Chromium oxide on silica

A supported catalyst for ethylene polymerization which requires no alkyl aluminum for activation was first claimed by the Phillips Petroleum Company (32). It consists of chromium oxide on silica, reduced with hydrogen. Krauss and Stach (93) showed that the active sites are Cr(II) centers. The presence of solvent, or even aluminum alkyls, diminishes... 

Table IV. Polymerization over Nickel Oxide—Chromium Oxide on Silica—Alumina...

Supported bis(triphenylsilyl) chromate is widely used as a low-activity substitute for chromium oxide in fluidized-bed reactors with gas-phase reactants. To generate sufficient activity, it is necessary to add an organoa-luminum compound (e.g., AlEt3 or AlEt2OEt) to reduce and alkylate the catalyst. The aluminum alkyl is usually impregnated onto the silica-supported bis(triphenylsilyl) chromate. These catalysts usually provide a broader MW distribution than simple catalysts made from chromium oxide on silica, and the two types are often contrasted with each other [150]. Elowever, catalysts made from chromium oxide on silica can be similarly impregnated with such cocatalysts (Section 17) and they then produce the same broad MW distribution [155-159]. 

The tetravalent chromium alkyl compounds were found to give catalysts that are somewhat more active than the catalyst made from the divalent chromium counterpart, under commercial reaction conditions (90-110 °C, 0.5-1.5 mol ethylene L ). Indeed, they were among the most active organochromium catalysts tested in our laboratory. Their overall 1-h yield was usually also superior to that observed with some of the best chromium oxide on silica-titania catalysts. Even when compared with chromium oxide systems used with a cocatalyst, the catalysts made with tetravalent chromium alkyls were equal or better in activity. Unfortunately, for commercial applications, these catalysts also tend to make some oligomers and wax as well. 

The data in Figure 185 allow a comparison of the MW distributions of polymers made from the 250 and 400 °C silicas, with polymer produced by chromium oxide on silica, as shown in Figure 19 or 24, for example. They are quite similar. This comparison supports the argument that both types of catalyst contain the same, or at least similar, active species. In both cases, this may be interpreted as the di-attached species shown in Scheme 38. 

H2 is often added to the reactor to decrease the polymer MW. The MW reduction is thought to occur by simple hydrogenolysis, as shown in Scheme 14. Chromium oxide on silica is not as sensitive to H2 as some other catalysts, such as Ziegler or chromocene catalysts. However, its H2 sensitivity is also not unusual, as many Ballard (zirconium or titanium) catalysts fall into the same category [297,376]. The sensitivity of chromium oxide catalysts can vary considerably, depending on the support, suggesting that various sites respond quiet differently. 

In 1964 a short investigation was made of EPR signals of platinum on alumina by F. Nozaki, D. Stamires and Turkevich.(70) The relation of catalytic activity of transition metal oxides to their EPR properties was studied. Thus in 1967 Kazanski investigated the chromium oxide on silica and its ability to carry out low temperature polymerization of ethylene. 

This technology accoxmts for an annual production of 4-5 million tons of HDPE. The process involves polymerization of ethylene at temperatures below the melting point of the polymer using a solid catalyst to form solid polymer particles suspended in an inert hydrocarbon diluent. Recovery of polymer (by filtration, centrifugation, or flashing) is economic. The chromium oxide-on-silica catalyst developed by Phillips yields polymers which can be easily extruded and blow-moulded. The process is unable to produce copolymers of density below 0.937 g/cm. For a density of 0.92 g/ cm or below, the polymer swells, becomes sticky, and starts to dissolve in the reaction diluent. 

