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Oxidative activation of light alkanes on dense ionic oxygen conducting membranes
Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.
655
Oxidative activation of light alkanes on dense ionic oxygen conducting membranes M. Rebeilleau a, A.C. van Veen a, D. Farrusseng a, J.L. Rousset a, C. Mirodatos a, Z.P. Shao b, G. Xiong b. aInstitut de Recherches sur la Catalyse, 2. Av. A. Einstein 69626 Villeurbanne Cedex France bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110,Dalian 116023, People's Republic of China ABSTRACT The oxidative dehydrogenation of ethane to ethylene (ODHE) has been studied in a catalytic membrane reactor (CMR) using a dense mixed ionic oxygen and electronic conducting perovskite membrane Ba0.sSr0.sCo0.8Fe0.203.e~. At 1080K, an ethylene yield of 66% was obtained with the bare membrane. After Pd cluster deposition, the ethylene yield reached 76% at 1050K. Ni cluster deposition led to a decrease of ethane conversion compared to the bare membrane without changing ethylene selectivity. 1. INTRODUCTION Extensive efforts are currently devoted to the chemical conversion of light alkanes, i.e. C2H6 and C3H8, to valuable chemicals, both being besides methane main constituents of natural gas. Oxidative dehydrogenation (ODH) of alkanes to alkenes is considered as an attractive alternative to the existing thermal steam cracking processes due to the lower temperature required and decreased coke formation. Unfortunately, yields obtained up to now in conventional reactors still remain by far too low for industrial developments. At low temperature (<873K), the best ethylene yield currently reported is below 40% and was obtained with a mixed oxide Mo0.y3V0.18Nb0.09Ox [1]. The best results for ODHE at high temperatures were close to 44-49% over mixed oxide catalysts [2, 3]. In fact, the reaction of ethane with gaseous oxygen, over all known catalyst is accompanied by the thermodynamically favored formation of carbon oxides, thereby decreasing selectivity at a given conversion. In this way, dense catalytic membrane reactors (CMR) are regarded as promising candidates for enhanced reactor configurations in partial oxidation
656 reactions. Here, dense ionic oxygen conducting membranes (IOCM) perform the selective separation of oxygen from air on one side of the membrane while providing activated oxygen to the other side being fed with the alkane. Thus, dense CMR combines in the same unit oxygen separation and alkane conversion, leading with high probability to breakthroughs in cost efficiency when mass transfer and reaction rate are tuned adequately. Recently, promising results using IOCM were reported for oxidative dehydrogenation of ethane (ODHE) [4, 5]. However, the temperature range used remains still at a too high level for an industrial application and so, it appears necessary to increase the yield of ethylene at lower temperature, keeping as reference the yield of 16% obtained at 973K as reported in [4]. The absence of catalyst on top of the membrane, on the reaction side, is certainly a feature to change when opting to increase conversion without decreasing selectivity. In fact, even if perovskites are known to catalyze many oxidation reactions, like oxidative coupling of methane or oxidative dehydrogenation of light hydrocarbons, the contact surface, i.e. the geometric surface of the membrane, is quite low to play a significant role taking parallel proceeding gas-phase reactions into account. In order to increase the surface activity and consequently the alkane conversion at lowered temperature, we decided to deposit a catalyst on the surface. Pd and Ni metallic clusters, known to be active for alkane activation, were deposited by laser vaporization of corresponding metallic rods to obtain a catalytic surface modification with high metal dispersion and uniform cluster size. For this purpose, we used a bare Ba0.sSr0.sCo0.8Fe0.203_e membrane comparable to that employed in [4], which showed good results in oxygen permeation and in partial oxidation of methane [6-8]. The performances in ODHE of the bare and surface modified membranes are compared in this paper, in order to i) demonstrate that the concept of CMR with dense IOCM is validated for ODHE, and ii) evaluate the influence of surface modification of the membrane on ethane conversion and ethylene selectivity. 2. EXPERIMENTAL
The Bao.sSro.sCoo.8Feo.203.~ (BSCFO) perovskite powder was prepared using an adapted variant of the so-called citrates method. In this method, stoichiometric amounts of Ba(NO3)2, Sr(NO3)2, Co(NO3)z'6H20 and Fe(NO3)3"6H20 (purity >99.5%) were fully dissolved in a small amount of distilled water, followed by addition of EDTA and Citric Acid with a molar ratio of Perovskite: EDTA: Citric Acid equal to 1:1.5:3. The obtained purple solution was heated to 373K, and a gel-like material was formed by evaporation of water after about three hours, which transformed into a brown foam by a heat treatment at 575K for three hours. This foam was calcined at 1173K for four hours in air yielding a perovskite powder, that was further homogenized by grinding in a mortar. Raw
