avior of romati rocar J
C. R. K I ~ N E YAND ELSIO DELBELI The Pennsylvania State University, State College, Pa.
T
M PTERIALB
RE pyrolytic behavior of aromatic hy(im:arboiis is of fun:lamental importance because most of the aromatic hydrocarbons of commerce are product,s of pyrogenic reactions such as the coking of coal. I n the by-product coke oven conditions are particularly severe because the vapors must travel for the most part through layers of incandescent c o l e and along o r e n \Tails a t temperatures of 800" t o 1000" C. or more. Although such conditions are generally co1iduciT.e t o the beat yields of t,he unsubstitut,ed aromatics reported upon in the present ~ o r l c ,it, is indeed surprising that any hydrocarbon can survive such treatment and it was for this reason an investigation of the behavior of aromatic hydrocarbons in the region of 800" to 1000" C. in contact with coke was unclert,alten. The high-temperature reactions of t,hese hydrocarbons are also of iniportance in searching for the origin of the nonvolatile pitch molecules which are found in pyrolytic tars such a' coal tar after condensation of the t a r vapors, as well ism of the growth of carbon cr omatic hydrocarbons. EsiensiT-e pyrolysis of benzene has been reported in the temperature range of 800" to 1000" C. (6):but little is lino~viiabout the behavior of the higher aromatics. The hydrocwbons sclect'ed for study are examples of the unsubstituted aromatics: benzene, naphthalene, anthracene, chryseiw, arid pyrenc. Uenzenc w:ts included for purposes of reference. Anthraceiie, chryene. and pgrene \!-ere chosen as esaniples of linear, angular, arid con lensed fused ring aroniatics. 1
HYDROCAEBOKS. The properties of the hydrocttiboris uqe,l ai c given in Table I. TABLE I. PROPERTIES OF HYUROCARBOAS 1Sydiocarbon Benzene Xaphthalene Antliracene
3Ielting Point, 5 79-80 116-1 16 263-259
C.
Reiiiaiks Thiophene-free Recrystallized froin alcolwl Faint yellow tinge Colorless. blue fluoresccncr
COS,I~. the rAte is very similar to that observed for benwne, but at !loo" and 1000" C. more carbon is formed from naphthalene than from benzene. dt 10-second contact time with coke packing the peicpiitage of the carbon of the naphthalene feed deposited as carbon is about 2, 43, and 76% at 800°, goo", and 1000" C., rcspectively. The heterogeneous nature of the reaction is shown by the different rates of carbon deposition at 1000" C. with varying nmounts of packing-Le., none, 50 grams, and 100 grams The solid products other than carbon obtained from naphthalene at 800' rvere of a light brown color. A benzene solution gave a \ ellom fluorescence when irradiated with ultraviolet light. On sublimation over 90% was found t o be unchanged naphthalene, and no higher condensation products or tarry mattrr were obw v e d . At 900" C. the results were similar but traces of tarry matter Fvere obtained. At 1000' C. 2,2'-hinaphthyl, perylene, a small amount of an unidentified dark red amorphous solid, and some tarry products were observed. No 1,l'- or 1,2'-binaphthyl was isolated at any of the reaction temperatures. The gaseous products, as from benzene, were almost entirely hydrogen and methane ( 1 4 ) with hut traces of acetylene. The conversion of anthracene to carbon proceeds 11ith far greater ease than with either benzene or naphthalene, as will be seen in Figure 5 . Even a t 800' e. nearly 70% of the carbon of the feed is deposited a t a contact time of 10 seconds with coke packing, compared with only a few per cent for benzene or naphthalene. At 900" C. nearly 80% and a t 1000" C. over 95% of the carbon is deposited. The reaction is also surface catalyzed, as shown by the curves obtained a t 1000" C. with 0, 50, and 100 grams of coke packing. Variable rates were also observed with silica brick, quartz, and mullite chips at 900" C. Quartz appeared to have the least effect. The carbon deposited on the walls of the reaction tube and the coke packing was shiny, hard, and gray in color, but in addition a
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Vol. 46, No. 3
100 K
e
90
u 80 0 + 0 W
+ i
70
5 w
0 0
3
60
50
L
s 40 E
?!
0 *0
c
e
30 20
c
t IO
a
0
IO Figure 6.
