The Improvement of the Bleaching of Peroxyformic Sugar Cane Bagasse Pulp by Photocatalysis and Photosensitization

Presented here is a new process in two stages for bleaching peroxyformic sugar cane bagasse pulp combining photosensitization and photocatalysis. The first stage consists of an irradiation (1h), with tungsten or mercury lamps, of the pulp under alkaline pH in aqueous suspension at a consistency of 5% with TiO2 and methylene blue (MB) or 3,4,9,10-perylenetetracarboxylic acid (PTCA) or Fe (II) 4,4’,4’’,4’’’-tetrasulfophthalocyanine (TSPC) (concentration <FONT FACE=Symbol>&raquo;</FONT> 10-5 mol L-1) in the presence of bubbling oxygen and hydrogen peroxide (2% pulp basis). The second photobleaching stage performed with hydrogen peroxide (3%) efficiently completed the delignification and the brightness gain, removing completely the sensitizer from the pulp. Under those conditions, efficient bleaching of the pulp was obtained, using uv/vis light and PTCA or MB (unbleached pulp: kappa number: 13, brightness: 44, viscosity: 900 dm³ kg-1; bleached pulp: kappa number: 1.2-1.3, brightness: 86, viscosity: 510-550 dm³ kg-1) or visible light with MB (kappa number: 1.7, brightness: 80, viscosity: 625 dm³ kg-1). The mechanisms of photobleaching to increase the selectivity and the efficiency of the process remain to be established

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