Ind. Eng. Chem. Res. 1988,27, 1387-1390
tivity of the quaternary ammonium groups. Interestingly, the activity, started from an almost zero value increased during recycle experiments and reached values close to that of the fully quaternized polymer after three experiments. FB reactors with ultrasonic mixer, on the other hand, reached a level of performance comparable to those obtained in a slurry reactor. The latter system possesses the drawback of uneven recycle of the catalyst because of the grinding and collapsing of the catalytic particles. It has been confirmed that the substitution of conventional stirring modes by an ultrasonic mixer decreases the activation energy of the reaction by at least 10%. Acknowledgment The authors thank the Italian National Research Council (C.N.R.) Fine and Secondary Chemistry Finalized Project for partial financial support. Registry NO.PAN, 140-29-4;TBA, 2052-49-5; TEBA, 56-34-8; C4H$r, 109-65-9NaOH, 1310-73-2;CBH,CH(C4H9)CN, 3508-983;
1387
IRA 904, 9050-98-0; Duolite A161, 64427-61-8; Duolite A171, 75366-89-1;Duolite A101, 92307-93-2. Literature Cited Balakrishnan, T.; Ford, W. T. Tetrahedron Lett. 1981, 44, 4377. Balakrishnan, T.; Ford, W. T. J . Org. Chem. 1983, 48, 1029. Dehmlow, E. V.; Dehmlow, S. S. Phase Transfer Catalysis; Verlag Chemie: Weinheim, 1980; p 32. Komeili-Zadeh, H.; Dou, H. J.-M.; Metzger, J. J . Org. Chem. 1978, 43, 156. Ragaini, V.; Saed, G. 2.Phys. Chim. N . F. 1980, 119, 85. Ragaini, V.; Verzella, G.; Ghignone, A.; Colombo, G. Znd. Eng. Chem. Process Des. Deu. 1986, 25, 878. Ragaini, V.; Chiellini, E.; D'Antone, S.; Colombo, G.; Barzaghi, P. Universiti di Milano, Department of Physical Chemistry & Electrochemistry, unpublished results, 1988. Solaro, R.; D'Antone, S.; Chiellini, E. J. Org. Chem. 1980,45, 4179. Steere, N. V. J . Chem. Ed. 1975, 52, A419.
Received for review July 20, 1987 Revised manuscript received January 21, 1988 Accepted March 21, 1988
Conversion of Methane to Oxygen-Containing Compounds by the Photochemical Reaction K o t a r o O g u r a , * C a t h a r i n a T. Migita, and M i n o r u F u j i t a Department of Applied Chemistry, Yamaguchi University, Ube 755, Japan
The oxidative photolysis of methane was studied in the presence of water vapor and air a t 100 "C and in atmospheric pressure. T h e major products were oxygen-containing compounds involving methanol, formaldehyde, acetic and peracetic acids, formic acid, and carbon dioxide. The reactions gave 4-16% conversions with a selectivity in methanol exceeding 33%. Hydroxyl radicals which are formed by the photolysis of water vapor activate methane by hydrogen abstraction in the first step in the synthesis, and the reaction pathway of the products is discussed. The activation of methane is a prevailing subject that commands considerable attention derived from interest in conversion of methane to a more worthy substance. Methane is the dominant ingredient of natural gas and occurs more widely throughout the world than raw petroleum. It is then desired to open up new avenues of use for methane except that as an energy source. Many approaches have been tried for methane activation, but a technologically promising process in which methane is directly used as a raw material under mild condition has not been developed. Recently, the monohalogenation of methane has been catalytically achieved over