Synthesis of N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) using solid acid catalyst

Abstract

A method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide from a mixture of 2,4-dinitroaniline, 4-nitrobenzoyl chloride and a solid acid catalyst in an organic solvent, wherein the solid acid catalyst is insoluble in the organic solvent and the solid acid catalyst is acidic clay, ion exchange resin, beta zeolite, sulfonated tetrafluoroethylene-based fluoropolymer-copolymer or some mixtures thereof.

Description

Background Art

[0001] FIELD OF THE INVENTION The present invention relates to an improved process for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) from 2,4-dinitroaniline (2,4-DNA) and 4-nitrobenzoyl chloride (4-NBC).

[0002] Description of the Related Art RU 2283307 (C1) discloses a method for synthesizing 5(6)-amino-2-(4'-aminophenyl)benzimidazole from N-(2,4-dinitrophenyl)-4-nitrobenzamide. The method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide comprises an acylation reaction of 2,4-dinitroaniline and 4-nitrobenzoyl chloride in a solvent medium selected from chlorobenzene, p-xylene or a mixture of xylene, toluene or ethylbenzene in the presence of ferric chloride as a catalyst, said to form 2,4',4-trinitrobenzanilide, which is the same as N-(2,4-dinitrophenyl)-4-nitrobenzamide. The reaction is carried out at an elevated temperature, and once the reaction is substantially complete, the solution is cooled, resulting in precipitation of TNBA from the solvent medium. The TNBA is then washed with a fresh solvent medium and subsequently with water to obtain the desired TNBA. Other publications such as RU 2394810 (C2) and RU 2547262 (C3) disclose similar processes for the preparation of TNBA in the presence of ferric chloride catalyst.

[0003] It has been found that the ferric chloride catalyst is problematic in that it is soluble in the solvent medium, which means that both the solvent system and the precipitated TNBA produced by the process will contain catalyst, an undesirable impurity that must be further removed. Specifically, a preferred method of removing the catalyst from TNBA, as disclosed in RU 2283307 (C1), is to first separate the TNBA from the solvent system, and then further wash the TNBA with a large amount of water to remove the catalyst. In addition, if the TNBA produced by the above-mentioned prior art method is subsequently used for other catalytic reactions, any residual ferric chloride present in the TNBA will poison any downstream catalyst or catalysts and / or give color to the final product. In addition, the ferric chloride catalyst used in the above-mentioned prior art method cannot be recovered and reused, and its use presents substantial handling problems, increasing the operating costs of the manufacturing process.

[0004] What is needed is a process that avoids all of the problems associated with using a catalyst that is soluble in an organic solvent system and that also allows flexibility in process design to either precipitate TNBA from the solvent system or use TNBA dissolved in the solvent system in further processing. Summary of the invention

[0005] The present invention also relates to a method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA), which is a monomer used in the manufacture of advanced polymers.

[0006] In some embodiments, the present invention relates to a method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide, the method comprising the following steps:

[0007] a) forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in an organic solvent;

[0008] b) reacting the first mixture at a temperature of at least 90° C. in the presence of a solid acid catalyst to form a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and the organic solvent, wherein the solid acid catalyst is insoluble in the organic solvent,

[0009] wherein the solid acid catalyst is acidic clay, ion exchange resin, beta zeolite, sulfonated tetrafluoroethylene-based fluoropolymer-copolymer or some mixture thereof;

[0010] c) cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamide as a solid; and

[0011] d) removing the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide from the second mixture. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A continuous process for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) is shown. DETAILED DESCRIPTION

[0013] The present invention relates to a method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide from a mixture of 2,4-dinitroaniline, 4-nitrobenzoyl chloride and a solid acid catalyst in an organic solvent, wherein the solid acid catalyst is insoluble in the organic solvent. This is an improvement over processes using catalysts soluble in organic solvents, such as ferric chloride (FeCl3); when such soluble catalysts are used, they contaminate the final reaction product and are difficult to remove from the reaction medium. Soluble catalysts such as ferric chloride (FeCl3) have traditionally been used because they are cheap and such homogeneous catalytic reactions of the Lewis acid type are known to be useful for the reaction.

