Synthesis of n-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) using solid acid catalysts

Abstract

A method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide from a mixture of 2,4-dinitroaniline, 4-nitrobenzoyl chloride, and solid acid catalyst in an organic solvent, wherein the solid acid catalyst is not soluble in the organic solvent, the solid acid catalyst being an acidic clay, an ion exchange resin, a beta zeolite, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, or some mixture of these.

Description

Background of the InventionField of the Invention. This invention relates to an improved method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) from 2, 4-dinitroaniline (2,4-DNA) and 4-nitrobenzoyl chloride (4-NBC).Description of Related Art. RU2283307(C1) discloses a method for synthesis of 5(6)-amino-2-(4'-aminophenyl) benzimidazole from N-(2 4-dinitrophenyl)-4-nitrobenzamide. Themethod for making the N-(2,4-dinitrophenyl)-4-nitrobenzamide involves an acylationreaction of 2,4-dinitroaniline and 4-nitrobenzoyll chloride in the presence of ferric chloride asa catalyst in a solvent medium chosen from chlorobenzene, paraxylene, or a mixture ofxylenes, toluene or ethylbenzene, which is said to form 2,4',4-trinitrobenzanilide, which isthe same as N-(2 4-dinitrophenyl)-4-nitrobenzamide. The reaction is conducted at anelevated temperature, and once it is essentially complete, the solution is cooled, resulting inthe precipitation of TNBA from the solvent medium. The TNBA is then washed with freshsolvent medium and subsequently with water to yield the desired TNBA. Other publicationssuch as RU2324810(C2) and RU2547262(C3) disclose similar processes for making TNBAin the presence of an iron chloride catalyst.It has been found that the ferric (iron) chloride catalyst poses a problem in that it issoluble in the solvent medium, meaning that both the solvent system and the precipitateTNBA produced by this process will contain the catalyst as an undesirable impurity thatmust further be removed. Specifically, the preferred method of removing the catalyst fromthe TNBA, such as disclosed in RU2283307(C1), is to first isolate the TNBA from thesolvent system and then further wash the TNBA with copious amounts of water to removethe catalyst. Furthermore, if the TNBA produced by the above prior art process issubsequently used in other catalyzed reactions, any residual ferric (iron) chloride present inthe TNBA can poison any downstream catalyst(s) and / or impart color to the final product. Inaddition, the ferric chloride catalyst used in the above prior art process cannot be recycledand reused, and its use represents a substantial disposal problem, increasing the operatingcost of the manufacturing process.What is needed is a process that avoids all the issues associated with the use ofcatalysts that are soluble in the organic solvent system and that also allows process designflexibility to either precipitate the TNBA from the solvent system or use the TNBA dissolvedin the solvent system in further processing.Brief Summary of the InventionThis invention also relates to a method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA), a monomer useful in the manufacture of advanced polymers.In some embodiments, this invention relates to a method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide comprising the steps of;a) forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in anorganic solvent;b) reacting the first mixture at a temperature of at least 90 °C in the presence of a solidacid catalyst, wherein the solid acid catalyst is not soluble in the organic solvent, toform a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and theorganic solvent,wherein the solid acid catalyst is an acidic clay, an ion exchange resin, a betazeolite, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, or somemixture thereof;c) cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamideas a solid; andd) removing the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide from the second mixture.Brief Description of the DrawingsFig. 1 illustrates one continuous method for making N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA) .DETAILED DESCRIPTION OF THE INVENTIONThis invention relates to a method of making N-(2,4-dinitrophenyl)-4-nitrobenzamidefrom a mixture of 2,4-dinitroaniline, 4-nitrobenzoyl chloride, and solid acid catalyst in anorganic solvent, wherein the solid acid catalyst is not soluble in the organic solvent. This isan improvement over processes that utilize catalysts such as iron chloride (FeCls) that aresoluble in organic solvents; when such soluble catalysts are used, it can contaminate thefinal reaction product and is difficult to remove from the reaction media. Soluble catalystssuch as iron chloride (FeCls) have traditionally been used because they are inexpensiveand such homogenous catalytic reactions of a Lewis acid type are known to be useful inthis reaction.The solid acid catalysts that are both suitable for the desired reaction and areinsoluble in the organic solvent used for the reaction include acidic clays, ion exchangeresins, beta zeolites, sulfonated