Chromium Catalysts for Ethylene Polymerization and Oligomerization

CHAPTER THREE

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 oligomerization catalysts widely applied for industrial production of polyethylene and 1-hexene. Phillips chromium catalyst is a well-known heterogeneous catalyst for commercial production of HDPE products, which accounts for more than 40% of world production 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

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 ethylene polymerization/oligomerization. In addition, the progress in mechanistic understanding 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 catalysts 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 producing more than 10 million tons of high-density polyethylene (HDPE) products 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 academic field in elucidation of the precise structure of active sites, the active oxidation states of chromium center, and the initiation mechanism for ethylene 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 support, (c) the multiple valence states of chromium center, (d) the instant encapsulation of active sites by produced polymer, (e) the super-fast polymerization 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 polyethylene chain (McDaniel, 2010). These special features bring its HDPE products 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 copolymerization 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 Phillips chromium catalyst, and the mechanistic studies of environmental-friendly nonchromium (molybdenum) catalyst for ethylene polymerization and two important chromium-based catalyst systems for ethylene-selective trimerization.

2. PHILLIPS CHROMIUM CATALYSTS FOR ETHYLENE POLYMERIZATION 2.1. Brief overview on Phillips chromium catalysts Phillips chromium catalyst is usually prepared by impregnation of chromic(III) acetate onto a silica gel carrier and a subsequent drying around 150C followed by a calcination between 500 and 900 C in dry air. During the calcination, chromic(III) acetate could be oxidized to chromic(VI) trioxide (CrO3) followed by subsequent anchoring of CrO3 onto the silica surface resulting in the formation of chromate species including monochromate, dichromate, or even polychromate. By contacting with ethylene monomer, the chromate species on the calcined Phillips Cr(VI)Ox/SiO2 catalyst could be reduced to lower valence states, usually Cr(II), showing high ethylene polymerization activity without using any organometallic cocatalyst (McDaniel, 1985). This unique feature of the Phillips chromium catalyst brings us a long-standing question: how is the first CrdC bond formed on the naked chromium active site? That is to say, the initiation mechanism of ethylene polymerization in terms of the formation of the first polymer chain over each chromium active site on the Phillips catalyst is the key problem waiting for elucidation. As shown in Scheme 3.1, three kinds of typical initiation mechanisms have been proposed in the literatures for the Phillips chromium catalyst: Cossee mechanism (Cossee, 1964), carbene mechanism (Ghiotti et al., 1979, 1988; Ivin et al., 1978; McDaniel and Cantor, 1983), and metallacycle mechanism (Ghiotti et al., 1991; Groppo et al., 2006a). Cossee mechanism was originally proposed by Cossee in 1964 for the Ziegler-Natta catalyst (Cossee, 1964). As for metal-alkyl-free catalysts, such as the Phillips chromium catalyst, Cossee mechanism could not rationalize the origin of an extra hydrogen atom, 132 Zhen Liu et al.

Scheme 3.1 Literature proposed initiation mechanisms for ethylene polymerization over the Phillips chromium catalyst. which is needed in the formation of the first metal-ethyl center. Carbene mechanism was proposed by Rooney et al. in 1978 for the stereospecific polymerization of olefins by Ziegler-Natta catalysts (Ivin et al., 1978). Zecchina et al. reported a very weak infrared band for carbene species on the Phillips chromium catalyst (Ghiotti et al., 1979). However, McDaniel et al. found no hydrogen scrambling taking place during polymerization on the Phillips chromium catalyst using deuterium-labeled ethylene, which was against the chain growing through carbene mechanism (McDaniel and Cantor, 1983). Metallacycle mechanism was proposed by Zecchina et al. in 1991 in the investigation of CO/C2H4 coadsorption and reaction on the Phillips chromium catalyst (Ghiotti et al., 1991). The subsequent in situ Fou- rier transform infrared (FTIR) spectroscopic studies by the same group showed the absence of methyl end-group during the initial stage of ethylene polymerization suggesting a metallacycle initiation on the Phillips chromium catalyst (Groppo et al., 2006a). Further investigations with more conclusive evidence are indispensable for a complete elucidation of the initiation mechanism of ethylene polymerization over the Phillips chromium catalysts. In the past decades, Phillips chromium catalyst has been attracting much attention from the academic and industrial fields with various techniques including spectroscopic characterizations, kinetic studies, model catalysts investigations, and molecular modeling simulations. The extensive experimental Ethylene Polymerization and Oligomerization 133 and theoretical studies on the Phillips chromium catalyst for ethylene polymerization will be shortly reviewed following these aspects and a more detailed review can be found in Cheng et al. (2013).

2.1.1 Spectroscopic and kinetic investigations Spectroscopy and kinetics are the most important traditional methods for investigations in the field of heterogeneous catalysis. As summarized in Fig. 3.1, many modern analytical techniques have been applied to

Magnetic SEM/EDS susceptibility UV-vis DRS EPR (ESR) AFM EPMA IR/FTIR/DRIFT In situ FTIR Solid-state NMR

Raman Phillips TPR In situ Raman Catalyst TPD-MS

XPS (ESCA) LA-MS TG-DTA XAS (XANES/EXAFS) In situ XAS PIXE RBS LDI-MS

SIMS XRD (XRPD)

Figure 3.1 Various methods for the characterization of the Phillips chromium catalyst: AFM (Thune et al., 2007), EPMA (Liu and Terano, 2001), EPR (Bensalem et al., 1997; Cimino et al., 1991a; Ellison et al., 1993; Groeneveld et al., 1979; Qiu et al., 2011, 2012), FTIR (Groppo et al., 2005a,b,c, 2006b, 2007, 2011; Demmelmaier et al., 2009; Zhong et al., 2012a; Rebenstorf and Larsson, 1981; Nishimura and Thomas, 1993; Barzan et al., 2012), LA-MS (Aubriet et al., 2006), LDI-MS (Aubriet et al., 2006), PIXE (Rahman et al., 1995), magnetic susceptibility measurement (Groeneveld et al., 1979), Raman (Damin et al., 2006; Dines and Inglis, 2003; Groppo et al., 2005d, 2011; Hardcastle and Wachs, 1988; Moisii et al., 2006; Richter et al., 1988; Vuurman et al., 1993; Zaki et al., 1986), RBS (van Kimmenade et al., 2004), SEM/EDS (Schmidt et al., 1996), SIMS (Ellison and Overton, 1993), solid state NMR (Cheng et al., 2010; Ellison and Overton, 1993; Xia et al., 2010), TG-DTA (Qiu et al., 2009), TPD-MS (Liu et al., 2002), TPR (Bensalem et al., 1997; Jozwiak and Dalla Lana, 1997), UV–vis DRS (Bensalem et al., 1997; Groeneveld et al., 1979; Groppo et al., 2011; Zaki et al., 1986), XAS (XANES-EXAFS) (Agostini et al., 2007; Demmelmaier et al., 2009; Ellison et al., 1988; Groppo et al., 2005e; Moisii et al., 2006; Zhong et al., 2012a), XPS (Best et al., 1977; Cheng et al., 2010; Cimino et al., 1976; Fang et al., 2005a; Liu and Terano, 2001; Liu et al., 2002, 2004a, 2005; Merryfield et al., 1982; Okamoto et al., 1976; Rahman et al., 1995), XRD (Jozwiak and Dalla Lana, 1997; McDaniel, 1981; Vuurman et al., 1993; Wang et al., 2000). 134 Zhen Liu et al. characterize the chromium states on the silica surface of the Phillips chromium catalyst (Groppo et al., 2005a; Weckhuysen et al., 1996). The characterizations based on these methods aim to provide a basic understanding of the surface chromate species formed during the catalyst calcination, and to further clarify the reaction mechanism for ethylene polymerization catalyzed by the Phillips chromium catalyst. Through long-term investigations from both industry and academia, it will be demonstrated that a valuable understanding has been achieved through (1) spectroscopic investigations on activation of the Phillips chromium catalyst by thermal calcination or reducing agents including CO, Al-alkyl, Al-alkoxy, ethylene, etc. and (2) kinetic studies of slurry or gas-phase polymerizations.

2.1.1.1 Spectroscopic investigations on activation of the Phillips chromium catalyst The chromium species of the Phillips catalyst were anchored onto silica surface to form various surface-stabilized chromates during catalyst preparation through thermal activation. However, the hexavalent chromate species must be first reduced to lower valance states before showing activity for ethylene polymerization. This process can be easily fulfilled through activation by ethylene monomer itself or using reducing agents, such as CO, Al-alkyl, Al-alkoxy, etc. During thermal activation, a highly dispersed chromate species, including monochromate, dichromate, and polychromate, could be generated through a redispersion cycle of sublimation, volatilization, spreading, depo- sition, and stabilization of bulk CrO3 onto the silica surface (McDaniel, 1985). McDaniel suggested that the initially formed species was monochromate at 200 C, the dichromate became dominant at 500 C, and polychromate might exist above 800 C(McDaniel, 1981). Panchenko et al. performed a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and diffuse reflectance spectroscopy (DRS) study of the Phillips chromium catalyst and confirmed that the reaction of CrO3 with silica dehydroxylated at 250, 400, and 800 C yielded monochromate, mono- and dichromate, and polychromate, respectively (Panchenko et al., 2006). Liu et al. studied the thermal activation of the Phillips chromium catalyst by means of X-ray photoelectron spectroscopy (XPS) measurements and suggested that two unfavorable situations might occur in the calcination process: the calcination induced reduction of Cr(VI) species into lower valence state (þ5, þ4, or þ3) and the formation of aggregated Cr2O3 even in the presence of oxygen or dry air (Fang et al., 2005a; Liu and Terano, 2001; Ethylene Polymerization and Oligomerization 135

Liu et al., 2002, 2003, 2004a,b). The irreversible formation of a-Cr2O3 is a common phenomenon for the Phillips chromium catalyst with chromium loading higher than 1 wt% or in the presence of trace amounts of moisture. The previous studies revealed that the formation of aggregated Cr2O3 on the silica surface with low-level chromium loading usually occurred in the later stage of calcination (Cimino et al., 1976, 1991b; Groeneveld et al., 1979; Rahman et al., 1995). Initially, explanation on the formation of aggregated Cr2O3 was considered as a thermal decomposition/reduction of bulk CrO3. However, according to the XPS results (Liu and Terano, 2001), all the Cr(III)Ox,surf species and one-seventh of the Cr(VI)Ox,surf species transformed into aggregated Cr2O3 at high temperature in the presence of trace amounts of moisture regardless of the oxygen or inert atmosphere. Consid- ering the trace amount of moisture from the simultaneous dehydroxylation of residual hydroxyl groups on silica surface, the formation of aggregated Cr2O3 microcrystals might be induced by trace amounts of moisture through cleavage of the Cr(III)Ox,surf species during calcination. In laboratory research, the hexavalent chromate species were usually reduced by CO or Al-alkyl cocatalyst in a separated preactivation step, or by ethylene monomer itself during the initial stage of polymerization (McDaniel, 1985). The activation by ethylene monomer is widely used in commercial processes because of the low production cost. Phillips chromium catalyst reduced by CO at 350 C shows instantaneous polymerization activity upon contacting with ethylene monomer. The CO prereduced catalyst is widely employed in spectroscopic characterization of chromium active sites of the Phillips catalyst (McDaniel, 1985; Groppo et al., 2005a). Through a comparison of the chromium oxidation states before and after the reduction by CO, the XPS study showed that about 63% of surface chromate species were reduced to Cr(II) species (Liu et al., 2004b). Furthermore, the chromium active sites are fully available for CO reduction at higher temperature at 600 C, as suggested by the DRS results (Weckhuysen et al., 1993). The activation of the Phillips chromium catalyst by Al-alkyl cocatalyst was also systematically studied by XPS and solid state nuclear magnetic resonance (NMR) (Liu et al., 2005; Xia et al., 2006). As indicated by XPS characterizations, there are four oxidation states including þ2, þ3, þ5, and þ6 of surface chromium species of triethylaluminum-modified Phillips catalyst. The observations from above experiments suggested that the active precursor of the chromium site may have the flowing form 2þ 6þ Cr 2Cr composed of one Cr(II)Ox,surf species and two Cr(VI)Ox,surf species, in which Cr(II)Ox,surf species acted as the real active chromium 136 Zhen Liu et al.

precursor and the residual Cr(VI)Ox,surf species acted as the neighboring ligand providing the electronic and steric environment (Liu et al., 2005). Concerning the activation by ethylene monomer, Liu et al. reported an extensive investigation on reduction of the hexavalent chromate by means of XPS, temperature-programmed desorption-mass spectrometry (TPD- MS) methods and found that surface chromium species might exist in three oxidation states: (1) surface chromate Cr(VI)Ox,surf species, (2) surface- stabilized trivalent Cr(III) species, and (3) surface-stabilized Cr(II) species (Liu et al., 2002). Some short alkenes including propylene and butylene as well as the reduction by-product of formaldehyde were confirmed based on TPD-MS characterizations (Liu et al., 2003). Formaldehyde is experimentally observed as a by-product of the redox reaction between ethylene and hexavalent chromate, which is believed to be reduced to Cr(II) species (Baker and Carrick, 1968; Liu et al., 2002, 2004c). Subsequently, the Cr(II) species coordinated with formaldehyde might act as the active precursor at lower temperature to produce the new short olefins with odd or even number of carbon atoms. These experimental evidences obtained in the early stage of ethylene polymerization cannot be rationalized by the classic Cossee-Arlman mechanism. It is worthy of note that the con- version of ethylene into higher olefins with odd or even number of carbon atoms is well understood through ethylene metathesis reaction (O’Neill and Rooney, 1972), indicating that the coordination of formaldehyde on the Cr(II) center might generate an active precursor for olefin metathesis rather than polymerization.

