Energy & Fuels 1988,2,574-577
574
Vapor-Phase Oxidative Coupling of Methane under Pressure Kenji Asami, Kohji Omata, 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 Received December 4, 1987. Revised Manuscript Received March 2, 1988
The noncatalytic oxidative-coupling reaction of methane to form ethane and ethylene was found to be remarkably enhanced under pressurized conditions up to 1.6 MPa in the temperature range from 650 to 800 "C. The effect of pressure was discussed in terms of the elementary reactions including bimolecular initiation to form methyl radicals and their termolecular recombination resulting in ethane formation. Higher yield and selectivity for C2+ hydrocarbon (>77%) were obtained by using nitrous oxide as an oxidant instead of oxygen.
Introduction Recently much attention has been given to the oxidative coupling of methane as a means to develop new uses of natural gas as a chemical feedstock. A number of studies on the catalytic reaction have been published.l-' The present authors have also reported that a lead oxide catalyst supported on MgO has high activity and selectivity for the reactionesgand that the active oxygen species for the coupling reaction is lattice oxygen in PbO.'O We also found that alkaline-earth-metal halides supported on alkaline-earth-metal oxides show higher selectivity to C2 hydrocarbons than the PbO catalyst." However, there are few reports dealing with the oxidative coupling of methane in a homogeneous gas phase. Iwamatsu et al. reported that a trace amount of C2hydrocarbon is obtained from methane at 750 "C in their blank test for catalytic reactiom6 It has also been claimed that C2H2and C2H4 are obtained by pyrolysis of methane at high temperature (in flame) above 1000 "C under atmospheric pressure.12 On the other hand, a number of investigations have been made on partial oxidation of methane into formaldehyde and methanol; several reviews have been published.13J4 These studies were performed a t a relatively lower temperature (below 600 "C) than that for the oxidative coupling of methane. In some cases, ethane and ethylene were obtained but their selectivity was low. (1) Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73, 9. (2)Hinsen, W.; Bytyn, W.; Baerns, M. Proceedings of the 8th Internotional Congress on Catalysis; Verlag Chemie: Weinheim, FRG, 1984; Vol. 3,p 581. (3)Otauka, K.;Jinno, K.; Morikawa, A. Chem. Lett. 1985, 499. (4)Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721. (5)Imai, H.;Tagawa, T. J. Chem. SOC.,Chem. Commun. 1986, 52. (6) Iwamatsu, E.;Moriyama, T.; Takasaki, N.; Aika, K. J. Chem. SOC., Chem. Commun. 1987, 19. (7) Matsuura, I.; Utsumi, Y.; Nakai, M.; Doi, T. Chem. Lett. 1986, 1981. (8)Asami, K.;Hashimoto, S.; Shikada, T.; Fujimoto, K.; Tominaga, H. Chem. Lett. 1986, 1233. (9)Asami, K.; Haehimoto, S.; Shikada, T.; Fujimoto, K.; Tominaga, H. Ind. Eng. Chem. Res. 1987,26, 1485. (10)Asami, K.; Shikada, T.; Fujimoto, K.; Tominaga, H., Ind. Eng. Chem. Res. 1987,26, 2348. (11)Fujimoto, K.; Hashimoto, S.; Asami, K.; Tominaga, H. Chem. Lett. 1987, 2157. (12)Union oil, U.S. Patent 2 679544,1954. (13)Gesser, H. D.; Hunter, N. R. Chem. Rev. 1985,85, 235. (14)Pitchai, R.;Klier, K. Catal. Reu.-Sci. Eng. 1986, 28, 13.
0887-0624/88/2502-0574$01.50/0
In the present work, the oxidative coupling of methane was found to proceed in the gas phase under pressurized conditions at temperatures as low as 750 "C. Several characteristic features of the reaction under pressure have also been examined.
