HZSM-5

Ind. Eng. Chem. Res. 2003, 42, 3203-3209

3203

Kinetics of Methane Nonoxidative Aromatization over Ru-Mo/ HZSM-5 Catalyst Maria C. Iliuta, Ion Iliuta, Bernard P. A. Grandjean,* and Faı1c¸ al Larachi Department of Chemical Engineering & CERPIC, Laval University, Que´ bec, Canada G1K 7P4

Kinetic study of oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 was carried out in a fixed-bed differential reactor under atmospheric pressure. The reaction temperature was varied between 873 and 973 K, and the methane feed fraction ranged from 50 to 90% v/v. A plausible reaction pathway involved in oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 was proposed. Different models based on LangmuirHinshelwood-Hougen-Watson reaction mechanisms were tested to correlate the kinetic data. Among these models, the one postulating that the surface dehydrogenation reaction of adsorbed methane is rate-determining was successful in terms of physical likeliness (thermodynamic and kinetic parameters) and statistical goodness of fit. The activation energy, adsorption enthalpy, and entropy frequency were evaluated by means of Arrhenius or Van’t Hoff relationships. Introduction The direct conversion of methane primarily into hydrogen, and to a certain extent to other commodity products, such as aromatics (benzene, toluene, naphthalene), constitutes an important challenge in heterogeneous industrial catalysis. Compared with methane oxidative coupling and methane partial oxidation, the advantages of methane nonoxidative conversion lie in its higher selectivity toward higher hydrocarbon products as well as easy separation of the reaction main product, i.e., benzene, from the methane feed. In addition, problems stemming from side reactions such as methane complete oxidation can be circumvented under nonoxidative reaction conditions. On the contrary, severe thermodynamic limitations resulting in low conversions, especially at lower temperatures, and sometimes catalyst deactivation, through coke formation and deposition, have been identified as the main setbacks for methane nonoxidative dehydrogenation and aromatization. The underlying mechanisms of methane aromatization are still not completely well understood, and further fundamental studies are required on this important topic.1 Over the years, much of the literature connected to nonoxidative dehydrogenation and aromatization of methane focused on improving potential catalytic materials. Compared to other metal-doped HZSM-5, molybdenum-containing HZSM-5 materials rank among the most promising catalytic systems for methane dehydrogenation and aromatization and have been under intensive scrutiny for the past 10 years.1-12 However, all these studies show that Mo/HZSM-5 catalyst activity and stability need to be improved. Modification of Mo/HZSM-5 by adding various transition metal promoters, e.g., Co, Cs, Cu, Fe, Ir, La, Li, Pd, Pt, Ru, V, W, and Zr, has been shown to enhance both stability and activity.13-19 The role of Pt as a promoter for Mo/HZSM-5 is somewhat controversial. Pt was reported not to affect the catalytic activity, while it enhanced the catalyst stability by reducing the amount * To whom correspondence should be addressed. E-mail: [email protected].

of coke deposit.15 Other workers, however, questioned the validity of the relationship between catalyst stability and coke, as they observed that addition of Pt prolonged catalyst stability, while it enhanced coke formation and slightly decreased methane conversion.16 Promotion with divalent copper improved to some extent both stability and catalytic performance.17 Vanadium addition was shown to be detrimental to both CH4 conversion and aromatics selectivity.18 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 coke reduction.9,19 Properly dispersed Ru over Mo/HZSM-5 proved to be a promising promoter for improving the conversion into benzene as well as the catalyst stability.12,13,20 In contrast, in the open literature, very little information is available about the kinetics of methane dehydrogenation and aromatization. Rival et al.21 developed a simplified phenomenological reaction-permeation kinetic model describing oxygen-free methane aromatization in a fixed-bed conventional configuration (i.e., without hydrogen permeation) as well as with hydrogen permeation in a membrane catalytic reactor configuration. However, no attempt was made to model the catalyst activity decline though the authors observed rapid deactivation of the catalyst under permeation conditions. Moreover, the model was descriptive of virtual steady-state conditions around the maximum methane conversion attained between the induction and the deactivation steps with the membrane in duty. Another restriction was that the kinetic model was validated based on conversion measurements at 873 K only, wherein benzene conversion was still rather low (2.5%). Based on 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 initially available in the zeolite channels likely as MoO3,22 or as (Mo2O5)2+ dimers23-26 is first converted into active Mo2C carbide species,6 which are believed to represent the key compound in methane activation.10 These active sites catalyze methane activation into CHx (x < 4) fragments, which are in turn transformed by