[0014] Solid acid catalysts suitable for the desired reaction and insoluble in the organic solvent used in the reaction include acidic clays, ion exchange resins, beta zeolites, sulfonated tetrafluoroethylene-based fluoropolymers-copolymers, and mixtures of these catalysts. To the surprise of the inventors, heterogeneous catalysis using Bronsted-type catalysts is effective for this reaction.

[0015] The method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) from a mixture of 2,4-dinitroaniline (2,4-DNA), 4-nitrobenzoyl chloride (4-NBC) and a solid acid catalyst in an organic solvent can be achieved by a batch process, a semi-batch process, a stepwise process or a continuous process. For example, a mixture of 2,4-DNA and 4-NBC in a solvent can be reacted in the presence of a catalyst to form TNBA in a continuous process that utilizes a fixed bed or packed bed catalytic reactor (essentially a container filled with a fixed catalyst) that does not require any stirring. Alternatively, using equipment such as a mechanically stirred autoclave, a continuous stirred tank reactor (CSTR) or a gas stirred reactor (such as a slurry bubble column reactor (SBCR)), in the presence of a catalyst, a batch process, a semi-batch process or a continuous slurry process can be used to react 2,4-DNA and 4-NBC in a solvent to form TNBA.

[0016] The claimed reaction is carried out in a solvent, preferably under an inert atmosphere. Suitable organic solvents are aromatic solvents in which 2,4-DNA, 4-NBC and the desired product TNBA are soluble at a temperature of 60°C or higher. Preferably, the boiling point of the solvent at atmospheric pressure is greater than 100°C, more preferably greater than 120°C, and most preferably greater than 140°C. In addition, it is preferred that the solvent can be azeotropically dried.

[0017] In some embodiments, suitable organic solvents include xylene, toluene, chlorobenzene, o-dichlorobenzene and trichlorobenzene or mixtures thereof; wherein preferred organic solvents are chlorobenzene and o-dichlorobenzene or mixtures thereof. In some embodiments, preferred organic solvent is chlorobenzene.

[0018] In order to obtain a good reaction yield, 2,4-DNA should be dry. This means that 2,4-DNA preferably contains less than 500 ppm of water by weight; and more preferably contains less than 200 ppm of water by weight. Similarly, 4-NBC should also be dry, preferably containing less than 500 ppm of water by weight; and more preferably containing less than 200 ppm of water by weight.

[0019] In addition, impurities in the ingredients can reduce the yield and can affect the purity of the resulting product mixture. Therefore, it is desirable that both 2,4-DNA and 4-NBC have a purity of at least 90%, preferably 95%, and more preferably greater than 99%. Specifically, it is preferred that 4-NBC is substantially free of organic impurities, containing less than 0.5 weight percent of 4-nitrobenzoic acid (4-NBA).

[0020] The mixture further comprises a solid acid catalyst that is insoluble in an organic solvent. "Insoluble in an organic solvent" means that there is no difference in the elemental analysis between the reaction mixture containing the catalyst and the same reaction mixture without the catalyst, or any such difference is within the experimental error of the instrument. Specifically, there is no difference in the elemental analysis between (1) a mixture containing 2,4-dinitroaniline, 4-nitrobenzoyl chloride and a solid acid catalyst in an organic solvent and (2) a mixture containing only 2,4-dinitroaniline, 4-nitrobenzoyl chloride and an organic solvent, or any such difference is within the experimental error of the instrument.

[0021] Catalyst is the material that changes the transition state of reaction to lower activation energy thereby improving reaction rate. "Solid acid catalyst" means the catalyst having proton or coordinated unsaturated cationic center on its surface, which has the required thermal stability to survive under reaction conditions, and is substantially insoluble in reaction medium under reaction conditions. The solid acid catalyst can be non-supported or can be supported on at least one catalyst support.

[0022] Solid acid catalysts which are suitable for the desired reaction and which are insoluble in the organic solvent used in the reaction include acidic clays, ion exchange resins, beta zeolite, sulfonated tetrafluoroethylene-based fluoropolymer-copolymers and mixtures of these catalysts.

[0023] In some embodiments, the preferred solid acid catalyst is a natural acidic clay mineral. Natural acidic clay minerals include kaolinite, bentonite, attapulgite and montmorillonite. Montmorillonite is a preferred acidic clay.

