H₂O₂ in CO₂/H₂O Biphasic Systems:  Green Synthesis and Epoxidation Reactions

Abstract

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 H 2 O 2 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 CO 2 as the solvent. Producing H 2 O 2 directly from H 2 and O 2 in the presence of a CO 2 -soluble Pd catalyst could potentially eliminate entire unit operations and reduce raw material costs significantly. Further, homogeneous reaction in liquid CO 2 allows for contact between significant concentrations of O 2 and H 2, 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 CO 2 -soluble or -dispersible Pd(0) catalyst. Finally, we have found that CO 2 /H 2 O 2 /H 2 O mixtures are, themselves, useful reagent systems. A biphasic aqueous H 2 O 2 /CO 2 mixture is an efficient epoxidizing system, where HCO 4 - is formed through various reactions of water, CO 2, and H 2 O 2 and transfers of oxygen to alkenes. From our results, it appears that H 2 O 2 can react directly with CO 2, producing more percarbonate ion than in situations that employ only bicarbonate as the precursor.