10.1021/ie030044r CCC: $25.00 © 2003 American Chemical Society Published on Web 06/11/2003

3204

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003

oligomerization into mainly ethylene and ethane on the acidic sites of the HZSM-5 crystallites.6,7,10,23 Such products are further converted into aromatics (benzene, toluene, naphthalene).27 In our previous works,12,20 the oxygen-free methane aromatization was studied in a membrane catalytic reactor at temperatures up to 973 K using a Ru-Mo/ HZSM-5 catalyst. Methane aromatization was evaluated under two sets of conditions: without hydrogen permeation in a conventional fixed-bed catalytic reactor and with hydrogen permeation in an inert membrane catalytic reactor. The role of hydrogen in the reversibility of deactivation in the membrane mode, and its effect on the nature of minor end products and byproducts, such ethane, ethylene, toluene, and naphthalene, was discussed. In this study, a kinetic study of oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 was carried out using kinetic data obtained in a conventional fixed-bed reactor under nondeactivating conditions. The reaction was conducted at atmospheric pressure. The reaction temperature was varied between 873 and 973 K, while the methane concentration ranged from 50 to 90% v/v in the feed stream. A detailed reaction pathway involved in oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 was proposed. The low conversions allowed assumption of a reasonable differential reactor approximation.28 Different model scenarios based on LangmuirHinshelwood-Hougen-Watson reaction mechanisms were tested. The kinetic models were restricted to the steady-state regime between the induction and the deactivation steps.

molar ratio was varied by changing the methane concentration between 50 and 90% v/v. In all these runs, the hourly gas space velocity (HGSV) was kept at 270 mL(STP)‚h-1‚g-1. The flow rates were regulated with a mass flow controller. All methane aromatization tests were run at atmospheric pressure. Analysis. The gaseous reaction products were analyzed on-line by means of a Perkin-Elmer gas chromatograph equipped with a flame ionization detector 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 connected to a Carboxen 1010 capillary column (30 m long and 530 µm i.d.) supplied by Supelco. All detected gaseous products in the reactor exit stream were quantified: benzene, toluene, ethylene, and ethane. Benzene was the dominant hydrocarbon product; the selectivity to benzene exceeded 90% in all kinetic tests. The coke deposition was found to depend strongly on the temperature, methane flow rate, and reaction time. In the reaction conditions used in this work (methane flow rate, HGSV ) 270 mL(STP)‚h-1‚g-1; temperature, T ) 873-973 K), the kinetic measurements were performed when the catalyst activity stabilized to a constant level (800-1400 min of reaction on stream). In these conditions, the carbon contributed by the coke deposits was found to be negligible and was discounted in the carbon balance. For instance, after ∼4000 min of reaction on stream, the increase in catalyst weight represented less than 0.1% of the carbon contained in the total amount of methane consumed at 873 K and ∼0.2% at 973 K. Taking into account that benzene was the major reaction product, methane conversion into benzene was calculated based on benzene concentration.

