an acid and a complexing agent, is more effective than a simple acid wash for the removal of transition metals. Furthermore, such a treatment would remove some of the easily degradable hemicelluloses, thus facilitating the accessibility of reactants to the interior of the wood structure. The removal of some of the hemicelluloses would also decrease the alkali requirement in subsequent pulping operations as the amount of acid degradation products is reduced.
The objective of prehydrolysis in the pretreatment procedure of the present invention is not to remove all the hemicellulose as in the preparation of dissolving pulps. The prehydmlysis process for production of dissolving pulps, as extensively described scribed in pulping handbooks, emphasizes the importance of running the prehydrolysis at high temperatures on the order of 170xc2x0 C. anda final delignification stage located downstream of the oxygen delignification stage.
Another group of catalysts, which includes transition metals such as V, Mo,W and Ti can promote the heterolysis of the oxygen-oxygen bond in hydrogen peroxide and alkylperoxides, the latter components formed during oxygen delignification. Acidic metal oxides such as MoO3, WO3 and V2O5 catalyze the formation of peracids from hydrogen peroxide. In these peracids the conjugate base of the acid provides an excellent leaving group for nucleophilic displacement For example, the oxidation of iodide, a preferred carbohydrate protector component in the present invention, by hydrogen peroxide is catalyzed by molybdenum compounds through the intermediacy of permolybdic acid.
Although metal complexes with designed coordination spheres and ligands offer a very large potential to promote the desired reactions in the oxygen