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Kinetic conjugation effects in oxidation of C1-C2 hydrocarbons: Experiment and modeling
Chemical Engineering Journal 370 (2019) 1210–1217
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Kinetic conjugation effects in oxidation of C1-C2 hydrocarbons: Experiment and modeling Vladimir Lomonosov, Yury Gordienko, Ekaterina Ponomareva, Mikhail Sinev
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Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin Street, Moscow 119991, Russia
H I GH L IG H T S
effects in C -C alkane oxidations are due to their free radical nature. • Conjugation inhibits oxidation of ethane as a radical scavenger. • Methane accelerates transformation of methane due to a high rate of chain branching. • Ethane improves the selectivity to ethylene due to the inhibition of its oxidation. • Methane • A dual role of catalyst consists in activation of reactants and trapping of radicals. 1
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A R T I C LE I N FO
A B S T R A C T
Keywords: Methane Ethane Oxidation Kinetic conjugation Free radicals Simulation
Oxidation of individual C1-C2 alkanes and their mixture (CH4:C2H6 = 8:1) was studied in an empty flow reactor and in the presence of two model catalysts used for oxidative coupling of methane (OCM) and oxidative transformations of C2+ alkanes, namely NaWMn/SiO2 and Ca-La/Al2O3. Ethane is much more reactive in homogeneous oxidation due to a high rate of free radical chain branching. Methane inhibits both homogeneous and catalytic oxidation of ethane, while ethane accelerates the conversion of methane. Methane reactivity is higher over the Ca-La/Al2O3 catalyst. In the presence of NaWMn/SiO2 ethane reacts faster, and the ‘conversion vs. contact time’ kinetic curves preserve an S-shape character typical for homogeneous branching chain processes. The observed features were rationalized using the numerical simulations in the framework of heterogeneous-homogeneous kinetic scheme that accounted both free-radical gas phase processes and reactions of molecular and radical species with catalytic active sites. It was shown that typical OCM catalysts play a dual role: they activate hydrocarbons by capturing H-atoms from C–H bonds and also efficiently terminate chain reactions by trapping free radicals. Consequently, over the catalyst that is more active in methane oxidation (Ca-La/Al2O3) ethane reacts slower due to the elimination of free radical chain branching and development. During the oxidation of ethane, methane – as an efficient radical scavenger – improves the selectivity to ethylene due to the inhibition of its secondary transformations (mainly to CO via HCO% radicals).
1. Introduction Light alkanes (LA) are the main constituents of natural and petroleum gases, as well as various technological gas mixtures. Their conversion into value-added products (semi-products for organic and petrochemical synthesis, synthetic fuels, etc.) is considered as one of the ways to efficiently use natural resources and reduce the industrial impact on the environment. The existing technologies of LA valorization are mainly based on their preliminary transformation to the mixture of hydrogen and carbon monoxide (synthesis gas). Developing alternative processes in which LA would be directly transformed into target
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products (chemicals or fuel components) is still a challenge. Light olefins are one of the main building blocks for basic organic chemistry and petrochemical synthesis. Nowadays they are produced by thermal and catalytic dehydrogenation and cracking of C2+ hydrocarbons. As a rule, these processes are endothermic and energy-intensive. Moreover, methane, as the most abundant component of hydrocarbon gas mixtures of various origin, cannot be involved in the existing processes of olefin production. These limitations can be avoided by using an oxidative conversion of LA. To date, three related processes of oxidative transformation in which light olefins are forming directly from LA are known:
Corresponding author. E-mail addresses: [email protected] (V. Lomonosov), [email protected] (E. Ponomareva), [email protected] (M. Sinev).
https://doi.org/10.1016/j.cej.2019.04.006
Available online 02 April 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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active sites and the heterogeneous-homogeneous free-radical nature of the LA oxidation processes. Although some important aspects of the LA oxidation mechanism are still being debated, or remain beyond consideration, at present, its general scheme is seen as follows. LA molecules are activated on the oxide catalysts by homolytic C–H bond cleavage resulting in surface OH-group formation and free alkyl radicals evolution. The final products of partial and deep oxidation are formed via a complex combination of secondary free-radical reactions, both homogeneous and surface-assisted. This means that the kinetic model for such reaction should include numerous elementary reactions of molecular and freeradical species, as well as surface active sites. Some of them are well studied, and their kinetic parameters are known and included into review articles and databases (see, for instance, [42,43]). On the other hand, some of elementary steps, including the most important heterogeneous reactions between catalyst active sites and gaseous molecular and free-radical species, are not accessible for direct observation and kinetic measurements. The main reason for the aforementioned discrepancies in the results reported by different authors may be due to the fact that the experimental data were obtained under different conditions. In the present work, the effects of the mutual influence of methane and ethane were studied under identical conditions over two model OCM catalysts, namely, NaWMn/SiO2 and Ca-La/Al2O3. Though they both are known as efficient OCM and ethane ODH catalysts, they are different in some important aspects, such as activity towards methane and ethane activation and product distributions. The experimentally observed conjugation effects were analyzed in framework of detailed heterogeneoushomogeneous kinetic model of C1-C3 hydrocarbon oxidation.
- oxidative coupling of methane (OCM); - oxidative dehydrogenation (ODH) of C2+ LA; - oxidative cracking of C3+ LA. These three processes have several common characteristics, such as optimum temperature ranges and the types of catalysts used. The main peculiarities of their mechanism (the initial reactant activation and free-radical character of reaction intermediates) are also the same. The general features of this type of processes, including the details of their mechanism and the development of catalysts for them, have been reviewed and analyzed elsewhere [1–12]. The main obstacle to the practical implementation of oxidative transformations of LA into valuable products is the unfavorable ratio of the reactivity of the initial hydrocarbons and the target products. As a consequence, the yield of the latter is lower than what is of practical interest, due to the sharp drop in selectivity at increasing conversion. Reduced selectivity also affects the thermal regime of the process as a large amount of heat is released during deep oxidation. In combination with the high optimal temperature (> 700 °C), this poses additional difficulties in terms of reactor operation. It should be noted that the separation of mixtures of various LA of natural or technological origin is also a difficult task. However, if their transformations into the same products can occur on the same catalysts and under similar conditions, it would be very attractive to involve them all in the process without prior separation. The effects of the reciprocal influence of methane and ethane during their simultaneous oxidation are of particular interest. First, ethane is present in natural gas. Besides that, ethane is always present in relatively high concentrations in the OCM reaction mixture as the primary product of methane coupling. The combined oxidation of methane and C2+ hydrocarbons has been investigated in a number of publications (see, for example, [13–23]). Though in some cases a positive effect in terms of increasing the selectivity and yield of olefins (not only ethylene, but also propylene) was demonstrated, in general, the data on the reciprocal influence of different LA during their simultaneous oxidation are contradictory. Some literature data indicate that methane can act as an inhibitor of the homogeneous ethane oxidation [13,21,23], while the results obtained under the conditions of catalytic OCM are conflicting. According to [14,17,20], the addition of ethane to the initial methane-oxygen mixture decreases the rate of methane conversion along with C2-hydrocarbon selectivity. The observed effect is attributed to the competition between methane and more reactive ethane either for the gas phase oxygen [14,17] or for the catalyst active sites [20]. By contrast, in [24,25] it was demonstrated that simultaneous presence of methane and ethane in the initial mixture noticeably increases the total conversion of both hydrocarbons. It should be noted that in the cited works only a qualitative estimate of reciprocal influence of methane and ethane was given. Anyhow, there is no doubt that in the case of simultaneous oxidation of several hydrocarbons, the effects of strong kinetic conjugation are manifested. On the one hand, this is not surprising, since the close similarity of the chemical structures and the mechanisms of transformation of various alkanes inevitably lead to the formation of the same intermediate active particles during their oxidation. And this is what obviously leads to conjugation. On the other hand, the chains of sequential-parallel transformations in the oxidation of hydrocarbons are so complex that conjugation can occur in different parts of their network, leading to very different effects depending on the specific conditions. Additional understanding of the essence of the observed effects can be achieved by analyzing reliable kinetic models reflecting the mechanism of the process. Several approaches to build such models have been proposed and discussed (see, e.g. [26–41]). To possess predictive power and successfully play the role of a tool in studying the mechanism, such models should account the main features of the catalyst
2. Materials and methods 2.1. Sample preparation NaWMn/SiO2 catalyst (0.8% Na, 3.2% W, and 2.0% Mn, 4.1 m2 × g−1) was prepared by incipient wetness impregnation of silica gel (Aldrich, Davisil grade 646) with aqueous solution of sodium tungstate followed by impregnation with aqueous solution of manganese nitrate after intermediate drying at 120 °C for 3 h. Ca-La/Al2O3 catalyst (20% Ca and 10% La, 3.5 m2 × g−1) was prepared by incipient wetness impregnation of α-Al2O3 with aqueous solution of calcium nitrate followed by impregnation with aqueous solution of lanthanum nitrate after intermediate drying at 120 °C for 3 h. The support was obtained by calcination of γ-Al2O3 (Sorbis Group) at 1200 °C for 15 h. Both catalysts were calcined at 600 °C for 2 h and at 900 °C for 6 h. 2.2. Experimental methodology The catalytic experiments were carried out in a quartz flow reactor with an internal diameter of 4 mm; the central part of the reactor was 8 mm in diameter and equipped with a quartz well for a movable thermocouple. A catalyst (0.25–0.5 mm fraction; 12 mg, or ∼0.03 ml, bed length ∼1 mm) was placed in the ring-shaped spacing between the walls of the reactor and the thermocouple well. No axial temperature gradient was detected along the catalyst bed. At the outlet, the reaction mixture passed through a trap chilled with a mixture of ice and saturated solution of sodium chloride to condense liquid products, and then the gaseous products were analyzed by on-line gas chromatography. The experiments on oxidation of methane, ethane and methane-ethane mixture were conducted in the temperature range 720–860 °C and at feed rate varied from 35 to 175 ml × min−1. The initial mixtures contained (in mol. %) 80 methane, 10 ethane, and 10 oxygen. Nitrogen was used as inert diluent to replace methane or ethane when oxidation of individual hydrocarbons was studied. A detailed description of the experimental setup and the chromatographic protocol were published elsewhere [44]. 1211
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Fig. 1. Oxygen conversion as a function of the inverse flow rate for oxidation of methane, ethane and their mixture over NaWMn/SiO2 (left) and Ca-La/Al2O3 (right) catalysts; solid lines – 760 °C; dashed lines 800 °C.