In the 1950s, almost two decades after the launch of LDPE, transition metal catalysts proved capable of producing unbranched linear low density polyethylene (LLDPE) and linear high-density polyethylene (HOPE), both of which had significantly different properties from LDPE. Remarkably, the discovery occuued nearly simultaneously in three different research groups using three different catalyst systems. First was Standard of Indiana s reduced molybdate on alumina catalyst in 1951, followed by Phillips with chromium oxide on silica ( chromox ) catalysts, and Ziegler s titanium chloride/ alkylaluminum halide systems in 1953 (only the latter two were widely commercialized). At about the same time, crystalline polypropylene (PP) was produced in the Phillips labs... 

A series of catalytic systems was also developed, in which the mechanism of action is similar to Ziegler-Natta catalysts. These are the oxides of nickel, cobalt, vanadium and molybdemun deposited on the surface of aluminum and of chromium oxide on silica gel. Such catalysts contain different promoters in the form of metal alkyl. They can be also used for the synthesis of polydienes. 

In catalytic toluene hydrodealkylation, toluene is mixed with a hydrogen stream and passed through a vessel packed with a catalyst, usually supported chromium or molybdenum oxides, platinum or platinum oxides, on silica or alumina. (50). The operating temperatures range from 500—595°C... 

Activity. Although pure CrO begins to decompose above 200 C into Oo and eventually Cr203,l a certain amount (0.4 Cr/nm ) is stabilized on silica up to 900 C in 02. And lower valent salts of chromium are easily oxidized on silica to the hexavalent form. This is probably due to the formation of a stable chromate ester on the silica surface as in (3). 

Chromium oxide on aluminophosphate produces polymers having a broader MW distribution than its Cr/silica counterparts, which is evidence of greater heterogeneity of Cr species on the catalyst surface. Organochromium compounds on aluminophosphate also produce polymers having broad MW distributions, and with these catalysts these same trends become unusually clear. Perhaps because the chromium tends to bind through only one link to the surface instead of two, it is often possible to obtain more detailed information about the catalyst from the resultant polymer. 

Low-pressure Processes. Three processes for the polymerization of ethylene have recently been developed. The commercial process of the Phillips Petroleum Company for the polymerization of ethylene is carried out at relatively low pressures (100-500 psi) in either fixed-bed or slurry-type operations. The catalyst consists of 2-3 weight per cent chromium as oxide on silica alumina, and the reaction temperature varies from 90— 180°C. In fixed-bed operation, purified ethylene and hydrocarbon solvent streams are passed downflow, liquid phase over the catalsrst bpd. Solvent and polymer are collected, and the solvent is flashed overhead. Unreacted gases are removed from the solvent, taken overhead, and metered the solvent is recycled to the reactor. The solvent and polymer in the first receiver are cooled to room temperature to precipitate the polymer, which is then filtered and dried in a vacuum oven. In the slurry-type operation (indicated in Fig. 15-33 by a proposed flow diagram), solvent and a small... 

Extruded reduced chromium oxide on a porous silica support... 

The user should be familiar with the Schlenk technique for handling air- and moisture-sensitive compounds.Prepurified argon or nitrogen are recommended as inert gas. The argon is dried by use of a column (100-cm length, 5-cm diameter) filled with molecular sieves 4 and 5 A. Traces of oxygen are removed by another column filled with chromium(II) oxide on silica gel, which was prepared as reported by reduction of CrOj on silica gel with carbon monoxide. 

Silica gel is simply impregnated with an aqueous solution of chromium (III) acetate to give about 1% chromium on the catalyst. This low chromium content gives the maximum yield per unit weight of catalyst used during operation, and avoids the clumping of uncomplexed chromium oxide on the surface. It has been estimated that at this stage only about one-third of the total chromium... 

The preferred catalyst is one which contains 5% of chromium oxides, mainly Cr03, on a finely divided silica-alumina catalyst (75-90% silica) which has been activated by heating to about 250°C. After reaction the mixture is passed to a gas-liquid separator where the ethylene is flashed off, catalyst is then removed from the liquid product of the separator and the polymer separated from the solvent by either flashing off the solvent or precipitating the polymer by cooling. 

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