Direct Synthesis of Hydrogen Peroxide from Oxygen and Hydrogen Using Carbon Dioxide as an Environmentally Benign Solvent and Its Application in Green Oxidation

摘要

Based on the concept of green chemistry, a new green chemistry metric was developed with the consideration of both quantity and quality (hazard) of chemicals involved in the synthesis of a chemical product. This new metric was used to evaluate the greenness of various synthetic methods in this study. Hydrogen peroxide (H2O2), a widely used green oxidant, is currently produced by an anthraquinone auto-oxidation (AO) process. This AO process is not environmentally benign because it generates wastes, uses large volumes of organic solvents, and has low efficiency and high energy consumption. Direct synthesis of H2O2 from O2 and H2 is a green technology to replace the AO process since it is the most atomic-efficient method by which H2O2 can be synthesized. Compressed CO2 has the potential to replace the conventional solvents used in this direct synthesis process. In this study, a method was developed to measure the amount of directly synthesized H2O2 in compressed CO2 over precious metal loaded titanium silicalite (TS-1) molecular sieve by using a selected indicator to react immediately with the in situ generated H2O2. The experimental results proved, for the first time, that H2O2 could be effectively generated from O2 and H2 using CO2 as the solvent. This in situ generated H2O2 in CO2 was then used in the epoxidation of propylene to propylene oxide (PO). A PO yield over 20% with vital selectivity was achieved, for the first time, using compressed CO2 as the solvent and small amounts of polar co-solvents. The addition of a selected weak base inhibitor effectively suppressed a number of common side-reactions, including the hydrogenation of propylene, the hydrolysis of PO and the reaction between PO and methanol.The oxidation of cyclohexene by in situ generated H2O2 in compressed CO2¡ªan alternative and green approach to the synthesis of adipic acid was also explored using precious metal loaded TS-1 and W-MCM-41 (a silicate mesoporous molecular sieve). Experimental results showed that the oxidation of cyclohexene by in situ generated H2O2 in CO2 was limited by the small pore size of TS-1, while W-MCM-41 was an effective catalyst in carrying out this reaction.