Ind. Eng. Chem. Res. 2007, 46, 1063-1068
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Detailed Kinetic Study of the Partial Oxidation of Methane over La2O3 Catalyst. Part 1: Experimental Results Matthieu Fleys, Wenjuan Shan, Yves Simon, and Paul-Marie Marquaire* Nancy-UniVersite´ , CNRSsDe´ partement de Chimie Physique des Re´ actions, ENSIC 1, Rue GrandVille, B.P. 20451 F-54001 Nancy Cedex, France
A complete kinetic study of the partial oxidation of methane has been performed in a “catalytic jet-stirred reactor”. The main goal of this study was to elucidate the interactions between gas-phase reactions and surface reactions and to propose a detailed heterogeneous-homogeneous mechanism. The catalyst used in this study was lanthanum oxide (La2O3). Various kinetic parameters were considered, including the temperature, the residence time, the contact time, and the amount of catalyst. The results revealed a complex behavior. In this first part of the study, the experimental results are given and discussed. 1. Introduction Energy is a key factor for development; however, traditional energy feed stocks, such as crude oil, are not infinite and are non-renewable. For this reason, much effort has been dedicated to finding new energy sources and new industrial processes. The general idea is to produce energy by various ways without modifying natural balances. The “sustainable development” concept implies limited and controlled pollution and is directly related to green chemistry. The interest in converting natural gas to valuable chemicals such as synthesis gas (syngas), which is a mixture of hydrogen (H2) and carbon monoxide (CO), is renewed, because the crude oil reserves continue to decrease. Hydrogen or syngas seems to be an ideal energy vector; however, its major drawback lies in its production. Traditionally, syngas is produced by steam reforming,1,2 but the large energy requirements, due to the highly endothermic reaction, limits its development. One possible alternative is the partial oxidation of methane (POM):
1 CH4 + O2 ) CO + 2H2 2
(∆rH0298K ) -38 kJ/mol) (1)
This alternative has several advantages: the reaction is slightly exothermic; it can be coupled to the steam reforming, to obtain an autothermal process; and the H2/CO ratio is close to 2, which is the desired ratio for Fischer-Tropsch (FT) synthesis. Most of the catalysts reported to be active in the POM reaction are noble metals or nickel-based catalysts. Much work has been focused on the experimental development of efficient catalysts,3,4 but less effort has been devoted to the mechanism itself. The goal of this paper is to elucidate and propose a detailed mechanism of the POM reaction by considering both gas-phase reactions and surface reactions at the same time. Experiments were performed using lanthanum oxide (La2O3) as a catalyst, which was previously used in our laboratory for various reactions such as the oxidative coupling of methane.5-10 It has also other applications, such as the reduction of NO.11,12 In this paper, the first part of the study, a complete experimental kinetic study has been performed. These data are used in the second part of the study13 to propose a suitable mechanism and compare simulations with experiments. The * To whom correspondence should be addressed. Tel.: +33 383 175 070. Fax: +33 383 378 120. E-mail address: paul-marie.marquaire@ ensic.inpl-nancy.fr.
