铬系催化乙烯选择性齐聚反应机理及新型催化剂开发研究
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摘要
线性α-烯烃是近年来迅速发展的重要化工原料,其中1-己烯和1-辛烯主要作为共聚单体用于生产高性能的高密度聚乙烯和线性低密度聚乙烯,是目前国内急需的产品。相较传统的全分布生产方式,乙烯选择性三聚和四聚生产1-己烯和1-辛烯具有很高的原子利用率,更符合绿色化学的要求。目前制约该领域发展的一个重要原因是乙烯选择性齐聚机理未明,包括反应路径、活性中心金属氧化态和电子自旋态以及配体效应等。
本文首先用基于密度泛函理论的计算方法研究了雪弗龙-菲利普公司的铬／吡咯体系催化乙烯三聚的反应机理。通过比较含有Cr(Ⅰ)/Cr(Ⅲ)和Cr(II)/Cr(IV)氧化还原循环的三聚反应路径中的自由能发现,Cr(Ⅰ)/Cr(Ⅲ)能更好地代表该体系中三聚活性中心的氧化态。同时还发现各个活性物种中的Cr-Cl距离、铬中心和氯原子的电荷随反应进程发生变化,说明氯原子在反应中表现出hemilabile特性,这一特性是该体系实现高1-己烯选择性的关键因素之一。
随后又使用理论计算对Cr-SNS乙烯三聚体系进行了研究。为了阐明实验研究中存在的有关活性中心的争议,本文设计了含有不同金属氧化态和不同SNS配体的催化剂活性中心模型。从三聚反应的自由能垒图中可以看出,该反应的速率决定步骤为铬金属七元环的形成。电子自旋态分析的结果显示反应中的活性物种的基态倾向于不同的自旋多重态。发生在铬金属五元环形成之前的自旋交叉使反应从5重态自由能面转换到3重态自由能面上进行,从而有效降低了反应速率决定步骤的活化能。通过比较各个活性中心模型上反应速率决定步骤的活化能,本文发现含有未去质子化SNS配体的Cr(Ⅰ)/Cr(Ⅲ)配合物最有可能是该体系中的三聚活性物种。
本文后半部分专注于新型乙烯三聚和四聚催化体系的开发。第一个体系的催化剂由Cr(acac)3和正吡咯基二苯基膦配体组成,助催化剂为甲基铝氧烷,体系的主要产物为1-己烯和1-辛烯,反应活性中等。通过改变反应条件(反应溶剂、助催化剂、催化剂浓度、配体/铬比例、反应温度和反应压力),液相产物中1-己烯/1-辛烯的比例可以从0.3调节至20,1-己烯和1-辛烯的选择性最高分别可达91%和74%。当使用脂肪族碳氢化合物为反应溶剂时,加入少量的甲苯能有效提高1-辛烯的选择性,使其成为主要液相产物；而当甲苯含量超过10vol-%后,体系逐渐失去三聚和四聚选择性,液相产物为符合统计学分布的一系列齐聚物。第二个体系的催化剂为一系列结构相似的含有吡啶-氨基膦配体的铬配合物,这些催化剂反应生成各种短链α-烯烃和聚乙烯时表现出不同的选择性和反应活性。配体结构对催化性能有显著影响,通过调节膦官能团上的取代基和配体中P-Cr-N角度可以调控C6或C8烯烃的选择性以及1-己烯和1-辛烯的纯度。含配体PyCH2N(Me)P'Pr2的铬配合物能催化乙烯生成大量的齐聚物和少量聚乙烯,其液相产物中C6和C8烯烃的总选择性高达99%,且1-己烯和1-辛烯的纯度很高。
本文结合实验方法和理论计算来探索和解决化学问题。相比于单独使用实验方法或分子模拟方法,两种方法的有效结合有助于对乙烯选择性齐聚机理进行更加全面和深入的认识。
Since the discovery of ethylene trimerization in1967, selective ethylene oligomerization has stimulated both industrial and academic research for its ability to selectively produce a-olefins for linear low density polyethylene (LLDPE) manufacture. Compared to the conventional process for1-hexene and1-octene production via non-selective ethylene oligomerization, ethylene trimerization and tetramerization are favored for high atom efficiency and reduction of separation costs. Over the last decade, numerous selective ethylene tri-and tetramerization systems have been developed, including the successfully commercialized Chevron-Phillips ethylene trimerization system. However, lack of theoretical understanding of the factors influencing the catalytic performance makes trial and error the main approach in the catalyst development. In this thesis, the intriguing but challenging topic of selective ethylene oligomerization has been studied with both experimental and theoretical approaches.
