HZSM-5 in a

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Methane Nonoxidative Aromatization over Ru-Mo/HZSM-5 in a Membrane Catalytic Reactor Maria C. Iliuta, Faı1c¸ al Larachi, Bernard P. A. Grandjean,* and Ion Iliuta Department of Chemical Engineering and CERPIC, Laval University, Ste-Foy, Que´ bec, Canada G1K 7P4

Abdelhamid Sayari Centre for Catalysis Research and Innovation (CCRI), Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5

Low-temperature oxygen-free methane aromatization was carried out over Ru-Mo/HZSM-5 in a catalytic membrane reactor. The 0.5% Ru-3% Mo/HZSM-5 catalyst, prepared by incipient wetness coimpregnation, was highly selective toward benzene production. Methane aromatization was evaluated under two sets of conditions: (i) without hydrogen permeation in a fixed-bed conventional catalytic reactor (CR) and (ii) with hydrogen permeation in a catalytic membrane reactor (CMR). In CR mode, the catalyst exhibited remarkable stability with no significant deactivation for 24 h on stream. Switching to CMR mode gave rise to a significant increase in conversion, which reached levels well beyond the thermodynamic conversion. The continuous withdrawal of coproduced H2 promoted the formation of carbonaceous species with a low H/C ratio and led to a decrease in benzene production. At a methane space velocity of 270 mL (STP)‚h-1‚g-1 and a temperature of 873 K, the CR mode yielded a maximum conversion of methane to benzene equal to 3.8%, i.e., 73% of the thermodynamic equilibrium conversion (5.2%). Under similar conditions, the maximum conversion to benzene attained in CMR mode was 9%. Alternating CR and CMR sequences under moderate feed flow rates proved to be a viable strategy for maintaining high catalyst activity toward benzene production for more than 100 h on stream. The CR step, through an increased hydrogen concentration, helped regenerate the active sites by hydrogenating the carbonaceous species, while the CMR step contributed in the overshoot of benzene conversion due to the equilibrium shift brought about by hydrogen withdrawal. A twoactive-site model was proposed to rationalize the experimental observations. Further tests dealing with hydrogen addition to the methane feed flow clearly demonstrated the beneficial influence of hydrogen on the catalyst activity. 1. Introduction The direct conversion of methane into hydrogen and other high-value-added products such as aromatics is receiving growing attention because of the major economic stakes and abundance of natural gas reserves. Methane aromatization catalyzed by Mo-supported HZSM-5 zeolites has been recognized as a promising route for producing aromatics in an oxygen-free atmosphere.1-10 Despite prolific research activity worldwide, progress in this field has been hampered by severe thermodynamic limitations and strong catalyst deactivation. The underlying mechanisms of methane aromatization are still not completely well understood, and further fundamental studies are required on this important topic.1 Several factors inherent to zeolite-based catalytic materials have been recognized to affect the performance of methane conversion into aromatics.7,8,10-12 These include the Brønsted acid site density, the zeolite * Corresponding author. Tel.: 418 656 2859. Fax: 418 656 5993. E-mail: [email protected].

channel structure, and the oxidation state and location of the molybdenum species. On the basis of several studies of the interaction between the Mo species and the zeolite, various modes of methane activation have been suggested.1 There is a widespread consensus that the hexavalent molybdenum, which is likely initially available in the zeolite channels as MoO313 or as (Mo2O5)2+ dimers,12,14-16 is first reduced by CH4 into active MoCx carbide species.6 These, in turn, are believed to provide the active sites for C-H bond activation into CHx (x < 4) fragments and their condensation into mainly ethylene and ethane.6,7,12,17 Such products are further converted into aromatics on the acidic sites via oligomerization, cracking, and cyclization.18 One important drawback to methane aromatization catalysts is the occurrence of coke, which is responsible for the decrease in conversion into useful products. Much of the current work is therefore directed at improving the conversion into aromatics and at lowering coke formation.1,19-28 Much of the literature related to the nonoxidative dehydrocyclization of methane is focused on improving

10.1021/ie010977s CCC: $22.00 © 2002 American Chemical Society Published on Web 04/23/2002

