α,β-双醚型木质素三聚体模化物热解机理模拟计算

Computational study on pyrolysis mechanism of an α,β-diether-type lignin trimer model compound

为了解木质素三聚体的热解机理,采用密度泛函理论方法,对同时含有β-O-4和α-O-4连接的木质素三聚体模型化合物(4-(1,2-二苯氧基)-丙基苯酚)的热解过程进行了理论计算研究,分析了三聚体中α-O-4和β-O-4醚键断裂的相互影响,以及三聚体的整体热解反应机理和产物形成途径。计算结果表明,Cα-O均裂是三聚体初步热解的最主要反应,Cβ-O和Cα-Cβ均裂则是竞争反应。三聚体的α-O-4和β-O-4醚键在初步热解过程中的相互影响作用很小;但是在后续热解过程中,α-O-4(β-O-4)醚键断裂产物继续发生β-O-4(α-O-4)醚键断裂的解离能却显著降低。基于Cα-O均裂反应的热解产物主要有苯酚、4-丙烯基苯酚、4-烯丙基苯酚和含双键的二聚体(大分子产物的前驱物)等;基于Cβ-O均裂反应的热解产物主要有苯酚、4-丙烯基苯酚和4-丙基苯酚等;基于Cα-Cβ均裂反应的热解产物主要有苯乙醚、4-羟基苯甲醛、苯、4-甲基苯酚和苯酚等。该文的研究结果将为深入了解木质素的热解机理提供理论依据。

In order to understand the pyrolysis mechanism of lignin trimer, 4-（1,2-diphenoxy）-propylphenol was selected as lignin trimer model compound containing both β-O-4 and α-O-4 linkages, and its pyrolysis processes were theoretically investigated by employing density functional theory method at M06-2X level with 6-31＋＋G（d,p） basis set. The equilibrium geometries of the reactant, intermediates, transition states and products in the pyrolysis processes were fully optimized. The activation energy in each pyrolysis pathway was calculated. Analyses were performed to reveal the interaction of the t-O-4 and α-O-4 linkages in their cleavages, as well as the overall pyrolytic mechanisms and the pathways of product formation. The calculation results indicated for the 6 primary homolytic ways of the lignin trimer model compound, and the bond dissociation energies were in the order of Cα-O 〈 Cβ-O 〈 Cα-Cβ 〈 Cβ-Ca 〈 C4,-0 〈 C4,-O, with the lowest energy of Cα-O （250.6 kJ/mol） and the second lowest energy of Cα-O （286.9 kJ/mol）, followed by that of Cα-Cβ （300.5 kJ/mol）. Hence, the homolytic cleavage of Cα-O bond should be the major reaction of primary pyrolysis for the trimer model compound, while the homolytic cleavages of Cα-O and Cα-Cβbonds were competitive pyrolysis reactions. The β-O-4 and a-O-4 linkages had little interaction effects in their primary cleavage reactions, because the bond dissociation energies of Cα-O and Cβ-O in the lignin trimer model compound were similar to those of the corresponding lignin dimer model compounds. However, after the initial cleavage of the a-O-4 （t-O-4） linkage, the formed intermediate had significantly reduced bond dissociation energy for the further cleavage of the t-O-4 （a-O-4） linkage. Based on the energy barrier of each reaction pathway for the primary homolytic cleavage of Cα-O bond, the major pyrolytic products of the model compound included phenol, 4-propenylphenol, 4-allyphenol, and dimer compound with double bonds （the precursor of large molecular products）. Based on the primary homolysis of Cβ-O bond, the major pyrolytic products included phenol, 4-propenylphenol and 4-propylphenol. From the primary homolysis of Cα-Cβ bond, the major pyrolytic products included phenetole, 4-hydroxy benzaldehyde, benzene, p-cresol and phenol. In conclusion, based on the 3 homolytic mechanisms, the pyrolysis of the lignin trimer model compound will produce phenol, 4-propenylphenol, 4-aUyphenol, and dimer compound with double bonds as the major products, as well as 4-propylphenol, phenetole, 4-hydroxy benzaldehyde, benzene andp-cresol as the competitive products. The research results will provide theoretical basis for the deep understanding of the pyrolysis mechanism of lignin.