Temperature alumina formation

Other PK variations include microwave conditions, solid-phase synthesis, and the fixation of atmospheric nitrogen as the nitrogen source (27—>28). Hexamethyldisilazane (HMDS) is also an excellent ammonia equivalent in the PK synthesis. For example, 2,5-hexanedione and HMDS on alumina gives 2,5-dimethylpyrrole in 81% yield at room temperature. Ammonium formate can be used as a nitrogen source in the PK synthesis of pyrroles from l,4-diaryl-2-butene-l,4-diones under Pd-catalyzed transfer hydrogenation conditions. 

Similar results were achieved over a Rh/alumina monolith catalyst " using catalytic POX for the reforming of a simulated JP-8 military feed containing 500 ppm of sulfur (as benzothiophene or dibenzothiophene). Stable performance for over 500 h with complete conversion of the hydrocarbons to syngas at 1,050°C, 0.5 s contact time, and LHSV of about 0.5 h was reported. At this high temperature, carbon formation was not reported and the sulfur exited as hydrogen sulfide. 

Studies of catalysts deactivation by coke are abundant in the literature most of them are usually conducted at high temperatures (around 500°C) using metal catalysts supported on oxides with low surface area such as silica, aluminas or silica-alumina [2 and references therein]. The deactivation by coke of zeolite catalysts has also been studied and such studies have mostly been done for high temperature reactions such as the conversion of n-hexane or the isomerization of xylenes [2,4]. However, low temperature coke formation (20-25°C) combining the effect of high acidity and size specificity for a high coking component such as nickel, has not yet been considered from the point of view of the presence of compounded effects of crystalline structure and location of metal particles. 

In the earlier studies [1] the phase 3-NiAl was found to be very oxidation resistant, forming a protective alumina scale in a wide range of temperature and oxygen pressure. However, in more recent studies [2-5] many critical features were detected in this alumina formation and even accelerated attack by intergranular and internal oxidation was observed under special conditions. 

For all these reasons an increase in chromium concentration may be the best solution to increase the sulphidation resistance of iron-aluminium alloys. The present work has shown that chromium has a very beneficial effect on the sulphidation resistance in oxygen deficient gases. Such improvement of sulphidation resistance in H2S environments has also been found in nickel-aluminium alloys, where the corrosion rate decreased significantly, when 8 wt% chromium were added [22], Whether chromium increases the rate of alumina formation or contributes to the formation of protective spinell type oxides Fe(Cr,Al)203 cannot be decided from the available results. It is also possible that chromium catalyses the transition from a less to a more protective type of A1203. It was for example found that chromium enhances the transition from 0-Al2O3 to a-Al203 in Ni-Al at temperatures above 1000°C [30],... 

Alloys that are exposed to aggressive atmospheres at high temperatures are usually designed to be heat resistant and one of the most serious problems is how exposure to a complex gas can cause the protective behaviour of scales to change. Usually, such alloys rely on scales based on chronua or alumina formation. Consequently, they rely on sufficiently high concentrations of chromium and aluminium being available in the alloy. This can be disrupted if internal sulphidation removes the chromium or aluminium from solution in the matrix. 

These normally utilize the low- and medium-temperature decomposition of inorganic aluminum salts and hydroxides, or metal-organic compounds of aluminum. Typical precursors include aluminum nitrate and aluminum hydroxides. Hydro-thermal conditions are often applied [8], but colloidal methods (sol-gel) have been extensively studied over the past three decades [9-11]. Recent efforts have been aimed at reducing the particle size of a-Al203, and decreasing the temperature of formation of a-Al203 from transient aluminas to <1000 °C [12]. Results similar to those in sol-gel can be achieved with the use of metal ion-polymer-based precursor solutions. Here, the precursor solution (e.g., nitrate salt) is mixed with a water-soluble polymer, which provides a matrix for the dispersion of cations [13]. 

Severed effects due to boron addition in supports were observed in this study Boron raises the temperature of formation of y alumina ... 

The sequence and temperature of thermal induced phase transformations in the activated samples are extremely sensitive to preliminary mechanical treatment duration (Fig.8). Mechanical treatment increases the upper temperature boundary of the y- and x- AI2O3 existence up to coexistence with a-AbOa, i.e. direct formation of a-AbOs proceeds. This stabilization of low-temperature aluminas never observed previously. 

