HEINL-GERHARD FRANCK
/-
Phenanthrene,
THE CHALLENGE IN COAL T A R
the second largest constituent, is but one example
of how coal tar chemicals are going to waste -an
estimated 250,000
tons is readib recoverable
composition of coal tar, at least qualitatively, is Thestartling, . and provides a fertile field for the chemical industry to exercise its imagination and ingenuity. It contains an estimated 10,000 chemical compounds of which only about 400 have been identified. Thus, even though remarkable progress has been made in recent years, a large unexplored area remains which may contain the key to new markets. Another interesting fact applies to coal tar. Many of its components are biologically active and therefore may lead to new materials such as pesticides, preservatives, and bactericides which can he put into large scale production. O n a smaller production scale, hut no less important, are undiscovered drugs which may have long-sought-for specific activities. The components which have been identified comprise only about 55% of coal tar. In addition, the tar contains another 2% of high molecular weight, soot-like compounds that cannot be dissolved or distilled; thus, the composition of this fraction cannot he determined by chemical methods. Until recently, the 55% of identified components in coal tar was the only source of aromatic compounds, but even so, only two-naphthalene and anthraceneare used on a large scale as pure products for chemical processes. Almost all the naphthalene is recovered and processed further, hut anthracene is used only to a limited extent, although its use has increased recently. The number of other constituents used widely in their pure forms is small. The list includes only phenol and its homologs, pyridine and its homologs, and quinoline. Because they contain the small reactive building blocks from which the end products are synthesized, phenol and the pyridine bases are used for materials such as synthetic fiben, resins, pesticides, dyestuffs, and drugs. However, most of the compounds in coal tar have relatively large molecules without functional groups, and are 38
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
quite inert chemically. This is why only two constituents find wide application. Naphthalene is an exception-it can be oxidized in high yield to phthalic anhydride. The composition of coal tar varies greatly, depending on type of coal and the coking process and temperature. Furthermore, certain changes occur during distillation, and yields are influenced by the type of distillation process. About 11 compounds exist in quantities greater than 1% and, except for methylnaphthalenes, all are nonsubstituted, aromatic substances without functional groups. Gas chromatography is ideal for investigating coal tar and its fractions. The data contained in this report were obtained hy that method, and pertain to high temperature tar from Ruhr soft coal processed by continuous distillation. Phenanthmne
Phenanthrene is the second largest constituent in coal tar, but still it is not used to any appreciable extent, although its skeletal structure underlies numerous physiologically active hydroaromatic compounds which are widely distributed in nature-e.g., resin acids, morphine, sterols, bile acids, digitalis glycosides, sex hormones, and antirachitic vitamins. The amount potentially available to the Western World is impressive-nearly 250,000 tons annually. This assumes that about 10 million tons of crude tar are distilled and that only half of the phenanthrene is recovered. However, because it has a higher boiling point, it is more difficult to recover than naphthalene and therefore will always be more expensive. But if produced on the same scale, its price would decrease dubstantially. The literature on chemical reactions of phenanthrene
. -
is rather extensive and numerous uses for the reaction products are proposed, such as in resins, dyestuffs,drugs, and plasticizers. Also, the solid chlorination products are proposed as non-flammable electrical insulators and impregnants. Like naphthalene, phenanthrene can be oxidized to diphenic acid or its anhydride which, if cheap enough, could be used for making synthetic resins and plasticizers. However, no simple commercial route has been devised and the known process gives not only diphenic acid but also considerable amounts of other oxidation products which present purification problems. Because of this and poor yields, price of the acid prohibits its use for all but specialized purposes. However, much research is being done, and if one application gains industrial importance, then the resulting price reduction will open up other markets. Fluwanlhena
