Phase equilibrium for the hydrogenation of polystyrene in CO2-swollen solvents

Polystyrene (PS) can be hydrogenated using a heterogeneous catalyst suspended in a solvent swollen by supercritical carbon dioxide (scCO2). Various phase equilibria are involved in this system. First, the application of scCO2 to a solution of PS can cause the polymer to precipitate. Therefore, the effect of CO2 pressure and temperature on the phase behavior of various solvents containing dissolved PS was investigated, leading to the selection of decahydronaphthalene (DHN) for in-depth study. It was found that the CO2 pressure required to precipitate PS from DHN increased with the temperature. The volume of solutions containing various concentrations of PS in DHN increased considerably as the CO2 pressure was increased. Volume expansions of 35-40% were obtained between 40 and 150 °C and between 3 and 9 wt.% PS. Moreover, calculations using the Peng-Robinson equation of state showed that the H2 concentration in the liquid phase was higher in CO2-swollen DHN than in the pure solvent, at a constant H2 partial pressure. The rate constant for PS hydrogenation was found to be higher in CO2-swollen DHN than in the pure solvent.