Study On Pyrolysis Kinetics Of Hexamethyldisiloxane Based On Reactive Molecular Dynamics Simulations

Hexamethyldisiloxane(HMDSO)widely exists in biogas produced from landfills and waste water treatment facilities and its combustion products will cause serious harm to power equipment.At the same time,HMDSO as a typical silicone has broad application prospects in the flame synthesis of nano silica particles.Therefore,an in-depth understanding of HMDSO pyrolysis process and mechanism is of great strategic significance for the development and utilization of biogas in landfills and wastewater treatment plants and the mitigation of energy crisis.It can also provide precise reaction control and corresponding theoretical guidance for the process of flame synthesis of nano silica particles with HMDSO as precursor.Therefore,the high-temperature pyrolysis process of HMDSO is deeply studied in this work.Using ReaxFF molecular dynamics simulation method,combined with gas chromatography and synchrotron radiation vacuum ultraviolet photoionization mass spectrometry,the pyrolysis products,reaction path and reaction rate constant are studied.Also,the effects of three different reaction force fields on the simulations are discussed and their reliability is analyzed.Pyrolysis simulations for the large-scale HMDSO system under different temperatures and pressures are carried out,and the pyrolysis kinetics is deeply analyzed.The main conclusions are as follows:(1)Gas chromatography shows that with the increase of pyrolysis temperature,the content of hydrocarbon products increases,and they are mainly the stable low-carbon hydrocarbons(mainly CH4 and C2H4).The trends of products variety and content are basically consistent with the results of ReaxFF molecular dynamics simulation,which verifies the accuracy of ReaxFF molecular dynamics simulation to a certain extent.In addition,Si/C/H products are found in the pyrolysis of HMDSO by synchrotron radiation mass spectrometry,which include SiH4,SiCH4,Si(CH3)3 and Si(CH3)4.Based on the above experimental results,through the comparison with molecular dynamics simulations under different force fields,the most suitable ReaxFF reaction force field is selected,which lays a foundation for the following research of molecular dynamics simulations.(2)The pyrolysis pathways of HMDSO are revealed through singlemolecule and multi-molecules ReaxFF molecular dynamics simulations.Among them,besides the known dissociation reaction of CH3 removal,the dissociation reaction of CH4 removal is found in the initial pyrolysis pathways of HMDSO via single-molecule molecular dynamics simulations.This enriches the understanding of HMDSO initial pyrolysis reactions.In addition,the main products of HMDSO pyrolysis are found to be the small hydrocarbon molecules(i.e.,CH3,CH4,H2,CH2O,etc.)and the Sicontaining products(i.e.,SiH4,SiH2,CH4Si,etc.)by molecular dynamics simulations.Through the evolution of the above products,the multimolecules reaction network for HMDSO pyrolysis is obtained.(3)Through ReaxFF molecular dynamics simulation under different temperatures and pressures,with the increase of temperature,Si(CH3)4 and(CH3)2SiO generated by Si-O bonds breaking and intermediate products such as SiO can be observed by ReaxFF molecular dynamics simulations,which theoretically explains the existing experimental measurement results and reaction pathways.According to the evolution of HMDSO molecule with time at different temperatures,the reaction rate constant of HMDSO initial pyrolysis is calculated,and the equivalent activation energy and preexponential factor are obtained.Besides,the increase of temperature enlarges the number of free radicals and active molecular fragments in the system,and the intermediate reaction becomes more intense,resulting in change of the type and structure of pyrolysis products and making the products tend to be fragmented.The variation of pressure leads to the change of molecule concentration and affects the possibilities of collision between molecules and the reaction occurrence.Under low pressure,the intensity of intermediate reaction is low,and there are more active small molecules that have not participated in the reaction in the system.The higher the pressure,the easier it is to form stable pyrolysis products and macromolecular clusters.