Calcined features

An interesting feature of these high-temperature-calcination apph-cations is the direct injeciion of either heavy oil, natural gas, or nue coal into the fluidized bed. Combustion takes place at well below flame temperatures without atomization. Considerable care in the design of the fuel- and air-supply system is necessary to take full advantage of the fluidized bed, which sei ves to mix the air and fuel. 

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Further dehydration of boehmite at 600 0 produces y-alumina, whose spectrum is shown in Figure 3b. There is a loss in surface area in going from boehmite to y-alumina. The sample shown here has a surface area of 234 m /g (this sample was obtained from Harshaw A23945 the calcined Kaiser substrate gave an identical infrared spectrum). The y-alumina sample shows two major differences from o-alumina. First, there is a more intense broad absorption band at 3400 cm" due to adsorbed water on the y-alumina. Second, the y-alumina does not show splitting of the phonon bands between 400 and 500 cm" as was observed for o-alumina. The y-alumina is a more amorphous structure and has much smaller crystallites so the phonon band is broader. The y-alumina also shows three features at 1648, 1516 and 1392 cm" due to adsorbed water and carbonate. 

W-Li absorption edges for WO3 and for several WOx-ZrOa samples calcined at 1073 K are shown in Figure 7. The absorption edge reflects the excitation of W 2s electrons by X-ray photons. A pre-edge feature at -5 eV is caused by 2s to 5d dipole-forbidden transitions, which become detectable in non-centrosymmetric compounds because of d-p orbital mixing [28]. 

The Phillips Cr/silica catalyst is prepared by impregnating a chromium compound (commonly chromic acid) onto a support material, most commonly a wide-pore silica, and then calcining in oxygen at 923 K. In the industrial process, the formation of the propagation centers takes place by reductive interaction of Cr(VI) with the monomer (ethylene) at about 423 K [4]. This feature makes the Phillips catalyst unique among all the olefin polymerization catalysts, but also the most controversial one [17]. 

Reduction of particle size was found to increase the activity as long as the crystallinity was maintained or restored by calcination, etc. Surface features such as steps that act as active sites were also found to increase activity [54,57,84]. 

Reynolds Metal A process for extracting aluminum from clay by leaching with nitric acid. An essential feature is the pelletizing of the clay by calcination with kaolin in order to pro-... 

Figure 9.28 RBS proves that chromate can indeed desorb from the model catalyst during thermal activation. A wafer with 10 Cr/nm2 loading features 7.0 Cr/nm2 after calcination at 550 °C. The desorbing chromate readily readsorbs on an empty silica surface placed opposite to the loaded wafer (courtesy of...

Figure 1.15 Diffuse reflectance UV-Vis spectra from a series of chromia/alumina catalysts after various treatments [115], All these spectra display a shoulder at about 16,700 cm-1 corresponding to the first d-d transition of Cr3+, but the main feature seen in the hydrated and calcined samples at about 26,000 cm-1 due to a Cr6+ charge transition is absent in the data for the reduced sample. This points to a loss of the catalytically active Cr6+ phase upon reduction. (Reproduced with permission from Elsevier.)...

LDHs can take up anion species from solution by three different mechanisms surface adsorption, interlayer anion-exchange and reconstruction of a calcined LDH precursor by the memory effect . The memory effect [130] of LDHs, discussed in 2.3 above, is one of their most attractive features as adsorbents for anionic species. Calcination allows the recycling and reuse of the adsorbents with elimination of organic contaminants as CO2 and water [ 131]. The main advantages of LDHs over traditional anionic exchange resins are their higher AEG values and the fact that LDHs are resistant to high temperature treatments. 

Other restricted media in which radical cations of organic substrates are formed are zeolites. Typical anisotropic features of the naphthalene radical cation were observed after exposure of naphthalene to aluminated H ZSM-5 (n = 3, 3.4) calcined under oxygen at 773 K. These spectra contain an overlapping feature, which was assigned to isolated electrons. These signals persist over several weeks at room temperature29. 

M(lI)AlSn-LDHs with M(II) being Mg, Ni or Co were synthesized by a coprecipitation method. The influence of Sn on the thermal transformations and redox properties were investigated in detail using XRD, TG/DTA, SEM, TPR, 1 l9Sn-MAS NMR and UV-visible diffuse-reflectance (DR) spectroscopy methods. Some of these samples calcined at 450 °C were tested as catalysts in the partial oxidation of methanol (POM) reaction. In this paper we discuss briefly the effect of Sn-incorporation on the structural features and reducibility of CoAI-LDH. The catalytic performance of Co-spinel microcrystallites derived from CoAl-, and CoAlSn-LDHs was also evaluated. 

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