Abstract:
A new set of additives to be sued in the preparation of inorganic materials; especially of perovskite nature is proposed. The chemical compositions of the perovskites prepared in the presence of the mentioned additives are found to be more homogenous, leading to better catalytic behavior, including higher selectivity and yields as compared to catalysts of identical formulations prepared through the conventional method of using EDTA/citrate (or other organic additive) method.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This applications claims priority of U.S. Provisional Patent Application No. 61/117,821, filed on Nov. 25, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods used for the preparation of perovskites. The invention more specifically relates to the methods for the preparation of oxygen permeable perovskites with a formula of ABO 3 , in which A is composed of rare and alkaline earth metal ions and B is a transition metal ion. The invention also relates to the application of such prepared compound as catalyst in different reactions. 
     BACKGROUND OF THE INVENTION 
     Nowadays, conversion of methane to more valuable products is of paramount importance, due to the existence of large gas resources throughout the world. Oxidative coupling of methane (OCM) to C 2  hydrocarbons (e.g. ethane and ethylene) is a well-known conversion process. The non-selective gas-phase reaction, however, leads to low C 2  selectivity and yields. 
     Several studies have been carried out in packed-bed reactors in co-feed operation mode according to which methane and oxygen were fed to the reactor at the same time. The results, however, were not so promising due to the low C 2  selectivity, which was caused by the fact that the oxidant of the process is gaseous molecular oxygen. 
     To overcome this problem, researchers have tested using perovskite membrane reactors, which have led to the indirect mixing of methane and oxygen during their transport. The major advantage of membrane reactors is preventing the direct mixing of oxygen and methane. This is because the perovskite membrane allows the permeation of ionic oxygen species produced under the operating conditions of the reaction, and keeps methane on the other side. Once the permeated ionic oxygen species reach the methane side, they readily react with the methane that is always in excess amounts due to the transportation mechanism of oxygen. This helps avoid the, side reaction of methane combustion, increasing the selectivity and, to some extent, the yield of the OCM reaction. 
     Oxygen permeable perovskites that can be used for this purpose are known to have the general formula of ABO 3  in which A and B are of rare and alkaline earth metal ions and transition metal ions respectively. 
     Substitution of alkaline-earth ions on the A-site affects the oxygen nonstoichiometry of the perovskite, while B-site is known to help optimize the catalytic properties of the perovskite-type oxides for oxidation reactions. 
     Dense membranes of the type of La x Sr 1-x Co y Fe 1-y O 3-δ  are conductors of both oxygen ion and electron. 
     La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  (LSCF) powders that are commonly used as membrane reactors, are prepared through complexation methods using ethylenediamine tetraacetic acid (EDTA) and an organic acid buffer, which can be later combusted, leaving no traces in the catalyst structure. 
     According to Pingying Zeng et al (J. of Mem. Sci. 302 (2007)), stoichiometric quantities of the desired metal salts are added to an EDTA, NH 4 OH aqueous solution under heating and stirring, and then followed by the addition of citric acid. The pH value of the system is controlled around 6. This is because at lower pH values EDTA precipitates, leading to the formation of non-homogenous La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  powders. The water content of the reaction mixture is then evaporated to yield a dark purple gel, which is then pretreated at 250° C. for several hours to form a solid precursor, which is then calcined at 800° C. for 5 h to obtain the oxide with the desired composition. 
     The La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  (LSCF) powders prepared through such conventional methods, however, are found to suffer the disadvantage of relatively low C 2  hydrocarbon selectivities and yield if used in OCM reactions. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to develop a complexation method including the application of a group of EDTA derivatives, comprising mono, di, tri, and/or tetra amide products of the amide formation reaction between EDTA and hydrazine, called EDNAD hereinafter. 
     The preferred compounds of the present invention, called Ethylene Diamine N-Acetyl Diamine (EDNADs) hereinafter, having a formula of: 
                                
wherein each of X, X′, X″, X′″ is independently selected from the group consisting of NH—NH 2 , OH, and O, with the proviso that at least one of X, X′, X″, X′″ is NH—NH 2  is found to lead to highly homogenized La 0.6 Sr 0.4 C 0.8 Fe 0.2 O 3-δ  (LSCF) perovskites in a very wide pH range and without the need to add organic buffers, which have excellent catalytic behavior in the oxidative coupling of methane (OCM) reaction.
 
