H2O2 in CO2/H2O biphasic systems:: Green synthesis and epoxidation reactions

Hydrogen peroxide is a "green" oxidant whose relatively high cost has prevented it from being generally applied to commodity chemical processing. In large part, the attributes of the current H2O2 process (the anthraquinone, or AQ, route) that contribute to its high cost also contribute to its nonsustainable features: byproduct streams, high energy input and solvent usage, and multiple required unit operations. We have explored the generation of hydrogen peroxide directly from hydrogen and oxygen using liquid CO2 as the solvent. Producing H2O2 directly from H-2 and O-2 in the presence of a CO2-soluble Pd catalyst could potentially eliminate entire unit operations and reduce raw material costs significantly. Further, homogeneous reaction in liquid CO2 allows for contact between significant concentrations of O-2 and H2, high rates of reaction, and ready recovery of the product via stripping into water. Both Pd(+2) and Pd(0) catalysts were explored for the reaction; our results suggest that future work should focus on the optimization of a CO2-soluble or -dispersible Pd(0) catalyst. Finally, we have found that CO2/H2O2/H2O mixtures are, themselves, useful reagent systems. A biphasic aqueous H2O2/CO2 mixture is an efficient epoxidizing system, where HCO4- is formed through various reactions of water, CO2, and H2O2 and transfers of oxygen to alkenes. From our results, it appears that H2O2 can react directly with CO2, producing more percarbonate ion than in situations that employ only bicarbonate as the precursor.