Hydrolysis of isocyanate

Related to this process is the hydrolysis of isocyanates or isothiocyanates" where addition of water to the carbon-nitrogen double bond would give an N-substituted carbamic acid (3). Such compounds are unstable and break down to carbon dioxide (or COS in the case of isothiocyanates) and the amine ... 

According to the literature (6) the formation of urea from amine and isocyanate proceeds much faster than the formation of urethane or the hydrolysis of Isocyanate. 

Piazzesi, G. (2006) The Catalytic Hydrolysis of Isocyanic Acid (HNCO) in the Urea-SCR Process, PhD. Thesis No. 16693, ETH Zurich. 

Different reactions pathways on Rh may explain the intermediate formation of ammonia. NH3 can be obtained via successive reaction steps between adsorbed NHX and dissociated hydrogen species [29]. Alternately, the formation of ammonia may occur via the hydrolysis of isocyanic acid (HNCO) [30]. Isocyanate species are formed by reaction between N and COads on metallic particles. Those species can diffuse onto the support leading to spectator species or alternately react with Hads yielding ultimately HNCO. Previous infrared spectroscopic investigations pointed out that isocyanate species predominantly form over rhodium-based catalysts [31]. 

Some of the reaction products of polymerisation and cure can be toxic, for example, aromatic amines from hydrolysis of isocyanates and bisphenol A from... 

Oxidation, hydrolysis, and photolysis are the three predominant chemical processes that may cause loss of simple cyanides in aquatic media are. Cyanides are oxidized to isocyanates by strong oxidizing agents the isocyanates may be further hydrolyzed to ammonia and carbon dioxide (Towill et al. 1978). However, it has not yet been determined whether such oxidation and subsequent hydrolysis of isocyanate is a significant fate process in natural waters known to contain peroxy radicals (EPA 1992f). 

Heating should be gradual and in a large reaction vessel as hydrolysis of isocyanate is accompanied by evolution of carbon dioxide and considerable foaming may occur. If excessive foaming occurs, the steam bath should be removed. 

Note Top reaction sequence represents hydrolysis of isocyanate functions to corresponding amines. The remainder of the reaction scheme represents some of the potential side reactions of hexamethylene diisocyanate and the hydrolysis products of hexamethylene diisocyanate. Extended repetition ofthese reactions results in the formation of high molecular weight polyurea compounds. 

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. 

Abstract. Infrared analysis of reacting foams has been used to determine the sequence of chemical reactions that occurs in the foaming process and to relate this sequence of reactions to the stages evident in the formation of foam measured by rate-of-rise techniques. A reaction sequence model has been developed in which at the earliest stage of foaming, out-of-phase carbamic acid and carbamic acid salts, formed by hydrolysis of isocyanate, are the main species present tending to stabilize the froth-like foam. These species are in turn converted into polyureas. In the latter stages of the reaction sequence, urethane formation becomes predominant as the foam develops substantial bulk modulus... 

Publication has occurred of patents dealing with (i) hydrolysis of isocyanates of the type CFs-CFR-NCO (R = CFs or CFaCl) with (CFs)2C0,6H20-KF-MeCN to provide CFs CR NH, " and the reversion CFs-CRiNH + COF2-KF-MeCN CFs-CFR-NCO (ii) conversions such as fCFs)aCO-KF-MeCN + CICN -> (CF3)2CF-NC0 + (CFs)2CF-N C(CF8)3, and the related procedure... 

Hauck P, Jentys A, Lercher JA (2007) On the quantitative aspects of hydrolysis of isocyanic acid on Ti02. Catal Today 127 165-175... 

Piazzesi G, Krocher O, Elsener M et al (2006) Adsorption and hydrolysis of isocyanic acid on Ti02. Appl Catal B 65(l-2) 55-61... 

Czekaj I, Krocher O (2009) Decomposition of Urea in the SCR Process Combination of DFT Calculations and Experimental Results on the Catalytic Hydrolysis of Isocyanic Acid on Ti02 and AI2O3. Top Catal 52(13) 1740-1745... 

Czekaj I, Krdcher O, Piazzesi G (2008) DFT calculations, DRIFT spectroscopy and kinetic studies on the hydrolysis of isocyanic acid on the Ti02-anatase (1 0 1) surface. J Mol Catal A 280(l-2) 68-80... 

The reaction scheme presented above, according to experimental measurements available in the literature [26], is characterized by different values of the kinetic constants depending whether they occur onto a catalyzed bed or not. Focusing on the urea thermal decomposition process, experiments [26] have shown that reaction (17.38) is very rapid and is not affected by the presence of a catalyzed bed of typical SCR systems, whereas the hydrolysis of isocyanic acid occurs only at high temperature levels. According to this analysis, the values of the kinetic constants that can be used for the simulation of urea thermal decomposition are ... 

To conclude, ammonia can be formed only during the regeneration steps of the NOx-trap, even if ammonia release can also occur during the subsequent lean phase. Two pathways are described the direct reaction of H2 with the stored NOx or with NOx present in the gas phase, and via the hydrolysis of isocyanates species. Hydrogen is reported as a more efficient reductant than CO, leading to a higher emission of ammonia [11, 12]. 

The HPLC analysis of the eleetrolysis carried out in DMF solvent showed the presence of both isoeyanate and earbammic acid, but no qualitative analysis was possible beeause earbammie aeid ean be also produced, by hydrolysis of isocyanate in the HPLC column. 

---