20
40
30
Contact Tlme (Seconds)
50
Rate of Conversion of Chrysene to Carbon
soft, sooty black form was deposited in the interstices of the packing which was not observed with the other hydrocarbons or at, least not in such quant,ity. These different forms of carbon may be related to those observed by Iley and Riley (9) in the range of 800" to 1300" C. The appearance of carbon in the iiiterstices of the coke packing and the high rate of deposit,ion imply that a part of the react,ion occurred in the vapor phase and not as a surface reaction. However, there appeared to be no tendency for the soft black carbon to be plated over with subsequent deposits of the hard shiny form, even though the spaces betmen the particles of packing were almost closed in some experiments. The solid products from anthracene, other than carbon, were light broivn in color and dissolved in benzene t o give a light brown solution with a green fluorescence in daylight similar to that of certain petroleums. By chromatographing on alumina the product ivas found to be largely unchanged anthracene with small quantities of a red-brown, high-mehing powder. Products ohtained at 900" and 1000" C. had the characteristic odor of naphthalene, but this substance could not be isolated or identified. Since chrysene has been shown to undergo hydrogenolysis when heated with hydrogen under pressure at, 460" C. to yield phenanthrene, napht,halene, benzene, and various homologs and hydrides of these substances (9),it, is assumed that traces of naphthalene may be formed by the hydrogenolysis of anthracene, since some hydrogen is ayailable under the conditions of the experiment. No bianthryl XTas isolated and if formed must have undergone further condensation to nonvolatile products; neither n-ere appreciable amounts of tar produced. The product gases as before were largely hydrogen and methane with traces of acetylene. Chrysene was condensed to carbon, as shown in Figure 6. Compared with anthracene chrysene is more stable, but it is less stable t,han naphthalene. At 10-second contact 13, 65, and 8870 of the carbon of the chrysene feed was deposited as carbon in the reaction tube at 800°, goo", and 1000" C., respectively. The carbon deposited in the tube was hard, shiny, and gray in color; frequently the coke packing was cemented together by the carbon deposited. I n the empty reaction tube, some soft black carbon m-as formed in addition to the hard gray carbon deposited on the nalh The solid products other than carbon were a light yellow color even a t 1000° C. Benzene solutions fluoresced under ultraviolet
light'. The product was largely unchanged chrysene, but a t 1000" C. a small amount of an unidentified colorless crystalline substance melting at 194" to 195" was obt,ained. This is the nieltiiig point of triphenylene, lvhich is isomeric with chrysene, but the unknown substance failed to form a picrate which is characteristic of triphenylene. I n addition, a small amount of a red-brown powder of high melting point was obtained, and traces of tar. The principal gases lvere hydrogen and methane with very small qumtities of acetylene. Pyrene produced carbon at rates very similar t,o those observed for chrysene (see Figure 7 ) . At' contacts of 10 seconds, 15, 57, and 90y0 of the carbon of the feed was deposited as carbon at 800°, Y O O O , and 1000" C., respectively. The carbon was hard, shiny, and gray in color. In the empty tube a t 1000" C., a small amount of soft, black carbon fell to the bottom of the tube, but most of the deposit' was the hard shiny carbon variety on the walls of the tube. As with the other hydrocarbons, the presence of coke packing increased the deposition of carbon. The solid reaction products condensed in the receiver were of a. brownish orange color. .4 benzene solution exposed to daylight was green by reflected light' and red by transmitted light. Large amounts of unchanged pgrene were separated from the product, by sublimation. The residue was an unidentified, high-melting, dark-red powder which was responsible for the fluorescenc'e. Segligible amounts of tar were produced. The gases formed w x e mainly hydrogen and methane, with but traces of acetylene. DISCUSSION
The pelcentages of carbon of the five unsubstituted aromatic hydrocarbons deposited at 1000" C. in contact with coke and at variouv contact times itre shown in Figure 8. At very short contact times. below about 3 second?, slightly less carbon is deposited from naphthalene than from any of the hydrocarbons ( 1 2 ) ,but at contact times greater than 3 seconds benzene is the least carbonized. Anthracene, chrysene, and pyrene are markedly less stable than benzene and naphthalene and form a separate group of interlacing curves a t the top of the figure. Extensive carbonization of the unsubstituted aromatic hydrocarbons occurs under conditions similar to those encountered in the coke oven ( 8 ) . In Table 111are given the percentages of car-
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0
-
OOOC., 5 0 g . o f Coke
-
I
I
Contact Time [Seconds) 30 Figure 7.
