Abstract
In order to understand the pyrolysis mechanism of β-O-4 type lignin dimers, a pyrolysis model is proposed which considers the effects of functional groups (hydroxyl, hydroxymethyl and methoxyl) on the alkyl side chain and aromatic ring. Furthermore, five specific β-O-4 type lignin dimer model compounds are selected to investigate their integrated pyrolysis mechanism by density functional theory (DFT) methods, to further understand and verify the proposed pyrolysis model. The results indicate that a total of 11 pyrolysis mechanisms, including both concerted mechanisms and homolytic mechanisms, might occur for the initial pyrolysis of the β-O-4 type lignin dimers. Concerted mechanisms are predominant as compared with homolytic mechanisms throughout unimolecular decomposition pathways. The competitiveness of the eleven pyrolysis mechanisms are revealed via different model compounds, and the proposed pyrolysis model is ranked in full consideration of functional groups effects. The proposed pyrolysis model can provide a theoretical basis to predict the reaction pathways and products during the pyrolysis process of β-O-4 type lignin dimers.
Highlights
Lignin is the most abundant renewable aromatic biopolymer in nature, biosynthesized through polymerization of three basic monolignols, i.e., p-coumaryl, coniferyl, and sinapyl alcohols which are interlinked by C–O bonds (e.g., β-O-4, α-O-4 and 4-O-5) and C–C bonds (e.g., β-1, β-5 and 5-5) [1,2,3,4]
Based on the possible pyrolysis reactions and previous studies [13,23,29], a pyrolysis model for β-O-4 type lignin dimers (LD) is proposed as shown in Figure 1, which considers the effects of hydroxyl, hydroxymethyl, and methoxyl groups on pyrolysis pathways
A pyrolysis model containing 11 initial pyrolysis mechanisms for β-O-4 type lignin dimers is proposed which comprehensively considers the effects of functional groups on the alkyl side chain and aromatic ring
Summary
Lignin is the most abundant renewable aromatic biopolymer in nature, biosynthesized through polymerization of three basic monolignols, i.e., p-coumaryl, coniferyl, and sinapyl alcohols which are interlinked by C–O bonds (e.g., β-O-4, α-O-4 and 4-O-5) and C–C bonds (e.g., β-1, β-5 and 5-5) [1,2,3,4]. Apart from different linkages, there are various functional groups on the alkyl side chain and aromatic ring of the lignin basic units, including methoxyl, hydroxyl, hydroxymethyl, carbonyl and more [5,6,7]. Pyrolysis is a promising thermo-chemical conversion method for the utilization of lignocellulosic biomass materials [8,9,10], through which lignin can be decomposed to form various value-added aromatic compounds [9,11,12]. Conventional pyrolysis is a non-selective process, and it has difficulty selectively controlling the lignin decomposition process towards specific aromatic compounds. Density functional theory (DFT) method is an effective tool to reveal the lignin pyrolysis mechanism at a micro level [13,14,15,16]
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