gas pressure was 0.1 MPa (1 bar) in all calculations. The initial concentrations of each form of active sites were determined in preliminary calculations to make them close to stationary during the entire process. For most cases, they were as follows:
2.3. Kinetic modeling The kinetic model used in this work consisted of two blocks: homogeneous and heterogeneous. In order to describe the processes that occur in homogeneous gas-phase, the kinetic scheme known as AramcoMech 2.0 was used. It was developed by the Combustion Chemistry Centre at NUI Galway [45,46]. In our opinion, among all published kinetic models, this scheme most fully implements the principles on which the kinetic model of LA oxidation must be based in order to serve as a reliable tool for studying the mechanism of the process [39]. It is well documented and verified on a large basis of experimental data obtained in a broad variety of reaction mixtures and reaction conditions [47]. In its basic version the AramcoMech 2.0 scheme operates with 493 species and 2716 reversible elementary reactions. For heterogeneous (surface) reaction description, a simplified scheme including 6 species and 50 elementary reactions was used. The main approaches to the construction of the heterogeneous part of the kinetic scheme are described elsewhere [32,36,39]. The list of included elementary steps and examples of modeling can be found in [48]. According to the main assumptions of the model [32,36,39], active sites can exist in three different forms:
[LO]0 = 0.05 [LX ]0 ; [LOH ]0 = 0.95 [LX ]0 ; [L]0 = 0. The calculations were performed using “Chemical Workbench” software developed and distributed by Kintech Lab company (Russia) [49]. 3. Experimental results For several reasons the experimentally determined parameter that allows one to directly compare the results obtained for the used reaction mixtures with different initial composition is the oxygen conversion. Oxygen is present in mixtures of all three compositions in the same concentration, which in all cases is lower than that required for the complete conversion of hydrocarbons according to the stoichiometry. The oxygen content limits the achievable conversion of hydrocarbons. Moreover, in the case of simultaneous oxidation of C1–C2 alkanes it is hardly possible to determine their conversions. On the one hand, ethane is consumed due to its oxidation, but it simultaneously forms via the OCM reaction. On the other hand, though the rate of methane formation from ethane is low and can be neglected, the methane conversion can hardly be determined due to its high concentration and low oxidation rate. Therefore, oxygen is the only reactant for which the degree of conversion can be determined distinctly; this value can serve as a comparative characteristic of the depth of the process. The data on oxidation of methane, ethane and their mixture at constant temperatures (760 and 800 °C) over NaWMn/SiO2 and Ca-La/ Al2O3 are presented in Fig. 1. These data indicate that during methane oxidation over NaWMn/SiO2 catalyst, in the range of flow rates used in this work the maximum oxygen conversion does not exceed 10% at 760 °C and 20% at 800 °C. In the case of ethane oxidation, almost total conversion of oxygen was reached at increasing contact time. Meanwhile, if inert gas in the initial mixture is replaced by methane, the rate of oxygen consumption drops substantially (mixture curve). One can conclude that simultaneous presence of methane and ethane in the initial mixture drastically changes the rates of oxidation of individual hydrocarbons. Obviously, the ethane oxidation is suppressed in the presence of methane. This effect is more noticeable at 760 °C compared to 800 °C. However, it should be noted that the methane conversion increases at rising temperature and its contribution to the oxygen conversion becomes more substantial. As a result, it is more difficult to trace the effect of methane onto the oxygen consumption in the oxidation of ethane.
- oxidized active form further denoted as LO (L – element of the oxide lattice, which is connected to active oxygen); - reduced hydroxylated form (LOH) – it arises during the homolytic abstraction of the hydrogen atom by LO from any gaseous particle; - reduced de-hydroxylated form (L). The active sites of different catalysts differ in the values of the main thermochemical parameters – the oxygen binding energy and the energy of O–H bond. The total concentration of these three forms ([LO] + [LOH] + [L] = [LX]) remains constant in the course of reaction. The kinetic parameters of the individual elementary steps taken from [48] were estimated using the thermochemical data (oxygen binding energy and strength of O–H bond in the [LOH]) obtained experimentally for the model Li/MgO catalyst. These values are similar to those for the Ca-La/Al2O3 catalyst used in this work. All calculations were carried out for an isothermal reactor of the calorimetric bomb type under conditions corresponding to those in which experimental data were obtained (see above). The reaction system was described in a quasi-homogeneous (or quasi-catalytic) approximation, i.e. catalytic active sites were added to the homogeneous gas mixture as free particles in different concentrations varied from 10 to 200 mol.% of the total concentration of gaseous particles; the total 1212
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The comparison of the data for two catalysts presented in Fig. 1 shows that in the case of oxidation of individual hydrocarbons not only the values of oxygen conversion achieved during a certain contact time, but also the shapes of the kinetic curves are different. In the presence of NaWMn/SiO2 catalyst, the rate of oxygen conversion is low when methane is oxidized. It is significantly higher when ethane is oxidized, and the kinetic curves are S-shaped. The latter is particularly noticeable at lower temperature. When methane is added to ethane, the S-shaped character of kinetic curves nearly disappears; also, the oxygen conversion is significantly reduced compared to what is observed during the oxidation of ethane. The Ca-La/Al2O3 catalyst is much more active in the oxidation of methane, whereas the rate of ethane conversion over this catalyst is lower. Thus, the oxygen conversion during the separate oxidation of methane and ethane becomes comparable at 760 °C, and the analysis of experiments on the oxidation of methane-ethane mixture becomes more complicated. Though the oxygen conversion is higher when both hydrocarbons are present in the feed, the total amount of oxygen consumed in separate oxidation of methane and ethane is higher compared to their simultaneous oxidation (Fig. 1, right panel). However, as the temperature increases to 800 °C, the difference in oxygen conversion during ethane and methane oxidation increases, while the oxygen conversion during mixture oxidation becomes lower compared to separate ethane oxidation. This likely indicates a decrease in the rate of ethane conversion in the presence of methane. In spite of clear differences in the oxidation of individual hydrocarbons in the presence of two studied catalysts, the shapes of kinetic curves observed during their simultaneous oxidation become very alike, as well as the amounts of oxygen consumed at the same contact times. Thus, it can be stated that the general kinetic patterns change when two hydrocarbons are present simultaneously in the initial mixture due to a strong conjugation between methane and ethane oxidation. In order to elucidate the effect of methane on different reaction paths in ethane oxidation we compared the concentrations of the main products obtained in the oxidation of the methane–ethane mixture with the sum of their concentrations in the separate oxidations of methane and ethane under the same conditions. As it is shown in Fig. 2, the amount of ethylene formed in the mixed feed over NaWMn/SiO2 catalyst increases along with the reverse flow rate and, correspondingly, at the increasing conversion of reactants. On the contrary, the total ethylene content obtained during separate oxidation decreases in the shown range of conversions, although the maximum ethylene yield can be obviously reached at lower conversion; the total amount of carbon oxides (COX) is substantially higher in the case of separate oxidation. Fig. 3 demonstrates that the amount of ethylene obtained in the mixed
Fig. 3. Concentrations of ethylene and COX as functions of temperature for oxidation of ethane (solid line) and methane-ethane mixture (dashed line) over NaWMn/SiO2 catalyst (flow rate – 125 ml/min).
feed over NaWMn/SiO2 catalyst constantly increases with the temperature at constant flow rate. At relatively low temperatures the amount of ethylene calculated as the sum from methane and ethane separate oxidation is significantly higher, but it passes through the maximum at 800 °C. Both dependencies (ethylene yield on contact time and on temperature) convincingly indicate that the presence of methane in the reaction mixture reduces the rate of ethylene formation, but at the same time suppresses the consecutive reactions of ethylene oxidation to carbon oxides. All in all, methane reduces the overall rate of ethane conversion and, in particular, it suppresses its oxidation to carbon oxides. At higher conversions, the ethylene consecutive oxidation is also restrained in the presence of methane.
