Dimerization of acrylonitrile to adiponitrile

The first sodium amalgam-induced dimerization of acrylonitrile to adiponitrile [29] was performed in aqueous solution, and the yield was discouragingly low (5%). Mainly, a base-induced addition of water resulting in p, -bis(cyanoethyl) ether occurred together with a reduction to propionitrile (PN). 

Processes 4, 5, and 6 in Figure 2.13 are based on acrylonitrile. Among them, process 6 is the oldest. Introduced in 1965 as a technical process by Monsanto [175], it involves the electrolytic dimerization of acrylonitrile to adiponitrile. The process is carried out in a system of electrolytic cells containing sulfonated polystyrene membranes. An aqueous solution of tetraethylammonium- -toluyl sulfonate is used as the catholyte and aqueous sulfuric add as the anolyte. Using a 40% aqueous solution of acrylonitrile at temperatures of 25 35°C, the following reaction sequences occur ... 

Figure 6.5 Flow sheet for Monsanto hydro dimerization of acrylonitrile to adiponitrile, AN, acrylonitrile ADN, adiponitrile QS, tetraethylammonium ethylsulphate.

Figure 6.7 Possible reaction pathways for the hydro dimerization of acrylonitrile to adiponitrile. The asterisk indicates that electron transfer can be from the cathode or from [CH2CHCN] in homogeneous solution.

Development of the industrial process for electrochemical conversion of acrylonitrile to adiponitrile led to extensive investigation into the mechanism of the dimerization process. Reactions of acrylonitrile radical-anion are too fast for investigation but the dimerization step, for a number of more amenable substrates, has been investigated in aprotic solvents by electrochemical techniques. Pulse-radiolysis methods have also been used to study reactions in aqueous media. 

The fact that the iR drop is a smaller problem for UMEs compared to microelectrodes has another straightforward advantage, the substrate concentration can be increased substantially this makes the performance of electroanalytical studies under conditions similar to industrial conditions possible. For instance, the industrially important hydrodimerization of acrylonitrile to adiponitrile takes place at high concentrations in aqueous medium in the presence of tetraalkylammonium salts that form an aprotic medium in the vicinity of the electrode surface. The mechanism consists of a dimerization reaction of the radical anions of acrylonitrile formed upon reduction of acrylonitrile in the aprotic tetraalkylammonium layer, followed by protonation of the dimer in the aqueous phase (Eq. 87). However, at low to moderate concentrations of acrylonitrile, a change in mechanism occurs in favor of a two-electron reduction of acrylonitrile to propionitrile (Eq. 88). 

It might appear likely that electroreduction processes take place directly by electronst on metals which have high hydrogen overpotential and by H(a) on low overpotential metals. Indeed, e.g., electroreduction of acrylonitrile to adiponitrile on Pb or Hg takes place by electronation, followed by proton addition and dimerization, while electrolytic olefin hydrogenation on Pt takes place by transfer of H(a) to the organics. Nevertheless, such... 

Hexamethylenediamine (HMDA), a monomer for the synthesis of polyamide-6,6, is produced by catalytic hydrogenation of adiponitrile. Three processes, each based on a different reactant, produce the latter coimnercially. The original Du Pont process, still used in a few plants, starts with adipic acid made from cyclohexane adipic acid then reacts with ammonia to yield the dinitrile. This process has been replaced in many plants by the catalytic hydrocyanation of butadiene. A third route to adiponitrile is the electrolytic dimerization of acrylonitrile, the latter produced by the ammoxidation of propene. 

Electrochemical processes are often touted as being green chemistry because electricity is considered inexpensive, and toxic metal reagents are usually avoided. Electrochemical processes have produced tons of bulk chemicals [37], the best-known of which may be adiponitrile from reductive dimerization of acrylonitrile (Figure 13.17) [38]. An electrochemical synthesis to manufacture fenoprofen is shown in Figure 13.18, with the magnesium provided as a sacrificial electrode [39], Flow cell technology has been used for these operations on a commercial basis. 

The hydrodimerization of acrylonitrile is only one of many possible chemical reactions in the catholyte and, indeed, several have been observed during electrolysis. The first step in the hydrodimerization is the reduction of acrylonitrile to its anion radical, and to form adiponitrile dimerization and protonation must... 

