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chains, and eventually aromatization in the channels. * To whom ... sissauga, Ontario, Canada L5K 1B3. .... (1996) indicated that HZSM-5 could only ad...
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Bifunctional Behavior of Mo/HZSM-5 Catalysts in Methane Aromatization Jun Shu,* Alain Adnot, and Bernard P. A. Grandjean Department of Chemical Engineering and CERPIC, Laval University, Ste-Foy, Quebec, Canada G1K 7P4

Catalytic functions of Mo/HZSM-5 toward methane aromatization are examined under various activation and reaction conditions. Molybdenum ions are necessary in the activation of methane. High benzene yields of around 7% are obtained with a selectivity of more than 96% at 973 K over a 2 wt % Mo/HZSM-5 catalyst in a tubular reactor. Pretreatment with hydrogen is found to be beneficial to the initialization of aromatization. The aromatization activity increases upon decreasing the Si/Al ratio in HZSM-5, indicating an acidity dependence. This is supported by the fact that a Mo catalyst supported on cesium-exchanged ZSM-5 losses its catalytic aromatization activity at all. The effective coordination between molybdenum ion sites and Brønsted acid sites in the framework of ZSM-5 is important in the activation of the stable C-H bonds in methane and the eventual aromatization to form benzene. The constant O1s binding energy in the XPS spectra of both fresh and used catalysts suggests that coke formation does not occur on the SiO4 tetrahedra but rather on the molybdenum sites supported on the external surface of ZSM-5. 1. Introduction Transition metal ion promoted zeolites, in particular, molybdenum-modified ZSM-5 catalysts, have been found to be effective in the transformation of lower alkanes to aromatic chemicals since the past decade. Catalytic performances of Mo-exchanged Y, mordenite, and ZSM-5 were studied by Howe and co-workers for propene oxidation (Howe and Huang, 1988) and for the reaction of toluene with methanol (Huang and Howe, 1987). It was found that the catalytic activity and the selectivity for propene oxidation depend on molybdenum content, location of Mo, and zeolite acidity. Since a few years, Mo/HZSM-5 catalysts have received growing attention due to their outstanding activity toward methane aromatization. This reaction is of both scientific interest and potentially industrial importance in upgrading natural gas resources. Currently, only a small portion of the vast natural gas reserve is used as chemical feedstock to produce synthesis gas which could be further converted into more valuable petrochemicals via methanol synthesis or Fischer-Tropsch processes. Due to the endothermic behavior, direct pyrolysis of methane requires so high a temperature (>1473 K) that the complete dissociation of methane into carbon and hydrogen cannot be avoided. Oxidative aromatization of methane was studied by using either nitrous oxide (Anderson and Tsai, 1985) or oxygen (Abasov et al., 1991; Claridge et al., 1992) as the oxidizing agent. However, the benzene selectivity was reported to be generally less than 20%. Bricker (1991) of UOP claimed a methane aromatization process using 1 wt % Ga/ ZSM-5 catalysts and phosphorus-containing alumina at a temperature ranging from 823 to 1023 K and a pressure below 1 MPa. To prevent the catalyst from coke deposition, a small amount of hydrogen (0.5 mol %) was * To whom correspondence should be addressed. Current address: ORTECH Corporation, 2395 Speakman Drive, Mississauga, Ontario, Canada L5K 1B3. Fax: (905) 822-9537. E-mail: [email protected].

