醚类给电子体对Cr/PNP乙烯选择性齐聚体系催化反应性能的影响

Effects of Ether Electron Donors on Cr/PNP-Catalyzed Ethylene Selective Oligomerization

采用实验和DFT计算相结合的方法,研究了4种醚类给电子体对Cr/PNP乙烯齐聚催化体系的影响.实验结果表明, C_6+C_8选择性和聚乙烯的选择性受给电子体种类影响各异. DFT计算表明,添加乙醚、甲缩醛、二噁烷和乙二醇二甲醚给电子体后,反应的速率决定步骤均从两分子乙烯氧化偶联成金属五元环转移到第四分子乙烯插入铬金属七元环.给电子体乙二醇二甲醚和甲缩醛的两个氧原子与铬中心在反应过程中发生单/双配位交替变化,其环状结构的大小和稳定性影响乙烯分子插入难易程度,从而影响反应选择性和活性.醚类给电子体对乙烯齐聚反应的影响是电子效应和位阻效应的协同作用,但位阻效应更加明显.另外,在甲基环己烷和甲苯两种溶剂下,乙烯齐聚体系能垒差小于1.5 kJ·mol^(-1),在本体系中可以忽略甲基铝氧烷(MAO)中微量甲苯对反应性能的影响.

1-Hexene and 1-octene,which are used to synthesize high performance polyolefin products,are important chemical raw materials with rapid development in recent years.The catalytic system based on PNP ligand with Cr active site has excellent ethylene oligomerization performance.The effects of the four ether compounds on the Cr/PNP system were studied by combined experiment and DFT calculation.The total selectivity of C6+C8 increased with the addition of 1,2-dimethoxyethane and dimethoxymethane,while the selectivity of polyethylene decreased with diethyl ether and dioxane.The reactivity and selectivity remained at a high level in the presence of dioxane or dimethoxymethane.DFT results showed the electron donors led to the shift of the rate-limiting step from the oxidative coupling of two coordinated ethylene molecules to metallacycle growth by insertion of a fourth coordinated ethylene.Alternating changes of single and double coordination occurred between two oxygen atoms of 1,2-dime-thoxyethane/dimethoxymethane and chromium center during the reaction.The size and stability of the ring formed by the coordination with chromium affect the ethylene insertion.Thus,the effects of ethers on ethylene oligomerization was a synergic effect of the electronic and steric effects.By calculating the buried volume percentage(%Vbur)with online software SambVca2,the steric hindrance of the ethers increased in the order of 1,2-dimethoxyethane>dioxane>diethyl ether>dimethoxymethane.Both the Solvent Model Density(SMD)model and the Real Solvent Conductor-like Shielding(COSMO-RS)model calculations showed that the energy barrier difference of ethylene oligomerization system in methylcyclohexane and toluene was less than 1.5 kJ·mol-1,suggesting the trace toluene in MAO might be neglected in this system.