tetrafluoroethylene-based fluoropolymer-copolymers, andmixtures of these catalysts. It was a surprise to the inventors that a heterogeneous catalyticreaction using a Bronsted-type catalyst would be effective with this reaction.The method of making N-(2 4-dinitrophenyl)-4-nitrobenzamide(TNBA) from a mixtureof 2,4-dinitroaniline(2,4-DNA), 4-nitrobenzoyl chloride(4-NBC), and solid acid catalyst in anorganic solvent can be achieved either via a batch process, a semi-batch or stepwiseprocess, or a continuous process. For example, the mixture of 2,4-DNA and 4-NBC in asolvent can be reacted to form TNBA in the presence of a catalyst in a continuous processthat utilizes a fixed bed or packed bed catalytic reactor (essentially a vessel withimmobilized catalyst packed therein) that does not require any agitation. Alternatively,batch, semi-batch, or continuous slurry processes can be used to react the 2,4-DNA and 4-NBC in a solvent to form TNBA in the presence of a catalyst using equipment such asmechanically stirred autoclave, continuously stirred tank reactor (CSTR) or gas agitatedreactors such as a slurry bubble column reactor(SBCR).The claimed reaction is carried out in a solvent, preferably under an inertatmosphere. A suitable organic solvent is an aromatic solvent in which 2,4-DNA, 4-NBC,and desired product TNBA are soluble, at a temperature of 60°C or greater. Preferably, theboiling point of the solvent at atmospheric pressure is greater than 100 °C, more preferablygreater than 120°C, and most preferably greater than 140°C. Further, it is preferred that thesolvent can be dried azeotropically.In some embodiments, suitable organic solvents include xylenes, toluene,chlorobenzene, o-dichlorobenzene, and trichlorobenzene, or mixtures thereof; withpreferred organic solvents being chlorobenzene and o-dichlorobenzene, or mixturesthereof. In some embodiments, the preferred organic solvent is chlorobenzene.For good yield of the reaction, the 2,4-DNA should be dry. This means the 2,4-DNApreferably contains less than 500 ppm of water on a weight basis; and more preferablycontains less than 200 ppm of water on a weight basis. Likewise, the 4-NBC should also bedry, preferably containing less than 500 ppm of water on a weight basis; and morepreferably contains less than 200 ppm of water on a weight basis.Further, impurities in the ingredients can lower the yield and can affect the purity ofthe resulting product mixture. Therefore, it is desirable that both the 2,4-DNA and the 4-NBC have a purity of at least 90%, preferably a 95% purity, and more preferably a purity ofgreater than 99%. Specifically, it is preferable that the 4-NBC be substantially free oforganic impurities, containing less than 0.5 weight percent 4-nitrobenzoic acid (4-NBA).The mixture further comprises a solid acid catalyst that is not soluble in the organicsolvent. By "not soluble in the organic solvent" it is meant there is no difference betweenthe elemental analysis of a reaction mixture containing the catalyst and the same reactionmixture not containing the catalyst, or any such difference is within experimental error ofthe instrument. Specifically, there is no difference between the elemental analysis of (1) themixture containing 2,4-dinitroaniline, 4-nitrobenzoyl chloride, and solid acid catalyst in theorganic solvent and (2) the mixture containing only 2,4-dinitroaniline, 4-nitrobenzoyl|chloride, and the organic solvent, or any such difference is within experimental error of theinstrument.A catalyst is a substance that modifies the transition state of a reaction to a loweractivation energy, increasing the rate of reaction. By "solid acid catalyst" it is meantcatalysts that have protons or coordinately unsaturated cationic centers on their surfacehaving the thermal stability required to survive reaction conditions and substantiallyundissolved in the reaction medium under the reaction conditions. The solid acid catalystsmay be unsupported, or may be supported on at least one catalyst support.The solid acid catalysts that are both suitable for the desired reaction and areinsoluble in the organic solvent used for the reaction include acidic clays, ion exchangeresins, beta zeolites, sulfonated tetrafluoroethylene-based fluoropolymer-copolymers, andmixtures of these catalysts.In some embodiments, the preferred solid acid catalysts are natural acid clayminerals. Natural acidic clay minerals include kaolinite, bentonite, attapulgite, andmontmorillonite. Montmorillonite is a preferred acidic clay.Suitable ion-exchange resins are crosslinked copolymer particles (e.g. beads), thathave been chemically treated to attach or form functional groups and have a capacity forion exchange. In one embodiment, the ion-exchange resin comprises an aromatic polymerthat further comprises crosslinked copolymers of styrene or substituted styrene and acrosslinker such as divinylbenzene. Further, such ion-exchange resins having sulfonic acidfunctional groups were found to be particularly active and selective. US Pat. Nos.2,366,007 and 2,500,149 describe the preparation of cation exchange resins based onsuch sulfonated styrene-divinylbenzene