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.