(329b) Depolymerization of Lignin for the Production of Phenolic Monomers

Lignin is an abundant aromatic polymer in the world, and the depolymerization of lignin into phenolic monomers is one of the main methods for its efficient utilization. However, due to its complex structure and low reactivity, achieving directional depolymerization and improving product yield and selectivity is considered to be one of the most important challenges in lignin applications.

In this paper, a metal-supported molybdenum carbide catalyst was developed to depolymerize lignin for the selective production of high value-added phenolic monomers. The characterization results of the catalysts showed that the Mo₂C was successfully synthesized by impregnation coupled with carburizing reaction method. And the introduction of metal could not only promote the formation of β-Mo₂C active catalytic center, but also improved the specific surface area and structure of the catalysts. Under the optimized conditions (260 ℃, 4 h, 20% methanol addition), the Ni-Fe supported molybdenum carbide catalyst can depolymerize lignin to obtain 89.56% liquid yield and 35.53% phenolic monomers, and among them the yield of 4-ethylphenol reached 14.77%. The results indicated that the efficient and selective depolymerization of lignin under mild conditions were realized. The depolymerization mechanism was studied by techniques such as NMR and HPLC-MS. The results showed that during the depolymerization of lignin, unstable intermediates were formed first. Afterwards methanol and active hydrogen would react with these intermediates to convert them into liquid products, thereby inhibiting the formation of coke. At the same time, Ni and Fe metals in the catalyst can inhibit the conversion of 4-ethylphenol to phenol by controlling the hydrogenolysis reaction of the lignin side chain, thus achieving the purpose of selectively production of phenolic monomers.

Finally, the depolymerization mechanism of lignin was studied by the density functional calculation (DFT). The results showed that the bimetallic NiFe(111)-Mo₂C(001) catalyst had better adsorption effect for the model compound than Mo₂C(001), and can reduce the bond energy by stretching the bond length of β-O-4, then promote the depolymerization reaction. At the same time, the catalysts had a lower adsorption energy for the partially hydrodeoxygenation products, which was beneficial for their rapid desorption from the catalysts surface, and then removed from the reaction. On the other hand, the catalysts had strong adsorption to the products containing C=O, which can facilitate the formation of target products by hydrodeoxygenation reaction. The study of the β-O-4 cleavage mechanism showed that the reaction energy barrier for H-adduct at the β position first followed by the β-O-4 cleavage was lower than that of the one-step hydrogenolysis of the β-O-4 bond, which was also beneficial for the hydrodeoxygenation of the products. For MME, the total reaction energy barrier of the depolymerization starting with the hydrogenation of C=O was higher than that of starting with the cleavage of β-O-4, indicating that the cleavage of β-O-4 prior to the hydrodeoxygenation of C=O is the most reasonable reaction route, which also verifies the experimental results from MME depolymerization.