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Chromium Catalysts for Ethylene Polymerization and Oligomerization

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Chromium Catalysts for Ethylene Polymerization and Oligomerization CHAPTER THREE Chromium Catalysts for Ethylene Polymerization and Oligomerization Zhen Liu*, Xuelian He*, Ruihua Cheng*, Moris S. Eisen†, Minoru Terano{, Susannah L. Scott}, Boping Liu* *State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, P.R. China †Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Technion City, Haifa, Israel { School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa, Japan } Department of Chemical Engineering, University of California, Santa Barbara, California, United States Contents 1. Introduction 129 2. Phillips Chromium Catalysts for Ethylene Polymerization 131 2.1 Brief overview on Phillips chromium catalysts 131 2.2 Characterization of microstructures of polyethylene chains 145 2.3 Polyethylene-based nanocomposites 149 3. Phillips Chromium Catalysts for Alkyne Cyclotrimerization 154 4. Molybdenum Catalysts for Ethylene Polymerization 162 5. Chromium Catalysts for Selective Ethylene Oligomerization 167 5.1 Cr-DME-mediated ethylene trimerization 169 5.2 Cr-SNS-mediated ethylene trimerization 175 6. Summary and Outlook 179 Acknowledgments 181 References 181 Abstract Chromium-based catalysts are the most important ethylene polymerization and oligo- merization catalysts widely applied for industrial production of polyethylene and 1-hexene. Phillips chromium catalyst is a well-known heterogeneous catalyst for com- mercial production of HDPE products, which accounts for more than 40% of world pro- duction annually. The Chevron-Phillips Cr-based homogeneous catalyst system is the first commercialized catalyst for the production of 1-hexene through selective ethylene oligomerization. Although a great success with these Cr-based catalysts has been achieved in industrial applications, there are still many debates in the academic field concerning the precise structure of active chromium species, the oxidation states of # Advances in Chemical Engineering, Volume 44 2014 Elsevier Inc. 127 ISSN 0065-2377 All rights reserved. http://dx.doi.org/10.1016/B978-0-12-419974-3.00003-8 128 Zhen Liu et al. chromium center, the effects of cocatalysts/ligands and the catalytic mechanisms. Dur- ing the last decades, a step-forward mechanistic understanding has been achieved through extensive and successive investigations on these Cr-based catalysts for ethyl- ene polymerization/oligomerization. In addition, the progress in mechanistic under- standing on alkyne cyclotrimerization by the same Phillips catalyst and ethylene polymerization over Mo-based catalyst are also covered. The later might be served as an alternative green catalyst for the industrial production of polyethylene. ABBREVIATIONS AFM atomic force microscope CB carbon black DFT density functional theory DME dimethyl ether DRS diffuse reflectance spectroscopy DSC differential scanning calorimetry EDS energy dispersive spectrometer EPMA electron probe microanalysis EPR electron paramagnetic resonance ESCR environmental stress-cracking resistance FTIR Fourier transform infrared HDPE high-density polyethylene HLMI High load melt index LA-MS laser ablation-mass spectrometry LDI-MS laser desorption-ionization mass spectrometry LLDPE linear low-density polyethylene MAO methylaluminoxane MECP minimum energy crossing point MWD molecular weight distribution NMR nuclear magnetic resonance PES potential energy surface PIBAO partially hydrolyzed tri-isobutylaluminum PIXE proton induced X-ray emission RBS Rutherford backscattering spectrometry SC step crystallization SCB short-chain branch SCBD short-chain branch distribution SEM scanning electron microscopy SIMS secondary ion mass spectroscopy SSA successive self-nucleation and annealing TG-DTA thermogravimetry-differential thermal analysis TMB trimethylbenzene TOF turnover of frequency TPD-MS temperature-programmed desorption-mass spectrometry TPR temperature-programmed reduction TREF temperature rising elution fractionation Ethylene Polymerization and Oligomerization 129 UV–vis DRS ultraviolet–visible diffuse reflectance spectroscopy XAS X-ray absorption spectroscopy XPS X-ray photoelectron spectroscopy XRD X-ray diffraction 1. INTRODUCTION In the 1950s, the world had witnessed two kinds of important cat- alysts successfully applied in industrial production of polyolefins including Ziegler-Natta catalyst and Phillips chromium catalyst (Groppo et al., 2013). After about 60 years of intensive researches and continuous innovations, these catalysts are widely used in a large scale in the commercial production of polyolefins. Nowadays, Phillips chromium catalyst is currently produc- ing more than 10 million tons of high-density polyethylene (HDPE) prod- ucts