2.1.1.2 Kinetic studies of slurry or gas-phase polymerizations Kinetic investigation through either experiments or mathematic modeling both for slurry and gas-phase polymerization is one of the most important ways to investigate the catalytic mechanisms, and thus to provide basic data for design of the polymerization reactor and developing of new process. Mathematic modeling of ethylene polymerization kinetics over the Phillips chromium catalyst has been demonstrated as a powerful tool for the precise evaluation of the basic kinetic parameters and to establish equations for structure-property regulation of polyethylene products through control of process parameters (Choi et al., 2004; Choi and Tang, 2004; Matos et al., 2004; Kissin et al., 2008). Polymerization kinetics of the Phillips chromium catalyst could be significantly affected by the reductive activation process for ethylene polymerization using different activators, such as ethylene, CO, Al-alkyl cocatalyst, or other reducing agents. Ethylene Polymerization and Oligomerization 137

The kinetics of the Phillips chromium catalyst using ethylene monomer as an activator for ethylene polymerization has been systematically investigated (McDaniel, 1985, 2010). Typically, a linearly built-up type kinetic curve would be presented with an induction period depending on the polymerization temperature and ethylene pressure. Reductive activation by CO only diminishes the induction period without changing the character of the built-up type kinetic curve. In recent years, activation of the Phillips chromium catalyst by Al-alkyl cocatalysts becomes one of the most important ways to improve the catalyst performance and to regulate the microstructures of the polyethylene products. The Al-alkyl cocatalysts could act as reducing agent, alkylation, poison scavenger, and thus have a significant impact on the polymer microstructures by control of the chain transfer and stereospecificity. Additionally, excess amount of Al-alkyl cocatalyst could deactivate the catalyst through over-reduction of the chromium active species. Ethylene polymerization with the Phillips chromium catalyst without using any organometallic cocatalyst is taken as strong evidence to support the monometallic active site mechanism. Therefore, Al-alkyl cocatalyst can be safely excluded as the active site contributor for the Phillips chromium catalyst. During the past decades, experimental reports on the combination of Al-alkyl cocatalyst with the Phillips chromium catalyst are very limited. Spitz et al. reported a significant effect of triethylaluminum on the Phillips chromium catalyst for the activity, kinetic, and 1-hexene incorporation during the ethylene/1-hexene copolymerization (Spitz et al., 1979). McDaniel et al. studied the effects of triethylborane on the polymerization kinetics of the Phillips chromium catalyst with different supports, such as AlPO4, SiO2, and Al2O3 (McDaniel and Johnson, 1986, 1987). Tait et al. investigated the effects of triisobutylaluminum on kinetics of the Phillips chromium catalyst and polymer morphology (Wang et al., 1991). Liu et al. studied the Phillips chromium catalyst combined with Al-alkyl cocatalyst and revealed that the polymerization kinetics could be significantly affected by the type of Al-alkyl cocatalysts as well as the timing for introducing cocatalyst in both slurry and gas-phase ethylene polymerization (Fang et al., 2005b, 2006; Li et al., 2013; Liu et al., 2004b, 2005; Xia et al., 2006, 2009). As shown in Fig. 3.2A, the kinetic curve is hybrid type kinetics and could be deconvoluted into two basic types of typical kinetic curves including one type with fast activation followed by fast decay and the other type with slow activation followed by slow decay, which might be derived from two different types of active sites. The kinetic curve (Fig. 3.2B) is only one single type kinetics with slow 138 Zhen Liu et al.

AB Polymerization rate Polymerization rate Polymerization

Reaction time Reaction time

Phillips chromium catalyst + AIR3 Phillips chromium catalyst + AIR3 Before slurry polymerization During slurry polymerization Phillips chromium catalyst + AIR2OR Before gas-phase polymerization Before/during slurry polymerization Before gas-phase polymerization Figure 3.2 Two types of kinetic curves for ethylene polymerization over Phillips chromium catalysts: (A) a typical hybrid kinetic curve and (B) a typical single kinetic curve.

activation followed by slow decay, which might be derived from one type of active sites. In the cases of activation of the Phillips chromium catalyst, the AlR3 cocatalyst added during slurry polymerization or before gas-phase polymerization (in catalyst preparation) showed a hybrid type kinetics (curve in Fig. 3.2A), while the AlR3 cocatalyst added before slurry polymerization (in catalyst preparation) showed a single type kinetics (curve in Fig. 3.2B). The activation of the Phillips chromium catalyst by AlR2OR cocatalyst could only present a single type kinetics (curve in Fig. 3.2B) regardless of the timing for introducing cocatalyst in both slurry and gas-phase ethylene polymerization. A basic understanding concerning the thermal activation during catalyst preparation, activation by CO or Al-alkyl cocatalyst, and activation by ethylene monomer during the induction period had been achieved through various spectroscopic methods and kinetic studies. Further emerging of new techniques applied in recent studies of the Phillips chromium catalyst could be observed, such as time-/temperature-resolved FTIR spectroscopy (Groppo et al., 2007), pressure-/temperature-resolved FTIR spectroscopy under in situ/operando conditions (Barzan et al., 2012; Lamberti et al., 2010), in situ XAS spectroscopy (Bordiga et al., 2013), laser ablation-mass spectrometry (LA-MS) and laser desorption-ionization mass spectrometry (LDI-MS) (Aubriet et al., 2006), etc. The characterization under close to the actual industrial conditions is still a challenge as well as an opportunity to shed Ethylene Polymerization and Oligomerization 139 some light on the related mechanisms. Investigation of the polymerization kinetics over the Phillips type catalyst combined with Al-alkyl cocatalyst could provide valuable information to guide the design and development of ethylene polymerization processes. More efforts should be devoted into the investiga- tionsofgas-phasepolymerizationkineticsthroughcombinationofexperiments with mathematical kinetic modeling and microkinetic modeling based on molecular simulations in the near future.

2.1.2 Model catalysts and molecular modeling Experimental studies on the traditional Phillips catalyst are hardly to give a clear image on the active sites and to unravel the mechanisms for ethylene polymerization. The difficulties are primarily induced by surface complexity of the Phillips catalyst including the heterogeneity of the porous amorphous silica support, the coexistence of mono-, di-, and polychromate species, the formation of Cr2O3 microcrystal, the low fraction of active chromium species in the total chromium loading, and so on (Groppo et al., 2005a). A step- forward understanding on the behavior of the active sites and the reaction mechanisms for the Phillips catalyst could be achieved through investigations on well-defined model catalysts as well as molecular modeling simulations. 1. Model catalysts. Model catalysts usually feature with well-defined structures and could provide much clearer information of the active site to understand the Phillips catalyst. Scheme 3.2 depicts some typical silica-supported heterogeneous model catalysts for the Phillips chromium catalyst. S-2 catalyst prepared by wet impregnation of bis(triphenylsilyl) chromate onto thermally pretreated silica gel could be considered as a

Scheme 3.2 Heterogeneous model catalysts for Phillips chromium catalyst. 140 Zhen Liu et al.

commercial heterogeneous model (1A) for the Phillips chromium catalyst (Carrick et al., 1972). The S-2 catalyst shows an increased activity after supporting onto silica gel compared with bis(triphenylsilyl)chromate and produces HDPEs with even broader MWD compared to that of the Phillips chromium catalyst. McDaniel (1982) reported a heterogeneous model catalyst (2A) via mild grafting of CrO2Cl2 at 200 C onto thermally pretreated silica, which generated a surface monochromate structure and showed activity for ethylene polymerization similar to those of the Phillips chromium catalyst. Scott and coworkers (Demmelmaier et al., 2008, 2009) prepared the similar catalysts via ambient anhydrous grafting of CrO2Cl2 onto silica pretreated at 200, 450, and 800C, respectively. The IR, XANES, and EXAFS spectroscopic results suggested that the higher polymerization activity over CrO2Cl2 grafted onto silica pretreated at 800 C was related to a more strained six- membered chromasiloxane ring (2A). A recent study by Scott et al. on the extremely air-sensitive divalent Phillips model catalysts showed that the coordination of the surface siloxane ligands on the divalent active site precursor and the calcination temperature were crucial for determination of the precise microstructures and coordination environment of the active chromium species of the Phillips catalyst (Zhong et al., 2012a). Tonosaki et al. (2011a) utilized two different starting materials 3 3 (Cr( -allyl)3 and Cr2( -allyl)4) to vary the surface chromate structures of the catalysts. It was found that the surface dichromate model catalyst (3A) produced more methyl branches in the polyethylene products compared with the monochromate model catalyst (2A). Model catalyst (4A) is a supported alkylidene complex reported by Scott et al. (Ajjou and Scott, 1997, 2000; Ajjou et al., 1998a,b; Scott and Ajjou, 2001; Scott et al., 2008) which may initiate ethylene polymerization through Green-Rooney alkylidene mechanism (Ivin et al., 1978). Monoi et al. (2003) and Ikeda et al. (2003) reported a trivalent model catalyst by the supporting of Cr(CH(SiMe3)2)3 onto silica pretreated at 200 C and suggested that model catalyst (5A) is the most plausible active site for ethylene polymerization. Thu¨ne et al. (1997) prepared a flat surface model catalyst (6A) by impregnating aqueous CrO3 on a flat Si(100) sub- strate covered by amorphous silica layer. Then, the obtained model catalyst with monochromate supported on the silica surface showed ethylene polymerization activity at 160 C, while the prereduced surface Cr(II) species failed to polymerize ethylene due to its extreme sensitivity to air and moisture (Agostini et al., 2007). Ethylene Polymerization and Oligomerization 141

Heterogeneous model catalysts with more uniform and well-defined structure of surface chromium species supported on silica gel had been demonstrated as a powerful strategy for the basic study of the Phillips chromium catalyst. However, the complexity is still coming from the heterogeneity of the porous amorphous silica support with more than 99% of the active sites existing on the inner surface within the micro- and mesopores of the silica support. In this regards, well-characterized homogeneous model catalysts have been developed in order to simplify such complexity of the traditional Phillips catalyst originating from the silica support. Theopold gave a microreview on the homogenous chromium catalysts for olefin polymerization mostly based on cyclopentadienyl chromium catalysts (Theopold, 1998). These compounds, however, exhibited little similarity compared with the traditional Phillips catalyst. Scheme 3.3 showed some typical homogeneous models of the Phillips chromium catalyst. Bis(triphenylsilyl)chromate (1B) was first proposed as a typical homogeneous model catalyst, which was able to polymerize ethylene without adding any organometallic cocatalyst at high temperatures (T>130C) and high pressures (P¼300–1500 atm) (Baker and Carrick, 1970). A polyhedral oligomeric silsesquioxanes (POSS) supported chromium complex [(cyclo-C6H11)7Si7O11(OSiMe3)]CrO2 (2B) developed by Feher et al. seemed to be a more realistic homogeneous model for the

Scheme 3.3 Homogeneous model catalysts for Phillips chromium catalyst. 142 Zhen Liu et al.