     EDNADs are formed during the amide formation reactions between EDTA and hydrazine and in case all of the X , X′, X″, X′″ branches in the above formula are NH—NH 2  the compound is called Ethylene Diamine N, N, N′, N′, Tetra N-Acetyl Diamine (EDTNAD), which is a preferred compound to be used in the process of the present invention. 
     According to another embodiment of the present invention the reaction mixture incorporating the EDNADs and the reactants leading to their formation can be directly used in the method of the present invention. 
     According to another embodiment of the present invention the mixture comprising EDNADs is used as an additive in the production of any other inorganic compound that requires the incorporation of an additive for the dispersion of the active ingredients. 
     According to a more preferred embodiment of the present invention La x Sr 1-x Co y Fe 1-y O 3-δ  perovskites (LSCF) are prepared through a complexation reaction in the presence of a solution of EDNADs, as additives. 
     According to another embodiment of the present invention inorganic compounds prepared through the complexation of the ingredients in the presence of EDNADs, enjoy better distribution of the active species throughout their structures (homogeneity) as compared to the EDTA/organic salt method, which can be inferred comparing the turbidity data of the reaction solutions in both cases (example 1 a and b) that is an indicator of the homogeneity of the reaction solutions, finally leading to more homogenous organic compounds. 
     In the methods of the present invention preparing an inorganic compound, the complexation method provides a reaction of two or more metal ions by dissolving two or more metal salts, preferably soluble metal salts, in an aqueous solution of EDNADs. Preferably the two or more metal salts are selected from the group consisting of salts of Ag, Ba, Sr, Ca, Pb, La, Y, Nb, Ni, Ta, Ir, Ti, Sn, Zr, Mn, Mo, Fe, Cr, Co, and V. Preferably, the metal salts are nitrate salts. 
     The methods of the present invention preparing an inorganic compound preferably further comprise the steps of heating the obtained compound, evaporating the solution to obtain a material, self-igniting the material, and sintering. 
     According to another preferred embodiment of the present invention, the reaction mixture comprising the EDNADs does not require buffering agent due to the wide solubility pH range of EDNADs. 
     According to another embodiment of the present invention the EDNADs solution used for the preparation of the perovskites has a concentration range of about 10 to about 25%, preferably about 15 to about 25% (W/V) with respect to the total amount of EDNADs. 
     According to another embodiment of the present invention the EDNADs in the solution used for the preparation of the perovskites are composed of a mixture of all mono, di, tri and tetra structural derivatives in a way that the average number of the —NH—NH 2  groups in the EDNADs mixture is about 2.0 to about 4.0. 
     According to a more preferred embodiment of the present invention the average number of the —NH—NH 2  groups in the EDNADs mixture is about 3.0 to about 3.9. 
     According to the most preferred embodiment of the present invention the average number of the —NH—NH 2  groups in the EDNADs mixture is about 3.5 about 3.9. 
     According to another embodiment of the present invention and due to the superior solubility pH range of the EDNADs, and their strong chelating effects as compared to EDTA, there will be no need for the presence of buffers (e.g. organic acids and/or salts like citric and/or citrate) in the reaction solution of the present invention. 
     According to another embodiment of the present invention stoichiometric amounts of the desired salts are dissolved in an aqueous solution of EDNADs of proper concentration and the obtained solution is heated at about 50 to about 80° C., preferably about 55 to about 70° C., more preferably about 60° C. for about 3 hours while stirring. 
     According to a more preferred embodiment of the present invention La x Sr 1-x Co y Fe 1-y O 3-δ  perovskites (LSCF) prepared in the presence of EDNADs have very high C 2  selectivities (of more than about 70%, preferably more than about 90%, more preferably of about 100%) and OCM reaction yields (of about 3% to about 6%, preferably about 5.0%), which makes them superior over those produced through the conventional EDTA method. The better catalytic performance in this case can also be associated with the more homogenous catalytic structures, prepared through the method of present invention as compared to the EDTA method. 
     According to another more preferred embodiment of the present invention, in the case of preparing an oxygen permeable OCM-catalytic membrane of La x Sr 1-x Co y Fe 1-y O 3-δ  perovskites (LSCF), stoichiometric amounts of Sr(NO 3 ) 2 , Co(NO 3 ) 2 .6H 2 O, Fe(NO 3 ) 3 .9H 2 O and La(NO 3 ).6H 2 O are first dissolved in a about 15 to about 20% (W/V) EDNADs aqueous solution and the obtained solution is heated at about 40 to about 60° C., and most preferably at about 50° C. for about 3 h while stirring. 
     According another embodiment of the invention the so-produced La 0.6 Sr 0.4 CO 0.8 Fe 0.2 O 3-δ  perovskite is used as a highly C 2  selective catalyst for the oxidative coupling of methane (OCM) reaction. 
     In one embodiment, the present invention provides a process for preparing a compound that is made by reaction of two or more metal ions comprising dissolving two or more soluble metal salts in a solution that is comprised of the following compound: 
                                
