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 Manufacture of Adiponitrile
  • Manufacture of Adiponitrile
  • Four basically different routes have been developed for the commercial scale manufacture of adiponitrile (adipic acid dinitrile, ADN):
    1. Dehydrative amination of adipic acid with NH3 in the liquid or gas phase going through diamide intermediate
    2. Indirect hydrocyanation of butadiene with 1,4-dichloro- butene intermediate
    3. Direct hydrocyanation of butadiene with HCN
    4. Hydrodimerization of acrylonitrile in an electrochemical process

    To 1:

    In the first method, a recently developed process, adipic acid is converted with NH3 through the intermediates diammonium adipate and adipic acid diamide into the dinitrile. The reaction is conducted in a melt at 200-300 °C, or with adiponitrile and the intermediates as solvent, in the presence of a catalyst such as H3PO4 which is soluble in the reaction medium:

    The older process is conducted in the gas phase at 300-350°C using, e. g., boron phosphate catalysts with a great excess of NH3. The disadvantage is the decomposition of adipic acid on evaporation, which limits the selectivity to only 80% and also necessitates a catalyst regeneration. The main improvement in this process has been the conversion from a fixed bed to a fluidized bed with an H3PO4/SiO2 catalyst and continuous regeneration. The selectivity to adiponitrile can reach 90%. However, many plants have been taken out of operation in recent years for economic reasons.

    To 2:

    The butadiene hydrocyanation can, as in the process developed by Du Pont in 1951, take place indirectly by chlorination of butadienc. Initially, the chlorination was carried out in the liquid phase; today it is usually conducted in the gas phase at 200-300°C without catalyst. A mixture of 3,4-dichloro-1-butene and cis- and trans- 1,4-dichloro-2-butene is obtained with a selectivity of about 96%:

    The dichlorobutcnes are then reacted with HCN or an alkali cyanide in the liquid phase at 80 °C to butenedinitriles. The formation of 3,4-dicyano-1-butene is not disadvantageous since, in the presence of the copper-cyano complex, an allyl rearrangement takes place under the hydrocyanation conditions. A mixture of the cis/truns isomers of 1,4-dicyano-2-butene is obtained with about 95% selectivity:

    The mixture can then be hydrogenated with 95-97% selectivity to adiponitrile at 300 °C in the gas phase using a Pd catalyst. Du Pont used this technology in two adiponitrile plants until 1983.

    To 3:

    The third method - the direct hydrocyanation - was also developed by Du Pont. The first step, HCN addition to butadiene, gives a mixture of isomers of pentene nitriles and methylbutene nitriles, which is then isomerized predominantly to 3- and 4-pentene nitriles. In the second step, adiponitrile is formed from the anti-Markovnikov addition of HCN:

    The reaction takes place at atmospheric pressure and 30-150 °C in the liquid phase using a solvent such as tetrahydrofuran. Nio-complexes with phosphine or phosphite ligands and metal salt promoters, e. g., zinc or aluminum chlorides, are suitable catalysts. The net reaction is reported to lead to adiponitrile in high selectivity.

    Du Pont produces adiponitrile in the USA mainly by this process, and, together with RhBne-Poulenc (Butachimie), has operated a plant in France for direct hydrocyanation with a capacity of 100000 tonnes per year since 1977.

    To 4:

    The fourth method is known as the Monsanto EHD process (electro-hydrodimerization). It is based on the hydrogenative dimerization of acrylonitrile (AN) to adiponitrile (ADN):

    The net cathodic reaction can be represented as follows:

    This reductive or cathodic dimerization was first practiced on a commercial scale in the USA in 1965. Other plants have been built in the USA and, since 1978, also in Western Europe. This lectrochemical approach was also developed to a commercial process by Phillips , and in Europe by ICI, RhÔne-Poulenc, and UCB. In Japan, Asahi Chemical has practiced its own technology in a 26000 tonne-per-year plant since 1971.

    Ion-exchange membranes were initially used to separate the anode and cathode regions. In the latest advances in electrolysis cells, developed by Asahi, BASF, and UCB as well as by Monsanto, no such mechanical separation is necessary. Instead, a finely divided two-phase emulsion is rapidly pumped through the cathode-anode system. The aqueous phase contains the conducting salt and a small amount of acrylonitrile (determined by its solubility), while the organic phase consists of acrylonitrile and adiponitrile. The loss of acrylonitrile in the aqueous phase by reaction is offset by the more facile transfer of acrylonitrile from the emulsified organic phase. Although graphite and magnetite (Fe3O4) can be used as cathode and anode, respectively, the most recent patents refer to an advantageous membrane-free procedure with a Cd cathode and Fe anode.

    The conducting salt - a tetraalkylammonium salt - screens the cathode so completely with its hydrophobic alkyl groups that no water electrolysis - with hydrogen formation - can occur. Thus the hydrogenation of acrylonitrile to propionitrile is almost entirely suppressed, and only the organophilic acrylonitrile can be dimerized at the cathode.

    After passing through the electrolysis cell, part of the organic phase - the unreacted acrylonitrile and adiponitrile product - is separated and distilled. The selectivity to adiponitrile is about 90%, with byproducts propionitrile and biscyanoethyl ether.


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