Adipic acid, precipitation

In the Dupont process, cyclohexane is reacted with air at 150 °C and 10 atm pressure in the presence of a soluble cobalt(II) salt (naphthenate or stearate). The conversion is limited to 8-10% in order to prevent consecutive oxidation of the ol-one mixture. Nonconverted cyclohexane is recycled to the oxidation reactor. Combined yields of ol-one mixture are 70-80%.83,84,555 The ol-one mixture is sent to another oxidation reactor where oxidation by nitric acid is performed at 70-80 °C by nitric acid (45-50%) in the presence of a mixture of Cu(N03)2 and NH4V03 catalysts, which increase the selectivity of the reaction. The reaction is complete in a few minutes and adipic acid precipitates from the reaction medium. The adipic acid yield is about 90%. Nitric acid oxidation produces gaseous products, mainly nitric oxides, which are recycled to a nitric acid synthesis unit. Some nitric acid is lost to products such as N2 and N20 which are not recovered. 

The nylon 66 salt is prepared by reacting the hexamethylenediamine and adipic acid in boiling methanol, the comparatively insoluble salt (melting point 190-191°C) precipitating out. 

In a 500-mL Erlenmeyer flask, 29.2 g (0.2 mol) of adipic acid is dissolved in 250 mL of warm ethanol, and the solution is cooled to room temperature. A solution of23.43 g (0.202 mol) of hexamethylenediamine in 50 mL of ethanol is added to the adipic acid solution with gentle stirring (rinse the flask containing the diamine with ethanol). The solution warms up by the exothermic reaction and immediate precipitation of the salt takes place. The suspension is allowed to cool, then filtered, washed with methanol, and air dried. The pH of a 1% solution of the salt in water is measured and should be 7.6. 

The aqueous layer remaining after extraction with n-butanol was acidified (to pH 1) by the addition of 50% sulfuric acid, giving a precipitate of adipic acid which was collected by filtration, washed with 120 parts of water in two equal portions, and dried at 110° C. The crude adipic acid obtained was recrystallized from twice its weight of water to provide adipic acid in 90.2% yield, which was pure enough to be used in the synthesis of adiponitrile. 

The diamine and diacid monomers used to make type AABB nylons are typically rather difficult to handle in their pure form. Diamines are liquids or semisolids at room temperature, while the diacids are crystalline solids. These monomers become much more manageable when they are combined to form nylon salts, as shown in Fig. 23.7 a). Nylon salts are solids that can be easily handled and ensure a stoichiometric balance between the diacid and diamine, which is necessary to produce high molecular weight polymers. In the case of nylon 66, the precursor salt is made by boiling adipic acid and hexamethylene diamine in methanol, from which the nylon salt precipitates. 

Aqueous NaOCl (10%, 400 ml) is added with stirring to the cycloalkanone (0.1 mol) and Aliquat (4 g, 10 mmol) at 10°C. The mixture is stirred and the pH is maintained at 12.0 by the addition of aqueous NaOH (0.5 M). On completion of the reaction, the aqueous phase is separated, washed with CH2C12 (200 ml), and acidified to pH 2.0 with HC1 (2M). The acidic solution is cooled to 0°C to cause precipitation of the dicarboxylic acids (e.g. cyclohexanone yields a mixture of adipic acid 63%, succinic acid 9%, glutaric acid 17%, and a,a-dichloroadipic acid 5%). 

The reaction mixture is poured into a beaker and on cooling the adipic acid crystallizes. It is filtered with suction (Note 5), and the precipitate is washed once with about 200 cc. of cold water and dried in the air. The yield of slightly yellow adipic acid is 375-386 g. (55-56 per cent of the theoretical amount). This product is pure enough for most purposes. However, to remove the yellow color and obtain a purer product, this crude acid may be recrystallized from 700 cc. of concentrated nitric acid (sp. gr. 1.42). The loss in this purification is less than 10 per cent. The recrystallized acid melts at 1530 (Notes 6 and 7). 

Davto, R., Villermaux, J Marchal, P. Klein, J. P. 1991 Crystallization and precipitation engineering - IV. Kinetic model for adipic acid crystallization. Chemical Engineering Science 46, 1129-1136. 

Interaction in the Cu -Pb -Na2Ad-H20 system (AdH2 = adipic acid) has been examined by precipitation and residual concentration methods the Cu and Pb adipates cocrystallize. For the copper(ii)-catalysed oxidation of ascorbic acid by dissolved dioxygen in the presence of nitrate at pH 2.0—3.5 it is proposed that cupric ascorbate dimers are the reactive species. Dioxygen binds across a Cu—Cu dinuclear... 

