Boehmite calcination

Further dehydration of boehmite at 600 0 produces y-alumina, whose spectrum is shown in Figure 3b. There is a loss in surface area in going from boehmite to y-alumina. The sample shown here has a surface area of 234 m /g (this sample was obtained from Harshaw A23945 the calcined Kaiser substrate gave an identical infrared spectrum). The y-alumina sample shows two major differences from o-alumina. First, there is a more intense broad absorption band at 3400 cm" due to adsorbed water on the y-alumina. Second, the y-alumina does not show splitting of the phonon bands between 400 and 500 cm" as was observed for o-alumina. The y-alumina is a more amorphous structure and has much smaller crystallites so the phonon band is broader. The y-alumina also shows three features at 1648, 1516 and 1392 cm" due to adsorbed water and carbonate. 

The alkali process uses sodium hydroxide and is well known as Bayer s process. It involves relatively simple inorganic and physical chemistry and the entire flowsheet can be divided into caustic digestion, clarification, precipitation and calcination. Although mineral assemblage in bauxites is extensive, processing conditions are primarily influenced by the relative proportions of alumina minerals (gibbsite and boehmite), the iron minerals (goethite and hematite), and the silica minerals (quartz and clays-usually as kaolinite). 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

Douhova et al. (2003) found that after mixing for 24-72 hours at 20 °C, calcinated synthetic hydrotalcite and calcinated natural boehmite were much more effective in removing As(V) from water than oxihumolite (i.e. humic-rich low-rank coals) (Douhova et al., 2003, 259). Dosages of 1 gL-1 of hydrotalcite or 2.6 gL-1 of boehmite removed 70.5 and 97.1 %, respectively, of 2 mmol L-1 (150 mg As L-1) of As(V) (Dou.vova et al., 2003, 265). The arsenic probably bonded with oxygens on the hydrotalcite and boehmite. Infrared spectra detected As-0 bonds in both inorganic sorbents (Dousova etal., 2003, 261, 263). In contrast, As-0 bonds were absent in the infrared spectra of the oxihumolite (Dou. ovd et al., 2003, 263). 

CoMo-124 B Cobalt was brought on uncalcined boehmite, 4 wt% in one step. Afterwards the sample was calcined at 650°C, impregnated with molybdenum oxide (12 wt%) and calcined at 650°C for a second time. The surface area was 241 m /g. 

Such an effect might be expected when boehmite supported cobalt is being calcined, viz. during the phase transition AIO(OH) - y-Al203. Figure 7 shows spectra of pyridine, adsorbed on the sample CoMo-124 B, which has been prepared in this way. Spectra for MoCo-122, -123 and -124, containing 2, 3 and 4 wt% CoO resp. are shown for comparison. All these catalysts have had a final calcination of 650°C. Comparison of the spectra of CoMo-124 B and MoCo-124 indicates that the intensity of the 1612 cm l band, which is introduced by the interaction of the cobalt ions and the molybdate layer, is lower for CoMo-124 B than for MoCo-124. The spectrum for CoMo-124 B resembles that of CoMo-123, indicating that a part of the cobalt ions does not participate in this interaction. 

A standard membrane as prepared by de Lange [45] consisted of a die-pressed a-alumina support, fired at 1360°C with a pore diameter of 160 nm on which a y-alumina membrane was coated with a home-prepared boehmite sol. The coated y-alumina layer was calcined at 600°C, had a thickness of 7 pm with a pore diameter of 5 nm. On top of this mesoporous membrane,... 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

Boehmite is itself decomposed at c. 400—450°C. As expected, the calcined products have much lower specific surface areas than the activated aluminas produced from the trihydroxides. However, the results of de Boer and his co-workers (de Boer, 1972) indicate that a sample prepared at 580°C was highly microporous and that up to this temperature there was only a small change in the external area. 

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. 

If water was added to the 15% Si02 co-gel to fill the pore voids a partially recrystallized boehmite was formed with a surface area of 464 m /g and with a pore volume of 1.8 oc/g. If water was added to the 15% Si02 co-gel to form a slurry and then dried and calcined at 500 a partially recrystallized bodunite was formed with a surface area of 334 m /g. steam treatment at 760 of this second, small pore, bodunite-like silica-alumina resulted in no change in the surface area. The gas oil cracking activity of the steamed sample was definitely hi er than that for the amorphoias co-gel, i.e., a Micro Activity Test (MAT) Activity Number of 38 (see Table 1.). 

Liquid phase modifications. Alternatively a porous membrane can be reduced in pore size by a liquid deposition prcx ess where the membrane is dipped into a solution or sol to form deposits inside the membrane pores. For example, a silicon nitride tube with a mean pore diameter of 0.35 pm is first immersed in a solution of aluminum alcoholate (aluminum isopropylate or 2aluminum tris(ethyl acetoacetate) or ethyl acetoacetate aluminum diisopropylate) in an organic solvent (hexane, cyclohexane, benzene, isopropanol, etc.). It is then treated with saturated water vapor to hydrolyze the alcoholate or chelate to form bochmite inside the pores, thus changing the pore diameter to as small as 20 nm [Mitsubishi Heavy Ind., 1984a and 1934b]. Upon calcining at 800X, boehmite transforms into transition-alumina. 

Contrary to SEM, TEM does not provide the striking three-dimensional images. Shown in Figure 4.5 are some well dispersed crystallites of boehmite as the precursor to a thin, unsupported partially calcined alumina membrane the TEM image of which is given in Figure 4.6. It is estimated from the TEM that many of the crystallites appear to be smaller than about 50 nm. The ordered suiicture of the very thin, partially calcined alumina membrane is evident in Figure 4.6. 

Al-sol can be prepared using various aluminum precursors, such as pseudo-boehmite, A10(0H) xH20, and aluminum alkoxide [48-50,55,60]. Xu et al. [55] described several possibilities. Additives, e.g., urea, or organic amines, e.g., hexamethylenetetramine (HMT), can be added to the sol in order to improve the quality of alumina obtained. The decomposition of these additives in heat treatment (calcination) may lead to a better porosity of the alumina. The reaction for urea decornposition is as follows ... 

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

In addition, there Is pressure to reduce the cost of the catalyst, Vashcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropDxlde or boehmite have been suggested to be useful materials to calcine [42]. Both of these precursors are not cheap, and less expensive raw materials would be desired,... 

If the initial microcrystalline boehmite gel slurry is aged at 40°C it is converted into bayerite, a crystalline form of Al(OH)3. If this product is filtered, dried and then calcined, a compound designated as ri-Al203 is formed. Calcination at yet higher temperatures produces another compound 6-AI2O3, which is converted into a-Al203 at temperatures exceeding 1100°C. 

A last complication results from the measurement of the thickness of supported layers. This can easily be done with SEM for calcined layers. Wet lyogel, or even dry xerogel, layers cannot be measured in this way. Reproducible thickness measurements on wet lyogel films could not be obtained with other easy-to-perform methods. Consequently layer thicknesses were measured after calcination. Estimates of the shrinkage in the thickness direction were made for supported alumina (boehmite) membranes dried at 40°C and 60% RH made with a standard precursor solution of 1 mol AlOOH/1 stabilised at pH = 4 by... 

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