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E. GIL-AV, J. SHABTAI, and F. STECKEL Daniel Sieff Research Institute, Weizmann Institute of Science, Rehovoth, Israel
Thermal Aromatization of 1,3=Butadiene When this reaction scheme i s applied to thermal aromatization of hydrocarbon oils, Q whole range of aromatics can derive from interaction of dienic breakdown products
HAGUE AND WHEELER aromatization scheme, based on Diels-Alder reactions between butadiene and lower olefins.
On the basis of intermediates found a t
T H E
550" C., the formation of aromatics from butadiene is assumed to proceed via dimerization to 4-vinylcyclohexene, followed by double bond and/or skeleton isomerization, and finally by a dehydrogenation step of cyclohexadienes to aromatics. I t has further been shown that a certain amount of C6 and C, aromatics are formed in addition to the CScompounds which constitute the bulk of the products at 550" C. Their formation is explained by the splitting off of one and two carbon atoms, respectively, subsequent to the dimerization of butadiene to 4-vinylcyclohexene.
R
has gained wide acceptance, but the individual steps involve considerable difficulty. Therefore a study of the thermal decomposition of pure butadiene was made, as a result of which a scheme cf aromatization is proposed, which differs from that of Hague and Wheeler.
O n the other hand, small amounts of Cg and C ~ aromatics O were found, their
Editor's Note: Detailed data for this article, including apparatus, procedure, graphs, tables, and full diaeussisn appear in the concurrent issue of the Journul of Chemicul and Engineering Dotu 5, 98 ( 1960).
The Reaction Products Were Worked Up According to This Scheme at 550" C. Butadiene-1,3
r
i Liquid
4
Low temperature fractionation and complementary Orsat gas analysis
4
H 2
J-
CHI CZHI CiHs CsHs C3& C4Ha C4Ha C4HlO
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Carbon
Repeated displacement chromatography over silica gel at Oo C.
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t
t
Aromatics
Cyclomono-olefin" and semicyclodienesb (mainly unconjugated)
1
4
1. Fractional distillation
1. Fractional distillation
Catalytical hydrogenation and dehydrogenation IR analysis 3. 4.
2.
2.
IR analysis
+
Cyclohexene Methylcyclohexenes Dimethylcyclohexenes 4-Vinylcyclohexene 4-Ethylidenecyclohexene 1-Vinylcyclohexene CSand CIOalkylcyclohexen
Contains also small quantities of hexenes and octenes.
Ca and C4 open-chain dienes.
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1
Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene styrene CQ and CIO alkylbenzenes
Includes both alkylidene- and alkenylcyclohexenes.
VOL. 52,
e
NO.
Contains some quantities of
1
e
JANUARY 1960
31
formation being ascribed to the interactions of butadiene with its highrr homologs, which \+'ere actuall) identified in the products. According to Hague and TVheeler, only ethylbenzene and styrene form from butadiene:
Results of the study reported here, however, shox that carbon skeleton komerization and carbon-carbon splitting also occur, forming benzene, toluene, and the xylenes. Therefore, contrary to the original assumption, the presence of olefins is not necessary to form these compounds from butadiene. O n the other hand, in formation of aromatics, the proposed reaction scheme does not involve dehydrogenation to convert a cyclohexene to the cyclohexadiene structure. Instead, the second double bond required for progressive evolution of a cyclohexene to the aromatic structure is already present in the molecule of intermediate compounds and needs only to he shifted into the ring by migration. Thus? thermal dehydrogenation of cyclohexenes to aromatics. a main difficulty of the Hague and \\'heeler hypothesis, is not involved. Preliminary results with 4-vinylcyclohexene subjected to the same conditions as butadiene are in accord with the reaction mechanism proposed. By generalizing these results and using literature data, it is believed that, in thermal aromatization of hydrocarbon oils under conditions similar to those used in these experimen is, butadiene and its higher homologs formed in the course of cracking should yield a whole range of aromatics by the same scheme. The extent of aromatization based on conjugated dienes is, of course, limited to the amounts of these compounds present in the breakdown products. Depending on the starting materials and conditions, other mechanisms also could operate.
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PARA COMPOUNDS
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The proposed scheme for formation of Cs aromatics from butadiene via 4-vinylcyclohexene differs from that of Hague and Wheeler Experimental
T h e apparatus consisted essentially of a reactor and packing, both of stainless steel. together with a system for drying and measuring the starting material, a combination of coolers, traps, stripping column. and a 250-liter gas holder for collection and preliminary separation of the products. Pure butadiene was treated at 5%' and 700" C., at 1.2- to 15.9-second contact time. Large amounts of intermediates were found in the product a t 550' C. The reaction mixtures were analyzed quantitatively by a combination of chromatography, fractional distillaticn, spectroscopy, and chemical analysis, as shown schematically (page 31). Thus. as many as 34 compounds or groups -~ of isomers were determined in the liquid formed a t 550' C. The reaction mix-
INDUSTRIAL AND ENGINEERING CHEMISTRY
tures were analyzed quantitatively. Steps in formation of aromatics were reconstructed on the bases of intermediate compounds and iariations with contact time of these compounds and the final products. T h e results were plotted as change in product concentration us. time. Thus. secondarv and Drimarv reactions could bc distin&hei by ex;rapolation to zero time. Acknowledgment
The authors are indebted to Shraga Pinchas, Weizmann Institute of Science, for carrying out the ultraviolet and infrared analyses, and to Chairn Greener and Ahuva Jakob for valuable laboratory assistance. R~~~~~~~ for review September 2 6 , 1958 ACCEPTEDJanuary 1, 1959
