Study on the Catalytic Performance of Nickel(II) Complexes with Distinct Triazine Support Structures in Ethylene Oligomerization via Different Experiment Designs

Covalent organic frameworks hold great promise for heterogeneous catalysis because of their porous structure for gas adsorption and tunable functionality. Two triazine support materials (MA<i>m</i>PA-COF and MA<i>o</i>PA-COF) were prepared by using melamine as the linked monomer and meta-phthalaldehyde (MPA) or ortho-phthalaldehyde (OPA) as the sub-construction monomer. Two nickel(II) complexes (Ni@MA<i>m</i>PA-COF and Ni@MA<i>o</i>PA-COF) based on the synthesized COFs were prepared to use for ethylene oligomerization. The nickel(II) complexes had good catalytic activities in ethylene oligomerization. Moreover, the substituent position of the aldehyde group in the sub-construction monomer had a certain influence on the specific surface area, morphology and catalytic activity. The morphology of Ni@MA<i>m</i>PA-COF was spherical, while Ni@MA<i>o</i>PA-COF exhibited layered stacking shapes and had a large specific surface area. Ni@MA<i>o</i>PA-COF has a higher catalytic activity and higher selectivity for low-carbon olefins in ethylene oligomerization due to its larger specific surface area and smaller pore width. Ni@MA<i>o</i>PA-COF has good recyclability and still had excellent catalytic activity after three cycles. Based on the gray correlation analysis and single factor experiment, the reaction pressure was the most important factor affecting the activity of Ni@MA<i>o</i>PA-COF in ethylene oligomerization, and the molar ratio of Al/Ni was the main important factor affecting the selectivity.