High-temperature calcination

The commonly used approach to drive off the guest molecules from the microporous frameworks is high-temperature (550 °C in air) calcination, which oxidizes and decomposes the organic molecules. However, this process is highly exothermic, and if inappropriately handled, the zeolite structures would be destroyed by the calcination. For instance, Da[1] found that calcination of zeolite BEA at 550 °C removes the template 

Chemistry of Zeolites and Related Porous Material - Synthesis and Structure Ruren Xu, Wenqin Pang, Jihong Yu, Qisheng Huo and Jiesheng Chen 2007 John Wiley Sons, (Asia) Pte Ltd 

In order to minimize the detrimental effects of high-temperature calcination on zeolite structures, Duan et al. suggested a two-step detemplating approach using microwave radiation. Again, BEA was used as the model for removal of the template TEA. First, the sample was heated at Tc 200 °C for 3 h, followed by treatment under microwave radiation at room temperature for 40 min, and then the sample was subject to a programmed 

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Calcium Pyrophosphate. Calcium pyrophosphate, Ca2P20y, is manufactured by high temperature calcination of DCP in a rotary calciner. Temperature is carefiiUy controlled to adjust the proper ratio of P- and y-forms. 

Calcined diatomite is produced from natural diatomite, which is then subjected to high temperature calcination in a rotary kiln at about 980°C. The calcined material is then again milled and classified to remove coarse agglomerates as well as extreme fines. 

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. 

A crystalline form of free silica, extremely hard and inert chemically very resistant to heat. Quartz in refractory bricks and amorphous silica in diatomaceous earth are altered to cristobalite when exposed to high temperatures (calcined). Cristobalite is extensively used in precision casting by the hot wax process, dental laboratory work, and certain speciality ceramics. 

Complete dissolution of plutonium residues, especially high temperature calcined plutonium dioxide contained in residues such as incinerator ash, continues to cause problems, despite continued research since the Manhattan Project (9). Methods to improve the Rocky Flats system include the use of additives (e.g., cerium) and electrochemistry, other solvents (HCl-SnCl2) as well as high-temperature fusion methods (10). High pressure dissolution, HF preleaching, fluorination, and other methods are being investigated. 

By minimizing the Fe concentration (i.e., avoiding extensive Fe-exchange), zeolites, or mesoporous compounds can be detemplated at low temperatures without the need for high-temperature calcination. This third concept refers to the low-temperature Fenton detemplation. Strictly speaking, Fenton requires thermal activation but always below 100 °C. We refer here to quasi room temperature as compared to the high temperatures usually applied for calcination. 

Most of the microporous and mesoporous compounds require the use of structure-directing molecules under hydro(solvo)thermal conditions [14, 15, 171, 172]. A serious handicap is the application of high-temperature calcination to develop their porosity. It usually results in inferior textural and acidic properties, and even full structural collapse occurs in the case of open frameworks, (proto) zeolites containing small-crystalline domains, and mesostructures. These materials can show very interesting properties if their structure could be fully maintained. A principal question is, is there any alternative to calcination. There is a manifested interest to find alternatives to calcination to show the potential of new structures. 

High-temperature calcination Two-step calcination [187e-f 190]... 

The first position can be safely excluded since a high temperature calcination, causing the removal of Fe atoms from the lattice, remarkably increases the a>site concentration [27]. Besides, a-sites can be prepared via the impregnation of a ready zeolite matrix [28], when the probability for Fe atoms to incorporate into the lattice is very low. a-Sites do not occupy also the 3rd type position deactivation of the outer zeolite surface by its covering with an inert Si02 layer affects neither catalytic activity no a-site concentration [29]. Thus, we may deduce that the active iron occupies the second type position in ZSM-S matrix and is either isolated Fe ions or small complexes inside the micropore zeolite space. 

In the studies conducted by Stuart et al. (1964), Stuart and Gaven (1967 1968) the beagles inhaled 144Ce oxide aerosols formed by high temperature calcination (400°C) or peroxide precipitation. The high... 

C02 — 12.5 mol % space velocity 6400 h 1, the CO conversion rate was found to depend very strongly on the calcination temperature of the catalyst. For example, a catalyst calcined at 500 °C achieved only 25% conversion by 350 °C, while a catalyst calcined at 900 °C achieved 80% conversion by 225 °C. The authors suggested that the Cu-Mn spinel oxide was more easily reducible after high temperature calcination treatment based on a TPR study, allowing for a greater number of highly dispersed Cu species, as verified by XRD. The optimum Cu/Mn ratio was found to be 1/2. 

More recently, dealumination was achieved by fluorination of zeolites at ambient temperature with a dilute fluorine-in-air stream, followed by high-temperature calcination (102). The suggested reaction mechanism involves the formation of different aluminum-fluorine compounds along with zeolites containing hydroxyl and fluorine nests. During the high-temperature calcination, it is assumed that silica insertion occurs, similar to the scheme in Figure IB. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts. (4)... 

In this last case, the product was left as a gel, but if the product is to be a glass, a powder, or crystalline material, a high temperature calcining step is required after... 

Like chalk, limestone is largely calcium carbonate and dissociates reversibly to lime and carbon dioxide at sufficiently high temperatures (calcination) ... 

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