Pathway of TCNE interaction with coal to enhance its solubility in the NMP-CS2 mixed solvent

Tetracyanoethylene (TCNE) is a very effective additive enhancing coal solubility in the N-methyl-2-pyrrolidinone (NMP)-CS2 (1:1, v/v) mixed solvents. It was found that TCNE generates 1,1,2,3,3-pentacyanopropene (PCNP) anion in NMP in the presence of water. The PCNP anion is not an electron acceptor but is as effective as TCNE at enhancing solubility. This rules out charge-transfer complexation between coal and TCNE as a main factor in increasing coal solubility. In addition, we have not found a correlation between the electron acceptor ability of additives and their ability to enhance coal solubility. These results do not support the proposal that formation of a charge-transfer complex between electron accepters and coal is responsible for the enhancement of coal solubility.