Twenty-Five Years’ Progress in Gas and Fuel Chemistry A symposium presented before the Division of Gaa and Fuel Chemistry a t the 56th Meeting of the American Chemical Society, Chicago, Ill., September 10 t o 15, 1933.
The Chemical: Coal H. H. LOWRY,Coal Research Laboratory, Carnegie I n s t i t u t e of Technology, Pittsburgh, Pa.
F
E W materials are used in sulfur, nitrogen, phosphorus, and Not only as a source of energy but in all other as large q u a n t i t i e s in other elements and inorganic fields of utilization, coal must undergo a series of chemical r e a c t i o n s as mineral matter. Most coals, chemical reactions to yield the end sought. except cannels, are macroscopicoal. Not only as a source of Our knowledge of the chemical nature of coal energy, but in all its other uses, cally and microscopically banded, is based on consideration of (1) its probable coal must undergo a series of bright and dull layers alternatchemical reactions to yield the ing with each other. Twigs and mode of origin, (2) its reactions with simple end sought. K n o w l e d g e aclarger plant parts are recognizreagents, and (3) the products obtained by c u m u l a t e d from the work of able in some coals, w h i l e the thermal decomposition. Solvent extraction is many investigators during the microscope reveals spores, cuticlassed as a n especially mild type of pyrolysis. last two decades has made evicles, etc. During the coalificaCorrelation of the results obtained in the past dent the qualitative chemical tion process the originally relanature of coal. Although it is tively simple molecules may be two decades in various investigations of coal by regarded as having condensed difficult to evaluate the intrinsic the three separate methods listed shows that convalue of such knowledge, credit and polymerized to form a much siderable advance has already been achieved tomore complex “coal molecule’’ for increased efficiency in the wards a n understanding of the general chemical from which in some cases the utilization of coal properly benature of coal. Because of the complexity of original simpler molecules may longs to the engineer who has so vastly improved equipment for b e r e g e n e r a t e d b y suitable the coal substance, the need for additional syshandling materials and controltreatment. T h i s s t a t em e n t tematic work, using all available methods of implies that any single lump of ling operations. It is to the deattack, is emphasized. velopment of the steam turbine, coal m a y be regarded a s a materials for withstanding higher molecule of enormous size and temperatures and pressures, the regenerative cycle of feed complexity, a point of view in accord with modern recogniwater heating, etc., and not to research on the chemical tion of the difficulty of distinguishing, in a solid, between the nature of coal, that, for instance, the decrease in pounds of forces holding atom to atom in a molecule and those holdcoal per kilowatt-hour from 4.4 in 1912 to 1.5 in 1932 is ing neighboring molecules. mainly due. With the approach to perfection of such meCoal would therefore be expected to contain as an essential chanical equipment, it seems evident that further advance part of its chemical structure the stable nuclei of the original in coal utilization must come from a better knowledge of the plant material joined together as a result of elimination of material itself. Coal, the chemical, remains a challenge to the more reactive groups. If we neglect plant constituents of chemist’s ingenuity. More work along the lines already de- lesser quantitative significance from consideration, cellulose veloped, new methods of attack, the best available tools, and and/or lignin may be regarded as the “mother” substance deep study are needed. Quantitative, rather than qualita- of coal. The early point of view of the geologist that coal tive, understanding of the chemical nature of coal appears was formed essentially from cellulose has received support essential before we will be able to obtain the most from this from the work of Bergius ( 3 ) and Berl (4.4) who have prepared very valuable natural resource. coal-like substances from cellulosic materials. On the other hand, it has been pointed out, particularly by Fischer and CHEMISTRY OF COALFORMATION Schrader (14) and by Thiessen (45), that bacterial action, Coal is known to have been formed from vegetable matter under the conditions existing in present peat bogs, reduces more or less consolidated by the action of heat and pressure, the cellulose content of peat to practically zero, leaving a Its chemical composition ranges over wide limits, depending residue in which is concentrated the lignin and other more on the nature of the original vegetation, on the conditions resistant plant parts. These facts and others have led to a in the early stages of its deposition, and on the magnitude controversy which is still activ-ne side supporting celluof the natural forces to which the deposit was later subjected. lose as the main source of coal, and the other, lignin. A The present existence of vegetable matter in all stages of third school, represented by Terres ( 4 4 , regards the protein coalification, from peat to graphite, supplies our primary matter of dead bacteria, which lived on the vegetation during qualitative fact regarding the chemical nature of coal and its early stages, as an important source material of coal, a t the same time greatly complicates the problem of quantita- primarily to account for the relatively constant and high tive understanding, since no two coals are identical. percentage of nitrogen found in coal. The work of Waksman Like the vegetation from which i t is derived, coal is hetero- and collaborators (46) on the formation of stable proteingeneous both chemically and physically. Besides carbon, lignin complexes during peat decomposition also has a bearing hydrogen, and oxygen, coal contains varying amounts of on this subject. It is doubtful whether the various points 133
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of view emphasized by these investigators are as irreconcilable as may first appear; as in many similar cases, some truth may be found in each. Particular significance is attached to the lignin-cellulose controversy owing to the fact that lignin is known to contain the benzene ring characteristic of aromatic compounds (do), while cellulose does not. This fact loses some of its importance, however, since the recent work of Hawley (24) shows that a lignin-like substance may be prepared directly from cellulose by relatively mild treatment. Similarly, since, in the presence of a large excess of organic matter, reducing
B
A FIGURE1
a direction which may be attributed to the appearance of some new agency; we may infer that to form these higher rank coals more heat and/or pressure is required than for the lower rank coals, and that sufficient methane is lost during the later stages of coalification to cause the observed curvature of the coal band. A consideration of Figure 2 will indicate the modification in these conclusions necessitated if one postulates the loss of carbon monoxide, ethane, or other hydrocarbons. The results obtained by Berl(4A) on treating cellulose a t high temperatures and pressures with varying concentrations of aqueous alkali may also be qualitatively explained by Figure 2. At high alkali concentrations, favoring decarboxylation, the product was similar to petroleum, while a t lower concentration-i. e., less decarboxylation and more dehydration-the product resembled coal. A straight line drawn from carbon dioxide, decarboxylation, through the composition of any coal will end on the line of zero oxygen or the hydrocarbon line. These same considerations indicate that the acidity of the coal seam during the coalification period may well modify the end composition as postulated by Taylor (49).
