Autothermal Oxidative Dehydrogenation of Ethane on LaMnO3- and Pt

Ind. Eng. Chem. Res. 2005, 44, 285-295

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Autothermal Oxidative Dehydrogenation of Ethane on LaMnO3- and Pt-Based Monoliths: H2 and CO Addition Francesco Donsı`,*,† Stefano Cimino,‡ Raffaele Pirone,‡ and Gennaro Russo†,‡ Dipartimento Ingegneria Chimica, Universita` di Napoli Federico II, Piazzale Tecchio 80, I-80125 Napoli, Italy, and Istituto di Ricerche sulla CombustionesCNR, Piazzale Tecchio 80, I-80125 Napoli, Italy

The autothermal oxidative dehydrogenation of ethane in short contact time reactors can be carried out with high ethylene yields on LaMnO3-based non-noble-metal catalysts. The performance of LaMnO3 honeycomb monoliths is evaluated in direct comparison with state-ofthe-art Pt catalyst in an autothermal reactor configuration at varying external heating, N2 dilution, C2H6/O2 ratio, and total flow rate. LaMnO3 and Pt are also compared after the addition of a secondary fuel. Evidence is present that H2 and especially CO addition are the means of increasing ethylene selectivity above 80% on LaMnO3, as preferentially this is oxidized in place of ethane. The lower activity of Pt toward CO oxidation makes CO addition ineffective on such a catalyst. 1. Introduction The oxidative dehydrogenation of ethane (ODH) for ethylene production has been subject to renewed interest in recent years, due to the evidence that olefins can be efficiently produced with high yields in structured reactors at high space velocity (short contact time reactors, SCTRs). Until recently, platinum emerged as the most active phase for ethylene formation.1-4 Furthermore, doping Pt with tin or copper resulted in improved performance for ethane ODH, with a significant increase in ethylene yield,5 which was even greater as a result of H2 addition as sacrificial fuel.3 It seems now widely accepted that ethylene is mostly formed in the gas phase via homogeneous radicalic reactions.6-11 We have recently shown, by means of a mathematical model,12 that large ethylene yields may be attained over a combustion catalyst, which ignites and sustains the gas-phase reactions through release of heat generated by the sacrifice of a fraction of the fuel.12 In support of this theory, the experimental results of Mulla et al.7 showed scarce differences in the conduction of ODH experiments at short contact times on catalytic and noncatalytic reactors (inert pellets) only when adequate preheat was applied to compensate for the absence of the catalyst. Analogously, the ethane ODH experiments of Beretta et al.,9,10 carried out on Pt under diluted and temperature-controlled conditions, showed that at low temperature only COx and H2O are formed via a catalytic route, while a certain ethylene yield is observed only above 650 °C, and can be entirely attributed to the gas phase reactions. Moreover, by sampling the gas composition downstream of a Pt foam catalyst, Henning and Schmidt reported that a large fraction of ethylene is formed outside the catalyst, while just at the exit mainly oxygenated products are found (COx selectivity ∼52 vs ∼44 of C2H4), suggesting that Pt is important to the process especially for its oxidation activity.11 * To whom correspondence should be addressed. Tel.: +39 0817682237. Fax: +39 081 5936936. E-mail: [email protected]. † Universita` di Napoli Federico II. ‡ Istituto di Ricerche sulla Combustione.

This consideration led to the indication that a good oxidation catalyst, even non-noble-metal based, can be employed in the process, as was recently demonstrated in a number of experimental works.8,13-17 Nevertheless, the good performance of Cr2O3-based foam monoliths was limited in time probably due to a severe deactivation process of the catalyst.13 Analogously the rapid coke deposition occurring on hexa-aluminate BaMnAl11O19 catalysts drastically reduced the reactor operation time.14 Among non-noble-metal active catalysts, LaMnO3 perovskite-like monoliths, developed in our research group for applications of high-temperature catalytic combustion,18,19 seem to represent a good compromise between thermal stability and oxidation activity in short contact time reactors for ethane ODH.16,17 It was shown that on perovskite-based honeycombs the reaction can be carried out with an ethane-air feed at moderate preheat and with performance comparable to that of noble-metal-based catalysts, without any sign of deactivation under many hours (up to 90) of reaction. In this work, we aim to extend the previous investigation of LaMnO3 perovskite-like monoliths to a wider range of conditions, evaluating the performance in a head-to-head comparison with a Pt-based catalyst, which at present may be considered the state-of-theart catalyst, under the same experimental conditions described in our more recent work,17 characterized by the higher degree of adiabaticity than in our first work on the subject.16 Furthermore, the effect of adding a secondary fuel is comparatively investigated on Pt and LaMnO3. Indeed, on Pt and Pt/Sn, H2 addition was profitably applied to increase reactor performance,3 starting from the consideration that hydrogen is one of the main byproducts of ODH and can be recycled to the reactor in a loop. In this work, the option of H2 addition is evaluated for perovskite-based systems too, and, in consideration of the well-known activity of perovskites toward its oxidation,20 also CO is investigated for the first time as a possible secondary fuel.