[0024] Suitable ion exchange resins are cross-linked copolymer particles (e.g., beads) that have been chemically treated to attach or form functional groups and have ion exchange capacity. In one embodiment, the ion exchange resin comprises an aromatic polymer that further comprises a cross-linked copolymer of styrene or substituted styrene and a cross-linking agent (e.g., divinylbenzene). In addition, such ion exchange resins with sulfonic acid functional groups have been found to be particularly active and selective. U.S. Patent Nos. 2,366,007 and 2,500,149 describe the preparation of cation exchange resins based on such sulfonated styrene-divinylbenzene copolymers.

[0025] Suitable sulfonated tetrafluoroethylene based fluoropolymer-copolymers include, for example NR50 substances.

[0026] In an embodiment, the solid acid catalyst is a supported acid catalyst. The support of the solid acid catalyst can be any solid material that is inert under the reaction conditions, including but not limited to oxides such as silicon dioxide, aluminum oxide, titanium dioxide, sulfated titanium dioxide and compounds thereof and combinations thereof; barium sulfate; calcium carbonate; zirconium oxide; carbon, especially acid-washed carbon; and combinations thereof. Acid-washed carbon is carbon that has been washed with an acid such as nitric acid, sulfuric acid or acetic acid to remove impurities. The support can be in the form of a powder, granules, pellets, etc. The supported acid catalyst can be prepared by depositing the acid catalyst on the support by many methods (such as spraying, soaking or physical mixing), followed by drying, calcining, and, if necessary, activation by methods such as reduction or oxidation. Based on the combined weight of at least one acid catalyst and at least one support, the loading of at least one acid catalyst on at least one support is in the range of 0.1-20 weight percent. Some acid catalysts perform better at low loadings (such as 0.1%-5%), while other acid catalysts are more likely to be useful at higher loadings (such as 10%-20%). In an embodiment, the acid catalyst is an unsupported catalyst with 100% acid catalyst unsupported, such as pure beta zeolite and ion exchange resin.

[0027] Both supported and unsupported solid acid catalysts can be in any physical form typical of the material, including but not limited to powder form, with a particle size of 0.01-150 μm. For fixed bed reactors, the catalyst is usually in the form of tablets, extrudates, spheres and / or engineered particles with a uniform size of 0.5-10 mm.

[0028] The general method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide comprises the following steps:

[0029] a) forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in an organic solvent;

[0030] b) reacting the first mixture at a temperature of at least 90° C. in the presence of a solid acid catalyst to form a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and the organic solvent, wherein the solid acid catalyst is insoluble in the organic solvent,

[0031] wherein the solid acid catalyst is acidic clay, ion exchange resin, beta zeolite, sulfonated tetrafluoroethylene-based fluoropolymer-copolymer or some mixture thereof;

[0032] c) cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamide as a solid; and

[0033] d) removing the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide from the second mixture.

[0034] In a continuous process, it may be advantageous to conduct the mixing step a) and the reacting step b) in separate vessels, such as a stirred tank for mixing and a fixed or packed bed catalytic reactor for reacting the ingredients. However, if desired, steps a) and b) can be conducted in a single vessel, conceivably as a continuous process or a batch process, a semi-batch process, or a semi-continuous process. For example, the combination of steps a) and b) can be accomplished in a batch process by forming a first mixture of 2,4-DNA, 4-NBC, and a solid acid catalyst in an organic solvent, and reacting the mixture, optionally with stirring, to form a second mixture comprising TNBA, a solid acid catalyst, and an organic solvent, plus any unreacted 2,4-DNA and 4-NBC.

[0035] Preferably, both steps a) and b) or the combined steps a) and b) are carried out in an inert atmosphere, such as under a nitrogen atmosphere. Preferably, there is some agitation in steps a) and b), either by mechanical stirring or the like, or a static mixer, or even fluid dynamics, such as turbulence created when pumping the mixture through a packed column or slurry reactor.

[0036] Although the reaction ideally requires equimolar amounts of 2,4-DNA and 4-NBC, an excess of 4-NBC is typically added to compensate for common impurities; 4-NBC is almost always contaminated with some amount of the corresponding acidic PNBA. In some preferred embodiments, the molar ratio of 4-NBC to 2,4-DNA is 1.05 to 1.07. In addition, in some preferred embodiments, the first mixture is formed by mixing at least 50 grams of 2,4-DNA per liter of solvent, and in some embodiments, even more preferably, 200 grams or more of 2,4-DNA per liter of solvent.

