Alumina chromia

Dehydrogenation ofy -menthadienes and a-piuene ia the vapor phase over catalysts such as chromia—alumina produces y -cymene (70). / -Menthadienes can be disproportionated over a Cu—Ni catalyst to give a mixture of yvmenthane andy -cymene (71). 

Chromia—alumina catalysts are prepared by impregnating T-alumina shapes with a solution of chromic acid, ammonium dichromate, or chromic nitrate, followed by gentie calciaation. Ziac and copper chromites are prepared by coprecipitation and ignition, or by thermal decomposition of ziac or copper chromates, or organic amine complexes thereof. Many catalysts have spiael-like stmctures (239—242). 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

The Physical-Chemical Properties of Chromia-Alumina Catalysts Charles P. Poole, Jr. and D. S. MacIver... 

Dehydrogenation Ethylbenzene Iron oxide or chromia alumina Styrene... 

Neopentane does not undergo isomerization 185) on chromia/alumina (non-acidic) at 537°C, the only significant reaction been hydrogenolysis to methane and iso-C4. However, the reality of isomerization is made clear from, for instance, the formation of xylenes from 2,3,4-trimethylpentane. For o- and p-xylene, the reactions are (24) and (25) 182, 93). These processes are formally quite analogous to those we have described in previous... 

The electron paramagnetic resonance spectrum of transition metal ions has been widely used to interpret the state of these ions in systems of catalytic interest. Major emphasis has been placed on supported chromia because of its catalytic importance in low-pressure ethylene polymerization and other commercial reactions. Earlier work on chromia-alumina catalysts has been reviewed by Poole and Maclver 146). On alumina it appears that the chromium is present in three general forms the S phase, which is isolated Cr3+ on the surface or in the lattice the 0 phase, which is clusters of Cr3+ and the y phase, which is isolated Cr5+ on the surface. The S and 0... 

DETOL [De-alkylation of toluene] A process for making benzene by de-alkylating toluene and other aromatic hydrocarbons. Developed by the Houdiy Process and Chemical Company, and generally similar to its Litol process for the same purpose. The catalyst is chromia/alumina. Licensed by Air Products Chemicals. In 1987,12 plants had been licensed. 

U.S. imports for consumption, 6 545t Chromia-alumina, catalytic aerogels, 7 763t... 

Figure 1.15 Diffuse reflectance UV-Vis spectra from a series of chromia/alumina catalysts after various treatments [115], All these spectra display a shoulder at about 16,700 cm-1 corresponding to the first d-d transition of Cr3+, but the main feature seen in the hydrated and calcined samples at about 26,000 cm-1 due to a Cr6+ charge transition is absent in the data for the reduced sample. This points to a loss of the catalytically active Cr6+ phase upon reduction. (Reproduced with permission from Elsevier.)...

Pines and Csicsery (90, 90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over non-acidic chromia-alumina. A similar, 1,3-methyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (90b). Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 

Stepwise Ce dehydrocyclization was observed over potassia-chromia-alumina as well as potassia-molybdena-alumina catalysts (9, 10). Higher operating temperatures (450°-500°C) of these catalysts facilitate the appearance of unsaturated intermediates in the gas phase. Radiotracer studies indicate a predominant Ce ring closure of C-labeled n-heptane over pure chromia (132,132a). 

Froment and BischofT (19) report a study of the dehydrogenation of 1-butene to butadiene on a chromia-alumina catalyst. Neglecting isomerization of 1-butene, the following steps are postulated ... 

Following a variation of the well-known Horiuti-Polanyi mechanism, we consider the following steps as possible for the system -butane- -butenes-hydrogen over chromia alumina catalyst ... 

In view of these considerations, a large amount of effort is reported in the scientific press on the development of a process to produce benzene from n-hexane by combined cyclization and dehydrogenation. w-Hexane has a low Research octane number of only 24.8 and can be separated in fair purities from virgin naphthas by simple distillation. Recently, an announcement was made of a process in the laboratory stage for aromatiza-tion of n-hexane (16). The process utilizes a chromia-alumina catalyst at 900° F., atmospheric pressure, and a liquid space velocity of about one volume of liquid per volume of catalyst per hour. The liquid product contains about 36% benzene with 64% of hexane plus olefin. The catalyst was shown to be regenerable with a mixture of air and nitrogen. The tests were made on a unit of the fixed-bed type, but it was indicated that the fluid technique probably could be used. If commercial application of this or similar processes can be achieved economically, it could be of immense help in relieving the benzene short-age. 

