I n d . Eng. C h e m . Res. 1987,26, 2344-2348
2344
plants required to convert the olefins back to gasoline has increased. As predicted from the ideal feedstock processing, hydrogen availability declines slightly when the Paragon Process is used. While hydrogen consumption has been reduced in the hydrocracker, the decline in hydrogen production in the reformer more than offsets this increase. So the net production declines slightly. Paragon greatly reduces the operating temperature of the reformer because other high octane blend stocks are added to the pool. Refiners are now faced with lead phasedown and are pushing their reformers harder. The production of high octane blend stocks from olefins can provide a way of reducing the load on this plant. A summary of this stock balance analysis shows that the Paragon Process can increase gasoline production and convert excess butanes to this product by alkylation, can also increase gasoline production with less of a demand for butanes by use of polymerization and MTBE manufacture, can decrease hydrogen availability slightly, and can substantially reduce the operating temperatures of an associated reformer. The ability of a refinery to use the Paragon Process depends on many factors which are specific to it, such as the availability of isobutane. However, it is a new approach to hydrocracking which may have application in some refineries. Summary The Paragon Process provides a unique way of using the paraffins in a hydrocracker’s feedstock It converts them to olefins. The olefin streams from it have many unique properties: very low levels of impurities (sulfur, nitrogen, and diolefins),high concentration of olefins (C,’s are 60-75 LV % propylene; C,’s are about 80% butenes), and rich s; and C6’s in isoolefins ((2,‘s are about 40% isobutylene; C contain about 50% etherable isoolefins). The yield and distribution of the olefins will depend on the amount of paraffins in the feed but can also be controlled by the process conditions. By providing a new source of olefins, it can increase gasoline production. If butanes are in excess, they can be
converted to gasoline by alkylation. If butanes are not available, the olefins can also be converted to gasoline by polymerization or MTBE manufacture. The usefulness of the Paragon Process will depend on details of the refinery being studied, such as the availability of butanes and existing facilities. By providing a new source of high octane blend streams (alkylate, polymer, or MTBE), it should reduce the demands on associated reformers. It can also improve the flexibility of a hydrocracker to make jet and diesel fuels from waxy crudes. It should be easy to commercialize since it is based on the marriage of two commercially proven processes: dewaxing and hydrocracking. In the near term, it can be viewed as a way of improving the manufacture of transportation fuels. But the olefin intermediates may also be viewed as a long-term source for chemical manufacture. Acknowledgment I thank Hazel Olbrich and Jerry Mayer for their help in developing this process; Don Hickson, Guy Raumer, and Bobby Russel for synthesis of the ZSM-5; Ron Wright for his advice in analysis of the process; and the management of Chevron Research for their permission to publish this information. Literature Cited Argauer, R. J.; Landolt, G. R. U.S.Patent 3702886, 1972. Chen, N. Y.; Garwood, W. E. Catal. Reu.-Sci. Eng. 1986, 28, 185-264. Flanigen, E. M. “Molecular Sieue Zeolite Technology-The First
Twenty-Five Years”, Proceedings of the Fifth International Conference on Zeolites; Rees, L. V., Ed.; Heyden and Son: London, U.K.,1980; p 760. Heinemann, H. Catal. Rev.-Sci. Eng. 1981,23, 315-238. O’Rear, D. J.; Mayer, J. F. US.Patent 4390413, 1983. Venuto, P. B.; Habib, E. T., Jr. Fluid Catalytic Cracking With Zeolite Catalysts; Marcel Dekker: New York, 1979; pp 110-117. Voge, H. H. Catalysis; Emmett, P. H., Ed.; Reinhold: New York, 1958; Vol. VI, p 433. Walsh, D. E.; Rollmann, L. D. J. Catal. 1979,56, 195.