CARBON O F FEED1I)EPOPITED A S C.4RBON O N S E C O X D COSTACT WITH 50 G R A M S OF C O K E P A C K I N G
TABLE 111.
C.
800'
3 2
70 13 16
I
50
Rate of Conversion of Pyrene to Carbon
boil of the hydrocarbon feed deposit,ed as carbon at contact times of 10 seconds and in contact with coke. Probably less carbonization occurs in the coke oven than is indicated by the results given in Table 111, because of the presence of hydrogen, which has a retarding action on the deposit'ion of carbon, but increased yields of light oil have been reported by several authors Jvhen the crown temperature of coke oveiis is kept below 800" C. For example, n'armuzinski ( I S ) found that the optimum temperature was 720' to 760" C. and that above t,his teinperature excessive graphite formation r i t h loss of light oil occurred. Cellan-Jones ( 6 ) reported that cooling the crown to 700' to 800" C. with cooled debenzolixed coke oven gas increased benzene yields as much as 20%. KO doubt these results are due to decreased carbonization of the simple aromatic8 at, temperatures helow 800" C.
Hydiocarhon Benzene Naphthalene Anthracene Chrysene Pyrene
I
40
10-
formed. Even using hydrogen gas as the carrier gxa, which should enhance the formation of gaseous hydrocarbon products, no increase in t'he production of these hydrocarbons from benzene a t 1000" C. and a contact t'ime of 22 seconds with coke packing wai; observed. However, carbon production was suppressed, falling from 66 to 49%. .4lthough acetylene is extensively decomposed in this temperature range ( 7 ) , more than mere trares should escape complete destruction if the primary decomposition of the aromatic hydrocarbons proceeded in this direction; also products peculiar to the decomposition of acetylene such as ethylene should be observed. The most likely mechanism for the production of carbon from the unsubstituted aromatics seems to be the multiple condensation of the aromatic structures with the elimination of hydrogen forming fused polynuclear aromatic hydrocarbons of ever-increasing size (4). The early stages of such condensations are well known, particularly for benzene, which has been found to produce the folloxing condensation products.
--
Carbon Weight % 9000 C. 1 0 0 0 ~c. 20 60 43 76 80 B .5 65 88 57 90
Considerable speculbtion appears in the literature on the source of the nonvolatile pitch niolecules present in coal tars. Presumably this t3pe of product is formed either b) the subsequent condensation oi polymeiization of reactive molecules produced during the pyrolvsis of coal or by the entrainment on nonvolatile material as fog. Although tarry products have been reported in the pyrolysis of aromatlc hydrocarbons, very little was observed in the present work in contact nith coke at 800" to 1000" C. Consequently, the formation of pitch by the pyrolytic decomposition of the unqubRtitutedaromatm In the coke oven does not seem likely. The inechanlsm of the carbonization of the uneubstituted aromatic hydrocarbons does not appear to involve the decomposition of carbon-to-carbon bonds, for the reason that large amounts of gaseous hydrocarbons, particularly acetylene, are not
+ Hz
+Hz
+Hz
1,2-Diphenylbenzene has been reported by some investigators ( 1 ) and denied by others ( b ) , but all have isolated the 1,3- and
1,4- isomers. Possibly the 1,Z-isomer has not been isolated in all cases because it is converted immediately into triphenylene, but this is also denied by Bachmann and Clarke ( d ) ,who reported that only biphenyl and carbon were formed on pyrolysis of the 1,2isomer. The conversion of each of the five hydrocarbons to carbon is catalyzed by the presence of contact surfaces, especially coke or carbon. No doubt the hydrocarbons are adsorbed on such surfaces, and undergo dehydrogenation, forming biradicals or even more complex condensation products which are desorbed with in-
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Figure 8. Comparison of Rates of Deposition of Carbon from Five Hydrocarbons At 1000° C. in contact with 50 grams of coke
creasing difficulty as their molecular weight incremes. Vhcu the vapor pressure of t,he product beconles negligible at high temperatures, it is then classified as "carbon." The suppression of the formation of carbon by the presence of hydrogen as the carrier gas is readily explained by its competition for the radicals which othervise would combine and lead eventually to the formation of carbon. For example, when biphenyl is heated with hydrogen (no catalyst) benzene is formed (8).
T.4BLE
a
Iv.