10.1021/ie0492911 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004

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2. Experimental Section Catalyst Preparation. Both Pt and LaMnO3 active phases are deposited on 400 cpsi cordierite honeycomb monoliths in the shape of disks with 1 mm channel hydraulic diameter, ∼18-mm external diameter, and 10mm length. In the case of Pt, the metal is directly deposited on bare cordierite by dry impregnation of the honeycomb monoliths with a diluted solution of H2PtCl6, as previously described by Huff and Schmidt.1 After drying, the catalyst sample is calcined for 3 h at 700 °C in air, and the cycle is repeated to reach a target Pt loading of 2.5% w/w, which was chosen according to the experimental evidence that above 1% w/w the catalyst loading does not affect the performance.21 The preparation of the perovskite-based honeycomb monoliths via active-phase deposition on cordierite is essentially identical to that described in our first work.16 Briefly, ceramic honeycombs are washcoated with alumina by repeated dipping in a slurry of submicronic γ-Al2O3 powder. The washcoat is stabilized with La2O3 (5% w/w) added by impregnation, followed by calcination at 800 °C for 3 h. The LaMnO3 active phase (30% w/w, monolith substrate excluded) is deposited on the washcoat by repeated coimpregnation with an equimolar aqueous solution of precursor salts, lanthanum nitrate (La(NO3)2‚6H2O), and manganese acetate (C4H6MnO4‚ 6H2O). After drying, the samples are calcined at 1000 °C for 3 h. Reactor Configuration. The reactor configuration, as already reported,17 has been adjusted to achieve a higher degree of adiabaticity with respect to that previously described.16 In particular, catalyst dimensions were switched from a 40-mm long monolith with square cross section and 49 channels to a 10-mm long disk with around 130 open channels.17 The catalytic monoliths were placed in a quartz reactor and tightly sealed with a ceramic wool paper to avoid gas bypass and to reduce heat loss by conduction. The quartz reactor itself was wrapped with ceramic wool and placed in an electric furnace, used only to light-off the reacting mixture, unless explicitly specified. Three K-type thermocouples (d ) 0.5 mm) were placed in the central channels of the front radiation shield (T1, upstream of the catalyst), of the catalyst (T2), and of the back radiation shield (T3, downstream of the catalyst), respectively, while a fourth thermocouple (T4, K type with d ) 1 mm) was placed downstream of the back heat shield to measure the gas exit temperature. Experimental Setup. High-purity gases calibrated via Brooks 5850 series mass-flow controllers were premixed and fed to the reactor at a gas hourly space velocity (GHSV) comprised between 18 000 and 480 000 h-1 as evaluated at standard conditions on the basis of monolith volume, corresponding to residence times between 200 and 7.5 ms. Feed gas composition was varied with a C2H6/O2 ratio between 1.5 and 2.5, usually with 30% N2 dilution. Reactor pressure was always kept between 1.2 and 1.3 atm, and feed composition was always above the upper flammability limit at room temperature. Additional fuels (H2 or CO), expressed in terms of fuel/O2 ratio, were fed in appropriate amounts holding all other flow rates constant (at C2H6/O2 ) 2), so that the total flow rate increased and N2 dilution decreased with secondary fuel. The presence of an additional fuel (CO or H2) in the feed moved the

