The teams led by Kai Wu and Xiong Zhou at the College of Chemistry and Molecular Engineering, Peking University, and Yongwang Li at Synfuels China Technology Co., Ltd. have employed, for the first time, innovative technology in surface chemistry for visualizing ethylene polymerization on the surface of ordered iron carbide at the molecular level. The key mechanistic steps in ethylene polymerization have been clarified, including chain initiation via molecular isomerization and chain propagation via molecular insertion. The research results were published online by the journal Science on March 10, 2022 (https://www.science.org/doi/10.1126/science.abi4407), with the title "Visualization of on-surface ethylene polymerization through ethylene insertion". Science also published a perspective by Prof. Joost Wintterlin entitled "Growing polymers, caught in the act" in the same issue (https://www.science.org/doi/10.1126/science.abo2194).
Polyethylene is the key raw material in the manufacture of plastic products, with a yearly output of 100 Mt worldwide, and is widely involved in our daily lives. Ethylene polymerization, which is divided into free radical polymerization and coordination polymerization, is a typical chain polymerization reaction that includes the fundamental steps of chain initiation, propagation, and termination. In coordination polymerization, chain propagation follows the Cossee-Arlman mechanism, that is, ethylene is inserted into the metal-carbon bond that connects the growing polyethylene chain, while the metal center in the catalyst facilitates monomer polymerization. Achieving chain initiation over Phillips catalysts for ethylene polymerization without alkyl aluminum as the initiator has been a long-lasting fundamental issue that eluded the polymer science community.
With in situ visualization technology, ethylene polymerization can be studied in real time, providing a great impetus for uncovering the ethylene polymerization mechanism. However, the chain anchored on the surface of the catalyst surface, through the active reaction center, which subsequently grows rapidly unidirectionally, makes it difficult to visually characterize well-ordered surfaces at the sub-molecular level. In combination with the long-term advanced technological accumulation in surface science by Kai Wu's group and the rich understanding of the Fischer-Tropsch synthesis by Li Yongwang's group, both groups became keenly aware that the Fischer-Tropsch synthesis could be envisioned as a polymerization of CH2, and its catalyst iron carbide could serve as a model catalyst for the exploration of on-surface ethylene polymerization. Thus, it is possible to visualize on-surface ethylene polymerization on a molecular scale.
The selected substrate was an atomically flat iron carbide thin film fabricated through carbon aggregation from a bulk Fe(110) single crystal. The iron carbide surface consisted of parallel domain strips separated by narrow boundary strips. Using state-of-the-art high-resolution scanning tunneling microscopy (STM), angle-resolved X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations, the domain structure was determined to best resemble the θ-Fe3C(102) surface. Subsequently, in situ STM images of the carburized Fe(110) surface under 1 × 10−8 mbar of ethylene (C2H4) at room temperature were continuously recorded at a rate of one frame per 35 s (Fig. 1). Initially, the protrusions appeared exclusively at the boundary and then grew into short chains, implying that these boundary-anchored species acted as initiators for polymerization. The number and length of the chains increased as the reaction proceeded. One end of the polyethylene chain was tethered to the boundary, and the other grew peristaltically, showing a typical unidirectional growth mode. Furthermore, it was noticed that once polymerization was initialized, its subsequent chain growth became very rapid, which emphasized the characteristics of the chain-reaction polymerization mechanism. Both teams systematically conducted low-temperature STM characterizations and DFT calculations to unambiguously identify surface species and reaction sites.
Fig. 1 Scanning tunneling microscopy (STM) snapshots of the carburized iron surface exposed to ethylene (C2H4) at room temperature. Video frames selected from the STM movie at 35, 245, 385, and 420 s.
All the experimental and computational results fit the scenario in which a C2H4 molecule is inserted into the initial CHCH3 intermediate at the triangular site to form a chain (Fig. 2). This also explains polyethylene formation via ethylene self-initiation over the Phillips catalyst without the need for an activator. These results confirm the molecular insertion mechanism for ethylene polymerization, reveal self-initiation through ethylene isomerization in the absence of an initiator, and clarify the scientific debate regarding the chain initiation process over Philips catalysts. Additionally, this study indicates that the interaction between polyethylene and the catalyst can be tailored and exploited to terminate the polyethylene chain at a specific length, thereby, greatly improving the product chain length selectivity and providing a method for controlling the product chain length distribution.
Fig. 2 Illustration of the ethylene polymerization mechanism on
the surface of ordered iron carbide.
Weijun Guo, a Ph.D. student at Peking University, and Junqing Yin, a Ph.D. student at the Institute of Coal Chemistry, are co-first authors of this paper. Prof. Kai Wu and Assoc. Prof. Xiong Zhou from Peking University are the co-corresponding authors. Part of this research was conducted by Weijun Guo at Synfuels China Technology Co., Ltd. This research was jointly supported by the National Natural Science Foundation, Ministry of Science and Technology, and the Beijing National Laboratory for Molecular Sciences in China.
Original link for the paper: https://www.science.org/doi/10.1126/science.abi4407.
