SiO2 Catalyst for the Simultaneous Production of C2H4

Jan 21, 2010 - New Co−La/SiO2 Catalyst for the Simultaneous Production of C2H4 and Syngas from CH4 with ... Catalysis Today 2014 228, 212-218 ...
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Ind. Eng. Chem. Res. 2010, 49, 2078–2083

New Co-La/SiO2 Catalyst for the Simultaneous Production of C2H4 and Syngas from CH4 with Na2WO4/Mn/SiO2 Songtao Ren, Song Qin, Jianqiang Zhu, Xiaoxi Peng, and Changwei Hu* Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan UniVersity, Chengdu, 610064, China

A new dual catalyst bed system with Na2WO4/Mn/SiO2 in the first layer and Co-La/SiO2 in the second layer has been applied in the conversion of methane to ethylene and syngas simultaneously. The effects of flow velocity and the ratio of methane to oxygen on the catalytic performance were investigated. The results showed that, when F ) 150 mL/min, CH4/O2 ) 2.5, the amount of Na2WO4/Mn/SiO2 was 0.27 g in the first bed, and that of Co-La/SiO2 was 0.1 g in the second bed, the expected mixture of C2H4/H2/CO ) 1/1/1 with a total yield of 20.6% for C2H4 + CO was achieved, which could be directly used as a feedstock to produce propanal by hydroformylation of ethylene. The new process provides an efficient method for the utilization of CO2, H2O, and thermicity. The Co-La/SiO2 catalyst on the second layer was found to possess a catalytic performance for both partial oxidation of methane and oxidative coupling of methane reactions. 1. Introduction To activate methane, the main component of natural gas, and convert it into more valuable chemicals is one of the research focuses worldwide. Up to now, different strategies have been proposed for the conversion of methane.1–7 Among those methods, oxidative coupling of methane (OCM) stands out as one potentially important method, for it can directly and effectively convert methane into value-added C2 hydrocarbons.8,9 Since 1982 when Keller and Bhasin10 released the first report, a large number of papers and patents have been published.11–14 Unfortunately, because the chemical activities of products are often much higher than that of methane, the single-pass combined yield of C2H4 and C2H6 (C2 products) is therefore limited to 25%.1 To overcome this restrictive problem, some novel and useful ways were developed by many researchers.15–20 In our previous work, a novel strategy of methane utilization has been proposed to convert methane to H2, C2H4, and CO simultaneously with similar mole concentrations, and the produced mixture can be directly used in propanal synthesis via hydroformylation of ethylene: C2H4 + H2 + CO f CH3CH2CHO.21–23 This way of methane utilization makes CO and H2, the byproduct in OCM, useful and favors the successive process. In our previous work, kinetically controlled free radical gas phase methane oxidation was combined with catalytic oxidative coupling of CH4 over Mn/Na2WO4/SiO2 to concomitantly acquire ethylene and syngas with close concentration,22 and the utilization or treatment of other residual gas (C2H6, CO2, CH4, and H2O) was also mentioned.21–24 Under the optimal reaction conditions, the value of CO:H2:C2H4 ) 1.0:1:0.9 in the product was obtained, but the ethylene yield was rather low (only 7.3%). In our previous work, a series of La-promoted 5 wt % Na2WO4/2 wt % Mn/SiO2 catalysts were prepared and applied in OCM.23 The yield of C2H4 reached the optimized value of about 17.6%, but the H2 mole ratio was inadequate with a C2H4/CO/H2 ratio of 1/1/0.3. Later, the dual catalyst bed system was used, and it was found that the dual catalyst bed can effectively adjust the distribution of the products of the reaction. H2, C2H4, and CO with similar mole concentrations * To whom correspondence should be addressed. Tel.: 86-2881801141. Fax: 86-28-85411105. E-mail: [email protected] or [email protected].