657 membrane discs were pressed by applying a pressure of 140 MPa for one minute. The complete densification of the raw discs was accomplished by sintering in dense alumina boats at 1425K for eight hours. A Nd-YAG laser was focused onto a metallic rod, which was driven in a slow screw motion. Short helium bursts, synchronized with laser pulses, cooled the plasma and a supersonic expansion occurred at the exit of the source, inducing the formation of a beam of clusters, containing neutral and ionized species. The masses of the charged clusters could be analyzed by a time of flight mass spectrometer, hence allowing for reproducible cluster size adjustment. These clusters were sent to a vacuum chamber and deposited either on the surface of the dense ionic oxygen conducting membrane or on amorphous carbon to allow an easy characterization by electron microscopy. The typical deposition rates monitored by a quartz microbalance, were 5 nm/cmVmin (6.1 ~tg/min) for the case of the non-porous materials used and the thickness of the deposition was close to 8 ~. Two rods were used with the Pdl00 and Nil00 composition. The formation of a pure perovskite structure was checked by X-ray diffraction using a Bruker D5005 system in the 20 range of 3 to 80 ~ a step width of 0.02 ~ a counting time of l s and Cu Kal+a2 radiation (1,54184 A). The bulk elementary composition was determined by ICP-OES analyzing a sample dissolved by heating at 523 to 573 K in a mixture of H2SO4 and HNO3. TEM micro graphs were taken with a JEOL JSM-840A electron microscope. The oxidative dehydrogenation of light hydrocarbons was studied using a reactor as described previously [8]. Discs (about 1 mm thick) were sealed in between two dense alumina tubes using gold rings as chemically inert sealant. Furthermore, the side wall of the disc was extensively covered with gold paste suppressing radial contributions to the oxygen flux permeating through the active cross-section of 0.5 cm 2. The sealing was carried out in the beginning of experimental runs by heating to 1073 K for one night. The oxygen-rich side was fed at a constant total pressure adjusted to 120 kPa and a constant total flow rate of 50 ml-min -t using a mixed stream of O2 and N2 (evaporated liquid N2) individually controlled by mass flow controllers. The reaction side was fed by a mixture of ethane and He individually controlled by mass flow controllers. On both membrane sides the introduced gases were preheated while passing the void space between the alumina tube and an inserted quartz tube and left afterwards rapidly via the tube center. Two pressure transducers were installed allowing to record the total pressure on each membrane side of the reactor. Ethane oxidation experiments were conducted within the temperature range of 973K to 1100K, with a flow of35mL-min -t C2H6/He and a CzH6partial pressure equal to 0.25atm. The reactant gases (02, N2, C2H6) and the product gases (H2, CH4, CO2, C2H4, C2H6 and H20) were analyzed by two on-line gas chromatographs, GC, connected to a PC based automated data collection and
658 analysis. The first GC was equipped with a TCD and a molecular sieve 13X column, allowed the detection of O2, N2, CH4, C O and H 2 0 . H2, CH4, CO2, C2H4, C2H6 and H20 concentrations were deduced from the second GC equipped with a Haye-Sep D column and TCD. Moreover, Ar was introduced as internal standard with the reactants in order to determine the flow increase caused by the molar expansion during the reaction. The carbon balance could be closed within 4%. Gas leakage due to an insufficient sealing or an incomplete densification of the membrane could in case be detected by monitoring the N2 concentration. 3. RESULTS
The effect of temperature on the ethane conversion and ethylene selectivity is represented in Fig. 1 for the bare membrane and those modified by Pd and Ni deposition, respectively. 3 . 1 0 D H E performances on bare membrane The ethane conversion increased from 11% to 72% within the temperature range of 975- 1080K. Selectivity decreased slightly from 96% to 92%, and the yield of ethylene reached a maximum value of 68% at 1080K. These results fully agree with those reported in [4]. During the experiment, we did not observed any oxygen in gaseous phase, indicating a total consumption of oxygen permeating through the membrane. The selectivity of the other carbon containing products (i.e. CH4, CO2, CO) was always below 5%. However, a large amount of hydrogen was detected (around 12 vol.%), indicating that direct dehydrogenation of ethane (C2H6-+ C2H4+ H2) occurs together with ODHE (C2H6 + 89 02--~ C2H4 + H20). By comparing i) the amount of oxygen that ODHE would require for the observed ethylene yield, and ii) the amount of oxygen effectively permeating through the membrane (from side permeation measurements), we estimated the contribution of the direct dehydrogenation of ethane to 80% and that of oxidative dehydrogenation to 20%. Furthermore, since no coke formation was observed during all the experiments, one major role of permeating oxygen is also to react with carbon deposits leading to the observed side CO• products. lOO
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Fig. 1 Ethane conversion (a) and ethylene selectivity (b) as a function of temperature for a bare BSCFO membrane and both Pd and Ni modified BSCFO membrane. Conditions: Air side Fair=50ml/min, Pair=l.2 arm; reaction side: Fetha,e/helium=37mL/min,Pethane--0.25atm.
