Ureas thermolysis

Urea thermolysis Isocyanic acid hydrolysis Overall reaction ... 

Urea thermolysis is usually considered a solely thermal reaction, whereas the intermediate isocyanic acid (HNCO) is stable in the gas phase but hydrolyzes on the SCR catalyst or on a dedicated hydrolysis catalyst [9]. Catalytic reactions wiU be discussed later this section is focusing on thermal decomposition. 

Basic investigations of urea thermolysis, including the formation and decomposition of byproducts, have been performed using thermogravimetric analysis (TGA) and/or differential scanning calorimetry (DSC) [11, 13, 36, 38]. The DSC data consistently show a sharp feature at 133 °C, the melting point of urea. Further features strongly depend on experimental conditions, like the type of sample administration [11, 38]. 

The presented studies [56, 58] will not be the last on catalytic urea decomposition, yet the results already clearly show that urea thermolysis is catalyzed and that catalytic urea hydrolysis is much slower than the hydrolysis of pure HNCO. Taking into account the risk of urea-induced deactivation of the SCR catalyst [12], the ongoing trend for lower exhaust gas temperatures and the good properties of TiOi for byproduct decomposition [37], using a dedicated hydrolysis catalyst may be a good option for some urea-SCR applications. 

Fang HL, DaCosta HPM (2003) Urea thermolysis and NOx reduction with and without SCR catalysts. Appl Catal B 46 17-34... 

Lundstrom A, Andersson B, Olsson L (2009) Urea thermolysis studied under flow reactor conditions using DSC and FT-IR. Chem Eng J 150 (2-3) 544—550... 

Bernhard AM, Peitz D, Elsener M et al (2013) Quantification of gaseous urea by FTIR spectroscopy and its application in catalytic urea thermolysis. Top Catal 56 130-133... 

A convenient method for the synthesis of these low boiling materials consists of the reaction of /V,/V-dimethy1iirea [96-31-1] with toluene diisocyanate to yield an aUphatic—aromatic urea (84). Alternatively, an appropriate aUphatic—aromatic urea can be prepared by the reaction of diphenylcarbamoyl chloride [83-01-2] with methylamine. Thermolysis of either of the mixed ureas produces methyl isocyanate ia high yield (3,85). 

The hydroxyl derivative of X-CN is cyanic acid HO-CN it cannot be prepared pure due to rapid decomposition but it is probably present to the extent of about 3% when its tautomer, isocyanic acid (HNCO) is prepared from sodium cyanate and HCI. HNCO rapidly trimerizes to cyanuric acid (Fig. 8.25) from which it can be regenerated by pyrolysis. It is a fairly strong acid (Ka 1.2 x 10 at 0°) freezing at —86.8° and boiling at 23.5°C. Thermolysis of urea is an alternative route to HNCO and (HNCO)3 the reverse reaction, involving the isomerization of ammonium cyanate, is the clas.sic synthesis of urea by F. Wohler (1828) ... 

The major problems with the substitution of the reducing agent ammonia for urea are on the one hand the homogeneous mixing of urea and exhaust gas and on the other hand the limited residence time in SCR systems for the different decomposition steps, i.e. the evaporation of water from the droplet, the thermolysis of urea to isocyanic acid and the following hydrolysis to ammonia [18]. 

Also the thermohydrolysis of the urea solution after the injection into the hot exhaust gas upstream of the SCR catalyst has been investigated at the diesel test rig. Urea solution was atomized about 3 m upstream of the SCR catalyst into the hot exhaust equivalent to a residence time in the pipe section of 0.1 s at 440°C. As expected for the thermolysis reaction, ammonia and isocyanic acid were found at the catalyst entrance at all temperatures (Figure 9.3). The 1 1 ratio of both components shows that only the thermolysis but not the hydrolysis is taking place in the gas phase. It can also be seen that the residence time of 0.1 s is not sufficient for the quantitative thermolysis of urea, as appreciable amounts of undecomposed urea were always found. The urea share even raises with lowering the flue gas temperature, although the residence time... 

Moreover, triuret, ammeline, ammelide, melamine and other products may be formed from isocyanic acid, biuret and combinations of them. If urea is heated up very fast, these reactions are suppressed and the decomposition into ammonia and isocyanic acid is the preferred reaction. Due to the high reactivity of isocyanic acid, its primary formation may subsequently lead to the formation of the aforementioned compounds of higher molecular weight. In order to avoid the formation of by-products, the heating-up must be carried out fast. Only then ammonia and isocyanic acid are obtained as sole products. In any case, local undercooling of the gas duct should be avoided and rapid dilution of the thermolysis products in the exhaust gas has to be ensured in order to avoid locally high concentrations of reactive compounds. 

The reaction usually used to produce cyanuric acid (48) is the thermolysis of urea (47) between 180 °C and 300 °C (Scheme 7.11) [58]. The reaction occurs with formation of ammonia, which itself can react with 48 to give secondary products. It is, therefore, necessary to eliminate NH3 and to operate with an open reactor. 

Tab. 7.5 MW-assisted thermolysis of urea (47) under solvent-free conditions (Scheme 7.11) [59].

