Evaluation of the contribution of lignin stilbene phenol units in the photoyellowing of peroxide-bleached lignin-rich pulps

In order to assess and quantify the contribution of stilbene phenols in photoreversion of bleached high-yield pulps, a method to reduce the stilbene double bond and quantify the formed diphenylethane was developed on 4-benzyloxy-3,3′-dimethoxy-4′-hydroxystilbene, as lignin monomer model, and also on a more sophisticated lignin polymer model constituted by a polystyrene framework containing 4,4′-dihydroxy-3,3′-dimethoxystilbene elements as pendent groups. The method used RhCl(PPh3)3 as soluble catalyst to get an efficient hydrogenation and AlCl3 in benzene to liberate the diphenylethane from the polymer framework. For the first time a semi-quantitative evaluation of the content of the stilbene phenols formed from β-1 units was given for high-yield pulps. The value of 2 p-stilbene phenols for 1000 C9 lignin units found indicates that they are present in very small quantities after the peroxide bleaching. UV irradiation of the polystyrene model adsorbed on solid cellulose matrix had revealed an efficient stabilization after the hydrogenation treatment. This was not the case for the peroxide bleached pulp showing that the p-stilbene phenols formed from β-1 units are not the main contributors of the rapid yellowing of bleached lignin-rich pulps