Influence of inherent alkali metal chlorides on pyrolysis mechanism of a lignin model dimer based on DFT study
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In order to understand the catalytic effects of inherent inorganic elements in biomass on the pyrolysis mechanism of lignin, density functional theory with a Gaussian method of M06-2X and basic set of 6-31 + G(d,p) was employed to simulate the pyrolysis pathways of a β-O-4 type lignin dimer model compound (1-methoxy-2-(4-methoxyphenethoxy)benzene) catalyzed by NaCl and KCl which are major inorganic constituents of biomass at microscale level. The calculation results indicate that cations (Na+ and K+) in alkali metal chlorides are facile to combine with the oxygen-containing functional groups in the lignin dimer model compound. Both cations increase the Cβ−O bond length and shorten the Cα–Cβ bond length, which will further affect their bond dissociation energies. In the initial pyrolysis process of the lignin dimer model compound, NaCl and KCl can promote the Cβ–O homolytic reaction and concerted decomposition reaction, while restrain the Cα–Cβ homolytic reaction. Therefore, the lignin dimer model compound decomposes mainly through the concerted decomposition and Cβ–O homolytic mechanisms under NaCl and KCl catalytic pyrolysis conditions, producing 1-methoxy-4-vinylbenzene, 1-ethyl-4-methoxybenzene, 2-methoxyphenol, catechol and 2-hydroxybenzaldehyde, among which NaCl and KCl have inhibitory effect on 2-hydroxybenzaldehyde, but have promoting effect on the other pyrolytic products.
KeywordsLignin Alkali metal chloride Catalytic pyrolysis mechanism Model dimer Density functional theory (DFT)
The authors thank the National Natural Science Foundation of China (51576064, 51821004), Beijing Nova Program (Z171100001117064), Beijing Natural Science Foundation (3172030), Grants from Fok Ying Tung Education Foundation (161051), and Fundamental Research Funds for the Central Universities (2016YQ05 and 2018ZD08) for financial support.
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