Hong Tong

Theoretical studies on pyrolysis mechanism of guaiacol as lignin model compound

The pyrolysis processes of guaiacol as lignin model compound were theoretically investigated by using density functional theory methods at B3LYP/6-31G++(d,p) level. The bond dissociation enthalpies (BDE) of the major bonds of guaiacol were calculated. Three possible pyrolytic pathways were proposed according to related experimental results, calculation results of BDE of guaiacol and research experiences. The standard kinetic parameters in each reaction pathway were calculated. In reaction pathway (1), the initial reaction step of guaiacol pyrolysis is the homolytic cleavage of the CH3-O bond and 1,2-dihydroxybenzene is the main pyrolysis product, of which the total energy barrier of formation is 312.9 kJ/mol. In reaction pathway (2), the initial reaction step of guaiacol pyrolysis is the homolytic cleavage of the O-H bond and the main pyrolysis products are o-cresol, 2-hydroxybenzaldehyde, o-quinonemethide, of which the total energy barrier of formation is 474.1 kJ/mol. In reaction pathway (3), addition of H to the carbon atom of benzene ring can effectively lower the activation energy of demethoxy reaction of guaiacol. Based on kinetic analysis results, we can infer that the main pyrolysis products are 1,2-dihydroxybenzene, methane, phenol, o-cresol, 2-hydroxybenzaldehyde, and coke.

Studies on pyrolysis mechanism of syringol as lignin model compound by quantum chemistry

Abstract The pyrolysis of syringol as lignin model compound was investigated using density functional theory methods at B3LYP/6-31G++ (d, p) level. Three possible pyrolytic pathways were proposed and the equilibrium geometries of the reactants, transition states, intermediate and products were fully optimized. The standard kinetic parameters in each reaction pathway were calculated and the formation and evolution mechanism of main pyrolysis products were analyzed. Bond dissociation energies calculation results show that the bond dissociation energy of CH 3 -O of syringol is the lowest and the order of all kinds of bond dissociation energy is CH 3 -O 3 O-C aromatic 2 -H aromatic aromatic -H. In reaction pathway (1) and (2), the main pyrolysis product is 3-methoxycatechol and 2-methoxy-6-methylphenol, respectively. The total energy barrier is 366.6 and 474.8 kJ/mol in pathway (1) and (2), respectively. For reaction pathway (3), the total energy barrier of o-methoxyphenol formation is as low as 21.4 kJ/mol, which shows that addition of hydrogen to the carbon atom connected with methoxyl can effectively lower the reaction energy barrier of demethoxy reaction of lignin model syringol.

Theoretical study of bond dissociation energies for lignin model compounds

Abstract The bond dissociation energies (EB) of C–O and C–C bond in 63 lignin model compounds for six prevalent linkages (β–O–4, α–O–4, 4–O–5, β–1, α–1 and 5–5) were theoretically calculated by using density functional theory methods B3P86 at 6–31 G (d,p) level. The effect of various substituents on EB and the correlation between the bond lengths and the corresponding EB were analyzed. The calculation results show that C–O bond is generally weaker than C–C bond, and the average bond dissociation energy of Cα–O (182.7 kJ/mol) is the lowest, and that of Cβ–O is second lowest. The substituent group on both the aromatic and alkyl groups can substantially weaken C–O bonds, and C–O bonds do not exhibit a strong correlation between C–O bond lengths and BDE. Compared with C–O bonds, EB of C–C bonds are little affected by the substituent on the aromatic groups, but affected obviously by the substituent on alkyl groups. There is a strong linear relationship between C–C bond lengths and BDE. The EB are weak when the C–C bond lengths are long.

