Pyrolysis of urea

Entry Conditionsab W (°q Time (min) Tield (%) Selectivity (%) Reaction rate (103 s ) 

This reaction has been studied with classical and MW heating under homogeneous and heterogeneous conditions [59]. Table 9.4 summarizes the results. 

When the reaction was conducted in the homogeneous phase at 200 °C (Table 9.4, entries 1-4), identical reaction rates and similar yields and selectivity were obtained for both heating modes. Kinetic data for the first-order equation were similar Ea (MW) = 159 3 kj mol-, Ea (A) = 160 + 3 kj mol i In A (MW) 35 + 1, 

In A (A) 34 + 1. In contrast, in the presence of graphite (57 graphite = 4 1, w/w), improved yield and selectivity were obtained under the action of MW irradiation compared with conventional heating (Table 9.4, entries 5-8 at the same bulk temperature). Chemat and Poux ascribed this phenomenon to localized superheating ( hot spots ) on the graphite surface (Section 9.4.2). 

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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 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. 

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 %). 

Operabihty (ie, pellet formation and avoidance of agglomeration and adhesion) during kiln pyrolysis of urea can be improved by low heat rates and peripheral speeds (105), sufficiently high wall temperatures (105,106), radiant heating (107), multiple urea injection ports (106), use of heat transfer fluids (106), recycling 60—90% of the cmde CA to the urea feed to the kilns (105), and prior formation of urea cyanurate (108). 

Beside continuous horizontal kilns, numerous other methods for dry pyrolysis of urea have been described, eg, use of stirred batch or continuous reactors, ribbon mixers, ball mills, etc (109), heated metal surfaces such as moving belts, screws, rotating dmms, etc (110), molten tin or its alloys (111), dielectric heating (112), and fluidized beds (with performed urea cyanurate) (113). AH of these modifications yield impure CA. 

Chemat et al. have reported several microwave reactors, including systems that can be used in tandem with other techniques such as sonication [68], and ultraviolet radiation [69]. With the microwave-ultrasound reactor, the esterification of acetic acid with n-propanol was studied along with the pyrolysis of urea. Improved results were claimed compared with those from conventional and microwave heating [68]. The efficacy of the microwave-UV reactor was demonstrated through the rearrangement of 2-benzoyloxyacetophenone to l-(2-hydroxyphenyl)-3-phenylpropan-l,3-dione [69]. 

Chemat, F. and Poux, M., Microwave-assisted pyrolysis of urea supported on graphite under solvent-free conditions, Tetrahedron Lett., 2001, 42, 3693-3695. 

By coupling an ultrasonic probe with a microwave reactor and propagating the ultrasound waves into the reactor via decalin introduced into their double jacket design, Chemat et al. studied the esterification of acetic acid with propanol and the pyrolysis of urea to afford a mixture of cyanuric acid, ameline and amelide (Scheme 9.19)136. Improved results were claimed compared to those obtained under conventional and microwave heating. The MW-US technique was also used to study the esterification of stearic acid with butanol and for sample preparation in chemical analysis137,138. 

Miscellaneous Pyrolysis of urea herbicide to phenylisocyanate in the injection heater of the gas liquid chromatograph, then detection by ECD [339,355]... 

The simultaneous decomposition of pentachlorophenol and regeneration of activated carbon, using microwaves was reported [46], claiming that the quality of the carbon was maintained or actually increased after several adsorption/microwave-regeneration cycles. Carbon, in graphite form, has also been used as a microwave absorbent for the microwave pyrolysis of urea [47]. 

In the countries where chlorine is expensive, such as Japan, isocyanuric acid is made by pyrolysis of urea, yielding ammonia as a byproduct. 

Cyamelide (16) is a colorless powder, which is insoluble in water and organic solvents. It depolymerizes when heated in the presence of acids (cf. Houben-Weyl, Vol. E4, p 923). Above 150 JC cyanic acid gives cyamelide, below this temperature cyanuric acid is mainly formed. Cyanuric acid can exist in both the oxo and enol form, the latter being found as the enolate in alkaline media (cf. Introduction). Several other syntheses of cyanuric acid have been described the principal industrial method is the pyrolysis of urea (see Section on Urea Fragments, p 692). 

Schaber et al. [48] showed that, in the pyrolysis of urea, no significant reaction is observed when heating from room temperature to 133 (urea melting point). Urea typically melts with difficulty, and usually a complete melt, within a reasonable time period, is not achieved until 135 °C. Mass loss begins in earnest at 140 °C and is primarily associated with urea vaporization. At 152 C, urea decomposition begins, as noted by vigorous gas evolution from the melt. 

Schaber, P. A. Colson, J. Higgins, S. Thielen, D. Anspach, B. Brauer, J. Thermal decomposition (pyrolysis) of urea in an open reaction vessel. Thermochimica Acta 2004, 424, 131-142. 

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