[0037] If steps a) and b) are performed in a single vessel, it is conceivable to use an unsupported catalyst, which is generally present in the first mixture in an amount of at least 5 weight percent of the amount of 2,4-DNA present in the first mixture. Preferably, the catalyst is present in the first mixture in an amount of at least 20 weight percent of the amount of 2,4-DNA present in the first mixture, and more preferably at least 30 weight percent of the amount of 2,4-DNA present in the first mixture.

[0038] Various active steps can be taken to avoid moisture in the individual reactants and / or the first mixture. For example, individual components (like 2,4-DNA) can be pre-dried (e.g., under vacuum; in a vacuum oven at 85°C-100°C using a small amount of nitrogen flow). In some process variants, such as batch, semi-batch or stepwise reactions, the combination of non-supported catalyst and 2,4-DNA can be dried together in a solvent by sparging nitrogen and heating the mixture in the reactor under reflux conditions. However, if these mixtures are heated under reflux conditions, the system needs to be cooled to 120°C or less before introducing (4-NBC). Alternatively, a mixture of 2,4-DNA, 4-NBC and an organic solvent can be formed, and then or simultaneously, before the catalytic reaction, the undesirable amount of water in the mixture is removed by heating or some other known techniques. Then, the dried mixture can be contacted with a solid acid catalyst for catalytic reaction.

[0039] The catalytic reaction of the first mixture forms a second mixture containing at least N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) in an organic solvent. It is advantageous to react the components at a temperature of at least about 90°C. In some embodiments, it is advantageous to react the components at a temperature of at least about 100°C. In some embodiments, the reaction temperature may be in the range of from about 90°C to about 135°C. In some embodiments, the reaction temperature may be in the range of from about 100°C to about 135°C. In some embodiments, the reaction temperature may be in the range of from about 100°C to about 125°C. In some embodiments, the temperature may gradually rise from one level to another and maintain a fixed time period, and the process may continue several times until the final temperature is reached and a fixed time period is maintained. For example, in an intermittent reaction, the holding time may be in the range of from 0.5 to 10 hours, preferably between 0.5 and 7.5 hours, and more preferably from 0.5 to 5 hours. Preferably, the byproduct hydrochloric acid gas produced in the reaction is removed from the reaction using an inert gas (typically nitrogen) covering the reaction. Particularly in batch reactions, stirring is required during the hold time and is continued until the reaction is complete or a sufficient amount of TNBA is formed. The extent of the reaction can be determined by the disappearance of the limiting reactant 2,4-DNA by analyzing a small aliquot of the reaction mixture using thin layer chromatography (TLC) or high performance liquid chromatography (HPLC).

[0040] If a non-supported solid acid catalyst is used for the reaction and is part of the TNBA product mixture together with the solvent, the solid acid catalyst can be removed by filtration, preferably by hot filtration. For example, TNBA has a very high solubility in the solvent chlorobenzene at high temperatures and a very low solubility at low temperatures. Therefore, it is advantageous to remove the solid acid catalyst at high temperatures to avoid removing any desired TNBA. The mixture of TNBA and solvent can then be cooled to crystallize and precipitate solid TNBA, and then TNBA can be separated from the solvent. Typically, this separation is accomplished by filtration. Preferably, the filtration is accomplished using a Rosenmund filter, a Nutsche filter, a filter press, or a drum filter. Finally, the solid TNBA is washed and dried in a vacuum oven. Specifically, the solid TNBA can be washed with an alcohol such as methanol, or otherwise treated to remove residual organic solvents. Similarly, if desired, the TNBA can be washed with a weak base aqueous solution such as sodium hydroxide to remove any acidic byproducts of the reaction, such as hydrochloric acid.

[0041] Examples

[0042] As used in the following examples, percent conversion (%) and percent yield are defined as:

[0043]

[0044] The amounts of TNBA, 4-NBC, and 2,4-DNA were determined by the area percentage technique using HPLC chromatography.

