Model experiments to study the inhibition mechanism of lignin self-condensation under tert-butyl alcohol and concentrated sulfuric acid conditions

The concentrated sulfuric acid/tert-butyl alcohol (TBA) hydrolysis system efficiently converts lignocellulose into monosaccharides and tert-butyl alcohol lignin (TBL), which exhibits thermal plasticity and a high solubility in common organic solvents. However, the reaction mechanism and the TBL chemical structure remain unclear. In this study, 2-methoxy-4-methylphenol (creosol, 4-methylguaiacol) was used as a model compound for the guaiacyl ring of lignin, and was treated with the concentrated sulfuric acid/TBA system to elucidate the chemical structure of TBL. At the beginning of this reaction, TBA rapidly reacted with 4-methylguaiacol to produce two main products. These products were fractionated using high-performance liquid chromatography and analyzed using nuclear magnetic resonance (NMR) spectroscopy to determine their chemical structures. The NMR spectra of the products revealed that the tert-butyl group was incorporated into either the ortho position of the phenolic OH group or the para position of the methoxyl group of 4-methylguaiacol. The former product was stable, whereas the latter gradually decomposed in the presence of sulfuric acid. This model experiment suggests that the consumption of anionic species on the lignin aromatic ring could be effective in preventing lignin self-condensation between the aromatic ring and the benzyl position of lignin under sulfuric acid treatment.