Site Requirements for the Oxidative Coupling of Methane on SiO2

Sep 19, 2006 - SrO·Al2O3 mixed oxides: A promising class of catalysts for oxidative coupling of methane. Tinku Baidya , Niels van Vegten , René Vere...
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Ind. Eng. Chem. Res. 2006, 45, 7077-7083

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Site Requirements for the Oxidative Coupling of Methane on SiO2-Supported Mn Catalysts Sicong Hou, Yuan Cao, Wei Xiong, Haichao Liu,* and Yuan Kou* Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Green Chemistry Center, Peking UniVersity, Beijing 100871, China

The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4 and C2H6) was examined on Mn/SiO2 and sodium salt-modified Mn/SiO2 catalysts containing different oxo anions, i.e., WO42-, MoO42-, SO42-, PO43-, P2O74-, CO32-, and SiO32-. The catalysts were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, and temperature-programmed reduction with H2. The structural and catalytic properties of the catalysts largely depend on the presence of the Na+ ions and the identity of the oxo anions of the salts. Mn/SiO2 consisted of Mn3O4 and amorphous SiO2 phases. The addition of the sodium salts to Mn/SiO2 led to the transformation of amorphous SiO2 exclusively to R-cristobalite, and the concurrent oxidation of Mn3O4 to different Mn species. Mn2O3 was the predominant species as the salts contained oxo anions of WO42-, MoO42-, SO42-, PO43-, and P2O74-, whereas the basic sodium salts of CO32- and SiO32led to the preferential formation of Mn4+ species. These effects on the formation of the Mn species were demonstrated indeed to require the coexistence of Na+ and the oxo anions. Compared to Mn/SiO2, the sodium salt-modified samples with the formation of Mn2O3 showed much higher reducibility, activities, and selectivities to C2 products. However, the samples with the formation of Mn4+ species exhibited very low OCM activities, as a result of the strong basicity of Na2CO3 and Na2SiO3 inhibiting the partial reduction of the Mn4+ species. The observed effects on the structures and catalytic performances suggest that Mn2O3 species act as the active sites responsible for the methane activation, which may provide the rationale for the design of new efficient catalysts for the OCM reaction. 1. Introduction The discovery of the large reserves of natural gas and the ever-increasing shortage of crude oil around the world have stimulated intense efforts to convert methane, the principal component of natural gas, to high-value fuels and chemicals during the past 2 decades. Methane conversion can be achieved by both direct and indirect approaches. Special attention has been paid to the direct routes, which can circumvent the expensive steps of synthesis gas production required for the established indirect routes.1 Some examples include selective oxidation of methane to methanol and formaldehyde, oxidative coupling of methane to ethylene and ethane, and aromatization of methane to aromatics and H2 in the absence of molecular oxygen.1-5 Among these processes, the catalytic oxidative coupling of methane (OCM) in terms of productivity and catalyst stability has emerged as a very promising one with distinct prospects of practical implementation, especially at the moment when the oil prices are now at an unprecedented high. In efforts to achieve efficient OCM reaction, a wide range of oxide catalysts have been explored since the pioneering work of Keller and Bhasin.6-16 Extensive evaluations of the catalysts have identified sodium-promoted manganese oxides on silica to be among the best ones;6,9,15,16 these manganese catalysts became more prominent since the finding of the Na2WO4-Mn/ SiO2 system by Li and co-workers.17-23 For example, 5 wt % Na2WO4-2 wt % Mn/SiO2 demonstrated a steady-state 35% CH4 conversion and 68% C2 selectivity at 1073 K. This excellent performance was easily reproduced by other researchers,24-30 * To whom correspondence should be addressed. Tel: 86-10-62757792; 86-10-6275-4031. E-mail: [email protected]; yuankou@ pku.edu.cn.

but there still exists noticeable disagreement or controversy concerning the active components or sites on those Na2WO4Mn/SiO2 catalysts. Li et al. related the excellent performance to WO4 tetrahedral structures containing WdO and W-O-Si bonds.19,20 This model was further developed by Kou et al. with emphasis on the combination of tetrahedral WO4 and octahedral MnO6 sites, respectively responsible for the activation of methane and the lattice oxygen transport.31 In contrast, Lunsford and co-workers suggested that Na-O-Mn species were attributable to the activation of methane, and the presence of tungsten ions appeared to improve the catalyst stability.25 Recently, Ji et al. claimed that both Na-O-Mn and Na-O-W species acted as the active sites for the OCM reaction.22 These conflicting results tempted us to clarify the nature of the active sites on the catalysts, which will be relevant to the rational design of new catalysts more efficient for the synthesis of C2 hydrocarbons from the OCM reaction. This was addressed by choosing a wide range of sodium salts to compare with Na2WO4. Here, we report the significant effects of sodium salts containing different oxo anions on the structures and catalytic behaviors of the Mn/SiO2-based catalysts. The oxo anions include MoO42-, SO42-, PO43-, P2O74-, CO32-, and SiO32-. The effects of this wide variety of oxo anions have not been rigorously examined in the literature,16,28 and systematic studies on these effects will provide deep insight into the role of WO42and the active site requirements for the OCM reaction. 2. Experimental Section 2.1. Catalyst Preparation. Supported catalysts were prepared by incipient wetness impregnation of silica (99.5%, Tianjin Hangu Haizhong) with aqueous solutions of Mn(NO3)2 (Beijing Chemicals) and various sodium salts. The sodium salts used in this work included Na2WO4, Na2MoO4, Na2SO4, Na3PO4,

