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Study of the Effect of Gas Space Time on the Combination of Methane Gas-Phase Oxidation and Catalytic Oxidative Coupling over Mn/Na2WO4/SiO2 Catalyst Haili Zhang, Jingjing Wu, Song Qin, and Changwei Hu* Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan UniVersity, Chengdu, 610064, China
The effects of gas space time on the product distribution in noncatalytic gas-phase methane oxidation and in the combination reaction of noncatalytic and catalytic oxidative coupling of methane (OCM) over Mn/Na2WO4/SiO2 catalyst were studied. For the gas-phase reaction, methane and oxygen conversions and syngas selectivity increase drastically with increasing gas space time (tg) from 6.2 to 7.7 s; however, the selectivity of C2 hydrocarbons decreases sharply in this range and the concentration of C2H4 in the producing mixture is much lower than that of CO and H2. For the combination reaction, there is a minimum of methane conversion and C2 hydrocarbon selectivity with tg increasing from 3 to 5 s resulting from the variation of the mixture composition reaching the catalyst bed. The dilution of the catalyst bed with silica chips in the combination reaction partially restricts the sequential oxidation of products and raises the selectivities of ethylene and hydrogen. Surface enrichment of Na, W, and Mn species is observed on the catalysts used both in the sole catalytic OCM and in the combination reaction. The combination reaction makes the enrichment of Na and W species more evident, and the aggregation of Mn species less obvious, than the OCM reaction. 1. Introduction Methane is the major constituent of natural gas. In 2004, worldwide proven reserves of natural gas were about 176 quadrillion cubic feet. Over past decades, extensive efforts have focused on the conversion of methane to value-added products, especially to easily transportable fuels. Currently, the most economically viable operating process for the utilization of methane is steam reforming over nickel-based catalysts to produce hydrogen or syngas, which can be utilized in the synthesis of ammonia, methanol, hydrocarbons, dimethyl ether, and acetic acid, and for oxo-synthesis.1 As a potential competitive process to utilize methane, the partial oxidation of methane to syngas (POM) with air as the oxygen source is attractive for its mild exothermic heat of reaction and the desirable H2/CO ratio for downstream processes, such as methanol and FischerTropsch synthesis compared with steam reforming of methane. It has long been recognized that a direct conversion process for methane to useful chemicals such as methanol would have many advantages over the indirect technology via syngas. Among the direct conversion processes, the partial oxidation of methane to useful oxygenates, particularly methanol, possesses great potential and is viewed as one of the biggest challenges in catalysis.2 However, the current state-of-the-art technology for the direct conversion process is not competitive with the indirect one via syngas. Oxidative coupling of methane (OCM) is one of the most important research projects for methane conversion. Most of the active and selective catalysts for the OCM reaction are composed of two or three irreducible oxides, e.g., alkali metal oxides, alkaline earth metal oxides, and rare earth metal oxides.3-6 Mn/Na2WO4/SiO2 catalyst, first reported as an OCM catalyst by Fang et al.,7,8 has received much attention for its good performance. On this catalyst, Lunsford and co-workers achieved a methane conversion of 20% and a C2 selectivity of * To whom correspondence should be addressed. Tel.: 86-2881801141. Fax: 86-28-85411105. E-mail:
[email protected] or
[email protected].
g80% at 1073 K and 1atm, using a CH4/O2 ratio of 8/1, with no diluents in the reagents.9 This catalyst was found stable for up to 97 h.10 Significant progresses have been achieved in the research and development of OCM,11 but it is still difficult to overcome the barrier of technical-economic problems in connection with the low selectivity for C2 hydrocarbons and the high heat of reaction released in this process. The short-contact-time, high-temperature catalytic processes (catalytic combustion and pyrolysis) permit kinetic control of the formation of products by isolation and trapping of reaction intermediates that are thermodynamically less stable,12 and Somorjai13 considered that these processes were the frontier areas of catalysis science in the 21st century. Schmidt and coworkers studied the catalytic oxidative dehydrogenation of alkanes in short contact times (10-5-10-3 s), and prospective results were achieved.14-16 Zanthoff17 and Chen18 simulated the homogeneous gas-phase reaction between methane and oxygen by considering the elementary reactions. The corresponding experiment was carried out with ethane and ethylene as the main products. There was good agreement between simulated and experimental data on product selectivity above 1000 K. Reyes at al.19,20 developed a homogeneous/heterogeneous kinetics of OCM over many metal oxide catalysts, and the simulations of this model led to the common conclusion that gas-phase reactions significantly contribute to the overall process performance. Also, the homogeneous paths predominate in the reaction scheme, especially in the routes to higher hydrocarbons; only the formation of carbon dioxide occurs mostly via surface reaction. Goralski et al.21 simulated the model including both homogeneous and heterogeneous chemistry of a high-temperature, catalytic short-contact-time syngas reactor, and the modeling demonstrated that there was significant interplay between homogeneous and heterogeneous chemistry; the homogeneous chemistry (unselective for syngas) was favored by higher pressure and higher preheat temperature. A novel strategy of methane utilization has been raised in our previous work;22,23 that is, methane is first converted to H2, CO, and C2H4 simultaneously with similar mole concentrations, and the produced mixture could be used in propanal
10.1021/ie060303n CCC: $33.50 © 2006 American Chemical Society Published on Web 09/15/2006
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Figure 1. Temperature profile of the reactor at 1073 K.