copolymers.Suitable sulfonated tetrafluoroethylene-based fluoropolymer-copolymers includesuch things as Nafion NR50.In an embodiment, the solid acid catalyst is a supported acid catalyst. The supportfor the solid acid catalyst can be any solid substance that is inert under the reactionconditions including, but not limited to, oxides such as silica, alumina, titania, sulfatedtitania, and compounds thereof and combinations thereof; barium sulfate; calciumcarbonate; zirconia; carbons, particularly acid washed carbon; and combinations thereof.Acid washed carbon is a 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 powder,granules, pellets, or the like. The supported acid catalyst can be prepared by depositing theacid catalyst on the support by any number of methods such as spraying, soaking orphysical mixing, followed by drying, calcination, and if necessary, activation throughmethods such as reduction or oxidation. The loading of the at least one acid catalyst on theat least one support is in the range of 0.1-20 weight percent based on the combinedweights of the at least one acid catalyst and the at least one support. Certain acid catalystsperform better at low loadings such as 0.1-5%, whereas other acid catalysts are more likelyto be useful at higher loadings such as 10-20%. In an embodiment, the acid catalyst is anunsupported catalyst having 100% acid catalyst with no support such as, pure beta zeolitesand ion exchange resins.Both the supported and unsupported solid acid catalyst can be in any physical formtypical for the material, including but not limited to powdered forms, with 0.01 - 150 umparticle size. For fixed bed reactors, the catalyst is generally in the form of tablets,extrudates, spheres, and / or engineered particles, having a uniform 0.5 - 10 mm size.The general method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide comprisesthe steps of:a) forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in anorganic solvent;b) reacting the first mixture at a temperature of at least 90 °C in the presence of a solidacid catalyst, wherein the solid acid catalyst is not soluble in the organic solvent, toform a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and theorganic solvent,wherein the solid acid catalyst is an acidic clay, an ion exchange resin, a betazeolite, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, or somemixture thereof;c) cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamideas a solid; andd) removing the solid N-(2,4-dinitrophenyl)-4-nitrobenzamide from the second mixture.In a continuous process, it can be advantageous to perform the mixing step a) andthe reacting step b) in separate vessels, such as stirred tank for mixing and a fixed bed orpacked bed catalytic reactor for reacting the ingredients. However, steps a) and b) can beperformed in a single vessel if desired, conceivably in either a continuous process or abatch, semi-batch, or semi-continuous process. For example, the combination of steps a)and b) can be accomplished, in a batch method by forming a first mixture of the 2, 4-DNA,4-NBC, and the solid acid catalyst in an organic solvent and reacting this mixture, optionallywith agitation, to form a second mixture comprising the TNBA , the solid acid catalyst, andthe organic solvent, plus any unreacted 2,4-DNA and 4-NBC.Preferably both steps a) and b), or the combined steps a) and b), are conducted inan inert atmosphere, such as under a nitrogen atmosphere. Preferably some sort ofagitation is present in steps a) and b), either via mechanical stirring or the like, or staticmixers, or even fluid dynamics, such as the turbulence that arises in the pumping of amixture through a packed column or a slurry reactor.While ideally equal molar amounts of 2,4-DNA and 4-NBC are needed for thereaction, generally an excess molar amount of 4-NBC is added to compensate for commonimpurities; 4-NBC is almost always contaminated with some amount of the correspondingacid PNBA. In some preferred embodiments, the molar ratio of 4-NBC to 2,4-DNA is 1.05 to1.07. In addition, in some preferred embodiments, the first mixture is formed by mixing atleast 50 grams of 2,4-DNA per liter of solvent, and in some embodiments mixing 200 gramsor more of 2,4-DNA per liter of solvent is even more preferred.If steps a) and b) are performed in a single vessel, conceivably using anunsupported catalyst, the catalyst is present in the first mixture in an amount that isgenerally 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 that is at least 20 weightpercent of the amount of 2,4-DNA present in the first mixture, and more preferably in anamount that is at least 30 weight percent of the amount of 2,4-DNA present in the firstmixture.Various active steps can be taken to avoid moisture in the individual reactantsand / or the first mixture. For example, the individual ingredients like the 2,4-DNA can bepre-dried (e.g., under vacuum; 85 - 100 °C in a vacuum oven using a small nitrogen bleed).In some process variants like a batch, semi-batch or step-wise reaction, the combination ofthe unsupported catalyst and the 2,4-DNA can be dried together in a solvent by spargingnitrogen