Transition alumina

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Transition aluminas are good catalyst supports because they are inexpensive and have good physical properties. They are mechanically stable, stable at relatively high temperatures even under hydrothermal conditions, ie, in the presence of steam, and easily formed in processes such as extmsion into shapes that have good physical strength such as cylinders. Transition aluminas can be prepared with a wide range of surface areas, pore volumes, and pore size distributions. 

The support needs to be iaert, which explains the choice of a-Al O most metal oxides, including transition aluminas, cataly2e unselective oxidation. The catalyst has a low surface area, about 1 m /g, and large pores to minimise the influence of intraparticle diffusion, which would reduce the selectivity. 

Alumina exists in several forms, among which only the a-form is crystalline. The others, called transitional aluminas, have spinel-like stmctures with different orders in which layers are stacked. In the spinel stmcture of, for example, MgAl204, oxygen ions are packed in a fee manner, while Mg occupies tetrahedral and A1 octahedral... 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

Characterization of the acid-base properties of transition aluminas by model reaction and correlation with IR study... 

Keywords transition alumina, cyclopentanol/cyclohexanone transformation, acid-base properties, IR study. 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

Among the several transition alumina phases, y-Al203 is the most important and most studied phase for catalysis [57, 58]. However, even nowadays, several aspects of its structural and surface chemistry are still not well understood, since y-Al203 is a poorly crystalline solid, showing some variation in its structural stoichiometry and a wide range of defects. In the last 50 years, several empirical models for y-AI2O3 surface have been reported, trying to explain the complexity of this surface... 

A review of the animal studies concluded that a fibronodular response has resulted only from intratracheal insufflation of catalytically active, low-temperature-range transitional aluminas and high-surface-area aluminas. In general, alumina is efficiently eliminated from the lung and has a low degree of fibrogenicity. 

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... 

As already mentioned, in spite of the widespread use of alumina in industry as adsorbent, catalyst, or catalyst support, there is only a limited understanding about the relationship between its surface properties and dehydroxylation-rehydroxylation behavior. The rehydration-dehydration behavior of transition aluminas containing controlled amounts of pentahedral A1 has been investigated by Coster et al. [185],... 

Although, the decomposition of aluminum hydroxide to oxide is usually written as one step, it can proceed through an intermediate mono-hydroxide [boehmite, A10(0H)], which often has a much higher stability than the hydroxide, decomposing at about 450°C and, if present, can reduce fire-retardant effectiveness. In addition, several so-called transition aluminas are formed upon heating of hydroxides, depending on the applied temperature.31 Boehmite formation can occur when the... 

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