Preparation of Chromia

Ordinary drying of the gel at room temperature or at 100° followed by heating to 200-400° in an inert atmosphere leads to the formation of a black material which is amorphous to X-rays and which has a surface area in the vicinity of 300 m /gm. The surface area is almost entirely in micropores about 10 A in width 27, 28). The adsorption of propane at — 78° fits a Langmuir adsorption isotherm with great accuracy 27). 

The rate of addition of base to a solution of Cr(H20)fi+ needs to be slow so that the high polymer described above can form properly. Abrupt addition of base leads to quite another product. 

Finally, condensation and elimination of water leads to 0 of CN = 4, but in amorphous gel, the average CN seems always distinctly less than 4. 

For illustrative purposes we give in Table II the water content of chromia as a function of pretreatment temperatures. In obtaining the results at the left, chromia packed in a tube was heated in hydrogen 

Water Content of Chromia Pretreated at Different Temjieraturea 

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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. 

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... 

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). 

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. 

As indicated in Section IV, one might worry that the surface of chromia would become oxidized upon drying in air at 100-110° and that reduction by hydrogen of the oxidized form contributes in some special way to the formation of surface coordinatively unsaturated Cr3+. A chromia gel prepared by the urea method (21) which was dried at room... 

Figure 9 suggests that careful control of conditions of activation would be needed for good reproducibility. The level of reproducibility which we secured was fair (see Table IV) but there are factors affecting catalytic activity which we have not identified. We have established that the activity of our samples of chromia did not depend upon trace impurities of other metals nor upon surface oxidation during drying the gel at 110° in the course of preparation of the catalyst. 

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. 

We mentioned above two copper catalysts produced by coprecipitation, viz., the Adkins catalyst (copper-chromia) and the copper-zinc oxide catalyst. The precursor of the two catalysts is produced by coprecipitation. The preparation of the catalysts involves selective removal of carbonate ions, water, and the oxygen atoms bonded to copper. The intimate mixing of the copper ions with the precursor of the supports and the strong interaction of copper with both zinc oxide and chromia furnish copper particles that are still small even after virtually complete reduction of the copper. 

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 %... 

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. 

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. 

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. 

Cutrufello MG et al (2005) Preparation, characterization and activity of chromia-zirconia catalysts for propane dehydrogenation. Thermochim Acta 434 62-68... 

During the preparation of this manuscript it came to our attention that a very similar technique had been utilized to prepare chromia pillared a-zirconium phosphate [28]. Very large interlayer spacings were achieved with very high surface areas. Thus, this exfoliation... 

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