Ammonium Carbamate formation

Dynamic Capture of Carbon Dioxide via Reversible Ammonium Carbamate Formation... 

Fig. 11 (a) C02 capture by reversible ammonium carbamate formation, (b) Example of linear solid oligomer formed upon reaction of 2-hydroxy-l,3,5-benzenetrialdehyde and at least 3 equiv. of diethylenetriamine in the presence of C02... 

The formation of ammonium carbamate and the dehydration to urea take place simultaneously, for all practical purposes ... 

The value has been extrapolated because, at temperatures above 170°C, the rate of reaction 2 rapidly iucreases and it is difficult to determine the carbamate vapor pressure owiag to the formation of water and urea and the consequent lowering of the partial pressure of ammonium carbamate. 

A simpler nonphosgene process for the manufacture of isocyanates consists of the reaction of amines with carbon dioxide in the presence of an aprotic organic solvent and a nitrogeneous base. The corresponding ammonium carbamate is treated with a dehydrating agent. This concept has been apphed to the synthesis of aromatic and aUphatic isocyanates. The process rehes on the facile formation of amine—carbon dioxide salts using acid haUdes such as phosphoryl chloride [10025-87-3] and thionyl chloride [7719-09-7] (30). 

Urea is dehydrated to cyanamide which trimerizes to melamine in an atmosphere of ammonia to suppress the formation of deamination products. The ammonium carbamate [1111-78-0] also formed is recycled and converted to urea. For this reason the manufacture of melamine is usually integrated with much larger facilities making ammonia and urea. 

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

Ammonium carbamate is prepared from dry ice and liquid ammonia [14]. These conditions are very similar to the conditions under which we have observed the formation of amine salts. To some readers, ammonium carbamate may seem to be an exotic compound. In fact, it is manufactured industrially on a multiton scale, because on heating (usually at 100-185°C) ammonium carbamate is converted to urea and water [14-16]. Urea is important for both the agricultural and the plastics industries. The ammonium carbamate is not always isolated during urea preparation. Instead, the reactions are carried out under conditions where the carbamate is just an intermediate. Ammonium carbamate is only moderately stable and it gradually loses ammonia in air. Although the data are sparse, the rate of decomposition of carbamates in solution seems to decrease as the volatility of the parent amine decreases [17]. Free carbamic acids in solution do not decompose spontaneously to free amine and C02. Instead, the acid ionizes by reaction with water the proton is transferred from the hydronium ion to the amine and then decomposition occurs [17]. Acids catalyze the decomposition. 

In contrast, the treatment of ammonium carbamate (514) with the equivalent amount of 142 gave TMS carbamate (515) which decomposed under formation of N,0-bis(TMS)-carbamate (516) (Scheme 80)28S. Intensive studies286 revealed... 

PEG as a support could enhance the reaction, which could be explained with the proposed mechanism as shown in Scheme 5.14. The reaction involves two steps, that is, formation of the ammonium carbamate and dehydration to the urea. In this reaction, PEG could form [R2NH2PEG]+ [R2NC02], and thus could increase the thermodynamic stability of [R2NH2]+. Increasing the basicity of the base with the help of PEG could also facilitate formation of the ammonium carbamate salt. 

Barzagli F, Mani F, Peruzzini M (2011) From greenhouse gas to feedstock formation of ammonium carbamate from C02 and NH3 in organic solvents and its catalytic conversion into urea under mild conditions. Green Chem 13(5) 1267-1274... 

Figure 7.12 outlines how hydantoin is produced during the synthesis of methionine depicted in Figure 7.11. Initially, the aminonitrile B is formed. With its amino group it adds to carbon dioxide, which—under the weakly basic reaction conditions—is followed by the formation of the carbamate ion D. In a fully analogous fashion, ammonia reacts with carbon dioxide to give ammonium carbamate (cf. Section 8.2). The intermediate D in Figure 7.12 permits the intramolecular and thus kinetically favored—i.e., fast—addition of the oxyanion to the C=N triple bond. The addition product is the negatively charged heterocycle F, which in...

The most important example of the reactions in this section s title is the second reaction of the urea synthesis (B) (Figure 8.15). It starts from ammonium carbamate (A) generated in situ. In a reversible acid/base reaction, A—to a minor extent—is transformed into NH3 and the unsubstituted carbamic acid C. In a reversal of its formation reaction the latter decomposes—... 

Fig. 8.15. Synthesis of urea. The overall reaction is shaded red. The mechanism of the initial formation of ammonium carbamate was discussed using the (unnumbered) formula on p. 351. Here we present the mechanism of the subsequent conversion of ammonium carbamate into the final product...

The Heat Recycle Urea Process (HRUP) was developed by Urea Technologies in the 1970 s and is now offered by Monsanto Enviro-Chem. This process is shown in Figure 11.5. In this process ammonia, recycled carbamate and about 60% of the make-up CO2 feed are charged to the top of an open-ended reactor coil at 3200 psig (220 bar). Ammonium carbamate is formed within the coil, exits the coil at the bottom and flows up and around it. The exothermic heat of carbamate formation in the coil drives the endothermic dehydration of carbamate to urea on the outside of the coil. This isothermal reactor design allows the conversion rate to reach 77%. The reactor has a uniform temperature profile that eliminates corrosion in the bottom of the reactor112. 

When primary aliphatic isocyanates that show the lowest reactivity compared to secondary or aromatic isocyanates are used in combination with hydrophilic (pre-) polymers, crosslinking may be performed in aqueous solution without the use of additional crosslinkers. At neutral pH, hydrolysis of isocyanates to carbaminic acid with subsequent decarboxylation yields amines. These amines react much more rapidly than water with isocyanates, resulting in crosslinking if the functionality per macromolecule is more than two [43], This crosslinking reaction can be quenched by adjustment of the pH value. At pH values above 10, carbamate formation is faster than decarboxylation, whereas at pH values below 3 an almost quantitative protonation of the formed amino groups results in the formation of ammonium. In both cases, chemical crosslinking is prevented. 

The reactor effiuent, consisting of urea, ammonium carbamate and unconverted reactants, is subjected, by altering the operating conditions, to a decomposition that converts part of the ammonium carbamate to carbon gaseous product is compressed and condensed. This leads to renewed formation of carbamate which is recycled to the reactor in aqueous s ution, while the excess ammonia is mixed with fresh ammonia. The entire operation is repeated to decompose aO the carbamate. The final solution obtained contains 72 to 76 per cent weight of urea, and the fmal purity desired can be obtained by a finishing treatment. 

Fulop and co-workers [124] have reported the synthesis of some new (ami-noalkyl) naphthols and (aminoalkyl) quinolinols 85 in good yields through a one-pot, microwave-assisted three-component modified Mannich reaction of naphthol or quinolinol with two equivalents of an aldehyde using ammonium carbamate or ammonium hydrogen carbonate as solid ammonia sources. It was observed that aliphatic aldehydes did not lead to the formation of the desired (aminoalkyl) quinolinols 85 (Scheme 64). 

The rates of formation and hydrolysis of carbamates were examined by Faur-holt . A recent, but inconclusive, attempt to confirm the occurrence of ammonium carbamate as an intermediate in the hydrolysis of urea catalysed by urease has been published. Caplow has reported rates of formation and breakdown of a number of carbamates and demonstrated that their formation involves a rate law like... 

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