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Figure 1. CR-CMR design.

potential catalytic materials. Compared to other transition-metal-doped HZSM-5 materials, molybdenumcontaining HZSM-5 zeolites rank among the best methane aromatization catalysts. Modification of Mo/HZSM-5 by the addition of various transition metal promoters, e.g., Co, Cs, Cu, Fe, Ir, La, Li, Pd, Pt, Ru, V, W, and Zr, has been shown to improve both stability (measured in terms of reduction of carbon deposition) and activity.22-28 However, the performance of a particular M-Mo/ HZSM-5 catalyst (where M is the second metal component) often depends on the preparation and pretreatment conditions. Properly dispersed Ru over Mo/ HZSM-5 has proved to be one of the most promising promoters for improving both the conversion into benzene and the catalyst stability.22 The role of Pt as a promoter for Mo/HZSM-5 is somewhat controversial. Pt was reported not to affect the catalytic activity but to enhance the catalyst stability by reducing the amount of coke deposit.24 Other workers, however, questioned the validity of the relationship between catalyst stability and coke, as they observed that the addition of Pt prolonged the catalyst stability while enhancing the coke formation and slightly decreasing the methane conversion.25 Promotion with divalent copper improved, to some extent, both the stability and the catalytic performance.26 For example, at 1023 K, Cu-Mo/ HZSM-5 yielded a maximum CH4 conversion of 10.1% with a benzene selectivity of 94.9%, versus a 7.4% conversion and a 92.7% selectivity for Mo/HZSM-5. Vanadium addition was shown to be detrimental to both CH4 conversion and aromatics selectivity.27 Conversely, addition of Zr, W, Fe, or Co to Mo/HZSM-5 was found to favor higher conversion and/or selectivity toward aromatics, often combined with a reduction in coke formation.9,28 As methane dehydrocyclization is an endothermic equilibrium-controlled reaction, temperatures in excess of 973 K are required to yield meaningful conversions into aromatics in the CR mode. However, such severe conditions not only are favorable for deactivation through coke deposition but also are detrimental to the integrity of the catalyst through sintering or sublimation, for example.19-23 Despite major efforts in investigating a large number of potential catalysts, literature data indicate that conversions obtained at 873 K or below have never exceeded 2.5%7,20,25,26,29-32 which is one-half of the thermodynamic equilibrium conversion, i.e., 5.2%.

To achieve economically viable methane conversions, it is therefore necessary that, in addition to improving the catalyst performance, the thermodynamics barrier be circumvented. To this end, we embarked on an investigation of nonoxidative methane dehydrocyclization using a catalytic membrane reactor (CMR) with the anticipation that continuous withdrawal of hydrogen through a permselective membrane will not only eliminate the thermodynamic constraints but also increase the reaction rate.33 In a previous study,32 we investigated low-temperature oxygen-free methane dehydrocyclization at 873 K in a catalytic membrane reactor containing a Ru-Mo/HZSM-5 catalyst. Selective withdrawal of coproduced hydrogen through the membrane did, indeed, lead to an overshoot in benzene conversion slightly above the thermodynamic value. However, despite the improvement in the methane conversion into benzene, a pronounced activity loss, associated with enhanced accumulation of coke in the absence of hydrogen, was obtained. The aim of the present study is to gain further insight into oxygen-free methane aromatization in a membrane catalytic reactor at low temperature using an improved Ru-Mo/HZSM-5 catalyst formulation. The current catalyst exhibited not only enhanced activity under both CR and CMR conditions, but also remarkable stability under CR operation. In addition, the role of hydrogen in the reversibility of deactivation in the CMR mode and its effect on the nature of minor products such ethane, ethylene, toluene, and naphthalene are discussed. 2. Experimental Section 2.1. Synthesis and Pretreatment of Ru-Mo/ HZSM-5 Catalyst. A 0.5% Ru-3% Mo/HZSM-5 catalyst was prepared by incipient wetness coimpregnation of the ammonium form of the zeolite [NH4ZSM-5] (Si/Al ) 15, supplied by Zeolyst) using the required amount of aqueous ammonium heptamolybdate [(NH4)6Mo7O24‚ 4H2O] and ruthenium chloride, both supplied by Aldrich. Subsequently, the catalyst was air-dried at ambient temperature for 12 h and then for 2 h at 393 K. It was finally calcined in air for 4 h at 873 K. The solid samples were pressed, crushed, and sieved to separate catalyst granules in the size range 20-35 mesh for subsequent use in aromatization reactions. Before exposure to reactant, the catalyst underwent gradual heating under argon in temperature-ramped