10.1021/ie060269c CCC: $33.50 © 2006 American Chemical Society Published on Web 09/19/2006

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Na4P2O7, Na2CO3, and Na2SiO3 (>99.0%, Beijing Chemicals). The samples were dried at 393 K for 4-6 h after impregnation and then calcined in air generally at 1123 K for 6 h. The resulting catalysts contained typically 2 wt % Mn and 5 wt % sodium salts, as reported by Li et al. and Lunsford et al.18,25 To keep consistent with the catalyst notations reported in the literature, the catalysts in this work were expressed in this way: sodium salt-Mn/SiO2, e.g., Na2WO4-Mn/SiO2. For comparison, sodium-free (denoted as Mn/SiO2) and Mn-free (denoted as sodium salt/SiO2, e.g., Na2WO4/SiO2) catalysts were also prepared in the same way by using Mn(NO3)2 with no sodium salts and the sodium salts with no Mn(NO3)2 in the impregnation solutions, respectively. 2.2. Catalyst Characterization. X-ray diffraction patterns were obtained in ambient air in the 2θ range 10°-70° on a Rigaku D/MAX-2000 diffractometer using Cu KR radiation operated at 40 eV and 100 mA. In situ X-ray diffraction patterns were obtained on a Rigaku D/MAX-PC2500 diffractometer using Cu KR radiation operated at 40 eV and 100 mA. Samples were mounted on a Pt plate with a thermocouple and heated at 0.167 K/s in air. The patterns were recorded at an interval of 100 K from 673 to 1073 K. XPS spectra were collected on an Axis Ultra spectrometer (Kratos, UK) using monochromatic Al KR (1486.71 eV) radiation at a source power of 225 W (15 mA, 15 kV). The binding energies were referred to the Si 2p peak at 103.4 eV. Surface compositions were calculated from peak areas using the sensitivity factors of the spectrometer attached. Raman spectra were recorded in ambient air in a Renishaw 1000 spectrometer equipped with an Ar+ laser (514.5 nm) and a CCD camera enabling microanalysis of a sample point. The resolution was 2 cm-1. Raman shifts for all the samples were measured in the range 100-1500 cm-1. Temperature-programmed reduction (TPR) measurements were performed in a flow unit (TP5000, Tianjin Xianquan) equipped with a TCD detector. Samples (100 mg, calcined at 1123 K in air) were placed in a quartz cell and heated linearly from 293 to 1123 K at 10 K min-1 in flowing 20% H2/N2 (40 cm3 min-1; Beijing Tianyuan, certified mixture). The H2 content in the effluent was measured using the TCD detector after the water formed during reduction was removed using a P2O5 trap at ambient temperature. The thermal conductivity response was calibrated from the reduction of pure CuO powder. 2.3. Catalytic Methane Reactions. Catalytic methane reactions were carried out in a fixed-bed quartz microreactor (i.d. 6 mm) using 0.2 g of catalysts (∼2.5 cm in height) in atmospheric pressure. The reactants consisted of CH4 (99.995%), O2 (99.995%), and N2 (99.999%) at a molar ratio of 2.5:1:2.5 and were introduced into the reactor using mass flow controllers (D07-11/ZM, Beijing Sevenstar) at a GHSV of 33000 mL g-cat.-1 h-1. The reactants and products were analyzed with two on-line gas chromatographs equipped with TCD detectors and two packed columns of 5A molecular sieve for O2, CH4, and CO and Porapak Q for CH4, CO2, C2H4, and C2H6. No products of C3 or larger were detected under the reaction conditions employed. 3. Results 3.1. Catalytic Methane Reactions. Table 1 presents the steady-state conversions and selectivities in oxidative coupling of methane (OCM) at 1073 K on Mn/SiO2 and a variety of Mn/ SiO2-based catalysts modified by sodium salts of Na2WO4, Na2MoO4, Na2SO4, Na3PO4, Na4P2O7, Na2CO3, and Na2SiO3, respectively. Blank test with only SiO2 as catalyst gave a CH4

Table 1. Catalytic Performances of Different SiO2-Supported Catalystsa catalysts Mn/SiO2 Na2WO4-Mn/SiO2 Na2MoO4-Mn/SiO2 Na2SO4-Mn/SiO2 Na3PO4-Mn/SiO2 Na4P2O7-Mn/SiO2 Na2CO3-Mn/SiO2 Na2SiO3-Mn/SiO2 Na2WO4/SiO2 Na2MoO4/SiO2 Na2SO4/SiO2 Na3PO4/SiO2 Na4P2O7/SiO2 Na2CO3/SiO2 Na2SiO3/SiO2 Mn-SO4/SiO2b

selectivity (%) conversion C2 yield of CH4 (%) C2H4 + C2H6 C2H4 COx (%) 20.6 35.5 30.7 35.7 31.0 33.5 8.9 9.4 15.2 14.0 4.0 5.0 5.1 3.2 3.1 20.9