added to the methane feed stream. At least 3 mol % C2 hydrocarbons and 5 mol % C6-C8 aromatics were obtained in a single pass at a gas hourly space velocity (GHSV) of 400-7500 h-1. In addition, Vasina et al. (1994) conducted the same reaction in a pulse system using a Ga-, Zn-, or Cr-modified ZSM-5 catalyst and obtained a yield of aromatic hydrocarbons of 11-14% at 1023 K. Wang et al. (1993) reported some promising results of methane aromatization over Mo/HZSM-5 catalysts prepared by a conventional impregnation method. They claimed a methane conversion of 7-8% with a benzene selectivity of 100% over an air-activated Mo/HZSM-5 catalyst at 973 K (the equilibrium methane aromatization conversion is 11.4% at 973 K and 1 bar). Further studies demonstrated that metal ions, strong acidic sites, and zeolite channel structure were crucial for a good catalytic activity of Mo/HZSM-5 (Xu et al., 1995a,b). The benzene selectivity of 100% obtained in the preliminary work of Wang et al. (1993) seems questionable considering the unavoidable coke formation during methane conversion at high temperature. Indeed, much lower benzene selectivities of around 60-70% under similar conditions were obtained by other researchers (Wang et al., 1995, 1997; Solymosi et al., 1997). In the literature, various activation procedures have been reported for aromatization catalyst tests. Wang et al. (1993) kept an air stream for 30-40 min at 973 K for catalyst pretreatment until the introduction of methane for aromatization. The role of the oxidative activation atmosphere remained unclear. Reductive atmospheres were also used sometimes for the pretreatment of Mo/HZSM-5 catalysts (Wang et al., 1997; Tan et al., 1997), without comparison with oxidative atmospheres. It is generally recognized that methane aromatization undergoes a bifunctional pathway. The very stable C-H bonds in methane need to be activated over some transition metal ion sites, followed by growth of carbon chains, and eventually aromatization in the channels

10.1021/ie990145i CCC: $18.00 © 1999 American Chemical Society Published on Web 08/31/1999

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of ZSM-5. Previously, Wang et al. (1993) reported that methane aromatization activity over Mo/HZSM-5 with SiO2/Al2O3 ) 50 was higher than that over Mo/HZSM-5 with SiO2/Al2O3 ) 25. This result was contradictory to the observation of Marczewski et al. (1995) who used alternative catalysts of MnOx-Na/SiO2-HZSM-5 and CuO-FeO/HZSM-5 and found that the methane aromatization conversion increased upon decreasing the SiO2/Al2O3 ratio over CuO-FeO/HZSM-5. The latter findings led to the conclusion of an acidity dependence, as the acidity of HZSM-5 is associated with the 3-coordinated Al sites. The lower the SiO2/Al2O3 ratio, the higher the acidity of the HZSM-5 catalysts is. There is, however, little evidence in the literature to further reveal the interaction between the active components in Mo/HZSM-5 catalysts. The emphasis of the present work is to examine roles of various catalytic functions through catalytic reaction tests and an X-ray photoelectron spectroscopic study (XPS) in order to gain a further understanding of the direct methane aromatization process. 2. Experimental Section 2.1. Preparation of Mo/HZSM-5 Catalysts. HZSM-5 was prepared from Na-ZSM-5 (Si/Al ) 8.33, 25 and 40) by three NH4+ exchanges with 1 M ammonium nitrate solution at 353 K (3 h per exchange), followed by airdrying at 373 K overnight and calcination at 823 K for 4 h. ZSM-5 zeolites with Si/Al ) 25 were used in most cases, except for the examination of the Si/Al effect, as described later. Similar ion exchanges were practiced for the preparation of Cs-ZSM-5 from Na-ZSM-5. This was done by repeated exchanges of Na-ZSM-5 with 1 M ammonium nitrate solution and then 1 M cesium nitrate solution. Molybdenum loading on HZSM-5 (up to 5 wt %) or Cs-ZSM-5 (2 wt %) was performed by wet impregnation of calcined zeolites with a suitable volume of ammonium molybdate solution ((NH4)6Mo7O24‚4H2O), followed by drying at 383 K overnight and calcination in air at 823 K for 4 h. To examine the zeolite function in aromatization, activated acidic γ-Al2O3 (purchased from Aldrich) was also used to support molybdenum (2 wt %) by wet impregnation. High-energy ball milling was applied to grind 2% Mo/ HZSM-5 using a hardened steel vial with a Spex 8000 laboratory ball mill at a rotating speed of 750 rpm. The milling was done in a nitrogen atmosphere at ambient temperature for 3 h. 2.2. Reaction Test. Catalytic activities of the prepared catalysts toward methane aromatization were evaluated under atmospheric pressure by using a conventional tubular reactor made of stainless steel. The stainless steel was confirmed to be sufficiently inert under the reaction conditions. Even with pure HZSM-5 packed in the reactor bed, no reaction was detected at 973 K. To catalyze the aromatization reaction, a total of 1.5 g of catalyst was packed in the reactor with quartz wool as the support for each test. For the pretreatment of catalysts, both a reductive activation procedure and an oxidative procedure were practiced. In the reductive procedure, the catalyst was first heated in argon to 973 K and then submitted to pretreatment in hydrogen at 973 K for 2 h. In the oxidative procedure, an air flow was kept during the pretreatment stage instead of use of argon and hydrogen. Aromatization was conducted by passing methane at a GHSV of 1000 h-1 through the catalyst bed at 973 K. Samples for XPS measurements

