2001 TAPPI Pulping Conference ACIDIC PEROXIDE DELIGNIFICATION OF SOFTWOOD KRAFT PULPS CATALYZED BY MOLYBDENUM- AND TUNGSTEN-BASED POLYOXOMETALATES Brian N. Brogdon Douglas G. Mancosky Lucian A. Lucia Senior Research Scientist Ph.D. Candidate Associate Professor Vinings Industries Inst. of Paper Sci. & Tech. (IPST) Inst. of Paper Sci. & Tech. (IPST) ABSTRACT 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 (P POM ). It was speculated that oxidized structures in the P POM 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 P POM residual lignin contained ~1.8 mmol quinones/g lignin. Methanol analysis of P POM 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 P POM pulps decreases to 0.15 when treated with alkali, possibly indicating the reformation of aromatic struc- tures from quinones. The molybdenum-based P POM stages afforded pulps with a lower brightness than tungsten-based P POM 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. INTRODUCTION New methods are currently being sought to delignify and bleach kraft pulps selectively while having minimum impact on the environment. Research has intensively focused on new bleaching technologies that will replace elemental chlo- rine, as well as possibly replacing chlorine dioxide. Various oxygen-based technologies have been explored as the likely candidates, however, most of these technologies have distinct limitations. Ozone is a very aggressive delignifi- cation agent, but is not sufficiently selective at reacting with lignin versus carbohydrates. Oxygen is an effective method of delignifying kraft pulps, but is typically limited to 40 to 60% to prevent excessive losses in pulp strength. Hydrogen peroxide by itself, in acidic or alkaline environments, has a very limited ability to delignify kraft pulps. Suchy and Argyropoulous [1] critically reviewed the scientific and patent literature on methods for increasing the re- activity of oxygen and peroxide towards lignin during bleaching delignification. These authors noted several studies for catalyzing acidic peroxide delignification using tungsten-or molybdenum-based compounds, which also included polyoxometalates (POMs). Polyoxometalates have been investigated by Weinstock and co-workers [2-6 ] as a delig- nification technology for the effluent-free mill. Limited research work has examined POMs as catalysts for peroxide bleaching. Jakr et al. [7] and DuPont [8 ] have examined 12-molybdosilicate ([SiMo 12 O 40 ] 4- ), and Evtuguin et al.