Theoretical Study On Atmospheric Oxidation Mechanism Of Furan Compounds

Furan and its derivatives such as furfural and alkyl furan compounds are heterocyclic aromatic hydrocarbon compounds.They are widely used and can be used as intermediates for synthesis of alkane fuel.However,furan compounds will inevitably be released into the atmosphere once they are used in large scale.Oxidation of these compounds in the atmosphere may lead to ozone and secondary organic aerosol formation.Therefore it is necessary to study their fates in the atmosphere and their impact on air quality in advance.There have been a few experimental studies on the fates of furan compounds in the atmosphere.However,it is still considerably difficult to obtain their oxidation mechanism in the atmosphere due to the experimental difficulty,particularly in identifying the reactive intermediate radicals.Alternatively,theoretical calculation is a powerful method in studying the kinetics of intermediate radicals.Combined experimental and theoretical studies are highly desirable in elucidating the oxidation mechanism of volatile organic compounds in the atmosphere.In this paper,we use quantum chemistry and kinetics calculations to study the hydroxyl radical-initiated oxidation mechanism of furan compounds in the atmosphere.We have used the density functional method M06-2X to optimize the structures of the reactants,transition state and products,and then used the ROCBS-QB3 method for single-point electronic energy calculation.Based on the quantum chemistry potential energy surfaces,we carried out kinetic calculations for the rate coefficients of reactions using traditional transition state theory and unimolecular rate theory coupled with master equation(RRKM-ME)for collisional energy transfer.Reaction products and their branching ratios under various conditions were predicted based on the kinetic calculations.The main results of our study are as follows:(1)Oxidation of Furfural Initiated by OH RadicalThe reaction between furfural and OH radical is initiated mainly by OH additions to C2 and C5 positions,forming R2 and R5 adducts,which are followed by rapid ring-breakage to R2 B and R5 B,respectively.All these radicals react rather slowly with O2 under the atmospheric conditions because the additions of O2 to these radicals are only slightly exothermic and therefore highly reversible.Alternatively,these radicals might react directly with the atmospheric trace species such as O3,NO2,and HO2/RO2 etc.Under typical atmospheric conditions,the main products expected include 2-oxo-3-pentene-1,5-dialdehyde(C5H4O3),5-hydroxy-2(5H)-furanone(C4H4O3),4-oxo-2-butenoic acid(C4H4O3),and 2,5-furandione(C4H2O3).These compounds are likely to remain in the gas phase and are prone to further photooxidation.(2)Oxidation of FuranReaction of furan and OH radical starts with the addition of OH radical to C2 position of furan,generating high-energy exited state adduct R2*,which will isomerize,rapidly to energetic R2-iso*.At 298 K,the OH radical addition to C3 location is negligible.O2 would add to deactivated R2-iso radicals to form peroxy radicals,which would eliminate HO2 at the rate of about 105 s-1 to produce a mixture of cis-structure and trans structures of 1,4-butenedial.We find the production yield of butenedial is 80.4% at 298 K and 760 Torr,which is independent to the concentration of NO.The yield increases with the decreased pressure and increased temperature.About 19.6% of deactivated R2 radical would react with O2 almost irreversibly,forming the peroxy radical,R2-5OO-s.This radical would react with NO to form alkoxy radical R2-5O-s.Finally,R2-5O-s would react with O2 to form 2-hydroxy-5-furanone and HO2.In general,furan reacts with hydroxyl radical at 760 Torr and 298 K to produce 1,4-butenediolial(about 80.4%),2-hydroxy-5-furanone(about 19.6% × 90%)and nitric acid(about 19.6% × 10%).(3)Oxidation of of Alkyl FuransThe reactions between alkyl furans and hydroxyl radical are similar to the oxidation of furan.Taking 2-MF as an example,the reaction stars with the addition of OH to the C2 and C5 positions of the furan ring,forming energetic adduct R2*and R5*.RRKM-ME calculations show that the R2* and R5* would undergo ring-opening isomerization rapidly,followed by reaction with O2,forming the mixture cis-structure and trans-structure of 4-oxo-2-pentenal(~37.1%).The ground state radials R2 and R5 would combine with oxygen in the atmosphere,and the peroxy radicals formed would react with NO/HO2-/-RO2 to form R2-5O-s(~18.6% × 90%)and R5-2O-s(~44.3% × 90%),of which R2-5O-s would lead to the formation of 2-hydroxy-2-methyl-5-furanone and HO2 and R5-2O-s to 2-hydroxy-5-furanone and methyl.The atmospheric oxidation mechanisms of 3-MF,2,3-DMF and 2,5-DMF are similar to 2-MF with some differences in the capability in opening the ring in the initial adducts due to the differences in the methyl substitution positions.Generally,the fraction of ring-opening in the initial adducts decreases with increased methyl substitution.The subsequent reaction paths after ring opening are similar to those in 2-MF,forming eventually the unsaturated carbonyl compounds.