REACTIONS AND
THE
CHEMICAL NATUREOF COAL
From a study of the probable chemistry of coal formation as outlined in the preceding paragraphs, two primary chemical B. facts are evident: Coal is a complex organic molecule, resulting from condensation and reaction of simpler molecules, conditions may be expected to exist in a peat bog, the sig- and contains, as an essential element of structure, the sixnificance of the lignin-cellulose controversy may be regarded membered carbon ring characteristic of aromatic compounds. as considerably lessened by the work of Schrauth (41) who These facts receive additional support from the data obtained showed that one of the main products obtained by Willstatter in investigations of the reactions of coal with simple chemical and Kalb (49) in the reduction of lignin, cellulose, and other reagents. Few of these reactions have received the syscarbohydrates possessed properties practically identical with tematic study necessary for them to furnish quantitative synthetic 9,10-perhydrobenzophenanthrene,Figure 1A. As information regarding the chemical nature of coal. As is a result of this study Schrauth (40) suggested that the coal- natural, their development has been largely towards empirical forming vegetable matter may first have been converted tests for judging the suitability of coals for specific uses. The reagents used by different investigators include into compounds of the nature shown in Figure 1B. A molecule of this type would be expected readily to undergo con- gaseous oxygen (11), ozone ( I % ) , nitric acid ( d ) , nitric acid densation in various ways, giving molecules of unlimited and potassium chlorate (25), concentrated sulfuric acid (26), size, and containing nitrogen and sulfur by reaction with sulfuric acid and dichromates (42), acid (IO) and alkaline (9) other compounds. Such condensation products would ap- permanganates, chlorine (SS), bromine ( d l ) , hydriodic acid proach coal both in composition and properties. To date, ( I S ) , sodium formate (15), carbon monoxide ( I 6 ) , hydrogen this appears to be the most satisfactory presentation given ( I ) , and many others (cf. summaries of citation 36). The literature citations are not complete, only certain of those referin any detail of a chemical mode of formation of coal. -4further and more general idea of the nature of coal and ences which consider the qualitative and quantitative chemiof the transformation of vegetable matter to coal may be cal aspects being listed. Detailed consideration of this mass gained by a consideration of the results of the analyses of of data is impossible in the limited space available. However, coals of all ranks. Many thousands of analyses of coals have we may group the reactions into three categories-oxidation, attempt to draw some been published and many methods proposed for their most halogenation, and reduction-and convenient presentation. The method illustrated by Figure general conclusions regarding the significance of the work in 2, in which analyses are recalculated so that the sum of the elucidating the chemical nature of coal. OXLDATION. Since peat, one of the earliest stages of percentages of carbon, hydrogen, and oxygen, on an ashand moisture-free basis, equals 100 (cf. citation 39) appears of coalification, is known to contain considerable amounts of value not only in connection with the changes undergone in material soluble in dilute alkalies and known as “humic acids” nature but also in the later reactions to which coal itself is or “ulmic acids,”it is not surprising that onmild oxidation with normally subjected. The “coal band” illustrated includes a gaseous oxygen (11), nitric acid (6),and concentrated sulfuric large majority of published analyses of coal and lies between acid (26), similar compounds may be regenerated from coal. a “line of dehydration,” starting a t the composition of water, Humic acids represent in all probability a group of acidic passing through a point representing the composition of compounds of high molecular weight, having an equivalent cellulose, and ending a t 100 per cent carbon, and a “line of weight of approximately 350 and a molecular weight of decarboxylation” (loss of carbon dioxide), starting a t the 1300 to 1400 (22). Fuchs (23) postulates, from his study of composition of carbon dioxide and, only for purposes of the reactions of humic acids, that they have a unit structure illustration, also passing through C6H1006,and ending on the indicated in Figure 3B which may combine as illustrated in line of zero oxygen. Since the coals all lie between these Figure 3C. While the structure illustrated does not satislines, it may be concluded that the initial stages of coalifica- factorily account for all the properties and analyses of humic tion are characterized by the loss of carbon dioxide and water, acids, it is of interest to note the similarity of the nuclear primarily the latter. Only when we reach the higher rank structure (Figure 3A) to that proposed by Schrauth and also coals does the slow progressive change in composition shift in the importance in the structure of the six-membered carbon A.