synthesis via the hydroformylation of ethylene process. Kinetically controlled free radical gas-phase methane oxidation was combined with its catalytic oxidative coupling over Mn/ Na2WO4/SiO2 catalyst to control the product composition, especially the concentration of the three target products (H2, CO, and C2H4). A mixture with CO/H2/C2H4 ) 1.0:1:0.9 was achieved under the optimal reaction conditions.22 To better understand and control the combination reaction, a detailed study of the gas-phase methane oxidation and its effects on the combination reaction is very necessary. This is the aim of the present work.
out: the combination reaction with 0.5 g of catalyst and tg ) 4.7 s was carried out at 1073 K for 10 h, and the sole catalytic reaction was performed under similar conditions with tg ) 0. The fresh catalyst and used ones were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray diffraction patterns were obtained with a Philips X’Pert ProMPD diffraction instrument using Cu KR radiation. Diffractograms were recorded for 2θ ) 10-90° with a step size of 0.02°. The XPS measurements were carried out with a Krotos XSAM800 spectrometer using Al KR radiation. The binding energy was corrected by C(1s) at 284.8 eV.
2. Experimental Section
3. Results and Discussion
2.1. Preparation of Catalyst. The OCM catalyst 2 wt % Mn/5 wt % Na2WO4/SiO2 was prepared by a two-step incipient wetness impregnation method according to the literature,9 and a detailed description of the preparation is given in our previous paper.22 2.2. Experiments. The gas-phase methane oxidation and combination reaction were carried out in a quartz tubular, fixedbed reactor (1.2 cm i.d. and 70 cm long) placed inside a vertical furnace (40 cm length) and being fed downstream. A quartz thermocouple well (0.8 cm o.d. and 35 cm long) was equipped inside the reactor from the bottom, and the end of the well was in touch with the catalyst bed. In addition, this quartz well decreases the disengaging volume of the reactor significantly. Figure 1 gives the temperature profile of the reactor at 1073 K. The height was measured from the bottom of the electric furnace. According to Figure 1, the constant-temperature segment locates in height between 16 and 30 cm, and the reactor volume in the constant-temperature segment is 15.4 mL. The catalyst in the combination reaction was loaded at the midpoint of the reactor, and there is a disengaging section above the catalyst bed. Other experimental equipment and reaction programs have been described in detail previously.22 The gas space time tg in both the gas-phase reaction and combination reaction was defined as V/F (V, gas-phase volume in constant-temperature segment of the reactor; F, volumetric flow rate at the inlet of the reactor). A mole triratio of the three target products was defined as ([CO]/[H2]):1:([C2H4]/[H2]). 2.3. The Characterization of Catalysts. To compare the structural differences between the fresh catalyst and the used catalysts both in the combination reaction and in the sole catalytic OCM reaction, two control experiments were carried
3.1. Effect of Gas Space Time on Gas-Phase Methane Oxidation. The effect of reaction temperature on the gas-phase methane oxidation has been investigated in our previous work22 between 973 and 1073 K at CH4/O2 ) 3. It was found that higher temperature favored the formation of carbon monoxide, decreased the selectivity of C2 hydrocarbons, and slightly increased the selectivity of carbon dioxide. To investigate the variation of product composition at different methane conversions, gas-phase methane oxidation was carried out with different gas space times (tg’s) at CH4/O2 ) 3 and T ) 1073 K (control temperature) in the present work. The reactant conversions and product selectivities versus tg are plotted in Figure 2. Ethane, ethylene, carbon monoxide, carbon dioxide, and hydrogen were produced in gas-phase methane oxidation within the range of tg values examined. Oxygen and methane conversions are at low levels (26.6%, 8.2%) at tg ) 3.5 s, and ascend continuously as tg increases from 3.5 to 9.2 s. As can be seen from Figure 2, this ascending trend is more notable from 6.2 to 7.7 s. The methane conversion mounts to 21.0% at tg ) 9.2 s, while oxygen is nearly exhausted. The carbon monoxide selectivity versus tg is almost parallel to that of hydrogen within the tg range 3.5-9.2 s, and both of them also show a sharply ascending trend between 6.2 and 7.7 s similar to methane and oxygen conversions. The value of CO/ H2 remains constant at about 1.3. Carbon monoxide becomes the dominant product at tg ) 9.2 s (63.7% selectivity). The curve of C2 hydrocarbon selectivity, however, shows an trend opposite that of carbon monoxide, which decreases from 44.5% to 23.3% with tg increasing from 3.5 to 9.2 s. The selectivity of carbon dioxide is about 10% between 3.5 and 7 s, and shows a slight increase with further increase of tg.