It is not widely recognized that fluoranthene is the third most abundant constituent in coal tar. If only half were recovered, about 165,OO tons would be available each year. It is frequently ignored in articles on organic chemistry or merely mentioned in marginal notes. Nevertheless, fluoranthene has an advantage over phenanthrene: like naphthalene, it can be readily obtained in high purity because its fraction contains no
significant amount of less soluble material or other chemicals which form mixed crystals. Fluoranthene is offered in a 97-98% pure technical grade and mass production could reduce its price considerably. Coal tar is now usually processed by continuous distillation. The fractions recovered, in the order of their boiling are: water, light oil, middle oil (carbolic oil), naphthalene oil, and pitch (distillation residue). Like the naphthalene fraction, other closely cut distillate fractions yielding concentrates of other main constituents can be separated in the primary distillation. However, because most coal tar distillate is used as creosote, a broad complicated mixture of compounds, this type of distillation is not profitable. Fluoranthene (lccurs in the high-boiling anthracene oil, and to a lesser extent in the pitch. Anthracene oil is distilled further, to yield a number of other fractions, the most important of which is pyrene. Boiling point of pyrene at 1 atm. is 393O C.; fluoranthene, 383.5'' C. Fluoranthene would cost less if the pyrene fraction could also be used. This co-product problem becomes more complex as the amount of the desired material in the tar becomes less. I n practice, only naphthalene and phenanthrene are free of this burden. The limited literature reports on halogenation, nitration, sulfonation, hydrogenation, oxidation, and condensation with phthalic anhydride and acid chlorides. Thus, even though the molecule is symmetrical, it is not chemically inert. Also fluoranthene is available in high purity. Then why has not an important use been found? Probably because it has not amacted the attention of chemists or because the ewe of recovery bas not been realized. The reaction behavior of fluoranthene suggests that it could be a starting point for synthesizing drugs and, particularly, dyestuffs. The oxidation-type reactions are not 89 promising because no novel multicarboxylic acids have been reported. V O L 5 5 NO. 5 M A Y 1 9 6 3
39
NCHa
Phenanthrene
0
Morphine
CH.
,
Testosterone
&-a
OH CHs
LAJ
HO
D :tv * i
II CH,OCCH&& OH
I
-
I
HO
H
Cortisone Cholic Acid
04
Thc b a m rmrchlrc of phmanthrme appem in many phynologically wfivc compowzdr
Ppne
Pyrene is used only on a modest scale, compared to the potential 100,000 tons a year which is available. Reactions such as halogenation, nitration, hydrogenation. oxidation, and sulfonation have been investigated, as well as a large number of condensation reactions which deserve special mention--e.g., condensationwith phthalic or acetic anhydride, benzoyl chloride, diazoacetic ester, dichlorodiphenylmethane, glycerin, cyanuric chloride, and formylmethylaniline. Like fluoranthene, pyrene is found in both the anthracene oil and pitch. However, recovery is more complicated because its fraction has some slightly soluble
components boiling close to pyrene, including 1,2- and 2,3-benzodiphenylene oxide. These are troublesome and expensive to remove. This explains why pyrene is offered in lea pure form ( 9 0 4 5 % ) than fluoranthene. So far pyrene is used mainly as a starting material for making dyestuffs and whether or not it will become important in other fields is difficult to predict. If fluoranthene is used successfully, then the market outlook
Pyrene
Franck is with the Gtseiischaft fiir Teerwcrwertungm.b.H., Duisburg-Mcidnich, West Germany AUTHOR Heinz-&hard
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
Q+
for pyrene would undoubtedly improve. At the present time, its price is too high. Even if recovered with fluoranthene, cost of pyrene would be reduced only slightly because its fraction is harder to upgrade than that of fluoranthene. However, pyrene gives some oxidation products such as 4,5-phenanthrenedicarboxylic and 7,4,5,8-naphthalenetetracarboxylic acid. These acids would be particularly promising if a simple oxidation process giving a high yield could be found.
The starting point for recovery is usually the distillate from manufacture of hard pitch, although pitch coke oil from the coking of hard pitch itself is also rich in chrysene. All of the compounds discussed up to this point should be usable on a large scale. Chrysene, however, has less chance, even though its occurrence in coal tar compares with that of fluorene and pyrene. Purification is more difficult and high cost is a serious handicap. Chrysene derivatives have been used to some extent in ultraviolet filters and sensitizers.