However, the most significant changes in nylon-6, 6 production lie in the newer routes to adiponitrile introduced by Monsanto and Du Pont into both the U.S.A. and Western Europe. The former entails the electroreductive dimerization of acrylonitrile, while Du Pont use the addition of HCN to 1,3-butadiene catalysed by Ni(0) complexes ... 

Hydrodimerization. The reductive dimerization of acrylonitrile can be done either chemically or electrochemically to form adiponitrile. Hydrodimerization with its derivatives also takes place. 

Acrylic textile fibers are primarily polymers of acrylonitrile. It is copolymerized with styrene and butadiene to make moldable plastics known as SA and ABS resins, respectively. Solutia and others electrolytically dimerize it to adiponitrile, a compound used to make a nylon intermediate. Reaction with water produces a chemical (acrylamide), which is an intermediate for the production of polyacrylamide used in water treatment and oil recovery. 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

About one third of all adiponitrile is made from acrylonitrile. In the electrodimerization of acrylonitrile a two-phase system is used containing a phase transfer catalyst tetrabutylammonium tosylate [(n-Bu)4bTOTs ]. The head-to-head dimerization may be visualized to occur in the following manner. 

Ruthenium-catalyzed hydrodimerization of acrylonitrile under hydrogen atmosphere to give adiponitrile (393) is a useful coupling reaction [154], Dimethyl hexenedioates (394a and 394b) are formed by dimerization of methyl acrylate by Pd, Ru and Rh catalysts. In particular the catalyst prepared by the treatment of RuC12 with Zn and... 

The initial drive for acrylonitrile (AN) production (6.2 Mt/a in 2004 worldwide) was the discovery, in the late 1930s, of the synthetic rubber Buna N. Today nitrile rubbers represent only a minor outlet for AN which is utilized primarily for polymerization to give textile fibres (50%) and ABS resins (24%), and for dimerization to adiponitrile (10%). Early industrial processes depended on the addition of hydrogen cyanide to acetylene or to ethylene oxide, followed by the dehydration of intermediate ethylene cyanohydrin. Both processes are obsolete and are now supplanted by the ammoxidation of propylene (Equation 34) introduced in 1960 by Standard Oil of Indiana (Sohio). The reason for the success stems from the effectiveness of the catalyst and because propylene,... 

To illustrate this technique, consider the dimerization of the acrylonitrile anion radical (AN") in DMF (26). The electroreductive hydrodimerization of AN is used commercially to produce adiponitrile [(ANH)2], a precursor in Nylon production. The proposed reaction mechanism, an E1.C2 reaction [Section 12.1.1(b)], is... 

It has been shown by studies of the electroreduction of acrylonitrile at Pb, Ni, and Pt cathodes in the presence of quaternary ammonium salts that the yield of adiponitrile depends on the electrode material, with little dependence on cathode potential. On a Pb cathode the yield of adiponitrile was found to be almost independent of quaternary ammonium salt concentration, whilst on a Ni cathode the yield increased with increasing concentration of quaternary salt. The yield of adiponitrile, when a Pt cathode was used, was low and this led to the conclusion that atomic H2 plays a part in the mechanism. The ratio V2/v (dimerization/propionitrile rates) from their common... 

TAA+ have been reported to enhance dimerization within the potential window . Specifically TAA+ favor the formation of adiponitrile from acrylonitrile 5-l9). However, since TAA+ are inactive in this potential region their involvement may be different than described above. 

In terms of sustainability, the process starting from propene would be preferable, since it avoids the risks connected with the use of HCN in the butadiene route, even if produced on demand. However, the butadiene route to produce adiponitrile (ADN) is more cost-effective, owing to the need to use an electrochemical reaction for acrylonitrile dimerization. The problem of cost, however, is highly dependent on several factors, including sensitivity to natural gas prices (which influences butadiene cost), the market for acrylonitrile, and soon. The acrylonitrile route is used by Solutia, BASF and Asahi Kasei. New plants to make caprolactam, using ADN as intermediate, are under construction in Asia. 

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