Aluminum-chromium oxides catalyst

Aluminum chromium oxide catalyst Cr203,Af203... 

We begin with the structure of a noble metal catalyst, where the emphasis is placed on the preparation of rhodium on aluminum oxide and the nature of the metal support interaction. Next, we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here, we are concerned with the composition and structure of the catalytically active surface, and how it evolves as a result of the preparation. In the final study we discuss the structure of chromium oxide catalysts in the polymerization of ethylene, along with the polymer product that builds up on the surface of the catalyst. 

The supported chromium oxide catalysts can be prepared by impregnating a silica-alumina support with a solution of chromium ions or by coprecipitating the oxides. The preferred impregnating solutions contain dissolved Cr(N03)s.9H20 or CrOs in nitric acid because catalysts made from chromium chlorides or sulfates retain some of the anions after calcination. The solid mixture of chromium-silicon-aluminum compounds is calcined in dry air at 400-700° C or higher to obtain the desired oxide. This probably results in the reaction of surface hydroxy groups in the support material with CrOs to form chromate (IV) and dichromate (V) species ... 

Because this chapter focuses on molecular transition metal complexes that catalyze the formation of polyolefins, an extensive description has not been included of the heterogeneous titanium systems of Ziegler and the supported chromium oxide catalysts that form HDPE. However, a brief description of these catalysts is warranted because of their commercial importance. The "Ziegler" catalysts are typically prepared by combining titanium chlorides with an aluminum-alkyl co-catalyst. The structural features of these catalysts have been studied extensively, but it remains challenging to understand the details of how polymer architecture is controlled by the surface-bound titanium. This chapter does, however, include an extensive discussion of how group(IV) complexes that are soluble, molecular species polymerize alkenes to form many different types of polyolefins. 

C and 19,600 kPa (2800 psi). The catalyst is a complex aluminum—ca dmium —chromium oxide that has high activity and exceptionally long life. The process is claimed to give a conversion of ester to alcohol of about 99% retaining essentially all of the original double bonds. 

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. 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

Titanium, vanadium or chromium oxides activated with chlorine-free organo-aluminum compounds, triethyl- or triisobutyl aluminum, have also been used as catalysts [285],... 

Hitachi Cable Ltd. (35) has claimed that dehydrogenation catalysts, exemplified by chromium oxide—zinc oxide, iron oxide, zinc oxide, and aluminum oxide—manganese oxide inhibit drip and reduce flammability of a polyolefin mainly flame retarded with ATH or magnesium hydroxide. Proprietary grades of ATH and Mg(OH)2 are on the market which contain small amounts of other metal oxides to increase char, possibly by this mechanism. 

The Econ-Abator system is a fluidized-bed catalytic oxidation system. Catalytic fluidized beds allow for destruction of volatile organic compounds (VOCs) at lower temperatures than conventional oxidation systems (typically 500 to 750°F). The technology uses a proprietary catalyst consisting of an aluminum oxide sphere impregnated with chromium oxide. 

Reforming Both thermal and catalytic processes are utilized to convert naphtha fractions into high-octane aromatic compounds. Thermal reforming is utilized to convert heavy naphthas into gasoline-quality aromatics. Catalytic reforming is utilized to convert straight-run naphtha fractions into aromatics. Catalysts utilized include oxides of aluminum, chromium, cobalt, and molybdenum as well as platinum-based catalysts. 

The direct formation of the S—C bond to form thiophenes is better illustrated by the conversion of 2-alkylthiophenols (1) to benzothiophenes (2) (48JA1561). Passing (1 n =2) over a chromium-aluminum oxide catalyst at 475 °C gave (2 R = R = H) in about 20-25% yield, while similar treatment of (1 n - 3) gave (2 R = H, R = Me) in only 10-12% yield. 2-Mercaptostyrene also cyclized to (2) over this same catalyst, and a number of substituted derivatives of (1), where n =2 and R = 5-NH2, 3-NH2, 5-CN or 5-C02Me, were also converted to benzo[6]thiophene derivatives in low yield (56JOC265). 

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... 

Treatment of butadiene or 1,4-butanediol with hydrogen sulfide over an alumina catalyst, or an iron sulfide/alumina catalyst, leads to the formation of thiophene. This method has been more useful in the benzothiophene series. Styrene with four equivalents of hydrogen sulfide, when passed over an iron sulfide/aluminum oxide catalyst at 600 °C for 20 seconds, gave a 60% mole conversion to benzo[6]thiophene (47JA2008). Similar treatment of ethylbenzene over a chromium oxide-alumina catalyst gave an 18% yield of benzo[6]thiophene, accompanied by the evolution of hydrogen (48JA2495). 

The catalyst may be prepared by impregnating a silica-aluminum support with an aqueous solution of chromium trioxide. After drying, the catalyst is usually activated in a stream of dry air at temperatures ranging from ca. 400°-800°C. Under these conditions catalysts can be prepared with a major percentage of chromium in the form of chromium trioxide. Evidently, the support is not just an inert diluent for chromium oxide by itself is stable only as Cr203 at these temperatures. There must be interaction between chromium trioxide and the support which stabilizes the former. 

Butadiene, a substance used industrially to make polymers, is prepared by thermal cracking of butane over a chromium oxide/aluminum oxide catalyst, but this procedure is of little use in the laboratory. 

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]. 

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