Polystyrene Hydrogenation in Supercritical Carbon Dioxide-expanded Decahydronaphthalene

The aromatic rings of polystyrene (PS) were hydrogenated in decahydronaphthalene (DHN) expanded with supercritical carbon dioxide (scCO2). Conducting the hydrogenation in scCO2-DHN offers the benefits of improved transport properties, specifically lower solution viscosity and a higher PS diffusion coefficient. Various Pd catalysts are effective for hydrogenating the aromatic rings of PS at temperatures in the region of 150C. Typically hydrogen pressure is 800 psig. When 1250 psig CO2 is added to the Pd-catalyzed PS hydrogenation system, carbon monoxide (CO), a catalyst poison, is formed via the reverse water-gas shift reaction (RWGSR). Previously, when 5% Pd/BaSO4 was used to hydrogenate PS in scCO2-DHN, aromatic ring hydrogenation still occurred after 5 hours. In contrast, when either 5% Pd/SiO2 or 5% Pd/Al2O3 is used, PS hydrogenation ceased within 15 minutes of the start of the reaction in scCO2-DHN. Compared to the Pd/BaSO4-catalyzed reaction, CO concentrations were on the order of 10-fold greater in the Pd/SiO2 counterpart and 20-fold larger with the Pd/Al2O3 system. The dramatic increases in CO concentrations are believed to be a result of the accessibility of catalytic sites to H2 and CO2. In this research, only a fraction of the PS molecules are able to reach the active sites in the interior of the Al2O3 and SiO2-supported catalysts. However, the smaller CO2 and H2 molecules can easily diffuse in and out of the catalysts’ pores and react to form CO. CO can be converted to methane using a Ni or Ru catalyst. Bimetallic catalyst systems containing Pd and a methanation catalyst were used to hydrogenate PS in scCO2expanded DHN. A physical mixture of catalysts reduced CO levels, but no dramatic increases in the degrees of PS hydrogenation were observed. A SiO2 catalyst co-impregnated with Pd and Ru was effective for both PS hydrogenation in scCO2-DHN and reduction of CO levels. INTRODUCTION Carbon dioxide-expanded liquids (CXLs) and supercritical carbon dioxide (scCO2) have been used as reaction media [1, 2, 3]. Oftentimes, chemical reactions in CXLs or scCO2 exhibit higher reaction rates, which are a result of the increased reactant diffusion coefficients and lower solution viscosity. The presence of CO2 also offers a way to tune reaction conditions. Despite these favorable characteristics, there have been reports of losses of selectivity and catalyst poisoning in reactions in CXLs and scCO2. For example, Minder et al. studied 5% Pt/Al2O3-catalyzed ethyl pyruvate hydrogenation at 35C [4]. They reported the cessation of hydrogen consumption upon CO2 addition to the reaction mixture. Their