Polystyrene Hydrogenation in Supercritical CO2-Decahydronaphthalene Using Porous Catalysts

Abstract

The heterogeneous hydrogenation of polystyrene (PS) was studied in a slurry batch reactor. Mixtures of supercritical carbon dioxide (scCO2) and decahydronaphthalene (DHN) were used as the solvent for the polymer. Several palladium-based porous catalysts were identified for PS hydrogenation at 150oC. Relatively high degrees of hydrogenation were obtained with monometallic palladium catalysts for the reaction conducted in neat DHN. However, when either palladium catalyst was used in scCO2-DHN, hydrogenation ceased within 15 minutes of CO2 addition to the reactor. Carbon monoxide (CO) formed via the reverse water-gas shift (RWGS) reaction and poisoned hydrogenation sites. Physical mixtures consisting of a hydrogenation catalyst and a methanation catalyst were effective in reducing CO levels. However, when the “salt-and-pepper†catalyst was used, aromatic ring hydrogenation levels in scCO2-DHN were consistently lower than those obtained in neat DHN.

A bimetallic catalyst in which the hydrogenation and methanation functions are located on the same support was successfully used to reduce CO levels and to hydrogenate PS in scCO2-DHN. The success of the bimetallic catalyst in hydrogenating PS in scCO2-DHN over the salt-and-pepper approach was attributed to the differences in internal mass transfer resistances for PS hydrogenation and the RWGS reaction.

Polymer size effects on heterogeneous PS hydrogenation were determined by varying polymer molecular weight and by using CO2 to tune polymer coil size in DHN. The ability to tune polymer coil size by varying CO2 concentration was demonstrated in high pressure dynamic light scattering experiments. The improvements in reaction rate in either neat or CO2-expanded DHN were found to be directly related to increases in PS diffusivity and decreases in polymer coil diameter, both of which are functions of polymer molecular weight and solvent quality.