Hydrogen peroxide is currently produced via the sequential hydrogenation and oxidation of a 2-alkylanthraquinone. Use of liquid CO2 as the process solvent could ameliorate several environmental and engineering problems inherent to the conventional process. Oxidation reactions of perfluoroether-functionalized anthrahydroquinones (FAQH2s) generated in situ from Pd-catalyzed hydrogenation of functionalized anthraquinones (FAQs) were conducted in a high-pressure batch reactor in liquid CO2. The reaction was found to be first order with respect to both O2 and FAQH2. We have also found that the reactivity of FAQH2 in the oxidation is not affected by the size of the “CO2-philic” tail or the nature of the linker between the aromatic rings and the CO2-philic tail. These results are correlated with our previous conclusions on the phase behavior of FAQs in CO2 and kinetic studies of hydrogenation of FAQs. Finally, an optimum structure of FAQ to be used in the process is proposed.
Pd-catalyzed hydrogenations of fluoroether-functionalized anthraquinones (FAQs) were conducted in liquid CO 2 (P ) 235 bar) at room temperature. The kinetics of the hydrogenation of the FAQs in liquid CO 2 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 H 2 , CO 2 , and FAQ formed a single phase. The 1 H 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 CO 2 was kinetically controlled.
H2O2 production via sequential hydrogenation−oxidation of anthraquinones (AQs) represents a potentially efficient process application of liquid or supercritical CO2 (ScCO2). First, the use of CO2 as the organic solvent in the process will eliminate the mass-transfer limitation during the hydrogenation and oxidation and also the contamination of the aqueous phase during isolation of hydrogen peroxide. Further, the hydrogen peroxide can be recovered from CO2 solution without depressurization by liquid−liquid extraction, helping to minimize the energy cost. The primary obstacle to use CO2 as a working fluid in the process is the poor solubility of conventional AQs in CO2. Therefore, we designed and generated CO2-philic AQs suitable to be used in the production of hydrogen peroxide. Both mono- and difunctionalized anthraquinones (FAQs) were synthesized by attaching CO2-philic polymers chains (−CF(CF3)CF2O−) either to mono- or diaminoanthraquinones or to (hydroxymethyl)anthraquinone. All FAQs synthesized are highly soluble in CO2 and present liquid−liquid phase behavior with minimum miscibility pressure between 170 and 210 bar. Cloud-point pressures were shifted to lower values by using non-hydrogen bonding linkers between AQ block and CO2-philic tails or by increasing the CO2-philic content of FAQs.
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