CATALYTIC REACTIOXS O F E T H Y L E S E BP HERBERT WARREN WhLKER
Objects of the Investigations: To study the products obtained when ethylene is passed over various solid catalysts at different temperatures with special reference to the liquid condensation products. To find out how each catalyst is affectingthe ethylene and to discover the catalyst which will yield a high percentage of polymerized ethylene in the form of a liquid oil. Introductory and Historical
,
The action of ethylene at high temperatures without the use of any specially prepared catalyst, save what catalytic effect the material of the reaction chamber may have contributed, has been studied by several investigators. Marchand’ who was the first to make a careful study obtained carbon and hydrogen from ethylene at a “white heat.” Magnus2 working at “red heat” obtained methane, hydrogen, carbon, and naphthalene. Berthelot3 in several investigations obtained acetylene, ethane, styrolene, and naphthalene. Sorton and Koyes4 passed a current of ethylene through a red-hot tube and examined the products. They succeeded in identifying benzene, naphthalene, anthracene (?), propylene, butylene, divinyl (butadiene), methane, ethane, and carbon. Only a trace of hydrocarbons of the acetylene series was reported. Day5 was the first of the workers on this subject t o get away from the use of such indefinite terms as “red heat’’ and “white heat” to denote the temperature at which he worked. His experiments were carried out at 400’. He obtained, if the gas was heated for sufficient time, marsh gas, ethane, hydrogen carbon, and liquid products. Lewes6 found the decomposition did not begin below 600’ and concluded that ethylene is primarily resolved into equal volumes of acetylene and methane as a result of 3CzH4 = zCzHz zCH4, and that the acetylene subsequently either polymerizes or is resolved into its elements according to the temperature. Bone and Coward’ starting with a known volume of ethylene determined the changes in the partial pressures of the gas. Ethylene, acetylene, ethane, methane, hydrogen, and aromatic hydrocarbons were present at definite
+
J. prakt. Chem., 26,478 (1842). *Pogg. Ann.pQ0, I (18j3). Compt. rend., 50, 8 o j (1860);62,94 (1866); Ann. Chim. Phys., (4) 16, 144 (1869) etc J. Am. Chem. Soc., 8 , 362 (1886). 5 J. Am. Chem. Sac., 8 . 66 (1886). 6Proe. Roy. Sac., 55, 90 (1894). 7 J. Chem. Soc., 93, 1216 (1908).
962
HERBERT WARRES WALKER
temperatures over certain time intervals. The primary effect of high temperatures (600°-9000)was to cause an elimination of hydrogen with a simultaneous loosening or dissolution of the bonds between the carbon atoms, These residues (a) form giving rise t o residues such as :CH2 and ICH. H,C:CH? and CHICH or (b) break down into carbon and hydrogen or (c) hydrogenate to methane in a n atmosphere rich in hydrogen. These possibilities may be realized simultaneously in proportions dependent on teyperature, pressure, and amount of hydrogen present, lingelder' discovered that a t temperatures below 500' ethylene was not decomposed appreciably under the conditions of the experiment, without a catalytic agent. Bradley and Parr? found that ethylene decomposed into a mixture of ethane and methane in the neighborhood of joo". Above 500' the ethane content gradually disappeared, with a resultant increase in methane. Hollings and Cobb3 observed that around 800" ethylene decomposed into methane and acetylene, while a t higher temperatures it went into methane and hydrogen. Concerning the action of catalytic agents on ethylene not so much data are available as for the pyrogenic decomposition. Sabatier and Senderens: in connection with their famous hydrogenation experiments, included an investigation of the decomposition of ethylene. With heating alone, ethylene was decomposed slowly in a tube above 3'5' giving carbon, methane, and free hydrogen. From our experience with the presence of air or oxygen in the ethylene we conclude that when Sabatier observed the apparent decomposition of ethylene at these low temperatures, some air for an actual combustion of the gas was probably present. With nickel, Sabatier and Senderens observed that if a current of the gas is passed over the reduced metal heated above 300' the nickel is seen to swell up into a voluminous black material which finally fills the tube and chokes it up; all the ethylene disappears and a gas is obtained containing ethane, methane, and hydrogen. I n contact with nickel the ethylene was decomposed into carbon and hydrogen, but the latter was taken up at once by a portion of the ethylene to form ethane which is more and more broken down to methane a t higher temperatures. With cobalt at 360' and even 42 jo the ethylene underwent slow carbonization without rapid swelling and much more ethylene survived. The cobalt acted less actively than nickel. Iron did not act till above 350° and gave 9 still slower decomposition, Platinum black and reduced copper did not have any pppreciable action on ethylene. Magnesium powder acting at 600' on methane, ethane, ethylene, and acetylene caused a 9 j per cent decomposition. Aluminum powder, near the fusion point of the metal, 660", caused a total decomposition, while platinum decomposed only 80 percent. Ethylene with anhydrous zinc chloride at, 2;s' and i o atmospheres pressure, gives a gas containing 36 percent ethylene, 3 percent hydrogen, and 6 1 per1
