T H E POLYMERIC CHARACTER OF BITUMINOUS COAL’ H. C. HOWARD Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pennsylvania Received J u n e l l 1 1 9 d 6
The bituminous coals occupy an intermediate position in what has been called (15) the “coal band” (figure 1). Such organic substances as lignin and humic acids lie a t one end of this band, and anthracite coaI and graphite at the other. Whether lignin or cellulose is looked upon as the essential progenitor of bituminous coals, it appears that in the coalification process the simple linear polymeric structure, which is generally accepted for cellulose and which has been proposed (11) for lignin, is modified in the sense of the formation of a tridimensional polymer by linkages between the linear units. Whatever the mechanism, there is no doubt that the process is characterized by increasing enrichment in ring structures (6). The higher the rank of the coal, the more complete is the condensation to such structures, until graphite, the limiting member of the series, is reached. The establishment of a building unit in such a polymer obviously presents great difficulties, and we can perhaps never obtain as satisfactory a picture for the structure of such a substance as we have for cellulose and the poly‘ meric esters, lactones, and anhydrides. The essentially “chemical” character of polymerization has been emphasized in recent years (7), and in those cases where the energetics of the process have been investigated, the reactions have been shown t o be exothermic; hence, one would expect elevated temperatures to displace the equilibrium toward depolymerization. Many such cases have long been known among the addition polymers such as styrene and rubber, and more recently the ready thermal reversibility of such purely condensation types as the polymeric laetones and anhydrides has also been pointed out (8). A typical bituminous coal from the Pittsburgh seam contains carbon, hydrogen, and oxygen in approximately the same ratio as coumarone, CSHGO, which has a normal boiling point of 170°C.; the coal contains somewhat less oxygen and more hydrogen and such polar groups as hydroxyl and carboxyl are present, if a t all, in very small amounts. Thus on the basis of composition alone, one would expect bituminous coal to distill completely a t moderate temperatures. Its low volatility is ob1 Presented before the Thirteenth Colloid Symposium, held a t St. Louis, Missouri, June 11-13, 1836. 1103
1104
H. C. HOWARD
viously due to high molecular weight, but thermal decomposition would be expected to give some clue to the nature of the building units as it has in the case of other high molecular weight substances. That the ordinary distillation processes do not, appears to be due to the fact that in coal the primary unit itself has a very low vapor pressure. Hence in pyrolysis the rate of decomposition of this primary unit into secondary products, some of greater molecular weight (coke) and some of less (gaseous and liquid hydrocarbons and low-boiling phenolic bodies), is faster than its rate of
100
0
LINEOF
Z E R O CARBoN
FIQ.1. The “coal band” and some typical carbon, hydrogen, a n d oxygen compounds evaporation. Certain methods of degradation, some purely thermal in character and others combining thermal and chemical effects, which yield significant information as to the nature of the building units in bituminous coal, have, however, been developed. These are (1) pyrolysis in the molecular still, (2) thermal decomposition in solvents a t elevated temperatures,, (3) hydrogenation, and (4) mild oxidation. PYROLYSIS IN T H E MOLECULAR STILL
When a bituminous coal is heated to 500-550°C. at pressures of 1 micron or less in a molecular still, where the distance between evaporating and
1105
POLYMERIC CHARACTER OF BITUMINOUB COAL
condensing surfaces is very much less than the mean free path of the evaporating molecules, and where the possibility of collision with surfaces hotter than the evaporating surface is eliminated, secondary thermal decomposition is largely avoided (13). The condensate obtained under these conditions contains, along with the usual liquid hydrocarbons and phenolic substances, appreciable amounts of brown, amorphous solids, neutral in character and readily soluble in such polar aromatic solvents as phenol, but almost completely insoluble in petroleum ether or ethyl ether. These substances have been designated “bitumens” by coal investigators. They can not be redistilled, even in the molecular still, without some decomposition, yielding gaseous and liquid hydrocarbons, phenolic bodies, bitumens, and a coke residue. The point of view that these neutral, ether-insoluble substances, the bitumens, constitute an important primary TABLE 1 Composition of condensates from atmospheric and vacuum distillations Temperature, 525°C.
I
ATMOSPHIRIC
Basic .....................................
Water and loss.. .......................... Total.. ....................................