659
Fig. 2. TEM image of Pd deposited on amorphous carbon (to allow imaging)
3.2 Effects of surface modification by Ni and Pd deposition. Laser deposition allowed to obtain an excellent dispersion of Ni and Pd particles with a homogeneous particle size (d -~ 4 nm) as shown in Fig. 2. In this case, the surface coverage with metal was close to 10 % (1 gg/cm2). The effects of Pd and Ni cluster deposition at the surface of the BSCFO membrane on both ethane conversion and ethylene selectivity are shown in Fig. 1. In the case of Pd deposition, the ethane conversion increased fourfold at 973K leading to an ethylene yield of about 38%, to be compared with the reference value of 16% obtained with the bare membrane. The selectivity reached almost 100% at 1100K. A slight decrease of ethylene selectivity was observed at high temperature with a minor increase in CO2 formation. The maximum value in yield of ethylene was close to 76% at 1050K (with ethylene selectivity 86%), to be compared with the maximum value of 68% obtained with the bare membrane at 1080K. In this way, the Pd deposition on the surface membrane allowed to increase significantly the ethylene yield above the level obtained for an unmodified membrane which was investigated at even higher temperatures. On the other hand, in the case of Ni deposition the conversion was lowered compared to that obtained with the bare membrane within all the temperature range. The ethylene selectivity was not affected by this modification, but the yield of ethylene reached only 37% at 1050K and 67% at 1100K. In both cases, like for the bare membrane, there was no oxygen detected in the effluent gas which indicates that all the oxygen crossing the membrane was consumed. Furthermore, no coke formation was observed during membrane reactor operation, while reference experiments in the absence of oxygen using a conventional tubular configuration indicated massive coke formation. 4. DISCUSSION The evaluation of the intrinsic activity of membrane and catalyst is not easily accessible due to many parameters. Firstly, in temperature range of 973-1173K the gas phase reactions (leading to direct dehydrogenation) are likely to play important role. Secondly, the oxygen flux crossing the membrane is a function of temperature and of the oxygen partial pressure gradient, and so, the effective oxygen flux crossing the membrane during the reaction cannot be not directly measured. However, from the hydrogen amount detected we could estimate in the case of the bare membrane the contribution of oxidative dehydrogenation
660 was about 20%. Concerning the experiments with the two catalysts deposited, we observed minor changes in oxygen permeation (as calculated from oxygenated products) which cannot explain the changes in ethane conversion. Thus, it seems that Ni plays an inhibitor role for the ethane activation while Pd allows to increase it. An explanation to the observed differences most likely relates to the different properties of metals used. Under the pertaining conditions, Pd is likely to stay predominately in metallic form favoring the direct dehydrogenation, or allowing for enhanced ODHE by surface oxygen supplied from the membrane. In the case of nickel, kinetics and thermodynamics would favor an oxidation of Ni to nickel oxide in the presence of oxygen crossing the membrane. Due to the expected mobility of the oxide phase at high temperature, a solid/solid reaction with the perovskite could have formed a kind of surface layer decreasing in turn the accessibility of the hydrocarbon to the active membrane surface. 