main goal of this study does not consist of developing a highperformance catalyst, but, rather, obtaining mechanistic information on the POM reaction. 2. Experimental Section 2.1. Experimental Setup. The POM reaction was performed in a perfectly stirred reactor that was developed and designed according to Matras and Villermaux’s criteria.14,15 The Matras et al. criteria15 were checked to ensure that the reactor could be considered to be ideal: that is, well-stirred without thermal or mass gradients. A schematic representation of the quartz reactor is given in Figure 1. The homogeneous reactions occurred in the hemispherical part of the reactor, which had a volume of 110 cm3 and a radius of 3 cm. The heterogeneous reactions occurred at the surface of the catalyst pellets. The catalyst pellets were laid on the surface of a removable cylindrical support. Because quartz is an inert material, no catalytic reactions occurred along the wall of the reactor. The reactor is said to be a “catalytic jet-stirred reactor”, because it is the kinetic energy of the inlet gas that enables good mixing in the gas-phase volume. The gas flow arrives in a cross-shaped injector that has four nozzles at its branch extremities. The inside diameter of the nozzles is 0.3 mm. Before entering the reactor, the reactant mixture was preheated. The temperature of the first preheating part was 100 °C lower than the reactor temperature, whereas the temperature of the second preheating part was equal to that of the reactor. The reactor was heated by Thermocoax resistor wires. The resistor wires were shaped so that they could be in direct contact with the quartz wall. The temperature of the gas phase, which is called the reaction temperature, was measured by a thermocouple located inside a quartz finger at the middle of the free volume. 2.2. Experimental Conditions. The reactant mixture was composed of methane (4%), oxygen (2%), and argon (94%). The reactants were highly diluted in argon, to better control the reaction temperature and to avoid hot spots. All flow rates were regulated by mass flow rate controllers (model RDM 280Air liquide). The outlet gas was analyzed by a micro-chromatograph (Agilent model 3000). Two different columns were used in parallel. One molecular sieve column was used to separate H2, O2, CH4, and CO, using helium as a carrier gas. The second
10.1021/ie060342z CCC: $37.00 © 2007 American Chemical Society Published on Web 01/18/2007
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Figure 2. Estimation of the activation energy at low conversions and at three different residence times: 1 s, 1.5 s, and 2 s.
recycling (IR) is defined as Figure 1. Schematic representation of the catalytic jet-stirred reactor.
column was a U-plot for CO2, C2H4, C2H6, and C2H2, using argon as a carrier gas. For both columns, a thermal conductivity detector was used. In the present study, the influence of several parameters was studied: the residence times (from 0.7 to 6 s) and the contact time, the number of pellets (from zero to eight), and the temperature (from 500 °C to 900 °C). All experiments were conducted at atmospheric pressure. The catalyst used for this study was lanthanum oxide (La2O3, 99.9% purity), which was provided by Alfa Aesar. Lanthanum catalysts can have different phases such as La2(CO3)3, La2O2CO3, La(OH)3, or La2O3.16 These different phases have different catalytic activities; however, at 850 °C, they all change to La2O3.17 Hence, to work with the same starting material, the catalyst was calcined at 900 °C for 8 h. After calcination, the powder was pelleted at a pressure of 1.6 kbar (20 kN) into a cylindrical shape. Using 0.45 g of powder gave a pellet that had dimensions of 12.6 mm for the diameter and 1.1 mm for the thickness. Prior to reaction, the pellets were heated at the reaction temperature with argon for 1 h. The Brunauer-Emmett-Teller (BET) specific surface areas of the calcined powder and the pellets were equal to 5 m2/g. 2.3. Formulas/Calculations. 2.3.1. External Diffusion. Two different methods were used to check the influence of the external diffusion: a theoretical method and an experimental method. In the theoretical method, the external resistance fraction, fe, was calculated according to
fe )
Cex - Csurf rpL ) kdCex Cex
(2)
The calculations were performed for a characteristic data point: that is, at Tr ) 800 °C, Pr ) 105 Pa, and for a residence time equal to 3 s. Moreover, all the physical properties were calculated with respect to methane and argon. To estimate the transfer coefficient kd, we used the classical correlation for fluid/solid transfer:
Sh )
kddp ) 2 + 1.8Re1/2Sc1/3 D
(3)
where dp ) 6L. From these calculations, we found that fe ) 0.1 when the internal recycling was not taken into account, where the internal
IR )
Q Q0
(4)
