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The first stage in the conventional trinitrotoluene (TNT) manufacturing process begins with the nitration of toluene in the presence of mixed sulfuric and nitric acids. Typically, the mononitrotoluene (MNT) isomers produced have the following distribution: 38% para (p-MNT), 58% ortho (o-MNT), and 4% meta (m-MNT). With subsequent nitration of MNT, the presence of m-MNT eventually leads to the generation of three undesirable TNT isomers. These TNT isomers have physical properties, that when present, degrade the performance of alpha-TNT in munitions. For this reason, these isomers must be removed from the mixture of TNT isomers to obtain mainly alpha-TNT. The ensuing purification process that uses sellite, a sodium sulfite solution, results in the generation of red water, a K047 hazardous waste. In recent years, researchers have discovered that nitration of toluene with nitric acid (HNO3) and an acidic zeolite catalyst (H-ZSM-5) will produce MNT with an isomer distribution of 81% p-MNT, 20% o-MNT, and no m-MNT. The use of low-cost, environmentally benign catalysts such as zeolites in the selective nitration of toluene has the potential to prevent undesirable isomers that ultimately lead to the generation of red water during the purification step.
The objective of this project was to develop an environmentally benign process to produce MNT without creating m-MNT, while simultaneously maximizing the yield of the p-MNT isomer.
The technical approach involves the nitration of toluene using zeolite catalysts and HNO3 as the nitrating agent. The current process, also known as the “mixed acids” process, is the conventional TNT manufacturing process requiring three nitration steps. Since the mixed acid process produces a considerable amount of m-MNT, extensive research on nitration methods that would eliminate this by-product has been conducted. To improve the selectivity for p-MNT and o-MNT over m-MNT, different catalysts and sources of nitronium ions (NO2+) have been tried. Only modest improvements have been realized. To overcome this problem, a new approach was considered using H-ZSM-5 catalysts. In the H-ZSM-5 catalyst, the protons present as ions balance the framework charge in the zeolite. This innovative synthesis technique, called regio-selective catalytic nitration of toluene using solid acid catalysts, was evaluated using more economical and widely available catalyst varieties. A semi-pilot-scale continuous process was also developed and optimized to further nitrate p-MNT and o-MNT to produce military grade TNT.
The results of this project indicate that nitration with three different nitrating agents (n-propylnitrate, N2O5, HNO3) and an acidic zeolite catalyst (H-ZSM-5) will produce MNT with an isomer distribution of 80% p-MNT, 20% o-MNT, and no m-MNT. Out of the three nitrating agents, HNO3 is the most economical. Zeolites are environmentally benign and economically feasible catalysts that eliminate the need for H2SO4. Researchers have demonstrated that toluene can be nitrated with HNO3 in the presence of H-ZSM-5 to generate primarily p-MNT and to suppress the formation of m-MNT. These laboratory batch studies identified H-ZSM-5 zeolite, with a silica-to-alumina ratio (SAR) of 1000:1, as the catalyst for high para- and very low meta-ring substitution of a nitro group on toluene. With this approach, excess toluene is recovered and recycled, zeolite catalyst is reused, and HNO3 is completely consumed, thus generating no waste in the process. However, the conversion of toluene to the corresponding MNT ranges from 45-48%. Hence, the focus was to design and develop a more ideal catalyst to increase the conversion rate of MNT where there is no m-MNT in the products, while maximizing the yield of the p-MNT isomer that will yield only alpha-TNT.
Building on this discovery, experiments were performed to optimize the shape-selective, zeolite-catalyzed nitration of toluene. Included in this work was the optimization of reaction conditions, optimization of H-ZSM-5 catalyst properties, and finally initial scale-up of the optimized reaction conditions to the semi-pilot scale. Through optimization experiments, it was found that the “ideal” ZSM-5 catalyst for toluene nitration should be one that meets the following criteria: (1) the molar SAR is in a range that provides an acceptable reaction rate (toluene conversion) at a reasonable catalyst to toluene weight ratio, (2) the particle size is small so that internal diffusion is minimized and conversion is optimized, and (3) the external acid sites are deactivated to minimize formation of undesirable by-products. Using H-ZSM-5 zeolites with these properties, it was determined that shape selective, zeolite-catalyzed nitration of toluene gave the highest conversions and highest para-selectivity under the following reaction conditions:
Using the optimized reaction conditions, at reaction scales of 0.3 L (based on the amount of toluene), researchers demonstrated the selective nitration of toluene in which levels of m-MNT were observed to be below the MilSpec level of 1.5% while producing low levels of benzaldehyde, an oxidative side product formed during the nitration of toluene. The observed selectivity for the production of o-MNT and p-MNT was achieved at conversions of 50-60% (based on HNO3).
Researchers then sought to reproduce the same selectivity achieved at the 0.3-L scale at a scale of 30 L. The successful demonstration of shape selective nitration of toluene at a scale of 30 L of toluene represents a scale-up of a factor of 100 compared to the H-ZSM-5 catalyzed process previously demonstrated. A semi-pilot scale toluene nitration reaction (30 L of toluene) was run using TDA’s 30-L, semi-pilot scale, glass Schott reactor system and a catalyst loading of 1.5 mL of toluene per gram of H-ZSM-5 catalyst. The overall yield of the reaction was 45% (based on HNO3 conversion), with an m-MNT selectivity of 0.5%, which is well below the MilSpec value of 1.5%. The para/ortho isomer ratio (p/o) was 5.2, which indicates that the reaction took place with good selectivity for the desired para isomer. The results are comparable to those obtained at the small scale.
Due to the similarity in results, as well as m-MNT levels being well below MilSpec, the reproduction of the small-scale results at a scale 100 times larger was successful. The large-scale toluene nitration represents a significant advance in the scale-up of this process. The successful development of a continuous MNT synthesis process based on H-ZSM-5 catalyst will enable cleaner, less expensive production of TNT while reducing hazardous waste streams.