Chromia-alumina catalyst preparation

It would be expected that chromia-alumina catalysts prepared by coprecipitation techniques should differ from impregnated catalysts, and this difference has been demonstrated by Eischens and Selwood (5) who measured the magnetic susceptibility of a chromia-alumina catalyst (35 wt % Cr) prepared by coprecipitation with ammonium hydroxide from a solution of aluminum nitrate and chromium nitrate. The susceptibility of the reduced catalyst indicated a much greater dispersion of the chromium than was characteristic of the impregnated catalysts. This was attributed to the presence of a three-dimensional dispersion of the chromium in the coprecipitated catalysts, as compared to a two-dimensional dispersion in the case of the impregnated catalysts. 

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). 

A detailed description of a chromia-on-alumina catalyst prepared by impregnation has been given elsewhere . Another supported nonmetallic catalyst widely used commercially is cobalt molybdate-on-alumina. The preparation of this catalyst using an alumina support with controlled pore-size distribution is as follows. Silica-stabilized alumina, with greater than 50% of its surface area in 3-8 nm pores and at least 3% of the total pore volume in pores greater than 200 nm in diameter, is impregnated with an aqueous solution of cobalt and molybdenum. The finished oxysulfide catalyst was tested for hydrodesulfurization of petroleum residuum at 370°C and 100 atm for 28 days and compared with a convential cobalt-molybdate catalyst having a major portion of the surface area in 3-7 nm pores. The latter catalyst and controlled pore catalyst maintained 57 and 80% activity, respectively. 

The method outlined applies equally well to supported oxides of transition metals. The familiar chromia-alumina catalyst is a good example. In such cases, the degree of attenuation of the supported oxide may be much greater than in the gel oxides, which may be considered to be self-supported. All the common paramagnetic oxides have been studied in this way, on a variety of supports, and as prepared by a variety of methods. A few oxides, such as molybdena, for one reason or another do not lend themselves to this method. But for most common catalyst components, the method has proved itself to be a useful supplement to x-ray diffraction. 

C. M. Cunningham and H. L. Johnston explained the zero-order kinetics of the heterogeneous liquid phase o-hydrogen conversion on chromia-alumina catalysts on the basis of the selective adsorption of o-hydrogen pointed out by Y. Sandler. This led us to the successful preparation of 95 %... 

The chromia-alumina catalysts of present concern are, for the most part, fairly high surface area, porous solids available from several manufacturers, generally in a pelleted form. Surface areas usually range from 50 to 300 m /gm, and the chromia contents vary from 5 to 20wt %. Depending upon the particular application, various promoters may be present in the catalyst, the most common of these being alkali metals or alkaline earths in concentrations up to about 2wt %. The preparation of chromia-alumina catalysts is the subject of an extensive patent and... 

When coprecipitation techniques are used to prepare a chromia-alumina catalyst the situation is somewhat more complicated in that not only must one insure a proper dispersion of the two oxide phases, but at the same time care must be taken to obtain the desired surface texture. The concentrations of metal ions in the precipitating solution, the pH and temperature at which the gel is precipitated, and the method of drying the gel are but a few of the many variables which influence the surface texture of the final catalyst. 

In summary, the oxygen chemisorption studies, described above, are consistent with the oxidation studies of Weller and Voltz (25) and the magnetic susceptibility measurements of Eisohens and Selwood (5) since they demonstrate that, in a chromia-alumina catalyst, the chromia tends to form clumps or crystallites on the alumina surface. The extent of chromia area is a function of the preparational method, as well as of the composition of the catalyst, and thus its measurement by oxygen chemisorption can be a useful guide in catalyst preparation. 

Another study of the structure of coprecipitated chromia-alumina catalysts was carried out by Kehl el al. (34,44). They prepared catalysts containing between 0 and 43.7 mole % OaOs which were calcined at 500, 750, 900, and 1400° after drying at 110°. The only phase observed in the coprecipitated samples after drying at 110° was boeh-mite, and the lines of its diffraction pattern were quite broad. With increasing chromium concentration this phase gradually disappeared. 