We determined that IR ) 200, and we calculated that, for IR > 30, we obtained fe < 0.05. The external resistance is considered negligible when fe < 0.05. To confirm this result experimentally, we assessed the apparent activation energy by performing experiments at low conversions and three different residence times. The results are reported in Figure 2; it is interesting to note that the plots are three straight lines, all parallel to each other, which means that, under these operating conditions, there was no external diffusional resistance. Indeed, diffusion limitations would be detected by a change of slope of the Arrhenius plot for a wide range of temperature. Moreover, the apparent activation energy is equal to 13 kcal/mol, which is quite far from zero. The apparent activation energy of the oxidative coupling of methane over La2O3 was determined experimentally to be equal to 16 kcal/ mol by Barbe et al.,6 17 kcal/mol with Sr/La2O3 by Xu et al.18 and 21 ( 5 kcal/mol with Sr/La2O3 by Feng et al.19 Toops et al.20 determined that the apparent activation energy of methane combustion over La2O3 was equal to 25 ( 2 kcal/mol. Islam et al.21 concluded, via atomistic computer simulations, that the {001} and {011} surfaces of the La2O3 catalyst are the most stable and probably have an important catalytic role. The apparent activation energy for O2 adsorption at surface oxygen vacancies on La2O3 {001} was estimated using gradientcorrected periodic density functional theory (DFT) calculations and was determined to be equal to 12.1 kcal/mol.22 It was estimated, by measuring the CH3‚ concentration in a fixed-bed reactor, that the apparent activation energy was ∼15 kcal/mol over a 1%Sr/La2O3 catalyst.23 Hence, The value of our work is consistent with that reported in the literature. From these results, it was concluded that the external diffusion is not the limiting process in this study. 2.3.2. Internal Diffusion. Considering the low value of the BET surface of the catalyst used in this study (5 m2/g), the latter could be said to be poreless, and the corresponding discussion about internal diffusion limitations may not be fundamental. Nevertheless, we estimated a value of the Weisz modulus to examine the importance of the internal diffusional resistance. The Weisz modulus, φWeisz, is defined as
rpL2 φWeisz ) DeCsurf
(5)
Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1065 Table 1. Influence of the Pelleting Pressure on the Geometric Size of the Catalyst Pellet, the Selectivities, and the Methane Conversiona pressure (kN)
thickness (mm)
available surface area (mm2)
XCH4
SCO
SH2
1 5 10 20
1.6 1.32 1.17 1.1
188 177 171 168
7.95 7.87 7.52 7.30
0.157 0.153 0.148 0.154
0.215 0.216 0.221 0.217
a
where De is estimated from the Bosanquet relationship, where both the molecular and the Knudsen diffusivities must be calculated. The average pore size was estimated by mercury porosimetry and was determined to be equal to 0.2 µm. The surface concentration of methane is equal to the bulk concentration, because no external diffusion limitations occurs. The estimated value of φWeisz was 3, and internal diffusion limitations are considered to be important when φWeisz > 1. This result means that the reactions occur at the surface of the catalyst only, not in the bulk of the catalyst. To check this assertion experimentally, the following experiment was done: the pelleting pressure was changed from 1 kN to 20 kN and the reaction was performed at 600, 700, and 800 °C, at a total flow rate of 4.8 cm3/s (at 298 K). Similar results were obtained for all temperatures. The effect of pelleting pressure has been already studied for the same purposes; however, only a few reports appear in the literature.6,24 The results obtained at 700 °C are given in Table 1. It can be observed that, as the pelleting pressure increases from 1 kN to 20 kN, the conversion decreased up to 10% (relative). However, the CO and H2 selectivities did not change. This result means that, when different increasing pelleting pressures were used, the thickness of the pellet decreased slightly, which had an influence on the number of available sites and led to a slight decrease in the conversion. It is noteworthy that the relative decrease of the geometric surface of the pellet was approximately equal to the relative decrease of methane conversion (10%). These results also confirm that the reactions occur mainly at the surface of the catalyst, not in its bulk. 2.3.3. Formulas. For all experiments, the carbon balance (CB) was checked, according to
∑i RiCi out (CHin 4 - CH4 )
(6)
where Ri is the number of carbons in the carbon products Ci. The CO selectivity was calculated in the following way:
SCO )
CO CO + CO2 + 2(C2H6 + C2H4 + C2H2)
(7)
The selectivity of H2 was calculated according to
S H2 )
H2 2XCH4CHin 4
(8)
The residence time is defined according to
τ)
V Q(Tr,Pr)
The pseudo-contact time is defined as
W Q(Tr,Pr)
(10)
which means that tc ) RWτ (where R is a constant; R ) 1/V). The pseudo-contact time is proportional to the product of the residence time, relative to the amount of catalyst. 3. Results and Discussion
Conditions: τ ) 7 s; tc ) 30 mg s/cm3.