Receiued for review December 30, 1986 Accepted July 17, 1987
Formation of Asymmetric Cellulose Acetate Membranes for the Separation of Carbon Dioxide-Methane Gas Mixtures? B h u p e n d e r S. Minhas,* Takeshi Matsuura,* and Srinivasa S o u r i r a j a n Division of Chemistry, National Research Council of Canada, Ottawa, Canada K I A OR9
Cellulose acetate reverse osmosis membranes cast under identical conditions were shrunk in hot water a t different temperatures and then dried by the solvent-exchange technique using various combinations of solvents. These membranes were tested at room temperature for the separation of carbon dioxide/methane gas mixtures. The mole fraction of carbon dioxide in the feed was varied from 0.107 to 0.9 and the operating pressure from 400 to 2300 kPa absolute. A unique variation in the separation factor (1-28) and in the product permeation rate (0-1.33 X kmol/(m2.s)) was observed. The effect of pressure and of the feed composition on the separation factor and the product permeation rate was also investigated. Formation of asymmetric cellulose acetate membranes was first reported by Loeb and Sourirajan (1960,1963) for Issued as NRC No. 28038. Research Division, W.R. Grace and CO., Columbia, MD 21044.
* Current address:
the purpose of seawater desalination. The reverse osmosis membranes, when dried in a manner to preserve their porosity and the surface pore structure, showed high permeation rates and significant separations for gaseous mixtures (Agrawal and Sourirajan, 1969, 1970). Many studies on the methods of membrane drying and the in-
QS8S-~~S5/87/2626-2344$01.50/0 Published 1987 by the American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2345 dustrial application in gas separations of the resulting dry membranes have also appeared in the literature (Cooley and Coady, 1978; Gantzel and Merten, 1970; MacDonald and Pan, 1974; Manos, 1978; Merten et al., 1968; Schell, 1979). Fundamental studies have been reported later on the permeation of different gases through dry reverse osmosis membranes of different polymeric material (Rangarajan et al., 1982,1984) and the separation of hydrogen/methane gas mixtures (Mazid et al., 1985; Minhas et al., 19841, establishing a valid means of treating gas permeations and separations through reverse osmosis membranes. In the above studies, the gaseous flow through the pores in the surface layer of asymmetric porous membrane was considered to be governed by Knudsen, slip, viscous, and surface flow mechanisms. The contributions of different flows, defined by different flow mechanisms, to the total flow through the membrane are dependent upon the pore size and the pore size distribution in the surface layer of the membrane. For achieving a membrane of appropriate pore size and pore size distribution which gives a high separation factor and high permeation rate, a firm cause and effect relationship has to be established between the variables involved in the membrane formation process and the performance data of membranes produced. Among the many variables involved in the formation of cellulose acetate membranes, particularly, we have identified that the evaporation period, the shrinkage temperature, and the solvent used for the replacement of water in the membrane during the drying process are some important factors affecting the ultimate pore size and pore size distribution of the membrane and consequently the membrane performance data (Minhas et al., 1984). Therefore, it was a natural consequence to study the effect of various solvents used in the solvent exchange during process of membrane formation on its subsequent performance of hydrogen/ methane gas mixtures (Minhas et al., 1985). A similar study has been made by Ohya et al. (1986) with respect to the pure gas permeation through cellulose acetate membranes and the separation of several gas mixtures. The pore size and the pore size distribution on the membrane surface which gives the highest separation and the highest permeation rate may, however, be different from one gas mixture to another. For example, the pore size distribution which optimizes the separation for H,/ CHI mixtures is expected to be different from that for C02/CH4. Hence, it is the objective of this study to investigate the effect of the variables involved in the membrane formation process on the membrane performance data regarding the separation of COZ/CH4gas mixtures. As such variables, the shrinkage temperature and the solvent used for the replacement of water are considered.
