T H E METHANE EQUILIBRIUM. I11 BY R. C. CANTELO
A. The Catalytic Decomposition of Ethylene The Materials used: Ethylene-The ethylene was prepared by the action of phosphoric acid upon absolute alcohol. The gas so prepared analyzed 96-98x C2H4. (2) Contact Materials: Of the various contact agents used, nickel reduced from the oxide was the only efficient one. I t alone will be considered here. The catalytic nickel was prepared according to the method described in the first paper of this series. (3) ThP Apparatus: This consisted of a “train” similar to that described in the previous paper. The reaction chamber was a Pyrex glass tube which was heated in a specially constructed gas oven. The temperature of the tube was taken by two thermometers placed in contact with the glass tube; and the temperature of the oven could be controlled easily to j degrees. (4) Experimental Method: In the first series of experiments the gas was passed but once over the catalyst; but in those experiments designed to test the stability of ethane a t 36ooC, the gaseous products were passed backwards and forwards over the catalyst a varying number of times a t the rate of one to two liters per hour. Hydrogen was used for the preliminary washing-out of the apparatus. The gaseous products were collected in every case over a I:I mixture of glycerol and water. ( 5 ) Method of Analysis: Carbon dioxide, unsaturated hydrocarbons, oxygen and carbon monoxide were determined by the usual absorption methods. Hydrogen was determined by combustion at 2 70-300°C with copper oxide in a small quartz tube, heated in a small gas oven the temperature of which could be controlled easily. To assist the rate of combustion of hydrogen, some metallic copper was always present a t the start of the combustion.’ The reduction in volume after combustion was equivalent to the C.C. of hydrogen in the gas. It was found, however, that when the percentage of hydrogen in the gas was small, there was some loss due to the combustion of some ethane. This was overcome by adding 1 5 to 2 0 C.C.of pure electrolytic hydrogen. Tests then showed the method to be satisfactory. The temperature for rapid and complete combustion of hydrogen alone varied slightly with the quartz tube used. For the tube finally used in this series, the average temperature was 280OC. (I)
Pease and Taylor: J. Am. Chem. Soc., 34, 2 1 7 9 ( 1 9 2 1 )
247
THE METHAKE EQUILIBRIUhl
Methane and ethane were determined in the residue from the combustion of the hydrogen. The quartz tube was heated directly by a free flame some 3 inches wide, and the temperature was that of a bright red heat. From the equations: COS (g) 2 HzO (1) 4 CU 4 CUO (s) CH4 (g) x C.C. x C.C. 2 COZk) 3 Ha0 (1) 7 CU (s) 7 CuO (SI C2H6 (g) y C.C. 2 y C.C. it follows that the increase in volume after combustion equals y C.C. = C.C. of C& and again x 2 y = C.C. of COafrom which x is easily determined. (6) Experimental Results: Experiments were carried out a t temperatures from I j o to 3 50°C. The results given in Table I are those for 300 to 350°C only. The analyses have been calculated to nitrogen free.
+
+ +
+
+
+
+
TABLE I Catalyst- -reduced nickel oxide Temperature
300 325
325 325 350 350
Analyses of products CO CH, C2Hs
COz
CIH,
0.6
1.6
0.8 0.4 0.4
18.4 5.9 6.0
0.4 0.4
0.0
0.9
0.0
0.9 1.3
0.9
0.0
1.7
0.0
02 0.0
0.0
1.3 0.8
0.8
32.9 33.1 30.j
30.5 42.3 40.2
Hz
16.6
47.0 29.9 40.4 40.4 34.1
21.6 21.4 21.4
36.7
20.5
17.0
From Table I, it is evident that above 3oo0C, methane is formed in addition to ethane by the catalytic decomposition of ethylene. Ethane, however, appears to be stable even up to 35oOC; and this is in direct, contradiction mith Sabatier's statement that with a sufficiently long tube or a sufficiently slow rate ethylene might be decomposed completely into met,hane, hydrogen and carbon.' It was necessary t o test the validit,y of this view as it conflicts with the theoretical conclusions drawn by the writer. The following experiments establish the stability of ethane u p to 36ooC in the presence of nickel catalyst. Ethylene analyzing 9jYc CzHl was passed back and forth from one gasometer t o another over a nickel catalyst heated to 36ooC. The data obtained were as follows:Volume of C2H4 used-2300 C.C. Rate-about one liter per hour. Total time gas circulated-j hrs. 15 min. Volume of resulting gas-1800 C.C. Sabatier: Compt. rend., 124, 616, I3:9 (1897); 131, 267 (1900).
248
R. C . CANTELO
The experiment was continued on the following day. Final volume of resulting gas-1800 C.C. Total additional time gas circulated-3 hrs. I j min. Total time-8.5 hrs. Analysis of final product (Calculated to nitrogen-free) CO? C2H4 01 co CHI C?HB H P 0.0 0.0 0.0 0.7 68.2 2j.2 5.6 It would be useless, therefore, to attempt to determine the equilibrium constant for the methane, carbon, hydrogen system at or below 360°C, when ethylene is used as the initial system.