The first part of the thesis provides some mechanistic insights into two typical ethylene trimerization systems, investigating experimentally unsolved issues exclusively with theoretical methods. For the landmark Chevron-Phillips ethylene trimerization catalyst, a detailed mechanistic study has been carried out by density functional theory (DFT) calculations on a Cr/pyrrole-based model system. Possible reaction pathways are located on the basis of the metallacycle mechanism. Consistent with experimental results, the trimerization route is proven to be energetically preferred compared to ethylene dimerization or further ring expansion of the chromacycloheptane intermediate. From detailed analyses of oxidation states and electronic configurations, the Cr(I)/Cr(III) redox couple is found to be responsible for1-hexene selectivity and all active species involved in the catalytic cycle are constantly under quartet spin state. The importance of the coordination of a pendant chlorine, as was observed in various crystal structures, has been investigated for different intermediates. Bond distances and angles and charge analyses clearly prove a hemilabile behavior of the chlorine, which is considered a key factor for1-hexene selectivity.
In the Cr-SNS ethylene trimerization system, long-term existing debates from experimental evidence, including the oxidation states of the active species and the occurrence of ligand deprotonation, have been examined by DFT methods. Gibbs free energy surfaces of full reaction cycles have been completely located, and formation of chromacycloheptane is identified as the rate-determining step. A detailed spin state analysis reveals that the ground states of the intermediates change along the redox cycle. A spin crossover occurs at the minimum energy crossing point before chromacyclopentane formation, which opens up a much lower energy pathway by spin acceleration. By comparison of the activation energies of the rate-determining step, Cr(I)/Cr(III) complexes bearing non-deprotonated ligands are proposed to be the most plausibly active species, which has been supported by experimental proof. Frontier orbital and natural population analyses have also been carried out to further elucidate the reason for high1-hexene selectivity in this system.
In the second part of the thesis, two novel ethylene tri-and tetramerization systems have been developed and explored. The first system consists of Cr(acac)3, N-pyrrolyldiphenylphosphine ligand and aluminum alkyls as co-catalysts. Upon activation with a co-catalyst, the system is capable of selectively producing1-hexene and1-octene with moderate catalytic activity. The1-hexene/1-octene ratio can be tuned from0.3to20by adjusting the reaction conditions (solvent, co-catalyst, catalyst loading, ligand/Cr ratio, temperature and pressure). And the highest1-hexene and1-octene selectivities can be achieved up to91%and74%, respectively. Furthermore, addition of a small volume percentage of toluene to a system running in aliphatic hydrocarbons has been found crucial to generate1-octene as the dominant liquid fraction, whereas using higher concentrations of toluene gradually switches the system non-selective, producing oligomers with a statistic distribution.
In the second catalytic system, a series of chromium catalysts containing pyridine-phosphine ligands with similar backbones have been synthesized and characterized. These catalysts show varying catalytic activities and selectivities towards the formation of a-olefins and polyethylene (PE). The ligand structure dramatically influences the catalytic behavior. Selectivities towards C6or C8as well as the purity of the1-hexene and1-octene can be controlled by subtle modifications of the substituents on phosphorous or the P-Cr-N bite angles. Ligand PyCH2N(Me)P'Pr2in combination with CrCl3(THF)3has been found to enable selective ethylene tri-and tetramerization, affording1-hexene and1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE.
