Ethylene chromium polymerization catalysts

In using a similar approach, Thiine et al. [29] applied static SIMS to show that Cr/SiC>2 model catalysts which are active for ethylene polymerization contain only monochromates. Secondary ions with more than one Cr ion in the cluster, such as Cr2C>4 and Cr203, disappeared from the spectra after the catalysts had been calcined only CrSiOx ions remained. Aubriet et al. [30] studied the anchoring of chromium acetyl acetonate, Cr(acac)3 to a planar SiO2/Si(100) model support with static SIMS. Chromium polymerization catalysts are discussed further in Chapter 9. 

Most chromium-based catalysts are activated in the beginning of a polymerization reaction through exposure to ethylene at high temperature. The activation step can be accelerated with carbon monoxide. Phillips catalysts operate at 85—110°C (38,40), and exhibit very high activity, from 3 to 10 kg HDPE per g of catalyst (300—1000 kg HDPE/g Cr). Molecular weights and MWDs of the resins are controlled primarily by two factors, the reaction temperature and the composition and preparation procedure of the catalyst (38,39). Phillips catalysts produce HDPE with a MJM ratio of about 6—12 and MFR values of 90—120. 

Fig. 1. Examples of the kinetic curves during ethylene polymerization by chromium oxide catalysts. Support—SiOs temperature—80°C polymerization at constant ethylene pressure in perfect mixing reactor. Curve 1—catalyst reduced by CO at 300°C. Curve 2— catalyst activated in vacuum (400°C) polymerization in the case of (1) and (2) in solvent (heptane) ethylene pressure 10 kg/cm2 02 content in ethylene 1 ppm, HsO 3 ppm. Curves 3, 4, 5, 6—catalyst activated in vacuum (400°C) polymerization without solvent ethylene pressure 19 (curve 3), 13 (curve 4), 4 (curve 5), and 2 (curve 6) kg/cm2 02 content in ethylene 1 ppm, HsO = 12 ppm.

The change of shape of the kinetic curves with monomer and inhibitor concentration at ethylene polymerization by chromium oxide catalysts may be satisfactory described 115) by the kinetic model based on reactions (8)-(14). 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

Although chromium-based ethylene polymerization catalysts have already been developed commercially, these processes are based on a heterogeneous catalyst... 

We begin with the structure of a noble metal catalyst, where the emphasis is placed on the preparation of rhodium on aluminum oxide and the nature of the metal support interaction. Next, we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here, we are concerned with the composition and structure of the catalytically active surface, and how it evolves as a result of the preparation. In the final study we discuss the structure of chromium oxide catalysts in the polymerization of ethylene, along with the polymer product that builds up on the surface of the catalyst. 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

Chromocene (CrCp2) supported on silica is used to generate certain chromium-based catalysts for the polymerization of ethylene (e.g., Phillips and Union Carbide catalysts). The nature of the organometallic species responsible for the catalysis is not known with certainty, though it is noteworthy that some Crm alkyls such as [Cp Cr(CH2Ph)(THF)2]+BPh catalyze the polymerization of ethylene.19... 

The first oxoaUcyl of chromium(V), ( -C5Me5)CrO(Me)2, was prepared via oxygen transfer from trimethylamine N-oxide to the Cr precursor ( -C5Me5)Cr(py)Me2. Its effective magnetic moment at 285 K, 1.76 J,b, is consistent with the presence of pentavalent chromium. At ambient temperature and pressure, this complex does not function as an ethylene polymerization catalyst however, it does react with the starting material to give the Cr dimer [( l -C5Me5)Cr(Me)2]2(Ai2-0). Another chromium(V) alkyl, namely, dinuclear [Cp Cr(0)Me]2(/tr-0), was produced by the reaction of [Cp Cr(/u.-Me)]2 with O2. 

Examples of supports modifying the properties of transition metal oxides have also appeared in the literature. Recent work points to iron oxide phases as important species in Fischer-Tropsch synthesis (3 ). Iron oxide supported on SiO2 (4 ) and TiO ( ) resist reduction under conditions in which bulk iron oxide easily reduces. Thus supported iron oxide catalysts are potentially interesting Fischer-Tropsch catalysts. The extensive studies on ethylene polymerization catalysts suggests that chromium (VI) species exist on a SiOp surface at temperatures above which bulk chromic anhydride (CrOg) decomposes ( ). 

In the literature, most of the early discussion of the "active" valence is in reference to silica-supported chromium oxide catalysts. However, many organochromium compounds of widely differing valence are also known to be active upon contact with a support and subsequent exposure to ethylene. For example, as early as 1961, Walker et al. showed that diare-nechromium(O) compounds polymerize ethylene when deposited onto silica or another support [280,281]. The Cr(0) is probably oxidized by silanol groups to Cr(I), consistent with the inference that it too can be an active precursor. 

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