Fluorene a
Wash oil boils at 230' to 300' C. and fluorene, occurring in tar in about the same proportion as pyrene, has the highest boiling point (297.9' C.). Acenaphthene and dibenzofuran (diphenylene oxide) boil slightly below fluorene. Therefore, considering these three compounds together is convenient. However, fluorene and dibenzofuran must be carefully separated by distillation because they form a continuous series of solid solutions which, of course, cannot be separated by crystallization. Formation of mixed crystals is common among the constituents of coal tar. Fluorene has a reactive group. The methylene group between the two benzene rings has highly reactive hydrogen atoms. Numerous reactions have been reported and several uses have been suggested, such as for making cleaning and wetting agents, textile auxiliaries, pharmaceuticals, disinfectants, pesticides, dyestuffs, liquid scintillators, and thermoplastic resins. However, attempts to promote wide-scale development of fluorene as a raw material have been unfruitful. There may be an outlet in drug synthesis because substances obtained from fluorenone, a product readily prepared from fluorene, are being tested.
Carbazole
Until recently, carbazole, which is recovered as a co-product in anthracene purification, has sold in large quantities for making dyestuffs and pesticides. Also, a plastic, based on poly-N-vinyl-carbazole, has good dielectric properties, good chemical resistance, high softening point, and thermal stability. This product is used commercially in the electric industry under the trade names, Luvican and Polectron. Unfortunately, these markets have declined considerably, and some manufacturers of dyestuffs now specify that the product supplied must contain less than a
Carbazole certain per cent of carbazole. Therefore, prognosis of the market is difficult. There is reason for certain optimism because carbazole is a co-product of anthracene and it does have a number of significant uses. Nevertheless, sales can be restored only if new uses are developed. 1- and 2-Methylnaphthalene
Chrysene
Chrysene, the most abundant constituent of coal tar pitch, is also found in smaller amounts in anthracene oil. However, because of a high boiling point (440.7' C . at 1 atm.) and the extraordinarily high melting point (255' C . ) ,it is difficult to recover in its pure form.
These two monomethylnaphthalenes occur in substantial amounts, with the 2-derivative predominant. In high temperature tar, the ratio of naphthalene to the monomethylnaphthalenes is about 4 to 1 ; however, as coking temperature is lowered, this ratio shifts in favor of the methyl and dimethyl derivatives. The methylnaphthalene fraction, a constituent of the wash-oil, is distilled after the naphthalene fraction and before diphenyl and the dimethylnaphthalenes. Its recovery therefore dovetails nicely with that of the other three components.
1-Methylnaphthalene (Continued on next page) VOL. 5 5
NO. 5
MAY 1963
41
Because of their availability, methylnaphthalenes have become more important recently, even though the quantities sold are still limited. Many uses have been proposed for each isomer but they frequently overlap uses for naphthalene. However, naphthalene is cheaper and therefore it has an advantage-e.g., 1-naphthylacetic acid, used as a growth promoter, can be made from 1-methylnaphthalene by chlorination, reaction with KCN, and subsequent hydrolysis. I t can be made also by chloromethylating of naphthalene followed by the same reactions. Thus, the route of synthesis becomes a matter of cost. 1-Methylnaphthalene has an unusually low freezing point (- 30.6' C.) which in the technical grade is lowered further by the presence of isomeric 2-methylnaphthalene. Because of this property and its high solvent power, the alpha isomer is used as a solvent and heat transfer oil. It can also be used as a carrier in the dyeing of polyester fibers, as a cetane-number indicator, and for determining the theoretical number of trays in distillation columns. Other suggestions include an extraction agent for sulfur, a constituent of liquid dielectrics, and a starting material in the manufacture of plasticizers, pesticides, plastics, and textile auxiliaries. However, these uses have not been developed to any significant extent.