.
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Nuclear structure found by Schrrtuth (41) in one of the main products of the reduction of lignin, cellulose, and other Carbohydrates “Parent” molecule of coal, suggested by Schrauth (40)
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ring. From the experimental work of many investigators, there can be little doubt that a similar unit forms an essential part of the coal substance. The stability of the unit is indicated by the fact that with suitably mild treatment, a large part of the coal substance, even in high-rank bituminous coals, may be made soluble in dilute alkali. Results obtained in our Laboratory (SI) indicate that the yield of “nitrohumic” acids from a residue obtained from the pressure extraction of coal with benzene is approximately equal in amount to that obtained from the original coal. From this observation i t may be concluded that the material extracted from coal contains nuclei not essentially different from the part of the coal which is insoluble in the solvent. Consideration of Figure 4 will indicate that a 100 per cent yield of humic acids would not be expected from any coal. I n Figure 4 the oxidation of a coal, K , having the composition [(C = 85) (H = 5 ) (0 = lo)] = 100 is assumed. Direct addition of oxygen along the line KO will not pass through the composition of humic acids. A line drawn starting from carbon dioxide and passing through the center of the circle representing the composition of humic acids intersects the line KO a t A. The distance a-humic acid is a measure of the loss of carbon dioxide necessary for coal K to be transformed into humic acids. For a lower rank coal, K’, the analogous point of intersection is B, indicating that less of the carbon of the coal need be lost as carbon dioxide in the regeneration of humic acids in this case and therefore a higher yield of alkali-soluble materials will be obtained. These facts may be correlated with the method of “rational analysis” of Francis and Wheeler (19): A determination of the amount of regenerated humic (ulmic) acids is merely an indirect way of determining the elementary composition of the the original coal, in accord with the experience of the U. S. Bureau of Mines as cited by Kester (Sa).
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stituent oxygen atoms. The high yield of benzene carboxylic acids is remarkable in view of the fact that by this type of oxidation all rings would be destroyed which contained as substituents either hydroxyl radicals or groups which on hydrolysis are converted to the hydroxyl radical. Furthermore, if the central benzene ring shown in Figure l A or one of the rings shown in Figure 3 d is assumed to persist in the benzene carboxylic acids, in both cases seven out of eighteen (38.9 per cent) carbon atoms would be the maximum yield recovered as monocarboxylic acids. Higher yields may be readily explained by the presence of polycarboxylic acids in the final product. When due consideration is given to the probable presence of side chains which might be oxidized to simpler acids, the yield obtained by Bone indicates a most efficient process.
+
TABLEI. OXIDATIONOF RESIDUES FROM COALS(6) AUSTRALIAN MORWELL CANADIAN ESTWAN BROWNCOAL BROWNLIQNITE Carbonic anhydride Carbonic anhydride Acetic Acetic Oxalic Oxalic Succinic Succinic Phthalic Phthalic Isophthalic Isophthalic Terephthalic Terephthalic Trimellitic Trimellitic Hemimellitic Hemimellitic Trimesic i;y’romellitic Pyromellitic Mellophanic -Mellophanic Beneenepentacarboxylic Benzenepentacarboxylic Mellitic Mellitic
TABLE11.
CARBOX
BALAXCEO F
ORIQINALCARBON OF COAL AS: SUBSTANCE APPEARINQ Carbonic anhydride Acetic acid Oxalic acid Succinic acid Benzenoid acids Total carbon accounted for
DURHAM BUSTY COKINQCOAL Carbonic anhydride Acetic Oxalic Fithalic Isophthalic Terephthalic Trimellitic P&bmellitic -Mellophanic Benzenepentacarborylic Mellitic
THE OXID.4TION (6)
MORWELL % ,41.3 2.5 7.7 0.8 47.4 99.7
BUSTY % ,42.4 1.7 6.5 Nil 48.8 99.4
On more severe oxidation by gaseous oxygen a t higher temperatures or pressures (II), by ozone (IW),or by alkaline permanganate (9),the coal substance is broken down to smaller, recognizable units. The most thorough work on this subject has been carried out by Bone, Horton, and Ward; Tables I and I1 summarize their work (6). In this case the oxidation was carried out a t 100” C. in aqueous alkaline permanganate, the coal having been first extracted with benzene under pressure. Other experiments ( 7 ) on oxidation of the extract have shown that similar products are obtained, indicating that the fundamental chemical nature of the extract may not be essentially different from the extracted residue, possibly differing mainly in the number of sub-
0
IO nzo 20
X,
40
LINEOF FIGURE 2.