4. Results of simulations The results of calculation performed using the AramcoMech 2.0 kinetic scheme demonstrate a significant difference in the main features of the homogeneous oxidation of methane and ethane in the studied range of parameters. At 800 °C oxygen conversion does not reach any noticeable value within 0.1 sec during the methane oxidation, whereas in the case of ethane it sharply increases after ∼5 msec and rapidly goes beyond 90% (see Fig. 4, curve [LX] = 0). In the case of a heterogeneous-homogeneous process, the
Fig. 2. Concentrations of ethylene and COX as functions of inverse flow rate for oxidation of ethane (solid line) and methane-ethane mixture (dashed line) over NaWMn/SiO2 catalyst (800 °C).
Fig. 4. Oxygen conversion as a function of time at varied concentration of active sites (oxidation of ethane, 800 °C). 1213
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effect can be attributed to the participation of methane in the primary activation of ethane molecules, because of the increase in the concentration of reaction products and the rates of secondary processes, which can also contribute to kinetic conjugation. Thus, the results of simulations show that the rate of methane conversion increases markedly, but the maximum rate of ethane conversion decreases if two hydrocarbons are present in the initial mixture. On the one hand, when ethane is added to the reaction mixture, it causes a rapid rise in the free radical generation and the development of chain reaction; methane becomes involved in oxidation process and, therefore, the rate of its oxidation increases noticeably. On the other hand, methane and ethane compete for the same active particles, and the rate of ethane conversion becomes lower in a certain range of conversions compared to its individual oxidation. As a result, the rate of oxygen consumption sharply increases compared to the individual oxidation of methane, but decreases compared to the ethane oxidation, which is accompanied with a qualitative change of the shape of kinetic curves (see Fig. 7). Interestingly, in the mixture of such composition (molar methane-to-ethane ratio 1:8) the rates of conversion of both hydrocarbons become almost equal in the whole range of [LX] from 0 to 2, which cannot be achieved during their separate oxidation. This result is similar to that obtained in [50]: if methane is used as a gas diluent during the ethane oxidation, the self-ignition character of the process is not manifested, but methane itself is involved into the formation of reaction products. Fig. 8 shows calculated concentration of ethylene and COX produced during the oxidation of ethane and methane-ethane mixture as a function of the reaction time. In the case of the separate ethane oxidation, the concentrations of ethylene and COX initially increase with reaction time. The slope of COX curve gradually decreases with reaction time, while concentration of C2H4 passes through the maximum at ∼0.04 s. The patterns change significantly in the case of simultaneous oxidation. Although the slope of the C2H4 formation curve decreases compared to separate oxidation, the ethylene concentration increases permanently over the entire range of the reaction time; finally, it exceeds the maximum concentration achieved during the separate oxidation of ethane. As for COX, their concentration is markedly lower in the case of the joint oxidation as compared to the separate ones. By comparing the results of experiments (Fig. 2) and simulations (Fig. 8), one can conclude that the observed patterns are identical, even if calculations are performed in the framework of the homogeneous scheme.
development of the oxidation of methane and ethane over time also exhibits very different trends. In the case of methane, the results obtained for [LX] ranged from 0.1 to 2 (from 10 to 200 mol.% of active sites relative to the total number of particles in the gas phase) demonstrate a gradual increase in the reaction rate; the ‘X(O2) vs. time’ curves are very slightly S-shaped, i.e. the rate of methane oxidation passes through a maximum at very low conversions. In the same range of [LX], the shape of kinetic curves for the oxidation of ethane changes dramatically. As can be seen from Fig. 4, at low concentrations of active sites ([LX] = 0.1–0.2) they are similar to the one observed for homogeneous oxidation. At increasing [LX] their shape gradually changes and approaches the same observed in the oxidation of methane. It is worth noticing that at low reaction times (< 10 msec at 800 °C, see insert in Fig. 4) and conversions the rate of reaction increases at increasing [LX]. However, at low [LX] the development of free-radical processes is much more pronounced, and high conversions are reached much faster. This is why at lower concentrations of active sites high conversions are achieved at shorter reaction times. As we noted above, it is rather difficult to experimentally measure the rates of conversion of methane and, moreover, of individual hydrocarbons during the oxidation of their mixture. In the case of modeling, it is possible not only to obtain the values of the overall rates (consumption and/or formation) for any substance, but also to determine the contribution of individual elementary processes to them. It was already mentioned that the methane conversion rate passes through a maximum at very low conversions. This holds true also for its oxidation together with ethane. However, if in the individual oxidation of methane the rate of its conversion smoothly increases from the value close to zero at [LX] = 0, the addition of 10 vol% of ethane sharply increases the rate of methane oxidation even in the absence of catalytic active sites (see Fig. 5). It further increases at increasing [LX], but the slope of the 'rate vs. [LX]' curve decreases at [LX] > 0.3, though the rate of methane conversion still remains higher in the presence of ethane. The family of curves shown in Fig. 6 demonstrates that the effect of methane on the rate of ethane oxidation is more complex. At [LX] < 1 the maximum rate of ethane oxidation is substantially lower in the presence of methane. However, at low conversions, i.e. during the period of the development of chain process, methane accelerates the rate of ethane conversion, and this effect becomes more and more pronounced at increasing concentration of catalytic active sites. The conversion of oxygen at which the change from promotion to inhibition occurs increases from ∼0.5 to ∼5% with an increase in [LX] from 0 to 2. It is interesting to note that with an increase in [LX], the rate of oxidation of ethane in a mixture with methane increases again as compared to separate oxidation. However, it’s not clear if the observed
5. Discussion The results of experiments performed in this work can be summarized as follows: 1. the main features of the catalytic oxidation of methane and ethane differ significantly; 2. the rate of simultaneous oxidation of two hydrocarbons is lower than the sum of the rates of their separate oxidations; 3. the formation of carbon oxides is suppressed more significantly than the formation of ethylene during the simultaneous oxidation of methane and ethane. It is worth noticing that the results of simulations not only reproduced the experimental observations (see, for instance, the data presented in Figs. 1, 2 and 7, 8), but also shed a new light onto the origin of experimentally observed regularities. Let us first compare the data presented in Figs. 1 and 4. It is evident that the shape of kinetic curves for ethane oxidation observed for the NaWMn/SiO2 catalyst is close to the one that is typical for simulated curves obtained at low concentrations of active sites. Whereas for the Ca-La/Al2O3 catalyst the ‘X(O2) vs. 1/V’ curves are more similar to those obtained at high concentrations of [LX]. These observations strongly correlate with the data obtained for the individual oxidation of
Fig. 5. Maximum methane oxidation rate as a function of [LX]: (1) – no ethane added; (2) – 10 vol% ethane added. 1214
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Fig. 6. Rates of ethane conversion at different [LX] and oxygen conversions; effect of methane.