Phillips chromium catalyst. This compound polymerized ethylene under mild conditions (20 C, 1 atm) in the presence of AlMe3 as cocatalyst (Feher and Blanski, 1990, 1993). Some bimetallic chromium siloxane complexes were also synthesized to model the Phillips catalyst. Abbenhuis pointed out that a 12-memberedinorganic heterocycle [Cr(═O)2{(OSiPh2)2O}]2 (3B) was a potential homogeneous catalyst for ethylene polymerization, but no further experiment was conducted on this bimetallic Cr-based compound (Abbenhuis et al., 1997). Recently, Qiu et al. reported a novel homogeneous triphenylsiloxy complex of chromium(II) model catalyst [(Ph3SiO) Cr(THF)]2(m-OSiPh3)2 (4B), which polymerized ethylene at low Al/Cr molar ratios (Al/Cr100) with methylaluminoxane (MAO) as cocatalyst. With further increasing Al/Cr molar ratio (Al/Cr>200), an interesting transformation of ethylene polymerization into ethylene nonselective oligomerization was observed (Qiu et al., 2011). A similar transformation was also presented over bis(triphenylsilyl)chromate combined with MAO as cocatalyst, but the critical point of Al/Cr molar ratio for the transformation was much higher (Qiu et al., 2012). 2. Molecular modeling. To simulate the behavior of the real heterogeneous catalyst, a reasonable molecular model must be first built to mimic the active sites anchored on the support. Figure 3.3 shows some typical molecular models for the active sites of the Phillips chromium catalyst. Espelid and Børve had done a series of systematic density functional theory (DFT) investigations on the active sites of the Phillips chromium

Figure 3.3 Molecular models for Phillips chromium catalyst. Ethylene Polymerization and Oligomerization 143

catalyst (Espelid and Børve, 2000, 2001, 2002a, b, c). Accordingly, the six-membered chromasiloxane ring (1C) was proposed as a key model of the active site of the Phillips chromium catalyst. 1C was then adopted as a model of active chromium species in many other theoretical investigations (Cheng et al., 2010; Damin et al., 2009; Demmelmaier et al., 2009; Tonosaki et al., 2011b). Demmelmaier et al. confirmed the validity of 1C as an ideal molecular model rather than the model with larger ring size for the Phillips chromium catalyst through a combination of experiments and theoretical calculations (Demmelmaier et al., 2008, 2009). Recently, Zecchina et al. reported the adsorption of probe molecules (CO, N2) on the cluster model 1C and found a good agreement between the experimental IR observations and the calculated vibrational frequencies by increasing the percentage of Hartree—Fock exchange in the hybrid density functional B3LYP (Damin et al., 2009). Interestingly, a clear mechanistic understanding on the transformation of a metathesis site into a polymerization site during the induction period of the Phillips chromium catalyst was achieved through DFT calculations with a comparison to the experimental findings (Liu et al., 2003; Zhong et al., 2012b). Furthermore, Liu et al. studied the effects of Ti-modification on the Phillips chromium catalyst using the six- membered chromacycles (1C, 2C, and 3C) and clarified some effects of Ti-modification over the Phillips chromium catalyst, such as the promotion of the polymerization activity, extension of MWD to the low molecular weight region, and improvement of the distribution of inserted comonomers (Cheng et al., 2010). Recently, Tonosaki et al. found that both the calculated activation energies for ethylene insertion and chain transfer were in good agreement using model 1C and an extended larger cluster (Tonosaki et al., 2011b). It was pointed out that the intrinsic origin of the broad MWD of the polyethylene produced by the Phillips chromium catalyst might be derived from the multiple coordination environments around the active chromium site on silica surface. It has long been recognized that the silica support is not an inert component of the catalyst which simply directs polymer particle morphology. The neglect of the real silica surface could introduce some artificial effects and provide unrealistic environment for the adsorption of monomer on active chromium center (Sautet and Delbecq, 2010). Nowadays, with the improvement of the computing resources and the development of quantum methodologies, high-level calculations using a large surface supported 144 Zhen Liu et al. model or a periodic model of silica gel surface can be performed. Very recently, Zhong et al. developed a surface model containing 37 Si atoms through anchoring of a six-membered chromasiloxane ring onto silica surface cutting from the b-cristobalite crystal structure (4C), and the modeling results were in good agreement with the experimental IR spectra concerning CO adsorption over prereduced Phillips chromium catalyst (Zhong et al., 2012a). Guesmi and Tielens (2012) reported an amorphous silica surface slab containing 120 atoms (Si27O5413H2O, 5C) represented the amorphous character of the hydroxylated silica surface containing different silanol types. Through a periodic DFT calculation, a higher stability of mono-oxo and di-oxo chromium species was confirmed in comparison with chromium-hydroxyl species. The main conclusion of their study came with a strong support of the six-membered chromasiloxane ring as a valuable molecular model on the amorphous silica surface for the Phillips chromium catalyst. Thus far, all the related theoretical calculations mentioned above agreed that the six-membered chromasiloxane ring could be served as a reasonable cluster model for the Phillips chromium catalyst, but the effects of the silica support should be considered as well. Based on rational design of these active sites, the theoretical studies on the mechanisms of ethylene polymerization by the Phillips catalyst was reviewed by us very recently (Cheng et al., 2013). The initiation mechanism of ethylene polymerization in terms of the formation of the first CrdC bond over the active site of the Phillips catalyst and the transformation mechanism for the reactions during the induction period of the Phillips chromium catalyst was elucidated by means of molecular modeling simulations (Espelid and Børve, 2000, 2001, 2002b,c; Zhong et al., 2012b). After all, it is still very difficult to obtain direct evidence for the real active sites of the Phillips catalyst and for the ethylene polymerization mechanisms through traditional kinetic studies. The rational design and utilization of novel model catalysts for the Phillips chromium catalyst could be expected to allow further progress with better understanding of the real and complex catalyst system. A step forward in this field requires the combination of model catalysts with more advanced and multiple characterization techniques especially in situ/operando techniques as well as molecular modeling. With fast growing of computing power and in-depth development of quantum packages, one could perform theoretical calculations based on more realistic surface models resembling the real Phillips chromium catalyst. The combination of experiments Ethylene Polymerization and Oligomerization 145 and theoretical calculations is crucial and would result in more profound and interesting findings.

2.2. Characterization of microstructures of polyethylene chains Phillips chromium catalyst could produce polyethylene with broad MWD, long-chain branches, and short-chain branches (SCBs). These special features bring its HDPE products with good mechanical properties, such as environmental stress-cracking resistance (ESCR) and yield stress. One of the most important application areas of its HDPE products is manufacture of high-grade polyethylene pipe materials like PE80 and PE100. The crucial factor to manipulate the long-term mechanical properties of these high- grade pipe materials is to control the short-chain branch distribution (SCBD) of the HDPE products made from Phillips chromium catalyst through copolymerization of ethylene with a-olefin, such as 1-butene and 1-hexene. Therefore, characterization of microstructures of polyethylene chains in terms of SCBD is very important in order to correlate catalyst behavior with structure properties of polyethylene products. The HDPE pipe materials are widely used for the transportation of natural gas, syngas, water, etc. It is generally agreed that the MW, MWD, and SCBD of ethylene/a-olefin copolymers are the most important key factors determining the mechanical properties of the HDPE pipe materials. The relationship of MW and MWD to the mechanical properties had been fully investigated in early time by many researchers such as Kennedy et al. (1994), Failla et al. (1994), and Jordens et al. (2000), etc. In this aspect, a relatively high MW and a very broad MWD are necessary for high-grade polyethylene pipe materials. Later on, a number of studies showed that the long-term mechanical properties of the pipe materials are mainly determined by the SCBD of the polyethylene chains (Fan et al., 2009; Garcia et al., 2008; Luruli et al., 2007; Mortazavi et al., 2010; Razavi-Nouri, 2006; Stadler et al., 2007). The report from the research groups, such as Soares et al. (2000), Hubert et al. (2001, 2002), and McDaniel (Deslauriers and McDaniel, 2007; Deslauriers et al., 2005) confirmed the important effect of SCBD of ethylene/a-olefin copolymers on the long-term mechanical properties of HDPE pipe materials. In general, the more SCBs on the high MW parts and the less SCBs on the low MW parts are beneficial for the high performance of high-grade polyethylene pipe materials (Krishnaswamy et al., 2008). In this regards, two different methods namely TREFþ 13C NMR and TREFþstep crystallization (SC) as effective methods to 146 Zhen Liu et al. characterize the SCBD of ethylene/a-olefin copolymers for HDPE pipe materials were successfully developed (Zhang et al., 2012a). Two HDPE pipe material samples PE-1 and PE-2 with different long- term mechanical properties were fully characterized by means of the above- mentioned two combined methods. The physical and mechanical properties of the two HDPE samples are listed in Table 3.1. Apparently, PE-2 sample showed a much larger ESCR value than that of PE-1. In order to analyze the SCBD of these two samples, the first step is to use temperature rising elution fractionation (TREF) to physically separate the two HDPE samples and get their fractions for the subsequent characterizations. The weight distribution of fractions versus crystalline temperature of the two HDPE samples is shown in Fig. 3.4. Both TREF curves are narrow as typical HDPE samples with very small amount of low and high temperature fractions. When the fractions were obtained by TREF method, 13C NMR was applied to characterize 1-hexene content in six selected fractions of each sample (PE-1 and PE-2). High temperature GPC characterization of the six selected fractions of each sample demonstrated that the fraction at higher crystallization temperatures with higher crystallinity showed higher MW.

Table 3.1 The physical and mechanical properties of PE-1 and PE-2 samples Density HLMI Yield stress Breaking Crystallinity ESCRa Sample (g/cm3) (g/10 min) (MPa) elongation (%) (%) (h) PE-1 0.9413 23.6 18.3 830 63.1 1.78 PE-2 0.9430 10.3 21.4 702 63.9 >4000 aAt 80 C and 4.6 MPa with F323 mm pipe.

Figure 3.4 Weight distribution curveP of fractions after TREF fractionation of the HDPE samples (A) PE-1 and (B) PE-2. a: Wi b: Wi/(△T). Ethylene Polymerization and Oligomerization 147

3 PE-1 PE-2

1 1-Hexene incorporation (mol%) 0 0.0 2.0 × 105 4.0 × 105 6.0 × 105 M W Figure 3.5 The 1-hexene content distribution curves of PE-1 and PE-2.

A B

a a

b b

c c

d d Heat flow (W/g) Heat flow (W/g) e e f f

60 70 80 90 100 110 120 130 140 150 60 70 80 90 100 110 120 130 140 150 Temperature (°C) Temperature (°C) Figure 3.6 DSC curves of different fractions of (A) PE-1 and (B) PE-2 after SC. a: 113 C (A)/112 C (B), b: 105 C (A)/103 C (B), c: 87 C, d: 70 C, e: 60 C, f: 30 C.