50
€0
70
80
93
liwH
ZERO CAREON
SCHEMATIC REPRESENTa4TIOZT O F THE COAL
BAXD
Summarizing, oxidation studies of coal have indicated the probable presence in coal of units of structure similar to the nuclei of the so-called humic acids. Little is known how these units are held together, but many data are available clearly indicating that within these units of structure there exist the stable six-membered carbon rings characteristic of aromatic compounds and graphite. If graphite may be regarded as the final result of the coalification process, i t is natural to assume that coalification must be a continued gradual condensation of such six-membered carbon rings with elimination of side groups and all elements except carbon. HALOGENATION. The halogens are commonly used to determine the degree of unsaturation of an organic substance. Little published work is to be found on a systematic study of the nature of the reaction of chlorine and bromine with coal. The data of McCulloch and of Eccles and collaborators (53) show clearly that the reaction progresses to a considerable extent with the evolution of heat and hydrochloric acid. Their main attention appears to have been directed towards the changes in properties in the chlorinated products when compared with the original coal. For more than two years a study of the reactions of a bituminous coal with chlorine and bromine has been under way in our Laboratory (cf. summaries of citation, 56). The coal was suspended in a solvent to absorb the heat evolved in the reaction. At 30” and at 65” C. the total bromine reacting is approximately the same a t times up to 200 hours. The reaction has not been carried to completion, and is still con-
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tinuing a t 30 O after 650 hours. The rate of reduction is almost independent of the particle size of the coal from lp to 100mesh. Comparison of results obtained on unextracted coal and on coal first extracted under pressure by benzene shows little difference in the extent of the reaction, indicating again chemical similarity of the extract to the original coal, More than 90 per cent of the reaction is by substitution, and may be attributed to the presence in the coal of very reactive hydrogen and not much unsaturation. At these temperatures approximately 1.5 grams of bromine react with each gram of the particular coal being studied. At 74’ in carbon tetrachloride
Vol. 26, No. 2
coniirms the importance of this unit in the structure of coal itself.
THERMAL DECOMPOSITION AND THE CHEMICAL NATURE OF COAL
Because of the great complexity of the coal molecule, it is natural that many attempts to learn more about its structure have started with thermal treatment in order to break down the coal into smaller, recognizable units. Contrary to usual classification of methods, solvent extraction will be included in this paper, with thermal decomposition. The fact that solvents may extract substances from coal without evolution of gaseous products of decomposition has led most investigators to assume prior existence of such substances in the coal. That this is an unnecessary assumption appears to be a direct conclusion of the following considerations: HO A solution process is analogous to a distillation process, no in so far as one substance will dissolve in another, only so long as its partial pressure in the solution is less than its A normal vapor pressure; when the partial pressure is equal n to the normal vapor pressure, the solvent is saturated with CWU respect to the solute. The vapor pressure difference reCOOH H HO quired for appreciable rates of distillation, even in a high HO CWH H vacuum, is of an entirely different order of magnitude than CWW that necessary for rapid solution, as is well illustrated by the COOU ready solution of cane sugar in water a t room temperature and its low rate of distillation (50). Furthermore, it follows directly from elementary electrostatics that the attraction between two oppositely charged bodies is less the higher the dielectric constant of the medium in which the bodies exist. FIGURE3 I n the presence of a solvent, therefore, the entire solid strucA . Nuclear structure of a unit in humic acid ture of coal is so loosened that less kinetic energy in the form B . Unit structure of humic acid proposed by Fucha (3s) C. Dimer of B. Fuchs (3s) of heat is required to rupture the bonds holding the solid together than in the absence of the solvent. Also if a bond a t higher concentrations of bromine, the reaction proceeds is once ruptured, the solvent offers an effective electrostatic about twice as far. The products soluble in carbon tetra- shielding and provides a certain molecular mobility not chloride are very complex. The bromine may be replaced present in the absence of the solvent. This explanation acwith CN, and, after hydrolysis, products similar in appear- counts for the greater effectiveness of pyridine (dielectric conance and molecular weight to humic acids are obtained. stant 9.4) than of benzene (dielectric constant 2.1) as a This observation simply confirms the oxidation work on the solvent for coal a t their normal boiling points, completely neglecting specific chemical reactions. existence in coal of a unit structure similar to humic acid. One advantage in so considering solvent extraction as an The chlorides formed a t 74” C. have been of too great molecular complexity to permit any conclusions regarding extremely mild type of thermal decomposition is that it their structure. At this temperature approximately 1.4 permits a direct correlation to be made between this method grams of chlorine react with each gram of the same coal used of studying coal and methods involving the direct application in the bromination work. Chlorination in antimony penta- of heat resulting in gas evolution and condensable vapors. chloride a t 200-220” C. yields, among the identifiable prod- Solvent extraction, vacuum distillation, and low- and highucts, hexachlorobenzene and hexachloroethane. I n this temperature carbonization may therefore be regarded as work, as in the permanganate oxidation work of Bone, the methods of studying the progressive thermal decomposition six-membered carbon ring appears an important structural of coal of increasing severity of thermal treatment. It is difficult to name a liquid that SOLVENT EXTRACTION. element of the coal. REDUCTION.Studies of the reduction