Reaction Pathways in the Oxidation Decomposition of Hydroxylamine over a Titanosilicates/H2O2 System

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 NH3 and 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/H2O2 system. 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 H2O2 and the free radicals formed by the activation of H2O2 with nonframework titanium species to produce N2O and N2 and (b) by active intermediate Ti-OOH species formed by the activation of H2O2 with 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 H2O2 determines 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/H2O2 ammoximation system to achieve a greener chemical process.