wherein each of X, X′, X″, X′″ is independently selected from the group consisting of NH—NH 2 , OH, and O, with the proviso that at least one of X, X′, X″, X′″ is NH—NH 2  to obtain a solution, and forming a complex by reaction of the metal ions and the above compound with each other in the solution.
 
     In one embodiment the present invention provides a process for preparing a perovskite comprising
         a) combining two or more salts, water and at least a compound of formula:       

                                
wherein each of X, X′, X″, X′″ is independently selected from the group consisting of NH—NH 2 , OH, and O, with the proviso that at least one of X, X′, X″, X′″ is NH—NH 2  to form a solution;
         b) heating the solution to obtain a complex compound of the metal ions and the EDNADs;   c) evaporating the solvent to obtain a gel-like residue.   d) heating the gel-like residue in vacuum (pressure of less than about 100 mmHg) (in the absence of O 2 ) to self-ignite, thereby obtaining a powder;   e) calcining the powder.       
     In one embodiment the present invention provides an oxygen permeable membrane, comprised of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ , prepared by the above process wherein the membrane when used as a membrane between methane and oxygen for production of C 2  products, exhibits a C 2  selectivity of about 100% and a yield of about 5% at a temperature of 1073-1173 K. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a fixed bed reactor using the perovskite membrane of the present invention, used for the comparison experiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A new facile complexation procedure in preparation of different perovskites is devised. The advantages of the method, especially in the case of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  perovskites, are revealed by the modified catalytic behavior of the dense membrane (La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ ) during the OCM reaction, their pH stability, and also their modified mechanical properties. 
     According to the embodiments of the present invention the La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  (LSCF) powders, or any other perovskite of desire, are prepared by complexation method using an aqueous solution of EDNADs, where EDNADs refer to compound according to the formula of 
                                
wherein each of X, X′, X″, X′″ is independently selected from the group consisting of NH—NH 2 , OH, and O, with the proviso that at least one of X, X′, X″, X′″ is NH—NH 2 . This compound is disclosed in EP 1 808 428, which is incorporated herein by process.
 
