Reaction Pathways in the Oxidation Decomposition of Hydroxylamine over a Titanosilicates/H₂O₂System

The green synthesis of oxime in a titanosilicates/H₂O₂(TS/H₂O₂) system is based on the synergistic effect of the in situ synthesis of hydroxylamine (NH₂OH) by the oxidation of NH₃and the subsequent noncatalytic oximation reaction. However, NH₂OH has a highly unstable character, leading to its easy decomposition to release greenhouse gas N₂O. Herein a comprehensive investigation was carried out to clarify the mechanism of NH₂OH decomposition in the -TS/H₂O₂system. The reaction pathway network with NH₂OH oxidation decomposition was established, and effective ways to inhibit the formation of byproduct N₂O were proposed. We proposed that the oxidative decomposition of NH₂OH was divided into two main competitive pathways: (a) by H₂O₂and the free radicals formed by the activation of H₂O₂with nonframework titanium species to produce N₂O and N₂and (b) by active intermediate Ti-OOH species formed by the activation of H₂O₂with framework titanium species to product only inert N₂. We found that promoting the formation Ti-OOH species by enhancing the Lewis acidity strength of the titanosilicates could significantly inhibit the pathway of NH₂OH oxidation decomposition to N₂O. Meanwhile, proper additives to stabilize Ti-OOH species through hydrogen bonding would achieve the same effect. The reaction behavior of H₂O₂determines the types of products of NH₂OH oxidation decomposition. By analyzing the decomposition pathways of NH₂OH deeply, we could realize clearly how the products of NH₂OH oxidation decomposition were generated, which can be applied to the actual industrial production of the TS/H₂O₂ammoximation system to achieve a greener chemical process.