The initial low-temperature alumina transition forms are ultradisperse and characterized by highly developed surface. Thermal induced transition to a-oxide is accompanied by sintering and significant reduction of specific surface area. The specific surface area of the samples after MA is lower that of initial ones in the temperature region studied (Fig. 9, A and B) due to the formation of strong aggregates and enhanced sintering in them. 

Our results on the La cation effect on the thermal and mechanical stability of alumina are described in [48]. Even the small amount of La (up to 5wt.% by La203) causes deceleration of the a-alumina formation. At higher concentrations transition to a- is completely suppressed and alumina is stabilized either in 0-form or in a mixture of y- and 8-forms. Observed results allow us to conclude that the La stabilizing effect occurs when the solid solution of La ions forms in the structure of the low temperatm-e aluminas. La incorporation into the alumina lattice prevents the diffusion of A1 ions and the rearrangements into the high temperature forms. 

Here N Dq is the oxygen permeability in the alloy matrix, Vgg are the molar volumes of alloy and oxide, Db is the diffusion coeffident of chromium or aluminium in the alloy, and g the critical volume fraction of oxide required to form a continuous layer. According to this equation, the critical value for external chromia formation in Ni-Cr at 1000°C is Nct = 0.29 and for alumina formation in Ni-Al at 1200°C is Nm = 0.11 [45], both in agreement with the experimental measurements. The requirement for these relatively large concentrations of Cr or A1 to form a complete protective scale will, in many cases, change other alloy propjerties, which limits the applicability of this approach, pjarticularly at lower temperatures. 

It has also been found that the presence of chromia aids in a - alumina formation, as well as limits the phase transformations during heating to temperatures below 1200 (Marple et al., 2001 Chraska et al, 1997). Therefore, chromium nitrate was added to the aluminum nitrate precursor to stabilize the a - alumina phase. 

Magnesium Hydroxide. Magnesium hydroxide [1309-42-8] is another metal hydrate that decomposes endothermically, accompanied by the formation of water. It decomposes at 330°C, which is 100°C higher than alumina trihydtate, and can therefore be used in polymers that ate processed at higher temperatures. 

Anhydrous, monomeric formaldehyde is not available commercially. The pure, dry gas is relatively stable at 80—100°C but slowly polymerizes at lower temperatures. Traces of polar impurities such as acids, alkahes, and water greatly accelerate the polymerization. When Hquid formaldehyde is warmed to room temperature in a sealed ampul, it polymerizes rapidly with evolution of heat (63 kj /mol or 15.05 kcal/mol). Uncatalyzed decomposition is very slow below 300°C extrapolation of kinetic data (32) to 400°C indicates that the rate of decomposition is ca 0.44%/min at 101 kPa (1 atm). The main products ate CO and H2. Metals such as platinum (33), copper (34), and chromia and alumina (35) also catalyze the formation of methanol, methyl formate, formic acid, carbon dioxide, and methane. Trace levels of formaldehyde found in urban atmospheres are readily photo-oxidized to carbon dioxide the half-life ranges from 35—50 minutes (36). 

Alkali metal haHdes can be volatile at incineration temperatures. Rapid quenching of volatile salts results in the formation of a submicrometer aerosol which must be removed or else exhaust stack opacity is likely to exceed allowed limits. Sulfates have low volatiHty and should end up in the ash. Alkaline earths also form basic oxides. Calcium is the most common and sulfates are formed ahead of haHdes. Calcium carbonate is not stable at incineration temperatures (see Calcium compounds). Transition metals are more likely to form an oxide ash. Iron (qv), for example, forms ferric oxide in preference to haHdes, sulfates, or carbonates. SiHca and alumina form complexes with the basic oxides, eg, alkaH metals, alkaline earths, and some transition-metal oxidation states, in the ash. 

Direct ammonolysis involving dehydratioa catalysts is geaerahy ma at higher temperatures (300—500°C) and at about the same pressure as reductive ammonolysis. Many catalysts are active, including aluminas, siUca, titanium dioxide [13463-67-7], and aluminum phosphate [7784-30-7] (41—43). Yields are acceptable (>80%), and coking and nitrile formation are negligible. However, Htfle control is possible over the composition of the mixture of primary and secondary amines that can be obtained. 

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