no pronounced S-shaped character of the oxygen conversion during the oxidation of ethane). The data presented in Fig. 9 demonstrate that, as we already mentioned, the rates of methane and ethane conversion when they both are present in the initial mixture are nearly the same. Fig. 9 also allows us to compare the rates of elementary steps that the most significantly contribute to the activation of methane and ethane. The major contribution is made by OH and HO2 radicals, H atoms, and also CH3 radicals in the case of ethane activation in the presence of methane. The rates of their interaction with two alkanes are also very close, which has a simple explanation: the values of rate constants for all these particles are about one order of magnitude higher in the case of ethane [42,43], but the concentration of the latter in the mixed feed is 8 times less, which compensates the difference in reactivities. One may assume that the patterns should be different for the other ratios of methane and ethane. The shape of the kinetic curves together with the obtained values of the concentrations of active radicals and the rates of elementary reactions show that during the oxidation of ethane the free radical chain branching, as a whole, is more intense compared to the methane oxidation. As a result, the rates of radical reactions during the developed chain process become much higher than the rates of the heterogeneous activation of ethane over catalyst active sites, if the concentration of the latter is not very high (as in the case of NaWMn/SiO2 catalyst). Therefore, at low degrees of conversion, ethane activation over catalytic active sites prevails, while the rate of homogeneous conversion becomes higher with the development of a chain process. Evidently, the catalyst plays a dual role: it increases the rate of the alkane molecule activation, but also acts as an efficient radical scavenger. The latter leads to the inhibition of ethane conversion by the catalyst active sites in the later stages of the reaction. These results are in good agreement with the experimental data obtained earlier. As it was shown in [24], the rate of ethane oxidation decreases in the presence of NaWMn/SiO2 catalyst as compared to its oxidation in an empty reactor. According to [19], several calcium-based OCM catalysts are able to inhibit the gas-phase oxidation of ethylene also due to the capture of radical chain-carriers by their active surface sites. As can be seen in Fig. 9, the rate of methane conversion is almost three orders of magnitude lower than that of ethane in the case of their separate oxidation. However, it increases by approximately two orders of magnitude in the presence of ethane at short reaction times. Meanwhile, the rate of ethane conversion decreases by about 3.5 times under the same conditions since the produced radicals are actively intercepted by methane molecules. Both experimental (Fig. 2) and simulation (Fig. 8) data demonstrate
Fig. 7. Oxygen conversion as function of contact time at 800 °C and LX = 0 during the oxidation of methane, ethane, and methane-ethane mixture.
Fig. 8. Concentration of ethylene and COX as functions of contact time for oxidation of ethane (solid line) and methane-ethane mixture (dashed line); 800 °C LX = 0.
methane: oxygen consumption over NaWMn/SiO2 in the same conditions is by far lower than over Ca-La/Al2O3. In other words, for both processes the two catalysts behave similarly: NaWMn/SiO2 – as a system with low concentration of active sites (low activity in methane oxidation, high rates and S-shaped curves of oxygen conversion vs. contact time in ethane oxidation), and Ca-La/Al2O3 – as a system with high concentration of active sites (higher activity in methane oxidation, 1215
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Fig. 9. Overall rates (in molec. × cm−3 × s−1) of methane and ethane conversion and partial rates of elementary reactions as functions of oxygen conversion: 1 – d [CH4]/dt; 2 – CH4 + OH < = > CH3 + H2O; 3 – CH4 + H < = > CH3 + H2; 4 – CH4 + HO2 < = > CH3 + H2O2; 5 – C2H6 + CH3 < = > C2H5 + CH4; 6 – LO + CH4 < = > LOH + CH3; 7 – d[C2H6]/dt; 8 – C2H6 + OH < = > C2H5 + H2O; 9 – C2H6 + H < = > C2H5 + H2; 10 – C2H6 + HO2 < = > C2H5 + H2O2; 11 – LO + C2H6 < = > LOH + C2H5; 12 – CH3 + CH3 + M < = > C2H6 + M; 13 – C3H8 + M < = > CH3 + C2H5 + M; A – oxidation of methane; B – oxidation of ethane; C – mixed feed.
that the product distributions substantially change in the case of simultaneous methane and ethane oxidation compared to their separate conversion, namely a significant shift from the formation of COX to ethylene is observed. This agrees well with earlier experimental observations (see, for example, [13,23]) that the rates of ethylene oxidation to carbon monoxide are markedly reduced in the presence of methane in the reaction mixture. The analysis of simulation results shows that the main path of ethylene transformation starts from its reaction with hydroxyl radical: C2H4 + OH = > C2H3 + H2O
(1)
while produced vinyl radical is then rapidly transformed into CO by reactions: C2H3 + O2 = > CH2O + HCO CH2O + X = > HCO + XH HCO + X = > CO + XH
(2) Fig. 10. Rates of elementary reactions responsible for ethylene conversion to carbon monoxide as functions of contact time: solid lines – oxidation of ethane; dashed lines – oxidation of methane-ethane mixture.
where X is any free radical, catalyst active site, or O2 molecule. As can be seen from Fig. 10, the rate of vinyl radical formation noticeably decreases in the presence of methane, which effectively intercepts hydroxyl radicals:
intensively during the oxidation of ethane participate in the conversion of methane, as a result of which the conversion rates of both hydrocarbons change dramatically under the same conditions. In the presence of catalytically active sites, alkane molecules are activated on them already at very low degrees of conversion and long before a surge in the reaction rate due to the development of a chain process. The particular manifestation of the reciprocal influence of free radicals and catalytic active sites depends on the concentration and specific activity of the latter. The elucidation of the reasons for the acceleration of ethane conversion in the presence of methane at the initial stages of the process requires a detailed analysis of the rates of all elementary stages in
CH4 + OH = > CH3 + H2O In the same way methane intercepts all other active species that are able to transform ethylene to vinyl radical. This is the reason for the inhibition of CO formation from ethylene via CH2O and HCO. 6. Conclusions The similarity of the chemical processes of free radical nature in the oxidation of methane and ethane is the main cause of the appearance of strong kinetic conjugation. The active particles generated more 1216
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which free radicals participate. Most likely, this is due to some differences in the transformations of methane and ethane, primarily the activity and the pathways of transformation of the oxy- and proxy-radicals formed in the initial stages of the transformation of alkane molecules.
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Funding This work was supported by the Russian Foundation for Basic Research (RFBR) [grant number 18-33-00798], and as a part of the Governmental task for ICP RAS 0082-2014-0007, state registration number AAAA-A18-118020890105-3. References [1] M.Y. Sinev, V.N. Korchak, O.V. Krylov, The mechanism of the partial oxidation of methane, Russ. Chem. Rev. 58 (1989) 22–34, https://doi.org/10.1070/ RC1989v058n01ABEH003423. [2] E. Wolf, Methane Conversion by Oxidative Processes: Fundamental and Engineering Aspects, Springer, Netherlands, 1992. [3] J.H. Lunsford, The catalytic oxidative coupling of methane, Angew. Chem. Int. Ed. 34 (1995) 970–980, https://doi.org/10.1002/anie.199509701. [4] E.N. Voskresenskaya, V.G. Roguleva, A.G. Anshits, Oxidant activation over structural defects of oxide catalysts in oxidative methane coupling, Catal. Rev. 37 (1995) 101–143, https://doi.org/10.1080/01614949508007092. [5] M.Y. Sinev, L.Y. Margolis, V.N. Korchak, Heterogeneous free-radical reactions in oxidation processes, Russ. Chem. Rev. 64 (1995) 349–364, https://doi.org/10. 1070/RC1995v064n04ABEH000154. [6] Y. Schuurman, C. Mirodatos, Uses of transient kinetics for methane activation studies, Appl. Catal., A. 151 (1997) 305–331, https://doi.org/10.1016/S0926-860X (97)00031-8. [7] O.V. Krylov, V.S. Arutyunov, Okislitel’nye Prevrashcheniya Metana (Oxidative Transformations of Methane), Nauka, Moscow, 1998. [8] M.Y. Sinev, Free radicals in catalytic oxidation of light alkanes: kinetic and thermochemical aspects, J. Catal. 216 (2003) 468–476, https://doi.org/10.1016/ S0021-9517(02)00116-1. [9] E.V. Kondratenko, M. Baerns, Oxidative coupling of methane, Handbook of Heterogeneous Catalysis, Wiley‐VCH Verlag GmbH & Co, 2008, pp. 3010–3023, , https://doi.org/10.1002/9783527610044.hetcat0152. [10] U. Zavyalova, M. Holena, R. Schlögl, M. Baerns, Statistical analysis of past catalytic data on oxidative methane coupling for new insights into the composition of highperformance catalysts, ChemCatChem. 