Chromium-based catalysts

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

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. 

A wide variety of chromium oxide and Ziegler catalysts was developed for this process (61,62). Chromium-based catalysts produce HDPE with a relatively broad MWD other catalysts provide HDPE resins with low molecular weights (high melt indexes) and resins with a narrower MWD (63,64). 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Cocatalysts, such as diethylzinc and triethylboron, can be used to alter the molecular-weight distribution of the polymer (89). The same effect can also be had by varying the transition metal in the catalyst chromium-based catalyst systems produce polyethylenes with intermediate or broad molecular-weight distributions, but titanium catalysts tend to give rather narrow molecular-weight distributions. 

Another highly active chromium-based catalyst family is the triazacyclohexane series (103).269 Activities are dependent upon the length of the alkyl substituents attached to the nitrogen donors, reaching 717 g mmol- 1h 1 bar-1 for R = n-dodecyl. With higher a-olefins, or when branched R substituents such as 3-propyl-heptyl are employed, these compounds behave as trimerization catalysts 270,271... 

Chromium(III) acetylacetonate, physical properties, 6 528t Chromium alloys, 6 468-523 Chromium alumina pink corundum, formula and DCMA number, 7 347t Chromium antimony titanium buff rutile, formula and DCMA number, 7 347t Chromium-based catalysts, 20 173 Chromium baths, 9 800-804... 

The basic chain transfer with chromium-based catalysts is a spontaneous p-hydrogen transfer to the transition metal leading to a vinyl chain end ... 

The commercial production of high-density polyethylene started almost at the same time in late 1956 by Phillips using a chromium-based catalyst in a medium-pressure process and by Hoechst using a Ziegler catalyst in a low-pressure process. Polypropylene production began in Montecatini and Hercules plants in 1957. Poly(l-butene) and poly(4-methyl-1-pentene) have been produced in small commercial quantities since about 1965. The commercial production of ethylene/propylene-based rubbers started in 1960 [241]. 

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 second step in the process given by Eq. (16) is catalyzed by chromium-based catalysts. The effect of oxyfluorides of chromium to carry out the addition reaction of HF to trifluoroethylene at mild temperatures has been described by Von Halasz (49). 

Due to increasing environmental problems, the use of metal catalysts will gradually be reduced in the future the reduction in the early use of lead- and chromium-based catalysts is evidence of this. With stricter regulations governing the release of metals, the cost of catalyst recovery and environmental remediation is quickly making noncatalytic processes for the production of carboxylic acids preferable. Coupled with recent advances in the field of biocatalysis, metal-mediated oxidations may give way to alternative processes as we enter a new millennium of chemistry. 

Process description The INNOVENE S process utilizes a proprietary vertical slurry-loop reactor, as shown in the flow diagram. Two reactors are used for bimodal capability. Isobutane is normally used as the hydrocarbon diluent in the process, although hexane may be used as an alternative. The diluent is used as a catalyst carrier and as the polymerization reactor suspension and heat transfer medium. Hexene-1 and/or butene-1 can be used as a comonomer. Hydrogen is used for molecular weight control when using the Zieglerg catalyst platform. Titanium-based and chromium-based catalysts are both used. 

Other chromium-based catalysts have been explored which, when supported, afford polyethylene with relatively narrow molecular weight distribution. These are based on mono(cyclopentadienyl) chromium alkyl complexes first explored by Theopold. These may be Cr(II) compounds such as [Cp -CrMe]2, Cr(III) oxo compounds such as Cp Gr(0)-Me2, neutral and cationic Cr(III) compounds such as Cp CrMe2(THF) and [Cp CrMe(THF)2][BPh4],227 mixed valence dimers such as Gp Gr( -GH2Ph)(M- 7 7 -CH2Ph)CrCp, or even anionic complexes such as [Li] [Cp Cr(CH2Ph)3] (Table... 

Manyik RM, Walker WE, Wilson TP (1997) Soluble chromium based catalysts for ethylene trimerization and polymerization. J Catal 47 197-209... 

Sasol reported the use of a chromium-based catalyst (Cr salt, 2,5-dimethylpyrrole ligand) for the trimerization of ethylene into 1-hexene (P= 5 MPa, T= 115 °C) [18, 19]. According to Sasol, the IL of choice is l-ethyl-2,3-dimethylimidazolium chloride/ AlEtj containing an excess of AlEtj. When compared to typical homogeneous catalytic systems, the activity and yield in 1-hexene are lower. 

The final category of chemical synthesis catalysts is the iron-based catalysts for ammonia and the copper- or chromium-based catalysts for methanol synthesis. The cost is relatively low but frie tonnage for ammonia alone is about one-fifth of total catalysts for chemicals. 

During the late 1970s, Union Carbide developed a low-pressure polymerization process (Unipol process) capable of producing polyethylene in the gas phase that required no solvents. The process employed a chromium based catalyst. In this process (Figure 4.1) ethylene gas and solid catalysts are fed continuously to a fluidized bed reactor. The fluidized material is polyethylene powder which is produced as a result of polymerization of the ethylene on the catalyst. The ethylene, which is recycled, supplies monomer for the reaction, fluidizes the solid, and serves as a heat-removal medium. The reaction is exothermic and is normally run at temperatures 25-50°C below the softening temperatures of the polyethylene powder in the bed. This operation requires very good heat transfer to avoid hot spots and means that the gas distribution and fluidization must be uniform. 

Reaction of tiichloroacetaldehyde with hydrogen fluoride in the presence of chromium-based catalysts gives CFs-CHO.HF which, on treatment with an alkoxysilane (RO)4Si or with ROH-SiCU (R = Me, Et, Pr, or n-CsHu), yields the corresponding hemiacetals CF3-CH(OH)OR (69— 92%) the acetal I-----------------------1... 

Today, the slurry polymerization process is being run all over the world with great success. The current installed capacity for the described process is around 6 million tons per year, which represents approximately one sixth of the worldwide PE-HD production. Other processes for PE-HD production are the Phillips process, operated with chromium-based catalysts, and other slurry processes also operated with Ziegler-type catalyst systems [60]. 

A typical chromium-based catalyst is prepared similarly. It is usually supported by a 9 1 of Si02 Al203 carrier. Either Cr(N03)3 9H20 or C1O3 solutions in nitric acid are used to impregnate the support. The nitrates are decomposed in air at 400 to 1000 An optimum chromium content... 

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