Pseudoboehmite alumina

Laser Raman spectra of steamed (760 C/5hr) pseudoboehmite aluminas metal loaded with solutions of vanadyl naphthenate in benzene are shown in Fig. 3 band positions are listed in Table 2. Some of these results have already been discussed in details elsewhere (3,4). At low V-loadings, the spectrum is characterized by an intense band near 925 cm 1, Fig. 3B. This band has been attributed to V=0 stretching modes resulting from the presence of isolated vanadyl ions on the alumina surface. Hydrogen bonding, resulting from steaming, is believed responsbile for the broad nature of this band. 

XRD, XPS, and Raman characterization of V supported on pseudoboehmite alumina or on amorphous aluminosilicate gels (metal loaded with a solution of V0+z Naphthenate in benzene) have indicated the presence of tetrahedrally and octahedrally coordinated vanadium (31) having speciation and dispersion that depends on vanadium concentration (and surface area) present on the steamed (760°C/5h) samples. 

In (pseudoboehmite) alumina, A1-0-A1 bonds are not easily broken by oxycations of V and condensed species, -(V-0-V)-, resistant to reduction are formed. In fact, it is believed that following the oxidative decomposition of the naphthenate precursor, V0+ cations... 

Pseudoboehmite alumina (Catapal-B) and 85 wt% H3PO4 were used exclusively as the aluminum and phosphorus starting materials. Aqueous (55 wtX) tetrabutylammonium hydroxide (TBA) and n-dipropylamine (DPA) were purchased from Alfa and Aldrich, respectively. 

Alumina promoted FCC catalysts are commercially viable if, and only if, the alumina component produces the desired properties without detrimentally affecting the attrition resistance and the cracking activity of the finished catalyst particle. Previous work (8.9.11) indicated that attrition resistant catalysts containing alumina could be formed only if a highly dispersed, pseudoboehmitic alumina was used. Other studies have demonstrated catalytic performance improvement without determining the attrition character of the catalyst (1-7). 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

Gel-Based Activated Aluminas. Alumina gels can be formed by wet chemical reaction of soluble aluminum compounds. An example is rapid mixing of aluminum sulfate [17927-65-0], Al2(S0 2 XH20, and sodium aluminate [1302-42-7], NaA102, solutions to form pseudoboehmite and a... 

Hydrolysis of aluminum alkoxides is also used commercially to produce precursor gels. This approach avoids the introduction of undesirable anions or cations so that the need for extensive washing is reduced. Although gels having surface area over 800 m /g can be produced by this approach, the commercial products are mosdy pseudoboehmite powders in the 200 —300 m /g range (28). The forming processes already described are used to convert these powders into activated alumina shapes. 

Membranes. Membranes comprised of activated alumina films less than 20 )J.m thick have been reported (46). These films are initially deposited via sol—gel technology (qv) from pseudoboehmite sols and are subsequently calcined to produce controlled pore sizes in the 2 to 10-nm range. Inorganic membrane systems based on this type of film and supported on soHd porous substrates have been introduced commercially. They are said to have better mechanical and thermal stabiUty than organic membranes (47). The activated alumina film comprises only a miniscule part of the total system (see Mel rane technology). 

In addition to gibbsite there are other routes to manufacture Al(OH>3 and the consecutive transition oxides. One is the precipitation of Al(OH)3 from aluminum salts by adjusting the pH between 7 and 12 by adding bases. Precipitation at elevated temperatures and high pH leads to formation of bayerite, whereas at lower pH pseudoboehmite and subsequently boehmite are formed. By heating, these materials can be converted to the active transition aluminas. 

Using such procedures, high purity pseudoboehmite is produced (PURAL01, Condca Chemic, Brunsbuttel). With this type of microcrystallinc pseudoboehmite spheres of y-alumina can be manufactured. PURAL can also be extruded to pellets with binders and then subjected to calcination to prepare y-alumina. The cx-trudation of various aluminum hydroxides and oxide hydroxides with binders to pellets is covered in Ref. [44], The properties of the different aluminum hydrox-... 

The procedure for the preparation of alumina spheres is as follows. Pseudo-boehmite powder (AIOOH.2H2O) is dispersed in an aqueous solution of urea and a monovalent inorganic acid, e.g. HNO3. The type of powder or powder mixture may exert considerable influence on the properties of the sol and the end product [12]. The type of acid which is used for the peptization of the pseudoboehmite powder is not very important [12], so nitric acid is used, being the most suitable one. 

