Degradation and recovery of fine chemicals through singlet oxygen treatment of lignin

Singlet oxygen mediated degradation of lignin was performed on lignin from steam exploded straw, organosolv lignin and alkali lignin. The kinetics of singlet oxygen degradation were obtained following the gel permeation chromatograms in the presence of an external standard. This procedure can not be used for alkali lignin considering its insolubility. In the case of steam exploded and organosolv lignins, the reactions follow zero order kinetics for the first 4–8 h; then, the reactions slow down and follow slower zero order kinetics. This behaviour can be explained assuming that easily degradable structures in the outer sphere of lignins are degraded in the first period, while the core lignin is degraded in the second one. The degradation in organosolv lignin is faster than in steam exploded lignin. Molecular weight distribution and the absence of guaiacyl units in steam exploded lignin can give rise to this behaviour. A similar behaviour was obtained performing singlet oxygen degradation on the pulp of steam exploded straw. Four-hour irradiation of steam exploded lignin from straw in the presence of singlet oxygen gave in low yield a lactone. A sample of lignin from pine was treated with singlet oxygen. Column chromatography of the residue showed the presence of six compounds: trans -sinapyl alcohol, 4-hydroxy-3,5-dimethoxybenzaldehyde, 4-hydroxy-3,5-dimethoxyphenylacetone, 4-hydroxy-3-methoxybenzaldehyde, cis -sinapyl alcohol, and sinapyl aldehyde. We obtained only compounds deriving from syringyl units in lignin in agreement with the hypothesis that guaiacyl units are more easily oxidable. The photochemical oxidation of lignin models in the presence of singlet oxygen was studied by using ab initio calculations. The treatment of the non-phenolic β- O -4 aryl ether derivatives (4-ethoxy-3-methoxy-2-(2-methoxyphenoxy)-acetophenone, 1-(4-ethoxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-3-hydroxy-1-propanone, 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)-3-hydroxy-1-propanol) gave products deriving from a formal β-C–O cleavage formation. The reaction occurred in low yields when a 2,6-dimethoxyphenol derivative is used. When the phenoxy part of the molecule showed a lower reactivity towards singlet oxygen (1-(4-hydroxy-3-methoxyphenyl)-2-(2,6-dimethoxyphenoxy)-3-hydroxy-1-propanol), the oxidation of the phenol moiety can occur. These results can be interpreted assuming a frontier orbitals control between the HOMO of the lignin model and the LUMO of singlet oxygen.