Activation of alkaline peroxide delignification using vanadium peroxo complex

HYDROGEN PEROXIDE HAS ALWAYS been a technologically attractive oxidant to the pulp and paper industry, with its significance becoming increasingly important as environmental regulations become more stringent. Beyond its ease of handling and application, hydrogen peroxide is also a versatile and benign compound whose decomposition products are water and oxygen. Among its major drawbacks are its high price, reduced selectivity, and its somewhat low efficiency compared to chlorine and chlorine dioxide. However, despite these drawbacks, recent accounts have shown that when using peroxide at high temperatures, the rate of reaction is significantly accelerated and the extent of lignin removal increases (1). The combination of a high-temperature peroxide state (PH T ) with an ozone stage allows oxygen-delignified kraft pulp to be bleached up to a brightness level of 90 ISO (2). High consistencies, high temperatures, and long retention times have also been claimed to assist delignification and brightness development during a P-stage (3, 4). Production limitations imposed by longer retention times induced research which demonstrated that using higher pressures during a peroxide stage can significantly reduce the time required to reach a desired brightness level (5). Molecular oxygen has a triplet ground state whose direct interaction with singlet-state organic molecules is a spin-forbidden transition. This fact limits the oxidative selectivity of molecular oxygen. Industrially, it would be extremely beneficial to fix molecular oxygen within organic or inorganic compounds capable of transferring it selectively to an organic substrate such as lignin. Peracids, such as Caro’s acid, peracetic acid, and performic acid, can be considered as organic molecules containing active oxygen. In fact, they have all been shown to be effective delignification and bleaching agents (6–9). An example of an inorganic molecule containing active oxygen is peroxymonosulfuric acid, which was shown to improve oxygen delignification of kraft pulps when used in two stages (10). For all of these molecules, however, one may again cite high prices and reduced selectivity. Dioxiranes also possess the ability to transfer a single activated oxygen atom onto aromatic and unsaturated substrates. As such, dimethyldioxirane (DMD) was tested as a novel and selective non-chlorine-containing bleaching agent for the production of fully bleached totally chlorine-free (TCF) pulp (11). Detailed studies of the reactions of DMD with softwood lignin model compounds showed that the compounds’ aromatic rings undergo ortho and para hydroxylation reactions, thus introducing new phenolic hydroxyl groups into the lignin structure (12). Another form of activated oxygen may also be present within peroxycarboximidic acid, which is thought to be an intermediate in the interaction of cyanamide with peroxide. For Activation of alkaline peroxide delignification using a vanadium peroxo complex