Influence of inherent alkali metal chlorides on pyrolysis mechanism of a lignin model dimer based on DFT study

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 C_β−O bond length and shorten the C_α–C_β 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 C_β–O homolytic reaction and concerted decomposition reaction, while restrain the C_α–C_β homolytic reaction. Therefore, the lignin dimer model compound decomposes mainly through the concerted decomposition and C_β–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.