Theoretical And Experimental Research On The Tri-/tetramerization Of Ethylene Catalyzed By Chromium-based Catalysts

Linearα-olefins are an important type of organic chemical raw materials.The selective oligomerization of ethylene to produce specificα-olefins by Cr-based catalysts is an important research direction that has been rapidly developed in recent years.Among them,the research of catalytic reaction mechanism is important for new and efficient The research and development of catalysts has an important guiding role.By combining theoretical calculations with experimental methods,a deeper understanding of the catalytic reaction process can be obtained.This article mainly carried out research work from the following three aspects:(1)Based on the experimentally reported single crystal structure of the rigid PNP ligand chromium catalyst,this paper constructs three theoretical models of monovalent chromium active centers.Based on density functional theory(DFT),by constructing the potential energy surface of the reaction process,the metal ring reaction mechanism of the three chromium-based catalysts catalyzed by the experimental report on the selective oligomerization of ethylene was studied at the atomic and molecular level,and the determination of the reaction was determined.The quick steps clarify the effect of the change of the ligand substituent on the rigid PNP ligand chromium catalyst.The research results can enable researchers to have a deeper understanding of the reaction mechanism of the selective oligomerization process of ethylene by this type of catalyst,so that the molecular design of the catalyst can be purposefully carried out,which greatly shortens the development cycle of the catalyst and reduces the development cost of the catalyst.(2)Based on the experimentally reported single crystal structure of the rigid PNP ligand chromium-based catalyst,we simplified the catalyst structure,replaced all the substituents on the phosphorus atom with methyl groups,and constructed a catalyst model D.A detailed theoretical study on the selective oligomerization of ethylene catalyzed by the rigid phosphorous ligand chromium catalyst is carried out by density functional theory(DFT).By constructing the potential energy surface of the reaction,the mechanism of the reaction was revealed from the atomic and molecular level,and the rate-determining step of the reaction was determined.On the other hand,the structure of the catalyst was adjusted by introducing silyl methyl groups into the framework of rigid phosphorus ligands,and a new type of rigid silyl methyl phosphorus ligand catalysts(model E)were predicted,and the effect of this catalyst on ethylene was investigated.The reaction mechanism of the selective oligomerization reaction catalyzed process.The results show that the introduction of silyl methyl group is more conducive to obtaining 1-octene.Compared with the simplified model of rigid phosphorous ligand chromium-based catalyst,rigid silylmethyl phosphorous ligand exhibits higher catalytic activity for ethylene tetramerization.(3)Multi-site ligands L1 and L2 were synthesized by using diphenylphosphonium chloride as a raw material and reacting with 2,4-diaminotoluene,3-aminobenzylamine,respectively.The catalytic system composed of these multi-site phosphine ligands,chromium acetylacetonate and methylaluminoxane(MAO)is used to catalyze the selective oligomerization of ethylene.The experimental results show that the catalyst activity of2,4-diaminotoluene and 3-aminobenzylamine catalysts for the selective oligomerization of ethylene and the selectivity of 1-C8~=are better,Both L1 and L2 catalyst systems are beneficial to the selective tetramerization of ethylene to produce 1-octene.Among them,the activity of the Cr/L2/MAO catalytic system can reach 6.58×10~5g/(mol Cr·h),and the selectivity to1-octene is as high as 62.89%.