Chemical Activation Reactions of Cyclic Alkane and Ether Ring-Opened Diradicals with O2: Thermochemistry, Reaction Paths, Kinetics Itsaso Auzmendi Murua, Jason Hudzik Joseph W. Bozzelli 7th International Conference on Chemical Kinetics, July 10-14, 2011 Department of Chemical Engineering
Introduction • Cyclic Aliphatic Hydrocarbons are major components in modern fuels: • - Present in reactants: • Commertial jet fuel contains: 26% cycloalkanes and alkylcycloalkanes • Commercial diesel fuel (up to 40%) and gasoline (up to 3%) • - Produced during the gas-phase processes • During combustion or pyrolisis processes, cycloalkanes can lead to formation of: • - Toxic compounds or soot precursors such as benzene (via dehydrogenation) • - Linear unsaturated species such as acrolein (via ring opening) • 3 to 6 member cyclic ethers are formed at early times by alkyl radical reactions with dioxygen in combustion and pre-combustion processes that occur at moderate T.
Introduction – s-butane oxidation Formation of Cyclic Ethers in Alkyl Radical Oxidation R. + O2 => ROO. Hydrogen atom transfer then Cyclic ether formed with OH elimination
Introduction – s- and t- isooctane oxidation Formation of Cyclic Ethers In Isooctane Radical + O2 Reactions
Introduction • Initial unimolecular dissociation reactions of cyclic alkanes and ethers in combustion systems are ring opening to form a di-alkyl radical. • Release of ring strain in small ( 3 to 5 member ring) and bicyclic molecules reduces the bond energy needed for bond cleavage - ring opening – Diradical Formation. • The initial ring opened di-radical or the peroxy – alkyl di-radical can undergo triplet – singlet conversion by: • Electronic state crossing • Collisions of the di-alkyl radical with the bath gas • Chemical activation reaction of one radical site via association with 3O2
Introduction • This study is an attempt to determine the importance of the diradicals reacting with dioxygen. • Quantum chemical calculations for thermochemical properties. • Statistical rate theory for the T and P dependence of the rate coefficient • Systems Studied : • Cyclic Alkanes : y(ccc), y(cccc) and y(ccccc) • Cyclic Ethers : y(cco), y(ccco) and y(cccco) • TCD (C10H16) Tri-cyclo Decane Tri-cycloDecane
Thermochemical Properties • Use of computational chemistry → calculate for radicals and molecules: - Heats of formation - Entropies - Heat capacities • Heat of formartion from Isodesmic work reactions: (*)Sirjean, B., et al. . J. Phys. Chem. A 2006, 110, 12693.
Rate Constants • Association and addition reactions are treated as: • Chemical activation reactions with: • Quantum Rice Ramsperger Kassel analysis for k(E) • Master Equation for fall-off (pressure dependant reactions) • Steady State Analysis for Activated Species • Input file for Chemaster: • Thermochemical information on reaction paths • Temperature and pressures desired for study • Frequencies of the species involved in the reactions • High Pressure Rate Constants • Lennard Jones Collision Parameters of reactants and the bath gas • ΔEdown and ΔEaverage for the determination of k(E)
Chemaster – QRRK and ME analysis • Excited (A)* can: • Dissociate back to reactants • Be stabilized by collisions • React to new products Diradical + O2 (A)* m = bath gas (N2, Air, Ar…) ks(m) (A)o Energy levels from one External Rotation included in density of states
P and T dependence of rate constants Chemical activation Di-radical + O2 only c.ccc. + o2
Reaction of the diradicals with O2 • Chemical activation analysis is used for reaction of the diradicals with O2 : - qRRK for k(E) - Master Equation Analysis for fall-off • Chemkin used for analysis of a reaction system of the diradical • Chemkin analysis includes: • Results (kinetics) from diradical with O2 (chemical activation association) • Triplet-Singlet conversion • Formation of oxygenated ring Hrxn = exothermic ~ 70 kcal mol-1 • Ring opening via cleavage of weak cyclic O-O bond ~ 45 kcal mol-1 • Unimolecular reactions of the diradical: • Intramolecular H transfer to form an stable olefin • β-scission to form olefins + New Radical • Reactions of stabilized intermediates • β-scission and Ring closure • …
Reaction Paths – Example - Cyclobutane - y(cccc) Unimolecular Dissociation Chemical Activation
Intramolecular H transfers and HO2 elimination reactions C.CCC.+ O2→ C.CCCQ.
CHEMKIN MODELING RESULTS
Reaction Paths – Cyclopropane – y(ccc) Unimolecular Dissociation Chemical Activation
Reaction Products – Cyclopropane – y(ccc) 1 atm Main reaction paths: → Ring closure then reaction to y(cco) + CH2O At higher temperatures: → Formation of ethylene becomes important by unimolecular dissociation of C.CC.
