Ultraviolet stimulation of hydrogen peroxide production using aminoindazole, diaminopyridine, and phenylenediamine solid polymer complexes of Zn(II)

Hydrogen peroxide (H2O2) is a valuable chemical commodity whose production relies on expensive and energy intensive methods. If an efficient, sustainable, and inexpensive solar-mediated production method could be developed from the reaction between dioxygen and water then the use of H2O2 as a fuel may be possible and gain acceptance. When concentrated at greater than 10 M, H2O2 possesses a high specific energy, is environmentally clean, and is easily stored. However, the current method of manufacturing H2O2 via the anthraquinone process is environmentally unfriendly making the unexplored nature of its photochemical production at high concentration from solar irradiation of interest. Towards this end, we studied the concentration and quantum yield of hydrogen peroxide produced in an ultraviolet (UV-B) irradiated environment using solid, Zn(II)-centered, complexes of amino-substituted isomers of indazole, pyridine, and phenylenediamine to catalyze the reaction. Aqueous suspensions in contact with air were exposed to 280–360-nm light from a low-power lamp. Of the ten complexes studied, Zn-5-aminoindazole had the greatest first-day production of 63 mM/day with a 37% quantum yield and p-phenylenediamine (PPAM) showed the greatest long-term stability. Isomeric forms of the catalysts’ organic components (e.g., amino groups) affected H2O2 production. For example, irradiation of diaminopyridine isomers indicated 2,3-diamino and 3,4-diamino structures were the most productive, each generating 32 mM/day H2O2, whereas the 2,5-diamino isomer generated no H2O2. A significant decrease in H2O2 production with time was observed for all but PPAM, suggesting the possibility of a catalyst-poisoning mechanism. We propose a reaction mechanism for H2O2 production based on the stability of the resonance structures of the different isomers.