supported solid acid such as FeO,ClY/Al2O3a t temperatures between 180 and 250 "C (Olah et al., 1985). Kitajima and Schwartz (1984) employ silica-supported rhodium complexes in the catalytic chlorination of methane, and their products were methyl chloride and hydrogen chloride with lesser amounts of other chlorinated methane. Photoactivation of methane has been paid attention to from the fundamental as well as the technological points of view. The mercury-sensitized photolysis of methane gives H2, C2H4, C2Hs,C3H8,etc., for which a mechanism involving "hot" hydrogen atoms is proposed due to the very small value of the temperature coefficient obtained (Mains and Newton, 1961). Rebbert and Ausloos (1968) have photolyzed methane with a helium resonance lamp and observed that about 95% of the protons absorbed lead to ionization according to CH4 + hv [CH4+]*+ e-. [CH,]* may dissociate into CH3+or
-
CH2+ or react with CHI to give CH6+and CH3. In the economic application of methane, however, photochlorination is one of the most encouraging processes of activation (Dumas, 1840; Chaktravarty and Dranoff, 1984; Ogura and Takamagari, 1986). However, this process lacks selectivity and generally yields mixtures of chloromethanes, which is similar to the case in thermal chlorination. Methyl chloride is chlorinated more rapidly than methane itself; so if methyl chloride is the desired product, a ratio of methane to chloride of at least 1O:l is required (Jones and Allison, 1919). Although the partial oxidation of methane is very important in the technological aspect, little fundamental work has been reported on the process. Liu and Lunsford (1982) and Liu et al. (1984) found that nitrous oxide is a fitting oxidant and molybdenum supported on silica is an operative catalyst for the conversion of methane to methanol and formaldehyde. Solymosi et al. (1985) report the partial oxidation of methane by N 2 0 over a Sn02-Bi20, catalyst which leads to formaldehyde with a selectivity of 95-84% at 1.7-2.7% conversion at 550 "C. The direct conversion processes of methane to a more valuable compound practiced industrially and proposed so far are confronted with many problems if methane is designed as a raw material on an extensive scale. The following points may be indicated: (i) severe reaction conditions; (ii) low selectivity and conversion efficiency; (iii) not a very worthy product; Le., oxygen-containing C2 species are seldom obtained.
0888-5S85/88/2627-1387$01.50/0 0 1988 American Chemical Society
1388 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 2
'J
Figure 1. Schematic diagram of photochemical reaction system: 1, reaction chamber; 2, thermometer; 3, low-pressure mercury lamp; 4, gas storage; 5 , manometer; 6, pentane slush trap; 7, water-cooled condenser; 8, solution storage; 9, pressure-adjusting tank containing NaC1-saturated solution; 10, pump; 11, flow meter; 12, water vapor inlet; 13, methane inlet; 14, air inlet; 15, vacuum.
We have undertaken the photochemical conversion of methane with the objective of mitigating these shortcomings, and a new activation process was developed in which methane is converted to oxygen-containing compounds at temperatures lower than 100 OC (Ogura and Kataoka, 1988). In the present study, the reaction apparatus was scaled up and the conversion reactions were investigated under more extensive conditions.