(C2H4/H2/CO ) 1.2/1/1.1) had been achieved.24 The partial oxidation catalyst Co/γ-Al2O3 was put on the first catalyst bed in order to obtain a certain amount of H2 and CO; after the gas went through Na2WO4/Mn/SiO2 catalyst on the second bed, C2H4 was generated. However, in such a dual catalyst bed system, the yield of C2H4 was not satisfactory, for a part of the O2 was consumed in the first Co/γ-Al2O3 catalyst and insufficient O2 remained over the second Na2WO4/Mn/SiO2 catalyst. On the basis of our previous work, we report here another dual bed system. The Na2WO4/Mn/SiO2 catalyst for oxidative coupling reaction was placed in the first bed, in order to obtain a better yield of C2H4. A new Co-La/SiO2 catalyst was prepared and used in the second layer. It is expected that the effluent from the first bed, that is, C2H6, H2O, and CO2 obtained, and unreacted CH4, may undergo further reaction to produce a certain amount of syngas in the second catalyst bed. 2. Experimental Section 2.1. Catalyst Preparation. Preparation of Na2WO4/Mn/ SiO2 Catalyst. The catalyst was prepared by wetness impregnation of silica (Nanjing Heyi Chemical Co., China, SBET ) 317 m2/g; particle size, 20-40 mesh) support with aqueous solution containing appropriate concentrations of Mn(NO3)2 for 24 h. After removal of the solvent by heating, the sample was dried at 393 K for 4 h. The obtained materials were impregnated with aqueous solution containing appropriate concentrations of Na2WO4 for 24 h, then dried at 393 K for 4 h, and finally calcined in a muffle furnace at 1073 K for 5 h. Preparation of Co-La/SiO2 Catalyst. The catalyst was prepared by a stepwise loading and using nitrate salt as the precursor of active components. Commercial silica as described above was used as the initial support. At first, the SiO2 was impregnated with an aqueous solution of La(NO3)3; after 24 h the sample was dried at 393 K for 4 h and subsequently calcined in air at 1073 K for 5 h. Co was further impregnated on La/ SiO2. The sample was finally calcined in air at 1073 K for 5 h, which kept the Co and La loading in the weight percentages of 2% and 4.3% in the catalyst, respectively. 2.2. Activity Test. The catalytic reaction using the dual catalyst bed was investigated in a tubular fixed-bed down-flow microreactor made of quartz (70 mm length, 8 mm i.d.) under

10.1021/ie9010468  2010 American Chemical Society Published on Web 01/21/2010

Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

atmospheric pressure. The temperature of the reactor was controlled by a thermocouple at a position corresponding to the center of the catalyst bed. In our dual catalyst bed system, Na2WO4/Mn/SiO2 catalyst was used in the first layer, followed by the Co-La/SiO2 catalyst. These two catalyst beds were separated by 0.2 mL of silica sand. The catalyst beds were first heated to 1073 K at the rate of 10 K/min in a flow of Ar (99.999%, 20 mL/min), and then the reactant gases (CH4 and O2) were introduced into the reactor without any dilution. The velocity of the reactant gases was dominated by mass flow controllers (D07-11A/ZM made by Beijing Sevenstar Huachuang Electronic Co. Ltd.). At the reactor outlet, the effluent gases after removal of H2O were detected online by a gas chromatograph (Agilent 1790 GC) equipped with a thermal conductivity detector, using a plot-C2000 capillary column to separate the effluent gases. All the activity results discussed in this article were obtained after the reaction reached its stabilization state in about 3 h, and the deviation of the activity was about (0.2%, obtained by at least three parallel tests. 2.3. Catalyst Characterization. To compare the structural differences between the fresh catalyst and the used catalyst, X-ray diffraction (XRD) was used to characterize the samples. The XRD patterns of the catalysts were obtained with a DX1000 CSC diffractometer using Cu KR radiation at 40 eV and 25 mA. The scattering angle of 2θ ranged from 10° to 70° with a step of 0.06 deg/min. 2.4. Calculations. The conversion of CH4, the selectivities of the products, and the yields and mole ratio of target products for the reaction were calculated by the following equations, respectively. XCH4 )

