Synthesis Of Schiff Base Late Transition Metal Catalysts And Their Catalytic Performance For Ethylene Oligomerization

Alpha-olefin is an important chemical raw material,widely used in lubricating oil,emulsifier,detergent,drilling fluid and other fields.The ethylene oligomerization is the main method for preparingα-olefins,and the key to its core technology is the study of catalysts.Structural factors such as ligand structure and the metal center of the complexes for ethylene oligomerization have a crucial influence on the catalytic performance.However,the research on structure-activity relationship between catalyst structure and catalytic performance of ethylene oligomerization is not exhaustive.Aiming at the above problems,a series of salicylaldimine nickel complexes with different ligand structures,a series of pyridine imine metal complexes with different metal centers and MOF supported pyridine imine nickel catalysts were synthesized in this paper.Combined with related density functional theory calculations,the catalytic performance of as-prepared catalysts for ethylene oligomerization was systematically investigated,and the structure-activity relationship between the catalyst structure and its catalytic performance of ethylene oligomerization was systematically studied.The research results have certain guiding significance for improving the catalytic activity and selectivity of catalyst for ethylene oligomerization.1.In order to study the effect of ligand structure of complexes on its catalytic performance of ethylene oligomerization,ethylenediamine,a series of salicylaldehyde derivatives with different substituents and nickel chloride hexahydrate were used as raw materials to synthesize a series of 10 kinds of Schiff base salicylaldimine nickel complexes containing electron donating groups and electron withdrawing groups on benzene rings.Fourier transform infrared spectroscopy（FT-IR）,hydrogen nuclear magnetic resonance spectroscopy（¹H NMR）,ultraviolet-visible spectroscopy（UV-Vis）,electrospray mass spectrometry（ESI-MS）and other characterization methods were used to confirm that the structure of the synthesized series of Schiff base salicylaldimine ligands and their complexes was consistent with the theoretical structure.The ethylene oligomerization results of the series of catalysts show that the Schiff base salicylaldimine nickel complexes all have good ethylene oligomerization activity,the catalytic activity of nickel complexes containing electron donating groups on the benzene ring of the ligand is higher than that of nickel complexes containing electron withdrawing groups on the benzene ring,and as the steric hindrance of the ortho-substituent of the phenolic hydroxyl group increases,catalytic activity and selectivity of the product to higher carbon number olefins decrease.2.In order to further study the relationship between the ligand structure of the catalyst and its catalytic performance,density functional theory calculations were used to perform molecular simulation calculations on the ethylene oligomerization process catalyzed by 10 Schiff base salicylaldimine nickel complexes.The calculation results of the highest occupied molecular orbital（HOMO）and lowest unoccupied molecular orbital（LUMO）show that the stronger the electron donating ability of the substituents on the benzene ring,the smaller the steric hindrance,the larger the HOMO value,and the larger the energy gap（ΔE）,the more favorable the formation of catalytic active centers and the reduction ofβ-H elimination rate in the reaction,and the catalytic activity would increase,product distribution would be broader.When the ortho substituent of the phenolic hydroxyl group is methyl group,the HOMO value of the nickel complex is-0.063,the LUMO value is-0.192,theΔE is 3.513 e V,and the catalytic activity is16.3×10⁵ g/mol Ni·h;When the ortho and meta positions of the phenolic hydroxyl group are chlorine atoms,the HOMO value of HOMO of nickel complex is-0.089,LUMO value is-0.213,ΔE is 3.368 e V,and catalytic activity is 0.3×10⁵ g/mol Ni·h.3.In order to study the effect of metal center of Schiff base imine transition metal catalysts on catalytic performance,based on the study of Schiff base salicylaldimine nickel complexes,a series of pyridine imine transition metal complexes with different metal centers were synthesized.The results of catalytic ethylene oligomerization show that the Schiff base pyridine imine iron complex has the highest catalytic activity,10.8×10⁵ g/mol Ni·h,and the Schiff base pyridine imine cobalt complex has the lowest catalytic activity,2.5×10⁵ g/mol Ni·h;Schiff base pyridine imine nickel complex has the highest high-carbon olefin content of 28.6%,and Schiff base pyridine imine iron complex has the lowest high-carbon olefin content of 0.2%.The results of density functional theory calculations show that the Schiff base pyridine imine iron complex has the highest energy gap value,and the catalyst structure is the most stable,which is more conducive to the oligomerization reaction;the N-M-N bond angle of Schiff base pyridine imine nickel complex is the smallest,which is beneficial to the growth of the chain.4.Based on the study of homogeneous catalysts,in order to study the influence of the support on the catalytic performance,a type of organometallic framework material supported nickel pyridine imine catalyst（Ni@MOF）was synthesized by hydrothermal method.Characterization methods including scanning electron microscopy（SEM）and inductively coupled plasma ionization（ICP-OES）confirmed that the supported catalyst exhibits a truncated pyramid morphology,and nickel content is about 1.7 wt%.Compared with the homogeneous nickel pyridine imine catalyst,the catalytic activity of Ni@MOF is lower,and the oligomer products are mainly C₆.The kinetic size of MAO（～1 nm）is close to the pore size of Ni@MOF（2.02 nm）.MAO may be interfered by the steric hindrance of the pore wall when it reaches the metal active site,and its catalytic activity will decrease.Simulated and calculated by density functional theory,the volumes of the main intermediates I,II,and III formed during the catalytic process of Ni@MOF are spectively 0.71 cm³/g,0.73 cm³/g,0.76 cm³/g.The pore volume of Ni@MOF（0.75 cm³/g）is slightly larger than that of I and II.Ethylene is easier to insert into I to form II,which is then eliminated byβ-H to form C₆ oligomers.5.Based on the Cossee mechanism,the mechanism of ethylene oligomerization catalyzed by nickel salicylaldimine complexes,metal pyridine imine complexes and MOF-supported nickel pyridine imine catalyst is proposed.In order to further study the microscopic reaction process of ethylene oligomerization,a molecular model of pyridine imine nickel complex and its catalytic ethylene oligomerization reaction path are proposed,and the structure and energy of reaction intermediates and transition states are calculated by DFT density functional theory.The results show that the molecular model of nickel pyridine imine complex has a lower energy barrier for ethylene insertion andβ-H elimination energy barrier,which proves that the molecular model of nickel pyridine imine complex has an active center of ethylene oligomerization.