Aluminum oxide catalysis

Matsumoto, K., Yamaguchi, T., Eujisaki, J., et al. (2008). Aluminum Oxidation Catalysis under Aqueous Conditions highly Enantioselective Sulfur Oxidation Catalyzed by Al(salalen) Complexes, Chem. Asian J., 3, pp. 351-358. 

The selective intercalation of guests into solid hosts offers the potential for application in catalysis and separation science. An excellent case in point is zeolites, which exhibit shape and size selective inclusion properties and are used for an enormous variety of processes [44,45]. Additionally, a munber of layered materials have been reported to possess selective intercalation properties, including layered metal phosphonates [46,47], montmorUlonite [48], magnesium aluminum oxide [49], and layered double hydroxides [50-59]. 

These results suggest that if we want to design a molecular sieve to separate a mixture of normal hexane and 2-methylpentane, we should use a zeolite with normal sinusoidal pores but small-diameter straight pores such a zeolite will preferentially accept normal -hexane and preferentially reject 2-methylpentane. More important, perhaps, these results have implications for selective catalysis. If we want -hexane to react but not 2-methylpentane, then we should make a zeolite where the catalytically active centers (aluminum oxides) are situated in the sinusoidal pores but not in the straight pores. Conversely, if we desire preferential catalysis for the branched isomer, we want a zeolite where the active centers are at the intersections between straight and sinusoidal pores. 

The molecular sieves are aluminosilicates of particular importance. They are crystalline materials that have open structures that contain pores and channels that have molecular dimensions. This family of materials includes the zeolites, which have numerous applications in heterogeneous catalysis, ion-exchange materials, absorption of molecular species, and gas separation. While some zeolites occur naturally, they are usually manufactured from silicon and aluminum oxides mixed with tetraaUcylammonium templates in a high-pressure autoclave (see Zeolites). 

Aluminum oxide clusters having the same stoichiometry as bulk alumina are able to oxidize CO and, subsequently, the reduced cluster can be reoxidized to produce the original active stoichiometry. These findings are relevant to the supply of oxygen to catalyst particles in supported heterogeneous catalysis. 

Proton catalysis of the dissolution rate of aluminum oxide. The logarithm of the dissolution rate, Rh, is plotted as a function of pH (A) and the logarithm of the surface concentration of hydrogen ion,... 

Studies of the dissolution rates of alum inum oxide and beryllium oxide (Furrer and Stumm, 1986) showed that the rates of these reactions are facilitated by increased concentrations of protons and various aliphatic and aromatic ligands at the oxide surface. The key link for determining the mechanism of reaction catalysis is establishing the relation between the concentration of these species in solution and the concentration at the solid surface. This relation is determined by potentiometric titration of the solution-solid mixture, just as described earlier for the surface catalysis of Mn oxidation. The relations between the dissolution rate of aluminum oxide and the hydrogen concentration both in solution and on the solid surface are presented along with the proposed mechanism in Fig. 9.9. The solution dependence of the dissolution rate on pH (Fig. 9.9A) has a fractional order of 0.4, which is similar to that of other oxide dissolution reactions (Table 9.4). But when the same relation is plotted as... 

Stone suggests that catalysis occurs because the ionized carboxylate group of MPT is able to specifically sorb to the positively charged aluminum oxide surface where subsequent attack of hydroxide ions in the diffuse layer occurs. 

For a simple example of heterogeneous catalysis, think of the catalytic converter in your car. It s made of a platinum-rhodium alloy that is finely deposited on aluminum oxide (specifically y-alumina) to decompose nitrogen oxides to less toxic nitrogen, as well as promote the combination of O2 with toxic CO and hydrocarbons to form CO2. 

Considering the longevity of alumina usage in the catalysis and adsorption industries, it is surprising to note how many misconceptions still exist concerning its physical-chemical properties. Because the surface structure of an adsorbent essentially determines its adsorptive characteristics, an understanding of the surface chemistry of aluminum oxides is necessary to comprehend selective adsorption properties. 

Aluminum oxides and related compounds have long been technologically important as abrasives (corundum) and in refractories and ceramics in the a-crystalline modification. In the y modification, a more open, defect structure, aluminum oxide becomes activated alumina and is useful in chromatography and in catalysis. A third modification occurs on the surface of the metal on exposure to air and serves as the well-known protective oxide. A more recent technological achievement is the production of remarkably uniform cylindrical fibers of AI2O3. These fibers can be incorporated in a variety of fabrics, papers, ropes, and so on, which gain the advantage of stability to very... 

Aluminum oxide (a-Al20s, corundum) has a large number of technological applications. Due to its hardness, its chemical and mechanical stability at high temperatures and its electronic properties as a widegap insulator it is used for the fabrication of abrasives, as a carrier for thin metal films in heterogeneous catalysis and in optical and electronic devices. 

Methylamine, dimethylamine and trimethylamine are produced by reaction of methanol vith ammonia [route (a) in Topic 5.3.4]. The reaction proceeds as a continuous gas-phase catalysis using form-selective, acidic silicon/aluminum oxides (mordenite, chabasite) as catalyst contacts. This catalyst choice favors formation of the most desired product dimethylamine (<10% trimethylamine) in contrast to the earlier applied amorphous silicon/aluminum oxide contacts. The reactor temperature is kept betv een 250 and 300 °C and the reaction pressure is betv een 10 and 30 bar. 

FIGURE 22.25 Certain minerals have catalytic properties. Aluminosilicate minerals are composed of aluminum oxide units interspersed with silicon oxide units, and can have pores for molecules to enter and react, (a) A common building block of aluminosilicates. (b) Part of a zeolite structure, which is a common type of aluminosilicate used in heterogeneous catalysis. 

A. Hess and E. Kemnitz, Characterization of catalytically active sites on aluminum oxides, hydroxyfluorides, and fluorides in correlation with their catalytic behavior, J. Catalysis, 149, 449 57 (1994). 

An early contribution to use of the transition-metal-catalyzed pyridine formation reaction was the synthesis of vitamin Be (124) via the crossed-cyclotrimerization reaction of the bis-stannylated diyne 122 with acetonitrile under cobalt catalysis (Scheme 7.26) [36a andb]. The underlying crossed [2 - - 2 - - 2] cycloaddition reaction here provided the fused pyridine 123 in 76% yield after a regioselective destannylation effected by treatment of the cycloaddition product with aluminum oxide. 

Some 0,0-dia]kyl 1-hydroxyalkylphosphonates have been prepared by the addition of diaJkyl phosphonates to aldehydes under dilferent reaction conditions [2], which can be summarized as follows (1) non-catalytic thermal addition [3, 4], (2) base-catalysis addition [5], and (3) solvent-free catalytic process using potassium fluoride, calcium fluoride, aluminum oxide, or others, as a catalyst [6-8]. 

Heterogeneous Catalysis. The main discovery of the 1980s was the use of titanium sihcaUte (TS-1) a synthetic zeoHte from the ZSM family containing no aluminum and where some titanium atoms replace siUcon atoms in the crystalline system (Ti/Si = 5%) (33). This zeoHte can be obtained by the hydrolysis of a siUcate and an alkyl titanate in the presence of quaternary ammonium hydroxide followed by heating to 170°C. Mainly studies have been devoted to the stmcture of TS-1 and its behavior toward H2O2 (34). The oxidation properties of the couple H2O2/TS-I have been extensively developed in... 

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

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