XO2 )

SCO2 )

SC2H4 )

SC2H6 )

SCO )

S H2 )

FCH4 in - FCH4 out FCH4 in FO2 in - FO2 out FO2 in

× 100%

× 100%

FCO2 FCH4 in - FCH4 out 2FC2H4 FCH4 in - FCH4 out 2FC2H6 FCH4 in - FCH4 out

× 100%

× 100%

× 100%

FCO × 100% FCH4 in - FCH4 out FH2

2(FCH4 in - FCH4 out)

× 100%

YC2H4+CO ) XCH4(SCO + SC2H4) × 100% C2H4 /H2 /CO ) FC2H4 out /FH2 out /FCO Fi is the flow rate of species i (mL/min). The carbon balance was calculated around each experiment by comparing the carbon outlet in the gas phase to the carbon inlet, which was expressed as follows:

C balance ) 2FC2H4 + 2FC2H6 + FCO + FCO2 + FCH4 out FCH4 in

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× 100%

3. Results and Discussion 3.1. XRD Characterization of Co-La/SiO2 Catalyst. Figure 1 shows the XRD patterns of the Co-La/SiO2 catalyst before and after reaction. The diffraction of amorphous silica is apparent for both samples. The diffraction peaks corresponding to Co3O4 (at 36.9°, 45.0°, 59.4°, and 65.3°) were observed for the fresh Co-La/SiO2 catalyst. Weak diffraction peaks of LaO (at 30.2°, 50.1°, and 59.5°) and quartz (at 28.3° and 41.5°) were observed in the catalyst before reaction, and the intensity of these peaks increased after reaction. This indicates that bigger LaO and quartz particles might form in the catalyst after reaction. As shown in Figure 1, the diffraction peaks corresponding to Co3O4 disappeared, and those for Co and CoO appeared after reaction. This indicated that Co3O4 was reduced in the reaction. Bouarab et al.25 investigated the addition of La promoter to Co/SiO2 on the structure of the catalyst, and found that crystalline La2O3 did not appear with 5% and 10% La added. It is reasonable that La2O3 crystalline was not detected, for the content of La was less than 5% in the present work. The appearance of LaO diffraction peaks might be caused by the higher calcination temperature in our research. 3.2. Effects of Dual Catalyst Bed. In our previous study,24 a dual catalyst bed with Co/γ-Al2O3 as the first layer aiming to produce syngas and Na2WO4/Mn/SiO2 catalyst as the second layer to produce ethylene was used to realize the simultaneous production of ethylene and syngas from CH4 and O2. A yield of 18.5% for the target products was obtained. In the present work, a reverse order of the target gas production strategy was used. As shown in Table 1, the Na2WO4/Mn/SiO2 catalyst exhibits good activity for OCM, and the low methane conversion in our experiment is similar to those reported in the literature.8,26,27 The ratio of C2H4/H2/CO was about 3.6/1/2.8, respectively; therefore the products obtained were unsuitable for the hydroformylation of ethylene to propanal as expected. When the Co-La/SiO2 catalyst was used solely, higher CO and H2 selectivity could be obtained. At the same time, a considerable amount of C2H4 with a selectivity of 18.1% was obtained, and the ratio of C2H4/H2/CO was about 0.2/1/1.1. Therefore, the products obtained over this catalyst bed were also unsuitable for the hydroformylation of ethylene to propanal. To understand the origin of C2H4 formation, a control experiment without catalyst was carried out as a blank test. As shown in Table 2, the conversion of CH4 over the Co-La/SiO2 catalyst was 15.2%, which is much higher than that obtained in the blank test (5.8%). The yields of C2H4 and C2H6 in the blank test were only about 2.0% and 1.4%, although the selectivities for C2H4 and C2H6 reached about 34.9% and 23.6%, respectively. When the Co-La/SiO2 catalyst was filled in the reactor, the yields of C2H4 and C2H6 increased to 2.8% and 1.7%, respectively. These results indicated that the Co-La/SiO2 catalyst exhibits not only partial oxidation of methane (POM) activity but also OCM activity in increasing C2 yield. At least, the increased amount of CO and H2 over Co-La/SiO2 might come from the increased conversion of methane, and the Co-La/SiO2 catalyst does not decrease the production of ethylene via POM. It is also possible that the amount of C2 hydrocarbon produced over Co-La/SiO2 by OCM is greater than those consumed over it by POM. It is known that Co has no OCM activity; these results show that La may play an important role for the OCM activity. This is in