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Figure 2. Effect of gas space time on the methane conversion and the products distribution in the gas-phase reaction. Reaction conditions: T ) 1073 K; CH4/O2 ) 3.
Coˆme et al.24,25 carried out gas-phase methane oxidation in a continuous flow stirred tank reactor between 973 and 1173 K in order to study the relative importance of gas-phase reaction and surface reaction in catalytic OCM reaction over La2O3. They suggested that hydrocarbons, carbon monoxide, and carbon dioxide were formed by gas-phase methane oxidation, and a simple mechanism was proposed that ethane and carbon monoxide came mainly from methyl radicals and ethylene came from ethane, while carbon dioxide came mainly from CO oxidation. Based on this simple mechanism and our experimental results as discussed above, the following reactions may occur in gas-phase methane oxidation:
CH4 + 0.5O2 f CO + 2H2
∆H298 ) -36 kJ/mol
2CH4 + 0.5O2 f C2H6 + H2O C2H6 + O2 f C2H4 + H2O CO + 0.5O2 f CO2 CO2 + H2 f CO + H2O
(1)
∆H298 ) -177 kJ/mol (2) ∆H298 ) -105 kJ/mol (3)
∆H298 ) -283 kJ/mol
(4)
∆H298 ) 41 kJ/mol
(5)
Zanthoff17 considered that there was an induction period required before the concentration of the reactive radicals was high enough to let any significant oxygen consumption occur. This may explain the slow increase of the conversion of methane and oxygen within the tg range from 3.5 to 6.2 s shown in Figure 2, and the inducing period for the present system may be tg e 6.2 s. After that, extensive reaction occurs between 6.2 and 7.7 s, and the conversion increases sharply. The flats on the curves of methane and oxygen conversion above 7.7 s may be resulting from the shortage of oxygen, which is nearly exhausted. According to the composition of the effluent and the temperature variation on the catalyst bed, it may be deduced that methane is converted mainly through reactions 1 and 2 leading to the similar selectivities of CO and C2 hydrocarbons at tg ) 3.5 s. The increase of the selectivities of carbon monoxide and hydrogen with tg and the corresponding decrease of C2 hydrocarbons indicate that reaction 1 is promoted and the contribution of reaction 1 is increased in the overall reaction with increasing tg. With the increased contribution of reaction 1 (∆H298 ) -36 kJ/mol), and the decreased contribution of
reaction 2 (∆H298 ) -177 kJ/mol) in the overall reaction, the exothermic heat per mole methane conversion of the overall reaction will decrease. According to the simple mechanism in ref 24, carbon dioxide comes mainly from the oxidation of carbon monoxide (reaction 4). However, the selectivity of carbon dioxide shows little change with carbon monoxide selectivity increasing from 45.9% to 63.7%. The possible explanation for this is that the produced carbon dioxide is converted according to other processes. As can be seen from Table 1, the ratio of CO/H2 ratio in the effluent mixture remains around 1.3, which is larger than the value of CO/H2 in reaction 1. It is speculated that reaction 5 may occur in this system, which decreases the concentrations of H2 and CO2 and increases the relative concentration of CO. Since this reaction is an endothermic process, it may be one of the reasons for the temperature decrease with tg increasing. In this work, the mole ratio of C2H4:C2H6 increases continuously while tg rises from 3.5 to 9.2 s, indicating that long contact time favors the oxidative dehydrogenation of ethane (reaction 3). The value of CH4/O2 in the effluent mixture augments quickly with tg, and it increases from 4.4 to 33.9 while tg rises from 3.5 to 9.2 s. Hence it may be concluded that tg obviously exerts an influence on the conversion of methane and oxygen, as well as on the product distribution. Therefore, it will be an effective way to control the product distribution in the combination reaction through adjusting the gas space time of the gas-phase reaction. 