Experimental Section Apparatus and Procedure. The photochemical reaction apparatus used in this experiment is shown schematically in Figure 1. The main parts of this apparatus consist of the reaction chamber, water-cooled condenser, pentane slush trap, and pressure-adjusting tank. The reaction chamber was constructed of a Pyrex glass tube (inside diameter, 90 mm; length, 900 mm), and the temperature was maintained a t 100 f 1 "C with a ribbon heater. A 100-W low-pressure mercury lamp (inside diameter, 10 mm; length, 1000 mm) made of synthetic quartz, which was used as the light source, was put in the reaction chamber. Methane and water vapor were admitted into the reaction chamber, and in some cases air was simultaneously added. After the whole system was filled with these gases in a given proportion, the mercury lamp was turned on. The methane gas (Seitetsu Kagaku Co.) used was 99.2% pure and had the following impurities: 0.5% COz, 0.2% Nz, and 500 ppm of 02.Water vapor was generated in a flask (1 dm3) containing pure water with a heating mantle and brought in the reaction chamber through a joint connected to the flask. The amount of water vapor fed to the chamber was regulated by changing the temperature of the flask. The products and water vapor were first cooled in a condenser attached to the reaction chamber and were then trapped with pentane slush (-130 "C). The effluent gas was again circulated with a pump. The total pressure in the reaction chamber was always kept a t atmospheric pressure by controlling the level of the NaC1-saturated solution in the pressure-adjusting tank. After the reaction was brought to completion, the pentane slush trap was restored to room temperature. The gases generated on this occasion were collected in the gas storage which was evacuated beforehand. Analyses. The reaction products were found in the condensed solution and gas phase for the most part, but the amounts detected in the water remaining in the flask were not negligible. The instruments employed for analyzing the products were a gas chromatograph (Shimadzu
Table I. Products (Imol) Obtained in t h e Photolysis of Methane" air, vol % 1.1 20.5 60.6 CHSOH 5556 (19.9)b 20577 (33.3)b 7 297 (13.0)b C2HSOH 501 (1.8) 224 (0.4) 7 7 16 i-C,HjOH 21 (0.1) 5 n-C3HjOH 34 (0.1) 7 tert-C4HgOH 9 sec-C4HgOH 12 i-CdHSOH 11 1 1 n-C,HSOH 528 (1.9) 71 (0.1) 3 CH3COCH3 4 32 (0.1) CZHbCOCH3 16 (0.1) CZH5COCzHj 3853 (6.2) 5795 (10.3) 476 (1.7) HCOOH CH3COOH + 534 (1.9) 18829 (30.5) 19145 (34.1) CHXOOOH HCOOCH3 23 (0.1) 96 (0.2) CH 3 C00CH 3 5 3 1 HCHO 925 (3.3) 13751 (22.2) 10762 (19.2) CHBCHO 5 33 (0.1) CZH, 18630 (66.7) 3 399 (5.5) COZ 610 (2.2) 957 (1.5) 13 137 (23.4) H2 49 387 16 680 5 580 HZ02 268 8 415 12 561 convsn of CH, 3.3 8.5 15.9 Reaction time, 3 h. Temperature in the reaction chamber, 100 "C. n(CH,)/n(H,O) = 0.07. bSelectivity, % .
GC-8A, JEOL JGC-1100), a steam chromatograph (Ohkura Model SSC-l), and a high-performance liquid chromatograph (HPLC, Hitachi 655A). The gas chromatograph was used at 100 "C with a flame ionization detector (FID) and a Porapak Q column or a t 30 "C with a thermal conductivity detector (TCD) and a molecular sieve 5A column. The steam chromatograph was employed at 130 "C with a FID and a Porapak R column, and the high-performance liquid chromatograph was employed at 60 "C with an UV monitor and a GL-C61OH column. Acetic acid, which was determined with HPLC, had the same retention time as peracetic acid, and the total amount of acetic acid plus peracetic acid was estimated by comparison with that obtained for a standard solution of acetic acid. The identification of peracetic acid was carried out by an ion chromatograph (Yokogawa, IC-100 type) and chemical analysis. Formaldehyde was determined by a colorimetric analysis using chromotropic acid (Thomas and Chamberlin, 1980). The absorption spectrum of the solution was obtained by means of a Hitachi 100-50 type double-beam spectrometer.
Results and Discussion Although methane has been conventionally assumed to be very inactive, the present results show that methane is reactive and can be converted to oxygen-containing C1-CI compounds. The conversion reaction took place at 100 "C and at atmospheric pressure, and the activation of methane is initiated by a radical reaction proceeding under the illumination of UV light. Methane and water vapor were simultaneously brought in the reaction chamber, and the products were trapped by the water-cooled condenser and the pentane slush trap. The effluent gas consisting of unreacted methane and untrapped products was again fed to the reaction chamber with the pump. The greater part of the products was found in the condensed solutions and gas phase. The analysis results are shown in Table I. In the presence of air at 1.1 vol %, the products are C1-C4 alcohols, ketones, acids, esters, aldehydes, hydrogen peroxide, carbon dioxide, and hydrogen gas. The major products derived from methane are methanol, ethanol,
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1389 30
lii: /
O@
-
, 20
I 40
I 60
I I 80
1
I 20
0
Air / volume %
I
I
I
40
60
80
Air / volume %
Figure 2. Products versus the volume percentage of air in the reaction chamber at 100 "C. n(CH4)/n(H20)= 0.07. Reaction time, 3 h. ( 0 )Acetic and peracetic acids, (0) methanol, (A)formaldehyde.