were prepared by conducting the reaction test for 2 h, followed by cooling in argon. The temperature of the catalyst bed was measured by using a fine thermocouple placed in the center of the reactor tube. Reaction products were analyzed by an on-line quadrupole mass spectrometer (Leybold Transpector Gas Analysis System) and a gas chromatograph (PerkinElmer 1020) using an FID detector. The GC separation column was a capillary GS-Q, supplied by J&W Scientific, for the complete analysis of C1-C10 hydrocarbons. External sampling was performed to calibrate GC peaks of both reactants and products. Methane conversion and product selectivity were calculated on the basis of a carbon balance. 2.3. Catalyst Characterization. Crystalline structures of Mo/HZSM-5 catalysts before and after reaction were examined by X-ray diffraction (XRD) using a Semens D500 X-ray diffractometer with Ni-filtered Cu KR radiation (λ ) 1.5406 Å) and a scintillation counter. Mo/ZSM-5 catalysts were also characterized by FT-IR (FTS-60 IR spectrometer from Bio-Rad Laboratories) in order to collect information on carbonaceous deposition upon reaction. XPS measurements of both fresh and used Mo/ZSM-5 catalysts were carried out by using a VG ESCALAB Mark II electron spectrometer with a hemispherical analyzer. Mg KR X-ray radiation (hν ) 1253.6 eV) was generated at 20 mA and 15 kV. Peaks of Si2p, Al2p, C1s, O1s, and Mo3d were recorded in a binding energy interval of 0.1 eV. The energy scale was corrected by using the binding energy of zeolitic Si2p at 103.0 eV as an internal standard. The spectrometer chamber had a residual gas pressure close to 10-9 mbar during data acquisition. XPS intensities used for the estimation of near-surface compositions of molybdenum species were calculated by integrating peak areas after a linear background subtraction. Peak decomposition was performed with the help of software based on the code distributed by Hughes (Hughes and Sexton, 1988) assuming fixed FWHM values, fixed binding energy distances between doublet peaks, and fixed Gaussianto-Lorentzian ratios (Paal and Schlogl, 1992). Peak positions were determined by the fitting procedure itself. 3. Results and Discussion 3.1. Catalytic Performance of Mo/ZSM-5 Catalysts. At the very beginning, 1.5 g of the 2 wt % Mo/ HZSM-5 catalyst (10-30 mesh in particle size) was packed in the reactor for reaction test under conditions similar to those used by Wang et al. (1993). After treatment in hydrogen at 973 K, the reaction product distribution was recorded with time by both GC and MS. In this case, where coarse catalyst particles were used, the obtained total methane conversion was below 8% (Figure 1). The effect of catalyst particle size was further checked by using the fine powder catalyst as synthesized, resulting in a methane conversion nearly twice as high as that with coarse particles. The conversion enhancement is not surprising from the consideration of the importance of crystalline channels inside ZSM-5 toward methane aromatization. Molecular diffusion inside the catalyst particles might become rate-limiting if large particles were used. Therefore, powder catalysts as-prepared were used in the study for further catalyst tests. Effects of both oxidative and reductive activation procedures on methane aromatization were compared

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Figure 1. Effect of catalyst particle size on methane conversion over 2 wt % Mo/HZSM-5 at 973 K, Si/Al ) 25.

