Chromia Preparations

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

Catalysts. Chromia prepared by the method of Burwell and others (3, 4). Weight 0.86 gram, Treated with continuous flow of hydrogen and thiophene for 2 hours at 400° C. before use. This particular catalyst deteriorated rather rapidly. 

The nitrogen isotherms in Figure 10.30 were determined on samples of another chromia preparation, gel B, which had been heated for different periods in air (A) or in vacuo (V). Temperature and duration of heat treatment are indicated for each sample. 

Isomerization is much faster than hydrogenation on microcrystalline chromia prepared by activating in hydrogen to 400°. This may be seen... 

Fig. 62. The volcano-shaped curves of the dehydrogenation (I) and the dehydration (II) of isopropyl alcohol, the adsorption potentials q, in kcal, and the heights of energy barriers —E, in kcal, for chromia prepared by different methods. The figures at the secants are the figures of the catalysts 462),...

Bevan et al. 19) studied the electrical conductivity a of pelleted chromia placed between platinum electrodes from room temperature to 750°. They found that oxidized chromia (i.e., chromia prepared in air) possesses a higher conductivity in the presence of air than after evacuation to 10 Torr oxygen pressure, as shown on Fig. 31. The conduc-... 

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). 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

Promoter deposition through different mechanisms can account for different catalyst properties. In particular, chromate depositing as chromia does not easily redissolve but, zinc oxide does redissolve once the leach front passes and the pH returns to the bulk level of the lixiviant. Therefore, chromate can provide a more stable catalyst structure against aging, as observed in the skeletal copper system. Of course, promoter involvement in catalyst activity as well as structural promotion must be considered in the selection of promoters. This complexity once again highlights the dependence of the catalytic activity of these materials on the preparation conditions. 

Unpromoted and chromia-promoted skeletal copper catalysts were prepared as described in detail previously (10, 11, 14, 15) by leaching a CUAI2 alloy, sieved to 106-211pm, in a large excess (500 mL) of 6.1 M NaOH, either alone or containing Na2Cr04 (0.004 M), for 24 hours at 5°C. 

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. 

A series of Chromia-Alumina aerogel catalysts containing different contents of chromium was prepared by autoclave method. The specific areas of the catalysis were measured with Ng at 77°K according to the BET method. Their structural properties were determined from the X ray diffraction patterns recorded on a philips diffractometer PW 1050/70. EPR measurements were performed with a 8ruker ZOO TT spectrometer at 77°K operating in X band. DPPH was used as the g value standard. Kinetic data were obtained in dynamic pyrex microreactor operating at atmospheric pressure as described elsewhere (ref. 3). 

Experimental Activation Energies 1 e Bond Energies of Atoms in the Reacting Molecules with the Catalyst Qak Adsorption Potentials q and the Heights of the Potential Barriers E kg. cat./mole on Chromias of Different Methods of Preparation-, the Subscripts Designate 1—Dehydrogenation of Hydrocarbons, II—Dehydrogenation of Alcohols and Acids, III—Dehydration of Alcohols ... 

The catalytic and structural properties of two chromia-pillared montmorillonites were compared in an effort to establish structure-reactivity relationships in these materials. The basal spacings of pillared products, prepared by reaction of Na+-montmorillonite with base-... 

It is noteworthy that components other than alumina often have detrimental chemical and physical effects on the catalyst. For example, Herman et al. (77) reported that addition of ceria to the Cu/ZnO catalyst lowered methanol conversion by a factor of 5, despite the presence of a large concentration of microparticulate copper metal. This effect was explained by the ability of ceria to drive copper from the active state in zinc oxide solution to inactive metallic copper. Chromia, which had been used as a component of catalysts for methanol for a considerable period of time, is a suitable structural promoter, but some preparations result in an increase of concentration of side products such as higher alcohols (39), dimethyl ether (47), or even hydrocarbons. 

Although the first technical plants for CFC manufacturing used the Swarts catalyst exclusively, heterogeneously catalysed processes are competitive in the situations described above. Metal(III) oxides, especially chromia and alumina, are frequently used as solid catalysts. Moreover, they have often been used mixed with traces of other, usually metal(II), oxides, to prepare catalysts that have perceived advantages. 

Chromia pillared and pillared-delaminated clays have been synthesized from different montmori I Ionites and characterized by a variety of methods. Chromia-sulfide pillared materials show a high activity and selectivity in thiophene HDS and the consecutive hydrogenation of butene. The use of different clays as starting materials for the preparation of Cr-PILC enables control of their textural properties and chromium loading and thus to tailor the activity of these catalysts. 

Chromia pillared montmorillonite with 1.0 -1.1 nm gallery height and a high specific surface area has been prepared successfully by using an elevated temperature (368 K) for the formation of hydroxy-chromium polycations [2]. This catalyst possesses significant activity in the hydrocracking of n-decane. Previously, we synthesized... 

Kratohvil S. and Matjevic, E. (1987) Preparation and properties of coated uniform colloidal particles. I. Aluminum (hydi ous) oxide on hematite, chromia and titania. Adv. Ceramic Mat. 2 798-803. 

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