VACUUM
20-40 Mesh
20-40 Meah
cad
wCoal*
per cant
per cent
per cent
2.78 7.51 2.45 0.40 3.98 17.12
7.11 8.61 2.56 0.20 2.32 20.80
17.10 8.20 1.37 0.16 1.50 28.33
coal
Neutral Ether-insoluble (bitumens). . . . . . . . . . . . . . Ether-soluble ............................ Phenolic and acidic ........................
I
* Prepared by grinding in a Szegvari pebble mill; particle size less t h a n 2p. thermal degradation product of bituminous coal, is supported by the experimental facts that in the thermal decomposition of a given coal a reduction in pressure, an increase in heating rate, and a decrease in particle size of the coal all tend to increase the yield of bitumens. The effects of reduction in pressure and the use of coal ground to particles less than 2 p , designated p-coal, are illustrated by the data of table 1. The increase in the amount of bitumen recovered in the condensate when the p-coal was employed is very striking and shows that in a 20- to 40-mesh coal particle considerable secondary decomposition takes place before the evaporating substances can escape. Since phenolic bodies have been shown (14) to be secondary decomposition products of the bitumens, the decreased yield of the former with increased recovery of the latter is to be anticipated.
1106
H. C. HOWARD THERMAL DECOMPOSITION IN SOLVENTS
That resinous or bituminous substances could be extracted by solvents from bituminous coals at a temperature much below that at which they could be recovered by distillation has been known for many years. The early experimenters, working a t low temperatures and recovering a few per cent of material, regarded the process as a simple solvent extraction, and considered the materials extracted to have been present as bodies which were of significantly different chemical constitution from, and hence much more soluble than, the bulk of the coal substance. With the development of pressure extractors and the use of new solvents (17) and of temperatures ranging from 250’ to 350°C., the yield of “extract” rose as high as 80 per cent, thus showing that products a t present not distinguishable from the resins and bitumens of the early investigators could be formed by thermal decomposition of the bulk of the coal substance. The point of view that solvent extraction a t elevated temperatures should be looked upon as a mild thermal decomposition has been emphasized by Lowry (15); Peters and Crenier (16) point out that the small variation in chemical composition between extract and residue can not account for their marked physical differences, and hence one must assume that polymerization is involved. A recent study ( 3 ) of extract and residue from a bituminous coal from the Pittsburgh seam has shown that insofar as present knowledge permits these substances to be characterized, there is little difference between extract and residue other than in molecular weight, and that the extract can properly be considered a primary building unit in the coal polymer, depolymerization having been effected by the elevated temperature and the action of the solvent. To what extent the degradation of coal by solvents a t elevated temperatures involves the formation of colloidal dispersions rather than molecular solutions is not certain. Dark-field examination of these solutions a t room temperature reveals colloid particles. The extracts, however, contain very little inorganic residue and it seems probable, if the phenomenon were chiefly peptization, that the peptized material would be of ash content similar to the coal. It appears more likely that the substances extracted are molecularly dispersed at the temperatures of the extraction process and associate or polymerize to the colloid units on cooling. Also, as will be seen later, all these products formed by solvent action a t elevated temperatures give freezing-point depressions in phenolic solvents which correspond to molecular units of moderate size. The yields of soluble products recovered by thermal decomposition of a bituminous coal from the Pittsburgh seam in benzene (l), phenol, and tetralin (2) at temperatures of 250-350OC. are shown in table 2. The much greater effectiveness of this type of degradation zls compared with vacuum distillation is strikingly illustrated, as is also the specific action of
1107
POLYMERIC CHARACTER OF BITUMINOUS COAL
such a polar solvent as phenol, with which, a t corresponding temperatures, a fivefold yield of soluble products over that with the non-polar benzene is obtained. Unfortunately attack by this method a t still higher temperatures is limited by the thermal instability of the solvents themselves. MILD HYDROGENATION
The effectiveness of mild hydrogenation in the primary breakdown of bituminous coal was demonstrated by F. Fischer, Peters, and Cremer (9), who found that by prolonged reaction a t temperatures as low as 26OoC., a German bituminous coal, ground to a particle size of less than 2 p, yielded 85 per cent of material soluble in hot benzene. It is significant that these investigators designated the products recovered by this mild hydrogenation as "pseudo-bitumens.$' TABLE 2 Decomposition in solvents at elevated temperatures COMPOBITION OF BXTRACTS
BOLYENT
TEMPERATURE
TIME
__ Phenolic
~
Ether-
______
__
___
Basic
Total
Neutral insolu-
ble
Ethersoluble
and acidic
___ -
"C.