As shown in Fig. 3.5, the 1-hexene comonomer content of PE-2 at low crystallinity and low MW fractions was less than that of PE-1 sample and kept even higher comonomer content at the high crystallinity and high MW parts. The SC method could be used to indirectly characterize the SCBD of different polymers. The differential scanning calorimetry (DSC) curves of the selected fractions of PE-1 and PE-2 are shown in Fig. 3.6. After the treat- ment of SC, the DSC curves showed a similar profile for each of the high temperature fractions above 87 C. Moreover, a strong single peak was 148 Zhen Liu et al. observed at about 132 C with a broad shoulder. The main difference appeared in the low temperature fractions, especially for the 30 C fraction. It was clear that the low temperature fractions of PE-1 sample showed more low temperature multiple peaks in the DSC curves, which suggested that there were much thinner lamella in low temperature fractions of PE-1 sample than those of PE-2. This result was consistent with the comonomer content distribution of PE-1 with higher 1-hexene incorporated in the low crystalline temperature and low MW parts as previously shown in Fig. 3.5. According to Thomas Gibbs equation, we could calculate the lamella thickness distribution from those endothermal peaks in the DSC curves through multiple peaks fitting method. The calculated lamella thickness distribution of the selected fractions of PE-1 and PE-2 are shown in Fig. 3.7. Generally speaking, the more incorporated comonomer would result in a thinner lamella thickness. Comparing the fractions of PE-1 and PE-2 obtained at 30 C, the thinner lamella thickness content of fractions of PE-1 is higher than that of PE-2 indicating that more incorporated 1-hexene in PE-1 sample in low temperature and low MW fractions. Although the DSC curves showed a similar profile of high temperature fractions of PE-1 and PE-2, the calculated lamella thickness distribution of the high temperature fractions of PE-2 was slightly thinner than that of PE-1. Therefore, it indicated that the SCBs in the low temperature fractions of

AB 112 °C 112 °C

105 °C 103 °C

87 °C 87 °C

70 °C 70 °C

60 °C

Relative percentage (%) Relative percentage (%) 60 °C

30 °C 30 °C

4 6 8 10121416182022 4 6 8 10121416182022 L (nm) L (nm) Figure 3.7 Lamella thickness distribution of different fractions of (A) PE-1 and (B) PE-2. Ethylene Polymerization and Oligomerization 149

PE-1 was slightly higher than that of PE-2, but the opposite situation occurred in the high temperature fractions of these two samples. It was demonstrated that lower the content of comonomer in the low MW parts and increase the content of comonomer in the high MW parts would be crucial in order to achieve much better long-term mechanical properties of the HDPE pipe materials. Two efficient combined methods have been developed and applied to analyze the SCBD of the HDPE pipe materials. Although 13C NMR could precisely and quantitatively characterize comonomer contents of each HDPE fraction, it is very time consuming and expensive, while SC method combined with TREF could effectively and qualitatively describe the relative comonomer contents and the SCBD of the HDPE pipe materials with low cost and much shorter time. More recently, a more convenient but similar method using TREF combined with successive self-nucleation and annealing (SSA) was successfully utilized to investigate the SCBD of the ethylene/1-hexene copolymers made by a novel inorganic and organic hybrid Cr-based catalyst (Zhang et al., 2012b, 2013). The relative SCBs contents for each PE fraction from TREF was qualitatively obtained from the lamella thickness measured by SSA. Characterization methodologies of the microstructures of polyolefins will surely be playing more and more important role in catalyst innovation in the near future.

2.3. Polyethylene-based nanocomposites HDPE pipe materials produced by the Phillips chromium catalyst have the following distinct advantages: low cost, lightness, good corrosion resistance, high ductility, and excellent mechanical properties (Galli and Vecellio, 2004). However, when long-term exposure to sunlight and air, ultraviolet resistance becomes very important characteristic for these HDPE pipe materials. Incorporation of CB into polyolefin matrix can enhance the ultraviolet resistance, weather adaptability, and therefore prolong the service life time (Hubert et al., 2001). The resistance to ultraviolet degradation is usually related to the morphology, particle size, and surface properties of the CB. However, the perfect dispersion of CB is hampered by the strong inter- particle forces of CB and weak polymer–CB interfacial interaction. On one hand, the ultraviolet resistance property of HDPE pipe materials will be determined by the homogeneous dispersion of CB. On the other hand, the long-term mechanical properties will be determined by not only the dispersion state of CB but also a strong polymer–CB interfacial interaction. 150 Zhen Liu et al.

The graft of polymers onto carbon nanoparticles is an effective method to improve polymer–carbon interfacial interaction and the dispersion quality of carbon in polymer matrix, and consequently enhance the physical properties and ultraviolet resistance (Paiva et al., 2004; Wang et al., 2005; Xu et al., 2006). Traditionally, there are three different approaches for grafting polymers onto CB surface: (a) “grafting from” method: the surface grafting of polymer chains was initiated from initiating groups introduced onto the CB surface; (b) “grafting onto” method: grafting onto the surface during the polymerization, initiated by a conventional radical initiator in the presence of CB; and (c) polymer reaction method: polymers having terminal functional groups were reacted with functional groups on the CB surface (Tsubokawa, 1992; Tsubokawa et al., 1995). However, all of these methods have similar limitations: (1) rigorous and complicated pretreatment; (2) cannot be applied effectively to the grafting of polymers onto carbons with few functional groups (e.g., furnace black and acetylene black). We recently reported the use of HDPE matrix-grafted CB (HDPE-g- CB) prepared by thermal mechanical technique to reinforce commercial HDPE pipe materials (He et al., 2012). The CdC a-bonds of the methine groups of the polyethylene chains encapsulated around the CB surface could be cleaved to produce chain radicals under the intensive thermal and mechanical effects (Hoang et al., 2006). The polymer chain radicals could be terminated either by other chain radicals or by the CB surface as a strong radical scavenger because of its polycondensed aromatic rings. The HDPE- g-CB pipe material was prepared by mixing the HDPE and CB at 140 C for 15 min to graft the molten nonpolar HDPE chains on the larger nonpolar surface of CB. The FTIR spectroscopy is a very useful measurement to study the functional groups chemically attached to the surface of the CB. Figure 3.8 shows the FTIR spectra of (A) pristine CB and (B) HDPE-g- CB after extraction. A peak at 1718 cm-1 occurred in the spectrum of HDPE-g-CB, indicating the presence of carbonyl groups (C]O) on the CB surface. It has been reported that the carbonyl (C]O, 1718 cm-1) groups were formed by an oxidation reaction in the presence of O2 atmosphere (Hoang et al., 2006). The peaks in the 2850–2912 cm-1 region are the stretching of CdH bond and the intensities were greatly enhanced after the grafting reaction. Other peaks, such as 477, 707, and 804 cm-1, were the fin- gerprint peaks of HDPE. Due to the stain derived from the strong entanglement of the polyethylene attached on the CB surface, the CB aggregates could be more easily broken down into smaller particles. In addition, the grafted polymer layer Ethylene Polymerization and Oligomerization 151

Figure 3.8 FTIR spectra of (A) pristine CB, and (B) HDPE-g-CB.

Figure 3.9 TEM micrographs of (A) pristine CB and (B) HDPE-g-CB. protected the new generated CB from van der Waal’s attraction to other CB particles. As a result, the HDPE-g-CB could retain nanodimensions at about 100 nm with visible textures of the CB specimens as shown in Fig. 3.9.In contrast, the particles in pristine CB retained a much larger agglomerate at the dimension of about 1–10 mm. The load of grafted polyethylene can be calculated quantitatively according to the TG measurement. The pristine CB and HDPE-g-CB showed 1.2 and 11.2 wt% total weight loss at 500 C, respectively. The weight percentage of grafted polyethylene in the HDPE-g-CB after extraction was about 10.1%. The HDPE/CB and HDPE/HDPE-g-CB composites were thus prepared by mixing the HDPE pipe materials with pristine CB and 152 Zhen Liu et al.

Figure 3.10 SEM micrographs of HDPE/pristine CB (A1, A2) and HDPE/HDPE-g-CB (B1, B2) with the CB content at 1.0 and 10.0 wt%, respectively.

HDPE-g-CB, respectively. The content of CB or HDPE-g-CB in the polymer composites was set to be 1.0, 5.0, 10.0, and 15.0 wt%. As an example, the scanning electron microscopy (SEM) images of the fracture surfaces of HDPE/CB and HDPE/HDPE-g-CB composites with CB or HDPE-g- CB content of 1.0 and 10.0 wt% are shown in Fig. 3.10. In the SEM photos of the HDPE/CB composites (Fig. 10A1, A2), there is quite a broad distribution of the particle size and large aggregates and agglomerates were clearly observed, indicating a large number of unbroken CB and a poor polymer-CB adhesion. On the other hand, the grafting process efficiently broke down the large aggregates and agglomerates of CB, leading to the homogeneous dispersion of the CB during composites blending. In the SEM photos of the HDPE/ HDPE-g-CB composites (Fig. 10B1, B2), the carbon particles were uniformly dispersed in the HDPE matrix, and the particle size was remarkably decreased. The results are in good agreement with respect to the transmission electron microscopy (TEM) images (Fig. 3.9) and indicate that polyethylene- grafted CB was effective in improving the dispersion states of the CB. More- over, it could be inferred that the HDPE matrix grafted onto the CB surface improved the compatibility between CB and the polyolefin matrix. Ethylene Polymerization and Oligomerization 153

It was found that the HDPE/HDPE-g-CB composites generally showed better tensile properties than that of the HDPE/CB composites. For the HDPE/HDPE-g-CB composite with CB content of 5.0 wt%, the tensile strength and ultimate strain were improved by 17% and 30%, respectively, as compared to HDPE/CB composite. The improvement of the tensile properties of the HDPE/HDPE-g-CB composites might be due to the enhanced crystallization, the improved dispersion state of CB in the matrix, and a more efficient load transfer from the pipe materials matrix to the CB nanoparticles induced by the HDPE-g-CB (Tang et al., 2003). The impact strengths of HDPE/HDPE-g-CB composites were improved by 13.4%, 4.3%, and 6.5% when the content of CB was 1.0, 2.5, and 10.0 wt%, respectively, compared with that of HDPE/CB composites. This is possibly caused by the smaller particle size and much better dispersion of the modified CB, which results in stronger interfacial interaction between CB and PE. It is well known that stress dissipation at the tip of a growing crack by a viscoelastic process results in strain resistance of the polymers. The strong interfacial interaction between HDPE-grafted CB and polymer chains renders efficient dissipation of the applied stress, and thereby higher resistance to external force. It is also noted that for composites containing HDPE-g-CB, there was a downturn in mechanical properties when the CB content was increased to 15.0 wt%. Therefore, even when the nanoparticles are grafted with polymer chains, it can still be difficult to homogeneously disperse the nanoparticles in the polymer matrix at higher nanoparticle loadings. The ultraviolet resistance after incorporation of CB into polyethylene matrix for both the HDPE/HDPE-g-CB and the HDPE/CB composites was further compared. As shown in Fig. 3.11, at the CB load of 1.0 wt%,

Figure 3.11 Absorption of ultraviolet and visible light of (A) HDPE/CB and (B) HDPE/ HDPE-g-CB composites with the content of 1.0 and 5.0 wt%, respectively. 154 Zhen Liu et al. the pipe materials mixed with HDPE-g-CB exhibited slightly higher adsorption of ultraviolet radiation than that adsorbed by HDPE/CB composite. As the CB load increased to 5.0 wt%, the ultraviolet adsorption of HDPE/HDPE-g-CB composites increased significantly. In addition, the HDPE-g-CB absorbed more ultraviolet and visible radiation with higher CB content. The polyethylene chains grafted onto carbon nanoparticles reduce the tendency of CB to aggregate and the nanoparticles can be dispersed more homogeneously in the polymer matrix (see Fig. 3.10). In the presence of HDPE-g-CB, ultraviolet light is more likely to be absorbed rather than scattered and the polyethylene ultraviolet degradation protective effect was enhanced. As a result, the ultraviolet resistance of composites containing HDPE matrix-grafted CB were superior to composites containing only pristine CB. The tensile strength, ultimate strain and toughness of the HDPE were all improved upon the incorporation of proper amount of HDPE-g-CB, and the incremental improvements of CB dispersion enhanced the ultraviolet energy absorption and thus would positively influence the expected lifespan of the HDPE pipe materials. The encapsulated polymer layer was effective in the reduction of the surface free energy of CB and improving the compatibility between CB and polymer matrix. The HDPE-g-CB was uniformly dispersed in the HDPE pipe materials, and the particle size was remarkably decreased. With much enhanced mechanical properties, such as tensile strength, elongation at break and impact strength, and the ultraviolet resistance, HDPE/HDPE-g-CB composite could possibly be applied for making high grade of HDPE pipe materials.

