Energy & Fuels 1988,2,235-236
235
Registry No. Co, 7440-48-4; Mo, 7439-98-1. 0
D. C. Elliott,* E. G. Baker Battelle, Pacific Northwest Laboratories Richland, Washington 99352
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J. Piskorz, D. S. Scott Department of Chemical Engineering University of Waterloo, Waterloo, Ontario, Canada
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Figure 2. Production cost aa a function of energy efficiency. Analysis of the liquid hydrocarbon product by gas chromatography-mass spectrometry demonstrates the complex nature of hydrocarbons (see Figure 1). The mix of saturated cyclic hydrocarbons and aromatics is similar to that reported earlier for wood-derived products. Small amounts of phenolics, like those reported earlier in wood-derived products, have also been identified. They are remnants of the feedstock that require separation for reprocessing or for use as chemicals. A group of saturated straight-chain hydrocarbons (C7-C2J are unique to the peat-derived product and are not produced from wood. They are also reported as present in the peat pyrolysis tar and are apparently derived from waxes that are widely reported to be present in peat. Distillation of the product indicates that 86% is recoverable below 225 OC, the ASTM limit for gasoline boiling range. This fraction contains primarily aromatics and saturated cyclics, which would be good gasoline blending components. The heavier fraction is primarily straight-chain paraffins and might be useful as jet or diesel fuel. The resulta of these testa are being used in a preliminary process design analysis as part of the IEA Direct Biomass Liquefaction Cooperative Project. More complete results will be available a t a future date. The overall energy efficiency of the process of pyrolysis and hydrotreating is 34.4% based on the higher heating value of the gasoline product and the peat feedstock. The mass yield is 6526 kg/h gasoline from 41667 kg/h of dry peat in a 1000 ton/day plant. Presently the energy efficiency of the pyrolysis is only 50.3% based on tar yield while the hydrotreatment is only 68.5% energy efficient based on gasoline from tar. Improvements through optimization studies could conceivably increase efficiencies to the 60% range for pyrolysis and 80-85% in the hydrotreatment resulting in an overall process efficiency of about 50%. Efficiency is translated into product cost in Figure 2, which suggests a present price of the gasoline product a t about $6OO/ton ($1.74/gallon) and an optimized price of about $400/ton ($1.16/gallon) based on an overall process efficiency increased to 50%. These costs are based on a pyrolysis plant cost of $50 million with a 20-year life and 8% interest. The range of the bar in Figure 2 represents ungrading costa at $10-20 million/year. Peat feedstock cost is $16/ton milled peat (50% moisture). As a result, one can conclude that moderate improvements in process efficiency through optimization of the processing parameters can have dramatic effeds on the projected cost of product. Acknowledgment. We thank the Finnish Ministry of Trade and Industry for financial support of this series of experiments.
* To whom correspondence should be addressed.
Y.Solantausta Fuels and Lubricants Laboratory Technical Research Centre, Espoo, Finland Received November 9, 1987 Revised Manuscript Received February 3, 1988 Oxidative Coupling of Methane over Perovskite-Type Oxides' Sir: A general consensus is being reached that abundant reserves of natural gas exist. Much of this gas, however, is in locations too remote from market areas to be recovered on a commercial basis by present methods. One approach to utilizing this remote gas is to convert the natural gas to liquid hydrocarbon fuels. A method that has the potential of being part of an economically viable process is direct partial oxidation step via oxidative coup1ing.l It involves the one-step conversion of methane to ethylene or ethane as illustrated in 2CH4 + O2 = CH2CH2 2H20 4CH4 + 0 2 = 2CH3CH3 2H20 Various catalysb/promoters that enhance these reactions, yet suppress the reactions producing CO and COz, have been recently reported.l+ We would like to report high activity and selectivityfor C2hydrocarbon formation using perovskite-type oxide catalysts in a flow reactor cofed with methane and oxygen. The perovskite-type oxides provide a system of compounds with similar crystal structures such that, with appropriate cation substitution, catalytic behavior can be correlated to chemical properties. The unsupported catalysts were synthesized by hightemperature, solid-state reactions. The nitrates of the lanthanides, carbonates of the alkali metals, and manganese oxide were used as starting materials. The amounts of each reagent required to give the desired metal ion stoichiometry were mixed, pressed into pellets, and fired in an alumina crucible in air at 1100 "C for 16 h with frequent grinding and refiring of the sample. Powder X-ray diffraction analysis confirmed the perovskite crystal structures. Exceptions to this preparation method were those used to prepare LaMnOBoand LaMnOB1. LaMnO,,o was prepared by firing the sample at 1200 "C in a nitrogen atmosphere. LaMnOal was prepared by firing the sample at 900 OC in an oxygen atmosphere.' By these preparation
+ +
'This is a work of an agency of the United States Government.
(1) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. Energy Fuels 1987, I , 12-16. ( 2 ) Otauka, K.; Jinno, K.; Morikawa, A. J. Catal. 1986,100,353-359. (3) Matauura, 1.; Utsumi, Y.; Nakai, M.; Doi, T. Chem. Lett. 1986, 1981-1984. (4) Sofranko, J. A.; Leonard, J. J.; Jones, C. A.; Gaffney, A. M.; Withers, H. P. Prepr.-Am. Chem. SOC., Diu. Pet. Chem. 1987, 32(4), 763-769. ( 5 ) Lo,M. Y.; Aganval, 5.;Marcelin, G. fiepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1987,32(4), 770-773.