Experimental Section

Experimental Results Assessment of Mass-Transfer Effects. In any kinetic model, the resistance due to film (external) and pore (internal) diffusion must be minimized in order to propose an intrinsic rate. Usually, in the submillimeter particle size range, internal mass-transfer retardation does not affect gas-phase catalytic reactions.28 Here it is considered that the oxygen-free methane dehydrogenation and aromatization reaction is internal diffusion free because the catalyst particle sizes range between 400 and 800 µm. For estimation of the external diffusion effects, two series of runs were undertaken at 873 K and 1 atm with different amounts of catalyst and a feed stream consisting of a 90:10 v/v% methane-to-argon mixture. In one series, the amount of catalyst introduced in the reactor was 3 g, and in the other, it was 1.5 g. The feed flow rate was correspondingly adjusted to yield constant HGSV for methane, FCH4/W, for the two amounts of catalyst. These resulted in HGSV evolving in the range 130-770 mL(STP)‚h-1‚g-1. The results showed that the conversions for both series at constant FCH4/W are feed flow rate independent, thus precluding the presence of any external mass-transfer resistance in the conditions tested in this work. Similar results also indicated that the external mass-transfer limitations can be considered negligible.29 To achieve meaningful conversion levels unaltered by diffusion, the following kinetic investigation was carried out at a methane hourly space velocity of 270 mL(STP)‚h-1‚g-1 for different temperatures and methane feed compositions. Catalyst Stability. The current Ru-Mo/HZSM-5 catalyst exhibited remarkable stability in the reaction

Synthesis and Pretreatment Method of the RuMo/HZSM-5 Catalyst. The method is similar to that described previously.12,20 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 air-calcined 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, i.e., methane, the catalyst underwent gradual heating under argon in temperature-ramped mode up to 873 K. It was maintained at this temperature for 4 h. Then, 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 at 573 K. Apparatus and Procedure. The experimental setup consisted of a 17-mm-i.d. fixed-bed reactor. This reactor assembly could be easily configured for the reaction test with permeation by a simple replacement of the inner tube with a membrane.12 The reactor loaded with 3 g of catalyst was connected to a gas feeding unit and analytical equipment. Kinetic runs were carried out at five different temperatures, viz., 873, 898, 923, 948, and 973 K. At each temperature, the methane-to-argon

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3205

Figure 1. Time evolution of methane conversion to benzene over Ru-Mo/HZSM-5 catalyst at different temperatures (873-973 K) and 270 mL(STP)‚g-1‚h-1.

Figure 3. Experimental reaction rate as a function of methane concentration at different temperatures.

creased temperatures,r methane inlet concentrations, or both have a significant effect on oxygen-free methane dehydrogenation and aromatization. For each temperature, the space-time conversion data have been analyzed and the rates of reaction were obtained by differential analysis of the plug-flow reactor equation:28

rexp ) dXCH4/d(W/FCH4)

(1)

Kinetic Modeling

Figure 2. Effect of temperature on methane conversion under different methane concentrations.

conditions used. As a matter of fact, no significant activity decline was detected during continuous operations that lasted for as long as 1400 min (Figure 1). In addition, the experimental tests showed that at the hourly gas space velocity used in this work coke formation can be practically ignored in the steady-state conditions of reaction for which the proposed kinetic model was validated. As reported in previous work, methane aromatization in an oxygen-free environment proceeds via an induction period prior to the formation of hydrocarbon products: ethylene, ethane, benzene, toluene, and naphthalene. Figure 1 shows that the induction period is not affected by temperature. However, the induction period is strongly dependent on methane space velocity (not shown here). The lower the methane hourly space velocity, the longer the induction period, in agreement with literature findings.7,21 Kinetic Data. The effect of temperature on methane conversion under different methane to argon molar ratios is depicted in Figure 2. An increase of temperature from 873 to 973 K brought about a significant effect on oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5. The figure also reveals that methane conversion is more pronounced the more diluted the feed stream. Figure 3 shows the experimental reaction rates versus methane concentrations at different temperatures. In-

Mechanism and Rate Equations. The process of methane activation is still the focus of debate in the literature, and consequently, many kinds of reaction mechanisms have been proposed. Some works suggested that the reaction intermediate is ethylenesthe sole primary product, which is further converted to benzene. Other works suggested that the key intermediate in the formation of benzene is ethane, which is dehydrogenated to ethylene, and ethylene is further converted to benzene. All elementary reaction pathways in the actual proposed mechanisms are speculative, but it seems however that there is an almost common agreement that ethylene is the sole reactant in benzene formation. Therefore, in the proposed mechanism, the coupling of CH2 species was considered the main reaction pathway in ethylene formation and this was the intermediate in benzene production. Possible steps related to the ethane transformation into ethylene, CH3 decomposition to CH2, or both have not been considered for the sake of simplicity. As shown in the Introduction, the complicated mechanistic picture of this nontrivial bifunctionally catalyzed reaction is one of the reasons why so little information about the reaction kinetics has been published. It was demonstrated that the active Mo2C carbide species catalyze the methane activation into CHx (x < 4) fragments, while the HZSM-5 acidic sites catalyze their oligomerization first toward ethylene (ethane), which are further converted into aromatics (benzene, toluene, naphthalene). To take into account the role played by both the Mo active sites and the zeolite acid sites in the reaction, a double-site mechanism might be beneficial for consideration in the model derivation. Considering S1 the active Mo sites and S2 the acid sites of the zeolite,