experimental conditions farther away from the upper flammability limit.3 The ignition of the reactor was achieved by heating the system in the electric furnace above the threshold temperature of the catalyst, prior to feeding the reactants. Once ignited, external heating is totally removed, unless for the evaluation of the effect of preheating. Product gases passed through a CaCl2 trap to selectively remove water, prior to splitting to a ABB continuous analyzer, employed to measure concentrations of H2 (Caldos17), CO, CO2, and CH4 (Uras14), and to an online gas chromatograph equipped with molecular sieve and Poraplot Q columns and TCD and FID detectors to measure C2H6, C2H4, C2H2, CH4, N2, O2, CO, CO2, and other eventual hydrocarbons up to C4. Nitrogen was used as an internal calibration standard. 3. Results In our previous work, ethane ODH tests were carried out over a less adiabatic LaMnO3-based monolithic reactor, operated with a very large N2 dilution (60% vol), thus always requiring external heating of at least 275 °C to keep the reactor ignited.16 In the new reactor configuration, the LaMnO3-based catalyst can selfsustain the ODH process autothermally, with large ethylene yields also in the absence of external heating thanks to the higher degree of adiabaticity, due to increased size and more accurate insulation of the catalytic reactor. Effect of External Heating. The effect of external heating on the autothermal performance of LaMnO3 catalyst is examined first. Results reported in Figure 1 in terms of ethane conversion, ethylene yield, and main products selectivity and Figure 2 (measured temperature profiles along the reactor) are obtained for a C2H6/ O2 ) 2 mixture flowing at 50 slph and at 30% vol N2 dilution, by varying the furnace temperature (indicated as external temperature in Figures 1 and 2) between room temperature and 650 °C, which is still below the ignition temperature of homogeneous reactions. Oxygen conversion is not reported as it is always complete in all of the experiments conducted. The reactor performance seems to strongly depend on catalyst temperature, which is in turn function of external heating. On increasing the furnace temperature from room temperature to 650 °C, ethane conversion rises from 66% to 89%, while ethylene selectivity decreases slightly from 71% to 65% (Figure 1). This leads to an overall increase in the C2H4 yield from 47% to 57%. On larger external heating, and hence at higher reactor temperature, again CH4, C2H2, H2, and CO selectivities increase, and CO2 and H2O selectivities decrease (Figure 1). Noticeably, on increasing furnace temperature by 600 °C, the temperatures measured in the reactor do not increase correspondingly (Figure 2). In particular, while the front shield and the downstream temperatures exhibit a more direct dependence on external temperature, the catalyst temperature rises by only ∼50 °C. Furthermore, as the reactor gets hotter due to increasing external heating, a larger fraction of endothermic reactions of dehydrogenation and cracking takes place, as revealed from the increase of selectivity of CH4 and C2H2, hence contributing to the reduction of the adiabatic temperature rise (∆Tad). In fact, Figure 2 shows that the adiabatic temperature Tadiab ) Texternal + ∆Tad, calculated from the measured product distribu-

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Figure 1. Ethane conversion, ethylene yield, and selectivity of main products on LaMnO3-based monolith as a function of external temperature. Experimental conditions: flow rate ) 50 slph, C2H6/O2/N2 ) 46.7/23.3/30.

Figure 2. Measured temperature profiles and calculated adiabatic temperature from product distribution for the conditions of Figure 1.

tion, increases by only 400 °C for a furnace temperature variation of 625 °C. Effect of Dilution. With a more adiabatic reactor configuration, nitrogen dilution could be varied to between 10 and 60% vol, with a C2H6/O2 ) 2 mixture flowing at 50 slph and absence of external heating, as it is possible for the reactor to stay continuously ignited. Results are reported in Figure 3, where ethane conversion and atom selectivity of the main products

are showed, and in Figure 4, which reports the measured temperature profiles. On increasing N2 dilution from 10% to 60%, ethane conversion falls drastically from ∼80% down to ∼50%, while ethylene selectivity is less severely affected, exhibiting a small maximum for a N2 dilution around 30% vol. The distribution of the main byproducts also changes, with a higher selectivity of CO2 and H2O with larger N2 dilution. This can be related to the temperature of the reactor, which becomes hotter at reducing N2 dilution, as shown in Figure 4. For instance, the temperature of the catalyst changes by more than 100 °C, reducing N2 dilution from 60% down to 10% vol. In concurrence with the increase in ethane conversion, the selectivity of CH4, C2H2, H2, and slightly of CO is also increased, as these products are favored at higher temperature. The small maximum in C2H4 selectivity is reasonable due to the fact that the temperature increase caused by the reduction of N2 dilution is beneficial to ethylene formation, but when temperature is too high the byproducts of the degradation of ethylene (mainly CH4 and C2H2) are formed. 3.1. Head-to-Head Comparison between Pt and LaMnO3. Once it has been proved that the new reactor configuration is able to improve the previously obtained performance of LaMnO3-based catalyst, such performance is evaluated in direct comparison with that of Pt. The head-to-head comparison between Pt and LaMnO3 is carried out under the same experimental conditions and with the same morphology of the catalyst, varying the C2H6/O2 ratio and the flow rate. The results obtained on Pt are consistent with the results reported in the literature,3,5 despite the different experimental

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Figure 3. Ethane conversion, ethylene yield, and selectivity of main products on LaMnO3-based monolith as a function of N2 dilution. Experimental conditions: flow rate ) 50 slph, C2H6/O2 ) 2.