3. PHILLIPS CHROMIUM CATALYSTS FOR ALKYNE CYCLOTRIMERIZATION

Phillips chromium catalyst has been primarily used as ethylene polymerization catalyst in a large industrial scale. However, the same catalyst could cyclotrimerize acetylene and methylacetylene into benzene and trimethylbenzene (TMB) rather than polyacetylene and polymethylacetylene, respectively (McDaniel, 2010). The mechanism has never been studied up to now, which also own particular academic interests in this field. Hogan and coworkers first reported alkyne cyclotrimerization catalyzed by the Phillips chromium catalyst only a few years later after they invented the Phillips chromium catalyst (Clark et al., 1959). In their report, the acetylene was found primarily cyclotrimerized into benzene, while a ratio of 0.18 of Ethylene Polymerization and Oligomerization 155

1,3,5-TMB to 1,2,4-TMB was obtained for the methylacetylene cyclotrimerization. A decade ago, Zecchina et al. revisited this catalytic system and found that 1,3,5-TMB is the only product of the cyclotrimerization of methylacetylene (Zecchina et al., 2003). Recently, we performed a theoretical investigation on the mechanism of acetylene and methylacetylene cyclotrimerization catalyzed by the Phillips chromium catalyst (Liu et al., 2012, 2013). The ground spin state of chromium(acetylene) adducts is known to be of quintet, and the most plausible reaction pathway on quintet surface needs to overcome two activation barriers to finish a single catalytic cycle, as depicted in Scheme 3.4. However, the reaction on quintet surface is prohibited by presenting a free-energy barrier of 31.1 kcal/mol that transforms two coordinated acetylene into the key intermediate 54D. Thus, the turnover of frequency (TOF) for the catalytic cycle on the quintet surface is 1.3610 9 h 1, which rules out the quintet reaction mechanism for acetylene cyclotrimerization by Cr(II)/SiO2 model catalyst. Next, the reaction crosses to the adjacent triplet surface via a minimum energy crossing point (MECP) 5-3CPI and the following reaction proceeds on the triplet surface. As shown in Fig. 3.12, a chromacyclopropene species 31D is generated immediately after 5-3CPI on the triplet surface without showing any transition state. The key intermediate chromacyclopentadiene species 34D is generated through a rather facile insertion of a second

Scheme 3.4 Catalytic cycle for acetylene cyclotrimerization by Cr(II)/SiO2 on the quintet surface. 156 Zhen Liu et al.

Figure 3.12 Gibbs free energy profiles for acetylene cyclotrimerization by Cr(II)/SiO2 cluster model. The Gibbs free energies are calculated at 298.15 K, 1 atm as default in Gaussian09. Also shown are the total energies in parentheses. The triplet reaction pathway is depicted in gray, while the quintet parts are in black. Energies are in kcal/mol and relative to 51C plus the corresponding number of acetylenes.

acetylene into the three-membered ring in 31D. The adsorption of a third acetylene through formation of a hydrogen bond and the subsequent [4þ2] cycloaddition with a moderate free-energy barrier of 13.7 kcal/mol leads to a coordinated cyclohexadiene-like species 36D. The catalytic cycle is finally finished on the quintet surface through another MECP 3-5CPII. The detachment of the coordinated benzene ring from 56D requires 7.4 kcal/mol of Gibbs free energy. Thus, the conclusion is reached with a two-state reactivity following the pathway 51C! 51D! 5-3CPI! 31D ! 32D! 34D! 35D! 36D! 3-5CPII! 56D! 51C0 (Liu et al., 2012). The proposed two-state mechanism for acetylene cyclotrimerization requires two spin-inversion processes, as shown in Fig. 3.12. The first MECP 5-3CPI is crucial for initiation of the acetylene cyclotrimerization. In this regards, the insertion of a second acetylene molecule into a three- membered ring on the triplet surface was found to be much more facile than that proceeds through oxidative coupling on the quintet surface. Rather than crossover to the quintet surface through 3-5CPII, the displacement of benzene ring by acetylene on the triplet surface was predicted to be ther- modynamically favorable with an exergonicity of 18.9 kcal/mol. Therefore, the acetylene cyclotrimerization initiates with coordination of an acetylene molecule on the quintet surface. After a spin-flipping at the quintet chromium(acetylene) complex, the following catalytic cycle favors a triplet [4þ2] cycloaddition pathway as shown in Scheme 3.5. The calculated TOF Ethylene Polymerization and Oligomerization 157

Scheme 3.5 Two proposed mechanisms for acetylene cyclotrimerization by Cr(II)/SiO2 cluster model: two-state reactivity versus a triplet catalytic cycle.

for the above-mentioned two-state catalytic cycle is about 53 h 1, which is much lower than that for the reaction pathway on a single triplet surface with a TOF of 728 h 1. The spin crossover phenomenon has also been reported in other transition-metal catalyzed alkyne cyclotrimerization reactions (Agenet et al., 2007; Gandon et al., 2006; Martinez et al., 2005; Xu et al., 2008). We further studied methylacetylene cyclotrimerization by Cr(II)/SiO2 model catalyst. Similar to acetylene cyclotrimerization, all the quintet reaction pathways for methylacetylene cyclotrimerization need to overcome much higher free-energy barriers leading to very low TOFs for the catalytic reactions. The feasibility of spin-flipping reaction was examined at the naked cluster model 51C, the most stable mono-methylacetylene-chromium complex 51Ea, and the most stable di-methylacetylene-chromium complex 52Ea, respectively. As shown in Fig. 3.13, the spin flipping reaction was predicted to take place at 51Ea through an MECP 5-3CPIII to its triplet ana- log 31E. After this transition, a raised reactivity for the catalytic cycle on the triplet surface is highly expected. As shown in Scheme 3.6, the first reaction pathway from 31E lead to the cyclic product 1,2,4-TMB through either an intermolecular [4þ2] cycloaddition pathway on potential energy surface (PES)–T1a or an insertion and reductive elimination pathway PES–T1b. The Gibbs free energy profiles are depicted in Fig. 3.14. The key intermediate 34Ea is formed through direct insertion of a second methylacetylene into the three-membered ring in 32Ea 158 Zhen Liu et al.

Figure 3.13 Gibbs free energy profile at 298.15 K for the spin crossover at 51Ea. The quintet parts are shown in black, while the triplet complexes are in gray. The MECPs are marked with solid cycle. Energies are in kcal/mol and relative to 51C plus the corresponding number of methylacetylenes.

Scheme 3.6 Mechanisms for cyclotrimerization of methylacetylene on the PES–T1. with a Gibbs free energy barrier of 5.7 kcal/mol. This is an exergonic process by 26.5 kcal/mol. A reductive elimination of 34Ea to generate a dimerization product 35Ea is predicted to be prohibitive by the presence of a high Gibbs free energy barrier of 26.6 kcal/mol. Alternatively, 36Ea is produced by adsorbing a third methylacetylene molecule in a methyl-group-down orientation through the formation of a weak hydrogen bond. This process requires Gibbs free energy of 4.0 kcal/mol. The rate-determining transition-state 3TS[6Ea–9Ea] on the concerted pathway PES–T1a was Ethylene Polymerization and Oligomerization 159

Figure 3.14 Gibbs free energy profile at 298.15 K of the triplet reaction pathway (PES–T1) for methylacetylene cyclotrimerization over the Cr(II)/SiO2 cluster model. The reaction pathway via intermolecular [4þ2] cycloaddition is depicted in black, while the stepwise pathway is in gray. The reaction to generate 1,3-dimethyle-cyclobutadiene 35Ea is in light black. Energies are in kcal/mol and relative to 51C plus the corresponding number of methylacetylenes. found to be 2.2 kcal/mol lower in Gibbs free energy than that of the transition-state 3TS[6Ea–7Ea] on the stepwise pathway. Therefore, the intermolecular [4þ2] cycloaddition is the most favorable pathway to produce 1,2,4-TMB on the PES–T1. The first catalytic cycle is then finished through the displacement of the 1,2,4-TMB in the complex 39Ea by a methylacetylene molecule. After the thermal replacement, a more stable complex 31E0 is regenerated by releasing a free 1,2,4-TMB arene and is ready for the next turn of the catalytic cycle. The coordination of an unsymmetrical methylacetylene molecule in a different orientation plays a key role in determining the regioselectivities of the cycloaddition products. As shown in Scheme 3.7, four kinds of di-methylacetylene-chromium complexes could be generated through the coordination of a second methylacetylene molecule on a Cs symmetric 31E. As discussed above, 32Ea leads to the first [4þ2] cycloaddition pathway PES–T1a. The other three [4þ2] cycloaddition pathways PES–T2a, PES–T3a, and PES–T4a are also depicted in Scheme 3.7. The pathway PES–T4a leads to production of 1,3,5-TMB, while the other three pathways generate 1,2,4-TMB. Figure 3.15 shows the full intrinsic reaction coordinate trajectories for the four [4þ2] cycloaddition pathways. The four pathways 3 proceed smoothly with two moderate activation barriers TS[2E–4E](a–d) 3 and TS[6E–9E](a–d). The reaction coordinate is finished within about 170 steps for PES–T2a and PES–T4a, and about 220 steps for PES–T1a and PES–T3a with a stepsize of 0.2 amu1/2Bohr, respectively. 160 Zhen Liu et al.

Scheme 3.7 Various reaction pathways for cyclotrimerization of methylacetylene on the triplet surface. The Gibbs free energies for the intermediates are listed in parentheses and the Gibbs free energies for the transition states are shown above the arrow. Energies are in kcal/mol and relative to 51C plus the corresponding number of methylacetylenes.

Figure 3.15 Intrinsic reaction coordinates for methylacetylene cyclotrimerization to give a cyclic product through a concerted [4þ2] cycloaddition. Also shown are the optimized geometries for the transition states 3TS[6E–9E].

Table 3.2 listed the calculated TOFs for the four [4þ2] cycloaddition pathways. As shown in Table 3.2, the temperature affects the magnitude of the TOF enormously with a slightly different selectivity between 1,3,5-TMB and 1,2,4-TMB. The ratio of the TOF for producing 1,3,5- TMB and 1,2,4-TMB is 0.48 at 363 K, which is larger than the experimental value of 0.18 (Clark et al., 1959). We further investigated the effect of silica support through building a large silica-supported cluster model, denoted as silica-A, for the most plausible reaction pathways. The TOF for each of the Ethylene Polymerization and Oligomerization 161

Table 3.2 The TOFs for the most plausible reaction pathwaysa via an intermolecular [4þ2] cycloaddition of methylacetylene into TMB 250 K (I/II/III)b 298 K (I/II/III) 363 K (I/lI/III) PES–T1a 0.0020/0.022/ 0.093/0.60/3.3106 140/570/1.0108 1.9106 PES–T2a 0.0037/0.034/ 0.16/0.85/6.3106 170/670/1.5108 4.9106 PES–T3a 0.012/0.20/ 0.50/4.3/3.0106 440/3200/9.6107 3.6105 PES–T4a 0.0082/0.092/ 0.30/2.0/2.8106 360/1700/1.1108 1.6 106 Ratio of TOF 0.46/0.36/0.22 0.40/0.35/0.22 0.48/0.38/0.32 (1,3,5-/1,2,4- TMB) aThe Gibbs free energy profiles employed for the calculation of TOFs are calculated at 250 K and l atm, 298.15K and l atm, and 363 K and 40.8 atm, respectively. TOFs are in h 1. b(I) Gibbs free energies calculated using model A; (II) Gibbs free energies calculated using model silica— A; (III) Gibbs free energies calculated using model silica—A with consideration of the dispersion interaction in the energetics. reaction pathways increased by a factor 9–17 at 250 K, 5.3–8.6 at 298.15 K, and 3.9–7.3 at 363 K, respectively. For instance, the TOFs calculated at 363 K using the six-membered cluster model A for the [4þ2] cycloaddition pathways PES–T1a, PES–T2a, PES–T3a, and PES–T4a are 140, 170, 440, and 360 h 1, respectively. When using the extended model silica-A, the calculated TOFs increased to 570, 670, 3200, and 1700 h 1, respectively. Interestingly, the ratio of 1,3,5-TMB to 1,2,4-TMB in the product decreased at all the three conditions indicating that 1,2,4-TMB is a major product on a silica-supported active site. Moreover, the dispersion corrections are considered for all the silica-supported species and the TOFs calculated using the corrected Gibbs free energies are also given in Table 3.2. 3 3 Since the transition states TS[2E–4E](a–d) and TS[6E–9E](a–d) are highly stabilized relative to starting complexes by dispersion corrections, the corresponding TOFs for all the four reaction pathways increased by about 4–8 orders of magnitude calculated at three different conditions. The ratio of 1,3,5-TMB to 1,2,4-TMB decreases to 0.22, 0.22, and 0.32 at 250, 298.15, and 363 K, respectively. The TOF is extremely sensitive to the calculated Gibbs free energies and depends exponentially on the activation energies (Kozuch, 2012; Kozuch and Shaik, 2006, 2008, 2011; Uhe et al., 2011). 162 Zhen Liu et al.

