Effect of pore size and nickel content of Ni-MCM-41 on catalytic activity for ethene dimerization and local structures of nickel ions

The catalytic activity of nickel ion-loaded mesoporous silica MCM-41 (Ni-M41) for ethene dimerization was investigated as a function of the pore size and the amount of nickel. In addition, the silica wall and the loading of the nickel species were characterized. The Ni-M41 samples with smaller pore size and higher Si/Ni ratio exhibited greater reaction rate constants. The Fourier transform infrared (FT-IR) spectra indicated the formation of 2:1 nickel phyllosilicate-like species along the pore wall. Furthermore, the IR band at approximately 570 cm ^-1 and the X-ray absorption fine structure (XAFS) spectra indicated the existence of five-membered rings consisting of Si-O on the M41 pore wall in addition to the typical six-membered ones. On the basis of the UV-vis-NIR diffuse reflectance (UV-vis-NIR DR), FT-IR, and XAFS data, we propose that the three- and four-coordinated Ni ²⁺ ions lie on the five- and six-membered Si-O rings of silica, respectively. Nitrogen monoxide was employed as a probe molecule in the FT-IR and UV-vis-NIR DR experiments and revealed that NO adsorbed as di- and mononitrosyl species on the three- and four-coordinated Ni ²⁺ ions. The intensity of the dinitrosyl species on the three-coordinated Ni ²⁺ ions correlated with the catalytic activity for ethene dimerization. Therefore, the three-coordinated Ni ²⁺ ions are proposed to act as the active site for the reaction.