Organo-chromium Catalysts

Catalysts derived from organo-chromium compounds supported on silica gel are veiy different to the original Phillips chromium oxide catalyst. In the original catalysts one molecule of chromium oxide is bound to two silica units, to form the silyloxy chromate sites. In contrast, organo-chromium compounds are usually only bound to a single site on the sihca support and the other coordination sites are still occupied by the organic ligands. For example, tris-7t-allyl chromium reacts with a hydroxylated support, with the evolution of propylene and the formation of a bis-a-allyl chromium siloxy derivative. 

The support for an organo-chromium catalyst must, therefore, be carefiilly calcined to lower the concentration of surface hydroxyl groups, leaving a sufficiently wide separation to avoid the possible linkage of the chromium to two units of silica.Excessive calcination has to be avoided as it removes too much water, leaving too few hydroxyl groups to achieve the required loading of chromium. 

A new catalyst in which chromocene was supported on silica gel was developed by Union Carbide for use in gas-phase polymerization reactions. A typical catalyst contained about 2.5% by weight of chromium, and was activated by the addition of small amounts of alkylaluminium alkoxides, such as diethyl-aluminium ethoxide. Use of such a catdyst formulation gave rise to a polymer with much narrower molecular weight distributions. The average molecular weight could be controlled by the addition of hydrogen, which caused termination of the growing chain. 

Mixed catalysts have been prepared by reaction of an activated chromium oxide silica catalyst with an organo-chromium compound. Mixed catalysts are 

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Fig. 24. Two types of sites are visible in the molecular weight distribution of polymers from three organo-Chromium catalysts run with hydrogen. Only the support was different. The low MW peak probably results from chromium associated with phosphate, while other sites are associated with aluminum.

Other chromium catalysts for ethylene polymerization employ chromo-cene [246] and bis(triphenylsilyl) chromate [247] deposited on silica-alumina. The catalyst support is essential for high activity at moderate ethylene pressures (200—600 p.s.i.). The former catalyst is activated further by organo-aluminium compounds. Polymerization rates are proportional to ethylene pressure and molecular weight is lowered by raising the temperature or with hydrogen (0.1—0.5 mole fraction) in the monomer feed wide molecular weight distributions were observed. 

The data of Table 55 show how the polymer composition varied with activation temperature. Such observations have been reported from this and other laboratories for catalysts made with several different organo-chromium compounds [301,640,644,654], and most recently by Bade et al. [311], who used chromium allyl to make their catalyst. Presumably, the calcination temperature of the silica resulted in the formation of two very different species. Cr(DMPD)2 reacted with silica treated at 250 and at 400 °C to yield di-attached or coordinated species, whereas it reacted differently with silica treated at 600 °C, because on that support only a single oxide attachment can form. Clearly, the higher OH group population has a major effect on the behavior of the site. 

Iron catalysts [20-24] have been used to make polyethylene-clay nanocomposites where the polyethylene had very broad molecular weight distribution (MWD). Ziegler-Natta [25, 26], organo-chromium (Phillips) [27], and bis(imino)pyridine iron and cobalt catalysts [28] have also been used to make polyolefin-late transition metal catalysts [29, 30], capable of producing highly branched polyethylene from only ethylene and of promoting the copolymerization of ethylene with polar comonomers, have also been apphed to make polyolefin-clay nanocomposites. 

Takashi Mono and coworkers [50] reported a silica-supported organo-chromium compound that provided catalysts that produce polyethylene with a very broad molecular weight distribution. 

Titanium, vanadium or chromium oxides activated with chlorine-free organo-aluminum compounds, triethyl- or triisobutyl aluminum, have also been used as catalysts [285],... 

Slurry processes may use a combination of organo-aluminum and organo-magnesium compounds with titanium tetrachloride. Phillips-type catalysts (supported chromium compounds) are also used. Pressures and temperatures are moderate. 

Catalysts used in the polymerization process. For example, commercial isotactic polypropylene is polymerized from a hetereogenous organo-aluminium-titanium complex (Ziegler-Natta process) [110,1260], or less frequently from metallic oxides of chromium, vanadium or molybdenum bonded to an inert support (e.g. the Philips process) [447]. Transition metal ion contents vary in different commercial samples (Table 2.4). 

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