Insights into the Coal Extractive Solvent N-Methyl-2-pyrrolidone plus Carbon Disulfide

A wide set of experimental and computational tools were used to characterize the N-methyl-2-pyrrolidone (NMP) + carbon disulfide mixed solvent in the full composition range. The interest in this solvent rose from its very efficient use for coal extraction through a mechanism still not fully understood. Thermophysical properties at ambient pressure together with pressure-volume-temperature (PVT) behavior were measured with the objective of providing the required data for the industrial use of the mixed fluid and to get insight into the fluid structure at the molecular level. NMR, FTIR, and solvatochromic studies were performed together with microwave dielectric relaxation spectroscopy (DRS) measurements, thus providing more information on the fluid's structure and allowing one to relate the molecular level behavior with the measured macroscopic properties. Moreover, density functional theory (DFT) and classical molecular dynamics simulations (MD) were used to obtain a detailed picture of the intermolecular interactions within the fluid, at short and long ranges, and of other relevant features leading to the structure of the studied system. The whole study leads to a fluid's picture in which carbon disulfide hinders the development of NMP/NMP intermolecular dipolar interactions, thus increasing the monomer population. We should remark that some properties reported in this work are in remarkable disagreement with previously reported studies, the most important one being the positive excess molar volume in the whole pressure-temperature range studied, which contrasts with the negative values reported in the literature. Previously reported properties are hardly justified with a coherent molecular level picture, whereas the whole collection of properties reported in this work leads to a more reasonable fluid's structure.