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Figure 1. Results of XRD characterization for Co-La/SiO2 catalyst. (a) Fresh catalyst; (b) used catalyst (2, amorphous silica; b, Co3O4; O, CoO; 9, Co; 4, quartz; 0, LaO).

accordance with some literature, where the La species was usually considered as OCM catalyst.11,12,28 We applied the Co-La/SiO2 catalyst to the second layer and the Na2WO4/Mn/SiO2 catalyst was placed in the first layer. It was expected that this approach might effectively enhance ethylene production and then increase the yield of target products. By comparison of the results over solely Na2WO4/ Mn/SiO2 catalyst with those over the dual catalyst bed system, as shown in Table 1, it was found that when the dual catalyst system was used, the conversion of CH4 increased slightly from 31.2% to 32.1% and the selectivities of H2 and CO increased from 3.1% and 17.3% to 4.6% and 18.7%, respectively. In contrast, the selectivities of CO2, C2H4, and C2H6 slightly decreased. This might be caused by the enhanced reforming reaction between CO2, H2O, C2H6, C2H4, and CH4 over the Co-La/SiO2 catalyst. As a result, in the dual catalyst bed system, we could obtain more desired result at the C2H4/H2/ CO ratio of about 2.4/1/2. In the dual catalyst bed system, different reaction conditions, such as gas space time and CH4/O2 ratio in the feed, resulted in different conversions of methane and product distribution. Thus those reaction parameters are also optimized. 3.3. Effect of CH4/O2 Ratio. Under the condition of F ) 120 mL/min, a series of experiments were carried out in terms of changing the CH4/O2 ratio to 2, 2.5, 3, 4, and 5 to tune the producing mixture composition over the dual catalyst bed. The conversion of methane and the selectivity of CO2 increased continuously with increasing amount of O2 in the feed, while the selectivity of C2H4 decreased from 51% to 33.9% continuously. Liu et al.27 investigated the influence of the CH4/O2 initial ratio on the methane conversion over the Na2WO4/Mn/SiO2 catalyst, and found that higher CH4/O2 inlet ratios resulted in lower methane conversion and increased the selectivity of C2 hydrocarbons. As shown in Table 3, we obtained similar results, namely, with the increase of the CH4/O2 ratio, the conversion of methane decreased continuously and the selectivity of C2H4 increased continuously. These were in agreement with the results obtained over Sr/La2O3,18 Mn-Na2WO4/SiO2,27 BaCO3/LaOBr,28 the SCMCR,15 and our previous dual catalyst bed system.24 The high selectivity of C2 hydrocarbons at high CH4/O2 ratio may be attributed to the lack of oxygen to further oxidize the products formed. In the scope of the experiment, regardless of

