Study On The Nitrolysis Mechanism Of DPT To Prepare HMX Based On Nitric Acid And Ammonium Nitrate System And Development Of New Processes

1,3,5,7-Tetranitro-1,3,5,7-tetraazacyclooctane(HMX)is one of the most popular and practically-used high explosive owing to its excellent detonation performance.The so-called“small-molecule route”using urea as the starting material to prepare HMX via3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane(DPT)is envisioned as a promising method for scale-up thanks to the much lower cost in comparison with traditional Bachmann's process.However,the nitrolysis of DPT afforded HMX inevitably with a lower yield,which limits the development of this method.The main reason is attributed to the ambiguity of the nitrolysis mechanism.Herein,a new mechanism for the nitrolysis of DPT was put forward.Then,based on this mechanism,a new route to prepare HMX from DPT was developed and optimized.The reaction of dimorpholinomethane in fuming HNO₃ was investigated for simplifing the nitrolysis model of DPT.The major product was identified as N-nitrosomorpholine and a key intermediate N-hydroxymethylmorpholine was detected during the reaction by ¹H NMR tracking.The effects of ammonium nitrate and water on the nitrosolysis of N-hydroxymethyldialkylamine were investigated through the experimental and theoretical methods.Observations show the key role of ammonium ions and a small amount of water in promoting the nitrosolysis reaction.In addition,it was found that ammonia,generated from the unfavourable balance with the ammonium nitrate,acts as Lewis base catalyst.A rigid bicyclic transition state established with N-hydroxymethyldialkylamine,ammonia and three water molecules through a hydrogen-bonding interaction leads to low activation energies,which promotes the nitrosolysis reaction.Two intermediate compounds,1-nitroso-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane(MNX)and 1-nitroso-3,5-dinitro-1,3,5-triazacyclohexane(DNMS),were isolated from the nitrolysis of DPT in fuming HNO₃ and ammonium nitrate.Based on the ¹H NMR monitor,it was identified that MNX acted as the essential intermediate for the formation of HMX while DNMS played as the intermediate for the formation of1,3,5-trinitro-1,3,5-triazacyclohexane(RDX).Then,the reaction kinetics of this reaction was studied.It was further verified that MNX is intermediate in the reaction,and the first-order reaction rate constants,the activation energies and pre-exponential factors of the nitrolysis of MNX to produce HMX(main reaction)and ring-opening byproducts(side reaction)were obtained respectively.In addition,the half life for the nitrolysis of MNX and the reaction time for the conversion of MNX to 99.0%at different temperatures were achieved through calculation.Furthermore,according to the transition state theory,the free enthalpy,free entropy and free energy of the main reaction and side reaction at 250 K were afforded severally.1-Acetoxymethyl-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane(PHX)was synthesized from DPT,and the ¹H NMR spectrum and ¹³C NMR spectrum of the active intermediate1-hydroxymethyl-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane were obtained by tracking the hydrolysis of PHX.The effects of ammonium nitrate,water and nitrogen tetroxide on the reaction of PHX and DPT in fuming HNO₃ were investigated.It was found that MNX is also an intermediate in the nitrolysis of PHX to form HMX,and certain amount of ammonium nitrate,water and nitrogen tetroxide in fuming nitric acid is benefit to the formation of MNX.Among which nitrosonium in nitration system has two resources:one is from the nitrogen oxides in fuming nitric acid;the other is from the redox reaction between nitric acid and formaldehyde,which was promoted by water in the fuming nitric acid.What's more,it was also found that the water in fuming nitric acid can promote the hydrolysis of the esterification intermediate,and simultaneously verified that1-hydroxymethyl-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane is a key active intermediate in the nitrolysis of DPT.However,the existence of ammonium nitrate,water and nitrogen tetroxide in the nitric acid is unfavorable to the conversion of MNX to HMX,which leads to the inefficient preparation of HMX from DPT through the traditional one-pot nitrolysis process.Similar,the effects of ammonium nitrate and amine hydrochlorides on the nitrolysis of DPT were explored with experimental and theoretical methods.The results show the formation of MNX can be facilitated by ammonia nitrate and some amine hydrochlorides with short alkyl substituents.And for the same alkyl substituent,the positive effect follows the order:primary amine>secondary amine>tertiary amine.Among which ammonia or amines,in situ generated from the unfavourable balance with their salts,act as Lewis base catalysts.The rigid bicyclic transition states established with1-hydroxymethyl-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane,ammonia(or amines)and three water molecules through a hydrogen-bonding interaction lead to very low activation energies,which promotes the synthesis of MNX.Therefore,a new reaction mechanism including nitrolysis,nitrosolysis and nitrolysis processes for the synthesis of HMX from DPT was put forward.Based on the new mechanism,a stepwise protocol for the synthesis HMX from DPT via intermediate MNX was developed.Among which three methods for preparing MNX by adding Na NO₂,N₂O₄ or CH₃NH₂·HCl were studied respectively.After optimization of the reaction conditions,MNX could be obtained with the highest yield of 84.0%,86.1%and78.5%respectively,and MNX could be efficiently and smoothly nitrolyzed in 100%HNO₃and afforded?-HMX with an excellent yield of 96.0%.Therefore,the overall yield of the stepwise procedure was 75.4%?82.7%,much higher than traditional one-pot nitrolysis.TG-DSC curves indicate that MNX has good thermal stability.In addition,the impact sensitivity and electrostatic sensitivity of MNX are lower than that of RDX and HMX,while the friction sensitivity is between RDX and HMX.The nitrolysis of 3,7-dipropionyl-1,3,5,7-tetraazabicyclo[3.3.1]nonane(DPPT)for preparing HMX was investigated.Two intermediate compounds,1,5-dipropionyl-3-nitroso-7-nitro-1,3,5,7-tetraazacyclooctane(DPNNO)and1,5-dipropionyl-3,7-dinitro-1,3,5,7-tetraazacyclooctane(DPDN),were isolated from the reaction mixture.According to the ¹H NMR monitor,it was identified that the synthesis of HMX from DPPT through the nitrolysis of DPNNO and DPDN successively.A stepwise method to prepare HMX from DPPT via intermediate DPNNO was developed.After optimization of the reaction conditions,DPPT could be converted to DPNNO with an excellent yield of 94.2%,and HMX could be obtained by the nitrolysis of DPNNO with a satisfactory yield of 85.0%.Therefore,the overall yield of the stepwise procedure was as high as 80.0%.A new reaction mechanism for the conversion of DPT to HMX was established in this dissertation.Based on this mechanism,two new stepwise protocols for the synthesis HMX were developed.The results not only break through the bottleneck of the low yield of HMX synthesized by small molecule method with the urea as starting material in recent decades,but also provide a template for studying the nitrolysis of compounds with similar structures to DPT.