3.2. Effect of Gas Space Time on the Combination Reaction. In our previous work,22 gas-phase methane oxidation was combined with OCM over Mn/Na2WO4/SiO2 catalyst to control the product distribution. The dose of catalyst (m) employed was varied to study the influence of the catalytic reaction on the combination reaction, and the results indicate that there is a monotonically increasing tendency of C2 hydrocarbon concentration in the producing mixture with catalyst dose. In the present work, the combination reaction is studied by varying tg to control the gas-phase methane oxidation. Different amounts of silica chips were loaded above the catalyst bed to decrease the volume of the disengaging section in the reactor, and tg varied from 3 to 5 s accordingly. The reactants’ total velocity and the catalyst dose (m) were kept constant in these experiments. The results of the combination reaction are shown in Table 2. Different from the monotonically ascending trend with tg in the gas-phase reaction, methane conversion in the combination reaction decreases along with increasing tg, reaches a minimum (10.7%) at tg ) 4 s, and then increases with further increase, while the selectivity of carbon dioxide shows the opposite tendency. C2 hydrocarbon selectivity decreases sharply with increasing tg from 3 to 4.5 s, and increases slightly to 5 s; however, the selectivity of ethylene has its minimum at tg ) 4 s. The selectivities of carbon monoxide and hydrogen increase continuously with increasing tg; nevertheless, the values of CO/H2 (mole ratio) produced show little variation around 1 ((0.2). The value of ethylene in the triratio deceases first with increasing tg and then remains constant above tg ) 4 s. The triratio of the three target products shows a promising value (∼1:1:1) with tg ) 4-5 s, and in this tg range the highest yield of the target products C2H4 + CO (10.0%) is obtained at tg ) 5 s. It can be deduced that the increasing of tg depresses methane conversion on the catalyst surface. As mentioned above (section 3.1), the effluent composition of the gas-phase reaction varies obviously with increasing tg, that is, the augmentation of
Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006 7093 Table 1. Composition of Gas-Phase Methane Oxidation Effluenta content (mol %) tg (s) 3.5 6.2 7 7.7 9.2 a
Tb
mole ratio
(K)
H2
O2
CO
CH4
CO2
C2H4
C2H6
CH4/O2
CO/H2/C2H4
1089 1081 1081 1068 1071
2.6 3.3 6.1 8.9 9.3
17.1 14.8 9.9 3.0 2.1
3.1 4.2 8.4 11.5 12.2
75.1 74.9 71.6 72.4 71.7
0.7 0.9 1.5 2.2 2.5
0.8 1.2 1.8 2.2 2.2
0.7 0.7 0.7
4.4 5.1 7.2 24.9 33.9
1.2:1:0.3 1.3:1:0.4 1.4:1:0.3 1.3:1:0.3 1.3:1:0.2
Reaction conditions: T ) 1073 K; CH4/O2 ) 3. b Actual temperature of gas phase when the control temperature is 1073 K.
Table 2. Effect of Gas Space Time on the Combination Reactiona tg (s)
C (%) CH4
CO2
CO
3 3.5 4 4.5 5
16.8 16.1 10.7 13.4 15.1
18.3 18.5 22.0 20.7 19.0
15.1 19.5 21.6 22.0 23.0
a
selectivity (%) H2 8.4 8.9 12.0 12.2 12.8
C2H4
C2H6
yield (%) CO + C2H4
C2H4/C2H6
45.7 44.7 35.5 39.9 43.2
20.8 17.2 20.9 13.8 14.9
10.2 10.3 6.1 8.3 10.0
2.2 2.6 1.7 2.3 2.9
mole ratio CO/H2/C2H4 0.9:1:1.4 1.1:1:1.3 0.9:1:0.8 0.9:1:0.8 0.9:1:0.8
Reaction conditions: T ) 1073 K; CH4/O2 ) 4.5; F ) 120 mL/min; m ) 0.15 g.