The viscosity of the sol, which is very important in relation with the dropping, will be controlled by the concentration of the powder, urea and nitric acid. Pseudoboehmite concentrations as high as 33 wt.% are possible. Urea addition lowers the viscosity of the sol and there is an optimum viscosity, depending on the amount of acid added. To obtain a good dispersion of the alumina particles in the sol, the sol must be made by using a high shear mixer. The sol of the correct viscosity is pumped through the orifices and the droplets will fall into the oil... 

Microsphere Formation. Because the microspheres were fabricated using a batch process, we monitored the viscosity and pH of the catalyst slurry as it aged. Figure 2 shows that the viscosity of the slurry was dependent on both the age of the slurry and the additive type. The reference formula was stable for 3 h, but the CP-alumina and pseudoboehmite formulations thickened or gelled in the same time period. A typical batch starting at pH 3.0 increased to about pH 3.3 before the onset of thickening (about 100 cP). For CP formulations, the onset of thickening may be related to the median particle size of the powder. 

The data in Figure 5 can be examined from three viewpoints. First, the curves of the Reference and CP-2 samples resemble that of a commercial material (Catalyst I) that has a low Attrition Index (5.3). Second, the CP-2 curve rises at a slower rate than the Reference. This indicates more attrition resistance in the CP-2 alumina formula. Finally, the pseudoboehmite formula produced fines much more rapidly than the CP-2 product. These findings suggest that the rehydratable alumina is a superior binder for some FCC formulations. 

Even though the differences between the Al NMR spectra of the transition aluminas are subtle, the technique has been used to smdy the thermal transformation sequences of the hydrated aluminas gibbsite, Al(OH)3 (Slade et al. 1991, Meinhold et al. 1993), boehmite, 7-AIOOH (Slade e a/. 1991a, Meinhold era/. 1993, Pecharromm eta/. 1999), pseudoboehmite (Meinhold et al. 1993) and bayerite, Al(OH)3 (Meinhold et al. 1993, Pecharroman et al. 1999). 

For the preparation of washcoated monoliths the suspensions of sol-aluminium hydroxide with pseudoboehmite structure have been used. This sol formed during the reaction between the hydroxide and nitric acid serves both as a binder and a source of y-A Oa in the final product after calcination. Salts of additives were introduced into sol. The influence of the following parameters on the formation of thermostable washcoated layer have been studied concentration of anhydrous alumina in the sol amount of added HNO3 dipping time number of dippings drying and calcination duration. 

The process of making alumina by alkoxide involves three steps hydrolysis of aluminium butoxide, formation of gel and finally pyrolysis of alumina, when aluminum secondary butoxide is hydrolysed at a temperature of about 80°C, it forms pseudoboehmite. The role of solvent and pH in this type of reactions is vital since they partially control hydrolysis and condensation. Therefore, the solvent mixture must dissolve intimately with the reactant species in order to obtain a greater interaction. 

Alumina from the hydrolysis of alcoholates is typically obtained in the form of boehmite or pseudoboehmite. It is important to mention that both processes give products of equivalent quality. 

The initial step of the process is the formation of aluminum triethyl from aluminum metal, ethylene and hydrogen. In a second step ethylene is added to the aluminum triethyl causing the carbon chains to grow in increments of two carbon atoms. After the chain growth reaction the aluminum alkyl is oxidized to an aluminum alkoxide. The alkoxide is then hydrolyzed with water, forming fatty alcohols and alumina slurry. The alcohols and the alumina slurry can be separated from each other and processed into the final products. After drying of the slurry the alumina is obtained in the form of a high purity aluminum oxide monohydrate of boehmite or pseudoboehmite structure. 

Boehmite type aluminas from hydrolysis of aluminum alkoxides are typically not fully crystallized. This is reflected in a broader X-ray pattern as well as in an overstoihcimetric water content. These so-called pseudoboehmites are of the same structure as boehmite, but have additional water incorporated in the crystal. There are different theories about the exact structure of pseudoboehmite [7]. However, the widely accepted model is that the additional water molecule are located between the boehmite layers (figure 6). The amount of additional water uptake can be adjusted by the processing conditions and determines the properties of an alumina to a great extent. 

The impact of crystallinity, however, goes beyond surface area and porosity properties. Phase transition temperatures and thermal stability of a product also change with the size of the primary crystals. This is demonstrated in figure 8, showing a typical phase transition diagram for a material of small and large crystallites, representing pseudoboehmite and boehmite alumina. 

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