Heterogeneous Catalytic Hydrogenation of Polymers Facilitated by Supercritical Carbon Dioxide

Abstract

The heterogeneous catalytic hydrogenation of polystyrene (PS) in decahydronaphthalene (DHN) was studied in a batch reactor using 5%Pd/BaSO4 as the catalyst. The effects of temperature and H2 pressure were investigated over the ranges from 90 to 180C and 250 to 1000 psig. At high H2 pressure, the rate of PS hydrogenation was approximately first order with respect to aromatic ring concentration, with an apparent activation energy of 59.6 kJ/mol. For the hydrogenation of 3wt% PS solution at 150C and with agitation above 2000 rpm, the resistance to gas-liquid mass transfer was negligible. Calculations indicated that influences of liquid-solid mass transfer and pore diffusion were minimal during hydrogenation of PS solutions at concentrations of 3wt% or less. The resistance to mass transport became severe in the hydrogenation of PS solutions at concentrations of 4.5wt% or higher.

Precipitation of PS from different solvents under high pressure CO2 was investigated. DHN was selected as the solvent for PS hydrogenation in the presence of supercritical CO2 (scCO2) due to the high precipitation pressure and the saturated structure of DHN. It was found that H2 solubility in CO2-swollen DHN was substantially higher than in pure DHN at a constant H2 pressure. Deactivation of 5%Pd/BaSO4 was observed during PS hydrogenation in CO2-swollen DHN above 150C. The analysis of the gas phase showed that about 50 ppm CO was formed at 150C, which could poison the catalyst. It was found that 65%Ni/Al2O3/SiO2 could effectively convert CO to CH4. Therefore, the bimetallic catalyst system (5%Pd/BaSO4 plus 3wt% 65%Ni/Al2O3/SiO2) showed improved resistance to CO poisoning during PS hydrogenation in CO2-swollen DHN.

The irreversible adsorption of PS on the surface of catalyst was found to lead to a lower hydrogenation rate. The adsorption of PS from DHN onto solid catalyst was studied. At equilibrium, the amount of adsorbed PS increased with increasing temperature. The presence of high pressure CO2 significantly reduced the adsorption of PS. Assuming PS adsorption to be irreversible, a kinetic model of PS adsorption was developed and was demonstrated to be consistent with the experimental results at temperatures from 25 to 150C.