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Figure 2. Hydrogen permeation obeying Sievert’s law. Table 1. Methane Conversion to Benzene in CR Mode at Different CH4 Hourly Space Velocities and Various Reaction Times at 873 K FM/W

tR (min)

XB (%)

130

65 1500a 46 700a 18 250a 20 140a

0.08 4.15a 0.35 3.80a 0.30 2.55a 0.63 1.85a

270 490 770 a

Figure 3. Methane conversion over Ru-Mo/HZSM-5 catalyst at different methane space velocities without hydrogen permeation (CR mode) and 873 K.

Maximum methane conversion obtained.

Table 2. Methane Conversion into Benzene and Rate of Benzene Formation at 873 K and FM/W ) 270 mL (STP)‚h-1‚g-1. Comparison with Literature Data catalyst

reactor

XB (%)

FB/W

source

0.5% Ru-3% Mo-HZSM-5

CR CMR CR CMR CR CR

3.8 9.0 2.5 5.8 2.6 2.8

1.49 3.92 1.10 2.70 0.22 0.23

this work

0.5% Ru-3% Mo-HZSM-5 1% Pt-HZSM-5 1% Os-HZSM-5

32 29 29

mode up to 873 K. It was maintained at this temperature for 4 h. Then, the temperature was increased to 973 K, and the catalyst was treated in flowing air for 30 min. The gas flow was then switched to argon, and the catalyst cooled to 573 K. Finally, pure hydrogen was used for 30 min for catalyst reduction. 2.2. Reaction. As shown in Figure 1, the experimental setup consisted of a 12-mm-i.d. commercial membrane reactor hosting four 178-mm-long and 2.38-mmo.d. Pd-coated tantalum/niobium alloy tubular membranes with high hydrogen permeability from REB Research & Consulting (Ferndale, MI). The reactor, loaded with 1.51 g of catalyst, was connected to a gas feeding unit and analytical equipment. The reaction temperature, 873 K, was controlled by a sheathed thermocouple embedded in a 35-mm-high catalyst layer. No higher temperatures were used because of the thermal stability limitations of the Pd-based membranes. After catalyst pretreatment, a 90:10 v/v% CH4/ Ar mixture was fed to the reactor at several mass-flowcontrolled rates sweeping the methane hourly space velocity range of 130-770 mL (STP)‚h-1‚g-1. All methane aromatization tests were run at atmospheric pressure. To assess the impact of hydrogen permeation, experiments were run in CR mode without H2 permeation, in

Figure 4. Maximum methane conversion over Ru-Mo/HZSM-5 catalyst versus methane space velocity in CR and CMR modes at 873 K.

CMR mode with H2 permeation, and in a series of CRCMR cycles. In CMR mode, a vacuum pump was connected to the permeation zone to maintain the pressure at ca. 0.2 Pa, thus providing a strong enough driving force for withdrawal of the produced hydrogen from the reaction zone. The gaseous reaction products were analyzed by means of a Perkin-Elmer gas chromatograph equipped with a flame ionization detector (FID) connected to a GS-Q capillary megabore column (30 m long and 530 µm i.d.) supplied by J&W Scientific and a thermal conductivity detector (TCD) connected to a Carboxen 1010 capillary column (30 m long and 530 µm i.d.) supplied by Supelco. All detected products in the reactor exit stream were analyzed: benzene, toluene, naphthalene, ethylene, and ethane. Benzene was the major hydrocarbon product, whereas naphthalene was detected in trace amounts mainly during CMR operation. Naphthalene was ignored in the carbon balance calculations. Also discounted in the carbon balance was the carbon contributed by the coke deposits, which were found to be negligible. After 4000 min of reaction in CR and CMR modes at 873 K, the increase of the catalyst weight represented less than 0.1% of the carbon contained in the total amount of methane used. Methane conversion was thus calculated based on the carbon

