Acidic Peroxide Delignification of Softwood Kraft Pulps Catalyzed by Molybdenum- and Tungsten-Based Polyoxometalates, 2001 Pulping Conference Proceedings

B. N. Brogdon, D. G. Mancosky, L. A. Lucia--Acid peroxide delignification of softwood kraft pulps catalyzed by molybdenum- and tungsten-based polyoxometa- lates (POMs) resulted in pulps with reduced kappa numbers but little residual lignin removed. The lignin/kappa num- ber ratio increased from 0.15 to 0.18 for acid peroxide POM pulps (PPOM). It was speculated that oxidized structures in the PPOM lignin, such as quinones, consumed less permanganate than unbleached residual lignin. Based on the change in the lignin/kappa ratio, it was estimated that the PPOM residual lignin contained ~1.8 mmol quinones/g lignin. Methanol analysis of PPOM effluents indicated that the molybdenum-based POMs afforded ~0.8 mmol o-quinones/g lignin, while the tungsten-based POMs yielded ~0.2 mmol o-quinones/g lignin. It was observed that the lignin/kappa ratio for PPOM pulps decreases to 0.15 when treated with alkali, possibly indicating the reformation of aromatic structures from quinones. The molybdenum-based PPOM stages afforded pulps with a lower brightness than tungsten-based PPOM stages, whereas the former had slightly lower amounts of residual lignin than the latter. The differences in brightness are likely related to the level of o-quinones in the residual oxidized lignin.