Figure 2. Effect of Mo loading over HZSM-5 on methane conversion at 973 K, Si/Al ) 25.

over the 2 wt % Mo/HZSM-5 catalyst. The results indicated that the induction period for methane aromatization over the oxidized catalysts was much longer than that over the reduced catalysts. The oxidative activation atmosphere gave a high initial methane conversion but a low benzene selectivity (e.g., 14% methane conversion with 20% benzene selectivity after 1 h of reaction at 973 K) compared to the reductive atmosphere (e.g., 10% methane conversion with 70% benzene selectivity under the same reaction conditions). Basically, the aromatization atmosphere in the absence of oxygen is reductive due to the presence of methane and the product hydrogen. It can be understood that the oxidative activation would extend the induction period for aromatization, although the oxidative activation might favor the dispersion of Mo species on the zeolite surface because of the increasing mobility of MoO3 near its melting point (1068 K). The 2 wt % Mo/HZSM-5 catalyst was found to be effective in catalyzing methane aromatization. To clarify the role of molybdenum, the effect of molybdenum loading was examined. Figure 2 shows the effect of Mo loading on methane conversion in the first 2 h of reaction. HZSM-5 itself showed no catalytic activity at all toward either methane decomposition or aromatization. Its original color remained unchanged after 2 h of reaction test at 973 K. This is different than its behavior in catalyzing aromatization of higher hydrocarbons. For example, HZSM-5 was reported to be active toward propane aromatization (Ono, 1992). In the latter case, Lewis acid sites consisting of tricoordinated aluminum species in HZSM-5 were suggested to abstract

Figure 3. Methane aromatization conversion (X) and selectivity (Y) as a function of reaction time over 2% Mo/HZSM-5 at 973 K, Si/Al ) 25.

a hydride ion from the saturated propane (Ono, 1992; Cumming and Wojciechewski, 1996). The inactive behavior of HZSM-5 toward methane activation can be attributed to the very stable C-H bonds in methane, which are difficult to be directly attacked by Lewis acid sites over HZSM-5. An FT-IR study conducted by Chen et al. (1996) indicated that HZSM-5 could only adsorb methane at temperatures below 213 K by an interaction between methane and bridging hydroxyl groups in HZSM-5 zeolites. When molybdenum was present in the catalyst, methane conversion was found to increase with the Mo content in the range of 0.1-2 wt %. The 2% Mo/ HZSM-5 catalyst was found to be most active and selective toward methane aromatization. However, an increase in Mo loading up to 5 wt % resulted in a decreased methane conversion, which was in agreement with the findings obtained by Chen et al. (1995). Such a conversion decrease with excessive Mo loading might be attributed to the partial blockage of zeolite channels by agglomerative molybdenum oxide particles. Thus there exists an optimal Mo loading (2 wt % in the present case) on the HZSM-5 substrate to form an active aromatization catalyst. It was noticed that methane conversion varied with time over Mo/HZSM-5 catalysts. The methane conversion after 5 min of reaction was over 10% in the case of 2 wt % Mo loading and then fell considerably until 40 min of reaction. In the product stream, components detected by GC included methane, benzene, and small amounts of ethylene, ethane, and naphthalene. Figure 3 shows the selectivities of typical products over 2% Mo/ HZSM-5 (right-hand axis). The selectivity to benzene at the beginning of the reaction test was only 40%. After 120 min, it reached a high level of over 90% (∼96% after 160 min). Interestingly, the yield of benzene, i.e., the product of methane conversion and benzene selectivity, was almost constant (around 6-7%) during evaluation. The selectivities to ethylene and naphthalene slowly increased with time, whereas that to ethane slightly decreased. From the simultaneous monitoring of the products with the mass spectrometer, a relatively large amount of hydrogen was detected at the beginning. It was the carbonaceous deposition of methane over the catalyst that made an important contribution to the initial high methane conversion. This was supported by the color change of catalyst from white to very dark during the reaction test. The gradual deactivation of catalyst in methane aromatization might be attributed to the blocking of

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Figure 4. Methane decomposition into hydrogen and carbonaceous species over pure MoO3 at 973 K.