     The compounds called Ethylene Diamine N-Acetyl Diamine (EDNADs), are formed during the amidification reactions between EDTA and hydrazine and in case all of the X, X′, X″, X′″ branches in the above formula are NH—NH 2  the compound is called Ethylene Diamine N,N, N′, N′, Tetra N-Acetyl Diamine (EDTNAD) which is a preferred compound to be used in the procedure of the present invention. 
     In the methods of the present invention preparing a perovskite, the complexation method provides a reaction of two or more metal ions by dissolving two or more metal salts, preferably soluble metal salts, in an aqueous solution of EDNADs. Preferably the two or more metal salts are selected from the group consisting of salts of Ag, Ba, Sr, Ca, Pb, La, Y, Nb, Ni, Ta, Ir, Ti, Sn, Zr, Mn, Mo, Fe, Cr, Co, and V. Preferably, the metal salts are nitrate salts. 
     The methods of the present invention preparing a perovskite compound preferably further comprise the steps of heating the obtained compound, evaporating the solution to obtain a material, self-igniting the material, and sintering. 
     According to the invention the best results with respect to the product homogeneity and later catalytic behavior are obtained when average number of the —NH—NH 2  groups in the EDNADs mixture is about 2.0 to about 4.0, preferably 3.0 to about 3.9, more preferably about 3.5 to about 3.9, but other mixtures of the compound can be suitably applied for the same purpose. 
     In the case of using an EDNADs solution for the production of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  to be used as OCM catalysts, stoichiometric amounts of each salt including Sr(NO 3 ) 2 , Co(NO 3 ) 2 .6H 2 O, Fe(NO 3 ) 3 .9H 2 O and La(NO 3 ).6H 2 O are first dissolved in a 10-about 25% (W/V), preferably about 15 to about 25% (W/V), and most preferably about 18% (W/V) aqueous EDNADs solution. It is noteworthy that the minimum amount concentration of the EDNADs in the solutions should constitute a 1:1 stoichiometric ratio between the total metal ions and the EDNADs, but any excess amount of EDNADs can also be used, since it will not interfere with the modified membrane properties and will bum in the self-ignition step. The obtained solution is then heated at about 50 to about 80° C., preferably about 55 to about 70° C. and most preferably about 60° C. for about 3 h while stirring. 
     The obtained gel-like dark-red material obtained after evaporating the solution at room temperature is then self-ignited at about 200° C. in a vacuum oven. Preferably, evaporating the solution is carried out at about 50° C. to about 60° C. under vacuum (at a pressure of less than about 100 mm Hg). The step of self-igniting preferably comprises heating to a temperature of about 120° C. to about 250° C., more preferably at about 150° C. to about 230° C., even more preferably at about 200° C. Preferably, the step of self-igniting is carried out in vacuum, at a pressure of less than 100 mm Hg. Then, the obtained gray powder is calcinated at about 950° C. to about 1200° C. for 5 h. The obtained black oxide powder is pressed into disk pellets under 400-600 MPa (4000-6000 bar) hydraulic pressure. Disks are sintered at about 1200° C. for 10 hours by heating and then cooling rate at 2° C./min. Both sides of the sintered membrane are then polished with about 1000 mesh SiC paper to give a final thickness of approximately 0.7-1.0 mm. The catalysts are then used in an OCM setup to check their catalytic effects during the oxidative coupling of methane (OCM) reaction. (See Journal of Molecular Catalysis A: Chemical 286 (2008) 79-86, incorporated herein by reference in its entirety). 
     It is also noteworthy that the solution of EDNADs as an additive can be extended to the production of other inorganic compounds that are produced through complexation methods. 
     The oxygen permeable membrane of the present invention comprised of La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  prepared as described above, has high selectivity and provides a high yield. Preferably the membrane when used as a membrane between methane and oxygen for production of C 2  products, exhibits a C 2  selectivity of more than about 70% and a yield of about 3% to about 6% at a temperature of 1073-1173K. More preferably the selectivity is more than about 90%, even more preferably about 100%, and the yield is about 5%. 
     EXAMPLES 
     The below examples are meant to elaborate on the subject-matter of the current invention, but the scope of the invention is not limited to the examples at all. 
     Example 1 
     In order to indicate the higher potential of EDNAD as a proper chelating reagent for the production of perovskites, two experiments are conducted for comparing the homogeneity and pH-stability of the metal complexes prepared though the EDNAD and EDTA methods. 
     a—EDTA/Citrate Method 
     8.068 gr of H 4 -EDTA (ethylene diamine tetra acetic acid) was dissolved in an ammonia solution (8.0 M). The initial pH of the solution is adjusted at about 8. Then stoichiometric amounts of metallic nitrates Sr(NO 3 ) 2 , La(NO 3 ) 3 .6H 2 O, Co(NO 3 ) 2 .6H 2 O and Fe(NO 3 ) 3 .9H 2 O were added, respectively so that the molar ratio of EDTA to total metal cation content was 1.5:1.0. After addition of the second salt, some coagulated materials formed in the solution. Indeed after addition of the metal salts, pH of the solution decreases considerably (becomes acidic). By heating and addition of excess amounts of ammonia solution coagulates dissolved. The same problem was faced with the addition of 3 rd  and 4 th  salts, in which small size particles were formed in the solution. Again, using heat and extra amounts of ammonia solution the problem was solved. These findings indicate that the complexes between the metal ions and EDTA do not show high stability with the changes in pH of the solution and the solution is not really homogeneous probably because of partial formation of metallic hydroxides at alkaline pHs and the insolubility of EDTA salts in acidic pH values. To check the homogeneity of the solution at high pH values, turbidity of the solutions at pH 8.0 and pH 12.0 were measured using a Hach laboratory turbidimeter. The results are summarized in table 1. 
     Turbidity of the solutions was obtained 95 and 57 NTU (Nephelometric Turbidity Units), respectively. At pH 3.0 the solution contained large amounts of coagulates and precipitates, which makes it not useable for the production of any homogenous organic structures. 
     b—EDNADs Method 
     A solution of metal ions/EDNADs with a molar ratio of EDNAD to total metal cation content of 1.5:1.0 was prepared. Using the EDNADs chelating agent solution none of the above problems were observed. The solution was clear and homogeneous at acidic and alkaline media. For example turbidity of the complex solutions of metal ions/EDNAD at pH 3.0, pH 8.0 and pH 12.0 were obtained 4, 2 and 1 NTU, respectively. The results are summarized in table 1. 
     Comparing the results of the experiments in the table shows that the products of the EDNADs have a much higher homogeneity due to the more homogenous reaction media, which has no turbidity as compared to the EDTA method that has relatively higher turbidity values. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of the turbidity (and hence homogeneity) of the 
               
