Coal, aliphatic structures

Growth of the 1700 cm.-1 band is accompanied by reduction in the intensity of all absorptions associated with nonaromatic C-H groups. Currently held views are that aliphatic structures are destroyed during the oxidation of coal, and the changes in absorption pattern support this. 

Aliphatic structures are still of major importance in the second group of resinites, those of the bituminous coals, but aromatic structures are present in significant amounts. The spectra of these resinites display the type of absorption pattern that has come to be associated with other coal macerals, particularly the sporinites and to a large extent the vitrinites. This pattern is established in the resinites of the high volatile bituminous coals. Furthermore, resinites of this group are reactive during carbonization and oxidation processes in which their behavior parallels that of similarly affected vitrinites of equivalent rank. 

A decade of work on the distribution of carbon (and also hydrogen) has now led to a fair assessment of the aromatic and hydroaromatic contents of coals although there may still be differences of opinion about the accuracy of these values (16). Our information on the aliphatic structures present in coal is, however, less complete. 

It is evident, therefore, that the aromatic carbon alone yields coke, and hydroaromatic carbon yields tar. Since neither appears to contribute substantially to the formation of gases (during the low temperature pyrolysis), it seems certain that the gases of low temperature pyrolysis owe their origin largely to the aliphatic structure in coal. At least it is now certain that methane formation is quite independent of the aromatic and hydroaromatic structures in coal. 

MAZIHADAft ET 41. Aliphatic Structure in Coal and Hydrogen undor Atmospheric Pressure... 

MAZUMDAR FT Al. Aliphatic Structures in Coal (Assussud From Pyrolysis) with Othor Forms of Carbon in Coal... 

A second requirement of the Tronov postulates—formation of acid anhydrides—is met by evidence from infrared spectra of oxidized coal samples periodically withdrawn from the reactor. As a matter of experimental record, it should be observed that such anhydrides did not appear until the later stages of reaction, by which time -COOH concentrations were already quite high. However, since initial carboxyl formation probably occurs in aliphatic structures present in the parent coal (9), appearance of anhydrides after rather... 

Product distribution data (Table V) obtained in the hydrocracking of coal, coal oil, anthracene and phenanthrene over a physically mixed NIS-H-zeolon catalyst indicated similarities and differences between the products of coal and coal oil on the one hand and anthracene and phenanthrene on the other hand. There were differences in the conversions which varied in the order coal> anthracene>phenanthrene coal oil. The yield of alkylbenzenes also varied in the order anthracene >phenanthrene>coal oil >coal under the conditions used. The alkylbenzenes and C -C hydrocarbon products from anthracene were similar to the products of phenanthrene. The most predominant component of alkylbenzenes was toluene and xylenes were produced in very small quantities. Methane was the most and butanes the least predominant components of the gaseous product. The products of coal and coal oil were also found to be similar. The most predominant components of alkylbenzenes and gaseous product were benzene and propane respectively. The data also indicated distinct differences between products of coal origin and pure aromatic hydrocarbons. The alkyl-benzene products of coal and coal oil contained more benzene and xylenes and less toluene, ethylbenzene and higher benzenes when compared to the products from anthracene and phenanthrene. The gaseous products of coal and coal oil contained more propane and butanes and less methane and ethane when compared to the products of anthracene and phenanthrene. The differences in the hydrocracked products were obviously due to the differences in the nature of reactants. Coal and coal oil contain hydroaromatic, naphthenic, heterocyclic and aliphatic structures, in addition to polynuclear aromatic structures. Hydrocracking under severe conditions yielded more BTX as shown in Table VI. The yields of BTX obtained from coal, coal oil, anthracene and phenanthrene were respectively 18.5, 25.5, 36.0, and 32.5 percent. Benzene was the most... 

One can imagine two plausible explanations for the observation that acidic fractions from H-coal consistently give greater amounts of diester products either the acidic fractions (which are phenolic) are more reactive and, thus, oxidize more completely than do the basic fractions, or the acidic components actually contain a greater number of methylene chains than do the bases. Either condition would be reflected in the amounts of diesters produced by oxidative degradation. This type of oxidative analysis provides both a means for characterizing the H-coal fractions and information that can be used to refine hypotheses about the aliphatic structures present in these materials. 

In low-rank coals, particularly in lignites, many of the aliphatic bridges, which participate in depolymerization, may be linked to noncondensed phenolic rings. The reactivity of an aliphatic structure linked to a phenolic ring is suf-... 

Formation of sub-bituminous coal seems to involve O loss through conversion of dihydroxy phenolic units (catechols) to monohydroxy units (phenols and alkylphenols), as shown in Fig. 4.7, based on the simple distribution of pyrolysis products, which are dominated by phenol, ortho-cresol (2-methylphenol) and 2,4-dimethylphenol (Hatcher 1990). Oxygenated aliphatic structures (alkyl hydroxyls and ethers) seem to be absent. Figure 4.8 shows the types of units present at various stages of biochemical coalification, based on a random hgnin polymer. 

Our knowledge of the aliphatic structures in coals is currently inadequate. In many low-rank coals, there is a significant amount of long-chain aliphatic material. There is a debate as to whether this material is bound to the coal macromolecular structure or whether it is simply tangled up with it and thus trapped. 

The concept of aliphatic structures being a predominant part of coal structure has been advocated on the basis of the oxidation of coal by sodium hypochlorite (Chakrabartty and Berkowitz, 1974, 1976). This particular concept requires that part or all of the coal carbon exist as sp carbon and invokes the three-dimensional adamantane system as the major building block of the coal structure. 

The concept was derived on the presumption that when organic structures are degraded by means of sodium hypochlorite the occurrence of carbon dioxide in the products indicates the presence of sp carbon. In addition, the requirement is that the majority of the carbon in coal (the exact proportion varying according to the rank of the coal) exists in the diamondoid aliphatic structure (Chakrabartty and Kretschmer, 1972). Thus, it was estimated that 59%-71% of the total carbon in a variety of coals (carbon content 76.1%-90.2%) occurred in the sp valence state (Table 10.3)... 

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