Experimental And Modeling Study Of Dimethyl Methylphosphonate (DMMP) Pyrolysis And Oxidation

At present,fire accidents have seriously affected people’s personal and property safety,and the use of flame retardants can effectively avoid the occurrence of fire accidents and reduce the harm of fire.However,with the increased emphasis on environmental protection,halon products,which are seriously damaging to the ozone layer,have been banned.Thus,it is important to find eco-friendly halon replacements.Dimethyl methylphosphonate(DMMP)is one of the organic phosphorous compounds(OPCs)with low toxicity,good effectiveness as a retardant,and environmental friendliness.Therefore,it is a great halon replacement.However,previous work lacked DMMP studies on the quantitative experiment,model development and validation,and calculation of reaction rates,leading to a poor understanding of the mechanism of DMMP pyrolysis and oxidation.Hence,this paper aims to study the experiment and mechanism for DMMP pyrolysis and oxidation.First,this work investigated the pyrolysis of DMMP at atmospheric pressure(1 atm)via synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUVPIMS)combined with a jet-stirred reactor(JSR).The experimental temperature range was from 900 K to 1200 K.For experimental methods,proper photon energies were selected to ensure the near-threshold ionization of species,which can avoid fragmentation.Photoionization efficiency energy curves were scanned for species determination.The oxidation experiments of DMMP were also carried out on the synchrotron vacuum ultraviolet photoionization mass spectrometry combined with a jet-stirred reactor.Three equivalent ratio conditions(phi=0.5/1.0/2.0)and a wide temperature range(810 K to 1170 K)were selected for the experiments.The mole fraction of DMMP and other products cannot be calculated without photoionization cross section(PICS)data of DMMP.Therefore,mass spectrometry was used to measure the photoionization cross section data of DMMP for species quantification.Finally,the concentration distribution of the reactants and products as a function of temperature was obtained by experimental studies of pyrolysis and oxidation,and P-containing products were detected in both pyrolysis and oxidation experiments.Previous theoretical studies on DMMP have some shortcomings.In detail,there was a lack of computations for the rate constants of the initial reaction channels of DMMP during previous computational studies.In this paper,the geometric optimization and vibration frequency analysis of DMMP unimolecular decomposition,isomerization reactions and subsequent dissociation reactions of DMMP isomers were carried out at the MN15/6-311+G(2df,2p)level of theory.Additionally,kinetic calculations were performed using the kinetic code of MESS.Finally,a more complete reaction network and potential energy surface for DMMP pyrolysis were obtained,and the rates of the atmospheric pressure and high-pressure limits for all the calculated pathways were provided in this paper.According to the calculated potential energy surface,DMMP isomerizes into P[OH]CH2[OCH3]2 with the lowest energy barrier,and DMMP mainly undergoes isomerization channels to form its isomers before decomposing into pyrolysis products.For DMMP unimolecular decomposition pathways,the CH3 radical leaving the OCH3 group has the lowest energy barrier.The theoretical study of this work provided more information on the DMMP pyrolysis reaction network and the rate constant corresponding to the reactions than before.Furthermore,the ionization energies and thermodynamic data of some P-containing species are also calculated in this work,which provides a basis for the determination of experimental conditions and the development and validation of kinetic models.Based on the previous kinetic model of DMMP and the results of present theoretical studies,the DMMP pyrolysis model has been updated and developed.Theoretical calculations showed that the isomerization reactions were the main reaction pathways before the decomposition of DMMP,which is different from the DMMP decomposition channels adopted in the literature models.Therefore,this work modified and improved the previous model,by adding the isomerization reaction pathways and their rate constants under atmospheric pressure,and by updating the rates of direct decomposition reactions for DMMP.To better describe the experimental measurements,some rate tuning of reactions by 5 or 10 factors has been tried in model development.Finally,kinetic simulation was conducted via CHEMKIN-PRO software.The new model established in this paper can predict DMMP pyrolysis significantly better than before.In addition,DMMP oxidation was also simulated and kinetic analysis was carried out.The results of sensitivity analysis and rate of production analysis showed that the H-abstraction reactions played an important role in DMMP oxidation.Finally,to further verify the reliability of the present model developed in this paper,the simulation results of the new model were compared with various experimental data reported in previous literatures,including the CO concentration distribution in the shock tube,flame velocity and product concentration distribution in the premixed flame.The verification results showed that the improved DMMP model is reliable.This work laid a foundation for the further study of combustion characteristics and the establishment of kinetic models of organophosphorus compounds.In the context of environmental protection and economic development,green flame retardant is an important issue in the field of fire protection today.In this paper,by carrying out experimental and theoretical research on DMMP,a green highefficiency flame retardant,we solve the problems of experimental quantification of DMMP and kinetic analysis of DMMP decomposition around the development demand of national green flame retardant.Conclusions and perspectives are presented at the end of this paper,including developing theoretical calculation methods to obtain more accurate rate constants,further study of the DMMP pyrolysis reaction network,and carrying out more DMMP pyrolysis and oxidation experiments under various conditions.