2-Methylnaphthalene
Acenaphthene
Crude tar contains about O.5yGacenaphthene but tar distillate contains 170, roughly the same as dibenzofuran. This is because acenaphthene is one of those few compounds which may be formed during distillation. The chemical reactions involved are not completely understood, but the most likely is dehydrogenation of 1,8-dimethylnaphthalene. However, this cannot be demonstrated quantitatively, and therefore it must be assumed that other compounds are converted. Like fluoranthene, acenaphthene is easily purified by crystallization and the technical grade usually is 9 7 4 8 % pure. Like pyrene, acenaphthene has been used as a starting material for dyestuffs, but on a scale which consumes only a fraction of that which could be produced. By means of catalytic gas-phase dehydrogenation, it can be converted to acenaphthylene which is easily polymerized with peroxide catalysts. Like copolymers containing acenaphthylene, the polyacenaphthylenes are noted chiefly for their good electrical properties and high melting points but have not yet attained practical importance. Catalytic gas-phase oxidation gives a high yield of naphthalic anhydride, used chiefly in the manufacture of dyestuffs. At a lower price, this product could probably be used for making synthetic resins. On the whole, the market outlook for acenaphthene is better than for its two co-products, dibenzofuran and fluorene. However, to reduce production costs, substantial uses for all three compounds must be found.
mcH3
Also numerous uses have been proposed for 2-methylnaphthalene, including production of dyestuffs, textile auxiliaries, growth inhibitors, detergents, emulsifiers, and wetting agents. The beta isomer is important because its 1,4-quinone is a starting product for making Vitamin K. Dibenzofuran (Diphenylene Oxide)
Dibenzofuran is the most abundant oxygen heterocyclic compound in coal tar. I n composition and occurrence its similarity to carbazole, the most important nitrogen heterocyclic, is striking. Dibenzofuran is the skeletal substance of morphine, and reactions such as halogenation, nitration, sulfonation, methylation, hydrogenation, and condensation are reported in the literature. Potential uses are also described, such as for making disinfectants, insecticides, preservatives for wood and other materials, textile auxiliaries, synthetic resins, high temperature lubricants, and additives for candle mixes. Dibenzofuran has been used to some extent as a dyestuff intermediate. Also, Benzofuran
0 42
because of its high thermal stability, it is suitable for heat transfer media, despite its high melting point (82' (2.). An interesting derivative is o,o '-bisphenol (2,2 'diliydroxy-diphenyl), obtained by fusion with caustic potash. This compound is used in making disinfectants and pesticides.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Indene
Indene, with its low boiling point, is the reverse of acenaphthene so far as recoverability is concerned. Further, its content in tar (nearly 1%) decreases rather than increases during distillation. In the presence of hydrogen it is easily converted into indan, a product of little value. The extent to which this occurs depends on the type of distillation-the longer the process and higher the temperature, the greater the decrease which can amount to more than 50%. Indene has a relatively high freezing point ( - 1.6' C.) and the pure form can be prepared by extreme cooling of the appropriate distillation fraction. .41though many uses have been proposed, pure indene is not isolated and used as such, even though it is the principal constituent of coumarone resins. More accurately these materials should be called indene resins. Diphenyl
The diphenyl-indole fraction occurs between the methyl and dimethyl naphthalene fractions, and recovery is conveniently combined with the methylnaphthalenes.
