Alumina layers

Fig. 18. Cross-sectional scanning electron micrograph of a three-layered alumina membrane/support (pore sizes 0.2, 0.8, and 12 p.m, respectively).

Figure 3A Pore size distribution of a four-layered alumina membrane (Hsieh, Bhave and Fleming 1988).

Most industrial catalysts based on mixed oxides are simply and economically prepared via co-precipitation in aqueous medium.20 For the preparation of hexaaluminates, this method was first reported by S. Matsuda and co-workers.21 La203 xAl203 samples were prepared starting from an aqueous solution of La and Al nitrates. The precipitation was carried out by the addition of NH4OH solution up to pH=8. After it was washed, filtered and dried, the precursor was calcined at different temperatures up to 1400 °C. For a La203/Al203 mole ratio 5/95, the formation of a layered-alumina phase was observed starting from 1000°C and samples with a surface areas of 30 m2/g and 8 m2/g have been obtained upon calcination at 1200 °C and 1400 °C for 2 h respectively. 

Effect of the Transition Metal Ions - Hexaaluminate materials, 1 >19, including transition metal ions in the structure (M = Mn, Fe, Cr, Co, Ni) were prepared both via the alkoxide15 and the coprecipitation route.23,24,25 For all the compositions investigated, monophasic samples with layered-alumina structure and surface area in the range 10-15 m2/g were obtained upon calcination at 1300 °C. 

Also in Fe-containing materials, promoting effect on the formation of the final layered-alumina phase was observed.24 In the completely substituted material BaFe120i9 a monophasic sample with magnetoplumbite structure was obtained at 700 °C. This low formation temperature was related to the greater mobility of oxygen and Ba ions in the lattice of Fe oxides than in A1 oxides. Indeed, the transition from y-+a alumina occurs in Fe oxides at temperatures hundreds of degrees lower than those required for phase transitions in A1 oxides. 

For composition near the extremes of the formation range of layered-alumina phases, samples with typical px (Ba poor composition) and pri (Ba-rich composition) structure were obtained. For the intermediate composition, BaAl12Oi9,... 

These data indicate that sintering resistance is related not only to the formation of a layered-alumina structure. It appears that a critical Ba content is required for the effective suppression of crystal growth along the c axis of crystallites. Indeed, in BaAli4022, sintering proceeded even after the formation of Ba-P-Al203. 

However, the extent of the activity enhancement cannot be related to the higher surface area of this material. Two possible explanations were proposed to account for the effect of mirror plane composition on combustion activity one is related to the different oxidation state of the cation in the mirror plane the other is associated with the crystal structure of layered-alumina materials (i.e., magne-toplumbite and (3-Al203) which have different population and co-ordination of the ions in the mirror planes. Both these electronic and structural factors can, in principle, affect the redox properties. 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

The crystal structure and the sintering behavior of hexaaluminates was widely investigated. The relation of sintering resistance to anisotropic ion diffusion in the layered alumina phase was clarified to a large extent. Other evidence suggests that combustion activity is obtained through a redox mechanism associated with reversible variation of oxidation state of the transition metal ions in the structure. Mn was the best and most stable active component. However, fundamental and applied studies are needed to better clarify the redox mechanism of the reaction and how it is related to the chemical and structural features of the Mn-containing layered-alumina phase. This could also provide useful information for the development of an optimum catalyst composition,... 

For multi-layered asymmetric or composite membranes where the pore sizes between layers are widely different, the analysis gives the pore size distribution of the densest layer (membrane) even if they may be only a small volume or weight fraction of the total membrane/support structure. Shown in Figures 4.14 (a) and (b) are the pore size distributions of two very similar multi-layered alumina membranes prepared by the sol-gel process and two distinctively different alumina membranes made by the anodic oxidation process. The comparison of pore size distributions in each case reflects the similarities and differences of the membrane samples. 

A special case arises when the "skin" (membrane) layer of a normal composite membrane element is immobilized with a catalyst and not intended for separating reaction species. Consider the example of an enzyme, invertase, for the reaction of sucrose inversion. Enzyme is immobilized within a twomembrane element by filtering an invertase solution from the porous support side. After enzyme immobilization, the sucrose solution is pumped to the skin or the support side of the membrane element in a crossflow fashion. By the action of an applied pressure difference across the element, the sucrose solution is forced to flow through the composite porous structure. Nakajima et al. [1988] found that the permeate direction of the sucrose solution has pronounced effects on the reaction rate and the degree of conversion. Higher reaction rates and conversions occur when the sucrose solution is supplied from the skin side. The effect on the reaction rate is consistently shown in Figure 11.6 for two different membrane elements membrane A is immobilized by filtering the enzyme solution from the support layer side while membrane B from the skin layer side. 

