THERMAL REACTIONS OF
ETHYLENE ROBERT E. BURK, BRUCE 0. BALDWIN, AND C. H. WHITACRE Western Reserve University, Cleveland, Ohio
A
The thermal reactions of deoxygenated ethylene were studied at 625" C. and with various addition agents chosen to elucidate the mechanism. These results, together with the vast amount of prior work, may be represented by the dual mechanism :
.
LTHOUGH the field of polymerization has become of
great practical importance not only to the petroleum industry but t o various other industries, no really clear and acceptable explanation of the mechanism of such reactions has been presented. The noncatalytic polymerization of ethylene should constitute a particularly simple and fundamental contribution to the general understanding of polymerization. Yet prior researches on the subject have been in disagreement in important respects. Previous results and theories are summarized in Table I. Investigations in which catalysts were employed are omitted. The prior work is arranged to show the various concepts which have been put forward relative to the mechanism of ethylene pyrolysis. They are: 1. Mechanisms involving the primary decomposition to car-
bon.
2. Mechanisms involving the primary formation of acetylene. 3. Mechanisms providing for the direct polymerization of ethylene to butylene as the primary step.
4. Mechanisms in which butadiene is thought to play an important part in the reactions and in some cases thought to be a primary product. 5. Other investigations where important observations were made such as the prominent formation of propylene but which were not definite as to mechanism, or where the primary formation of cyclobutane was thought possible, or where a free radical mechanism was assumed.
Not only have various mechanisms been proposed for ethylene pyrolysis, but the experimental observations are not in agreement. One cause for the latter is no doubt that more than one primary reaction is occurring, different primary reactions being brought out under the varying conditions of the investigations. A second possible cause is the fact, recently emphasized by Storch, that traces of oxygen, difficult to remove by conventional means, catalyze the reactions. A third cause of discrepancies in experimental observations is faulty analytical procedure. This is improved each year.
Experimental Procedure A flow system was used consisting of a Pyrex glass reaction vessel 29 cm. long and 2.2 cm. i. d., which fitted snugly in the middle of an electrically heated copper casting, 2.6 cm. i. d., 9 cm. 0. d., and 61 cm. long. The reaction vessel contained a Pyrex glass thermocouple well at its axis which extended throughout its length. Capillary spaces extending for 15 cm. in the inlet and
+
CZHP CzHa + C4Ha
I or I1 may be emphasized by suitable choice of conditions. Gaseous paraffins forrried in small quantity are accounted $or by hydrogenation of the corresponding olefin or as thermal decomposition products. outlet ends of the furnace, and for 17.5 cm. beyond the outlet end, provided for rapid heating and cooling of the gases. The reaction time was about 12.5 seconds. The reaction vessel temperature was measured with a Leeds & Northrup Micromax recorder and with a platinum and platinum-rhodium thermocouple. It was held constant by means of a Leeds & Northrup Micromax controller. From 70 to 150 liters of anesthetic ethylene were passed during an experiment; it analyzed 99.7 per cent C2H4. One-third of the impurities boiled higher than ethylene. Traces of oxygen were removed by admixing about 1 per cent of carbon monoxide and passing the mixture over a commercial Hopcalite catalyst at 100-llOo C. The gases from the catalyst passed through an allglass line to the reaction vessel without coming into contact with stopcocks or joints, The absence of oxygen was demonstrated by the acetone fluorescence test of Damon (6),which shows the presence of approximately 0.01 per cent oxygen. Practically all of the carbon monoxide introduced was recovered, and increasing its roportion did not produce effects which were not substantial& duplicated by dilution with nitrogen, with minor exceptions which will be explained. The carbon monoxide thus did not participate prominently in the reaction beyond serving as an oxygen remover. The work was aimed rimarily a t establishing what reactions occurred. The extent otreaction was therefore limited, generally to about 2.5 per cent. Products heavier than ethylene were concentrated and collected as liquid currently in an automatically controlled Davis column, the top of which was held at the boiling point of ethylene. Products lighter than ethylene, to ether with most of the unchanged gas, were collected periodicafiy in a gas holder. Each fraction was then further fractionated quantitatively in an analytical column of the Podbielniak type. At least 10 cc. of liquid were used in each Podbielniak distillation. The permanent gases and Cz, CS, and C , fractions were separately nnalyzed in Orsat equipment. The reagents used in the Orsat equipment are shown in Table 11. 326
MARCH, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLEI.