Experimental Section Methane oxidation was performed with a flow type reaction apparatus under pressurized conditions. The tubular type reactor of Incolloy H is shown in Figure 1. Quartz tubes were inserted into the reactor to prevent direct contact of the reactant gas with the metal surface of the reactor. Methane and oxygen were mixed and subjected to a reaction in the hatched zone. The volume of the reaction zone was 3.5 mL. The standard reaction conditions were as follows: temperature, 750 "C; CH4:O2:N2= 14:1.684.4; flow rate, 350 mL/min. The explosive range of this reactant mixture under these conditions is estimated above 4-5 MPa. All the reactants and products were analyzedby gas chromatographs. Oxygen, nitrogen, and hydrogen were analyzed with TCD detectors, while CO and COzwere analyzed with an FID detector with hydrocarbons after conversion into CHI by a methanator placed between the separation column and the detector. Results and Discussion Methane Oxidation under Pressure. (a) Effect of Total Pressure. Figure 2 shows the effects of the reaction pressure under standard conditions. While no reaction was observed without catalyst under atmospheric pressure, conversions of methane and oxygen increased, producing both C2+ hydrocarbon and CO, (mostly composed of carbon monoxide) as the pressure was raised. Hydrogen formed besides water, and the molar ratio H20:H2was usually between 4.2 and 4.3. Conversions of CHI and O2 reached 10.2%and 78.7% at 1.6 MPa, respectively, similar to those obtained by the catalytic reaction over 20 wt % PbO/MgO (1g) at the same temperature under atmospheric p r e s s ~ r e With . ~ ~ ~the increase in pressure, the selectivity of the C2+ hydrocarbons decreased from 60% (0.35 MPa) to 45% (1.6 MPa) due to a rapid increase in CO formation. The ratio C2H4:C2H6increased with increasing pressure, which may be attributed to the prolonged residence time due to the higher pressure. At 1.1 MPa and above, the selectivity to ethylene was higher than that to ethane. The major part of the C3+ hydrocarbon was propylene; the balances were propane, butenes, butanes, and butadiene. Carbon dioxide selectivity was low (4-8%), irrespective of the reaction conditions, which is 0 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 4,1988 575
Oxidative Coupling of Methane CHq
02tN2
stainless tube
I [I
1
=
quartz
Figure 1. Reactor. -
5
6p
4
0 E 1
4
4J
a
TJa
0
3
0
4 J M
!-la s c c o oo m m
2
a 0 c4J L:
1
U
I 0
Total Pressure
(MPa)
Figure 2. Effect of total pressure. Conditions: temperature, 750 OC; CH4:02:N2= 141.684.4;flow rate, 350 mL/min. 12 I
-
10
Temperature
order to separate the two effects, the residence time was varied by changing total pressure at constant mass flow rate (350 mL STP/min) and by changing mass flow rate at constant pressure (0.6 MPa), as shown in Figure 3. Methane conversion in the latter experiments, shown by open circles, increased linearly with the residence time, while that of the former experiments increased sharply with an S type curve. The increase in the product formation with increased pressure, therefore, can be attributed to an increase in the residence time of reactants to a small degree. (c) Considerations. It was demonstrated that C2 formation from methane on a PbO catalyst proceeds exclusively through the abstraction of hydrogen atom from methane by lattice oxygen.l0 The gas-phase oxidation of methane to give carbon oxides and water at high temperature, on the other hand, probably proceeds by a radical-chain mechanism;13-15included are the main elementary reactions described by eq 1-12. Ethane is formed by the recombination of the
CH,' CHSOO'
6p
a-
0
1
2 Residence Time
3
CHI
CH,OO'
CHSOOH
+ CH3'
+ OH'
CH,O'
HCHO
+ H'
-+
+ OH' + (M) H2O + (M)* OH' + CHI H 2 0 + CH,' HCHO + 0 2 HCO' + HO2' HCHO + OH' H2O + HCO'
flow rate (ml/min)
m
O2
CH,O'
c
6-
CH3' + HOC
CH300H
0
.rl Y]
+ + -
CHI + O2
1
-
("C)
Figure 4. Transition border of the reaction order with respect to the recombination of methyl radicals.
H'
HCO'
4
(s)
Figure 3. Effect of residence time. Conditions: temperature, 750 "C; CH,:02:N2= 14:1.6:84.4;(0)flow rate, 350 mL/min; (0) total pressure, 0.6 MPa.
in sharp contrast to the catalytic reaction under atmospheric pressure, where large amounts of C 0 2 were
(b) Effect of Residence Time. Residence time of reactants in the reaction zone is prolonged when the reaction pressure is raised, and thus the effect of the increase in total pressure includes that of the residence time. In
-
+0 2
+ HCHO CO + OH'
CO + HOz'
HOz'
HCO' +
+ H202
COZ + H'
methyl radicals (eq 13). Hydrogen (H2)is formed by the
CH3'
+ CH3' + (M)
-
C2HG + (M*)
(13)
recombination of the atomic hydrogens or hydrogen ab(15) Semenov,N.N.SomeProoblems of Chem. Kinetics and Reactiuity; Suga, T.; Akita, S.,Eds.;Iwanami: Tokyo, 1963.
Asami et al.
576 Energy & Fuels, Vol. 2, No. 4, 1988
-
12
12
9
9
dp
c 0
';I 6
6
I4
3
e
0
u
2"
3
3
U
1
0
3
2 p
*
R.T.
4
0
5
(MPa'S)
straction by H'from methane or intermediate radicals such as HCO' and CH30'. The fact that oxidative coupling of methane is enhanced under the pressurized conditions might be interpreted by some mechanisms. One is the increased effect of a third body (M), which stabilizes a vibrationally excited ethane molecule formed through reaction 13. If the stabilization does not proceed efficiently, the excited ethane should decompose to methyl radicals quickly because of its excess energy, which is enough to dissociate the carbon-carbon bond of ethane. Figure 4 indicates the transition of the reaction order of the recombination of methyl Above the line the recombination proceeds with second-order kinetics, whereas it proceeds with third-order kinetics below the line. The transition pressure at 750 "C, where the experiments were carried out, is about 0.4 MPa, and ethane formation under atmospheric pressure (0.1 MPa) is inferred to be difficult, because the chances of third-body collision are quite small. On the other hand, the reaction proceeds easily under the pressure above 0.4 MPa since the removal of the excess energy from excited ethane goes smoothly. However, the difference between two lines in Figure 3 could not be understood by the third-body effect because the experiments were performed above 0.4 MPa. Another possible mechanism is a promotion of the initiation reaction (eq l),which is a bimolecular reaction. The rate of methane conversion in the reaction zone (rc ) is expressed as the product of the collision frequency (C%) of CHI and O2and the probability that they react (a) (eq 14). The = (CF)a
(14)
CF is in proportion to the product of partial pressure of methane (P(CH4)),partial pressure of oxygen ( P ( 0 2 ) )and , the residence time (RT) (eq 15). As the probability (a) CF is constant,
PCH, rcH,
QC
(P(CH4))(P(0,)) (RT)
can be expressed as eq 16. = k(P(CH&)(P(Oz))(RT)
(15)
Methane (16)
conversion is defined as the quotient of the r C H , over the feed rate of methane in the reaction zone, which is in
I.