how the oxygen content was increased, the O2 conversion was 100% (under our reaction conditions, the detected oxygen conversion was 100%), so the increased oxygen was mainly consumed in the deep oxidation. This can be proven by the changing trend of the selectivities of CO and CO2 which increased monotonically with increasing O2 in the feed gases. 3.4. Influence of Flow Velocity on the Results. Based on the results obtained above, the reaction was carried out by passing a mixture of CH4 and O2 over the dual catalyst bed system at different flow velocities while keeping the CH4/O2 ratio of 2.5 constantly in the feed at 1073 K. Adjusting the total flow rate of reagents (F) could alter the contact time of reactants with the catalysts, and tune the distribution of the effluent from the dual bed system. From Table 4, it was found that higher flow velocity favored the formation of CO and decreased the selectivity of C2H4. This is in agreement with the results in our previous report.29 The selectivity of H2, decreasing along with the increasing flow velocity, reached a minimum of 4.6% at F ) 120 mL/min and then increased with further increase of flow velocity. It seems that when F e 120 mL/min, CO2 and H2O stay a longer time in the second catalyst bed, and then the reforming reaction C2H6 + 2H2O f 2CO + 5H2 and C2H6 + 2CO2 f 4CO + 3H2 occurred more sufficiently. Consequently, the selectivity of hydrogen could reach 7.1% in F ) 60 mL/min, and then decreased with increasing flow velocity. Similar observations were also made in Marquaire’s30 and Zheng’s31 investigations on the influence of contact time on the reforming reaction, in which they proposed the requirement of sufficient contact time for the reforming reaction to occur significantly. On the other hand, Liu et al.27 considered that, under high gas hourly space velocity conditions, the gas phase reaction became intensive and a significant amount of reactants and C2 products were deeply oxidized to CO2. Hence, owing to the more intensive gas phase reaction when F increased from 120 to 180 mL/min, the selectivity of C2H4 decreased along with the increasing of the flow velocity and reached the minimum of about 24.1% at F ) 180 mL/min. The selectivity of CO and H2 increased continuously with the increasing of flow velocity, which might also be attributed partially to the gas phase reaction. As shown in Table 4, the molar ratio of C2H4/H2/CO in products could be successfully controlled by tuning the flow velocity. At F ) 150 mL/min, the ratios of target products were about C2H4/H2/CO ) 1/1/1, respectively, and this was suitable as the feedstock for the production of propanal. The total yield of CO + C2H4 reached about 20.6%, which was higher than our previous result (18.5%).24 3.5. Role of Co-La/SiO2 Catalyst in the System. Although the Co-La/SiO2 catalyst for reforming reaction was tested in the above section, it was not sufficient to ascertain the detailed performance of the Co-La/SiO2 catalyst in the dual catalyst system. In order to clarify this problem, several controlled experiments were carried out to probe the catalytic performance of La/SiO2 for different feeds. Since C2H6 could be formed in the first catalyst layer and then transformed to other species, such as CO, H2, and C2H4, we focused our attention on the reaction of C2H6 over the Co-La/SiO2 catalyst. Because the effluent gas from the first layer contained about 5% C2H6, the feed diluted with Ar (Ar/C2H6/CO2 ) 90/5/5) was first employed for simulating the feeds for the second layer. We also noticed that water can be also generated from the first layer, so the feed of Ar/C2H6/H2O ) 90/5/5 was also used. As supplementary experiments, the tests using the feeds of Ar/CH4/CO2 ) 90/5/5 and Ar/CH4/H2O ) 90/5/5 were also performed to probe the

Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010 Table 1. Catalytic Performance of Single Na2WO4/Mn/SiO2 Catalyst Bed and the Dual Catalyst Bed conversion, %

selectivity, %

catalyst bed

CH4

O2

C2H4

CO

H2

CO2

C2H6

C2H4 + CO yield, %

Na2WO4/Mn/SiO2 dual catalyst bed

31.2 32.1

100 100

44.9 43.2

17.3 18.7

3.1 4.6

25.8 23.4

15.0 13.8

19.4 19.9

a

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a

mole ratio C2H4/H2/CO

C balance, %

3.6/1/2.8 2.4/1/2

100.9 99.7

Reaction conditions: T ) 1073 K; CH4/O2 ) 2.5; F ) 120 mL/min; M1:M2 ) 0.27 g:0.1 g (1, Na2WO4/Mn/SiO2; 2, Co-La/SiO2).