CB )
tc )
(9)
Before performing experiments, the catalyst stability was tested for more than 350 h. Figure 3 shows the results at 850 °C and τ ) 3 s for one catalyst pellet. Methane and oxygen conversions, as well as hydrogen and carbon monoxide selectivities, are plotted as a function of reaction time. This figure clearly shows that the catalyst was very stable over this period of time. 3.1. Influence of Temperature with Different Amounts of Catalyst. 3.1.1. Methane and Oxygen Conversions. The conversions of methane (XCH4) and oxygen (XO2), as a function of temperature, are given in Figures 4 and 5 for different amounts of catalyst (from zero to eight pellets). All experiments were performed for a constant residence time of 3 s. The global trends show that conversions increase with temperature and catalyst loading. At 700 °C, the ratio XO2/XCH4 is equal to ∼2 for all experiments; this ratio increases with temperature. For example, at 700 °C with four pellets of La2O3, XCH4 ) 0.12 and XO2 ) 0.25; however, at 900 °C, we determined that XCH4 ) 0.2 and XO2 ) 0.6. This result demonstrates that the chemistry of the reaction changed with temperature. It shows that, as the temperature increased, the oxygen conversion increased much faster than the methane conversion. This phenomenon was observed for higher residence times, where the oxygen conversion was ∼100% and the methane conversion was 875 °C, the methane conversion became higher without a catalyst than when a catalyst was used. Several reasons can explain this behavior. First, one can imagine that the catalyst surface has an inhibiting role by favoring new radical termination reactions. Second, the adsorption and dissociation of the reactants is strongly dependent on temperature. It was reported by Huang et al.11 that the adsorbed amount of oxygen on La2O3 surfaces was low at room temperature, attained a maximum value at ∼250 °C, then decreased noticeably at higher temperatures. Moreover, the heats of adsorption of methane and oxygen over La2O3 at 500-700 °C were determined to be equal to 20 and 30 kcal/mol, respectively.20 These results show that oxygen adsorption is favored in this temperature range, compared to methane, and this accounts for the large oxygen conversion values, compared to methane, in the presence of a catalyst. For example, at 900 °C, the ratio XO2/XCH4 was 3 when the catalyst is used. As a consequence, it is expected that, in the presence of this La2O3 catalyst, the methane conversion was reduced, because of the high oxygen consumption. 3.1.2. Gas Composition at the Outlet of the Reactor. For methane conversions of >10%, we determined that CB > 95%. Hence, all the major products were detected and quantified. The outlet composition is detailed in Figure 6 at two different conditions: at 700 and 900 °C in the presence of four La2O3 pellets and without a catalyst for both temperatures. At 700 °C, the methane conversion was close to zero when no catalyst was
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Figure 7. Carbon monoxide selectivity versus temperature at different catalyst loadings (zero, one, two, four, and eight pellets) at constant residence time (τ ) 3 s). Figure 3. Catalytic activity and stability of lanthanum oxide at 850 °C, τ ) 3 s, using one pellet.
Figure 4. Methane conversion versus temperature at different catalyst loadings (zero, one, two, four, and eight pellets) at constant residence time (τ ) 3 s).
Figure 5. Oxygen conversion versus temperature at different catalyst loadings (zero, one, two, four, and eight pellets) at constant residence time (τ ) 3 s).
Figure 6. Outlet composition at two different temperatures (700 and 900 °C) without catalyst or with four catalyst pellets at constant residence time (τ ) 3 s).
used, whereas it was ∼12% in the presence of the catalyst, showing the catalytic effect of La2O3 at low temperatures. At 900 °C, the outlet composition of the products was larger when no catalyst was used than that produced in the presence of a catalyst, except for CO2. The catalyst La2O3 favors the products of the total oxidation, which leads to a higher conversion of oxygen, compared to the methane conversion.
Figure 8. Hydrogen selectivity versus temperature at different catalyst loadings (zero, one, two, four, and eight pellets) at constant residence time (τ ) 3 s).