3 (2011) 1935–1947, https://doi.org/10. 1002/cctc.201100186. [11] S. Arndt, T. Otremba, U. Simon, M. Yildiz, H. Schubert, R. Schomäcker, Mn–Na2WO4/SiO2 as catalyst for the oxidative coupling of methane. What is really known? Appl. Catal., A 425–426 (2012) 53–61, https://doi.org/10.1016/j.apcata. 2012.02.046. [12] V.I. Lomonosov, M.Y. Sinev, Oxidative coupling of methane: mechanism and kinetics, Kinet Catal. 57 (2016) 647–676, https://doi.org/10.1134/ S0023158416050128. [13] J.C. Mackie, J.G. Smith, P.F. Nelson, R.J. Tyler, Inhibition of C2 oxidation by methane under oxidative coupling conditions, Energy Fuels 4 (1990) 277–285, https:// doi.org/10.1021/ef00021a011. [14] H. Mimoun, A. Robine, S. Bonnaudet, C.J. Cameron, Oxypyrolysis of natural gas, Appl. Catal. 58 (1990) 269–280, https://doi.org/10.1016/S0166-9834(00) 82295-2. [15] J. Sanchez-Marcano, C. Mirodatos, E.E. Wolf, G.A. Martin, Inhibition of the gas phase oxidation of ethylene by various solids and influence of their addition on the catalytic properties of lanthanum oxide towards the oxidative coupling of methane, Catal. Today 13 (1992) 227–235, https://doi.org/10.1016/0920-5861(92)80146-E. [16] C.A. Mims, R. Mauti, A.M. Dean, K.D. Rose, Radical chemistry in methane oxidative coupling: tracing of ethylene secondary reactions with computer models and isotopes, J. Phys. Chem. 98 (1994) 13357–13372, https://doi.org/10.1021/ j100101a041. [17] Q. Chen, P.M. Couwenberg, G.B. Marin, The oxidative coupling of methane with cofeeding of ethane, Catal. Today 21 (1994) 309–319, https://doi.org/10.1016/ 0920-5861(94)80152-5. [18] Y.-D. Zhang, S.-B. Li, Y. Liu, J.-Z. Lin, G.-G. Lu, X.-Z. Yang, J. Zhang, Cofeeding of methane and ethane over Na2WO4-Mn/SiO2 catalyst to produce ethylene, Stud. Surf. Sci. Catal. 119 (1998) 599–604, https://doi.org/10.1016/S0167-2991(98) 80497-7. [19] L. Yu, W. Li, V. Ducarme, C. Mirodatos, G.A. Martin, Inhibition of gas-phase oxidation of ethylene in the oxidative conversion of methane and ethane over CaO, La2O3/CaO and SrO–La2O3/CaO catalysts, Appl. Catal., A 175 (1998) 173–179, https://doi.org/10.1016/S0926-860X(98)00208-7. [20] A. Machocki, A. Denis, Simultaneous oxidative coupling of methane and oxidative dehydrogenation of ethane on the Na+/CaO catalyst, Chem. Eng. J. 90 (2002) 165–172, https://doi.org/10.1016/S1385-8947(02)00077-3. [21] N.M. Pogosyan, M.D. Pogosyan, L.N. Strekova, L.A. Tavadyan, V.S. Arutyunov, Effect of the concentrations of methane and ethylene on the composition of the products of their cooxidation, Russ. J. Phys. Chem. B 9 (2015) 218–222, https:// doi.org/10.1134/S1990793115020104. [22] R.N. Magomedov, A.Y. Proshina, B.V. Peshnev, V.S. Arutyunov, Effects of the gas medium and heterogeneous factors on the gas-phase oxidative cracking of ethane,
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Kinetic conjugation effects in oxidation of C1-C2 hydrocarbons: Experiment and modeling Vladimir Lomonosov, Yury Gordienko, Ekaterina Ponomareva, Mikhail Sinev
T
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Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin Street, Moscow 119991, Russia
H I GH L IG H T S
effects in C -C alkane oxidations are due to their free radical nature. • Conjugation inhibits oxidation of ethane as a radical scavenger. • Methane accelerates transformation of methane due to a high rate of chain branching. • Ethane improves the selectivity to ethylene due to the inhibition of its oxidation. • Methane • A dual role of catalyst consists in activation of reactants and trapping of radicals. 1
2
A R T I C LE I N FO
A B S T R A C T
Keywords: Methane Ethane Oxidation Kinetic conjugation Free radicals Simulation
Oxidation of individual C1-C2 alkanes and their mixture (CH4:C2H6 = 8:1) was studied in an empty flow reactor and in the presence of two model catalysts used for oxidative coupling of methane (OCM) and oxidative transformations of C2+ alkanes, namely NaWMn/SiO2 and Ca-La/Al2O3. Ethane is much more reactive in homogeneous oxidation due to a high rate of free radical chain branching. Methane inhibits both homogeneous and catalytic oxidation of ethane, while ethane accelerates the conversion of methane. Methane reactivity is higher over the Ca-La/Al2O3 catalyst. In the presence of NaWMn/SiO2 ethane reacts faster, and the ‘conversion vs. contact time’ kinetic curves preserve an S-shape character typical for homogeneous branching chain processes. The observed features were rationalized using the numerical simulations in the framework of heterogeneous-homogeneous kinetic scheme that accounted both free-radical gas phase processes and reactions of molecular and radical species with catalytic active sites. It was shown that typical OCM catalysts play a dual role: they activate hydrocarbons by capturing H-atoms from C–H bonds and also efficiently terminate chain reactions by trapping free radicals. Consequently, over the catalyst that is more active in methane oxidation (Ca-La/Al2O3) ethane reacts slower due to the elimination of free radical chain branching and development. During the oxidation of ethane, methane – as an efficient radical scavenger – improves the selectivity to ethylene due to the inhibition of its secondary transformations (mainly to CO via HCO% radicals).
1. Introduction Light alkanes (LA) are the main constituents of natural and petroleum gases, as well as various technological gas mixtures. Their conversion into value-added products (semi-products for organic and petrochemical synthesis, synthetic fuels, etc.) is considered as one of the ways to efficiently use natural resources and reduce the industrial impact on the environment. The existing technologies of LA valorization are mainly based on their preliminary transformation to the mixture of hydrogen and carbon monoxide (synthesis gas). Developing alternative processes in which LA would be directly transformed into target
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products (chemicals or fuel components) is still a challenge. Light olefins are one of the main building blocks for basic organic chemistry and petrochemical synthesis. Nowadays they are produced by thermal and catalytic dehydrogenation and cracking of C2+ hydrocarbons. As a rule, these processes are endothermic and energy-intensive. Moreover, methane, as the most abundant component of hydrocarbon gas mixtures of various origin, cannot be involved in the existing processes of olefin production. These limitations can be avoided by using an oxidative conversion of LA. To date, three related processes of oxidative transformation in which light olefins are forming directly from LA are known:
Corresponding author. E-mail addresses: [email protected] (V. Lomonosov), [email protected] (E. Ponomareva), [email protected] (M. Sinev).
https://doi.org/10.1016/j.cej.2019.04.006
Available online 02 April 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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active sites and the heterogeneous-homogeneous free-radical nature of the LA oxidation processes. Although some important aspects of the LA oxidation mechanism are still being debated, or remain beyond consideration, at present, its general scheme is seen as follows. LA molecules are activated on the oxide catalysts by homolytic C–H bond cleavage resulting in surface OH-group formation and free alkyl radicals evolution. The final products of partial and deep oxidation are formed via a complex combination of secondary free-radical reactions, both homogeneous and surface-assisted. This means that the kinetic model for such reaction should include numerous elementary reactions of molecular and freeradical species, as well as surface active sites. Some of them are well studied, and their kinetic parameters are known and included into review articles and databases (see, for instance, [42,43]). On the other hand, some of elementary steps, including the most important heterogeneous reactions between catalyst active sites and gaseous molecular and free-radical species, are not accessible for direct observation and kinetic measurements. The main reason for the aforementioned discrepancies in the results reported by different authors may be due to the fact that the experimental data were obtained under different conditions. In the present work, the effects of the mutual influence of methane and ethane were studied under identical conditions over two model OCM catalysts, namely, NaWMn/SiO2 and Ca-La/Al2O3. Though they both are known as efficient OCM and ethane ODH catalysts, they are different in some important aspects, such as activity towards methane and ethane activation and product distributions. The experimentally observed conjugation effects were analyzed in framework of detailed heterogeneoushomogeneous kinetic model of C1-C3 hydrocarbon oxidation.