Figure 4. Measured temperature profiles in LaMnO3-based reactor for the conditions of Figure 3.

setup and catalyst morphology (honeycomb vs foams), as highlighted in the Discussion and Conclusion section. Effect of the C2H6/O2 Ratio. The C2H6/O2 ratio is varied between 1.5 and 2.5 maintaining constant N2 dilution (30% vol) and the total flow rate (50 slph, corresponding to a mean residence time of 35 ms, calculated at the average temperature of 900 °C). Figure 5 compares the effect of the C2H6/O2 ratio on LaMnO3 and on Pt. Ethane conversion and ethylene selectivity are always larger in the perovskite-based reactor under all of the experimental conditions tested (Figure 5). It must be noted that significantly larger quantities of CO are produced on Pt rather than on perovskite: CO selectivity ranges from 27% to 24% on

Pt, while it ranges from 17% to 10% on LaMnO3 (Figure 5). Correspondingly, CO2 selectivity is always lower on Pt, where it decreases from 10% to 6% on decreasing C2H6/O2 ratio, than on LaMnO3, where it decreases from 15% to 12% (Figure 5). Instead, Pt produces more H2O (but less H2) than LaMnO3. The temperature profiles reported in Figure 6 for LaMnO3 and Pt reveal that the temperature of the front shield in the Pt-based reactor is always higher than the corresponding temperature of the LaMnO3-based sample. Conversely, the peak temperature, always measured on the catalytic monoliths, attains quite similar values for both systems. A very likely explanation is that the reactants fed to the reactor are heated to the threshold temperature of the catalyst before reaching the reaction front: for Pt, such a temperature is lower than for LaMnO3 (∼400 °C) due to its higher specific activity. Consequently, on LaMnO3, the front section of the catalytic monolith is colder and works only as heat exchanger, while the reaction front is postponed. However, the temperature profiles appear only shifted one with respect to the other along the axial coordinate, with identical maximum temperatures due to very similar heat losses in the two systems. It must be highlighted that the differences in temperature in the catalyst bed as well as in the upstream mixing section and the downstream outlet section of the reactor may affect ethylene selectivity and yield due to the strict coupling between homogeneous and heterogeneous reactions in this autothermal system. Hence, the comparison must be intended between two whole reactor systems, thus including also the different


Figure 5. Comparison of Pt- (solid lines) and LaMnO3-based (dashed lines) monolith reactors: ethane conversion and selectivity of main products as a function of C2H6/O2 ratio. Experimental conditions: flow rate ) 50 slph, N2 ) 30% vol.

Figure 6. Measured temperature profiles in LaMnO3- (a) and Pt-based (b) reactors for the conditions of Figure 5.

temperature profiles the two catalysts induce due to their specific activity features.

Effect of the Flow Rate. Space velocity is varied by changing the total flow rate of a reacting mixture at

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Figure 7. Comparison of Pt- (solid lines) and LaMnO3-based (dashed lines) monolith reactors: ethane conversion and selectivity of main products as a function of total flow rate. Experimental conditions: C2H6/O2 ) 2, N2 ) 30% vol.

fixed composition C2H6/O2/N2 ) 46.7/23.3/30 between 25 and 125 slph, corresponding to a GHSV ) 1.2-6 × 104 h-1. This corresponds to a contact time interval ranging from 15 to 75 ms, evaluated at an average temperature of 900 °C. Figure 7 compares the performances observed on Pt and on LaMnO3. Similarly as with C2H6/O2 variation, ethane conversion and ethylene selectivity are always better in the perovskite-based reactor under all of the experimental conditions tested, due to a more favorable product distribution with larger amounts of CO2 on LaMnO3. Both on Pt and on LaMnO3, upon increasing the flow rate from 25 to 50 slph, ethane conversion and ethylene selectivity significantly rise, while above 75 slph they are almost unchanged by flow rate. The drastic temperature reduction observed for both catalysts at the lowest flow rate (Figure 8) confirms that the system tends to lose adiabaticity, because of the insufficient

power generated, as previously discussed:16,19 a flow rate of at least 50 slph is required to apply the short-contacttime concept to our experimental reactor. On increasing the flow rate above 75 slph, the temperature increase becomes progressively less relevant, because above 900 °C the mechanisms of heat dissipation by radiation become efficient. The axial temperature profiles measured along the reactor (Figure 8) reveal that by increasing the flow rate, the front radiation shield is drastically cooled on LaMnO3 from 775 to 425 °C, and from 780 to 650 °C on Pt, due to the higher activity of the noble metal that maintains the reaction front closer to the inlet face of the catalytic monolith. Due to its lower activity, the LaMnO3-based reactor undergoes extinction above 150 slph. For this reason, the comparison between Pt and LaMnO3 is carried out at longer residence times than reported in the literature1,3-5 (∼15 vs ∼5 ms). Nevertheless, it must be noted that moderate preheating above the threshold


Figure 8. Measured axial temperature profiles in LaMnO3- (a) and Pt-based (b) reactors for the conditions of Figure 7.

