Production of Hydrogen Peroxide in Liquid CO2. 2. Catalytic Hydrogenation of CO2-philic Anthraquinones

Abstract

Pd-catalyzed hydrogenations of fluoroether-functionalized anthraquinones (FAQs) were conducted in liquid CO2 (P = 235 bar) at room temperature. The kinetics 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 varying the catalyst loading and catalyst particle size. The pressures employed were such that H2, CO2, and FAQ formed a single phase. The 1H NMR analysis of the FAQs after a hydrogenation−oxidation cycle showed no indication for “deep” hydrogenation or degradation of the linker. True kinetic constants, diffusion coefficients, and effective diffusivities were determined by simultaneous regression of the kinetic data. The diffusion coefficients of the FAQs decreased as the length of the fluoroether tail increased. Small catalyst particles and high stirring rates totally eliminated the external transport limitations. The nature of the linking group and the spacer (between the tail and the anthraquinone block) affected the reactivity of FAQs in the hydrogenation process. Using FAQs with relatively short fluoroether tails, we could readily achieve conditions where hydrogenation in CO2 was kinetically controlled.