J. Phys. Chem., 21, 676 (1917).
* Chem. Met. Eng., 27,
737 (1922).
3
J. Gas Lighting, 126, 9;.
4
Sabatier-Reid: "Catalysis in Organic Chemistry" 326 ( 1 9 2 2 ) .
CATALYTIC REACTIONS O F ETHYLENE
963
cent saturated hydrocarbons and a complex liquid of which 85 percent is pentane and hexane without any methyl-cyclobutane. The remainder consists of numerous hydrocarbons including unsaturated hydrocarbons boiling above 145’ and naphthenes which are particularly abundant around z 5 0 ° . * Berthelot and Gaudechon2 submitted ethylene to the action of ultraviolet light produced by a mercury arc lamp. An oily liquid polymer formed, boiling a little above 100’. Yield--I1 percent. The residual gas was pure ethylene as analyzed by combustion. Engelder (loc. cit .) incidentally observed that ethylene is not decomposed by alumina but that titania catalyzes the decomposition to a small extent at 490°-i.e. 4 percent for a rate of flow of gas of one liter per hour. As has been stated, to find out how different catalysts split ethylene at different temperatures is one of the purposes of our study. The field for the most part is unploughed. Yo one could have foretold that zinc oxide would be the best catalyst to accomplish the synthesis of methyl acohol from carbon monoxide and hydrogen. Fischer and Tropsch3 had to discover empiricall$ with years of work that weakly alkalized iron and mixtures of iron and oxides of metals like chromium, zinc, beryllium, etc., as catalysts would serve in the synthesis of petroleum and solid paraffins from carbon monoxide and hydrogen. This apparent success of Fischer at synthesizing gasoline interests us because we hope to obtain a synthetic fuel by polymerizing ethylene. At present, however, Fischer cannot be counted on to let the outside world know in just what ratios he mixes his materials and how he prepares them t o get the yields he says he does at ordinary atmospheric pressure.
Theoretical The decomposition of ethylene by heat and catalytic substances may take place in the following ways: (a) by splitting off two hydrogen atoms yielding hydrogen and acetylene: CgHa = CzHz H?, (b) by the dissolution of the bonds between the carbon atoms to form :CH2 groups, or iCH groups, considering the intermediate product acetylene, (c) by the direct breaking down into carbon and hydrogen: C2Ha= Z C 2Hz. The acetylene formed as a product of the splitting off of two hydrogen atoms as in (a) may in turn, and probably does, decompose into carbon and hydrogen. Thus the carbon and hydrogen become the products of a consecutive reaction. I t will be necessary to point out that acetylene is as unstable as, or better, more unstable than ethylene under some given conditions. The polymerization of ethylene takes place according to the equation: xCzH4 = (C2H4)x-a reaction in which the bonds between the carbon atoms are opening up to allow linkage of two or more molecules of ethylene, but not
+
+
Sabatier-Reid: “Catalysis in Organic Chemistry” z I I (1922). *Compt. rend., 150. 1169(1910). Ber., 59,830 (1926).
964
H E R B E R T W A R R E N WALKER
broken so that the carbon atoms separate to bring about a decomposition of the ethylene. I t will be noted there is a marked change in volume during the process of polymerization, especially if the polymerized product be considered a liquid. Pressure on the reactor should have considerable influence on the quantity of ethylene polymer formed and should diminish the decomposition into C2H2 and H? (see page 993). K i t h the acetylene formation decreased to a negligible amount or entirely prevented, t,he carbon produced by its decomposition will be correspondingly eliminated. Elimination of carbon deposition is most essential for a continuous polymerization process because it renders the catalyst inert.