28.0 54.0 45.7 56.4 50.5 52.5 49.6 44.9 62.4 59.3 44.4 39.5 34.3

15.0 41.0 31.0 41.1 38.6 36.9 19.0 21.7 33.0 21.5 6.3 6.5 13.3

72.0 42.8 52.8 42.4 48.7 47.2 50.3 55.2 33.3 38.7 54.9 58.5 65.0

29.7

17.1

68.6 g-cat.-1

5.8 19.2 14.0 20.1 15.7 17.6 4.4 4.2 9.5 8.3 1.8 2.0 1.7 6.2 h-1;

1073 K; CH4:O2:N2 ) 2.5:1:2.5; GHSV ) 33000 mL 0.2 g of catalyst. b SO42- anion was introduced using (NH4)2SO4 as precursor. a

conversion of less than 3% (not shown in Table 1) under the reaction conditions. The Mn/SiO2 catalyst was active for OCM (20.6% conversion), but the selectivity to C2 hydrocarbons (C2H6 + C2H4) was as low as 28.0%, and the selectivity to COx (CO and CO2) was as high as 72.0%. The identity of the sodium salts, specifically the corresponding oxo anions, largely affected the catalytic performance for Mn/ SiO2-based catalysts, as shown in Table 1. Compared to the Mn/SiO2 catalyst, addition of Na2WO4 significantly increased the CH4 conversion to 35.5%, and the C2 selectivity to 54.0% as a result of the increase in C2H4 selectivity from 15.0% to 41.0%. The catalyst containing Na2MoO4 showed a CH4 conversion of 30.7% and a C2 selectivity of 45.7%, which were much higher than those of Mn/SiO2 but lower than those of Na2WO4-Mn/SiO2. When Na2SO4 was introduced to the Mn/SiO2 catalyst, both CH4 conversion and C2 selectivity were noticeably improved. As shown in Table 1, Na2SO4-Mn/ SiO2 showed a CH4 conversion of 35.7% and a C2 selectivity of 56.4%, which were even slightly better than those on Na2WO4-Mn/SiO2, one of the best catalysts reported so far, under identical conditions. Similar promoting effects were observed in the case of PO43and P2O74-. The CH4 conversion and C2 selectivity increased to 31.0% and 50.5% on Na3PO4-Mn/SiO2, and 33.5% and 52.5% on Na4P2O7-Mn/SiO2, respectively, as compared to 20.6% and 28.0% on the Mn/SiO2 catalyst. These results show that this series of nonmetal oxo anions can promote the OCM performance of Mn/SiO2 as effectively as WO42- can. However, addition of Na2CO3 and Na2SiO3 to Mn/SiO2 caused a remarkable decrease in the OCM activity (Table 1). The CH4 conversions on the two catalysts were ∼10% lower than that on Mn/SiO2, implying that these kinds of nonmetal oxo anions are not appropriate additives for the Mn/SiO2 catalyst to achieve a good catalytic performance. Such negative effect of Na2CO3 on the activity was also observed by Wang et al., as added to Na2WO4-Mn/SiO2.25 To elucidate the modification role of these sodium salts on the above catalysts, only the sodium salts were supported on SiO2 and the catalytic properties in OCM were compared under identical reaction conditions. It can be easily seen from Table 1 that Na2WO4/SiO2 and Na2MoO4/SiO2 gave C2 selectivities of 62.4% and 59.3% at 15.2% and 14.0% conversion levels, respectively. Different from these transition metal oxo anions,

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Figure 1. XRD patterns for (a) Mn/SiO2, (b) Na/SiO2 (R-cristobalite), (c) Na2WO4-Mn/SiO2, (d) Na2MoO4-Mn/SiO2, (e) Na2SO4-Mn/SiO2, (f) Na3PO4-Mn/SiO2, (g) Na4P2O7-Mn/SiO2, (h) Na2CO3-Mn/SiO2, (i) Na2SiO3-Mn/SiO2. (3) Mn3O4; (0) Mn2O3; (b) sodium salts added.