(60-95 " C ) to obtain different porous structure on the membrane surface and subsequently dried by a multiplestage solvent-exchange technqiue used for gas separation experiments. In the drying technique, water in the membrane is replaced by a water-miscible solvent (called the first solvent) which is a nonsolvent for the membrane material. Then, the first solvent is replaced by a second solvent which is nonpolar, volatile, and miscible in the first solvent. The second solvent is air evaporated to obtain the dry membranes. A number of combinations of solvents were used to dry the membranes. Two solvents, isopropyl alcohol and tert-butyl alcohol, were used as the first solvents, and the second solvents used were pentane, hexane, heptane, toluene, isopropyl ether, triethylamine, and carbon disulfide. Water in the membranes was replaced by a first solvent in four stages, i.e., first water in the pores was replaced by an aqueous solution containing 25 vol % of the first solvent, which was then replaced by another aqueous solution containing 50 vol % of the first solvent. In the third stage, the aqueous solution was replaced by another aqueous solution containing 75 vol % of the first solvent, which in the fourth stage is replaced by the pure first solvent. Membranes dried by the combination of different solvents were numbered in the form of CA-mn for the purpose of membrane identification, where CA is cellulose acetate, m is the number given to the first solvents, and n is the number given to the second solvents. Numbers given to the first solvents are as follows: tert-butyl alcohol, 1;isopropyl alcohol, 2. The second solvents were numbered as follows: pentane, 1; hexane, 2; heptane, 3; toluene, 4; triethylamine, 5; isopropyl ether, 6; and carbon disulfide, 7. The equipment and the details of the experimental procedure have been reported previously (Minhas et al., 1984,1985). Namely, the experimental data for the permeation rate, [PR], and the separation factor for carbon dioxide/methane gas mixture were obtained. The flow rate of permeate gas was measured by a soap bubble meter. The separation factor is defined as SI2= (Xpermeate/ Xfe,),/(Xpermeate/Xfeed)2 where X indicates mole fraction. Subscripts 1and 2 represent COz and methane. Carbon dioxide mole fraction in the feed was varied from 0.107 to 0.9. The feed and permeate gas compositions were measured by gas chromatography using Model TRACOR MT160/220 equipped with a Porapak Q column. The results of gas analysis were reproducible within f l % accuracy. Gases used were 99.9% pure and were supplied by Matheson Canada Limited. All experiments were conducted at room temperature, and the pressure was varied from 400 to 2300 kPa absolute. The effective area of the membrane used in the permeation study was 10.2 x 10-~ m2.
Experimental Section
Results and Discussion
The method used in the present investigation for the preparation of cellulose acetate membranes was similar to that described in our earlier work (Minhas et al., 1985). All membranes were cast at 30 OC, atmospheric temperature, and 65%, atmospheric relative humidity, using a casting solution of the following composition (wt %): cellulose acetate (Eastman 398-3), 17; acetone, 69.2; magnesium perchlorate, 1.45; and water, 12.35 (Pageau and Sourirajan, 1972). Casting solution temperature was maintained at 10 "C. All membranes were cast on glass plates of equal nominal thickness and were gelled in an ice-cold water bath after 60 s of evaporation time. The membranes were then shrunk at different temperatures
Cellulose acetate membranes, shrunk at temperatures between 60 and 95 "C and dried by the solvent-exchange technique by using various combinations of first and second solvents, showed a wide variation in the separation factor for carbon dioxidefmethane gas mixtures, ranging from 1to 28. A wide variation in the permeation rate was kmol/(m2-s). also found ranging from 0 to 1.33 X These variations indicate that the shrinkage temperature and solvents used in the solvent-exchangedrying of membranes strongly influence the membrane performance for the separation of carbon dioxide/methane gas mixtures. Table I shows the effects of solvents used in solventexchange drying of membranes on the separation factor