and simultaneously heating the mixture in the reactor under reflux condition.However, if these mixtures are heated under reflux condition, it is desirable to cool thesystem to 120 °C or less prior to the introduction of the (4-NBC). Alternatively, a mixture ofthe 2,4-DNA , 4-NBC, and the organic solvent can be formed, followed by, or concurrentwith, the removal of undesirable amounts of water in the mixture by heating or by someother known technique, prior to the catalyzed reaction. This dried mixture can thensubsequently contact the solid acid catalyst for the catalyzed reaction to take place.The catalyzed reaction of the first mixture forms a second mixture comprising atleast the N-(2,4-dinitrophenyl)-4-nitrobenzamide (TNBA) in the organic solvent. It isadvantageous to react the ingredients at a temperature of at least about 90 °C. In someembodiments, it is advantageous to react the ingredients at a temperature of at least about100 °C. In some embodiments, the reaction temperature can range from about 90 °C toabout 135 °C. In some embodiments, the reaction temperature can range from about 100°C to about 135 °C. In some embodiments, the reaction temperature can range from about100 °C to about 125 °C. In some embodiments, the temperature can be ramped graduallyfrom one level to another and held for a fixed period of time and this process can becontinued several times until the final temperature is attained and held for a fixed period oftime. For example, in a batch reaction the hold time can range from 0.5 to 10 hours,preferably between 0.5 to 7.5 hours, and more preferably from 0.5 to 5 hours. Preferably,byproduct hydrochloric acid gas that evolves from the reaction is swept from the reactionusing the inert gas blanketing the reaction (typically nitrogen gas). Especially in a batchreaction, agitation is desired during the hold time and is continued until the reaction iscompleted or an adequate amount of TNBA is formed. The extent of reaction can bedetermined by the disappearance of 2,4-DNA, the limiting reactant, determined byanalyzing a small portion of aliquot from the reaction mixture using thin layerchromatography (TLC) or high-performance liquid chromatography (HPLC).If an unsupported solid acid catalyst is used for the reaction and is part of the TNBAproduct mixture along with the solvent, the solid acid catalyst can be removed by filtration,preferably by hot filtration. For example, TNBA has very high solubility in the solventchlorobenzene at high temperatures, and very low solubility at low temperatures.Therefore, it is advantageous to remove the solid acid catalyst at high temperatures toavoid removing any of the desired TNBA. The mixture of TNBA and solvent can thenbe cooled to crystallize and precipitate solid TNBA, and then the TNBA can be separatedfrom the solvent. Typically, this separation is accomplished by filtration. Preferably, thefiltration is accomplished using a Rosenmund filter, Nutsche filter, filter press or rotary drumfilter. Finally, the solid TNBA is washed and dried in vacuum oven. Specifically, the solidTNBA can be washed with an alcohol such as with methanol or otherwise treated toremove residual organic solvent. Likewise, if desired, the TNBA can be washed with aweak aqueous base, such as sodium hydroxide, to remove any acidic byproducts of thereaction, such as hydrochloric acid.ExamplesAs used in the following examples, percent (%) conversion and percent yield aredefined as:% Conversion =(mols of 2,4-DNA charged - mols of 2,.4-DNA remaining)x 100mols 2,4-DNA chargedmols of TNBA formed% Yield = x 100mols of 2,4-DNA chargedThe amounts of TNBA, 4-NBC, and 2,4-DNA were determined using the areapercent technique using HPLC chromatograms.Comparative Example A and Examples 1-4Comparative Example A and Examples 1 through 4 illustrate a batch method formaking N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA).15 grams (0.082 mol) of 2,4-dinitroaniline (2,4-DNA), dried overnight in vacuum oven at 100 °C under a smallcontinuous bleed of nitrogen, 17.2 grams (0.092 mol) of 4-nitrobenzoyl chloride (4-NBC),5.0 grams of the catalyst, and 200 ml of chlorobenzene were placed in a 500 ml four neckreaction flask fitted with a thermocouple, mechanical stirrer, nitrogen inlet, and acondenser. The reaction mixture was heated to 120 °C and stirred for desired reaction time.After the reaction was carried out for the time on stream reported in the Table below, it wascooled overnight to room temperature. Then, 100 ml of dimethylformamide was added inthe reaction vessel, heated to 80 °C and stirred for 1 hour. The reaction mixture was filteredhot using 0.2p fritted filter and the solution was analyzed by HPLC. The conversions of 2,4-DNA and the yields of n-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA) for the differenttypes of solid acid catalysts and times on stream have been delineated in the Table below.Table 1Example Catalyst Time Conversion Yield(hours) (%) (%)A None 12 94.6 92.71 Montmorillonite 6 99.7 gr7.2Clay2 CP-814E 12 99.9 90.6Beta Zeolite3 Nafion 8 85.9 84.6NR504 Dowex 10 99.8 98.2MarathonCH+Note: Conversion and Yield based on HPLC area %.A calculation of the rate of reaction in these Examples illustrates the rate of reactionin the absence of catalyst is the lowest, with the relative rates of reaction being as follows:No Catalyst (7.8 h-1) < CP-814E Beta Zeolite (8.3 h-1) < Dowex Marathon CH+ (10 h-1) < Nafion NR 50 (10.7 h-1) < Montmorillonite clay (16.6 h-1).Example 5This example illustrates a stepwise method for making N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA). 