SiO2-supported sodium salts containing nonmetal oxo anions, SO42-, PO43-, and P2O7,4- exhibited very poor OCM performances with CH4 conversions as low as ∼5%, while the CO32and SiO32- containing samples were almost inactive for CH4 conversion under the same conditions. 3.2. Characterization of Catalysts. X-ray Diffraction. Figure 1 shows the XRD patterns for Mn/SiO2, Na/SiO2, and the sodium salt-containing Mn/SiO2 catalysts. On Mn/SiO2, a broad peak around 2θ ) 22° and two weak peaks at 2θ ) 32.3° and 36.1° were observed, which are attributed to the typical amorphous SiO2 phase28 and Mn3O4,32 respectively. On Na/ SiO2, only R-cristobalite phase was detected. For Na2WO4Mn/SiO2, a new peak appeared at 2θ ) 32.9° in addition to the characteristic peaks of the R-cristobalite phase, showing the presence of Mn2O3 phase.22 Similarly, R-cristobalite and Mn2O3 phases were observed for the catalysts containing the sodium salts of MoO42-, SO42-, PO43-, and P2O74-. These sodium salts were also detected by XRD, but Na3PO4 rather than Na4P2O7 was detected on Na4P2O7-Mn/SiO2 (Figure 1, curve g), indicating the decomposition of Na4P2O7 to Na3PO4 during the catalyst preparation. On the catalysts containing Na2CO3 and Na2SiO3, however, only R-cristobalite was present, and no Mn2O3, Na2CO3, and Na2SiO3 phases were detected. To get the information of the active phases at the reaction temperatures, some of the samples were chosen and characterized by in situ XRD at higher temperatures in air. Figure 2 shows the XRD patterns for Na2WO4-Mn/SiO2 and Na2SO4-Mn/SiO2 (after calcination at 1123 K and exposure to ambient air) measured at 293, 673, 773, 873, 973, and 1073 K. At 673 K, R-cristobalite changed from tetragonal to cubic phase. For Na2WO4-Mn/SiO2, the diffraction signals of Na2WO4 (mp 971 K) became weaker from 773 K and disappeared at temperatures higher than 973 K. However, the peak of Mn2O3 remained unchanged at all the temperatures employed (Figure 2A). Similar phenomena were observed on Na2MoO4-Mn/SiO2. On the Na2SO4-Mn/SiO2 sample, the diffraction signals of Na2SO4 (mp

Figure 2. XRD patterns for (A) Na2WO4-Mn/SiO2 and (B) Na2SO4Mn/SiO2 catalysts at 293 K and 673-1073 K in air. (0) Mn2O3; (b) sodium salts; (1) tetragonal R-cristobalite; (3) cubic R-cristobalite.

1157 K) became weaker from 973 K and at 1073 K; only about 10% of the added Na2SO4 remained in crystalline forms (calibrated by a series of self-mixed Na2SO4 and SiO2 samples). While Mn2O3 remained stable at all the tested temperatures (Figure 2B). Similar phenomena were observed on Na3PO4Mn/SiO2 and Na4P2O7-Mn/SiO2. The weakening or disappearance of the diffraction signals of the corresponding sodium salts might be related to the deformation of the lattice structures at the tested temperatures. Further discussion will be given below. The structural changes of the samples after they were treated in different atmospheres were also probed by XRD. Figure 3

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Figure 3. XRD patterns for Na2WO4-Mn/SiO2 before (a) and after treatment in CH4 (30% CH4 in N2) at 1073 K for 2 h (b), followed by treatment in O2 at 1073 K for 2 h (c), and after OCM reaction (40% CH4/ 16% O2/N2) at 1073 K for 4 h (d). (b) Na2WO4; (0) Mn2O3; (1) MnWO4; (3) Na4WO5.

shows the corresponding XRD patterns for Na2WO4-Mn/SiO2 as an example. With reference to the fresh sample (Figure 3, curve a), the diffraction peak for Mn2O3 at 2θ ) 32.9° vanished, and new peaks were detected at 2θ ) 18.3, 23.5, 24.0, 29.8, and 30.2° upon treatment with CH4 at 1087 K for 2 h (Figure 3, curve b). These new peaks are attributed to MnWO4.22 Subsequent treatment with O2 led to the recovery of the Mn2O3 peak along with the disappearance of the MnWO4 peaks, indicating the reversible change between Mn2O3 and Mn2+ species at 1087 K (Figure 3, curve c). Reduction of Mn2O3 partly occurred after the OCM reaction in the CH4-rich conditions, as evidenced from the slightly reduced intensity of the Mn2O3 peak and the weakly observed MnWO4 peaks (Figure 3, curve d). For Na2WO4 on Na2WO4-Mn/SiO2, its XRD peaks (at 2θ ) 16.8, 27.6, and 32.5°) nearly disappeared by treatment with CH4, and then their intensity was restored by subsequent treatment with O2, concurrent with the change of the MnWO4 peaks. After O2 treatment, two weak peaks appeared at 2θ ) 17.6 and 23.4°, which are assigned to Na4WO5, indicating the conversion of Na2WO4 to Na4WO5. X-ray Photoelectron Spectroscopy. Table 2 shows the XPS results of the surface concentrations and the binding energies for Na and Mn on the catalysts before and after OCM reaction. For Mn/SiO2, the observed binding energy of Mn 2p3/2 was 641.1 eV, indicating that Mn was present as Mn2+ and Mn3+ most likely in the form of Mn3O4.25 This assignment is consistent with the characterization results from XRD (Figure 1) and Raman (Figure 4, as described below). The binding energy of Mn 2p3/2 for Na2WO4-Mn/SiO2 shifted to 641.8 eV, which is assigned to Mn3+ in the form of Mn2O3, in combination with the XRD and Raman results. Similar binding energies of Mn 2p3/2 in the range 641.5-641.9 eV were observed for the samples containing Na2MoO4, Na2SO4, Na3PO4, and Na4P2O7, demonstrating the existence of Mn2O3 as the main Mn species on the surfaces. However, Na2CO3-Mn/SiO2 and Na2SiO3-

