Polystyrene hydrogenation in supercritical carbon dioxde-decahydronaphthalene using porous catalysts

The heterogeneous hydrogenation of polystyrene (PS) was studied in a slurry batch reactor. Mixtures of supercritical carbon dioxide (scCO 2) and decahydronaphthalene (DHN) were used as the solvent for the polymer. Several palladium-based porous catalysts were identified for PS hydrogenation at 150°C. Relatively high degrees of hydrogenation were obtained with monometallic palladium catalysts for the reaction conducted in neat DHN. The 5% Pd/SiO2 was twice as active in hydrogenating PS as 5% Pd/Al 2O3. However, when either palladium catalyst was used in scCO2-DHN, hydrogenation ceased within 15 minutes of CO2 addition to the reactor. Detected in the gas phase at the end of CO2 -containing reactions, carbon monoxide (CO) was formed via the reverse water-gas shift (RWGS) reaction and poisoned hydrogenation sites. Physical mixtures consisting of a hydrogenation catalyst (5% Pd/Al2O 3 or 5% Pd/SiO2) and a methanation catalyst (65% Ni/SiO 2/Al2O3 or 5% Ru/Al2O3) 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 (e.g., 1.6% Ru/4.2% Pd/SiO2 or 5% Pd/5% Ru/SiO2) 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. Because of their smaller molecular diameters, CO2 and H2 are able to access the active sites in the catalyst interior and form CO. However, the much larger polymer molecules had very little or no access to the catalyst interior and could not react on the poisoned sites. When the methanation function was proximate to the hydrogenation metal, as in the case of the bimetallic catalyst, CO was methanated before it poisoned hydrogenation sites.;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 CO 2 concentration was demonstrated in high pressure dynamic light scattering experiments. A nearly two-order-of-magnitude increase in the rate of hydrogenation in neat DHN was observed when PS molecular weight decreased from 160 kDa to 24 kDa. For a 160 kDa PS, the rate of hydrogenation increased with CO 2 concentration; an order-of-magnitude change was observed when CO 2 pressure was increased from 0 psig to 2200 psig. 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.