Investigation of phenolic ionic liquids as efficient nitric oxide absorbents

Abstract

A series of phenolic ILs with different cations were synthesized and used for the capture of nitric oxide (NO) in this work. These phenolic ILs showed excellent absorption performance because of the chemical interaction between the phenolate anion and the acidic NO. In particular, their absorbability for NO was largely affected by the cationic bond-donating ability. Tetrabutylphosphonium phenolate ([P4444][PhO]) was thus highlighted with the cation of weak bond-donating ability, reaching a maximum solubility of 1 mol NO/mol IL at 1 bar and 313.2 K. With the assumption of complex formation between NO and phenolic ILs, the reaction equilibrium thermodynamic model (RETM) was constructed to correlate the experimental solubilities. Therefore, thermodynamic parameters such as the Henry’s law constant Hm, the reaction equilibrium constants K, the enthalpy ΔHSOL and the entropy ΔSSOL could be achieved to evaluate the absorption performance of NO in the phenolic ILs. Furthermore, through a combination of experimental solubilities, RETM fitting results and spectroscopic investigations, the absorption mechanism was proposed that there was a chemical interaction between NO and the O-site on the phenolate anion, with the reaction stoichiometry of 1:1 and 2:1. All these data showed that the phenolic ILs may be used as a kind of promising absorbents for NO capture.