Table 2. Catalytic Performance of the Co-La/SiO2 Catalyst and the Blank Test without Catalysta conversion, %

selectivity, %

yield, %

catalyst bed

CH4

O2

C2H4

CO

H2

CO2

C2H6

C2H4

C2H6

blank test Co-La/SiO2

5.8 15.2

33.2 42.5

34.9 18.1

27.1 44.6

15 20.0

13.9 24.1

23.6 11.0

2.0 2.8

1.4 1.7

a

mole ratio C2H4/H2/CO

C balance, %

0.6/1/0.9 0.2/1/1.1

100.0 99.7

Reaction conditions: T ) 1073 K; CH4/O2 ) 2.5; F ) 120 mL/min; 0.1 g of Co-La/SiO2 catalyst.

Table 3. Effect of CH4/O2Ratio on the Reactiona selectivity, %

ratio of CH4/O2 in reagents

conversion, % CH4

C2H4

CO

H2

CO2

C2H6

C2H4 + CO yield, %

5 4 3 2.5 2

20.9 24.6 28.2 32.1 37.0

51.0 48.4 48.3 43.2 33.9

10.7 11.6 16.3 18.7 25.1

6.6 4.9 4.8 4.6 6.7

16.6 20.9 23.1 23.4 29.2

20.5 19.6 14.9 13.8 8.6

12.9 14.8 18.2 19.9 21.9

a

mole ratio C2H4/H2/CO

C balance, %

1.9/1/1.8 2.5/1/1.2 2.5/1/1.7 2.4/1/2 1.3/1/1.9

99.7 100.1 100.7 99.7 98.8

Reaction conditions: T ) 1073 K; F ) 120 mL/min; M1:M2 ) 0.27 g:0.1 g (1, Na2WO4/Mn/SiO2; 2, Co-La/SiO2).

Table 4. Influence of the Total Flow Rate of the Feeda selectivity, %

F, mL/min

conversion, % CH4

C2H4

CO

H2

CO2

C2H6

C2H4 + CO yield, %

60 90 120 150 180

32.1 33.5 32.1 32.5 34.9

50.9 48.2 43.2 41.1 24.1

14.7 16.9 18.7 22.1 32.3

7.1 5.0 4.6 10.5 26.0

26.6 23.3 23.4 23.2 24.9

10.7 13.5 13.8 14.3 10.9

21.1 21.8 19.9 20.6 19.7

a

mole ratio C2H4/H2/CO

C balance, %

1.8/1/1 2.4/1/1.7 2.4/1/2 1/1/1 0.2/1/0.6

100.9 100.6 99.7 100.2 97.3

Reaction conditions: T ) 1073 K; CH4/O2 ) 2.5; M1:M2 ) 0.27 g:0.1 g (1, Na2WO4/Mn/SiO2; 2, Co-La/SiO2).

Table 5. Composition of the Effluents with Different Feed over the Co-La/SiO2Catalysta content, mol %

conversion, %

feed composition

H2

CO

CH4

CO2

C2H4

C2H6

C balance, %

C2H6 or CH4

CO2

Ar/C2H6/CO2 Ar/C2H6/H2O Ar/C2H6/CO2, no catalyst Ar/CH4/CO2 Ar/CH4/H2O

40.6 58.7 31.8 0.4 1.1

36.9 15.5 0 0.4 0.4

1.7 2.5 3.0 54.3 98.6

7.7 0 36.2 44.9 0

11.7 19.8 21.9 0 0

1.3 3.5 7.1 0 0

98.2 98.5 86.5 99.6 98.9

94.5 89.4 79.8 0.4 0.4

70.6

a

0 0.4

Reaction conditions: T ) 1073 K; F ) 120 mL/min; 0.1 g of Co-La/SiO2 catalyst; feed composition, Ar/X/Y ) 90/5/5.