Although the calculated ratio of 1,3,5-TMB to 1,2,4-TMB of 0.32 at 363 K is larger than the experimental value, it is predictive for the selectivity. The 1,2,4-TMB is preferred as a dominant product for the cyclotrimerization of the methylacetylene catalyzed by the Phillips chromium catalyst. It is worthy of note that the titanium-catalyzed methylacetylene cyclotrimerization gives a similar ratio of 0.33 of 1,3,5-TMB to 1,2,4-TMB (Pierce and Barteau, 1994). The acetylene and methylacetylene cyclotrimerization follow [4þ2] cycloaddition mechanism, which rules out the proposed [2þ2þ2] mechanism by Zecchina et al. (2003). After reduction of chromium to divalent state, the model catalyst and all the chromium(alkyne) adducts showed a quintet ground spin state. However, the following reaction on the quintet surface is inhibited by the oxidative coupling of the two coordinated alkynes to yield a five-membered ring. The spin-flipping to the triplet surface is hardly to occur at the naked chromium(II) in the model catalyst as the MECP lies much higher in energy. The coordination of an alkyne on the cluster model lowers the energy gap of the two adjacent states of the chromium(alkyne) complex and the spin-flipping reaction is thus determined with great feasibility. After the spin transition, the CrdC bond is formed immediately in triplet chromacyclopropene species and the following insertion of a second alkyne is easy to proceed. Therefore, the first CrdC bond could not be formed on the quintet surface, but is formed immediately after the spin-flipping to the triplet surface at the chromium (alkyne) complex. As it can be seen, how is the spin surface crossing phenomena affect the catalytic reactivity, which has been frequently encountered in Cr-catalyzed alkene polymerization and alkyne cyclotrimerization, is still worthy of great research attention in the field of transition metal-catalyzed coordination chemistry.

4. MOLYBDENUM CATALYSTS FOR ETHYLENE POLYMERIZATION

The traditional Phillips chromium catalyst was prepared with highly toxic CrO3. Although a lot of efforts had been devoted in replacing CrO3 with much less poisonous chromate(III) acetate, the obtained catalyst after calcination was still in the form of highly toxic hexavalent chromate species (McDaniel, 1985, 2010). Furthermore, the chromium compounds contained in wastewater and solid dust produced during the catalyst preparation and the chromium residues in polyethylene products might cause Ethylene Polymerization and Oligomerization 163 significant damage to the environment and were very harmful to human’s health. With increasing concerns in the Cr-induced human health and environment problems, a green catalyst is highly expected as a potential alternate of the Phillips chromium catalysts in the future (McDaniel, 2013). Sited with chromium in the same group 6B in the Periodic Table of Elements, molybdenum attracts many efforts in the study of ethylene polymerization. As early as in 1950s, Indiana Standard Oil Company discovered supported molybdenum oxide catalyst was active for ethylene polymerization (Field and Feller, 1957). However, the catalyst was then abandoned because of its poor catalytic performance. Our preliminary experiments on the MoOx/SiO2 catalyst also showed very low reactivity with comparison to the CrOx/SiO2 catalyst for ethylene polymerization. In order to improve the activity of Mo-based catalyst for ethylene polymerization, the active valence states of molybdenum sites, and the mechanism of the catalytic reaction should be first elucidated. We performed a detailed theoretical study combined with experiments to investigate the active oxidation states of molybdenum and the effects of surface hydroxyl on the polymerization activity of supported Mo-based catalysts (Cao et al., 2010). In the experiments, Al/Mo molar ratio was changed from 2.5 to 30 to explore the optimum condition for the highest activity of MoOx/SiO2 catalyst for ethylene polymerization. The results showed that the highest catalyst activity of 2.4 g-PE/g-cat/h was obtained at Al/Mo¼5. Figure 3.16 shows the DSC and FTIR profiles of the polymer produced by the MoOx/SiO2 catalyst. The melting point of the polymer is 134.8 C indicating that the polymer might be polyethylene. The FTIR spectroscopy further confirmed that the obtained polymer is polyethylene.

Figure 3.16 DSC and FTIR profiles of the polymer produced by MoO3/SiO2 catalyst (The catalyst residue was not removed from the polymer.) 164 Zhen Liu et al.

Two typical symmetry and anti-symmetry vibration frequencies of CH2 group were observed at 2848 and 2920 cm 1, respectively. Thus, MoOx/SiO2 catalyst was confirmed to be active for ethylene polymerization although the activity was relatively low. The activity of MoOx/Al2O3 catalysts for ethylene polymerization was 179 g-PE/g-cat/h, which was much higher than that of the MoOx/SiO2 catalyst prepared in this work. It was concluded that MoOx/Al2O3 and MoOx/SiO2 catalysts were all active for ethylene polymerization, but a direct comparison of these two catalyst systems was quiet difficult only using experimental techniques. In order to develop green and highly efficient Mo-based polyethylene catalysts, the corresponding models of the active sites for MoOx/Al2O3 and MoOx/SiO2 catalysts were established in the following computational modeling work. As reported in the literature, Mo species mainly existed in an isolated form in the low molybdenum content catalyst. In our work, four kinds of isolated molybdenum models of the active sites were built with a consideration of different valence states, as shown in Scheme 3.8. Models F and G represent molybdenum centers attached to two Al atoms that were bridged

Scheme 3.8 Mo active site models with different valence states supported on Al2O3 or SiO2. For the model catalysts F, H and J,X¼OH; for the model catalysts G, I, and K,X¼H. Ethylene Polymerization and Oligomerization 165 by two coordinated hydroxyl groups, while H and I were the simplified models of F and G with one H2O molecule eliminated, and two Al atoms were connected by one oxygen atom. Models J and K were built for representing active sites of the MoOx/SiO2 catalysts. In the models of F, H, and J, the support atoms (Si or Al) were saturated by hydroxyl (OH) groups to stand for fully hydroxylated support surface, while the support atoms in G, I, and K were saturated by hydrogen (H) atoms to represent the dehydroxylated support surface. The oxidation state of the molybdenum was noted in the label of each molecular model. For instance, 5F–5K represent the Mo active sites with the oxidation states of þ5. According to Cossee mechanism (Scheme 3.1)(Cossee, 1964), the metal-C center was proposed as the active site for olefin polymerization. The energy barriers of ethylene insertion into Mo centers with different valence states supported on Al2O3 or SiO2 are listed in Table 3.3. For all the models of the Mo active sites, a general tendency of the increasing of polymerization activity on Mo valence states was found to be as follows: þ þ þ þ Mo4

Table 3.3 Energy barriers (kcal/mol) for ethylene insertion into the ModC bond in different catalyst models Model Insertion Model Insertion Model Insertion Model Insertion 5F 26.1 4F 29.9 3F 27.1 2F 22.7 5G 26.3 4G 28.3 3G 27.0 2G 22.4 5H 25.3 4H 30.5 3H 26.0 2H 22.4 5I 25.7 4I 26.3 3I 26.0 2I 22.3 5J 23.7 4J 27.6 3J 24.8 2J 20.7 5K 23.3 4K 27.9 3K 24.6 2K 21.7 166 Zhen Liu et al.

Scheme 3.9 Ethylene coordination on the Mo2þ active site models (2F0, 2H0, and 2J0) with hydroxyl coordination. Energies in kcal/mol. catalysts, hydroxyl group was usually considered as poison to the active sites in commercial polymerization process over the Phillips chromium catalyst (Augustine and Blitz, 1996). Surface hydroxyl on support was eliminated during catalyst preparation, for example, by fluorination, calcination, to þ obtain highly active catalysts. Because Mo2 models of the active sties showed the highest activity for ethylene polymerization, we further investigated the effects of surface hydroxyl using the divalent models 2F–2K.As shown in Scheme 3.9, three new models of the active site, namely 2F0, 2H0, and 2J0, were obtained with the hydroxyl group coordinated on the Mo center through the formation of a weak ModO bond. The ethylene insertion energy barriers for 2F0, 2H0, and 2J0 models with hydroxyl coordination were 25.3, 28.4, and 26.9 kcal/mol, respectively. These activation energies are higher than their corresponding models (2F, 2H, and 2J) without hydroxyl coordination. On the basis of the experiments, we built 24 kinds of molybdenum active sites with the Mo oxidation state ranging from þ5toþ2. The effects of surface hydroxyl groups were also considered in the theoretical work. DFT results showed that ethylene insertion barrier for SiO2-supported catalysts were slightly lower than that for alumina-supported catalysts. For the same þ kind of support, Mo2 active sites showed the lowest energy barrier of eth- þ ylene insertion, while Mo4 sites presented a larger activation energy. In addition, the coordination of hydroxyl on Mo center could decrease the electron deficiency of molybdenum center leading to an increase in the ethylene insertion energy barrier. It had been demonstrated that pre-reduction of hexavalent Mo into lower valence state þ2 and elimination of surface hydroxyl groups during catalyst preparation were the key factors to obtain highly efficient Mo-based catalysts for ethylene polymerization. Through the combination of experiments and theoretical calculations, a better Ethylene Polymerization and Oligomerization 167 understanding on the Mo-based supported polyethylene catalysts has been achieved so far. The highly toxic Cr-based Phillips type catalysts could be possibly substituted by Mo-based catalyst through further successive modi- fications in the future.

5. CHROMIUM CATALYSTS FOR SELECTIVE ETHYLENE OLIGOMERIZATION

The polyethylene products produced by the Phillips chromium catalysts account for a large share of the polyolefin consumer market and hence require a large-scale industrial production (McDaniel, 2010). As a kind of highly valuable linear alpha olefin, 1-hexene is greatly demanded as comonomer for application in the production of HDPE and LLDPE products. The HDPE and LLDPE products produced with 1-hexene as comonomer showed much improved physical properties in diverse applications compared with that produced with 1-butene as comonomer. However, 1-butene copolymer had been dominant in the polyethylene market in the past few decades because of the high production cost of 1-hexene. Tra- ditionally, 1-hexene is produced by metal catalyzed ethylene non-selective oligomerization (Shell, BP Amoco, Chevron Phillips, et al.) or by Fischer- Tropsch technology (Sasol, et al.), along with a broad range of linear alpha olefins. A successive distillation processes for separation leads to a relatively high cost of 1-hexene production (Dixon et al., 2004). In 1967, Manyik et al. in UCC first discovered 1-hexene as a major ethylene oligomer by-product along with the polymerization of ethylene catalyzed by Cr(III)(2-EH)3 with hydrolyzed tri-isobutylaluminum as an activator (Manyik et al., 1967). This is the first report on transition-metal catalyzed ethylene trimerization. After about 40 years of intensive research from both industry and academia, the first commercial plant established by Chevron-Phillips finally came on stream for producing comonomer grade 1-hexene by selective ethylene trimerization using Cr(III)(2-EH)3 homogeneous catalyst system (Dixon et al., 2004). The main improvements of the Phillips ethylene trimerization catalyst system are mainly concerning the aluminum-alkyl cocatalysts, such as triethylaluminum and diethyl aluminum chloride, together with some promoters like 2,5-dimethylpyrrole. In the past decades, the selective olefin oligomerization has been exten- sively reviewed (Agapie, 2011; Dixon et al., 2004; McGuinness, 2011). It is well-known that there are three alternative mechanistic pathways leading to 168 Zhen Liu et al. the production of 1-hexene from ethylene. The first pathway is through a b-H transfer from the metal-alkyl linear propagating chain based on Cossee- type mechanism. This mechanistic pathway is believed to be able to produce a full range of linear a-olefins with even number of carbon atoms including 1-hexene with a Schulz-Flory or Poisson distribution. The second pathway is through ethylene metathesis based on Chauvin-type mechanism. This mechanistic pathway could only make a full range of olefins with both even and odd number of carbon atoms including 1-hexene. The above- mentioned two mechanistic pathways are not applicable for selective ethylene trimerization. The third pathway is through b-H transfer from the metallacyclic propagating chain also based on Cossee-type mechanism. This so-called metallacyclic intermediates mechanism is most popularly accepted nowadays for the transition-metal catalyzed selective ethylene trimerization to give 1-hexene. There are two key aspects for the metallacyclic mechanism: (a) during the metallacyclic chain propagation, the insertion step must be faster than the decomposition of the metallacyclopentane to 1-butene; (b) the liberation of 1-hexene must be faster enough than further ethylene insertion into the metallacycloheptane to give a larger nine-membered ring. Regarding the above-mentioned metallacyclic mechanism, there are also three different routes proposed based on the specific b-H transfer from the metallacyclic propagating chain. These three pathways are depicted in Scheme 3.10 as path (a), (b), and (c), respectively. Path (a) is proposed by Manyik indicating a direct b-H transfer to ethylene monomer from the metallacyclopentane species. Thus, 1-hexene is generated through a