Figure 9. Comparison of Pt- (solid lines) and LaMnO3-based (dashed lines) monolith reactors: ethane conversion and C-atom selectivity of main products as a function of H2 addition. Experimental conditions: H2 progressively added to a C2H6/O2/N2 ) 46.7/23.3/30, 50 slph mixture.

temperature of LaMnO3, which Figure 1 shows is undoubtedly beneficial in terms of ethylene yield, represents a viable way to stabilize the LaMnO3-based reactor also at larger space velocities. 3.2. Fuel Addition. It was previously reported3 that the addition of H2 as secondary fuel to the C2H6/O2 feed significantly improved the ethylene selectivity of Pt and Pt/Sn catalyst, as H2 is oxidized in the reactor in place of ethane to generate the heat that sustains the gasphase dehydrogenation to ethylene. H2 addition is hence investigated on LaMnO3 in comparison with Pt. Moreover, the use of CO as additional fuel in ethane ODH is evaluated for the first time, based on the consideration that CO is another valuable byproduct which can be recovered and recycled back to the reactor

to increase the overall ethylene yield. Such an option appears particularly promising with LaMnO3-based catalysts, due to its higher carbon deep oxidation activity with respect to Pt. H2 Addition. The effect of H2 addition is investigated for a H2/O2 ratio between 1 and 3 added to a C2H6/O2/ N2 ) 46.7/23.3/30 mixture flowing at 50 slph. Figure 9 compares the experimental results obtained co-feeding H2 on Pt and on LaMnO3. As observed also in the absence of additional fuel, ethane conversion is always higher on LaMnO3 (Figure 9) and progressively drops for both systems as a response to larger hydrogen additions. On the other hand, the Pt catalyst rapidly fills the initial gap in ethylene selectivity with LaMnO3 in the presence of hydrogen, and already for H2/O2 ) 1

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Figure 10. Comparison of Pt- (solid lines) and LaMnO3-based (dashed lines) monolith reactors: ethane conversion and selectivity of main products as a function of CO addition. Experimental conditions: CO progressively added to a C2H6/O2/N2 ) 46.7/23.3/30, 50 slph mixture.

ethylene, selectivity is the same for both catalysts (Figure 9), and increases with the same pace up to 80%. Because most of the oxygen is consumed for hydrogen oxidation, increasing H2 addition, both CO and CO2 selectivities drop, with the first one again higher on Pt and the second one higher on LaMnO3. The reactor temperature is almost unaffected by H2 addition (and hence not shown), as the main parameter in determining reactor temperature appears to be the O2 flow rate, which is unchanged. O2 is indeed the limiting reactant for the oxidation reactions occurring on the catalyst, thus determining the extent of heat production. Coherently with what has been previously discussed in this case, the front temperature on Pt is still higher than that on LaMnO3. CO Addition. Based on the evidence of good activity of CO oxidation of perovskite-based catalysts,20 CO is added to a C2H6/O2/N2 ) 46.7/23.3/30 mixture flowing at 50 slph, for a CO/O2 ratio ranging from 1 and 3, analogously to H2 addition. The performance of LaMnO3 catalyst upon addition of CO qualitatively does not differ from the case of H2 addition. Ethane conversion drops, while ethylene selectivity rises on increasing the additional fuel, as shown in Figure 10. On the other hand, it must be noted that the conversion loss on Pt (from 57% to 47%) is more severe than on LaMnO3 (from 66% to 58%), while the increase in ethylene selectivity is quite small at CO/O2 ) 1, and absent at higher CO/O2 ratios. On the contrary, LaMnO3 is able to oxidize CO much more efficiently than Pt, and hence CO can be fully used as sacrificial

fuel in place of ethane, leading ethylene selectivity to rise up to 80% (Figure 11b). Also in this case, it can be noted that, on Pt, higher selectivity of H2O and lower selectivity of H2 are obtained than on LaMnO3. Pt performance after CO addition exhibits a negligible improvement probably because most of the CO passes unreacted through the catalytic reactor. In addition, the low activity of the noble metal toward CO oxidation under the conditions investigated (rich mixtures) leads to a rapid catalyst deactivation, thus reducing catalyst lifetime to a few hours. In the case of LaMnO3, instead, catalyst could be used for more than 100 h under a variety of experimental conditions, including CO addition, without showing any sign of deactivation and always giving very repeatable results. It was already shown by means of spectroscopic studies with deuterium that, in the case of H2 addition, H2 reacts on the catalyst before the ethane, thus producing heat that drives the ethane dehydrogenation and roughly forms the same amount of fed H2.22 To shed light on the role played by CO when co-feeding it with ethane on LaMnO3 catalysts, and to prove that, in analogy with H2, CO too reacts on the catalyst rather than passing through it unchanged, the transient lightoff behavior of the catalytic reactor has been analyzed. To this aim, the LaMnO3-based monolith is placed in a furnace heated at 350 °C with a C2H6/O2/N2 ) 46.7/23.3/ 30 mixture flowing at 50 slph. The system does not ignite with this external heating, as the catalyst tem-


