Depolymerization of Coal

Coal macromolecules contain ether, ethylene, and aryl-alkyl linkages that may be cleaved ionically as described in the following equations. 

These acid-catalyzed reactions occur at temperatures lower than those used in conventional liquefaction processes. Phenol induces acid-catalyzed solvolytic reactions at temperatures as low as 120°C. The liquefied products contain chemically bound phenolic groups, a portion of which can be recovered by cracking. 

The most attractive feature of the halide catalysts is that they can liquefy coal at temperatures much lower than those of conventional liquefaction. Unfortunately, they are difficult to recover and recycle, and they are very corrosive. The halides may also react with sulfide or hydroxide, thus losing their acidity. Their acidity may also be neutralized with strong stable N-bases. In spite of their molten state, a large amount of halides is required. 

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The reaction may contain hydrogenation, alkylation, and depolymerization of coal molecules assisted by the liquefying solvent. Through these reactions, coal molecules can be converted to be fusible or soluble in the solvent. 

Hofrichter, M., and Fritsche, W., Depolymerization of Low-Rank Coal by Extracellular Fungal Enzyme Systems. 3. In Vitro Depolymerization of Coal Humic Acids by a Crude Preparation of Manganese Peroxidase From the White-Rot Fungus Nematoloma Frowardii B19. Applied Microbiology and Biotechnology, 1997. 47(5) pp. 566-571. 

Solid/liquid separation is important at all stages and depends on the extent of coal cleaning during pretreatment, depolymerization of coal macromolecules at the dissolution stage, and catalyst durability through the catalytic process. 

The formation of the coal extract was interpreted as a free-radical chain reaction leading to the depolymerization of coal and aromatization of some of the hydroaromatic structures. It was suggested that phenanthrene possibly plays the role of a chain carrier in this process. 

Acid catalyzed depolymerization of coal with phenol affords a means for dissolution of coal under relatively mild conditions (185°C, ambient pressure). Once dissolved, separation of ash constituents and unreacted char is accomplished by filtration or centrifugation (also under mild conditions). Depolymerized coal recovered as a low ash product from excess phenol could be dissolved in a coal derived solvent and hydrogenated to stable liquids. It might be anticipated that access to hydrogen and contact with the catalyst would be more efficient in the case of the solubilized coal substance than for coal particle slurries. Hydrogenation might proceed more efficiently and with less... 

Table IV. Depolymerization of Coals of Different Ranks with Phenol-...

Depolymerization of coal is a technique which, it is claimed, solubilizes coal by cleaving methylene bridges in the coal. The methylene chains joining aromatic groups can be cleaved at the ring and the free alkyl group and then alkylate another aromatic substrate (Larsen et al., 1981 Mastral-Lamarca, 1987 Sharma, 1988) ... 

Thus, the depolymerization of coal (Heredy et al., 1965 Heredy, 1981 Wender et al., 1981 Given, 1984) by means of a phenol/boron trifluoride mix is reputed to proceed by the cleavage of aryl-alkyl-aryl systems (Figure 12.11). On this basis, the extent of the polymeric nature of coal is proportional to the number of methylene bridges in the coal, which can be determined from the concentration of di(hydroxyphenyl)methane in the reaction products. 

Baranov, S. N. Zheryakova, G. L Kochkanyan, R. O. Formation of aurin in acidic depolymerization of coal. Dopovidi Akad. Nauk Ukr, SeriyaB GeoL, Khim. Biol. Nauk. 1980, 38 0 Chem. Abstr. 1980, 93, 50114. 

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