| Biomass with renewable capabilities has great potential for growth under the historical background of the "carbon neutral".Products such as bio-oil that prepared using low-temperature thermal conversion of biomass is considered as one of the significant utilization of biomass energy.By adding moderate CaO in the biomass pyrolysis process,the volatile reforming process and the secondary reaction can be controlled and promoted,respectively.In this way,excess oxygen in the bio-oil will be removed,achieving the purpose of improving the quality of bio-oil and so on.Therefore,it is essential to explore the pyrolysis mechanism in order to make better use of biomass energy and regulate product quality.Over the past 100 years of pyrolysis reaction research,researchers have confirmed that the low-temperature pyrolysis reaction of biomass is dominated by free radical reactions.Since free radical reactions follow the chain reaction mechanism,the number of which is usually calculated by tens of thousands,so the free radical reaction system is quite complicated.As a result,reaction lump is truly an optimal perspective to explore the common laws of pyrolysis basic reactions.For starters,reaction lump is regarded as the lumped reaction categories.From the elementary reaction process,the radical reaction involves only the basic steps of induction and addition.Secondly,it is the aggregation of reaction components.As for biomass,the possible lumped way is chemical bonds,owing to the angle of chemical bonds,the pyrolysis reaction involves only nine kinds of covalent bonds,namely,Car=Car,Car-Car,Cal-Cal,Car-Cal,Car-O,Car-H,Cal-O,Cal-H and O-H,while S and N elements that have lower abundance are excluded.In this way,the infinite reaction situations are classified into several limited situations,which is convenient for discussion,and more importantly,it also simplifies the pyrolysis mechanism of complex systems.To sum up,the reaction lump is an innovation point of this research.In this article,it focuses on the theme of" A DFT Study on the Effect of CaO on the Covalent Bonds Breaking of Biomass Pyrolysis Process".Starting from the angle of reaction lump,it constructs a molecular model of reactants and transient state for the induction and addition reaction of free radicals and covalent bonds.In addition,the energy of these molecules is obtained through the optimization of quantitative program and frequency calculation.Finally,the general law of the reaction between free radicals and covalent bonds with or without the action of CaO is acquired.The results of studies on free radical-induced reactions show that:(1)The steric hindrance remains master influence on the energy barrier of free radical-induced reactions,while the influence of covalent bonds is relatively minor.(2)With no presence of the·OH and·H identical group-induced exchange reaction,the induced energy barrier of·OH is around 40 kJ/mol higher than that of·H,and the induced energy barrier of·CH3 is around 50 and 90 kJ/mol higher than that of·OH and·H,respectively.(3)With the presence of the·OH and·H identical group-induced exchange reaction,an increase in the energy barrier of about 70 kJ/mol will be caused.The results of the study on the addition reaction of free radicals show that:(1)The energy barrier of the Car=Car bond added by different radicals is sequenced as follows:·CH3>·H>·OH.(2)Substituent group(category,position)and addition position have little effect on the added energy barrier.(3)On average,the energy barriers of ·CH3,·OH and·H added Car=Car bond are certain values,which are about 130,80 and 90 kJ/mol,respectively.(4)The difficult degree of reactions of different radicals added to olefins is as follows:·CH3>·OH>·H,where the addition reaction of the·H and the Car=Car bond is more superior in the aspect of thermodynamics.The results of comparative studies on induction and addition reactions between covalent bonds and free radicals with or without CaO show that:(1)The addition of CaO can markedly increase the energy barrier of covalent bond cleavage reaction induced by free radical,but will not change the strong and weak ability order of the same covalent bond fracture induced by different free radicals.Nor does it change the difficulty and ease order of the different covalent bonds to be broken induced by the same free radical.(2)To systems containing benzene ring π electrons or systems with Car=Car bond affected by benzene ring electrons,CaO basically has no effect on the reaction energy barrier of free radical addition of Car=Car bond.On average,there was no significant difference in the reaction energy barriers of free radical addition with or without CaO.The energy barriers of·CH3,·OH and·H addition Car=Car bond are about 150 kJ/mol,80 kJ/mol and 100 kJ/mol respectively.(3)CaO can facilitate the reaction of radicals adding Car=Car bond and inhibit the reaction of radicals inducing C-O bond,but have little effect on the reaction of radicals inducing C-H and O-H bond. |
