Aluminum oxide, dehydration

Aluminum chloride, 60, 61 Diels-Alder catalyst, 144 Friedel-Crafts acylation catalyst, 91-100 MA complex, 212 Aluminum, diethyl chloride, 345 Aluminum oxide, dehydrating catalyst, 87 Aluminum phenoxides, MA polymerization reactant, 273... 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

Dehydration of gibbsite under pressure in moist air produces boehmite (aluminum oxide mono-hydrate). An infrared spectrum of boehmite (Kaiser substrate grade alumina) is shown in Figure 3c. 

Many reactions are catalyzed by aluminum oxide, A1203, which is also known as alumina. In the solid, there are sites on the surface where a strongly acidic aluminum ion is available to bond to an electron pair donor. One such reaction involves the dehydration of alcohols to produce alkenes. This process can be represented as follows ... 

For example, the reaction of nitronates (123) with a zinc copper pair in ethanol followed by treatment of the intermediate with aqueous ammonium chloride a to give an equilibrium mixture of ketoximes (124) and their cyclic esters 125. Heating of this mixture b affords pyocoles (126). Successive treatment of nitronates (123) with boron trifluoride etherate and water c affords 1,4-diketones (127). Catalytic hydrogenation of acyl nitronates (123) over platinum dioxide d or 5% rhodium on aluminum oxide e gives a-hydroxypyrrolidines (128) or pyrrolidines 129, respectively. Finally, smooth dehydration of a-hydroxypyrrolidines (128) into pyrrolines (130f) can be performed. 

F NMR of Fluorine-Doped -Alumina. The samples studied 115) were high surface area aluminum oxides doped with fluorine by addition of aqueous HF to alumina and subsequent dehydration. A suflScient number of paramagnetic impurities were present in the samples to give relaxation times of the order of 0.01 second. The BET surface areas of most of the samples examined were within 20% of 250 meterVgram. 

Several reagents reduce aldehydes preferentially to ketones in mixtures of both. Very high selectivity was found in reductions using dehydrated aluminum oxide soaked with isopropyl alcohol and especially diisopropylcarbinol [755], or silica gel and tributylstamane [756]. The best selectivity was achieved with lithium trialkoxyalumimm hydrides at —78°. In the system hexanal/ cyclohexanone the ratio of primary to secondary alcohol was 87 13 at 0° and 91.5 8.5 at —78° with lithium tris(/er/-butoxy)aluminum hydride [752], and 93.6 6.4 at 0° and 99.6 0.4 at —78° with lithium tris(3-ethyl-3-pentyl-oxy)aluminum hydride [752],... 

Assessment of the toxicity of aluminas has been complicated by the chemical and physical variants of the compounds and inconsistencies in the nomenclature used to describe them. The group of compounds referred to as aluminas is composed of various structural forms of aluminum oxide, trihydroxide, and oxyhydrox-ide. As these aluminas are heated, dehydration occurs, producing a variety of transitional forms temperatures between 200 and 500°C result in low-temperature-range transitional... 

Most alcohols also will dehydrate at fairly high temperatures in the presence of solid catalysts such as silica gel or aluminum oxide to give alkenes or ethers. The behavior of ethanol is reasonably typical of primary alcohols and is summarized in the following equations ... 

At 1650, both tosyloxy groups in 108 were replaced with inversion, and the bis(dimethylamino)-D-glucitol derivative was obtained. On heating isosorbide (3) or its diacetate (111) in the presence of such dehydrating agents as aluminum oxide in a Pyrex-glass tube above 400°, the doubly unsaturated compound 112 is formed in —50% yield194 (see Scheme 23). 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

Spherical alumina can also be formed from commercial, low cost aluminum-oxides or even from aluminum-hydroxides. In the latter case energy of the plasma should provide not only the enthalpy of melting but that of dehydration and subsequent phase transformations of alumina as well. Under the aforementioned conditions particles below 45 pm have a good chance to be spherodized. Presumably the wide particle size distribution of starting gibbsite powder accounts for the less spheroidization rate of 70%. 

Thus aluminum chloride aluminum oxide, phosphoric oxides, stannic oxide, etc., are Lewis acids and can catalyze dehydration reactions. Thus ... 

Some of the gas dissolves in the aqueous solution, and the gas pressure developed within the apparatus forces the mixture out through the nozzle, which is directed onto the flame. If some heavy molasses or similar gluelike material is added to the sodium carbonate solution and some aluminum sulfate is added to the sulfuric acid, the solution discharges from the nozzle in the form of a foam in which the carbon dioxide gas is trapped. This heavy blanket of foam containing carbon dioxide settles over and around the burning object. The aluminum sulfate hydrolyzes to form aluminum hydroxide, which, at the temperature of the flame, dehydrates to produce a crust of aluminum oxide over the flame. Both effects serve to extinguish the flame. The common Foamite fire extinguisher is an example of this type. 

TLC MeOH Aluminum oxide C HC1 j-dehydrated EtOH (95 5) Dilute K, iodobismuthate TS-Kl/L (1 1) CP, other alkaloids, isolated form root of Scopolia tangutica Maxim. [1138]... 

Benzo[c]thiophene may be prepared by low-pressure (20 mm) vapor-phase catalytic dehydrogenation of l,3-dihydrobenzo[c]thio-phene (Section III,A) at 330° under nitrogen,5,8 by decarboxylation of benzo[c]thiophene-1 -carboxylic acid (Section III,C) with copper in quinoline16,38 or by dehydration of l,3-dihydrobenzo[c]thiophene 2-oxide (Section VI,A) in acetic anhydride or over aluminum oxide at 20 mm Hg and 100°-125° in a sublimation tube.52 A trace of water appears to be beneficial to the first reaction, and it has been suggested53... 