zeolite channels by the development of carbon deposits. This blocking effect should be similar to the overloading of Mo on HZSM-5. Our experiment showed that an oxidation treatment of deactivated catalysts in an air stream could significantly recover the catalytic activity of Mo/HZSM-5 catalysts toward methane aromatization. The catalytic activity of pure MoO3 was examined in order to further understand the role of molybdenum in methane aromatization. Figure 4 shows the result of methane conversion with time on-stream. At the beginning, methane was converted in a large portion (e.g., 31.6% at 5 min) with no benzene production. Then the conversion decreased dramatically to a level of ca. 5% with a minor production of ethane (the selectivity varies between 2.6% and 7.7%). The formation of abundant hydrogen in the beginning, revealed by MS, indicated that MoO3 was able to catalyze methane decomposition but unable to further produce aromatics. The quick deposition of carbonaceous species on the catalyst surface resulted in a dramatic methane conversion drop. This evidence should be useful in understanding the fact that MoO3 had no activity for the aromatization reaction. 3.2. Effects of Catalyst Acidity and Zeolite Channels. The 2% Mo/HZSM-5 catalyst showed a high catalytic activity toward methane aromatization, while its support HZSM-5 was inactive. This does not simply imply the inertness of the support. When an activated acidic γ-Al2O3 was used as the support instead of HZSM5, the methane conversion over the 2 wt % Mo/γ-Al2O3 catalyst decreased dramatically with reaction time, and the benzene yield was much lower than that over 2% Mo/HZSM-5 (Figure 5). The latter experiment further demonstrated that methane aromatization over Mo/ HZSM-5 catalysts requires a combination of functional Mo species and zeolitic HZSM-5 surrounding. To examine the importance of acidic functions in ZSM-5 toward aromatization, Cs-ZSM-5 was used in lieu of HZSM-5 to support the Mo component. This substrate is sometimes regarded as a basic catalyst due to the replacement of Brønsted acid sites in ZSM-5 by basic Cs+ ions. A 2 wt % Mo/Cs-ZSM-5 catalyst was thus tested in conducting methane aromatization. The catalyst showed a relatively high initial methane conversion of around 7% at 5 min (Figure 6), and the rapid conversion decrease thereafter to a very low level until 40 min. Almost no benzene was produced during the reaction test. After the reaction test, the catalyst was found to be dark in color. This implies that the aromatization could not be initialized over Mo/Cs-ZSM-5 due

Figure 5. Methane aromatization conversion (X) and selectivity (Y) as a function of reaction time over 2% Mo/γ-Al2O3 at 973 K.

Figure 6. Methane aromatization conversion (X) and selectivity (Y) as a function of reaction time over 2% Mo/Cs-ZSM-5 at 973 K.

to the disappearance of the acidic function in the catalyst while the carbonaceous deposition over molybdenum ions still occurred. The catalytic activity of acidic ZSM-5 zeolites is often related to their Si/Al ratio. This is fundamentally understandable considering the origin of Brønsted acid sites in HZSM-5:

The lower the Si/Al ratio, the higher the Brønsted acidity of the catalyst due to the contribution of protonated [AlO4] to the Brønsted acidity. Experimental measurements conducted by Marczewski et al. (1995) support this relationship for HZSM-5. It has been established that for various reactions the activity of ZSM-5 is associated with the presence of protonated tetrahedral Al in their framework (Haag et al., 1984). The validity of this relationship was examined for methane aromatization over Mo/HZSM-5 catalysts. Two more HZSM-5 substrates with Si/Al ) 8.33 and 40 were tested, and the results are shown in Figure 7. It can be seen that the total methane conversion over Mo/HZSM-5 with Si/Al ) 8.33 is higher than the one with Si/Al ) 40. The selectivity toward benzene in the former case reaches a slightly higher level in the early stage of

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Figure 7. Effect of the Si/Al ratio in HZSM-5 on methane conversion (X) and benzene selectivity over 2 wt % Mo/HZSM-5 at 973 K.