(6) Bartek, J. P.; Hupp, J. M.; Brazdil, J. F.; Grasselli, R. K. Prepr. -Am. Chem. SOC., Diu. Pet. Chem. 1987,32(4), 114-778. (7) Voorhoeve, R. J. H.; Remeika, J. P.; T r i b l e , L. E.; Cooper, A. S.; Disalvo, F. J.; Gallagher, P. K. J. Solid State Chem. 1976,14,395-406.
This article not subject to U.S.Copyright. Published 1988 by the American Chemical Society
236 Energy & Fuels, Vol. 2, No. 2, 1988
Communications
Table I. Conmarison of Activity and Selectivity of Catalysts for the Oxidative Coupling of Methane" conversion, Cz selectivity, catalvst surface area, m2/a mol % of CH, mol % of C Cz yield, mol % of C CzH4/C2Hs 1.11 0.56 11.0 10.1 3.4 LaMn03,,, 5.06 0.81 37.2 13.6 6.8 LaMnO., 1.2 8.03 41.8 19.2 2.6 13.3 21.0 63.4 2.1 10.1 14.1 71.6 0.54 2.53 84.4 3.0 9.39 1.3 48.4 19.4 8.52 0.67 19.1 44.6 ~~
4Conditions: 1 atm pressure; 820 "C = catalyst bed temperature; CH4/02/He = 5/1/4;17.2 cm3/min NTP total flow rate; 0.25 g of catalyst.
procedures, LaMn03 was prepared in a nearly stoichiometric, orthorhombic state and in an oxygen-rich rhombohedral state. The oxygen-rich state contains a greater amount of Mn4+. This Mn4+requires charge compensation leading to predominantly La cation vacancies. As a result, the general formula is more properly written as Lal,n,Mn03 where indicates vacancy. A fixed-bed microreactor with the reactor body made of quartz was used at atmospheric pressure. A 0.25-g sample of catalyst (28-48 mesh) was held in place by quartz wool, and a thermocouple sheathed in a quartz thermowell was located in the bed. Produds were analyzed by gas chromatography. Surface areas were measured by the Brunauer-Emmett-Teller (BET) method using nitrogen adsorption. The experimental variables and the results of various catalysts are presented in Table I. Conversion is reported as mole percent of methane converted. Selectivity is based on mole percent of the reacted methane being converted to a particular product. The products observed were carbon monoxide, carbon dioxide, ethane, ethene, and water. For L%.gNa,,lMn03, the C2 selectivity increased with increasing temperature up to the maximum value of 63.4% at 820 OC. Further increase in temperature resulted in increased formation of carbon oxides and thus lower C2 selectivity. A total methane conversion of 21.0% and a total oxygen conversion of 93.2% were observed at 820 OC. The ethene/ethane ratio increased with temperature. Of all the oxides examined, La,,.gNao.lMn03 showed the highest yield of C2hydrocarbons. To our knowledge, the only other perovskite-type oxide that has been examined as a catalyst for oxidative coupling of methane was LaA103. For similar conditions (CH4/02 = 5/1; continuous flow, cofeeding), Imai and Tagawa report a C2 selectivity of 48.4% for LaA103 with a total methane conversion of 25.3% at 710 0C.8 An empty reactor test was performed to study the effects of reactor walls (quartz) and temperature. At temperatures below 900 "C, no significant amount of products was detected. Perovskite-type oxides have attracted great attention as catalysts for the complete oxidation of hydrocarbons, particularly related to exhaust ~ o n t r o l . ~ These systems offer an isostructural series of mixed oxides in which the properties of the surface oxygen atoms can be modified, (8) Imai, H.; Tagawa, T.J.Chem. SOC.,Chem. Commun. 1986,52-53. (9)Seiyama, T.; Yamazoe, N.; Eguchi, K. Ind.Eng. Chem. Prod. Res. Deu. 1986,24,19-27.
leading to catalytic promoter effects. A further benefit in using perovskites containing lanthanum is the high thermal stability of these oxides.1° Recently, specific surface area has been shown to be an important factor in the catalytic oxidative coupling reaction." Decreasing surface area resulted in higher C2yields. So in order to compare the catalytic behavior and attribute the effects to chemical properties, the surface areas of the catalysts must not be widely different. In this work, all of the samples except Mn02 have surface areas within a small range ( (La,K)Mn03> (La,n)MnO,. From this, it can be observed that higher binding energies for oxygen are correlated with higher selectivity to C2 hydrocarbons. We conclude that sufficiently strong binding of oxygen to a surface site is evidently necessary to selectively produce higher hydrocarbons from methane as opposed to producing carbon oxides. Indirect support for this conclusion is provided by catalytic studies on complete combustion of methane over perovskite-type oxides? For deep oxidation, weakly bonded oxygen is considered to be effective. The ideal catalyst for oxidative coupling of methane must have (1)surface oxygen sites with sufficient binding energy and (2) a minimal number of weakly bonded oxygen sites. (10)Coutures, J. P.;Badie, J. M.; Berjoam, R.; Coutures, J.; Flamand, R.; Rouanet, A. High Temp. Sci. 1980,13,331-336. (11)Iwamatsu, E.; Moriyama,T.; Takasaki, N.; Aika,K.J.Chem. SOC., Chem. Commun. 1987,19-20. (12)Scurrell, M. S.Appl. Catal. 1987,32,1-22. (13)Oak Ridge Associated Universities Research Fellow.
J. E. France, A. Shamsi,* M. Q. AhsanI8
Morgantown Energy Technology Center U S . Department of Energy, P.O. Box 880 Morgantown, West Virginia 26507 Received December 7, 1987 Revised Manuscript Received February 2, 1988
