Theoretical Investigation And Kinetic Modeling For The Thermal Cracking Process Of Dimethyl Ether

Dimethyl ether,often abbreviated DME,has great potential as an oxygenated additive blended with diesel fuel due to its high blending cetane number,low auto-ignition temperature,and good combustion performance.The displacement of hydrocarbons by DME leads to a significant reduction of soot particulate,nitrogen oxides,and noise emissions,which has become a research hotspot in energy,environment,and chemical engineering today.To optimize combustion process of DME and promote the development of DME-fueled engine and related equipment,understanding the complex pyrolysis mechanism and constructing the kinetic model have important practical significance.The present work will systematically investigate the thermal cracking mechanism and kinetics of DME using high-level quantum chemical calculations and various versions of transition state theory based on the fundamental law of free radical chain process.The main research contents and conclusions are as follows:The mechanism and kinetics for the initial cracking step(C-O bond rupture)of DME are well elucidated employing the M06-2X/MG3S,MP2/TZVP,MP4/TZVP,and B2PLYP/TZVP levels.Tunneling coefficients are approximated using the Wigner and Skodje-Truhlar methods.The calculated results indicate that,in the process of the C-O bond rupture of DME,there exists a transition state lying above the reactant CH3 and CH3O in energy by 50-90 kJ mol-1,and the heat of reaction is 324-360 kJ mol-1,agreeing well with the experimental results.Tunneling produces little influence on the calculations of rate constants.The activation energies based on the MP2/TZVP calculations including the Wigner tunneling correction exhibit a little temperature dependence(increasing with the temperature),and the relationship between activation energy and temperature is nonlinear.The present work performs high-level theoretical chemical calculations rigorously using the M06-2X/MG3S,BMK/MG3S,and B2PLYP/TZVP levels to obtain molecular structure changes and establishs a comprehensive kinetic model at temperatures 200-2600 K for the hydrogen-abstraction from DME by the CH3 radical employing various versions of transition state theory(TST),which include conventional TST,canonical variational TST(CVT),and improved CVT(ICVT),and tunneling corrections are calculated using multidimensional zero-curvature tunneling(ZCT)and small-curvature tunneling(SCT)methods.The calculated results indicate that hydrogen-abstraction from the DME molecule by the CH3 radical needs to pass through transition states(having reactant-like character)to attack the out-of-plane and in-plane H sites of DME,The transition states for the two channels can be interchanged by hindered internal rotation which produces a profound influence on the kinetic model.Tunneling appears to make minor(at high temperatures)but significant contributions at low temperature to the rate constant evaluation.Variational effects on the computed rate constants are found to be negligibly small.The ICVT/CT/SCT model with M06-2X/MG3S calculations including anharmonic torsion and SCT tunneling correction agrees well with most of available measurements to validate and complement previous experimental investigations.The M06-2X/MG3S,BMK/MG3S,and B2PLYP/TZVP levels and the CBS-QB3,G4,and G4MP2 composite methods in combination with multi-structural canonical variational transition-state theory(MS-CVT)are employed to study the hydrogen-abstraction from DME by the methoxy radical.The calculated results indicate that only one transition state is located with the hydrogen-abstraction reaction occurring at the out-of-plane hydrogen atoms which directly connects the pre-reaction DME-CH3O and post-reaction CH3OCH2-CH3OH adducts lying below the corresponding separate reactants DME + CH3O and products CH3OCH2 +CH3OH in energy.The transition structure has the reactant-like and earlier character.Direct dynamics calculations demonstrate that tunneling appears to be important at low temperatures and variational effects on the computed rate constants at temperatures 200-2800 K are found to be negligibly small.The multiple-structure anharmonicity provides the crucial contribution to the MS-TS values at low temperatures,and with increasing temperature,contributions from torsional anharmonicity also become increasingly important.The MS-CVT/SCT rate constant based on the M06-2X/MG3S calculations including torsional and multiple-structure anharmonic effects and tunneling correction will accelerate the experimental researchs.