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Boping Liu, Yuwei Fang, Minoru TeranoSchool of Materials Science, Japan Advanced Institute of Science and Technology1-1 Asahidai, Tatsunokuchi, Ishikawa, 923-1292, JapanTel: +81-761-51-1622, Fax: +81-761-51-1625, E-mail: boping@jaist.ac.jp
As an important catalyst for olefin polymerization, the Phillips CrOx/SiO2 catalyst has achieved spectacular success in the industrial field since early 1950s. With its unique polymerization behaviors and low production cost, it is still quite competitive in some application areas like blow molding HDPE products compared with Ziegler-Natta and metallocene catalysts. The unique properties and processibility of its HDPE was derived from the unique primary structure of polymer chains featuring both short chain branching (SCB) and long chain branching (LCB) with ultra-broad molecular weight distribution (Polydispersity usually locates around 10 ~ 30.). Further improvements of this catalyst system are in high demand in the industrial field but still remained as a challenging work due to poor mechanistic understanding even after 50 years of great research efforts in this field.1, 2) Therefore, Phillips CrOx/SiO2 catalyst is still extracting both academic and industrial interests. Phillips catalyst (Cr(VI)Ox/SiO2) is usually prepared through a thermal-activation (isothermal calcination) process in dry air at temperature usually around 600 ~ 800 ºC for several hours using silica gel and bulky CrO3 as starting raw-materials.. The addition of Ti-compounds during the thermal activation process can significantly improve the polymerization performance of the calcined catalyst. Subsequently, the calcined catalyst can be further activated into divalent Cr precursor (Cr(II)Ox/SiO2) for ethylene polymerization by either ethylene monomer or CO or alkyl-Al cocatalyst, whereas, the specific activation mechanisms with respect to the formation of active sites with different catalytic properties during either preparation or polymerization processes have not been clarified yet. In this work, the following two activation processes of Phillips catalysts were investigated through combination of experimental and theoretical methods: 1) Thermal activation; 2) Activation by monomer for ethylene polymerization. Recently, we have performed a series of experimental studies on the thermal activation and monomer activation for ethylene polymerization processes utilizing XPS and TPD-MS methods. 3~6) The specific procedures and instrumental conditions of XPS and TPD-MS measurements for Phillips catalyst have been described in detail in our previous report.[3~6] Hereafter only a simplified introduction was given. XPS data were obtained on a Physical Electronics Perkin-Elmer Model Phi-5600 ESCA spectrometer with monochromated Al Kα radiation (1486.6eV) operated at 300W. TPD-MS spectra were obtained from a Multi-task TPD instrument produced by Bel Japan, Inc. equipped with a quadrupole mass spectrometer (MS). Theoretical methods used in this work include paired interacting orbital (PIO) method, which was developed by Fujimoto et al.7), and density functional theory (DFT) method, which was established by Kohn 8). The unique feature in this study is to adopt a combination of experimental and theoretical investigations on these complicated reaction mechanisms related to the thermal activation and polymerization processes on the Phillips CrOx/SiO2 catalyst. Much more profound understanding on mechanisms relating to these two processes will be demonstrated in detail.References [1] M. McDaniel, Adv. Catal. 1985, 33, 47. [2] B. M. Weckhuysen, R. A. Schoonheydt, Catal. Today 1999, 51, 215. [3] B. Liu, M. Terano, J. Mol. Catal. A: Chem. 2001, 172, 227. [4] B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A: Chem. 2002, 184, 387. [5] B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A: Chem. 2003, 201, 189. [6] B. Liu, Y. Fang, M. Terano, J. Mol. Catal. A: Chem. 2004, 219, 165. [7] H. Fujimoto, K. Fukui, J. Am. Chem. Soc. 1985, 107, 6157. [8] W. Kohn, P. Hohenberg, Phys. Rev. 1964, 136, B864.论文来源：Asia Polymer Symposium(APOSYM/2004)October 10-16 |
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