Mn/SiO2 had a binding energy around 642.3 eV, which can be attributed to Mn4+.25 The binding energies of Mn 2p3/2, specifically for the samples containing Mn2O3, decreased slightly by about 0.2-0.3 eV after the OCM reaction (Table 2), apparently due to the slight reduction in the CH4-rich reaction conditions. By comparison with the bulk compositions, it was noted that the surface concentrations of Mn ions for all the samples (Table 2) were very similar to the corresponding bulk values of 2.0%. This indicates that the identity of the oxo anions of the sodium salts did not affect the dispersion of Mn species on the surfaces. The surface concentrations of Na+ on all these samples were about 3-7 times greater than the bulk concentrations, showing the significant enrichment of Na+ ions on the surfaces. These surface elemental concentrations, as shown in Table 2, slightly changed after the OCM reaction. Laser Raman Spectroscopy. Figure 4 shows the Raman spectra for the Mn/SiO2 and sodium-containing Mn/SiO2 catalysts in the range 350-1050 cm-1. Mn/SiO2 showed a single band at 655 cm-1 that is assigned to the vibration of MndO in Mn3O4.33 Two Raman bands appeared at 775 and 408 cm-1 on all the sodium-containing Mn/SiO2 catalysts. These bands correspond to the formation of R-cristobalite,22 which, by comparison with the spectrum for Mn/SiO2, confirms the role of Na+ in promoting the phase transformation of amorphous SiO2 to R-cristobalite. The observed bands at 690, 620, 573, and 515 cm-1 on Na2WO4-Mn/SiO2, taken together with the above XRD and XPS results, are assigned to Mn2O3.23 The same Raman bands for Mn2O3 were detected on the samples containing Na2MoO4, Na2SO4, Na3PO4, and Na4P2O7. The samples containing Na2WO4 and Na2MoO4 also showed several Raman bands in the range 800-1000 cm-1. These bands are attributed, respectively, to the vibrations of tetrahedral WO4 (951 and 925 cm-1)22,34 and MoO4 (954 and 888 cm-1).34 An additional weak band at 651 cm-1 for Mn3O4 was also observed on Na2SO4Mn/SiO2, Na3PO4-Mn/SiO2, and Na4P2O7-Mn/SiO2. The much higher Raman sensitivity to Mn3O4 than to Mn2O333 allows us to conclude from the weak feature of the 651 cm-1 band that Mn2O3 species were predominant on these samples, consistent with the XRD results that only Mn2O3 peak was detected on these samples (Figure 1). The band at 994 cm-1 on Na2SO4Mn/SiO2 showed the presence of Na2SO4.34,35 In contrast, for Na2CO3-Mn/SiO2 and Na2SiO3-Mn/ SiO2, no band corresponding to manganese oxides appeared. Reduction of the Catalysts in H2. Figure 5 shows H2-TPR profiles for Mn/SiO2, Na2CO3-Mn/SiO2, and Na2SO4-Mn/SiO2 as well as Na2SO4/SiO2 for comparison. The Mn/SiO2 sample shows a single peak at around 923 K and the H2/Mn ratio for this peak is 0.36, corresponding to the reduction of Mn3O4 to MnO.32 The Na2SO4-Mn/SiO2 sample shows two peaks at 720 and 945 K. The H2/Mn ratio for the peak at 720 K is 0.17, corresponding to the reduction of Mn2O3 to Mn3O4.32 The peak at 945 K is very intense, apparently as a result of both the reduction of Mn3O4 and SO42- by comparison with the reduction behavior of the Mn/SiO2 sample and pure Na2SO4/SiO2 sample (Figure 5). However, no obvious reduction was detected for the Na2CO3-Mn/SiO2 up to 1123 K. These data suggest that the reducibility of the Mn species on Na2SO4-Mn/SiO2 is higher than that on Mn/SiO2, and Na2CO3-Mn/SiO2 is the least reducible. 4. Discussion The structural properties of the Mn/SiO2-based catalysts and their catalytic performances in the OCM reaction largely depend

Ind. Eng. Chem. Res., Vol. 45, No. 21, 2006 7081 Table 2. XPS Results of Binding Energies (BE) and Surface Compositions (at. %) for Mn/SiO2 and Na-Containing Mn/SiO2 Catalysts before and after OCM Reaction Na 1sa

Mn 2p3/2a

Na 1sb

Mn 2p3/2b

catalysts

BE (eV)

at. %

BE (eV)

at. %

BE (eV)

at. %

BE (eV)

at. %

Mn/SiO2 Na2WO4-Mn/SiO2 Na2MoO4-Mn/SiO2 Na2SO4-Mn/SiO2 Na3PO4-Mn/SiO2 Na4P2O7-Mn/SiO2 Na2CO3-Mn/SiO2 Na2SiO3-Mn/SiO2

1071.8 1071.8 1072.0 1071.9 1071.8 1072.0 1071.8

5.3 4.2 5.7 6.8 5.5 6.2 6.3

641.1 641.8 641.9 641.5 641.6 641.6 642.3 642.4

1.9 2.2 2.1 2.0 2.4 2.3 1.8 2.1

1071.8 1071.8 1071.9 1071.9 1071.8 1071.9 1071.9

5.1 3.8 6.4 7.9 6.0 5.3 5.6

641.2 641.5 641.7 641.3 641.4 641.4 642.2 642.2

2.5 2.5 2.4 2.3 3.0 2.8 1.9 2.1

a For fresh samples. b After the OCM reaction for 4 h (1073 K; CH : 4 O2:N2 ) 2.5:1:2.5 (molar ratio); GHSV ) 33000 mL g-cat.-1 h-1).

