Adipic acid from nylon-6,6 hydrolysis

Hydrolysis of Nylon 6,6 and Nylon 4,6 The depolymerization of nylon 6,6 and nylon 4,6 involves hydrolysis of the amide linkages, which are vulnerable to both acid- and base-catalyzed hydrolysis. In a patent granted to the DuPont Company in 1946, Myers [33] described the hydrolysis of nylon 6,6 with concentrated sulfuric acid, which led to the crystallization of adipic acid from the solution. Hexamethylene diamine (HMDA) was recovered from the neutralized solution by distillation. In a later patent assigned to the DuPont Company by Miller [34], a process was described for hydrolyzing nylon 6,6 waste with aqueous sodium hydroxide in isopropanol at 180°C and 2.2 MPa pressure. After distillation of the residue, HMDA was isolated and on acidification of the aqueous phase, adipic acid was obtained in 92% yield. Thorbum [35] depolymerized nylon 6,6 fibers in an inert atmosphere at what was reported to be a supeiatmospheric pressure of up to 1.5 MPa and at a temperature in the range of 160-220°C in an aqueous solution containing at least 20% excess equivalents of sodium hydroxide. 

Adipic acid and HMDA are obtained from nylon-6,6 by die hydrolysis of die polymer in concentrated sulfuric acid (Fig. 10.7). The AA is purified by recrystallization and the HMDA is recovered by distillation after neutralizing die acid. This process is inefficient for treating large amounts of waste because of die required recrystallization of AA after repeated batch hydrolyses of nylon-6,6 waste. In a continuous process,5 nylon-6,6 waste is hydrolyzed with an aqueous mineral acid of 30-70% concentration and the resulting hydrolysate is fed to a crystallization zone. The AA crystallizes and the crystals are continuously removed from the hydrolysate. Calcium hydroxide is added to neutralize the modier liquor and liberate the HMDA for subsequent distillation. 

A synthesis of great industrial interest is the electrochemical anodic reductive dimerisation of two molecules of acrylonitrile to give adiponitrile, from which adipic acid and 1,6-hexanediamine are prepared by hydrolysis and reduction, respectively, of the two nitrile groups. Polycondensation of the resulting products leads to Nylon 66 (Scheme 5.27). 

Nylon 66 is a polymer manufactured by the condensation of adipic acid with hexamethylcnediamine, and in excess of I million ton year are manufactured in the USA atone. Adiponitrile is a convenient intermediate for the production of both hexamethylenediamine and adipic acid, the former by hydrogenation and the Latter by hydrolysis. In addition, acrylonitrile is available from the gas phase, catalytic oxy-amination of propylene. Hence, to complete the manufacture of nylon from propylene and ammonia, a procedure for the conversion of acrylonitrile to adiponitrile is required (Fig. 6.1). 

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