hours
per cent
per Cenl
per cenl
Benzene.. . . . . . . . . . . . . . . . . . . . 260
44
10 '26
4 93
0.16
0.10
15.45
260 300 350
136 126 108
0.71 0.32 0.74 8% -1.77
0.17 0.15 0.18 0.50
33.8 17.0 30.8 81 6
0.13
66.7
Tetralin
Phenol.. . . . . . . . . . . . . . . . . . . . / 250
1
44
20 12 20 52
0 3 2 5
57 6
12 4 9 26
90 34 65
7 0
0.80
p e r cent per cent
- __ -
Work with a coal from the Pittsburgh seam has led to similar results, but a higher temperature, 350-4OO0C., has been found necessary. Under these conditions approximately 80 per cent of the American coal can be recovered in the form of high-boiling oils and bitumens. MILD OXIDATION
When a bituminous coal is refluxed with dilute nitric acid, e.g., 1 N , it is rapidly converted to organic acids, some of which are of low enough molecular weight to be soluble in water or dilute acid, and others, the socalled regenerated humic acids, which are soluble in or peptized by alkali, but precipitate on acidification. The humic acids formed in this way usually represent not less than 60 per cent of the carbon of the original coal,
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H'.
C. HOWARD
although this is a function of the period of oxidation, since these acids are the primary oxidation product and the water-soluble acids of lower niolecular weight are secondary products (21). The rate of formation of both types of acids is a function of the rank of the bituminous coal, lower rank coals reacting with greater velocity. The humic acids obtained by oxidation of bituminous coal dry to brownish-black or jet-black scales. They do not melt and on pyrolysis yield only low molecular weight volatile products, carbon dioxide, and water, and a carbonaceous residue. They form brown solutions with dilute alkalies, in which dark-field examination discloses many colloid particles. Their aqueous alkaline solutions diffuse only partially through parchment. The alkali humates formed from coal show certain analogies t o soaps: both form colloidal aggregates in aqueous solution; both are molecularly dispersed in solvents of the type ROH, where R is aliphatic in the case of TABLE 3 Ultimate composition o.f bitumens and original coal BOURCB
1
__
NITRO-
CARBON
BYDROQEN
>XYGES
per ceni
oer eeni
ne? eeni
5 69
11 56
1.16
6 32 5 06 488 7 27 5 19 5 66
5 71 10.00 9 32 4.88 5.53 6 03
1.59 1.16 1.52 1.88 1.63 1.78
ASH
QEN ~
Molecular still, 525°C.. . . . . . . . . . . . . . 81.15 Pressure extraction: Benzene, 250°C.. . . . . . . . . . . . . . . . . . . 84.70 Tetralin, 300°C.. . . . . . . . . . . . . . ..,. . 82 21 Phenol, 250°C.. , . , . , . , . . , . , , , . . , . . 81.15 Hydrogenation*. . . . . . . . . . . . . . . . . . . . . 85.25 Original c o a l . . . . . . . . . . . . . . . . . . . . . . . . 78.27 Ash-free.. . . . . . . . . . . . . . . . . . . . . . . . . 85.50
9er
cent
9er
Cent
0.44
0.68 0.43 0.89 0.42 0.98 1.03 __ * Hydrogenation of the residue from a benzene extraction a t 250°C. ~
ier cent
0 0 1 08 1 14
2 24 0 30 8 39
__
soaps and aromatic in the case of the humates; both are dispersing agents in aqueous solution; and both furnish aqueous solutions of high viscosity, although this is much less marked in the case of the humates because of plate-shaped rather than thread-like particles. COMPOSITIOK O F T H E BITUMEKS AND T H E HUMIC ACIDS
Ultimate compositions of bitumens obtained by different methods of degradation are shown in table 3. All contain, along with carbon and hydrogen, significant amounts of oxygen and small quantities of nitrogen and sulfur. Group tests indicate the absence of carboxyl, ester, carbonyl, or alkoxy1 groups. Determinations of hydroxyl oxygen have given variable results depending upon the sample of bitumen employed and the method of methylation. Usually not more than half of the oxygen can, however, be accounted for as hydroxyl groups, and the balance must be
1109
POLYMERIC CHARACTER OF BITUMINOUS COAL