3206

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003

the following series of elementary steps can be proposed:

1.

CH4 + S1 T CH4S1

(2)

2.

CH4S1 T CH2S1 + H2

(3)

3.

CH2S1 T CH2 + S1

(4)

4.

CH2 + S2 T CH2S2

(5)

5.

1 CH2S2 T C2H4 + S2 2

(6)

6.

1 1 1 CH T CH + H 2 2 4 6 6 6 2 2

(7)

However, it was noticed that assuming the surface reaction 3 as the rate-determining step (as will be demonstrated later) led to a rate equation expression similar to the one obtained considering a single-site mechanism. Moreover, in the case of a double-site mechanism, it was not possible to determine all equilibrium constants associated to the proposed model. Also, considering steps 3-5 (eqs 4-6) as the ratedetermining steps is a much more difficult task, especially due to the lack of information related to the quantitative relation between the S1 and S2 catalytic sites. Because the actual experimental data do not allow proving or disproving which one of these mechanisms is the more plausible, we decided to choose the simplest one, i.e., the single-site mechanism. The kinetic model was validated in the steady-state conditions, when no evident deactivation of the catalyst was observed. In the absence of gas-film or pore diffusional resistance, a mechanism involving adsorption/ desorption steps and surface reactions will control the rate of catalytic reaction. Different models based on Langmuir-Hinshelwood mechanisms were used to correlate the kinetic data using the approach suggested by Hougen and Watson.30 The following assumptions were made to establish the model equations: (i) all active sites (S) whereon surface reaction occurs are identical; (ii) all elementary steps involve partial first order with respect to every respective reactant; (iii) external mass transfer and intraparticle resistances are negligible; (iv) no irreversible loss of active sites occurs; (v) the rate equations for the reaction steps were derived on the basis of the mass action law.31 The final reaction rate expressions were obtained by manipulation of the following series of elementary steps:

1.

K1

CH4 + S 798 CH4S CH4S 798 CH2S + H2

3.

1 CH2S 798 C2H4 + S 2

4.

K4 1 1 1 C2H4 798 C6H6 + H2 2 6 2

K3

3′. 3′′.

K5

CH2S 798

K3 ) K6/K5

(10)

(11)

Step 3 was obtained by merging CH2 dimerization and C2H4 desorption steps for the sake of simplification of

(12) (13)

Many possible rate equations can be written for the oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 catalyst based on single-site mechanism. In the absence of any experimental evidence allowing capture on line of the true intermediate species during this reaction, a reaction pathway such as the one depicted by steps 1-3 is considered as a good starting point for the conceptualization of this reaction. A series of submodels can be derived from this reaction pathway depending on the rate-limiting step to assume the physical likeliness of the resulting kinetic and thermodynamic parameters and also the statistical goodness of fit (least-squares criterion). After several attempts, three plausible models have been screened out and will be discussed in what follows.

model 1: rate-determining step is assumed to be the methane adsorption (8) 1 1/6 P P3/2 K P C 6H 6 H 2 (14) r 1 ) k1 K3 K3 1/6 1/6 3/2 1/2 1+ P P + P P K2K4 C6H6 H2 K4 C6H6 H2 PCH4 -

model 2: rate-determining step is assumed to be the surface reaction 9 1 1/6 P P3/2 K P C 6H 6 H 2 r2 ) k2 K3 1/6 1 + K1PCH4 + P P1/2 K 4 C 6H 6 H 2 PCH4 -