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Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987
Table I. Effect of Solvents Used i n Solvent-Exchange Drying Technique on Cellulose Acetate Membraneo Performance for a Feed Containing 0.9 Mole Fraction of Carbon Dioxide in Carbon Dioxide/Methane Gas Mixture membrane CA-21
CA-22
CA-23
CA-24
CA-26
CA-27
CA-12
CA-16
CA-14
solvent first second isopropyl pentane alcohol -
isopropyl alcohol
isopropyl alcohol
isopropyl alcohol
isopropyl alcohol
isopropyl alcohol
tert-butyl alcohol
tert-butyl alcohol
tert-butyl alcohol
PRI,
pressure, kPa abs. 466.8
SIz kmol/tm24 1.14 1.91 x 10-5
hexane
859.8 1349.3 1742.3 2190.4 466.8
1.14 3.85 X 1.1 7.08 x 10-5 1.1 9.78 x 10-5 1.1 13.32 X 27.0 0.14 X
heptane
873.5 1335.5 1763.0 2170.0 466.8
25.0 20.0 17.0 14.0 8.3
toluene
873.5 1335.5 1763.0 2170.0 466.8
5.6 5.9 3.7 2.8 1.2
2.60 X 5.20 x 10-5 6.94 x 10-5 8.11 X 0.33 X
859.8 1349.3 1742.3 2190.4 466.8
1.2 1.2 1.2 1.13 6.5
0.66 x 1.19 x 1.65 x 2.25 X
859.8 1349.3 1742.3 2190.4 466.8
6.0 4.6 4.14 3.5
isopropyl ether
carbon disulfide
hexane
isopropyl ether
toluene
859.8 1349.3 1742.3 2190.4 466.8
28.0 27.0 28.0 1.2
0.30 X 0.50 x 0.82 x 1.17 X 1.27 x
low5 10-5
10-5 10-5
10-5 10-5 10-5 1O-j
0.66 X
1.32 X 2.38 X 3.18 X 4.35 x 10-5 0.11 x 10-5 0.21 x 10-5 0.40 X 0.55 X 0.79 X 2.47 X
859.8 1349.3 1742.3 2190.4 466.8
1.1 4.63 x 1.1 7.39 x 1.1 9.92 x 1.1 12.40 X 1.9 0.35 x
859.8 1349.3 1742.3 2190.4 1349.3
1.7 1.6 1.5 1.4 1.3
10-5 10-5
10-5 10-5
0.72 X 1.36 X 1.93 X 2.71 X 0.06 X lom5
a All membranes were prepared with an evaporation time of 60 s and were shrunk at 80 "C.
and the total permeation rate for a feed containing 0.9 mole fraction of carbon dioxide. All the membranes reported in Table I were shrunk at 80 "C. The results show that when the second solvent used is the same, membranes dried using isopropyl alcohol as the first solvent gave higher separation factors as compared to those dried using tert-butyl alcohol as the first solvent, though toluene as a second solvent was an exception. Similar observations were also made in our previous investigation (Minhas et al., 1985) for the separation of hydrogen from hydrogenlmethane gas mixtures. Membranes dried using the second solvents hexane and carbon disulfide, when the first solvent used was isopropyl alcohol, showed a high separation factor, whereas those using pentane and toluene showed little separation, if any; on the other hand, heptane
and isopropyl ether as the second solvents are reasonably good. When tert-butyl alcohol was used as the first solvent, separation factors obtained were very low irrespective of the second solvent used. Table I shows that the experimental separation factors obtained generally decreased with an increase of pressure for a given membrane. The membrane dried using the isopropyl alcohol/ hexane combination (CA-22) showed a drop in separation factor from 27 at 467 kPa absolute to 14 at 2170 kPa absolute pressure. However, for the same level of pressure increase, the total permeation rate increased from 0.14 X to 1.17 X kmol/(m2-s). For the same level of pressure increase, the separation factor obtained for a membrane dried using isopropyl alcohol and carbon disulfide solvents (CA-27) remained almost constant at 28, which corresponds to a separation index (defined as the relative permeation rate of carbon dioxide to methane) of 256, whereas the total permeation rate inkmol/(m2.s). Thus, creased from 0.11 X 10" to 0.79 X the membrane CA-27 showed greater separation at high pressure though the total permeation rate was lower when compared to the performance data of the membrane CA22. Judging from the stable separation factor with changing operating pressures, CA-27 membrane is the first choice for carbon dioxide separation from methane or natural gas. Furthermore, the comparison of the data obtained from membranes CA-23 and CA-26 indicates that the former film shows higher separation factors and higher fluxes at operating pressures of 467 and 1336 kPa absolute. This shows that the