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Figure 5. Time evolution of methane conversion to benzene over Ru-Mo/HZSM-5 catalyst at 270 mL (STP)‚g-1‚h-1 in cycled CRCMR modes.

content of benzene, toluene, ethylene, and ethane. The calculated methane conversion can be thought of as an integral conversion arising solely from the gaseous organic products. It is important to mention that blank experimental tests have shown that no CH4 conversion occurs on the wall of the membrane in the absence of the catalyst. 2.3. Permeation Test. Hydrogen permeability was investigated experimentally to verify the selectivity of the membrane toward hydrogen. Permeation rates were measured using the experimental setup described in the previous section at 673, 773, and 873 K with a H2 partial pressure in the permeation side equal to 101 kPa. Figure 2 illustrates that the hydrogen permeation rate, J, is proportional to the difference between the square roots of hydrogen partial pressure in the reaction and permeation sides, thus obeying Sievert’s law

J ) J0eED/RT

(x

r PH 2

xP

p H2

)

(1)

where ED ) 20.4 kJ‚mol-1 and J0 ) 1.07 × 10-4 m3 (STP)‚s-1‚m-2‚Pa-0.5. These results compare quite well with those reported in our earlier work:32 ED ) 21.2 kJ‚mol-1 and J0 ) 1.12 × 10-4 m3 (STP)‚s-1‚m-2‚Pa-0.5. 3. Results and Discussion Tables 1 and 2 summarize some of our main results, along with the few data available in the open literature on methane aromatization at 873 K. Note that the thermodynamic equilibrium conversion, XeB, of methane into benzene at 873 K is 5.2%. As shown in Table 1, the maximum methane conversion into benzene XB,

i.e., the main gaseous product, decreases with an increase in methane space velocity, FM/W. From Table 2, two important features of this study are also worth noting. First, the current synthesis and pretreatment conditions of 0.5% Ru-3% Mo/HZSM-5 yielded higher maximum XB values than in our previous study: 73% XeB versus 48% XeB in CR mode and 173% XeB versus 112% XeB in CMR mode.32 Second, the maximum benzene conversion achieved in CMR mode in the present work outperformed the maximum conversions previously published for the same temperature,29,32 whether in CR or CMR mode and irrespective of the catalyst. 3.1. CR Mode. As reported in previous works, methane aromatization in an oxygen-free environment proceeds via an induction period prior to the formation of hydrocarbon products, namely, ethylene, ethane, benzene, toluene, and naphthalene. Figure 3 shows that, once the induction period is over, the Ru-Mo/HZSM-5 catalyst activity levels off at conversions parametrized by the methane space velocity (or contact time). In accordance with literature findings,7,32 it is seen that the lower the methane space velocity, the longer the induction period but the higher the ultimate methane conversion. In our previous study,32 the Ru-Mo/ HZSM-5 catalyst underwent deactivation regardless of the methane space velocity. However, the current catalyst exhibited remarkable stability. In fact, no significant activity decline was detected during a continuous operation that lasted for as long as 1400 min. At the highest methane space velocity, i.e., 770 mL (STP)‚h-1‚g-1, the methane conversion decreased only slightly from its maximum value of 1.85-1.75% over a period of 1400 min (Figure 3). 3.2. CMR Mode. The evolution of the maximum methane conversion as a function of methane space velocity is depicted in Figure 4 in which the CR and CMR modes are compared. As can be seen, the effect of hydrogen permeation on methane conversion is quite modest at high methane space velocity. Above ca. 600 mL (STP)‚h-1‚g-1, the methane conversions for the two modes are almost identical. The CMR mode becomes increasingly advantageous when the methane space velocity decreases, particularly below ca. 400 mL (STP)‚h-1‚g-1. Therefore, most of the reaction tests reported hereafter were carried out at a methane space velocity of 270 mL (STP)‚h-1‚g-1 or lower. The typical time evolution of the benzene conversion during six successive CR-CMR cycles spanning a time interval of 6200 min is presented in Figure 5. One CR step followed by one CMR step defines a cycle. In the first cycle, immediately after switching from CR to CMR, the benzene conversion climbs sharply to culminate at 9%. As long as H2 was bled across the membrane, the benzene conversion decreased steadily from 9 to 7.4% after 700 min (see inset of Figure 5). Thus, over time, the beneficial effect of H2 permeation in shifting the equilibrium toward benzene and hydrogen production will diminish as a result of increased deactivation. In all likelihood, under hydrogen-starved conditions during CMR operation, carbonaceous species accumulate at a higher rate, blocking catalytically active sites and thus causing gradual deactivation.35 The availability of hydrogen to hydrogenate these surface species or to adversely affect their formation during the CR mode might explain the slow deactivation rate under these conditions, as discussed earlier. The presence of hydrogen in the reaction medium enables the catalyst

