Ammonia pyrolysis

Okamura and co-workers (18) have taken air-cured PCS polymer and, through pyrolysis in the presence of ammonia, prepared essentially carbon-free silicon oxynitride fibers (equation 13). However, if the PCS polymer fiber is cured by electron beam radiation (to prevent oxygen addition), the same ammonia pyrolysis conditions provide nearly stoichiometric quantities of silicon nitride fibers (equation 14). 

Dove and Nip (1979) studied this reaction as part of their investigation of ammonia pyrolysis. Using mass spectrometric analysis, they monitored the rates of formation of NH behind reflected shock waves in NH3/Kr mixtures with and without added H2 (6% NH3 in Kr 6% NH3, 1.2% H2 in Kr). The difference in the early time rates of formation was attributed to the NH-removal reaction... 

In another study of ammonia pyrolysis, Roose (1981) monitored NH behind incident shocks using uv emission (336 nm) and was able to infer k from the NH concentration record at early times. The initial formation of NH was attributed to the slow reaction (Section 4.1). 

In a third shock tube study of ammonia pyrolysis, Yumura and Asaba (1981) monitored H atoms in dilue NH3/Ar mixtures using atomic resonance absorption and were able to infer k through comparisons with computer simulations. The critical reactions in the simulation were reactions (1), (3), and (4) together with... 

Ammonia is used in the fibers and plastic industry as the source of nitrogen for the production of caprolactam, the monomer for nylon 6. Oxidation of propylene with ammonia gives acrylonitrile (qv), used for the manufacture of acryHc fibers, resins, and elastomers. Hexamethylenetetramine (HMTA), produced from ammonia and formaldehyde, is used in the manufacture of phenoHc thermosetting resins (see Phenolic resins). Toluene 2,4-cHisocyanate (TDI), employed in the production of polyurethane foam, indirectly consumes ammonia because nitric acid is a raw material in the TDI manufacturing process (see Amines Isocyanates). Urea, which is produced from ammonia, is used in the manufacture of urea—formaldehyde synthetic resins (see Amino resins). Melamine is produced by polymerization of dicyanodiamine and high pressure, high temperature pyrolysis of urea, both in the presence of ammonia (see Cyanamides). 

A hquid-phase reaction in which TiCl is reacted with hquid ammonia at —35 C to form an adduct that is subsequendy calcined at 1000°C has also been proposed (35). Preparation of titanium nitride and titanium carbonitride by the pyrolysis of titanium-containing polymer precursors has also been reported (36). 

Manufacture. Dicyandiamide is converted into melamine by heating. Simple pyrolysis above the melting point leads to an exothermic reaction however, deammoniation occurs, forming products containing two or three triazine rings as well as melamine. After it was discovered in 1940 that deammoniation can be counteracted by conducting the reaction under ammonia pressure, various methods were developed to control the exothermic reaction on an industrial scale. 

A convenient laboratory synthesis of high purity CA is hydrolysis of cyanuric chloride (7). On a commercial scale, CA is produced by pyrolysis of urea [57-13-6]. When urea is heated at - 250 ° C for about an hour, it is converted to crude CA with evolution of ammonia. 

Cyanuric acid can also be prepared by pyrolysis of urea derivatives. Biuret and triuret give less aminotria ines due to reduced ammonia evolution. Urea cyanurate also provides a higher assay product. 

By-product formation can also be reduced by use of a stripping gas or vacuum to faciUtate removal of ammonia (88) however, sublimation of urea becomes excessive if the pressure is too low. Addition of ammonium salts (eg, CU, NO7, or ) (89—91), acids, or pyrolysis of preformed urea salts, eg,... 

Chemical and physical solvent losses can render a process uneconomical. Desirable solvents are good solvents for urea, poor solvents for CA, high boiling, and stable to pyrolysis intermediates, ammonia, oxygen, and heat. Although no perfect solvent has been identified, some solvents, eg, dinitriles (94), pyrrohdinones (95,96), and sulfones (97) largely meet these requirements. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

Catalytic reduction of thiophenes over cobalt catalysts leads to thiolane derivatives, or hydrocarbons. " Noncatalytic reductions of thiophenes by sodium or lithium in liquid ammonia leads, via the isomeric dihydrothiophenes, to complete destructions of the ring system, ultimately giving butenethiols and olefins. " Exhaustive chlorination of thiophene in the presence of iodine yields 2,2,3,4,5,5,-hexachloro-3-thiolene, Pyrolysis of thiophene at 850°C gives a... 

Pyrolysis of hexakis(dimethylamido)dialuminum with ammonia at 200-250°C at 1 atm. 

The pyrolysis of aluminum-nitrogen organic complexes, such as diethyl aluminum azide [(C2H5)2A1N3], is also used successfully at low deposition temperatures (450-870°C).0 l Another metallo-organic, hexakis(dimethylamido)dialuminum, reacting with ammonia allows deposition at 200-250°C at atmospheric pressure. 1... 

These polymers may be used in the preparation of quite pure silicon nitride if the pyrolysis is carried out in a stream of ammonia (a reactive gas) rather than under nitrogen or argon. The ammonia reacts with the... 

The pyrolysis of bis(trimethylsilyl)aminodichloroborane to B-trichloro-N-tris(trimethylsilyl)borazine, contrary to literature (in. did not take place in boiling xylene. Temperatures above 150 C were necessary for trimethylchlorosilane elimination. The highest yield of the relatively pure product was around 20%. The transformation of the B-trichloro-N-tris(trimethylsilylJborazine to B-triamino-N-tris(trimethylsilyl)borazine proceeded readily using liquid ammonia. 

The thermal degradation of B-trianilinoborazine, when conducted in ammonia even below 215°C, produced 87.7% of the available aniline. This was brought up to 92.3% by further pyrolysis at 275-299 C which compares with the maximum of 53% observed under... 

The polysilazanes were also melt spun, cured, and pyrolyzed to give silicon carbonitride fibers (Eq. 7). The carbon content of these fibers depends on the molecular composition of the polysilazane and the pyrolysis gas. When ammonia is used as reactive gas pure silicon nitride fibers will be obtained (Eq. 8) [14]. 

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

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