Figure 5. Temperature-programmed reduction profiles for (a) Mn/SiO2, (b) Na2CO3-Mn/SiO2, (c) Na2SO4-Mn/SiO2, and (d) Na2SO4/SiO2 (for comparison).

Figure 4. Raman spectra for (a) Mn/SiO2, (b) Na2WO4-Mn/SiO2, (c) Na2MoO4-Mn/SiO2, (d) Na2SO4-Mn/SiO2, (e) Na3PO4-Mn/SiO2, (f) Na4P2O7Mn/SiO2, (g) Na2CO3-Mn/SiO2, and (h) Na2SiO3-Mn/SiO2.

on the sodium salt added, i.e., both the Na+ ions and oxo anions, as clearly shown by the above results. Na+ is known to be a required component of the Mn/SiO2-based catalysts, particularly for achieving the good C2 selectivities in the OCM reaction.16,25,28 The promoting role of the Na+ ions was previously attributed to their beneficial effects on the transformation of amorphous SiO2 to R-cristobalite and the enhancement of the surface basicity.16,28 It is known that basic surfaces tend to interact more weakly than acidic surfaces with C2H4, thus favoring the desorption of C2H4 to reduce its combustion and increase its selectivity. Amorphous SiO2 indeed catalyzed the combustion of the C2 products while R-cristobalite was inert in the OCM reaction.28 These previous findings are consistent with our results; addition of the sodium salts to Mn/SiO2, irrespective of the identity of their anions, led to the exclusive transformation of amorphous SiO2 to R-cristobalite (Figure 1), and consequently to the 2-3 times greater C2H4 selectivities, while the C2H6

selectivities remained essentially unchanged on the efficient catalysts such as Na2WO4-Mn/SiO2 and Na2SO4-Mn/SiO2 (Table 1). Upon the addition of sodium salts to Mn/SiO2 that consisted of amorphous SiO2 and Mn3O4, as amorphous SiO2 transformed exclusively to R-cristobalite, Mn3O4 underwent oxidation to form Mn2O3 or Mn4+ species, depending on the identity of the oxo anions of the salts. When the salts containing the stronger acid anions such as WO42-, MoO42-, SO42-, PO43-, and P2O74were used, Mn2O3 preferentially formed on the catalysts, and the dramatically improved catalytic performances for the OCM reaction were obtained as compared to Mn/SiO2. However, the sodium salts containing CO32- and SiO32- favored the formation of Mn4+ species on the surfaces, and the corresponding catalysts showed the poor activities (Figures 1 and 5, Tables 1 and 2). These observed effects imply the importance of the Mn2O3 species to the superior performances of the catalysts containing WO42-, MoO42-, SO42-, PO43-, and P2O74-. Therefore, Mn2O3 can be regarded as the active site or to be directly involved in the formation of the active sites for the OCM reaction. With respect to the nature of the active sites on the Mn-based catalysts, no consensus has been reached, albeit the extensively performed studies to date. For example, Jones et al. observed the disappearance of Mn3O4 on Mn/SiO2 after addition of Na+ and attributed this to the manganese-sodium interaction.16 Baronetti et al.13 reported that a KCl-promoted Mn2O3/R-Al2O3 catalyst exhibited a good catalytic performance in the OCM reaction. Wang et al.25 suggested the importance of Na-OMn species on supported Na2WO4-Mn catalysts, while Jiang et al.19 and Ji et al.23 emphasized the importance of W-containing species and WO4 tetrahedrons in activating CH4, respectively. For similar catalysts, Kou and co-workers paid attention to the cooperation between the octahedral MnO6 and tetrahedral WO4 species.31 However, by carefully examining these previous studies, it is not difficult to get the connection between the genesis of the active sites and the Mn2O3 phase on their catalysts