[0045] Comparative Example A and Examples 1-4

[0046] Comparative Example A and Examples 1 to 4 illustrate the batch method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA). 15 g (0.082 mol) of 2,4-dinitroaniline (2,4-DNA) was dried overnight in a vacuum oven at 100 °C under a small continuous nitrogen stream. 17.2 g (0.092 mol) of 4-nitrobenzoyl chloride (4-NBC), 5.0 g of catalyst, and 200 ml of chlorobenzene were placed in a 500 ml four-necked reaction flask equipped with a thermocouple, a mechanical stirrer, a nitrogen inlet, and a condenser. The reaction mixture was heated to 120 °C and stirred for the desired reaction time. After the reaction time reported in the following table for the runs, it was cooled overnight to room temperature. Then, 100 ml of dimethylformamide was added to the reaction vessel, heated to 80 °C, and stirred for 1 hour. The reaction mixture was hot filtered using a 0.2 μ porous filter, and the solution was analyzed by HPLC. The conversion of 2,4-DNA and the yield of N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) for different types of solid acid catalysts and run times have been described in the following table.

[0047] Table 1

[0048]

[0049] Note: Conversion and yield are based on HPLC area %.

[0050] The calculation of the reaction rate in these examples shows that the reaction rate is the lowest without a catalyst, and the relative reaction rates are as follows: without catalyst (7.8 h-1) < CP-814Eβ zeolite (8.3 h-1) < Marathon TM CH+ (10 h-1) < NR 50 (10.7 h-1) < montmorillonite clay (16.6 h-1).

[0051] Example 5

[0052] This example illustrates a step-by-step method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA). 30 g (0.164 mol) of 2,4-dinitroaniline (2,4-DNA) and 200 ml of chlorobenzene were placed in a 1000 ml four-necked reaction flask equipped with a thermometer, a magnetic stir bar, a nitrogen inlet, and a Dean-Stark water separator. The mixture was stirred for 5 minutes under a nitrogen atmosphere. The mixture was heated to reflux, and 7 ml of chlorobenzene condensate was collected to help remove any residual moisture. After recovering 7 ml of chlorobenzene condensate, 10 g of solid montmorillonite (K-10) catalyst was added.

[0053] The solution temperature was cooled to 120 ° C, and 32.1 grams (0.168 mol) of 4-nitrobenzoyl chloride (4-NBC) was added using a powder funnel. Once the 4-NBC addition was complete, the reaction mixture was heated to reflux and stirred for 5 hours. After cooling overnight to room temperature, a tan solid precipitated from the solution. The solid was collected by filtration. The collected solid was washed with 200 ml of methanol in a beaker, filtered and washed with 500 ml of water, and the pH was adjusted to 7.0 with 0.1 N NaOH. The solid was recovered and dried overnight in a vacuum oven at 100 ° C. The reaction produced 51.56 grams of N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) (94.7% yield based on 2,4-DNA). Since montmorillonite was not removed in the process, the TNBA weight was corrected according to the weight of montmorillonite.

[0054] Examples 6-8

[0055] 15 grams (0.082 mol) of 2,4-dinitroaniline (2,4-DNA) was dried overnight in a vacuum oven at 100°C under a small continuous stream of nitrogen, and the specific amount of catalyst reported in Table 2 and 100 ml of 1,2-dichlorobenzene were placed in a 500 mL four-necked reaction flask equipped with a thermocouple, a mechanical stirrer, an addition funnel with a nitrogen inlet, and a condenser. The reaction mixture was heated to 120°C, and 17.2 grams (0.092 mol) of 4-nitrobenzoyl chloride (4-NBC) dissolved in 100 mL of 1,2-dichlorobenzene were added continuously from the addition funnel over 30 minutes under constant stirring. The reaction mixture was maintained at 120°C and stirred for the desired reaction time. After the reaction was run for the time reported in the table below, it was cooled overnight to room temperature. Then, 100 ml of dimethylformamide was added to the reaction vessel, heated to 80°C and stirred for 1 hour. The reaction mixture was filtered hot using a 0.2μ porous filter and the solution was analyzed by HPLC. The conversion of 2,4-DNA and the yield of N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) for different types of solid acid catalysts, amounts of catalysts and run times have been described in the table below.

[0056] Table 2

[0057]

[0058] Note: Conversion and yield are based on HPLC area %.