The topics presented in this paper include a description of the bench-scale system, the experimental approach, and the results of degradation testing. Also included are the results of batch precipitation experiments designed to study coprecipitation of adipic acid in scrubber waste solids. 

In order to further characterize this mechanism for adipic acid loss, two series of batch precipitation experiments were performed. The tests were designed to study ... 

Table IV. Summary of Test Results for Addition of Adipic Acid Before and After Solids Precipitation...

Solids from the batch precipitation tests were also examined by scanning electron microscopy. In tests where no adipic acid was added, the calcium sulfite solids formed a single platelet crystal. However, upon addition of 3,000 ppm adipic acid prior to solids precipitation, the calcium sulfite crystals formed as platelet clusters or rosettes. As the concentration of adipic acid was increased the crystals became smaller and less plate-like until at 10,000 ppm adipic acid in the slurry solution the crystals were submicron in size and resembled popcorn shaped spheres (5). These results suggest that adipic acid effects the nuclea-tion rate of calcium sulfite and certainly can drastically change the particle size distribution and crystal morphology of precipitated solids. 

The batch precipitation tests show dramatic effects of adipic acid slurry concentration and solid phase oxidation fraction on coprecipitation of adipic acid in scrubber solids. Real world scrubbers would probably never operate at adipic acid concentrations as high as those tested and would also not likely ever produce pure phase calcium sulfite hemihydrate. Therefore, the magnitude of the results observed is somewhat a product of the laboratory test conditions. The results do, however, establish the potential importance of adipic acid coprecipitation and, hence, the need for analysis of scrubber solids for adipic acid when determining adipic acid chemical degradation rates by a mass balance calculation approach. 

The nylon-6,6 salt (melting point 190-191°C) is prepared by reacting hexamethylenediamine and adipic acid in boiling methanol, so that the comparatively insoluble salt precipitates out. A 60% aqueous slurry of the salt together with a trace of acetic acid to limit the molecular weight to the desired level (9000-15,000) is heated under a nitrogen blanket at about 220°C in a closed autoclave under a pressure of about 20 atmospheres (atm). The polymerization proceeds to approximately 80-90% without removal of by-product water. The autoclave temperature is then raised to 270-300°C, and the steam is continuously driven off to drive the polymerization to completion. 

Salt dehydration. Direct esterification requires high purity materials in equimolar amounts because esterifications rarely go beyond 98% completion in practice. To overcome this, hexamethylene diamine and a dibasic acid such as adipic acid can be reacted to produce a nylon salt, hexamethylene diammonium adipate. A solution of 0.5-mol diamine in a mixture of 95% ethanol (160 cm ) and distilled water (60 cm ) is added to 0.5-mol diacid dissolved in 600 cm of 95% ethanol over a period of 15 min. The mixture is stirred for 30 min during which time the nylon salt precipitates as a white crystalline solid. This can be recrystallized and should melt at 456 K. The pure salt can be converted into a polyamide by heating it under vacuum in a sealed tube, protected by wire gauze, at about 540 K in the presence of a small quantity of the diacid, e.g., 10 g salt to 0.55-g adipic acid is a suitable mixture. If a lower molar mass is desired, a monofunctional acid can replace the adipic acid and act as a chain terminator. 

Nylon 6,6 is a condensation product of hexamethylenediamine and adipic acid. This polyamide was originally synthesized in 1935 and first produced conunercially in 1938. It is still one of the major commercial nylons produced today. Because high molecular weight is required for such polymers to possess good physical properties, it is necessary to follow exact stoichiometry of the reactants in the condensation. To achieve that, the practice is to initially form a nylon salt prior to the polymerization. To do this, equimolar quantities of adipic acid and hexamethylenediamine are combined in aqueous environment to form solutions of the salt. The end point is controlled electrochemically. An alternate procedure is to combine the diacid with the diamine in boiling methanol. A1 1 adduct precipitates out, is filtered off, and dissolved in water. 

A solution containing 21.9 g of adipic acid (adipat Al = 8.8), lOOmL deionized water, and 20 g of NaOH was stirr in a flask. The solution containing 13.13g of aluminum nitrate and 17.92g of magnesium nitrate in lOOmL water (mole ratio of Mg /Al =2) was added dropwise from a separation funnel. After complete addition of the metal nitrate solution, the mixture (pH- 9-10) was heated at 323K for 4 h. The precipitate was ffltoed, washed with deionized water, and dried at 323 K. Similarly, a series of compounds with adipate/Al + mole ratio varied from 1 to 8.8, and Mg /Al mole ratio varied from 1 to 4 was prepared. 

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