Silica-aluminas definition

The silica-alumina surface is still more strongly acidic than the alumina surface. The acidity is less sensitive to poisoning by water. There has been much discussion whether the acidity of silica-alumina is caused by Bronsted or by Lewis acid sites. This matter has not been. settled definitely, although there is evidence that both types of acidity are present. This would explain the observation that the catalytic efficiency in different reactions may be selectively poisoned by different reagents. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

If water was added to the 15% Si02 co-gel to fill the pore voids a partially recrystallized boehmite was formed with a surface area of 464 m /g and with a pore volume of 1.8 oc/g. If water was added to the 15% Si02 co-gel to form a slurry and then dried and calcined at 500 a partially recrystallized bodunite was formed with a surface area of 334 m /g. steam treatment at 760 of this second, small pore, bodunite-like silica-alumina resulted in no change in the surface area. The gas oil cracking activity of the steamed sample was definitely hi er than that for the amorphoias co-gel, i.e., a Micro Activity Test (MAT) Activity Number of 38 (see Table 1.). 

On a silica-alumina catalyst poisoned by the exchange of the protonic sites for Na+ the 420 band is eliminated, but the 330 m/x. one remains. This definitely disproved the assignment of the latter to the carbonium ion 92). It is more likely that just this band does belong to the charge-transfer optical transition sought for. In fact it is situated in the same spectral range where charge-transfer bands of a variety of intermolecular and 7r-complexes are known to lie. 

It is also interesting to note that a small but definite increase in stereoselectivity was always observed in the presence of strong Lewis sites, that is when using silica-alumina cogels or Cu systems. The interaction of the carbonyl group with the Lewis site probably favours the formation of the more stable isomer. 

The number of active sites on M-46 determined by the TPD technique (2 to 4 X 10 sites/cm ) is much smaller than estimated from the conventional measurements of acidity. However, it agrees well with the recent estimates for silica-alumina by Peri (less than 10 sites/cm ) 23) and by Leftin and Hermana (9 X 10 sites/cm ) 24). It also agrees well with the estimate of Lewis acid centers on M-46 by Leftin and Hall (4 to 6 x 10 sites/cm ) 25). While it may therefore be tempting to conclude that the sites observed by the TPD technique are Lewis acid sites, not enough information is available to draw definitive conclusions. 

Matsuishi, T. Shimada, T. Morihara, K. Definitive evidence for enantioselective catalysis over molecular footprint catalytic cavities chirally imprinted on a silica (alumina) gel surface. Chem. Lett. 1992, 1921-1924. 

The chemical structure and physicochemical properties of asphaltenes and resins are not well understood. The operational definitions of asphaltenes and resins are based on their solubility in different diluents. Asphaltenes are defined as the fraction of crude oil insoluble in excess normal alkanes such as n-pentane but soluble in excess benzene and toluene at room temperature. Resins are defined as the fraction of crude oil insoluble in excess liquid propane at room temperature. Resins are adsorbed on silica, alumina, or other surface-active material. Figure 5.8 shows the precipitation when a bitumen oil is mixed with various diluents. The normal alkanes used are n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane. According to the operational definition stated above, the asphaltene content of the bitumen... 

In accordance with the above definitions, a summarized list of solid acids and bases is given in Tables 1.1 and 1.2. The first group of solid acids in Table 1.1 includes naturally occurring clay minerals. The main constituents are silica and alumina. Various types of synthetic zeolites such as zeolites X,Y,A, ZMS-5, ZSM-11, etc. have been reported to show characteristic catalytic activities and selectivities. The well-known solid acid, synthetic silica-alumina, is listed in the seventh group, which also includes the many oxide mixtures which have recently been found to display acidic properties and catalytic activity. In the fifth and sixth groups are included many inorganic chemicals such as metal oxides, sulfides, sulfates, nitrates, phosphates and halides. Many have been found to show characteristic selectivities as catalysts. 

Allophane and halloysite were the subject of detailed consideration by Ross and Kerr [1934]. They employed optical. X-ray, thermal dehydration, and chemical analysis to investigate a number of selected samples of allophane from different localities. They found wide ranges of chemical compositions (Si02, 25.19 to 33.96% AI2O3, 30.41 to 36.53% H20 , 12.84 to 21.20% H20, 14.43 to 20.28%) and of indices of refraction (1.472 to 1.496). From these results and from those of other publications they concluded that allophane is an amorphous material commonly associated with halloysite. It has no crystal structure and no definite chemical composition. The name allophane should be restricted to mutual solid solutions of silica, alumina, water, and minor amounts of bases, but the specimen should include all such materials, even though the proportions of these constituents may differ. This broad definition by Ross and Kerr of the range of properties of allophane is generally accepted. 

Type I isotherms are characterized by a plateau which is nearly or quite horizontal, and which may cut the p/p° = 1 axis sharply or may show a tail as saturation pressure is approached (Fig. 4.1). The incidence of hysteresis varies many Type I isotherms exhibit no hysteresis at all (Fig. 4.1), others display a definite loop, and in others there is hysteresis which may or may not persist to the lowest pressures ( low-pressure hysteresis ) (Fig. 4.2). Type 1 isotherms are quite common, and are no longer restricted, as seemed at one time to be the case, to charcoals. Many solids, if suitably prepared, will yield Type 1 isotherms the xerogcls of silica, titania, alumina... 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

A soluhon explored to definitely eliminate the problem of interachons with free silanol groups was the use of non-silica based stahonary phases. Many studies recently reviewed [9] were conducted on this class of stahonary phases to test their usefulness for UpophiUcity prediction. A few of these phases including polybutadiene-coated alumina [4] and octadecyl-bonded alumina show intereshng results. Conversely, several of them show poor correlahons or correlahons not better than those obtained on ODS stahonary phases between the retenhon parameter and log Pod-... 

In addition to the crystalline clays described earlier, there are some materials that act like clays but do not have crystalline structure. Amorphous clays do not have a definite X-ray diffraction pattern and are differentiated from the crystalline clays on this basis. They are composed of mixtures of alumina, silica, and other oxides and generally have high sorptive and cation exchange capacities. Few soils contain large amounts of amorphous clays [2],... 

But [said he] it was still necessary to learn the base of the salt. Its solution could not be precipitated either by tartaric acid in excess or by platinum chloride. Consequently it could not be potassium. I mixed another portion of a solution of the same salt with a few drops of pure potash, but without its becoming cloudy. Therefore it contained no more magnesia hence it must be a salt with soda for a base. I calculated the quantity of soda which would be necessary to form it but it always resulted in an excess of about 5 parts in 100 of the mineral analyzed. Therefore, since it seemed probable to me that the different substances might not have been well washed, Or that the analysis might not have been made with sufficient precision in other respects, I repeated it twice more with all the care possible, but always with results very little different. I obtained. Silica. 78 45, 79.85, Alumina 17 20, 17.30 Sulfate 19.50, 17.75. At last, having studied this sulfate more closely, I soon found that it contained a definite fixed alkali, whose nature had not previously been known (21). 

There are two main types of alumina (bauxite) ores used as the primary sources for aluminum metal and aluminum chemicals aluminum hydroxide [Al(OH)3] (gibbsite) and a mixed aluminum oxide hydroxide [AIO(OH)] (boehmite). Thus, bauxite is a term for a family of ores rather than a substance of one definite composition. An average composition of the ores used by industry today would be alumina (A1203), 35 to 60% silica (Si02), 1 to 15% ferric oxide (Fe203), 5 to 40% and titanium dioxide (Ti02), 1 to 4%. 

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