Catalysts chromium

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. 

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. 

A silver-gauze catalyst is still used in some older processes that operate at a relatively higher temperature (about 500°C). New processes use an iron-molyhdenum oxide catalyst. Chromium or cohalt oxides are sometimes used to dope the catalyst. The oxidation reaction is exothermic and occurs at approximately 400-425 °C and atmospheric pressure. Excess air is used to keep the methanol air ratio helow the explosion limits. Figure 5-6 shows the Haldor Topsoe iron-molyhdenum oxide catalyzed process. 

In polymerization by one-component catalysts [chromium oxide catalyst (75), titanium dichloride 159) at ethylene concentrations higher than 1 mole/liter and temperatures below 90°C the transfer with the monomer is a prevailing process. The spontaneous transfer, having a higher activation energy, plays an essential role at higher temperatures and lower concentrations of the monomer. 

Supported Rhodium Catalysts Alkali Promoters on Metal Surfaces Cobalt-Molybdenum Sulfide Hydrodesulfurization Catalysts Chromium Oxide Polymerization Catalysts... 

The data on the rate of reaction of o-, m-, and p-xylene over vanadium oxide catalyst and of m-xylene over mixed vanadium oxide catalysts (chromium-vanadium and antimony-vanadium) were correlated with the reaction scheme below by the following rate expressions, which are based on the Langmuir-Hinshelwood mechanisms where the adsorption of m-xylene is strong. 

As can be seen from Table 3, the product distribution is different due to the catalyst. Chromium(III) oxide leads, in the case of l,l,2-trichloro-l,2,2-trifluorocthanc and 1,2-dichlo-ro-l,l,2.2-tetrafiuoroethane, to products with roughly the same amounts of fluorine atoms attached to both carbons whereas aluminum trifluoridc leads to products with a high fluori-nation degree on one carbon and a low one on the other carbon. Similar results are obtained with an aluminum trifluoride catalyst prepared from aluminum oxide and chlorofluoroethanes.25... 

Unlike Ziegler catalysts, chromium oxide based catalysts are extremely sensitive to minor changes in the preparation or calcining history. The active sites no doubt respond to the local electronic environment, which determines the molecular weight distribution of the polymer. Therefore, replacing the... 

The modification of the Ni Al alloy by addition of molybdenum or chromium has a significant effect on the properties of the Raney nickel catalyst in the reaction of hydrogenation of valeronitrile. In the case of molybdenum, the catalytic properties may be correlated to the physico-chemical characteristics of the catalysts. Chromium is an effective promoter for initial activity and for selectivity. The mechanism for promotion of chromium in Raney nickel is not known exactly. 

Italogenation catalyst. Chromium carbonyl catalyzes the monohalogenation of cyclohexane by CC14 (78% yield). Other cycloalkanes undergo the same reaction, liioniination can be effected in this way with CBrCl,. Other metal carbonyl complexes arc less active. Cr(CO)f, is actually more efficient than di-/-butyl peroxide. A free indical mechanism is involved. 

Polymers of 1,4-dienes are obtained in the presence of titanium, and also with Co, Ni, and Rh, where allyl complexes can be isolated. 1,2-Polybutadiene can be produced in the presence of Pd, which is not generally regarded as a Ziegler catalyst. Chromium and molybdenum systems have also been used. Whereas structural isomerism is controlled by the metal in the catalyst center, the geometric isomerism is determined by the ligands and counterions. 

Cobalt-molybdenum sulfide hydrodesulfurization catalysts Chromium oxide polymerization catalysts... 

Fig. 1. Steps in the formation of an olefin polymerization catalyst. Chromium is thought to bind the high-surface-area carrier by reaction with hydroxyl groups. Activation is accomplished by calcining the support at a temperature of 600° C or higher, which removes much of the excess hydroxyl group population.

The copper surface areas of fresh (S ) and used (S ) catalysts are demonstrated in Table l. The ratio of S1/S0 exhibits the extent of copper surface area reduced after reaction. The copper surface areas reduce after dehydrogenation reaction. This indicates that sintering occurs in reaction process for all of the catalysts. Chromium promoted catalysts have higher fresh copper surface areas than the unpromoted one as shown in Table 1. The previous results [5] indicated that the catalyst with Cr/Cu molar ratio of 1/10 had the highest stability for unsupported catalyst nevertheless, the catalyst with Cr to Cu molar ratio of 1/40 is the most stable one in Si02-supported case. The stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases. 

Description Catalyst or metallocene is injected directly into the reactor as received in the case of Ziegler-Natta or after activation for chromium catalyst. Chromium catalysts give broad molecular-weight products and supported Ziegler-Natta catalysts produce narrow molecular-weight distribution products. The BP metallocene catalysts offer exceptional strength and clarity normally associated with metallocenes but combined with easy processing. 

Furan has also been labeled with heavy water on supported catalysts (chromium, zinc, and manganese oxides promoted with K2C03) at a temperature of 350°.117 Deuterated furan has also been obtained from the vapor phase decarbonylation of furfural over mixed oxide catalysts in the presence of heavy water. Both of these systems utilize extreme experimental conditions and the methods outlined in Table XII are to be preferred for preparative labeling. 

One is left to ponder initiation by other organochromium catalysts. Chromium allyls or 2,4-dimethylpentadienylchromium(II) could conceivably rearrange into p-l coordination upon addition of ethylene. However, chromocene must initiate the first chain in some other way, because the site must retain the ring. Thus, for chromocene catalysts, the initiation problem is similar to that described for chromium oxide. The diarene-chromium(O) and Cr(0)(CO)6 catalysts may also have this problem. Perhaps this is why these catalysts sometimes initiate polymerization more sluggishly than the chromium alkyls. However, there is also some evidence that the Cr(0) compounds can be oxidized by surface OH groups to leave a Cr-H group, which could also be considered an alkylated species. 

Other Solid-add Catalysts - Chromium on an aluminophosphate support, which is supposed to be a polymerization catalyst, was used in the oligomerization of light olefins and ethylene. For ethylene oligomerization, the catalyst exhibited activity for dimerization, with around 60% conversion to C4 fractions. In another patent the support and the active phase were the same as in the previous patent, except that the support had been treated with a solution of triethylaluminium in toluene before being contacted with the chromium compound. The catalyst system showed selectivity towards trimerization. When... 

Besides substitution reactions, alkanes also can undergo dehydrogenation reactions in which hydrogen atoms are removed and the product is an unsaturated hydrocarbon. For example, in the presence of a catalyst [chromium(lll) oxide] at high temperatures, ethane can be dehydrogenated, yielding ethylene, C2H4. 

Catalyst - chromium/silica catalysts, Ziegler-Natta catalysts, or metallocene ... 

Peher PJ, Blanski BLL Polyhedral oligometallasilasesquioxanes as models for sihea-supported catalysts chromium attached to two vicinal siloxy groups, J Chem Soc Chem Commun 22 1614-1616, 1990. 

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