*2.4. Mechanism Analysis*

The results of characterization illustrated parts of modified Ni were incorporated into the TS-1 skeleton as replacements of some Si sites and interacted with Ti active site and improved the electrophilicity of the Ti center [26], which is beneficial to adsorb H2O2 in the reaction system. the mechanism of liquid-phase ketoammoximation is hydroxylamine mechanism [36]. According to literature reports and our experimental results, we proposed the mechanism of Ni-TS-1 catalyzed the ammoximation of MEK (Scheme 2). Firstly, the four-coordinated titanium species combines with NH3·H2O to become the six-coordinated state, further forming titanium peroxide under the action of H2O2. Then titanium peroxide reacts with NH3·H2O to form ammonium peroxide, which would easily release NH2OH. Eventually, NH2OH would react with MEK to produce MEKO by non-catalytic oxidation [37]. The addition of Ni reduced the electron cloud density of the titanium active center, improved its electrophilicity, enhanced the adsorption capacity for H2O2, and thus increased the reaction rate of the control step of ketoammoximation, i.e., the formation rate of hydroxylamine. In addition, the increase of basicity of the Ni-modified catalyst was beneficial to controlling the alkaline environment of the reaction system and e ffectively inhibiting the occurrence of side reactions. Therefore, Ni-TS-1 exhibited a good catalytic e ffect in the ammoximation of MEK.

**Scheme 2.** Reaction mechanism of ammoximation of MEK catalyzed by Ni-TS-1.