Combining experiments and theoretical calculations, this thesis provides an example of effective teamwork in exploring and resolving chemical problems. Although both experimental and computational studies can give helpful suggestions on their own, the results presented in this thesis show that a more comprehensive understanding regarding selective ethylene oligomerization can be obtained by merging these two tools.
本文首先用基于密度泛函理论的计算方法研究了雪弗龙-菲利普公司的铬／吡咯体系催化乙烯三聚的反应机理。通过比较含有Cr(Ⅰ)/Cr(Ⅲ)和Cr(II)/Cr(IV)氧化还原循环的三聚反应路径中的自由能发现,Cr(Ⅰ)/Cr(Ⅲ)能更好地代表该体系中三聚活性中心的氧化态。同时还发现各个活性物种中的Cr-Cl距离、铬中心和氯原子的电荷随反应进程发生变化,说明氯原子在反应中表现出hemilabile特性,这一特性是该体系实现高1-己烯选择性的关键因素之一。
随后又使用理论计算对Cr-SNS乙烯三聚体系进行了研究。为了阐明实验研究中存在的有关活性中心的争议,本文设计了含有不同金属氧化态和不同SNS配体的催化剂活性中心模型。从三聚反应的自由能垒图中可以看出,该反应的速率决定步骤为铬金属七元环的形成。电子自旋态分析的结果显示反应中的活性物种的基态倾向于不同的自旋多重态。发生在铬金属五元环形成之前的自旋交叉使反应从5重态自由能面转换到3重态自由能面上进行,从而有效降低了反应速率决定步骤的活化能。通过比较各个活性中心模型上反应速率决定步骤的活化能,本文发现含有未去质子化SNS配体的Cr(Ⅰ)/Cr(Ⅲ)配合物最有可能是该体系中的三聚活性物种。
本文后半部分专注于新型乙烯三聚和四聚催化体系的开发。第一个体系的催化剂由Cr(acac)3和正吡咯基二苯基膦配体组成,助催化剂为甲基铝氧烷,体系的主要产物为1-己烯和1-辛烯,反应活性中等。通过改变反应条件(反应溶剂、助催化剂、催化剂浓度、配体/铬比例、反应温度和反应压力),液相产物中1-己烯/1-辛烯的比例可以从0.3调节至20,1-己烯和1-辛烯的选择性最高分别可达91%和74%。当使用脂肪族碳氢化合物为反应溶剂时,加入少量的甲苯能有效提高1-辛烯的选择性,使其成为主要液相产物；而当甲苯含量超过10vol-%后,体系逐渐失去三聚和四聚选择性,液相产物为符合统计学分布的一系列齐聚物。第二个体系的催化剂为一系列结构相似的含有吡啶-氨基膦配体的铬配合物,这些催化剂反应生成各种短链α-烯烃和聚乙烯时表现出不同的选择性和反应活性。配体结构对催化性能有显著影响,通过调节膦官能团上的取代基和配体中P-Cr-N角度可以调控C6或C8烯烃的选择性以及1-己烯和1-辛烯的纯度。含配体PyCH2N(Me)P'Pr2的铬配合物能催化乙烯生成大量的齐聚物和少量聚乙烯,其液相产物中C6和C8烯烃的总选择性高达99%,且1-己烯和1-辛烯的纯度很高。
本文结合实验方法和理论计算来探索和解决化学问题。相比于单独使用实验方法或分子模拟方法,两种方法的有效结合有助于对乙烯选择性齐聚机理进行更加全面和深入的认识。
Since the discovery of ethylene trimerization in1967, selective ethylene oligomerization has stimulated both industrial and academic research for its ability to selectively produce a-olefins for linear low density polyethylene (LLDPE) manufacture. Compared to the conventional process for1-hexene and1-octene production via non-selective ethylene oligomerization, ethylene trimerization and tetramerization are favored for high atom efficiency and reduction of separation costs. Over the last decade, numerous selective ethylene tri-and tetramerization systems have been developed, including the successfully commercialized Chevron-Phillips ethylene trimerization system. However, lack of theoretical understanding of the factors influencing the catalytic performance makes trial and error the main approach in the catalyst development. In this thesis, the intriguing but challenging topic of selective ethylene oligomerization has been studied with both experimental and theoretical approaches.