30 grams (0.164 mol) of 2,4-dinitroaniline (2,4-DNA) and 200 ml|of chlorobenzene were placed in a 1000 ml four neck reaction flask fitted with athermometer, magnetic stir bar, nitrogen inlet and a Dean-Stark trap. The mixture wasstirred for 5 minutes under a nitrogen atmosphere. The mixture was heated to reflux and 7ml of chlorobenzene condensate was collected to help remove any residual moisture. 10Grams of solid Montmorillonite (K-10) catalyst were added after the 7 ml of chlorobenzenecondensate was recovered.The solution temperature was cooled down to 120°C and 32.1 grams (0.168 mol) of4-nitrobenzoyl chloride (4-NBC) was added using a powder funnel. Once the 4-NBCaddition was completed, the reaction mixture was heated to reflux and stirred for 5 hours.After cooling overnight to room temperature, a tan solid precipitated from solution. The solidwas collected by filtration. The collected solid was washed in a beaker with 200 ml ofmethanol, filtered and washed with 500 ml water, which was adjusted to pH 7.0 with 0.1 NNaOH. The solids were recovered and dried in a vacuum oven at 100°C overnight. Thereaction yielded 51.56 grams of N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA) (94.7%yield based on 2,4-DNA). The TNBA weight was corrected for the weight of theMontmorillonite since it was not removed in this procedure.Examples 6 - 815 grams (0.082 mol) of 2 4-dinitroaniline (2,4-DNA), dried overnight in vacuum ovenat 100 °C under a small continuous bleed of nitrogen, a specified amount of catalyst asreported in the Table 2, and 100 ml of 1,2-dichlorobenzene were placed in a 500 mL fourneck reaction flask fitted with a thermocouple, mechanical stirrer, addition funnel with anitrogen 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-dichlorobenzenefrom the addition funnel was continuously added in 30 minutes under constant stirring. Thereaction mixture was maintained at 120 °C and stirred for the desired reaction time. Afterthe reaction was carried out for the time on stream reported in the Table below, it wascooled overnight to room temperature. Then, 100 ml of dimethylformamide was added inthe reaction vessel, heated to 80 °C and stirred for 1 hour. The reaction mixture was filteredhot using 0.2 fritted filter and the solution was analyzed by HPLC. The conversions of 2,4-DNA and the yields of N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA) for the differenttypes of solid acid catalysts, amounts of catalysts, and time on stream have beendelineated in the Table below.Table 2Example Catalyst Amount of Time Conversion YieldComparative Example B illustrates the prior art use of iron chloride catalyst, which issoluble in the solvent. The batch procedure of Examples 1 to 4 was used with FeCls as thecatalyst. Specifically, 68 mg of FeCl; catalyst was added, and the reaction was run for 2.5hours. A 97.8% yield of TNBA was obtained for 99.6% conversion of 2,4-DNA (limitingreactant), which is comparable to the inventive examples; however, additional steps wereneeded to remove the FeCls catalyst from the final product.Typically, a process such as the following is necessary to remove iron chloride fromthe final product. After the complete conversion of 2,4DNA, TNBA wet cake is separatedfrom mother liquor. The wet cake is first washed with methanol four times (~1.4 x wt. of wetcake in each wash), followed by washing one time with an aqueous ammonium hydroxidewash with 0.25 wt% ammonia (~2.5 x wt. of wet cake), and finally washed 2 times withdemineralized water washes (2 x wt. of wet cake in each wash).Example 9This example illustrates one continuous method for making N-(2,4-dinitrophenyl)-4-nitrobenzamide) (TNBA) as shown in Fig. 1. The process utilizes a mixing vessel 1 and afixed bed catalytic reactor 2 containing a solid acid catalyst. Reactants 2,4-DNA and 4-NBCand solvent chlorobenzene are introduced into the mixing vessel 1 via an entry point 3,where they are mixed. The mixing vessel 1 is inerted using nitrogen introduced throughentry point 8 and purged out of the vessel through exit point 9. The mixture of 2, 4-DNA and4-NBC in chlorobenzene is pumped to the catalytic reactor 2 via a pump 4. The catalyticreactor is fitted with a multipoint thermocouple 10 to maintain and control the temperaturein the catalyst bed. The desired TNBA generated in the fixed bed reactor in the organicsolvent exits the process via exit 7. A part of the exit stream from the fixed bed reactor isrecycled back into the process via pump 5. The catalytic reactor 2 is provided with nitrogenvia entry point 6. In this arrangement, the nitrogen is supplied into a recycle stream via therecycle line, but it can be supplied directly to the catalytic reactor if desired.

