Hydrogen-Donor-Assisted Solvent Liquefaction of Lignin to Short-Chain Alkylphenols Using a Micro Reactor/Gas Chromatography System

The benefit of using hydrogen donor solvents in lignin solvolytic conversion was studied using a micro reactor coupled to online gas chromatography/mass spectrometry with a flame ionization detector. This system is able to achieve very high heating rates of reactants and capable of analyzing the reaction products immediately after the reaction is completed, thus allowing for probing semi time-resolved reactions occurring during lignin conversion. These features have been impossible for the batch reactors typically employed in previous solvent liquefaction studies. The study showed that the hydrogen donor solvents tetralin and isopropanol were effective in converting lignin into short-chain alkyl phenols (SCAP) at high yield. The yield of SCAP increased with an increasing reaction temperature over the range of 300400 degrees C and reaction time over the range of 515 min, which is attributed to stabilization of lignin-derived products by hydrogen abstraction from the hydrogen donor solvents. The molecular weight distributions of solvolysis products revealed that molecular weight significantly decreased with increasing reaction times in the presence of a hydrogen donor solvent. In contrast, in the absence of a hydrogen donor solvent, the molecular weight of products increased with increasing reaction times. Also, the peak assigned to oligomers increased and shifted to higher molecular weights at the same reaction conditions. Overall, the solvolytic conversion of lignin involves thermal cleavage of lignin macromolecules, followed by secondary reactions, including cracking and repolymerization, among the primary products. The presence of a hydrogen donor was found to suppress repolymerization reactions by stabilizing the primary products to alkyl-substituted phenols and promoting demethoxylation.