Kinetics and OH yield measurements to constrain energy barriers in the

CH3OCH2 + O2 reaction

Arkke Eskola, Scott Carr, Robin Shannon, Mark Blitz, Mike Pilling, Struan Robertson,

Paul Seakins and Baoshan WangUniversity of Leeds, UK

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

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

0.8 1.0 1.2 1.4 1.60.1

1

10

100 Pure DME, = 2.0Ig

nitio

n de

lay

time

/ ms

1000 K / T

RCM 7 atm ST 13 atm ST 30 atm

Poor agreement(delay time is log scale)

Data and modelling from Curran

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

CH3OCH2 + O2 Potential Energy Surface

CH3OCH2 + O2

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

CH3OCH2 → HCHO + CH3

CH2OCH2O2H → OH + 2HCHOOH + HCHO → HCO + H2O

H + O2 → HO2

CH3OCH2 + O2 → CH3OCH2O2

OH + CH3OCH3 → H2O + CH3OCH2

CH2OCH2O2H + O2 → O2CH2OCH2O2H

CH3OCH2O2 → CH2OCH2O2H

Sensitivities to Ignition DelaysAt 850 K (Zhao et al. 2008)

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

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

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

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

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

+ M

CH3OCH2O2* OH + 2H2COCH3OCH2 + O2

CH3OCH2O2

kC

kM[M]

(R2b)

(R2a)

Scheme 1.

CH2OCH2OOH*

]He)[/(1

]He)[/(1

OCHCH

OCHCH

cHe

refcHe

ref023ref

023

kk

kk

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

])He[1(1

c

Heref k

k

Determination of yields via kinetics

• Monitor OH decays in the presence of DME and DME/O2. In latter case OH is regenerated

CH3OCH2 OH + 2H2COCH3OCH3 + OH

CH3OCH2O2

kR2b

O2, [M]

Scheme 2.

Initiationt-C4H9OOH + 248nm

CH3CO + O2

Cl + CH3OCH3 + O2

k1

kR2a

O2

OH is recycled, if O2 present

Determination of yields via kinetics (2)

331121 OCHCHO kkk 331332121 OCHCH1OCHCHOO kkk

1

21 O1

k

k

Calculations ab initio

• Potential energy calculated at CBS-QB//mpw1k/avtz level. Main channel shown:

CH3OCH2 + O2

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

-34.8kcal

-9.8

-25.0

-3.0

Calculation – Master Equation

• Data (kinetics AND yields) simulated using MESMER

• RRHO approximation with treatment of hindered rotors in CH3OCH2O2

• Vibrational frequencies from ab 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

Fits to the experimental data

6.4 5.5

4.6 3.7

2.8

1.91.5

1.5

1.9 2.8

3.7

4.6 5.5

1.2 1.1

-15.0 -14.8 -14.6 -14.4 -14.2 -14.0 -13.8 -13.6 -13.4 -13.2 -13.0-9.0

-8.8

-8.6

-8.4

-8.2

-8.0

-7.8

-7.6

-7.4

-7.2

-7.0T

S2

TS1

Parameters

Parameter Ab initio value MESMER value

CH3OCH2O2 -34.8 kcal -33.6 kcal

TS1 -9.8 -13.8

CH2OCH2OOH -25.0 -25.0

TS2 -3.0 -8.3

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 Ed

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

Conclusions (1)

Objectives• Study the kinetics and branching ratio of CH3OCH2 + O2

as a function of T, p monitoring OH productionDone 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 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

Acknowledgments

Thanks to:EPSRC for research funding and studentship for

Scott CarrNERC for studentship for Robin Shannon

NCAS for supporting Dr Mark BlitzFinnish Government for partial support for Dr

Arkke Eskola

0.8 1.0 1.2 1.4 1.60.1

1

10

100 Pure DME, = 2.0Ig

nit

ion

del

ay t

ime

/ ms

1000 K / T

RCM 7 atm ST 13 atm ST 30 atm

Poor agreement(delay time is log scale)

Data and modelling from Curran

CH3OCH2 + O2

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

CH3OCH2 + O2

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

+ M

CH3OCH2 + O2

CH3OCH2O2

TS1

TS2

2HCHO + OH

CH2OCH2OOH

-34.8kcal

-9.8

-25.0

-3.0

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

G4//B3LYP G4//MP2 CBS-QB3 CBS//MP2 CBS//mpw1k APNO//mpw1k

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