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since the effective catalysts reported in those papers contained Mn2O3 as required species. While these Mn2O3 species were stable in air at the OCM reaction temperatures as demonstrated by high-temperature XRD (Figure 2), they underwent reduction, and the extent of reduction depends on the relative ratios of CH4 to O2. In the CH4-rich reaction conditions, only slight reduction was observed, as verified from the slightly weakened Mn2O3 peak (Figure 3) and a ∼0.2-0.3 eV shift to lower Mn 2p3/2 binding energy (Table 2). For Na2WO4-Mn/SiO2, as shown in Figure 3, Mn2O3 reduced to Mn2+ species in the form of MnWO4, with no detectable reduction of WO42-. This reflects the view that the redox cycles between Mn3+ (Mn2O3) and Mnδ+ (2 e δ < 3) may be involved in the OCM mechanism; δ could not be smaller than 2, based on the H2-TPR data as shown in Figure 5. This redox mechanism for the OCM reaction was proposed by Jones et al. on Na4P2O7-Mn/SiO2.16 Consistent with this redox mechanism, the observed catalytic activities of the catalysts parallel their reducibilities (Table 1 and Figure 5). It is clear in this work that the sodium salts containing different oxo anions led to the remarkable differences in the formation of active sites and improvement of OCM activities; however, the anion effects appear to be overlooked in the literature. Only the role of WO42- on Na2WO4-Mn/SiO2 has been extensively studied.19-23,25,31 As discussed above, WO42was proposed by Li et al. to be the active sites for the OCM reaction,19-22 while Lunsford and co-workers suggested that WO42- contributed to the stability of the catalyst in cooperation with the active sites of Mn-O-Si species.25 Our results are consistent with the latter proposition in terms of the Mn species as the active sites. Na2WO4 itself after being supported on SiO2 was active for OCM reaction (Table 1), but the observed stable activity of the Na2WO4-Mn/SiO2 with almost no detectable W species after a long time test on stream5,31 indeed precluded the crucial role of WO42- as the active sites in the OCM reaction. MoO42- has a tetrahedral structure and its catalytic properties resemble those of WO42-. Table 1 showed that the two anions containing catalysts exhibited similar catalytic performances. The lower C2H4 selectivity concurrently with higher CO2 selectivity on Na2MoO4-Mn/SiO2 than on Na2WO4-Mn/SiO2 reflects higher activity of C2H4 combustion on Mo-based sites than on W-based sites. These results do not intend to unambiguously exclude the role of WO42- as active sites, but do show that WO42- is not unique for the OCM reaction. This is further confirmed by the catalysts containing SO42-, PO43-, and P2O74-, on which the observed catalytic performances were as good as those for the WO42- and MoO42- cases due to the formation of the same Mn2O3 phase on these catalysts. The sodium salts of these three nonmetal oxo anions, when used alone (Table 1), in contrast to WO42- and MoO42-, were almost inactive, which can thus rigorously preclude the contributions of these nonmetal anions alone as active sites to the activation of CH4. On the other hand, the addition of the salts containing CO32- and SiO32anions dramatically suppressed the OCM activity for Mn/SiO2 (Table 1) due to the presence of Mn4+ rather than Mn2O3 species. When all these results are taken together, it is clear that different catalytic performances obtained with these oxo anions are not mainly due to the difference in the catalytic activities of the anions themselves, but due to the difference in the Mn species induced by the anions. The effective catalysts require the presence of Mn2O3 rather than Mn3O4 or Mn4+ species. It was further noted that the anions alone in the absence of Na+ do not impart any effects on improvement of the catalytic

performance for Mn/SiO2-based catalysts, as shown by the example denoted as Mn-SO4/SiO2 in Table 1. The activity and C2 selectivity of the Mn-SO4/SiO2 containing no Na+ were essentially the same as those on Mn/SiO2, but were much lower than those on Na2SO4-Mn/SiO2 (Table 1). A similar phenomenon was observed by Palermo et al. for the Na2WO4-Mn/ SiO2 system.28 This reflects that the coexistence of Na+ and oxo anions with commensurate effects is a prerequisite for achieving the promoting effects. The different Mn species formed on the catalysts are tentatively attributed to the effects of surface basicity. Surface basicity appears to facilitate the formation of Mn species with higher oxidation states. Mn3O4 existed on Mn/SiO2, probably caused by the weak surface acidity of the amorphous SiO2 support. The basicity of Na2CO3 and Na2SiO3 most likely led to the formation of Mn4+ species on Na2CO3-Mn/SiO2 and Na2SiO3-Mn/SiO2. The structures of the Mn4+ species have not yet been determined, but these species could not be MnO2 due to its thermal instability.32 They could possibly be sodium manganate-like mixed oxides as a result of the strong interactions between acidic MnO2 and basic Na2O, Na2CO3, or Na2SiO3. The basicity tends to inhibit the reduction of the transition metal oxides in H2 by strengthening the metal-oxygen bonds, as reported for MoOx.36 Such inhibiting effect prevented the reduction of Mn4+ species (Figure 5) required for the redox OCM mechanism on the Na2CO3-Mn/SiO2 and Na2SiO3-Mn/ SiO2 surfaces. Other sodium salts used in this work, such as Na2WO4 and Na2SO4 containing stronger acid anions, may lead to the catalyst surfaces with balanced basicity and acidity. Such commensurate effects of Na+ and oxo anions could be the reason that these salts favor the formation of Mn2O3 and the required redox cycles between Mn3+ (Mn2O3) and Mnδ+ (δ < 3) for the OCM reaction. In addition, as mentioned above, the weakening or disappearance of the diffraction signals of the sodium salts on the active catalysts, as shown by the high-temperature XRD results (Figure 2), might be related to the deformation of their lattice structures at the temperatures near or higher than their melting points, although it is known that supporting a component on a surface and/or mixing it with a second component generally may also destroy the crystallinity. Tammann found that the deformation of the lattice structures could lead to the formation of bulk metal or oxide phases far below their melting points.37 The influence of the melting points on the design of working catalysts was first discussed by Bond, who emphasized that supported metal catalysts may work near their melting points, perhaps in their semi-fluid states under working conditions.38 Na2WO4 (mp 971 K), Na2MoO4 (mp 960 K), Na2SO4 (mp 1157 K), Na3PO4 (mp 1613 K), and Na4P2O7 (mp 1153 K) have low melting points, relative to the OCM temperatures. In accordance with the so-called Tammann temperatures, we tentatively propose here that one of the characteristics of these sodium salt promoters for the Mn/SiO2-based catalysts appear to be associated with the following: they are melting but without decomposition, low-dimensional but well-organized with the active Mn2O3 sites, and highly diffusional on the catalyst surfaces under the OCM reaction conditions. These observed effects of sodium salts on the formation of the active sites and the catalytic performances thus provide new insights into the understanding of the Mn/SiO2-based catalysts and will be helpful for the synthesis of more efficient OCM catalysts.

