Experience in the Chemical Destruction of Organic Matter in Gas-Type Bituminous Coal

P. F. Volodin

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

The object of the present investigation is bituminous coal from the Leninsk deposit in Kuzbass, belonging to the gas type. The elemental composition of the organic matter of this coal is as follows: carbon—82.2%; hydrogen—4.8%; nitrogen—2.7% and oxygen—10.3%. The ash content of the dry coal is 6.24%. The yield of volatile substances on combustible mass is 40%. The coal cakes well.

In carrying out the investigation, two aims were pursued:

1) a theoretical aim—to elucidate the chemical nature of the organic matter of coal, and

2) a practical aim—to reveal the possibility of obtaining, by chemical destruction, low-molecular products for organic synthesis.

The working hypothesis of the present investigation is the conception of the organic matter of coal as an amorphous and high-molecular substance. It is assumed that the formation of macromolecules of the organic matter of coal occurred under natural conditions by the type of polycondensation reactions.

If one proceeds from the general scheme of formation of the organic matter of bituminous coals through humic acids, then, in accordance with the properties of the latter as oxy acids, it may be assumed that in the course of transformation at the brown-coal and bituminous-coal stages they enter into a polycondensation reaction through the interaction of hydroxyl groups with carboxyl groups. As a result of such a reaction, ester linkages arise in the macromolecules of the organic matter of coal—

$$ \mathrm{O{-}CO}\ldots $$

The probability that amino acids, formed during the decomposition of plant protein substances, are present in the products of transformation of dead plant remains also permits the hypothesis that, in the process of coal formation, polycondensation reactions proceed both among the amino acids themselves and between amino acids and humic acids, which in both cases should lead to the formation of an amide linkage.

On the same grounds, taking into account the phenolic character of humic acids and the possibility of formation of aldehydes in the process of transformation of dead plant remains, it may also be assumed that, during coal formation, polycondensation reactions of humic acids with aldehydes occur. In this case, linkages of the type of those in phenol-aldehyde resins will appear in the macromolecules of the organic matter of coal.

For the three types of linkages cited, and similar ones, it is characteristic that they are unstable under the action of certain chemical agents and may readily be destroyed by them. Phenol–aldehyde resins, for example, are unstable with respect to alkalis, under the action of which the bonds between phenol and aldehyde residues in the macromolecule are destroyed. In polyamides, amines and carboxylic acids may produce a similar action, and in polyesters, carboxylic acids.

In accordance with the hypothesis set forth concerning the chemical nature of coals, experimental work was carried out on the chemical destruction of gas-type bituminous coal.

Chemical destruction of the organic matter of coal by hydrolysis

In order to clarify the question of whether hydrolyzable bonds are present in the macromolecules of the organic matter of the coal under study, and what their nature is, experiments were conducted in which coal samples were treated with solutions of alkalis and acids, with determination of the yield of soluble products [1].

The first experiments were performed by treating weighed portions of dry coal of about 1 g on funnels with a glass porous filter successively with two-normal solutions of soda, caustic soda, and hydrochloric acid in water.

After repeated treatment on the funnel with hot solutions of each of the listed reagents, the weighed portion of coal was washed with hot water and dried. The loss in weight of the coal was determined after treatment with each of the three indicated solutions.

The loss in weight, calculated for ash-free coal, was: upon treatment with soda, 1.28% and, in a parallel experiment, 1.24%; upon subsequent treatment with a solution of caustic soda, respectively 0.96% and 1.12%; and upon treatment with hydrochloric acid, 1.06 and 0.95%; in total, 3.30% in the first experiment and 3.31% in the second, i.e., practically coinciding figures.

In preparative organic chemistry, two-normal solutions of soda, caustic soda, and hydrochloric acid are used to extract from a complex mixture of organic substances, respectively, carboxylic acids, phenols, and organic bases. There is therefore some basis for assuming that the products extracted from coal by the solutions contain carboxylic acids, phenols, and amines formed as a result of hydrolytic cleavage of macromolecules at the types of bonds mentioned in the presentation of the working hypothesis.

However, the small quantity of products extracted from coal by aqueous solutions of soda, caustic soda, and hydrochloric acid gives no idea of the extent to which these bonds are distributed in the organic matter of coal or of their role in the construction of the macromolecules. Therefore the idea arose of carrying out experiments on hydrolysis with solutions of alkali and acid in organic liquids, specifically in alcohol and in a mixture of alcohol with benzene. In this case, the wetting of the coal was to be better effected, with deeper penetration of the solution into the micropores of the coal particles. In addition, the solubility of the hydrolysis products in organic liquids could be better.

