Azeotropic nitration

To solve some of the environmental problems of mixed-acid nitration, we were able to replaee sulfuric acid with solid superacid catalysts. This allowed us to develop a novel, clean, azeotropic nitration of aromatics with nitric acid over solid perfluorinated sulfonic acid catalysts (Nafion-H). The water formed is continuously azeotroped off by an excess of aromatics, thus preventing dilution of acid. Because the disposal of spent acids of nitration represents a serious environmental problem, the use of solid aeid eatalysts is a significant improvement. 

For the process step involving handling of spent sulfuric acid, several patents have been issued in which improvements in this step were a main claim. The azeotropic nitration of benzene essentially eliminates the need to reconcentrate sulfuric acid (10,11). The nitration step is carried out at higher than usual temperatures (120—160°C). Because excess benzene is used, the higher temperature allows water to be removed as a water—benzene azeotrope. The water is separated and the benzene phase, containing approximately 8% nitrobenzene, is recycled back into the reactor. The dry sulfuric acid is then reused continuously. 

The solid superacidic Nafion-Hhas also been found to catalyze effectively nitration reactions with various reagents.503 The nitrating agents employed were n-butyl nitrate, acetone cyanohydrin nitrate, and fuming nitric acid. In nitric acid nitrations, sulfuric acid can be substituted by Nafion-H and the water formed is azeotropically removed during the reaction (azeotropic nitration).503... 

Azeotropic nitration of benzene is conducted at 120-160 °C with excess benzene. Water generated during the reaction is removed as a water-benzene azeotrope and may be separated from the benzene, which is recycled to the reactor. The sulfuric acid remains dry and can be used for longer periods between regeneration. 

In view of these considerations and the limitations of polystyrenesulfonic acids, we studied superacidic Nafion-H perfluorosulfonic-acid-resin-caialyzed nitration of aromatics with nitric acid under conditions of azeotropic removal of water (azeotropic nitration) [12]. The azeotropic removal of water in Nafion- H -catalyzed nitration allows utilization of nitric acid to a significantly greater extent than do conventional conditions of nitration. Both fuming and concentrated nitric acid are effective (Table VII). 

Table VII. Azeotropic Nitration of Aromatic with Nilric Acid over Nafion-H Catalyst...

Because of the advantages of using solid superacidic catalysts in electrophilic aromatic nitration and in acid-catalyzed reactions in general, Olah et al. have examined the mercury (Il)-promoted azeotropic nitration of aromatics using Nafion-H solid superacidic catalyst [142]. Azeotropic removal of water accelerates the rate of reaction by mitigating the dilution of nitric acid in a static reaction system. The yield of nitroaromatics varies from 48-77% (Table XXVIII). 

As the water formed is removed axeotropically, the mercury impregnated Nafion-H catalyst can be recovered by filtration without any loss of activity and can be recycled. Comparison of data with nitration in the absence of mercury catalyst shows that formation of less hindered isomeric nitroarenes are favored. It is interesting to note that attempted azeotropic nitration of ethylbenzene with nitric acid/Nafion-H yielded only acetophenone via side-chain oxidation, whereas in the presence of mercury salt under similar reaction conditions, niiroethylbenzenes were obtained in good yield with only 13% of product of side chain oxidation. 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

Nitric acid-water Maximum-hoiling azeotrope Sulfuric acid, magnesium nitrate for salt process Sulfuric acid process relies heavily on boundary curvature... 

To a stirred and refluxing solution of 40 parts of benzene and 35 parts of dimethylformamide (both solvents previously dried azeotropically) are added successively 1.6 parts of sodium hydride and 7.7 parts of Ct-(2,4-dichlorophenyl)imidazole-1-ethanol, (coolingon ice is necessary). After the addition is complete, stirring and refluxing is continued for 30 minutes. Then there are added 7.8 parts of 2,6-dichlorobenzyl chloride and the whole is stirred at reflux for another 3 hours. The reaction mixture is poured onto water and the product 1-[2,4-dichloro-/3 (2,6-dichlorobenzyloxy)phenethyl] imidazole, is extracted with benzene. The extract is washed twice with water, dried, filtered and evaporated in vacuo. The bese residue is dissolved in a mixture of acetone and diisopropyl ether and to this solution is added an excess of concentrated nitric acid solution. The precipitated nitrate salt is filtered off and recrystallized from a mixture of methanol and diisopropyl ether, yielding 1-[2,4-dichloro- (2,6-dichlorobenzyl-oxv)phenethyl] imidazole nitrate melting point 179°C. 

