Alumina surface acidity

They represent an improvement over earlier platinum on alumina catalysts in their abiHty to resist coke fouling when operated at low pressures. Dehydrogenation and hydrogenation occur on the active metal sites isomerization takes place on the acidic alumina surface. 

Gas-phase methylation of catechol by methanol was studied on y -AI2O3 modified by the basic elements K, Li, Mg and Ca. Addition of 7.5 at.% Mg to y-AljOa was optimal and increased the 3-methyl catechol selectivity from 0.26 to 0.65. X-ray diffraction experiments showed the diffusion of Li and Mg cations into the y -AI2O3 bulk. This induces a change in the surface species (XPS data) and the surface acid-base properties (TPD experiments). Ca and K addition to y-alumina was ineffective due to formation of basic oxide layers on the sur ce. 

Alumina is not widely used in modem HPLC [48]. Porous gamma alumina is prepared by dehydration and thermal treatment of crystalline bayerite [8,49]. It is available in several types with pore diameters from 6-lS nm, surface areas 70-250 m /g and pore volumes 0.2-0.3 ml/g. After conditioning with acid or base its apparent surface pH can be adjusted between pH 3-9. The alumina surface is more heterogeneous than silica containing both hydroxyl... 

The acid-base oxides such as aluminas were used as catalysts, adsorbents or catalyst supports and it was interesting to know the surface acid-base properties of these catalysts. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Prediction of interaction between metal clusters with oxide surface The HSAB principle classifies the interaction between acids and bases in terms of global softness. In the last few years, the reactivity index methodology was well established and had found its application in a wide variety of systems. This study deals with the viability of the reactivity index to monitor metal cluster interaction with oxide. Pure gold cluster of a size between 2 and 12 was chosen to interact with clean alumina (100) surface. A scale was derived in terms of intra- and intermolecular interactions of gold cluster with alumina surface to rationalize the role of reactivity index in material designing [43]. 

The basic and acid sites on alumina surfaces have been represented graphically (43, 72). In order for the acid and the basic sites of the alumina to attack tra s-l,4-cyclohexanediol from different planes of the catalyst surface it is necessary for the dehydration to be restricted to submicroscopical holes or crevices or to occur between channels of those particles. Since the basic and acid sites of the alumina have to surround the cyclohexanediol, as in the solvolytic reaction, the alumina therefore can be considered as a pseudosolvent for such dehydration reactions. 

The effect of fluoridation with hydrofluoric acid on the. surface acidity of catalytic alumina was also studied by Ballou e< al. (340), About 1 % of fluoride was taken up by the catalyst. The surface acidity was estimated using a titration technique developed by Benesi (341a). After addition of varying amounts of butylamine in benzene solution, the color changes... 

Silica-alumina mixtures are of great technological importance in the oil industry as catalysts for petroleum processing. The cracking activity is closely linked to surface acidity. Other typical reactions catalyzed by silica-alumina are the dehydration of alcohols and the polymerization of olefins. 

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. 

Surface compounds other than saltlike compounds on the surface of silica-alumina would not be expected to be very stable due to the strong surface acidity. Traces of water would decompose such compounds. 

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. 

SILICA ALUMINA GEL ACIDIC SURFACE FOR CRACKING LARGE MOLECULES... 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The preparation of precious metal supported catalysts by the HTAD process is illustrated by the synthesis of a wide range of silver on alumina materials, and Pt-, Pt-Ir, Ir-alumina catalysts. It is interesting to note that the aerosol synthesis of alumina without any metal loading results in a material showing only broad reflections by XRD. When the alumina sample was calcined to 900°C, only reflections for a-alumina were evident. The low temperature required for calcination to the alpha-phase along with TEM results suggest that this material was formed as nano-phase, a-alumina. Furthermore, the use of this material for hexane conversions at 450°C indicated that it has an exceptionally low surface acidity as evidenced by the lack of any detectable cracking or isomerization. 

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