2,4,6-trinitrotoluene (TNT), an important military explosive, is produced commercially by the nitration of toluene, initially using mixtures of concentrated nitric and sulfuric acids to give an isomeric mixture of dinitrotoluenes and ultimately with mixtures of nitric acid and oleum (sulfuric acid containing up to 44% free sulfur trioxide) to convert the dinitrotoluene mixture to TNT. A major shortcoming in the overall process is the production of asymmetrical TNT isomers, which are generated through nitration in the 3 or meta- position of the toluene ring. The presence of these compounds in the final product results in TNT that has too low a melting point for military use, so the asymmetrical isomers must be removed by treatment with bisulfite. This bisulfite treatment generates a by-product waste-stream known as “redwater,” which is difficult and costly to treat. A further problem with current TNT manufacturing processes is the generation of an undesirable by-product, tetranitromethane, and nitrogen oxide off-gases, which require remedial treatment at extra cost.

Redwater Pollution at a TNT Production Facility

The objective of this project was to develop a process for the manufacture of military grade TNT that eliminates the generation of redwater by removing the need for bisulfite treatment. The project also focused on ways to carry out the nitration of dinitrotoluenes to TNT more cleanly in order to reduce the production of pollutants. A principal aim was to reduce the proportion of meta-substituted nitrotoluenes to levels below 1.3% (instead of the 4-5% levels in current processes); a subsidiary aim was to reduce the formation of tetranitromethane in the final nitration stage (dinitrotoluenes to TNT) from current levels of around 0.5 pounds per 100 pounds of TNT produced.

The project investigated methods to remove the need for bisulfite treatment by increasing regioselectivity in the nitration of toluene so that the formation of meta-substituted nitrotoluenes, especially in the step producing mononitrotoluenes, is suppressed. Such isomers give rise, when fully nitrated, to asymmetrical isomers of trinitrotoluene.  The use of a flow nitration system, operated at sub-ambient temperature, for the first stage of nitration (toluene to mononitrotoluenes), in conjunction with dinitrogen pentoxide (N₂O₅) solution in dichloromethane (DCM) as a mild nitrating agent, reduced the meta-nitrotoluene content to below 1.3%, sufficient to allow the synthesis of TNT with a solidification point of 80.2°C or higher (which meets the military specification for Type I TNT). The need to use oleum in the final nitration stage was eliminated by utilizing, in a pot reactor, a novel nitrating system consisting of one of the following: (1) N₂O₅ dissolved in sulfuric acid; (2) this system with a Bronsted acid activity enhancer if necessary (e.g., trifluoromethane-sulfonic acid); or (3) a system based on the use of inert perfluorocarbon “bulking liquids” with conventional mixed acids, which was already demonstrated to be effective in preliminary studies of toluene nitration. The project was comprised of laboratory-scale synthesis and technical demonstrations, supported by robust analysis methods.

Experimental work indicated that TNT with a set point within 0.3°C of the Mil. Spec. (80.2°C) can be made by the QinetiQ process, without the need for sulfite washing. One recrystallization raises this figure to 80.6°C. Other established manufacturing processes to meet the Mil. Spec. either start from toluene and rely on sulfiting or use pre-formed 2-nitrotoluene as a feedstock, which raises supply problems for large quantities of this chemical. Environmental impact aspects of the chemistry of the proposed processes have been addressed, and it was found that the chemicals proposed for use in the new process would not be environmentally hazardous. Preliminary discussions with a North American explosives manufacturer indicated that the process would, subject to availability of funding, be capable of exploitation particularly if economies of scale came into play regarding the production of N₂O₅. A possible exploitation pathway could involve N₂O₅ as a nitrating agent for a range of products, from nitroaromatics through nitramines to nitrate ester plasticizers, thus achieving the desired economies of scale with enhanced atom efficiency and cleanliness of the respective processes.

The primary benefit of this project is the avoidance of the production of redwater during the manufacture of TNT without incurring additional environmental problems. A subsidiary outcome would be cleaner production of TNT using nitration methodologies that reduce the formation of pollutants such as nitrogen oxide off-gases and tetranitromethane. The synthetic routes are aimed at being directly applicable to manufacturing scale operations.