Alumina preparation

Figure 9.11. Microstructures of porous sintered alumina prepared undoped (right) and when doped with magnesia (left). Optical micrographs, originally 250x (after Burke 1996).

Tonerde-hydrat, n. hydrate of alumina (aluminum hydroxide). -kali, n. potassium aluminate. -lack, m. alumina lake. -metaU, n. aluminum, -natron, n. sodium aluminate. -prMparat, n. alumina preparation, tonerderelch, a, rich in alumina, aluminous. Tonerde-galz, n. aluminum salt, -stein, m, alumina brick, -sulfat, n, sulfate of alumina... 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

Lippens (48) has studied the texture of the catalytically active aluminas by means of diffraction and adsorption techniques. He concluded that the structure of tj-alumina formed from bayerite consists of lamellae with an average thickness of about 15 A and a distance of about 25 A. y-Alumina prepared from gelatinous boehmite is composed of fibrillar-shaped particles of about 30 X 30 A. Both structures can easily account for the pseudosolvent effect of alumina, which will be referred to in more detail in the forthcoming discussion. 

The dehydration of menthols over alumina, prepared from aluminum isopropoxide and having intrinsic acidic sites, was accompanied by double bond migration of the cycloalkenes produced. The isomerization was, however, suppressed by the preferential neutralization of the strong acid sites with bases. The neutralization of acidic sites thus preventing isomerization was confirmed by von Rudloff (61) who evacuated pyridine-treated alumina for 6 hours, when about 0.8% base was retained. 

Pillai and Pines (84) found that neopentyl alcohol, mixed with 10% by weight of piperidine and passed over alumina prepared from aluminum isopropoxide, yielded 2-methyl-l-butene and 2-methyl-2-butene, in a maximum ratio of 3, and small amounts of 1,1-dimethylcyclo-propane. However, lert-pentyl alcohol yielded these two olefins in a maximum ratio of only 1.4, and none of the cyclopropane was produced (Table VI). Because of these facts a carbonium ion mechanism which is applicable to ferf-pentyl alcohol is not adequate to explain the rearrangement taking place during the dehydration of neopentyl alcohol,... 

A refers to alumina prepared from aluminum isopropoxide. A(10% pip.) refers to reaction on catalyst A where the alcohol feed was mixed with 10% by weight of piperidine. B2 refers to alumina prepared from sodium aluminate and washed twice. 

Tab. 3.6. Dispersion data of Pd catalysts on alumina prepared with and without ultrasound.

An experimental diffraction contrast image of model catalysts of Ag/alumina, prepared by evaporation, is shown in figure 5.5(a) at 200 kV. The corresponding EDX spectra from and off the metal particle are shown in figures 5.5(b) and (c), respectively. An HRTEM image of Pt/alumina at 400 kV in figure 5.6. 

Adkins and Nissen, ] for example, found that alumina prepared in different ways exhibited very varying catalytic activity towards the decomposition of formic acid. This could hardly be due simply to the different surface areas of the several preparations, since the variation was not confined to the total speed of reaction but affected also the relative speeds of the two alternative decompositions which formic acid undergoes. Moreover, the apparent heats of activation varied from one kind of alumina to another. J... 

Cement, dry process Cement, wet process, 44% water Limestone calcination Dolomite calcination Alumina preparation Barium sulfide preparation Ignition of inorganic pigments Iron pyrite roasting... 

Of immediate concern is the purity of the materials, since carbon surfaces are prone to adsorption of trace impurities. Commercial alumina preparations often contain deagglomerating agents, which should be avoided. Some workers have used diamond dust preparations, which are often suspended in an or-... 

Finally, Majda has investigated a novel inorganic membrane-modified electrode [32]. The membrane used was a microporous alumina prepared by anodizing metallic aluminum in an acidic electrolyte [33]. Majda et al. lined the pores of these membranes with polymers and self-assembled monolayers and studied electron and ion transfer down the modified pore walls to a substrate electrode surface [32]. Martin and his coworkers have used the pores in such membranes as templates to prepare nanoscopic metal, polymer, and semiconductor particles [34],... 

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

A parameter that determines the performances, as outlined also in patents, is the mean diameter of Pd or doped-Pd particles. This is also one of the claims in Headwaters Nanokinetix Inc. patents. It seems that a maximum in the activity/ selectivity as a function of the particle size is present. Figure 8.11 reports the effect of the Pd-particle size (Pd supported on alumina, prepared by deposition-precipitation method) in the direct synthesis of H202 in water at atmospheric pressure [77]. Low... 

Kim, Y., Kim, C., Choi, I. el al. (2004) Arsenic removal using mesoporous alumina prepared via a templating method. 

Table 2. Properties of different precursors used for active alumina preparation.

Figure 4. Inhomogeneity of silica-aluminas prepared by various methods. A series of 17 commercial samples of silica-aluminas from seven different producers was submitted to microanalysis. All of them showed considerable fluctuations of composition at the scale of several tens of nanometers to several micrometers. These samples were prepared by coprecipitation or by the sol-gel method. It is not known whether some of these samples were prepared from alkoxides. Smaller but significant fluctuations at the micrometer scale were also observed for two laboratory samples prepared from alkoxides. The samples were dispersed in water with an ultrasonic vibrator. A drop of the resulting suspension was deposited on a thin carbon film supported on a standard copper grid. After drying, the samples were observed and analyzed by transmission electron microscopy (TEM) on a JEOL-JEM 100C TEMSCAN equiped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPM A). The accelerating potential used was 100 kV.

Li /3-alumina, prepared from Na jS-alununa by ion exchange in LiCl at 650 °C, has the one of the highest Li+ conductivities reported in a crystalline material, 3 x 10 2scm at25°Cin a single crystal sample. 

Important trends in N2 isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without ]4iysical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Frenkel-Halsey-Hill (BdB-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained from the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2 isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po ca. 0.5 and sharp lifting-up hysteresis loop at P/Po > 0.8. This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-free from alumina framework. 

Figure 2. Niuogcn adsorption-desorption isotherms of the porous alumina prepared using stearic acid and/or PS bead as a template (curve are shitted for clarity). The inset shows the corresponding pore size distribution from the desorption branch.

Calcination of aluminum hydroxide to alumina Preparation of aluminum sulfate from bauxite Preparation of fluorides aluminum trifluoride, uranium tetra- and hexafluorides... 

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. ... 

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