Figure 3. Products versus the volume percentage of air in the reaction chamber a t 100 OC. n(CH,)/n(HzO) = 0.14. Reaction time, 3 h. ( 0 )Acetic and peracetic acids, (0) methanol, (A)formaldehyde.
acetone, formic acid, acetic and peracetic acids, formaldehyde, and carbon dioxide. Hydrogen peroxide and a part of the hydrogen gas result from the photolysis of water vapor since the formation of these compounds was also observed in the photochemical reaction of pure water vapor. The quantities of the products depend considerably on the volume percentage of added air. Methanol, formic acid, acetic acid plus peracetic acid, formaldehyde, carbon dioxide, and hydrogen peroxide increase markedly with an increase of air, but other products generally decrease. Especially in the presence of a large quantity of air, C3 and C4 compounds become negligibly small. The conversion of methane increases steadily with increasing the volume of air. In Figure 2, the mixtures of methane, water vapor, and air were circulated a t the velocity of 1dm3 min-l, and the molar ratio of methane to water consumed [n(CH,)/n(H20)] was 0.07. The three products, which were most abundant, are displayed versus the volume percentage of added air. The addition of a small quantity of air leads to the conspicuous enhancement of the products, and in particular the effect is notable in acetic and peracetic acids. The products show a maximum amount at the volume percentage of air: 20% (methanol), 35% (formaldehyde), 35% (acetic and peracetic acids). The appearance of the maximum is obviously due to the decrease of the concentration of methane in the reaction chamber. A similar plot is shown in Figure 3 where n(CH4)/n(HzO)was equal to 0.14. When this result is compared to that in Figure 2, the amount of acetic and peracetic acids is comparable in the whole volume percentage of air, but both methanol and formaldehyde are smaller. The volume percentage of air leading to the maximum amount of the product is almost identical with that obtained in Figure 2. In Table 11,the amounts of methanol, formic acid, acetic and peracetic acids, and formaldehyde are shown where n(CH,)/n(H,O) was varied but the volume ratio of methane to air was always kept at 3:l. As seen from this table, the former ratio has an extreme influence on the formation of acetic and peracetic acids. At n(CH4)/n(H20)= 0.02, acetic and peracetic acids are slightly formed, but a t 0.14 their amounts are 26.6 mmol, which is largest among the products. The amount of carbon dioxide increases certainly as the ratio increases, but the formation of formaldehyde and methanol is rather favorable at lower values of n(CH,)/n(HzO). Thus, the photochemical conversion
Table 11. P r o d u c t s (mmol) Obtained in t h e Photolysis of Methane"
12.6 17.2 1.1 2.2 0.1 18.1 19.6 16.6 0.3 0.7 1.9 4.1
CH30H HCOOH CH3COOH + CH3COOOH HCHO CZH6 COZ convsn of CH,
5.4
9.9 2.1 26.6 12.3
8.9
13.6 2.8 23.3 10.9
11.7 2.3 21.5 10.9
3.7
3.7
4.8
8.4
8.3
7.8
"Reaction time, 3 h. Temperature in the reaction chamber, 100 "C. CHI, 75%; air, 25%.