- oxidative coupling of methane (OCM); - oxidative dehydrogenation (ODH) of C2+ LA; - oxidative cracking of C3+ LA. These three processes have several common characteristics, such as optimum temperature ranges and the types of catalysts used. The main peculiarities of their mechanism (the initial reactant activation and free-radical character of reaction intermediates) are also the same. The general features of this type of processes, including the details of their mechanism and the development of catalysts for them, have been reviewed and analyzed elsewhere [1–12]. The main obstacle to the practical implementation of oxidative transformations of LA into valuable products is the unfavorable ratio of the reactivity of the initial hydrocarbons and the target products. As a consequence, the yield of the latter is lower than what is of practical interest, due to the sharp drop in selectivity at increasing conversion. Reduced selectivity also affects the thermal regime of the process as a large amount of heat is released during deep oxidation. In combination with the high optimal temperature (> 700 °C), this poses additional difficulties in terms of reactor operation. It should be noted that the separation of mixtures of various LA of natural or technological origin is also a difficult task. However, if their transformations into the same products can occur on the same catalysts and under similar conditions, it would be very attractive to involve them all in the process without prior separation. The effects of the reciprocal influence of methane and ethane during their simultaneous oxidation are of particular interest. First, ethane is present in natural gas. Besides that, ethane is always present in relatively high concentrations in the OCM reaction mixture as the primary product of methane coupling. The combined oxidation of methane and C2+ hydrocarbons has been investigated in a number of publications (see, for example, [13–23]). Though in some cases a positive effect in terms of increasing the selectivity and yield of olefins (not only ethylene, but also propylene) was demonstrated, in general, the data on the reciprocal influence of different LA during their simultaneous oxidation are contradictory. Some literature data indicate that methane can act as an inhibitor of the homogeneous ethane oxidation [13,21,23], while the results obtained under the conditions of catalytic OCM are conflicting. According to [14,17,20], the addition of ethane to the initial methane-oxygen mixture decreases the rate of methane conversion along with C2-hydrocarbon selectivity. The observed effect is attributed to the competition between methane and more reactive ethane either for the gas phase oxygen [14,17] or for the catalyst active sites [20]. By contrast, in [24,25] it was demonstrated that simultaneous presence of methane and ethane in the initial mixture noticeably increases the total conversion of both hydrocarbons. It should be noted that in the cited works only a qualitative estimate of reciprocal influence of methane and ethane was given. Anyhow, there is no doubt that in the case of simultaneous oxidation of several hydrocarbons, the effects of strong kinetic conjugation are manifested. On the one hand, this is not surprising, since the close similarity of the chemical structures and the mechanisms of transformation of various alkanes inevitably lead to the formation of the same intermediate active particles during their oxidation. And this is what obviously leads to conjugation. On the other hand, the chains of sequential-parallel transformations in the oxidation of hydrocarbons are so complex that conjugation can occur in different parts of their network, leading to very different effects depending on the specific conditions. Additional understanding of the essence of the observed effects can be achieved by analyzing reliable kinetic models reflecting the mechanism of the process. Several approaches to build such models have been proposed and discussed (see, e.g. [26–41]). To possess predictive power and successfully play the role of a tool in studying the mechanism, such models should account the main features of the catalyst
2. Materials and methods 2.1. Sample preparation NaWMn/SiO2 catalyst (0.8% Na, 3.2% W, and 2.0% Mn, 4.1 m2 × g−1) was prepared by incipient wetness impregnation of silica gel (Aldrich, Davisil grade 646) with aqueous solution of sodium tungstate followed by impregnation with aqueous solution of manganese nitrate after intermediate drying at 120 °C for 3 h. Ca-La/Al2O3 catalyst (20% Ca and 10% La, 3.5 m2 × g−1) was prepared by incipient wetness impregnation of α-Al2O3 with aqueous solution of calcium nitrate followed by impregnation with aqueous solution of lanthanum nitrate after intermediate drying at 120 °C for 3 h. The support was obtained by calcination of γ-Al2O3 (Sorbis Group) at 1200 °C for 15 h. Both catalysts were calcined at 600 °C for 2 h and at 900 °C for 6 h. 2.2. Experimental methodology The catalytic experiments were carried out in a quartz flow reactor with an internal diameter of 4 mm; the central part of the reactor was 8 mm in diameter and equipped with a quartz well for a movable thermocouple. A catalyst (0.25–0.5 mm fraction; 12 mg, or ∼0.03 ml, bed length ∼1 mm) was placed in the ring-shaped spacing between the walls of the reactor and the thermocouple well. No axial temperature gradient was detected along the catalyst bed. At the outlet, the reaction mixture passed through a trap chilled with a mixture of ice and saturated solution of sodium chloride to condense liquid products, and then the gaseous products were analyzed by on-line gas chromatography. The experiments on oxidation of methane, ethane and methane-ethane mixture were conducted in the temperature range 720–860 °C and at feed rate varied from 35 to 175 ml × min−1. The initial mixtures contained (in mol. %) 80 methane, 10 ethane, and 10 oxygen. Nitrogen was used as inert diluent to replace methane or ethane when oxidation of individual hydrocarbons was studied. A detailed description of the experimental setup and the chromatographic protocol were published elsewhere [44]. 1211
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Fig. 1. Oxygen conversion as a function of the inverse flow rate for oxidation of methane, ethane and their mixture over NaWMn/SiO2 (left) and Ca-La/Al2O3 (right) catalysts; solid lines – 760 °C; dashed lines 800 °C.
gas pressure was 0.1 MPa (1 bar) in all calculations. The initial concentrations of each form of active sites were determined in preliminary calculations to make them close to stationary during the entire process. For most cases, they were as follows:
2.3. Kinetic modeling The kinetic model used in this work consisted of two blocks: homogeneous and heterogeneous. In order to describe the processes that occur in homogeneous gas-phase, the kinetic scheme known as AramcoMech 2.0 was used. It was developed by the Combustion Chemistry Centre at NUI Galway [45,46]. In our opinion, among all published kinetic models, this scheme most fully implements the principles on which the kinetic model of LA oxidation must be based in order to serve as a reliable tool for studying the mechanism of the process [39]. It is well documented and verified on a large basis of experimental data obtained in a broad variety of reaction mixtures and reaction conditions [47]. In its basic version the AramcoMech 2.0 scheme operates with 493 species and 2716 reversible elementary reactions. For heterogeneous (surface) reaction description, a simplified scheme including 6 species and 50 elementary reactions was used. The main approaches to the construction of the heterogeneous part of the kinetic scheme are described elsewhere [32,36,39]. The list of included elementary steps and examples of modeling can be found in [48]. According to the main assumptions of the model [32,36,39], active sites can exist in three different forms:
[LO]0 = 0.05 [LX ]0 ; [LOH ]0 = 0.95 [LX ]0 ; [L]0 = 0. The calculations were performed using “Chemical Workbench” software developed and distributed by Kintech Lab company (Russia) [49]. 3. Experimental results For several reasons the experimentally determined parameter that allows one to directly compare the results obtained for the used reaction mixtures with different initial composition is the oxygen conversion. Oxygen is present in mixtures of all three compositions in the same concentration, which in all cases is lower than that required for the complete conversion of hydrocarbons according to the stoichiometry. The oxygen content limits the achievable conversion of hydrocarbons. Moreover, in the case of simultaneous oxidation of C1–C2 alkanes it is hardly possible to determine their conversions. On the one hand, ethane is consumed due to its oxidation, but it simultaneously forms via the OCM reaction. On the other hand, though the rate of methane formation from ethane is low and can be neglected, the methane conversion can hardly be determined due to its high concentration and low oxidation rate. Therefore, oxygen is the only reactant for which the degree of conversion can be determined distinctly; this value can serve as a comparative characteristic of the depth of the process. The data on oxidation of methane, ethane and their mixture at constant temperatures (760 and 800 °C) over NaWMn/SiO2 and Ca-La/ Al2O3 are presented in Fig. 1. These data indicate that during methane oxidation over NaWMn/SiO2 catalyst, in the range of flow rates used in this work the maximum oxygen conversion does not exceed 10% at 760 °C and 20% at 800 °C. In the case of ethane oxidation, almost total conversion of oxygen was reached at increasing contact time. Meanwhile, if inert gas in the initial mixture is replaced by methane, the rate of oxygen consumption drops substantially (mixture curve). One can conclude that simultaneous presence of methane and ethane in the initial mixture drastically changes the rates of oxidation of individual hydrocarbons. Obviously, the ethane oxidation is suppressed in the presence of methane. This effect is more noticeable at 760 °C compared to 800 °C. However, it should be noted that the methane conversion increases at rising temperature and its contribution to the oxygen conversion becomes more substantial. As a result, it is more difficult to trace the effect of methane onto the oxygen consumption in the oxidation of ethane.