Historically, the first major advance in our understanding of the physical-chemical structure of a chromia-alumina catalyst resulted from a series of magnetic susceptibility studies carried out by Selwood and Eischens (5). They prepared catalysts by both coprecipitation and impregnation techniques, and measured the magnetic susceptibility as a function of catalyst composition. From the results they were able to draw important conclusions concerning the valence state of the chromium, and the manner in which this chromium was combined with the diamagnetic alumina. The same technique has since been applied by other workers with considerable success. 

The technique of optical and ultraviolet reflectance spectroscopy, as described earlier, provides still another physical-chemical technique which can be used to study chromia-alumina catalysts. Like ESR and NMR, it has the advantage of being applicable to powdered, commercial catalysts without the necessity of special sample preparation. In the present section, some typical reflection spectra of chromia-alumina catalysts will be presented, and it will be shown that these spectra provide a measure of the amount of chromium present in the 6-f oxidation state. Reflectance spectroscopy is particularly useful here because the Cr + ion is diamagnetic, and hence cannot be detected by the usual magnetochemical or ESR techniques. In addition, it turns out that the intensity of the reflectance spectrum of a chromia-alumina catalyst is directly related to the extent to which the surface of the catalyst is covered with chromia, and thus the surface composition of such a catalyst can be roughly estimated from its optical spectrum. 

The statistical model explains a large number of data on chromia-alumina catalysts in the range of low chromia concentrations. The deviations that occur at high chromium contents are believed to result from the sample preparation process. At low chromium concentrations the great majority of the surface chromium ions are adsorbed randomly from the solution. At high chromium concentrations, on the other hand, the majority of the surface chromium atoms are deposited from the... 

Attention will now be directed to several miscellaneous preparations and measurements, each of which yields a little more information concerning the chromia-alumina catalyst system. 

Chromia-alumina catalysts are often prepared by procedures other than the method of impregnation. A precipitated chromia was prepared as follows y-alumina was suspended in 25 per cent ammonium hydroxide solution. The mixture was stirred rapidly while chromic nitrate solution was added from a buret. The resulting mixture was then dried, ignited, and reduced in the same manner as for impregnated samples. A total of four samples was prepared. The susceptibility isotherm for this series is of the same general form as for the impregnation series except that point I is virtually absent. But the most striking... 

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). 

For the case of impregnated chromia-alumina, the area and pore size distribution are primarily those of the alumina, and the amount of chromia or the presence of chemical promoters such as potassium do not ordinarily have a major influence on either. The method of preparing the alumina is of prime concern, not only because this determines the surface texture of the final catalyst, but also because the size of the pores has a strong influence on the physical distribution of the impregnated component on the support surface. 

Comparison of catalytic activities for samples containing different percentages of nickel was done by the method which had previously proved convenient for testing the dehydrocydization of n-heptane over chromia-alumina. In all activity tests the total nickel concentration was set at 3.33 per cent nickel. In all but the most dilute sample this was achieved by making a mechanical dilution of the original prepared catalyst. The diluting i ent was y-alumina identical with that used for impregnation. 

Several analyses are indicated in Table 20-19. The other type of catalyst, i.e., chromia-alumina, appears to be most useful for the dehydrogenation of propane and the butanes. Alumina prepared by calcining specially crystallized alumina trihydrate is used at 600 to 650 C for propane or isobutane. Chromium oxide in the form of its active gel is highly selective in its action but is destroyed by temperatures exceeding 500 C. If alumina is used as the carrier for 10 to 20 per cent chromium oxide, the dehydrogenation reaction proceeds at even 50 to 70 C but the life of the catalyst is short. Both the activated alumina and the alumina-chrome catalysts may be regenerated by careful oxidation of the impurities in a... 

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. 

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. 

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