reaction, although the trends reverse slightly after 100 min. If the benzene yield is counted as the product of methane conversion and benzene selectivity, it can be found that Mo/HZSM-5 with a low Si/Al ratio is more active toward methane aromatization. Although there might be a small fraction of extraframework Al in ZSM-5 with a low Si/Al ratio, the contribution of the protonated tetrahedral Al to the Brønsted acidity was still confirmed in an XPS study with pyridine chemisorption (Borade et al., 1990). The above findings together with the effect of Cs+ exchange imply the direct involvement of Brønsted acid sites of HZSM-5 in the methane aromatization. An FT-IR examination of the used 2 wt % Mo/HZSM-5 revealed the presence of C-H stretch bands on the catalyst surface. Crystalline structures of the Mo/ZSM-5 catalysts were determined by XRD and were found to be unchanged upon reaction at 973 K. There was no indication that the deposited carbon species was in the form of graphite from the XRD observation. From the effect of Mo overloading on HZSM-5 and the gradual deactivation of catalyst by carbonaceous deposition, it is understood that the open zeolite channels are critical for methane aromatization to take place. The fresh 2% Mo/HZSM-5 catalyst was further treated under high-energy milling for 3 h. A recent study (Zielinski et al., 1995) showed that such a treatment could transform a crystalline zeolite into an amorphous-like material. Upon milling, the secondary crystalline structure of zeolites, or the supercage, would be destroyed, although the zeolite β-cage would still remain. The methane aromatization result over the milled 2% Mo/HZSM-5 catalyst is shown in Figure 8. It can be seen that the deformation of the HZSM-5 framework resulted in a significantly decreased methane conversion. 3.3. XPS Results. XPS O1s spectra of both fresh and coked Mo/ZSM-5 catalysts are shown in Figure 9. The O1s binding energy in both Mo/HZSM-5 and Mo/CsZSM-5 is almost constant (located at 532.2 ( 0.1 eV) and independent of coke deposition. This is different from the findings of Kulkarni and co-workers (1987), who found decreased O1s binding energies (about 1 eV shift) in the coked HZSM-5 catalysts and attributed the shift to an electron charge donation from coke to the framework atoms of Si and O. In their work, the coke was formed on HZSM-5 by conducting a reaction of toluene with ethanol. It was found that HZSM-5 was catalytically active toward the reaction between toluene and ethanol. Their results suggested that the coke was deposited on the SiO4 tetrahedra of HZSM-5. However,

Figure 8. Methane conversion over fresh and milled 2 wt % Mo/ HZSM-5 catalysts at 973 K.

Figure 9. O1s spectra of fresh and used Mo/ZSM-5 catalysts: MoH-1, fresh impregnated 2 wt % Mo/HZSM-5; MoH-2, used impregnated 2 wt % Mo/HZSM-5; MoCs-1, fresh 2 wt % Mo/CsZSM-5; and MoCs-2, used 2 wt % Mo/Cs-ZSM-5.

our reaction test showed that HZSM-5 was not active at all for methane conversion at 973 K. Thus the disagreement of the XPS results of O1s binding energies between ours and that of Kulkarni et al. suggests that the coke is not deposited directly on the SiO4 tetrahedra of the ZSM-5 framework during methane aromatization, leaving the framework oxygen surrounding in ZSM-5 undisturbed with respect to the electron interaction. Figure 10 shows C1s spectra of both fresh and used Mo/ZSM-5 catalysts, where two kinds of carbon species are distinguished. On all fresh catalysts, the binding energy of C1s is located at 284.7 ( 0.1 eV with relatively small intensities. This kind of carbon species is in agreement with the literature data corresponding to carbon contamination. The self-consistence of this C1s binding energy to the literature data is helpful in checking the XPS binding energy scale. Another kind of carbon species is found on the used catalysts with a binding energy of 283.6 ( 0.1 eV, which has been attributed to the formation of Mo2C species in the literature. Solymosi et al. (1997) and Wang et al. (1997) found that the Mo2C species was active in the activation of methane. It seems that the reduction from Mo6+ to Mo2C-like species results in coordinatively unsaturated molybdenum serving as the activation sites for methane decomposition. Mo3d spectra of both fresh and used Mo/ZSM-5 catalysts are rather complicated, due to the multiple states of molybdenum. Since there are significant state