separation and flux are unrelated; that is, higher separation factor does not mean lower permeation rate and vice versa. A similar conclusion was also obtained in our previous investigation (Minhas et al., 1985). Figure 1 shows the effect of shrinkage temperature for membranes obtained using isopropyl alcohol as the first solvent and different second solvents on the membrane performance for the separation of carbon dioxide from a feed containing 0.9 mole fraction of carbon dioxide at an operating pressure of 2200 kPa absolute. In the cases of hexane, isopropyl ether, and triethylamine, an optimum shrinkage temperature was in the range of 75-85 "C, for obtaining higher separation. In the case of toluene, however, an increase in shrinkage temperature reduced the separation factor. The highest separation factor of 12 was obtained when the membrane was shrunk at 60 "C. The separation factor in the case of carbon disulfide increased to 28 with an increase of shrinkage temperature from 60 to 80 "C. At 85 "C, the flux obtained was too small to measure. The latter membrane also showed the highest flux at 80 "C, where the separation factor was the highest. With respect to other membranes, the flux was higher at lower separation factors and vice versa. These results indicate that shrinkage temperature is an important variable for the formation of cellulose acetate membranes for the separation of the gas mixtures. Figure 2 shows the effect of the feed composition on the total permeation rate through different membranes at 2200 kPa absolute. Permeation rate increased with an increase of carbon dioxide concentration in the feed, which is expected since carbon dioxide is the faster permeating gas. However, the permeation rate increase was not uniform for all the membranes; in some cases, an increase in permeation rate was higher than other membranes. This indicates that enhancement in the permeation rate depends on the porous structure of the membrane surface. Figure 3 shows the effect of feed composition on the separation factors for various membranes. Generally, the change in the separation factor is little, if any. In some
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2347
I
,CA - 12(80 1
0
0.2 0.4 0.6 0.8 1.0 MOLE FRACTION OF CARBON DIOXIDE IN FEED. X i 2
2 8 nl
E
26 E
Y
2 4 n I
2
0
Figure 2. Effect of COPmole fraction in the feed gas mixture on the product permeation rate. Membranes: material, cellulose acetate 398-3;evaporation time, 60 s; shrinkage temperature and solvents used in solvent-exchange drying, as specified in the figure. Operating conditions: pressure, 2200 kPa absolute; temperature, room.
+ +
+
+
+
CA -27(80)
5
0
012 014 Oh 1lO MOLE FRACTION OF CARBON DIOXIDE IN FEED, X i 2
SHRINKAGE TEMPERATURE, OC
Figure 1. Effect of shrinkage temperature and of second solvents used in solvent-exchange drying of membranes on the product permeation rate and the separation factor. Membranes: material, cellulose acetate 398-3;evaporation time, 60 a; first solvent, isopropyl carbon disulfide, (X) hexane, (v)toluene, alcohol; second solvent, (0) (A) isopropyl ether, (0)triethylamine. Operating conditions: pressure, 2200 kPa absolute; temperature, room; COzmole fraction in feed, 0.9.
cases, however, the separation factor increases with an increase of carbon dioxide concentration in the feed.
Conclusion The performance of asymmetric cellulose acetate membranes for the separation of carbon dioxidelmethane gas mixtures is strongly influenced by the shrinkage temperature and by the solvents used for solvent-exchange drying of membranes. An appropriate shrinkage temperature and a proper combination of solvents can be found to prepare an asymmetric cellulose acetate membrane of a pore size and pore size distribution on the membrane surface which gives a high separation factor and high product rate. It was found that the separation factor, in general, decreases with an increase of pressure, whereas the total permeation rate increases. The permeation rate also increases with an increase of carbon dioxide concentration in the feed; however, the variation in separation factor, in general, was found to be negligible.