               
                 EDTA and EDNADs reaction solutions at different pH values 
               
             
          
           
               
                   
                 EDTA method 
                   
                 EDNADs method 
                   
               
             
          
           
               
                   
                 pH 
                 Turbidity (NTU) 
                 pH 
                 Turbidity (NTU) 
               
               
                   
                   
               
             
          
           
               
                   
                 3 
                 Insoluble 
                 3 
                 4 
               
               
                   
                 8 
                 95 
                 8 
                 2 
               
               
                   
                 12 
                 57 
                 12 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     Also after three days the EDTA solutions were even more turbid, while the EDNADs solutions were totally transparent, which is another indication of the homogeneity of perovskite-preparation metal complex solutions in the presence of EDNADs. 
     Example 2 
     Preparation of a Perovskite OCM Catalyst Using EDNADs 
     3 gr of a La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ  perovskite OCM catalyst was prepared through the EDNAD method as describe below: 
     To an 18% W/V solution of EDNADs having an average —NH—NH 2  content of 3.5-3.9 (which was prepared by diluting the 70% W/V solution) were added 1.1415 gr of Sr(NO 3 ) 2 , 3.5037 gr of La(NO 3 ) 3 .6H 2 O, 3.2046 gr of Co(NO 3 ) 2 .6H 2 O and 1.1007 gr of Fe(NO 3 ) 3 .9H 2 O respectively, with a 30 minute interval before the addition of each salt. The resulting dark-red solution was evaporated to form a gel, which was further dried and oxidized in a vacuum oven in 200° C. The resulting powder was calcinated for 5 hours in 950° C. with a heating rate of 2° C./min. The obtained black oxide powder is pressed into disk pellets under 400 MPa (4000 bar) hydraulic pressure. Disks were sintered at 1200° C. for 10 h by heating and then cooling rate at 2° C./min. Both sides of the sintered membrane are then polished with 1000 mesh SiC paper to give a final thickness of approximately 0.7-1.0 mm. The catalysts are then used in an OCM setup to check their catalytic effects during the oxidative coupling of methane (OCM) reaction. 
     Example 3 
     Preparation of a Perovskite OCM Catalyst Using the EDTA/Citrate 
     3 gr of a La 0.6 Sr 0.4 Cu 0.8 Fe 0.2 O 3-δ  perovskite OCM catalyst was prepared through the EDNAD method as describe below: 
     To a solution containing 12.0514 gr EDTA, 36.0854 gr NH 4 OH, and 84.4285 gr of deonized water, having a pH of 8-9 were added 1.1415 gr of Sr(NO 3 ) 2 , 3.5037 gr of La(NO 3 ) 3 .6H 2 O, 3.2046 gr of Co(NO 3 ) 2 .6H 2 O and 1.1007 gr of Fe(NO 3 ) 3 .9H 2 O respectively, with a 30 minute interval before the addition of each salt, and the final pH was fixed at 6-6.5 using nitric acid and/or ammonia solutions. Then the pH was adjusted at 6.0 using citric acid (8.6222 gr). The resulting dark-red solution was evaporated to form a gel, which was further dried and oxidized in a vacuum oven in 200° C. The resulting powder was calcinated for 5 hours in 950° C. with a heating rate of 2° C./min. The obtained black oxide powder is pressed into disk pellets under 400 MPa (4000 bar) hydraulic pressure. Disks were sintered at 1200° C. for 10 h by heating and then cooling rate at 2° C./min. Both sides of the sintered membrane are then polished with 1000 mesh SiC paper to give a final thickness of approximately 0.7-1.0 mm. The catalysts are then used in an OCM setup to check their catalytic effects during the oxidative coupling of methane (OCM) reaction. 
     Example 4 
     Reactor Tests of the Two Catalysts 
     OCM reactions were carried out in the LSCF dense membrane reactors prepared according to examples 2 and 3 according to Scheme 1. 
     In both cases a mixture of He and CH 4  was fed on one side and oxygen on the other side. Experimental results obtained from the membrane reactor were compared for two procedures of LSCF preparation. Experimental conditions and catalyst characteristics are shown in Tables 2. 
     For LSCF membrane catalyst prepared by both EDTA/Citrate and EDNADs methods, the best results including C 2  selectivity, conversion and yield were obtained at a temperature range of 1023-1173 K. Below 1023 K the extent of oxygen permeation is low and above 1240 K the sealing of membrane reactor was lost, hence the values of C 2  selectivity is only reported for the above optimum temperature range. A maximum methane conversion of 5.01% was obtained for the membrane reactor prepared through the EDNADs method.
 