Experimental J l a t e r i a l s . The ethylene is commercial ethylene made from ethyl alcohol and is procured in tanks put up by The Y.P.Industrial Chemical Co. The catalytic substances are described in detail below. A p p a ra t us a n d Procedure. Fig. I shows the essential parts and arrangement of the apparatus. The general procedure was to pass a quantity of gas a t a measured rate of flow from a graduated j liter cylindrr through a purifying and drying chain into the heated tube over the catalyst, and to collect the gaseous products in the graduated cylinder R , connected beyond the furnace. 'Ihe confining liquid in the cylinder mas saturated sodium chloride solution which ran from an over-head beaker through a siphon tube t,o displace the ethylene and to maintain sufficient pressure to force the gas through the apparatus. The flow of gas was regulated by the stop-cock D. ?'he purifying chain consisted of a Friedrichs wash-bottle containing alkaline pyrogallol prepared as for regular oxygen absorption in gas analysis (see below), and a similar bottle filled with concentrated sodium hydroxide solution for carbon dioxide removal and to act as an indicator of the approximate rate of gas flow. The drying tubes coniprised one of anhydrous calcium chloride, beyond the bottle containing the sodium hydroxide solution, and three phosphorus pentoxide tubes. X Jena or Pyrex glass tube was used in the, furnace K in which to heat the catalyst. Where possible the catalytic material was measured out in a porcelain boat and the boat inserted in the tube. In some instances the material was placed directly in the tube before setting the tube in place in the heating unit. All heating was done in a multiple-unii electric resistance furnace. A side-arm tube L with an inner seal served, with a test tube J attached, to collect the liquid products. Beyond this stood a manometer b I for meaeuring decomposition and polymerization by presssure changes under what might be called static conditions. The gas-receiving bottle B contained as confining liquid saturated sodium chloride solution in which the gases are only slightly soluble. This bottle was provided with a manometer and ontlet tube for the confining liquid. In each run a rapid stream of ethylene, four or five litrrs per hour from a twelve-liter bottle, was started through the apparatus and at the same time the heating current turned on. The catalyst was heated gradually from room
965
CATALYTIC REACTIOSS OF ETHYLESE
temperature. By the time the heating unit reached a temperature of 100' all traces of residual air and gases had been swept out and ethylene alone remained in contact with the catalyst. Now the 3-liter cylinder was connected in place of the bottle and a quantity of ethylene a t a measured rate of flow passed through the apparatus. A . very uniform temperature could be maintained by regulating the rheostat. Temperatures were determined by a
P a
C
E
k
I 1 FIG.I
chromel-alumel thermocouple inserted in the furnace and protected by a PLrex tube. The millivolt readings on the Leeds and Korthrup potentiometer box So. 82184 gave an accuracy of plus or minus five degrees within the values given in Hoskins' catalogue table of milllvolts and temperature equivalents for chrome1 and alumel. The catalysts, in general, were prepared by calcining the nitrates deposited on pumice and subsequently reducing by hydrogen, if metals. If oxides, they were precipitated as the hydroxides in dilute solution with ammonium or sodium hydroxide. The gelatinous or flocculent precipitates were washed thoroughly by decantation until no test for electrolyte in the filtered superThe dry natant liquid was obtained, filtered and dried in a air oven at I 20'. solid was powered in an agate mortar to a zoo-mesh fineness.
