Heteropolyanion (HPA) Catalyzed Ozone Delignification in Organic Solvents: Extremely Effective and Selective Pulp Bleaching Approach pp. 261-280
Authors: (Anatoly A. Shatalov, Forest Research Center, Technical University of Lisbon, Institute of Agronomy, Tapada da Ajuda, Lisbon, Portugal)
Abstract: The highly efficient and selective environmentally benign bleaching approach using polyoxometalate (POMs) catalyzed ozonation of chemical pulps in organic solvent reaction media has been developed. A number of low-boiling polar aprotic and protic organic solvents showed a well-defined capacity for ozonation improvement in the presence of -Keggin-type mixed-addenda heteropolyanions (HPAs), particularly molybdo-vanado-phosphate HPAs of series PMo(12-n)VnO40(3+n)-. Aqueous acetone solution was found to be the best suited reaction media for ozone delignification catalyzed by PMo7V5O408- (HPA-5) polyanion. The effect of solvent and catalyst concentration, pH and ionic strength on ozone bleaching of commercial eucalypt kraft pulp has been examined. The solvent proportion in solution and medium acidity were the principal factors affecting catalytic efficiency of HPA during ozonation. Under optimized conditions, the brightness improvement of bleached pulp by ca. 15% ISO, with additional lignin removal by ca. 39% and increase in intrinsic viscosity by ca. 3% was observed in comparison with conventional (solvent/catalyst free) ozonation. The elimination of the Donnan effect by increase in ionic strength (cation concentration) of pulp suspension substantially improved delignification capacity of HPA/O3 bleaching system. The catalyst recovery/re-oxidation by ozone has been examined using partially reduced polyanion species (HPA-5red or heteropoly blue). It was shown that the bulk portion (ca. 90%) of HPA-5red is readily re-oxidized by ozone during first reaction minutes thereby confirming the closure of redox catalyst cycle, as a required condition for successful HPA-catalyzed oxidative delignification.