Calcination silica-alumina

Capping. Capped silica-alumina was prepared by vapor phase transfer of hexamethyldisilazane (> 99.5%, Aldrich) onto calcined silica-alumina until there was no further uptake, as indicated by stabilization of the pressure. The reactor was evacuated and the material heated to 350°C under dynamic vacuum for 4 h to remove ammonia produced during the capping reaction. 

Other solid cocatalysts that are sometimes used include strong solid Lewis acids. For example, calcined silica-alumina, or sulfate- or fluoride-treated alumina or silica-alumina, can adsorb poisons in the reactor, including redox by-products such as formaldehyde. It is surprising how much more activity Cr /silica can produce when tested together with one of these materials. 

It is not possible to decide at present whether the acidity of calcined silica-alumina is due to coordination of water with four-coordinated or with six-coordinated aluminum ions. In summing up all the experimental evidence, the conclusion appears to be that the active centers of... 

A calcined silica-alumina catalyst was also subjected to ion exchange with potassium ion (Table V). For this calcined cataljrst as little as 0.04 milliequivalents of potassium ion per gram of catalyst causes a severe loss of activity. To bring about a given decrease of catal3rtie activity a... 

On reviewing these data it is apparent that the most probable structure of calcined silica-alumina catalysts is a mixture of silica and alumina particles with the silicon and aluminum ions sharing oxygen ions at the points of contact. If this structure is actually present, the chemical properties of alumina in its various crystal forms will be the main controlling factor of the behavior of the mixed oxide system. The crystal habits of silica can be expected to be of merely secondary importance in determining the nature of the catalyst. 

We also see (Fig. 6) that raising the calcination temperature of rare earth modified silica-aluminas from 500 to 600°C, may slightly sinter the rare earth oxide. Although the x-ray pattern... 

We measured the dispersion of Pt (impregnated from a chloroplatinic acid precursor, calcined at 450 C and reduced at 500 C) on a series of Nd203-loaded silica-aluminas (Fig. 8). We find, unexpectedly, that dispersion increases with increasing rare earth oxide loading up to about 18% Nd203, where it plateaus at between 40 and 50%, compared to 10% with unmodified Si-Al. This compares with dispersions of -60-80% measured on similarly Pt-loaded transitional AI2O3 catalysts. Transmission electron micrographs confirmed the decrease in particle size with rare earth content on Si-Al. 

The mordenite zeolites used in this study were purchased from both PQ Corporation (CBV-20A, silica/alumina molar ratio 20, Na20 content 0.02 wt%, surface area 550 m2/g, in ca. 1.5 mm extruded form) and from Union Carbide Corporation (LZM-8, silica/alumina molar ratio 17, Na20 content 0.02%, surface area 517 m2/g in powder form). All samples were calcined at 540 °C prior to use. 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

The SEA approach can be applied to a novel system in three steps (1) measure the PZC of the oxide (or carbon) and choose a metal cation for low-PZC materials and an anion for high-PZC materials, (2) perform an uptake-pH survey to determine the pH of the strongest interaction in the appropriate pH regime (high pH for low PZC and vice versa), and (3) tune the calcination/reduction steps to maintain high dispersion. Highly dispersed Pt materials have been prepared in this way over silica, alumina, and carbon. Other oxides can be employed similarly. For bimetallics, the idea is to first adsorb a well-dispersed metal that forms an oxide intermediate with a PZC very different to the support. In this way the second metal can be directed onto the first metal oxide by SEA. Reduction may then result in relatively homogeneous bimetallic particles. 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

Materials. Methyltrioxorhenium, NH4Re04 and Re207 were purchased from Aldrich and used as received. The silica-alumina was Davicat 3113 (7.6 wt.% Al, BET surface area 573 mVg, pore volume 0.76 cmVg), provided by Grace-Davison (Columbia, MD). For reactions involving MeReOs, silica-alumina was pretreated by calcination for 12 h under 350 Torr O2 at 450°C to remove adsorbed water, hydrocarbons, and carbonates, then allowed to cool to room temperature under dynamic vacuum. The silica was Aerosil 200 (BET surface area 180 mVg, with no significant microporosity) from Degussa (Piscataway, NY). 

Ermakova and co-workers manipulated the Ni particle size to achieve large CF yields from methane decomposition. The Ni-based catalysts employed for the process were synthesized by impregnation of nickel oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium dioxide, zirconium oxide and magnesia). The optimum particle size (10 0 nm) was obtained by varying the calcination temperature of NiO. The 90% Ni-10% silica catalyst was found to be the most effective catalyst with a total CF yield of 375 gcp/gcat- XRD studies by the same group on high loaded Ni-silica... 

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. 

Figure 4. Nitrogen adsorption and desorption isotherm curves and pore size distribution curve (inset) from the adsorption branch of (a) calcined mesoporous silica sphere and (b) calcined mesoporous alumina sphere.

Two pigment production routes are in commercial use. In the sulfate process, the ore is dissolved in sulfuric acid, the solution is hydrolyzed to precipitate a microcrystalline titanium dioxide, which in turn is grown by a process of calcination at temperatures of ca 900—1000°C. In the chloride process, titanium tetrachloride, formed by chlorinating the ore, is purified by distillation and is then oxidized at ca 1400—1600°C to form crystals of the required size. In both cases, the raw products are finished by coating with a layer of hydrous oxides, typically a mixture of silica, alumina, etc. 

Figure 18. Influence of the nature of the support on the reducibility of NiO. Silica aluminas (SA) containing the percentage of Si02 indicated in the designation of the sample (e.g. SA-55 55wt% Si02) were impregnated by nickel nitrate using pore volume impregnation NiO content 10wt%. The samples were dried for 2 h at 383 K., and calcined at 773 K for 6h. Reduction in pure H2 (100 kPa) was carried out at 598 K a represents the fraction of NiO reduced (88).

Table III shows XRD and porosimetry data for calcined USY and AFS zeolites. All samples show shrinkage of the unit cell to comparable values following calcination. As a result, calcined samples are compared at similar silica-alumina framework ratios. All calcined samples have well developed microporous structures and comparable total pore volumes. These porosimetry data confirm that the hydrothermally dealuminated materials contain a significant fraction of mesopores relative to chemically dealuminated materials. The extensive washing given to AFS-1 results in higher micropore surface area and volume compared to AFS-2 and suggest that AFS-2 contains occluded fluoroaluminate and fluorosilicate compounds within the microporous structure.

Following calcination, both USY and AFS materials undergo framework changes but continue to show differences. The 29 spectra show an enhancement of the n-0 peak in both USY and AFS spectra but indicate that silicon distributions are different in AFS and USY materials. Silica-alumina framework ratios calculated from... 

XRD measurements show that calcined AFS and USY zeolites have comparable unit cell sizes. Upon steaming, the unit cell sizes for both AFS and USY reduce to identical values. Hence, framework silica-alumina ratios equilibrate to comparable levels independent of the method by which the zeolites were originally dealuminated. 

As-synthesized AFS zeolites do not contain extraframework aluminum as evidenced by Al NMR. As-synthesized USY zeolites contain appreciable amounts of extraframework material as seen by comparing framework and bulk silica-alumina ratios and by examining 27A1 spectra. Upon calcination both AFS and USY materials contain extraframework aluminum. The amount of extraframework aluminum in both AFS and USY materials increases on steaming. 

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