5. C O N C L U S I O N The dehydrogenation of the ethane to ethylene was studied in a dense membrane reactor made of Ba0.sSr0.sCo0.gFe0.zO3_e before and after modification of the surface on the reaction side. In agreement with Wang et al. [4], a per pass ethylene yield of 68% with an ethylene selectivity of 92% was obtained at 1080K. After deposition of Pd clusters on top of the membrane, a major improvement of the CMR performance was achieved with a maximum per pass ethylene yield of 76% with 86% ethylene selectivity at 1050K. In contrast, a decrease of ethane conversion was observed over all range of temperatures after the Ni deposition without change in ethylene selectivity. The concept of dense membrane reactor as an alternative to the conventional thermal dehydrogenation process seems therefore validated. High and stable productivity is established by the achieved formation rate of ca. 7x102 kgc~m/gpd/h and an overall system efficiency of ca. 4x10 -~ kgc~I44/kgmembrane/h. Further works work on mechanism understanding and process modeling are subjects of ongoing work. ACKNOWLEDGEMENTS Financial support by the EC program CERMOx (G55RD-CT-2000-0035). REFERENCE [ l ] S.J Conway, D.J. Wang, J.H. Lunsford, Appl. Catal., 79 (199 l) L 1. [2] O.J. Velle, A. Andersen, K.-J. Jens, Catal. Today, 6 (1990) 567. [3] L. Ji., J. Liu, Chem. Commun, 1996, 1203. [4] W. Wang, Y. Cong, W. Yang, Catalysis Letters 84 (2002) p. 101-106. [5] F.T. Akin, Y.S. Lin, J. Membr. Sci., 209 (2002), 457. [6] Z. Shao, W. Wang, Y. Cong, H. Dong, J. Tong, G. Xiong, J. Mcmbr. Sci., 172 (2000) 177. [7] Z. Shao, G. Xiong, H. Dong, W. Yang, L. Lin, Separation and Purification Technology, 25 (2001) 97. [8] A.C. van Veen, M Rebeilleau, D. Farrusseng, C. Mirodatos, Chem. Commun. 2002, 32.
655
Oxidative activation of light alkanes on dense ionic oxygen conducting membranes M. Rebeilleau a, A.C. van Veen a, D. Farrusseng a, J.L. Rousset a, C. Mirodatos a, Z.P. Shao b, G. Xiong b. aInstitut de Recherches sur la Catalyse, 2. Av. A. Einstein 69626 Villeurbanne Cedex France bState Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110,Dalian 116023, People's Republic of China ABSTRACT The oxidative dehydrogenation of ethane to ethylene (ODHE) has been studied in a catalytic membrane reactor (CMR) using a dense mixed ionic oxygen and electronic conducting perovskite membrane Ba0.sSr0.sCo0.8Fe0.203.e~. At 1080K, an ethylene yield of 66% was obtained with the bare membrane. After Pd cluster deposition, the ethylene yield reached 76% at 1050K. Ni cluster deposition led to a decrease of ethane conversion compared to the bare membrane without changing ethylene selectivity. 1. INTRODUCTION Extensive efforts are currently devoted to the chemical conversion of light alkanes, i.e. C2H6 and C3H8, to valuable chemicals, both being besides methane main constituents of natural gas. Oxidative dehydrogenation (ODH) of alkanes to alkenes is considered as an attractive alternative to the existing thermal steam cracking processes due to the lower temperature required and decreased coke formation. Unfortunately, yields obtained up to now in conventional reactors still remain by far too low for industrial developments. At low temperature (<873K), the best ethylene yield currently reported is below 40% and was obtained with a mixed oxide Mo0.y3V0.18Nb0.09Ox [1]. The best results for ODHE at high temperatures were close to 44-49% over mixed oxide catalysts [2, 3]. In fact, the reaction of ethane with gaseous oxygen, over all known catalyst is accompanied by the thermodynamically favored formation of carbon oxides, thereby decreasing selectivity at a given conversion. In this way, dense catalytic membrane reactors (CMR) are regarded as promising candidates for enhanced reactor configurations in partial oxidation
