Experimental and theoretical study of the catalytic effects of potassium and calcium additives on the pyrolysis of β-O-4 type lignin model compounds

In this work, to investigate the catalytic effects of potassium and calcium additives on lignin pyrolysis, two β-O-4 type model compounds for lignin containing different oxygen-containing functional groups (a carbonyl group or a hydroxyl group) on Cα were prepared, and their catalytic fast pyrolysis experiments were carried out on Py-GC/MS. Furthermore, the possible catalytic mechanism of potassium and calcium additives is further investigated using density functional theory (DFT) calculations. The experimental results show that the addition of potassium acetate and calcium acetate is beneficial to the formation of phenol, while the promotion effect of potassium addition is stronger than that of calcium addition. Moreover, considering the contributions of temperature and activation entropy ΔS‡, the activation Gibbs free energy ΔG‡ is required as the basis to explain the pyrolysis mechanism of lignin. It is determined that the Cβ-O homolytic reaction performs the most favorable in the initial depolymerization of both dimers. By comparing the ΔG‡, it is confirmed that the addition of K+ and Ca2+ are beneficial to the Cβ-O homolytic reaction and further promote the conversion of the intermediate to phenol. Furthermore, the different effects of K+ and Ca2+ additives are compared on the side chains cleavage through homolytic reaction and concerted decomposition. The calculation results show that K+ is more conducive to the simple homolytic reaction, while Ca2+ is more conducive to the concerted decomposition.