**4. Conclusions**

Five kinds of transition metal-modified TS-1 catalysts were prepared and employed to catalyze the ammoximation of MEK. The catalytic activity of various transition metal-modified TS-1 was diverse. Compared with TS-1 and other M-TS-1, 3 wt % Ni-TS-1 showed the best catalytic activity. The conversion of MEK and the selectivity of MEKO reached 99.0% and 99.3%, respectively. Furthermore, the catalyst was fully recovered without considerable decrease of MEK conversion and MEKO selectivity. The characterization of Ni-TS-1 showed that the introduction of nickel species did not change the skeleton structure of TS-1. Meanwhile, the nickel entering the framework of TS-1 e ffectively changed the electron environment of the Ti active sites and increased the electrophilicity of TS-1. In addition, the surface nickel species reduced the surface acidity of the catalyst, inhibiting the deep oxidation of the MEKO. We believe the obtained metal-modified TS-1 catalyst is very promising for the ammoximation of MEK and relative reactions for industrial applications.

**Author Contributions:** S.J. and X.X. conceived and designed the experiments and guided the research. D.Y., H.W., W.W., S.P., and X.Y. performed characterization and catalytic studies. All authors analyzed and discussed the results. D.Y., S.J., and X.X. analyzed the data and wrote the paper.

**Funding:** This research received no external funding.

**Acknowledgments:** This research was financially supported by the science and technology development plan of Shandong Province (2013GZX20109)**.**

**Conflicts of Interest:** The authors declare no conflict of interest.