Ind. Eng. Chem. Res., Vol. 41, No. 10, 2002 2375 Table 3. Dual-Active-Site Proposal active sites S1

active sites S2

active under CR and CMR modes of operation rate of benzene formation over S1 is higher under CMR than under CR mode of operation after reaching steady state under CR mode, S1 sites do not deactivate under both working conditions

active only under CMR mode of operation deactivated by carbonaceous deposit during CMR mode of operation deposit reacts slowly under CR mode

Figure 6. Peak (CMR) and plateau (CR) methane-to-benzene conversions of Figure 6.

Figure 8. Time evolution of methane conversion to benzene over Ru-Mo/HZSM-5 catalyst at 270 mL (STP)‚g-1‚h-1 in cycled CRCMR modes: effect of hydrogen on stream.

Figure 7. Time evolution of methane conversion to benzene over Ru-Mo/HZSM-5 catalyst at 130 mL (STP)‚g-1‚h-1 in cycled CRCMR modes.

to self-clean its surface so that a longer catalyst lifetime can be achieved before regeneration is required.36 Similar interpretations were put forth in the study of ethane conversion over Zn/HZSM-5, where the hydrogen formed during aromatization was believed to play a key role in the scission of fragments from the carbonaceous species.37 The second cycle in Figure 5 commenced with a switch back to the CR mode, i.e., H2 permeation was again suspended. At this stage, methane conversion into benzene, XB, collapsed sharply until it reached the same value as obtained in the continuous CR mode at the same methane space velocity of 270 mL (STP)‚h-1‚g-1 discussed earlier (Figure 3, open square symbols). Returning again to H2 permeation gave rise to a sudden increase in methane conversion, without however, reach-

ing the same level as in the previous cycle. Again, because of hydrogen impoverishment, XB started to diminish. Sequential CR and CMR operations over a long period of time led to a saw-toothed conversion profile as shown in Figure 5. It is noted that, once permeation was off, XB invariably reached the same plateau corresponding to the continuous CR operation at the same methane flow rate (Figure 6). On the contrary, under CMR conditions, the maximum methane conversion decreased from one cycle to the next (Figures 5 and 6). This indicates that, even though the methane conversion reaches the same level under CR conditions, the catalyst surface does not necessarily recover the same state. Otherwise, the maximum conversion under CMR operations would also have been the same for different cycles. Consistent with this inference, the maximum conversion during CMR operation was found to depend on the length of the preceding CR period. Figure 5 shows that, when the CR steps are relatively short, the maximum conversion of methane to benzene during successive CMR operations decreases gradually, most probably because the surface is not allowed enough time under CR conditions to be fully restored to its original state, i.e., the surface state at the end of the very first CR step. This important statement will be further supported later. On the contrary, the steady-state conversion under CR mode does not seem to depend on the preceding CMR step, as practically the same conversion is reached regardless of the number of cycles and the length of the CMR steps. At first glance, these two observations do not seem to be compatible with each other. For example, because deactivation occurs under CMR mode, one would expect a decreasing methane conversion under successive CR operations. To provide a reasonable explanation of such