Diphenyl is well known as a constituent of heat transfer oils. .&I eutectic mixture with diphenyloxide is sold under the trade name Dowtherm A and Diphyl. Diphenyl is also used as a preservative--e.g., for impregnating citrus fruit wrappers. It is the basic substance of benzidine dyestuffs, although these compounds are normally produced from other starting materials. Compared to potential production, very little coal tar diphenyl is used commercially. It suffers a handicap because diphenyl is available from synthetic sourcesas a by-ppduct in the synthesis of phenol by the chlorination process and pyrolysis of benzene. Therefore the market is divided. Indole
Indole, the nitrogen analog of indene, is one of the most interesting constituents of coal tar. It cannot be separated from diphenyl by simple distillation because the two compounds have vapor pressure curves which lie close together; also they form an azeotropic mixture which has a boiling point a few degrees lower than the pure products at 1 atm. (boiling point of indole, 254.7"C.; diphenyl, 255.6'C.). Separationisachieved by taking advantage of the slight acidity of the imide group and isolating indole via its potassium compound by fusion with caustic potash. Diphenyl then can be recovered from the indole-free oil by fractionation and crystallization. Another method of separation is by adding a third component, diethylene glycol. Azeotropic mixtures of diphenyl- and indole-diethylene glycol have boiling points differing by 12' C. (230.4' and 242.6O C., respectively). The azeotropic effect is so pronounced that separation by distillation does not require particularly efficient columns.
Indole is used commercially in several fieldsof chemistry. In nature it occurs as a constituent of jasmine and orange blossom oils, and has long been used as a perfume fixative. Extracting of indole sufficiently pure to satisfy perfume manufacturers is a chemical achievement because even trace amounts of impurity falsify the aroma. Indole is a starting material for growth-promoting substances and for amino acids. 3-Indoleacetic acid (indole-3-acetic acid), one of the first growth-promoters used, is known commercially as heteroauxin. In one method of production, indole is condensed with formaldehyde and hydrochloric acid followed by condensation with potassium cyanide and hydrolysis of the resulting nitrile. Amino-3-indole-propionic acid, known as tryptophane, is an integral component of many types of protein and one of the vital amino acids. 3(Dimethylaminomethy1)indole (gramine), obtained by a Mannich reaction of indole with formaldehyde and dmethylamine, is an intermediate in the synthesis of tryptophane which is made by allowing gramine to react with acetaminomalonic ester and followed hy saponification. The quantity of indole used is still small, but improvement in the recovery process bas resulted in steadily increased sales. Even so, its potentialities have not been fully explored. Intermediates for dyestuffs and drugs can be made by reaction of the hydrogen atom attached to the nitrogen. Of course, the amount of indole in coal tar is rather small, and extraction-purification techniques are complicated. Therefore, in terms of price it can never compete with mass volume products. 2-Phenylnaphlkalene
Little can be said about this compound. Formerly it was considered a rare constituent, but actually it is rather abundant and can be recovered in substantial amounts. The fraction boils below fluoranthene; thus, recovery must be combined with that of fluoranthene and pyrene. 2-Phenylnaphthalene is produced only in laboratory quantities, but should demand develop, the methylphenanthrenes, which boil below phenylnaphthalene, would be more accessible. There are considerable quantities of these in tar. Iroquinoiina Quinaldine
Indole acetic acid H
Tryptophane
The problem of separating tar bases from their coproducts is even greater than for neutral hydrocarbons. Crude bases are extracted from distillation fractions by mineral acid from which they are subsequently liberated by adding caustic soda. However, the fractions contain other materials and further, formation of azeotropic mixtures with hydrocarbons causes the tar bases to be distributed over a broad distillation range. Invariably, the resulting product is a mixture of several compounds. Isoquinoline and quinaldine, the most important compounds occurring with quinoline, have boiling points above that of quinoline and occur in the distillation residue. Should a demand develop, considerable amounts can be recovered. Literature on these two compounds is extensive. Isoquinoline is used as an auxiliary solvent in dyeing VOL 5 5
NO. 5
M A Y 1963
43
I'I
Thianaphlhene Diknzothiophene (Diphenylene Sulfide)
Quinaldine
Isoquinoline
and as a starting product in preparing isoquinoline red and other cyclic dyestuffs, photographic sensitizers, drugs, pesticides, and vulcanization accelerators. Quinaldine can be used similarly, and the most important dyestuffs from this material include quinoline yellow, quinoline red, and ethyl red, as well as sensitizing dyes. Quinaldine can also be used as an inhibitor for metals and as a seed disinfectant. Acridine Phenanthridine 7,.&Benroquinoiine
These three compounds are the most important and easily recoverable tar bases in anthracene oils. None are used on a large scale and prices are still high. However, acridine, the best-known, occurring in the largest amount, is attracting interest. In fact, it is the base for numerous dyestuffs and drugs, but in practice dyestuffs are usually made from 1,3diaminobenzene and aliphatic or aromatic aldehydes. Even well-known acridine derivatives, such as the drugs Trypaflavin (3,6diamino-10-methylacridinium chloride) and Rivanol (2-ethoxy-6,9diaminoacridine)usually are not made from acridine. The extent to which 7,s-benzoquinoline occurs in coal tar has been realized only recently, and its only use is in gas chromatography-i.e., in separating m- and p-cresol. As with acridine, several drugs can be derived from phenanthridine. For example, quarternary salts of diaminophenanthridium series have good trypanocidal properties.