Inasmuch as methyl nitrate is very sensitive to mechanical action, it was found much safer to use it in methanol soln. Such solns, called Myrol, may be obtained directly in the methyl nitrate manufg process, since all that is necessary is to use an excess of methanol. One of the most suitable solns proved to be an azeotropic mixt consisting of about 75% methyl nitrate and 25% methanol. This mixt has a bp of 57.5°. Myrols contg at least 25% methanol will not evaporate to leave 100% methyl nitrate... 

A violent explosion ocurred during prolonged azeotropic drying at 105-110°C of a 75 wt% benzene solution of the salt. Traces of a nitrate ester may have been formed from a slight excess of nitrate ion. 

Azeotropic benzene nitration, 2 7 255 Azeotropic composition, 24 684 Azeotropic and extractive distillation, 8 786-852 28 647 20 751 22 44-45 23 550. See also Distillation(s) Extractive distillation(s) binary mixtures, 8 824-825 extractive, 8801-815... 

To a flask containing 34.5 gm (0.53 mole) of sodium azide in 70 ml of water and 25 ml of methanol is added 68.5 gm (0.50 mole) of n-butyl bromide while stirring at room temperature. The resulting mixture is heated and stirred on a steam bath for 24 hr. The bottom layer of n-butyl bromide disappears after this time and a top layer of crude n-butyl azide forms. The crude azide is separated and then treated overnight with alcoholic silver nitrate to remove traces of butyl chloride. The mixture is then filtered, washed with water, and distilled behind a safety barricade to yield 40.0 gm (90 %) of n-butyl azide, b.p. 106.5°C (760 mm), n 9 5 1.4152, d29-5 0.8649. (NOTE n-Butyl azide and methanol form an azeotrope (b.p. 60°C) from which the azide is liberated by the addition of a saturated solution of calcium chloride.)... 

Explain from the standpoint of the law of mass action why dry sodium nitrate (saltpetre) and a concentrated sulphuric acid solution are taken to prepare a concentrated nitric acid solution. Why does the reaction mixture have to be heated, but carefully What are the boiling points of sulphuric and nitric acids How do nitric acid solutions of various concentrations behave when heated What is the composition of an azeotropic mixture of nitric acid with water ... 

In this process, the material to be nitrated is dissolved or suspended in an inert solvent which form azeotropes with water (such as petr eth, CC14, etc) and treated with coned nitric acid under such conditions that water formed during nitration is removed as an azeotrope. Dilution of nitric acid is thus avoided. The azeotrope which distills at the temp of nitration is cooled by condensation and the solvent minus the water is continuously returned to the nitrating vessel. 

This method was investigated at PicArsn, Dover, NJ and found to have only a very limited application (Ref 1). The azeotropic method seems to be suitable for nitration of benzene (Ref 2)... 

Azeotropic distn can be employed in the explosives industry as, for instance, for removal of water formed during nitration (See Autogen Company Nitration) and also for decn of water in various liquids (See under Aquametry)... 

Table I summarizes the effect of such nitrates on the azeotropic composition in the nitric acid-water system. Addition of potassium nitrate increases the azeotropic composition, but other nitrates decrease it, the amount by which the azeotrope is displaced being proportional to the amount of nitrate added. The azeotrope is eliminated completely at salt concentrations of 45, 48, 54, or 64% by weight for addition of magnesium, zinc, lithium, or calcium nitrates respectively.

The displacement of the azeotropic composition by progressive addition of magnesium nitrate has been shown in Table I above. Vapor-liquid equilibria have been determined (3, 4). Figure 5 depicts equilibrium vapor compositions in the ternary system at the boiling point, while Figure 6 shows boiling points in the system at 760 mm Hg. 

The demand for concentrated nitric acid 98+% strength, primarily for use in nitration, led to the use of sulfuric acid in the extractive distillation of nitric acid. The maximum boiling azeotrope of 68 wt % nitric acid prevents distilling the 55-60 wt % nitric acid produced in an ammonia oxidation plant (AOP) to a strength greater than 68 wt % unless extractive distillation is used. 

Because of the inherent difficulties of eliminating water azeotropically during nitration reactions, due to the nitric acid - nitric oxides equilibria, we searched for more convenient insitu techniques. 

Moisture Determination in Ammonium Nitrate by Azeotropic Distillation A370-R to A371-L... 

An important point is that for most uses concerned with chemical production, the acid must be concentrated above its azeotropic point to greater than 95%(wt). Conversely, the commercial manufacture of ammonium nitrate uses nitric acid below its azeotropic point in the range 50-65%(wt). If the stronger chemical grade is to be produced, additional process equipment appropriate to super-azeotropic distillation is required. 

Belson DJ. A distillation method of aromatic nitration using azeotropic nitric acid. Ind Eng Chem Res 1990 29 1562-1565. 

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