Pulp bleaching by hydrogen peroxide activated with copper 2,2′-dipyridylamine and 4-aminopyridine complexes

Oxygen-prebleached kraft pulp (OKP) was bleached with H2O2 activated with a copper complex coordinated with 2,2′-dipyridylamine (dpa) or 4-aminopyridine (4-ap) under alkaline conditions. Bleaching of OKP by H2O2 activated with the Cu(II)-dpa complex decreased the kappa number (k) and viscosity (v) of the pulp by 26 and 0.7%, respectively. In contrast pulp bleaching without the coordinator resulted in a decrease in k and v values of 25 and 7.8%, respectively. Thus, selectivity for delignification, expressed by k/v, was increased 12-fold by the coordination with dpa. ESR demonstrated that the coordination with dpa suppressed the production of OH by 57%. These results support the involvement of a hydroperoxo complex of Cu(II) formed by the reaction of −OOH with [Cu(II)(dpa)(H2O)3]2+. When the OKP was bleached with H2O2 activated with a Cu(II)-4-ap complex, selectivity (k/v) in pulp bleaching increased by 2.6-fold and production of OH decreased by 47%. We conclude that Cu(II)-dpa is a catalyst potentially applicable to totally chlorine free (TCF) bleaching sequences due to its high selectivity for delignification.