Coprecipitates silica-alumina

Coprecipitates. Silica-alumina coprecipitates are immensely complicated. As a result of structural disorder and substitution of Al(III) for Si (IV) on tetrahedral sites, they exhibit cation exchange behavior (34). The fraction of total Al which occurs on tetrahedral sites and the CEC vary widely in response to the conditions of precipitation and subsequent sample history, especially thermal history. In general, the fraction... 

Surface acidity and catalytic activity develop only after heat treatment of a coprecipitated mixture of amorphous silicon and aluminum oxides. Similar catalysts can be prepared by acid treatment of clay minerals, e.g., bentonite. The acidity is much stronger with silica-alumina than with either of the pure oxides. Maximum catalytic activity is usually observed after activation at 500-600°. At higher temperatures, the catalytic activity decreases again but can be restored by rehydration, as was shown by Holm et al. (347). The maximum of activity was repeatedly reported for compositions containing 20-40% of alumina. 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

In Figure 10 the experimental ZPCs of hydrous silica-alumina coprecipitates (64) are compared with those of dried and ignited silica-alumina catalysts (49), some of the previously discussed aluminosilicate minerals, and the composition dependence derived from Equation 18 assuming ... 

Figure 4. Inhomogeneity of silica-aluminas prepared by various methods. A series of 17 commercial samples of silica-aluminas from seven different producers was submitted to microanalysis. All of them showed considerable fluctuations of composition at the scale of several tens of nanometers to several micrometers. These samples were prepared by coprecipitation or by the sol-gel method. It is not known whether some of these samples were prepared from alkoxides. Smaller but significant fluctuations at the micrometer scale were also observed for two laboratory samples prepared from alkoxides. The samples were dispersed in water with an ultrasonic vibrator. A drop of the resulting suspension was deposited on a thin carbon film supported on a standard copper grid. After drying, the samples were observed and analyzed by transmission electron microscopy (TEM) on a JEOL-JEM 100C TEMSCAN equiped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPM A). The accelerating potential used was 100 kV.

The catalysts studied were prepared and purified by methods previously described1. Briefly, the silica-alumina support was prepared by a coprecipitation method on to the support and possessed a Si/Al ratio of 25. Ni2+ ions were introduced from an aqueous solution of nickel chloride by ion exchange at reflux. The final catalysts contained about 1.5 mass% Ni, with some residual sodium and chlorine, corresponding to a CI Ni mol ratio of <0.1, and a Ni Al mol ratio close to 1.0 (calculated with the assumption that all residual sodium is associated with some of the Al sites in the material (Na/Al mol ratio typically ca. 0.5)). 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

Catalysts. Two types of silica support were used in these experiments. Davison grade 952 silica had a pore volume of 1.6 cc/g and a surface area of about 280 nr/g. The other support was a coprecipitated silica-titania (3.3 wt% Ti02) having a pore volume of 2.5 cc/g and a surface area of about 450 m /g. Ordinarily both supports were first treated with chromium (III) acetate to yield 1 wt Cr. Activation was accomplished in a shallow bed fluidized by air or another gas predried through alumina columns. Gases other than air were also deoxygenated through columns of specially reduced Cr/silica-alumina 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 ... 

The preparation of synthetic silica-alumina catalysts is a relatively simple one, involving the coprecipitation or cogelation of the two hydrous oxides from mixed solutions of sodium silicate and aluminum sulfate. Depending on how the solutions are mixed and on the pH and concentration of the resulting mixture, the combined hydrous oxides will be formed as a coprecipitate, which separates from a greater part of the aqueous phase, or as a true hydrogel, which embraces the entire solution volume. 

It should be pointed out that the coprecipitation or cogelation of silica-alumina from sodium silicate and aluminum salts (sulfate, chloride, or sodium aluminate) results in the formation of a product having strong zeolitic properties. It is necessary to remove the sodium ion by exchanging with another ion such as H+, NH4+ or A10+, and this is usually done by treatment of the precipitate or hydrogel with a dilute solution of ammonium chloride (or sulfate) or of aluminum sulfate. After the sodium is exchanged out, the material is washed free of electrolytes, dried, and calcined (700°C.). 

A series of Co-Fe/silica-alumina catalysts, prepared by a coprecipitation of cobalt and iron species on silica-alumina, had decreasing selectivity (Sjq) and Dc with increasing amount of Fe addition when they were reduced at 500°C for Ih, while the selectivity of the catalysts reduced at 400°C for Ih was increased by Fe addition of... 

Figure 3. DTG-in-Hj profiles of the Co-Fe/silica-alumina precursors prepared by coprecipitation of Co and Fe(III) nitrates.

