Pseudoboehmite

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). 

Stmctural order varies from x-ray indifferent (amorphous) to some degree of crystallinity. The latter product has been named pseudoboehmite or gelatinous boehmite. Its x-ray diffraction pattern shows broad bands that coincide with the strong reflections of the weU-crystallized boehmite. 

Commercial production of bayerite is relatively small and employs CO2 neutralization of caustic aluminate Hquor obtained from either Bayer or sinter processes. The product obtained is about 90% crystalline bayerite having small amounts of gibbsite, pseudoboehmite, and amorphous aluminum hydroxides. 

AIPO4-5 molecular sieve (BET surface area, 299 m g ) used in this study was prepared by the method of Wilson et al. [1j. VjOg/AIPO -S samples were prepared by impregnating with aqueous solutions of ammonium vanadate after removing templating agent in the AIPO -S by calcining at 530°C. The samples were dried at 110 C for 24 h then calcined in air at 550°C for 2 h. VAPO -S was prepared from gel mixture of phosphoric acid, pseudoboehmite, vanadium pentoxide, tri-propylamine and water (1.3 PrjN x AljOj PjOj 40 HjO) by crystallization at 165°C for 3 - 7 days [7. ... 

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... 

Depending on the kinetics of the different elementary processes involved in the formation of the precipitate, a temperature increase might lead to an increase in crystallite size, as was observed for the crystallization of pseudoboehmite [24] or iron molybdates [25]. However, in other cases no influence of the precipitation temperature on the crystallite size of the final catalyst was reported [26], or a decrease was reported, as for the ZnO system [27],... 

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-... 

Phase gibbsite pseudoboehmite, bayerite, nordstrandite, gibbsite pseudoboehmite... 

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. 

Materials. The components of the biphasic synthesis mixture were hexanol (HEX), tetraethyl orthosilicate (TEOSi), orthophosphoric acid (85%), Pr N and Pr4N-OH (25%), all from Janssen Chim., pseudoboehmite (Vista, 70% Al20,f 30% h20) and water. These compounds were mixed in the rollowing way. To hexanol first TEOSi was added and subsequently phosphoric acid, pseudoboehmite, the template and water. The synthesis mixtures were transferred into autoclaves with a capacity of 120 ml. Agitation was performed by rotating the autoclaves at 50... 

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... 

Boehmite is of considerable interest to the surface scientist. It was pointed out by Lippens and Steggerda (1970) that a clear distinction should be made between crystalline boehmite and the gelatinous forms of pseudoboehmite, which always contains some non-stochiometric, interlamellar water. Pseudoboehmite is the main constituent of European bauxites and can be easily prepared by the neutralization of aluminium salts, but hydrothermal conditions are required for the formation of crystalline boehmite. 

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). 

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. 

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