Reaction Paths – Cyclobutane - y(cccc) Small C4 system : 3 kcal mol-1 barrier to beta scission is low Unimolecular Dissociation Chemical Activation
Reaction Products – Cyclobutane – y(cccc) 1 atm Unimolecular dissociation to two ethylene moieties is the most important channel under both temperatures. At 500 K → Oxidation to two formaldehyde plus ethylene is next most important At 1200 K →Intramolecular H transfer to form stable butene is most important Formation of oxitane (cy- CCCO) → Some importance at 500K → Negligible at 1200K.
Reaction Paths – Cyclopentane – y(ccccc) Unimolecular Dissociation Chemical Activation
Reaction Products – Cyclopentane – y(ccccc) 1 atm (*) At 500K pentene and y(cccco) are major product and overlap At 1200K, pentene is mayor product and ch2o, y(cccco), y(ccc) and ch2ch2 are all similar At 500 K → Formation of pentene and cyclopropane are the main reaction paths. → Formation of two CH2O plus ethylene and singlet diradical1CH2 are also important At 1200 K →Intramolecular H transfer - Formation of pentene is the dominant reaction path
Reaction Paths – Oxirane Cyclic ether – y(cco) Unimolecular Dissociation Chemical Activation
Reaction Products – OxiraneCyclic ether – y(cco) 1 atm Formation of CH2O and HCO2. are dominant at both temperatures At 500 K → Ring closure resulting on y(coo) has some importance At 1200 K→ Formation of a formaldehyde and the singlet diradical1CH2
Reaction Paths – Oxetane Cyclic ether – y(ccco) Unimolecular Dissociation Chemical Activation
Reaction Products – Oxetane Cyclic ether – y(ccco) 1 atm Formation of a formaldehyde plus ethylene → most important at both temperatures At 500 K → Ring closure of stabilized intermediate o.cco. has importance At 1200 K → Formation of coc=c has importance
Reaction Paths – Cyclic Ethers – y(cccco) It can β-scission to form two different diradicals
Reaction Paths – Cyclic Ethers – y(cccco) 1 Unimolecular Dissociation Chemical Activation
Reaction channels – Cyclic Ethers – y(cccco) 1 (*) At 500K ch2o and y(cccoo) are dominant and overlap At 1200K ch2o and y(ccc)are dominant and overlap At 500 K → Formation of formaldehyde and ring closure to form y(cccoo) most important At 1200K → Formation of two formaldehyde plus cyclopropane becomes the dominant path
Reaction Paths – Cyclic Ethers – y(cccco) 2 Unimolecular Dissociation Chemical Activation
Reaction channels – Cyclic Ethers – y(cccco) 2 (*) At 500K ch2o and y(cocco) are dominant and overlap At 1200K ch2ch2 and y(cco) are dominant and overlap At 500 K→ Formation of formaldehyde and ring closure to form y(cocco) most important At 1200 K → Ring closure to form formaldehyde plus the three memebered cyclic ether becomes the dominant reaction path.
JP10 – C10H16 - Tri-cyclodecane (TCD) • Main component of the synthetic fuel JP10, widely used in aircraft • Unimolecular decomposition of TCD is initiated by: • Breaking of a C-H bond • Opening of one of the rings, which forms a diradical • - If the diradical is formed, this will: • Further dissociate (β-scission and intramolecular H transfer) • Chemical activation reactions with molecular oxygen
Reaction Paths – JP10 – C10H16 - Tri-cyclodecane Unimolecular Dissociation
Reaction Paths – JP10 – C10H16 - Tri-cyclodecane Chemical Activation
Reaction Products – JP10 – C10H16 - Tri-cyclodecane 1 atm Both T → Formation of YC5YC5E is the main reaction path → Formation of butadioene (C=CC=C) has some importance → Formation of 1,4 pentadiene (C=CCC=C) some importance Lower T → Formation of YC5.PN=O. important reaction path → Formation of YC5=OYC5OH some importance
Conclusions • Reformation of cycle → fast function of Ring-Opening → Further reactions • Most ring opening occurs at high temperature → β-scission • β-scission and intramolecular H transfer reactions with low barriers exist → these dominant • C.CCC.→ 2 C2H4 Ea = 3.0 kcal mol-1 • O.CCCCO.→ O=CCCCOH Ea = 2.3 kcal mol-1 • Where β-scission and intramolecular H transfer reactions are typical (Ea ~ 14-20 kcal mol-1) → Reactions with O2 become important at low T • Ring closure from chemical activation intermediate species
Future Work • Further study of intramolecular H transfers for diradicals • Development of unimolecular and chemical activation kinetics for TCD • ACKNOWLEDGMENTS • Naval Office of Research • Basque Government