of coal have been has not been used as a solvent for coal, ranging, for example, largely directed towards the preparation of a maximum yield from liquid sulfur dioxide to high-boiling oils. I n the latter of a petroleum substitute. With the exception of the work case it is extremely difficult to distinguish between solution of Heyn and Dunkel, Ormandy and Craven, Pertierra, and and dispersion. With few exceptions most of the several Varga and von Makray ( I ) , no report appears to have been hundred investigations have been directed towards the exmade of the individual chemical compounds resulting from traction and characterization of a hypothetical “coking the reaction. General classification of the nature of the principle” so as to be able to measure the coking quality of products has been made largely on solubility in certain sol- coals. Nevertheless, several reports on the separation of vents, However, benzene, toluene, and xylene, and their the extracts into relatively pure individual constituents hexahydro derivatives, naphthalene and its tetra- and deca- throw some light on the chemical nature of coal. The hydro derivatives, phenanthrene, and certain p a r a f i s solvents which have received most attention from this point and hydroaromatics have been isolated, in addition to of view are pyridine and benzene. Wheeler and collaborators (SO) elaborated a solvent method aromatic bases, phenol, and higher phenols. The percentage yield of any single compound based on the original coal is for separating the pyridine extract into several fractions. so small that its presence adds little to our knowledge of the The pyridine soluble-chloroform insoluble fraction was shown chemical nature of the coal substance. However, the pre- to be “ulmic” or humic in nature. The remaining fractions dominance of compounds containing aromatic nuclei again contain hydrocarbons, phenolic esters, and resins. Char-
n
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February, 1934
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acterization of the pyridine extract was carried much farther by Hofmann and Damm (a“),who isolated from the neutral ether-soluble portion many saturated and unsaturated hydrocarbons. Paraffins from CZIH, through C27H56, unsaturated compounds of the series Cd&,-(d, 6 , 8, IO, 12, 14, 16, or id, bicyclic naphthenes from CI1 to C16, and tri- or polycyclic ranging from naphthenes of the series CnH2n-((4,6 , 8, or C1, to C24were isolated. Methylanthracene was definitely identified. Many of the compounds isolated by Hofmann and Damm had previously been isolated from the benzene extract of a high-volatile Saar coal by Pictet and collaborators (37) ; 5200 kg. of coal, on extraction for 4.5 days in two portions, yielded a total of 13.30 kg. of extract a t the normal boiling point of benzene. The part soluble in petroleum ether consisted essentially of hydrocarbons with small amounts of bases and alcohols. The hydrocarbon portion was made up of about 25 per cent saturated and 75 per cent unsaturated compounds. The saturated hydrocarbons isolated (ranging from CloHmthrough C13H26) were all monocyclic naphthenes, as indicated by their formula C,Hz,. I n addit>ion,a compound C3oH@was isolated and assumed to be a member of the same series. The unsaturated hydrocarbons isolated were dihydrotoluene, dihydro-m-xylene, dihydromesitylene, dihydroprehnitol, C11H16, C14HlB,hexahydrofluorene, dihydrofluorene, and C1,Hm. As in the case of the pyridine extract, most of the compounds contained a six-membered carbon ring. Compounds of the same type as were found by Pictet from the extraction of coal by benzene a t its normal boiling point have been found in the extract obtained with the same solvent at higher temperatures by Fischer (13) and Bone (8) and their collaborators, and also in the extract obtained by Berl and Schildwachter ( 5 ) using tetralin as solvent. Since the extract obtained from coal represents, in most cases, a very small fraction of the original coal sample, the chemical structure of the extract necessarily does not reveal much about the general chemical structure of coal. The relatively large proportion of hydrocarbons in the extract does suggest, however, that units of structure containing the least oxygen are least firmly bound in the coal molecule. This is in accord with the knowledge that the presence of an oxygen atom in an organic molecule is, in general, accompanied by increased “association,” higher boiling point, and higher melting point,. The presence of paraffins in the extract in the quantities found is not an argument against an essentially six-membered carbon ring structure for coal, since in the coalification process one may assume that the condensation reactions were not confined to the cellulosic and/or ligninic components, but included the waxes, resins, gums, etc., present in the original vegetation. VACUUMDISTILLATION.Although many of the substances found in the solvent extract of coal have very high vapor pressures a t t,emperatures below the “initial decomposition temperatures of coals” (18)-characterized by gaseous evolution-no appreciable yield of condensable material is obtained even in a vacuum of mm. of mercury until active gaseous evolution begins (29). This fact may be used to support the classification of solvent extraction as a very mild process of thermal decomposition. Most of the vacuum distillation work reported in the literature (38) has been carried out to determine the “primary” products of pyrolysis and in this way to learn the source of the compounds found in normal coal tar. It should be pointed out that none of the investigators has used sufficiently low pressures to avoid secondary, or a higher order, decomposition, the pressures ranging from 2 to 25 mm. of mercury. The vacuum tar so obtained resembles closely the ‘Lprimary”tar obtained in low-temperature carbonization (see next section). At pressures a thousand times or more lower than this, the tar ob-
137
tained from a Pittsburgh coal a t 525’ C. amounts to over 20 per cent by weight of the original coal and, when fresh, is yellow to orange in color and solid a t room temperature (29). Although the general composition-i. e., phenols, acids, bases, neutral, etc.-does not differ greatly from a Fischer retort tar (17) obtained from the same coal at the same temperature, the vacuum tar has a higher average molecular weight (29).
