real world of
edited by W. C. FERNELIUS
Kent State University, Kent OH HAROLDWITTCOFF
Benzene-The Polymer Former Harold Wittcoff Koor Chemicals Ltd., Beer-Sheva, Israel. P.O.B. 60 About 75% of all the petrochemicals manufactured in the U S . are based on ethylene, propylene, and henzene. Previously the chemistry of ethylene and propylene have been disc~ssed?.~ Now let us look a t benzene's chemistry. But first, let us find out where it comes from. Ethylene came from ollly one source, the steam cracking of hydrocarbons, a process intended to vield olefins. Prowlene comes from two sources. It is a ~ o - ~ r b d uwhenever ct kihylene is produced by stkam cracking, hut it is also produced by catalytic cracking, a process intended to convert the large molecules found in petroleum to smaller ones useful for gasoline. Benzene's origin is more complicated. Sources of Benzene As Table 1 indicates. henzene's most important source is catalytic reforming, a pkxess in which alkanes, primarily the Cn-CI? found in the naphtha fraction of pe-- hvdrocarbons . troleum, are cyclized to cycloalkanes and then dehydrogenated to aromatics (see figure). If the feed contains cycloalkanes, then only dehydrogenation is necessary. An important reaction is the isomerization of substituted cycloalkanes, such as methvlcvclooentane. To accommodate both . . to cvclohexane. . t h k reaction and the dehydrogenation, a dual phase catalyit is used. 'l'his means that one ourtion of the catalvst brines about isomerization and the other part effects the-dehydrieeuation. If the catalvsts were used separatelv an eauilihrium ~ o u kmxjn l he reach& in the isomeruition siepand the yield would he low. When the two ratnlysts are used together the dehydrogenation of the isomerized material takes place as soon as it is formed, driving the equilibrium to the right. The major products produced are henzene, toluene, and the isomeric xylenes which in the refinery are referred to as BTX. Reference to Tahle 1indicates that a lame amount of henzene also comes from a process known as toluene hydrodealkylation. Catalvtic reforminr is also related tu this suurce since it produces the toluene. ~ydrodealkylation,which is a free iadical reaction, proceeds according to the following equation.
Table 1.
Sources of Benzene (1976)
Source
X of Total
Catalytic Reforming Toluene hydrodealkyiation Steam cracking (pyrolysis gasoline) Coal tar distillates Toluene disproportionation irnpons (net)
50.4 27.3 12.3 8.2 0.7 1.1 100%
~
Methyleydowntane
Cyelohexane
Benzene
,.
F
a
Toluene
Benzene
The reaction may he carried out a t about 600°C with a chromia-alumina catalyst or a t about 700"without a catalyst. What motivated the use of toluene as a source of benzene, oarticularlv since a carbon atom is wasted? The answer. of course, is economics. Traditionally there have been emharrassine excesses of toluene. Conversion of toluene to the more expensive benzene has heen economical and hydrodealkylntion has been the largest use for tuluene. Pure toluene, how. evrr, is now used ro increase the ortane numher of non-leaded gasoline. This was originally nrcomplished by the use of the H'I'X fraction which also iscalled catalytic reformate. But as 270
Journal of Chemical Education
Methyleydohexane
Toluene
Catalytic reforming: typical reactions.
-
.
the suonlv of naohtha becomes more critical., netroleum companies have seized on pure toluene as an octane improver. Pure toluene results whether we want it or not when BTX is distilled to get the more valuable henzene and xylenes. The use of pure toluene in gasoline has caused its price to rise and has made it less desirable as a source of benzeue.3 The third source of benzene. as reference to the firure indicates, is the steam cracking process which yields &fins. In this process a by-product results called pyrolysis gasoline. It is rich in aromatics and the production of 100 pounds of ethylene may also provide 15-29 pounds of henzene. If toluene becomes a less desirahle source of henzene because of its use in non-leaded gasoline, pyrolysis gasoline will become a more desirable source. Reference to Tahle 1 indicates that approximately 8%of benzene comes from the volatile material which is generated from the coking of coal. A very small source of benzene is a dis~roportionationreaction in which two moles of toluene are rearranged to provide one mole of henzene and one mole of mixed xylenes. .A
-
'Wittcoff, H., J. CHEM.EDUC..56,385 (1979). *Wittcoff,H., J. CHEM.E D U C . . ~ (1980). ~, %rice this artiile was submitted,the price of toluene has increased sufficiently to make it an uneconomieai source of benzene.
Table 2.
