Comparative studies on the delignification of pine kraft-anthraquinone pulp with hydrogen peroxide by binucleus Mn(IV) complex catalysis

Pine kraft-anthraquinone (kraft-AQ) pulp was bleached in alkaline solution with hydrogen peroxide catalyzed by either [L1Mn(IV)(mu-O)(3)Mn(IV)L-1](PF6)(2)] (C1) or [LMn(IV)(2)(mu-O)(3)] (ClO4)(2) (C2) at 60 and 80 degreesC for 120 min with a catalyst charge of 10 ppm on pulp. The resulting bleached pulp was hydrolyzed with cellulase to obtain insoluble and soluble residual lignins. The alkaline bleaching effluents were acidified to precipitate alkaline-soluble lignins. These lignin preparations were then characterized by 2D heteronuclear multiple-quantum coherence (HMQC) NMR spectroscopic techniques. The results showed that biphenyl (5-5) and stilbene structures of the residual lignin in the pulp are preferentially degraded in both the C1- and C2-catalyzed bleachings, whereas beta-O-4, beta-5, and beta-beta structures undergo degradation to a lesser extent. In both cases, the degradation of the residual lignin increased with the increase in reaction temperature from 60 to 80 degreesC. Thus, the result of C1-catalyzed delignification is not in agreement with the observed decrease in the disappearance rate for substrates in the C1-catalyzed oxidation of lignin model compounds with hydrogen peroxide when the reaction temperature is increased from 60 to 80 degreesC. In addition, the resulting residual lignins in the C2-catalyzed bleaching at 80 degreesC are less degraded than the corresponding lignins in the C1-catalyzed bleaching at both 60 and 80 degreesC. Thus, C1 is more effective than C2 as catalyst in the binucleus Mn(IV) complex-catalyzed bleaching of pine kraft-AQ pulp with hydrogen peroxide.