650
700
(16) Laider, K.J. Reaction Kinetics; Pergamon: New York, 1963; Vol.
(17) Kistiakowsky, G.B.;Roberts, E.K. J . Chem. Phys. 1953,21,1637. (18)Dodd, R.E.; Steacie, E. W. R. h o c . R. SOC.London, A 1964,233, 283.
800
750
Temperature
Figure 5. Relationshipbetween P(RT) and methane conversion. Conditions: temperature, 750 O C ; CH4:02:N2= 141.684.4;( 0 ) flow rate; 350 mL/min; (0) total pressure, 0.6 MPa.
rCHl
s3 /
("C)
Figure 6. Effect of reaction temperature. Conditions: total pressure, 1.1 ma;CH4:OZ:Nz= 141.684.4;flow rate, 350 ml/min.
"
0
25
50
P(02)
75
100
(kPa)
Figure 7. Effect of partial pressure of oxygen. Conditions: total pressure, 1.1 MPa, P(CH4),160 kPa; Nzbalance; flow rate, 350 mL/min; temperature, 750 "C. proportion to P(CH4). Considering this and that P(0,)is in proportion to the total pressure (P),methane conversion should be in proportion to the product of P and RT (eq 17). Figure 5 demonstrates the relationship between CH4 conversion a P(RT) (17) P(RT) and methane conversion. The results obtained by the above two experiments (Figure 3) are expressed by the same line, which means that the promotion effect of the initiation of the chain reaction (eq 1) is supposed to be essential in a higher pressure range. Effect of Operational Factors under Pressure. (a) Temperature. Figure 6 demonstrates the effect of reaction temperature at 1.1 MPa. The yield of each product increased with the increase in temperature. Although C2+ selectivity slightly increased between 650 and 750 "C (50-54%), it decreased at 800 "C (47%). The selectivity to ethylene increased as the reaction temperature was raised and became superior to that of ethane above 750 "C. The temperature effect at 1.6 MPa was also examined. Both conversions of methane and oxygen were higher at every temperature than those at 1.1 MPa. However, the C2+ selectivity decreased to 40-45%, which was accompanied by a marked increase in the selectivity to carbon monoxide. (b) Partial Pressure of Oxygen. Figure 7 demonstrates the effect of the partial pressure of oxygen ( P ( 0 2 ) ) where the total pressure and the partial pressure of methane (P(CH4))were kept constant at 1.1 and 0.16 MPa,
Energy & Fuels, Vol. 2, No. 4, 1988 577
Oxidative Coupling of Methane
Table I. Oxidative Coupling of Methane in the Gas Phase by Oxidants" convn, % selectivity, % P(oxid), kPa P(CH4),kPa CHI oxid CzHB CzHl C,+
P, MPa
oxid 0 2
N2O
NO
co
coz
1.4
36.4 41.3
4.0
2.6 6.4 0 0
8.2 18.9 37.0 23.8
3.0 3.5 40.9 34.1
84 150
0.6 3.5
3.0 28.5
45.9 26.1
12.1 27.2
0
1.1
10 18
0.6 1.1 0.6 1.1
24 44 24 44
84 150 84 150
1.8 3.8 0.1 0.2
7.3 17.0 1.0 1.9
56.9 33.4 22.1 30.5
29.3 37.8 0 11.6
0.6
5.6
Conditions: temp, 750 O C , flow rate, 350 mL/min; balance gas was Nz or He. 81
I
respectively, while those by nitrous oxide were 1.8% and 3.8% , respectively. Since each partial pressure of oxidant and methane is doubled when the total pressure is doubled, the rate of chain initiation increases by four times in the case of a bimolecular reaction while it is doubled in the case of unimolecular reaction. The initiation step in methane oxidation by N20 is a unimolecular thermal decomposition of N 2 0 to form nitrogen and oxygen atoms as described in eq 18; the reaction rate should be less N2O Nz + 0 (18) sensitive to total pressure. In contrast, the initiation step in the case of dioxygen is a bimolecular reaction (eq 1). Therefore, methane oxidation by O2 should be more strongly dependent on total pressure. As for the selectivity, N20 gave a much higher amount of Cz hydrocarbon (>77%) than O2(