Longer life and better activity were obtained with catalysts composed of chro-mia and alumina.151 While pure alumina has little dehydrogenation activity, the incorporation of as little as 3% or as much as 60% chromia provides effective catalysts the most widely used commercial catalyst usually contains 20% chromia. Chromia-alumina is used in the dehydrogenation of C2—C4 hydrocarbons to the corresponding alkenes.152-154 1,3-Butadiene may also be manufactured under appropriate conditions (see Section 2.3.3). 

Butadiene and Isoprene. Butane may be transformed directly to 1,3-buta-diene on chromia-alumina (Houdry Catadiene process).144-146 172 The most significant condition is operation under subatmospheric pressure (0.1-0.4 atm), which provides an improved yield of 1,3-butadiene. Operating at about 600°C, the process produces a mixture of butenes and 1,3-butadiene. After the removal of the latter, the remaining butane-butenes mixture is mixed with fresh butane and recycled. Extensive coke formation requires regeneration of the catalyst after a few minutes of operation. 1,3-Butadiene yields up to 63% are obtained at a conversion level of 30 40%. 

Much higher butadiene yields may be obtained in a two-step process developed by Phillips in which butane is first converted to butenes with the chromia-alumina catalyst, and the butenes are then further dehydrogenated to 1,3-butadiene.144 173 The butene selectivity in the first step is about 80-85% (600°C, atmospheric pressure). The butenes recovered from the reaction mixture undergo further dehydrogenation in the presence of excess steam (10-20 mol) over a mixed... 

Catalytic reforming has become the most important process for the preparation of aromatics. The two major transformations that lead to aromatics are dehydrogenation of cyclohexanes and dehydrocyclization of alkanes. Additionally, isomerization of other cycloalkanes followed by dehydrogenation (dehydroisomerization) also contributes to aromatic formation. The catalysts that are able to perform these reactions are metal oxides (molybdena, chromia, alumina), noble metals, and zeolites. 

Of the metal oxides, chromia-alumina is the preferred and most widely studied catalyst.202 The temperature required for the aromatization of cyclohexanes is much higher (400-500°C) than that over noble metals. 

Chromia and molybdena were found to effect dehydrocyclization of alkanes under reaction conditions similar to those of aromatization of cyclohexanes.96 Because of its great practical significance in refining (hydrorefining), chromia- alumina was extensively studied in the dehydrocyclization of alkanes. 

In a series of experiments carried out by Pines and coworkers,202 the intrinsic acidity of alumina was neutralized before use to avoid ionic-type skeletal isomerization. Radiotracer studies showed that toluene formed from [l-l4C]-heptane over chromia-alumina contained only 18-32% labeling in the methyl group, that is, less than 50% required by 1,6 carbon-carbon closure.204 This indicates that direct 1,6 carbon-carbon closure is not the only path of cyclization of /(-heptane. 

Dienes undergo isomerization due to shifts of the double bonds. The reversible isomerization of allenes to acetylenes is catalyzed characteristically by basic reagents (see Section 4.2.2). Nonconjugated alkadienes tend to isomerize to conjugated alkadienes the conversion is usually accompanied by polymerization. Among other catalysts, activated alumina and chromia-alumina may be used to catalyze the formation of conjugated dienes.89,106-108... 

Matsunaga (15) applied the magnetic techniques of Eischens and Selwood and the chemisorption and chemical techniques of Voltz and Weller to a series of chromia-alumina catalysts. He found that in the limit of low chromia contents, where Eischens and Selwood deduced a two-dimensional distribution of chromium ions, treatment with oxygen at 450°C. resulted in an average valence number of six for all of the chromium ions in the sample. 

Hydrodealkylation, for example, of toluene to benzene, is promoted by chromia-alumina with a low sodium content. 

G-41 A chromia-alumina catalyst, used for hydrodealkylation and dehydrogenation reactions G-S8 Palladium-on-alumina catalyst, for selective hydrogenation of acetylene in ethylene G-52 Approximately 33 wt % nickel cm a refractory oxide support, prereduced. Used for oxygen removal from hydrogen and inert gas streams... 

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