assumed to be in ether linkages or heterocycles. Hydrogenation a t 425OC. of bitumens obtained by benzene pressure extraction, followed by dehydrogenation to aromatics and isolation of the crystalline picrates of the aromatics has resulted in evidence for the presence of condensed cyclic structures as large as four or five rings (4). Drastic oxidation of coal bitumens is reported (5) to result in high yields of benzene polycarboxylic acids. There thus appears little doubt that the nucleus of these bitumens consists of condensed C6 ring structures with occasional heterocyclic rings containing oxygen, nitrogen, or sulfur. The ultimate compositions and equivalent weights of humic acids prepared by mild oxidation of: (1) the whole coal, (2) bitumens extracted by heating with benzene a t 250°C., and (3) the insoluble residue from the benzene extraction are shown in table 4. Carboxyl and hydroxyl groups can be determined by methylation; the latter are present in smaller amounts than reported for “humic acids” from other sources. There is TABLE 4 Ultimate composition and equivalent weights of humic acids SOURCE
per cent
Original coal.. . . . . . . . . . . . . . . 61.46 Bitumen* .................... 57.17 Residue?.. . . . . . . . . . . . . . . :. . . 60.17
per cent per cent
3.21 3.71 3.89
30.4 34.7 31.4
PET
cenl
per cent
per cenl
0.65 0.34 0.55
1.1 0.9 1.0
3.36 3.09 3.04
240 192 244
also evidence for the presence of isonitroso groups, but not more than onehalf to two-thirds of the total oxygen can be accounted for in all functional groups together, indicating the pres.ence of ether or heterocyclic oxygen. Little information is available as to the nature of the nucleus of these humic acids. Hydrogenation of the alkali humates by heating with sodium formate (10) a t 360°C. results in simultaneous decarboxylation and conversion of the nucleus to high-boiling oils and bitumens. The latter are not distinguishable from bitumens obtained by degradation of the original coal. MOLECULAR W E I G H T MEASUREMENTS
These preparations of primary degradation products of coal are, of course, not homogeneous. Determinations of average values of molecular weight have, however, led to interesting results. As has been pointed out, the bitumens are readily soluble in polar aromatic solvents such as phenol and catechol, and much less soluble in aromatic non-polar types such as
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H. C. HOWARD
benzene and diphenyl. Only two solvents suitable for cryoscopic measurements on the regenerated humic acids have been found, namely, catechol (20) and p-nitrophenol. The results of cryoscopic molecular-weight determinations on bitumens and regenerated humic acids, prepared in a number of ways from a coal from the Pittsburgh seam, are shown in table 5. The significant feature of these data is the low and relatively constant values obtained in the solvents, catechol and p-nitrophenol. Considering the different methods of degradation employed, the relative constancy of these values argues strongly for the presence in this bituminous coal of TABLE 5 Molecular weights of bitumens and humic acids SOLVENT
__ SOLUTE
?Nitro phenol
-Bromc phenol
__ Bitumen from: klolecular s t i l l . , . . . . . . . . . . . . . . . . . 250 Benzene e x t r a c t . . . . . . . . . . . . . . . . . . . 285 Tetralin e x t r a c t . . . . . . . . . . . . . . . . . . . 335 Phenol extract.. . . . . . . . . . . . . . . . . . . 360 Hydrogenation of residue'b) . . . . . . . 320 Hydrogenation of humic acids(c), . 260 Humic acid from: Original coal.. . . . . . . . . . . . . . . . . . . . . Bitumencd) . . . . . . . . . . . . . . . . . . . . . . . . Residue(e1. . . . . . . . . . . . . . . . . . . . . . . . .
235 215 240
220
320
320 550 540 440 550
Tribrornophenol
Pyrene
Diphenyl
360 650
>1000
510
>1000 > 1000
(8)
230 280
-
(a) Incompletely soluble. (b) Insoluble residue from benzene pressure extraction, hydrogenated a t 350-400". (e) Humic acids from original coal, hydrogenated a t 350-400". (d) Bitumen from benzene pressure extract. ( e ) Residue from benzene pressure extract.