of methane is dependent on the value of n(CH4)/n(H20) as well as on the volume ratio of methanol to air. As described previously (Ogura and Kataoka, 1988),the hydroxyl radical accomplishes a key role in the activation of methane. The generation of the hydroxyl radical is given by the following reaction (McNesby et al., 1962; Getoff and Schenck, 1968; Simons, 1984; Bersohn, 1984): HzO
hv
'H
+ *OH
(1)
The formation of methanol is attributed to the reaction of methane with the hydroxyl radical: CH4
+ 'OH
-+
CH30H
+ 'H
(2)
CH3 radicals are produced by the reactions of methane with OH radicals and H atoms (Wilson, 1972; Fristrom and Westerberg, 1965):
+ *OH CH4 + 'H
CH,
-
-
'CH3 + HzO 'CH3
+ Hz
(3) (4)
In the presence of low concentrations of air, the formation of ethane was largest among all the products except hydrogen gas, but the inhibition effect of air against the ethane formation was prominent. On the other hand, the formation of methanol and formaldehyde was markedly enhanced by the addition of air. These results indicate that the reaction of CH, radicals with methane leading to ethane is suppressed by oxygen, and instead the following formation reactions of methanol and formaldehyde are dominant:
1390 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988
'CH3
+ O2
-
-
CH302*
+ 'OH CH300H + 'CH3 CH30' + *OH CH30H + 'CH,
CH302* HCHO
-
+ CH4
CH302'
CH300H CH30'
+ CH4
-
(5) (6) (7) (8)
(9)
The photolysis of the mixtures of methane, water, and air gave no traces of nitrogen-containing species, and the nitrogen molecule is inert under these conditions. Photolysis of 02-H20 mixtures at 185 nm produces both ozone and perhydroxyl radicals as follows (DeMore and Tschuikow-Roux, 1974): 20
0 2
0 + O2
O3 + 'H 0 2
-
+ 'H
-
(10)
O3
(11)
*OH + O2
-
HO,'
(12)
(13)
As shown in Table I, the amount of hydrogen peroxide obtained increases with increasing the addition of air, while that of hydrogen gas diminishes on the contrary. It is therefore suggested that hydrogen molecules were consumed by the reaction with H 0 2 radicals to form hydrogen peroxide: HO2'
-
+ H2
H202
+ 'H
(14)
Porter and Noyes (1959) have studied the photochemical decomposition of methanol vapor at wavelengths below 200 nm, and the principal products are hydrogen, formaldehyde, and ethylene glycol. They suggest two primary processes occurring in the photolysis of methanol vapor: CH30H CH30H
-
HCHO 'CH,OH
+ H2
(15)
+ 'H
(16)
Ethylene glycol was not detected here, and the coupling reaction of two CH20H radicals is negligible. In the present case, CH20H radicals can be also produced by H abstruction by OH radicals. As shown in Table I, the yield of formaldehyde exceeds that of methanol in the system 39.4% CH4-60.6% air, which is probably due to the scavenging of CH20H radicals by O2 to form formaldehyde (Kurylo et al., 1986): 'CH20H
+0 2
-
HCHO
+ HO,'
(17)
Ethane is the main product in lower concentrations of air, and the ethanol formation may be ascribed to the oxidation of ethane:
-
2'CH3 C2H6
+0
C2H6
CH3CH2OH
(18) (19)
In fact, ethanol formation became less effective as the O2 supply increased as shown in Table I. The mechanism of the formation of acetic acid and peracetic acid is not obvious; however, acetaldehyde produced by the oxidation of ethanol may play an important role in it (McDowell and Thomas, 1950):
-
+ 0 CHSCHO + 'OH, CH3CHO + 0 2 CH,CO' + HO2' CH,CO' + 02 CH3CO3' CH3CO3' + CH4 CHBCOBH + 'CH, CH,CO,H + 'H CH3COOH + *OH CH3CH20H
+
-
(20) (21)
(22)
(23) (24)
Alternative pathways may involve the reduction reaction of peracid by formaldehyde (Imamura, 1968) and the photolysis of peracid: CH3CO3H + HCHO CH3COOH + HCOOH (25)
-
+ 'OH CH3COOH + 'CH,
CH3C03H-k CH3COO'
-
(26)
CH3COO' + CH, (27) The production of C3 and C4 compounds was favorable in lower concentrations of air. Isopropyl alcohol and acetone may be formed by the reaction of CH,CHOH radicals with methane and further abstraction of hydrogen atoms from isopropyl alcohol, respectively. These reactions are competitive with the oxidation of ethanol to acids described above, and such competition is obviously observed in Table I. It was therefore revealed that methane can be converted to alcohol, acid, and aldehyde by the photochemical reaction with water vapor, although the elucidation of the detailed mechanism must be expected in a further investigation. This process is completely different from the traditional one and has a possibility that methane is extensively used as a raw material for the chemical industry. Registry No. CHI, 74-82-8; HzO, 7732-18-5; CH,OH, 67-56-1; HCOZH, 64-18-6; HSCCOZH, 64-19-7; H,CCOzOH, 79-21-0; HCHO, 50-00-0; CZH,, 74-84-0; COZ, 124-38-9.
