Theoretical Investigation On The Degradation Of Typical Phenols Of Lignin Pyrolysis Products Initiated By Ozone

Biomass combustion or the use of biomass fuels has brought opportunities to the development of the new resources of energy,but the resulting environmental issues cannot be ignored.As one of the essential elements of natural wood,the thermal decomposition of biomass lignin can generate high concentrations of phenolic molecules.In general,these aromatic compounds with unsaturated>C=C<are easily oxidized by oxidants in the air,such as OH radicals,NO3 radicals,Cl atoms and ozone.These compounds undergo complex atmospheric chemical processes,leading to the formation of haze and secondary organic aerosols.Furthermore,as known to all,ozone is a significant reactive oxidant in the troposphere,which plays an important role in the removal of air pollutants.Lignin-pyrolysis products enter the gaseous atmosphere with wood smoke,and some water-soluble organic products keep in the the aqueous phase,such as fogs,clouds.During condensation,the semi-volatile organic products are largely retained in the particle phase produced by biomass combustion.Considering the natural processes such as atmospheric deposition and water circulation,these compounds could be harmful to the water environment.To date,most studies have focused on the rate constants using experimental methods in this area.Due to the limitations of the experimental methods,it is difficult to have a deep understanding of the oxidative degradation process of lignin-pyrolysis products.Therefore,six lignin pyrolysis products with simpler structure such as methoxybenzene(MB),catechol(1,2-DB),3-methylcatechol(3-MC),4-methylcatechol(4-MC),syringol(2,6-DMP),and 4-methylguaiacol(4-MG),was selected to investigated the structure information,atmospheric degradation process,kinetic properties and the aquatic toxicity in this paper.Some important conclusions are drawn as follows.(1)The gas-phase ozonolysis of MBThe detailed reaction mechanisms and kinetics of the ozonolysis of MB were investigated using DFT methods[M06-2X/aug-cc-pVDZ//M06-2X/6-31+g(d,p)]and Rice-Ramsperger-Kassel-Marcus(RRKM)theory.The results show that O3-addition to the methoxy-substituted carbon dominates the whole reaction process of MB with O3.At 298 K and 1 atm,the total rate constant of the ozonolysis of MB is calculated to be 2.67 x 10-21 cm3 molecule-1 s-1.Within the temperature and pressure range studied,the total rate constants and six initial reaction rate constants show positive dependence on temperature and negative dependence on pressure.In addition,the atmospheric lifetime of MB with respect to O3 is 16.41 years.Consequently,ozonolysis of MB is not important degradative pathway.In other words,MB is difficult to degrade via ozonolysis in the gas phase.(2)O3-initiated oxidation mechanisms and kinetics of 1,2-DBWe examined the ozone-initiated degradation of 1,2-DB,a lignin pyrolysis intermediate,using theoretical methods at the M06-2X/aug-cc-pVDZ//M06-2X/6-31+G(d,p)level.Six initial ozonolysis pathways of 1,2-DB and the futher reactions of Criegee intermediates(CIs)are considered.The detailed reaction processes of CIs in the presence of NO and H2O are also elucidated.The predicted products in our study such as malealdehyde,oxalic acid and carbon dioxide were also detected in the experiment.Moreover,kinetics of ozone-initiated degradation of 1,2-DB were calculated by the RRKM theory.Under atmospheric conditions,the total rate constant of the ozonolysis of 1,2-DB is 1.37 × 10-18 cm3 molecule-1 s-1,which is in good agreement with experimental data.The bimolecular rate constants reveal positive dependence on temperature and negative dependence on pressure.The atmospheric lifetime of 1,2-DB determined by O3 was estimated to be 12.07 days.The O3-initiated degradation of 1,2-DB is more likely to occur in aqueous phase.This work provides a comprehensive consideration of the ozonolysis of 1,2-DB.(3)Ozonolysis of 3-MC and 4-MC in the gas and aquase phase3-MC and 4-MC were selected as a representative congenator of lignin pyrolysis product to study its environmenatal fate both in the gas and aqueous phase.This work aimed to summarize the ozonolysis rules of monomeric phenolic compounds.The contribution of such previously unconsidered reactions to the SOA formation from methylcatechols was evaluated.We provided a guideline for the optimization of a body of water of such pollutants.Quantum chemical calculations was utilized in the investigation of the O3-initiated oxidation of methylcatechols,including six initial channels and the further reaction pathways of the CIs with nitric oxide(only in the atmop gas phase),water and oxygen.Using kinetic simulation,the calculated total rate constant(at 298 K and 1 atm)of ozonolysis of 3-MC and 4-MC are 7.89 × 10-17 and 1.11 × 10-17 cm3 molecule-1 s-1,respectively.For the methylcatechols involved in this thesis,atmospheric lifetime comparison shows that the reactivity of these compounds takes the order 3-methylcatechol>4-methylcatechol>catechol>methoxybenzene.The aquatic toxicity assessment shows majority of the toxic copmpunds are converted into harmless small molecule compounds after ozonolysis reactions.(4)Heterogeneous ozonolysis of 2,6-DMP and 4-MG on grapheneWe studied the heterogeneous ozonolysis mechanism of 2,6-DMP and 4-MG onto graphene(GP)sheets using density functional theory(DFT)methods.The heterogeneous degradation of 2,6-DMP and 4-MG initiated by ozone is likely to occur in the high humidity atmospheric environment.Moreover,Criegee intermediates generated from the primary ozonolysis steps will undergo subsequent reactions in the presence of NO and H2O.The total rate constants for the ozonolysis of 2,6-DMP are 4.62 × 105 and 4.77 × 107 M-1 s-1 in the aqueous atmosphere and aerosol particles,respectively(298 K,1 atm).The total rate constants for the ozonolysis of absorbed 4-MG isl.14 × 105 M-1 s-1.Based on toxicity assessments,most transformation products are aquatic organisms-friendly.