Transition aluminas, preparation

Ishikawa T., Ohashi R., Nakabayashi H., Kakuta N., Ueno A., Furuta A. Thermally stabilized transitional alumina prepared by fume pyrolysis ofboehmite sols. J. Catal. 1992 134 87-97 Ishiguro K., Ishikawa T., Kakuta N., Ueno A., Mitarai Y., Kamo T. Characterization of alumina prepared by sol-gel methods and its application to M0O3-C0O-AI2O3 catalyst. J. Catal. 1990 123 523-533... 

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. 

Alumina and /-alumina are also called active aluminas. These materials are seldom phase pure and contain other transition aluminas as impurities. Their properties strongly depend on the type of starting materials, the procedure chosen for thermal treatment, and the operation parameters such as temperature and pressure. Highly active y-alumina has been prepared by shock calcination followed by rehydration [43],... 

The less compact transition aluminas (y-type) are highly porous, more reactive and do not occur in nature. They are prepared by the heat treatment of Al(OH)3 or AlOOH at intermediate temperatures and undergo an irreversible change to a-Al203 at high temperature. Their BET areas are typically 300-400 m2 g 1 and they are widely used as catalysts and catalyst supports. 

We will next consider the case of a lew silica content co-gel. A 5% silica-content silica-alumina was prepared by precipitation of aluminum isdsutoxide and tetraethoxv-silane as described for the silica-free gel. After gelation water was added just sufficient to fill the pore voids of the gel. The added water led to formation of a boehmite-rich hase during recrystallization. After drying at 120 and calcination at 500 0 for 16 hours, a transitional alumina hase is formed with a surface area of 410 m /g and a pore volume of 1.9 oc/g. This silica-alumina had an average pore diameter of 18 nm, similar to the silica-free material discussed previously. Steam treatment of this 18 nm pore diameter silica-alumina at 870°C (1600 ) in 90% H20-10% N2 for 16 hours resulted in a material with surface area of 196 m /g. This surface area is much hi er than expected for an amori ous gel and is consistent with silica enrichment of the outer surface during the recrystallization step vhere water was added to the pores of the amoridious gel. Silica stabilization of bodunite alumina by formation of a surface Aiase complex has been reported in recent work (9). ESCA analysis also indicates silica surface enrichment vhen compared to the amori ous gel. 

The catalytic activity of transitional aluminas (y-, T)-, 5-, 6-AI2O3) are undoubtedly mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as weU as to the high ionicity of the surface Al—O bond [101]. Alumina s Lewis sites have been well characterized by adsorption of several probes. They are the strongest among metal oxides. The number of such very strong Lewis sites present on transitional alumina surfaces depend on the dehydroxylation degree (depending on the activation temperature) and on the particular phase and preparation. 

The support generally used for observation of catalysts in transmission electron microscopy comprises a thin film of amorphous carbon with holes, itself supported by a metallic grid. These carriers allow the observation of several zones of a preparation, of a few hundred square micrometres, without interference. We have pointed out the need to work in transmission electron microscopy on samples with a thickness not much above a hundred nanometres. In certain cases, which include that of reforming catalysts, this criteria is naturally satisfied due to the nature of the support. Transition aluminas are effectively made up of very fine platelets that can be separated by simple grinding. The preparation is then dispersed in a liquid and a drop is collected and placed on the carrier. 

Montanaro L. and Saracco G., Influence of some precursors on the physico-chemical characteristics of transition aluminas for the preparation of ceramic catalytic filters. Ceramics International 21 A3 (1995). 

The most common catalyst supports are the transitional aluminas, particularly y alumina, which is best prepared by heating v)/ boehmite. This process gives a y alumina having a surface area between 150-300 m /g, a pore volume between 0.5-1 cm /g and a large number of pores in the 3-12 nm range. The Y aluminas prepared from other sources have significantly lower surface areas and pore volumes. In contrast, a alumina, the most dehydrated form of alumina, is essentially non-porous with surface areas between 0.1 and 5 m /g. ... 