B. Methane Equilibrium with Ethylene as the Initial System The apparatus used was the same as that described in the first paper of the series. The ethylene was prepared in the usual way from absolute alcohol by the dehydrating action of phosphoric acid. The experimental method, however, was varied slightly, in that at the end of an experiment, before t,he next one was started one-half of the nickel catalyst was removed and replaced by freshly prepared material. It was hoped that in this way, the life of the catalyst would be lengthened. The method of analysis was varied slightly. After a preliminary analysis to determine the absence of et,hane, the analysis was repeated using hhe direct combustion method for the determination of methane and hydrogen. Preliminary experiments showed that the successive analyses of the product differed after each passage over the catalyst until a point was reached where the analyses remained constant, i. e. until the equilibrium point was reached. This was found to correspond fairly well with the point where further passages produced no further increase in volume. Accordingly the gaseous products were circulated until the change in volume became zero, and were then circulated for two to three hours in addition. It seemed probable that continued circulation of the gas would lead to lower values for Kp than those calculated upon the basis of amorphous carbon alone. It is inevitable that some graphitization of the carbon will occur when it is subjected to a high temperature over a long period of time. In the actual equilibrium experiments, every attempt was made to reproduce as nearly as possible the same system in the duplicate experiments. The experimental results follow in Table 11. Table I11 is a summary, in which the writer’s values (C) are compared with the average values of Mayer and Altmayer‘ (M & A). The initial system is specified in each case. To complete the table, calculated values are given (Saunders’ Equat.ion for amorphous carbon),Z as are also the determined values of Coward and Wilson3 (C & W) for temperatures above 8oo0C.
’ Ber., 40, 2134 (1907). J. Phys. Chem., 28, 1151 (1924). J. Chem. Soc., 115, 1380 (1919)
THE METHASE EQUILIBRIUM
2
49
TABLE I1 Temperature “C. CO?
02
Analyses
co
CHI
38.5 34.6 57.7
1.1
1.3
72.2
0.43
77.1 81. j 82.7
0.34 0.19 0.18
0.67 0.67 0.35
jI0
0.0
0.0
2.6
510
I .o
0.0
2.1
565 615 61j 670 670
0.9
0.0
0.0
0.0
1.8 1.5
jo.4 jj.8 36.1 22.6
0.0
0.0
0.0
20.3
0.0
0.0
1.2
12
0.0
0.0
0.9
12.3
.6
TABLE I11 Temperature “C.
KP:
Calc d.
510
536 536 565 565 567 567 577
607 61j 615
625 670 670 670 670 770
850
C
PCH, F, P?H* 3.4 4.6
(Calc.)
3.9 3.9
0.35
c 8- w
3.6
508 j06 510
hI & A Average
Hz
1.8
3.9 3.9
3.4 4.6 2.8 1 . j
1.3
1.1
1.3
0.95 I .9
1.3 0.78 0.57
0.67 0.67
0.43 0.34 0.34 0.43(?)
0.35 0.35 0.35 0.35
0.I 8
0 .I2
0.I 2
0.SI(?)
0.19
0.05
0.025
1000
0.02
0.011
1000
0.02
0.OIj
IO00
0.02
0.011
I100
0.01
I100
0.01
0.006 0,006
Table I11 shows that the results of Mayer and Altmayer are of the same order as those determined by the writer. In addition it shows that the determined results are as a rule lower than the calculated. The results obtained with ethylene as the initial system probably represent the equilibrium between amorphous carbon, methane and hydrogen more
250
R. C. CANTELO
closely than any of the other values. These results are strictly comparative. I n each of these experiments fresh catalyst was used; and the carbon was the amorphous material precipitated by the thermal decomposition of ethylene. It must be admitted, however, that some graphitization of this material would take place over the long period during which the gases were circulated.
Summary The catalytic decomposition of ethylene was studied and it was found that the special nickel-nickel oxide catalyst was most effective in bringing about this decomposition. At 3ooOC and higher temperatures, methane is found in the gaseous decomposition products, the percentage of methane following one passage of the gas over the catalyst, increasing with the temperature. Experiments in which the gaseous products were passed repeatedly over the catalyst proved that ethane was stable in the presence of a nickel catalyst up t o 360' C. Experiments in which the gaseous products of the catalytic decomposition of ethylene (at 5ooOC and above) were circulated repeatedly over a nickelnickel oxide catalyst showed that the methane equilibrium should be attained in this manner. Equilibrium constants for the methane equilibrium at 500 to 770°C were obtained by repeatedly circulating the products of the catalytic decomposition of ethylene over the nickel catalyst. These constants are in general agreement with those obtained by other investigators. Department o j Chemistry, University o j Cinnnnati, Cincinnati. Ohzo.