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impregnation, where only Mo6+ was found on the fresh catalyst. It seems that a strong electronic interaction between Mo6+ and Cs+ occurs in Mo/Cs-ZSM-5. Despite the different forms of molybdenum species, however, carbonaceous deposition occurred on both Mo/HZSM-5 and Mo/Cs-ZSM-5 catalysts. The role of well-coordinated acid sites in ZSM-5 should be determinative in the eventual aromatization process, which will be discussed in the next section. 3.4. Discussion of the Mechanism of Methane Aromatization. Regarding the origin of catalytic activity of Mo/HZSM-5, Wang et al. (1993) believed that methane was activated via an acid-assisted heterolytic splitting of the C-H bond to form carbonium ion species CH3+:

CH4 + Mo6+(s) f CH3+ + [Mo-H]5+ Figure 10. C1s spectra of fresh and used Mo/ZSM-5 catalysts. Refer to Figure 9 for denotation. Table 1. XPS Peak Positions of Mo/ZSM-5 Catalysts XPS peak Si2p O1s C1s Mo6+3d5/2 Mo6+3d3/2 Mo5+3d5/2 Mo5+3d3/2 Mo4+3d5/2 Mo4+3d3/2 Mo2C3d5/2 Mo2C3d3/2 Cs3d5/2

Mo/HZSM-5 fresh used

Mo/Cs-ZSM-5 fresh used

103 532.3 284.7 233.0 236.1

103 532.3 284.9

103 532.3 283.6 233.2 236.3 230.6 233.9

231.4 234.6 229.4 232.4

228.1 231.1 724.9

103 532.2 283.6 232.8 236.2 231.1 234.2 227.2 230.1 725.2

Table 2. Relative Contents of Mo Species Calculated from XPS Mo3d Doublets Mo/HZSM-5

Mo/Cs-ZSM-5

species

fresh

used

fresh

used

Mo6+ Mo5+ Mo4+ Mo2C

100 0 0 0

24.8 6.7

0 85.7 14.3 0

14.4 33.6

68.5

52.0

changes in Mo/ZSM-5 catalysts, decomposition of Mo3d peaks is preferable in order to gain a further understanding. Table 1 gives detailed peak position data discovered upon decomposition. In the fresh sample prepared by the impregnation method, molybdenum is found in the form of Mo6+ with doublet binding energies located at 233.0 and 236.1 eV, being consistent with the results of Wang et al. (1995). After the reaction test, a large portion of molybdenum is converted into Mo2Clike species, coexisting with Mo6+. In both cases, the Mo5+ species is found to be in a small portion. Table 2 lists the relative contents of different Mo species for a semiquantitative analysis. The coexistence of various molybdenum species might suggest that the Mo2C-like species is in the form of molybdenum oxycarbide, as evidenced previously by Ledoux et al. (1995). Strangely, no Mo6+ species was detected on the fresh Mo/Cs-ZSM-5 catalyst prepared by impregnation. Instead, a major portion of Mo5+ was found, together with a small amount of Mo4+ in this case. After the reaction test, Mo2C-like species and Mo5+ were detected on the catalyst surface with very little Mo6+ species. This is quite different from Mo/HZSM-5 also prepared by

(2)