- oxidized active form further denoted as LO (L – element of the oxide lattice, which is connected to active oxygen); - reduced hydroxylated form (LOH) – it arises during the homolytic abstraction of the hydrogen atom by LO from any gaseous particle; - reduced de-hydroxylated form (L). The active sites of different catalysts differ in the values of the main thermochemical parameters – the oxygen binding energy and the energy of O–H bond. The total concentration of these three forms ([LO] + [LOH] + [L] = [LX]) remains constant in the course of reaction. The kinetic parameters of the individual elementary steps taken from [48] were estimated using the thermochemical data (oxygen binding energy and strength of O–H bond in the [LOH]) obtained experimentally for the model Li/MgO catalyst. These values are similar to those for the Ca-La/Al2O3 catalyst used in this work. All calculations were carried out for an isothermal reactor of the calorimetric bomb type under conditions corresponding to those in which experimental data were obtained (see above). The reaction system was described in a quasi-homogeneous (or quasi-catalytic) approximation, i.e. catalytic active sites were added to the homogeneous gas mixture as free particles in different concentrations varied from 10 to 200 mol.% of the total concentration of gaseous particles; the total 1212
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The comparison of the data for two catalysts presented in Fig. 1 shows that in the case of oxidation of individual hydrocarbons not only the values of oxygen conversion achieved during a certain contact time, but also the shapes of the kinetic curves are different. In the presence of NaWMn/SiO2 catalyst, the rate of oxygen conversion is low when methane is oxidized. It is significantly higher when ethane is oxidized, and the kinetic curves are S-shaped. The latter is particularly noticeable at lower temperature. When methane is added to ethane, the S-shaped character of kinetic curves nearly disappears; also, the oxygen conversion is significantly reduced compared to what is observed during the oxidation of ethane. The Ca-La/Al2O3 catalyst is much more active in the oxidation of methane, whereas the rate of ethane conversion over this catalyst is lower. Thus, the oxygen conversion during the separate oxidation of methane and ethane becomes comparable at 760 °C, and the analysis of experiments on the oxidation of methane-ethane mixture becomes more complicated. Though the oxygen conversion is higher when both hydrocarbons are present in the feed, the total amount of oxygen consumed in separate oxidation of methane and ethane is higher compared to their simultaneous oxidation (Fig. 1, right panel). However, as the temperature increases to 800 °C, the difference in oxygen conversion during ethane and methane oxidation increases, while the oxygen conversion during mixture oxidation becomes lower compared to separate ethane oxidation. This likely indicates a decrease in the rate of ethane conversion in the presence of methane. In spite of clear differences in the oxidation of individual hydrocarbons in the presence of two studied catalysts, the shapes of kinetic curves observed during their simultaneous oxidation become very alike, as well as the amounts of oxygen consumed at the same contact times. Thus, it can be stated that the general kinetic patterns change when two hydrocarbons are present simultaneously in the initial mixture due to a strong conjugation between methane and ethane oxidation. In order to elucidate the effect of methane on different reaction paths in ethane oxidation we compared the concentrations of the main products obtained in the oxidation of the methane–ethane mixture with the sum of their concentrations in the separate oxidations of methane and ethane under the same conditions. As it is shown in Fig. 2, the amount of ethylene formed in the mixed feed over NaWMn/SiO2 catalyst increases along with the reverse flow rate and, correspondingly, at the increasing conversion of reactants. On the contrary, the total ethylene content obtained during separate oxidation decreases in the shown range of conversions, although the maximum ethylene yield can be obviously reached at lower conversion; the total amount of carbon oxides (COX) is substantially higher in the case of separate oxidation. Fig. 3 demonstrates that the amount of ethylene obtained in the mixed
Fig. 3. Concentrations of ethylene and COX as functions of temperature for oxidation of ethane (solid line) and methane-ethane mixture (dashed line) over NaWMn/SiO2 catalyst (flow rate – 125 ml/min).
feed over NaWMn/SiO2 catalyst constantly increases with the temperature at constant flow rate. At relatively low temperatures the amount of ethylene calculated as the sum from methane and ethane separate oxidation is significantly higher, but it passes through the maximum at 800 °C. Both dependencies (ethylene yield on contact time and on temperature) convincingly indicate that the presence of methane in the reaction mixture reduces the rate of ethylene formation, but at the same time suppresses the consecutive reactions of ethylene oxidation to carbon oxides. All in all, methane reduces the overall rate of ethane conversion and, in particular, it suppresses its oxidation to carbon oxides. At higher conversions, the ethylene consecutive oxidation is also restrained in the presence of methane.
4. Results of simulations The results of calculation performed using the AramcoMech 2.0 kinetic scheme demonstrate a significant difference in the main features of the homogeneous oxidation of methane and ethane in the studied range of parameters. At 800 °C oxygen conversion does not reach any noticeable value within 0.1 sec during the methane oxidation, whereas in the case of ethane it sharply increases after ∼5 msec and rapidly goes beyond 90% (see Fig. 4, curve [LX] = 0). In the case of a heterogeneous-homogeneous process, the
Fig. 2. Concentrations of ethylene and COX as functions of inverse flow rate for oxidation of ethane (solid line) and methane-ethane mixture (dashed line) over NaWMn/SiO2 catalyst (800 °C).
Fig. 4. Oxygen conversion as a function of time at varied concentration of active sites (oxidation of ethane, 800 °C). 1213
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effect can be attributed to the participation of methane in the primary activation of ethane molecules, because of the increase in the concentration of reaction products and the rates of secondary processes, which can also contribute to kinetic conjugation. Thus, the results of simulations show that the rate of methane conversion increases markedly, but the maximum rate of ethane conversion decreases if two hydrocarbons are present in the initial mixture. On the one hand, when ethane is added to the reaction mixture, it causes a rapid rise in the free radical generation and the development of chain reaction; methane becomes involved in oxidation process and, therefore, the rate of its oxidation increases noticeably. On the other hand, methane and ethane compete for the same active particles, and the rate of ethane conversion becomes lower in a certain range of conversions compared to its individual oxidation. As a result, the rate of oxygen consumption sharply increases compared to the individual oxidation of methane, but decreases compared to the ethane oxidation, which is accompanied with a qualitative change of the shape of kinetic curves (see Fig. 7). Interestingly, in the mixture of such composition (molar methane-to-ethane ratio 1:8) the rates of conversion of both hydrocarbons become almost equal in the whole range of [LX] from 0 to 2, which cannot be achieved during their separate oxidation. This result is similar to that obtained in [50]: if methane is used as a gas diluent during the ethane oxidation, the self-ignition character of the process is not manifested, but methane itself is involved into the formation of reaction products. Fig. 8 shows calculated concentration of ethylene and COX produced during the oxidation of ethane and methane-ethane mixture as a function of the reaction time. In the case of the separate ethane oxidation, the concentrations of ethylene and COX initially increase with reaction time. The slope of COX curve gradually decreases with reaction time, while concentration of C2H4 passes through the maximum at ∼0.04 s. The patterns change significantly in the case of simultaneous oxidation. Although the slope of the C2H4 formation curve decreases compared to separate oxidation, the ethylene concentration increases permanently over the entire range of the reaction time; finally, it exceeds the maximum concentration achieved during the separate oxidation of ethane. As for COX, their concentration is markedly lower in the case of the joint oxidation as compared to the separate ones. By comparing the results of experiments (Fig. 2) and simulations (Fig. 8), one can conclude that the observed patterns are identical, even if calculations are performed in the framework of the homogeneous scheme.