Pictet and collaborators (58) carried out a separation of a vacuum tar, obtained from the distillation of 1500 kg. of a Montrambert coal at 450’ C. and about 20 mm. of mercury, using the same technic already mentioned in connection with their work on a solvent extract. I n many cases the same individual chemical compounds were identified. KO data are given by the authors regarding the relative amounts of the chemical individuals in the extract and vacuum tar, although internal evidence suggests they were not essentially different. Since the yield of vacuum tar was approximately sixteen times that of the extract, it may be tentatively concluded that the mechanism of thermal breakdown is not essentially different in the two cases. It may be postulated
FIGURE 4. SCHEMATIC REPRESENTATION OF THE REGENERATION OF HUMIC ACIDSFROM COALBY OXIDATION that the higher temperature deoxygenates certain structural units by elimination of water, carbon dioxide, etc., which thereby become less firmly held by the coal substance and are free to evaporate. As the distillate is formed by breaking bonds holding it originally to the solid coal substance, it appears safe to assume that the residue simultaneously polymerizes until it becomes so condensed in structure that further heating is no longer effective in yielding molecules condensable into tar. Low- AND HIGH-TEMPERATURE CARBONIZATION. Distillation of coal a t normal pressures yields products of a composition and nature which depend primarily on the mechanical features of the system and on the h a 1 temperature. Both of these factors may determine the temperature gradients through the coal sample and the amount of cracking of the primary decomposition products. Carbonization at tem-
138
Ole es Octene Aonene
Bases Aniline Pyridine a-Picoline 2,4-,2,6-Lutidine
Ben ene Homologues Benzene Toluene o m &p-Xylenes *hmene 1,2,4-Trimethylbenzene
llapht enes Methylcyclohexane lJ3-,1,4-Dimethylcyclohexane 1,2,4-Trimethylcyclohexane
A consideration of the data reported in the literature on the probable mode of formation of coal leads to the conclusion that the coal molecule has resulted from condensation and polymerization of polynuclear six-membered carbon ring compounds containing hydrogen, oxygen, nitrogen, sulfur, and other elements found in coal. It is suggested, on the basis of the nature of the products obtained from the mild oxidation of coal that an important unit of structure has essentially the same nuclear structure as humic acids. More severe oxidation, halogenation, and reduction of coal yield derivatives of a six-carbon cyclic nucleus confirming its
Phenolic
perature carbonization are discussed from the point of view of an increasing severity of thermal treatment. The progressive increase in molecular complexity and the persistence of the six-carbon ring from high-
Anales 60c. espafi. f6s. quim., 28, 792 (1930). Shatwell, H . G., and Graham, J. I., Fuel, 4, 25, 76, 127 (1925). Shatwell, H. G., J . SOC.Chern. Ind., 44, 471 (1925). Shatwell, H. G., and Bornen, A. R., Fuel, 4, 252 (1925). Skinner, D. G., and Graham, J. I., Fuel, 4, 474 (1925), 6, 74 (1927), 74 (1927), 7, 543 (1925). Varga, J., Brennst0.f-Chem., 9, 277 (1928). Varga, J., and von Makray, I., Ibid., 12, 21 (1931). von Makray, I.,Ibid., 11, 61 (1930). Baranov, A., and Francis, W., Fuel, 1, 219 (1922); Charpy, G., and Decorps, G., Compt. rend., 178, 1588 (1924). Francis, W., and Wheeler, R. V., J. Chem. Soc., 127, 2236 (1925). Fuchs, W., ”Die Chemie der Kohle,” Springer, 1931. Fuchs, W., and Horn, O., Brennstof-Chem., 1 2 , 6 5 (1931). Hendrickson, A. V., Fuel, 2, 103, 356 (1923). Hilpert, S.,Keller, K.,