(15)

model 3: rate-determining step is assumed to be the CH2 dimerization and desorption (10) 1 1/6 P P3/2 K P C 6H 6 H 2 r3 ) k 3 PH2 + K1PCH4PH2 + K1K2PCH4 PCH4 -

(16)

where

(

K4 ) exp Kp )

(9)

1 CHS 2 2 4

K6 1 1 C2H4S 798 C2H4 + S 2 2

(8)

K2

2.

the kinetic model:

3/2 PC1/66H6,e PH 2,e

PCH4,e

)

∆G4 RT

(

) exp -

(17) ∆G RT

)

(18)

Estimation of Kinetic Parameters. The above rate equations contain three unknown parameters: (k1, K2, K3) or (k2, K1, K3) or (k3, K1, K2). These parameters were estimated by treating the rate data for each temperature separately. The amounts of methane, benzene, and hydrogen were used in the parameter estimation. The molar amounts of methane and benzene were determined from chromatographic analysis, and the amount of hydrogen was stoichiometrically calculated from the global reaction (CH4 T 1/6C6H6 + 3/2H2). As the rate

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3207

equation is nonlinear with respect to these unknown parameters, a nonlinear regression program based on Marquardt’s algorithm was used to obtain a mathematical fit for the above rate equations by minimizing the objective function for the residual sum of squares:32 n

F)

2 - rexp ∑k (rcalc k k )

(19)

The equilibrium constant Kp for the global reaction was calculated using the following expression for the Gibbs free energy (NIST thermodynamic tables33):

∆G ) - (575 - 0.427T ) × 1000/6

(20)

The equilibrium constant K4 was calculated using the following relationships for the Gibbs free energy, heat, and entropy for ethylene to benzene equilibrium reaction 4 (eq 11):

∆G4 ) ∆H4 - T∆S4

(21)

where

∆H4 ) - 12296 + 5.456(T - 298) + 1.516 × -3

2

2

-5

3

3

10 (T - 298 ) - 0.36725 × 10 (T - 298 ) (22) ∆S4 ) 0.434 + 5.456 ln(T/298) + 3.033 × 10-3(T - 298) -0.55087 × 10-5(T 2 - 2982) (23) Discussion The magnitude of the objective function for the residual sum of squares (19) had almost the same value for all proposed models. The kinetic models were therefore tested by two different methods: (i) physicochemical constraints and (ii) residual analysis. Analysis of the experimental data was performed on a mathematical basis, and therefore, it did not account for the thermodynamic significance of the kinetic parameters. From thermodynamic considerations, the rate constant, the activation energy, and the adsorption constants should have positive values. Models 1 and 3 were therefore disqualified on the basis of improper trends with the temperature of the kinetic and adsorptions constants. The fitting results were further analyzed using the statistical criteria suggested by Froment and Bischoff34 in order to evaluate the adequacy of model 2 and the accuracy of the kinetic constants. The residual analysis was based on the following expression of the relative residuals:

RR ) (rexp - rcal)/rexp

(24)

The representation of the relative residuals as a function of methane concentration showed that the values of RR were normally distributed with almost zero mean and exhibited no trend. We can conclude that the rate-determining step for the oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 is the surface reaction of dehydrogenation of adsorbed methane, and model 2 can represent the kinetic data well. The kinetic and equilibrium constants associated with model 2, estimated by nonlinear regression, are tabulated in Table 1. It is evident from this table that with an increase in temperature the rate constant k2 in-

Figure 4. Van’t Hoff and Arrhenius plots for equilibrium and rate constants. Table 1. Estimated Kinetic and Equilibrium Constants Associated to Model 2 temp, K

k2, mol/gcat‚h‚atm

K1, atm-1

K3, atm-1/2

873 898 923 948 973

0.00717 0.0102 0.014 0.019 0.025

2.877 2.197 1.675 1.280 1.029

2.359 2.870 3.020 3.185 3.300

creases. The adsorption constant K1 exhibits, as expected, a decreasing trend with an increase in temperature. The equilibrium constant of CH2 dimerization and C2H4 desorption merged steps (K3) increases with the increase of temperature. Taking into account the definition expression of this constant, K3 ) K6/K5, it is clear that this trend is in agreement with expectation. The ratio between K6 and K5 increases with temperature, even though the adsorption constant (K6) decreases with the increase of temperature, and this is due to the fact that the equilibrium constant of the CH2 dimerization step decreases with temperature, CH2 dimerization being an exothermic process. The kinetic and adsorption constants are temperature-dependent parameters. These constants evaluated and tabulated at various temperatures were used to determine the activation energy, the frequency factor, the adsorption enthalpy, and entropy using Arrhenius and Van’t Hoff relationships:

k2 ) k0,2 exp(- Ea/RT)