some fundamental unit of the order of magnitude indicated. Wherever solubility permitted, measurements were made in several solvents. In such cases a rough correlation was observed between dipole moment of the solvent and apparent molecular weight, solvents of higher moment giving lower molecular weights. The differences noted between different solvents are much greater than has been found for known cases of molecular association. Two explanations appear possible to the writer: (1) association to colloid particles in the non-polar solvents or (2) actual degradation of a polymer by the polar solvents. Such physical properties as vapor pressure and solubility indicate that both bitumens and regenerated humic acids are bodies of high molecular weight. It is known, however, that polar groups in certain positions
POLYMERIC CHARACTER OF BITUMINOUS COAL
A
1111
reduce volatility to a surprising degree. p-Hydroxybenzoic acid, for example, of molecular weight 138, is stated (19) to have a vapor pressure of 0.3 micron at 100°C., a value a thousandfold less than that for the o-hydroxy acid, salicylic acid. Further, while an average molecular weight of 300 appears low, an aromatic hydrocarbon of seven closely condensed rings has a molecular weight of only 302, and it should be noted that the molecular weight of the largest condensed aromatic structures which have been actually isolated from coal or pitch is smaller than this. It is also of interest that Fuchs (12), in picturing a hypothetical humic acid molecule, suggests a structure containing five condensed rings as the building unit, while Schrauth’s (18) building element contains four six-membered and three five-membered condensed rings. The presentation of the picture of bituminous coal as a polymer, built up of units of moderate molecular weight, naturally arouses the question as to how these units are in turn held together to form the high molecular weight body we know as coal. Unfortunately, very little can be said on this point a t the present time. Considering the probable method of genesis of coal, many of the known types of polymeric structure could conceivably be present. Its relatively great resistance to hydrolytic agents appears, however, to render improbable some of these, such as ester, anhydride, acetal, or lactone. An ether-linked structure, such as has been suggested for lignin, a Bakelite type, or an addition polymer formed from unsaturated units seems more likely. The determination of the exact character of the building units in bituminous coal and the nature of the forces which hold these units together present problems which as yet have been scarcely touched and offer a fertile field for chemical and physical investigation. SUMMARY
Evidence is presented for the point of view that bituminous coal is a polymer. Cryoscopic measurements in catechol on the degradation products of a typical coal from the Pittsburgh seam indicate that the unit is of moderate molecular weight, 250 to 350. Significant yields of this fundamental building unit are obtained by thermal decomposition in the molecular still, by the action of solvents such as benzene, tetralin, or phenol a t elevated temperatures, by hydrogenation, and by mild oxidation. The chief difference between the primary degradation products obtained by mild oxidation, the regenerated humic acids, and those recovered by thermal decomposition or hydrogenation, the bitumens, appears to lie in the presence in the humic acids of carboxyl groups, which confer alkali solubility and hydrophilic properties. The probable chemical nature of these primary degradation products and the possible methods by which the units combine are discussed.
1112
H. C. HOWARD
REFERENCES ( 1 ) ASBURY:Ind. Eng. Chem. 26, 1301 (1934).
(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (1.5) (16) (17) (18) (19) (20) (21)
ASBURY:Ind. Eng. Chem. 28, 687 (1936). BIGGS:J. Am. Chem. Soc. 68,1020 (1936). BIQGS:J. Am. Chem. SOC.68, 487 (1936). BONE,HORTON,A N D WARD:Proc. Roy. Soc. London 127A, 508 (1930). SAPIRO,AND GROOCOCK: Proc. Roy. SOC.London 148A, 521 BONE, PARSOBS, (1935). CAROTHERS: Chem. Rev. 8, 354 (1931). CAROTHERS: Chem. Rev. 8, 388, 405 (1931). FISCHER, F., PETERS, . ~ N D CREYER:Brennstoff-Chem. 14, 181 (1933). FISCHER~ F., A N D SCHRADER: Ges. Abhandl. Kenntnis Kohle 6, 470 (1920). FRECDENBERG: Ber. 63, 2713 (1930). FUCHS:Die Chernie der Kohle, p. 445. Springer, Berlin (1931). JUETTNER A N D HOWARD: Ind. Eng. Chem. 26, 1115 (1934). JUETTNER AND H O W A R DInd. : Eng. Chem. 26, 1117 (1934). ASBURY:Ind. Eng. Chem. 26, 1306 (1934). LOWRY:Ind. Eng. Chem. 26, 321 (1934). PETERS AND CREMER: Z. angew. Chem. 47, 576 (1934). POTT,BROCHE,A N D SCHEER:Fuel 13, 91 (1934). ASBURY:Ind. Eng. Chem. 28, 687 (1936). SCHRAUTH: Brennstoff-Chem. 4 , 161 (1923). A N D EUBASK:J. Chem. soc. 117, 396 (1920). SIDGEWICK ShrITH AND HOWARD: J. Am. Chem. s o c . 67, 512 (1935). JUETTBER, SMITH,A N D HOW.4RD: J. Am. Chem. s o c . 67, 2322 (1935).