Uses for Benzene % of Total Benzene used
We Use Benzene to Make
Mired Xylenes
How much henzene do we use for chemicals? In 1977 we used 11.25 billion pounds which was slightly below the 13.4 billion pounds of propylene used. The Uses of Benzene
From benzene we make primarily intermediates for polymers and Table 2 shows its major uses. The largest use for benzene is to make ethylbenzene which in turn is dehydrogenated to styrene. CH,-CH,
CH=CH, I
-
I
Ethylbenzene
Styrene
Styrene is an important monomer for both homopolymers and copolymers of which the most important elastomer, styrenebutadiene copolymer, is an example. The second largest use of henzene is to make phenol via the cumene hydroperoxide process which in addition provides acetone as the following series of equations indicate.
Ethylbenzene (for styrene) Curnene (for phenol &acetone) Cyclahexane (for adipic acid & caprolactam for nylons) Aniline (for phenyl isocyanate for urethanes) Chlorabenzene ~ a l e i cAnhydride (for Alkyds and Polyesters) Detergent Alkylate Others
51 17 15 4 4 3 3 3 100%
-
pounds of adipic acid are manufactured yearly by this route. Caprolactam lor nylon 6 is manufactured yearly in the United m tradlStates to the excent of ahout 900 m ~ l l ~pounds.'l'he tional route to caprolactam starts with cyclohexauone, as the followingequations indicate and proceeds via oxime formation and ~ e c k m i n nrearrangement
H
I
H,C-C-CH,
Cyclohexanone
I
NOH
+ CB--CH=CH, Propylene
Cumene OOH
I
?f'
E e!. 7C-M 1C Cumene hydroperoxide
6
+ a-c-.Ha
Phenol
II
0 Acetone
This reaction too has been discussed in a previous a r t i ~ l eI.t~ was the first example of industrial chemistry which yields two products from one set of reactions. Cyclohexane
The third largest use for henzene is for the manufacture of cyclohexane by hydrogenation. Cyclohexane is the all-important raw material for adipic acid for n y l ~ n . ~ T h eare r e two common types of nylon, nylon 6,6 and nylon 6. Caprolactam or its hydrolysis product, 6-aminocaproic acid, is the monomer for nylon 6. The synthesis of adipic acid is cumbersome. Direct oxidation of cyclohexane to adipic acid does not seem to he feasible since low yields result. Accordingly, a two-step oxidation is used as shown in the following equations. CH
H,C'
Cyeloherane
This certainlv seems like straiehtfonvard chemistw. However. The hydrGylamine is formed by aprocess there is a which eives a large amount of ammonium sulfate. Similarly, the ~ e c k m a n nrearrangement is done aith sulfurir acid. The caprolactam is ohtnined as the sulfate which is converted to the free capn~lactamby treatment with ammonia. Thus, nltogether, 4.5 pounds ot ammmiurn sulfate are produced per nwmd ofcaorolactam.This means that the hv-vroduct credit - -~~~~~ .. from the ammonium sulfate, which is primarily a fertilizer inzredient. is a verv economic factor. The companv - im~ortant . . " manufacturing caprolactam by this classical route must also he in the fertilizer business. This unwanted by-product stimulated the search for other routes to caprolact&. There are a t least four additional routes for the synthesis of caprolactam. A process, developed in Japan, involves treatment of cyclohexane with nitrosyl chloride to obtain cyclohexanone oxime hydrochloride in one step.
.~
~
~~~a
OH
9 &'F
I AH
HO
HA-$
'&H
,CH, ,CHz CHa CH, Cydohexanone C~elohexanol +
Benzene
C H> Caprolactam
Cyelohexanone oxime
ha
Cyclohexsne
H ':, HS,
e
'CH,
XH,
CHZ Cydohexanone Oxime Hydrochloride
HNO,.
HWC-!CHI),-COOH Adipie Acid
This is one of the very few examples of a large scale oxidation which is not carried out completely with air. About 2 billion 3Wittcoff, H., J. CHEM.EDUC., 56,810 (1979). the source of hexamethylenediamine see Wittcoff, H., J. C H E MEDUC..56,654 (1979). Tor
This elegant reaction is initiated by light, and a major problem was the devising of a mercury photochemical lamp to provide the proper wavelengths in the proper concentration so that a reasonable yield of caproladam is ohtained per kilowatt hour of energy expended. Of all the other processes, one of the most interesting was devised by an Italian company, Snia Viscosa, and is based on toluene. Prior to the increase in the price of toluene this was believed to be the most economical process. Volume 58
Number 3 March 1981
271
I -
Toluene
Benzoic Acid
1
HC,
HG' CH, 'NH-H,SO,
HC,
,CH: CH, Caprolactam
I t generates about half as much ammonium sulfate as the traditional process. In a newer version of the process the caprolactam sulfate is converted to caprolactam not with ammonia, hut by hydrolysis with water. From the hydrolysis mixture the caprolactam can be extracted with an alkylphenol. lsocvanates Reference to Table 2 indicates that there are several lesser uses for henzene. One of the most interesting is the conversion of benzene to aniline. In the conventional process benzene is nitrated and the nitrohenzene is reduced. Aniline, for many years, has been an important constituent of dyes. Its major use now is as a raw material for phenyl isocyanate as shown in the following equation.