Table 3. Effect of Silica Dilution on the Combination Reactiona V (mL)
C (%) CH4
CO2
CO
0 1.3 1.7
16.0 15.8 15.1
20.6 19.6 19.0
21.9 20.1 23.0
a
selectivity (%) H2 8.4 10.1 13.3
C2H4
C2H6
yield (%) CO + C2H4
C2H4/C2H6
39.6 42.6 43.2
18.0 17.7 14.9
9.8 9.9 10.0
2.2 2.4 2.9
mole ratio CO/H2/C2H4 1.3:1:1.2 1.0:1:1.1 0.9:1:1.0
Reaction conditions: CH4:O2 ) 4.5:1; F ) 120 mL/min; m ) 0.15 g.
Figure 3. X-ray diffraction pattern of Mn/Na2WO4/SiO2. (a) Fresh catalyst; (b) used catalyst in the combination reaction for 10 h; (c) used catalyst in the sole catalytic OCM reaction for 10 h. O, Na2WO4; 0, R-cristobalite; 4, Mn2O3.
reductive gas (CO and H2) and the increasing of CH4/O2 ratio. In the combination reaction, the composition of the gas mixture reaching the catalyst bed will show the same tendency with tg, and these variations decrease the methane conversion over the catalyst surface. Thus, methane conversion is dominated first by the catalytic OCM reaction over Mn/Na2WO4/SiO2 catalyst between tg ) 3 and 4 s, and then by the gas-phase reaction between tg ) 4 and 5 s. The tendencies of the ratio of CO/H2 and their selectivities with tg are very similar to those in the gas-phase reaction. It can be deduced that CO and H2 mainly form in the gas phase, and the sequential oxidation of them over the catalyst occurs to a low degree. Therefore, the gasphase reaction at an appropriate degree is effective to control the product composition in the combination reaction.
3.3. Effect of Silica Dilution of the Catalyst on the Product Distribution. The main reactions of the catalytic oxidative coupling of methane are exothermic, and the existence of hot spots in the catalyst bed during the OCM process arrests its development to a certain extent. The diluter, inert to reactants and the catalyst, makes the heat removal from the catalyst bed easier, and the temperature along the catalyst bed becomes more uniform. In the case of the mixtures containing more than 25% of the catalyst, there were no clear depressing of the conversion and selectivity of catalytic OCM reaction;26 however, further decreasing the catalyst content will cause a slight decrease of methane conversion. The catalyst was diluted by silica chips in the combination reaction in order to decrease the temperature grade in the catalyst bed, which is also assumed to weaken the sequential oxidation of CO and H2 produced before the catalyst bed. In this series of experiments, the amount of catalyst used was kept at 0.15 g, the volume of which was 0.2 mL. The catalysts were diluted with different amounts of silica chips as 0, 1.3, and 1.7 mL corresponding to 100%, 13%, and 10% catalyst content (vol %), while the total volume of catalyst bed was kept identical by adding different amounts of silica chips above the catalyst bed, so tg did not vary in these three experiments. As can be seen from Table 3, the dilution of the catalyst decreases the methane conversion slightly, as well as the selectivity of carbon dioxide. The dilution of the catalyst bed will decrease the probability of carbon monoxide colliding with the catalyst surface, so the sequential oxidation of it will be constrained. The value of C2H4/C2H6 increases with the volume of the diluting silica, and it can be deduced that the dilution of the catalyst could promote ethane dehydrogenation in this combining process. When the catalyst was diluted by 1.7 mL of silica chips, the best performance was obtained in terms of the selectivity of the three target products (CO, H2, and C2H4) and their relative ratio (0.9:1:1). 3.4. Catalyst Characterization. 3.4.1. XRD results. Representative samples of the fresh and used catalysts in the
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Table 4. Observed XPS Binding Energy (eV)a O(1s)
a
catala
Na(1s)
W(4f)
Mn(2p)
Si(2p)
MOx
SiO2
CO32-
a b c
1072.2 1071.9 1072.0
35.5 35.5 35.5
642.2 642.0 642.3
103.1 103.1 103.0
530.9 530.6 530.6
532.7 532.7 532.9
531.7 531.7
Catalyst a, fresh catalyst; b, used catalyst in the combination reaction for 10 h; c, used catalyst in the sole catalytic OCM reaction for 10 h.