Claims

1. A method of making N-(2,4-dinitrophenyl)-4-nitrobenzamide comprising the steps of: forming a first mixture comprising 2,4-dinitroaniline and 4-nitrobenzoyl chloride in an organic solvent; reacting the first mixture at a temperature of at least 90 °C in the presence of a solid acid catalyst, wherein the solid acid catalyst is not soluble in the organic solvent, to form a second mixture comprising N-(2,4-dinitrophenyl)-4-nitrobenzamide and the organic solvent, wherein the solid acid catalyst is an acidic clay, an ion exchange resin, a beta zeolite, a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, or some mixture thereof; cooling the second mixture to precipitate the N-(2,4-dinitrophenyl)-4-nitrobenzamide as a solid; and 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) includes 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, organic solvent, and solid acid catalyst and subsequently 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 of claim 2 or 3 wherein step b) includes agitating the first mixture.

6. The method of claim 2 or 3 wherein after step b) but prior to step c¢) the solid acid catalyst is removed from the second mixture.

7. The method of claim 6 wherein the solid acid catalyst is removed by filtration.

8. The method of any one of claims 1 to 3 wherein the acidic clay is a montmorillonite clay.

9. The method of any one of claims 1 to 3 wherein the ion exchange resin is an ion- exchange resin having sulfonic acid functional groups.

10. The method of any one of claims 1 to 3 wherein at least step b) is conducted under an inert atmosphere.

11. The method of any one of claims 1 to 3 wherein prior to step a) the 2,4-dinitroaniline is combined with organic solvent and moisture is removed from the combination.

12. The method of any one of claims 1 to 3 wherein the organic solvent is chlorobenzene.

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

14. The method of any one of claims 1 to 3 wherein step a) is conducted at a temperature of about 100 °C to about 135 °C.