The first part of the thesis provides some mechanistic insights into two typical ethylene trimerization systems, investigating experimentally unsolved issues exclusively with theoretical methods. For the landmark Chevron-Phillips ethylene trimerization catalyst, a detailed mechanistic study has been carried out by density functional theory (DFT) calculations on a Cr/pyrrole-based model system. Possible reaction pathways are located on the basis of the metallacycle mechanism. Consistent with experimental results, the trimerization route is proven to be energetically preferred compared to ethylene dimerization or further ring expansion of the chromacycloheptane intermediate. From detailed analyses of oxidation states and electronic configurations, the Cr(I)/Cr(III) redox couple is found to be responsible for1-hexene selectivity and all active species involved in the catalytic cycle are constantly under quartet spin state. The importance of the coordination of a pendant chlorine, as was observed in various crystal structures, has been investigated for different intermediates. Bond distances and angles and charge analyses clearly prove a hemilabile behavior of the chlorine, which is considered a key factor for1-hexene selectivity.
In the Cr-SNS ethylene trimerization system, long-term existing debates from experimental evidence, including the oxidation states of the active species and the occurrence of ligand deprotonation, have been examined by DFT methods. Gibbs free energy surfaces of full reaction cycles have been completely located, and formation of chromacycloheptane is identified as the rate-determining step. A detailed spin state analysis reveals that the ground states of the intermediates change along the redox cycle. A spin crossover occurs at the minimum energy crossing point before chromacyclopentane formation, which opens up a much lower energy pathway by spin acceleration. By comparison of the activation energies of the rate-determining step, Cr(I)/Cr(III) complexes bearing non-deprotonated ligands are proposed to be the most plausibly active species, which has been supported by experimental proof. Frontier orbital and natural population analyses have also been carried out to further elucidate the reason for high1-hexene selectivity in this system.
In the second part of the thesis, two novel ethylene tri-and tetramerization systems have been developed and explored. The first system consists of Cr(acac)3, N-pyrrolyldiphenylphosphine ligand and aluminum alkyls as co-catalysts. Upon activation with a co-catalyst, the system is capable of selectively producing1-hexene and1-octene with moderate catalytic activity. The1-hexene/1-octene ratio can be tuned from0.3to20by adjusting the reaction conditions (solvent, co-catalyst, catalyst loading, ligand/Cr ratio, temperature and pressure). And the highest1-hexene and1-octene selectivities can be achieved up to91%and74%, respectively. Furthermore, addition of a small volume percentage of toluene to a system running in aliphatic hydrocarbons has been found crucial to generate1-octene as the dominant liquid fraction, whereas using higher concentrations of toluene gradually switches the system non-selective, producing oligomers with a statistic distribution.
In the second catalytic system, a series of chromium catalysts containing pyridine-phosphine ligands with similar backbones have been synthesized and characterized. These catalysts show varying catalytic activities and selectivities towards the formation of a-olefins and polyethylene (PE). The ligand structure dramatically influences the catalytic behavior. Selectivities towards C6or C8as well as the purity of the1-hexene and1-octene can be controlled by subtle modifications of the substituents on phosphorous or the P-Cr-N bite angles. Ligand PyCH2N(Me)P'Pr2in combination with CrCl3(THF)3has been found to enable selective ethylene tri-and tetramerization, affording1-hexene and1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE.
Combining experiments and theoretical calculations, this thesis provides an example of effective teamwork in exploring and resolving chemical problems. Although both experimental and computational studies can give helpful suggestions on their own, the results presented in this thesis show that a more comprehensive understanding regarding selective ethylene oligomerization can be obtained by merging these two tools.
引文
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