Density functional theory study on the effect of C_α-OH functional group modification on the homolytic cracking reaction routes during the pyrolysis of lignin dimer

Density functional theory method was used to calculate the bond dissociation energies of the C_aromatic-C_α, C_α-C_β, C_β-O bond, and C_aromatic-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 C_β-O bond is the main reaction in the initial pyrolysis of dimer, whereas the homolysis of C_α-C_β bond is a competitive reaction. After the oxidation and acetylation of C_α-OH, the bond dissociation energy of C_β-O bond decreases, whereas the dissociation energy of C_α-C_β bond increases, ccompanied with an increase in the probability of the C_β-O bond dissociation and a decrease in the competitive ability of C_α-C_β 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 C_α-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.