Scheme 3.10 Metallacyclic mechanisms for selective ethylene trimerization to 1-hexene proposed by Manyik et al. (1977) (path (a)), Briggs (1989) (path (b)), and Hessen (Deckers et al., 2001, 2002) (path (c)). Ethylene Polymerization and Oligomerization 169 subsequent reductive elimination (Manyik et al., 1977). Path (b) is proposed by Briggs supposing a one-step 1-hexene formation through a b-H transfer directly to the other (a–C) in the metallacycloheptane (Briggs, 1989). Path (c) is proposed by Hessen stating a b-H transfer to the chromium center from the metallacycloheptane (Deckers et al., 2001, 2002). Therefore, 1-hexene is released in a two-step manner rather than a direct hydrogen transfer comparing to path (b). According to the experimental evidence of metallacycle species and C2H4 isotope experimental findings, path (b) is the most plausible mechanistic pathway for selective ethylene trimerization to generate 1-hexene. The current understanding on the mechanism of selective ethylene trimerization has been approaching to a relatively clear image although many key points regarding the oxidation states of the chromium center and the role of the ligands are still unclear. As a first reported catalyst for ethylene trimerization, Cr(2-EH)3/partially hydrolyzed tri-isobutylaluminum (PIBAO) system is of most interests, which transforms from a polymerization catalyst into a trimerization catalyst after adding dimethyl ether (DME). The main challenge for designing and developing new catalyst for ethylene trimerization is to make it clear for the triggering mechanisms for those active sites transformation between ethylene polymerization and trimerization. More importantly, the competition between two oxidative coupling cycles Cr(II)/Cr(IV) or Cr(I)/Cr(III) also attracted much attention of many researchers. The mechanism of ethylene trimerization regarding catalyst initiation, effect of activators, chromium oxidation states, and reaction mechanism was investigated by experiments and molecular modeling. The contributions from our group to understand the active sites transformation of the Cr(2-EH)3/PIBAO system and the effects of ligand deprotonation of the Cr-SNS (SNS¼RS(CH2)2N(H)(CH2)2SR) system will be shortly discussed and reviewed in the following two subsections.

5.1. Cr-DME-mediated ethylene trimerization

Manyik et al. first patented Cr(2-EH)3/PIBAO system as ethylene polymerization catalyst with low ethylene trimerization reactivity (Manyik, 1967). Later on, the research from the same group showed that the activity and selectivity could be improved by adding DME to the Cr(2-EH)3/ PIBAO system (Manyik et al., 1977). In 1989, Briggs reported that the activity could be drastically increased to 2086 g/gCr per hour with much improved selectivity of 73% for ethylene trimerization catalyzed by 170 Zhen Liu et al.

Cr(2-EH)3/PIBAO/DME system (Briggs, 1989). In 2003, the Cr(2-EH)3/ 2,5-dimethylpyrrole/Et3Al/Et2AlCl catalyst system has been finally commercialized by Phillips Petroleum Company for producing comonomer- grade 1-hexene (Dixon et al., 2004). The most interesting thing for the Cr(2-EH)3/PIBAO catalytic system is the transformation from a polymerization catalyst into a trimerization catalyst after adding DME. Our recent work reported a theoretical study on the mechanism of transformation from ethylene polymerization into ethylene trimerization on the Cr(2-EH)3/PIBAO catalyst with or without DME coordination (Qi et al., 2010). Since the industrial homogeneous chromium-based catalyst for selective ethylene trimerization contains three components including the precursor Cr(2-EH)3, the ligand DME, and the cocatalyst PIBAO, we built five molecular models (L–P) representing the possible active species of the Cr(2-EH)3/PIBAO catalyst system without adding DME ligands and another five models (L0–P0) for the active species of the Cr(2-EH)3/PIBAO/DME catalyst system by consideration of the possible oxidation states of the chromium center, as listed in Table 3.4. According to the metallacycle mechanism for selective ethylene trimerization first proposed by Briggs (1989), the catalytic cycle including side reactions for ethylene dimerization and polymerization was depicted in Scheme 3.11. The reaction starts with coordination of two ethylene monomers to give 1X species, and then the key intermediates chromacyclopentane species 2X was generated by an oxidative-coupling reaction. The 1-butene and 1-hexene products could be released in two different ways: (1) one-step path via an agostic-assisted b-hydride shift to give 4X, or (2) two-step path through a Cr-H intermediate followed by reductive elimination. The formation of nine-membered chromacycle

Table 3.4 Molecular models for active chromium species of the Cr(2-EH)3/PIBAO catalytic system (R denotes isobutyl) Oxidation states Model Without DME Model With DME þ þ Cr(I)/(III) L Cr L0 Cr /DME M CrOR M0 CrOR/DME N CrR N0 CrR/DME þ þ Cr(II)/(IV) O Cr OR O0 Cr OR/DME þ þ P Cr R P0 Cr R/DME Ethylene Polymerization and Oligomerization 171

Scheme 3.11 Proposed catalytic cycle for ethylene trimerization over the Cr(2-EH)3/PIBAO catalyst system. (X¼L.Cr(I)þ; L0.Cr(I)þ/DME; M.Cr(I)OR;M0. Cr(I)OR/DME; N.Cr(I)R;N0.Cr(I) R/DME; O.Cr(II)þOR; O0.Cr(II)þOR/DME; P.Cr(II)þR; P0.Cr(II)þR/DME).

Figure 3.17 Gibbs free energy profile of model Cr(I)þ at 298.15 K. The solid line in black shows the metallacycle growth pathway; the solid line in gray shows 1-butene elimination pathway; the dotted line in gray shows b-hydrogen transfer pathway to give chromium hydride species 7L; the dotted line in black shows the b-agostic hydrogen shift pathway to give 1-hexene. Energy differences (kcal/mol) are expressed with respect to 1L corrected for the corresponding number of ethylene molecules. Energy barriers are indicated in italics. species 10X represents a ring expansion pathway leading to the production of oligomers and polymers. The Gibbs free energy profile for the reaction pathways by model L,a þ simple monovalent cationic species Cr(I) , is shown in Fig. 3.17. The liberation of 1-butene was prohibited by showing two successive activation 172 Zhen Liu et al. barriers of 14.7 and 11.6 kcal/mol. Alternatively, an ethylene coordinated chromacyclopentane species 5L was formed immediately through the exoergic coordination of a third ethylene. Therefore, the chromacycloheptane species 6L was generated with a relatively low energy barrier of 15.4 kcal/mol. Similarly, further coordination of a fourth ethylene on þ Cr(I) was also a fast exergonic process by releasing 12.2 kcal/mol. There- þ fore, site L Cr(I) without DME coordination preferred ethylene polymerization rather than ethylene trimerization. The ethylene polymerization catalyzed by Cr(2-EH)3/PIBAO would be transformed into selective trimerization by adding DME component reported by Manyik et al. (1977) and Briggs (1989). In order to investigate the role of DME for the transformation mechanism between ethylene polymerization and trimerization, we further studied the possible reaction using þ the model L0, a cationic model of Cr(I) with DME coordinated on the chromium center. As shown in Fig. 3.18, the liberation of 1-butene can also be safely excluded by shown two high activation barriers on the Gibbs free energy profile. Interestingly, the steric effects of DME ligand prevent the

Figure 3.18 Gibbs free energy profile of model Cr(I)þ/DME at 298.15 K. The solid line in black shows the metallacycle growth pathway; the dotted line in gray shows 1-butene elimination pathway; the solid line in gray shows 1-hexene elimination pathway via two- step route; the dotted line in black shows 1-hexene elimination pathway via b-agostic hydrogen shift. Energy differences (kcal/mol) are expressed with respect to 1L0 corrected for the corresponding number of ethylene molecules. Energy barriers are indicated in italics and heat absorption is shown in parentheses. Ethylene Polymerization and Oligomerization 173 further coordination of a fourth ethylene molecule, which results in an end- ergonic process by 8.7 kcal/mol. 1-Hexene liberation would follow a one- step path of agostic-assisted-hydrogen transfer rather than a two-step route. The direct reductive liberation of 1-hexene only requires overcoming an activation barrier of 16.7 kcal/mol, which is lower than the ring expansion þ with a barrier of 19.2 kcal/mol. Therefore, site L0 Cr(I) /DME with DME coordination preferred ethylene trimerization rather than ethylene polymerization. Table 3.5 lists energy differences of ethylene coordination and insertion for metallacycle expansion and 1-hexene liberation via agostic-assisted b-hydrogen transfer (TS[6X–8X]) in each model. The role of DME on the transformation of ethylene polymerization into ethylene trimerization can be understood readily. Before adding DME, the coordination of extra ethylene molecules occurred spontaneously with exoergic effect (cationic models L, O, and P) or with a negligible endoergic effect for both neutral models M and N. As a result, the formation of metallacycle was determined

Table 3.5 Energy differences (kcal/mol) of crucial steps in catalytic cycle of each model Ring5!Ring7 Ring7!Ring9

a b c d e Models Coord Insertion Coord Insertion 1-C6 þ Cationic L Cr 17.2 15.4 12.2 16.4 15.5 þ L0 Cr /DME 1.2 9.0 8.7 10.5 16.7 þ O Cr OR 15.9 12.5 8.3 15.5 10.6 þ O0 Cr OR/DME 20.9 10.2 24.8 11.6 13.4 þ P Cr R 5.5 10.7 0.6 13.8 14.3 þ P0 Cr R/DME 17.2 10.6 20.9 11.8 17.2 Neutral M CrOR 5.1 13.2 5.7 18.4 18.2 M0 CrOR/DME 24.4 5.2 29.1 7.4 29.8 N CrR 4.3 20.0 7.8 22.8 29.8 N0 CrR/DME 29.4 4.1 34.9 5.7 32.8 aHeat absorption required for the coordination of a third ethylene molecule, 2X!5X. bEnergy barrier required for the insertion of a third ethylene to form chromium seven-membered ring, TS[5X–6X]. cHeat absorption required for the coordination of a fourth ethylene molecule, 6X!9X. dEnergy barrier required for the insertion of a fourth ethylene to form chromium nine-membered ring, TS[9X–10X]. eEnergy barrier of 1-hexene liberation via agostic-assisted b-hydrogen transfer, TS[6X–8X]. 174 Zhen Liu et al.

Table 3.6 Apparent energy barriers (kcal/mol) of nine-membered ring formation (P) and 1-hexene liberation via agostic-assisted b-H transfer (T) and the differences between the two barriers for each cationic model Without DME With DME Pa Tb D(TP) Pc Td D(PT) þ þ L Cr(I) 16.4 27.7 11.3 L0 Cr(I) /DME 19.2 16.7 2.5 þ þ O Cr(II) OR 15.5 18.9 3.4 O0 Cr(II) OR/DME 36.4 13.4 23.0 þ þ P Cr(II) R 13.8 14.9 1.1 P0 Cr(II) R/DME 32.7 17.2 15.5 aEnergy barrier of TS[9X–10X]. bEnergy difference between TS[6X–8X] and 9X. cEnergy difference between 6X and TS[9X–10X]. dEnergy barrier of TS[6X–8X]. by the ethylene insertion step. After adding DME, the energy required for ethylene coordination increased enormously by more than 18 kcal/mol in all cases, which became a major determining factor for further metallacycle expansion to large ring size. The neutral sites (M: CrOR and N: CrR) could be safely excluded for Cr(2-EH)3/PIBAO catalyst system because ethylene trimerization was favored without adding DME compound, which dis- agreed with experimental findings. Table 3.6 summarizes the apparent energy barriers of nine-membered ring formation (representing ethylene polymerization) and 1-hexene liberation (ethylene trimerization) and the difference between the two barriers for each cationic model before and after addition of DME compound. For the cationic models L, O, and P, the path for ethylene polymerization is open by metallacycle growth to give a nine-membered ring, while DME coordination on these models increased the ethylene coordination energy and the reaction transformed from ethylene polymerization into ethylene trimerization. Therefore, cationic chromium models L, O, and P might be the most plausible active sites for the Cr(2-EH)3/PIBAO catalyst system with or without DME coordination. Thus, neutral models could be safely excluded from the active sites of the Cr(2-EH)3/PIBAO/DME system because ethylene trimerized into 1-hexene without adding DME. Before adding DME, the cationic models tend to produce polyethylene through further ring expansion on the chromacycloheptane species. However, the polymerization reaction on the cationic models was transformed into ethylene trimerization reaction after adding DME due to the steric and electronic effects of the DME ligands. All the cationic active site models might be the most plausible active sites Ethylene Polymerization and Oligomerization 175

for the Cr(2-EH)3/PIBAO catalyst system with or without DME coordination.