Figure 11. Transient behavior upon CO addition to the feed on LaMnO3-based monolith. Measured temperature profiles (a), C2H6, C2H4, CH4, and O2 molar fractions measured by GC (b), and CO, CO2, and H2 molar fractions measured by continuous analyzers (c). Experimental conditions: flow rate ) 50 slph, C2H6/O2 ) 2, N2 ) 30% vol, preheat temperature ) 350 °C, step addition of CO (CO/O2 ) 2) at 2.4 min.

perature is below its threshold temperature (∼400 °C for LaMnO3). At time t ) 2.4 min, a CO step (CO/O2 ) 2) is added to the mixture and the transient behavior is followed in time until a steady state is reached, continuously monitoring temperatures at four locations (Figure 11a) and CO, CO2, and H2 concentrations (Figure 11c) by means of the on-line analyzers. The other species are measured by GC analysis. Nevertheless, because GC analysis is at least 15 min long, each experimental point of Figure 11b corresponds to a single transient experiment; that means that as many transient CO addition runs as points in Figure 11b were followed, resulting in a very good repeatability. Such circumstance confirms that LaMnO3 catalyst does not seem to undergo any deactivation upon CO addition, differently from Pt. As soon as, at 2.4 min, the CO step is added to the feed, the reactor is first of all cooled due to the increased flow rate (see in particular T1 in Figure 11a). Because threshold temperature for CO oxidation on LaMnO3 is lower than that for ethane oxidation, upon CO addition the reactor ignites: after about 1 min from its addition, the CO step exhibits a negative peak, highlighting its consumption. The negative peak in CO concentration is accompanied by a positive peak of CO2 concentration and by the steep increase of catalyst and back shield

temperatures. At this point (t ) 4 min), all of the O2 has already disappeared, the reactor is very hot (above 800 °C), and also the ethane dehydrogenation reactions are ignited. Figure 11c shows the increase of H2 concentration, and Figure 11b shows the drop of C2H6 concentration accompanied by the formation of C2H4. As time elapses, the reactor temperatures and species concentration attain steady values, reached for t e 15 min. 3.3. Recycling of the Secondary Fuel to the Reactor. The option of co-feeding a secondary fuel may be more advantageous if the same amount of secondary fuel is recovered as that which is fed to the reactor, to perform a closed recycling loop. Figure 12 reports the ethylene yields attained on Pt and LaMnO3 and compares the moles of H2 and CO recovered at the exit of the reactor with the moles fed for the two kinds of catalysts. For H2 addition, on both catalysts the gain in ethylene selectivity compensates for the loss in ethane conversion in terms of ethylene yields, which is constant for LaMnO3 and increases for Pt (but it is always 5% points lower than LaMnO3). Furthermore, results suggest that on LaMnO3 an inlet H2/O2 ratio above 2 is impractical under the conditions investigated, unless a H2 source is available, because more H2 is consumed than fed: for such a level of H2 addition, ethylene selectivity is 76%. On Pt, the highest H2/O2 ratio to attain H2 parity is 1, corresponding to an ethylene selectivity of only 71% under the conditions investigated (Figure 12), and is explainable through the higher activity for H2 oxidation. In accordance with the higher oxidation activity of LaMnO3 toward CO,20 the amount of CO recovered versus CO fed is larger on Pt. Indeed, most of the CO passes unreacted through the Pt monolith under any conditions (Figure 12), while on LaMnO3 more CO is consumed than fed already from CO/O2 ) 1. Any further CO addition is anyway not beneficial on LaMnO3, because ethylene yield exhibits a maximum at CO/O2 ) 1: the large rise of ethylene selectivity (up to ∼80%) overcomes the drop in conversion and gives a small maximum in yield, which is higher than the highest yield attained upon H2 addition. On Pt instead, the drop in conversion does not correspond to a comparable increase in selectivity, thus reducing the ethylene yield on increasing CO, as CO is not efficiently oxidized by the catalysts, thus leading to its rapid deactivation. 4. Discussion and Conclusion This work represents the prosecution of the investigation of LaMnO3-based honeycomb monolith for ethane ODH, reported in previous papers.16,17 The process is conducted autothermally with high ethylene yield, adopting more adiabatic configurations and conditions than the previous ones. The changes induced by external heating and dilution on ethane conversion and product distribution appear to be explainable through the temperature changes they cause. Thus, external heating and absence of dilution both increase the catalyst temperature, which in turn increases ethane conversion. Ethylene selectivity does not vary monotonically with catalyst temperature, but exhibits a maximum. Indeed, at low catalyst temperature it increases, as dehydrogenation reactions leading to ethylene are sustained by the sacrifice of a smaller fraction of ethane to COx to produce heat at increasing temperature. At high catalyst temperature, ethylene

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Figure 12. Comparison of Pt- (solid lines) and LaMnO3-based (dashed lines) monolith reactors upon fuel addition: ethylene yields and moles of H2 (CO) measured at the exit of the reactor as a function of H2 (CO). Experimental conditions are as in Figures 9 and 10.