development of the oxidation of methane and ethane over time also exhibits very different trends. In the case of methane, the results obtained for [LX] ranged from 0.1 to 2 (from 10 to 200 mol.% of active sites relative to the total number of particles in the gas phase) demonstrate a gradual increase in the reaction rate; the ‘X(O2) vs. time’ curves are very slightly S-shaped, i.e. the rate of methane oxidation passes through a maximum at very low conversions. In the same range of [LX], the shape of kinetic curves for the oxidation of ethane changes dramatically. As can be seen from Fig. 4, at low concentrations of active sites ([LX] = 0.1–0.2) they are similar to the one observed for homogeneous oxidation. At increasing [LX] their shape gradually changes and approaches the same observed in the oxidation of methane. It is worth noticing that at low reaction times (< 10 msec at 800 °C, see insert in Fig. 4) and conversions the rate of reaction increases at increasing [LX]. However, at low [LX] the development of free-radical processes is much more pronounced, and high conversions are reached much faster. This is why at lower concentrations of active sites high conversions are achieved at shorter reaction times. As we noted above, it is rather difficult to experimentally measure the rates of conversion of methane and, moreover, of individual hydrocarbons during the oxidation of their mixture. In the case of modeling, it is possible not only to obtain the values of the overall rates (consumption and/or formation) for any substance, but also to determine the contribution of individual elementary processes to them. It was already mentioned that the methane conversion rate passes through a maximum at very low conversions. This holds true also for its oxidation together with ethane. However, if in the individual oxidation of methane the rate of its conversion smoothly increases from the value close to zero at [LX] = 0, the addition of 10 vol% of ethane sharply increases the rate of methane oxidation even in the absence of catalytic active sites (see Fig. 5). It further increases at increasing [LX], but the slope of the 'rate vs. [LX]' curve decreases at [LX] > 0.3, though the rate of methane conversion still remains higher in the presence of ethane. The family of curves shown in Fig. 6 demonstrates that the effect of methane on the rate of ethane oxidation is more complex. At [LX] < 1 the maximum rate of ethane oxidation is substantially lower in the presence of methane. However, at low conversions, i.e. during the period of the development of chain process, methane accelerates the rate of ethane conversion, and this effect becomes more and more pronounced at increasing concentration of catalytic active sites. The conversion of oxygen at which the change from promotion to inhibition occurs increases from ∼0.5 to ∼5% with an increase in [LX] from 0 to 2. It is interesting to note that with an increase in [LX], the rate of oxidation of ethane in a mixture with methane increases again as compared to separate oxidation. However, it’s not clear if the observed
5. Discussion The results of experiments performed in this work can be summarized as follows: 1. the main features of the catalytic oxidation of methane and ethane differ significantly; 2. the rate of simultaneous oxidation of two hydrocarbons is lower than the sum of the rates of their separate oxidations; 3. the formation of carbon oxides is suppressed more significantly than the formation of ethylene during the simultaneous oxidation of methane and ethane. It is worth noticing that the results of simulations not only reproduced the experimental observations (see, for instance, the data presented in Figs. 1, 2 and 7, 8), but also shed a new light onto the origin of experimentally observed regularities. Let us first compare the data presented in Figs. 1 and 4. It is evident that the shape of kinetic curves for ethane oxidation observed for the NaWMn/SiO2 catalyst is close to the one that is typical for simulated curves obtained at low concentrations of active sites. Whereas for the Ca-La/Al2O3 catalyst the ‘X(O2) vs. 1/V’ curves are more similar to those obtained at high concentrations of [LX]. These observations strongly correlate with the data obtained for the individual oxidation of
Fig. 5. Maximum methane oxidation rate as a function of [LX]: (1) – no ethane added; (2) – 10 vol% ethane added. 1214
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Fig. 6. Rates of ethane conversion at different [LX] and oxygen conversions; effect of methane.
no pronounced S-shaped character of the oxygen conversion during the oxidation of ethane). The data presented in Fig. 9 demonstrate that, as we already mentioned, the rates of methane and ethane conversion when they both are present in the initial mixture are nearly the same. Fig. 9 also allows us to compare the rates of elementary steps that the most significantly contribute to the activation of methane and ethane. The major contribution is made by OH and HO2 radicals, H atoms, and also CH3 radicals in the case of ethane activation in the presence of methane. The rates of their interaction with two alkanes are also very close, which has a simple explanation: the values of rate constants for all these particles are about one order of magnitude higher in the case of ethane [42,43], but the concentration of the latter in the mixed feed is 8 times less, which compensates the difference in reactivities. One may assume that the patterns should be different for the other ratios of methane and ethane. The shape of the kinetic curves together with the obtained values of the concentrations of active radicals and the rates of elementary reactions show that during the oxidation of ethane the free radical chain branching, as a whole, is more intense compared to the methane oxidation. As a result, the rates of radical reactions during the developed chain process become much higher than the rates of the heterogeneous activation of ethane over catalyst active sites, if the concentration of the latter is not very high (as in the case of NaWMn/SiO2 catalyst). Therefore, at low degrees of conversion, ethane activation over catalytic active sites prevails, while the rate of homogeneous conversion becomes higher with the development of a chain process. Evidently, the catalyst plays a dual role: it increases the rate of the alkane molecule activation, but also acts as an efficient radical scavenger. The latter leads to the inhibition of ethane conversion by the catalyst active sites in the later stages of the reaction. These results are in good agreement with the experimental data obtained earlier. As it was shown in [24], the rate of ethane oxidation decreases in the presence of NaWMn/SiO2 catalyst as compared to its oxidation in an empty reactor. According to [19], several calcium-based OCM catalysts are able to inhibit the gas-phase oxidation of ethylene also due to the capture of radical chain-carriers by their active surface sites. As can be seen in Fig. 9, the rate of methane conversion is almost three orders of magnitude lower than that of ethane in the case of their separate oxidation. However, it increases by approximately two orders of magnitude in the presence of ethane at short reaction times. Meanwhile, the rate of ethane conversion decreases by about 3.5 times under the same conditions since the produced radicals are actively intercepted by methane molecules. Both experimental (Fig. 2) and simulation (Fig. 8) data demonstrate
Fig. 7. Oxygen conversion as function of contact time at 800 °C and LX = 0 during the oxidation of methane, ethane, and methane-ethane mixture.
Fig. 8. Concentration of ethylene and COX as functions of contact time for oxidation of ethane (solid line) and methane-ethane mixture (dashed line); 800 °C LX = 0.
methane: oxygen consumption over NaWMn/SiO2 in the same conditions is by far lower than over Ca-La/Al2O3. In other words, for both processes the two catalysts behave similarly: NaWMn/SiO2 – as a system with low concentration of active sites (low activity in methane oxidation, high rates and S-shaped curves of oxygen conversion vs. contact time in ethane oxidation), and Ca-La/Al2O3 – as a system with high concentration of active sites (higher activity in methane oxidation, 1215
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Fig. 9. Overall rates (in molec. × cm−3 × s−1) of methane and ethane conversion and partial rates of elementary reactions as functions of oxygen conversion: 1 – d [CH4]/dt; 2 – CH4 + OH < = > CH3 + H2O; 3 – CH4 + H < = > CH3 + H2; 4 – CH4 + HO2 < = > CH3 + H2O2; 5 – C2H6 + CH3 < = > C2H5 + CH4; 6 – LO + CH4 < = > LOH + CH3; 7 – d[C2H6]/dt; 8 – C2H6 + OH < = > C2H5 + H2O; 9 – C2H6 + H < = > C2H5 + H2; 10 – C2H6 + HO2 < = > C2H5 + H2O2; 11 – LO + C2H6 < = > LOH + C2H5; 12 – CH3 + CH3 + M < = > C2H6 + M; 13 – C3H8 + M < = > CH3 + C2H5 + M; A – oxidation of methane; B – oxidation of ethane; C – mixed feed.
that the product distributions substantially change in the case of simultaneous methane and ethane oxidation compared to their separate conversion, namely a significant shift from the formation of COX to ethylene is observed. This agrees well with earlier experimental observations (see, for example, [13,23]) that the rates of ethylene oxidation to carbon monoxide are markedly reduced in the presence of methane in the reaction mixture. The analysis of simulation results shows that the main path of ethylene transformation starts from its reaction with hydroxyl radical: C2H4 + OH = > C2H3 + H2O
(1)
while produced vinyl radical is then rapidly transformed into CO by reactions: C2H3 + O2 = > CH2O + HCO CH2O + X = > HCO + XH HCO + X = > CO + XH
(2) Fig. 10. Rates of elementary reactions responsible for ethylene conversion to carbon monoxide as functions of contact time: solid lines – oxidation of ethane; dashed lines – oxidation of methane-ethane mixture.
where X is any free radical, catalyst active site, or O2 molecule. As can be seen from Fig. 10, the rate of vinyl radical formation noticeably decreases in the presence of methane, which effectively intercepts hydroxyl radicals:
intensively during the oxidation of ethane participate in the conversion of methane, as a result of which the conversion rates of both hydrocarbons change dramatically under the same conditions. In the presence of catalytically active sites, alkane molecules are activated on them already at very low degrees of conversion and long before a surge in the reaction rate due to the development of a chain process. The particular manifestation of the reciprocal influence of free radicals and catalytic active sites depends on the concentration and specific activity of the latter. The elucidation of the reasons for the acceleration of ethane conversion in the presence of methane at the initial stages of the process requires a detailed analysis of the rates of all elementary stages in
CH4 + OH = > CH3 + H2O In the same way methane intercepts all other active species that are able to transform ethylene to vinyl radical. This is the reason for the inhibition of CO formation from ethylene via CH2O and HCO. 6. Conclusions The similarity of the chemical processes of free radical nature in the oxidation of methane and ethane is the main cause of the appearance of strong kinetic conjugation. The active particles generated more 1216
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which free radicals participate. Most likely, this is due to some differences in the transformations of methane and ethane, primarily the activity and the pathways of transformation of the oxy- and proxy-radicals formed in the initial stages of the transformation of alkane molecules.
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