Silica-alumina, acid-base properties

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

The acid-base properties of amorphous mixed metal oxides can be varied by choosing different metal oxide constituents at diflerent concentrations and by changing the treatment of the sample (44). Thus, it appears that, by properly choosing the aforementioned variables, mixed oxides could be used to develop new catalysts with desired acid-base properties. The use of micro-calorimetric adsorption measurements to quantify the acid-base properties of metal oxides and mixed metal oxides has been limited, to date, to a few systems. However, for some of these solids, for example, silica, alumina, and silica-alumina, several investigations have led to a satisfactory description of their acidity and acid strength. We present here a compendium of those measurements and discuss some of the important properties observed. 

Hydrogen-bonding, steric effects, and acid-base properties of phenols are involved in their retention on silica gel and alumina with benzene and isopropylether as eluents. 

One of earliest methods of obtaining information about the acid-base properties of the catalyst surface, in particular of the silica-alumina, was to adsorb from nonaqueous solvents on to them a series of known organic indicators and to notice the color change.J... 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

Umansky et al. [106] showed that the intensive factor (acid site strengths) dominates over the extensive factor (concentration of the active sites) in controlling acid-base properties and the catalytic activity. The conclusion was reached by comparing the activity to convert isobutane of silica-aluminas, Y, ZSM-5, MOR, and b-zeolites, as well as sulfated zirconia and SbFs-promoted silica-alumina to the acidity expressed as Ho values of the strongest acid sites, which were determined from the spectral shift of adsorbed 4-nitrotoluene and 4-nitrofluorobenzene. 

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. 

Nonmodified silica gel is used most commonly for the separation of substances of medical interest. The separation is based on the interactions (hydrogen bonding, van der Waals forces, and ionic bonding) between the molecules of drugs, lipids, bile acids, etc., and the silica gel. Alumina has similar properties but is rarely used. Successful separation of endogenous substances, drugs, or their metabolites can also be achieved using physically or chemically modified silica gel. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

The alumina or silica-alumina supports used in bifunctional catalysts have been shown to be acidic in nature. The acidic properties are readily demonstrated by the affinity of these solids for adsorption of basic compounds such as ammonia, trimethylamine, re-butylamine, pyridine, and quinoline (01, R5). Furthermore, adsorption of certain acid-base indicators such as butter yellow gives a coloration similar to that observed in acid media (B3, B4). With regard to the origin of the acidity, Tamele (Tl) has suggested in the case of silica-alumina that aluminum atoms replace silicon atoms in the surface of the silica structure, giving rise to surface sites of the form... 

Cobalt, copper and nickel metal ions were deposited by two different methods, ionic exchange and impregnation, on an amorphous silica-alumina and a ZSM-5 zeolite. The adsorption properties towards NH3 and NO were determined at 353 and 313 K, respectively, by coupled calorimetric-volumetric measurements. The average acid strength of the catalysts supported on silica-alumina was stronger than that of the parent support, while the zeolite-based catalysts had (with the exception of the nickel sample) weaker acid sites than the parent ZSM-5. The oxide materials used as supports adsorbed NO in very small amounts only, and the presence of metal cations improved the NO adsorption [70]. 

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