High-Pressure, Global Phase Behavior for the Guaiacol–CO2 System

The global phase behavior of guaiacol, a model lignin monomer, with CO₂, has been determined. Lignin, an abundant, naturally occurring source of aromaticity in biomass, can potentially be used as an inexpensive replacement for petroleum-derived polymers; however, phase-behavior measurements of lignin or lignin model compounds with supercritical fluids are practically nonexistent. Guaiacol was chosen for this study, as it contains both methoxyl and hydroxyl groups on the aromatic ring, as do most lignins. Furthermore, guaiacol is a liquid at ambient conditions, and our research program with lignin carries out the fractionation of lignin in a “liquid-lignin” phase. In this work, a variable-volume, high-pressure view cell was used to observe both liquid–liquid cloud points as well as bubble points along the liquid–liquid–vapor (LLV) three-phase line for this system. CO₂ + guaiacol was found to exhibit Type III phase behavior according to the classification system of Scott and Van Konynenburg, indicating that even at high pressures the mutual solubility of the two components is relatively low. However, the system does exhibit a pressure minimum in the liquid–liquid critical curve at 15 °C and pressures just under 500 bar.