Union Carbide Chromium Catalysts

Chromocene catalyst has excellent hydrogen response for molecular weight control. 

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Figure 5.5 Structures of chromium compounds used in supported Union Carbide PE catalysts 67...

Table 5.25 Union Carbide chromium-based catalysts for the manufacture of HDPE (1968).

Union Carbide Corp. also uses a siUca-supported chromium catalyst in their extremely low cost Unipol gas-phase linear low density ethylene copolymer process, which revolutionized the industry when it was introduced in 1977 (86—88). The productivity of this catalyst is 10 —10 kg polymer/kg transition metal contained in the catalyst. By 1990, the capacity of Unipol linear low density polyethylene reactors was sufficient to supply 25% of the world s total demand for polyethylene. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

In 1951 Robert Banks and Paul Hogan of Phillips Petroleum discovered that ethylene could be polymerized under rather mild conditions of temperature and pressure to afford high molecular weight polyethylene using chromium trioxide as the catalyst. This invention laid the foundation for both the Phillips and Union Carbide processes for ethylene polymerization (both use heterogeneous chromium catalysts). 

Most catalysts are based on chromium that has been studied for this purpose since the mid-seventies, probably started by Union Carbide Corporation. Chromium is the metal of the Phillips ethene polymensation catalysts and presumably it was discovered accidentally that under certain conditions 1-hexene was obtained as a substantial by-product. Neither the precise catalytic cycle nor the intermediate complexes or precursors are known. It is generally accepted that an alkyl aluminium compound first reduces the chromium source and that coordination of two molecules of ethene is followed by cyclometallation, giving a chromocyclopentane. During the cyclometallation the valence of chromium goes up by two and thus a starting valence of either one or zero seems reasonable. This cyclic mechanism explains why such high selectivity is obtained [5],... 

Union Carbide (95, 96) developed chromium catalysts that need activation by aluminum alkyls, but which have the advantage that molecular weight can be controlled with hydrogen, as in the case of the titanium-based catalysts. To prepare the catalysts, organosilanols are combined with chromium trioxide to afford silylchromate. The silylchromate is then deposited on a silica support and activated with an aluminum alkyl (see Fig. 13). 

The chemical anchoring of complexes to a solid, such as silica-supported chromium catalyst, has been successfully used by Union Carbide for ethylene polymerization (see Section 6.2). 

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... 

Another example for a defined pincer carbene chromium(III) catalyst for the oligomerisation of ethylene also comes from McGuinness et al. [482]. It is an example of a nonmetallocene polymerisation catalyst [483] that traditionally falls into the categories of chromium based Phillips and Union Carbide systems [484], nickel based SHOP catalysts [485] and titanium/aluminium based Ziegler-Natta catalysts [29], The chromium(III) pincer carbene catalysts are highly active for the oligomerisation of ethylene and produce mainly a-oleflns. 

Though Phillips catalysts are by far the most important supported chromium catalysts for polyethylene, there are other commercially important examples of such catalysts. These were developed primarily in the 1970s by the Union Carbide Corporation (6), now part of the Dow Chemical Company. UCC chromium catalysts for polyethylene will be discussed in section 5.4. 

Figure 5.5 Compounds used to produce supported chromium catalysts developed by Union Carbide for use in gas phase processes for LLDPE and HOPE. Catalysts must be supported, usually on silica, for optimal performance. Chromocene catalyst is used without a cocatalyst BTSC uses diethylaluminum ethoxide as cocatalyst.

Supported chromium catalysts were developed by Union Carbide Corporation in the 1970s using different chromium precursors than are used in standard Phillips catalysts (6,11). The most important of these are based on chromocene and bis(triphenylsilyl)chromate, depicted in Figure 5.5. These catalysts are used in the Unipol gas phase process for LLDPE and HOPE and are different from standard Phillips catalysts in several respects ... 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

A competitor of Phillips catalyst, based on chromium oxide supported on silica, is the Union Carbide catalyst, which is prepared by the reaction of chromocene with silica. When chromocene, [Cp2Cr ], reacts with SiO2-(800)> it gives [(=SiO)Cr(Cp)] according to mass balance analysis (Scheme 42 and Table 12), and this surface complex is highly active in ethylene polymerization. ... 

The use of fluidized beds for gas-phase polymerization started in 1968 with the UNIPOL process, which was developed by Union Carbide to make high-density polyethylene. This process has now been adapted to produce other grades of polyethylene as well as polypropylene and various copolymers. The fluid bed is composed of porous particles, which are aggregates of polymer containing fine grains of titanium, chromium, or other metal catalyst. Polymerization takes place at the polymer-catalyst interface, and the particles grow larger over a period of several hours. Some of the polymer is withdrawn continuously or at intervals to maintain the bed... 

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. 

Gas-phase polymerization n. A polymerization process developed by Union Carbide for high-density polyethylene, particularly for a grade for making paper-like films. Purified ethylene and a highly active chromium-containing catalyst in dry-powder form are fed continuously into a fluidized-bed reactor. The resin forms as a powder, thereby avoiding the gel, discoloration, and contamination problems often associated with conventional polymerization processes. 

The selective trimerization and tetramerization of ethylene to form 1-hexene and 1-octene has become an important process to generate monomers for the synthesis of LLDPE. 1-Hexene was detected as a byproduct in the polymerization of ethylene catalyzed by homogeneous chromium complexes. Chromium complexes have now been identified that catalyze this oligomerization to form 1-hexene with remarkably high selectivity. Phillips patented the combination of 2,5-dimethylpyrrole, triethylalu-minum, and diethylaluminum chloride with a chromium(lll) salt, and researchers at Union Carbide patented a catalyst generated from chromium(lll) 2-ethylhexanoate and hydrolyzed triisobutylaluminum. - Researchers at BP have described a particularly active and selective catalyst based on a chromium(lll) precursor, a bis(diphosphino)amine ligand, (o-MeO-C H ),PN(CH,)P(o-MeOC H ) and methylaluminoxane (MAO). " ... 

In the late 1970s, Union Carbide introduced its UNIPOL process, which provided a much lower pressure and temperature route to low density polyethylene. The patent by I. J. Levine and Frederick J. Karol claims a supported, suitably modified, chromium catalyst that has been evaluated for use in a fluid bed process. The UNIPOL process jdelds linear low density polyethylene by the reaction of ethylene and an a olefin in a fluidized-bed reactor at 300 psi and 100°C (86). Although this process has many economic and technical advantages over the traditional high pressure polyethylene process, the new product has not displaced the older one. Nevertheless, the UNIPOL process has been a great success, and linear low density polyethylene is extensively used in films. 

Part II - Chromium-Based Catalysts Developed by Union Carbide... 

In the 1960s, scientists at the Union Carbide Corporation developed two additional silica-supported, chromium-based catalysts that are used in a gas-phase process for the manufacture of HDPE. One catalyst is based on... 

A catalyst based on chromocene, Bis(cyclopentadienyl)chromium, was developed in the 1960s at Union Carbide by G. L. Karapinka and cowork-ers and was the first commercial chromium-based catalyst that was prepared with an organochromium compound containing Cr-carbon bonds in the starting material. In addition, the starting material based on Cr(II), did not need to be oxidized to a Cr(VI) species to obtain a high activity ethylene polymerization catalyst. 

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