Analysis of the Gas Mixtures The methods used in analyzing the gas mixtures are given below. The sample was withdraxn from the receiving bottle and measured in a Hernpel burette containing saturated sodium chloride solution as the confining liquid The various constituents contained in the mixtures were dptermined as follows in the order given:
966
HERBERT WARRES WALKER
I. Carbon Dioxide. This was absorbed in I : I KOH contained in a Hempel single pipette. The pipette contained rolls of iron wire gauze to give a greater absorption surface. 2. Ethylene and Acetylene. In the earlier part of the work Tucker and Moody's method' for determining ethylene in the presence of acetylene, by first passing the mixture into ammoniacal silver chloride solution to remove the acetylene, and then into dilute bromine water for absorption of ethylene, was used. We found that the quantity of ethylene absorbed by the ammoniacal silver chloride varied so with the time the mixture was shaken as to make the method uncertain and impractical. The modified method of Ross and TrumbulP for the analysis of mixtures' of ethylene and acetylene was tried. Consistent and accurate results were obtained with known mixtures such as could not be obtained by the foregoing method, and so we adopted this means of determining et'hylene and acetylene in the presence of one another. I n brief, the scheme is to absorb both ethylene and aectylene in fuming sulphuric acid and then to remove the SO3 fumes by contact in the KOH pipette. The ethylene and acethylene were determined separately as follows: Ethylene. The percentage of ethylene is taken as the difference between the total percentage absorbed in fuming sulphuric acid and the value obtained for the percentage of acetylene in the sample. Acetylene. A separate sample of about j o cc. of the gas is passed into a bulb over silver nitrate solution. After closing a stop-cock in the bulb stem the bulb is disconnected from the burette and shaken until absorption is complete or until there is no further rise of the solution into the bulb. The stop-cock is opened, bulb rinsed, and the acid liberated by the reaction:
C2Hz
+ BAgN03
CzAgz.AgN03
+ ZHXO~,
titrated in the flask supporting the bulb with standard alkaline solution. Standard alkali is added until the brown color of silver oxide appears. The excess silver in solution is precipitated with 5-10 cc. neutral 20 percent sodium chloride solution. The excess alkali is titrated wit'h standard acid, using methyl orange as indicator. I t can be calculated that I cc. 0.1 S S a O H solution is equivalent to 1 . 1 2 0 cc. acetylene at standard conditions of oo C. and 760 mm. pressure. The volume of gas taken as a sample is therefore reduced to the volume it would occupy under standard conditions and compared with the volume of acetylene shown to be present by the alkali required to neutralize the nitric aicd liberated. 3 . Oxygen. Alkaline pyrogallol prepared by the method of Anderson3 i. e. dissolving 1 5 g. of pyrogallol in I O O cc. of a solution of KOH of specific gravity 1.55, removed all the oxygen with one minute shaking or fourpassages of gas back and forth. 4. Carbon Monoxide. The reagent for this constituent was ammoniacal cuprous chloride. This was made by adding zsg. of Cu?,Clzto I O O CC. of strong J. Am. Chem. SOC.,23. 671 (1901).
* J. Am. Chem. SOC.,41, 3
1180 (1919'.
J. Ind. Eng. Chem., 6 . 989 (1914'.
CATALYTIC REACTIONS OF ETHYLENE
967
T\",OH to which had previously been added 20 g. of NH4C1. After three minutes shaking in a double pipette, the ammonia fumes given off by the reagent were removed by passing the gas into a pipette containing 5 percent HzSOa. 5. Hydrogen. This gas was determined by fractional combustion over CuO heated to z 5 0 - z 7 0 ° . The contraction in volume gives directly the percentage of hydrogen. Before admitting the gas the air contained in the CuO tube must be displaced with nitrogen and a measured volume of nitrogen must be passed through the tube at the end of the determination to sweep out all the hydrocarbon residue for the subsequent combustion analysis. 6. Methane and Ethane These gases were burned with a measured volume of oxygen in a Dennis combustion pipette. From the contraction in volume observed after the combustion and the carbon dioxide formed, the percentages of methane and ethane can be calculated. The formulas are: C Z H ~= (2/3) (zCO2 - T. C.), CH1 = COP - zCzH6,where T. C. represents the total contraction observed after combustion. Zanetti, Suylam, and Offner' have shown that ethylene acted on by heat at 750' yields a maximum quantity of butadiene to the extent of 0.0096 liters of butadiene per liter of ethylene or approximately I per cent. While this is very interesting theoretically and it is possible that butadiene may be formed at lower temperatures under the influence of certain catalysts the actual yield seemed too small for us to make the long analysis for butadiene for each run when our catalysts did not seem to approach the reaction necessary for giving butadiene as a product. Of course, we, like everybody else who appreciates the role butadiene may play in the synthesis of rubber, would delight in obtaining a 90- I 00 percent yield of butadiene instead of polymerized ethylene, these hydrocarbon gases mentioned above, and carbon. The Action of Heat, Thermal Decomposition of Ethylene The results of passing ethylene through a clean, empty Jena glass tube at different temperatures and at different rates of gas flow are shown in Table I. -1rapid stream of gas was started through the apparatus at room temperature to sweep out any residual air and the heating current turned on. At definite fixed temperatures the heating was kept constant, the rate of gas flow diminished to a slow constant speed, and a measured sample of the gas was run through and collected for volume measurement and analysis. Any condensation of liquid products was observed as well as any deposition of carbon on the walls of the tube where heated. Below 600' no deposition or polymerization occurred. Each sample of collected gas at temperatures below and including 600' showed no change in composition from the ethylene used initially. That is to say, under the conditions of the experiment, ethylene is stable toward the action of heat unless 1