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Figure 9. Time evolution at 270 mL (STP)‚g-1‚h-1 in cycled CR-CMR modes of (a) benzene-to-ethylene ratio (BYR), (b) benzene-toethane ratio (BAR), (c) ethylene-to-ethane ratio (AYR), and (d) benzene-to-toluene ratio (BTR).

observations, it is proposed that two active sites, S1 and S2, are present. The main properties of these sites are provided in Table 3. During the very first CR step, the transformation of methane into benzene takes place on S1 sites with a steady-state conversion of 4%. During the subsequent CMR operation, the sudden increase in methane conversion is due to the combined effect of an increase in the reaction rate over S1 sites and the occurrence of methane aromatization over S2 sites. This is followed by a gradual decrease of methane conversion that can be attributed to the selective deactivation of S2 sites by carbonaceous deposits. This is consistent with the experimental observation that conversion under the CMR mode of operation tends to level off at about 5.5% regardless of the number of cycles (Figure 5). This level would correspond to the conversion of methane over S1 sites under CMR conditions, S2 sites being completely deactivated. When the operation is switched back to the CR mode, the conversion drops quickly to its steady-state level of 4%. It is believed that, under these conditions, only S1 sites are active in

converting methane to benzene, whereas S2 sites are inactive. However, because of the increased hydrogen concentration in the gas phase, S2 sites are slowly regenerated via hydrogenation of the carbonaceous deposit. Even if this reaction generates benzene, its rate is assumed to be much slower than that of methane aromatization over S1, and it will not have any significant effect on the observed methane conversion. The resistance to deactivation of S1 sites provide a straightforward explanation of the fact that the steady-state conversion of methane to benzene under CR conditions is constant regardless of the number of cycles and the length of the CMR steps. Because the regeneration of S2 sites under CR conditions is slow because of the relatively low partial pressure of hydrogen, if this step is not long enough for complete restoration of such sites, the maximum conversion in the following CMR step will be adversely affected. This assertion is consistent with the data shown in Figure 5. As discussed hereafter, to further substantiate this contention, two additional series of

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experiments were designed and carried out. The first series of experiments consisted simply of carrying out CR-CMR cycles of reactions with much longer CR steps, i.e., about 1000 min. Figure 7 presents the results obtained at 873 K, using for illustration the lowest methane space velocity, i.e., 130 mL (STP)‚h-1‚g-1, because as shown in Figure 4, this corresponds to the greatest difference between the maximum conversions in CR and CMR modes. Similar behavior was observed when a methane space velocity of 270 mL (STP)‚h-1‚g-1was used. It is seen that, provided that the CR steps are long enough, the maximum conversion of methane to benzene during the CMR steps reaches the same value of 13.5%. This experiment provides strong evidence that the activity of the S2 sites can be fully restored if the CR operation is long enough. The second set of experiments is based on the idea that the rate of restoration of the S2 sites must be dependent on the hydrogen concentration in the gas phase during the CR operation. From a more practical point of view, to maximize process productivity in a CRCMR cycling operation, it would be highly desirable to shorten, as much as possible, the CR period required to regenerate the S2 sites. This could, in principle, be achieved by purposely adding hydrogen into the feed for a very short period of time during the CR steps. A series of CR-CMR cycles was carried out, as described above, at 873 K using a methane space velocity of 270 mL (STP)‚h-1‚g-1. However, the CR step consisted of using as the feed a mixture of 45% CH4, 5% Ar, and 50% H2 for 5 or 30 min before restoring the standard flow of 90:10 CH4/Ar. As shown in Figure 8, the maximum methane conversion of 9% during CMR was obtained almost immediately, indicating that, in the presence of high concentrations of hydrogen, S2 sites can be readily regenerated. Expectedly, adding too much hydrogen to the gas phase would result in shifting the equilibrium in the undesired direction. This is why, in Figure 8, XB fell below the CR activity plateau of 4% before leveling off. Even though the precise nature of the proposed active sites remains largely unknown, it is possible to provide tentative assignments based on previous work in this field. There are two widely accepted ideas regarding oxygen-free methane aromatization over Mo/HZSM-5 supported by some compelling experimental evidence. These are that (i) under reaction conditions, Mo occurs as molybdenum carbide (say Mo2C) and (ii) the reaction is bifunctional, i.e, requires Mo2C and Brønsted acid sites. In addition, any active site would exhibit a higher intrinsic activity under CMR than under CR conditions, because of the negative effect of hydrogen pressure on the reaction rate. Our proposed active sites S1 and S2 are no exceptions, i.e., each site is a combination of some Mo atoms and Brønsted acid sites. The difference between them is that S1 might correspond to the more active Mo atoms than S2. Borrowing from catalysis over metals, it is suggested that the S1 sites involve Mo atoms located on corners and edges of Mo2C crystallites, whereas the S2 sites correspond to less active atoms on flat surfaces prone to carbon deposit. Thus, S2 sites are triggered only under hydrogen-starved conditions. 3.3. Minor Aromatization Coproducts. As mentioned earlier, regardless of whether the catalyst is used under CR or CMR operation, the selectivity to benzene calculated on the basis of organic products in the gas