Table 5. Near-Surface Compositions (atom %) of Catalysta O(1s)
a
catala
Na(1s)
W(4f)
Mn(2p)
Si(2p)
MOx
SiO2
CO32-
a b c
4.75 7.64 7.46
1.56 2.35 2.15
0.93 1.81 2.32
28.51 23.30 23.70
14.13 19.17 17.23
50.12 32.28 33.40
13.46 13.74
Catalyst a, fresh catalyst; b, used catalyst in the combination reaction for 10 h; c, used catalyst in the sole catalytic OCM reaction for 10 h.
combination reaction or in the sole catalytic OCM reaction for 10 h are examined by X-ray diffraction. Figure 3 shows the XRD patterns. The precursor silica support was amorphous, and the catalysts are calcined at about 1073 K for 5 h during the preparation. The diffraction of R-cristobalite is apparent in all of the XRD patterns of Figure 3a-c. Thus the initially amorphous silica underwent complete conversion to highly crystalline R-cristobalite during the calcination at a temperature far below the normal transition temperature of 1773 K. This result is in good agreement with that from ref 27, which demonstrated that this phase transition improved the anchoring and dispersion of the WO4 stabilizing Na component, and was important for the catalyst activity and stability. Comparing with the XRD pattern of the fresh catalyst, the diffractions of Na2WO4 on the catalysts after reaction become strong, while that of Mn2O3 become weak. These results indicate that the crystal of Na2WO4 sinters to a certain extent during the reaction, but Mn species becomes more dispersed, and this tendency is more serious for the catalyst used in the sole catalytic OCM reaction for 10 h. In the combination methane oxidation, gas-phase reaction produces the reductive gases (CO and H2), and decreases the value of the CH4/O2 ratio in the mixture reaching the catalyst bed. This should be responsible for the structure differences between these two kinds of used catalysts. Comparing with the fresh catalyst, the structure of the used catalyst in the combination reaction shows less change than that in the catalytic OCM reaction, and the combination methane oxidation gave good stability during the 10 h testing.22 It can be concluded that the Mn/Na2WO4/SiO2 catalyst is more stable when used in combination methane oxidation than in the sole catalytic OCM process. 3.4.2. XPS Results. Tables 4 and 5 present the observed binding energies and surface compositions of the fresh and used Mn/Na2WO4/SiO2 catalysts. The 2 wt % Mn/5 wt % Na2WO4/ SiO2 samples have an average bulk atomic composition of 0.75% Mn, 0.7% Na, 0.35% W, 31.9% Si, and 66.3% O. Compared with the bulk composition, the active components (Na, W, Mn) enrich over the surface of both the fresh and used catalysts, and this enrichment of Mn species is slighter to the catalyst used in the combination reaction, while that of Na and W species varies oppositely. The concentration of surface O in MOx (M ) Mn, W, and Na) is bigger on the surface of the catalyst used in the combination reaction. The surface Na/W atom ratio is about 3 in the catalysts tested, and increases slightly according to the order the fresh catalyst < the used catalyst in combination reaction < the used catalyst in sole catalytic reaction. These facts indicate that a part of surface sodium presents in a form other than Na2WO4, which
may be Na2O and Na2O2 for the fresh catalyst.9 The O(1s) binding energy of 531.7 eV illuminates that the slight increase of Na/W on the used catalyst surface results from the formation of Na2CO3. The surface Na/W atom ratio of the used catalyst in the combination reaction is lower than that in the sole catalytic reaction, which shows that the combination reaction constrains the decomposition of Na2WO4 to a certain degree. The Mn(2p) binding energy of 642.0 ( 0.3 eV on the surface of both the fresh and used catalysts is attributed to MnO2; however, Mn species exists as Mn2O3 in the catalyst bulk before and after reaction. Lunsford9 considered that the active site of the Mn/Na2WO4/SiO2 catalyst consisting of a Na-Mn-O species and surface Mn species was believed to be responsible for the activation of O2, and sodium was essential for preventing the complete oxidation of methane, possibly by isolating the Mn ions. From the results of XPS and XRD, it is possible that O2 is activated by a reduction-oxidation mechanism of Mn4+/ Mn3+. From the results of XRD and XPS characterizations of the two used catalysts, it is shown that Na and W species enrich more notably on the surface of catalyst used in the combination reaction, which also constrains the tendency of Na2WO4 sintering in the catalyst bulk. Compared with the characteristic results of the fresh catalyst, the Mn species show less change both in the bulk and on the surface of the catalyst used in the combination process than that used in the sole catalytic reaction. 