Genesis of MgCl₂‐based Ziegler‐Natta Catalysts as Probed with Operando Spectroscopy

Abstract

Ziegler-Natta catalysts for olefin polymerization are intrinsically complex multi-component systems. The genesis of the active sites involves several simultaneous and sequential steps, making the individual steps and interconnections difficult to be unraveled in an unambiguous manner. In this work, we combine X-ray diffraction and spectroscopy to probe each step of the birth and life of a MgCl -based Ziegler-Natta catalyst, namely the formation of high surface area MgCl by dealcoholation of an alcoholate precursor, the TiCl grafting, and the subsequent activation by triethylaluminum as co-catalyst. The so-prepared catalyst was tested towards ethylene polymerization, leading to the production of mainly crystalline high-density polyethylene. The use of operando characterization techniques allowed probing the transient details that are difficult to be dissected in the aftermath, but can radically affect the overall catalytic process.

Disclosing the Interaction between Carbon Monoxide and Alkylated Ti3+ Species: a Direct Insight into Ziegler–Natta Catalysis

“…Spectroscopic techniques have the potential to make up for this lack, unveiling the different species on the catalyst and monitoring their evolution during the catalyst synthesis and under reaction conditions. This is valid in general for heterogeneous catalysts, and it has been recently demonstrated to be a valuable approach also for characterizing Ziegler–Natta catalysts through a step-by-step approach, so as to isolate the contribution of each component, thus overcoming the intrinsic complexity of the catalyst . In this respect, FT-IR spectroscopy of adsorbed CO has been proved to be a powerful technique to probe the MgCl 2 surface as well as the distribution of the titanium species in Ziegler–Natta precatalysts, − and more recently it has been used to assess the overall condition of some industrial Ziegler–Natta catalysts and predict their effectiveness in catalyzing olefin polymerization …”

“…The catalyst was synthesized as discussed in ref , where each step of the synthesis was characterized by multiple spectroscopic techniques. Briefly, a MgCl 2 –6MeOH precursor was prepared by azeotropic distillation and successively dealcoholated under a dynamic vacuum at 200 °C to get a high-surface-area MgCl 2 (100 m 2 /g) .…”

“…This is valid in general for heterogeneous catalysts, 13 and it has been recently demonstrated to be a valuable approach also for characterizing Ziegler−Natta catalysts through a step-by-step approach, so as to isolate the contribution of each component, thus overcoming the intrinsic complexity of the catalyst. 14 In this respect, FT-IR spectroscopy of adsorbed CO has been proved to be a powerful technique to probe the MgCl 2 surface as well as the distribution of the titanium species in Ziegler−Natta precatalysts, 15−17 and more recently it has been used to assess the overall condition of some industrial Ziegler−Natta catalysts and predict their effectiveness in catalyzing olefin polymerization. 18 Potentially, the use of a CO probe might be further exploited to get direct information on the titanium active sites and their structure and reactivity, as widely done for many other heterogeneous catalysts, 19,20 including Cr-based catalysts for olefin polymerization.…”

Monitoring the Kinetics of Internal Donor Clean-up from Ziegler–Natta Catalytic Surfaces: An Integrated Experimental and Computational Study

“…MgCl 2 -supported Ziegler–Natta catalysts (ZNCs) dominate the ever-growing production of isotactic polypropylene, thanks to an exceedingly high stereoselectivity (isotacticity index I.I. > 98%) and a productivity in the order of several tons of polymer per gram of Ti. − These systems consist of a solid precatalyst and a soluble cocatalyst: the former is typically obtained by reacting a MgCl 2 precursor with TiCl 4 in the presence of a first Lewis base (“internal donor”, ID), whereas the latter consists of AlEt 3 and a second Lewis base (“external donor”, ED). , Surface modification by the electron donors is essential for the stereoselectivity; decades of experimental − and computational modeling − studies demonstrated that ID and ED molecules, chemisorbed in the vicinity of the active Ti sites, shape the catalytic pocket similarly to the ancillary ligands of molecular catalysts.…”