J. Am.Chem. SOC.,44, 2036 (1923).
968
HERBERT WARREN WALKER
TABLE I Ethylene through an empty No. of run I Temp. of tube (deg. C.) 500 Duration of run, in min. 50 Cc. gas passed through I 480 Cc. gas envolved 1500 Cc. gas per min. 29.6 Percent CzH4before 98.5 Percent C2H4after 99.4 Percent decomposition 00.0 nil Condensation of oil
Carbon deposition
Analysis cf gas: Ethylene Acetylene Hydrogen Methane Ethane Total
Jena combustion tube. I1
I200
I11 600 65 I 180
5 50 IOj
IV
v
650
6jo 45 550 530
6; I100
I220
I200
11.4 98.5 98,;
18.2 98.5 99.0
00.0
00.0
nil
nil
nil
nil
nil
very small amt.
99 4
98.7
99,o __
83 5 3.9 3.'
67.5
4.2
I2 .o
~
__
.
99 4
__
__
98.7
99.0
970 17.0
97.8 83 j 14.6 small amt.
12.2
97.8 67.5 31 . o drops red amber oil small amt .
8.2
4.4
2.9
6.5
97.6
98.6
the temperature exceeds 600". This confirms what Lewes (loc. cit.) found and is of the order of magnitude of temperature required before ethylene begins to be decomposed as observed by Engelder (loc. cit.) and Bradley and Parr (loc cit.). Above 600°, about 6 1j", see Table 11, ethylene begins t o decompose and to polymerize. Xs would be expected, the slower the rate of gas flow or longer the contact in the heated chamber, the greater the extent of decompsition and the more incresed the polymerization. There is markcd increase in the acetylene yield and a corresponding decrease in the ethylene content of the gaseous products with a 1 2 cc. per minute rate over that for the 17 cc. per minute rate. With two-thirds the rate of flow, as it happened, there was over twice the decomposition. Bone and Coward (loc. cit.) have studied the thermal decomposition of ethylene a t higher temperatures, performing experiments as high as I 180'. Our interest for the time stops where the temperature range in which glass tube exerts an effect begins. lye can now tell whether it is the glass tube or thc catalytic material in question that is exerting an influence and we have the problem of finding what it is in the glass tube that is bringing about this deroniposition and, more particularly, the polymerization.
969
CATALYTIC REACTIONS O F E T H Y L E N E 'r.4BLE 11 Ethylene through empty Jena tube. Rate of gas flow-1.5 liters per hour Temperatiire gradually increased from room temperature.
Condensation of oil
Temperature in degrees C.
Deposition of carbon
nil nil nil nil nil Light brown oil
400
455 510
580 600
615
nil nil nil nil nil Small amount
Attached manometer, passed gas into tube under slight pressure, and closed off tubc. Temperature in degrees C.
Time interval
Time
Reading of lower Hg col., in cm.
Reading of higher Hg col., in cm.
Diff., press. above atm.
16.90 19.15
26.90
I O . 00
24.50
5.33 -4.6;
5 :20
61.5 615
5
: 3 2
12
min.
Pressure change during interval (I)
I n order to account for the gaseous products obtained, it may be said the ethylene must have been split between the carbon and hydrogen
atoms, thus yielding hydrogen and acetylene:
H H
-
- --'
--
1 -
- .- =
H-C:C - H
CzHz
+ HZ.
We shall s h o w a little later that acetylene is more unstahle toward heat than ethylene and since carbon is deposited on the heated tube wall, the acetylene, in part anyway, is in turn breaking down into carbon and hydrogen:
HH
+
= 2C H?. This progressive break-down shows a n increase in C j C volume of the products over the reactants, but ( 2 ) the ethylene, simultaneously with the decomposition into acetylene and hydrogen, is polymerizing. The unsatisfied bonds between the carbon atoms are opening up so that one molecule may unite with another until a large aggregate or polymer is formed which exists in the liquid state under ordinary conditions. H H !