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5. Conclusions Sodium salts containing different oxo anions largely influence the structures, reducibility, and catalytic performances of the Mn/SiO2-based catalysts in the oxidative coupling of methane reaction. The addition of sodium salts to Mn/SiO2, which contained Mn3O4 and amorphous SiO2 phases, led to the exclusive transformation of amorphous SiO2 to R-cristobalite, and the concurrent oxidation of Mn3O4 to different Mn species. As the salts containing WO42-, MoO42-, SO42-, PO43-, and P2O74- anions were used, Mn2O3 was formed predominantly, and much higher reducibility, OCM activities, and C2 selectivities were obtained, as compared to Mn/SiO2. However, the basic sodium salts of CO32- and SiO32- led to the preferential formation of Mn4+ species and to the very low reducibility and OCM activities. These observed effects suggest that Mn2O3 species can be regarded as the active sites required for the methane activation. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 20443010) and State Key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University. The authors acknowledge the assistance of Dr. Jinglin Xie and Mr. Xudong Chen with the XPS and Raman measurements, respectively. Literature Cited (1) Lunsford, J. H. Catalytic Conversion of Methane to More Useful Chemicals and Fuels: A Challenge for the 21st Century. Catal. Today 2000, 63, 165. (2) Otsuka, K.; Wang, Y. Direct Conversion of Methane into Oxygenates. Appl. Catal., A 2001, 222, 145. (3) Lee, J. S.; Oyama, S. T. Oxidative Coupling of Methane to Higher Hydrocarbons. Catal. ReV.-Sci. Eng. 1988, 30, 249. (4) Lunsford, J. H. The Catalytic Oxidative Coupling of Methane. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (5) Li, S. Reaction Chemistry of W-Mn/SiO2 Catalyst for the Oxidation Coupling of Methane. J. Nat. Gas Chem. 2003, 12, 1. (6) Keller, G. E.; Bhasin, M M. Synthesis of Ethylene via Oxidative Coupling of Methane. 1. Determination of Active Catalysts. J. Catal. 1982, 73, 9. (7) Ito, T.; Wang, J. X.; Lin, C. H.; Lunsford, J. H. Oxidative Dimerization of Methane over a Lithium-Promoted Magnesium-Oxide Catalyst. J. Am. Chem. Soc. 1985, 107, 5062. (8) Otsuka, K.; Jinno, K.; Morikawa, A. The Catalysts Active and Selective in Oxidative Coupling of Methane. Chem. Lett. 1985, 499. (9) Otsuka, K.; Liu, Q.; Hatano, M.; Morikawa, A. Synthesis of Ethylene by Partial Oxidation of Methane over the Oxides of Transition Elements with LiCl. Chem. Lett. 1986, 903. (10) Burch, R.; Squire, G. D.; Tsang, S. C. Comparative Study of Catalysts for the Oxidative Coupling of Methane. Appl. Catal. 1988, 43, 105. (11) Burch, R.; Squire, G. D.; Tsang. S. C. Role of the Chlorine in Improving Selectivity in the Oxidative Coupling of Methane to Ethylene. Appl. Catal. 1989, 46, 69. (12) Baronetti, G. T.; Lazzari, E.; Garcia, E. Y.; Castro, A. A.; Garcia Fierro, J. L.; Scelza, O. A. Oxidative Coupling of Methane Over Supported Oxides Promoted with Alkali Metals. React. Kinet. Catal. Lett. 1989, 39, 175. (13) Baronetti, G. T.; Scelza, O. A.; Castro, A. A. Structure of Unpromoted and Alkali-Metal Promoted MnOx-Based Catalysts for Oxidative Coupling of Methane. Appl. Catal. 1990, 61, 311. (14) Mleczko, L.; Baerns, M. Catalytic Oxidative Coupling of Methane - Reaction-Engineering Aspects and Process Schemes. Fuel Process Technol. 1995, 42, 217. (15) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. The Oxidative Conversion of Methane to Higher Hydrocarbons. J. Catal. 1987, 103, 302. (16) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. The Oxidative Conversion of Methane to Higher Hydrocarbons over Alkali-Promoted Mn/ SiO2. J. Catal. 1987, 103, 311.

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ReceiVed for reView March 6, 2006 ReVised manuscript receiVed August 12, 2006 Accepted August 21, 2006 IE060269C