Theoretical Study Of Excited States And Oxidation Mechanisms Of Radicals And Intermediates From Pyrolysis Of Furan Biofuels

Bioenergy,produced from the biomass,is an important type of renewable energy sources suitable for gasoline or diesel engines.Furan molecular systems as the second-generation biofuels may play a crucial role in reducing the use of fossil fuels,owing to their adavantages of high energy density,high capacity of explosion resistance,and high boilling point.In the combustion process of furan biofuels,a large number of free radicals,intermediates,and even excited species will be produced.However,it is still not clear about the excited-state properties of these reactive species and their influences on the combustion process at present,and the further study is highly required.In addition,the combustion of furan fuels involves the low-temperature oxidation process.It is helpful for the establishment and further improvement of the combustion model of furan biofuels to figure out the reaction mechanism and the reaction kinetics.Here the structural and electronic spectral properties of the main free radicals and intermediates in the pyrolytic process of furan biofuels have been studied by extensive calculations,and the mechanisms and kinetic properties of the low-temperature oxidation process of 2-methylfuran are discussed in detail.The main research contents and results of this dissertation are summarized as follows:(1)Based on the density function theory(DFT)and the sophisticated wave function theory(WFT)calculations,the electronic structures,stabilities and electronic spectra of intermediates and radicals without the furan-ring opening in the pyrolysis of 2,5-dimethylfuran have been investigated.The computational results show that these radicals from the methyl dehydrogenation in 2,5-dimethylfuran and 2-methylfuran are more stable than those from the ring dehydrogenation.Two C4H3O radicals from the furan dehydrogenation have similar stability.For the H-addition of furan,C4H5O-2 is more stable than C4H5O-3.The predicted electronic absorptions show that the lowest excited state of furan,2-methylfuran,and 2,5-dimethylfuran is the Rydberg state(3s),and presence of the methyl on the furan ring can reduce the corresponding excitation energy.Several low-lying valence excited states and Rydberg states of the pyrolytic species appear in the visible light region and they may be involved in combustion of furan biofuels.(2)The low-lying valence excited states and Rydberg states of the radical species from the ring-opening reactions in pyrolysis of furan biofuels have been determined by extensive density functional theory and sophisticated wave function theory calculations.The radicals 1-C4H5O-2,2-furylCH2,and 4-C6H7O with the delocalized?-type single electron are predicted to be most stable among the reactive species here for furan,2-methyfuran,and 2,5-dimethylfuran,respectively.Some among the electronic excitations to low-lying states can take place in the visible light region,and they may be involved in the combustion process.Taking the most stable ring-opening radical 1-C4H5O-2 of furan as an example,farther surface hopping dynamics simulations on the excited states reveal that 89.9%sampling trajectories at the initial excited state of 22A"(?1?*2)decay to the 12A1(n1??2)state within an average of 384 fs,and then 81.2%trajectories at the 12A' state go to the ground state within an average of 114 fs.At the end of the simulation for 1000 fs,18.8%trajectories still stay on the excited states of 22A" and 12A',suggesting that the reactive radicals in the ground state are mainly responsible for the combustion chemistry of furan biofuels,although there is contribution from the excited-state species to some extent.(3)The low-temperature oxidation(LTO)mechanisms of the 2-methylfuran(2-MF)biofuel and corresponding thermodynamical and kinetic properties have been explored by the density functional theory(DFT)and composite G4 methodologies as well as kinetic simulations.The O2 addition to the main furylCH2 radical from the methyl dehydrogenation in 2-MF forms three peroxide radicals PO1,PO2 and PO3,respectively.Through the hydrogen transfer followed by dehydroxylation,these nascent products evolve into stable aldehydes and cyclic ketones,which may further decompose into smaller species under the action of OH.Based on the G4-calibrated thermodynamic parameters,the temperature and pressure dependences of the rate constants and the three-parameter Arrhenius coefficients for all reactions considered here have determined by using the transition state theory(TST)and Rice-Ramsperger-Kassel-Marcus(RRKM)methods.Calculations and simulations show that the product P1 from the dehydroxylation of PO1 has a dominant population(higher than 96%)among the final products,although the temperatures and pressures may influence the species profiles and rate constants to some extent.