(25)

∆Sads,i ∆Hads,i R RT

(26)

ln Ki )

Arrhenius and Van’t Hoff plots are shown in Figure 4. The activation energy was calculated to be 88.2 kJ/ mol, and the frequency factor k0,2 was 1.368 × 103 kmol/ kgcat‚h. The adsorption enthalpy for K1 was -73.4 kJ/ mol, and the corresponding adsorption entropy was -75.3 J/mol‚K. This entropy value is coherent with the Boudart et al.35 chemical-kinetic criteria:

∆Sads < 0

(27)

10 < -∆Sads < 12.2 - 0.0014∆Hads

(28)

A very good correlation is obtained between the observed and predicted rates (MARE ) 4.5%) and

3208

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 ki ) controlling-step rate constant, kmol/kgcat‚h‚atm (i ) 1, 2) or kmol/kgcat‚h (i ) 3) Ki ) equilibrium constant Kp ) equilibrium constant for global reaction, atm2/3 MARE ) mean absolute relative error, MARE ) N - 1∑|1 - ycalc,i/yexp,i| Pj ) partial pressure of component j, atm ri ) reaction rate, kmol/kgcat‚h R ) constant of ideal gas, 8.314 J/mol K ∆Sads ) adsorption entropy, J/mol‚K ∆Si ) entropy of reaction i, J/mol‚K T ) temperature, K W ) mass of catalyst, kg XCH4 ) methane conversion y ) kinetic parameter (as introduced in the expression of MARE)

Figure 5. Experimental versus calculated reaction rate.

Subscripts e ) equilibrium

between observed and predicted methane conversion (MARE ) 1.6%). The experimentally observed rate of reaction and the theoretically predicted rate values at the five temperatures are plotted in Figure 5. Hence, the proposed kinetic model fits very well with our experimental observations. Conclusion Kinetic behavior for the oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 was studied experimentally in a fixed-bed differential reactor under nondeactivating conditions, at atmospheric pressure, reaction temperature varying between 873 and 973 K and methane feed fraction ranging from 50 to 90% v/v. A detailed reaction pathway was proposed, and different models based on Langmuir-Hinshelwood-Hougen-Watson reaction mechanisms were applied to correlate the kinetic data. The results of kinetic modeling suggest that the rate-determining step for oxygen-free methane dehydrogenation and aromatization over Ru-Mo/HZSM-5 is the surface reaction of dehydrogenation of adsorbed methane. The proposed model was found to be in conformity with the trend shown by the kinetic and adsorption constants in the temperature range 873-973 K. A value of 88.2 kJ/mol was calculated for the reaction activation energy. The calculated enthalpy of methane adsorption on the Mo active sites was -73.4 kJ/mol and the corresponding adsorption entropy was -75.3 J/mol K. The kinetic model based on the proposed reaction mechanism predicts very well the measured conversion and reaction rate values. Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the “Fonds pour la formation de chercheurs et d′aide a` la recherche” (Que´bec) is gratefully acknowledged. Nomenclature Ea ) activation energy, J/mol F ) objective function FCH4 ) molar flow rate of methane, kmol/h ∆G ) Gibbs free energy, J/mol ∆Hads ) adsorption enthalpy, J/mol ∆Hi ) heat of reaction i, J/mol