I
xiH - C : C - H)= (CZH,),. When this polymerization reaction is taking place, undoubtedly some of the free hydrogen and : CH, and ' j C H groups, whose source is given below, link u p with the "nascent" ethylene and help build the liquid polymer. T h r formation of this liquid polymer from the gaseous rthylene is accompanied b). a
970
HERBERT WARREN WALKER
large decrease in volume and that the net result is a contraction in volume and not an increase as the formation of hydrogen from ethylene would in itself require. I n addition, (3) the ethylene is resolved into :CH* groups by a division H /H I I I
of the molecule between the carbon atoms: H - C : C - H +z :CH2, or the acetylene formed by
(I)
is splitting bewteen the carbon atoms to pro-
I
duce iCH groups: H
- C i,'C - H
= z ; C H I because methane is among the
products. It is known that methane is one of the products of the pyrogenic decomposition of acetylene. The methane results from the hydrogenation of these :CH* and iCH groups by the hydrogen previously liberated from the ethylene in the acetylene formation. This hydrogenation, whether i t 3Hz = zCH4 takes place according to: z : CH2 zH2 = zCH4or z iCH or both, produces a decrease in gas volume, which effect is added to that occurring with polymerization to the liquid. I t is plain to see that any ethane in the product is a result of the hydrogenation of some of the undecomposed ethylene. Table I1 shows what happens if a stream of ethylene at the rate of about 1.5 liters per hour is passed through the Jena tube and a t the same time the tube heated gradually from room temperature. That a temperature of 6 1 jo is required before the ethylene is decomposed or polymerized confirms the results in Table I. At this temperature a manometer was attached to the heating tube, some ethylene passed into the tube to produce a pressure of 10.0cm. above that of the atmosphere and the tube closed off a t the intake. Over an interval of 1 2 minutes there was a decrease of 4.65 cm. mercury caused by the polymerization and decomposition of the gas.
+
+
Thermal Decomposition of Acetylene In order to determine the relative stability of ethylene and acetylene toward heat under the conditions of our experiments and to find out at what temperature acetylene is breaking down into carbon, hydrogen, and i CH groups, we subjected acetylene to the heat treatment in the clean, empty Jena tube. In Table I11 are tabulated the measurements made with acetylene a t different temperatures and for different rates of gas flow through the tube. This study shows acetylene to be stable toward heat at 400' at as low a rate of flow as 13.8 cc. per minute. At 4 j o o with this same rate of flow acetylene polymerized, as indicated by the appearance of a brown fluorescent liquid condensing at the cooler end of the tube in minute drops. If at the same temperature the rate of gas flow were increased approximately twice or to z j cc. per minute no polymgrization occurred. At 4 j o o slight decomposition was taking place but the larger tendency mas for the acetylene to polymerize as work at higher temperatures shows better.
97 1
CATALYTIC REACTIONS O F ETHYLESE
TABLE 111 Acetylene through an empty Jena glass tube
I1
I11
IV
450
5 00
500
65 1630 1630
80
45
I55
IO00
I 500
2200
I180
450 70 970 920
I220
50
00
I80
00
5.2
00
675 325 32.5
1320
00
980 44,3
20
13.8 98.5 94.0 4.6
nil
min. amt.
Carbon deposition
nil
V.
I200
98.5 98.4 00
98.4 __
__ 98.4
12.0
25
12.5
98.5 96.3
98.5 93.8 4.8
33 98.5 93.8 4.8
nil
brown liq.
less than
nil
small amt.
small amt.
94.0
96.3
5 . 2
2.2
93.8 3.4 1.3 0.7 0.3 99.5
93.8 5'3
2.2
small amt.
Analysis of gas: Acetylene Ethylene Hydrogen Methane Ethane Total
V
VI
I
400 60
S o . of run
Temp. of tube (deg. C.) Duration of run, in min. Cc. gas passed through Cc. gas envolved Difference Percent contraction Cc. gas per min. Percent C2Hzbefore Percent C2H2 after Percent decomposition Condensation of oil
99 ' 2
98.5
Iv
__ __ 99.1
550
14.2
98.5 87.6 11.1
red ambei liq. small amt
87.6 7.8 1.9
trace trace 97.3
h t 500' and j j o " we get more marked polymerization and decomposition and these are decreased with increased rate of flow or with shorter contact of the gas with the walls of the tube-orthodox behavior. What is interesting, however, is the tendency acetylene has at those temperatures to polymerize rather than to decompose. The heat effect, combined with any catalytic effect of the glass tube,in opening up the bonds between carbon atoms to allowformation of larger molecules is greater than in splitting off hydrogen and yielding varbon. It is interesting also to analyze and consistently find ethylene among the gaseous products. The decomposition of acetylene up to 5 joo does not take place to a large extent. With a rate of gas flow of 14.2 cc. per minute at 550' 87.6 percent acetylene remained undecomposed. What eflects we do get are these. First, H H there is an elimination of the hydrogen atoms depositing carbon:--i-- - 1.- = 2C
+ H2.
