Effect of Fe3+ ions on transformation of hydrolysis lignin in water under sub- and supercritical conditions

The transformation of hydrolysis lignin in water under sub- and supercritical conditions (320-800(degrees)C, 10.0 MPa) in the presence of iron(iii) ions was studied. At temperatures above 400(degrees)C, two processes are most significant: deep depolymerization of lignin to phenolic monomers and condensation of aromatic moieties of lignin into carbonaceous deposits. In the presence of Fe3+ ions, the content of monomeric phenols increases 3-4-fold, which is indicative of the catalytic effect of iron in lignin depolymerization. When the reaction temperature increases from 400 to 660(degrees)C, the content of monomeric products (mainly, phenol and guaiacol) increases from 1.8 to 7.5 wt.%. Under the same conditions, considerable gasification of lignin monomers and oligomers to a hydrogen-rich mixture (up to 38 vol.%) takes place. The yield of carbon-containing gaseous products is independent of the presence of Fe3+ ions, retaining a value of 27% in terms of carbon. Further temperature rise up to 800(degrees)C decreases the content of the monomeric products of depolymerization and increases the contribution of gaseous products. In this case, the condensation of phenolic moieties to give carbonaceous deposits becomes the predominant reaction. The interaction of the Fe3+ ion with the beta-O-4-dimer of coniferyl alcohol was studied by quantum chemical calculations. According to the calculation results, in the presence of iron, the electron density of the highest occupied molecular orbital of the dimer partially shifts towards the iron ion, which facilitates cleavage of the C-bond.