656 reactions. Here, dense ionic oxygen conducting membranes (IOCM) perform the selective separation of oxygen from air on one side of the membrane while providing activated oxygen to the other side being fed with the alkane. Thus, dense CMR combines in the same unit oxygen separation and alkane conversion, leading with high probability to breakthroughs in cost efficiency when mass transfer and reaction rate are tuned adequately. Recently, promising results using IOCM were reported for oxidative dehydrogenation of ethane (ODHE) [4, 5]. However, the temperature range used remains still at a too high level for an industrial application and so, it appears necessary to increase the yield of ethylene at lower temperature, keeping as reference the yield of 16% obtained at 973K as reported in [4]. The absence of catalyst on top of the membrane, on the reaction side, is certainly a feature to change when opting to increase conversion without decreasing selectivity. In fact, even if perovskites are known to catalyze many oxidation reactions, like oxidative coupling of methane or oxidative dehydrogenation of light hydrocarbons, the contact surface, i.e. the geometric surface of the membrane, is quite low to play a significant role taking parallel proceeding gas-phase reactions into account. In order to increase the surface activity and consequently the alkane conversion at lowered temperature, we decided to deposit a catalyst on the surface. Pd and Ni metallic clusters, known to be active for alkane activation, were deposited by laser vaporization of corresponding metallic rods to obtain a catalytic surface modification with high metal dispersion and uniform cluster size. For this purpose, we used a bare Ba0.sSr0.sCo0.8Fe0.203_e membrane comparable to that employed in [4], which showed good results in oxygen permeation and in partial oxidation of methane [6-8]. The performances in ODHE of the bare and surface modified membranes are compared in this paper, in order to i) demonstrate that the concept of CMR with dense IOCM is validated for ODHE, and ii) evaluate the influence of surface modification of the membrane on ethane conversion and ethylene selectivity. 2. EXPERIMENTAL
The Bao.sSro.sCoo.8Feo.203.~ (BSCFO) perovskite powder was prepared using an adapted variant of the so-called citrates method. In this method, stoichiometric amounts of Ba(NO3)2, Sr(NO3)2, Co(NO3)z'6H20 and Fe(NO3)3"6H20 (purity >99.5%) were fully dissolved in a small amount of distilled water, followed by addition of EDTA and Citric Acid with a molar ratio of Perovskite: EDTA: Citric Acid equal to 1:1.5:3. The obtained purple solution was heated to 373K, and a gel-like material was formed by evaporation of water after about three hours, which transformed into a brown foam by a heat treatment at 575K for three hours. This foam was calcined at 1173K for four hours in air yielding a perovskite powder, that was further homogenized by grinding in a mortar. Raw
657 membrane discs were pressed by applying a pressure of 140 MPa for one minute. The complete densification of the raw discs was accomplished by sintering in dense alumina boats at 1425K for eight hours. A Nd-YAG laser was focused onto a metallic rod, which was driven in a slow screw motion. Short helium bursts, synchronized with laser pulses, cooled the plasma and a supersonic expansion occurred at the exit of the source, inducing the formation of a beam of clusters, containing neutral and ionized species. The masses of the charged clusters could be analyzed by a time of flight mass spectrometer, hence allowing for reproducible cluster size adjustment. These clusters were sent to a vacuum chamber and deposited either on the surface of the dense ionic oxygen conducting membrane or on amorphous carbon to allow an easy characterization by electron microscopy. The typical deposition rates monitored by a quartz microbalance, were 5 nm/cmVmin (6.1 ~tg/min) for the case of the non-porous materials used and the thickness of the deposition was close to 8 ~. Two rods were used with the Pdl00 and Nil00 composition. The formation of a pure perovskite structure was checked by X-ray diffraction using a Bruker D5005 system in the 20 range of 3 to 80 ~ a step width of 0.02 ~ a counting time of l s and Cu Kal+a2 radiation (1,54184 A). The bulk elementary composition was determined by ICP-OES analyzing a sample dissolved by heating at 523 to 573 K in a mixture of H2SO4 and HNO3. TEM micro graphs were taken with a JEOL JSM-840A electron microscope. The oxidative dehydrogenation of light