths. Appl Catal 70:29–42 11. Couwenberg PM, Chen Q, Marin GB (1996) Kinetics of a gas-phase chain reaction catalyzed by a solid: The oxidative coupling of methane over Li/MgO-based catalysts. Ind Eng Chem Res 35:3999–4011 12. Lintuluoto M, Nakamura Y (2004) Theoretical study on the adsorption of methane on MgO and Lidoped MgO surfaces. J Mol Struct 674:207–212 13. Tang L, Yamaguchi D, Wong L, Burke N, Chiang K (2011) The promoting effect of ceria on Li/MgO catalysts for the oxidative coupling of methane. Catal Today 178:172–180 14. Aika K, Lunsford JH (1978) Surface reactions of oxygen ions. 2. Oxidation of alkenes by O-1 ion on MgO. J Phys Chem 82(16):1794–1800 15. Korf SJ, Ross JA, De Bruin NA, Van Ommen JG, Ross JRH (1990) Lithium chemistry of lithium doped magnesium oxide catalysts used in the oxidative coupling of methane. Appl Catal 58:131–146 16. Myrach P, Nilius N, Levchenko SV, Gonchar A, Risse T, Dinse KP, Boatner LA, Frandsen W, Horn R, Schlcgl R, Freund HJ, Scheffler M (2010) Temperature-dependent morphology, magnetic and optical properties of Li-doped MgO. Chem Cat Chem 2:854–862 17. Lunsford JH (1995) The catalytic oxidative coupling of methane. Angew Chem Int Ed 34:970–980 18. Van Kasteren JMN, Geerts JW, Van der Wiele K (1990) The role of hetrogeneous reaction during the oxidative coupling of methane over Li/MgO. Catal Today 6:497–502 19. GuczI L, Van Santen RA, Sarma KV (1996) Low-temperature coupling of methane. Catal Rev Sci Eng 38(2):249–296 20. Heitz S, Epping JD, Aksu Y, Driess M (2010) Molecular heterobimetallic approach to Li-containing MgO nanoparticles with variable Li-concentrations using lithium-methylmagnesium alkoxide clusters. Chem Mat 22:4563–4571

123


385

21. Shigapov AN, Novoshilova MA, Vereshchagin SN, Anshits AG, Sokolovskii VD (1988) Peculiarities in oxidative conversion of methane to C2 hydrocarbons over CaO–CaCl2 catalysts. React Kinet Catal Lett 37(2):397–402 22. Otsuka K, Suga K, Yamanaka I (1988) Electrochemical enhancement of oxidative coupling of methane over LiCl-doped NiO using stabilized zirconia electrolyte. Catal Lett 1(12):423–1988 23. Kumar CP, Gaab S, Muller TE, Lercher JA (2008) Oxidative dehydrogenation of light alkanes on supported molten alkali metal chloride catalysts. Top Catal 50(1–4):146–167 24. Mcnamara DJ, Korf SJ, Seshan K, Van Ommen JG, Ross JRH (1991) The effect of Nb2O5 and ZrO additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane. Can J Chem Eng 69:883–890 25. Otsuka K, Hatano M, Komatsu T (1988) Synthesis of C2H4 by partial oxidation of CH4 over transition metal oxides with alkali chlorides. Stud Surf Sci Catal 36:383–387 26. Kus S, Otremba M, Torz A, Taniewski M (2002) Further evidence of responsibility of impurities in MgO for variability in its basicity and catalytic performance in oxidative coupling of methane. Fuel 81(13):1755–1760 27. Choudhary VR, Mulla SAR, Uphade BS (1999) Oxidative coupling of methane over alkaline earth oxides deposited on commercial support precoated with rare earth oxides. Fuel 78(4):427–437 28. Istadi I, Amin NAS (2006) Synergistic effect of catalyst basicity and reducibility on performance of ternary CeO2-based catalyst for CO2 OCM to C2 hydrocarbons. J Mol Catal A 259:61–66 29. Edwards JH, Tyler RJ, White SD (1990) Oxidative coupling of methane over lithium-promoted magnesium oxide catalysts in fixed-bed and fluidized-bed reactors. Energy Fuels 4:85–93 30. Roos JA, Bakker AG, Bosch H, Van Ommen JG, Ross JRH (1987) Selective oxidation of methane to ethane and ethylene over various oxide catalysts. Catal Today 1(1–2):133–145 31. Tung WY, Lobban LL (1992) Oxidative coupling of methane over lithium/magnesia: kinetics and mechanisms. Ind Eng Chem Res 31(7):1621–1625 32. Sadeghzadeh Ahari J, Ahmadi R, Mikami H, Inazu K, Zarrinpashne S, Suzuki Sh, Aika K (2009) Application of a simple kinetic model for the oxidative coupling of methane to the design of effective catalysts. Catal Today 145:45–54

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