YeildC 2 =Conversion CH 4 ×S C     2    
 
Where S C2  is the selectivity of the reaction towards the C 2  product, over the catalyst. It must be mentioned that in the OCM reaction the low conversion is not a limiting factor, instead, the higher C 2  selectivity is of paramount importance because in contrast to the fixed-bed catalyst, the membrane catalyst showed no methane combustion (CO x  formation reactions).
 
     C 2  selectivity, as the most important parameter for OCM reaction, for the LSCF dense membrane prepared through the EDNADs method was found to be about 100% over the used temperature range of 1073-1153 K. 
     
       
         
           
             
               Selectivity 
               
                 C 
                 2 
               
             
             = 
             
               
                 
                   2 
                   ⁢ 
                   
                     ( 
                     
                       
                         
                           C 
                           2 
                         
                         ⁢ 
                         
                           H 
                           4 
                         
                       
                       + 
                       
                         
                           C 
                           2 
                         
                         ⁢ 
                         
                           H 
                           6 
                         
                       
                     
                     ) 
                   
                 
                 
                   [ 
                   
                     CO 
                     + 
                     
                       CO 
                       2 
                     
                     + 
                     
                       2 
                       ⁢ 
                       
                         ( 
                         
                           
                             
                               C 
                               2 
                             
                             ⁢ 
                             
                               H 
                               6 
                             
                           
                           + 
                           
                             
                               C 
                               2 
                             
                             ⁢ 
                             
                               H 
                               6 
                             
                           
                         
                         ) 
                       
                     
                   
                   ] 
                 
               
               × 
               100 
             
           
         
       
     
     The results obtained for the catalyst prepared through the EDTA/citrate method are also given in table 2. As it is clear, the catalyst prepared through the method of the present invention is superior to the EDTA/citrate catalyst with respect to selectivity and yield. 
     
       
         
               
             
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Reactor test results for the catalysts prepared through the 
               
               
                 conventional EDTA method and the method of the present invention 
               
             
          
           
               
                 Membrane (EDNADs-method) 
                 Membrane (EDTA-method) 
               
               
                   
               
             
          
           
               
                 x 
                 0.6, 0.8 
                 x 
                 0.6 0.8 
               
               
                 y 
                 0.6, 0.8 
                 y 
                 0.6, 0.8 
               
               
                 Preferable δ range 
                 0.00-0.15 
                 Preferable δ range 
                 0.00-0.18 
               
               
                 Most preferable δ 
                 0.00 
                 Most Preferable δ 
                 0.15 
               
               
                 Preferable Temp. 
                 1023-1173 
                 Preferable Temp. 
                 1023-1173 
               
               
                 range (K) 
                   
                 range (K) 
               
               
                 Most preferable 
                 1073-1153 
                 Most Preferable 
                 1123 
               
               
                 Temp (K) 
                   
                 Temp (K) 
               
               
                 O 2  partial pressure 
                 0.1-1.0 bar 
                 O 2  partial pressure 
                 0.1-1.0 bar 
               
               
                 Thickness 
                 0.5-1.0 mm 
                 Thickness 
                 0.5-1.0 mm 
               
               
                 C 2  sel. % 
                 100 
                 C 2  sel. % 
                 52.33 
               
               
                 Yield % 
                 5.01 
                 Yield % 
                 2.26 
               
               
                   
               
               
                 Catalyst formula: A 1−x A′ x B 1−y B′ y O 3−δ  (A:La; A′:Sr; B:Co, B′:Fe)