+
HCHO
Formaldehyde
-
NCO
NM)
@CH** Methylene p h e n ~ l diisoevanate (MDI)
Trimers and tetramers are produced also and the product of commerce is a mixture. Nitration. althoueh it is a time-honored operation. is also a difficult one. Mghr there he a better way ti, make aniline? An estimated 700 million noundsof aniline was consumed in 1977, and usage was growing. Recent patents5 tell us that we can make aniline by combining benzene and ammonia directly. This chemistry is indeed surprising. Temperatures necessary to carry out this reaction are in the range of 250-500°C, and pressures vary from 300-700 atm. The catalyst is apparently nickel and stoichiometric amounts of nickel oxide are included to react with the hydrogen that is released. Rare earth oxides such as zirconium oxide annear .. to facilitate the reaction. This chemistry has not heen used commerrially. It is included here for two reasons. First of all. it is rertainlv. a good example of . the imagination industrial chemists bring to bear. ~ l s ~al-i t lows us to point out. as we see below, that problems may have several solbtions. lnthe manufacture of phenyl isocyanite the nitration step is certainly cumbersome, Equally cumhersome, however, is the reaction of the amine with phosgene to provide the isocyanate. Both reactions are dangerous and the latter is ~-wasteful of chlorine. Arco decided to work on the phase of the problem that utilized phosgene. The result was some remarkable chemistw indeed. Thev found that a carhamate can rewlt from the interactiutt of nitrohenzene with C'O and ethyl alcuhol in the presence of Se or S catalvst at 100-200°C and 10-100 atm df pressure. The carhamate is reacted with formaldehyde and pyrolyzed to obtain the MDI as the following equations indicate.
272
~
~
NHCOOR
NHCOOR
This process for making isocyanates has far-reaching implications, for it may also he useful for the preparation of the largest volume isocyanate, toluene diisocyanate.
+?
(Mixture of 2.4 and 2.6 isomers.)
kc0
Maleic Anhydrlde Another use for benzene, as reference to Figure 2 indicates, is as a raw material for maleic anhydride.
Phenyl Isocyanate
This in turn is converted to methylene phenyl diisocyanate (MDI) with formaldehyde as the following equation indicates.
~
I
I
,CH, CH?
Cyclohexane Carboxylie Acid
Aniline
NHCOOR
NO I )
~
Journal of Chemical Education
0 ~ a l e i Anhydride c
This is a classical route for the production of maleic anhydride whose largest use is for the synthesis of unsaturated polyester resins. Obviously there is a problem with this synthesis, for two carbon atoms of benzene are wasted. Might there he a better route? In fact, 2-hutene can be oxidized to maleic anhydride. CH,-CH-CH-CH,
$Co
+ 30, %
/O
. + 3H20
CH-C
This reaction is analogous to the formation of acrylic acid from pr~pylene.~ The 2-butene oxidation does not proceed smoothly, perhaps because there are two sets of allylic hydrogens. The reaction is markedly improved when n-butane is used. Why does n-butane give better results than 2-butene? The answer is obscure. Here is an example of empiricism in industrial chemistry that is crying for theoretical interpretation. Summary To summarize, over half of the 11.25 billion pounds of benzene manufactured in 1977 was used to make ethylhenzene, the precursor for styrene. 83% of the henzene manufactured was used to make ethylbenzene, phenol, and cyclohexane. All of these materials are precursors of important polymers and copolymers. Products manufactured in lesser volumes, aniline and maleic anhydride, are also polymer precursors. The chemistry involved varies all the way from classical Friedel-Crafts reactions (ethylbenzene, cumene) to the theory-defying conversion of nitrobenzene to ethyl phenylcarhamate. SSquire,E.N., U. S. Patent 3,919,155,assignedtoE. I. DuPontde Co., Nov. 11,1975, Chern. Abstr. 84,43586 (1976);Del Pesco. T. W.. U. S. Patent 4.031.10fi. assiened to E. I. DuPont de & do., ~ u n 21,1971, e Chern.'~hsir.87,134468~(1977). Nemours &
emo ours
