Catalytic Performance Of Transition Metal Catalyst Anchoring Via Covalent Binding Of PAMAM In Multiwalled Carbon Nanotubes For Ethylene Polymerization

Catalysts play an important role in olefin polymerization,and late transition metal catalysts are used widely in olefin polymerization due to the advantages with easy synthesis,low cost,better tolerance to polar groups and lower oxygen affinity.However,the late transition metal homogeneous catalysts used in olefin polymerization have the disadvantages with the difficulty in separation,poor thermal stability and poor recycling.Based on the problem,series of salicylaldimine nickel homogeneous catalysts with active hydroxyl groups were synthesized in the paper,and then these homogeneous catalysts were anchored covalently to carbon nanotubes to obtain series of carbon nanotube supported transition metal catalysts with good thermal stability and recyclability.The catalytic performance of series of homogeneous catalysts and series of heterogeneous catalysts in ethylene polymerization were investigated to explore the loading effect and the application of the carbon nanotube-supported transition metal catalysts in ethylene polymerization.1.Series of diamine-bridged hydroxyl salicylaldimine ligands（（AD-L1）-（AD-L3））and series of dendrimer-bridged hydroxyl salicylaldimine ligands（PAMAM-L1）-（PAMAM-L3）)were synthesized by Schiff base reaction with aliphatic diamine and low-generation dendrimers of amine terminal groups as the ligand skeleton and with 4-hydroxysalicyaldehyde as raw material,and their yields were above 60%.Series of nickel catalysts based on diamine-bridged hydroxyl salicylaldimine（（AD-C1）-（AD-C3））and series of nickel catalysts based on dendrimer-bridged hydroxyl salicylaldimine（（PAMAM-C1）-（PAMAM-C3））were synthesized by coordination reactions with the synthesized hydroxyl salicylaldimine ligands and nickel acetate tetrahydrate as raw materials,and their yields were above 78%.The structure and thermal stability of the synthesized ligands and the synthesized catalysts were characterized by elemental analysis,FT-IR,UV-vis,¹H NMR,¹³C NMR,XRD and TG.And the results showed that their structures were consistent with the designed structure and the ligand skeleton had an effect on the thermal stability.2.The catalytic results of aliphatic diamine-bridged hydroxyl salicylaldimine nickel catalysts（AD-Cn）with MAO as cocatalyst and cyclohexane with solvent showed that the catalysts had the good catalytic activity for ethylene polymerization.The main oligomers were low carbon number olefins and no polyethylene produced.Moreover,the oligomer distribution was wide.With the increase of the length of the alkyl chain in the ligand skeleton,the catalytic activity decreases slightly,and the selectivity of C₈⁺increases slightly.When the amount of Ni for AD-C2 was 5μmol,Al/Ni was 500,the reaction temperature was 25℃,the ethylene pressure was 0.7MPa and the reaction temperature was 25℃,the catalytic activity AD-C2 was 1.99×10⁵ g/（mol Ni·h）,and the selectivity of C₈ and C⁺₁₀ products were 61.25%.When the amount of Ni for PAMAM-C1 was2μmol,the Al/Ni ratio was 500,the reaction temperature was 25℃and the ethylene pressure was0.5MPa,series of nickel catalysts based on the PAMAM-bridged hydroxyl salicylicaldimine（PAMAM-C1）-（PAMAM-C3）had the good catalytic activity.The catalytic activity of PAMAM-C1was up to 4.44×10⁴ g/（mol Ni·h）and the main oligomers were C₄ and C₆ olefin.Moreover,the catalytic activity increases slightly with increasing of the length of the alkyl chain in the ligand skeleton.3.Two types of carbon nanotube supported nickel catalysts（（CNT-AD-C1）-（CNT-AD-C3）and（CNT-PAMAM-C1）-（CNT-PAMAM-C3））were synthesized with the above homogeneous catalysts with reactive hydroxyl groups as raw materials and carbon nanotubes as support.The results showed that the homogeneous catalysts were covalently loaded on carbon nanotubes were confirmed by the characterization of elemental analysis,FT-IR,TG,SEM,et al.The polymeric result of series of the synthesized heterogeneous cataysts in ethylene polymerization showed that and CNT-PAMAM-Cn had the good catalytic activity.The catalytic activity and the selectivity of high carbon decreased with increasing of the length of the alkyl chain in the bridging group for CNT-AD-Cn.However,the catalytic activity decreased and the selectivity of high carbon increased with increasing of the alkyl chain in the bridging group for CNT-PAMAM-Cn.The optimized process of ethylene polymerization catalyzed by CNT-AD-C2 showed that when the Ni dosage for CNT-AD-C2 was 5μmol,Al/Ni was 500,the reaction temperature was 35℃and the ethylene pressure was 0.7MPa,the catalytic activity was 5.25×10⁵ g/（mol Ni·h）,and the total selectivity of C₈ and C₁₀+products were 53.42%.The results of the optimization ethylene polymerization process of CNT-PAMAM-C1 showed that when the Ni dosage for CNT-PAMAM-C1 was 4μmol,Al/Ni was 500,reaction temperature was 25℃and ethylene pressure was 0.7MPa,the catalytic activity was 3.75×10⁴ g/（mol Ni·h）,and the selectivity of C₈ and C₁₀+was 3.86%.Under the same conditions,the catalytic activity of CNT-PAMAM-C3 was 2.34×10⁴ g/（mol Ni·h）,and the selectivity of C₈ and C₁₀+were 17.43%.The selectivity of the supported catalyst to high carbon had been improved due to the special pore structure of carbon nanotubes.The purpose of separating the supported catalyst from product by filtering only,which effectively improved the recyclability.