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139
and Lepsius, R., Ges. Abhandl. Kenntnis Kohle, 1, 22 (1915Heathcoat, F., Fuel, 9,452 (1930). 16). Lilly, V. G., and Garland, C. E., Fuel, 11, 392 (1932). Hilpert, S.,Keller, K., and Lepsius, R., Ges. Abhandl. K e n n t n i s (3) Bergius, F., J. Gasbeleucht, 54, 748 (1912) ; ~ ~ u t u r w i s s e n s c h a f t e n , Kohle, 1, 22 (1915-16). Kreulen, D. J. W., Brennstof-Chem., 16, 1 (1928). 8, 149 (1927). (4A)Berl, E., Proc. 3rd Intern. conf. Bituminous Coal, 2, 820 (1931). Hofmann, F., and Damm, P., Ibid., 3, 73, 81 (1922), 4, 65 (4B) Berl, E., Schmidt, A, and Koch, H., Z. angew. Chem., 43, (1923). 1018 (1930). 44. 329 (1931). 45. 517 (1932). Berl. E.. and Holroyd, R., and Wheeler, R. V., J. Chem. SOC.,1928, 3197. schmiht, A'.; Ann., 493, 97,' i24,.135 (1932); 496, 283 (1932). Howard, H. C., and Juettner, B., to be published. Berl, E., Schmidt, A., Biebesheimer, H . , and Dienst, W., Jones, D. T., and Wheeler, R. V., J. Chem. SOC.,105, 141, 2562 S a t u r w i s s e n s c ~ q f t e n ,20, 652. (1932). Berl, E., and Keller, (1914); 107, 1313 (1915); 109, 707 (1916). Stopes, M., and H., A n n . , 501, 84 (1933). Wheeler, R. V., Fuel., 3, 63, 399, 439 (1924). Cockram, C., Berl, E., and Schildwachter, H., Brennstoff-Chem., 9, 105, 121 and Wheeler, R. V., J . Chem. SOC.,1927, 700; 1931, 854. (1928). Juettner, B., unpublished data. Bone, W. A., J. Koy. SOC.A r t s , 79, 77 (1930). Kester, E. B., Bur. Mines, Information Circ. 6486, 15 (1931); Bone, W. X., Horton, L., and Ward, S. G., Proc. Roy. Soc. Fuel, 10, 284 (1931). (London), A127,508 (1930). Marsh, A , , McCulloch, A , , and Parrish, E., J. SOC.Chem. Ind., Bone, W. X., Pearson, A. R., Sinkinson, E., and Stockings, W. 46, 167T (1929). Eccles, A., and McCulloch, A., Ibid., 49, E., Ibid., A100, 582 (1922). Bone, W. A., Pearson, A4.R., 377T, 383T (1930). Eccles, A., Kenyon, G. H., and McCulloch, .4., Fuel, 10, 4 (1931). Eccles, A., Kay, H . , and and Quarendon, R., Ibid., i1105, 608 (1924). Bone, W. A., Horton, L., and Tei, L. J., Ibid., A120, 523 (1928). McCulloch. A. J . SOC.Chem. Ind.. 51.49T (1932). Kav. H.. Bone, W. A , , and Quarendon, R., Ibid., A110, 537 (1926). and McCulloch, A., Ibid., 51, 186T (1932); 52,'47T (i933). Bone, W.A,, Horton, L., and Ward, S. G., Ibid., A127, 480 Morgan, G. T., Ibid., 51, 67T (1932). (1930). Morgan. J. J., and Soule, R. P . , Chem. & M e t . Eng., 26, 1025 Charpy, G., and Decorps, G., Compl. rend., 173, 807 (1921). (1922). Kreulen, D. J. W., Brennstof-Chem., 10, 397 (1929). Siggemann, H., Ges. Abhandl. K e n n t n i s Kohle, 1, 1 (1915-16). Charpy, G., and Godchot, M., Compt. rend., 163, 745 (1916). Mensel, A., Ibid., 9, 308 (1929). Fischer, F., and Schrader, H., Ges. Abhandl. Kenntnis Kohle, Pictet, A., Ann. chim., [9] 10, 249 (1918). Pictet, A., and 4, 342 (1919), 5, 200 (1920). Fischer, F., and Schneider, Ramseyer, L., Chem.-Ztg., 35, 865, 907 (1911), 40, 211 (1916); W.,Ibid., 5, 135, 186 (1920). Fischer, F., Schrader, H., Ber., 44, 2486 (1911). Pictet, A., Ramseyer, I,., and Kaiser, and Treibs, W., Ibid., 5, 267 (1921). Fischer, F., Peters, K., O., Compt. rend., 165, 358 (1916). and Cremer, W., Brennstof-Chem., 14,184 (193311. Francis, W., Pictet, h.,Bnn. chim., [9] 10, 249 (1918). Pictet, A., and and Wheeler, R.. V., J . Chha. Soc., 127,112 (1925), 1927,2958; Ramseyer, L., Ber., 44, 2486 (1911). Pictet, A., and Bouvier Safety in M i n e s Research Board (London), 28 (1926). Kreulen, M., Ibid., 46, 3342 (1913), 48, 926 (1915); Compt. rend., 157, D. J. W., Brennstnff-Chem., 8, 241 (1927) ; Chem. W e e k b h d , 779, 1436 (1913); Chem.