Role of Noncovalent Bonding in Swelling of Coal

Citations Over TimeTop 15% of 1998 papers

Abstract

The effects of coal−solvent interactions and coal−coal interactions on coal swelling were investigated to clarify the role of noncovalent bonds in coal swelling in polar and nonpolar solvents. It is found that the swelling of coal is different from that of a covalently cross-linked polymer and is caused by both covalent cross-links and noncovalent cross-links (or some physical associations). Coal−solvent hydrogen bonding plays an important role in determining the swelling of coal in polar solvents. Coal swelling increases with stronger hydrogen-bonding solvents and with more hydroxyl sites in coal. Increased swelling is accomplished by conversion of carboxylate to carboxyl in lignite through acid washing and by hydrolysis of weak ether and ester bonds in bituminous coal to increase hydroxyl content. Coal swelling in nonpolar solvents is mainly controlled by coal−coal interactions. The low swelling of lignite in nonpolar solvents is attributed to strong ionic forces. On the other hand, hydrogen bonds, charge-transfer interactions, and π−π interactions are the predominant noncovalent coal−coal interactions in bituminous coal and restrain the solvent swelling of the coals. Some coal−coal interactions can be dissociated by acid washing or solvent extraction, e.g., acid pretreatment removes the ionic forces, solvent extraction with hydrogen bonding solvents dissociates hydrogen bonds, etc. For the cyclohexanone-extracted coal, swelling reaches the maximum value at carbon contents of 81.6 and 82.3 wt % for Fugu and Shuangyashan coal, respectively, which implies that the coal−coal interactions are the weakest in this rank of coal. In addition, the decrease of swelling after CS2−N-methyl-2-pyrrolidinone (NMP) extraction of Shenbei lignite was found to be attributed to the coordination of N and O in NMP with metal ions in the coal.

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