Ammonia evolution

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. 

Ammonium salts of the zeolites differ from most of the compounds containing this cation discussed above, in that the anion is a stable network of A104 and Si04 tetrahedra with acid groups situated within the regular channels and pore structure. The removal of ammonia (and water) from such structures has been of interest owing to the catalytic activity of the decomposition product. It is believed [1006] that the first step in deammination is proton transfer (as in the decomposition of many other ammonium salts) from NH4 to the (Al, Si)04 network with —OH production. This reaction is 90% complete by 673 K [1007] and water is lost by condensation of the —OH groups (773—1173 K). The rate of ammonia evolution and the nature of the residual product depend to some extent on reactant disposition [1006,1008]. 

Figure 6. TG, DTG curves and rate of ammonia evolution (arbitrary units) for melamine hydrobromide. Conditions as Figure 2.

The reactants are phthalic anhydride, urea and copper(n) chloride, which are heated in a high-boiling aromatic solvent such as 1,2,4-trichlorobenzene, nitrobenzene or m-dinitrobenzene in the presence of a catalyst, usually ammonium molybdate. The solvent also acts as a heat-transfer medium. On heating to 120 °C an exothermic reaction begins and this temperature is maintained for about an hour. The temperature is then raised to 160-180 °C and kept constant for 6-12 hours. During this time ammonia and carbon dioxide are evolved, together with some solvent the reaction is complete when ammonia evolution ceases. The remaining solvent is then removed by either steam or vacuum distillation. The yield is 90-95%. For many years the solvent process was in almost exclusive use. 

A maximum in catalytic activity occurred in samples which had been calcined at temperatures above that at which ammonia evolution had ceased. Venuto et al. (160) found that the maximum activity for alkylation... 

Nitrogen determination by perchloric acid digestion of coal without catalysts is reportedly safe and much more rapid than the conventional Kjeldahl procedure. Nitrogen is determined in the digest by the usual ammonia evolution and titration. However, extreme care should be taken whenever perchloric acid is used. Nuclear reactions have been applied successfully to the determination of nitrogen in coal. 

Co(NH3)61WS4 Green-yellow Slow decomposition with ammonia evolution an formation of black color... 

Figure 8.58. EGA curves of rCc em3]Cl3 (171). a, gas evolution b, ethylenediamine evolution c. ammonia evolution A HC1 evolution.

Figure 8.59. EGA and MS curves of Co(en)j] (N03)3 (172). a, total gas evolution b, ethy-lenediainine evolution c. ammonia evolution. Spectrum on right is that of gases evolved at 410°C.

The total carbon balance traces in the top panel reveal interesting transient effects. Addition of water (b -> c or a c) results in periods during which the carbon balance is in excess of the input quantity (2000 ppm). This indicates that a surface species has been reacted to produce carbon dioxide. Peaks in ammonia evolution occur over die same periods and die calcidated N2 concentrations are then strongly negative as expected if the quantities of NH3 (plus N2O) being produced exceed that expected from the NO conversion. Deletion of water (d a) is followed... 

Synthesis of phthalocyanine-2,9,16,23-tetracarboxylic acid cobalt(II) complex 48 (R = -COOH M = Co(II)) 0.26 g (2 mmol) water-free cobalt chloride, 1.54 g (8 mmol) 1,2,4-benzenetricarboxylic acid anhydride, 4.8 g (80 mmol) urea and around 20 mg ammonium molybdate are intensively mixed and heated under inert gas in a 100-mL glass vessel at 270 °C for 1 h. The blue-colored reaction mixture was pulverized, stirred in 400 mL 6 M aqueous HCl, filtered, intensively washed with 6 M HCl and then water and acetone. The product was treated in a Soxhlet apparatus with acetone and dried. The phthalocyanine tetracarboxylic acid amide obtained was saponified to the tetracarboxylic acid as follows. 1 g of the tetracarboxylic acid amide was heated in a mixture of 6 g KCl and 15 mL 2 M aqueous KOH at 90 °C until the ammonia evolution became weak. The filtered product was dissolved in a fairly small amount of 2 M NaOH and after filtration from byproducts separated with 2 M HCl and isolated by centrifugation. Purification was achieved by dissolving in 0.05 M NaOH, separation with 2 M HCl and centrifugation until the filtrate is free of chloride ions. Yield from 1 g tetracarboxylic acid amide 0.56 g (38%). IR (KBr) 1698, 1373, 1326, 1150, 1091, 743 cm . 

As far as ammonium is concerned, it is normally present in zeolites as the cation, taken up by cation-exchange, especially for preparing intermediate compounds, to be transformed by heating into the corresponding catalytically active H-forms. DTA of NH4-exchanged zeolites usually results in an overlapping of the dehydration and deammoniation effects, which are not resolvable by TG. Ammonia evolution, the amount of which is a measure of the cation-exchange completeness, may be monitored by the thermo-gas titrimetric (TGT) technique, which allows quantitative evaluation of NH3 to be obtained by a potentiometric acid-base titration [23]. 

Hay (1968) [162] used DSC to study the thermal reactions of PAN. Volatiles were not detected until the temperature reached above 230°C, when least three distinct thermal processes competing with one another were found coloration by cyanide group polymerization, ammonia evolution from the coloring structures, and chain scission and decomposition. 

Cobaltocene. To 0.5 mole of sodium cyclopentadienide in 400 ml of tetra-hydrofuran (see Method 2 for the preparation of sodium cyclopentadienide) is added 75 gm of Co(NH3)6Cl2 and the temperature is raised to 35°—40° C, whereupon ammonia evolution commences. When ammonia evolution is virtually complete, the mixture is refluxed at 65° C for 2 hr. The solvent is removed under vacuum and the dry residue transferred to a sublimation apparatus, where the cobaltocene is isolated in the form of dark violet crystals which are very rapidly oxidized by air. The yield is between 42 and 47 gm (86-96% of theoretical yield based on sodium cyclopentadienide). 

Formation of the residue is considered to be a condensed-phase contribution of melamine, whereas ammonia evolution dilutes the flame with noncombustible gases. 

Silylation. Alcohols, amines, and thiols can be trimethylsi-lylated by reaction with hexamethyldisilazane (HMDS). Ammonia is the only byproduct and is normally removed by distillation over the course of the reaction. Hydrochloride salts, which are typically encountered in silylation reactions employing chlorosilanes, are avoided, thereby obviating the need to handle large amounts of precipitates. Heating alcohols with hexamethyldisilazane to reflux is often sufficient to transfer the trimethylsilyl group (eq 1). Completion of the reaction is indicated by either a change in the reflux temperature (generally a rise) or by the cessation of ammonia evolution. 

---