The method of treatment was also changed. Dry weighed portions of coal of about 1 g were placed in a flask and covered with 50 cm³ of solution. The mixture was boiled on a water bath with a reflux condenser for two hours. The contents of the flask were then filtered on a Fritted funnel. The coal on the funnel was first washed with a small amount of the corresponding liquid (i.e., alcohol or alcohol–benzene), which was added to the filtrate; the filtrate was then removed, and the coal was thoroughly washed with hot water, dried, and weighed. The loss in weight of the coal as a result of its treatment was determined.

To take into account the influence of the organic liquids themselves on the solubility of the coal, experiments were carried out in which coal samples were treated with alcohol, benzene, and a mixture of alcohol and benzene in a 1:1 ratio by volume. Treatment with alcohol and benzene gives practically no appreciable loss in weight, but the mixture shows a rather significant effect, expressed as approximately 2 percent loss of the organic matter of the coal.

The best results in coal weight loss are obtained by treatment with a two-percent solution of caustic potash in alcohol–benzene—14 percent. Treatment with a solution of caustic potash in alcohol gives a smaller loss (about 11 percent), and treatment with a solution of hydrochloric acid in alcohol–benzene still less (only about 6 percent). The following curious fact should be noted: sequential treatment of one coal sample first with a solution of hydrochloric acid and then with a solution of caustic potash in alcohol–benzene gives, in total, the same result as treatment with the solution of caustic potash alone. This fact may be regarded as evidence of the identical nature of the action of the reagents, although their effectiveness differs.

The substances extracted from the coal by the alcohol–benzene solution of caustic potash were tested for solubility. For this purpose the filtrates containing extracts of these substances, obtained as a result of a series of experiments, were combined, evaporated to a small volume, and poured into a two-normal aqueous solution of hydrochloric acid. The extract thereby separated into two parts—one (the larger) precipitated, while the other (the smaller) remained dissolved in the hydrochloric acid. The precipitate, after being washed on a fritted-glass funnel, was dried and weighed. Then, on the same funnel, it was successively treated with two-normal solutions of soda and caustic soda, ether, alcohol, benzene, acetone, chloroform, and pyridine, with determination of the weight loss in it after each solvent.

The solubility of the destruction products, calculated on the combustible substance of the coal, in the listed solvents, in percent, is as follows:

Chemical destruction of the organic matter of coal by treating it with certain chemically active organic compounds

High-molecular organic compounds of the polyamide type, obtained by condensation of carboxylic acids with amines; of the polyester type, obtained by condensation of alcohols with carboxylic acids; of the phenol–aldehyde resin type, and others, are readily subject to destruction when acted upon by low-molecular compounds of the same chemical structure as the original monomeric products from which they were obtained. Thus, for example, polyamides may undergo destruction when heated with amines or carboxylic acids; polyesters—when heated with the corresponding alcohols or carboxylic acids; phenol–aldehyde resins—when heated with phenol, etc.

Studies of these destructive processes have shown that their mechanism is the same as in hydrolysis. Accordingly, they have received the names: destruction with the use of an amine—aminolysis; with the use of a carboxylic acid—acidolysis; with the use of alcohols—alcoholysis; with the use of phenol—phenolysis [2].

In accordance with the hypothesis concerning the nature of the organic substance, inves-

it could be assumed for coking coal that the indicated destructive processes are also possible in it. Therefore experiments were carried out in which coal samples were heated by boiling mixtures for two hours with an amount, twenty times the weight of the coal, of aniline, phenol, benzoic acid, as well as pyridine and carbazole.

The experiments were carried out in two variants: in the first variant, after heating was complete, the reaction mixture was filtered through a funnel with a porous glass filter, and the residue on the funnel and the filtrate were treated accordingly. In the second variant, the reaction mixture was poured into a suitable liquid in which neither the residual coal nor the destruction products dissolve, while the reagent used does dissolve. Then the reaction mixture was filtered on a funnel, the residue was washed, dried, and weighed. Thereafter the residue was treated successively on the funnel with various solvents. The results of the experiments are given in the table.

Table

Many experiments were carried out with phenol by both variants. In these experiments, according to the first variant, the coal on the funnel was washed with a small amount of alcohol, and then with hot water; the phenol solution was brought to a small volume and poured into cold water. The filtered-off precipitate, as well as the residual coal, was also washed first with a small amount of alcohol and then with hot water. The loss in weight of the coal, regardless of the method of treatment, fluctuated between 20 and 23%.

In the experiments described, attention is drawn to the fact that there is no clearly expressed dependence of the loss in weight of the coal on the boiling point of the reagent used. In fact, the boiling temperatures of the reagents are quite different: for pyridine 115, for aniline 184, for phenol 181, for benzoic acid 250, and for carbazole 354. Meanwhile, the differences in the destructive action of reagents that differ sharply in boiling temperature are not noticeable. Moreover, the differences in the effectiveness of the action of the reagents are hardly greater than the differences yielded by deviations in the conditions of conducting one and the same destructive action.