Pinnavaia et al. [69,72] prepared a hexagonally packed alumina through the neutral templating approach. In the presence of a polymer surfactant, (PEO)i3(PPO)3o(PEO)i3, an alumina with a d spacing of 63 A and a surface area of 420 m /g was obtained [72]. It was also mentioned that non-layered alumina can be synthesized using octyl or dodecyl amine as template and a neutral aluminum alkoxide precursor. 

Work on one particular ferritic steel, Fecralloy, for fabrication of catalyst substrates was pioneered by the United Kingdom Atomic Energy Authority at Harwell and Johnson Matthey, in collaboration with Resistalloy, which developed technology for producing thin strip [32]. This and related alloys, in addition to iron, chromium, and aluminum, contain low levels of elements such as yttrium (0.1-3.0%), thought to enhance the protective properties of the surface alumina layer. Alumina forms by the oxidation of bulk... 

Porous and thermally stable washcoating layer on mechanically strong support is an important component in both oxidative and three-way catalysts used for car exhaust gas cleaning. The washcoat provides a high and stable surface area for dispersion of the active component of the catalysts consisting of platinum and /or paladium. Usually for the preparation of this layer aluminas modified by La, Ce, Zr, Si etc. are used [1-3]. As it was shown in [4-6] the properties of modified aluminas depend on the method of introduction of the additives In this work we present the results on the preparation and study of model alumina systems modified by La, Ce and Zr as well as of monolith supports washcoated by optimal compositions of alumina and additives. 

Illite [(K,H30)(AlMg,Fe) (Si,Al)p ((0H), (Hp))] is a non-expanding micaceous mineral. It occurs as aggregates of small monoclinic grey to white crystals. It is layered alumina-silicate consisting of poorly hydrated potassium Cation, which is responsible for the poor swelling behavior. Its structure consists of repetition of tetrahedron- octahedron - tetrahedron (TOT) layers. The cation exchange capacity (CEC) of illite is comparatively better than kaolinite (20-30 meq/100 g). 

The TLC separation of plant extracts is described in different pharmacopeias, and is usually performed on silica layers, and sometimes on silica hydrocarbon (C8, Cl8) bonded layers. Alumina and other stationary phases are not excluded. 

In a different approach, a tris(acetylacetonato)-l,10-phenanthroline terbium(III) complex [Tb(acac)3phen] was immobilized on a thin layer alumina plate [88]. Figure 3 shows the quenching of the terbium 04 Fj transitions (compare... 

Of the sorbents available on prepared layers, alumina is one of the most unique. It can be made into different forms so that it not only has the basic character, but also neutral and acidic versions. This allows a great versatility for selectivity where the acid, neutral, or basic character need not be controlled with buffers in the mobile phase, certainly a disadvantage if doing TLC-MS work lest a buffer interfere in some manner. It also is a complex sorbent with hydroxyl groups, partial positive and negative surface charges, onto which water is also attracted. Only TLC precoated... 

Solvent I benzene layer silica gel-silver nitrate (75 + 25) [186], Solvent II petrol ether (30—60° C)-diethyl ether (anhydrous) (75 + 25) layer alumina G-silver nitrate (75 30) [334]. 

I. Layer Alumina without binder, analytical grade, manufactured by Lachema CSN. 68513.1 activity grade III, pH 8.6 grain size 0.075 mm 0.6 mm thick layer. Solvent benzene-95% ethanol (95 + 5) [141]. 

Fig. 181. Thin-layer chromatogram of chlorinated pesticides [67]. Detection of pesticide residues in milk. Layer alumina G solvent n-heptane (according to [51]). A and E 0.2 jxg endrin and aldrin B and F aldrin, heptachlor, DDT, lindane and heptachlor epoxide 0 2 g milk with added aldrin, heptachlor, lindane, heptachlor epoxide (all 0.1 ppm) and DDT (0.6 ppm) D 2 g milk with added eni-in and dieldrin (0.1 ppm) G 5 g milk with added aldrin, heptachlor, lindane, heptachlor epoxide (all 0.04 ppm) and DDT (0.2 ppm) H 5 g milk with added endrin and dieldrin (0.04ppm) J 50 g milk with spot of heptachlor epoxide I milk blank...

Layer Alumina G Solvent n-Heptane Temperature 23—24 C Relative... 

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