ROLE OF NONCOVALENT BONDING IN SWELLING OF COAL

The effects of coal-solvent interactions and coal-coal interactions on coal swelling were investigated to clarify the role of noncovalent bo nds in coal swelling in polar and nonpolar solvents. It is found that the swelling of coal is different from that of a covalently cross-link ed polymer and is caused by both covalent cross-links and noncovalent cross-links (or some physical associations). Coal-solvent hydrogen bon ding plays an important role in determining the swelling of coal in po lar solvents. Coal swelling increases with stronger hydrogen-bonding s olvents and with mole hydroxyl sites in coal. Increased swelling is ac complished by conversion of carboxylate to carboxyl in lignite through acid washing and by hydrolysis of weak ether and ester bonds in bitum inous coal to increase hydroxyl content. Coal swelling in nonpolar sol vents 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 pi-pi interactions are the predominant noncovalent coal-coal interacti ons in bituminous coal and restrain the solvent swelling of the coals. Some coal-coal interactions can be dissociated by acid washing or sol vent extraction, e.g., acid pretreatment removes the ionic forces, sol vent extraction with hydrogen bonding solvents dissociates hydrogen bo nds, etc. For the cyclohexanone-extracted coal, swelling reaches the m aximum value at carbon contents of 81.6 and 82.3 wt % for Fugu and Shu angyashan coal, respectively, which implies that the coal-coal interac tions are the weakest in this rank of coal. In addition, the decrease of swelling after CS2-N-methy-2-pyrrolidinone (NMP) extraction of Shen bei lignite was found to be attributed to the coordination of N and O in NMP with metal ions in the coal.