Kinetics and OH yield measurements to constrain energy barriers in the CH 3 OCH 2 + O 2 reaction Arkke Eskola, Scott Carr, Robin Shannon, Mark Blitz, Mike Pilling, Struan Robertson, Paul Seakins and Baoshan Wang University of Leeds, UK

Introduction – DME as a potential fuel Dimethylether, CH 3 OCH 3 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

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

Introduction – Origin of negative temperature dependence OH + CH 3 OCH 3  H 2 O + CH 3 OCH 2 CH 3 OCH 2 + O 2 + M  CH 3 OCH 2 O 2 + M CH 3 OCH 2 O 2  CH 2 OCH 2 OOH CH 2 OCH 2 OOH  2HCHO + OH CH 2 OCH 2 OOH + O 2  chain branching precursor Competition between CH 2 OCH 2 OOH reactions determines NTC CH 3 OCH 2  CH 3 + HCHO can also play a role

CH 3 OCH 2 + O 2 Potential Energy Surface CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH Sensitivities to Ignition Delays At 850 K (Zhao et al. 2008)

Objectives Study the kinetics of CH 3 OCH 2 + O 2 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

Experimental Reactions carried out in conventional slow flow, laser flash photolysis system with OH detection by laser induced fluorescence CH 3 OCH 2 Br + h (248 nm)  CH 3 OCH 2 + Br Eskola et al. Chem Phys Lett (2010) OH detected by off-resonance fluorescence Stainless steel cell heated for K Cooled by immersion for K

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

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: CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH + M

Results – Yields (2) A plot of 1/β vs [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

Determination of yields via kinetics Monitor OH decays in the presence of DME and DME/O 2. In latter case OH is regenerated

Determination of yields via kinetics (2)

Calculations ab initio Potential energy calculated at CBS- QB//mpw1k/avtz level. Main channel shown: CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH kcal

Calculation – Master Equation Data (kinetics AND yields) simulated using MESMER RRHO approximation with treatment of hindered rotors in CH 3 OCH 2 O 2 Vibrational frequencies from ab initio calculations ILT used to generate microcanonical rate coefficients for reverse reaction, RO 2 → R + O 2 Fitting kinetics and yields without hindered rotors gave inconsistent ∆E d

Fits to the experimental data

Parameters ParameterAb initio valueMESMER value CH 3 OCH 2 O kcal-33.6 kcal TS CH 2 OCH 2 OOH-25.0 TS EdEd 200 cm -1

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  E d G4//B3LYP G4//MP2CBS-QB3CBS//MP2CBS//mpw1kAPNO//mpw1k TS TS

Conclusions (1) Objectives Study the kinetics and branching ratio of CH 3 OCH 2 + O 2 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

Conclusions and outlook Hindered rotor removes the need for temperature dependent  E d, 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 CH 3 OCH 2 O 2 becomes important Decomposition of CH 3 OCH 2 will become important Uncertainties around mechanism of QOOH + O 2 Points to be addressed in current application with Klippenstein and Curran on DME chemistry

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

Data and modelling from Curran

CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH

CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH + M

CH 3 OCH 2 + O 2 CH 3 OCH 2 O 2 TS1 TS2 2HCHO + OH CH 2 OCH 2 OOH kcal

TS TS TS TS TS TS G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k G4//B3LYP G4//MP2CBS-QB3CBS//MP2CBS//mpw1kAPNO//mpw1k TS TS