cic
Secondly, to a far smaller extent a t this temperature, there is the division between the carbon atoms to give iCH groups, for ethylene is not acted upon at this temperature acd methane is among the products. The
H E R B E R T W A R R E S TV.4LKER
972
hydrogen which is set free does three things. It hydrogenates ( I ) the aeetylenc part way to ethylene, (Ross, Culbertson, and Parsons' prepared ethl-lene by 'the hydrogenation of acetylene), ( 2 ) the iCH groups to methane, and (3) some of the acetylene to ethane. I n comparing the behavior of acetylene with that of ethylene we observe that acetylene is much less stable toward heat than cthylene-the former begins to polymerize and decomposc at 450' while a temperature of 615' is required to polymerize and decompose ethylene. I t is to be noted that acetylene is much more inert toward decomposition 100' above the temperature at which it is first acted upon by heat than ethylene is 30Oabove the temperature at which thermal decomposition and polymerization begin. This merely shows it is easier to open up the bonds between the carbon atoms of acetylene than to split off and eliminate the hydrogen atoms.
TABLE I\Acetylene through an empty Jena glass tube. Tube heated rapidly from room temperature to j 4 5 O . Rapid stream of acetylene flowing through, 3 liters per hour. Manometer attached, passed acetylene into tube under slight pressure and closed off tube at intake. Temperature in degrees C.
Time
545
5 :04
545
5 :
I1
Time interval
Reading lower Hg col., in em.
Reading higher H g col. in cm.
Diff., or press above atm.
17.20
7 min.
29.90
26.60 17.65
-8.3j
Pressure change during interval
9,40
-17.75
Ran acetylene through tube heated to 545' for 20 hours at rate of one lit'er per hour, attached manometer, passed in gas under slight pressure and closed off tube at intake. Temperature in degrees C.
Time
Time interval
545 1 1 : 39 530 1 1 : 49 omi in. Pressure change during interval
Keading lower Hg col., in em.
r6,6j 18.65
Reading higher in cm.
Diff., or press. above atm.
27.10
10.45
H g col.,
25.05
6.40
-4.oj
K e tried the experiment of running the acetylene continuously through the tube heated to 540' at a rate of flow of one litre per hour. Carbon deposited on the inside walls of the tube, formed a thin layer of lamp black after about 2 0 hours. The pressure measurements in Table IT' show how markedly the catalytic action of the glass is cut down by this carbon deposition, thus indicating that contact catalysis plays a part in the polymerization and that J. Ind. Eng. Chem., 13, 7 1 5 ( 1 9 2 1 ) .
CATALYTIC REACTIONS OF ETHYLESE
973
activation of the acetylene is not due to heat alone. There was a reduction of I 7 . j 5 cm. of mercury in the pressure over a period of 7 minutes at j.+jo when the gas was first passd through the tube, but after 2 0 hours heating and passage, over an interval of I O minutes we got a diminution of only 4 . o j cni. mercury pressure. Part of this final reduction is due to a contraction of the gas with the 15 degree drop in temperature, but even so the value shows that the deposition of carbon is rendering the walls inert. During the 2 0 hours about 3 cc. of a red brown oil were collected. In addition there was the appearance of a heavy viscous oil and some small quantities of naphthalene were found. KOeffort to analyze this small yield was made, but some analyses of an oil obtained from a gaseous mixture of equal parts ethylene and acetylene passed through the heated tube throw light on the composition of this oil. Ethylene and Acetylene in Equal Parts through the Empty Jena Tube The mixture of gases was run to learn the mutual effect of the gases on the liquid product in the presence of one another. Temperature-600". Rate of flow of gases-1200 cc. per hour, A yellow-amher and a deep-red oily liquid condensed on the walls of the tube just beyond the heating until a I j cc. portion of the product was collected in something like 8 to I O hours and fractionally distilled under a reduced pressure of 2 j mm. mercury. The distillation was carried out in a Claissen flask with a side-arm filled with glass beads and heated in an oil bath. Reduced pressure was obtained by a large water-suction pump. To prevent frothing