annually throughout the world (McDaniel, 2010). Since the discovery in 1951, Phillips chromium catalyst was soon patented in 1958 (Hogan and Banks, 1958) and has been attracting tremendous researches from both industrial and academic fields during the last 50 years. The catalyst is famous for its high activity for ethylene polymerization without using any organometallic cocatalyst. This self-alkylation characteristic of the Phil- lips chromium catalyst is often described as “unique” when compared to the other important Ziegler-Natta and metallocene catalysts (McDaniel, 2013). Although Phillips chromium catalyst has achieved a great success in diverse commercial applications, there are still many debates in the aca- demic field in elucidation of the precise structure of active sites, the active oxidation states of chromium center, and the initiation mechanism for eth- ylene polymerization (Groppo et al., 2005a; McDaniel, 1985, 2008, 2010). The difficulties for fundamental studies of the Phillips chromium catalyst are mainly derived from the following aspects: (a) the low percentage of active chromium species, (b) the complexity of the amorphous silica sup- port, (c) the multiple valence states of chromium center, (d) the instant encapsulation of active sites by produced polymer, (e) the super-fast poly- merization rate, (f ) the existence of many side reactions like active sites deactivation and various chain transfer reactions, etc. As a general agreement is far from being reached, much deeper and clearer basic understanding on the Phillips chromium catalyst is still highly expected (McDaniel, 1985). 130 Zhen Liu et al. The HDPE products by the Phillips chromium catalyst usually have the following characteristics: (a) ultra-broad molecular weight distribution (MWD) with a typical polydispersity index larger than 10, (b) a small amount of long chain branches, and (c) a vinyl end-group for each polyeth- ylene chain (McDaniel, 2010). These special features bring its HDPE prod- ucts good mechanical properties and high melt strength, which are of key importance in blow molding process. In the past decades, the market demand of the HDPE products made by the Phillips chromium catalyst shows a dramatic increase in many diverse fields including gasoline tanks of automobile industry, ultra large size plastic containers, high-grade pipe materials like PE80 and PE100, and so on. The increasing market of the HDPE, medium density polyethylene (MDPE), and linear low-density polyethylene (LLDPE) products requires large amount of short a-olefins as comonomer for copolymerization with ethylene. Although copolymer- ization with 1-hexene could bring the HDPE products much improved mechanical properties, 1-butene had been dominant in the polyethylene market in the past few decades because of the high cost for conventional 1-hexene production. Only until 2003, the first plant established by Chevron-Phillips started the commercial production of the comonomer grade 1-hexene with a relative low cost through selective ethylene trimerization (Dixon et al., 2004). This technology was originated from the first discovery of Cr(2-EH)3 (EH, ethylhexanoate) system for ethylene polymerization with a small amount of trimerization product reported by Manyik et al. (1977). Recently, the newly invented catalysts for selective ethylene oligomerization including trimerization and tetramerization are mainly based on chromium catalysts, including bi- and tridentate chromium complexes with a ligand providing N, S, O, or P coordination (Agapie, 2011; Dixon et al., 2004; McGuinness, 2011). There are several reviews in the field of the Phillips chromium catalyst that have been published during the past decades including, to name a few, the review by Zecchina and coworkers in 2005 (Groppo et al., 2005a) and the reviews by McDaniel in 2008 and 2010. In the field of ethylene trimerization, Morgan and coworkers have written a review as early as in 2004 (Dixon et al., 2004), and McGuinness published another review very recently (McGuinness, 2011). In this contribution, we will present a short overview on the Phillips chromium catalyst for ethylene polymerization concerning spectroscopic characterizations, kinetic studies, model catalysts investigations, and molecular modeling simulations. Then, we will include recent progresses in the field of Phillips chromium catalyst with particular Ethylene Polymerization and Oligomerization 131 emphasis on the recent studies from the authors’ groups, including the microstructure characterization of the polymer chains and the grafting of HDPE onto carbon black (CB) focused on high-grade pipe materials of improved long-term mechanical and ultraviolet resistance properties, the mechanistic investigation on the alkyne cyclotrimerization catalyzed by the same
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