This conclusion is confirmed in another fact. When treated with the same coal sample successively with two or three different reagents—for example, aniline, phenol, and benzoic acid—the loss in weight of the coal occurred only upon treatment with the first of them, while the second and third produced almost no additional effect.

Chemical Destruction of the Organic Matter of Coal by Treating It with Abietic Acid or Rosin

The use of abietic acid extracted from rosin, or of rosin itself, containing other acids of analogous structure, was of interest because, as was shown in the previously cited studies of other authors, it gives a high effect in the destruction of the organic matter of bituminous coals of the gas, fat, and coking types [3].

The experiments here were also carried out in two variants. In the first variant, a coal sample weighing about 1 g was heated with a twentyfold amount of abietic acid or rosin until the mixture boiled and was kept at the boiling temperature for two hours. The reaction mixture was then dissolved with benzene and filtered. The precipitate on the filter was washed with benzene, alcohol, and hot water, dried, and weighed. The filtrate was evaporated to a small volume and precipitated in light gasoline. The precipitate that separated in the gasoline was filtered off, washed on the filter with benzene, alcohol, and hot water, dried, and weighed.

In the second variant, the reaction mixture was precipitated in gasoline; the precipitate was filtered off, washed on the filter with gasoline, alcohol, and water, dried, and weighed. Next, the dried treated coal was washed on the filter with benzene until the solution ceased to be colored. The undissolved residue was washed with alcohol and water, dried, and weighed.

The purpose of treatment with gasoline was to remove rosin oil, into which abietic acid and rosin were converted under the conditions of the experiment, while treatment with benzene was intended to dissolve the destruction products.

As in the case of treating coal with abietic acid, so also in the case of treatment with rosin, its solubility in benzene was high, reaching 80–85 percent, calculated on the initial organic matter (more precisely, on the combustible mass). The total weight of the residual coal and benzene extract always considerably exceeded the initial weight of the coal taken for treatment.

It should be noted here that in the preceding experiments with aniline, phenol, and benzoic acid, the treated coal dissolved only very little in benzene. Exhaustive extraction of the destruction products was achieved only by successive treatment with acetone, chloroform, and pyridine. Thus, the already easy solubility of the destruction products in benzene when abietic acid and rosin are used indicates a different character of the process in these cases compared with the destruction process when aniline, phenol, and benzoic acid are used.

It is also interesting to note the fact that the benzene extract from coal treated with rosin sharply changes its solubility after treatment with a two-percent solution of caustic potash in alcohol-benzene. Its solubility after such treatment is as follows, in percent:

On the basis of the experiments described, one may conclude that the interaction of the organic matter of coal with the resin acids of rosin consists not only in the destruction of the coal macromolecules and the attachment of acid ions to the “fragments,” but also in the simultaneous occurrence of an esterification reaction of the hydroxyl groups of the “fragments” with the acid groups of the rosin components, similar to what takes place in the modification of novolac phenol-formaldehyde resins with rosin.

Novolacs possess insufficient polarity to dissolve in water or soda, but their polarity is too great for them to dissolve in such nonpolar solvents as benzene and to be compatible with oils. In order to lower their polarity they are treated with rosin. The reactions that occur in the interaction of rosin with novolacs are regarded as ester-forming reactions. It may therefore be assumed that, in the interaction of the organic matter of coal with rosin, along with destructive processes, there also occurs the formation of esters that are readily soluble in benzene. When the benzene extract is treated with an alcohol-benzene solution of caustic potash, however, the reverse process takes place—saponification with destruction of the esters [4].

Analysis of the results of the investigation carried out on coal from the Leninsk deposit (Kuzbass) leads to the following general conclusions with respect to the aims set in this study.

1. The data from experiments on hydrolysis with an alkali solution in alcohol-benzene, on treating coal with substances containing functional groups, and especially from experiments on treating coal with abietic acid and rosin, confirm the validity of the hypothesis concerning the chemical nature of the organic matter of the coal studied as a high-molecular-weight product formed under natural conditions from products of the transformation of the remains of dead plants by polycondensation reactions.

2. The substantial yields, in aqueous solutions of acids and alkalis as well as in organic solvents, of products of chemical destruction in the experiments described indicate the fundamental possibility of obtaining low-molecular-weight compounds for subsequent use in organic synthesis.

LITERATURE

1. HOWARD, K. I. In: Chemistry of Solid Fuel. 1951.
2. KORSHAK, V. V. Chemistry of High-Molecular Compounds, 1950.
3. ZABAVIN, V. I., SURKOVA, V. L. In: Proceedings of the Institute of Fossil Fuels, Academy of Sciences of the USSR, vol. III, 1954.
4. BARG, E. I. Technology of Synthetic Plastic Masses. Goskhimizdat, 1954.

Corrections and Noted Misprints

Izv. TPI, Vol. 102.