Theoretical studies on pyrolysis mechanism of O-acetyl-xylopyranose

Abstract In order to understand the pyrolysis mechanism of hemicellulose and to identify the formation pathways of key products during pyrolysis, the pyrolysis processes of O-acetyl-xylopyranose are investigated by using density functional theory methods at B3LYP/6–31 G ++ (d, p) level. In the pyrolysis, O-acetyl-xylopyranose firstly decomposes to form acetic acid and IM1 with an energy barrier of 269.4 kJ/mol, and then IM 1 is converted to acyclic carbonyl isomer IM2 with a low energy barrier of 181.8 kJ/mol. IM2 further decomposes to form all sorts of small molecules through four possible pyrolytic reaction pathways. The equilibrium geometries of the reactants, transition states, intermediate and products were fully optimized, and the standard thermodynamic and kinetic parameters of every reaction pathway were calculated. The calculation results show that reaction pathways (2) and (4) are the major reaction channels in pyrolysis of O-acetyl-xylopyranose and the major products are low molecular products such as acetic acid, acetaldehyde, glycolaldehyde, acetone, CO, CO2 and CH4, which is according with related analysis of experimental results.

Molecular dynamic simulation study on pyrolytic behaviour of xylan

Xylan is the most relevant component in hemicellulose, and in order to understand the mechanism of thermal decomposition of hemicellulose, the pyrolysis process of xylan as a hemicellulose model compound has been simulated by molecular dynamics method. The simulation results show that the pyrolysis process of xylan can mostly be divided into three stages: low, intermediate and high temperature pyrolysis stage. The hydroxyl bonds begin to break down when temperature rises to about 400 K. The glycosidic bonds on side groups begin to break down at about 550 K, and those on main chain begin to break down at about 600 K. Thus, the whole molecule depolymerises and all kinds of fragments are formed. Based on the related experimental results in references, the possible formation pathways of major products have been analysed.

Theoretical studies on thermal degradation reaction mechanism of model compound of bisphenol A polycarbonate

Density functional theory methods (DFT) M062X have been used to investigate the thermal degradation processes of model compound of bisphenol A polycarbonate (MPC) and to identify the optimal reaction paths in the thermal decomposition of bisphenol A polycarbonate (PC). The bond dissociation energies of main bonds in MPC were calculated, and it is found that the weakest bond in MPC is the single bond between the methylic carbon and carbon atom and the second weakest bond in MPC is the single bond between oxygen atom and the carbonyl carbon. On the basis of computational results of kinetic parameters, a mechanism is proposed where the hydrolysis (or alcoholysis) reaction is the main degradation pathways for the formation of the evolved products, and the homolytic cleavage and rearrangement reactions are the competitive reaction pathways in the thermal degradation of PC. The proposed mechanism is consistent with experimental observations of CO2, bisphenol A and 1,1-bis(4-hydroxyphenyl)-ethane as the main degradation products, together with a small amount of CO, alkyl phenol and diphenyl carbonate.

Density functional theory study on bond dissociation enthalpies for lignin dimer model compounds

In order to enhance understanding of the mechanism of lignin pyrolysis, homolytic bond dissociation enthalpies (BDEs) in ten lignin dimer model compounds were calculated by using density functional theory methods at B3LYP/6-31G++(d,p) level and B3P86/6-31G++(d,p) level. The BDE values calculated at B3LYP/6-31G(d,p) level are about 20 kJ/mol lower than that at B3P86/6-31G(d,p) level, but the variation trends of BDE values at two levels are the same and the B3P86 functional was found to yield accurate BDEs. The calculation results show that the order of the BDE values for the β-O-4 type of linkage is as follows: Cβ-O < Cα-Cβ < C4-O < C1-Cα, and the substituents (methoxyl, carbonyl, and hydroxyl) on both the aromatic and alkyl groups in model compounds 1 have an important effect on the BDEs. It is found that the BDEs of Caromatic(1,4,5)-C(or O) in lignin model compounds are higher, but the BDEs of Calkyl(α, β)-Calkyl(α, β)(or O) are lower. Therefore, the initial step in pyrolysis of lignin is the homolytic cleavage of the Calkyl(α, β)-Calkyl(α, β)(or O) bond. The possible formation pathways of major products in the pyrolysis process of model compounds have been analysed.