Direct synthesis of hydrogen peroxide from hydrogen and oxygen: An overview of recent developments in the process

Hydrogen peroxide (H2O2) is an important commodity chemical and its demand is growing significantly in the chemical synthesis due to its “green” character. Currently, H2O2 is produced almost exclusively by the anthraquinone auto-oxidation (AO) process. The AO process involves indirect oxidation of hydrogen and thus avoids potentially explosive H2/O2 mixture. However, this large-scale process presents significant safety issues associated with the transport of bulk H2O2. Moreover, the AO process can hardly be considered an environmentally friendly method. In view of this, more economical and environmentally cleaner routes have been explored for the production of H2O2. The liquid-phase catalytic direct synthesis of H2O2 from H2 and O2 offers an attractive green technology for small-scale/on-site production of H2O2. However, the direct synthesis process suffers from two major drawbacks: (i) potential hazards associated with H2/O2 mixtures and (ii) poor selectivity for H2O2 because the catalysts used for H2O2 synthesis are also active for its decomposition and hydrogenation to water as well as for H2 combustion. These serious issues and the recent developments in the direct H2O2 synthesis are discussed in this review. The roles of protons (H+) and halide ions in promoting the H2O2 selectivity are also examined in detail.