Recycling nanoparticles stabilized in water-in-CO2 microemulsions for catalytic hydrogenations.

Catalytic hydrogenations of olefins took place effectively in supercritical CO2 with Pd0 nanoparticles dispersed in the fluid phase using a water-in-CO2 microemulsion consisting of water, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant, and 1-octanol as a cosolvent. The hydrogenated products dissolved in supercritical CO2 can be separated from the octanol solution containing AOT microemulsions with Pd0 nanoparticles by phase separation (upper phase, supercritical CO2 with hydrogenated products; lower phase, 1-octanol containing AOT microemulsions with Pd0 nanoparticles) accompanied by reduction of CO2 pressure. After collecting the hydrogenated products by flowing the upper CO2 phase to a collection vessel, the Pd0 nanoparticles remaining in the lower phase can be redispersed into supercritical CO2 by pressurizing the system to a pressure where a homogeneous phase is attained. The redispersed nanoparticles can be reused as catalysts for the next runs of the hydrogenations. Triphenylethylene was hydrogenated to 1,1,2-triphenylethane at conversions of 100% (1st-3rd runs), >99% (4th run), and >96% (5th run) using the recycled Pd0 nanoparticles. The feasibility of using other organic solvents as cosolvents is also studied in the present paper.