Photochemistry of Lignocellulosic Materials

Fifty years of research has shown that light-induced yellowing of mechanical and ultra-high yield pulps proceeds through a photooxidative discolouration of lignin in the fibre wall. At least four reaction pathways were identified: (1) direct absorption of uv light by conjugated phenolic groups to form the phenoxyl free-radical, (2) abstraction of phenolic hydroxyl hydrogen as a result of aromatic carbonyl triplet excitation to produce a ketyl and phenoxyl free-radical, (3) cleavage of non-phenolic phenacyl-α-O-arylethers to phenacyl-phenoxyl free radical pairs, and (4) abstraction of the benzylic hydrogen of the guaiacylglycerol-β-arylether group to form the ketyl free-radical which in turn undergoes cleavage of the β-O-4 aryl ether bond to produce an enol and phenoxy free radical. Alkoxyl and peroxyl free-radicals produced from the reaction of oxygen and lignin free-radicals react with the phenoxy free-radical formed to produce the quinonoid coloured chromophore.