Study On The Nitration Of Alkanes With NO₂ In “Red Oil” Components

Spent fuel reprocessing refers to the process of treating nuclear fuel chemically in the reactor,and it is an essential part of the nuclear fuel cycle.The PUREX process is generally used for the post-processing of spent fuel.In the post-processing process,tributyl phosphate（TBP）,diluent and nitric acid are exposed to high temperature and strong radiation environment,producing"red oil"which may cause explosion accident happened.The main component of the diluent is n-dodecane,which is decomposed to short-chain alkanes,branched-chain alkanes and cycloalkanes under high temperature and strong radiation conditions.Nitric acid is also easily decomposed into NO₂ under high temperature and strong radiation conditions.NO₂ has a strong ability of nitration.Generally,studying the nitration of alkanes with NO₂ in"red oil"component is of great significance for understanding explosion accidents.Taking n-hexane nitration as the model reaction,explore the effects of reaction conditions on the nitration of n-hexane with NO₂.When the reaction temperature was 120℃,the ratio of n-hexane to NO₂ was 1:2,and the reaction time was 4 h,the conversion of n-hexane can reach 85.9%.C₆～C₁₂ straight-chain alkanes,branched-chain alkanes and cycloalkanes,the representative degradation products of n-dodecane under radiation conditions,were further selected to explore the characteristics of the nitration reaction.As the carbon chain growed,the conversion rate of linear alkanes decreased,and nitration reaction was more difficult to occur.Nitration reaction appeard to occur on the tertiary carbon preferentially.The nitration of cycloalkanes exhibited to have higher selectivity,and it was easy to produce oxidation by-products without C-C bond breakage.The nitration reaction of alkanes appeared to be the free radical reaction in the preliminary exploration.The semi-empirical method（MO-PM2）and density functional theory（DFT）were used to calculate the possible reaction pathways of nitration of n-hexane with NO₂.The results showed that the rate-determining step of the nitration reaction is that the O atom in NO₂attacks the H in n-hexane.It showed that the breakage of the C-H bond and the formation of O-H is a coordinated process with the result of molecular geometry,atomic charge,IR vibration frequency data.The thermal hazard of the n-hexane-NO₂ system was further studied.Dibutyl phosphate（DBP）,the main degradation product of TBP,was selected as an additive to explore the effect of DBP on the n-hexane-NO₂ system.The thermal hazards of n-hexane-NO₂,DBP-NO₂ and DBP/n-hexane-NO₂ were explored by adiabatic accelerated calorimeter（ARC）.The thermal characteristic parameters,such as adiabatic temperature rise and etc.,of the n-hexane-NO₂and DBP-NO₂ systems increased with the increase of the amount of NO₂.The apparent activation energy（Ea）decreased,and the corresponding initiation temperature when the maximum temperature rise rate reaches 24 h(T_D24)decreased,indicating thermal hazards increased.When the ratio of n-hexane/NO₂ substance reached 1:3,it will cause more severe thermal runaway.It showed that the introduction of DBP has little effect on the initial exothermic temperature of n-hexane-NO₂ system,however,the T_D24 of the n-hexane-NO₂system was reduced,indicating the thermal hazards decreased.Through gas-mass spectrometry and infrared spectroscopy analysis,it showed that the cleavage of the C-C bond,as well as some complicated reactions such as nitration,oxidation and esterification occurred in the n-hexane-NO₂ system because of high temperature.In addition to 1-,2-,and3-nitrohexane,other short-chain nitro,alcohol,ketone,carboxylic acid,ester,cyano and amide compounds were obtained.The O-P cleavage of DBP under the influence of NO₂produced butanol and phosphoric acid.Butanol was further oxidized and dehydrated to produce butyric acid,butyl ether and various ester compounds.