Energy & Fuels 1989,3,727-730
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Oxidative Coupling of Methane over Perovskite-Type Oxides and Correlation of T,,, for Oxygen Desorption with C2Selectivity Abolghasem Shamsi" and Khurram Zahirt United States Department of Energy, Morgantown Energy Technology Center, P.O. Box 880, Morgantown, West Virginia, 26507-0880 Received May 25, 1989. Revised Manuscript Received August 28, 1989
Oxidative coupling of methane co-fed with oxygen to C2+hydrocarbons has been investigated both in the absence and in the presence of L%.9N%.lMn03at 1and 3.6 atm pressures. It was found that residence time, temperature, methane-to-oxygen ratio, and pressure are the major factors affecting the conversion and selectivity both in the empty reactor and in the presence of the catalyst. Significant gas-phase reactions were observed at higher pressure, 02:CH4ratio, and contact time. The results suggest that the activation of methane occurs both in the gas phase and on the surface of the catalyst, and the surface reactions appeared to be important in oxidizing the intermediates CH,', CzH4, CzH6, and CO to carbon dioxide. Temperature-programmed-desorptionstudies on a series of perovskite-type catalysts showed that T,,, the temperature at which maximum O2 desorbs, correlates very well with the Cz selectivity for oxidative coupling of methane, and these results suggest that a strong binding of oxygen to the surface site is essential to selectively produce C2+hydrocarbons from methane as opposed to complete oxidation leading to carbon dioxide.
Background Methane is the principal component of natural gas, which is found in porous reservoirs generally associated with crude oil. Some of these abundant reserves of natural gas exist in locations too remote from market areas to be recovered on a commercial basis by present methods. A promising new methane conversion technology uses catalytic oxidative coupling as the first step in the The oxidative-coupling step converts natural gas to olefins, which could be subsequently converted to higher hydroc a r b o n ~ . ~A key to developing this technology is optimization of the oxidative-coupling step, such as using novel catalysts. We have shown that, by proper cation substitution in perovskite-type oxides, active and selective catalysts for the oxidative coupling of methane to higher hydrocarbons can be obtaineds5s6 Recent studies showed that gas-phase reactions, especially at higher pressures, play a significant role in the partial oxidation of methane.7-9 Furthermore, there appears to be a discrepancy between the results obtained by Ito et al.l0 and the results obtained by Yates and Zlotin7 for empty reactors. Therefore, in order to gain further insight into this catalytic and noncatalytic system, the oxidative coupling of methane was investigated at several reaction conditions, in both the presence and absence of the perovskite-type oxide Lao,9N%lMn03.
One of the most important factors that govern the catalytic oxidation process is the metal-oxygen bond strength.'l For metal oxide catalysts, usually three different types of oxygen may be detected, namely, lattice oxygen, adsorbed oxygen, and absorbed oxygen.12 These activated oxygen species have widely different properties in catalytic oxidation and can affect the activity of a particular catalyst. In the case of metal oxide catalysts, reactivity of these oxygen species depends largely upon the kind of component metal cation present. Temperature-programmed desorption (TPD) is one of the techniques that can be used to obtain useful inforOak Ridge Associated Universities Postdoctoral Research Fellow.
mation on the existence of oxygen species in discrete states and/or over a broad distribution of energies.lZ-l4 Such information when combined with the results of catalytic reaction studies, carried out under identical conditions, can provide an insight into the catalytic-oxidative conversion of methane to hydrocarbons. In this paper, we report the oxygen-sorptiveproperties of perovskite-type catalysts in relation to their catalytic activity and selectivity.
Experimental Section The details of preparing the perovskite-type oxides have been reported el~ewhere.~ A 0.235-m-long downflow alumina reactor tube (9.53-mm o.d., 6.35-mm i.d., calculated heated volume = 2.0 cm3) with a quartz thermowell was used as a fixed-bed reactor with 0.5 g of catalyst (-28/+48 mesh) held in place by qua& wool. Electronic mass-flow controllers were used to feed methane (99.99%), oxygen (99.8%), and helium (99.999%). The reactor was electrically heated to reaction temperatures under a flow of helium. All transfer lines and valves were 316 stainless steel and held at 150 "C. Product analyses were performed by on-line gas chromatography (GC)and mass spectrometry (MS). The products
(1) Ekstrom, A.; Lapszewicz, J. A. J.Am. Chem. SOC.1988,110,5226. (2) Labinger, J. A.; Ott, K. C. J. Phys. Chem. 1987, 91, 2682. (3) Zhang, H.-S.; Wang, J.-X.; Driscoll, D.; Lunsford, J. H. J. Catal. 1988,112, 366. (4) Jezl, J. L.; Michael, G. 0.; Spangler, M. J. US.Patent 4,754,091, June 28, 1988. (5) France, J. E.; Shamsi, A.; Ahsan, M. Q. Energy Fuels 1988,2(2), 235. (6) France, J. E.; Shamsi, A.; Headley, L. C.; Ahsan, M. Q.Energy R o g . 1988,2(4), 185. (7) Yates, D. J. C.; Zlotin, N. E. J. Catal. 1988, 111, 317. (8) Lane, G.; Wolf, E. E. J. Catal. 1988, 113, 144. (9) Asami, K.; Omata, F.; Fujimoto, K.; Tominaga, H. 0. J. Chem. SOC.,Chem. Commun. 1987, 1287. (10) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J. Am. Chem. SOC. 1985, 107, 5062. (11) Kung, H. H. Ind. Eng. Chem. R o d . Res. Deu. 1986, 25, 171. (12) Seiyama, T.; Yamazeo, N.; Eguchi, K. Ind. Eng. Chem. Prod. Res. Deo. 1985, 24, 19. (13) Iwamoto, M.; Yoda, Y.; Yamazoe, N.; Seiyama, T. J. Phys. Chem. 1978, 82(24), 2564. (14) Falconer, J. L.; Schwartz,J. A. Catal. Reo. Sci. Eng. 1983,25,141.
