Effect of Single/Mixed Model Solvents on the Ternary Phase Diagrams of Pyrolytic Lignin, Model Solvent, and Water

This paper reports the dissolution and phase behavior of pyrolytic lignin (PL) in the mixtures of water and various single/mixed model solvents at 25 °C. The model solvents are formulated from the five key model components, including acetic acid (AA), hydroxyacetone (HA), furfural (FU), phenol (PH), and methanol (ME), that represent the five key chemical families in the water-soluble fraction of bio-oil. FU is completely miscible with AA, HA, and ME but forms two different homogeneous binary systems with water, i.e., an organic-rich system at low water contents (i.e., <5 wt %) and a water-rich system at high water contents (i.e., >96 wt %). In contrast, PH has very high solubilities (i.e., >95 wt %) in AA, HA, and ME but only forms homogeneous binary systems with water at high water contents (i.e., >93 wt %). Thus, it is important to ensure that the solvent/water mixture is homogeneous to study PL dissolution in these mixed model solvents. Among three single-component solvents (i.e., AA, HA, and ME), ME is the best solvent for PL dissolution, indicating that methanol addition is the most effective strategy to enhance the phase stability of bio-oil. While the addition of a small amount of water can slightly increase the PL solubilities in the AA/water and HA/water mixtures, a further increase in the water content leads to the reduction of PL solubilities in those solvents. In contrast, increasing the water content continuously reduces the PL solubilities in the ME/water mixtures. The ternary phase diagrams for the two-component solvents clearly demonstrate that both FU and PH have negative effect on PL dissolution in model solvent/water mixtures. An increase in the FU or PH content in the solvent further reduces the PL solubility in the solvent/water mixture. In comparison to PH, FU has a slightly stronger effect to suppress the PL dissolution in the solvent/water mixture. The ternary phase diagrams for the three-/four-component solvents show that the PL solubility decreases as the total content of FU and PH in the model solvent increases. Clearly, removal of FU and PH from bio-oil can be another effective strategy to enhance bio-oil phase stability. Furthermore, FU and PH have to be included in the mixed solvent to develop useful ternary phase diagrams for predicting bio-oil phase stability.