High temperature phases, laboratory preparation

In general, high-temperature phases are prepared in the laboratory by... 

Commercially, pyridine is prepared by the gas phase, high-temperature reaction of crotonaldehyde, formaldehyde, steam, air, and ammonia over a silica-alumina catalyst in 60-70% yield. However, in the laboratory, the challenge is in the preparation of substituted pyridine derivatives in a process that allows one to control regioselectivity and chemoselectivity in the most efficient manner. In this regard the utility of palladium-catalyzed cross-coupling reactions has enabled synthetic chemists by providing the ability to construct highly diversified pyridine derivatives in an efficient fashion [2]. 

Catalytic liquid phase semihydrogenation of acetylenes is an important industrial and laboratory reaction, especially in fine chemical synthesis [1]. The use of supported metal catalysts for this selective hydrogenation readily facilitates the separation of organic products from the catalyst. However, liquid phase reactions with supported catalysts tend towards mass transport limitation [2] and, therefore, the support particles should be between 1 and 10 pm in size this avoids transport limitations and separation problems. With support particles of this size high temperature reduction in a flow of H2 gas is very difficult and to avoid this step it is possible to prepare supported metal particles by decomposing organometallic compounds under mild conditions [3-5]. 

Pyridine is prepared commercially by the gas-phase, high-temperature reaction of crotonaldehyde, formaldehyde, steam, air, and ammonia over a silica-alumina catalyst in 60-70% yield. We will illustrate some of the common laboratory methodologies for the construction of substituted-... 

Boron nitride fibers have been prepared in the laboratory by chemical vapor infiltration of boron oxide glass fibers with ammonia (Equation 8), a process that may alternatively be considered to be a nitridation of B2O3 precursor fibers [31]. The precursor fibers, in turn, are melt spun at a low temperature (480 C) from a viscous melt. Thus, the nitridation of a boron oxide fiber could alternatively be considered to be derived from a solid precursor fiber, a topic otherwise discussed in Chapters 8 to 12. In this process, the final step is the chemical conversion of a given precursor fiber at a high temperature in a highly reactive vapor phase environment. 

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