and bumping, nitrogen was allowed to escape through a capillary beneath the surface of the liquid being fractionated. The first fract,ion consisted of a clear light yellow oil less viscous than the initial product. Temperature-39'-40°. At j8" heavy vapors came over which solidified in the condenser, due possibly to polymerized styrene. Aftpr a second light yellow distillate a heavy viscous oil of deep-red color was left as residue. From this a third fraction was obtained, light yellow in color, before the residue became very viscous, dark colored, and small in amount. Analysis of Fraction I by organic combustion. Percent carbon-88.97 Percent hydrogen-9.08 Ratio carbon : hydrogen-9.8 : I . 2 ) Percent carbon-85.73 Percent hydrogen--8.5 I Ratio carbon : hydrogen-Io : I . In 2 ) some unburned vapors escaped because the copper oxide in the conibustion tube was not hot enough to burn the vapors carried over by the dry oxygen. Ratio of carbon to hydrogen in acetylene-Iz : I I1 1'1L 11 " ethylene-6 : I If two parts acetylene to one part ethylene are in the polymerized mixture the ratio is calculated t o be I O : I . This fraction is very likely polymerized ethylene and polymerized acetylene mixed together. I)
ji
974
HERBERT WARREX WALKER
Analysis of Fraction I11 by organic combustion, Percent carbon-92.08 Percent hydrogen-; .26 Ratio of carbon : hydrogen-12.7 : I . I n acetylene the ratio of carbon to hydrogen is 1 2 : I , hence this fraction might well be polymerized acetylene. The condensation products of acetylene and ethylene heated in a Jena glass tube at 600’ may then be said t o consist of polymerized acetylene and polymerized ethylene in varying proportions. What is in the Jena glass which is catalyzing the polymerization of ethylene? Can we prepare some single catalytic material in the laboratory that will accomplish the opening up of the bonds between the carbon atoms of ethylene at a lower temperature than that at which the empty tube acts’? The composition of Jena glass’ is as follows: SiOs - 6 4 . 5 8 Percent. S a & - j .38 Percent. A1203 - 6 . 2 8 R?O3 - 10.03 ZnO - 1 1 . 7 8 ‘I Fe20s - 0 . I O CaO - 0 . 0 8 &O - trace NgO - 0 . I 2 X n O - trace. The following substances as catalytic materials were then tried: I)
Silica Gel a s Catalyst Preparation: joo cc. 40 percent water glass or sodium silicate solution mere diluted to 1 2 0 0 cc. and heated to boiling. 1 7 5 cc. concentrated HCI were diluted to 300 cc. and added slowly to the hot silicate solution with stirring. The precipitated silica was allowed to settle, the supernatant liquid decanted, and a hot water portion added to wash out the S a C l and excess acid. This extraction was repeated until the filtered liquor gaveno turbidity withAgS03. Filtered and dried over night in an air oven at 100’. The dry silica was then pulverized in an agate mortar to pass a zoo-mesh sieve.
TABLE I’ Ethylene over silica gel Temperature equivalent in deg. C.
Time
Potentiometer reading in millivolts
9 : 40 : 47 : 56
1 2 . j
305
14.3
350
IO
: 04 : 08
16.8 20.7 22.0
410
Change in silica
nil
nil
Li
I:
It
I1
Ll
II
500
530 560
: 12 : 21
23.0 24.6
: 26
2j.j
615
: 33
26.0
625
1
Condensation of oil
590
Hodkin and Cousen: “Glass Technology”.
II
I(
II
II
I1
slightly grey more grey (carbon) carbon depositing
v. small amt. v. small (fog)
975
CATALYTIC REACTIOXS O F ETHYLENE
Approximately I g. of the silica mas placed in a Jena glass tube in the center of the heating unit, the tube connected with the gas source and a rapid stream 2f ethylene started through the tube. When all the free air had been swept out and replaced by the ethylene the rate of gas flow was reduced to one liter per hour and the heating current turned on. As the temperature increased gradually from the temperature of the room the observations tabulated above were made. The results are typical of many runs in Jena and Pyrex glass tubing. Using a Pyrex tube as a container for the silica similar results were obtained. They are herewith tabulated.
TABLE VI Time
Potentiometer reading in millivolts
5 :04
12.8
: 06 : 08
14.8
: I8 : 21
: 30 : 34 : 38
16.5 18.; 21.5
24.0 2j.0
26.3
Temperature equivalent in deg. C.
(londensation
Change
of oil
in
silica
nil
nil
3'0 360
li
il
ii
(1
N
il
li
I