Figure 13. Ethylene selectivity as a function of ethane conversion for LaMnO3 and Pt monoliths, and for experimental data reported for Pt and Pt/Sn.3,5

selectivity is reduced by the onset of gas-phase reactions of degradation of ethylene to other byproducts such as C2H2 and CH4. This issue is addressed in more detail in our modeling paper.12 The LaMnO3 monolith, in a head-to-head comparison with the Pt monolith, always shows better ethane conversion and selectivity under a wide variety of conditions, that is, varying the C2H6/O2 ratio, the total flow rate, and also for addition of a secondary fuel, giving ethylene yields as high as 58% with ethylene selectivity >80%. Experimental results suggest that the differences between the two catalysts may be explainable in terms of temperature profiles and catalyst activity toward

ethane oxidation. Providing that most of the ethylene is produced in the gas phase,6-12 the catalyst contributes by driving the oxidation of a fraction of ethane to generate the heat required for the homogeneous dehydrogenation reactions. LaMnO3 appears to be more effective than Pt in optimizing heat production because it tends to drive the oxidation toward CO2 rather than toward CO, due to the well-reported high activity of the perovskites toward the oxidation of CO.20 This is analogous to what is reported for doped rare earth oxides by Mulla and coworkers.7 On the other hand, Pt is more active toward hydrogen oxidation, so that H2O selectivity is always higher than on LaMnO3.


Intuitively, it is clear that the more deeply carbon and hydrogen are oxidized, the more heat is developed per mole of oxygen fed and the less ethane is wasted. The difference in CO2 selectivity over the two catalysts is always larger than the difference in H2O selectivity, so that for a given ethane conversion, a ∼10% points higher ethylene selectivity is observed on LaMnO3, as shown in Figure 13, which compares all of the experimental data obtained in terms of ethylene selectivity as a function of ethane conversion. Upon secondary fuel addition, the different catalyst oxidation activities play a different role. In the case of H2 co-feeding, the higher hydrogen oxidation activity of Pt compensates for the gap with LaMnO3, and the two catalysts end up giving similar ethylene selectivity (but lower conversion on Pt). In the case of CO addition, LaMnO3 gives a much higher ethylene selectivity than Pt (∼80% vs 65%). Figure 13 also compares the performance on the LaMnO3 and the Pt monolith obtained in our experimental apparatus with results reported in the literature. Regarding the Pt-based catalysts, a good agreement is found with the data obtained by other authors.3,5 Only the Pt/Sn catalyst exhibits a selectivity similar to that of LaMnO3, but a shorter catalyst lifetime due to Sn volatilization. On the contrary, LaMnO3 catalyst was tested for more than 100 h in a large variety of operating conditions and repeated cycles of ignition and extinction, without showing any sign of deactivation and with good reproducibility in the ODH reaction in our lab-scale reactor, and, analogously, it has been proven to be stable in the catalytic combustion of methane under 100 h of operation in ignited conditions at 1000 °C.23 The ethane ODH process has a simpler reactor system and fewer fractionation columns than the corresponding ethane cracker, which results in about 20-25% lower estimated investment cost. On the other hand, estimated production costs are somewhat higher (10-15%), mainly due to the additional cost of oxygen. The development of a durable, non-noble-metal-based, high performance catalyst undertaken in this work may be beneficial to making catalytic ethane ODH more economically viable and represents a further step toward precommercial implementation of this technology. LaMnO3-based catalyst is a very robust catalyst for ethane ODH in SCTRs, giving greater ethylene selectivity and yields than Pt under a wide variety of conditions. Furthermore, it has been demonstrated for the first time that the practice of adding carbon monoxide as secondary fuel is a viable option on LaMnO3-based catalyst (but not on Pt) to increase ethylene selectivity above 80% keeping high yields, and it may give even better results than H2 addition on both Pt and perovskite systems. Analogously to H2 on Pt, it was shown on LaMnO3 that CO, which can be recovered from byproducts and recycled to the reactor, is preferentially oxidized in place of ethane, which can instead undergo dehydrogenation reactions to ethylene with extremely high selectivity. Literature Cited (1) Huff, M.; Schmidt, L. D. Ethylene formation by oxidative dehydrogenation of ethane over monoliths at very short contact times. J. Phys. Chem. 1993, 97, 11815.