possibility of the reaction of CH4 over the Co-La/SiO2 catalyst. The results are listed in Table 5. As shown in Table 5, it is interesting to note that in the reaction of C2H6 with H2O, or with CO2, a rather high concentration of C2H4, 19.8% and 11.7%, respectively, was obtained. To elucidate the origin of C2H4 produced, a blank test without catalyst was done for the reaction of C2H6 with CO2. The concentration of C2H4 in the blank test reached about 21.9%, while no conversion of CO2 was observed. The results indicated that C2H4 mainly came from C2H6 dehydrogenation (C2H6 f C2H4 + H2) at high temperature. While there was no CO yielded in this blank test, it proved that CO was mainly produced from the reforming reaction over the Co-La/SiO2 catalyst. By comparing the concentration of C2H4 and C2H6 over the Co-La/SiO2 catalyst with those obtained in the blank test, it was found that the Co-La/SiO2 catalyst improved C2H6 conversion. Baerns et al.32 made a comparative study on noncatalytic and catalytic oxidative dehydrogenation of ethane to ethylene, and also found the noncatalytic oxidative dehydro-

genation of C2H6 at high temperature (about 80% C2H6 conversion) and the enhancement of C2H6 conversion by the catalyst Sr1.0La1.0Nd1.0Ox at the same temperature. When we used the Co-La/SiO2 catalyst in different feeds, syngas was the main component in the products. This showed that the Co-La/SiO2 catalyst generally exhibited good reforming activity in our system. The XRD results showed that some Co3O4 was reduced to Co0 in the reaction process, and Co0 is usually33–35 considered as the active site and responsible for the significant reforming activity. The concentration of H2 in the product gases reached 58.7% and 40.6% in the reforming reactions of C2H6 by H2O and CO2, respectively. By contrast, the concentration of CO in the product was only 15.5% and 36.9%, respectively. Hence, it is reasonable to consider that H2 not only comes from those two reactions over the Co-La/SiO2 catalyst (C2H6 + 2H2O f 2CO + 5H2 and C2H6 + 2CO2 f 4CO + 3H2) but also comes from C2H6 dehydrogenation (C2H6 f C2H4 + H2). In the absence of catalyst, the C balance was only 86.5%, and carbon aggregations

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Table 6. Catalytic Performance of Single Na2WO4/Mn/SiO2 Catalyst Bed or Co-La/SiO2 Catalyst Bed and the Dual Catalyst Beda conversion, %

selectivity, %

catalyst

CH4

O2

C2H4

CO

H2

CO2

C2H6

C2H4 + CO yield, %

Co-La/SiO2 Na2WO4/Mn/SiO2 dual catalyst bed

14.5 30.7 32.5

41.2 100 100

14.8 43.4 41.1

45.3 18.2 22.1

19.0 4.3 10.5

25.6 25.6 23.2

10.0 14.7 14.3

8.7 18.9 20.6

a

mole ratio C2H4/H2/CO 0.2/1/1.2 2.5/1/2.1 1/1/1

C balance, % 99.4 100.6 100.2

Reaction conditions: T ) 1073 K; CH4/O2 ) 2.5; F ) 150 mL/min; M1:M2 ) 0.27 g:0.1 g (1, Na2WO4/Mn/SiO2; 2, Co-La/SiO2).

