Sublimation thermal degradation

The thermal degradation of TsHs and other TsRs species (R = Me, /Bu, nCsHiy, Ph) in air and an inert atmosphere has been studied by thermogravimetric analysis and shows that for TsHs incomplete sublimation tends to occur, and, in air, oxidation competes with volatalization. ... 

Lyophilization, although a relatively expensive proeess, can offer advantages for relatively imstable eompounds. In lyophilization, the solvent (usually water) is frozen and then removed by sublimation in a vaeuum environment. The low temperature maintained during the entire process minimizes thermal degradation of the drug compound. 

The goal of a commercial sublimation is the separation of a valuable material from nonvolatile ones at temperatures low enough to avoid thermal degradation. The preservation of cell structure (and taste) is a deciding factor in the choice of freeze drying, a special instance of sublimation, foods, pharmaceuticals, and medical products. 

Preservation of cell structure, food taste, and avoidance of thermal degradation are reasons for the removal of moisture from such materials by sublimation. The process is preceded by quick freezing which forms small crystals and thus minimum damage to cell walls, and is likely to destroy bacteria. Some of the materials that are being freeze dried commercially are listed in Table 19.9(b). 

The alkoxides and phenoxides of boron are all generally readily distillable or sublimable compounds and tend to be monomeric in solution.1 However, the r-butoxide undergoes thermal degradation to produce an alkene, presumably via a carbonium ion intermediate.160... 

Amorphous Films. An amorphous film is generally prepared by solvent evaporation of a dry organic solution of Chi on a solid substrate surface. The vacuum sublimation technique, which is widely employed for most synthetic dyes, is not applicable to Chi due to possible thermal degradation of the pigment. 

The Ln(tritox)3 system underlies a thermal degradation reaction (Sect. 6.3). Attempted sublimation yielded Ln(ditox)3, the dimeric composition of which... 

The more stable of the two organic ligands is completely volatilized below 200°C., and even the zinc complex is fully sublimed at 325°C. On the other hand, the zinc coordination polymer has only lost 68% by weight at 1000°C. and must be heated at this elevated temperature for about 8 hours before it is essentially completely volatilized. The zinc polymer is unique in this respect because the metallic zinc formed on thermal degradation is volatile at 1000°C. in vacuo, while the metallic residues remaining after decomposition of the other coordination polymers are not volatile under these conditions. 

Sublimation. Sublimation is less commonly used than it used to be, but can still be a valuable technique for the purification of small quantities of air sensitive organometallics. Its main disadvantages are that it is difficult to scale sublimations up to multiple gram quantities, and that thermal degradation typically results in significant loss of material during purification. Sublimation can be carried out using any of the commercially available sublimation... 

For both samples, the anatase—rutile structural transition, associated with a well-defined exothermic peak, occurs at 485°C. Such fact suggests that the temperature of the structural transition is associated with the mean crystaUite sizes, which have, for both samples, the same value. From 600°C to 800°C, an endothermic peak is observed for both samples, associated with a minor mass loss step, probably due to the sublimation of a minor amount of powder or the thermal degradation of the last alkoxide molecules, trapped into the three dimensional oxide structure. As an example, the TG—DTA curve for the DMF-synthesized powder is shown in Fig. 4.11. 

The aqueous aroma composition (solid, liquid, paste) includes components/supports able to form by freezing an amorphous matrix or a gel to immobilize aroma compounds. It will be transformed into an aroma powder, without modification of aroma, for easy storage. After freezing of the aroma composition, there is drying by sublimation of ice, usually under vacuum. The driving force for sublimation is the difference in pressure between the water vapor pressure at the ice interface and the partial pressure of water vapor in the chamber. Because of the low processing temperatures, thermal degradations reactions are excluded. 

Sublimation at 76°C light sensitive thermal degradation into phenol and CO2 coloration with iron salts 2-97... 

Many coumarins may be purified by high vacuum sublimation, but this should be avoided in work with potentially new coumarins. Alloisoimperatorin (291) is probably an artefact from thermal rearrangement of the prenyl ether, isoimperatorin (254) 153), while angelical (80) is now known to arise from thermal degradation of angelol (98) 253). It is not yet known whether crenulatin [ = angelical, (80)], recently isolated from Hesperathusa crenulata, is also an artefact. 

A high percentage 01 water remains after the sublimation process, present as adsorbed water, water of hydration or dissolved in the diy amorphous solid this is difficult to remove. Usually, shelf-temperature is increased to 25 to 40°C and chamber pressure is lowered as far as possible. This stiU does not result in complete diying, however, which can be achieved only by using even higher temperatures, at which point thermally induced product degradation can occur. 

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