T h i s article not subject t o U S . Copyright. Published 1989 by the American Chemical Society
728 Energy & Fuels, Vol. 3, No. 6,1989
Shamsi and Zahir
Table I. Comparison of Activity and Selectivity of Oxidative Coupling of Methane in the Presence and Absence of Catalyst at 845 "C, Alumina Reactor, CH4/He/02 = 20/20/10 cma/min N P T Flow Rates conversion, mol % selectivity, carbon mol % catalyst/conditions empty, contact time 2.4 s, 1 atm empty, contact time
0 0
10 -
W
-10
5
5
2
5-
01 1
-Ec E
I
I
3
1
I
I
5
I
7
9
P(CH,)P (02)
F i g u r e 2. Effect of methane-to-oxygen ratio on conversion and selectivity over L%,9N%,lMn03in an alumina reactor: 1.0 atm pressure; 830 "C; CH4/He/02= 20/20/10 "T flow rates; contact time = 0.4 s; 0.5-g catalyst.
96.8%, respectively. The product distribution in this case was 11.2% CzH4,4.6% C2H6,15.8% C 0 2 ,and 68.4% CO. The effects of temperature and pressure on conversions and selectivity to C2+hydrocarbons, over L%.9N%.iMn03 catalyst, are shown in Figure 1. At 3.6 atm pressure and temperatures below 780 OC, higher methane and oxygen conversions and higher selectivity to C2+hydrocarbons were observed compared to those at 1atm. However, when the temperature increased, the C2+ selectivity at low pressure (1atm) increased to a maximum value, while a t the higher pressure, the Cz+selectivity slightly decreased. Furthermore, a higher molar ratio of CPH4:C2H6 was observed at higher pressure. In the presence of the catalyst, the major portion of the carbon oxides was carbon dioxide, indicating that the catalyst played a significant role in the reaction sequences and altered the product distribution. The dependence of conversion and selectivity on the methane-to-oxygen ratio over L%.9N%.lMn03was also studied (Figure 2). Lower conversion of methane and higher selectivity to C2+hydrocarbons were obtained when
Energy & Fuels, Vol. 3, No. 6, 1989 729
Oxidative Coupling of Methane
250
450
650
850
1050
Temperature ("C)
Figure 3. Temperature-programmed desorption of ox gen (m/e = 32): heating rate = 1 "C/s; He flow rate = 0.5 cm1/s; 0.25-g catalyst. 80
-2?
70
E
60
-0
Gdo,Na, ,MnO,
C
e
50
0
.-5 .-xm
40
I
30
VJ
cr
the gas-phase contribution. At the reaction conditions (820 "C, 1 atm pressure, and p(CH4)/p(02)= 5/11, less than 290 methane was converted in the absence of the catalyst and C2 yield was about 190. The results presented in Figure 4 show that T,, for a-oxygen desorption correlates very well with the C2 selectivity. The catalyst Gdo.gNa,,lMn03 with a-oxygen desorbing at the highest temperature (865 "C) showed the highest selectivity for C2products whereas LaMnO,, which desorbs at the lowest temperature (695 "C), is the least selective for the conversion of methane to C2 product^.^ It is interesting to note that optimum conditions (higher C2 selectivity) for conversion of methane over La+gN%.lMn03catalyst correspond to the temperature (820 "C) that matches very nicely with the T,, for that catalyst. For the perovskite-type catalysts examined, it has been speculated earlier that the oxygen binding energy decreases in the order (La,Na)Mn03> (La,K)Mn03> (La,o)MnO3.l5 Present TPD studies provide an experimental basis for that trend in oxygen binding energies for these compounds and also similar information on other catalysts. For example, when comparing LaMn03 with La,,gK,,lMn03 and La,,gN%,lMn03, it becomes apparent that it is the nature of the metal-oxygen bond that controls the catalytic activity of oxide catalysts. Therefore, we suggest that, in catalytic oxidation of methane to higher hydrocarbons, the most important factor may very well be the binding energy of oxygen to a surface site of the catalyst. This observation also agrees well with the earlier claim that some of the common selective-oxidation catalysts (e.g., V203,Moo3, and Bi2MoO6)did not contain weakly adsorbed oxygen.13
20
LaMnOl 10
690
730
770
810
850
T m a x (OC)
Figure 4. Correlation of Cz selectivity with the temperature at which maximum O2desorbs (2"): 0.25-g catalysts tested at 820 OC; C H 4 / 0 2ratio = 5.
