Applications of transition-metal catalysts to textile and wood-pulp bleaching

From an economic perspective, textile and paper bleaching are amongst the most important oxidation processes. The removal of unwanted chromophores, be it stains on cloths or residual lignin in wood pulp, consumes more than 60 % of the world production of hydrogen peroxide. However, existing technologies have their limitations. At ambient temperature, hydrogen peroxide gives little stain bleaching and is used inefficiently. Hence the high product dosages and washing temperatures required limit its application to predominantly European markets, to the exclusion of the majority of the world's population. In paper manufacture, the use of chlorine-based oxidants results in the formation of chlorinated waste products, which show poor biodegradability. On the other hand, hydrogen peroxide requires higher temperatures, longer reaction times and is more expensive. Transition-metal catalysts offer an alternative. This review discusses the main classes of known bleach catalysts and their possible modes of action.