were observed in the tube after reaction. When the Co-La/ SiO2 catalyst was used under the same reaction conditions, the carbon balance was improved to 98.2% and no detectable carbon was found in the tube after reaction. Thus, it is obvious that the presence of the Co-La/SiO2 catalyst altered the reaction in two aspects, that is, enhancing the conversion of C2H6 and facilitating the removal of coke deposited. Chen et al.36 investigated the LaFeyNi1-yO3 supported nickel catalysts for steam reforming of ethanol, and found that La2O3 can react with CO2 and form La2O2CO3, which can eliminate the carbon deposited on the metallic surface through the reaction La2O2CO3 + C f La2O3 + 2CO. Thus the La species in the Co-La/SiO2 catalyst might be beneficial to carbon removal. To further understand the effect of the Co-La/SiO2 catalyst for achieving our target, several experiments were done under the condition of F ) 150 mL/min. From Table 6, it was found that when the Co-La/SiO2 catalyst is used in the second bed, the selectivities of C2H4 and C2H6 decreased slightly from 43.4% and 14.7% to 41.1% and 14.3%, respectively, while the selectivities of H2 and CO increased significantly from 4.3% and 18.2% to 10.5% and 22.1%, respectively. Those results shown that the Co-La/SiO2 catalyst exhibited reforming activity. The above results are in accordance with those obtained with a flow velocity of 120 mL/min. Hence, after the Co-La/ SiO2 catalyst was accommodated in this dual catalyst bed system, we obtained the desired result of C2H4/H2/CO ) 1/1/1 with a total yield of CO + C2H4 of about 20.6%. In addition, the stability test was carried out at CH4/O2 ) 2.5 and F ) 150 mL/min within 14 h. The conversions of reactants and the yields of products over the dual catalyst bed system with time on stream are depicted in Figure 2. The results illustrated that the system of dual catalyst bed exhibited considerable stability for the reaction process. As shown in Figure 2, there is no remarkable change in catalytic activity of the present dual bed system, for the conversions of CH4 remained constant about 32% within 14 h. Except that the

yield of C2H4 decreased slightly within 2 h, other product distributions remained constant with time on stream. As a consequence, the ratio of C2H4/H2/CO remained a constant 1/1/1 and therefore its yield had no distinguishable variation after 2 h. In summary, it can be induced that the catalytic performance of Co-La/SiO2 may exhibit in the following two aspects. First, the Co-La/SiO2 catalyst possessed a good reforming activity for the reactions between the C2H6, H2O, and CO2 obtained from the first catalyst bed in situ (C2H6 + 2H2O f 2CO + 5H2 and C2H6 + 2CO2 f 4CO + 3H2). Hence H2 and CO would be increased to achieve the same mole concentration of C2H4 without obvious sacrificing of C2H4 yield. Second, the Co-La/ SiO2 catalyst might inhibit the coke formation or enhance the carbon removal reaction, and then the carbon aggregations on the catalyst could be eliminated, and therefore the dual catalyst bed system remains stable. 4. Conclusions The dual catalyst bed used in the present work could effectively give our target products. Under optimized conditions (F ) 150 mL/min, CH4/O2 ) 2.5, the first layer catalyst Na2WO4/Mn/SiO2 of 0.27 g, and the second layer catalyst Co-La/SiO2 of 0.1 g), the desired result of C2H4/H2/CO ) 1/1/1 with a total yield of 20.6% for C2H4 + CO could be obtained. The combination process can utilize not only the CO2 and H2O yielded from methane oxidative coupling for the reforming reaction directly to produce syngas, but also the exothermicity from the methane oxidative coupling to support the reforming reaction. The new process provides an efficient method for the utilization of CO2, H2O, and thermicity. The Co-La/SiO2 catalyst has not only good reforming activity, but also exhibits significant OCM activity. In addition, the Co-La/SiO2 catalyst favors keeping the dual catalyst bed system stable. Acknowledgment The authors are grateful for financial support from the NNSFC (No. 20976109), the Special Research Foundation of Doctoral Education of China (No. 20090181110046), and the characterization of the catalysts from Analytical and Testing Center of Sichuan University. Literature Cited

Figure 2. Conversion of CH4 and yields of C2H4, CO, and H2 with time on stream. Reaction conditions: T ) 1073 K, CH4/O2 ) 2.5, F ) 150 mL/ min, M1:M2 ) 0.27 g:0.1 g (×, conversion of CH4; O, yield of C2H4; ], yield of CO; 4, yield of H2).

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ReceiVed for reView June 30, 2009 ReVised manuscript receiVed November 24, 2009 Accepted December 28, 2009 IE9010468