3.1.3. CO and H2 Selectivities. The selectivity of CO, as a function of temperature, is given in Figure 7. When a catalyst was used, a minimum in CO selectivity (SCO ) 0.2) was observed at ∼650-700 °C. Moreover, the CO selectivity was approximately half the value of that with a catalyst, compared to that without a catalyst, even though the conversions were different. When the temperature was 850 °C), the C2H6 concentration decreased with residence time. This is due to the following reactions, where ethane is converted to ethylene and oxidized compounds (COx):
C2H6 f C2H5‚ f C2H4
(11)
4. Conclusions An experimental kinetic study was conducted in a continuous jet-stirred reactor with a constant gas-phase volume and a variable number of catalyst pellets. The analysis of experimental results, such as compositions, conversions, selectivities, and yields, gave some insights into the general behavior of the catalyst during the POM reaction. The main conclusions are as follows: (1) Homogeneous and heterogeneous reactions are coupled and the coupling effect became more important as the temperature increased. (2) La2O3 is active, with respect to the partial oxidation of methane (POM) reaction, at low temperatures (such as 600 °C). (3) At temperatures of e700 °C, gas-phase reactions are of minor importance; the global reaction was mainly driven by the heterogeneous reactions. (4) At g800 °C, gas-phase reactions became increasingly important. (5) The total oxidation products, CO2 and H2O, were favored by the presence of the catalyst. Moreover, the production of these compounds was mostly driven by the heterogeneous reactions at all temperatures. (6) The catalyst was also active for oxidation of H2 and CO, which is a drawback for obtaining high H2 and CO selectivities. In the second part of this study,13 a detailed mechanism, including more than 500 homogeneous reactions and 33 heterogeneous reactions is presented. From this mechanism, it is possible to precisely determine the relative importance of gasphase reactions and surface reactions, the main intermediate species, and the relative importance of the different routes. Therefore, it is possible to predict the catalyst behavior. Acknowledgment The authors gratefully acknowledge the Re´gion Lorraine and the Centre National de la Recherche Scientifique (CNRS) for financial support. The authors also thank Prof. Alain Kienne-
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mann (LMSPC-Strasbourg) for interesting and helpful discussions and Maryse Bacri for BET measurements. Professor Robert W. Thompson (Worcester Polytechnic Institute (WPI)) is gratefully acknowledged for helpful comments and corrections. Nomenclature 0 ∆rH(298K) ) standard enthalpy at 298 K fe ) external resistance fraction φWeisz ) Weisz modulus τ ) residence time (s) tc ) pseudo contact time (mg s/cm3) rp ) Apparent rate of the heterogeneous reaction (mol‚s-1‚cm-3 of catalyst) L ) characteristic size of the catalyst pellet (thickness) (cm) kd ) mass-transfer coefficient (m/s) Cex ) concentration in the bulk phase (mol/L) Csurf ) concentration at the catalyst surface (mol/L) D ) average molecular diffusivity in the reactant mixture (m2/s) De ) effective diffusivity of methane in the mixture (m2/s) V ) gas-phase volume of the reactor; V ) 110 cm3 Tr ) temperature of the reactor (K) Pr ) pressure of the reactor (Pa) Q ) total flow rate at a distance R ) 3 cm from the nozzle (cm3/s) Qo ) Total flow rate at the outlet of the nozzle (cm3/s) Q(Tr,Pr) ) Flow rate calculated at the temperature and the pressure of the reactor (cm3.s-1) W ) Weight of the catalyst (mg) CHin 4 ) Inlet methane concentration entering the reactor (mol/L) CHout 4 ) Outlet methane concentration (mol/L) Sh ) Sherwood number Re ) Reynolds number Sc ) Schmidt number IR ) internal recycling XCH4 ) methane conversion XO2 ) oxygen conversion CB ) carbon Balance SCO ) CO selectivity SH2 ) H2 selectivity YCO ) CO yield YH2 ) H2 yield C2 ) hydrocarbons that contain two carbons
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ReceiVed for reView March 21, 2006 ReVised manuscript receiVed August 20, 2006 Accepted October 11, 2006 IE060342Z