Superscripts calc ) calculated exp ) experimental

Literature Cited (1) Xu, Y.; Lin, L. Recent Advances in Methane Dehydroaromatization over Transition Metal Ion-modified Zeolite Catalysts under Non-oxidative Conditions. Appl. Catal., A 1999, 188, 53. (2) Zhang, C.-L.; Li, S.; Yuan, Y.; Zhang, W.-X.; Wu, T.-H.; Lin, L.-W. Aromatization of Methane in the Absence of Oxygen over Mo-based Catalysts Supported on Different Types of Zeolites. Catal. Lett. 1998, 56, 207. (3) Weckhuysen, B. M.; Wang, D.; Rosynek, M. P.; Lunsford, J. H. Conversion of Methane to Benzene over Transition Metal Ion ZSM-5 Zeolites. I. Catalytic Characterization. J. Catal. 1998, 175, 338. (4) Solymosi, F.; Csere´nyi, A.; Szo¨ke, A.; Ba´nsa´gi, T.; Oszko´, A. Aromatization of Methane over Supported and Unsupported Mo-Based Catalysts. J. Catal. 1997, 165, 150. (5) Liu, W.; Xu, Y.; Wong, S.-T.; Wang, L.; Qiu, J.; Yang, N. Methane Dehydrogenation and Aromatization in the Absence of Oxygen on Mo/HZSM-5: A Study on the Interaction between Mo Species and HZSM-5 by using 27Al and 29Si MAS NMR. J. Mol. Catal., A 1997, 120, 257. (6) Wang, D.; Lunsford, J. H.; Rosynek, M. P. Catalytic Conversion of Methane to Benzene over Mo/ZSM-5. Top. Catal. 1996, 3, 289. (7) Xu, Y.; Liu, S.; Wang, L.; Xie, M.; Guo, X. Methane Activation without Using Oxidants over Mo/HZSM-5 Zeolite Catalysts. Catal. Lett. 1995, 30, 135. (8) Chen, L.; Lin, L.; Xu, Z.; Li, X.; Zhang, T. Dehydrooligomerization of Methane to Ethylene and Aromatics over Molybdenum/HZSM-5 Catalysts. J. Catal. 1995, 157, 190. (9) Wang, L.; Tao, L.; Xie, M.; Xu, G.; Huang, J.; Xu, Y. Dehydrogenation and Aromatization of Methane under Nonoxidizing Conditions. Catal. Lett. 1993, 21, 35. (10) Solymosi, F.; Erdo¨helyi, A.; Szo¨ke, A. Dehydrogenation of methane on supported molybdenum oxides. Formation of benzene from methane. Catal. Lett. 1995, 32, 43. (11) Liu, S.; Wang, L.; Ohnishi, R.; Ichikawa, M. Bifunctional Catalysts of Mo/HZSM-5 in the Dehydroaromatization of Methane to Benzene and Naphthalene XAFS/TG/DTA/MASS/FTIR Characterization and Supporting Effects. J. Catal. 1999, 181, 175. (12) Iliuta, M. C.; Larachi, F.; Grandjean, B. P. A.; Iliuta, I.; Sayari, A. Methane Nonoxidative Aromatization over Ru-Mo/ HZSM-5 in a Membrane Catalytic Reactor. Ind. Eng. Chem. Res. 2002, 41, 2371. (13) Shu, Y.; Xu, Y.; Wong, S.-T.; Wang, L., Guo, X. Promotional Effect of Ru on the Dedrogenation and Aromatization of Methane in the Absence of Oxygen over Mo/HZSM-5 Catalysts. J. Catal. 1997, 170, 11.