The one exception where certain fillers can provide electrical property improvement is in arc resistance. Here hydrated aluminum oxide and hydrated calcium sulfates will improve arc resistance if cure is sufficiently low to prevent dehydration of the filler particles. Electrical-grade fillers generally improve the arc resistance of cured epoxy systems, as indicated in Table 9.10. 

The direct condensation of ethanol to form n-butanol was investigated by Dolgov and Vol nov in 1933, using titanium oxide promoted with iron-aluminum oxides on charcoal as the catalyst (71). They suggest, with insufficient proof, that ethanol is dehydrated to form ethylene which then reacts with more ethanol to form the butanol. 

Summary Aluminum oxide can be readily prepared in a similar manner as for iron-III-oxide utilizing an open cell in an identical manner. To prepare aluminum oxide, a solution of pickling salt is electrolyzed using aluminum electrodes. During the electrolysis process, a fine white precipitate of mixed hydrated aluminum hydroxides is formed. Thereafter, this precipitate is collected by filtration, and then dried in the usual manner. The dried mass is then roasted at high temperature for several hours to facilitate formation of aluminum oxide, which is formed by the dehydration of the mixed hydrated aluminum hydroxides. 

As in the previous procedures, to prepare aluminum oxide, all you need to do is place the dried mass of hydrated aluminum hydroxides (prepared in step 1) into a crucible and then heat at 600 to 800 Celsius using a typical Bunsen burner for about 3 to 4 hours. During the heating process, water is volatized and removed, and the hydrate aluminum hydroxides are dehydrated forming a white powder. After the roasting process, the aluminum oxide is cooled, and then stored in any suitable container. This aluminum oxide is well suitable for use as a filtering aid, and for use in silica gel columns for filtration purification. 

These isomerizations, rearrangements, and cleavages are best explained by a carbonium-ion mechanism. Vapor-phase dehydration of alcohols over aluminum oxide greatly reduces the tendency for isomerization and rearrangement. The alcohol vapors are passed over the catalyst at 300-420°. In this manner, pure 1-butene is prepared from re-butyl alcohol and t-butylethylene is obtained from methyl-/-butylcatbinol (54%). The relative rates of dehydration of the simpler alcohols over alumina have been studied. The main side reaction is dehydration to ethers (method 118). 

Preparation of dienes is accomplished by dehydration of diols or ole-finic alcohols. Pinacol, (CHjljCOHCOHfCHjlj, is converted to 2,3-di-methyl-1,3-butadiene by heating with 48% hydrobromic acid or by passing the vapors over activated alumina at 420-470°, Yields of the diene are 60% and 86%, respectively. Aniline hydrobroniide is used as a catalyst in the dehydration of 3-methyl-2,4-pentanediol to 3 methyl-l,3-penta-diene (42%). An excellent laboratory preparation of isoprene from acetone in 65% over-all yield has been described. The last step involves catalytic dehydration of dimethylvinylcatbinol over aluminum oxide at 300° to give isoprene in 88% yield. ... 

Hydrolysis of 2,3-dihydropyran by dilute hydrochloric acid gives 5-hy-droxypentanal (7S>%), which is readily reduced to 1,5-pentanediol. The 2,3 dihydropyran is prepared by dehydration and rearrangement of tetra hydrofurfuryl alcohol over aluminum oxide. ... 

The precipitated aluminum hydroxide is then dehydrated by ignition (heating to a high temperature), and the purified aluminum oxide is ready for addition to the electrolyte. 

Aluminum Trihydrate. The utility of aluminum trihydrate as a flame retardant is based on its endothermic dehydration to aluminum oxide and water. In absorbing some of the heat of combustion and lowering the temperature of the substrate near the flame, the hydrate functions as a chemical heat sink. The water vapor provided by such action dilutes the gaseous reactants in the flame until all the water of crystallization is exhausted. 

Dihydropyran (DHP), having an atmospheric boiling point of 86 °C and a very limited solubility in water (3 g in 100 g of water at 20 °C), can be produced from tetra-hydrofurfuryl alcohol by dehydration and ring expansion over an aluminum oxide catalyst at... 

The temperature required to produce a suitable weighing form varies from precipitate to precipitate. Figure 12-6 shows mass loss as a function of temperature for several common analytical precipitates. These data were obtained with an automatic thermobalance, an instrument that records the mass of a substance continuously as its temperature is increased at a constant rate (Figure 12-7). Heating three of the precipitates—silver chloride, barium sulfate, and aluminum oxide—simply causes removal of water and perhaps volatile electrolytes. Note the vastly different temperatures required to produce an anhydrous precipitate of constant mass. Moisture is completely removed from silver chloride at temperatures higher than 110°C, but dehydration of aluminum oxide is not complete until a temperature greater than 1000°C is achieved. Aluminum oxide formed homogeneously with urea can be completely dehydrated at about 650°C. 

Even in the case of a less drastic final step (dehydration as opposed to reduction), Dalmai et al. found that irradiation infiuenced the catalytic properties of the product 182). Aluminum oxide produced from gibbsite, A1(0H)3, by heating at 290-340° was considerably more active in the decomposition of formic acid if the hydroxide had been irradiated in a reactor to 10 nvt or with about 6 X 10 i ev/gm of y-rays. The changes in catalytic activity were accompanied by somewhat complex effects of radiation on the rate of decomposition of the gibbsite 182a). Thus, irradiated samples decomposed more rapidly at low temperatures and less rapidly at high (above 210°) than unirradiated blanks, although the differences were relatively small. 

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