Hydrogen-Donor Solvent Assisted Thermal Decomposition of Lignin

Lignin makes up ~30% of lignocellulosic biomass, one of the world’s most abundant renewable materials, and it is also separated in the wood pulping process while manufacturing paper. Phenols and benzenes can be produced from lignin by cracking, making it the source of valuable chemicals. When lignin decomposes into bio-oil, biochar and non-condensable gases, it generates free radicals that lead to secondary reactions and the formation of large molecular weight compounds. To address this problem, we hypothesized that a hydrogen-donor solvent assisted thermal decomposition of lignin at high temperature and pressure represents a possible pathway to avoid these reactive functional groups in bio-oil.

Experiments were made in batch reactors using corn stover organosolv lignin as raw material and 2-propanol as a solvent varying the reaction times. The molecular weight distribution of the bio-oil recovered was analyzed using GPC (Gel permeation chromatography) and a compositional analysis was made by GC/MS (Gas chromatography/Mass spectrometry). These results were compared with experiments conducted in a sealed glass capsule micro reactor coupled with GC/MS.

It was found that, during solvolysis, phenols with unsaturated C=C bonds on side chains react with hydrogen atoms from 2-propanol, and convert into short chain alkyl phenols. The yield of these compounds was increased with reaction time. These results were shown in the GC/MS analysis for both batch and micro reactors. The GPC results showed an increase in bio-oil compounds. Consequently, it is possible to say that thermal decomposition of lignin in hydrogen-donor solvent could produce bio-oil with a substantial amount of stable monomeric phenols.