Density functional theory study on the effect of Cα-OH functional group modification on the homolytic cracking reaction routes during the pyrolysis of lignin dimer
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摘要: 采用密度泛函理论方法，对四种β-O-4型二聚体木质素模型化合物2-（2-甲氧基苯氧基）-1-苯基乙烷-1-醇、2-（2-甲氧基苯氧基）-1-苯基乙烷-1-酮、1-甲氧基-2-（2-甲氧基-2-苯基乙氧基）苯、2-（2-甲氧基苯氧基）-1-苯乙基乙酸酯的Caromatic-O、Caromatic-Cα、Cα-Cβ、Cβ-O键均裂解离能进行了理论计算，并对所述二聚体的热解均裂历程进行了理论计算研究，分析了不同二聚体的热解产物形成途径。结果表明，Cβ-O键均裂是二聚体初次热解的主要反应，Cα-Cβ键均裂是竞争反应。Cα-OH官能团被氧化、乙酰化修饰后，Cβ-O键均裂解离能降低，而Cα-Cβ键的键解离能升高，Cβ-O键裂解概率增大，Cα-Cβ键均裂竞争性降低。基于上述四种模型化合物热解的主要芳香族产物有苯甲醇、甲苯、苯甲醛和愈创木酚等，Cα-OH官能团的选择性修饰可调控热解产物种类，其中，氧化修饰后的二聚体的热解产物种类变少，产物选择性增强；甲基化、乙酰化修饰后的二聚体热解可产生苯乙烷和甲苯。Abstract: Density functional theory method was used to calculate the bond dissociation energies of the Caromatic-Cα, Cα-Cβ, Cβ-O bond, and Caromatic-O bonds in four lignin dimer model compounds, viz., (2-(2-methoxyphenoxy)-1-phenylethan-1-ol, 2-(2-methoxyphenoxy)-1-phenylethan-1-one, 1-methoxy-2-(2-methoxy-2-phenylethoxy)benzene, and 2-(2-methoxyphenoxy)-1-phenylethyl acetate; the homolytic cracking reaction during pyrolysis of these dimers was then invetigated and the formation pathways of pyrolysis products of different dimers were analyzed. The results show that the homogenization of Cβ-O bond is the main reaction in the initial pyrolysis of dimer, whereas the homolysis of Cα-Cβ bond is a competitive reaction. After the oxidation and acetylation of Cα-OH, the bond dissociation energy of Cβ-O bond decreases, whereas the dissociation energy of Cα-Cβ bond increases, ccompanied with an increase in the probability of the Cβ-O bond dissociation and a decrease in the competitive ability of Cα-Cβ bond homolysis. For the pyrolysis of four model compounds, the main aromatic products include benzyl alcohol, toluene, benzaldehyde, guaiacol, etc. The selective modification of the Cα-OH functional groups can regulate the types of pyrolysis products. In particular, the product types for the pyrolysis of model compounds modified by oxidation become less, accompanied with an increase in the selectivity to ceratin products. Ethylbenzene and toluene can be produced from the hydrolysis of dimers modified by methylation and acetylation.
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Key words:
- lignin /
- pyrolysis /
- homolysis /
- Cα-OH functional group /
- β-O-4 bonding /
- density functional theory
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图 2 木质素模型化合物(BG1、BG2、BG3、BG4)的几何优化构型(unit: nm)
Figure 2 Optimized geometries of lignin model compounds (BG1, BG2, BG3, and BG4; the unit for bond length is nm)
图 3 木质素二聚体热解过程中Cα-Cβ键均裂后续反应路径示意图
Figure 3 Proposed reaction pathways after the homolytic cleavage of Cα-Cβ bond
图 4 木质素二聚体热解过程中Cβ-O键均裂后续反应路径示意图
Figure 4 Proposed subsequent reaction pathways after the homolytic cleavage of Cβ-O bond
图 5 木质素二聚体热解过程中Cβ-O键、Cα-Cβ键均裂后续反应路径的势能剖面图
Figure 5 Potential energy profiles along subsequent reaction pathways after the homolytic cleavage of Cβ-O and Cα-Cβ bonds
图 6 Cα-Cβ键均裂的后续反应路径中产物、中间体和过渡态的优化几何构型(单位: nm)
Figure 6 Optimized structure of the products, intermediates and transition states in the subsequent reaction pathways after the homolytic cleavage of Cα-Cβ bond (the unit of bond lenthes is nm)
图 7 Cβ-O键均裂的后续反应路径中产物、中间体和过渡态的优化几何构型(单位: nm)
Figure 7 Optimized structure of the products, intermediates and transition states in the subsequent reaction pathways after the homolytic cleavage of Cβ-O bond (the unit of bond lenthes is nm)
表 1 四种β-O-4型木质素二聚体各主要键的解离能和键长
Table 1 Bond dissociation energies and bond lengths of four β-O-4 linkage lignin model compounds
Bond type Dissociation energy E/(kJ·mol-1) Bond length d/nm BG1 BG2 BG3 BG4 BG1 BG2 BG3 BG4 Caromatic-Cα 411.4 401.5 402.6 402.1 0.15141 0.15004 0.15167 0.15135 Cα-Cβ 302.7 333.2 302.6 323.6 0.15213 0.15253 0.15263 0.15304 Cβ-O 270.7 234.6 271.2 254.2 0.14214 0.14054 0.14294 0.14274 Caromatic-O 417.5 426.7 416.3 403.3 0.13668 0.13740 0.13673 0.13689 
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