4. Conclusions In the gas-phase methane oxidation reaction, gas space time exerts an obvious influence on methane conversion and product distribution. With increasing gas space time, carbon monoxide selectivity increases, but the selectivity of C2 hydrocarbons decreases while the selectivity of carbon dioxide increases slightly. However, the concentration of C2H4 in the producing mixture is much lower than that of CO and H2. Gas space time exerts an influence both on the gas-phase reaction and the next catalytic reaction simultaneously in the combination reaction. With decreasing gas space time, methane conversion and C2 hydrocarbon selectivity decrease to a minimum at tg ) 4 s. The dilution of the catalyst bed with silica chips can weaken the sequential oxidation of syngas produced in the gas-phase reaction. However, higher reaction temperature will promote the sequential oxidation. R-Cristobalite is formed during preparation, and Na2WO4 and MnO2 are well dispersed in both reactions. Na2CO3 forms during the reaction. Under similar reaction conditions, Na and W species enrich more notably on the surface of catalyst used in
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the combination reaction, which also constrains the tendency of Na2WO4 sintering in the catalyst bulk; however, the Mn species show less change both in the bulk and on the surface of the catalyst used in the combination process than that in the sole catalytic reaction. Acknowledgment The authors are grateful for financial support from the NNSFC (No. 20243006), The Special Research Foundation of Doctoral Education of China (No. 2000061.28), and the TRAPOYT (2002). Literature Cited (1) Urasaki, K.; Sekine, Y.; Kawabe, S.; Kikuchi, E.; Matsukata, M. Catalytic activities and coking resistance of Ni/perovskites in steam reforming of methane. Appl. Catal., A 2005, 286, 23. (2) Raja, R.; Ratnasamy, P. Direct Conversion of Methane to Methanol. Appl. Catal., A 1997, 158, L7. (3) Ito, T.; Lunsford, J. H. Synthesis of ethylene and ethane by partial oxidation of methane over lithium-doped magnesium oxide. Nature (London) 1985, 314, 721. (4) Maiti, G. C.; Baerns, M. Dehydration of sodium hydroxide and lithium hydroxide dispersed over calcium oxide catalysts for the oxidative coupling of methane. Appl. Catal., A 1995, 127, 219. (5) Martin, G. A.; Mirodatos, C.; Yu, C. Y.; Li, W. Z. Oxidative coupling of methane over calcium chloride-promoted calcium chlorophosphate. Appl. Catal., A 2001, 205, 253. (6) Maksimov, N. G.; Selyutin, G. E.; Anshits, A. G.; Kondratenko, E. V.; Roguleva, V. G. The influence of defect nature on catalytic performance of Li, Na-doped MgO, CaO and SrO in the oxidative coupling of methane. Catal. Today 1998, 42, 279. (7) Fang, X. P.; Li, S. B.; Lin, J. Z.; et al. The Preparation and Characterization of the W-Mn Catalyst of Oxidative Coupling of Methane. J. Mol. Catal. (China) 1992, 6 (4), 255. (8) Fang, X. P.; Li, S. B.; Lin, J. Z.; et al. Oxidative Coupling of Methane on W-Mn Catalyst. J. Mol. Catal. (China) 1992, 6 (6), 427. (9) Wang, D. J.; Rosynek, M. P.; Lunsford, J. K. Oxidative Coupling of Methane over Oxide-Supported Sodium-Manganese Catalysts. J. Catal. 1995, 155, 390. (10) Pak, S.; Lunsford, J. H. Thermal effects during the oxidative coupling of methane over Mn/Na2WO4/SiO2 and Mn/Na2WO4/MgO catalysts. Appl. Catal., A: Gen. 1998, 168, 131. (11) Lunford, J. H. Catalytic conversion of methane to more useful chemicals and fuels: a challenge for the 21st century. Catal. Today 2000, 63, 165. (12) Somorjai, G. A.; Mccrea, K. Roadmap for catalysis science in the 21st century: a personal view of building the future on past and present accomplishments. Appl. Catal., A: Gen. 2001, 222, 3. (13) Goetsch, D. A.; Schmidt, L. D. Microsecond Catalytic Partial Oxidation of Alkanes. Science 1996, 271, 1560.
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ReceiVed for reView March 13, 2006 Accepted August 16, 2006 IE060303N