the methane-to-oxygen ratio was increased. This result indicates that methane and higher hydrocarbon products, at higher oxygen partial pressures and in the presence of the catalyst, were increasingly converted to carbon dioxide. Temperature-Programmed Desorption. Temperature-programmed-desorption studies on a series of perovskite-type catalysts with general formula Al-,B,Mn03 (where A is Gd, Sm, La, or Ho, B is either Na or K, and x = 0 or 0.1) were carried out, and the temperature a t which maximum oxygen desorbed (T,,,) was correlated with the C2 selectivity. The TPD chromatograms for oxygen desorption are shown in Figure 3 as plots of mass spectrometer signal (desorption rate) for oxygen ( m / e = 32) versus temperature. All chromatograms are characterized by the appearance of two types of oxygen. The first type, a-oxygen, desorbs at a lower temperature compared to the second type, @-oxygen,which desorbs a t higher temperature. The values for T,, corresponding to the maxima for a-oxygen desorption, are plotted in Figure 4 along with the C2 selectivities obtained in the earlier ~ t u d i e s . ~The , ~ values for T,, ranges from 860 "C for Gdo,gNa,,lMn03to 695 O C for LaMnO, and is sensitive to the presence of alkali-metal ion B as well as to substitution a t site A. In the earlier studies, the catalysts were tested under identical experimental conditions such as temperature, pressure, methane-to-oxygen ratio, and contact time. The C2selectivities for perovskite-type oxides, plotted in Figure 4,were 19-21% except for H O ~ . ~ N % . ~ which M ~ O was ~, 15.3%. An empty reactor test was performed to measure
Conclusions We have shown that, by proper cation substitution in perovskite-type oxides, active and selective catalysts for the oxidative coupling of methane to higher hydrocarbons can be obtained. The results show that surface reactions are important in oxidizing the intermediates CH3', C2H4, C2H6, and CO to carbon dioxide. It was found that contact time, temperature, methane-to-oxygen ratio, and pressure were the major factors affecting the conversion and selectivity in the presence and absence of the catalyst. Furthermore, the high activity observed by Yates and Zlotin7for the empty reactor could be a result of the higher contact time and possible back pressure (caused by the capillary tube) when compared with the results reported by Ito et al.1° The activation of methane appeared to be occurring both in the gas phase and on the surface of the catalyst. In the gas phase, methane was possibly activated by diatomic oxygen. The oxidation of methane in the gas phase by diatomic oxygen has been discussed in other reports.16J7 The types of oxygen species on the surfaces of the catalyst responsible for the activation of methane are not well-defined. However, the activation of methane by surface oxygen species has been proposed by several researchers. Liu et a1.18 have shown 0- ions; Driscoll et have proposed [Li+O-] centers; and Otsuka and Jinno20 have proposed adsorbed diatomic oxygen, as being re(15) Voorhoeve, R. J. H.; Remeika, J. P.; Trimble, L. E.; Cooper, A.
S.;Disalvo, F. J.; Gallagher, P. K. J . Solid State Chem. 1975,14, 395. (16)Bone, W. A. Proc. R. SOC. London, Ser. A 1932,137, 243. (17)Walker, R. W. Reaction Kinetics; The Chemical Society, Bur-
lington House: London, 1975; Vol. l, pp 161-211. (18) Liu, H.-F.; Liu, K. Y.; Johnson, R. E.; Lunsford, J. H. J . Am. Chem. SOC.1984,106,4117. (19)Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. 1985,107, 58. Chem. SOC. (20)Otsuka, K.; Jinno, K.; Morikawa, A. J. Catal. 1986, 100, 353.