.kX.4LYSIS O F C A R R O N S P R O D U C E D AT
Souroe Sephthalene Anthracene Average of two analyses,
c, s
w,%
96.89 97.92
0.57 0.70
1000"
c.B
~~
The gaseous products from all of the hydrocarbons and at all temperatures were essentially hydrogen and methane. The yield of both gases increased with temperature. The increase in yield of methane in the pyrolysis of benzene above 750" C. has been attributed by Bolton and coworkers (5) t o the carbon-to-carbon decomposition of the benzene molecule. This: however, does not appear as likely as that the methane is split out from the more or less randomly oriented condensed aromatic rings, characteristic of the first stages of "carbon" format,ion, as the rings tend to shdt into more nearly graphitic structures n.ith furt,her red' 11 angements and condensations. Consequently. it seems very likely that, during these changes hydrogen atoms might shift t o and accumulate on certain carbon atoms, which would then be eliminat,ed a3 methyl or methylene radicals which eveniually become methane. In fact it is difficult to explain the amall amount of hydrogen remaining in the 1000° C. carbons 9hon-n in Table IV on any other basis. These two carbons xere deposited on the 13-allsof the empty tube and were of the hard, shiny, gray variet,?. X-ray diffraction pat'terns showed that, in spite of their very low hydrogen content, their crystallinity was very poorly developed and their layer planes n-ere irregularly spaced. No doubt t,he balance of the carbons as analyzed was oxygen or other gases combined or adsorbed Then the eamples were n-ithdrawn from the reaction tube. 1
CONCLUSIONS
Extensive conversion oi thc unsubstituted aromatic hydrocarbons to carbon occurs under conditions existing in by-product, coke oven?. Benzene is the most resistant to carbonization of t,he hydrocarbons at most contact t,imes, followed in order by naphthalene, chrysene and pyrene, and anthracene. However, direct comparison of the relative thermal stability of these hydrocarbons as measured by the amount of carbon formed, is unsatisfactory because they appear not to be decomposed directly to carbon and hydrogen, but rather t o undergo stepwise condensation of the aromatic structures with elimination of hydrogen until a noiivolat~ilecarbon remains. As the carbon undergoes further illternal rearrangements small amounts of methane are split out, as :L characteristic product. As the temperature is raised, the greater the quantitp of methane split out. The almost complete absenoc of pitch molecules in the products from the pyrolysis of thew hydrocarbons suggests that the pitch present in high-temperaturc coal tar is derived from some other source. LITERATURE CITED ( I ) Andiianov, IC., Iivitner, F., a n d T i t o v a , V.,U i g . Chena. L m l . (L7.S.S.R.).4,161 (1937). ( 2 ) Bachmann, W. E., a n d Clarke, H. T., J . Am. Chem. Soc., 49, 2091 (1927). (3) Bolton,'K., Cullingmorth, J. E., Ghosh, B. P., a n d Cobb, J. IT., J . Chem. Soc., 1942, 252. (4) Brooks, B.T., IND. ESG. CHEM.,18, 521 (1920). (6) Cellan-Jones, G., Coke Smokeless-Fuel Age, 8 , 8 (1946). (6) Cobb, J. W., a n d Dufton, S. F., Gas World, 69, 127 (1918); Gas J., 143,482 (1918). ( 7 ) Eglob, G., "Reactions of P u r e Hydrocarbons," p. 405, Nen. York, Reinhold Publishing Corp., 1937. ( 8 ) Hofmann, F., a n d Lang, IC,Brmnstofl-Chem., 10, 203 (1929). (9) I l e s , R., a n d Riley, R. L., J . Chem. SOC.,1948, 1362. (10) Orlov, N. A , , a n d Lichachev, N. D., B e i . , 62B,719 (1929:. (11) Peytral, E., Bull. SOC. chim., (4) 29, 44 (1921). cheev, 31. D., J . A p p l . C h m . (L'.LS.E.), 12, 105 (1939). (13) Warmuzinski, J., Pfxeglad Gdmicag, 7, 413 (1951). ENG.CIIEM.,1 0 , (14) Whitalrer, A f . C., a n d Suydam, J. R . , J. IND. 431 (1918). (15) Zanetti, J. E., a n d EglofT, G., Ibid., 9, 350 (1917).
ACCEP.CED November 5 , 1953. RVCEIVED for review May 6 , 1958 Presented before the Division of Gas and Fuel Chemistry, A x E n I c A N CHEMICAL SOCIETY, State College, Pa., hIay 3 and 6, 1032.