Figure 3. Effect of COz mole fraction in the feed gas mixture on the separation factor. Membranes: as specified for Figure 2. Operating conditions: pressure, 470 kPa absolute; temperature, room.
Acknowledgment We are grateful to NRC Bioenergy Project for supporting this work. B.S.M. thanks NSERC for a visiting fellowship. Registry NO.CHI,74-82-8; C02,124-38-9;H&CH(OH)CH3, 67-63-0;(H&)BCOH, 75-65-0;H&(CH,)&HS, 109-66-0; H&(CH2)&H3, 110-54-3; H3C(CH2),CH3, 142-82-5;C6H5CH3, 108-883; (H3C)&HOCH(CH3),, 108-20-3;CS2,75-15-0;cellulose acetate, 9004-35-7.
Literature Cited Agrawal, J. P.; Sourirajan, S. J. Appl. Polym. Sci. 1969, 13, 1065. Agrawal, J. P.; Sourirajan, S. J. Appl. Polym. Sci. 1970, 14, 1303. Cooley, T.E.;Coady, A. B. U.S. Patent 4130403, Dec 19, 1978. Gantzel, P. K.;Merten, U. Ind. Eng. Chem. Process Des. Dev. 1970, 9,331. Loeb, S.;Sourirajan, S. Report 60-60, July 1960; Department of Engineering, University of California,Los Angeles. Loeb, S.; Sourirajan, S. Adv. Chem. Ser. 1963, 38, 117. MacDonald, W.;Pan, C. Y. U.S.Patent 3842515, Oct 22, 1974. Manos, P. US. Patents 4068387 (Jan 171, 4080743 (March 28), 4080744 (March 28),4 120098 (Oct 17), 1978. Mazid, M. A.; Rangarajan, R.; Matsuura, T.; Sourirajan, S. Znd. Eng. Chem. Process Des. Dev. 1985,24, 907.
Ind. Eng. C h e m . Res. 1987, 26, 2348-2353
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Merten, U.; Beach, S.; Gantzel, P. K. U.S.Patent 3415038, Dec 10, 1968. Minhas, B. S.; Matsuura, T.; Sourirajan, S. Am. Chem. SOC.,Symp. Ser. 1985,281, 451-466. Minhas, B. S.; Mazid, M. A.; Matsuura, T.; Sourirajan, S. Proc. Fifth Bioenergy R&D Seminar; National Research Council of Canada: Ottawa, 1984; pp 183-188. Ohya, H.; Mase, A.; Negishi, Y.; Matsumoto, K. Maku 1986,11,169. Pageau, L.; Sourirajan, S. J. Appl. Polym. Sci. 1972, 16, 3185. Rangarajan, R.; Mazid, M. A.; Matsuura, T.; Sourirajan, S. Proc.
Fourth Bioenergy R&D Seminar; National Research Council of Canada: Ottawa, 1982; pp 435-440. Rangarajan, R.; Mazid, M. A.; Matauura, T.; Sourirajan, S. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 79. Schell, W. J. U.S. Patent 4 134 742, Jan 16, 1979. Received for review December 16, 1986 Revised manuscript receiued July 7, 1987 Accepted August 10, 1987
Oxidative Coupling of Methane over Lead Oxide Catalyst: Kinetic Study and Reaction Mechanism Kenji Asami, Tsutomu Shikada, Kaoru Fujimoto,* and Hiro-o Tominaga Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan
Lattice oxygen of PbO was found to be the active species for the oxidative coupling of methane, whereas adsorbed oxygen was supposed to be active for the carbon oxides formation. The coupling reaction proceeded through a redox cycle between Pb(0) and Pb(I1) which was facilitated by being supported on MgO catalyst. The kinetic data suggested that the oxidation of Pb(0) by oxygen was much faster than the reduction of PbO by methane and that the rate-determining step of the oxidative coupling reaction should be the formation of methyl radical via the reaction of methane and surface oxide. The synthesis of ethane and ethylene by the oxidative coupling of methane is a current subject of great significance in developing a novel chemical use of natural gas. Several catalysts have been reported to be active for the reaction (Keller and Bhasin, 1982; Hinsen et al., 1984; Otsuka et al., 1985, 1986a,b; Ito and Lunsford, 1985; Ito et al., 1985; Imai and Tagawa, 1986). We have also reported that an MgO-supported PbO catalyst has high activity and selectivity for the formation of C2+ hydrocarbons (Asami