Chromium-based catalyst systems

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. 

Table 4 Iron-, cobalt-, and chromium-based catalyst systems for CCTP of ethylene...

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). 

Nickel-chromium based catalytic systems can promote cydoisomerization of 1,6- or 1,7-enynes [21]. To achieve enough catalytic activity, the catalyst is attached to an insoluble phosphinylated cross-linked polystyrene. Although the mechanism remains unclear, the reaction seems to proceed similar to the Pd(0)-Pd(II) system and is effident in the formation of both five- and six-membered rings. A... 

The difference in polymerization mechanism between one-component metal oxide catalysts and traditional Ziegler-Natta two-component catalysts seems to exist only in the initiation stage, while the mechanism of continued propagation of polymer chain has many common features for all the catalyst systems based on transition metal compounds. Thus most studies of the chromium oxide catalyst system, for example, deal either with the nature... 

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. 

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]. 

The Nobel Prize in Chemistry was awarded to Karl Ziegler and Giulio Natta in 1963 for their research in developing olefin polymerization catalysts primarily based on titanium compounds and aluminum alkyl compounds required for the catalyst initiation process. However, by 1963 the details on the discovery of the Cr-based catalyst system in which the chromium compound was supported on amorphous silica were widely published and commercially important for the manufacture of HDPE. In the view of this author, the 1963 Nobel Prize awarded in chemistry should also have included two additional scientists, John P. Hogan and Robert L. Banks from Phillips Petroleum. 

Karol and coworkers [34] reported silica-supported chromium-based catalysts based on Bis(indenyl) chromium (II), Bis(fluorenyl) chromium (II) and Bis(9-methyIfluorenyI) chromium (II) in place of chromocene. These catalysts were prepared at room temperature by reacting the dehydrated silica with a hexane solution containing one of the Cr(II) compounds. The Bis(indenyl)-based catalyst exhibited good activity under some polymerization conditions, while the Bis(fluorenyl)-based catalyst was significantly less active than the chromocene-based catalyst. Table 3.8 summarizes some polymerization data for these catalyst systems. 

The particular features of phosphonium salts were exploited for a number of synthetic applications in 2014. Phosphonium chloride salts found applications as chlorine source and as modifiers for homogeneous catalyst systems. As an example, Muller, Rosenthal and co-workers reported the study of a chromium-based catalyst for the selective tri-merization of ethylene. A phosphonium precursor of the type i cyclo-(PR2CH2CH(OH) )2][Br]2) was used for the preparation of iron(n) complexes containing unsymmetrical P-N-P pincer ligands (Scheme 5). The group of Prof. Morris tested these compounds as catalysts for the asymmetric hydrogenation of ketones and imines. ... 

Qi Y, Dong Q, Zhong L, et al Role of 1,2-dimethoxyethane in the transformation from ethylene polymerization to trimerization using chromium tris(2-ethylhexanoate)-based catalyst system a DFT study, Organometallics 29(7) 1588-1602, 2010. 

Twelve-membered rings have been obtained using coordination catalysts. The transJmns,ds-cyc. ododec2Lti ien.e has been prepared with a tetrabutyl titanate—diethylalurninum chloride catalyst (48,49) and with a chromium-based system (50). The trans,trans,trans-isom.e-i. has been prepared with a nickel system. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

The annual production of various polymers can be measured only in billion tons of which polyolefins alone figure around 100 million tons per year. In addition to radical and ionic polymerization, a large part of this huge amount is manufactured by coordination polymerization technology. The most important Ziegler-Natta, chromium- and metallocene-based catalysts, however, contain early transition metals which are too oxophiUc to be used in aqueous media. Nevertheless, with the late transition metals there is some room for coordination polymerization in aqueous systems [1,2] and the number of studies published on this topic is steadily growing. 

Aside from the recently described Cu/Th02 catalysts, copper on chromia and copper on silica have been reported to catalyze methanol synthesis at low temperatures and pressures in various communications that are neither patents nor refereed publications. It is not feasible to critically review statements unsupported by published data or verifiable examples. However, physical and chemical interactions similar to those documented in the copper-zinc oxide catalysts are possible in several copper-metal oxide systems and the active form of copper may be stabilized by oxides of zinc, thorium, chromium, silicon, and many other elements. At the same time it is doubtful that more active and selective binary copper-based catalysts than... 

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