-Ztg., 38, 1025 (1914). Pictet, A, Kaiser, O., and Labouchhre, A., Compt. rend., 165, 113 (1917). 25, 642 (1928). Lynch, C. S., and Collett, A. R., Fuel, 11,408 Burgess, M. J., and Wheeler, R. V., J. Chem. Soc., 99, 649 (1932). Mabler, P., Compt. rend., 150, 1521 (1910), 165, 634 (1917). Mabler, P., and Charon, E., Ibid., 150, 1604 (1910). (1911), 105,1131(1914); Fuel, 5,65 (1926). Holroyd, R., and Pearson, A. R., Fuel, 3, 297 (1924). Wheeler, R. V., J. Chem. Soc., 1928, 2669, 3197, 1929, 633; Fischer, F., Ges. Abhandl. K e n n t n i s Kohle, 1, 26 (1915-16); Fuel, 9, 40, 76, 104 (1930). Jones, D. T., and Wheeler, R. Ber., 49, 1472 (1916). Fischer, F., and Niggemann, H., Ges. V., J. Chem. SOC.,105, 140, 2562 (1914), 107, 1318 (1915). Abhundl. K e n n t n i s Kohle, 1, 30 (1915-16). Fischer, F., Tideswell, F. W.,and Wheeler, R. V., J. Chem. SOC.,115, and Tropsch, 'H., Ibid., 2, 160 (1917). 619 (1919). Fischer, F., Broche, H . , and Strauch, J., Brennstoff-Chem., 5, Ralston, 0. C., Bur. Mines, Tech. Paper 93 (1915). 299 (1924), 6, 349 (1925). Broche, H., and Bahr, T., Ibid., Schrauth, W., Brennstnff-Chem., 4, 161 (1923). 6, 349 (1925). Fischer, F., and Gluud, W.,Ber., 49, 1460 Schrauth, W.,2. angew. Chem., 36, 571 (1923). (1916). Fischer, F., and Schneider, W., Ges. Abhandl. Simon, L. J.. Comvt. rend., 178, 495, 775 11924). K e n n t n i s Kohle, 1, 204 (1915-16), 3, 287, 315 (1918). Fischer, Taylor, E. M., Fuel, 5, 195 (1926); 6, 359 (1927); 7, 66, 127, F., and Kleinstuck, M., Ibid., 3, 301 (1918). Fischer, F., 129, 130, 227, 228, 230 (1928). Peters, K., and Cremer, W., Brennstqf-Chem., 13, 364 (1932). Terres, E., PTOC.3rd Intern. Conf. Bituminous Coal, 2, 797 Fischer, F., and Schrader, H., Ibid., 2, 37 (1921), 3, 65, 341 (1931); Terres, E., and Steck, W., Gas u. Wasserfach, 73, (1922). Fischer. F.. iYaturwissenschaften. 9. 958 11921): Special No. l(1930). brennst0.f-Chew.; 5, 132 (1924), 14, 1 4 7 (1933); Z . deut. p o l : Thiessen, R., Bur. Mines, BulZ. 117 (1920); Trans. Am. I n s t . Ges., A77, 534 (1925). Mining M e t . Engrs., 71, 35 (1925), 88, 419 (1930); Prnc. 2nd Fischer, F., and Schrader, H., Brennstof-Chem., 2, 161 (1921). Intern. C o n f . Batuminous Coal, 1, 695 (1928). Ibid., 2, 257 (1921). Waksman, S.A., Brennstoff-Chem., 13, 241 (1932). W'aksman, Fischer, F., and Schrader, H . , G E S .Abhandl. K e n n t n i s Kohle, 5, S. A., and Stevens, K. R., Soil Sci., 26, 113, 239 (1928); 55 (1920). 27, 271, 389 (1929); 28, 315 (1929). Waksman, S. d.,and Fischer, F., and Tropsch, H., Ibid., 2, 154 (1917). Reusser, H. W., Ibid., 33, 135 (1932). Waksman, S. A., Francis, W.,and Wheeler, R . V., J. Chem. SOC.,1929, 586. and Purvis, E. R., Ibid., 34, 95 (1932). Waksman, S. A., and Freudenberg, K., Ber., 63, 2713 (1930). Gerretsen, F. C., Ecology, 12, 33 (1931). Waksman, S. A., Fuchs, W., Brennsto.f-Chem., 9, 348 (1928). Hilpert, S., and Iyer, K . R . N., J. W a s h . Acad. Sci., 22, 41 (1932). Keller, K., and Lepsius, R., Ges. Abhandl. K e n n t n i s Kohle, Washburn, E. W., BUT.Standards J. Research, 2, 482 (1929). 1, 22 (1915-16). Pearson, A. R., Fuel, 3, 297 ;1924). Weiler, J. F., unpublished data. Fuchs, W., Brennstoff-Chem., 10,438 (1929). Willstatter, R., and Kalb, L., Ber., 55, 2637 (1922). Ibid., 12, 266 (1931). Hawley, L. F., and Harris, E. E., Isn. EN+.CHmf., 24,873 (1932). RECEIVED September 12, 1933.