phase exceeds 90%. However, the relative amounts of minor products depend strongly on the prevailing working conditions. The removal of hydrogen during CMR operation should favor the formation of unsaturated hydrocarbons, i.e., ethylene and polynuclear aromatics such a naphthalene.33 Figure 9 shows, in order, the evolution of the benzene-to-ethylene ratio (BYR), the benzene-to-ethane ratio (BAR), the ethylene-to-ethane ratio (YAR), and the benzene-to-toluene ratio (BTR) under the same cycling conditions as in Figure 5. It is seen that, within a CR-CMR cycle, more ethylene (Figure 9a) and less ethane (Figure 9b) was detected in CMR than in CR mode. However, in all cases, ethane was 3-20 times more abundant than ethylene (Figure 9c). At such a high temperature (873 K), in the presence of a noble metal, hydrogenation is strongly favored both kinetically and thermodynamically, even under very low hydrogen partial pressure. Figure 9c shows that, upon switching from CR to CMR conditions, the amount of ethylene relative to ethane undergoes a sudden burst and then decreases. Because, under CMR operation, the partial pressure of hydrogen is very low, but constant, the decrease in YAR is presumably due to enhanced hydride transfer from the accumulating carbonaceous deposits. Finally, although they formed in very small quantities, toluene (Figure 9d) and naphthalene (data not shown) were detected in larger amounts during CMR operation. 4. Conclusion The oxygen-free methane dehydrocyclization into benzene was carried out at 873 K over a Ru-Mo/ HZSM-5 catalyst in a catalytic membrane reactor (CMR). In the absence of permeation (CR mode) at a methane space velocity of 270 mL (STP)‚h-1‚g-1, the methane to benzene conversion reached 73% of its thermodynamic equilibrium value. Compared to literature data, the current catalyst exhibited not only higher activity but also, most remarkably, excellent stability. In a test of about 100 h of continuous CR operation, the catalyst showed little, if any deactivation. In CMR mode, hydrogen permeation entrained first an increase in benzene conversion up to 173% of the equilibrium value, followed by a relatively sharp decrease due to the accumulation of carbonaceous species under very low hydrogen pressure. This deactivation was found to be fully reversible under a subsequent CR step, provided that it lasted for a long enough period of time. Catalyst activity was envisioned using a two-site concept, providing a rational explanation of all of the experimental observations. Symbols FB/W ) rate of benzene formation, mL (STP)‚h-1‚g-1 FM/W ) CH4 hourly space velocity, mL (STP)‚h-1‚g-1 tR ) reaction time (min) XB ) methane conversion into benzene XeB ) thermodynamic equilibrium methane conversion into benzene

Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds pour la formation de chercheurs et d’aide a` la recherche (Que´bec), and the Natural Gas Technologies

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Received for review December 5, 2001 Revised manuscript received March 8, 2002 Accepted March 12, 2002 IE010977S