[0059] Comparative Example B

[0060] Comparative Example B illustrates the use of ferric chloride catalyst in the prior art, which is soluble in solvent. The intermittent procedure of Examples 1 to 4 was used, using FeCl3 as a catalyst. Specifically, 68 mg of FeCl3 catalyst was added, and the reaction was allowed to proceed for 2.5 hours. For 99.6% conversion of 2,4-DNA (limited reactant), a 97.8% yield of TNBA was obtained, which is comparable to the examples of the present invention; however, an additional step is required to remove the FeCl3 catalyst from the final product.

[0061] Typically, the following process is required to remove ferric chloride from the final product. After complete conversion of 2,4 DNA, the TNBA wet cake is separated from the mother liquor. The wet cake is first washed four times with methanol (about 1.4 x wt. of the wet cake each time), then washed once with aqueous ammonium hydroxide solution, washed with 0.25 wt% ammonia (about 2.5 x wt. of the wet cake), and finally washed twice with demineralized water (2 x wt. of the wet cake each time).

[0062] Example 9

[0063] This example illustrates a Figure 1 A continuous process for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) is shown. The process utilizes a mixing vessel 1 and a fixed bed catalytic reactor 2 containing a solid acid catalyst. Reactants 2,4-DNA and 4-NBC and solvent chlorobenzene are introduced into the mixing vessel 1 through an inlet point 3, where they are mixed. The mixing vessel 1 is inertized using nitrogen introduced through an inlet point 8 and purged from the vessel through an outlet point 9. A mixture of 2,4-DNA and 4-NBC in chlorobenzene is pumped into the catalytic reactor 2 through a pump 4. The catalytic reactor is equipped with a multi-point thermocouple 10 to maintain and control the temperature in the catalyst bed. The desired TNBA produced in the fixed bed reactor in an organic solvent leaves the process through an outlet 7. A portion of the outlet stream from the fixed bed reactor is recycled back into the process through a pump 5. Nitrogen is provided to the catalytic reactor 2 through an inlet point 6. In this arrangement, nitrogen is supplied to the recycle stream through a recycle line, but it can be supplied directly to the catalytic reactor if desired.

Claims

1. A method for preparing N-(2,4-dinitrophenyl)-4-nitrobenzamide, the method comprising the following steps: a) forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in an organic solvent; b) reacting the first mixture at a temperature of at least 90° C. in the presence of a solid acid catalyst to form a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and the organic solvent, wherein the solid acid catalyst is insoluble in the organic solvent, wherein the solid acid catalyst is acidic clay, an ion exchange resin having sulfonic acid functional groups, beta zeolite, a fluorine polymer-copolymer based on sulfonated tetrafluoroethylene, or some mixture thereof; c) cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamide as a solid; as well as d) removing the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide from the second mixture.

2. The method of claim 1, wherein: The first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in an organic solvent formed in a) comprises the solid acid catalyst.

3. The method of claim 2, wherein: The first mixture including the solid acid catalyst is formed by premixing the 2,4-dinitroaniline, an organic solvent, and a solid acid catalyst, and then adding 4-nitrobenzoyl chloride.

4. The method of claim 1, wherein: The solid acid catalyst is part of a continuous fixed bed reactor.

5. The method according to claim 2 or 3, wherein: Step b) comprises stirring the first mixture.

6. The method according to claim 2 or 3, wherein: After step b) but before step c), the solid acid catalyst is removed from the second mixture.

7. The method of claim 6, wherein: The solid acid catalyst was removed by filtration.

8. The method according to any one of claims 1 to 3, wherein: The acidic clay is montmorillonite clay.

9. The method according to any one of claims 1 to 3, wherein: At least step b) is carried out under an inert atmosphere.

10. The method according to any one of claims 1 to 3, wherein: Prior to step a), the 2,4-dinitroaniline is combined with an organic solvent and water is removed from the combination.

11. The method according to any one of claims 1 to 3, wherein: The organic solvent is chlorobenzene.

12. The method according to any one of claims 1 to 3, wherein: In step d) the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide is removed by filtration.

13. The method according to any one of claims 1 to 3, wherein: Step a) is carried out at a temperature of about 100°C to about 135°C.

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