PHOTODEGRADATION OF LIGNIN - A PHOTOCHEMICAL STUDY OF PHENOLIC CONIFERYL ALCOHOL LIGNIN MODEL MOLECULES

A better understanding of the involvement of coniferyl alcohol structures in the yellowing of high-yield pulps exposed to daylight was approached by studying the photophysics and the photochemistry of coniferyl alcohol (1) and two phenolic lignin models (2 and 3) incorporating a 3-hydroxy-1-propenyl residue and a biphenyl structure. Their photochemical reactivity was examined in both liquid (CH2Cl2 and t-BuOMe) and solid media (adsorbed on fitter paper and incorporated in 2-hydroxyropylcellulose (HPC) films). The influence of oxygen during the irradiation was also investigated. In solution we observe efficient photolysis of the models 1, 2 and 3 yielding essentially the photoisomers and in large amount a complex mixture of oligomers which include coloured substances. The expected aldehydes 4. 5, 6 and 6a have not been detected. The structure and titration (GC-MS analysis) of the non-oligomeric photoproducts indicate that addition of water on the double bond (provided by the formation of a conjugated quinone methide) and elimination of formaldehyde are important processes of the photodegradative steps. Phenoxy radicals generated by direct photolysis of the phenols appear to be the main source of the oligomers. Thioacidolysis of the latter shows a large proportion of quinoid structure which might partly account for the colour of the irradiated solutions. Irradiation of the models adsorbed on filter paper shows rapid consumption of the starting materials and reticulation of the coloured photoproducts with the cellulosic framework. Irradiation of the models included in HPC indicates a loss of aromatic structure at 285 nm and a yellowing of the films. These results are reminiscent to those obtained in the photochemistry of 2-methoxyhydroquinone and confirm the role played by the phenolic groups which are photolysed when the aromatic pi system is able to absorb above 300 nm.