et al., 1986). However, there are few reports on the reaction mechanism. For example, Keller and Bhasin (1982) have reported that the formation of C2 hydrocarbons on a-Alz03-supported metal oxide catalysts may involve the following steps: 2CH4 + 2M@")+0, 2M(2"-1)+0,-1+ 20H(a) + 2CH3(a) (1) 2M(2"-2)+0n-l + O2 2M(2")+0, (2) Namely, the active oxygen species for C2formation is the bulk oxygen of metal oxides, and the reaction proceeds by a redox cycle between metal oxide and reduced metal oxide. In the case of a lithium-promoted magnesium oxide catalyst, Ito and co-workers have demonstrated that methane is activated by [Li+O-]centers which are present on the surface, and the resulting CH,' radicals recombine, largely in the gas phase, to yield C2He(Ito and Lunsford, 1985; Ito et al., 1985). Lin et al. (1986) have also mentioned that a sorbed form of oxygen, e.g., 02-, may be the active species of C, formation on La203. It is interesting, therefore, to clarify which species is active and plays a key role in the coupling reaction. In the present work, attempts were made to identify the active oxygen species by measuring the transient responses of periodic oxidation-reduction reactions of supported PbO catalysts and also by measuring X-ray diffraction patterns of treated catalysts.
-
-
Experimental Section Catalysts employed in this study were prepared by impregnating the carriers with an aqueous solution of Pb(NO,),, followed by drying in air at 120 "C for 12 h. The 0888-5885/87/2626-2348$01.50/0
loading of PbO was 20% by weight. They were activated in flowing air at 750 "C for 1h before use. Commercially available reagent grade MgO (Kanto Chem. Co. Inc., purity >99%, specific surface area (S.A.) 4.6 m2/g) and JRC ALO-4 7-AlZO3(Catalysis Society of Japan, S.A. 177 m2/g, pore volume (P.V.) 0.66 cm3/g) were employed as carriers and subjected to air calcination at 800 "C for 2 h before the impregnation. Methane conversions were conducted with a flow-type reaction apparatus. A quartz tube (6-mm i.d.), which contained 1.0 g of catalyst (20-40 mesh), was employed as a reactor for the mixed gas (CHI, 02,and N2)reactions and the periodic reactions. The feed program of the reactants for the periodic reaction is shown in Figure 1. Reaction conditions were 750 "C, 4.3 g.h/mol, and atmospheric pressure. Effluent gas was analyzed on line by gas chromatographs with six-way sampling valves and multisample holders. Data of the mixed gas reactions were taken after the reaction reached steady state (24 h). Although a small amount of lead was lost from the catalyst due to its vaporization, the activity and the selectivity were scarcely affected. Experimental conditions for the kinetic study were as follows. Twenty milligrams of PbO/MgO (40-60 mesh) was diluted to 1 g with a fused SiOz of the same size as the catalyst. The reactor was a quartz tube with 6-mm inner diameter. Feed rate of the reactant gas mixture was about 420 mL/min ( W / F = 0.02 gh/mol). CO and COz were analyzed by a gas chromatograph with an FID detector after being converted to methane by passing through a methanator (catalyst Ru/A1,03, 450 "C, and efficiency 199%). X-ray diffraction (XRD) patterns of the catalysts were determined with a Rigaku Denki Ru-200 diffractometer with Ni-filtered Cu Ka radiation.
Results and Discussion Elucidation of Active Oxygen Species for C2 Formation. ( a ) Suspension of O2 Feed in the Steady State. It has been reported that MgO has a catalytic activity for the oxidative coupling of methane (Ito et al., 0 1987 American Chemical Society