Thus, heptafulvalene (522) was isolated in 33 and 65% yield after thermolysis of 517 in diglyme and its photolysis in THF, respectively [193]. An almost quantitative yield of 522 resulted when a mixture of 1-, 2- and 3-chloro-l,3,5-cycloheptatriene (518a) was treated with KOtBu in THF [206]. Even on variation ofthe concentration of the starting material and the temperature of the reaction, 522 turned out to be the exclusive product [207]. Also, the treatment of (trimethylsilyl)tropylium tetrafluoro-borate (519) with tetrabutylammonium fluoride [208] and the gas-phase pyrolysis of 7-acetoxynorbornadiene and 7-acetoxy-l,3,5-cycloheptatriene [209] afforded high yields of 522. Further, 522 was observed on FVT of N-nitroso-N-(7-norbornadienyl)-urea at 350 °C, which is believed to be converted into 7-diazonorbornadiene initially. Its decomposition should proceed via 7-norbornadienylidene to bicyclo[3.2.0]hepta-l(2),3,6,-triene (514) (Scheme 6.103) and then on to 5 [210]. The intermediacy of 514 is also suspected in the formation of 522 from 7-acetoxynorbornadiene. 

Typical SCR systems may achieve on-road NOx conversion efficiencies of 60-70% [18], However, the thermolysis of urea is an endothermic reaction that is favoured at high temperatures SCR is inefficient at temperatures below around 200°C [19], Hence, NOx emissions from SCR-equipped vehicles can often increase during urban driving where traffic conditions result in low exhaust temperatures [20]. Supplementary systems, such as EGR, are required to maintain acceptable emission-control performance under such conditions. 

Finally, polymer-supported oxime 12 has served as a reagent for trapping primary amines after treatment with phosgene [22] to furnish intermediate oxime carbamates 13 (Scheme 5) [23], Thermolysis in the presence of amines results in release of ureas into solution. 

Carbodiimides are a unique class of reactive organic compounds having the heterocumu-lene structure R—N=C=N—R. They can be formally considered to be the diimides of carbon dioxide or the anhydrides of 1,3-substituted ureas, and they are closely related to the monoimides of carbon dioxide, the isocyanates. The substituent R can be alkyl, aryl, acyl, aroyl, imidoyl or sulfonyl, but nitrogen, silicon, phosphorous and metal substituted carbodiimides are also known. The unsubstituted carbodiimide HN=C=NH is isomeric with cyanamide, H2NCN. Mono substituted carbodiimides, generated in the thermolysis of 1-substituted tetrazoles, can be isolated at liquid nitrogen temperature but isomerize to the cyanamides at higher temperatures. ... 

When an o-nitroaniline is acylatcd by ethyl chloroformate and then catalyti-cally reduced, thermolysis of the reduction product (33) gives a 1-substituted 2-benzimidazolone (34) (Scheme 2.1.15) f99J. Presumably the carbamates (33) eliminate ethanol as they cyclize. and so the reactions bear similarities to those which proceed through isocyanates (see Scheme 2.1.18). In the presence of magnesium chloride, which appears to activate the urea carbonyl group to solvolysis and condensation, some benzimidazolones are converted into 2-alkyl- and 2-arylbenzimidazoles [100],... 

Under conditions of thermolysis, photolysis and hydrogenation certain isoxazoles can be converted into imidazoles. For example, merely heating 5-amino-3,4-diaUcylisoxazoles at 180-190°C gives 40-65% yields of 4,5-dialkylimidazolln-2-ones in what initially appears to be a Dimroth-type rearrangement. Compounds such as 4-mcthyl-5-propyl-, 4-butyl-5-propyl, 5-benzyl-4-methyl-, 4,5-dimethyl- and 4-ethyl-5-methylimidazoles can be formed in the same way, but only if urea is present and if the reaction is carried out in the condensed phase. Without the added urea (or an arylamine) the yields are only 40-65% with urea they reach 70-90% [29-31]. 

Formation of the urea derivative C(0)(NHFc)2 is also observed, in addition to FC-NH2, in the thermolysis of Fc-NCO in organic solvents [57]. 

Phenyl azide and di-iron nonacarbonyl react rapidly in benzene at room temperature (as compared with thermolysis of PhNg alone which occurs at temperatures of 140-170° ). The principal product was the orange phenyl nitrene-complex (387) which decomposed spontaneously in solution to give the urea-based complex (388). Also obtained in low yield was the orange complex (389). The yield of azobenzene was reported to be negligible. C)n the other hand, when the decomposition was carried out in benzene under reflux in the presence of Fe3(CO)i2j a significant amount of azobenzene was found . ... 

The nonphosgene production of isocyanates takes place through the thermolysis of the corresponding carbamate. The carbamate synthesis may involve a number of possible alternative ways, such as the reaction of a nitrocompound with CO, or the reaction of an amine with CO and O2, with urea and alcohol, or with a carbonic ester. Among these routes, the reaction of DMC or DPC with aliphatic amines is a very efficient way to produce carbamates. 

An approach to a 1,3,5,2-triazarsinane starts from the reaction of the N,N -bis(trimethylsilyl)urea (315) with AsClj, which forms the eight-membered heterocycle (316), the structure of which was established by x-ray. Thermolysis of (316) at 140-150°C yields the crystalline 2-chloro-l,3,5-trimethyl-l,3,5,2-triazarsinane-4,6-dione (317) (Scheme 55) <80CB382i>. Compound (317) can also be obtained by reaction of N,N, N"-trimethyl-N,N"-bistrimethylsilylbiuret with AsCh <78ZN(B)756>. 

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