Ethylene, as an important basic chemical raw material, is mainly used to produce various polymers and copolymers. Catalyst is required in the polymerization of ethylene. Cr-based catalysts which are highly efficient and selective can produce products such as high density polyethylene (HDPE). The preparation, activation and mechanism of Cr-based catalysts have always been the hot research topics in the scientific community and industry.

Cr-based catalysts are prepared by the chromium compounds supported on the oxide carriers with a high specific surface area and large pores. Currently, there are three types of Cr-based catalysts that are most widely used in commercial applications: Cr-based catalyst of Phillips Company, S-2 Cr-based catalyst and S-9 Cr-based catalyst of US UCC company.

The Cr-based catalyst of Phillips Company is made of CrO₃ or other hexavalent chromium compounds supported on silica gel. During the polymerization process, it does not have catalytic activity which requires further processing (such as heating activation and roasting). During the roasting process of the catalyst, Cr⁶⁺ reacts with the hydroxyl groups on the surface of the carrier to produce stable chromate and dichromate structures, which are anchored on the surface of the silica gel. During the process of activation, Cr⁶⁺ is oxidized to Cr⁴⁺ by oxygen or air, and then reduced to the low-valent Cr²⁺ or Cr³⁺ by CO or metal alkyl compounds. This process can be used for one of the three Cr-based catalysts that are used most in commercial applications. With this catalyst, we can produce HDPE with a wide relative molecular mass distribution, and adjust its side chain length and distribution.

The S-2 Cr-based catalyst uses bistriphenylsilyl chromate as the active component. The compound is very stable and can reduce the valence of chromium during the olefin polymerization process to generate aldehydes and Cr²⁺ or Cr³⁺. The preparation process of the catalyst is as follows: make the silane chromate compound supported on the preactivated silica gel; use an alkyl aluminum compound (e.g. diethyl aluminum) to reduce Cr⁴⁺ to the low-valent chromium through preactivation. This catalyst can be used to produce the HDPE with long side chains and broad relative molecular mass distribution for preparing films and pipes.

For the S-9 Cr-based catalyst, chromocene is the active component. The chromocene part is connected to the surface of the dehydroxylated silica gel. During the supporting process, one active group in the cyclopentadienyl ligand is released, with the other part still connected to chromium. With different properties from other catalysts, this catalyst uses the gas phase method to obtain the HDPE with narrow relative molecular mass distribution. The chromium in chromocene has a zero-valence, which is obviously different from the positive sexavalence of CrO₃ and silane chromate, and its catalytic mechanism is also different from these two catalysts.

The supporting and activation process of Cr-based catalysts is a key step that affects its catalytic performance, which mainly includes three stages: supporting, oxidation, and reduction. During the supporting process, chromium interacts with the hydroxyl groups on the surface of the carrier to form stable chromate and dichromate structures, with each Cr⁶⁺ being directly connected to the carrier. During the process of oxidation, Cr³⁺ is oxidized to Cr⁴⁺ by oxygen or air, and then reduced to the low-valent Cr²⁺ or Cr³⁺ by CO or metal alkyl compounds. During the reduction process, Cr⁶⁺ is reduced to low-valent Cr²⁺ or Cr³⁺ by ethylene or alkyl aluminum compounds. Different spectroscopic techniques can be used to characterize the type, valence state and distribution of chromium species, such as infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy.

The polymerization mechanism of Cr-based catalyst refers to the process of reactions such as chain initiation, chain growth, chain transfer, and chain termination between the chromium active center and ethylene. The polymerization reaction of chromium catalyst generally has two stages: induction period and stable period. During the induction period, the Cr-based catalyst undergoes a redox reaction with ethylene to generate low-valent active chromium center and by-products such as formaldehyde or acetaldehyde. At the same time, ethylene also experiences a disproportionation reaction to generate unsaturated hydrocarbons such as propylene or 1-butene. After these reactions, two types of active sites are produced on the catalyst surface: active sites of disproportionation and active sites of polymerization. The active sites of disproportionation can generate products such as 1-butene through β-H elimination or metal cyclization mechanisms, while the active sites of polymerization can generate polyethylene chains through the insertion mechanism. During the stable period, the polymerization reaction rate of the Cr-based catalyst reaches the maximum value and remains unchanged over time. The polymerization reaction rate depends on the concentration of active sites and the reaction environment. The length of the polymer chain is determined by the chain growth reaction rate. It has little to do with the chain transfer reaction rate. Chain growth and chain transfer processes are highly sensitive to the environment of the active chromium center. The chain termination process is generally achieved through the β-H transfer process. It produces a vinyl group at one end of the chain and a methyl group at the other end of the chain. Thanks to the diversity of active species and the change of polymerization reaction rate over time, the supported CrOx catalyst can produce HDPE with wider relative molecular mass distribution than Ziegler-Natta catalyst or metallocene catalyst.

To sum up, this article systematically reviews the catalytic systems, supported activation processes and mechanisms of catalyzing ethylene polymerization of the three most widely used Cr-based catalysts in commercial applications. It analyzes and discusses the Cr-based catalyst in details from three aspects: preparation, activation and mechanism, and introduces the application of different spectroscopic techniques in characterizing the types, valence states and distribution of chromium species, as well as the applications of different theoretical and calculation methods in exploring the structure and reaction path of chromium active center. The supporting and activation process of Cr-based catalysts is a key step that affects its catalytic performance, which mainly includes three stages: supporting, oxidation, and reduction. The polymerization mechanism of Cr-based catalyst refers to the process of reactions such as chain initiation, chain growth, chain transfer, and chain termination between the chromium active center and ethylene. The polymerization reaction of chromium catalyst generally has two stages: induction period and stable period. During the induction period, the Cr-based catalyst undergoes a redox reaction with ethylene to generate low-valent active chromium center and by-products such as formaldehyde or acetaldehyde. At the same time, ethylene also experiences a disproportionation reaction to generate unsaturated hydrocarbons such as propylene or 1-butene. After these reactions, two types of active sites are produced on the catalyst surface: active sites of disproportionation and active sites of polymerization. The active sites of disproportionation can generate products such as 1-butene through β-H elimination or metal cyclization mechanisms, while the active sites of polymerization can generate polyethylene chains through the insertion mechanism. During the stable period, the polymerization reaction rate of the Cr-based catalyst reaches the maximum value and remains unchanged over time. The polymerization reaction rate depends on the concentration of active sites and the reaction environment. The length of the polymer chain is determined by the chain growth reaction rate. It has little to do with the chain transfer reaction rate. Chain growth and chain transfer processes are highly sensitive to the environment of the active chromium center. The chain termination process is generally achieved through the β-H transfer process. It produces a vinyl group at one end of the chain and a methyl group at the other end of the chain. Thanks to the diversity of active species and the change of polymerization reaction rate over time, the supported CrOx catalyst can produce HDPE with wider relative molecular mass distribution than Ziegler-Natta catalyst or metallocene catalyst.

In addition, there are still some problems and challenges in current research on the precise structure of the chromium active center, the intermediates and transition states involved in the polymerization mechanism, and how to control the structure and property of the polymer. We hope the study in future can utilize more advanced experimental and theatrical approaches, and deeply and fully reveal the catalytic mechanism of Cr-based catalyst, providing strong support for the high-performance polyethylene material.

References: Huang Fuling, Gao Yuxin, Li Wenpeng, et al. Progress of Research on the Mechanism of Ethylene Polymerization Catalyzed by Silica Gel Supported Cr-based Catalyst [J]. Synthetic Resins and Plastics, 2023, 40(2):72-77.