Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction

1. Kinetics and OH yield measurements to constrain energy barriers in the CH3OCH2 + O2 reaction ArkkeEskola, Scott Carr, Robin Shannon, Mark Blitz, Mike Pilling, Struan Robertson, Paul Seakinsand Baoshan Wang University of Leeds, UK

2. Introduction – DME as a potential fuel • Dimethylether, CH3OCH3 has great potential as a fuel • DME can be used as a neat fuel in compression ignition engines or additive to diesel • Compatible with current engine technologies and can be distributed through LPG networks • Potential for manufacture from methane or biomass

3. Introduction – DME combustion • DME is ideally suited to HCCI engines (homogeneous charge, compression ignition) ‘HCCI can be characterized as a controlled chemical auto-ignition process and an important feature is the unusually large role that fuel chemistry plays in determining combustion characteristics when compared to diesel or SI engines’ Westbrook and Curran • The relatively low temperatures of DME combustion minimise NOx production • DME shows the classic negative temperature dependence, but the mechanism is different from alkanes Data and modelling from Curran

4. Introduction – Origin of negative temperature dependence OH + CH3OCH3 H2O + CH3OCH2 CH3OCH2 + O2 + M  CH3OCH2O2 + M CH3OCH2O2  CH2OCH2OOH CH2OCH2OOH  2HCHO + OH CH2OCH2OOH + O2  chain branching precursor • Competition between CH2OCH2OOH reactions determines NTC • CH3OCH2  CH3 + HCHO can also play a role

5. CH3OCH2 + O2 Potential Energy Surface CH3OCH2 + O2 TS2 TS1 Sensitivities to Ignition Delays At 850 K (Zhao et al. 2008) 2HCHO + OH CH2OCH2OOH CH3OCH2O2

6. Objectives • Study the kinetics of CH3OCH2 + O2 as a function of T, p monitoring OH production • Quantify the fraction of OH production as a function of T, p • Model kinetics and yields using Master Equation, based on ab initio PES • Do measurements allow constraints on the barriers on PES and allow extrapolation beyond experimental conditions? • Higher temperature measurements and studies of chain branching to follow

7. Experimental • Reactions carried out in conventional slow flow, laser flash photolysis system with OH detection by laser induced fluorescence • CH3OCH2Br + h (248 nm)  CH3OCH2 + Br • Eskola et al. Chem Phys Lett (2010) • OH detected by off-resonance fluorescence • Stainless steel cell heated for 298 - 450 K • Cooled by immersion for 195 - 298 K

8. Results - Kinetics • Reactions carried out under pseudo-first-order conditions ([O2] >> [CH3OCH2]). Fits to traces give k’ • Bimolecular rate coefficients obtained from a plot of k’ vs[O2] • Stabilization of initially formed CH3OCH2O2* chemically activated adduct requires 3rd body and hence kinetics are pressure dependent • Note, not the characteristic ‘Lindemann’ curve as chemically activated CH3OCH2O2* can decompose to 2HCHO + OH

9. Results - Yields • The height of the signal proportional to OH yield • The OH yield will increase with decreasing pressure and should → 1 • The relative yield, β, is given by: CH3OCH2 + O2 TS2 + M TS1 2HCHO + OH CH2OCH2OOH CH3OCH2O2

10. Results – Yields (2) • A plot of 1/βvs [He] should be a straight line • Make reference pressure close to zero (5 Torr) so extrapolation is short. • Assumes no other channel other than OH production at zero pressure

11. Determination of yields via kinetics • Monitor OH decays in the presence of DME and DME/O2. In latter case OH is regenerated

12. Determination of yields via kinetics (2)

13. Calculations ab initio • Potential energy calculated at CBS-QB//mpw1k/avtz level. Main channel shown: CH3OCH2 + O2 -3.0 -9.8 TS2 -25.0 -34.8 kcal TS1 2HCHO + OH CH2OCH2OOH CH3OCH2O2

14. Calculation – Master Equation • Data (kinetics AND yields) simulated using MESMER • RRHO approximation with treatment of hindered rotors in CH3OCH2O2 • Vibrational frequencies fromab initio calculations • ILT used to generate microcanonical rate coefficients for reverse reaction, RO2→ R + O2 • Fitting kinetics and yields without hindered rotors gave inconsistent ∆Ed

15. Fits to the experimental data

16. Parameters

17. Discussion points • Simultaneous fitting of yields and kinetics constrain parameters • Significant difference between fitting and ab initio, but: • Variation of energies with methods suggests spin contamination issues • Use of hindered rotor removes the need for a temperature dependent Ed

18. Conclusions (1) Objectives • Study the kinetics and branching ratio of CH3OCH2 + O2 as a function of T, p monitoring OH production Done 195 – 450 K. Higher temperature work to follow. • Model kinetics and yields using Master Equation, based on ab initio PES. • Do measurements allow constraints on the barriers on PES? Yes, but still uncertainties • and allow extrapolation beyond experimental conditions? No, currently uncertainties on PES and density of states calculations too great Done

19. Conclusions and outlook • Hindered rotor removes the need for temperature dependent Ed, but: • Requires calculation of potential for hindered rotation • Treatment of other low frequency modes? • Uncertainties around potential energy surfaces preventing wider application Outlook • At higher temperatures, thermal production from stabilized CH3OCH2O2 becomes important • Decomposition of CH3OCH2 will become important • Uncertainties around mechanism of QOOH + O2 • Points to be addressed in current application with Klippenstein and Curran on DME chemistry

20. Acknowledgments Thanks to: EPSRC for research funding and studentship for Scott Carr NERC for studentship for Robin Shannon NCAS for supporting Dr Mark Blitz Finnish Government for partial support for Dr Arkke Eskola

21. Data and modelling from Curran

22. CH3OCH2 + O2 TS2 TS1 2HCHO + OH CH2OCH2OOH CH3OCH2O2

23. CH3OCH2 + O2 TS2 + M TS1 2HCHO + OH CH2OCH2OOH CH3OCH2O2

24. CH3OCH2 + O2 -3.0 -9.8 TS2 -25.0 -34.8 kcal TS1 2HCHO + OH CH2OCH2OOH CH3OCH2O2

25. G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k TS1 -8.8 -13.3 -11.5 -16.0 -11.3 -10.4 TS2 -0.1 7.2 -3.6 9.4 -3.3 -1.8 TS3 1.0 1.1 0.4 0.5 0.20 1.6 TS4 2.3 5.1 1.4 3.0 0.0 0.6 TS5 - -6.0 - -5.3 -0.6 -0.1 TS6 -64.2 -64.1 -64.8 -64.9 -65.1 -63.3