Effects of phenolic hydroxyl functionality on lignin pyrolysis over zeolite catalyst

摘要

The phenolic hydroxyl group (Phe-OH), a representative functional group in the lignin macromolecule, was found to play an important role in lignin pyrolysis in the presence of zeolite catalyst. We prepared lignin samples with different Phe-OH content by chemoselectivity controlled stepwise methylation. These methylated lignins had almost the same structures as the original lignin except for their Phe-OH content. However, the instantaneous decomposition rate of methylated lignins by exposure to heat increased with increasing degree of methylation due to restriction of intermolecular hydrogen bonding of the Phe-OH. It suggested that Phe-OH content changed the thermal decomposition behavior of lignin. Upon pyrolysis in the absence of catalyst, methoxylated aromatics compounds such as p-methy-, p-ethyl-, and p-vinylanisole (evidence of methylation) were released from methylated lignins. Pyrolyzing methylated lignins with zeolite Y catalyst produced benzene, toluene, xylenes, and naphthalenes as main aromatic hydrocarbons, and their yields showed. A positive correlation with the degree of methylation. In addition, oxygenated functional groups in pyrolysis products were completely decomposed with zeolite Y catalyst. We obtained 39% enhanced aromatic hydrocarbons yield in totally methylated lignin compared with the original lignin. In this study, we propose plausible pathways for methylated lignin pyrolysis over zeolite Y on the basis of our observations.