hydrocarbons was studied using a reactor as described previously [8]. Discs (about 1 mm thick) were sealed in between two dense alumina tubes using gold rings as chemically inert sealant. Furthermore, the side wall of the disc was extensively covered with gold paste suppressing radial contributions to the oxygen flux permeating through the active cross-section of 0.5 cm 2. The sealing was carried out in the beginning of experimental runs by heating to 1073 K for one night. The oxygen-rich side was fed at a constant total pressure adjusted to 120 kPa and a constant total flow rate of 50 ml-min -t using a mixed stream of O2 and N2 (evaporated liquid N2) individually controlled by mass flow controllers. The reaction side was fed by a mixture of ethane and He individually controlled by mass flow controllers. On both membrane sides the introduced gases were preheated while passing the void space between the alumina tube and an inserted quartz tube and left afterwards rapidly via the tube center. Two pressure transducers were installed allowing to record the total pressure on each membrane side of the reactor. Ethane oxidation experiments were conducted within the temperature range of 973K to 1100K, with a flow of35mL-min -t C2H6/He and a CzH6partial pressure equal to 0.25atm. The reactant gases (02, N2, C2H6) and the product gases (H2, CH4, CO2, C2H4, C2H6 and H20) were analyzed by two on-line gas chromatographs, GC, connected to a PC based automated data collection and
658 analysis. The first GC was equipped with a TCD and a molecular sieve 13X column, allowed the detection of O2, N2, CH4, C O and H 2 0 . H2, CH4, CO2, C2H4, C2H6 and H20 concentrations were deduced from the second GC equipped with a Haye-Sep D column and TCD. Moreover, Ar was introduced as internal standard with the reactants in order to determine the flow increase caused by the molar expansion during the reaction. The carbon balance could be closed within 4%. Gas leakage due to an insufficient sealing or an incomplete densification of the membrane could in case be detected by monitoring the N2 concentration. 3. RESULTS
The effect of temperature on the ethane conversion and ethylene selectivity is represented in Fig. 1 for the bare membrane and those modified by Pd and Ni deposition, respectively. 3 . 1 0 D H E performances on bare membrane The ethane conversion increased from 11% to 72% within the temperature range of 975- 1080K. Selectivity decreased slightly from 96% to 92%, and the yield of ethylene reached a maximum value of 68% at 1080K. These results fully agree with those reported in [4]. During the experiment, we did not observed any oxygen in gaseous phase, indicating a total consumption of oxygen permeating through the membrane. The selectivity of the other carbon containing products (i.e. CH4, CO2, CO) was always below 5%. However, a large amount of hydrogen was detected (around 12 vol.%), indicating that direct dehydrogenation of ethane (C2H6-+ C2H4+ H2) occurs together with ODHE (C2H6 + 89 02--~ C2H4 + H20). By comparing i) the amount of oxygen that ODHE would require for the observed ethylene yield, and ii) the amount of oxygen effectively permeating through the membrane (from side permeation measurements), we estimated the contribution of the direct dehydrogenation of ethane to 80% and that of oxidative dehydrogenation to 20%. Furthermore, since no coke formation was observed during all the experiments, one major role of permeating oxygen is also to react with carbon deposits leading to the observed side CO• products. lOO
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%~ ' ,;0 '10'00 ',&0 ',0',0 ',0;0 ',0'~0'
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Temperature (~
Fig. 1 Ethane conversion (a) and ethylene selectivity (b) as a function of temperature for a bare BSCFO membrane and both Pd and Ni modified BSCFO membrane. Conditions: Air side Fair=50ml/min, Pair=l.2 arm; reaction side: Fetha,e/helium=37mL/min,Pethane--0.25atm.