Synthesis of high-performance polyethylene

“…图1 典型的前过渡催化剂体系 [9~17] Figure 1 Typical transition metal catalyst system [9][10][11][12][13][14][15][16][17] 1995年，Brookhart课题组 [18] 发明的含大位阻取代基团的α-二亚胺钯系、镍系催化剂，以高活性制 备了从线性到超支化、再到极性单体共聚的一系列聚乙烯，解决了之前后过渡金属催化剂只能催化乙 烯寡聚的问题。与前过渡金属催化剂不同，后过渡金属催化剂在制备支化聚烯烃时，可通过-链行走‖ 机理，以乙烯为唯一单体制备支化甚至超支化聚烯烃(图2a) [19~21] 。与前过渡金属相比，Ni、Pd等 后过渡金属具有更低的亲氧性，对O、S、N等杂原子具有容忍性，不易被毒化，因而具有良好的稳定 性。Mecking课题组 [22,23] 通过配体修饰的后过渡镍系催化剂首次实现了聚烯烃的水相聚合；并基于乳 液聚合的思想，将生长的分子链利用乳化效应彼此分开，进而获得了理想的聚乙烯纳米晶体，结晶度 可达97%(图2b)。 然而，后过渡金属催化剂在进行极性单体共聚时催化剂活性不够高、分子量大幅 下降，产品应用价值欠佳；催化剂热稳定性差，在60℃以上的聚合反应中很快失活。对于强放热的乙 烯聚合过程，如此低的聚合温度无法满足工业生产时反应热撤热的要求，这也限制了这类催化剂的工 业应用。Guan课题组 [24,25] 报道了含环番基团的α-二亚胺镍催化剂，催化剂的热稳定性和聚合物的分子 量与经典的α-二亚胺镍催化剂相比都有显著提升 (图2c) 。 Brookhart课题组 [26] 等人 [38] 采用离子交换技术将VCl 3 负载在金属有机框架材料(MOF)的金属节点上。MOF框架的骨架 结构实现了V活性中心的单分子负载，聚乙烯产物分子量分布可缩小至3.0。Diesing等人 [39] 采用交联 聚苯乙烯(CL-PS)微球负载茂金属催化剂。CL-PS在甲苯中溶胀，形成了众多由溶剂小分子填充的 空穴。这些空穴为聚乙烯的链增长提供了弹性空间，构建了拟均相的聚合微环境，可窄化聚合产物的 分子量分布。空穴周围的CL-PS分子链能够控制乙烯传质速率，使催化剂活性平稳释放。 图3 非均相催化剂活性中心的多样性 [35~37] Figure 3 The heterogeneity of active species in the heterogeneous catalyst [35~37] 目前，非均相催化剂的工作大多集中于通过活性中心负载微环境设计，调控聚烯烃分子结构，极 少涉及聚集态结构的调控，这在一定程度上限制了聚烯烃高性能化的发展。Rastogi课题组 [40~42] 将FI 催化剂负载于纳米无机颗粒表面(单壁纳米碳管和TiO 2 ) 。纳米颗粒上有限的负载位能够降低链交叠 的发生概率；同时，25°C的聚合温度保证了初生链段的快速结晶，有利于降低初生态聚乙烯的链缠 结程度 [43] 。该过程避免了反应器粘壁现象，节省了助催化剂用量。低缠结的超高分子量聚乙烯 (UHMWPE)能在低于熔点温度下(125~145°C)进行单轴超倍拉伸(188倍) [44,45] 。然而，为保证 纳米颗粒在聚合溶剂中良好的分散，纳米颗粒需经复杂的改性过程，大大增加了催化剂的制备成本和 难度。此外，受制于纳米颗粒几何结构、分散性和有限的负载位等因素，催化剂大多负载于纳米粒子 团聚体的表面，且负载量低，致使催化剂的活性低、产物灰分高(载体残留量达3wt%~10wt%) 。本 课题组 [46~49] Figure 5 Reverse deduction method for the analysis of active species [50~52] 红外光谱和拉曼光谱等振动光谱对物质的原子类型、化学键、几何构型等特点极其敏感，对极低 浓度的物质也有很好的响应，可以在较低的活性中心负载浓度下解析活性中心的多样性，是近年来逐 步发展起来的活性中心结构解析手段。Potapov等人 [61] 同时采用漫反射红外光谱和拉曼光谱，以 MgCl 2 /TiCl 4 二元催化体系和 MgCl 2 /TiCl 4 /ID三元体系为对象，系统研究了给电子体的作用机理。以邻 苯二甲酸二丁酯(DBP)体系为例，其主要特征吸收峰为苯环C=O伸缩振动区间1650~1700 cm -1 和邻 苯二甲酰氯化物C=O伸缩振动峰1750 cm -1 。当三乙基铝(TEA)活化后，1750 cm -1 处邻苯二甲酰氯 化物峰强度大幅下降，表明该物质基本被TEA除去。与此同时，1700 cm -1 处吸收峰位置发生红移， 表明有不同结构的DBP-M复合物生成，这与拉曼光谱观察到的实验结果一致。近些年，Groppo课题 组 [62,63] 采用直接钛化法，以代表 MgCl 2 /TiCl 4 /乙醇体系的表面模型催化剂为研究对象，成功跟踪了从…”