Production of hydrogen peroxide in liquid CO2. 2. Catalytic hydrogenation of CO2-philic anthraquinones

Pd-catalyzed hydrogenations of fluoroether-functionalized anthraquinones (F AQs) were conducted in liquid CO2 (P = 235 bar) at room temperature. The ki netics of the hydrogenation of the FAQs in liquid CO2 was investigated in a high-pressure batch reactor under a 10-fold excess of hydrogen, while vary ing the catalyst loading and catalyst particle size. The pressures employed were such that H-2, CO2, and FAQ formed a single phase. The H-1 NMR analys is of the FAQs after a hydrogenation-oxidation cycle showed no indication f or "deep" hydrogenation or degradation of the linker. True kinetic constant s, diffusion coefficients, and effective diffusivities were determined by s imultaneous regression of the kinetic data. The diffusion coefficients of t he FAQs decreased as the length of the fluoroether tail increased. Small ca talyst particles and high stirring rates totally eliminated the external tr ansport limitations. The nature of the linking group and the spacer (betwee n the tail and the anthraquinone block) affected the reactivity of FAQs in the hydrogenation process. Using FAQs with relatively short fluoroether tai ls, we could readily achieve conditions where hydrogenation in CO2 was kine tically controlled.