1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) is a high-density, thermally stable high explosive with low sensitivity to shock stimuli. TATB is used in fuze and booster systems where high-energy, insensitive energetic compositions are required. Approximately 10,000-20,000 lbs of TATB are used annually in Department of Defense (DoD) applications. Until recently, TATB was manufactured for DoD in the United Kingdom. A shift in policy has caused production of TATB for customers outside of the United Kingdom Atomic Weapons Establishment to be suspended, leaving DoD without a production source for qualified TATB.
TNT is used extensively in bomb fill because of its relatively low melting point and processing ease. Current production requirements call for a minimum of 1,800,000 kg/year of virgin TNT for various munitions applications. TNT is manufactured in the United States from a mixed acids nitration process that has changed little in more than 60 years. This production method provides a quick and inexpensive route to TNT but is burdened with an environmentally problematic waste stream. The so-called red water waste is a result of the treatment of the TNT process stream with sellite (sodium sulfite) to remove off-isomers of nitrotoluenes from the TNT. This TNT red water is a Resource Conservation and Recovery Act (RCRA) regulated waste, and while much has been done to remediate this waste, it remains a practical and environmental concern for DoD.
The objective of this project was to use methylated phloroglucinols as the starting materials for environmentally benign syntheses of TATB and TNT. Synthesis of TATB from microbe-synthesized tri-O-methylphloroglucinol would eliminate the waste streams associated with orthoformates used during TATB synthesis. Microbial synthesis interfaced with chemical catalysis would provide a route to 2-methylphloroglucinol. Synthesis of TNT from 2-methylphloroglucinol would eliminate red water waste streams.
The approach to using biosynthesized phloroglucinols begins with microbial conversion of glucose to malonyl-CoA. Phloroglucinol synthase then catalyzes the conversion of malonyl-CoA into phloroglucinol. Phloroglucinol O-methyl transferees (POMT) then catalyzes the reaction of S-adenosylmethionine with phloroglucinol to form 5-methoxyresorcinol (mono-O-methylphloroglucinol). Orcinol O-methyl transferase catalyzes the sequential methylations leading to 3,5-dimethoxyphenol (di-O-methylphloroglucinol) and 1,3,5-trimethoxybenzene (tri-O-methylphloroglucinol).
In route to the microbial synthesis of O-methylphloroglucinols, POMT from Rosa chinensis var. spontanea has been successfully de novo synthesized in codon-optimized form for expression in E. coli, which is the host currently used for microbial synthesis of phloroglucinol from glucose. The specific activity of heterologously expressed, codon-optimized POMT is 5-fold higher than the specific activity of POMT purified to homogeneity from Rosa chinensis var. spontanea petals. Efforts to create a microbe that converts methoxyresorcinol to dimethoxyphenol were also made. Protein sequence of orcinol O-methyltransferase from Rosa hybrida was obtained from GenBank, codon optimized, and back-translated into DNA sequence for E. coli expression. The resulting OOMT1 gene was synthesized, cloned into the expression vector pET22b, and assayed for methylase activity using methoxyresorcinol as substrate. The crude extract specific activity of this construct BL21(DE3)/pKIT1.009 was found to be 0.006 U/mg. The production of dimethoxyphenol in the reaction mixture was further characterized by GC/MS.
The synthesis of mono-, di-, and tri-O-methylphloroglucinol was achievable in high yields by reacting phloroglucinol with methanol for 2 hr at 140ºC in a sealed tube in the presence of an acid catalyst such as H2SO4-. The process provided greater than 90% of O-methylated phloroglucinol with 53% of mono-O-methylphloroglucinol, 35% of di-O-methylphloroglucinol, and 3% of tri-O-methylphloroglucinol. These O-methylated phloroglucinols were successfully converted to TATB by scientists at ATK Space Systems with yields as competitive as what had been reported previously, while reducing toxic reagent use and minimizing the amount of waste byproducts generated.
The successful conversion of bioderived phloroglucinol to 2-methylphloroglucinol also provides a green approach to the synthesis of TNT. Under optimized reaction conditions, the conversion of phloroglucinol to 2-methylphloroglucinol is achievable in a yield of 60% with about 33% of the phloroglucinol remaining unreacted and a small amount of dialkylated byproduct, 5-methoxyresorcinol, being formed. Scientists at ATK Space Systems found that TNT can be easily made by first treating the bioderived 2-methylphloroglucinol with aqueous hydroxylamine to generate 2,4,6-trioxime-methylcyclohexane followed by refluxing this material in concentrated nitric acid. The process shows that TNT can be made successfully without the use of toxic nitration reagent and eliminates the problematic red water issues.
These results indicate that phloroglucinol can potentially play an important role in the synthesis of high energetic materials such as TNT and TATB without generating much of the toxic wastes associated with the traditional method. Additional research is needed to improve and enhance the product yields and selectivity of this process.