659
Fig. 2. TEM image of Pd deposited on amorphous carbon (to allow imaging)
3.2 Effects of surface modification by Ni and Pd deposition. Laser deposition allowed to obtain an excellent dispersion of Ni and Pd particles with a homogeneous particle size (d -~ 4 nm) as shown in Fig. 2. In this case, the surface coverage with metal was close to 10 % (1 gg/cm2). The effects of Pd and Ni cluster deposition at the surface of the BSCFO membrane on both ethane conversion and ethylene selectivity are shown in Fig. 1. In the case of Pd deposition, the ethane conversion increased fourfold at 973K leading to an ethylene yield of about 38%, to be compared with the reference value of 16% obtained with the bare membrane. The selectivity reached almost 100% at 1100K. A slight decrease of ethylene selectivity was observed at high temperature with a minor increase in CO2 formation. The maximum value in yield of ethylene was close to 76% at 1050K (with ethylene selectivity 86%), to be compared with the maximum value of 68% obtained with the bare membrane at 1080K. In this way, the Pd deposition on the surface membrane allowed to increase significantly the ethylene yield above the level obtained for an unmodified membrane which was investigated at even higher temperatures. On the other hand, in the case of Ni deposition the conversion was lowered compared to that obtained with the bare membrane within all the temperature range. The ethylene selectivity was not affected by this modification, but the yield of ethylene reached only 37% at 1050K and 67% at 1100K. In both cases, like for the bare membrane, there was no oxygen detected in the effluent gas which indicates that all the oxygen crossing the membrane was consumed. Furthermore, no coke formation was observed during membrane reactor operation, while reference experiments in the absence of oxygen using a conventional tubular configuration indicated massive coke formation. 4. DISCUSSION The evaluation of the intrinsic activity of membrane and catalyst is not easily accessible due to many parameters. Firstly, in temperature range of 973-1173K the gas phase reactions (leading to direct dehydrogenation) are likely to play important role. Secondly, the oxygen flux crossing the membrane is a function of temperature and of the oxygen partial pressure gradient, and so, the effective oxygen flux crossing the membrane during the reaction cannot be not directly measured. However, from the hydrogen amount detected we could estimate in the case of the bare membrane the contribution of oxidative dehydrogenation
660 was about 20%. Concerning the experiments with the two catalysts deposited, we observed minor changes in oxygen permeation (as calculated from oxygenated products) which cannot explain the changes in ethane conversion. Thus, it seems that Ni plays an inhibitor role for the ethane activation while Pd allows to increase it. An explanation to the observed differences most likely relates to the different properties of metals used. Under the pertaining conditions, Pd is likely to stay predominately in metallic form favoring the direct dehydrogenation, or allowing for enhanced ODHE by surface oxygen supplied from the membrane. In the case of nickel, kinetics and thermodynamics would favor an oxidation of Ni to nickel oxide in the presence of oxygen crossing the membrane. Due to the expected mobility of the oxide phase at high temperature, a solid/solid reaction with the perovskite could have formed a kind of surface layer decreasing in turn the accessibility of the hydrocarbon to the active membrane surface. 5. C O N C L U S I O N The dehydrogenation of the ethane to ethylene was studied in a dense membrane reactor made of Ba0.sSr0.sCo0.gFe0.zO3_e before and after modification of the surface on the reaction side. In agreement with Wang et al. [4], a per pass ethylene yield of 68% with an ethylene selectivity of 92% was obtained at 1080K. After deposition of Pd clusters on top of the membrane, a major improvement of the CMR performance was achieved with a maximum per pass ethylene yield of 76% with 86% ethylene selectivity at 1050K. In contrast, a decrease of ethane conversion was observed over all range of temperatures after the Ni deposition without change in ethylene selectivity. The concept of dense membrane reactor as an alternative to the conventional thermal dehydrogenation process seems therefore validated. High and stable productivity is established by the achieved formation rate of ca. 7x102 kgc~m/gpd/h and an overall system efficiency of ca. 4x10 -~ kgc~I44/kgmembrane/h. Further works work on mechanism understanding and process modeling are subjects of ongoing work. ACKNOWLEDGEMENTS Financial support by the EC program CERMOx (G55RD-CT-2000-0035). REFERENCE [ l ] S.J Conway, D.J. Wang, J.H. Lunsford, Appl. Catal., 79 (199 l) L 1. [2] O.J. Velle, A. Andersen, K.-J. Jens, Catal. Today, 6 (1990) 567. [3] L. Ji., J. Liu, Chem. Commun, 1996, 1203. [4] W. Wang, Y. Cong, W. Yang, Catalysis Letters 84 (2002) p. 101-106. [5] F.T. Akin, Y.S. Lin, J. Membr. Sci., 209 (2002), 457. [6] Z. Shao, W. Wang, Y. Cong, H. Dong, J. Tong, G. Xiong, J. Mcmbr. Sci., 172 (2000) 177. [7] Z. Shao, G. Xiong, H. Dong, W. Yang, L. Lin, Separation and Purification Technology, 25 (2001) 97. [8] A.C. van Veen, M Rebeilleau, D. Farrusseng, C. Mirodatos, Chem. Commun. 2002, 32.