Calcination porosity

Catalyst performance depends on composition, the method of preparation, support, and calcination conditions. Other key properties include, in addition to chemical performance requkements, surface area, porosity, density, pore size distribution, hardness, strength, and resistance to mechanical attrition. 

Soft-burned lime is a quicklime that is calcined at a relatively low temperature. It is characterized by high porosity and chemical reactivity. 

Ib/ft ). Again, dolomitic limes average about 4% denser than their high calcium counterparts. The severity of the calcination process largely determines the porosity of a quicklime the higher the temperature of calcination and the longer its duration, the more the porosity declines. 

Magnesia Calcination temperature, °C Surface area, m /g CrystaUite size, p.m Porosity, %... 

Catalyst films for electrochemical promotion studies should be thin and porous enough so that the catalytic reaction under study is not subject to internal mass-transfer limitations within the desired operating temperature. Thickness below 10 pm and porosity larger than 30% are usually sufficient to ensure the absence of internal mass-transfer limitations. Several SEM images of such catalyst films have been presented in this book. SEM characterization is very important in assessing the morphological suitability of catalyst films for electrochemical promotion studies and in optimizing the calcination procedure. 

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. 

Overall platelet dimensions of mineral aurichalcite did not appear to change during calcination, but became polycrystalline and porous. By dark field Imaging in the TEM, the ZnO particles were observed to be uniformly and highly dispersed. The porosity can be accounted for by the approximately threefold increase in density of Zn atoms upon decomposition of aurichalcite to ZnO. For this density change to occur with a constant overall platelet volume, pores must form. 

On this basis the porosity and surface composition of a number of silicas and zeolites were varied systematically to maximize retention of the isothizolinone structures. For the sake of clarity, data is represented here for only four silicas (Table 1) and three zeolites (Table 2). Silicas 1 and 3 differ in their pore dimensions, these being ca. 20 A and 180 A respectively. Silicas 2 and 4, their counterparts, have been calcined to optimise the number and distribution of isolated silanol sites. Zeolites 1 and 2 are the Na- and H- forms of zeolite-Y respectively. Zeolite 3 is the H-Y zeolite after subjecting to steam calcination, thereby substantially increasing the proportion of Si Al in the structure. The minimum pore dimensions of these materials were around 15 A, selected on the basis that energy-minimized structures obtained by molecular modelling predict the widest dimension of the bulkiest biocide (OIT) to be ca. 13 A, thereby allowing entry to the pore network. 

It corresponds to the cobalt initially exchanged into the HMOR porosity. Nevertheless, a fraction of cobalt oxide - Co304 - is produced after calcination, as previously seen in the case of Cat I, on the surface of zeolite grains. 

The capaciatance C depends on the gas-electrode-electrolyte interline "area" but not on the total electrode surface area S. If the porosity of all the electrode catalysts used is the same, which is a reasonable assumption since they were all prepared by the same calcination procedure, it follows that the interline "area" is proportional to the flat electrolyte surface area A, i.e. the constant X equals X A, where X is another constant which does not depend on any macroscopic dimension. 

Many gallophosphate molecular sieves are unstable to calcination in air or in the presence of moisture, which limits the utility of these materials because at room temperature (RT) the templates stuff the channels thus limiting access to the porosity. The ULM-16 structure is interesting because it is stable to 800°C under argon and up to 350-400°C in the presence of oxygen. This stability may reflect the fully connected framework of ULM-16. 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

An interesting modification of the Stober silica process has been described by Unger et al. (50). By using a mixture of TEOS and an alkyltriethoxysilane they were able to synthesize monodispersed porous silica particles. The porosity is created by the alkyl groups, which act like space holder. After calcination/burnout of the organics, a well-defined porosity is left behind in the silica particles. The materials are used for very fast high-pressure liquid chromatography. 

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