The 8-phase chromium resonance in chromia-alumina has been observed by several workers. It is dependent upon the alumina support since it does not appear in samples of chromia-silica, although it is observed in the case of chromia-silica-alumina. The isolated Cr + ions could conceivably be situated either in the bulk of the support, or on its surface. There is some evidence that both situations prevail. Nuclear magnetic resonance studies to be discussed presently (121) indicate that for impregnated chromia-alumina catalysts calcined at 500 the majority of these ions are on the surface. However, when such catalysts are heated above 600°, the 8 phase is greatly enhanced because of diffusion of chromium ions from the -phase into interior sites in the alumina lattice. The same situation arises with coprecipitated chromia-alumina catalysts (34). The S-phase resonance intensity of coprecipitated chromia-alumina was found to increase with calcination temperature, indicating an increasing three-dimensional dispersion of Cr + ions. In general, the S-phase resonance dominates the ESR spectra of chromia-alumina catalysts at low chromium concentrations, and therefore it... 

Ni supported on silica-alumina using three different techniques homogeneous deposition precipitation, impregnation and coprecipitation... 

Nickel incorporated onto silica-alumina using various procedures impregnation, coprecipitation and homogeneous deposition precipitation, was tested in propene oligomerization. Homogeneous deposition precipitation was found to be the most active. At a conversion of around 90% of the propene, the products were mainly dimers ca. 60%) and trimers ca. 25%). The product spectrum was unaffected by pressure changes, while higher temperatures resulted in more heavy products. 

The excess sodium is the amount of sodium ion (in milliequivalents per gram of silica-alumina) above the amount equivalent to the sulfate ion contained in the gels. Since at a pH of 7 both silica gel and alumina gel adsorb small amounts of ions from solution, the values for excess sodium ion were corrected by subtracting the small amounts of sodium ion adsorbed by a mechanical mixture of silica gel and alumina gel corresponding to both components in the coprecipitated gel. It should be... 

The reactions involved in the preparation of synthetic zeolites of the type described above are relatively complex. The silicate and the aluminum salts form their respective hydroxides at rates dependii on pH and concentration. There is no reason to assume a coprecipitation in the sense of an intermolecular mixing of the components. A more tenable concept is that-of micellar growth and intermicellar linkages which lead to gel formation, as proposed by Flank (28). Evidently, the alumina micelles or particles are sufficiently influenced by the surrounding silica So that they behave as an acid and thus combine with the positive ions present, in this case sodium ion, to give a ratio of one sodium ion to one aluminum ion as long as the amounts of alumina do not exceed 30 per cent by weight of the silica-alumina. 

In the system Al20 >Si02, the coprecipitation of alumina into silica causes first a d rease of the surface area which seems to stabilize between 25% and 75% of alumina. Al and PO in silica reduce the micropores number and consequently the surface area. 

Oxides of Cr, Mo, and W are usually used for catalysts as mixed oxides with other oxides such as alumina and silica which are prepared by coprecipitation, impregnation, etc. They are seldom put to practical use as simple oxides. Principal reactions catalyzed by these oxides, unlike those observed for silica-alumina or zeolites, often involve redox-type reaction steps, and during these steps reaction intermediates having covalent carbon-metal bonds are formed. Examples of those reactions are dehydrogeneration, hydrogenation and skeletal isomerization of hydrocarbons, and polymerization of olefms, as well as metathesis of olefins and hydrodesulfurization. Therefore, acid-base properties of catalysts usually play secondary roles in catalysts. 

Wright, A. C. S., and A. J. Metson, 1959. Soils of Raoul Island. N,Z, Soil Bur, Bull 10. 50 pp. Yamada, H., and S. Kimura, 1962. Coprecipitation of alumina and silica gels and their transformation at higher temperatures. J. Ceram, Assoc, Japan 70 65-71. 

The removal of sterols, vitamin E vitamers, carotenoids, and other interfering material from the unsaponifiable fraction of food samples has been achieved using one or more of the following techniques coprecipitation of sterols with digitonin (91), precipitation of sterols from a methano-lic solution (195,209), adsorption chromatography on open columns of alumina (70,91,96), thin-layer chromatography on silica plates (209), and solid-phase extraction on silica (68,100) and reversed-phase (210) cartridges. 

Catalyst-supporting materials are used to immobilize catalysts and to eliminate separation processes. The reasons to use a catalyst support include (1) to increase the surface area of the catalyst so the reactant can contact the active species easily due to a higher per unit mass of active ingredients (2) to stabilize the catalyst against agglomeration and coalescence (fuse or unite), usually referred to as a thermal stabilization (3) to decrease the density of the catalyst and (4) to eliminate the separation of catalysts from products. Catalyst-supporting materials are frequently porous, which means that most of the active catalysts are located inside the physical boundary of the catalyst particles. These materials include granular, powder, colloidal, coprecipitated, extruded, pelleted, and spherical materials. Three solids widely used as catalyst supports are activated carbon, silica gel, and alumina ... 

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

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