5.2. Cr-SNS-mediated ethylene trimerization

Cr(2-EH)3/2,5-dimethylpyrrole/Et3Al/Et2AlCl system is the first commercialized industrial catalyst for ethylene trimerization into 1-hexene (Dixon et al., 2004). Many experimental and theoretical works have been conducted in the past decades in order to elucidate the mechanisms of ethylene trimerization (Agapie, 2011; McGuinness, 2011). Although the metallacycle pathway (Briggs, 1989) of ethylene trimerization has been widely accepted in recent years due to the support from experimental (Agapie et al., 2004, 2007; Emrich et al., 1997) and theoretical reports (Bhaduri et al., 2009; Blok et al., 2003; Blom et al., 2007; Budzelaar, 2009; de Bruin et al., 2003; Qi et al., 2010; Tobisch and Ziegler, 2003, 2004a,b, 2005; van Rensburg et al., 2004; Yu and Houk, 2003), some key problems such as the oxidation state of the active site and effects of ligands are still subject to debate. In 2003, McGuinness et al. reported a remarkable ethylene trimerization catalyst SNS-CrCl3 (SNS¼RS(CH2)2N(H)(CH2)2SR) activated by MAO (McGuinness et al., 2003). The oxidation states of the active chromium species and the deprotonation of the SNS ligands remains controversial problems for this efficient catalyst system (Agapie, 2011; Jabri et al., 2006; McGuinness, 2011; McGuinness et al., 2006; Temple et al., 2006, 2007). McGuinness et al. suggested the SNS ligands would undergo deprotonation in the early stages during the activation by MAO. However, Gambarotta et al. isolated an inactive deprotonated SNS-Cr(II) complex for ethylene trimerization (Jabri et al., 2006). We recently reported a theoretical work on the SNS-Cr system in order to elucidate the oxidation state of the active chromium species with a consideration of the deprotonation of the SNS ligands (Yang et al., 2011). As depicted in Scheme 3.12, four molecular models Q–T were constructed for the typical SNS-Cr catalyst system. Cr(I) and Cr(II) were considered as the two most plausible oxidation states of the chromium center. Models Q and R are two active species without ligand deprotonation, while S and T were designed with consideration of SNS ligand deprotonation. The ethylene trimerization was calculated on the basis of the metallacycle mechanism first proposed by Briggs (1989). The catalytic cycle including the formation of 1-butene (7X), 1-hexene (6X), and metallacycle expansion 176 Zhen Liu et al.

Scheme 3.12 Schematic representation of molecular models Q, R, S, and T for the SNS-Cr system (SNS¼MeS (CH2)2N(H) (CH2)2SMe).

Scheme 3.13 Proposed catalytic cycle (X¼Q, R, S, and T).

(8X) was shown in Scheme 3.13. As previously reported in Ti (Blok et al., 2003; de Bruin et al., 2003, 2008; Tobisch and Ziegler, 2003, 2004a,b, 2005), Ta (Yu and Houk, 2003), and Cr (Bhaduri et al., 2009; Budzelaar, 2009; Klemps et al., 2009; Qi et al., 2010; van Rensburg et al., 2004) ethylene trimerization systems, the formation of 1-butene and 1-hexene from the metallacyclopentane and metallacycloheptane species, respectively, could follow two different routes: (1) one-step route that through a direct intramolecular-hydrogen transfer to the opposite a-carbon atoms; (2) two- step route that via a Cr-H intermediate followed by reductive elimination. Ethylene Polymerization and Oligomerization 177

However, the Cr-H species could not be located because of the steric effect of the SNS ligand and the search of the corresponding transition state for the two-step reaction path to generate the dimerization product 1-butene and the trimerization product 1-hexene failed in all cases. The direct b-hydrogen transfer in the more strain chromacyclopentane ring was failed to occur, which completely shutdown the side reaction to give dimerization product 7X. In the meanwhile, the fourth ethylene was repelled from chromium center by the steric effects from both the SNS ligands and the large metallacycloheptane ring, thereby stopping the route to go further metallacycle expansion. The only remaining path is to go through a direct b-hydrogen transfer in the chromacycloheptane spices 5X to give the trimerization product 6X. The Gibbs free energy profile of the reaction pathway by model Q was shown in Fig. 3.19. The coordination of the first ethylene to the unsaturated chromium center in the sextet ground state is slightly exoergic by 1.0 kcal/mol, which is followed by an endoergic coordination process of

Figure 3.19 Calculated Gibbs free energy profile for ethylene trimerization on model Q: MeS(CH2)2N(H)(CH2)2SMe-Cr(I). Energetic barriers are indicated in italics and heat absorption energies are exhibited underlined. Black lines and gray lines illustrate free energy surfaces under quartet and sextet respectively. Round points represent MECPs, and the corresponding free energies are shown in bold together with their optimized geometries. 178 Zhen Liu et al. the second ethylene. The subsequent oxidative coupling (oxidation from Cr(I) to Cr(III)) of the two coordinated ethylene on the sextet surface was prohibited by showing a very high activation barrier of 38.4 kcal/mol. Interestingly, the presence of an MECP between the sextet and quartet surfaces facilitates the reaction and the whole barrier for this step is about 24.0 kcal/mol. Therefore, the formation of the metallacyclopentane species undergoes a spin state change from sextet to quartet via an MECP lying 9.7 kcal/mol above the sextet 2Q. The final reduction of Cr(III) to Cr(I), generating 1-hexene, experiences another spin state change from quartet back to sextet with an activation barrier of 18.5 kcal/mol. The whole catalytic cycle by model Q was thus completed through two readily accessible MECPs, which lowered the activation energies of the transition states presented in the rate-determining step. In the following work, Gibbs free energy surfaces under all possible spin states of the other three models R–T were completely searched. It was found that spin surface crossing occurred between two higher spin states on each model. On models Q, S, and T, the spin surface crossing took place during the formation of metallacyclopentane and the generation of 1-hexene, while on model R, it occurred during the formation of metallacycloheptane. According to the calculated results, the rate-determining steps are the formation of metallacyclopentane and metallacycloheptane species on the four models and the corresponding activation barriers without and with consid- ering the spin-crossover phenomenon are listed in Table 3.7. If spin surface crossing is not considered, assuming all reactions take place on the surface of the highest spin state (the resting state of the reactants), the monovalent model S with ligand deprotonation would be the most possible active site

Table 3.7 Influence of spin surface crossing on the activation barriersa (kcal/mol) of metallacyclopentane (MCP) formation and metallacycloheptane (MCH) formation on each model Without spin surface crossing With spin surface crossing Model MCP MCH MCP MCH Q 44.6 28.7 30.2 30.4 R 41.9 22.5 41.9 22.5 S 37.2 37.9 20.8 40.4 T 46.2 27.1 28.3 32.9 aThe activation barrier includes the binding energy of ethylene coordination and the energetic barrier of metallacycle formation. Ethylene Polymerization and Oligomerization 179 model in the SNS-Cr system. However, as a matter of fact, spin surface crossing is definitely revealed by our DFT calculations for the SNS-Cr system. By taking into account spin surface crossing, model Q turns out to be the most favorable, which means the Cr(I)/Cr(III) catalytic cycle with a nondeprotonated ligand is the most plausible active site in the SNS-Cr system. The Cr(I)/Cr(III) cycle is also reported to be responsible for selective ethylene oligomerization in some other Cr-based systems (Albahily et al., 2011a,b; Jabri et al., 2008; Licciulli et al., 2010; Skobelev et al., 2010; Vidyaratne et al., 2009). The nondeprotonation of the SNS ligand by MAO for this catalyst system is also supported by the experimental report on a similar SNS-Cr system by Gambarotta, Duchateau, et al. (Jabri et al., 2006). Cr(I)/Cr(III) catalytic cycle with a nondeprotonated ligand might be the most plausible active site for ethylene trimerization catalyzed by the SNS-Cr system. A plausible spin surface crossing between Cr(I)/sextet and Cr(III)/ quartet was found to play a spin acceleration effect by lowering the activation energy of the rate-determining step in Cr-based ethylene trimerization catalysis. These theoretical results provided much deeper insight into understanding the highly selective trimerization mechanism and for further development of new catalysts with high performance as well. The spin crossover phenomenon was also found to be very important in the Cr-catalyzed ethylene selective oligomerization similar to the findings in Cr-catalyzed ethylene polymerization and alkyne cyclotrimerization. In recent years, more and more attention from both academic and industrial circles is starting to be put on the Cr-mediated ethylene tetramerization into 1-octene, a more important comonomer for production of value-added polyolefins compared with 1-hexene (Bollmann et al., 2004; van Leeuwen et al., 2011).

6. SUMMARY AND OUTLOOK

In the past decades, extensive investigations on the Phillips chromium catalyst have been conducted through spectroscopic and kinetic characterizations, model catalysts, and molecular modeling, usually through combination of experiments and theoretical calculations. A step-forward understanding of the nature of active sites and polymerization mechanisms has been achieved with particular interests in the following aspects concerning catalyst activation by thermal calcination or reducing agents (like CO, Al-alkyl cocatalysts, and the ethylene monomer itself, etc.), promotion effects of Ti-modification, spin-crossover phenomenon and its effects on the 180 Zhen Liu et al. catalytic reactivity, analysis of the microstructures of polyethylene chain in terms of SCBD, improvements of the mechanical and ultraviolet-resistant properties of CB-reinforced HDPE pipe materials through synthesis of HDPE-g-CB and so on. In spite of the progress achieved so far, the long-standing key question concerning the precise structure of the active sites and the initiation mechanism in terms of the formation of the first CrdC bond on the Phillips chromium catalysts have not been completely elucidated yet. Although the theoretical calculations suggested that the formation of the first CrdC bond was accelerated by the spin-crossover phenomenon, the suggested mechanism still need the support from experiments. Since selective ethylene trimerization was first reported more than 40 years ago, the researches in the field of selective ethylene oligomerization have been conducted tremendously during the last decade. The increased research activity mainly originated from the fast growth of industrial demand for more highly-valuable comonomers, in particular 1-hexene and 1-octene. Although the chromium-based system for the production of 1-hexene was commercialized 10 years ago, a clearer mechanistic understanding and further cost-effective improvements of the catalyst system is highly demanded. Meanwhile, the industrial production of 1-octene and mechanistic understanding on the Cr-ligand-mediated ethylene tetramerization is still in its infancy. The understanding on the structure and oxidation states of active chromium species, the role of organic ligands and the corresponding reaction mechanisms for various chromium-based catalysts is still facing many uncertainties. A clear elucidation of these basic questions requires the combination of multiple techniques especially those in situ/operando methods as well as theoretical molecular modeling. The continuous development of highly efficient new catalysts is driven by the desire from both industry and academia, including the environmental- friendly nonchromium-like molybdenum catalyst for ethylene polymerization and novel catalyst systems for selective ethylene oligomerization, in particular ethylene trimerization and tetramerization. The spin crossover phenomenon, which was found to be generally existing in the Cr-catalyzed ethylene polymerization, acetylene cyclotrimerization, and ethylene trimerization should be further explored in terms of its intrinsic relationship with the catalytic reactivity through both experimental and theoretical investigations. A state-of-the-art catalyst design with greatly improved efficiency based on experimental and computational high- throughput screening techniques would be highly expected in the Ethylene Polymerization and Oligomerization 181 polyolefin field in the near future. Bridging the two most important structure–property relationships between catalysts and polyolefins will become more and more indispensable for product innovations in the development of novel catalysts and polymers with high performance as well as for academic approaches in the understanding of the nature of active sites and polymerization/oligomerization mechanisms, which calls for persistent efforts and tight collaboration of scientists with different expertise from all over the world.

ACKNOWLEDGMENTS We gratefully thank the financial supports by the National Natural Science Foundation of China (No. 21004020, 21104019, 21174037, 21274040, 21304033 and 51003027), the National High Technology Research and Development Program 863 (2012AA040306), and the Shanghai Science and Technology Commission (Key Project for Basic Research 10JC1403700). This work is also financially supported by the Fundamental Research Funds for the Central Universities, research program of Introducing Talents of Discipline of university (B08021).

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