The mesoporous aluminas synthesized using a nonionic templating method are thermally stable not only to template removal, but also to prolonged heating at elevated temperature. Therefore, these aluminas would be able to maintain their unique structural features in fairly demanding catalyst preparations and catalytic applications. Unlike sol-gel-derived aluminas, the synthesis temperature used for the hydrolysis and condensation of the aluminum alkoxide did not affect the resulting thermal evolution from the aluminum hydroxide to transitional alumina and the subsequent thermal stability of the transitional alumina. The only observed effect of synthesis temperature was the impact on median pore diameter and pore volume.[231]... 

Sol-gel chemistry offers flexible methods for the preparation of porous metal oxides such as the transition aluminas used as catalyst supports. The physical properties of sol-gel materials depend on the nature of the reactants, the rate of mixing and the conditions of drying. High surface area aluminas have been prepared from various alkoxide and salt solutions and their textures have been examined extensively. The interest in the use of these chemically prepared materials is largely for catalyst and absorbent applications [1]. The first user of alkoxide precursor was Teichner who prepared pure aluminas by the water vapour action on aluminimn methoxide [2] and reported materials with surface area of 200 m /g. Harris and Sing [3] reported gels prepared from aluminium isopropoxide with water and... 

Among the numerous transition aluminas (as illustrated in Fig.1),gamma, delta and theta A O are the most used. These disordered spinels can be prepared by dehydration of boehmite. Fig. 2 shows the main ways of producing such aluminas. 

In Chapter 3, Busca summarizes the current state of knowledge of aluminas, the various polymorphs of which constitute some of the most commonly used catalyst components. The author starts with a discussion of the bulk structures of transition aluminas, which are the intermediate phases formed in the thermal transformation of aluminum oxyhydroxides into the thermodynamically most stable modification, a-alumina. Crucial are the definitions of the various phases, which are based on the methods of preparation rather than on the structural properties. The understanding of many alumina structures is incomplete, and progress, even with modem analytical methods and theory, is hampered by the defective and disordered nature of these materials. The stabilities of the various phases are governed by both thermodynamics and kinetics, either of which can be affected by impurities. The uncertainties in the surface stmctures are even greater than those of the bulk stmctures. Numerous models of alumina surface stmctures have been formulated over decades, but the tme stmctures seem to become even more elusive. Busca concludes his chapter with a list of research needs. 

Colloidal sols can be prepared from aluminum chloride (or nitrate) by dissolving AlCb (or AI(N03)3-9H20) and aluminum pellets in water under reflux, followed by filtration [27]. The hydrolysis/polycondensation reaction is continued until the viscosity of the precursor is suitable for dry spinning. The green fibers are dried, prefired to 800°C and sintered at 1300°C in air. Crystallization of the gel occurs above 700°C. First transition aluminas are formed and then converted to a-alumina [27],... 

The preparation of continuous transition alumina fibers requires the use of liquid precursors with a higher viscosity, e.g. 30- 100 Pa.s. The control of the viscosity of the spinning dope is achieved through (1) the use of high molecular weight polymers and (2) proper vaporization of the solvent in a vacuum [37]. After heat treatment, the fibers are composed of one or several transition aluminas, depending on the sintering temperature. 

The precursor of a similar fiber was prepared by polycondensation of an organoaluminum compound, such as monoisopropoxydiethyl aluminum, dissolved in ethylether [60]. Some isopropoxy groups were presumably replaced by a phenoxy group, such as ethyl 0-hydroxybenzoate, in order to improve the spinnability of the final dope. The polyaluminoxane was dissolved in benzene, the ether was distilled off and ethyl silicate was added. After concentration, the dope was dry spun and the green fibers were aged in a humid atmosphere and calcined. The fibers (Table II) had a glassy appearance and were composed of a nanocrystalline Al-Si spinel phase (or r /y-transition alumina) in an amorphous silica based matrix [33] [53]. After mullitization that starts at 1150 C and is complete after 2 min at 1400 C, the fibers were composed of mullite and corundum [33]. 

As shown in Figure 5 of Chapter 4, the modulus increases as the alumina content of the fibers increases. For a given composition, the stiffness of alumina-silica fibers produced by the sol-gel route depends on the residual porosity and hence the processing conditions. For example, the modulus of alumina fibers with a 96 AbOrSiOz (wt.%) composition increases from 110 GPa for the highly porous r -transition alumina fiber, to 280-300 GPa for the dense 5-AI2O3 fiber prepared at higher temperature. 

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