Polycondensation reaction and its mechanism during lignocellulosic liquefaction by an acid catalyst: a review
The increase in the residue content resulting from polycondensation would be adverse to the utilization of lignocellulose and to the quality of products obtained from liquefied lignocellulosic material. The yield of the residue formed from liquefaction and the mechanism of polycondensation were reported mainly by Lin, Yamada and Kobayashi. The major products of cellulosic liquefaction are levulinic acid and hydroxymethylfurfural (HMF) derivatives under polyhydric alcohols and phenolated compounds under phenols. The cleavage of the β-O-4 bonds is the major reaction pathway of lignin liquefaction under various liquefying reagents regardless of whether they contain acid catalysts or not. The break up compounds by decomposition are polymerized to substances with high molecular weight by polycondensation in lignocellulosic liquefaction. The molecular weight of condensed residues increases almost linearly as a function of liquefaction time at the later stage of lignocellulosic liquefaction. The longer the time required, the greater the content of new residue generated by polycondensation during the entire process of liquefaction. We conclude that the condensed residues may stem from the interaction of degraded lignin and cellulose components in wood or from the products of two major components reacting with liquefying reagents.