(2) Huff, M. C.; Schmidt, L. D. Elementary step model of ethane oxidative dehydrogenation on Pt-coated monoliths. AIChE J. 1996, 42, 3484. (3) Bodke, A. S.; Henning, D.; Schmidt, L. D.; Bharadwaj, S. S.; Maj, J. J.; Siddall, J. Oxidative dehydrogenation of ethane at millisecond contact times: Effect of H2 addition. J. Catal. 2000, 191, 62. (4) Schmidt, L. D.; Huff, M. Partial oxidation of CH4 and C2H6 over noble metal-coated monoliths. Catal. Today 1994, 21, 443. (5) Yokoyama, C.; Bharadwaj, S. S.; Schmidt, L. D. Platinumtin and platinum-copper catalysts for autothermal oxidative dehydrogenation of ethane to ethylene. Catal. Lett. 1996, 38, 181. (6) Lødeng, R.; Lindvåg, O. A.; Kvisvle, S.; Reier-Nielsen, H.; Holmen, A. Short contact time oxidative dehydrogenation of C2 and C3 alkanes over noble metal gauze catalysts. Appl. Catal., A 1999, 187, 25. (7) Mulla, S. A. R.; Buyevskaya, O. V.; Baerns, M. Autothermal oxidative dehydrogenation of ethane to ethylene using SrxLa1.0Nd1.0Oy catalysts as ignitors. J. Catal. 2001, 197, 43. (8) Mulla, S. A. R.; Buyevskaya, O. V.; Baerns, M. A comparative study on noncatalytic and catalytic oxidative dehydrogenation of ethane to ethylene. Appl. Catal., A 2002, 226, 73. (9) Beretta, A.; Ranzi, E.; Forzatti, P. Oxidative dehydrogenation of light paraffins in novel short contact time reactors. Experimental and theoretical investigation. Chem. Eng. Sci. 2001, 56, 779. (10) Beretta, A.; Ranzi, E.; Forzatti, P. Production of olefins via oxidative dehydrogenation of light paraffins at short contact times. Catal. Today 2001, 64, 103. (11) Henning, D. A.; Schmidt, L. D. Oxidative dehydrogenation of ethane at short contact times: Species and temperature profiles within and after the catalyst. Chem. Eng. Sci. 2002, 57, 2615. (12) Donsı`, F.; Caputo, T.; Di Benedetto, A.; Pirone, R.; Russo, G. Modeling ethane oxy-dehydrogenation over monolithic combustion catalysts. AIChE J. 2004, 50, 2233. (13) Flick, D. W.; Huff, M. C. Oxidative dehydrogenation of ethane over supported chromium oxide and Pt modified chromium oxide. Appl. Catal., A 1999, 187, 13. (14) Beretta, A.; Forzatti, P. High-temperature and shortcontact-time oxidative dehydrogenation of ethane in the presence of Pt/Al2O3 and BaMnAl11O19 catalysts. J. Catal. 2001, 200, 45. (15) Sadykov, V. A.; Pavlova, S. N.; Saputina, N. F.; Zolotarskii, I. A.; Pakhomov, N. A.; Moroz, E. M.; Kuzmin, V. A.; Kalinkin, A. V. Oxidative dehydrogenation of propane over monoliths at short contact times. Catal. Today 2000, 61, 93. (16) Donsı`, F.; Pirone, R.; Russo, G. Oxidative dehydrogenation of ethane over a Perovskite-based monolithic reactor. J. Catal. 2002, 209, 51. (17) Donsı`, F.; Pirone, R.; Russo, G. Catalyst investigation for applications of oxidative dehydrogenation of ethane in short contact time reactors. Catal. Today 2004, 91-92, 285. (18) Cimino, S.; Lisi, L.; Pirone, R.; Russo, G.; Turco, M. Methane combustion on perovskites-based structured catalysts. Catal. Today 2000, 59, 19. (19) Cimino, S.; Pirone, R.; Russo, G. Thermal stability of perovskite-based monolithic reactors in the catalytic combustion of methane. Ind. Eng. Chem. Res. 2001, 40, 80. (20) Cimino, S.; Di Benedetto, A.; Pirone, R.; Russo, G. CO, H2 or C3H8 assisted catalytic combustion of methane over supported LaMnO3 monoliths. Catal. Today 2003, 83, 33. (21) Bodke, A. S.; Bharadwaj, S. S.; Schmidt, L. D. The effect of ceramic supports on partial oxidation of hydrocarbons over noble metal coated monoliths. J. Catal. 1998, 179, 138. (22) Henning, D. A. Partial oxidation of ethane to ethylene. Ph.D. Thesis, University of Minnesota, 2002. (23) Cimino, S.; Di Benedetto, A.; Pirone, R.; Russo, G. Transient behaviour of perovskite-based monolithic reactors in the catalytic combustion of methane. Catal. Today 2001, 69, 95.

Received for review August 5, 2004 Revised manuscript received October 29, 2004 Accepted November 9, 2004 IE0492911

Autothermal Oxidative Dehydrogenation of Ethane on LaMnO3- and Pt

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