\ Acridine
/
:Ha
\
Thianaphthene and dibenzothiophene are the most abundant sulfur compounds found in coal tar. Thianaphthene is recovered with naphthalene, but separation is difficult because its boiling point is close to that of naphthalene. Separation is further complicated by formation of mixed crystals. However, separation of dibenzothiophene from phenathrene is not as difficult because difference in boiling points is greater. Nevertheless, because it distills just prior to phenanthrene its usage is limited by the restricted phenanthrene market. Even though both thianaphthene and dibenzothiophene can be recovered from coal tar in substantial amounts, the cost would still be high. Both have found few uses. Thianaphthenes has been used to some extent in the manufacture of drugs, and other suggested uses include production of thio-indigoid dyestuffs and herbicides. Diknzothiophene may possibly be a starting point for preservatives having a fungicidal and bactericidal effect. O t k r compound.
The dimethylnaphthalene fraction, which constitutes about 2% of the tar and lies between the diphenyl and acenaphthene fractions, contains nine identified components: 1,2- through 1,7-, and 2,3-, 2,6-, and 2,7dimethylnaphthalene. The 1,6- and 2,6- components can be recovered relatively easily, but the isolation of the other compounds is very costly and complicated. Anthracene oil has two interesting methyl homolog fractions: first, preceding the phenanthrene fraction is the methylfluorene fraction which accounts for 0.8% of the tar and includes 1-, 2-, 3-, 4-, and 9-methylfluorene; and second, following the carbazole fraction, the methylphenanthrene fraction which accounts for 1.8% and consists mainly of 1-, 2-, 3-, and 9 methylphenanthrene as well as 4,5-methylenephenanthrene. The benzofluorene fraction, being the highest boiling portion of the anthracene oil, also deserves mention. Distilling after the fluoranthene fraction, it accounts for 1.60/, of the tar and consists mainly of I ,Z-,2,3-, and 3,4benzofluorene, as well as 5,12-dihydrotetracene. Coal tar pitch fractions offer also a large reservoir of quadrinuclear and multinuclear aromatics. Although recoverable in large quantities, the processes involve difficulties which would necessitate high prices.
SOME IMPORTANT CONSTITUENTS OF COAL TAR
PW
c1
Cornparmi
Naphthalene Phmanthrene Fluoranthenc
pv==
Acriflavinehydrochloride 44
I N D U S T R I A L AND ENGINEERING CHEMISTRY
Fluorene Chrysme Anthracene Carbazole 2-Mahyinaphthalene Dibenzafuran I-Methylnaphthalent
Per
cot
cornpound
a":
10 5 3.3 2.1 2.0 2.0 1 .E 1 ,5 1.4 1.0
Diphenyl Indole 2-Phenylnaphthalenc Isaquioaline
0.4 0.2 0.3 0.2 0.2 0.6 0.2 0.2 0.3 0.3
Quipddinc
Acridine Phmanthridine 7,8-&nzoquinolinc Thianaphthmc Diphcnylene Sdfide
