Experimental and mechanism exploration on the separation of methanol-containing azeotropic compounds from biodiesel by phosphate esters ionic liquids

Abundant coal resources in China have led to an excess of methanol production. This work explores the separation strategy of dimethoxymethane-methanol (DMM-MeOH) and dimethyl carbonate (DMC)-MeOH binary azeotropes, which are easily produced in the production of diesel additives. Implementing energy-saving, emission reduction, and low carbon emission strategies is important. Three types of phosphate esters ionic liquids (ILs) with different chain lengths were prepared to realize the separation of DMM-MeOH and DMC-MeOH binary azeotrope. Based on the vapor-liquid equilibrium experiments, the vapor-liquid equilibrium data of the DMM-MeOH and DMC-MeOH binary azeotropes using phosphate esters ILs of three different chain lengths were measured at 101.3 kPa. The effect of phosphate esters ILs with three different chain lengths on the separation of DMM-MeOH and DMC-MeOH azeotropes was investigated. The separation mechanism of the azeotropes was studied at the molecular level. The electron density difference explores the optimal configuration between molecules. The atomic theory of molecules analysis directly displays the characteristics of bond critical points and explores the influence of hydrogen bond strength on the intermolecular bond length. The important roles of hydrogen bonds and steric hindrance in the separation process were investigated by reduced density gradient and independent gradient model. Quantum chemical calculation results showed that phosphate esters ILs have a significant effect on the molecular interactions of azeotropes, which is the main factor for phosphate esters ILs in reducing the relative volatility between azeotropes.