Principal Products
PYROLYSIS OF
ETHYLENE
CzHz (1270) CZH8 CHI Hz tar- decreasing liquid, inlreasidg Cd4, d with increasing C$?:k,
CHI, CrHe, oil, some C
Theoretical H:, CZHZ,polymers
++ ++
CzHtn mainly, some Hz and paraffins Nonaromatic liquids in high yields, olefins, paraffins naphthenes HI Unsatd. aliphatic hydrobarbons and some isoparaffins, C, H I ; no aromatics, polymethylenes, or cyclohexanes CaHa, butylene, pentenes, higher olefins Butylene CsHa cyclobutane, hexenes, cyclopentane. no 'Hz or. satd. hydrocarbons Up t o 92.5(0/0, C?Hz, HZ CHI, CzHe, CaHe, butylene, butadiene, sLtd. hydrocarbons Aromatics, olefins, butadiene, paraffin gases, C Tar Hz, butadiene, olefins; no CzHz. (CzHaCiHs fraction of natural gas cracked)
570-580' C. Circulating apparatus: 700720°, 800°, 950°, 1120O C. static, porcelain tubes Jena glass, stream 600-650' C.
+
bzn2L2C HI [C? + CzHz Hz]. CzHo =CH radicals postuiated CeHz 6
1000-1410° C.; 0.1 to 1 atm.; 0.004 to 0.005 sec.
Mechanisms Involving Primary Polymerization 350-450' C glass tube High-frequ&cy discharge 0' t o -10' C. 450-600° C.. atm. and rehuced pressures, Pyrex tube, static and flow 350-500° C.. 2.5 t o 10 atm. pressure, copper bomb, static
+
1000 lb./sq. in. (70 kg./sq. cm.), 386-400° C.
HZ+ 2 C p2(ECH)
++HzHz + CZH4
i
-+ oil \2(=CHz) &*Ha + (CZH4)Z. Jena glass thought t o catalyze? [CZH4 + CZHZ Hzl ( 1 ) ; unimolecular reaction CZH4 + CZHZ HZ CZH4 + HZ CzHz; CzHz -+ polymers; polymers He + hydrogenated polymers t o Butylene Polymerization a t 350' C. 2c1Ha -+ C4Ha 2CzHa -.L C4Hs: bimolecular but not oonventional 2CzH4 2CzH4 + C4Hs< C4Hs + CaH16
+ + ++
..... .
377O, 393O C.; 142 cm., 1-3 hr.; static, Pyrex tube 141.5 om., 377O C., 1-3 hr., static, Pyrex tube
Approximately seaond order: 2CzH4 --c butylene 2CzHa + CaHs; propylene, not primary
Polymeriaation assumed t o be primary 3-mm. porcelain tube, 1100-1400° C., 0.001 sec., 50 mm. Mechanisms Involving Formation of Butadiene Direct formation of aromatics from ethylene Low red heat, hard glass tube Silica tube, 550-950° C.
Hz, CH4, CZHZ,butadiene, butylene, CsHs, naphthalene CIHS, CzHs, CHI, Hn, C4Ha (liquids); not more than traces of CzHz Hz, butadiene, CaHe, butylene,, acetylene (initial products by extrapolation)
Stream a t 600-850° C. or static at 400-700° C.; quartz tube Quartz tube, 650-900° C., 10-20 sec.
sure
+ +
2CzH4 + butadiene Hz, t o account for the butadiene; butadiene CzHi + liquids 2CzHa + butadiene Hz; butadiene a probable intermediate in formation of aromatics Same as following
+
+
__
+
CH=CHThese radicals then become
......
A review, no exptl. work
Pressure change only was obsvd.
+
2CzH4 + CaHs; C4Ha -+ CnHe Hz; C4He -+ liquids 2CaH4 + CpHe Hz; C4He +,liquids; CaHe a primary produat; reaction order empirically
