Direct Hydrogen Cyanide Synthesis

The second reaction used to illustrate various features of adsorptive reactors is the direct synthesis of hydrogen cyanide from ammonia and carbon monoxide  

This overall reaction is already carried out industrially via the intermediate forma-mide (HCONH2) which, together with the use of disparate operating conditions (high pressures and mild temperatures for formamide synthesis, vacuum and high temperatures for its decomposition), overcomes the unfavorable thermodynamics of 

In addition to the much higher temperatures needed, this reaction is also more challenging than the Claus process with respect to the greater demands placed on adsorbent selectivity in the presence of another weak(er) acid (HCN) and the importance of an unwanted side-reaction (2CO = C + CO2 AHR = -172 kj) which is also encouraged by CO2 removal. 

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Closer inspection reveals that this somewhat superficial and largely self-evident evaluation is by no means exhaustive, and concrete experimental studies on adsorptive reactors expose both additional pitfalls and benefits that are often specific for a particular reaction system and decisive for the success or otherwise of adsorptive reactor concepts. Before illustrating this point with the help of four examples with which the author is personally acquainted - the Claus reaction, the direct hydrogen cyanide synthesis from ammonia and carbon monoxide and, to a lesser extent, the water-gas shift reaction and the Deacon process - it is worthwhile briefly reviewing other reaction systems for which the potential of adsorptive reactors has been examined (Tab. 7.2). 

Direct Hydrogen Cyanide Synthesis and Water-gas Shift Reaction... 

Direct hydrogen cyanide (HCN) gas in a fuel oil gasification plant to a combustion unit to prevent its release. 4. Consider using purge gases from the synthesis process to fire the reformer strip condensates to reduce ammonia and methanol. 5. Use carbon dioxide removal processes that do not release toxics to the environment. When monoethanolamine (MEA) or other processes, such as hot potassium carbonate, are used in carbon dioxide removal, proper operation and maintenance procedures should be followed to minimize releases to the environment. 

Two synthesis processes account for most of the hydrogen cyanide produced. The dominant commercial process for direct production of hydrogen cyanide is based on classic technology (23—32) involving the reaction of ammonia, methane (natural gas), and air over a platinum catalyst it is called the Andmssow process. The second process involves the reaction of ammonia and methane and is called the BlausAure-Methan-Ammoniak (BMA) process (30,33—35) it was developed by Degussa in Germany. Hydrogen cyanide is also obtained as a by-product in the manufacture of acrylonitrile (qv) by the ammoxidation of propjiene (Sohio process). 

A few chemicals are based on the direct reaction of methane with other reagents. These are carbon disulfide, hydrogen cyanide chloromethanes, and synthesis gas mixture. Currently, a redox fuel cell based on methane is being developed. ... 

The present procedure is a modification of the original synthesis.3 Hydrogen cyanide tetramer can be prepared directly from hydrogen cyanide.4... 

The Stacker reaction has been employed on an industrial scale for the synthesis of racemic a-amino acids, and asymmetric variants are known. However, most of the reported catalytic asymmetric Stacker-type reactions are indirect and utilize preformed imines, usually prepared from aromatic aldehydes [24]. A review highlights the most important developments in this area [25]. Kobayashi and coworkers [26] discovered an efficient and highly enantioselective direct catalytic asymmetric Stacker reaction of aldehydes, amines, and hydrogen cyanide using a chiral zirconium catalyst prepared from 2 equivalents of Zr(Ot-Bu)4, 2 equivalents of (R)-6,6 -dibromo-1, l -bi-2-naphthol, (R)-6-Br-BINOL], 1 equivalent of (R)-3,3 -dibromo-l,l -bi-2-naphthol, [(R)-3-Br-BINOL, and 3 equivalents of N-methylimida-zole (Scheme 9.17). This protocol is effective for aromatic aldehydes as well as branched and unbranched aliphatic aldehydes. 

A typical example is seen in the addition of hydrogen cyanide to an imine to yield a cyanoamine (Fig. 4-29). Many of these reactions have been used to best advantage in the synthesis of macrocyclic ligands and complexes, and as such are considered in Chapter 6. A simple example of such a reaction is seen in the addition of HCN to the cobalt(m) complex indicated in Fig. 4-30. The starting complex is also readily prepared by a metal-directed reaction. 

The Strecker amino acid synthesis, which involves treatment of aldehydes with ammonia and hydrogen cyanide (or equivalents) followed by hydrolysis of the intermediate a-amino nitriles to provide a-amino acids (Scheme 1), was first reported in 1850 [1], This method has been applied on an industrial scale toward the synthesis of racemic a-amino acids, but more recently interest in nonproteinogenic a-amino acids in a variety of scientific disciplines has prompted intense activity in the asymmetric syntheses of a-amino acids [2]. The catalytic asymmetric Strecker-type reaction offers one of the most direct and viable methods for the asymmetric synthesis of a-amino acid derivatives. It is the purpose of this Highlight to disclose recent developments in this emerging field of importance. 

Since HCN and aldehydes were produced directly from the electric discharge in the Miller s experiment [33], the Strecker reaction was very early proposed as a likely pathway for the prebiotic synthesis of amino acids. This reaction discovered in 1850 [34] is the most anciently known abiotic synthesis of a-amino acids, it originally consisted in the formation of an a-aminonitrile 1 from a carbonyl compound (either aldehyde or ketone), ammonia and hydrogen cyanide in moderately alkaline aqueous solution followed by aminonitrile hydrolysis in strong acid. 

Fig. 7.11. Catalytic activities and deactivation in the adsorptive direct synthesis of hydrogen cyanide due to unfavorable redox conditions.

Hydrogen cyanide is often obtained as a by-product of acrylonitrile manufacture (see Section 11.4). However, its availability by this method is steadily decreasing, in view of the improved yields of techniques for producing acrylonitrile and die faster growth of markets for methyl methacrylate. It is therefore produced by direct synthesis from hydrocarbons, according to three main schemes ... 

Hydrogen cyanide can also be converted to cyanoacetylene and hydrogen cyanate, both precursors of pyrimidines. These reactions were reproduced in the laboratory. In fact, in 1828, F. Wohler made urea from hydrogen cyanate and ammonia, the first synthesis of an animal substance from inorganic materials. Very likely, all these processes occurred primarily in an aqueous environment where and OH" ions acted as specific-acid or specific-base catalysts. It is particularly impressive that the three major classes of nitrogen containing biomolecules, purines, pyrimidines, and amino acids are formed by the hydrolysis of the oligomers formed directly from... 

The alkylation of Hagemann s ester (76) with )S -bromopropionic ester and subsequent decarboxylation and saponification on boiling with hydriodic acid led to the keto acid (77) [894, 895]. This could not be cyclized directly into the hydrindandione (81) [894], and therefore another method of synthesis was tried. The addition of hydrogen cyanide, alkaline hydrolysis, and esterification enabled the cis-diester (78) to be obtained, and this was then converted by Dieckmann cyclization and decarboxylation into the cis-hydrindandione (81) [896]. The same product was obtained by another route, from ethyl y-acetobutyrate (82). Its subsequent condensation with cyano-acetic ester, hydrogen cyanide, and acrylonitrile led to compound (83), which, on acid hydrolysis and esterification, gave the tetraester (84). The Dieckmann cyclization of this with the simultaneous formation of two rings and subsequent decarboxylation yielded the diketone (81) [897]. 

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