Special Carbon Preparations

Fig. 3.23 shows pore volume distributions of some commercially important porous materials. Note that zeolites and activated carbon consist predominantly of micropores, whereas alumina and silica have pores mainly in the me.sopore range. Zeolites and active carbons have a sharp peak in pore size distribution, but in the case of the activated carbon also larger pores are present. The wide-pore silica is prepared specially to facilitate internal mass-transfer. 

Why is carbon so special Why does it merit an entire chapter of its own Carbon is special because of the millions of compounds that it can form. Some of these compounds are naturally occurring others are prepared synthetically. The chart below contains the names of some natural and synthetic compounds. Notice how many of these compounds support life as we know it. 

We will not discuss here models for pores in carbons, as this topic is treated in Chapter 5, and elsewhere in specialist [15] or general reviews [106, 107]. For similar reasons, we will not discuss porosity control [44, 108] in detail. However, porous carbons prepared by the template technique, especially the ordered ones, deserve special attention. Ordered mesoporous carbons have been known to scientists since 1989 when two Korean groups independendy reported their synthesis using mesoporous silicas as templates [109, 110]. Further achievements have been described in more recent reports [111, 112]. One might have expected that the nanotexture of these materials would merely reflect the nature of the precursor used, namely phenol-formaldehyde [109] or sucrose [110] in the two first ordered mesoporous carbon syntheses (as is well known, these two precursors would have yielded randomly oriented, isotropic carbon had they been pyrolyzed/activated under more conventional conditions). However, the mesopore walls in some ordered mesoporous carbons exhibited a graphite-like, polyaromatic character [113, 114], as described in Chapter 18. This information was obtained by nitrogen adsorption at low relative pressures, as in classical... 

The carbons prepared in this way were highly microporous, even though no activation at all was included in the preparation process. In Table 11, some data on pore structure parameters of the thus prepared materials are listed. The carbon prepared by carbonization of poly(furfuryl alcohol) (PFA) followed by CVD of propylene at 973 K and then heat-treated at 1173 K for 3 h under Nj flow shows a special pore structure an exceptionally high BET surface area of 3600 m g" and a large amount of micropores of about 1.5 cm g without detectable mesopores. More recently [100], the preparation has been simplified by... 

This book has been written in eight chapters, which cover activated carbons their surface structure the adsorption on solid surfaces and the models of adsorption adsorption from solution phase the preparation, characterization of, and adsorption by carbon molecular sieves important applications of activated carbons with special emphasis on medicinal and health applications and the use of activated carbons in environmental clean up. 

Mixers or agitators (Figure 7-30) are very important components of mineral and chemical process plants. They are used in various stages such as flotation circuits, leaching circuits, gold adsorption on carbon, preparation of special chemicals such as milk of time, preparation of feed for pipelines, and fmal storage where sedimentation is hkely to occur. 

Various carbon materials are described more in detail in Section 18.7.3.1. LGH linear graphite hybrid is a special graphitic carbon preparation. 

More recently, attention has been focused upon carbon anodes which have been prepared specially with a fluorocarbon binder or coating. Such electrodes have shown relatively high current efficiencies (>35%) when evolving ozone at 35% volume concentration at 10°C in 48% vv/u HBF4 at 400mAcm . Ozone collected above the anodes must be immediately diluted by air to lower the ozone concentration below explosion limits. Normally, a concentration of 15% wt is considered safe. 

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

B) Nouveau-styles red, prepared by carbonic maceration of the grapes, giving a special flavor mshed to market Beaujolais nouveau, California (etc) nouveau... 

Synthetic manganese carbonate is made from a water-soluble Mn (IT) salt, usually the sulfate, by precipitation with an alkafl or ammonium carbonate. The desired degree of product purity determines the quaUty of manganese sulfate and the form of carbonate to be used. For electronic-grade material, where the content of K O and Na20 cannot exceed 0.1% each, the MnSO is specially prepared from manganese metal, and ammonium bicarbonate is used (26) (see Electronic materials). After precipitation, the MnCO is filtered, washed free of excess carbonate, and then, to avoid undesirable oxidation by O2, dried carefljlly at a maximum temperature of 120°C. 

Vinylidene Chloride Copolymer Latex. Vinyhdene chloride polymers are often made in emulsion, but usuaUy are isolated, dried, and used as conventional resins. Stable latices have been prepared and can be used direcdy for coatings (171—176). The principal apphcations for these materials are as barrier coatings on paper products and, more recently, on plastic films. The heat-seal characteristics of VDC copolymer coatings are equaUy valuable in many apphcations. They are also used as binders for paints and nonwoven fabrics (177). The use of special VDC copolymer latices for barrier laminating adhesives is growing, and the use of vinyhdene chloride copolymers in flame-resistant carpet backing is weU known (178—181). VDC latices can also be used to coat poly(ethylene terephthalate) (PET) bottles to retain carbon dioxide (182). 

Lakes. Lakes are a special kind of color additive prepared by precipitating a soluble dye onto an approved iasoluble base or substratum. In the case of D C and Ext. D C lakes, this substratum may be alumina, blanc fixe, gloss white, clay, titanium dioxide, 2iac oxide, talc, rosia, aluminum ben2oate, calcium carbonate, or any combination of two or more of these materials. Currentiy, alumina is the only substratum approved for manufactuting FD C lakes. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

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