The Structure of Active Centers and the Ethylene Polymerization Mechanism on the Cr/SiO 2 Catalyst: A Frontier for the Characterization Methods E. Groppo, C. Lamberti, S. Bordiga, G. Spoto, and A. Zecchina* Department of Inorganic, Physical and Materials Chemistry and NIS Centre of Excellence, University of Torino, Via P. Giuria 7, 10125 Torino, Italy Received July 12, 2004 Contents 1. Introduction 115 2. The Surface of Silica Support 118 2.1. OH Groups at the SiO 2 Surface: Experimental and Theoretical Approaches 118 2.2. Dehydroxylation of the SiO 2 Surface 119 2.2.1. Experimental and Theoretical Evidences on the Formation of Surface Strained Siloxane Groups 119 2.2.2. Hydroxyl/Siloxane Equilibrium 122 2.2.3. Summary and Basic Classification of Surface Rings 125 3. Since the Beginning until 1985: A Historical Review on the State of the Art 126 3.1. Anchored Process and the Structure of Anchored Chromium 126 3.2. Reduction Process and the State of Reduced Chromium 127 3.3. Modifications of Cr/SiO 2 128 4. Spectroscopic Characterization of the Structure of Chromium Sites: A Review of the More Recent Literature 130 4.1. Diffuse Reflectance UV-Vis Spectroscopy 130 4.1.1. Cr(VI) Species 130 4.1.2. Cr(II) Species 132 4.1.3. Theoretical Calculations 135 4.2. IR Spectroscopy 136 4.2.1. Interaction of CO 136 4.2.2. Interaction of NO 144 4.3. Raman Spectroscopy 145 4.3.1. Standard Raman Studies 145 4.3.2. New Highlights 148 4.4. XAS (XANES and EXAFS) 149 4.4.1. XANES 149 4.4.2. EXAFS 151 4.5. Other Techniques 152 4.5.1. XPS and SIMS 152 4.5.2. Distribution of Surface Chromium Species 154 4.6. Comprehensive Points on the Characterization Techniques 155 5. Catalytic Activity 157 5.1. Polymerization on the Phillips Catalyst: A Review of Literature, Problems, and Perspectives 157 5.1.1. Industrial Catalyst (Ethylene Reduced Catalyst) 157 5.1.2. CO-Reduced Model Catalyst 162 5.1.3. Modifications of Cr/SiO 2 Catalyst 164 5.1.4. Initiation Mechanism as Investigated by IR Spectroscopy in the 1980s 164 5.2. Ethylene Polymerization Mechanism on Cr/SiO 2 Catalyst 166 5.2.1. Introduction 166 5.2.2. Cossee Model for Initiation and Propagation 170 5.2.3. Carbene Model for Initiation and Propagation 170 5.2.4. Metallacycles Model for Initiation and Propagation 171 5.3. Is It Possible to Identify the Polymerization Mechanism? Recent Results and Reflections for the Future 173 5.3.1. C 2 H 4 Initiation Mechanism as Investigated by IR Spectroscopy: More Recent Results 173 5.3.2. General Considerations about the Possibility to Identify the Precursor Species 176 6. Open Questions and Perspectives 177 7. Abbreviations 178 8. Acknowledgments 179 9. References 179 1. Introduction The discovery of olefin polymerization catalysts in the early 1950s by Ziegler and Natta represents a milestone in industrial catalysis. This discovery was of outstanding relevance for the industrial synthesis of polyolefins. Tremendous evolution has taken place since that moment: today, fourth generation Zie- gler-Natta catalysts and metallocene-based “single- site” catalysts display activity and stereoselectivity close to those of enzymatic processes optimized by nature over millions of years. The production of polyolefins is nowadays a multibillion dollar indus- trial activity. Among all of the synthetic polymers, PEs have the highest production volumes. 1 Global production cur- rently stands at just over 40 million tons annually, making PE by far the most widely used commodity polymer. 2 PEs have been and still are so successful in competition with other plastic materials because they have clear advantages and key success factors. PEs have an excellent chemical resistance, a high impact strength, and stiffness even at low tempera- ture. The industrial processes work at low costs and * To whom correspondence should be addressed. Tel: +39011- 6707860. Fax: +39011-6707855. E-mail: adriano.zecchina@unito.it. 115 Chem. Rev. 2005, 105, 115-183 10.1021/cr040083s CCC: $53.50 © 2005 American Chemical Society Published on Web 01/12/2005