Some detailed discussions on this mechanism appeared in their recent publications (Wong et al., 1996; Pierella et al., 1997). The above expression seems to be farfetched in explaining the great difference of support effect between HZSM-5 and other supports. It should be pointed out that the assumption of carbonium ion species in methane aromatization is still controversial. Other researchers argued for the formation of CH3• free radicals in the activation of methane over molybdenum sites (Vasina et al., 1994; Chen et al., 1995; Solymosi et al., 1997). In discussing the methane aromatization mechanism, a straightforward perception is that the stable inert gas-like configuration of the methane molecule makes it difficult to be activated. This is indeed true at low temperature. The tetrahedral C-H bond has a bond dissociation energy of 435 kJ/mol (Koerts et al., 1992) at 298 K, exhibiting neither magnetic moment nor polar distortion to be chemically attacked. However, this general information may be misleading for methane conversion if the temperature is elevated. One should keep in mind that the standard Gibbs free energy change for methane decomposition into graphite and hydrogen becomes negative over 821 K. In the practice of methane aromatization over Mo/HZSM-5 catalysts, reaction temperatures are often maintained over 973 K. Under such conditions, methane has a thermodynamic tendency to form carbon and hydrogen if a suitable catalyst is present. It has been proven that many supported transition metal or metal oxide catalysts are capable of catalyzing this decomposition reaction via a radical mechanism. Different carbonaceous species could be formed as the decomposition intermediates depending on the temperature. High temperature might lead to the formation of an inactive Cγ carbonaceous species (Garnier et al., 1997). In the present study, the catalytic activity of a 2% Ni/HZSM-5 catalyst prepared by impregnation was examined in the methane activation. The catalyst was too active to be controlled in catalyzing methane decomposition. The catalyst bed path was gradually reduced with reaction owing to the rapid coke deposition on the catalyst at 973 K. With the rapid covering of the decomposition sites by the deposited coke, the methane decomposition is gradually inhibited. In this sense, methane decomposition behaviors over MoO3, Mo/γ-Al2O3, Mo/Cs-ZSM-5, and even milled Mo/HZSM-5 seem to obey the radical mechanism. On the outer surface of the ZSM-5 support, such a methane decomposition process would continue until a massive coke deposit is formed.

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However, the benzene yield over 2 wt % Mo/HZSM-5 was almost constant during the evaluation test, despite the rapid decrease in methane conversion. This constant aromatization phenomenon was not observed at all over 2 wt % Mo/Cs-ZSM-5. ZSM-5 has characteristic internal channels which are limited so that only a restricted carbon chain growth is allowed. The restricted carbon chain growth is not sufficient to explain the different aromatization behaviors between Mo/HZSM-5 and Mo/ Cs-ZSM-5. The Brønsted acid sites in the framework of HZSM-5 must have participated in the activation toward the eventual aromatization. From the variation trends of ethane and ethylene in the product distribution, ethylene is likely a conversion intermediate in methane aromatization, while ethane is a by-product of carbonaceous deposition. The ethane formation in this case is somewhat similar as that in the two-step methane conversion sequence catalyzed by transition metals (Garnier et al., 1997). In the aromatization process, methane is probably converted into ethylene by dehydrodimerization over molybdenum sites in the first step. Whether ethane is an intermediate to produce ethylene during methane aromatization is still not clear. Once ethylene is formed inside the ZSM-5 channels, this intermediate is ready to undergo aromatization with the help of the acidic sites of zeolite. This can explain the acidity dependence of methane aromatization. Therefore, our findings seem to support the reaction scheme proposed by Chen et al. (1995), that is, Mon+

CH4 98 CH3• + H• Mon+

2CH3• 98 C2H4 + H2 HZSM-5

3C2H4 98 C6H6 + 3H2

(3) (4) (5)

No doubt, both molybdenum and acidic functions must coordinate each other very well to demonstrate a good aromatization activity. 4. Conclusions Methane aromatization was conducted over Mo/ HZSM-5 catalysts under various activation and reaction conditions. The activity was found to increase with Mo loading to a maximum at 2% in weight. High benzene yields around 7% were achieved with selectivities of more than 96% at 973 K in a tubular reactor. An excessive Mo loading on HZSM-5 resulted in a decreased benzene yield, probably due to partial zeolite channel blocking. The aromatization activity also increased upon decreasing the Si/Al ratio of HZSM-5, indicating an acidity dependence. A Mo catalyst supported on cesiumexchanged ZSM-5 lost catalytic activity at all toward methane aromatization. XPS analysis of Mo/ZSM-5 catalysts was performed in order to explain their catalytic aromatization activity. The effective coordination between molybdenum ion sites and the Brønsted acid sites in the framework of ZSM-5 was found to be important in the activation of the stable C-H bonds of methane and the eventual aromatization to form benzene. The constant O1s binding energy in both fresh and used catalysts suggested that coke formation was not on the SiO4 tetrahedra but rather on the molybdenum sites supported on the external surface of ZSM-5.

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Received for review February 25, 1999 Revised manuscript received July 12, 1999 Accepted July 13, 1999 IE990145I