Energy & Fuels 1989,3,730-734
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sponsible for the activation of methane. From the TPD studies, we can conclude that, for the perovskite-type catalysts examined, a strong binding of oxygen to the surface site is essential to selectively produce Cz+hydrocarbons from methane as opposed to complete oxidation leading to undesirable carbon dioxide. Present results suggest a possibility that TPD technique can be
utilized to find more effective catalysts by selecting a proper combination of A and B site substitution in perovskite-type catalysts. Registry NO.L%$I%.lMn03,1133784&6; CHI,7482-8;CZH4,
74-85-1; c ~ H 74-84-0; ~ , coz,124-38-9;co,63048-0;HCHO, 5000-0;Sq9N%,lMn03, 123183-49-3; H O ~ , ~ N % , ~123183-50-6 M~O~, Gdo.9N~,lMn03, 123183-51-7; La,,&,Mn03, 113378-47-5.
Investigation of Structure/Property Relationships of Selected C5-Clo Hydrocarbons Using Canonical Correlation Analysis of Multisource Data Barbara L. Hoesterey,? Henk L. C. Meuzelaar,*vf and Ronald J. Pugmire1 Center for Microanalysis and Reaction Chemistry and Department of Fuels Engineering, University of Utah, Salt Lake City, Utah 84105 Received October 26, 1988. Revised Manuscript Received August 21, 1989
Twelve physicochemicaland thermodynamic properties including molecular weight, boiling point, flash point, density, refractive index, volumetric, gravimetric, and molar heats of combustion, atomic H/C, carbon number, hydrogen number, and the fuel-related threshold sooting index for 47 C5-Cl0 hydrocarbons were subjected to factor analysis. Two factors with eigenvalues greater than 1.0 were found, accounting for 95% of the variance. The major groups of variables were interpreted as arising from either molecular size (boiling point, flash point, molar heat of combustion, molecular weight, and carbon number) or degree of unsaturation (density, refractive index, volumetric heat of combustion, threshold sooting index, H/C, and gravimetric heat of combustion). Canonical correlation analysis of the properties factor space with mass spectrometry/Kovats retention index data showed that characteristic mass spectral variables correlated closely with the degree of unsaturation by differentiating aliphatic from aromatic compounds. The Kovats retention index variable, not unexpectedly, modeled molecular-size-related parameters such as carbon number.
Introduction There is current interest in determining structure/ property relationships for fuels and fuel components. Structure/property relationships have typically been determined for homologous series of hydrocarbons, such as n-alkane~.'-~Properties have been correlated with topological indices by using both raw data and logarithmic transforms of one or both of the parameter^.^^^ This structure/property correlation approach has been used widely in the form of structure/activity relationships for assessing the effectiveness of drugs of similar carbon skeleton.6J Many of the techniques are general and may apply to fuels. Especially important to fuel studies are correlations related to combustion properties such as between octane number,* heats of formation,l heats of combustion, and various topological indices or correlations related to sooting properties and structural parameters derived from spectroscopic measurement~.~J~ We have recently become interested in extending multivariate analysis techniques to properties of hydrocarbons and their relationship to structure. Our approach is somewhat different from that of most other procedures in which intuitively derived topological indices are tested against selected properties. We first determine the int Center
for Microanalysis and Reaction Chemistry.
* Department of Fuels Engineering.
terrelationships among the properties using multivariate analysis techniques. This is useful in a way directly related to structure since it helps decide how many structural parameters may be needed to fully describe the suite of properties of interest. Factor analysis creates linear combinations of variables based on correlation. A data set can be constructed by using the properties of interest, and this can be examined for correlations among the properties. This can also be done for a set of structural parameters derived from topological indices, spectroscopy, or other sources. Then, from canonical correlation analysis, the properties data and the structural data can be related. Canonical correlation analysis determines the relationship between a group of variables (e.g. physicochemical properties) and another (1)Wiener, H. J. Am. Chem. SOC.1947,69,17-20. (2)Rouvray,D. H.; Pandey, R. B. J. Chem.Phys. 1981,85,2286-2290. (3)Trinajstic, N.Chemical Graph Theory; CRC Press: Boca Raton, FL,1983;Vol. 2. (4)Rouvray, D. H.; Crafford, B. C. S. Afr. J. Sci. 1976, 72, 47-51. (5)Hansen, Peter J.; Jurs, Peter C. Anal. Chem. 1987,59,2322-2327. ( 6 ) Jurs, Peter C. Science 1986,232,1219-1224. (7)Rouvray, D. H.In Chemical Applications of Topology and Graph Theory; King, R. B., Ed., Elsevier: Amsterdam, The Netherlands, 1983; pp 159-177. (8) Balaban, A.; Motoc, I. Math. Chem. 1979,5,197. (9)Pugmire, Ronald J.; Grant, David M. Unpublished data. (10)Hamblen, D. G.; Solomon, P. R.; Tarentul, K. S.; Carangelo, R. M. Prep?.-Am. Chem. SOC.Diu. Pet. Chem. 1987,32(2),530-533.
0887-0624/89/2503-0730$01.50/00 1989 American Chemical Society