Literature Cited Bersohn, R. J . Phys. Chem. 1984,88, 5145. Chaktravarty, D.; Dranoff, J. S. AIChE J . 1984, 30, 986. DeMore, W. B , Tschuikow-Roux, E. J . Phys. Chem. 1974, 78, 1447. Dumas, J. B. Ann. Chim. Phys. 1840, 73, 94. Fristrom, R.; Westerberg, A. A. In Flame Structure; McGraw-Hill: New York, 1965. Getoff, N.; Schenck, G. 0. Photochem. Photobiol. 1968, 8, 167. Imamura, J. Yuki Gosei Kagaku Kyokai Shi 1968, 26, 661. Jones, G. W.; Allison, V. C. Ind. Eng. Chem. 1919, 11, 639. Kitajima, N.; Schwartz, J. J . Am. Chem. Soc. 1984, 106, 2220. Kurylo, M. J.; Ouellette, P. A.; Laufer, A. H. J . Phis. Chem. 1986, 90, 437. Liu, R. S.; Lunsford, J. H. J . Chem. SOC.,Chem. Commun. 1982, 78. Liu, H. F.; Liu, R. S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H. J . Am. Chem. SOC.1984, 106, 4117. McNesby, J. R.; Tanaka, I.; Okabe, H. J . Chem. Phys. 1962,36,605. Mains, G. J.; Newton, A. S. J . Am. Chem. SOC. 1961, 65, 212. McDowell, C. A.; Thomas, J. H. J . Chem. SOC.1950, 1462. Olah, G. A.; Gupta, B.; Farina, M.; Felberg, J. D.; Ip, W. M.; Husain, A.; Karpeles, R.; Lammertsma, K.; Melhotra, A. K.; Trivedi, N. J . J. Am. Chem. SOC.1985, 107, 7097. Ogura, K.; Takamagari, K. Nature (London) 1986, 319, 308. Ogura, K.; Kataoka, M. J . Mol. Catal. 1988, 43, 371. Porter, R. P.; Noyes, W. A. J . Am. Chem. SOC.1959, 81, 2307. Rebbert, R. E.; Ausloos, P. J . Am. Chem. SOC.1968, 90, 7370. Solymosi, F.; Tombicz, I.; Kutsin, G. J . Chem. Soc., Chem. Commun. 1985, 1455. Simons, J. P. J . Phys. Chem. 1984, 88, 1287. Thomas, L. C.; Chamberlin, G. J. In The Colorimetric Analytical Method; Tintometer Ltd.: Salisbury, 1980. Wilaon, W. E. J . Phys. Chem. Ref. Data 1972, 1 , 535.
Received for review June 19, 1987 Revised manuscript received January 4, 1988 Accepted March 23, 1988