Ind. Eng. Chem. Res., Vol. 42, No. 14, 2003 3209 (14) Zeng, J. L.; Xiong, Z. T.; Zhang, H. B.; Lin, G. D.; Tsai, K. R. Nonoxidative Dehydrogenation and Aromatization of Methane over W/HZSM-5-based Catalysts. Catal. Lett. 1998, 53, 119. (15) Chen, L.; Lin, L.; Xu, Z.; Zhang, T.; Li, X. Promotional Effect of Pt on Non-oxidative Methane Transformation over MoHZSM-5 Catalyst. Catal. Lett. 1996, 39, 169. (16) Tan, P.; Xu, Z.; Zhang, T.; Chen, L.; Lin, L. Aromatization of Methane over Different Mo-supported Catalysts in the Absence of Oxygen. React. Kinet. Catal. Lett. 1997, 61, 391. (17) Li, S.; Zhang, C.; Kan, Q.; Wang, D.; Wu, T.; Lin, L. The Function of Cu(II) Ions in the Mo/CuH-ZSM-5 Catalyst for Methane Conversion under Non-oxidative Conditions. Appl. Catal., A 1999, 187, 199. (18) Wang, L.; Xu, Y.; Wong, S.-T.; Cui, W.; Guo, X. Activity and Stability Enchancement of Mo/HZSM-5-based Catalysts for Methane Non-oxidative Transformation to Aromatics and C2 Hydrocarbons: Effect of Additives and Pretreatment Conditions. Appl. Catal., A 1997, 152, 173. (19) Liu, S.; Dong, Q.; Ohnishi, R.; Ichikawa, M. Remarkable Non-oxidative Conversion of Methane to Naphthalene and Benzene on Co and Fe Modified Mo/HZSM-5 Catalysts. Chem. Commun. 1997, 1455. (20) Iliuta, M. C.; Grandjean, B. P. A.; Larachi, F. Methane Nonoxidative Aromatization over Ru-Mo/HZSM-5 at Temperatures up to 973 K in a Palladium-Silver/Stainless Steel Membrane Reactor. Ind. Eng. Chem. Res. 2003, 42, 323. (21) Rival, O.; Grandjean, B. P. A.; Guy, C.; Sayari, A.; Larachi, F. Oxygen-Free Methane Aromatization in a Catalytic Membrane Reactor. Ind. Eng. Chem. Res. 2001, 40, 2212. (22) Solymosi, F.; Szo¨ke, A.; Csere´nyi, J. Conversion of Methane to Benzene over Mo2C and Mo2C/ZSM-5 Catalysts. Catal. Lett. 1996, 39, 157. (23) Borry, R. W., III; Kim, Y. H.; Huffsmith, A.; Reimer, J. A.; Iglesia, E. Structure and Density of Mo and Acid Sites in Mo-Exchanged H-ZSM5 Cataysts for Nonoxidative Methane Conversion. J. Phys. Chem. B. 1999, 103, 5787.

(24) Li, W.; Meitzner, G. D.; Borry, R. W.; Iglesia, E. Raman and X-Ray Absorption Studies of Mo Species in Mo/H-ZSM5 Catalysts for Non-Oxidative CH4 Reactions. J. Catal. 2000, 191, 373. (25) Kim, Y. H.; Borry, R. W.; Iglesia, E. Catalytic Properties of Mo/HZSM-5 for CH4 Aromatization. J. Ind. Eng. Chem. 2000, 6, 72. (26) Kim, Y. H.; Borry, R. W.; Iglesia, E. Genesis of Methane Activation Sites in Mo-exchanged H-ZSM-5 Catalysts. Microporous Mesoporous Mater. 2000, 35, 495. (27) Ding, W.; Li, S.; Meitzner, G. D.; Iglesia, E. Methane conversion to aromatics on Mo/H-ZSM5: Structure of Molybdenum Species in Working Catalysts. J. Phys. Chem. B 2001, 105, 506. (28) Levenspiel, O. Chemical Reaction Engineering; John Wiley & Sons: New York, 1999. (29) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall Int.: New York, 1992. (30) Carberry J. J. Chemical and catalytic reaction engineering; McGraw-Hill: New York, 1976. (31) Temkin, M. I. Kinetics of some industrial heterogeneous catalytic reactions. Adv. Catal. 1979, 28, 173. (32) Marquardt, D. W. An algorithm for least-squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math. 1963, 11, 471. (33) NIST. Chemical Thermodynamics Database Version 1.1, U.S. Department of Commerce, National Institute of Standards and Technology, Gaithesburg MD 20899, 1990. (34) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; John Wiley & Sons: New York, 1990. (35) Boudart M.; Mears, D.; Vannice, M. A. In Congres International Chimie Industrielle. Ind. Chim. Belge 1967, 32, 281.

Received for review January 17, 2003 Revised manuscript received April 28, 2003 Accepted May 6, 2003 IE030044R